Plastics Technology

JUL 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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EXTRUSION Single-screw performance is one of the most critical but least understood factors in processing. To be an expert takes some very specialized training and experience. However, to understand the basics of single-screw performance requires only a simple hand-held calculator. In June's column, we simplified extruder screw output using the drag- flow equation for a screw having a standard-pitch metering section. The calculation was simplified to: 0.02258 (D 2 )HN = Output (in. 3 /sec), where: D = Screw diameter, in. H = Channel depth, in. N = Screw rpm The result (in. 3 /sec) can be converted to lb/hr by multiplying 130 × melt specific gravity of the polymer. In June, we also showed the full formula if the pitch is other than standard. With these calculations, remember that these outputs are for open dis- charge or no head pressure. When I'm asked to evaluate the performance of a screw, the first thing I do is see how its actual performance stacks up to its calculated drag flow. It's unusual for a screw to significantly exceed the calculated drag flow unless the metering section is very short or the feed section is grooved. So, the calculated drag flow is generally the maximum output that would be expected. If the actual output is very close to the calculated and is stable, you can assume the screw has a balanced design. That is, the feed, melting and pumping sections are all working properly to achieve output of the metering section. If the output is well under the drag flow, then there is a series of things to check and the first is the effect of the head pressure. When there is head pressure—as in an actual extrusion operation—the output is generally less than the drag flow. In the accompanying illustration, A shows pure drag flow where the velocity at the top of the channel equals the velocity A Simple Way to Evaluate Your Screw Performance of the barrel relative to the screw and the bottom is zero velocity as the resin is stuck to the screw; while B shows the effect of head pressure. The drag flow is still there, but the head pressure is causing a backward flow near the bottom of the channel. The net flow is a combination of the drag flow minus the pressure flow. The effect of head pressure is a less exact calculation than the drag flow for several reasons. First, it depends on the polymer viscosity in the metering section; and second, there is usually a pressure at the beginning of the metering section that can offset the head pressure just like two pressures in a pipe. Neither of these variables is easy to determine precisely, even with advanced computer simulation. Get more insights on Extrusion from our expert authors. short.ptonline.com/extrudeKH Learn more at PTonline.com KNOW HOW EXTRUSION A hand-held calculator and some basic equations can help determine whether your screw is up to snuff . By Jim Frankland PART 2 In this illustration of an "unwrapped" screw channel, A shows pure drag flow where the velocity at the top of the channel equals the velocity of the barrel relative to the screw and the bottom is zero velocity as the resin is stuck to the screw; while B shows the effect of head pressure. A B Vb Vo Vb Vb Vo 24 JULY 2018 Plastics Technology PTonline.com K now How

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