Extrusion Screws for Thermoplastic Composites

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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 are pertinent to both.

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Extrusion Screws for Thermoplastic Composites

  1. 1. Extrusion Screws for Thermoplastic Composites By Timothy W. Womer Xaloy Inc. Introduction In the solids conveying function of the screw, the most critical phenomenon is theThere are two different areas of screw design relative coefficient of friction (COF) of thefor thermoplastic composites: the screw polymer. In the feed section there are threedesign which is being used to make the different coefficients of friction at work: thethermoplastic compound and the screw COF between the pellet and the barrel wall,design which is used to process the the COF between the pellet and the screwcompounded thermoplastic. This paper root and the COF between pellet and pellet.mainly discusses the latter of the two types,but many of the issues are pertinent to both. Although a given polymer may feed very well in the in the neat, or unfilled, state, theThe use of various fillers and additives, addition of fillers often causes awhich are compounded into thermoplastic considerable change in its COF and,resins, has become more and more prevalent therefore, in the screw’s solids conveyingin recent years in the plastics industry. The performance. For example, any time micaaddition of fillers such as talc, mica, calcium is the filler or additive in the base material,carbonate and nanoclays can increase the the COF drops dramatically. Therefore, thestiffness and/or strength and the utilization of screw may need to have a longer feedvarious plastics. These fillers have also had section of constant depth in order to developan effect on the performance and life of the enough feed pressure before material entersequipment that processes them. This paper the transition section or barrier section.will discuss some of the issues that these Also, to enhance the solids conveying ofcomposites have on the screw design, plus such a material, it might require changingscrew and barrel wear. the temperature profile by raising the temperature in the first barrel zone to Screw Design increase the COF between the pellet and the barrel wall. This would allow the polymerMost screws used in the extrusion of to start to tackify or stick to the barrel so thatthermoplastic resins primarily deal with the it can be conveyed forward. Poor orissues of feeding, melting and pumping of the unstable solids conveying will translateresin. directly into low throughput rates and cause significant surging of the process.Feeding the Polymer Melting the PolymerIn the feed section of the screw where solidsconveying takes place, the feeding The most important aspect of screwmechanism for a thermoplastic composite geometry affecting melting is the volumetricmay be quite different from that of the very compression ratio. This is determined by thesame thermoplastic polymer without filler. change in channel volume that takes place in
  2. 2. the transition section or barrier section of the Pumping the Polymerscrew, which is typically located directlyafter the feed section of the screw. Pumping in an extrusion screw is also very critical because of its effect on processWhen fillers are added to resins, it increases stability. In contrast to injection molding,their specific gravity. For example, a neat, where the screw pumps melted polymeror unfilled, 2 MFR polypropylene has a through a non-return valve to accumulatespecific gravity of 0.92, whereas the same the next injection shot, the extrusionpolymer with 40% talc filler has a specific process requires steady, stable andgravity of 1.24. This is an increase of 35% in consistent output. Yes, it is important thatdensity and also a 40% reduction in the the resin is fed consistently and is meltingamount of polymer that needs to be melted uniformly, but it is in the metering section ofduring processing. Since the filler is taking the screw where the pumping has to beup volume in the screw channels and does steady.not melt, compensations must be made inscrew design. Typically, resins which have fillers have a higher viscosity than the same resin withoutAs mentioned, since the filler does not fillers. Increased viscosity actually helpstypically compress or change its volume due pumping, but it also generates higher headto temperature change, the channel depths pressure if the extrusion die has not beenmust take that fact into account. For designed for the more viscous polymerexample, the screw for an unfilled compound.polypropylene typically has a the volumetriccompression ratio in the 3.5 to 3.75:1 range, Barrel Temperature Overrideversus 2.75 to 3.25:1, depending on screwsize. for a 40% talc-filled polypropylene. If a screw designed for “neat” or unfilledIn the case of a barrier-type screw, for a resin is used to process a filled resin, it maythermoplastics composite, the design must appear to be processing the material in antake into account not only the channel depths acceptable manner, but inside the barrelbut also the barrier flight clearance. other things are happening.On a barrier type screw design, as the Normally, the first sign of problems will bepolymer melts along the barrel wall, it must temperature overrides in the barrel zones.freely flow through the barrier flight gap as it This is due to the non-compressibility of theleaves the solids channel and is collected in filler. The cause of the temperature overridethe melt channel of the barrier section. is typically due to the fact that viscousAgain, since the composite polymer has non- heating is taking place in a particular area ofcompressible fillers in its matrix, the barrier the screw, therefore, in turn causing thegap needs to be more generous to allow the barrel zone to overheat. Typically,free flow of the melted material. Otherwise, whenever a barrel zone temperaturehigh pressure differentials between the solids overrides due to viscous heating, the basicchannel and melt channel will occur and cause is that the material is still too stiff orcould in turn cause barrel temperature viscous to flow smoothly through thatoverride in the middle barrel zones. portion of the screw channel.
  3. 3. Normally, the first processing technique thatshould be used is to increase all of the barrelzones prior to the zone that is overriding.This should help raise the temperature of theresin, lower its viscosity and in turn allow itto flow more easily through the portion of thescrew where the temperature override washappening. In most cases, this is only atemporary fix, and a long-term solution needsto be implemented with a properly designedscrew. Figure 1By correcting the screw design so that thegeometry takes into account the amount of The main reason that the majority of thefiller in the thermoplastic compound, wear takes place in the areas mentioned istemperature overrides can be eliminated, and that the resin is still in a pellet form in thisscrew wear can be reduced. portion of the screw and the composites are in turn then near the outer surface of the Wear pellets and are rubbing against the unprotected steel root of the screw. OnceScrew and barrel wear are two areas of the resin starts to melt, a film of meltedconcern in the use of composite material begins to help lubricate the areathermoplastic resins. between the pellets and the root of the screw.Screw WearThe other evidence of improper screw designfor composite thermoplastic resins is in thearea of screw wear. If the volumetriccompression ratio has not been optimized forthe composite thermoplastic, extreme wearwill be evident in the root of the screwchannel. Different fillers cause the wear tooccur in different areas of the screw. Thetype of screw wear caused by fillers isabrasive wear, as shown in Figure 1.Fillers like mica and fiberglass will cause Figure 2aggressive wear in the feed section of the Typically, an inexpensive method to reducescrew. Normally this wear will start to be this type of root wear is to have the screwseen in the third and fourth turn of the screw nitrided. Nitriding totally case-hardens theand will extend in to the third, fourth and root and sides of the flight to a depth offifth turn of the transition. Typically, the 0.015” to 0.020” and to a hardness of 60 Rc.wear is on the push side of the flight and The one downside to nitriding a screw is thatappears as the photo shown in Figure 2. since the thickness is only 0.015” to 0.020” thick, in due time it too will wear away; and
  4. 4. when it does, the composite thermoplastic New larger screws can be protected by J-will erode away the base metal like “a hot groove welding or by totally encapsulatingknife through butter.” The problem with this their root. This is done by machining theis that the operator will not be able to tell original screw channel profile deeper thanwhen this will happen unless that screw is the final finish geometry and then buildingpulled from the extruder on a regular basis up the root and the sides of the flight with aand visually examined. harder material. Figure 4 schematic shows encapsulation.This type of wear is repairable. The onlyproblem will be at the area where the newlywelded material meets the original base metalthat has been nitrided because many pinholeswill appear. These are caused by nitriding Figure 4gases “boiling” out of the base metal duringthe welding procedure. There is basically This schematic shows two differentnothing that can be done to prevent such materials applied to the base metal of thepinholes. They are mere cosmetic screw. The yellow area in the figureimperfections and do not affect the represents a hardfacing that is easier toprocessing performance of the screw.. machine such as Stellite 6, which has a hardness of 38-42 Rc. The red areaAnother method of helping to prevent represents a screw tip hardfacing such aspremature wear on the “push” side of the Xaloy X830, which is a tungsten-carbideflight is to have the feed section and material in a corrosion resistant base matrix.beginning of the transition section using a J-groove weld in the problem area. This type The final method of screw protectionof procedure is schematically shown in covered here involves spray welding andFigure 3. fusion with a tungsten carbide suspended material such as the patented Xaloy X8000 J-Groove (US Patent 5,198,268). This application is shown in Figure 5. Figure 3Another method of protecting the root ofsmaller screws is to coat them with tungsten-carbide. Such protection can be applied byseveral different methods such as HVOF(High Velocity Oxy-Fuel) or spray weldedand fused. Both of these methods protect theroot of the screw better than nitrided coating.Even though this is a better means ofprotection, it does not come without anadditional cost. Figure 5
  5. 5. All of the mentioned forms of screw be operated at much higher screw speeds,protection will have additional costs to the which also can cause high barrel and screwpurchase of the new screw, but the extended wear.and consistent longevity they provide greatlyoutweigh their initial cost. ConclusionBarrel Wear When extruding thermoplastic composite materials, screw design and screw and barrelAs mentioned with screw wear, abrasion construction are crucial considerations. Ifwear also occurs with the barrel liner of the they are addressed properly up front, theextruder. With today’s bimetallic technology processing of thermoplastic compositethat is used to spin-cast the bimetallic liners materials will be successful.onto the barrel base metal, improved barreldurability can be obtained. In today’s The main lesson to learn here is that if thetechnology, bimetallic liners can have proper research is done up front whentungsten-carbide particles suspended in the considering processing thermoplasticbimetallic base metal. This can be seen in composite materials, the learning curve willthe photograph shown in Figure 6. be much quicker. References (1) Chung, “Extrusion of Polymers – Theory and Practice”, Hanser Gardner Publishing, Inc., Cincinnati, Ohio. (2) Bernhardt, "Processing of Thermoplastic Materials", Robert E. Krieger Publishing Company. (3) Rauwendaal, "Polymer Extrusion", Hanser Publishers. Figure 6 (4) Tadmor and Gogos, “Principles of Polymer Processing”, John Wiley and Sons,The small particles seen in this photo are New York.small pieces of tungsten-carbide. Thistechnology was originally patented by XaloyInc. under the US Patent 3,836,341. Theincorporation of the tungsten-carbide into thebimetallic liner material will increase the lifeof the barrel by four to five times over theolder original bimetallic liners which wereavailable to the plastics industry 20 yearsago. Not only do these types of barrels allowfor the extrusion of more highly filledpolymers, but they also allow the extruders to

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