Page 31 - Plastics News May 2019

FEATURES

A Processor’s Most Important Job, Part 10: Fiber-Length Retention

Michael Sepe
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n previous articles in this series, we have reviewed three WKH ÀEHUV DUH SXOOHG WKURXJK D EDWK RI PROWHQ UHVLQ DQG
Iaspects of material composition that the processor has then cut to the desired length. But the process conditions
a role in preserving or optimizing: molecular weight, that these raw materials experience during molding will
crystal structure (if it is expected), and molded-in stress. KDYH D VLJQLÀFDQW LQÁXHQFH RQ WKH OHQJWK RI WKH ÀEHUV
The fourth aspect of processing that is designed to ensure LQ WKH ÀQDO SURGXFW $QG WKLV LV WUXH IRU ERWK VKRUW DQG
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many polymers to increase the strength and modulus of the
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and by the plasticating of the material during screw
rotation. These are necessary aspects of the process, but
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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
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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,
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changed between the two lots of parts. But the ash test
did reveal an important difference that accounted for
improved level of creep resistance and fatigue resistance. the reported variation in performance. In the crucible on
the left are an as-received part and the resulting residue
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part to a property known as the aspect ratio, which from the ash test for the good lot. The crucible on the
LV HVVHQWLDOO\ WKH UDWLR RI WKH ÀEHU OHQJWK WR WKH ÀEHU right contains the analogous parts and residue from the
defective lot. Note that the ash from the good part still
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a number of factors, it is known that with increasing aspect has a shape that is close to that of the molded part,
ratio, properties such as strength and modulus improve. while the ash from the bad part is a pile of disconnected
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entangled and will maintain the overall envelope that
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the compound when it is pelletized and packaged, the Flow paths that are too restrictive produce higher shear
ÀQDO SURSHUWLHV RI WKH PROGHG SDUWV SURGXFHG IURP WKH rates and shear stresses. This is true for the entire melt-
PDWHULDO GHSHQG XSRQ WKH OHQJWK RI WKH ÀEHUV LQ WKRVH delivery system and extends to the cavity itself. Material
parts. Melt processing that employs a screw will generate
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D VLJQLÀFDQW DPRXQW RI PHFKDQLFDO ZRUN DQG WKLV KDV WKH colorant being added to the material. Titanium dioxide,
SRWHQWLDO WR EUHDN WKH JODVV ÀEHUV 7KLV LV ZK\ ORQJ JODVV the most common pigment used to produce white and
materials are not compounded using extrusion. Instead,

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