Page 44 - Plastics News October 2018

FEAturES

Materials: A Processor’s Most Important Job, Part 3

Michael Sepe

Processors are often expected to compensate for ill-advised decisions made earlier in the product-develop-
ment process. In the case of shrinkage, one of the most common ‘fixes’ is to simply reduce the mold tem-
perature.

few years ago, I received a call from one of my clients, orientation. These are part-design, mold-design, or
A a processor with a reasonably good understanding of material-selection issues—not processing issues.
the way plastic materials behave. They had a customer But too often the processor is expected to compensate for
who was upset with them over the way they had “solved” ill-advised decisions that are made earlier in the product-
a warpage problem with a part molded in a glass-fiber- development process. And one of the most common “fixes”
reinforced PBT polyester. They had started out with a for excessive shrinkage is to simply reduce the mold
high mold temperature of 200 F (93 C). The parts came temperature. This is also a common strategy for reducing
out of the tool and very quickly warped as they cooled. overall shrinkage when the actual shrinkage turns out to
They dealt with this problem the way many molders do: be greater than the value that was used to cut the tool.
They turned the mold temperature down. When the mold If an outer diameter or an overall length is too small,
temperature reached about 95 F (35 C) the parts were just turn down the mold temperature and the parts get
straight. Problem solved.
larger. It seems simple and harmless because the lack of
Unfortunately, the parts have to operate in their crystallization is not a defect that can be seen by those
customer’s application at 180 F (82 C). So, the customer inspecting the parts.
qualified the parts by putting them into an oven for two
hours at 200 F (93 C). When they conducted this test
on these parts, they observed that the parts warped to
almost the same degree that they did when they came
out of the hot mold. The parts were rejected, which my
client felt was unfair.
I pointed out to them that they had corrected the warpage
problem by suppressing the ability of the PBT to crystallize.
Crystallization is accompanied by shrinkage. Crystals are
well-ordered domains within the polymer structure and
they occupy less space than the amorphous regions. The
more a material crystallizes, the more it shrinks. The
initial mold temperature was above the glass-transition
temperature (Tg) of the polymer and the resin had the
opportunity to crystallize to a degree that was reasonably
close to the maximum that was achievable. The reduced
mold temperature was below the Tg and the degree of
crystallinity was correspondingly lower. This reduced the
shrinkage of the material. The only way to detect the problem is by doing what
my client’s customer did, heat the parts to operating
Warpage is a symptom of shrinkage that occurs at different
rates in different areas of the part. It can have a number conditions and check for dimensional stability. If the parts
of specific causes, such as varying wall thickness, different had crystallized to an appropriate level, the oven exposure
cooling rates due to uneven heat removal from the part, would not have created any new crystalline structure, and
and different degrees of shrinkage caused by glass-fiber the part size and shape would have remained essentially
the same. But in this case, the as-molded structure and

44
44
Plastics News October 2018ews October 2018ews October 2018 46
Plastics N
Plastics N