Page 32 - Plastics News March 2019

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A Processor’s Most Important Job, Part 8: Molded-In Stress

Michael Sepe
The article focuses on how processing adjustments can control molded-in stress, warpage is one of the
mechanisms by which stress is relieved.

tress arises from two primary influences: Differences part. The level of this orientation will decrease as we
Sin pressure and differences in the rate at which the move to the center of the part. Not only is the gradient
polymer cools within the part. Flowing molten polymer in orientation between the surface and the core a source
exhibits a loss in pressure through the system that is of stress, but in fiber-reinforced materials, anisotropic
related in part to the distance and the cross-sectional shrinkage is caused by the limitations that the oriented
area through which the material flows. fibers place on the dimensional change in the polymer as
it cools. This limitation is not observed in the direction
Therefore, there will be a measurable pressure gradient
across any molded part as the material flows from the transverse to flow, or at least not to the same extent.
gate to the last places in the cavity to fill. This variation The Melting Process Effect
in pressure will produce variations in the degree to which Consequently, any fabricated part will contain some
the material shrinks.
level of internal stress that arises naturally from melt
The Cooling Rate Effect processing. In many cases, these stresses will appear as
warpage. Warpage is the natural result of shrinkage that
Cooling rate also influences the shrinkage of material.
Even when the cooling-line layout in a mold is done with varies in magnitude within a part, whether it be due
meticulous attention to detail, which rarely happens, to volumetric considerations or driven by orientation.
differential rates of cooling are still inevitable due to
wall-thickness considerations. Material close to the
mold surface will cool faster than the material in the
part core. The greater the wall thickness of the part,
the more problematic this will become. When molders
fail to deal with root causes and elect to either fixture
a part to prevent movement while it cools, or alter the
processing conditions to reduce warpage, they are often
building in even higher levels of stress.
Orientation is another factor associated with the
behavior of the different layers of material flowing
through the mold. Polymer chains naturally form an
entangled network where the molecules are coiled up.
However, when molten polymer flows, differences in the
shearing forces between the various layers of flowing
material cause the chains to straighten and orient in But even if warpage is not evident in the part, stresses
the direction of flow. This is the mechanism that causes are still present. Often, they do not show up until the
non-Newtonian behavior in polymers. part is in the field and experiences an environmental
Shear rates are at their highest just below the part condition such as elevated temperature, contact with
surface, and this is also the layer that cools most rapidly. a certain chemical, or an impact event. Elevated levels
This rapid cooling limits the time that the polymer chains of stress will result in a less-than-optimal response to
have to relax from the oriented state to the coiled state. these application influences.
Therefore, the material near the wall will exhibit a Warpage is one of the mechanisms by which stress is
relatively high degree of retained orientation in the final relieved. When molders fail to deal with root causes

Plastics News March 2019 32