Plastics Technology

OCT 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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Page 53 of 75

Here's how to find out what's happening to your screw and what to do about it. Screw Wear: Understanding Causes, Effects, and Solutions Feedscrew and barrel wear is a serious matter for maintenance and engineering departments in both extrusion and injection molding, and rightfully so. Excessive wear in a given plasticating unit poses a litany of serious perfor- mance issues, naturally affecting cost and overall quality. In an article that appeared in Plastics Technology's February 2018 issue, "What Is Your Extruder Trying to Tell You?," authors Wes Hobson and Mike Rutkowske from The Dow Chemical Company illustrated the need for baseline data and wear-monitoring procedures to avoid excessive loss in productivity. Hobson and Rutkowske did a great job demon- strating how to diagnose excess screw wear by moni- toring quality and perfor- mance issues. Now that we have some idea of what our extruders are "telling us" from a processor's point of view, the next step is to explore what is happening to the screw and what can be done about it. In this article we will look at how screw wear affects melt quality and the cost-effec- tiveness of rebuilding vs. replacing worn equipment. TYPICAL FEED- SCREW WEAR Feedscrew wear in extrusion and injection molding is similar: The majority of wear occurs in areas of high pressure—in particular, the transition section and last few turns of metering. Processors and suppliers com- monly theorize that wear in these areas is caused by metal-to-metal contact between the flight OD and barrel ID, due to screw flexion under these high pressures. This theory is unlikely because the internal pressure is generally the same all around the screw. A pressure differential is needed to generate a force deflecting the screw, not a general high pressure. Metal- to-metal contact can occur, but it is a result of mechan- ical conditions and is a symptom of potentially larger problems: misalignment, gearbox damage, bent screw, bent barrel, inadequate foundation, etc. Abrasion testing performed across many industries shows that increased pressure and temperature leads to increased friction. Since the transition section and last few turns of the metering section typically experience the highest pressures and temperatures, we can predict that this is where the bulk of OD wear occurs. This rings true empirically after inspecting and rebuilding thousands of screws. Much like flight OD wear, root wear is typically most severe in these high-pressure areas. Wear is most extreme on the forwarding side of the flight vs. the trailing side. The severity of root wear is greatly impacted by the resin or resins processed. When processing abrasive fillers, such as glass fibers or calcium carbonate, it is possible to see severe wear in the feed section as pressure is being developed by the forwarding side of the flight, with limited lubrica- tion provided by the unmelted pellets. The amount of filler present and particle shape strongly influence the intensity of wear. Larger particles with sharper corners tend to wear more than smaller, rounder particles. Unfortunately, the desired physical properties of the molded part dictate the filler to be used. Since wear is more By Stephen Surley R. Dray Mfg. The majority of wear occurs in areas of high pressure, in particular the transition section and last few turns of the metering section. Extreme root wear, primarily on the forwarding side of the flights, causes severe chipping of welded hardface inlay. FIG 1 Colmonoy Number 56 hardfacing applied to the flight OD after re-welding during the screw rebuilding process. FIG 2 52 OCTOBER 2018 Plastics Technology Tips and Techniques

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