Page 25 - Plastics News June - 2020

Annealing Tips for Amorphous Polymers
Mike Sepe
In amorphous polymers, annealing is performed to draw But even with a high mold temperature, the cooling rate
down the internal stresses to a level not achievable of a polymer during the injection molding process is on
within the conditions of a normal molding process. But a the order of 150-300° C/min (270-540° F/min). With
few parameters are important to achieving the desired such a rapid change in temperature, some level of
results internal stress is inevitable. In situations where the
application environment involves some combination of
Amorphous polymers are susceptible to failure by elevated temperature, extended lifetime, stresses that
environmental stress cracking (ESC). We understand this may exceed the proportional limit, and exposure to
mechanism to be essentially a mechanical failure that is
certain chemicals, even relatively low levels of internal
accelerated by the presence of a chemical that locally stress may result in premature failure due to ESC.
plasticizes the polymer in an area where a small defect Failure-analysis meta studies have shown that ESC is the
has been created.
leading cause of field failures in plastic parts and this
The defect can be an inclusion such as a piece of metal or failure mode primarily affects amorphous polymers.
carbon char or it can be a notch created by incidental
In amorphous polymers, annealing is performed to draw
damage. It may also be due to a design defect such as a down the internal stresses to a level not achievable
sharp corner or a rapid change in the wall thickness of within the conditions of a normal molding process. There
the part that creates a locally elevated stress level. Or it are a few parameters that are important to achieving
can be promoted by an elevated level of internal stress the desired results. The first of these is the temperature
caused by molding conditions. High levels of internal
of the annealing process. Typically, the recommended
stress are caused by rapid cooling of the polymer.
annealing temperature is indexed to the glass-transition
A processing strategy that involves rapid cooling can also temperature (Tg) of the polymer. This can be readily
influence short-term properties, most notably ductility. measured by analytical techniques such as differential
This is a concern since many amorphous polymers, such scanning calorimetry (DSC) or dynamic mechanical
as ABS and PC, are used in large part because of their analysis (DMA). DMA has the advantage of measuring the
excellent toughness. Figure 1 shows the results of a physical properties of the polymer, therefore it provides
study on the effects of melt and mold temperature on more information about the range of temperatures that
the impact resistance of ABS. This shows that the molded can be used to relax the internal stresses in the part.
specimens exhibit very low energy to break when the Failure-analysis meta studies have shown that ESC is the
mold temperature is set relatively low. As the mold
leading cause of field failures in plastic parts and this
temperature is increased the impact resistance rises failure mode primarily affects amorphous polymers.
dramatically.

Effect of the Mold Temperature

Figure 2 provides a plot of the elastic modulus as a
function of temperature for a typical PC. The Tg occurs
in the temperature region where the elastic modulus of

June 2020 25 Plastics News