Plastics Technology

SEP 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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Page 43 of 91 E X T R U S I O N K now How 42 SEPTEMBER 2018 Plastics Technology Get instant access to processing specs for thousands of injection molding plastics from more than 100 suppliers with the Mobile Specs for Injection Molding app. And it's FREE! CONVENIENT Search for supplier, product, family or grade QUICK Get instant results in the palm of your hand COMPLETE Access detailed processing notes from suppliers CURRENT Benefit from info that is constantly updated Download the Mobile Specs for Injection Molding app on your Android or IOS device today at MOBILE SPECS Shear stress at wall= ΔP•R/2L L is either the radius of a circular orifice, or the gap between the slit opening (H) or the gap in an annular die (R 0 -R i ). Proper heating of the downstream tooling is an important part of controlling head pressure. The tooling temperatures should be kept as close to the melt temperature as possible. That prevents developing the scale or static layer of colder, more viscous polymer on the inner walls that simply narrows the flow passage and causes increased pressure drop. It's difficult to reduce melt temperature by cooling the tooling because of its effect on raising the head pressure and because of the low thermal conductivity of the polymer. When assembling an extrusion line for any new shape, the estimation of head pressure from the tooling should be consid- ered in the output and cooling calculations. That's done by mini- mizing the length of the flow passages and holding the wall shear stress to as close to 10 psi as possible by adjusting the size of the flow passage. ABOUT THE AUTHOR: Jim Frankland is a mechanical engineer who has been involved in all types of extrusion processing for more than 40 years. He is now president of Frankland Plastics Consulting, LLC. Contact or (724)651-9196. Pressure Drop Shear Rate Circular flow passage: ΔP = Qμ (8L)/πR 4 Υ = 4Q/ πR 3 Slit flow passage: ΔP = Qμ (12L)/WH3 Υ = 6Q/ WH3 Annular passage: ΔP = Qμ (12L)/π(R 0 +R i )(R 0 -R i )3 Υ = 6Q/ π(R 0 -R i )3 Pressure Drop & Shear Rate for Different Flow-Channel Geometries P = Pressure Q = Output μ = Viscosity L = Length R = Radius R 0 = Outer radius R i = Inner radius W = Width H = Height Υ = Shear rate Source: Processing of Thermoplastic Materials, Ernest Bernhardt, Reinhold, 1959, p. 249 It may even cause degradation of the layer on the wall with ther- mally sensitive polymers. Shear stress at the wall is a simple formula but depends on the pressure drop, as calculated top right:

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