A method for purging and cleaning a screw and barrel


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A method for purging and cleaning a screw and barrel

  1. 1. A Method for Purging and Cleaning a Screw and Barrel By Timothy W. Womer, CTO Xaloy, IncorporatedIntroduction procedure to include the use of an acetyleneWhen it comes to maintaining screw and torch. This is the biggest mistake that canbarrels, the most common question asked is, be done which will destroy not only the“What is the best method to purge and clean metallurgical properties of the base metalour screws and barrels?” but also affect the closely machined tolerances of the screw.Hopefully, this paper will address this verycommon question and help companies bemore productive during their productionchangeover and routine maintenanceprocedures.Many times it is more cost effective to pullthe screw out of the extruder or injectionmolding machine than to use a purgingcompound when it is time to do a productchangeover. For extrusion applications, Most screws that are built for extrusiontypically, for extruders that are 90mm (3.5”) applications are made of 4140 H.T. steel.in diameter and smaller, it is often easier to Whenever isolated heat from an acetylenepull the screw out of the barrel and pristine torch is used to remove plastics from thethe system to insure that there is no root of the screw, it will cause the metal tocontamination for the next application. expand on that side of the screw and in turnWhen the process is Injection Molding, cause the screw to bend. Once the screwtypically, for machines that have screws that cools it is very doubtful that the straightnessare smaller than 75mm (3.0”) in diameter, of the screw will return to what it wasthese too are easier to remove the screw originally. Also, the isolated heat,from the barrel and clean the components especially if the steel is heated to a pointoutside the machine. where the isolated area turns to a permanent blue, will cause metallurgical changes to theWhat NOT to do! base metal. There have been occasions where the isolated area actually causes aPlasticating screws are manufactured to very delamination of the base metal and a largeclose tolerances. Most of the tolerances on portion of steel will separate itself from thethe screw are within +/- .025mm (.001”), main portion of the screw.straight within .1mm (.004”) and have ahighly polished finish. Many, many times it In the case of injection molding screws,is very common for a company’s most of these screws are manufactured tomaintenance department’s screw cleaning
  2. 2. withstand abrasive wear from the resins that system. For example, on a 50mm (2”)are being processed. The types of steels diameter screw it will require about 5 kg (10used to combat the abrasive wear are lb) of material, and on a 90mm (3.5”)typically various types of tool steels. During diameter screw it may require 15kg (33 lb)the manufacturing stages of building tool of HDPE material.steel screws, the base metal has to be heattreated to increase the hardness of the steel.If isolated heat from an acetylene torch isused to remove plastic from the root of thescrew, it will definitely anneal the basemetal and reduce the wear resistance in thatarea of the screw. Also, annealing the toolsteel with the use of isolated heat will reducethe yield strength of the steel, and this willincrease the likelihood of screw breakagedue to torque. Therefore, the basicstatement here is: Figure 1 Do Not Use Isolated Direct Heat!!! Depending on the type of extrusion process,Purging the Screw it may be necessary to remove the die or head tooling to reduce over pressuring theThe first step to pristine the plasticating end of the extruder. This is why it is verysystem is to purge the screw. First, close off important during the purging process thatthe flow of the resin that is being processed. extreme caution be used and two operatorsThis is typically done by closing the slide perform this portion of the process. Onegate at the bottom of the resin hopper. operator should be at the control panel (See Figure 2) to observe the screw speed andThe screw rotational speed needs to be drive load meter to insure that the drive doesreduced to approximately 15 to 25 rpm andoperated at this speed until the polymerstops flowing from the end of the die or outof the nozzle on an injection moldingmachine.All of the barrel zones should be set to about200°C (400°F), and the machine needs tocome to this temperature before continuingthe purging process.Once the barrel zones have reached the200°C (400°F) temperature settings, then thepurging process can begin. It is this author’sexperience that a fractional melt (.35 MI)HDPE is used as a purging compound, seeFigure 1. The screw size will determine the Figure 2amount of HDPE that is needed to purge the
  3. 3. not overload. The second operator needs to • Safety glassesobserve the head pressure gauge to make • Hot glovessure that the system is not over pressured. • Brass putty knifeAll the while, the screw should be rotating at • Brass gauzeabout 15 to 20 rpm. • A round wire brush approximately the diameter of the barrel boreIf the process allows the die to remain on the mounted on long rodextruder, then the purging process should • Stearic acid flakecontinue until a complete change in the • Electric drillextrudate is noticed. Figure 3 shows the • Several cotton ragschangeover from the processing resin to theHDPE purging resin. Figure 4 Figure 3 Stearic acid is used as an ingredient inPurging should continue until the extrudate making candles, plastics, dietary supple-has completely transformed from the ments, oil pastels, and cosmetics; forprocessing resin to the purging resin. softening rubber; and to improve the fusion of PVC. It is also used to harden soaps,Once the die has been completely purged, particularly those made with vegetable oil.the screw rotation can be completelystopped so that the die on the extruder or the NOTE! DO NOT USE STEEL SCREWendcap on the injection molding machine DRIVERS, SCRAPERS, OR PRY BARScan be removed and the end of the screw can WHEN CLEANING SCREWS ANDbe exposed. With the die or endcap BARRELS.removed, the screw can be restarted androtated at about 10 rpm to allow the Cleaning the Screwremaining HDPE purging resin to bepumped out of the remaining portion of the Once the resin has stopped extruding fromscrew. the screw, then the screw needs to be removed from the machine. For an extruderTools and Cleaning Materials (Figure 4) using a screw cooling system, needs to be removed, see Figure 5, before installing theThe tools and cleaning materials needed to screw extraction components.pristine a screw and barrel are very few andsimple. They consist of the following:
  4. 4. With the first portion of the screw exposed, now the HDPE resin can be cleaned from the channels of the screw as shown in Figure 8, using the brass putty knife and brass wire brush. Figure 5Once the hoses, rotary union, and piping areremoved from the back of the extruder, thescrew extractor mechanism can be attachedto the extruder gearbox. Using the screwextractor, the screw can be pushed down Figure 8most of the distance of the barrel. (SeeFigure 6) As the exposed portion of the screw has the HDPE removed, then another 4 to 5 turns of the screw can be pushed forward using the screw extractor, and the cleaning process is continued. Once the large amounts of HDPE are removed with the brass putty knife and wire brush, the stearic acid should be sprinkled onto the root of the hot screw, and the brass gauze should be used to remove the remaining HDPE residue. (See Figure 9) Figure 6The screw should now be pushed forwarduntil about 4 or 5 turns of the screw can beexposed for cleaning. (See Figure 7) Figure 9 Once the entire screw has been polished using the brass gauze, a final clean-up can be done using a soft cotton rag. In Figure Figure 7
  5. 5. 10, the screw channels and flight outside plastic, but also very good for protecting thediameter can be cleaned to the pristine screw when it is being stored between usescondition to insure that there is no in production.contamination in the next production run. Cleaning the Barrel Cleaning the barrel is much easier than the cleaning of the screw but just as important. (See Figure 12) Figure 10Once the screw has been completely cleanedit can be set aside (Figure 11) until the barrel Figure 12 With the barrel temperatures still set at the purging temperatures, the barrel is ready for its cleaning process. The first step of the barrel cleaning process is to assemble the round wire brush, the long extension rod, and the electric drill as a complete assembly. Once these components have been assembled, the copper gauze (See Figure 11 Figure 13) can be wrapped around the outside diameter of the wire brush.has been cleaned, or it can be placed on thescrew rack until it is needed for the nextproduction run requiring its use. If thescrew is place on the storage rack, then itshould be sprayed and wiped down with alight oil, such as WD-40 or PB Blaster inorder to prevent rusting of the base metal tooccur.Also, it is good to mention, that chromeplating is not only an excellent surfacecoating which will help in preventingplastics from building up on the root of the Figure 13screw and also improve the feeding of the
  6. 6. Before inserting the brush/gauze assembly the barrel clear of the HDPE and stearic acidinto the bore of the barrel, a handful of residue. After the rags have been passedstearic acid can be thrown into the bore of back and forth several times and returnthe plasticating barrel. Stearic acid can also totally clean, the barrel cleaning process isbe sprinkled over the copper gauze before complete.the brush/gauze assembly is inserted andthen pushed into the bore of the barrel. (See ConclusionFigure 14) By following the procedure described in this paper, the entire screw and barrel assembly will be totally pristine and ready for the next production run. Many times it can be much more effective and efficient to do a total cleaning process as described here versus using a commercially available purging compound. Sometimes the plasticating process prohibits cleaning the screw and barrel described here; these are processes, such as large blow molding Figure 14 equipment and extremely large cast film or laminating lines.Once the brush/gauze assembly is insertedinto the bore of the barrel, the electric drill is If the proper equipment is used, screws andused to rotate the brush/gauze assembly in barrels up to 150mm (6”) in diameter can beand out of the barrel bore until the thoroughly cleaned in one to two hours andbrush/gauze assembly moves easily in and save enormous amounts of time andout of the bore. It may be necessary to use materials that it would take otherwise.additional stearic acid before the borecleaning process is thoroughly completed.Once the brush/gauze assembly is removedfrom the barrel bore, a bundle of cotton ragsneeds to be pushed back and forth the (SeeFigure 15) length of the barrel bore to swab Figure 15