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Basic Screw Geometry

“Things Your Screw Designer Never Told You About Screws!!”ByTimothy W. WomerXaloy, Inc. Introduction L/D Ratio In the plastics industry today and as hasbeen since the start of plastic extrusion, theend user has depended on the OriginalEquipment Manufacturer (OEM) and/orscrew manufacturer to supply them with theproper screw design for their material andprocess. Most processors have learned overthe years a few critical points pertaining toscrew design, but never totallyunderstanding the reason why their suppliershave recommended certain aspects to thescrews that they have purchased. Hopefully,this paper will explain some of the basicknowledge needed in order for an end-userto make the proper decisions when using orpurchasing a new single screw for a smoothbore application. Some OEMs define their particular L/D ratio(or Length per Diameter ratio) differentlyfrom one to another. Some manufacturersdefine it as being the “enclosed” portion ofthe screw, or they measure the flightedlength from the front side of the feed port tothe end of the screw. Some measure theflighted length from the center of the feedport opening, while others measure theactual “effective” length or the entireflighted length when determining the L/Dratio. How they define the L/D ratio can beone issue, but the actual amount that thescrew manufacturer has to machine isdetermined as shown in Figure 1. Anexample how to determine the L/D of a 2.5”screw would be as follows:Screw Dia. = 2.5”Flighted length = 63” Therefore,L/D = 63” / 2.5” = 25.2 L/DAn OEM would classify this machine as a24:1 L/D extruder, but the screwmanufacturer will typically cost it as a 25.2L/D because that is the actual amount ofmachine work which the screwmanufacturer must do to complete theproduct.Typical extruder L/Ds are 24:1 and 30 or32:1, but there are special applicationswhere extruders are built as short as 10:1L/D and as long as 50:1 L/D. The properL/D is determined by the process andapplication that is being satisfied. Nomenclature Before we get started we need to definesome of the basic components of the singleflighted screw. These terms are shown infigure 1: Flighted Length = F.L. Screw Dia. = D Feed Transition Meterin g L/D =Flighted Length (F.L.)Screw Diameter ( D ) Figure 1
Paper presented at ANTEC 2000, the Annual Technical Conference of the Society of Plastics Engineers Inc. Copyright © 2000, Society of Plastics Engineers Inc. All rights reserved. When the paper was prepared, Mr. Womer was affiliated with NewCastle Industries Inc., acquired by Xaloy in 2003.