Page 45 - Plastics News October 2018

FEAturES

the intended structure were quite different, and when the it under controlled conditions in the DSC until it melts.
polymer was exposed to a temperature that re-established The melting event will be observable as a peak that is
the mobility needed to form new crystals, it did what it associated with a property known as the latent heat of
was intended to do. fusion. This is a measure of the degree of crystallinity.
The greater the heat of fusion, the more crystal structure
If the dimensional changes associated with this additional
crystallization occur once the part is in the field, the is present in the molded part.
results can be unpleasant, to say the least. Cracks can The melted sample is then cooled back to the solid
occur at attachment points; parts that are designed state, producing a peak that is a mirror image of the
to move relative to one another can bind; components melting event. This is the heat released by the process
designed to act as sealing surfaces can allow fluids to of crystallization. This heating and cooling process erases
leak from a valve or a housing. In this case, the warpage the thermal imprint of the molding process. When we heat
was an indicator that there was a problem that needed the sample a second time, we should see a new heat of
to be addressed, not covered up by making an ill-advised fusion that will be higher than the one we saw on first
process adjustment. heat. This is because the cooling rate in the DSC is much
slower than that which the material experienced during
It is important to understand that the cooling rates
associated with injection molding will never allow attaining the molding process. Figures 1 and 2 show the results for
the maximum achievable level of crystallinity. Consider a this type of DSC test performed on a PP part.
nylon 66 heated to a melt temperature of 550 F (288 C) The heat of fusion on first heat is 89.72 J/g, while the
entering a mold cavity set at 210 (99 C), a temperature second heat of fusion is 99.43 J/g. When we divide the
above the Tg of the nylon that will produce a relatively value from first heat by the value from second heat we get
high degree of 90.2%. That is considered to be the degree of crystallinity
crystallinity. If that we achieved relative to what was theoretically
the part cools possible if we could somehow cool the polymer very
to an ejectable slowly in the mold and convince our customer that it was
temperature acceptable to produce 15 parts/hr. This is the point. In
in 30 sec, then the real world of manufacturing there is a compromise
the co o ling between achieving the perfect structure in a material and
rate for the making a part at a price that is competitive. A value of
material is 680° 90% is considered to be a good outcome in most cases. It
F (378° C)/min! is important to understand that this result does not mean
When we test that the polypropylene in the molded part is 90% crystals.
materials under The value is a percentage of what was attainable.
l abo r ato r y The absolute degree of crystallinity in this part is
conditions, we actually a little less than 50%, which is quite normal in
traditionally use heating rates of 10° or 20° C/minute. polypropylene. Because PP is one of those materials that
Therefore, we can achieve a level of perfection in the does not crystallize to a significantly greater degree as
lab that cannot be attained at the press, even when mold temperature is increased, there is little danger in
we do things as well as possible. We also know that the running a mold temperature that is relatively low. In this
cooling rate at the part surface is faster than in the case the mold temperature was 100 F (38 C). But as we will
middle of the wall, unless the nominal wall is very thin. see in subsequent articles, sometimes cooling down the
But for processors, the objective is to balance production mold causes us to miss the target by a much larger amount.
efficiency with achieving a condition in the polymer that This usually occurs in the high-performance materials.
ensures good, but not perfect, stability.
ABOUT THE AUTHOR: Mike Sepe is an independent, global
We can perform a test called differential scanning materials and processing consultant
calorimetry (DSC) to check our work. This involves
preparing a sample from the molded part and heating Courtesy: Plastics Technology

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