Isostatic compaction of`
PTFE powders
Technical Service Note F14
2
Contents
INTRODUCTION
Page
8
SECTION 1. ISOSTATIC COMPACTION
AS A PROCESS
Comparison with other fabrication techniques
Uniformity of physical properties
Complexity of moulded shapes
Costs
Moulding tolerances
Cycle times
Summary of isostatic compaction as a process
Articles produced by isostatic compaction
Solid articles
Hollow articles
Encapsulation
In-situ lining
9
9
9
9
9
10
10
10
10
10
18
18
SECTION 2. PROCESS TECHNIQUES
The basic process
Multi-part tooling
Wet-bag and dry-bag compaction
19
19
21
23
SECTION 3. MOULD DESIGN
Shape of the preform
Accuracy of shape
Stability of compaction
Good reproducibility in shape
Surface finish of the preform
Direction of pressing
Considerations of detail
Flexible bag thickness
Bag fixing
Avoidance of damage to bags
Avoidance of damage to PTFE preform
Good compaction of PTFE
Mould configuration and dimensions
Mould configuration
Mould dimensions
24
24
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25
26
26
26
28
28
28
28
28
30
32
32
32
SECTION 3. MOULD DESIGN (cont)
Bag fixing/mould sealing
Wet-bag compaction
Dry-bag compaction
Mould materials
The rigid member
The flexible member
Improving the shape imparted by the bag
The production of sharp corners
Improvements in shape near the area
of bag attachment
Disposable mould parts
Special moulds - dry-bag compaction
Mould design procedure
Examples of moulds
9
34
34
36
36
36
39
39
39
39
41
41
41
44
SECTION 4. PTFE POWDERS
Choice of powder
Granular powders available
Unfilled powders
Filled powders
End properties
Page
48
48
48
48
48
48
SECTION 5. PROCESSING
The processing cycle
Mould assembly
Mould filling
Powder compaction
Mould disassembly and preform removal
Sintering
Speed of processing
Dimensional tolerances of mouldings
Testing procedures
Special processing techniques
Machining of PTFE preforms
Composite mouldings
The avoidance of bag buckling
Encapsulation with PTFE
In-situ lining
Rubber crumb technique
49
49
49
49
49
50
50
51
51
51
51
51
51
52
52
55
55
SECTION 6. PROCESS EQUIPMENT
Mould
Isostatic compaction unit
Pressure vessel
Means of fluid pressurisation
Means of controlling compression and
decompression of the fluid
Fluid reservoir
Oven
Services
Vibrator
Vacuum pump
Lifting equipment for heavy moulds
Sintering mandrels
Ancillary equipment
56
56
56
56
56
57
SECTION 7. PTFE HANDLING PRECAUTIONS
58
APPENDIX 1. TYPICAL PRODUCTION
OF A PTFE BOTTLE
59
57
57
57
57
57
57
57
57
APPENDIX 2. SIMPLE ISOSTATIC MOULDING
61
FLUON® TECHNICAL LITERATURE
FURTHER INFORMATION
62
62
3
4
List of photographs
Figure 1. Plain thin-walled tube
List of tables
Page
11
Table 1. Typical volume shrinkage during
Page
32
sintering, for Fluon® G307
Figure 2. Expansion joint/bellows
11
Figure 3a. Electrical insulator shed
12
Figure 3b. Complete electrical insulator
12
Figure 4a. Nose-cone and radome
13
Figure 4b. Laboratory ware
13
Figure 5. Complex shapes: pump Iiner, and a
high voltage switch component
14
Figure 6. Pipe fittings (T-piece and elbow)
15
Figure 7. T-piece and bottle (both threaded)
16
Figure 8. Pump liner, and gate valve liner
17
Figure 9. 10-inch diameter butterfly valve flap
18
Figure 10. Valve body and valve plug
18
Figure 11. Flanged T-piece PTFE-lined in-situ
18
Figure 16. Effect of bag buckling and rippling
25
Table 2. The effect of longitudinal compaction
33
restraint on the shrinkage during sintering
of tubes made from Fluon® G307
Table 3. Typical compaction ratios for
Fluon®
33
G307
Table 4. Typical properties of various
bag materials
39
Figure 37a. Wet-bag mould for a screw-top bottle 44
Figure 37b. Wet-bag mould for a threaded T-piece 45
Figure 37c. Wet-bag mould for a test piece
46
Figure 38. Dry-bag mould for a T-piece
47
*Fluon® is a trademark of the Asahi Glass Company
5
6
List of diagrams
Figure 12. Basic isostatic compaction process
Page
19
Figure 13. Two ways of moulding PTFE tube
20
Figure 14. Mould for the production of a beaker
using an inner flexible bag and outer rigid parts
21
Figure 15. Mould for the production of a beaker
using an outer flexible bag and inner rigid part
22
Figure 17. Preforms with undercuts
27
Figure 18. The choice of area of bag attachment
to give good preform shape
29
Figure 19. Limits on the size of holes and grooves 29
to avoid over-extension and failure of bags
Figure 30. The beaker mould of Figure 14
re-designed for dry-bag operation and
provided with a detachable mandrel
Page
38
Figure 31. Directly dependent and
interdependent seals for the mould shown
in Figure 30
38
Figure 32. The use of bag ridges to produce
sharp corners
40
Figure 33. The use of elastomeric materials to
improve preform shape in the area of bag fixing
40
Figure 34. The design of the Figure 14 beaker
mould modified to improve the shape in the area
of bag attachment, using a elastomeric ring
41
Figure 35. A preform having an internal
undercut, and of such thickness that it cannot
be produced by pressing from the inside
41
Figure 36. Pseudo dry-bag process
42
Figure 39. The application of a vacuum to
the PTFE powder in the production of a
thick-walled tube
50
Figure 40. Bag pre-tensioning for the
production of a thick-walled tube
52
Figure 41. Encapsulation with PTFE: importance
of free expansion of PTFE during sintering
53
Figure 42. Encapsulation of a metal bar
with PTFE
53
Figure 20. Avoidance of damage to PTFE
preform at changes of section
30
Figure 21. Possible cracking of preform by
bag rubbing during compression
30
Figure 22. Breaking of preform feature by bag
recovery during decompression
31
Figure 23. Moulds having inserts which will
deform during compaction
31
Figure 24. A mould design requiring
compaction into a deep groove
31
Figure 25. Limiting dimensions of grooves in
rigid mould parts to avoid preform thinning,
etc. for Fluon® G307, compaction ratio 2.7:l
32
Figure 26. The effect of clamping-bolt positions
on fluid sealing
35
Figure 43. Rubber crumb technique
54
60
Figure 27. The use of fluid pressure to
improve end-plug sealing
35
Figure 44. Full-scale sectional drawing of
experimental mould and flexible bag used
to produce a PTFE bottle having an
approximate weight of bottle 250g
Figure 28. Improved methods of bag clamping
and sealing for the beaker mould shown in
Figure 14
36
Figure 45a. Wet bag
61
Figure 45b. Dry bag
61
Figure 29. Split moulds
37
7
Introduction
Isostatic compaction is a method of pressing granular
powders, such as PTFE, which enables both simple
and complex preforms to be made close to the
required dimensions with a high degree of uniformity
in physical properties. These inherent characteristics
are attributable to powder compaction by means of a
pressurised fluid, the powder being separated from
the fluid by an impermeable
membrane sufficiently flexible to transmit the
pressure.
Alternative names for isostatic compaction are
'hydrostatic compaction' (limited to the use of liquids
as the pressurising medium) and 'rubber bag
pressing'. The flexible impermeable membrane is
normally called a 'flexible bag' or simply a 'bag'.
Isostatic compaction has been successfully applied to
PTFE and many other materials, having been used for
powder metallurgy and ceramics since 1913. Although
the optimum compacting pressures differ between
materials, the basic technique and equipment
design are similar, especially for materials of like
compaction ratios.
8
Section 1. Isostatic compaction as a process
Isostatic compaction can offer cost savings compared
with alternative methods of manufacture. As cycle
times are no greater, and may be less than those
obtainable with alternative methods, this process
should be considered in all areas of production for
which other processes have not been specifically
designed.
COMPARISON
TECHNIQUES
WITH
OTHER
FABRICATION
The following are the main points of comparison
between isostatic compaction and other methods of
forming PTFE.
Uniformity of physical properties
There is little pressure decay (i.e. pressure gradient
between the various parts of a preform during
pressing) in true isostatic compaction, because of the
absence of die wall friction. Therefore the process
gives a homogeneous, stress-free preform which
exhibits even shrinkage on sintering and produces a
physically uniform, distortion free component.
Furthermore, for a given compaction pressure, the
void content of the preform and its shrinkage on
sintering are lower and its density and physical
strength are higher than those obtained by uniaxial
compaction. Alternatively, for a given level of
properties, lower compaction pressures can be used.
Another practical advantage of isostatic compaction,
arising from the absence of pressure decay, is that a
great range of shapes and sizes can be made.
Although the use of rigid parts in moulds (see p.21)
prevents true isostatic compaction and introduces
some pressure decay effects, these effects are usually
small, so that uniformity of the preform remains
greater than with uniaxial compaction.
Complexity of moulded shapes
The application of pressure in all directions enables
complex shapes to be made whilst the use of a flexible
bag facilitates removal of the preform. Intricate details
such as threads, grooves and under-cuts can readily be
introduced. Sometimes isostatic compaction is the
only way in which an article can be produced, for
example where certain components are to be
encapsulated in PTFE.
Costs
Isostatic compaction can be the only method of
forming PTFE closely to certain required shapes - e.g.
beakers and bottles; in such instances it offers
considerable cost savings, both in material and labour,
compared with the alternative of machining parts from
the solid. Furthermore, isostatic moulds and
equipment are often cheaper than those required for
other methods of fabrication.
Moulding tolerances
It is not possible to have absolute control over the
shape of preforms which have been produced by a
flexible bag. Moreover, good definition is not always
obtainable because sharp corners cannot be produced
by the bag (see pages 24, 25), and the surface formed
against the bag will not be perfectly smooth. The bag,
being soft and deformable, will tend to follow the
contours of the PTFE granules during compaction. To
obtain a satisfactory finish and dimensional accuracy
the surface formed against the flexible bag may
require machining.
Straightforward symmetrical shapes, such as tubes,
can be made to a tolerance better than ± 2% of wall
thickness; more complex and asymmetric shapes can
be made to a wider tolerance.
9
Cycle times
ARTICLES PRODUCED BY ISOSTATIC COMPACTION
Cycle times equivalent to, or shorter than those of
other methods of compaction can generally be
achieved for comparable shapes. Cycle times as short
as 12 seconds are possible using multi-station
automatic isostatic compaction (see p.51). Cycle times
are increased with the complexity and size of
mouldings but in such instances isostatic compaction
may be the only possible method of manufacture.
Isostatic compaction is used for the production of a
wide range of articles made from PTFE as outlined
below.
Summary of isostatic compaction as a process
Solid articles
Low cost of tooling and uniform properties of
mouldings are the reasons for using isostatic
compaction for the production of solid shapes such as
rods and spheres.
To summarise, isostatic compaction is a process which
complements other methods of compaction, being of
benefit where one or more of its advantages are
significant. For example, the advantages of more
uniform properties and cheaper moulds make it
attractive for the manufacture, especially in small
quantities, of large items such as tubes greater than
about 300mm (12 inches) in diameter, rods or tubes
with high length/diameter ratios, thin-walled shells,
and composite mouldings. The possibilities of making
complex mouldings and of reducing material/labour
costs are attractive for the manufacture of many
components - e.g. articles with a closed end such as
beakers.
A wide range of rod sizes can be made, there being
little restriction on size or proportions, and moulds
being relatively inexpensive. Similarly, various sizes of
spheres can be produced.
Isostatic compaction is less suitable than uniaxial
compression moulding (manual or automatic) for the
production of relatively small, simple, symmetrical
shapes, and less suitable than extrusion for the large
quantity production of long rods or tubes.
(2) Simple open shapes:
Significant material and labour cost savings for items
such as:
(a) Flanged tubes
(b) Expansion joints/bellows - Figure 2 (p.11)
(c) Electrical insulators - Figure 3 (p.12)
Hollow articles
Hollow shapes can be produced isostatically with
significant cost savings, especially for mouldings with
closed ends.
(1) Plain tubes:
Small quantities can be made relatively
inexpensively - Figure 1.
(3) Closed-end shapes:
for example:
(a) Radomes and nose cones - Figure 4a (p.13)
(b) Laboratory ware - Figure 4b (p.13)
(c) Closed-end bellows
(4) Complex shapes:
for example:
(a) Pump liners, high-voltage switch components
- Figure 5 (p.14)
(b) Pipe fittings - Figure 6 (p.15)
(c) T-pieces, bottles - Figure 7 (p.16)
(d) Pump liners, gate valve liners - Figure 8 (p.17)
10
Figure 1. Plain thin-walled tube
Figure 2. Expansion joints/bellows
11
Figure 3a. Electrical insulator shed
Figure 3b. Complete electrical insulator
12
Figure 4a. Nose cone and
radome
Figure 4b. Laboratory ware
13
Figure 5. Complex shapes: Above, pump liner, 150mm long.
Below, high-voltage switch component
14
Figure 6. Pipe fittings (T-piece and elbow)
15
Figure 7. T-piece and bottle (see Appendix 1, p.59)
16
Figure 8. Above: Pump liner, Below: Gate valve liner
17
Encapsulation
In-situ lining
Isostatic compaction is often the only successful way
of encapsulating an article with PTFE (see p.52).
Typical examples are: (1) Magnetic stirrers,
thermocouples, (2) Butterfly valve flaps - Figure 9,
and (3) Valve plugs - Figure 10.
PTFE can be moulded in-situ inside an article by
isostatic compaction, thus forming a lining (see p.55).
A flanged T-piece which has been lined in this way is
illustrated in Figure 11.
Figure 10. Valve body and valve plug
Figure 9.
10-inch
diameter
butterfly
valve
flap
Figure 11. Flanged T-piece, PTFE lined in-situ
18
Section 2. Process techniques
The use of a single flexible bag in isostatic compaction
makes possible the production of a variety of solid
articles. However, of far greater significance is the
production of hollow articles for which two or more
mould parts must be used, not all of them necessarily
flexible. Cycle times can be shortened by using the
rigid part(s) of the mould to resist the pressurising
fluid, thus giving a dry or 'dry-bag' process (see p.23);
however, mould costs are then considerably
increased. These possibilities are now considered in
more detail.
THE BASIC PROCESS
In its simplest form the isostatic compaction of PTFE
consists of filling a flexible mould with granular PTFE,
closing and sealing the mould, inserting this in a fluid
contained by a pressure vessel, sealing the vessel and
pressurising the fluid; the process is completed by
dwelling at pressure, decompression, removal of the
mould from the vessel and removal of the preformed
PTFE from the mould. This is depicted in
Figure 12 below.
Figure 12. Basic isostatic compaction process
Top sealing plug, clamped in place
,,
,,
,,
,,
(a) Flexible bag mould,
filled and sealed
Clamp ring
Flexible bag
PTFE powder
End plug
,,
,,
,,
,,
O-ring seal
Pressure vessel
Pressurising fluid
,,
,,
,,
,,
(c) Mould after compaction,
showing PTFE preform
Dump valve
Pump
(b) Mould in pressure vessel ready for compaction
19
,,,
,,,,,,,,,,,,
,,,,,,,,,,,, ,,,
,,,
,,,,,,,,,,,,
,,,,,,,,,,,,
,,,,,,,,,,,,
Polished metal cylindrical tube
Seal
Section
This will have a smooth outer surface
(b) Inner flexible bag, outer rigid cylinder
,,,,,,,,,,,, ,,,
,,,
,,,,,,,,,,,,
,,,
,,,,,,,,,,,,
,,,,,,,,,,,,
,,,,,,,,,,,,
Polished metal rod/mandrel
This will have a smooth inner surface
(b) Outer flexible bag, inner rigid rod
Figure 13. Two ways of moulding PTFE tube
20
Section
The shape of the preform will be similar to that of the
mould used, and its dimensions will depend on the
compaction ratio of the PTFE. The sintering process is
the same for isostatically compacted preforms as for
those produced by other methods of compaction (see
p.50).
Two multi-part tooling systems for the production of a
beaker in the manner of (1) and (2) are shown in
Figure 14 (below) and Figure 15 (overleaf)
respectively; the flexible bag is supported in its relaxed
state on a mandrel or in a container/cage
(see p.26, ‘Good reproducibility in shape’).
MULTI-PART TOOLING
Although the introduction of a rigid mould surface
gives die wall friction effects and thus prevents true
isostatic compaction, the effect in most instances is
small, so that good uniformity of properties is still
achieved.
The basic process as described above gives solid PTFE
mouldings, such as rods and spheres; it is rather
limited in application, although quite complex forms
can be made. Where hollow shapes such as tubes and
beakers are required, multi-part tooling is usually
necessary. For example, a solid rod can be formed
using a cylindrical flexible bag with bungs for end
sealing, as indicated in Figure 12, whereas a tube
requires two main mould parts to form its inner and
outer surfaces; two different ways of forming a tube
are illustrated in Figure 13 on the opposite page:
(1) by an inner flexible bag and an outer rigid cylinder
with spacers for location and end sealing
(2) by an outer flexible bag and an inner rigid rod with
spacers for location and end sealing.
The presence of a rigid part improves the stability of
compaction giving quite good control of shape
(see p.25).
Beaker made by the type of mould
shown in Figure 14 below
A1
A
see page 35
CLAMPING FORCE
,,,,,,
,,,,,,
,,,,,,
,,,,,,
,,,,,,
,,,,,,
Top filling cover
and rigid
mould part
O-ring seals
Mandrel for bag support
and bottom cover
B
see page 35
High pressure
liquid supply
B
see page 35
Outer rigid
mould part
PTFE powder
Flexible bag
Ring for clamping bag
and forming lip
of beaker
CLAMPING FORCE
1
Fluid transmission
passages
Figure 14. Mould for the production of a beaker using an inner flexible bag and outer
rigid parts
21
Gap for filling
Top filling cover
O-ring seal
PTFE powder
Outer rigid
mould part
CLAMPING FORCE
,,,,,,
,,,,,,
,,,,,,
,,,,,,
,,,,,,
,,,,,,
High pressure
liquid supply
Spider to support
rigid mould part
and enable filling
Ring for clamping
bag and forming
tip of beaker
Inner rigid
mould part
Flexible bag
CLAMPING FORCE
Clamping ring
Spider
Container
(cover removed)
One of the three openings
formed by the spider for
PTFE powder filling
Cavity between the inner rigid mould
part and the flexible bag to receive
the PTFE powder
Figure 15. Mould for the production of a beaker using
an outer flexible bag and inner rigid part
22
WET-BAG DRY-BAG COMPACTION
Two techniques in isostatics have been developed, and
are known respectively as 'wet-bag' and 'dry-bag'
compaction. They are identical in principle but
different in operation.
The basic process (see p.19) of filling a mould which is
then sealed and immersed in a pressurising fluid is
known as the 'wet-bag' method because the operator
is visibly working with liquids and wet moulds.
The dry-bag compaction technique is a modification of
this basic process whereby the functions of mould and
pressure vessel are combined in one component and
the bag is fixed in position. The beaker moulds of
Figures 14 and 15 would be operated in a dry-bag
fashion by introducing pressurising fluid through a
high pressure hydraulic coupling (see Figure 30, p.38)
the whole mould being sufficiently strong to contain
the internally applied pressure. The use of the name
'dry-bag' is attributable to the apparent absence, to the
operator, of liquids and wet mouIds.
In dry-bag compaction the wet-bag process operations
of mould sealing/opening and pressure vessel
sealing/opening have been combined, whilst the
operations of mould immersion/removal and mould
drying have been eliminated. This gives a reduction in
the cycle time which is quite significant for simple
shapes where the overall cycle time is already short
(see p.50). Furthermore, because the process has
effectively been made dry there is no danger of
introducing contamination from the pressurising fluid.
Thus the dry-bag process is well suited to the large
quantity batch production of components, and lends
itself readily to automation.
However, because the mould must be suitably
designed and manufactured to act as a pressure
vessel, dry-bag moulds are much more expensive than
wet-bag moulds which do not need to support
significant differential pressures; furthermore, dry-bag
moulds are difficult or impossible to modify without
reducing their safe working pressure.
23
Section 3. Mould design
The use of flexible moulding components, together
with other variables, increases the tolerances which
must be provided for in the shape of the compacted
PTFE. These tolerances can be minimised by good
design but their presence must be accepted and a
compromise
between
mould,
process,
and
modification costs made.
preform is determined directly by that of the flexible
bag at compaction pressure and by that of the rigid
mould surfaces, if any.
It is suggested that a simple approach be taken
to new designs, refinements being introduced
on an empirical basis at a later stage.
(1) Shape of the mould system
(2) Finish of rigid mould surfaces
(3) Bag thickness, modulus of elasticity and original
state of stress
(4) Area of attachment of bag to rigid members
(5) Properties of PTFE being compacted
(6) Uniformity of mould filling
(7) Rate of pressurisation and compaction pressure
(8) Degree of stability of compaction
Although there are differences between moulds for
wet bag and dry-bag compaction, the basic principles
of mould design are similar and can be considered
together. Certain fundamental factors must be
considered throughout the design of moulds,
examples being:
Shape of the preform
Surface finish of the preform
Direction of pressing
Need to avoid damage to compacted powder
Need for uniform compaction
Ease of mould filling
Ease of preform removal
Choice of PTFE powder
Where it is not possible to satisfy all requirements
simultaneously, a compromise must be sought on the
basis of the relative importance of the various factors
for a particular moulding.
The dimensions of moulding surfaces can be
calculated from the required finished article
dimensions, using the properties of the PTFE powder
(compaction ratio and shrinkage) and an assumed bag
movement. Other dimensions are chosen on the basis
of general strength for wet-bag moulds and strength to
withstand the compaction pressure for dry-bag
moulds. Some of the design work relates to pressure
vessels and because of potential hazards it is
recommended that it be done by specialists in this
type of equipment.
Rigid members will, of course, impart a definite shape.
In contrast, the shape assumed by the flexible bag is
affected by factors such as:
A definite, reproducible shape would be achieved only
by keeping all the variable factors constant and
ensuring that the process of compaction was
inherently stable, ie bag and powder movements such
that a small increase in pressure tended to diminish
rather than increase any irregularities in shape which
might develop. In practice complete reproducibility of
shape is prevented by those variable factors which
cannot be kept entirely constant.
For example, the bags themselves may differ slightly
in thickness and elasticity, and will change with use;
and mould filling is subject to variation. The size of the
variations depends very much on the complexity of
the mould and the extent of the bag movements, or,
more accurately, the strains induced in the bag; thicker
sections of PTFE and higher compaction ratio powders
give greater variability.
A flexible bag is not inherently capable of producing
sharp corners on preforms, but if sharp corners are
required there is a technique for reproducing them
(see p.39). Where the bag is attached to rigid or solid
mould components there will be a reduction in the
effective flexibility which will affect the shape of the
preform (see Figure 13, p.20 for example).
However, some factors affect the basic configuration
of a mould and must be considered before any
detailed design can begin. The most important of
these are:
Although dimensional accuracy is not a feature of
preforms produced by flexible bags, good
approximations can be made, especially for simple
shapes.
Shape of the preform
Surface finish of the preform
Direction of pressing
Usually excess PTFE must be included to allow for the
likely deviations from the required shape and this may
be allowed to remain or may be removed from the
preform at a later stage. It may be possible to reduce
this excess by further development of moulds (see
p.39), but the cost involved must be balanced against
the estimated saving in PTFE.
SHAPE OF THE PREFORM
Accuracy of shape
The overall shape of an isostatically compacted PTFE
24
Stability of compaction
When a gradually increasing pressure is applied to a
flexible bag and powder system, two kinds of
behaviour are exhibited. At the lower pressures
powder mobility is high and the powder presents very
little resistance to bag movements which are, at this
stage, governed mainly by the geometry and elasticity
of the bag. At the higher pressure the partly compacted
powder presents a relatively high resistance to bag
movements which it therefore governs.
The lower the resistance to movement presented by
the bag (i.e. the thinner the bag and the lower its
modulus of elasticity) the greater the effect of powder
in controlling movements and determining the final
shape; this effect can be used to advantage if uniform
filling and low powder flow during compaction can be
achieved (see p.49). Isostatic compaction by the
expansion of a flexible bag, i.e. pressing outwards from
the inside as depicted in Figure 14 (p.21) is inherently
stable both for the bag and for the powder because the
powder particles move apart and phenomena such as
'bridging' cannot occur.
However, isostatic compaction by the contraction of a
flexible bag, i.e, pressing, inwards from the outside as
depicted in Figure 15 (p.22) is an unstable process for
the bag, which without the presence of powder would
exhibit 'buckling' and 'rippling' effects ('buckling' for
thicker bags and 'rippling' for thinner ones - see Figure
16). Stability is achieved only by the resistance of the
powder during compaction but as such resistance is
not generated until the bag has moved, the initial
stages of compaction are unstable.
These unstable conditions will be minimised if bag
movements are lessened by reducing the design
thickness of the PTFE preform and by using lower
compaction ratio powders. For thin-walled sections of
PTFE, the direction of pressing makes very little
difference to stability, and therefore to preform shape.
Significant differences are not observed until thicker
sections are moulded and/or high compaction ratio
powders are used.
In an all-flexible system bag movement and therefore
preform shape depends very much on the
characteristics of the bag, so that any irregularity, such
as in bag elasticity, will be reproduced. Although fairly
good control on shape can be obtained when pressing
solid sections very little control can be exercised when
using two flexible bags to press hollow sections.
It is recommended that wherever possible a flexible
bag be used in conjunction with a rigid member, the
bag preferably pressing outwards from the inside.
Figure 16. Effect of bag buckling and rippling
25
Good reproducibility in shape
DIRECTION OF PRESSING
Good reproducibility in shape can be achieved by
using as stable a system of compaction as possible
and by minimising the variable factors such as the
properties of the bag, the mould filling and the
compaction cycles.
Solid preforms must be made by pressing inwards
from the outside. However, when producing hollow
preforms, there is a choice in the direction of pressing.
In terms of stability it is preferable to press outwards
from the inside but many other factors affect the
choice and may be sufficiently important to override
stability considerations.
One of the principal factors affecting the preform
shape is the initial shape of the bag, which governs the
distribution of the PTFE on filling. Variations in bag
shape can quite easily occur when the bag is
unsupported, especially where the bag is not
sufficiently rigid to support itself. Thus, it is very
desirable to support the bag, either on a mandrel when
pressing from the inside (see Figure 14 p.21) or in a
container/cage when pressing from the outside (see
Figure 15 p.22). Passages must be provided in the
mandrel or container for the transmission of the
pressurising fluid to the surface of the bag. The
number and size of these passages must be chosen to
give adequate fluid flow at low fluid speeds, otherwise
local deformation of the bag will occur, causing
powder flow and poor preform shape in the region of
each passage. The passages should be greater than 2
mm (5/64 inch) diameter to minimise the possibility of
blocking.
However, merely to provide support for a bag will not
eliminate the permanent variations in bag shape (i.e.
permanent strain) which can occur as a result of
stretching during use; the bag simply becomes loose
on the mandrel or wrinkles in the container/cage. This
problem can be alleviated by holding the bag against
the mandrel or container by means of a vacuum
applied via the fluid passages whilst filling the mould.
(see Figures 14 and 15, p.21 and 22 respectively). If
this is to be normal procedure the application of a
vacuum can be facilitated by making the bag slightly
oversize or undersize, according to direction of
pressing, so that in the relaxed state there is a small
gap between the bag and its support. By these means,
provided the thickness of bags is uniform, good
control on the shape of the mould cavity during filling
can be achieved.
SURFACE FINISH OF THE PREFORM
Two kinds of surface must be considered, namely that
formed against the rigid members and that formed by
the flexible bag. The preform surface finish imparted
by rigid members is largely controlled by the forming
surface, the best finish being obtained with a polished
surface (see p.36). Given a polished rigid forming
surface, the preform surface finish will be dependent
on the particle size of the PTFE powder, small particles
tending to give the smoothest surfaces.
The preform surface finish given by the flexible bag
depends mainly on the hardness of the bag, which
always tends to follow the surface contours of the
PTFE particles; harder bags will give smoother
surfaces.
26
For example, the direction of pressing determines the
position of the relatively poorly defined surface, i.e.
that formed against the bag. If the component is to be
made to close tolerances all over, this surface would
require machining and the ease of such an operation
would probably depend on its location. With long,
small diameter tubes, external machining is far easier
than internal machining whilst for the T-piece of Figure
7 (p.16) the converse applies. Alternatively, machining
may be unnecessary in which case the location of
surfaces, and thus the direction of pressing, might be
chosen on the basis of appearance or function. For
example, when moulding a beaker, considerations of
appearance suggest that the poorly defined surface
should be on the inside, as in Figure 14 (p.21); but if
function were considered there would be a strong case
for having the better surface finish on the inside,
giving ease of cleaning. There are some instances
where a coarse surface might be necessary, e.g. if
another material is to be bonded or set onto the PTFE.
Certain processing factors must also be considered.
The ease of preform removal is important. Undercuts,
convolutions and similar features can easily be
generated by the bag which, being flexible, facilitates
removal of the preform. On the other hand a rigid part,
if on the outside, must be split into two portions, and
if on the inside, split into many portions or made to be
disposable, (see Figure 17), thus increasing cycle
times and mould costs. Such considerations are very
important for dry bag compaction.
Similarly, ease of filling must be considered, since this
is dependent on the mould configuration; for example,
the beaker mould of Figure 14 (p.21) which is pressed
from the inside, is easier to fill than that of Figure 15
(p.22) which is pressed from the outside.
Finally, it is not always possible to press in a particular
direction. For example, there is a lower limit on the
internal dimensions which can be achieved by
pressing outwards from the inside; this is imposed by:
(1) The maximum allowable tensile strain in the
flexible bag to avoid permanent deformation or
rupture.
(2) The compaction ratio of the PTFE powder.
(3) The PTFE section thickness to be produced, the
limitation being more severe for thicker sections.
,,
,,
,,
,,
,,
,,
,,
,,
,,,,,,,
,,,,,
,,,,,,,
,,,,,
,, ,,
,,
,,
,,
,, ,,
,,
,,,,
(i) (a) Preform with undercut
on outside
,,
,,
,,
,,
,,,,
,,,,,
,,,,,
,,,,,
,,,,,
PTFE
powder
Flexible
bag
Rigid
part
(b) Mould with undercut formed
directly by bag
(i) (d) Preform with undercut
on inside
(c) Mould with undercut formed
directly by rigid part which
must be split to permit
preform removal
PTFE
powder
Flexible
bag
Rigid
part
(f) Mould with undercut formed
directly by rigid part which
must be ’disposable’ to
permit removal
(e) Mould with undercut formed
directly by bag
Figure 17. Preforms with undercuts
(C - 1)
= kpD
minimum possible preform internal diameter
external diameter of tube
maximum allowable bag strain
compaction ratio of PTFE used
practical limiting factor depending on bag
material and PTFE used
∋
Even with an ideal bag (ie infinite possible strain)
there is a limit on the internal diameter given by:
dm = D
(C - 1)
C
1
2
= kiD
where the symbols have the same significance as
above and kj = ideal limiting factor depending on
PTFE used.
This is illustrated in the following three examples
based or the same O.D. of 100 mm.
Ki =
2.7 - 1
1
2
=
2.7
1
1.7
2
= 0.79
2.7
If a filled natural rubber bag with a limiting strain, m
of 7 (700%) is used,
Kp =
1
64 x 1.7
2
=
64 x 2.7 - 1
1
109
2
= 0.80
172
whereas, if a bag of another material, having a
limiting strain m of 1 is used,
Kp =
4 x 1.7
1
2
=
4 x 2.7 - 1
1
6.8
2
= 0.83
9.8
Thus for a 100 mm OD tube, D = 100 and dm = 79 mm
for an ideal bag, 80mm for a bag with m of 7 and
83 mm for a bag with m of 1, giving maximum
possible wall thicknesses of 10.5 mm, 10 mm and
8.5 mm respectively. It can be seen that the use of
very large bag extensions will not greatly increase the
maximum possible wall thickness.
∋
where
dm =
D
=
m =
C
=
kp
=
2
2
∋
=D
1
∋
dm
∋
(1 + ∋ m)
(1 + m) 2 (C - 1)
When pressing Fluon® G307, having a compaction
ratio C of 2.7 : 1,
∋
For a tube the minimum internal diameter that can be
produced is given by:
27
In the case of a 10 mm OD tube, the maximum possible
wall thickness is only about 1 mm; thus a significant
limitation is presented on wall thicknesses for smaller
sizes when pressing is from the inside.
When pressing from the outside the external
dimensions which can be produced are limited by:
(1) The overall size of the mould (either by chamber size
for wet-bag compaction or cost for dry-bag
compaction). (2) The PTFE section thickness that it is
required to produce, the limitation being more severe
for thicker sections.
For a tube the maximum external diameter that can be
produced is, given by:
Where
Dm = maximum possible preform external diameter
Dm =
Di
d
C
1
C
D i 2 + (C - 1) d 2
1
2
= maximum internal diameter of bag
= internal diameter of preform
= compaction ratio of PTFE used
This is illustrated in the following example: It is desired
to make a 100 mm OD tube of wall thickness 25 mm
from Fluon® G307 (compaction ratio 2.7 : 1) in a 200 mm
diameter wet bag pressure vessel. Is this possible?
Pressing from the inside will not produce a tube of this
wall thickness (see previous example). Therefore
pressing from the outside is the only possible method.
Allowance must be made for a 'cage', if used, and the
thickness of the bag. A 5 mm thick bag in a 2 mm thick
cage, with a diametral allowance of 6 mm between the
cage and the pressure vessel, gives:
Thus, the desired 100 mm tube could be made, one as
large as 117 mm being possible if required, assuming
D i = 200 -
6 + 2 x (5 + 2)
= 180mm
d = 100 - 2 x 25 = 50mm
C = 2.7
Dm =
1
2.7
= 10
180
2
+ (2.7 - 1) x 50 2
1
Generally a bag with a thickness equal to about 4% of
its dimension (e.g. diameter, for a tube) is
recommended. Whilst a thicker bag is more rugged,
giving longer life and often being self-supporting, it has
a greater effect on the final shape, cannot give as good
definition and higher compaction pressures must be
used. Furthermore, its recovery on decompression and
its subsequent removal are more likely to damage the
preform, especially if intricate details have been
incorporated (see p.50). Sometimes greater uniformity
of thickness and elasticity can be achieved in a thick
bag, thus improving preform shape; but this is not
always so. A thicker bag can, however, improve
stability of shape when pressing from the outside,
being less prone to buckling (see below).
Bag fixing
The bag must be attached to a rigid part of the mould,
both in wet-bag and dry-bag compaction, in order to
obtain reproducible positioning and a seal between the
liquid and the powder. The restraint which this
attachment exercises on the bag's movement, i.e. the
discontinuity in bag flexibility, must be considered in
the design of moulds. A typical effect of the
discontinuity is shown in Figure 13 (p.20). Almost
certainly it will be impossible to obtain the desired
preform shape in this region so that, ease of filling and
other factors apart, the area of attachment should if
possible be placed where accuracy of shape is least
important.
Figure 18 shows how the area of bag attachment for a
beaker mould (Figure 14 p.21) can be chosen to give
good shape, whilst facilitating post-machining and
minimising excess PTFE.
Avoidance of damage to bags
Rigid mould surfaces which the bag contacts and may
move against should have a fine-machined finish better
than 0.8 µm (32µ inch) Ra to minimise bag abrasion
and a corners should be rounded to at least 1mm (0.04
inch) radius to avoid bag tearing. In addition, mould
features such as deep holes and grooves, which during
powder compaction could cause over-extension and
failure of the bag, should be avoided whenever
possible, the limitations thus imposed are indicated in
Figure 19.
2
Avoidance of damage to PTFE preform
367
1
2
= 117mm
2.7
that bag buckling problems are not encountered (see
p.52).
CONSIDERATIONS OF DETAIL
A few details must be considered before finally
deciding the mould configuration and dimensions.
28
Flexible bag thickness
Powder shear during compaction should be minimised
because it will often cause cracked preforms which in
turn give cracked parts on sintering. Therefore the
combination of sharp edges and powder flow along the
rigid mould surface, such as may occur at changes of
PTFE section, should be avoided; if sharp edges are
essential, powder flow must be minimised by
modifying the bag shape and therefore the preform
shape. Figure 20 (p.30) illustrates this.
,,
,,
,,
PTFE
powder
Clamp
ring
,,
,,
,,
,,
,,
,,
Thinning
Poor shape and possibly
poor compaction
Part-section of resulting preform
Flexible bag
Mandrel for bag support
,,
,,
,,
Part-section of beaker mould with poor design for bag attachment area
,,
,
,,
,,
,,
Small amount of
PTFE to be removed
by matching
Flexible bag
Bag clamp ring
Mandrel for bag support
Part-section of resulting preform
Part-section of beaker mould (as in Figure 14) giving preform of better shape
Figure 18. The choice of area of bag attachment to give good preform shape
Having been compacted, the PTFE may be broken
by the mould itself in one of two ways:
,,,
,,,
,,,
,,,
,,,
t
PTFE
powder
R
m
m
> 1 .2t
0
R
> 2t
Rigid
part
t = wall thickness of
preform in the area
either side of the
hole/groove
Limits for ’Fluon’ G307
— compaction ratio 2.7:1
Flexible bag
< 1.8t
Figure 19. Limits on the size of holes and grooves
to avoid over-extension and failure of bags
(1) The bag may, on recovery during
decompression, rub along the surface of the
preform and shear it as indicated in Figure 21. Or
the bag may, on recovery, take a route different
from that of compression, possibly breaking the
features that it has just imparted to the PTFE, as
illustrated in Figure 22. This problem can be very
significant when using thicker, more rigid bags.
Furthermore, there is a greater possibility of
damage to preforms when removing such bags.
Therefore for solid sections and for intricate
preforms it may be necessary to use thin bags with
good flexibility.
(2) It is possible for a nett force to act on rigid inserts,
such as core pins, during compression. If such
inserts deform significantly, on subsequent recovery
they will either break the compacted PTFE or make
its removal very difficult; examples of this are
depicted in Figure 23.
29
Good compaction of PTFE
It has already been mentioned that as soon as rigid
parts are introduced into isostatic moulds uniformity
of properties is reduced, although a high degree of
uniformity is still achieved with good mould designs.
,,,
,,,
,,,
,,,
,,,
,,,
,,,
Sharp edge
PTFE
powder
Designs
requiring
compaction
into
deep
holes/grooves, as illustrated in Figure 24 are very
likely to give die wall friction effects and should thus
be avoided if possible.
,,
,,
,,
,,
,,
,,
,,
t
Likely powder
flow during
compaction
(a) Mould with change of section
and sharp edge
Sharp edge
PTFE
powder
(d) Improved bag shape if
sharp edges are essential
,,,
,,,
,,,
,,,
,,,,
,,,,
,,,,
R > 1 mm
R
0.2t
Shear
crack
likely
(b) Resulting preform
PTFE
powder
(c) Improved mould with
rounded edges
R > 1 mm
R
0.2t
PTFE
to be
removed
PTFE
powder
(f) ideal mould and bag
(e) Resulting preform
Figure 20. Avoidance of damage to PTFE preform at changes of section
End plug
PTFE powder
Flexible bag
,,,
,,,
,,,
,,,
,,,
,,,
(a) Mould filled prior
to completion
,,,
,,,
,,,
,,,
,,,
Preform
(b) Mould at compaction
pressure
,,,
,,,
,,,
,,,
,,,
(c) Mould recovery
on decompression
Figure 21. Possible cracking of preform by bag rubbing during decompression
30
Longitudinal bag
movement during
decompression bag rubbing over the
preform surface
Possible cracks
PTFE
powder
Rigid
part
,,,,
,,,,
,,,,
,,,,
,,,,
,,,,
,,,, ,,,, ,,,,
Shear
crack
Preform
Original bag
position
(a) Mould filled prior
to completion
(b) Mould at compaction
pressure indicating
change in bag position
(c) Possible shear crack or
break in preform caused
by bag recovery
Figure 22. Breaking of preform feature by bag recovery during decompression
Corepin
O-ring
seal
,,,,,
,,,,,
,,,,,
,,,,,
Compaction
pressure
Poorly fitting,
weak,
hemispherical
insert
Exaggerated deformation of insert
at compaction pressure
Exaggerated deformation of core-pin
at compaction pressure
,,,,
,,,,
,,,,
,,,,
PTFE
powder
Flexible Bag
Rigid
part
(a) Mould with core-pin deformation
— removal of core-pin after
decompression will be difficult
(b) Mould with poorly insert
— deformation will make
preform removal difficult
Figure 23. Moulds having inserts which will deform during
compaction
Flexible bag
Rigid part
Deep groove
PTFE powder
,,
,,
,,
,,
,,
(a) Mould
,,
,,
,,
,,
,,
,,
,,
,,
,,
,,
Figure 24. A mould design requiring compaction into a deep
groove
,,
,,
,,
,,
,,
Thinning
Poor compaction
is likely here as
a result of die
wall friction
(b) Preform
31
,,,
,,,
,,,
,,,
,,,
,,,
,,,
,,,
,,,
,,,
t
t
PTFE
powder
Rigid
part
> 5t
Rigid
part
Flexible
bag
t = wall thickness of preform in
area either side of groove
PTFE powder
> (2 x 2.7t) + (2 x bag thickness)
Flexible bag
(b) Deep groove - bag must follow contour to
avoid thinning and die wall friction effects
(a) Shallow groove
Figure 25. Limiting dimensions of grooves in rigid mould parts to avoid preform thinning, etc. for Fluon®
G307, compaction ratio 2.7 : 1
Such designs, apart from giving poor compaction, are
unsatisfactory because they restrict powder flow,
giving poor shape and, possibly, shear cracking as
indicated in Figures 18 and 20 (p.29 and 30)
respectively. These effects can usually be avoided if
the bag contour follows that of the rigid part as
recommended in Figures 18 and 20 (p.29 and 30) and
Figure 25.
Mould dimensions
The mould dimensions can be divided into two kinds,
those which directly affect the preform shape and
those which do not.
(1) Dimensions which directly affect the preform
shape: These must be considered separately for the
rigid parts and for the flexible parts.
MOULD CONFIGURATION AND DIMENSIONS
Mould configuration
The configuration of the mould, i.e. the shape, the area
of bag attachment, the area of filling etc. must be
decided on the basis of the considerations outlined on
p.24 (Section 3. Mould design) and on p.28
(Considerations of detail). Usually a compromise will
be necessary and this will be determined by
considering the relative importance of each
requirement.
(a) Dimensions of rigid parts:
The dimensions of rigid parts are dictated directly by
the required dimensions of the finished PTFE
component, allowance being made for shrinkage
during sintering and for final machining if necessary.
For true isostatic moulding the shrinkage during
sintering is uniform and similar in all directions. Thus
the cube root of the volumetric shrinkage can in
principle be used in calculations of linear dimensions;
typical values for Fluon® G307 are given in Table 1.
Table 1. Typical volume shrinkage during sintering, for Fluon® G307
Measured on tubes 245 mm (10 inches) long, 37 mm (11/2 inch) OD, 26 mm (11/16 inch) ID,
pressed from the outside onto a rigid mandrel.
Volume shrinkage
%
100
200
1425
2850
10.1
2.9
2.2
1.4
300
4275
1.8
1.2
400
5700
- 1.8
- 1.2
500
7125
- 1.4
- 1.2
(Negative quantities indicate expansion)
32
√ Volume shrinkage
%
Preforming pressure
Ibf/in2
kgf/cm2
3
Table 2. The effect of longitudinal compaction restraint on the shrinkage
during sintering of tubes made from Fluon® G307
Measured on tubes 245 mm (l0 inches) long, 37 mm (11/2 inch) OD, 26 mm (11/16 inch) ID,
pressed from the outside onto a rigid mandrel.
Preforming pressure
Ibf/in2
kgf/cm2
100
200
300
400
500
1425
2850
4275
5700
7125
Measured shrinkage %
Length
Wall thickness
5.7
3.6
3.7
2.8
3.0
Diameter
Outside
0.3
- 1.9
- 3.7
- 3.2
- 6.2
2.9
1.7
0.6
0.2
- 0.2
Inside
4.1
3.0
2.6
1.6
2.0
(Negative quantities indicate expansion)
Although the presence of a rigid part prevents true
isostatic moulding, non-uniformity of shrinkage is slight
for well-designed moulds. However, in practice,
movement of powder in certain directions may be
restrained, as in the moulds of Figure 13 (p.20) - axial
restraint - and Figure 14 (p.21), where movements along
the mould surface are negligible, in which case
shrinkage will depend on direction, as indicated in
Table 2, and on preform geometry. Comparison of
Tables 1 and 2 shows that considerable anisotropy
exists in the shrinkage of simple isostatically-moulded
tubes when pressing takes place only in the radial
direction. Such anisotropy of course also occurs in the
fabrication of PTFE by normal compression moulding,
with the shrinkage again substantially greater at right
angles to the pressing direction than in that direction.
Shrinkage is affected not only by the powder and mould
restraints - it also depends on the preforming pressure,
the efficiency of transmission of this pressure (i.e. the
dwell time, and the part thickness), and on sintering
conditions (mainly the rate of cooling from sintering
temperature). Therefore shrinkages for use in mould
design should be obtained experimentally by
isostatically compacting simple shapes and sintering
them under the same conditions as will be used in
production.
(a) Dimensions of rigid parts:
The dimensions of flexible parts are dictated by the
shape and dimensions of the relevant surfaces of the
component which is to be produced. The initial bag
shape which would be required to produce these
surfaces must first be estimated but because of the
many variable factors involved bag movements cannot
be easily predicted. The initial shape must therefore be
decided on the basis of experience, or simply guessed,
consideration being given to factors such as bag
restraints (e.g. areas of fixing), and the desirability of
minimising powder flow in all directions other than
perpendicular to the bag (see p.24, Shape of the
preform - Accuracy of shape, and p.28, Avoidance of
damage to the PTFE). For thin-walled sections of PTFE
where bag movements will be minimal it is reasonable
to assume that no powder flow will occur along the
mould/bag surface, and to design the bag to follow the
contours of the rigid part. However, such assumptions
are less valid for thick-walled sections, and experiment
will probably be necessary to produce the desired
shape.
When the bag shape has been decided its dimensions
can be calculated from the effective compaction ratio
and shrinkage of the powder to be used; typical values
of the compaction ratio for Fluon® G307 are given in
Table 3.
Table 3. Typical compaction ratios for Fluon®
G307
For tubes as in Tables 1 and 2
Preforming pressure
kgf/cm2
Ibf/in2
Volumetric compaction
ratio
100
200
300
400
500
2.40
2.60
2.65
2.70
2.70
1425
2850
4275
5700
7125
The compaction ratio depends on moulding conditions
such as the initial state of the powder (loose,
disagglomerated, vibrated or compacted), the
compaction pressure and the extent of pressure decay
(negligible for well-designed moulds). Therefore, as for
shrinkage, values of compaction ratio for mould design
should be obtained experimentally, by isostatically
compacting simple shapes and sintering them under
the same conditions as will be used in production.
For thin-walled sections where it is reasonable to
assume that all compaction occurs in a direction
perpendicular to the surface of the mould, the
volumetric compaction ratio of the powder gives a
direct measure of the ratio thicknesses of uncompacted
and compacted powder.
For thick-walled sections with unrestrained movement
calculations of dimensions should be made on an
overall volume basis (i.e. the ratio of the initial volume
in the mould to the volume of the preform), assuming
equal compaction in all directions, i.e. true isostatic
moulding. If movement in one direction is restrained, as
when using a mould with fixed ends to form a rod (see
Figure 13, p.20) the volumetric compaction ratio of the
powder gives a direct measure of the ratio of areas in
the unrestrained plane.
33
Allowance can be made for shrinkage in the same way
as when calculating the dimensions of the rigid mould
parts (see above). Uniformity of shrinkage can be
assumed unless thin-and thick-walled sections are
present together, or unless compaction of the powder
was restrained in one direction.
Because of the emphasis on speed, closure is usually
by means of a plain end plug, the whole mould being
placed between the platens of a hydraulic press, or in
a robust yoke or 'C' frame which holds the end plug in
place against the force exerted by the pressurised fluid
during isostatic compaction.
For both the rigid and flexible parts, because
compaction ratio and shrinkage depend on
mould restraints and preform geometry, precise
values for a particular component cannot be predetermined; moulds must be designed on the
basis of values obtained for simple symmetrical
shapes, and subsequently modified to achieve
precise dimensions where this is necessary. If the
rigid part is made slightly undersize when pressing
from the inside, and slightly oversize when pressing
from the outside, final adjustments to dimensions can
usually be made by further machining of this part
instead of making a new one. A similar course can be
followed when dipped bags are used, final
modifications being made by further machining of the
dipping tool, if necessary.
After its closure the vessel must be supplied with fluid
under high pressure and the incorporation of an
appropriate fitting must be considered in the design.
For complex components it is recommended that a
combination of design and experiment be used,
simple moulds being made initially, and the degree of
complexity then being increased, if necessary, as
experience is gained.
BAG FIXING / MOULD SEALING
(2) Dimensions which do not directly affect the
preform shape:
These dimensions, which include wall thickness, must
be calculated on the basis of strength and durability
according to the materials of construction and subject
to possible limitations on factors such as size and
weight. The detailed design depends very much on
whether wet-bag or dry-bag compaction is being
considered.
(a) Wet-bag compaction:
In wet-bag compaction most of the rigid mould parts
experience no significant nett force. In general the part
need only be strong enough to form a rigid unit
capable of maintaining a seal between the fluid and
the powder (see next column). However, if the mould
is so designed that some parts will experience a nett
force - for example if movements in any direction are
restrained or if rigid mating parts are a loose fit (see
p.28, Avoidance of damage to PTFE preform), mould
breakage or permanent deformation is likely to occur,
unless the relevant parts are of adequate strength .
(b) Dry-bag compaction:
In dry-bag compaction the rigid mould is required to
act as the pressure vessel for the fluid and therefore
must be able to withstand safely the intended working
pressures. Thus, the vessel must be carefully designed
by stress analysis, in a suitable material such as a high
grade, quality controlled steel, for the intended
working pressures and temperatures; features such as
sharp edges, fatigue resulting from pressure cycling
and the need for corrosion resistance must
be considered.
34
It is strongly recommended that safety aspects of the
design should be handled by experts. The mould
should be tested to at least 50% above the required
working pressure, and it must be remembered that
such tests may be invalidated by subsequent
modifications which may well lower the safe working
pressure. Any moulds that have undergone a large
number of cycles (~ 1000) should be proof-tested by
ultrasonics or magnaflux (crack detection with iron
filings and magnetic field) in order to obtain early
warning of any failure mechanisms, such as cracks,
which may have developed.
Wet-bag compaction
By its very nature, in isostatic compaction there is
always fluid on one side of the bag and PTFE powder
on the other; the fluid and the powder must be
separated by means of a seal, otherwise the powder
would become wet, full compaction would be
impossible and expensive material and time would be
wasted. The seal must be made around the edge of the
bag and is normally made directly by the bag, bag
sealing and bag fixing being combined (see Figure 14,
(p.21).
In addition, to prevent leakage during compaction, fluid
seals must be incorporated at any joins in the rigid
mould parts which communicate between the cavity
and the exterior. Covers almost always require a seal,
as indicated in Figure 14 (p.21); in addition, if holes are
incorporated (e.g. for clamping purposes), these will
require a seal at joint faces unless, as is preferable, they
are placed outside the relevant joint face seal, see
Figure 26).
The bag seal and cover seal should never be combined
because the repeated stressing of the bag caused by
mould opening and closing would cause weakening and
early failure; in addition such a combined seal gives
movements of the bag and therefore variations in shape
prior to filling. In the beaker mould (see Figure 14, p.21)
a ring has been incorporated, whose sole function is to
provide the bag seal; the bottom cover seal is formed
between this ring and the rigid mould part. Nitrile rubber
'O' rings and gaskets are suitable for sealing the joins
between rigid members; other methods such as
pressure sensitive tape and sealing compounds can
only be considered as temporary measures for use in
development or experimental work.
Fluid seepage around bolt
is kept out by O-ring seal
Fluid seepage
around bolt
,,
,,
,,
,,
O-ring
seal
Rigid
part
Rigid
part
PTFE powder
(b) Mould with clamping-bolt outside seal
— leakage to powder cannot occur
Figure 26. The effect clamping-bolt positions on fluid sealing
Sealing pressure can be applied using the elasticity of
the bag alone, as in Figures 12 (p.19) and 13 (p.20) or by
external means such as clips, clamps, bolts and in the
case of dry-bag compaction, the platens of a press.
Whatever method is employed all seals should be
designed so that the application of fluid pressure
improves the clamping; for example in the case of the
mould of Figure 14 (p.21), during compaction the fluid
pressure acts on the annulus AA1 of the top cover and
the annulus BB1 of the bottom cover thus exerting a
clamping force on the corresponding seals; similarly,
for the end plugs in Figure 13 (p.20) this effect can be
used to advantage as indicated in Figure 27.
Sometimes it is not possible to obtain satisfactory bag
fixing using simple combined clamping and sealing
because the bag moves at the area of attachment as a
result of repeated stressing during isostatic
compaction; this may occur where the bag attachment
relies solely on friction at the clamp. (See the beaker
mould, Figure 14, p.21). In such instances either an
improved design must be used as in Figure 28 (a)
(p.36)
or
separate
clamping
and
sealing
employed as in Figure 28 (b); in the latter instance the
means of clamping and sealing must be independent
and not interdependent, as shown in Figure 28 (c),
otherwise it will be difficult to obtain simultaneous
operation. Sometimes special features can be
incorporated in the bag for clamping and sealing
purposes as depicted in Figure 28 (d).
The clamp should be outside the seal. Most methods
of clamping within the seal (e.g. with a ring and bolts)
will provide a seepage path for fluid (see Figure 26,
above).
Whenever possible cover seals should be designed so
that they can be quickly made and broken and do not
disturb the powder on mould closure prior to
compaction. For the mould of Figure 14 (p.21)
incorporation of the part RRI in the top cover should be
avoided to prevent powder disturbance during mould
closure. Intersecting seals as depicted in Figure 29 (a)
should be avoided because of the difficulty in
obtaining satisfactory operation. Figure 29 (b) shows
a better method of sealing (see p.37).
End plug
PTFE
powder
,,,
,,,
,,,
(a) A plain end plug
PTFE powder
Flexible bag
Flexible bag
(a) Mould with clamping bolt inside seal
— leakage will occur at high pressure
,,,
,,,
,,,
,,,
End plug
Fluid pressure
will reinforce
sealing pressure
Flexible bag
,,,,
,,,,
,,,,
Bag deforms into groove
under fluid pressure
forming a good seal
Flexible bag under
fluid pressure
(b) An end plug design to further improve sealing
Figure 27. The use of fluid pressure to improve end-plug sealing
X
35
,,
,,
,,
,,
,,
,,
,,
,,
Flexible
bag
Bag clamping/sealing ring
— wedge shape improves clamping
Bag clamping ring
PTFE
powder
Flexible
bag
Bag sealing ring
,,
,,
,,
,,
(b) Separate independent bag clamping
and sealing
(a) Improved clamping ring design
Bag clamping ring
,,
,,
,,
PTFE
powder
PTFE
powder
Flexible
bag
Bag seal
(c) Separate interdependent bag clamping
and sealing - simultaneous operation
will be difficult to obtain
Bag clamping ring
PTFE
powder
Flexible
bag
Bag sealing
(d) Bag with O-ring features incorporated for
clamping and sealing purposes
Figure 28. Improved methods of bag clamping and sealing for the beaker mould shown in Fig. 14 (p.21)
Dry-bag compaction
As with wet-bag compaction a liquid/powder seal must
be provided at the periphery of the bag. In addition,
the mould must be sealed on the liquid side to prevent
escape of pressurised liquid to the atmosphere.
Usually the liquid/powder, liquid/atmosphere seals are
combined using the bag for both, as with the moulds
of Figures 14 (p.21) and 15 (p.22) when designed for
dry-bag use. Bags are clamped to the bottom of the
rigid mould part when pressing from the inside (see
Figure 14, p.21) or to the top inner wall of the rigid
mould part when pressing from the outside (Figure 15,
p.22) in order to facilitate removal of the preform from
the top of the mould.
Sometimes separate liquid/powder, liquid/atmosphere
seals are necessary as in the mould depicted in Figure
30, (p.38). Here the mandrel is made to be detachable
from the rigid mould, either for reasons of
construction or to facilitate bag replacement; in this
instance the two seals can be independent or directly
dependent, but not interdependent, (see Figure 31,
p.38) to facilitate simultaneous sealing.
36
The seals which, for wet-bag compaction, are required
at those joins in the rigid mould communicating
between the cavity and the exterior, are unnecessary
for dry-bag moulds, but these joins should fit together
well to minimise flash and prevent, under extreme
conditions, ejection of powder from the mould during
compaction. It may be considered wise to incorporate
seals at these joins as a precaution against the hazard
of possible high pressure fluid leaks should internal
sealing break down (e.g. as a result of bag failure).
However, except in small mould cavities, should
internal seals fail the pressure would usually be
dissipated within the mould.
MOULD MATERIALS
The rigid member
The mould materials which can be used for rigid
members differ widely for wet-bag and dry-bag
compaction, the requirements of the latter being far
more stringent. Moulding surfaces should be polished
to a finish between 0.1 - 0.2 µm ( 4 - 8 µ inch) Ra to
ensure a good corresponding preform surface and to
facilitate preform removal.
,,,,,
,,,,,
,,,,,
,,,,,
,,,,,
,,,,,
Flexible bag
Sections below
Container
Mandrel
Flexible bag
O-ring seal
top and bottom
,,,,
,,,,
,,,,
,,,,
,,,,,
,,,,,
,,,,,
,,,,,
,,,,,
,,,,,
PTFE
powder
Mandrel
Seal between
container halves
Intersecting
vertical
O-seal
,,,,,
,,,,,
,,,,,
,,,,,
(a) Interacting seals - bad practice
(b) Suggested method
Figure 29. Split moulds
(1) Wet-bag compaction:
In wet-bag compaction no significant differential
pressures or forces are imposed on the rigid member
(but see p.28, Avoidance of damage to the PTFE
preform) and therefore many materials may be used,
the choice depending on considerations such as ease
of construction, durability in service and accuracy of
dimensions. Plated or surface-hardened steels are
suitable for production moulds. For small production
batches and development work, materials such as
aluminium, epoxy resins etc. are quite satisfactory;
epoxy resins, of course, have the advantage of being
easily formed by casting and in this way many
moulds can be made relatively cheaply from one
master mould. Wet-bag moulds can be supplied by
various manufacturers.
(2) Dry-bag compaction:
In dry-bag compaction a suitable high grade steel
must be used to withstand safely the loads imposed.
A high tensile steel equivalent to the BS 970: grade
420 S37, T condition, (nearest US equivalent AISI
420), is recommended, since it has the necessary
tensile strength and ductility and is corrosion
resistant. Corrosion resistance is an important factor
when, as is usual, water is the pressurising medium,
even when soluble oil corrosion inhibitors are
incorporated. For working pressures below
350 kgf/cm2 (34.5 MN/m2; 5000 Ibf/in2) high strength
aluminium designed with a sufficient section
thickness may be adequate, although this material is
relatively soft and susceptible to mechanical damage
and to metal 'pick-up' (welding) between mating
parts.
Whatever material is used, safe operation at the
intended working pressures must be a prime
consideration as outlined on p.34 (Dry-bag
compaction), and it is strongly recommended that the
relevant design and construction work be placed in
the hands of specialists.
37
CLAMPING FORCE
,,,,,,
,,,,,,
,,,,,,
,,,,,,
,,,,,,
,,,,,,
Top filling cover
Bag seal
Bag clamp
PTFE powder
Flexible bag
Mandrel
Liquid/atmosphere
mandrel/seal
Possible seepage path
prevented by
mandrel seal
CLAMPING FORCE
Bag and mandrel seals
are independent
Figure 30. The beaker mould of Fig. 14 (p.21) re-designed for dry-bag operation and provided with a detachable mandrel
,,
,,
,,
,,
Bag seal
Mandrel seal
(a) Directly dependent seals
Mandrel seal
,,
,,
,,
,,
Bag seal (not necessarily successful)
(b) Interdependent seals — poor design — simultaneous
operation will be difficult to obtain
Figure 31. Directly dependent and interdependent seals for the mould shown in Fig. 30
38
Simultaneous operation
depends on this
distance being correct
The flexible member
modifications is likely to be justified when large
quantities of mouldings are to be made, and when
they can give further reductions in the amount of PTFE
required.
This must be resilient, exhibit uniform elastic
behaviour in all directions and be unaffected by the
liquid used for pressurisation which is usually water +
5% soluble oil corrosion inhibitor. In addition, if
pressing is from the inside, or if the bag is to be used
in tension, it must have a good tear strength. The
materials used may be natural and synthetic rubbers,
room temperature vulcanised (RTV) silicones, and
plasticised PVC. Ideally the bag should have a
hardness greater than 45 Shore A, and good abrasion
resistance. Fillers may be incorporated to modify the
hardness. For PVC, variation of plasticiser content also
provides a means of controlling hardness.
The production of sharp corners
If desired, bags can be made to impart fairly sharp
corners during compaction by the incorporation of
ridges, as depicted in Figure 32 overleaf. However,
this is not usually recommended because means will
have to be provided for accurate bag location in the
region concerned, which will probably increase mould
complexity and production times. Furthermore, sharp
corners should be avoided wherever possible in PTFE
components which are likely to be subjected in service
to mechanical stresses, especially those of a cyclical
nature.
The choice of bag materials can be made on the basis
of physical properties, cost, quality, and availability of
manufactured items. The process by which bags can
be made depends on the material used and affects to
a great extent the cost and quality; for example,
although dipping, where applicable, is generally the
quickest and cheapest method of making bags, very
close control on thickness can only be achieved by
processes such as casting and moulding. For
production purposes, where very large quantities of
bags are required, it is possible to justify the cost of
sophisticated bag moulds which permit the
incorporation of sealing, clamping and location
features (see Figure 28 (d), p.36). Table 4 gives typical
properties of various materials from which bags can
be made.
Improvements in shape near the area of bag attachment
The shape of the preform produced near the area of
bag attachment can be improved by incorporating
elastomeric or compressible components in this
region; Figure 33 shows a mould modified in this way
for the production of tubes (see overleaf).
The closer the behaviour of the elastomeric
component to that of the PTFE powder during
compression, the smaller the effect of the discontinuity
in the area of fixing, although it can never be
eliminated completely, (see Figures 33 (b) and (c)).
If the elastomeric component cannot also act as a seal,
a separate rigid part must be incorporated for this
purpose, as in Figure 33, where the discontinuity at the
area of bag fixing has simply been moved away from
the PTFE being compacted. Figure 34 shows this
system adopted in the design for the mould of Figure
14 (p.21). This improvement may well be worthwhile
when large quantities are being produced because,
although extra development work is necessary, the
production cycle time will be unchanged and a
significant material saving may be obtained, especially
for thicker sections.
The processes of bag manufacture are best undertaken
by specialists, although simple dipping and casting
methods can be used to produce small quantities for
experimental and developmental work.
IMPROVING THE SHAPE IMPARTED BY THE BAG
On page 24 (Accuracy of shape) it was emphasised
that the desired shape of the preform can rarely be
produced exactly because of the many variable factors
involved. However, improvements can be made by
suitable mould modifications. The cost of such
Table 4. Typical properties of various bag materials
Property
Natural
rubber
(moulded)
Natural
rubber
(latex)
Nitrile
Neoprene Butyl
Polyurethane Silicone
(RTV)
PVC
kgf/cm2
210
210
105
140
140
280
70
140-210
MN/m
20.5
20.5
10.5
13.5
13.5
27.5
7
13.5-20.5
Ibf/in2
3000
3000
1500
2000
2000
4000
1000
2000-3000
Hardness range
30-90
40
40-95
40-95
40-75
75-90
40-85
-
Tear resistance
Very good
Very good
Fair
Good
Good
Excellent
Poor
Fair to good
Abrasion resistance
Excellent
Good
Good
Good
Good
Excellent
Poor
Poor
ResiIience
Excellent
Excellent
Fair
Good
Bad
Good
Excellent
Bad
Compression set
Good
Fair
Good
Good
Fair
Poor
Fair
Poor
Tensile strength:
2
39
,,
,,
,,
,,
Sharp ridge
Rigid
part
,,,
,,,
,,,
,,,
Rigid
part
PTFE
powder
Sharp
corner
Flexible
bag
(a) Mould
Sharp
ridge
Preform produced
,,
,,
,,
,,
(b) Mould
,,
,,
,,
,,
PTFE
powder
Sharp
corner
Flexible
bag
Preform produced
Figure 32. The use of bag ridges to produce sharp corners
O-ring
seal
,,
,,
,,
,,
,,
,,
,,
,,
,,
,,
,,,,
,,,,
,,,,
,,,,
,,,,
,,,,
,,,,
Elastomeric
collar
PTFE
powder
Flexible
bag
Rigid
part
(rod)
,,,,
,,
,,,,
,,
,,,,
,,
Separate end plug
Deformed
elastomeric
collar
Reduced
end effect
Combined elastomeric
collar/end plug
(a) Mould ready for compaction
showing two methods of
end sealing
(b) Mould at compaction pressure
(c) Improved preform
shape obtained
Figure 33. The use of elastomeric materials to improve preform shape in the area of bag fixing
40
,,
,,
,,
,,
Rigid part
Bag clamp/seal
Elastomeric
seal
PTFE powder
Flexible bag
Mandrel
Shape of bag
at compaction
pressure
Figure 34. The design of Fig. 14 beaker mould (p.21) modified to improve the shape in the area of bag
attachment, using an elastomeric ring
DISPOSABLE MOULD PARTS
There are occasions where it is desirable or essential
to press onto a mandrel from the outside; for example,
for functional purposes or because it is impossible to
press from the inside (see p.26. Direction of Pressing),
as with the article in Figure 35.
,,,
,,,
,,,
,,,
,,,
,,,
,,,
,,,
,,,
,,,
,,,,,,
With such mouldings it is usually not possible to
produce undercuts or similar features because the
rigid insert could not then be removed. However,
where significant material or labour savings would be
obtained by introducing such features, this can be
done by using disposable mandrels which are
subsequently dissolved, melted or vaporised for
removal. Care must be taken to use materials which
will not contaminate the PTFE and the cost of the
disposable mandrel must be considered.
SPECIAL MOULDS: DRY-BAG COMPACTION
Dry-bag compaction is well suited to the relatively fast
production of simple shapes. However, it suffers from
the disadvantage of lack of flexibility - one mould will
only produce one kind of component and each mould,
being a pressure vessel, is relatively expensive;
furthermore, substantial mould modifications are
virtually prohibited (see p.34, Dry-bag compaction).
It is possible to obtain some of the advantages of a wet
bag system by using a dry-bag process: in this process
a plain cylindrical pressure container is used to
provide support for closely fitting mould inserts, as
depicted in Figure 36 overleaf, for the mould of
Figure 14 (p.21).
50mm dia
100mm dia
Figure 35. A preform having an internal undercut
and of such thickness that it cannot be produced
by pressing from the inside
These inserts can be made fairly easily and quickly
from almost any rigid material, as for wet-bag
compaction (see p.34); a range of inserts gives a range
of moulds which can be used in dry-bag fashion, only
one pressure container being required.
41
Pressure container
top plate
Spacer
Rigid
mould
insert
Pressure
container
,,,,,,
,,,,,,
,,,,,,
,,,,,,
,,,,,,
,,,,,,
Mould top plate
temporarily fixed to
pressure container
top plate
PTFE powder
Flexible bag
Mould mandrel/bottom
plate temporarily fixed
to pressure container
bottom plate
Pressure container
bottom plate
High pressure fluid
Figure 36. Dry-bag process
MOULD DESIGN PROCEDURE
The factors which should be considered in mould
design are inter-related in a complex manner, which
puts difficulties in the way of a logical design
procedure.
Furthermore, some factors are specific to only one
kind of mould whilst others can immediately be
disregarded. However, a list of considerations and
related factors, which might prove useful as a guide to
mould design, is given below.
Considerations
Some of the factors involved
(1) Details of PTFE article to be produced.
Dimensions, tolerances, surface finish, physical/
chemical properties, special conditions and quantity
required.
(2) Approximate cost evaluation - isostatic compaction
in comparison with alternative methods of production.
Comparison of isostatic compaction with other
techniques (see p.9) and quantity required.
(3) PTFE powder, compaction and sintering conditions
to be used.
Consideration (1), and suitability for processing (see
PTFE powders p.48).
(4) Compaction by wet- or dry-bag technique.
(5) Possible directions of pressing. If both pressing
from the inside and pressing from the outside are
possible, consider each separately.
42
Estimated equipment cost (may be partly offset
against future work), mould cost, cycle time, quantity
required and manufacturing costs.
See p.26, ‘Direction of pressing’
Considerations (continued)
Some of the factors involved
(6) Mould configuration i.e. basic shape and layout of
components.
Good uniform compaction of PTFE (see p.30)
Compromise between accuracy of shape and
complexity of mould (eg. p.34)
Ease of mould filling and preform removal (from bag
and rigid part)
Bag support (mandrel or container - see p.26)
Good sealing (see p.34)
Avoidance of PTFE and bag damage (see p.28)
Possible problems and need for special techniques
(see p.51).
(7) Detail design of mould rigid parts (except bag
support) - dimensions, material, location of parts,
seals and clamping.
(a) Moulding surface dimensions:
Dimensions and tolerances of required article
(consideration 1)
Allowing for PTFE shrinkage (consideration 3)
Possible need for machining (consideration 10)
(b) Material and overall dimensions (see pages 32
and 36); General strength,subject to size limitations
for wet-bag compaction; pressure vessel design for
dry-bag compaction.
(c) Seals and clamping (see p.34)
(8) Detail design of flexible part (bag) - dimensions,
material and thickness.
(a) Dimensions: (see p.31) Assumed bag movement,
compaction ratio and shrinkage for the PTFE
(consideration 3) and rigid moulding surface
dimensions (consideration 7).
(b) Material and thickness (see pages 28 and 39).
Preform surface finish, ease of preform removal,
need for special techniques (eg. pre-tensioning see
p.52) and cost.
(9) Detail design of bag support (mandrel/container)
- dimensions, bag sealing and clamping.
(a) Dimensions: Bag size and thickness (consideration
8); pressurising fluid flow rate for transmission
channel dimensions.
(b) Bag sealing and clamping: (see pages 28 and 34)
Avoidance of bag damage.
(10) Processing of PTFE after sintering eg. machining.
Dimensional tolerances and surface finish.
(11) Detail estimate of costs.
Estimated equipment cost, mould cost, cycle time,
quantity scrap rate and manufacturing cost.
(12) Estimate of time delay before full-scale
production.
Time for prototype or development work, and to
obtain equipment, mould set.
(13) Choose most suitable powder and direction of
pressing if choice still remains.
Cost, quality and time delay before full-scale
production.
(14) Prototype
(15) Modification
Cost of modification, in relation to cost savings.
(16) Final design
43
Figure 37a. (above and right). Wetbag mould for a screw top bottle
(see Appendix 3, p.62 for worked
example, producing this bottle.
This bottle is also shown in Figure
7, p.16)
EXAMPLES OF MOULDS
A typical wet-bag mould (for a screw-top bottle) is shown on this page - Figure 37a.
Figure 37b : T-piece (p.45)
Figure 37c : Test piece (p.46)
Figure 38 shows a typical dry-bag mould for a T-piece (p.47)
44
Figure 37b. Wet-bag mould for a threaded T-piece
45
Figure 37c. Wet-bag mould for a test piece
46
Figure 38. Dry-bag mould for a T-piece
47
Section 4. PTFE powders
The choice of PTFE powder for isostatic compaction
will depend on its suitability for processing and on
the properties which are required in the finished
component. In principle any PTFE powder may be
used, but processing difficulties are likely to be
encountered with, for example, those of higher
compaction ratio (e.g. 4 : 1).
CHOICE OF POWDER
The most important powder properties affecting
processing are flow, compaction ratio, resistance to
cracking during the compaction cycle, preform
strength, and shrinkage on sintering. However,
certain specific properties required for the finished
article, such as a smooth surface finish, will restrict
the choice of powder and may well override
processing considerations.
GRANULAR POWDERS AVAILABLE
Free-flowing or agglomerated powders are generally
preferred in isostatic compaction, because of their
better powder flow, lower compaction ratio, and
lower shrinkage at the recommended preforming
pressure. Compaction ratio and shrinkage values for
Fluon® G307 are given in Table 3 (p.33) and Table 1
(p.32) respectively. Fine particle powders such as
Fluon® G163 may be used to achieve certain desired
properties in the finished article, but their use is not
generally recommended because the associated high
compaction ratios are likely to give processing
difficulties (see p.25, Stability of Compaction and
p.52, The Avoidance of Bag Buckling).
Filled powders
Filled PTFE granular powders can be isostatically
compacted without difficulty; the free-flowing grades
should be used except for the production of thicker
walled articles (> 10mm; 3/8 inch) when non-free-flow
grades may be adequate.
Unfilled powders
END PROPERTIES
Unfilled PTFE granular powders currently available
may be divided into two broad categories:
(1) Free-flowing agglomerated powders e.g.
Fluon® G307 or G320
(2) Fine particle powders, eg. Fluon® G163
48
Some of the physical properties (e.g. dielectric
breakdown strength, gas permeability) which can be
obtained in the finished article depend mainly on the
type of PTFE granular powder which is used,
although slight changes can be obtained by
modifying the processing conditions.
Section 5. Processing
The processing conditions for the isostatic compaction
of PTFE are very similar to those for other methods of
compaction. Because pressure decay is minimal and
uniform compaction is possible, lower preforming
pressures can be used, and a wider working range of
pressure is therefore available. However, the higher
the moulding pressure, the lower will be the shrinkage
of the PTFE on sintering; thus it may not always be
advisable to aim for the lowest pressure consistent
with acceptable end properties, as this could give
processing difficulties.
The sintering process is the same as that for preforms
compacted by any other method.
Furthermore, in order to obtain good reproducibility
between mouldings, the average density of fill must be
constant. Good filling can be achieved if a free-flowing
powder is used together with controlled vibration or
tapping. To reduce the risk of preform cracking,
tamping should be avoided except with fine powders
such as Fluon® G163 which will withstand it. Not only
does vibration assist good mould filling, it also has the
desirable effect of increasing the bulk density of the
powder, thus reducing the effective compaction ratio.
Constant average fill densities can be ensured by
vibrating a fixed weight of powder into a certain fixed
volume or to a fixed level, such as the top of the
mould.
THE PROCESSING CYCLE
Processing basically consists of mould assembly,
mould filling, powder compaction, mould disassembly
and preform removal, followed by sintering. In order
to ensure reproducibility, all processing conditions
must be kept constant.
Mould assembly
All moulding surfaces and mating parts of the mould
must be clean and dry before assembly in order to
avoid contamination of the powder and to ensure
good assembly and operation (e.g. sealing). In
addition, it is advisable to keep the exterior of wet-bag
moulds clean because surface dirt will be transmitted
to the fluid used (see also p.57, ‘Fluid reservoir’).
Mould filling
Good mould filling is essential in isostatic compaction
because the initial powder distribution and density
determine, to a large extent, the bag movements and
thus the preform shape. It is important not only to
achieve filling of all cavities but also to obtain as
uniform a density as possible throughout the powder.
The use of bag supports and the application of a
vacuum to the liquid side of the bag during filling (see
p.26, ‘Good reproducibility in shape’) will ensure a
fixed bag shape and a reproducible mould volume.
Powder compaction
The pressure used to compact the powder should be
chosen to give the optimum physical properties in the
sintered moulding. However, bag wear and dry-bag
mould costs can be minimised by using the lowest
satisfactory pressure. The length of the compaction
cycle depends mainly on the thickness of section to be
moulded, thicker sections requiring longer cycle times.
In general the fluid pressure can be increased as
quickly as possible up to the recommended
compaction pressure, provided a smooth rate of
increase is achieved. Thicker sections, however, may
require a slower rate of pressure increase in order to
ensure smooth compaction: suitable rates depend on
the pressurisation system and the mould concerned
and must be determined experimentally.
49
When the chosen compaction pressure, in the range
100 - 500 kgf/cm2 (10 - 50 MN/m2; 1425 - 7125 Ibf/in2),
has been reached, a dwell is necessary, its extent
depending on the section thickness; as a rough guide
a dwell time of five seconds should be allowed for
every millimetre (0.04 inch) of section thickness,
although in practice no dwell is necessary for sections
less than 3mm (0.12 inch) in thickness. After a suitable
period of dwell the pressure is reduced to zero at a rate
depending on the wall thickness, the intricacy of the
compacted PTFE and the PTFE powder used. Too fast
a pressure release may give cracking of the preform as
the result of one or more of the following mechanisms:
(1) Trapped air:
The uncompacted PTFE contains air in the voids
between particles. During compaction much of this air
migrates to the surface of the preform although some
will remain trapped in small voids. If pressure is
released suddenly, rapid expansion of the trapped air
may cause cracking: slow release of pressure permits
migration of air and elastic deformation of the PTFE
surrounding the void. The problem is more severe for
thicker sections.
(2) Stored energy in the PTFE:
Elastic energy is stored in the PTFE when under
pressure. Sudden release of this energy may cause
cracking especially in thinner, more intricate preforms.
(3) Stored energy in the bag:
The bag, by virtue of its extension, contains stored
energy which, if suddenly released, may be sufficient
to break intricate parts of the preform. Thus the
pressure must be released gradually to avoid cracking,
a slower release being necessary for thicker sections,
thin intricate preforms, and powders which are prone
to crack easily.
Because of the number of variables and different
possible cracking mechanisms involved, the
permissible rate of decompression must be
determined experimentally, but this is only necessary
when problems are encountered. As a rough guide,
plain, thin-walled sections may be decompressed at a
rate of 100 kgf/cm2 (10 MN/m2; 1425 Ibf/in2) per second
whilst a 50mm (2 inch) diameter rod must be
decompressed at a rate less than 20 kgf/cm2
(2 MN/m2; 285 Ibf/in2) per second. Cracking problems
arising from trapped air may be overcome by applying
a vacuum (see p.57, ‘Vacuum pump’) to the powder
immediately after filling, thus removing air before
compaction (see Figure 39); this will almost certainly
give a more compact moulding with a minimum of
flaws. The vacuum must be applied slowly and over a
period of time in order to avoid sudden uneven bag
movements and to remove as much air as possible.
Mould disassembly and preform removal
For wet-bag compaction, liquid must be wiped from
the mould during disassembly in order to keep the
preform dry.
50
To vacuum
,,,,,
,,
,,
,,,,,
,,
,,
,,,,,
,,
,,
,,,,,
,,
,,
,,,,,
,,
,,
Open/close
valve or clamp
Filling
end plug
Filter
PTFE powder
Flexible bag
Bag movement
caused by
evacuation
Rigid part
(mandrel)
Final line
of preform
Mould before compaction
Figure 39. The application of a vacuum to the PTFE
powder in the production of a thick-walled tube
Care must be exercised when removing intricate
preforms from the bag and mandrels or core pins from
the preform, in order to avoid cracking or other
damage. Problems of bag removal can often be
overcome by the application of a vacuum to the liquid
side of the bag after compaction.
Sintering
The preform is sintered in the normal way, as if it had
been produced by any other method of compaction.
The main factor governing sintering cycles is the PTFE
section thickness (see Technical Service Note F1/F8).
If air has not been evacuated from the powder before
compaction it is advisable to leave thicker sections
several hours in normal atmospheric conditions before
sintering in order to allow the escape of any air which
has been trapped under pressure; otherwise the high
temperatures used for sintering will significantly
increase the pressure of the trapped air, preventing
closure of any voids which are present and possibly
causing cracking.
For sintering mouldings such as tubes and T-pieces it
is good practice to use a mandrel as a support in order
to prevent the PTFE from distorting under its own
weight whilst in the gel state. Allowance should be
made for PTFE shrinkage and ease of removal when
designing the mandrel. It is bad practice to shrink the
moulding onto the mandrel, using the latter in an
attempt to fix or change dimensions, because
subsequent removal of the PTFE would be difficult and
the PTFE would then gradually change its dimensions
under the residual stresses. If all processing conditions
are maintained constant, it is possible to reproduce
dimensions within close limits.
SPEED OF PROCESSING
The total cycle time required for isostatic compaction
can be divided into two parts, namely:
(1) Powder filling, compaction and decompression
(2) Mould assembly, disassembly and preform
removal.
The time required for (1) depends entirely on the
powder properties and the part being moulded,
whereas the time for (2) depends on the compaction
technique (ie, wet-bag or dry-bag), the complexity of
the mould and the degree of automation employed.
The use of dry-bag instead of wet-bag compaction will
usually reduce the time required for the second part
quite significantly, especially for simple shapes. The
application of automation to the dry-bag compaction
of simple shapes can further reduce this time to such
an extent that with, for example, an automatic multistage moulding machine, the cycle time is determined
by the rate of either powder filling or powder
compaction/decompression. For simple thin-walled
shapes powder compaction and decompression can
be achieved in less than 10 seconds, and multi-stage
automatic machines which will give cycle times of
half-a minute are commercially available. Manual drybag compaction of fairly simple shapes can be
achieved with an overall cycle time of less than one
minute, provided an automatic powder feed system is
used to fill the mould.
In comparison, the wet-bag compaction of large or
complex shapes is relatively slow, and overall cycle
times greater than five minutes are usual.
The dye penetrant test, by which any cracks or
porosity which may be present as a result of
unsuitable processing conditions may be detected, is
one of the most useful ways of determining that good
quality sintered PTFE parts have been made. Almost
certainly good physical properties will have been
achieved if the component is satisfactory according to
this test, although the relevant properties should
subsequently be checked by direct measurement.
The dye penetrant test requires complete immersion
of a finished sample in a penetrant dye, such as
'Ardrox' 996PA, or 'Ardrox' 970P24*. The sample is
then removed from the bath, washed in water to clean
the surface, and finally inspected; any cracks or
porosity which communicate with the surface will
have been penetrated by the dye and will thus be
clearly visible.
SPECIAL PROCESSING TECHNIQUES
Machining of PTFE preforms
PTFE preforms may be machined or trimmed with
sharp tools before sintering, but the preform, being
relatively soft, cannot give as good a surface finish on
machining as do sintered parts.
Care is needed in holding and cutting the PTFE to
avoid indentation, cracking,tearing, or other damage,
the degree of care which must be exercised depends
on the strength of the PTFE preform, which may be
affected by the type of PTFE powder used and by the
processing technique, as well as by the shape of the
moulding.
Composite mouldings
However, if the pressure vessel is sufficiently large,
production rates can be increased by compacting
several items simultaneously, and effective cycle times
can be reduced by filling some moulds whilst others
are in the pressure vessel.
DIMENSIONAL TOLERANCES OF MOULDINGS
With constant processing conditions, fairly close
tolerances can be achieved on the dimensions of those
surfaces which have been formed against the rigid
parts of the mould. However, only with simple
symmetrical shapes, such as tubes, can close
tolerances be achieved on the dimensions of those
surfaces which have been formed against the bag (see
p.24, ‘Shape of the preform’). Thus, although overall
tolerances of less than ±2% of section thickness can be
achieved for simple thin-walled symmetrical shapes,
such close tolerances cannot be maintained on all
dimensions when the shape is more complex.
As with other preforming techniques, composite
mouldings consisting of more than one grade of PTFE,
such as unfilled and filled, can be isostatically
moulded in one operation, a good interface bond
usually being achieved without difficulty. However, if
the optimum compaction pressures of the powders
differ widely, a compromise must be sought. For a
filled/unfilled composite the compaction pressure is
usually limited to the maximum which the unfilled part
can withstand without cracking, so that this part will be
well-pressed and the filled part may be under pressed
and possibly porous. This problem is less acute for
isostatic compaction than for other methods because
of the wider range of suitable compaction pressures
which are acceptable (see p.49, ‘Processing’). A certain
degree of porosity may be acceptable in the filled part,
but, if not, the porosity can be reduced by sintering in
an inert gas atmosphere such as nitrogen, a practice
frequently employed for filled PTFE.
TESTING PROCEDURES
*Ardrox Dyes supplied in the UK by:Chemetall plc, 65 Denbeigh Road, Bletchley, Milton Keynes, MK1 1PB
Tel: 01908 649333 Fax: 01908 361872 www. chemetall.com
(mid Eurpoe: Chemetall GMBH, Frankfurt a. M. Tel: + 49 697165-0)
Supplied in the USA by:
(Ardox Tel. toll free 1-800-297-3208)
51
The avoidance of bag buckling
Bag buckling may be encountered when forming thick
walled sections by pressing from the outside,
especially when high compaction ratio powders are
used. If this occurs three possible solutions can be
considered, namely:
(1) The use of a thicker bag
(2) Pre-compaction of the PTFE
(3) Pre-tensioning of the bag
(1) The use of a thicker bag:
Thicker bags require a greater compressive strain to
initiate buckling and will accommodate greater
movements when pressed from the outside before
exhibiting this phenomenon.
(2) Pre-compaction of the PTFE:
Some PTFE powders may be pre-compacted in order
to reduce their effective compaction ratio. This can be
done by hand tamping but is better performed in a
separate operation using, for example, uniaxial
compression moulding at low pressures; incremental
pressing can be employed with some powders, the
filling and pre-compaction being performed in several
stages.
For the successful operation of this technique the
powder must give a good preform strength at the very
low pre-compaction pressures used, of the order of 10
kgf/cm2 (1 MN/m2; 140 Ibf/in2), yet form a good bond
with surfaces which have already been precompacted: Fluon® G163 is very good in this respect.
(3) Pre-tensioning of the bag:
If the bag is initially tensioned the subsequent
compressive stress and strain occurring during
isostatic compaction will be reduced. By the
application of sufficient pre-tensioning to the bag, the
compressive strain can either be eliminated or reduced
to such a level that bag buckling will not occur. Pretensioning is normally effected by holding an
undersize bag against the internal walls of a solid
outer support, the latter imparting the desired initial
bag configuration; a vacuum is applied to the liquid
side of the bag prior to mould filling and is slowly
released immediately after filling and sealing; the
sequence of operations is depicted in Figure 40.
For this technique it is desirable to use thin bags,
about 1mm (0.04 inch) thick, in order to ensure that the
bag can be easily stretched by the desired amount.
Encapsulation with PTFE
Encapsulation with PTFE is a process in which a
continuous layer of PTFE is formed around a rigid
object, the PTFE forming a protective/insulating
covering. The rigid object is usually made of metal to
withstand the PTFE compaction pressure and sintering
temperature.
To vacuum
Open/close
valve or clamp
Flexible bag
under action
of vacuum
Mandrel
PTFE powder
partially
compacted
after release
of bag from
vacuum
End plug
(a) Mould before filling
Figure 40. Bag pre-tensioning for the production of a thickwalled tube
52
,,
,,
,,
,,
,,
,,
,,
,,
,,
,,
,,,,
Open/close
valve or clamp
Thin flexible
bag in relaxed
state
Rigid
outer bag
support
To vacuum
(b) Mould after filling and before final
compaction (vacuum released)
Final line
of preform
Flexible bag
Expansion of
PTFE in this
direction is
impossible
Preformed PTFE
,,
,,
,,
,,
,,
,,
,,
,,
,,
,,
Object
Poor fit
Shaped flexible bag
Object
Expansion of
PTFE is
possible
Sintered PTFE
(a) Encapsulated object
before sintering PTFE
PTFE
(b) Encapsulated object
after sintering PTFE
showing poor fit in
the area where expansion
in one direction was
restrained
,,
,,
,,
,,
,,
Object
Ensure this
is at least
(2 x 2.7t) +
(2 x bag
thickness)
(c) Improved design
Figure 41. Encapsulating with PTFE: importance of free expansion of PTFE during sintering
Spider and electromagnet
to position object to be
encapsulated and to allow
mould filling
,,,,
,,,,
,,,,
,,,,
Metal object
for encapsulation
PTFE powder
flexible bag
Plan of spider
,,,
,,,
,,,
,,,
,,,
Top plug
(b) Spider and magnet removed, mould
filling completed and top plug
inserted ready for compaction
(a) Mould partly filled using spider for
positioning the magnet and the object
Figure 42. Encapsulation of a metal bar with PTFE
53
Typical examples of encapsulated items are magnetic
stirrers, butterfly valve flaps and valve plugs.
The covering layer of PTFE may be formed by
compaction of granular PTFE and subsequent
sintering. The isostatic process is well suited to this
kind of compaction and in many instances is the only
possible method. The main problems of encapsulation
arise from differential expansion of the PTFE and the
substrate and the overall shrinkage of the PTFE during
the sintering cycle.
The overall shrinkage of the PTFE causes tensile
stresses to be induced in the covering. These stresses
can be sufficiently high to crack the covering,
especially if small cracks are already present (see p.49,
Powder compaction: cracks can also be induced by
features such as sharp corners on the encapsulated
object). Therefore special precautions need to be
taken:
PRESSING LOAD
Compression mould
top plug
Rubber crumb
Uniaxial
compression
moulding
cylinder
Compression mould
bottom plug
,,,,,,
,,,,,,
,,,,,,
,,,,,,
,,,,,,
,,,,,,
Figure 43. Rubber crumb technique
54
Bag clamp ring
PTFE powder
Flexible bag
Isostatic mould
rigid part
Isostatic mould
’top’ cover
(1) Air should be evacuated from the PTFE powder
before compaction (see p.50).
(2) During sintering the rate of temperature change
(rise and fall) near the gel point (i.e. at about 320° to
350°C; 608° to 662°F) should be kept low (eg. 10 - 20°
C/h; 18 - 36° F/h)to minimise thermal stresses resulting
from the high volume expansion of PTFE on melting.
(3) A PTFE powder having as low a shrinkage as
possible should be used to minimise the induced
tensile stresses. Thus, a free-flowing powder like
Fluon® G307, pressed at about 300 kgf/cm2
(30 MN/m2; 4250 Ibf/inv), is more suitable than a fine
particle powder like Fluon® G163 pressed at about 150
kgf/cm2(15 MN/m2; 2125 Ibf/in2), the former having a
lower shrinkage that more than compensates for its
slightly poorer physical properties.
(4) To ensure a uniform thickness and thus a uniform
distribution of stresses on sintering, the PTFE covering
should be carefully machined in the unsintered state.
inside a rigid object which normally acts as a support;
the material of the object must be able to withstand
the PTFE sintering temperature. The wet-bag
technique is normally used, as the object to be lined is
very unlikely to be strong enough to act as a dry-bag
pressure-vessel, capable of withstanding the full
compaction pressure. The principal application for this
process is the provision of liners for pipes, pipe
fittings, valves and pumps; a lined T-piece is illustrated
in Figure 11 (p.18).
In contrast to the process of encapsulation the main
problem encountered with in-situ lining is to prevent
PTFE shrinkage giving a loose-fitting and thus
unsupported liner. Details of a technique to overcome
this problem will be supplied on request.
Rubber crumb technique
If a good PTFE covering can be achieved, residual
stresses will slowly decay by stress relaxation, a
process which can be accelerated by heating the
article to a temperature, e.g. 250°C (482°F) below the
gel point, followed by slow cooling (not faster than 30°
C/h; 54° F/h).
In this method soft rubber crumb (or granules) is used
as the pressurising medium instead of a fluid; this
enables normal uniaxial compression moulding
equipment to be employed for isostatic pressing. An
isostatic mould of the wet-bag design is inserted in a
uniaxial compression moulding cylinder and covered
with rubber crumb, as depicted in Figure 43.
Another precaution which must be taken is to ensure
that the PTFE covering is not restrained from
expansion in any one direction during sintering,
otherwise all expansion will take place in the free
directions, with consequent change in the shape of the
covering and bad fit in the finished article (see Figure
41, p.53).
The pressure produced by compacting the rubber
crumb uniaxially is transmitted isostatically to the wetbag mould and thus to the PTFE. It must be ensured
that there is sufficient rubber to accommodate the
required PTFE compaction, but, because a liquid is not
used the seals necessary for wet bag compaction are
not required.
Typical processing stages in encapsulation, using a
magnetic stirrer as an example, are depicted in
Figure 42.
The technique is useful for the isostatic moulding of
small quantities of preforms when normal isostatic
facilities are not available. However, it is relatively slow
and problems may be encountered in obtaining the
desired shape since the rubber crumb, unlike most
liquids, affects the manner of bag movements. It is not
essential to use rubber only in the form of crumb or
granules. Indeed, the isostatic mould and pressurising
medium may be combined in the form, for example, of
a suitably shaped rubber block.
The main difference between encapsulation and
normal isostatic compaction is at the filling stage
where a method of locating the part to be surrounded
must be incorporated. Usually the part is held in
position in a manner that permits almost complete
filling; filling is completed after removal of the support
when the encapsulated part is held in position by the
powder which surrounds it.
In-situ lining
In this process a continuous layer of PTFE is formed
Note: The coating of items with PTFE by certain isostatic
compaction techniques is the subject of a number of
patents. Anyone intending to use this method should satisfy
themselves that they are not infringing any such patents.
55
Section 6. Process equipment
The following equipment is essential for the isostatic
compaction and subsequent sintering of PTFE:
A lifting device is usually provided to aid the removal
of the plug.
(1)
(2)
(3)
(4)
Many fluids are suitable for pressurisation but water
containing a soluble oil corrosion inhibitor is the
most widely used since it is cheap, relatively clean
and has a low viscosity. The use of gases such as
air or nitrogen is not advisable because of the
great amount of stored energy in pressurised
compressible fluids.
Mould
Isostatic compaction unit
Oven
Services - electricity, gas, water, compressed air.
In addition the following equipment may be required:
(5)
(6)
(7)
(8)
(9)
Vibrator
Vacuum pump
Lifting equipment for heavy moulds
Sintering mandrels
Ancillary items
Moulds have already been considered in Section 3,
Mould design (p.24 onwards). Again it must be
emphasised that the mould for dry-bag compaction is
itself a pressure vessel and must be designed to
operate safely as such.
The safety aspects in the design, manufacture and
operation of high pressure equipment on pages 34,
36 and 37 (Dry-bag compaction) must not be
neglected. For example, if the closure mechanism
fails during pressurisation, the stored energy, even in
a relatively incompressible fluid at only 100 - 200
kgf/cm2 (10 - 20 MN/m2; 1425 - 2850 Ibf/in2) pressure,
is sufficient to eject the usually very heavy plug at
high speed. Because of this potential hazard it is
strongly recommended that the design and
manufacture of the complete pressure vessel be
undertaken by experts.
ISOSTATIC COMPACTION UNIT
Means of fluid pressurisation
In its simplest form the compaction unit consists of:
Either pneumatically or electrically actuated pumps
may be used for pressurising the fluid. Electrically
actuated pumps have higher fluid flow outputs giving
faster pressurisation; they are easier to control, or to
link to - pressure cycle controllers; but they are
usually more expensive than pneumatic ones.
Refinements such as multi-stage pumping, giving
high fluid flow outputs at low pressures and low fluid
flow outputs at high pressures, are available if the
pressurisation time is very important. However,
except for automated dry-bag compaction and large
mouldings, the pressurisation time is likely to be
small compared with operations such as mould
assembly and filling.
MOULD
(a) Pressure vessel
(b) Means of fluid pressurisation
(c) Means of controlling compression and
decompression of the fluid
(d) Fluid reservoir
Pressure vessel
In dry-bag compaction the pressure vessel is already
provided by the mould. For wet-bag compaction the
pressure vessel usually consists of a chamber with a
removable top plug as described in Section 2,
‘Process techniques’ (p.19). This plug must be
provided with a good rugged seal and if speed is
important the opening and closing mechanism
should be as simple as possible. Various types of
closure, such as bolted, pin and plug, thread or
interrupted thread, can be used, provided that the
associated local stresses are allowed for in the design
of the vessel.
56
Pressure intensifiers, whereby pressurisation is
obtained very quickly by one stroke of a piston, may
be used but care must be taken to avoid compression
rates which are fast enough to generate pressure
waves in the fluid which will almost certainly damage
the PTFE. The pressure intensifier may be
incorporated in dry-bag moulds, a conventional
hydraulic press being used both for actuation and
mould clamping.
Means of controlling compression and decompression
of the fluid
This consists of valves, either directly or remotely
controlled, and switches, together with high pressure
and low pressure pipework, for the controlled
transmission of the fluid between the reservoir and the
pressure vessel. Automatic control of the
pressurisation cycle can be incorporated and is
particularly useful for dry-bag compaction.
Fluid reservoir
SERVICES
The necessary services must be provided and suitably
located.
VIBRATOR
A table vibrator having a small amplitude, high
frequency, oscillation is very suitable for the vibration
of moulds during filling. However, large heavy moulds
may be more conveniently vibrated by means of a
pneumatic hammer.
This acts as a store for fluid and must be provided with
filters to prevent circulation of particles which may
have entered the system, usually in the form of PTFE
or dirt on the exterior of moulds. Fluids under high
pressure gradients travel through the pipes at
sufficiently high velocities for any contained particles
to cause serious damage by erosion.
VACUUM PUMP
The whole compaction unit must be enclosed in a
cabinet to give protection to personnel should high
pressure fluid leaks occur and to guard the pipes,
valves and pumps from inadvertent blows or damage.
The wet-bag process lends itself to the construction
of
proprietary
units.
Several
specialist
manufacturers,who have supplied the metallurgical
and ceramics industries with such units for many
years can supply similar units for the compaction of
PTFE.
Lifting equipment will be necessary for heavy moulds
and preforms. In addition a cage is useful for the
lowering of moulds into, and their retrieval from, wetbag pressure vessels, especially when several moulds
are pressurised at the same time. Such a cage also
serves to protect the pressure vessel from possible
damage by any sharp protrusions on the moulds.
OVEN
An oven is required for the sintering of PTFE, the main
requirements being:
(a) Uniform distribution of temperature with a
maximum spread of 5°C (9°F)
(b) Good control over temperature to within ± 2 - 3°C
up to 400°C (± 4 - 5°F up to 750°F)
(c) Good exhaust ventilation to atmosphere for gases
which may be produced (see Section 7, p.58).
This will be necessary if it is desired to apply a vacuum
to either or both sides of the bag before compaction; a
simple pump is quite adequate.
LIFTING EQUIPMENT FOR HEAVY MOULDS
SINTERING MANDRELS
Mandrels may be required for the support of articles
during sintering. These should be made from stainless
steel, and the surfaces which will contact the PTFE
should be ground to a finish of 0.2 - 0.3 µm (8 - 12 µ
inch) Ra.
ANCILLARY EQUIPMENT
Various jigs and tools can be made to facilitate and
speed up mould assembly, disassembly and preform
removal stages.
Devices such as pneumatic nut runners are very useful
for wet-bag moulds where bolts are used for clamping.
57
Section 7. PTFE handling precautions
Within its working temperature range Fluon® is a
completely inert product, but when heated to its
sintering temperature it gives rise to decomposition
products which can be toxic and corrosive. These
fumes start to be produced during processing: for
example, when the material is heated to sinter it or
when brazed connections are being made to cables
insulated with PTFE. The inhalation of these fumes is
easily prevented by applying local exhaust ventilation
as near to the source of the fumes as possible.
58
Smoking should not be permitted in workshops where
Fluon® is handled because smoking tobacco
contaminated with PTFE will give rise to polymer
fumes. It is therefore important to maintain a good
standard of personal cleanliness and to avoid
contamination of clothing, especially the pockets, with
polymer dust.
More detailed information on these points is included
in the APME publication 'Guide for the Safe Handling
of Fluoropolymers,' and in the relevant Fluon®
Material Safety Data Sheet.
Appendix 1. Typical production of a PTFE bottle
The following is an account of the stages involved,
with the approximate time required for each, in the
production of a PTFE bottle using the mould shown
in Figure 37a, p.44, and detailed overleaf in Figure
44. Such a bottle is also shown in Figure 7, p.16.
MOULD CLAMPING (4 min.)
Secure the mould in the holder, place the retaining
plate in position and seal by tightening the stud nuts.
PRESSING (4 min.)
It should be noted that this is an experimental mould
made from relatively soft aluminium alloy ('Dural'HE
15B). A production mould would require to be made
from a harder material to avoid damage during
handling. Also, certain features, e.g. bag centring
during filling and mould clamping, could be
improved to give more consistent mouldings and
shorter cycle times.
Place the mould assembly, bag mouth uppermost, in
the pressure vessel of an Olin 6-24-15 isostatic press
using water as the pressurising fluid, plus 5% soluble
oil as a corrosion inhibitor. Close the pressure vessel
and bleed the air from the system. Raise the pressure
during 5 seconds to 300 kgf/cm2 (29.5 MN/m2, 4275
Ibf/in2), hold at that level for about 5 seconds and
reduce to zero over a period of 10 seconds.
MOULD CLEANING (3 min.)
Clean the mould thoroughly with a damp cloth and
then with a dry cloth. Take special care that the nitrile
rubber 'O'-ring seals are properly seated.
MOULD ASSEMBLY (5 min.)
Insert the flexible rubber bag into the mould base so
that the shaped neck of the bag fits over the
constriction in the mould base. Insert a tightly fitting
rubber stopper into the neck of the bag so that a
watertight seal is formed between the bag and the
mould base. A 3 - 4mm hole is required bored
through the stopper to allow entry of the pressurising
liquid into the bag.
Assemble the remaining mould parts - excepting the
top filling cover - and ensure that the 'O'-ring seal
between the outer casing of the mould and the mould
base is seated correctly.
MOULD FILLING (6 min.)
Fill the mould with PTFE powder and take care that
the bag remains in the central position. Place the
filled mould on a vibrating table to settle the powder
and add further powder to top up. Remove from the
vibrator and draw a straight-edge across the top of
the mould to scrape off surplus PTFE powder,
ensuring that no loose powder remains on the top
sealing face. Fit the top filling cover and place the
complete mould into a holder (shown in Figure 37a,
p.44) making sure that the mould parts are not
separated, so that powder leakage is avoided.
Remove the closure from the pressure vessel and lift
the mould assembly out - still with the mouth of the
bag uppermost. (It has been found that if the mould
is inverted during its removal from the pressure
vessel a partial vacuum is caused between the bag
and the moulding by liquid pouring from inside the
bag. This causes cracking at the base of the bottle
preform. Therefore special care has to be taken not to
empty the liquid from the bag at this stage, nor
during the subsequent dismantling process).
DISMANTLING (8 min.)
Dry the mould thoroughly, e.g. with paper towel, to
prevent contamination of the PTFE surfaces with
water containing soluble oil which might cause
discolouration of the PTFE during sintering.
Plug the hole in the rubber stopper to retain the
liquid inside the bag. Lay the mould, still in its holder,
on its side. Remove the stud nuts and retaining plate
and push the mould from the holder. Remove the top
filling cover, slide off the outer casing, remove the
inner split moulds and so expose the PTFE bottle
preform and allow the free entry of air between the
preform and the rubber bag.
Drain the liquid away from the rubber bag through
the hole in the rubber stopper and remove the bag
(still attached to the mould base) from the inside of
the bottle.
Total time: 30 minutes
59
127mm (5.0 inches) diameter
,,,,,,,,
,,,,,,,,
,,,,,,,,
,,,,,,,,
,,,,,,,,
,,,,,,,,
,,,,,,,,
,,,,,,,,
,,,,,,,,
,,,,,,,,
,,,,,,,,
,,,,,,,,
,,,,,,,,
,,,,,,,,
,,,,,,,,
196 mm(7.7 inches)
Initial sag
at end of
flexible bag
Additional sag
of bag after
filling with PTFE
Filling cover
’O’-ring seal
Rigid mould part
(two halves)
Outer casing
Flexible bag
(dipped natural
rubber)
PTFE powder
Position of
finished moulding
Highly polished
mould surface
Locating dowel
or the two
mould halves
Locating dowel
Two halves of
thread mould
’O’-ring seal
Mould base
All metal mould parts made from
aluminium alloy ’Dural’ HE 15B
Rubber stopper with
central hole
Figure 44. Sectional drawing of experimental mould and flexible bag used to produce a PTFE bottle
having an approximate weight of 250g. (See also Figure 37a, p.44).
60
Appendix 2.
This technique has been proven on moulds of 100 mm
and 200 mm diameter.
SIMPLE ISOSTATIC MOULDING
A technique has been developed which enables PTFE
fabricators to operate an isostatic process by making
minor modifications to standard direct pressing
equipment.
N.B. Users should satisfy themselves that operating
their moulds in this manner meets all relevant safety
requirements.
The technique avoids the need for a special isostatic
press and the only modifications needed to equipment
are to fit 'O'-ring seals on the pressing pieces of a
standard mould.
Figure 45(a) illustrates the system set up for 'wet bag'
pressing and Figure 45(b) suggests a modification
which enables a 'dry bag' process to be operated.
Press platen
Pressing tube
Operating procedures for the 'wet bag' process are:
1. Fit lower pressing piece into mould with 'O'-ring
seal and fill mould with water plus soluble oil (to act
as a corrosion preventative).
,,
,,
,,
,,
2. Fill rubber mould with Fluon® granular powder and
seal.
3. Immerse filled rubber mould in water/oil solution
and fit top pressing piece with 'O' ring seal. Using
the air vent, lower the top pressing piece until all air
is excluded, then close vent.
4. Press in the normal way as for a directly moulded
preform. Pressures are calculated on the total cross
sectional area of the mould.
5. Open air vent, remove top pressing piece and
rubber mould containing preform.
Vent valve
End pressing
pieces with
’O’ ring seals
Mould case used as
pressure valve
Wet bag mould
Water with soluble oil
Figure 45(a). Wet Bag
6. Sinter normally.
Press platen
CLAMPING FORCE
Vent valve
Top filling
cover
Bag seal
Dry bag mould
Bag clamp
Interconnecting pipe
,,,,,,
,,,,,,
,,,,,,
,,,,,,
,,,,,,
,,,,,,
PTFE powder
Flexible bag
Mandrel
Liquid/atmosphere
mandrel/seal
CLAMPING FORCE
Figure 45(b). Dry Bag
61
Further information
Fluon® technical literature
Information contained in this publication (and
otherwise supplied to users) is based on our general
experience and is given in good faith, but we are
unable to accept responsibility in respect of factors
which are outside our knowledge or control. All
conditions, warranties and liabilities of any kind
relating to such information, expressed or implied,
whether arising under statute, tort or otherwise are
excluded to the fullest extent permissible in law. The
user is reminded that his legal responsibility may
extend beyond compliance with the information
provided. Freedom under patents, copyright and
registered designs cannot be assumed.
The following is a comprehensive list of Technical
Service Notes on Fluon® PTFE. They are available
from the AG Fluoropolymers sales office.
Fluon® grades are general industrial grades. It is the
responsibility of the purchaser to check that the
specification is appropriate for any individual
application. Particular care is required for special
applications such as pharmaceutical, medical devices
or food. Not all grades are suitable for making finished
materials and articles for use in contact with
foodstuffs. It is advisable to contact the AG
Fluoropolymers sales office for the latest position.
Users of Fluon® are advised to consult the relevant
Health and Safety literature which is available from the
AG Fluoropolymers sales office. Users of Fluon® must
also refer to the relevent Material Safety Data Sheet
from AG Fluoropolymers.
F1
The Moulding of PTFE granular powders
F2
The Extrusion of PTFE granular powders
F3/4/5 The Processing of PTFE coagulated
dispersion powders
F6
Impregnation with PTFE aqueous
dispersions
F8
Processing of filled PTFE powders
F9
Finishing processes for
polytetrafluoroethylene
F11
Colouring of polytetrafluoroethylene
F12/13 Physical properties of unfilled and filled
polytetrafluoroethylene
F14
Isostatic compaction of PTFE powders
F15
Cast Film from Fluon® PTFE dispersion GP1
FTI500 Fluon® - A Guide to Applications, Properties
& Processing
FTI800 Potential Material & Equipment Suppliers
This edition © AGFP September 2002
62
63
UK
®
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