Page 33 - Plastics News November 2018

FeAtures

approximately 50% of its room-temperature modulus with a Tg of approximately 0 C, will show only a marginal
above the Tg. This is exactly what is observed for the increase in crystallinity at higher mold temperatures,
specimen molded at the highest mold temperature. The while crystallization time increases significantly, slowing
samples molded at the lower mold temperatures exhibit the cycle time. The nylons, with Tg’s of 65-70 C, will
progressively larger modulus reductions as the mold reach “full” crystallization using mold temperatures of
temperatures used to produce the samples decline. At 90-100 C, and it is evident that in this temperature range
the lowest mold temperature, the modulus decline is the crystallization time is essentially constant. As it rises
nearly 75%. This will have implications for long-term above this point, the crystallization time slowly increases,
performance characteristics such as creep resistance once again extending cycle time.
and fatigue resistance, even at temperatures below the However, note how differently the other two polymers
glass transition.
behave.The PET
It is unfortunate that some suppliers of high- evaluated here is
performance semi-crystalline materials tell molders that a non-nucleated
full crystallization is only important if the part is intended mater ial, so
for use at temperatures above the glass transition. This the behav io r
is not at all true, something we will address in a later documented here
article. is not typical of
the highly filled
Another notable difference between the sample molded at
the highest mold temperature and the other three samples semi-crystalline
can be observed at the conclusion of the glass transition. grades used for
The part molded at the highest mold temperature enters injection molding.
a slowly declining plateau region between the end of The MXD6 is one
the glass transition and the onset of the melting point of the partially
near 300 C (572 F). This is typical behavior for a properly aromatic nylons
crystallized polymer. The other three samples show that we have been
another symptom of incomplete crystallization. As soon referring to. These materials, and others like them,
as the glass transition is complete, the modulus begins such as PPS and PEEK, display a much different behavior
to increase, even though the temperature is increasing. than the polypropylene and the aliphatic nylons. With
This is caused by the formation of crystals that were these materials the rate of crystallization decreases
intended to be produced during the molding process. But significantly once the mold temperature declines below a
because these mold temperatures are too low to allow certain point that is related to the Tg of the polymer. The
for a complete level of crystallinity, the shortfall has to effect is considerable. For the MXD6 the crystallization
be made up in the solid state once the mobility needed time is 25 sec at a mold temperature of 160 C, while
to form crystals has been re-established. This upturn it increases to 180 sec at a mold temperature of 120
in the modulus is also associated with the previously C. For these materials, not only does the lower mold
discussed dimensional changes that always accompany temperature prevent the achievement of the desired
crystallization. level of crystallization, it increases the time required
to reach an ejectable modulus, which in turn increases
It is not a coincidence that the dividing line between the cycle time. Understanding the consequences of changing
properly crystallized sample and the ones that fall short the mold temperature requires knowledge of the specific
is the glass transition temperature of the polymer being polymer being processed.
molded. Maintaining a temperature above the Tg provides
the molecular mobility needed to form crystals. Figure 2 In our next article we will look at a semi-crystalline
shows the relationship between mold temperature and polymer that does not follow the rule that maintaining a
the rate of crystallization for various materials. mold temperature above the Tg is the only requirement
for achieving appropriate crystallinity.
Three of the profiled materials—PP, nylon 6, and nylon
66—are materials with glass-transition temperatures ABOUT THE AUTHOR: Mike Sepe is an independent, global
below the temperature range shown in the graph. The PP, materials and processing consultant

33 November 2018 Plastics News