Page 19 - Plastics News September 2020

Annealing Tips for Semi-Crystalline Polymers: Part 4

While it is technically possible to forgo the elevated molded.
mold temperatures normally associated with best
While this may work in some cases, it is an approach
practices for processing high-performance semi- fraught with downsides. These can be appreciated by
crystalline materials, it is very risky and likely to yield illustrating what occurs structurally when a material
parts that dramatically under-perform expectations. that “wants” to be semi-crystalline is molded in an
Mike Sepe amorphous state and is then crystallized later. PEEK is a
High-performance semi-crystalline polymers typically good example of a material that can be fabricated into
have high glass-transition temperatures. This results in an amorphous or a semi-crystalline structure, depending
the need for high mold temperatures to ensure that an upon the rate at which the material is cooled. In fact,
appropriate level of crystallinity is established in the some data sheets for unfilled PEEK give a density for both
the amorphous and the semi-crystalline form of the
part. Polymers such as PPS, PEEK, PPA, SPS, and even
some PET polyesters fall into this high-performance class material. As a semi-crystalline material, the density is
where mold temperatures of at least 120 C (248 F) and as given as 1.30 g/cm3. In amorphous form it is 1.26 g/cm3.
high as 200 C (392 F) are needed in order to achieve the Most of us know that semi-crystalline polymers shrink
desired structure. Some PET suppliers have developed more out of the mold than amorphous materials. A
nucleating technologies that allow for development of typical mold shrinkage value for an unfilled amorphous
an appropriate level of crystallinity while still using mold polymer is 0.005 mm/mm or about 0.5%. Unfilled PEEK in
temperatures that can be achieved with traditional semi-crystalline form shrinks approximately 1.5%. This
water heating. may lead a processor seeking to “anneal in” the crystal
structure of PEEK to calculate that the molded part
In the case of PPA, almost all suppliers of this class of
materials have developed fast-crystallizing grades as an dimensions will be reduced by approximately 1% during
alternative to the original materials that required the the annealing process. However, the difference in
more demanding mold temperatures. However, this density between the amorphous and the semi-crystalline
improvement was achieved by manipulating the form suggests a volume change of over 3%. Why the
discrepancy?
chemistry of the polymer so that the glass-transition
temperature (Tg) is reduced. This is something that As shown here, the fast-crystallizing grade sacrifices
material suppliers neglect to mention, and while the over 50° C of performance, illustrating the continual
molder may be very pleased to be able to produce parts trade-off between processability and performance that
without resorting to extreme mold temperatures, the we see in the world of plastics.
end user may not be so enthusiastic.
The accompanying graph shows a plot of modulus as a
function of temperature for a high-performance PPA and
a fast-crystallizing grade. The fast-crystallizing grade
sacrifices over 50º C (90º F) of performance. This is an
excellent illustration of the continual tradeoff between
processability and performance that we see in the world
of plastics. If a change is made to a material that makes
life easier for the processor, it almost certainly involves
a reduction in field performance.
The converse is also true, and with most high-
performance semi-crystalline materials, achieving the
optimal structure requires the use of mold temperatures
that cannot be achieved with traditional water heaters.
Instead, pressurized water, oil, or heater cartridges are
required. This can be a significant barrier to entry for What many of us forget is that mold-shrinkage values are
some processors. Consequently, some processors calculated from the way that parts shrink after they
attempt to anneal into the parts most of the crystallinity have been formed under significant pressure. As a
that is required. Instead of using the annealing process material cools in a mold, we continue to apply pressure
to produce the last 10% of the achievable crystallinity, to compensate for the relatively large change in volume
they run the material at a low mold temperature that that would occur if the part were to cool in an
essentially results in an amorphous structure and then unrestrained state. Amorphous polymers typically
they “bake in” the crystal structure after the parts are exhibit mold shrinkage values of approximately 0.5%,

September 2020 19 Plastics News