Page 30 - Plastics News December2018

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A Processor's Most Important Job, Part 5

Michael Sepe
Using a mold temperature above a polymer’s Tg ensures a degree of crystallinity high enough to provide
for dimensional stability, even if the part must be used at elevated temperatures. But POM is an exception.
Why?.

hree commonly used semi-crystalline polymers, chilled water through a mold should be sufficient to ensure
Tpolyethylene, polypropylene, and acetal—more adequate crystallinity. But the suppliers of these materials
commonly referred to these days as polyoxymethylene recommend use of relatively high mold temperatures:
(POM)—have glass-transition temperatures below room 80-85 C (176-185 F) for copolymers and 90-95 C (194-203
temperature. This means they continue to crystallize even F) for homopolymers. The design and processing guides
when they have cooled to room temperature. Processors from the resin suppliers have treated this subject in great
who have had to mold parts in these materials to close detail to quantify the complex relationship between
tolerances have experienced this continued crystallization mold temperature, part wall thickness, and post-mold
as a prolonged period over which the molded part dimensional changes at various application temperatures.
continues to shrink.
Figure 1 shows such a review for POM homopolymer provided
Most of the time a molded part will cool to room by DuPont, which invented the material in 1960. The
temperature and become dimensionally stable within 30- three graphs plot the
90 min, depending upon the polymer, the wall thickness relationship between
of the part, and the dimension being measured. But in the applicatio n
these three materials, dimensional changes can continue temperature and
for 24-48 hr. This continued shrinkage is physical evidence subsequent post-mold
that crystallization is continuing. shrinkage that may
Fortunately, most parts reach stable dimensions within occur as a function of
this extended time. However, if the mechanical properties the mold temperature
of the polymer are measured, a progressive change in used when the parts
strength, modulus and impact resistance will be observed were produced. This
that can continue for weeks. relationship is shown
for three different
One of my clients reported that when they shipped parts wall thicknesses; 0.8
fabricated in POM on a just-in-time basis, they would get mm (1/32 in.), 1.6 mm
complaints that the parts did not “feel stiff enough.” (1/16 in.), and 3.2 mm
They did not receive these complaints on parts that had (1/8 in.).
been in the warehouse for several weeks before being
shipped. This behavior is particularly troublesome in The thickest wall
POM because of the need for dimensional precision and provides the greatest
stable properties in functional parts such as gears. The degree of dimensional
problem becomes magnified if the part must operate at stability. At this thickness, if the parts are never exposed to
an elevated temperature, since this will promote even conditions above room temperature, then the subsequent
more post-mold shrinkage. dimensional change is 0.001 in./in. if the part is molded
at 38 C (100 F). As the mold temperature increases, this
We have already discussed a principle that employing a post-mold change drops essentially to zero when the mold
mold temperature above the glass-transition temperature temperature reaches 121 C (250 F). However, if the part
(Tg) of the polymer ensures a degree of crystallinity high is exposed to elevated temperatures, the dimensional
enough to provide for dimensional stability, even if the change due to post-mold shrinkage increases significantly.
part must be used at elevated temperatures. But this
rule appears to break down when it comes to POM. The For the part molded at 38 C, exposure to an application
Tg of POM is near -80 C (-121 F). Therefore, running even environment of 100 C results in a dimensional change of

Plastics News December 2018 30