Plastics Technology

NOV 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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In previous articles in this series, we have reviewed three aspects of material composition that the processor has a role in pre- serving or optimizing: molecular weight, crystal structure (if it is expected), and molded-in stress. The fourth aspect of processing that is designed to ensure good retention of mechanical properties is fiber-length retention. Glass and carbon fibers are incorporated into many polymers to increase the strength and modulus of the base material. Other ben- efits of this modification are an improved level of creep resistance and fatigue resistance. The reinforcing capability of fibers is related in large part to a property known as the aspect ratio, which is essentially the ratio of the fiber length to the fiber diameter. While the critical aspect ratio is dependent upon a number of factors, it is known A Processor's Most Important Job that with increasing aspect ratio, properties such as strength and modulus improve. This is the rationale for the development of long-fiber compounds, where the initial length of the fibers is 11-12 mm instead of the 1-2-mm lengths typical of short fibers. While these may be the fiber lengths incorporated into the compound when it is pelletized and packaged, the final prop- erties of the molded parts produced from the material depend upon the length of the fibers in those parts. Melt processing that employs a screw will generate a significant amount of mechanical work and this has the potential to break the glass fibers. This is why long-glass materials are not compounded using extrusion. Instead the fibers are pulled through a bath of molten resin and then cut to the desired length. But the process conditions that these raw materials experience during molding will have a significant influence on the length of the fibers in the final product. And this is true for both short- and long-glass fiber compounds. Mechanical work involves shear stress that is generated by the flow of the material during the mold-filling process and by the plasticating of the material during screw rotation. These are neces- sary aspects of the process, but when it comes to fiber-reinforced materials the process conditions associated with these factors must be selected with an awareness of how they affect fiber length. The accompanying photos show the result of ash tests performed on two parts molded in glass-fiber rein- forced nylon. Parts from one lot were reportedly stronger and stiffer than those from the second lot, and the natural conclusion was that the two lots had been made from materials with different levels of reinforcement. However, the ash tests showed that the glass-fiber content had not changed between the two lots of parts. But the ash test did reveal an important difference that accounted for the reported variation in performance. In the crucible on the left are an as-received part and the resulting residue from the ash test for the good lot. The crucible on the right contains the analogous parts and residue from the defective lot. Note that the ash from the good part still has a Glass and carbon fibers are often used to increase material strength and modulus. To maintain these properties, the aspect ratio of these fibers must be maintained. PART 10 Get more insights on Materials from our expert author: Learn more at KNOW HOW MATERIALS By Mike Sepe Nylon parts before and after ash tests. Good part on left, bad part on right. 20 NOVEMBER 2018 Plastics Technology K now How MATERIALS

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