Extrusion Screws for Thermoplastic Composites
By Timothy W. Womer Xaloy Inc. Introduction There are two different areas of screw design for thermoplastic composites: the screw design which is being used to make the thermoplastic compound and the screw design which is used to process the compounded thermoplastic. This paper mainly discusses the latter of the two types, but many of the issues arepertinent to both. The use of various fillers and additives, which are compounded into thermoplastic resins, has become more and more prevalent in recent years in the plastics industry. The addition of fillers such as talc, mica, calcium carbonate and nanoclays can increase the stiffness and/or strength and the utilization of various plastics. These fillers have also had an effect on theperformance and life of the equipment that processes them. This paper will discuss some of the issues that these composites have on the screw design, plus screw and barrel wear. Screw Design Most screws used in the extrusion of thermoplastic resins primarily deal with the issues of feeding, melting and pumping of the resin. Feeding the Polymer Melting the Polymer In the feed section of the screw wheresolids conveying takes place, the feeding mechanism for a thermoplastic composite may be quite different from that of the very same thermoplastic polymer without filler. The most important aspect of screw geometry affecting melting is the volumetric compression ratio. This is determined by the change in channel volume that takes place in In the solids conveying function of the screw, the most criticalphenomenon is the relative coefficient of friction (COF) of the polymer. In the feed section there are three different coefficients of friction at work: the COF between the pellet and the barrel wall, the COF between the pellet and the screw root and the COF between pellet and pellet. Although a given polymer may feed very well in the in the neat, or unfilled, state, the addition of fillers oftencauses a considerable change in its COF and, therefore, in the screw’s solids conveying performance. For example, any time mica is the filler or additive in the base material, the COF drops dramatically. Therefore, the screw may need to have a longer feed section of constant depth in order to develop enough feed pressure before material enters the transition section or barrier section. Also, toenhance the solids conveying of such a material, it might require changing the temperature profile by raising the temperature in the first barrel zone to increase the COF between the pellet and the barrel wall. This would allow the polymer to start to tackify or stick to the barrel so that it can be conveyed forward. Poor or unstable solids conveying will translate directly into low throughputrates and cause significant surging of the process.
the transition section or barrier section of the screw, which is typically located directly after the feed section of the screw. When fillers are added to resins, it increases their specific gravity. For example, a neat, or unfilled, 2 MFR polypropylene has a specific gravity of 0.92, whereas the same polymer with 40% talc filler has a specificgravity of 1.24. This is an increase of 35% in density and also a 40% reduction in the amount of polymer that needs to be melted during processing. Since the filler is taking up volume in the screw channels and does not melt, compensations must be made in screw design. As mentioned, since the filler does not typically compress or change its volume due to temperature change, the channel depths musttake that fact into account. For example, the screw for an unfilled polypropylene typically has a the volumetric compression ratio in the 3.5 to 3.75:1 range, versus 2.75 to 3.25:1, depending on screw size. for a 40% talc-filled polypropylene. In the case of a barrier-type screw, for a thermoplastics composite, the design must take into account not only the channel depths but also the barrier...
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