General purpose metering screw

GATT General Agreement on Tariffs and GPMS general purpose metering screw... 

General purpose screws can be used, for example, with an L D ratio of about 18 1 and a constant pitch of ID. The feed zone should be approximately 0.5L, the compression zone 0.3L and the metering zone 0.2L a compression ratio of about 2 1 should be sufficient. The screw is usually fitted with a nonreturn valve but valves are not essential on the nozzle unless high back pressures are involved. The best results have been obtained by using spring loaded or hydraulically operated types decompression or suck-back is also used. High speed low torque screw motors are often preferred to achieve short screw recovery times. 

Molten plastic imiformity is required for optimum processing performance hence, screw specification is another strategic issue. The standard general-purpose screw is known to produce immelted sohds (26,27). These unmelted solids clog gates and produce inferior parts. Molten plastic imiformity can be achieved with state-of-art screws and metering sections as represented in Figure 23. 

Extrusion. For processing of TPEEs by extrusion, the single-screw machines are recommended. The screws should have hardened surfaces for good wear resistance. A general purpose extruder barrels may be used with preferable length to diameter (L/D) ratio 24/1 to provide the best melt quality for precision extrusion. The barrel should have at least four independently controlled heating zones. Compression ratios should be between 2.5/1 and 3.5/1, as determined by the depth of the feed zone divided by the depth of the metering zone channel. 

You should know all the specifics relative to the screw. You need to know is it specific to your resin or is it a general-purpose screw. The UD of the barrel and screw plus the compression of the screw should be shown. Does the screw have the capability to be internally cooled or heated, and how many flights does it contain in the feed section, transition section, and the meter zone or is it of constant depth is the information you should know. 

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