Plastics Technology

SEP 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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In our review of characteristics that can be controlled to a signifi- cant extent through processing, the next item on the list is internal stress in the part, also referred to as molded-in stress. Stress arises from two primary influences: differences in pressure and differences in the rate at which the polymer cools within the part. Flowing molten polymer exhibits a loss in pressure through the system that is related in part to the distance and the cross-sectional area through which the material flows. Therefore, there will be a measurable pres- sure gradient across any molded part as the material flows from the gate to the last places in the cavity to fill. This variation in pressure will produce variations in the degree to which the material shrinks. Cooling rate also influences the shrinkage of a material. Even when the cooling-line layout in a mold is done with meticulous attention to detail, which rarely happens, differential rates of cooling are still inevi- table due to wall-thickness considerations. Material close to the mold surface will cool faster than material in the part core. The greater the wall thickness of the part, the more problematic this will become. 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 the direction of flow. This is the mechanism that causes non-Newtonian behavior in polymers. Shear rates are at their highest just below the part surface, and this is also the layer that cools most rapidly. This rapid cooling limits the time that the polymer chains have to relax from the oriented state to the coiled state. Therefore, the material near the wall will exhibit a relatively high degree of retained orientation in the final part. The level of this orientation will decrease as we move to the center of the part. Not only is the gradient in orientation between the surface and the core a source of stress, but in fiber- reinforced materials, anisotropic shrinkage is caused by the limitations that the oriented fibers place on the dimensional change in the polymer as it cools. This limitation is not observed in the direction transverse to flow, or at least not to the same extent. Consequently, any fabricated part will contain some level of internal stress that arises naturally from melt processing. In many cases these stresses will appear as warpage. Warpage is the natural result of shrinkage that varies in magnitude within a part, whether it be due to volumetric consid- erations or driven by orientation. But even if warpage is not evident in the part, stresses are still present. Often, they do not show up A Processor's Most Important Job How processing adjustments can control molded-in stress. PART 8 By Mike Sepe When ABS specimens were prepared in a mold set at a relatively low temperature of 29 C, the energy required to break the test specimens was only 1.5 N-m (1.1 ft- lb), a value associated with a very brittle material. Altering the melt temperature across a fairly wide range did not appreciably change this result. However, when the mold temperature was increased, the impact resistance improved dramatically. Effect of Melt and Mold Temperature On Impact Performance of ABS 85 71 57 43 29 Mold Temperature, C Melt Temperature, C Dart Impact, N-m 50 45 40 35 30 25 20 15 10 5 0 218 232 246 260 274 30 SEPTEMBER 2018 Plastics Technology PTonline.com K now How MATERIALS

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