INNOVATION AND TECHNOLOGY FROM PORTUGAL
PORTUGUESE PROCEEDINGS AT SPE ANTEC CONFERENCES 1989 - 2002
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“ THIS PAPER IS COPYRIGHTED BY THE SOCIETY OF PLASTICS
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ANTEC, MAY 2001”
universidade do minho
FRICTION PROPERTIES OF THERMOPLASTICS IN INJECTION
MOLDING
Ferreira, E.C., Neves, N.M., Muschalle, R. and Pouzada, A.S.
Departamento de Engenharia de Polímeros, Universidade do Minho,
4800-058 Guimarães, Portugal
Abstract
In the ejection stage of parts injection molded over
cores the knowledge of the friction properties between the
mould surface and the part are important to optimize the
ejection system solution. The coefficient of friction
depends strongly on the mould surface and the temperature
at the moment of ejection. Prototype equipment was
developed to measure the friction properties in as-molding
conditions, and methods developed to perform the testing.
Data will be presented for two thermoplastics
(polycarbonate and polypropylene).
Introduction
Injection molding is one of the leading technologies
for the production of engineering plastic parts. The
molding surfaces of the injection molds are usually
obtained from steel blocks by various processes like spark
erosion or milling. Those techniques have limitations in
terms of attainable finishing smoothness of the surface.
In the ejection stage of injection molding the parts are
mechanically forced to separate from the molding surfaces
(especially from the cores). The efficiency of the ejection
is related with a number of factors that are of concern to
the designer. Namely, the draft angles of the core and its
surface finish the properties of the plastic material at the
ejection temperature and the dimensioning of actuation
devices (such as hydraulic or pneumatic cylinders).
Aesthetics and functionality of products may require
the use of small draft angles. Small draft angles lead to an
increase of the ejection forces.
Good surface finishing is obtained by time consuming
techniques like polishing, leading to more expensive
molds. Economy in the mold making industry puts
pressure to the use of not so smooth surface finish of cores
(molding the inner part of moldings that are normally
invisible). In addition, it is known that very good polished
surfaces (mirror-like) can be difficult to separate by the
local formation of vacuum. To minimize these problems it
is common practice however to make the finishing in the
ejection direction.
Productivity in injection molding requires the
minimization of the cooling time at the cost of leading to
higher ejection temperatures and poorer mechanical
properties of the polymer materials. These additional
factors further contribute against an easy and safe ejection
of the parts from the mold.
Moreover, the thermal expansion coefficient of
thermoplastics is considerably different (0.6-1.4×10-4 ºC-1)
than that of steel (12×10-6 ºC-1). Thus, after cooling from
melt temperature, the plastic tends to stick over the surface
of the cores, reproducing closely its surface finish. This
unusual circumstance could lead to significant variation of
the coefficient of friction, since in common standard test
methods this condition is never considered. That was the
motivation to develop a prototype equipment to study the
effective coefficient of friction under those conditions.
In this work the prototype equipment is presented and
some preliminary results obtained using PP and PC
introduced.
Solid friction
Friction is normally understood as the resistance
offered by bodies in contact to relative motion. In injection
molding the bodies in contact are steel molding surfaces
and polymer moldings.
Coulomb (ca. 1781) experimentally confirmed the
fundamental theory that governs the friction behavior of
materials. Leonardo da Vinci and Amontons preceded his
work, the latter being the first to propose the use the
coefficient of friction and to establish a distinction
between static and kinetic coefficients of friction. Those
laws can be stated in very simple terms as:
• Static friction may be larger than kinetic
(dynamic) friction
• Friction force is proportional to normal force
• Friction force is independent of the contact area
The conclusions of Coulomb’s work are used in many
practical situations with good results. However, there are
some situations in which laws of friction are not in
agreement with experimental observations [1].
Friction is caused by forces acting at the interface
between the surfaces of contacting bodies. The magnitude
of those forces is related to the properties of the two
surfaces in contact and the properties of the two materials.
These forces are usually difficult to predict because the
surface properties continuously change over time by
deformation, wear, segregation of components or
oxidation. Moreover, the real contact area between the
bodies is also different than the apparent area of bodies
owing to the roughness of the contact surfaces.
The friction properties of pairs of materials are usually
represented by the coefficient of friction, µ. The
coefficient of friction is defined (e.g. in ASTM G40 [2]
test standard) as:
µ=
F
N
(1)
In which,
F – friction force
N – normal contact force
The same standard defines a coefficient of static
friction, µs, corresponding to the maximum force that must
be overcome to initiate macroscopic motion between the
two bodies.
A coefficient of kinetic friction is obtained from the
average friction force necessary to maintain the
macroscopic relative motion between the two bodies. It is
represented by µk. In our case, because of the draft angles
used in injection molds, this parameter is relatively less
interesting.
The coefficients of static friction range from 0.03 in
specially lubricated bearings to 0.5 - 0.7 in the case of dry
sliding [3]. Between very smooth and clean metal surfaces
in vacuum, values of the coefficient of static friction
exceeding 5 were obtained. Typical values for the
coefficient of friction of various polymers in contact with
steel are listed in Table 1.
Friction in the ejection stage of injection
molding
Menges et al. [4] first attempted the determination of
the coefficient of friction relevant for the prediction of
ejection forces in the early 80´s. In their work a mould was
developed enabling the study of the effect of different
molding conditions on the coefficient of friction. The
surface roughness and the presence of a release agent were
observed to be the most important parameters. The melt
temperature and the mould temperature were identified as
second order parameters concerning its influence over the
coefficient of static friction. Some results of the maximum
coefficient of static friction obtained for PE, PP, PS, ABS
and PC were reported. However, the scatter in their results
was pointed out as a problem limiting the broadness of the
conclusions obtained.
Vaziri and co-workers studied the dynamic friction
between polymers and steel [5]. This property is less
interesting for the ejection stage in injection molding
owing to the draft angles used in this case.
In the late 80´s Malloy et al. [6] studied the design of
ejector pins to be used in ejection moulds. In this work the
same difficulty of prediction of the coefficient of friction
was identified as one of the key problems to design the
ejection system in injection moulds. Later in the early 90´s
Burke et al. [7] used the thermal expansion coefficient, the
stiffness at the temperature of ejection and coefficients of
friction obtained from [4] to predict ejection forces. The
error in the predictions was of the order of 16 % for ABS
and HDPE. Later, Malloy et al. [8] reported on a standard
test procedure but with a temperature controlled chamber.
Steel, nickel-plated steel and PTFE/nickel plated steel
specimens were studied in contact with PS, PP,
PC/polyester alloy and 10 % glass fiber reinforced PC
specimens. The effect of the test temperature on the
coefficient of friction was also analyzed. Plating and the
use of release agents were reported as the most effective
ways of decreasing the coefficient of friction
polymer/steel.
Recently, Dearnley [9] reported a study on the friction
force developing between a ring of polymer molded over a
ring of steel with different surface roughness. Good
correlation was observed between the roughness of the
steel and of the polymer ring. Coatings of TiN, CrN and
MoS2 were studied in terms of the friction force against
acetal (POM). CrN coatings in P20 steel lead to the lowest
observed friction forces in spite of the slightly higher
surface roughness. This result was attributed to the
chemical behavior of the coating at the interface.
Prototype equipment
It was designed and developed a new equipment
enabling to study the effect of different parameters on the
coefficient of friction relevant for the ejection of plastic
parts from moulds. The concept selected is illustrated in
Figure 1.
The specifications for the equipment were:
• Range of operating temperatures (20-180ºC)
• Range of testing speeds (1 - 100 mm/min)
• Replication of the surface roughness of the molding
surface into the plastic specimen
from GE Plastics. The testing specimens, half of the
moldings, were cut from those moldings.
• Control of the normal contact force between molding
surface and specimen
The normal contact force applied by the pneumatic
cylinder was set to ca. 440 N.
• Control of the evolution of the friction force with time
The testing speed during friction test was kept at
10 mm/min.
To meet the specifications various functional systems
were used: temperature control, control of contact pressure
for replication of surface and for testing and guiding
system. A brief description of the functional systems will
follow.
Temperature control system
The control of the temperature is important to enable
good replication of surface at temperatures close to melt
temperature of semi-crystalline materials or above the
glass transition temperature in the case of amorphous
materials under study. It is also important to ensure
stability of temperature during the friction test.
Cartridge heaters mounted into the structure allow to
raising the temperature from room temperature up to the
replication temperature within a reasonable time (typically
5 minutes). Additionally, 5 mm insulating plates were used
to minimize heat losses.
The control is made with a Fe/Constantan
thermocouple connected to a temperature controller. The
depth between the tip of the thermocouple and the steelmolding surface is 2 mm.
Cooling down from replication temperature to the
testing temperature is obtained by circulating water in the
cooling circuit.
The testing routine included the following stages:
1.
Heating of the molding surface up to the
replication temperature
2.
Stabilization of the temperature
3.
Application (during 60 s) of the contact
pressure (1.4 MPa) to obtain surface
replication
4.
Cooling down to testing temperature
5.
Stabilization of temperature (60 s)
6.
Friction test (at 10 mm/min cross-head
speed)
For each condition seven specimens were tested. The
cycle time for the complete routine is of about 17 minutes
for PC specimens and 15 minutes for PP.
The testing parameters were adjusted to the specific
properties of the materials to be tested and are listed in
Table 2.
Results and discussion
System of control of contact pressure
A pneumatic cylinder is used to actuate the contact
pressure. The control of the pressure obtained with a
piezo-resistive pressure sensor.
System of control of friction force
The use of a tensile test machine was considered
easiest and more reliable way to control and acquire
data of the friction force during the test. Thus,
prototype apparatus (Figure 2) was designed to
mounted and work with a tensile test machine.
The material of the molding surface in contact with
the polymer specimen in all cases was tool steel (DIN
Ck 45).
the
the
the
be
Experimental
Two thermoplastic materials were used to produce
0.002 m thick injection-molded square testing specimens
(0.0625×0.0625 m2): a polypropylene HIFAX BA238G3
from MONTELL and a polycarbonate LEXAN 141 R
The results presented (Table 3) show the expected
pattern for the evolution of the force during the friction
tests. The force rises up to a maximum, corresponding to
the force used in the calculation of the static coefficient of
friction.
Over that maximum, the force tends to level off at
lower value, enough to keep the relative motion between
the two surfaces. That lower force is not constant and it
may oscillate in a periodic way.
The equipment developed seems
reproducible results as shown in Figure 3.
to
produce
The polycarbonate specimens show a coefficient of
static friction at 80ºC, with polishing transversal to the
testing direction of 0.47 (Figure 4). This is a relatively
high value when compared with values reported in
literature [3]. However, one must keep in mind that the
testing conditions are not comparable.
In similar testing conditions but at 50ºC,
polypropylene specimens show (Figure 5) a much lower
coefficient of static friction (0.19). This result is
considerably lower than previously published coefficients
of friction for this material [3] using standard tests.
The effect of testing temperature over the coefficient
of static friction is shown in Figure 6. Higher testing
temperature seems to increase the value of the coefficient
of static friction of polycarbonate.
Conclusions
8. Balsamo, R., Hayward, D. and Malloy, R., Proceedings
of Antec'93, 2515-2521, 1993
9. Dearnley, P. A., Wear, 225:229, 1109-1113, 1999
10. ASM Handbook, Appendix: Static and Kinetic Friction
Coefficients for Selected Materials, ASM, 73, 1992
Table 1 Coefficients of friction between various polymers
and steel [10]
µs
µk
0.36
0.26
PC
0.31
0.38
ABS
0.40
0.27
PA 6
0.54
0.37
Polymer
New prototype equipment was developed to study the
coefficients of static friction relevant for the ejection of
injection-molded parts.
PP
*
Preliminary tests indicate that the equipment is able to
replicate the surfaces of molding surfaces into plastic
specimens.
Tests using PC and PP seem to lead to very different
behaviors. At 25ºC the coefficient of static friction of PC is
consistent with previously published data. At 80ºC the
coefficient of static friction is larger than at room
temperature.
The coefficient of static friction obtained at 50ºC for
PP is lower than the values reported in literature.
* Mild steel/polycarbonate
Table 2 Testing parameters
Parameter
PP
PC
150
170
50
25 and 80
App. contact area (mm )
300
300
Roughness, Ra (µm)
0.5
0.5
Replication temp. (ºC)
Test temp. (ºC)
Acknowledgments
The Fundação para a Ciência e a Tecnologia (FCT)
for their support to the Project Mouldfriction, the Institute
of Materials (U. Minho) and the EU program
Socrates/Erasmus for the grant to Mr. R. Muschalle.
References
1. Pontes, A. J., Pinho, A. M., Miranda, A. S. and
Pouzada, A. S., O Molde, 10:34, 25-34,1998
2
Table 3 Summary of coefficients of friction
Material
Temperature (ºC)
Literature
[10]
25
80
PP
-
0.19
0.36
PC
0.32
0.47
0.31
2. G 40 test standard, Annual Book of ASTM Standards,
ASTM
3. ASM Handbook, 18, Friction, Lubrication and Wear
Technology, ASM, 1992
Surface
replication
Contact pressure
4. Menges, G. and Bangert, H., Kunststoffe German
Plastics, 71:9, 552-557, 1981
5. Vaziri, M., Stott, F. H. and Spurr, R. T., Wear, 122,
313-327, 1988
6. Malloy, R. and Majeski, P., Proceedings of Antec'89,
New York, 1231-1235, 1989
7. Burke, C. and Malloy, R., Proceedings of Antec'91,
Montreal, p. 1781-1787, 1991
Friction
force
Controlled
temperature
Figure 1 Illustration of the concept for the development of
the prototype equipment
250
200
Load (N)
150
100
50
0
0.0
Figure 2 Mounting of the testing apparatus in a tensile
testing machine
0.5
Displacement (mm)
1.0
Figure 5 Polypropylene, friction test result with surface
replication, roughness Ra=0.5 µm machined in the across
testing direction, 50ºC test temperature (µs=0.19)
250
250
80ºC (0.47)
200
200
25ºC (0.32)
Load (N)
150
Load (N)
150
100
100
50
50
0
0.0
0
0.0
0.5
Displacement (mm)
1.0
Figure 3 Reproducibility of testing results (PC)
250
200
Load (N)
150
100
50
0
0.0
0.5
Displacement (mm)
1.0
Figure 4 Polycarbonate friction test result with surface
replication, roughness Ra=0.5 µm machined in the across
testing direction, 80ºC test temperature (µs=0.47)
0.5
Displacement (mm)
Figure 6 The effect of testing temperature over the
coefficient of friction of PC
1.0