Plastics Technology

OCT 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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EXTRUSION In my June and July columns, I discussed how to use drag-flow and pressure-flow equations to analyze the performance of an extruder. Those same equations can be used to analyze certain output stability problems. If the output has a rhythmic surge, the cause is almost always a par- tially filled metering section. A specific length of fill in the metering section is required to provide stability. That length depends on the polymer viscosity, screw design and the head pressure. The mechanism of such a surge is an almost perfect cyclic response between the head pressure and the length of fill in the metering section. A partially filled metering section fills backward until it can balance the head pressure. When that occurs, the drag flow minus the pressure flow equals the actual output. Remember that the pressure flow is proportional to both the length of the fill and the viscosity for any particular screw design. The cyclic surge then develops as the metering section back-fills to balance the head pressure; but because there is a delay between the equalization and the response, there is a slight change in output. This becomes a harmonic variation or cyclic Head Pressure and Output Stability pulse that will continue until something changes to provide more back-fill, which dampens the response. Using the drag flow, pressure flow, and actual output, a measure of the metering-section fill can be obtained: Fill Length = [Q P / (Q D -Q A )]L, where Q P = Pressure flow, Q D = Drag flow, Q A = Actual output, L = Length of metering section. The amount of fill necessary to prevent a cyclic surge of this type depends on the polymer viscosity, channel depth, and head pres- sure, so there is no fixed fill length to eliminate such a surge. However, two flights are certainly a minimum; and with lower viscosities and lower head pressures, that can be as many as four flights. The cause of such a surge can be confirmed by simply changing the head pressure. Lowering the head pressure will result in an increase in the frequency and magnitude of the surge. Conversely, increasing the head pressure will result in lower frequency and magnitude. Pressure flow is proportional to both the length of the fill and the viscosity for any particular screw design. Use drag- and pressure-flow equations to analyze fluctuating output. By Jim Frankland Get more insights on Extrusion from our expert authors. short.ptonline.com/extrudeKH Learn more at PTonline.com KNOW HOW EXTRUSION The cause of cyclical surges can be confirmed by simply changing the head pressure. Lowering the head pressure will result in increased frequency and magnitude of the surge. Increasing the head pressure will result in lower frequency and magnitude. Confirming the Cause of Cyclical Surges Time, min Time, min Time, min Pressure, psi Pressure, psi Pressure, psi 3000 2500 2000 1500 1000 500 3000 2500 2000 1500 1000 500 3000 2500 2000 1500 1000 500 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 Initial Pressure Increased Pressure Lowered Pressure 32 OCTOBER 2018 Plastics Technology PTonline.com K now How

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