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Extrusion Screws for Thermoplastic Composites

ByTimothy W. WomerXaloy Inc. Introduction There are two different areas of screw designfor thermoplastic composites: the screwdesign which is being used to make thethermoplastic compound and the screwdesign which is used to process thecompounded thermoplastic. This papermainly discusses the latter of the two types,but many of the issues are pertinent to both.The use of various fillers and additives,which are compounded into thermoplasticresins, has become more and more prevalentin recent years in the plastics industry. Theaddition of fillers such as talc, mica, calciumcarbonate and nanoclays can increase thestiffness and/or strength and the utilization ofvarious plastics. These fillers have also hadan effect on the performance and life of theequipment that processes them. This paperwill discuss some of the issues that thesecomposites have on the screw design, plusscrew and barrel wear. Screw Design Most screws used in the extrusion ofthermoplastic resins primarily deal with theissues of feeding, melting and pumping of theresin. Feeding the Polymer Melting the Polymer In the feed section of the screw where solidsconveying takes place, the feedingmechanism for a thermoplastic compositemay be quite different from that of the verysame thermoplastic polymer without filler. In the solids conveying function of thescrew, the most critical phenomenon is therelative coefficient of friction (COF) of thepolymer. In the feed section there are threedifferent coefficients of friction at work: theCOF between the pellet and the barrel wall,the COF between the pellet and the screwroot and the COF between pellet and pellet.Although a given polymer may feed verywell in the in the neat, or unfilled, state, theaddition of fillers often causes aconsiderable change in its COF and,therefore, in the screw’s solids conveyingperformance. For example, any time micais the filler or additive in the base material,the COF drops dramatically. Therefore, thescrew may need to have a longer feedsection of constant depth in order to developenough feed pressure before material entersthe transition section or barrier section.Also, to enhance the solids conveying ofsuch a material, it might require changingthe temperature profile by raising thetemperature in the first barrel zone toincrease the COF between the pellet and thebarrel wall. This would allow the polymerto start to tackify or stick to the barrel so thatit can be conveyed forward. Poor orunstable solids conveying will translatedirectly into low throughput rates and causesignificant surging of the process. The most important aspect of screwgeometry affecting melting is the volumetriccompression ratio. This is determined by thechange in channel volume that takes place in