Page 34 - Plastics News March 2019

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and elect to either fixture a part to prevent movement longer if the mold temperature is increased. However,
while it cools, or alter the processing conditions to this objection can be answered by observing what occurs
reduce warpage, they are often building in even higher when the melt temperature is reduced.
levels of stress. Most processors know that reducing Lower Melt Temperatures
the temperature of the mold is a very effective way of
reducing warpage. We have discussed the temporary While lowering the melt temperature had little to no
nature of this improvement when talking about semi- effect on part performance when the mold was cold, the
crystalline polymers. But what about materials that do same adjustment produced an additional improvement
not crystallize? in impact resistance when the mold temperature was
increased. The best results, impact resistance of 48
How do amorphous polymers like polycarbonate, ABS, and nanometers (35 foot-pounds), were achieved with
polysulfone react to the faster cooling rate associated the lowest melt temperature and the highest mold
with a lower mold temperature? A study performed temperature. By reducing the melt temperature, the
on ABS some years ago offers some insights into this increased mold temperature can be managed without
question. The accompanying graph shows the results of changing the cycle time. Any fabricated part will contain
this study; an evaluation of the effect of mold and melt some level of internal stress that arises naturally from
temperatures on the impact resistance of test specimens melt processing.
molded from a particular grade of the polymer. ABS is
generally considered to be a reasonably tough material, The best way to remove heat quickly is not to introduce
and materials in this class will typically produce falling it in the first place. The lower melt temperature also
dart energies-to break in the range of 30-40 nanometers reduces the risk of polymer degradation. This behavior
(22-30 foot-pounds) and exhibit a ductile failure mode. can be verified by using another measure of molded-in
However, note that when specimens were prepared in a stress, resistance to stress cracking. When molding some
mold set at a relatively low temperature of 84°F (29°C) clear PC parts using a relatively high melt temperature
the energy required to break the test specimens was only of 610°F (321°C) and a low mold temperature of 75°F
1.5 nanometer (1.1 foot-pounds), a value associated with (24°C), it was observed that the parts developed cracks
a very brittle material. when exposed to olive oil. In fact, parts molded at these
conditions spontaneously developed cracks even without
Altering the melt temperature across a fairly wide exposure to the olive oil if enough time was allowed to
range from 425°F (218°C) to 525°F (274°C) did not pass after the parts were produced.
appreciably change this result. However, when the
mold temperature was increased, the impact resistance When the mold temperature was raised to 221°F (105°C)
improved dramatically. With the mold temperature set and the melt temperature was reduced to 530°F (277°C)
at 185°F (85°C), the impact resistance improved to 30 the stress cracking was eliminated without lengthening
nanometers (22 foot-pounds) with the melt temperature the cycle time. The part surface also had a better
set at the upper end of the experimental range. This appearance because the higher mold temperature
improvement is attributable to the smaller difference in delayed the rate at which the surface material solidified.
cooling rate between the surface material and the core These are anecdotal results, and in the world of plastics,
material, coupled with a reduction in the thickness of the everyone has a good story or two. But this does not pass
oriented layer of material. Both factors reduce the level for science.
of molded-in stress in the part, making it more capable In our next article, we will elaborate on the foundation
of managing an externally applied stress. principles behind these improvements and translate
The natural response of most processors, when presented them to the process conditions that ensure optimal
with these data, is to protest that the cycle time will be performance for the molded part.

Plastics News March 2019 34