Most injection molding screws are manufactured from too […]
Most injection molding screws are manufactured from tool steel so that they withstand abrasive wear from the resins being processed. The base metal of such screws has been heat-treated to increase hardness. If isolated heat from an acetylene torch is used to remove plastic from the root of the screw, it may anneal the base metal and reduce the wear resistance in that area of the screw.
First, here’s what not to do: It is very common for maintenance departments to use an acetylene torch as part of the screw-cleaning procedure. This is a big mistake. Plasticating screws are manufactured with exceeding precision. Most of the tolerances on the screw are within ±0.001 in. (0.025 mm). Screws are straight to within 0.004 in. (0.1 mm) and have a highly polished finish. Use of a torch will not only affect these closely machined tolerances but may destroy the metallurgical properties of the base metal.
The first step to cleaning the plasticating system is to purge the screw. Begin by closing off the flow of the resin that is being processed, typically by closing the slide gate at the bottom of the resin hopper. Next, reduce the screw rotational speed to approximately 15 to 25 rpm and let it operate at this speed until polymer stops flowing from the end of the extrusion die or out of the injection nozzle.
Most screws built for extrusion are made of 4140 hardened and tempered steel. When isolated heat from an acetylene torch is used to remove plastic from the root of the screw, it will cause the metal to expand on that side of the screw and thus cause the screw to bend. Once the screw cools it is highly doubtful that it will ever be as straight as it was originally. And if the steel is heated to a point where the isolated area turns a permanent blue, there will be metallurgical changes to the base metal. On some occasions it actually causes a delamination of the base metal, with a large portion of steel separating itself from the main body of the screw.
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