The extrusion process is one that predates plastic itself but it’s one that has become synonymous with plastic applications today. To get the perfect extruded part, plastic manufacturers use several techniques, machines and methods.
To help extruders save money, yield higher quality products, and use equipment more efficiently, industry expert Allan L. Griff put together 10 important principles to keep in mind. We take a brief look at these principles in this three-part post.
Power costs are relatively unimportant
Despite the real problems on a plant level with rising power costs, the power needed to run an extruder is still a very small proportion of total manufacturing cost. This will always be so because material cost is much higher.
Pressure at the screw tip is important
Measuring pressure is important for safety reasons if it gets too high, the head and die might blow off and hurt or damage people or machines nearby. Pressure is good for mixing, especially in the last (metering) zone in single-screw systems. However, higher pressure also means more energy is taken through the motor which may dictate the pressure limit.
Output = displacement of the last flight, +/- pressure flow and leakage
The displacement of the last flight is called the drag flow, and depends only on screw geometry, screw speed, and melt density. It is modified by the pressure flow, which consists of the effect of the resistance (indicated by head pressure) to reduce output, and the effect of any overbite in the feed to increase output. Leakage over the flights may also be in either direction.
Shear rate plays a key role in viscosity
All common plastics are shear-thinning, which means that the viscosity gets lower as the plastic moves faster and faster.
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Source: Allan L. Griff