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SPECIAL RENAISSANCE OF FOAM PROCESSES Injection Molding
[VEHICLE ENGINEERING] [MEDICAL TECHNOLOGY] [PACKAGING] [ELECTRICAL & ELECTRONICS] [CONSTRUCTION] [CONSUMER GOODS] [LEISURE & SPORTS] [OPTICS]
A Little Extra?
Process Combinations Open up New Horizons for Foam Injection Molding
The trend towards lightweight construction and the desire to further exploit the savings potential in plastics processing are ensuring the increasing adoption of thermoplastic foam injection molding. By selectively combining
this with other processes, it is possible to positively influence the mechanical and tactile properties, and surface
appearance of foamed parts.
I
n thermoplastic foam injection molding, plastic melt charged with a blowing
agent is injected into the cavity. The
foaming process occurring there replaces
the holding pressure required in nonfoam injection molding and leads to a
homogeneous pressure level. Since no
pressure gradient forms between the
gate and flow path end, the warpage tendency is significantly reduced. In addition,
the choice of sprue position and wall
thickness need not be chosen so as to
­allow the holding pressure to be transferred even to areas remote from the
sprue. The wall thicknesses can thus be
reduced in many applications, in addition
injection from “thin to thick” is possible [1].
Modeled on Nature: a Similar
­Structure to Bones and Tree Trunks
The cross-section of a foamed part shows
a sandwich-like structure: the nonfoamed outer layers and the fine-celled
foam structure on the inside, which are
responsible for the reduced part weight,
among other things. With most materials,
however, streaking occurs on the part
surface, since small bubbles occur at the
melt front during injection (Fig. 1).
The sandwich-like structure often improves the weight-specific properties of
the part, i. e. the property data that are
quoted per kilogram of material. Examples of this can also be found in Nature.
For example, human bones and a tree
trunk each have structure similar to that
of a foamed part, in which a non-foamed
outer layer surrounds a cellular core structure. In both cases, the core transmits the
A combination of variotherm mold temperature control with a thermoplastic foam injection
molding process such as MuCell allows foamed parts to be manufactured with high surface
quality. Engel demonstrated together with Roctool the potential of this technology combination
at the Fakuma 2014 with the production of middle console components.
shear forces or the compressive loads,
while the flexural properties are for the
most part derived from the non-foamed
outer layer.
Based on calculations [2, 3], it can be
shown how the tensile and flexural stiff-
nesses behave with decreasing density in
relation to non-foamed parts (Fig. 2). The
wall thicknesses of the parts are not
changed with these considerations. It can
be seen that foamed parts initially have a
very steep decline of tensile strength
© Carl Hanser Verlag, Munich Kunststoffe international 12/2014
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Injection Molding RENAISSANCE OF FOAM PROCESSES
Fig. 1. The surface quality is usually streaky with unfilled amorphous plastics (left). Depending on the material type used, filled semicrystalline
plastics have a higher surface quality (right)
Different Process Goals in
­Combination with Negative Coining
According to Steiner’s law, the flexural
strength is the higher, the more distant
the outer layers are from the neutral
phase. In practice, a greater distance of
the outer layers can be achieved by
combining thermoplastic foam injection molding with negative coining. In
thermoplastic foam injection molding,
plastic melt charged with a blowing
agent is injected into the cavity with
the basic wall thickness h 0. After cavity
filling and the subsequent formation of
the non-foamed outer layer, the cavity
or partial regions of the cavity are increased to the wall thickness h1 (Fig. 3) –
either by means of a movable core or
by selective opening of the mold. With
the use of Engel duo injection molding
machines, the mold is opened with
controlled parallelism to a preset end
position.
The effect of greater wall thicknesses
with constant part weight on the mechanical properties can be accurately estimated. Graphs determined from calculations [2, 3] make it clear that the flexural
stiffness increases with increasing wall
thickness (Fig. 4). A doubling of the wall
thickness, e. g. from 2 to 4 mm, is not rare
in the production of dashboard backings
in the automotive industry. It is possible
to make use of the possibility of improving the flexural strength without increasing the weight of the non-foamed part.
If the focus is not on greater flexural
stiffness but on lower part weight, the
initial wall thickness h 0 of the unfoamed part is reduced and the mold is
opened after the injection of the blowing-agent-charged melt to the extent
necessary to achieve the required flexural stiffness of the foamed part. In this
manner, the part weight can be reduced more strongly than with conventional foaming methods.
In implementing this process combination, users should pay particular attention to two points: the plastic used and
the part design. The process sequence illustrates the high requirements on the
material (Fig. 5).
WW In step 1, the cavity, with a basic wall
thickness of 2 mm, is filled.
WW In step 2, a non-foamed outer layer of
0.3 mm in each case is set.
WW In step 3, the mold is opened so far
that the finished part has an end wall
thickness of 6 mm.
When the mold is opened, the foaming
process starts in the molten interior of the
part – in this example over a thickness of
1.4 mm. Here, the typical cell structure develops, which is increasingly stretched as
the opening stroke progresses. The requirement on the material used consists
in stabilizing the fine cell structure during
the opening movement and not tearing
it open.
For the part design, it should be noted that surfaces that are not at a right angle to the opening direction undergo »
1.0
Relative tensile/flexural strength
with decreasing density. Because of the
sandwich-like structure, the flexural stiffness behaves differently and decreases
less sharply in the range relevant to the
applications. A density reduction of 10 %
reduces, for example, the tensile strength
by about 20 %. In this example, however,
the flexural strength decreases by less
than 10 %.
Besides the density, the thickness of
the non-foamed outer layer is also suitable for changing the part properties, as
regards the process technology, within
certain limits. For example, a lower gas
content usually involves the formation of
a thicker non-foamed outer layer. The
mold temperature, or melt temperature,
and the injection velocity influence the
sandwich structure and therefore the
properties of the part. Detailed investigations on the influence of the processing parameters are described, for example, in [4].
0.8
0.6
0.4
Tensile strength
Flexural strength
Practical field of
application
0.2
0
0
10
20
30
Weight reduction
40
%
50
© Kunststoffe
Fig. 2. Change of mechanical properties with part weight for constant wall thickness: at first, the
relative tensile strength drops steeply with increasing density reduction. For example, with a
density reduction of 10 %, the tensile strength falls by about 20 %. The flexural stiffness falls less
steeply because of the sandwich structure
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21
22
SPECIAL RENAISSANCE OF FOAM PROCESSES Injection Molding
Step 1
Step 2
Step 3
Cross-section
Step 1
Cross-section
Step 2
Cross-section
Step 3
Foam Injection Molding
Thermoplastic foam injection molding
can be subdivided into two parts depending on the type of blowing agent addition:
For example, in chemical foam injection
molding, the plastic melt is charged by the
decomposition of a chemical blowing
agent added to the molding compound.
In physical foaming, a physical blowing
agent, usually nitrogen, in rare cases carbon dioxide, is introduced directly into the
melt via an injector. In the mixing sector of
the screw, blowing agent and melt are homogenized until a single-phase mixture is
present in the front region of the plastication unit.
In physical foaming, there are a number of
process variations, with MuCell being the
most widespread. The MuCell method is
based on developments made at the Massachusetts Institute of Technology (MIT) in
Boston, and is marketed by Trexel, headquartered in Wilmington, both in Massachusetts/USA. The machine manufacturer
Engel offers completely integrated injection molding systems for the MuCell foam
injection molding process from a single
source under the name “Engel foammelt.”
The Authors
Dipl.-Ing. Robert Endlweber is technology manager in process technology
development at Engel Austria GmbH,
Schwertberg, Austria,
[email protected]
Dipl.-Ing. Josef Giessauf heads the
process technology development department at Engel; [email protected]
Service
References & Digital Version
BB You can find the list of references
and a PDF file of the article at
www.kunststoffe-international.com/948056
German Version
BB Read the German version of the
article in our magazine Kunststoffe or at
www.kunststoffe.de
h1
h0
© Kunststoffe
Fig. 3. In negative coining, the cavity is volumetrically filled in the first step (left). Due to the delay
time, the thickness of the compact outer layer can be adjusted (center). The subsequent opening
stroke initiates the foaming process (right)
a lower expansion of the wall thickness
during mold opening (Fig. 6). Consequently, the degree of foaming is not equal everywhere and the mechanical properties
can vary. To estimate the mechanical
properties that can be expected, the
above-mentioned graph (Fig. 4) can be
used.
Controlling the Haptics:
The Negative Coining of TPE
The above-described combination of
foaming and subsequent negative coining permits the tailored design of mechanical properties and supports lightweight construction and reduced consumption of raw material. It is often used
for manufacturing sheet-like hard-foam
beams. If, instead of a thermoplastic, a
soft material such as TPE is used, the softness or haptics of the part can be adjusted by means of the opening stroke and
other process parameters.
This principle is used, for example, in
the Dolphin process [5], in which, on an
Engel duo combi M injection molding
machine with rotary table, a thermoplastic beam can be produced, and overmolded with blowing-agent-charged TPE
in a second process step. In this case, the
soft-touch effect is selectively adjusted
by means of the opening stroke.
For Visible Applications: Overmolding
of Films and Textiles with Foam
The advantages offered by the foaming
of thermoplastics lead to the ever increasing use of foam processes such as “Engel
foammelt” (see Info box) for visible parts,
e. g. panels or various covers. Several
combinations of foam injection molding
with other processes make this possible
now.
Decorative materials such as film or
textiles are often overmolded with blowing-agent-charged melt. The characteristic streaky structure of the foamed part is
thereby easily concealed behind a
non-transparent insert part and is not further relevant for the surface quality. In
many cases, the film also forms a heat
barrier to the inner wall of the cavity [1], so
that the polymer melt does not solidify
immediately after the filling process and
streaking cannot occur on the part surface. Especially when amorphous plastics
are used, transparent films can thus be
produced with high surface quality.
The process combination using the
blowing-agent-charged melt hardly differs from the process without blowing
agent as regards the requirements on
product design, material selection, etc. It
is only necessary to consider the following additional points:
© Carl Hanser Verlag, Munich Kunststoffe international 12/2014
© Carl Hanser Verlag, München. Der Nachdruck, auch auszugsweise, ist nicht gestattet und muss beim Verlag gesondert beauftragt werden.
Injection Molding RENAISSANCE OF FOAM PROCESSES
10
Relative flexural stiffness (EI/(EI0)
Relative tensile strength (EA/(EA0)
1.00
0.95
0.90
0.85
0.80
0.75
0.70
0.65
0.60
1.0
1.5
2.0
Final wall thickness/initial wall thickness
2.5
8
6
4
2
0
1.0
1.5
2.0
Final wall thickness/initial wall thickness
2.5
© Kunststoffe
Fig. 4. Change of mechanical properties with wall thickness at constant part weight: with negative coining, only the tensile strength values are
reduced (left). In the case of the flexural stiffness, on the other hand, the greater wall thickness leads to a significant increase (right)
WW The films inserted in the mold must
permit outgassing of the blowing
agent. If the film forms a gastight barrier, bubble formation or delamination
of the film from the carrier may occur.
WW Because of the low cavity pressure
during foam injection molding, the
thickness of the film or of the textile
can be frequently reduced in many
cases. Due to the lower cavity pressure
effect, the soft decorative material is
compressed less strongly.
The elimination of sink marks due to the
foam pressure can be used to achieve
new design possibilities for ribs and bosses. Consequently, parts can be produced,
which have advantages in their surface
quality and mold complexity, compared
to conventionally produced parts.
Variotherm Process Control: High
Gloss without Secondary Finishing
The combination of the TSG process with
variotherm mold temperature control is
already used for the series production of
optically high quality panels in automotive engineering. In the first step, the cavity surface is heated. There are different
possibilities for this: the entire mold, a
mold insert or ideally only the surface of
the cavity can be heated. Known heat
sources are induction, infra-red or laser
radiation, ceramic resistance heaters or
conformal fluid temperature control with
water or steam [6].
Once the cavity surface has been
heated, the blowing-agent-charged polymer is injected. While the occurring
streaks solidify immediately on a cold
mold wall, they are eliminated again on
the preheated surface. Cooling takes
place parallel to the foaming process.
Parts manufactured with this process
combination have a high surface quality.
Due to the foaming, rib/wall thickness ratios of 1 : 1 are implemented without sink
marks becoming apparent on the surface
of the final part. With laser texturing,
matte and high-gloss surfaces can be
combined (Title figure).
»
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23
SPECIAL RENAISSANCE OF FOAM PROCESSES Injection Molding
Cross-section
Step 2
Cross-section
Step 1
cal foaming, about 10 bar is required, and
50 to 100 bar when physical blowing agent
is used. Subsequently, the blowing-agentcharged melt is injected and the nitrogen
previously introduced can escape from
the cavity again via a relief valve.
Due to the injection against the high
pressure, foaming is prevented directly at
the melt front and streaking is thus prevented [7]. The pressure level required for
injection, however, limits the possible applications to very simple geometries,
since it is very difficult to seal movable
­elements, such as slides. Elongated parts,
in which the gating point and gas inlet
and outlet are opposite one another are
suitable for this process. Only with this arrangement the gas cushion can be
through the gas outlet without uncontrolled gas inclusions occurring.
Cross-section
Step 3
0.3 mm
h0 = 2 mm
h1 = 6 mm
1.4 mm
© Kunststoffe
Fig. 5. In this example, in the first step, the cavity is volumetrically filled with a basic wall
t­ hickness of 2 mm (left). Then a 0.3 mm outer layer forms (center). The 1.4 mm-thick plastic core
is strongly stretched in step 3 (right) – making high demands on the plastic
100
Effective foam stroke
[% of opening stroke]
24
Direction of the opening
p
g stroke
80
A
60
40
Summary
20
B
0
0
10
20
30 40 50 60 70
Angle to opening stroke
°
90
© Kunststoffe
Fig. 6. All part surfaces, as in detail A, are not always oriented normal to the opening direction.
The region of the part shown in detail B is inclined by, for example, 60°. The wall thickness here is
enlarged by about 50 % of the actual opening stroke (figures: Engel)
The prerequisite for a high surface
quality is often good venting, since, in the
thermoplastic foam injection molding
process, injection is generally very fast to
prevent foaming occurring already during
injection. In addition, the gas cushion that
forms on injection due to outgassing
blowing agent requires good venting.
Foaming with Gas Counter Pressure
only for Simple Geometries
During foaming with gas counter pressure,
the pressure in the cavity is first increased
with nitrogen. The level of this varies depending on the blowing agent and blowing agent charge that are used. For chemi-
In thermoplastic foam injection molding, users profit from holding-pressure-free process
control, efficient machine dimensioning,
high freedom of design and advantageous
part properties. In addition, the process is often used to minimize the part weight.
If the foam process is combined with
negative coining, the part weight can be
reduced even more significantly than
with conventional thermoplastic foam injection molding process. In addition, this
is accomplished without loss of the flexural stiffness. In combination with the
overmolding of films and textiles, variotherm process control, or the gas counter
pressure process, even the challenges of
manufacturing high-quality visible parts
can be mastered. W
© Carl Hanser Verlag, Munich Kunststoffe international 12/2014
© Carl Hanser Verlag, München. Der Nachdruck, auch auszugsweise, ist nicht gestattet und muss beim Verlag gesondert beauftragt werden.