Basic Screw Geometry: Things Your Extruder Screw Designer Never Told You About Screws

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Basic Screw Geometry: Things Your Extruder Screw Designer Never Told You About Screws

  1. 1. Basic Screw Geometry “Things Your Screw Designer Never Told You About Screws!!” By Timothy W. Womer Xaloy, Inc.Introduction L/D RatioIn the plastics industry today and as hasbeen since the start of plastic extrusion, the Some OEMs define their particular L/D ratioend user has depended on the Original (or Length per Diameter ratio) differentlyEquipment Manufacturer (OEM) and/or from one to another. Some manufacturersscrew manufacturer to supply them with the define it as being the “enclosed” portion ofproper screw design for their material and the screw, or they measure the flightedprocess. Most processors have learned over length from the front side of the feed port tothe years a few critical points pertaining to the end of the screw. Some measure thescrew design, but never totally flighted length from the center of the feedunderstanding the reason why their suppliers port opening, while others measure thehave recommended certain aspects to the actual “effective” length or the entirescrews that they have purchased. Hopefully, flighted length when determining the L/Dthis paper will explain some of the basic ratio. How they define the L/D ratio can beknowledge needed in order for an end-user one issue, but the actual amount that theto make the proper decisions when using or screw manufacturer has to machine ispurchasing a new single screw for a smooth determined as shown in Figure 1. Anbore application. example how to determine the L/D of a 2.5” screw would be as follows:Nomenclature Screw Dia. = 2.5”Before we get started we need to define Flighted length = 63” Therefore,some of the basic components of the singleflighted screw. These terms are shown in L/D = 63” / 2.5” = 25.2 L/Dfigure 1: An OEM would classify this machine as a Flighted Length = F.L. 24:1 L/D extruder, but the screw manufacturer will typically cost it as a 25.2 Screw Dia. = D L/D because that is the actual amount of machine work which the screw manufacturer must do to complete the product. Feed Transition Metering Typical extruder L/Ds are 24:1 and 30 or Flighted Length (F.L.) 32:1, but there are special applications L/D = Screw Diameter ( D ) where extruders are built as short as 10:1 L/D and as long as 50:1 L/D. The proper Figure 1 L/D is determined by the process and application that is being satisfied.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.
  2. 2. Feed Section - Depths (COF) of the polymer must be greater at the barrel wall than it is at the root of the screw.Referring to Figure 1, the feed section of the Therefore, some polymers inherently havescrew is the first element of the screw where better COFs then others. So in the case ofthe polymer is introduced to the screw. these resins long feed section lengths are notTypically, on smooth bore extruders, this is needed. Typically, for most resins a feedthe deepest portion of the screw. On smaller section length of four to five diameters pastscrews, 2.5” diameters and smaller, special the feed throat opening will allow enoughattention needs to be given to this section of pressure to be developed to convey thethe screw in order to reduce the risk of material forward.twisting the screw in half due to overtorquing of the screw. Sometimes it is In the case of poor feeding resins oradvisable to have small screws materials with low COF, a feed sectionmanufactured out of 17-4 Ph stainless steel length of eight to ten diameters may be used.or other high yield material in order to Typically, one of the reasons for longer feedreduce the risk of this type of failure. sections is to allow for more heat to be introduced to the solid form of the resin,As a rule of thumb, the feed section of a causing it to stick to the barrel, which thenscrew should not be deeper than: will help develop the pressures needed for good solids conveying. It should also be Fd max = .2( ScrewDiameter ) Eq. 1 mentioned at this point that in the case of poor feeding materials, it is also beneficialThis is for screws that are 4.5” diameter and to use internal screw cooling in order tosmaller. For example, a 2.5” screw would keep the root of the screw cool and tobe: improve the COF between the resin and the steel at the screw root. Fdmax = .2*(2.5”) = .500” Eq. 2 It should be mentioned here, that in order toIf the screw design should require the screw find a means to improve solids conveying onto have a feed depth of more that this Fdmax material which have poor COF, groove feedvalue, then proper torque calculations should technology was developed in Europe duringbe done. Then, if the yield strength of the the early 1960s. This technology has yet toscrew is going to exceed more than a 2:1 be totally accepted in the United States, butsafety factor of the original steel for which it is slowly becoming an importantthe screw is being manufactured, then a processing technique.higher yield strength steel needs to bechosen. Transition SectionFeed Section – Lengths The transition or compression section of a conventional screw is where most of theThe main function of the feed section of a melting of the polymer takes place. This isscrew is to perform the function of solids the portion of the screw that “transcends”conveying. The basic theory of solids from the feed depth to the metering depthconveying is “the plastic must stick to the and where work is done on the resin causingbarrel and slip on the screw in order for the melting to occur. In this section of thepolymer to be moved forward”. For this screw, the root of the screw graduallyfunction to happen the coefficient of friction becomes shallower forcing the material
  3. 3. towards the barrel wall where the melting approximate amount of throughput desired.takes place. This is true primarily for low headpressure applications only.Example 1: Compression Ratio .006” 1” Compression Ratio is probably the most misused, misunderstood, but widely used F =.006” / 1” = .006 in./in term of the screw terminology. Most people understand the definition of compressionThe most important factor that must be ratio as shown in Figure 2:acknowledged in the designing of thetransition section is that the slope of the hf hmtransition should match the melting rate ofthe material as closely as possible. In orderto maximize the throughput rate of theextruder and reduce the amount of wear thatwill occur to the screw and barrel Feed Transition Meteringcomponents, this computation is verycritical. Figure 2Later in this paper compression ratio will be Therefore:discussed and will be tied into this section. hf Compression Ratio = h Eq. 4 mTypically, for a 24:1 L/D screw thetransition section will be between five andten diameters long, depending on the type of An example of this would be if a 2.5” screwpolymer being processed. had a feed depth (hf) of .300” and a metering depth (hm ) of .100” then the compressionMetering Section ratio would be defined as being:The metering or pumping section of the Example 2:screw is where the melting of the polymer iscompleted and pumping to overcome the CR = .300” / .100” = 3:1 Eq. 5headpressure takes place. Simplecalculations such as: But, there also could be a 2.5” screw with a feed depth (hf) of .450” and a metering depth Rate = 2.3*D2*hm*SG*N Eq. 3 of (hm ) = .150” and it would have a compression ratio of:Rate = throughput (lb/hr)D = Screw Diameter (inches) Example 3:hm = Metering Depth (inches)SG = Specific Gravity of Polymer (gm/cc) CR = .450” / .150” = 3:1 Eq. 6N = Screw Speed (RPM)…can be used to estimate a screw’s Both screws have a compression ratio of 3:1throughput rate or by back-calculating a but they are totally different. The first onemetering depth could be determined for an will have a much higher shear rate, plus it
  4. 4. will also only have approximately 2/3 the em = Width of the main flight in the Mtg.throughput rate. The second screw will D = Outside Diameter of the screwhave a much lower shear rate and be able toprocess more shear sensitive materials and it This formula is more complex, but it giveswill have a higher throughput rate. more accurate value for the true compression ratio. This formula determinesPlus, the slope of the transition hasn’t even the amount of cross sectional area there is inbeen considered in this case. Two screws the feed section and compares it with thecould have different compression ratios; but volume of the screw’s cross-section in theif the length of the transition section was metering section of the screw.different, they could still have the samemelting rates. As is true with standard metering screws, it is also important to evaluate the trueWhen describing the geometry of a screw, compression ratio of a barrier type screw. Inmake sure that all of the details are order to determine the true compressionexamined. ratio of a barrier type screw it is necessary to compare the cross-sectional area of the feedAccurate pumping capacities can be with the combined cross-sectional area at thecalculated by using more complex formulae, end of the barrier section. This is shown inbut a more thorough understanding of the Figure 3:polymer flow must be a major factor to the Wf W m W sscrew designer. hsCompression Ratio for Barrier Screws hf hmAs just mentioned, most individuals Cross-Section Cross-Section @understand “compression ratio” as being the of Feed End of Barrierfeed depth divided by the metering depth.This is what is normally referred to as Figure 3“Depth Compression Ratio”, but there is amore accurate method to calculate truecompression ratio and that is referenced to The Volumetric Compression Ratio of aas “Volumetric Compression Ratio”, this is barrier screw can be shown mathematicallyshown in the following formula: as follows: [h f ] (L f − n f e f ) ∗ ( − h f D ) (W f ∗ h f )VCR = Eq. 7 VCR = Eq. 8 [hm (Lm − n m em )]∗ (D − hm ) (Wm * hm ) + (Ws * hs ) VCR = Volumetric Compression RatioVCR = Volumetric Compression Ratio Wf = Channel Width in Feed Sectionhf = Feed Depth hf = Channel Depth in Feed Sectionhm = Metering Depth Wm = Width of Melt Channel at the end ofLf = Lead in the Feed Section the Barrier Section.Lm = Lead in the Metering Section h f = Depth of the Melt Channel at the endnf = Number of flights in the Feed section of the Barrier Section.nm = Number of flights in the Mtg. Section Ws = Width of Solid Channel at the end ofef = Width of the main flight in the Feed the Barrier Section.
  5. 5. h s = Depth of the Solid Channel at the end extruder is two-thirds of a similar L/D non- of the Barrier Section. vented extruder because on a vented extruder, a 100% melt has to be developedThis method of comparing one section to by the time the resin reaches the ventedanother will give a more accurate section of the screw. If the resin is notcomparison. completely melted at the vent section of the two-stage screw, it is possible that theIt should be noted here that depending on moisture or gases may be trapped inside anwho the screw designer might be, the unmelted pellet and therefore are not able tothroughput rate of the screw may be escape out though the vent port of the barrel.governed by the capacity of the barriersection or the pumping capability of the There are cases where a large amount ofmetering section. It totally depends on what devolitization needs to be accomplished andthe designer has in mind for the screw’s it will require a second vent to be installedperformance. in the barrel wall. Therefore a triple-stage screw will need to be used. These types ofTwo Stage Screws extruders are usually 36:1 L/D or longer.Two stage screws are basically two single As mentioned earlier, the two-stage screw isscrews placed end to end and performing simply two single stage screws in tandem,two different functions. Figure 5, shows the the first stage compression ratio is determinetypical nomenclature used on two stage in the same manner as a single stage screw.screws: It should be mentioned that in some cases barrier sections could be used in the first Flighted Length = F.L. stage of a two-stage screw. Vent Location = 67% F.L. The primary difference in designing the first hf hm hv hp stage of the two-stage screw is that the metering section does not need to be designed to overcome any type of back- pressure. Since there is no die or downstream restriction, the first metering First Second only has to complete the melting process Feed Trans. Metering Dec. Vent Comp. Metering and pump the resin into zero or negative First Stage Second Stage pressure vent section. Figure 4 In some vented applications, when highlyThe primary reason extruders are vented is viscous resins are being processed, it isfor devolitization of moisture or gases. necessary to install a vacuum pump to theToday, most two-stage extruders that are vent port of the barrel to assist in thedesigned for devolitization are typically 30:1 devolitization of the resin.or 32:1 L/D minimum. In the rubberindustry, injection molding, and early in the Finally, the primary purpose of the secondextrusion industry, shorter L/D screws are stage is to allow an area for devolization toand were used; but today’s extrusion take place and then to pump the resintechnology requires longer screws. through the die.Normally, the throughput rate of a vented
  6. 6. In the past this has been referred to as the rheology, and process data from the existing“pump ratio” and typically a multiplier of screw.1.5:1 to 1.6:1 was used. Finally, the main purpose of thisExample 4: presentation was to help the audience better understand the mechanisms of each of the hp screw section functions. With a better PumpRatio = = 1.6 : 1 Eq. 9 understanding of each of the screws section hm functions, the process engineer can more easily troubleshoot a process or improve anThis method will typically work for existing screw design.applications where very viscous resins arebeing processed. ReferencesA better method of designing the second (1) Chung, “Extrusion of Polymers – Theorystage metering depth is to calculate the net and Practice”, Hanser Gardner Publishing,flow of the second stage versus the net flow Inc., Cincinnati, Ohio.of the first stage using the “Drag Flow-Pressure Flow” equations for best results. (2) Bernhardt, "Processing of ThermoplasticThe second stage metering section or Materials", Robert E. Krieger Publishingpumping section must be designed so that it Company.can over pump the first stage metering by aminimum of 25% in order to keep the vent (3) Rauwendaal, "Polymer Extrusion",section from flooding and cause resin to Hanser Publishers.pumped out the vent port. (4) Tadmor and Gogos, “Principles ofThe second method will be very successful Polymer Processing”, John Wiley and Sons,for keeping the vent port from flooding if New York.good resin rheology is used in the DragFlow-Pressure Flow equation. (5) Spirex Corporation, "Plasticating Components Technology", ©1992Finally, to determine the depth of the vent Youngstown, Ohio.section, normally a simple 2:1 to 2.5:1 ratioto the second metering will be sufficient tokeep the resin from back flowing out of thevent port in the barrel.ConclusionThe purpose of this paper was nothingrevolutionary, but was meant to explainsome of the thought processes that a screwdesigner uses to determine how he is goingto approach a design of a screw.As always, it is very important for thecustomer to furnish the screw designer withgood equipment information, resin data or

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