The following are important principles to keep in mind regarding extrusion. They should help save money, yield higher quality products, and use equipment more efficiently.
7. Power costs are relatively unimportant.
Despite the popular fascination and 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, an extruder is an efficient system, and if excess energy is introduced the plastic will soon get too hot to process properly.
8. Pressure at the screw tip is important.
This pressure reflects the resistance of everything downstream of the screw: screens and contamination, breaker plate,adapter, transfer tubes, static mixers (if used), and the die itself. It depends not only on the geometry of these components, but also on the temperatures in the system, which in turn affect resin viscosity and throughput rate. It does not depend on screw design, except as it affects temperature, viscosity, and throughput.
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—thus higher melt temperature—which may dictate the pressure limit. In twin screws, the intermeshing of the two screws is a more efficient mixer, so pressure isn’t needed for this purpose.
In making hollow items, such as pipe with a spider die that uses arms to hold the central core in place, high pressure must be generated in the die to help the split streams weld together again. Otherwise, the product may be weaker along these weldlines and could fail in service.
9. 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 really 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.
It is also useful to calculate output per rpm, as this shows any deterioration of the screw’s pumping capacity with time.Another related calculation is the output per hp or kW of power used. This is the efficiency and enables estimation of the production capacity of a given motor and drive.
10. 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. Some plastics show this effect dramatically. Some PVCs, for example, flow 10 or more times as fast with just a doubling of the push. LLDPE, by contrast, does not shear -thin as much, and the same doubling of the pushing force increases its flow by only three to four times. The reduced shear -thinning effect means higher viscosity at extrusion conditions, which in turn means more motor power is needed. This explains why LLDPE runs hotter than LDPE.
Flow is expressed in terms of shear rate, which is around 100 sec -1 in the screw channels, between 100 and 1000 sec-1 in most die lips, and much more than1000 sec -1 in the flight-to-wall clearances and some tiny die gaps. Melt index is a common measure of viscosity but is inverted (i.e., flow/push instead of push/flow). Unfortunately, it is measured at shear rates of 10 sec -1 or less and may not be a true measure in an extruder where melt is flowing much faster.
11. The motor opposes the barrel, the barrel opposes the motor.
I started out with the 10 key principles of extrusion, but this one was so important that I had to include it, too. The Eleventh Law is why barrel control isn’t always as effective as desired or expected, especially in the metering zone. If the barrel is heated, the layer of material at the barrel wall becomes less viscous and the motor needs less power to turn in this more lubricated barrel. Motor current (amps) goes down. Conversely, if the barrel is cooled, the melt at the barrel wall becomes more viscous, the motor must work harder, amps go up, and some of the heat removed through the barrel is put right back again by the motor. Usually, the barrel controllers do have the effect on the melt that is desired, but nowhere as much as the zone change amount. It’s best to measure melt temperature to really understand what is happening.
Post time: May-27-2017