Page 26 - Plastcs News January 2019

FeAtures

A Processor's Most Important Job, Part 6

Michael Sepe

The importance of mold temperature to the development of the desired polymer crystalline structure
becomes absolutely crucial in the case of high-performance materials.

s we have indicated in recent articles, the rate molded. Many processors consider this a barrier to entry,
Aat which semi-crystalline polymers develop their and material suppliers want to sell resin.
crystalline structure tends to correlate with the level Therefore, sometimes there is a mixed message in
of performance that the materials can achieve in the the advice coming from the suppliers. The processing
solid state. Materials like PE and PP, often considered community is sometimes told that full crystallization
to be commodities, crystallize readily due to their low is only important if the part being molded will need to
glass-transition temperatures (Tg). The cooling rate of perform at temperatures above the Tg of the polymer. The
the material can be adjusted to change the degree of reasoning goes something like this: An under-crystallized
crystallinity in the molded part.
part will never experience performance problems if it is
However, these changes tend to be relatively small and not exposed to a temperature high enough to promote
the longer cycle times that may be needed to manage the additional crystallinity that was not achieved due to
the slower rate of heat removal may not be justified. quench cooling. According to this advice, the unstable
As we move up into the semi-crystalline polymers of
intermediate performance—aliphatic nylons, acetals,
and polyesters—the ability of the processor to affect the
degree of crystallinity becomes greater, and maintaining
the correct mold temperature is more important to part
performance.

But it is in the high-performance realm, materials
such as polyphthalamides, PPS, and PEEK, where the
importance of mold temperature to the development of
the desired structure becomes absolutely crucial, and
missing the correct mold-temperature setting by even 10-
15° F (5-8° C) can result in a part that falls far short of the
intended performance of the material. In most cases the
suppliers of these materials provide clear instructions on
how the mold temperature must be set to ensure adequate
crystallinity. However, occasionally a material supplier will
try to let the processor off the hook and in doing so they state associated with incomplete crystallization will only
give some very bad advice. come into play if the part becomes hot enough to undergo
continued crystallization.
we have already discussed the importance of maintaining
a mold temperature high enough to ensure a satisfactory Unfortunately, it is not that simple. Figure 1 shows a
level of crystallization in these high-performance comparison of modulus vs. temperature behavior for two
materials. Without exception, these mold temperatures specimens produced from a 40% glass fiber-reinforced
cannot be achieved with standard water-circulating units. PPS. The mold temperature used for the cold-molded part
They require the use of hot oil, electric cartridges, or was 180 F (82 C) while the mold temperature used for the
pressurized water since the required mold temperatures hot-molded part was 300 F (149 C).
are in the range of 250-400 F (121-205 C), depending The recommended mold temperature in the processing
upon the polymer and the wall thickness of the part being guides for achieving optimal crystallinity is 275 F (135 C).

Plastics News Januar y 2019 26