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screw setting & processing problem

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Author	Topic: screw setting & processing problem
slkoli Senior Member Posts: 7 From:pune,maharastra,INDIA. Registered: Jan 2005	posted July 07, 2005 03:52 AM 1.we have a parallel counter-rotating twins screw extruder (theysohn made)for pvc sheets processing.we periodically remove screw for cleaning and we load the screw in the barrel by taking radial &axial reading.But I wanted to know from the experts what are the correct procedure for screw setting? Is there different method for parallel & connical screw setting? 2.How important is the rear barrel temp. for processing pvc rigid &foam sheets? we have a operator who solve the processing problem (any king of problem) by changing rear barrel temp.,so pl.tell me the importance of the rear barrel temp.? IP: Logged
2beeps Moderator Posts: 94 From:Silver City, NM, USA Registered: Jun 2001	posted July 07, 2005 10:44 AM The procedures to "set" the screws to proper clearances--front-to back,should be obtained from the extruder manufacturer's engineering tech service people. Also, they should advise the proper flight/barrel clearances so you know when the screw has been worn to the point of rebuilding.Conical and parallel screws are set the same way, as I recall. I do not know the specific clearances however. The rear barrel heat is important because it affects the degree of fusion or gellation of the powder blend at the vent. A "cheesy",crumbly consistency is best for a good vacuum to remove volatiles without sucking powder---also if material is too fused and wrapped around the screws,vauum will not work as well, and the material will probably be too hot in the metering zone. Generally, a "reverse" barrel temp. profile--hotter rear tapering to a cooler front works best for rigid PVC. IP: Logged
slkoli Senior Member Posts: 7 From:pune,maharastra,INDIA. Registered: Jan 2005	posted July 11, 2005 01:12 AM Thanks for your prompt reply.I have one more question to ask you about screw temperature,how significant is screw temp. for processing?what will happen if we process pvc or other material without screw temperature ? IP: Logged
2beeps Moderator Posts: 94 From:Silver City, NM, USA Registered: Jun 2001	posted July 11, 2005 10:47 AM Screw temperature control---ie circulating oil from front-to-back---helps transfer frictional heat from the metering zone back to the feed zone (zone 1)to aid in the start of fusion,as mentioned in the earlier post. I have found that if the screw oil temp. was equal to the barrel metering zone temp., a smoother ID in pipe, or a more homogeneous PVC melt resulted. Screw temp. control provides another tool to control melt quality---without it (in my opinion), extrusion quality could suffer, since your only "handles" are barrel heaters and screw RPM's (also starve feeding). IP: Logged

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