Page 28 - Plastics News February 2019

FeAtures

Nucleation allows crystallization to begin at more sites however quality checks still involved cutting open the
simultaneously, producing a larger number of small thick section in almost 5% of our production to inspect
crystals as opposed to a smaller number of large ones. for voids, since non-destructive imaging techniques were
This will alter the properties of the material, increasing not readily available at that time. And even with all the
stiffness while typically reducing impact resistance. appropriate process strategies in place, we struggled with
It will also reduce the amount of shrinkage that the certain lots of material.
material exhibits as it cools, and it allows for a reduction When we analyzed our quality data as a function of lot-to-
in cycle time since the crystallization process begins and lot behavior, we noticed that the problem lots were the
ends at higher temperatures. While nucleation is often ones manufactured to the high end of the melt-flow-rate
accomplished intentionally with additives, it also can range. The nominal MFR specification was 11 g/10 min
occur unintentionally with the addition of certain fillers with a low end of 9 and a high end of 13.
and colorants.
Lots that came in below 11.5 g/10 min afforded us the
Another material property that influences the rate of needed processing window without producing voids. Voids
crystallization is molecular weight (MW). This is an often- occur due to excessive, localized shrinkage; and when the
forgotten relationship. Higher-MW grades of a given crystallization rate was faster this shrinkage occurred to
polymer will crystallize more slowly than their lower-MW a greater degree. The higher-viscosity lots shrank less.
counterparts. Again, if it is understood that crystallization
is promoted by chain mobility, this makes perfect sense. We may have been the first processor to ever contact a
In higher-MW systems, chain entanglement occurs to a material supplier and ask for a material that did not flow
greater degree and this limits mobility, suppressing the as well. The technical-service representative for the
tendency for the material to form crystals. resin company even tried to dissuade us from going in
this direction, arguing that we needed an easier-flowing
Too much crystallinity will reduce the toughness of the material so that we could pack more effectively. But the
material and can result in brittle failures.
experimental data confirmed our approach and by cherry
We used this principle many years ago to solve a difficult picking lots of higher viscosity we essentially eliminated
problem with voids that formed in the thick section of a the defect. We used this approach many times to solve
part molded from a glass-fiber-reinforced PBT polyester. internal void issues in thick walls.
The part had been designed with a nominal wall thickness We have probably exhausted the crystallinity conversation
of 3/16 in. but had an area approximately 3 in. from the at this point. Next time we will delve into another defect
gate where the wall thickness increased to almost 1 in.
that can be difficult to detect but is often driven by
This area was prone to form voids as the part cooled, molding conditions: internal stress.
a defect that was completely unacceptable to the end Mike Sepe is an independent, global materials and
user. Process development had minimized the problem, processing consultant

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