Page 32 - Plastics News November 2018

FeAtures

Materials: A Processor’s Most Important Job, Part 4

Michael Sepe

Engineering polymers require higher mold temperatures to achieve their ideal structure. The temptation to
turn down the mold temps can hurt part performance.

enerally, higher performance in polymers is associated to evaluate the temperature-dependent behavior of
Gwith the ability of a material to withstand exposure the material. This is provided as a plot of the elastic
to elevated temperatures. In semi-crystalline polymers, modulus of the material as a function of temperature.
two important transitions characterize this capability: The different levels of crystallinity that the polymer was
the glass-transition temperature (Tg) and the melting able to achieve at these various mold temperatures are
point (Tm). Polypropylene, which we discussed at the evident from the response of each sample.
end of the last article, is generally considered to be a Notice that the initial modulus of the material trends
commodity material with a Tg that is sub-ambient, near upward with increasing mold temperature. The modulus
0 C, and a Tm for most grades at 320-329 F (160-165 C).
of the sample molded at 57 C is approximately 9000
This temperature difference between the two transitions MPa (1300 ksi) while the sample molded at 159 C has a
is typical for most semicrystalline materials. Aliphatic room-temperature modulus of 10,350 MPa (1500 ksi).
nylons such as nylon 6 and nylon 66, and semi-crystalline This is a 15% improvement. The largest increase occurs
polyesters such as PBT and PET, form the next tier on the between the samples molded at 125 C and 159 C. But a
performance ladder, with Tg’s of 131-194 F (55-90 C) and much greater difference in performance is evident as
melting points of 437-500 F (225-260 C). the test temperature increases. As the temperature of
the tests reaches 130 C (266 F) the modulus of all four
Unlike PP and polyethylene, which can be run at relatively
low mold temperatures, these engineering polymers
require higher mold temperatures to achieve their ideal
structure. It is with these materials that the temptation to
turn down the mold temperature can become problematic
for part performance.
As we go up the performance scale to SPS, PPS, partially
aromatic nylons—sometimes referred to as PPA, and very
high-performance materials such as PEEK, the stakes get
higher. These materials have Tg’s that exceed the boiling
point of water (230-302 F). Therefore, maintaining mold
temperatures high enough to allow these materials to
crystallize to a satisfactory level requires the use of
pressurized water, oil or electric cartridges. Not all
processors are willing to adopt these technologies. But at
this high-performance level the consequences of failing
to achieve the mold temperatures required to promote
adequate crystallinity become especially severe. samples begins to decline rapidly. This is the onset of the

Figure 1 shows the behavior of test specimens molded glass transition. An exact measurement of the Tg for this
using the four different mold temperatures noted in the material gives a value of 145 C (293 F).
legend: 57, 90, 125, and 159 C (135, 194, 257 and 318 F). Semi-crystalline polymers exhibit a significant decline
The material is a partially aromatic nylon (PPA) reinforced in modulus as they pass through the glass transition.
with 35% glass fiber. Specimens from each sample group When polyamides (nylons) are filled with this level of
were tested using dynamic mechanical analysis (DMA) glass fiber it is expected that the material will retain

Plastics News November 2018 32