Plastics Technology

JUL 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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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. As a result, pressure flow is an estimate based on a viscosity from shear-rate/viscosity curves at the specific melt tempera- ture, and an assumption of the pressure at the beginning of the metering section. Estimation of the viscosity can be pretty accurate if you know the exiting melt temperature and have the shear-rate/viscosity data for that polymer. Estimation of the pressure at the beginning of the metering section, on the other hand, involves quite a few variables and takes experience. Pressure development in the screw is proportional to the screw speed, section length, viscosity, flight pitch, and diameter; and is inversely proportional to the square of the channel depth. Even with considerable experience, it's difficult to determine much more than an approximation of the pressure entering the metering section without pressure transducers placed along the barrel. As a result, it's typically ignored and the discharge or head pressure is used to calculate the pressure flow. The calculation for the effect of head pressure is again based on the work done at Western Electric in the 1950s and is still widely used today: F p π D H 3 P W (sin Θ)2/(12Lμ) = Pressure flow For most screws, W (percent of channel) is 0.9 or 90%, since most flights are 10% of the pitch; and F p is 1.10 for standard pitch, except for very deep screw channels, essentially cancelling each other. This, then, simplifies the equation to: π D H3 P (sin Θ)2/(12Lμ) or 0.02163 D H3 P/Lμ, where D = screw diameter, in. H = Channel depth, in. P = Head or discharge pressure, psi L = Metering-section length, in. μ = Viscosity, lb-sec/in. 2 Since most viscosity data is expressed in PaS or poise it can be converted by: 1 poise = 0.0000145 lb-sec/in. 2 and 10 poise = 1 PaS. Then the other terms can be in English units. As an example of how to use this, let's take a 3.5-in. screw turning at 100 rpm with a head pressure of 30 psi. The screw has a standard pitch (3.5) and standard flight width (0.35 in.); the metering length is 30 in., and the channel depth is 0.150 in. The viscosity is 10,000 poise at the shear rate in the channel and the discharge temperature. So: (0.02258) D 2 H N-(02163) D H 3 P/L μ = Net output, in. 3 /sec 4.149-0.176 = 3.973 in. 3 /sec Multiply by 130 × melt specific gravity for LDPE: 3.973 × 130 × 0.75 = 387.4 lb/hr The complete formula shown above can be used where the flight pitch, flight width and F p are greater due to exceptionally deep channels. In this example, the actual screw output was 375 lb/hr. That's close enough to the calculation that you can assume the screw is performing basically as it should for this application. However, if the actual output is, say, 250 lb/hr, then there is something causing the screw to perform below its calculated net flow. It could be a feeding issue, a melting issue, or poor screw design for the particular polymer, resulting in a screw design that is not balanced throughout. It's unusual for a screw to signifi- cantly exceed the drag flow unless the metering is very short, or the feed section is grooved. 1(905)507-9000 I I I Mississauga, ON, Canada Newest MacroPack-FP Die Fastest Purge Time Most Versatile Up to 13 layers Up to 30 layers 26 JULY 2018 Plastics Technology K now How

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