11 Fundamental Principles of Plastic Extrusion (2)

06 Materials cost the most

In some cases, material costs can account for as much as 80 percent of production costs—more than all other factors combined—except for a few products where quality and packaging are particularly important, such as medical catheters.

This principle naturally leads to two conclusions: processors should reuse scrap and scrap as much as possible in place of raw materials, and adhere to tolerances as closely as possible to avoid deviations from target thicknesses and product problems.

 

07 Energy costs are relatively unimportant

Although the attractiveness of a plant is on the same level as the real problems and rising energy costs, the energy required to run an extruder is still a tiny fraction of the total production cost.

Because the material cost is very high, the extruder is an efficient system, and if too much energy is introduced the plastic can quickly become so hot that it cannot be processed properly.

 

eva poe solar film extrusion line 1

 

08 The pressure at the end of the screw is very important

This pressure reflects the resistance of everything downstream of the screw: screen and contamination crusher plates, adapter ducts, stationary agitators (if any), and the mold itself.

It depends not only on the geometry of these components but also on the temperature in the system, which in turn affects resin viscosity and throughput speed. It is independent of screw design, except when it affects temperature, viscosity and throughput.

For safety reasons, it is important to measure the temperature – if it is too high, the die and die can explode and injure nearby people or machinery.

 

Pressure is advantageous for stirring, especially in the last zone (metering zone) of single screw systems. However, high pressure also means more power from the motor – and therefore higher melt temperature – which can dictate the pressure limit.

In a twin screw, the two screws intermesh is a more efficient agitator, so no pressure is required for this purpose.

 

When making hollow parts, such as tubes made with spider molds that use stents to position the core, high pressures must be created within the mold to help separate streams recombine. Otherwise, the product along the weld line may be weak and problems may arise during use.

 

pc pp pvc corrugated sheet extrusion line

 

09 output = displacement of last thread +/- pressure flow and leakage

The displacement of the last flight is called positive flow and depends only on screw geometry, screw speed and melt density. It is regulated by the pressure stream, which actually includes the drag effect that reduces the output (indicated by the highest pressure) and any over-occlusal effect in the feed that increases the output. Leaks on threads can be in either direction.

 

It is also useful to calculate the output per rpm (revolution) as this represents any drop in the pumping capacity of the screw over time. Another related calculation is the output per horsepower or kilowatt used. This represents efficiency and enables an estimate of the production capacity of a given motor and drive.

 

10 Shear rate plays a major role in viscosity

All common plastics have shear drop properties, meaning that the viscosity decreases as the plastic moves faster and faster.

This effect is particularly pronounced for some plastics. For example some PVCs will increase the flow rate by a factor of 10 or more when the thrust is doubled.

In contrast, the LLDPE shear force did not drop too much, and its flow rate only increased by a factor of 3 to 4 when the inference was doubled. The reduced shear reduction effect means higher viscosity under extrusion conditions, which in turn means more motor power is required.

 

This may explain why LLDPE operates at a higher temperature than LDPE. The flow is expressed as shear rate, which is about 100s-1 in the screw channel, between 100 and 100s-1 in most die dies, and greater than 100s-1 in the thread-to-barrel clearance and some small die gaps.

 

Melt coefficient is a common measure of viscosity but is reversed (eg flow/thrust instead of thrust/flow). Unfortunately, its measurement at shear rates of 10 s-1 or less and in extruders with fast melt flow rates may not be a true measurement.

 

pp hollow board extrusion line 4

 

11 The motor is opposed to the cylinder, and the cylinder is opposed to the motor

Why is the control of the cylinder not always as expected, especially in the measurement area?

If the barrel is heated, the material layer at the barrel wall becomes less viscous, and the motor requires less energy to operate in this smoother barrel. Motor current (amps) drops.

Conversely, if the barrel cools, the melt viscosity at the barrel wall increases, the motor must turn harder, the amperage increases, and some of the heat removed through the barrel is sent back by the motor.

Generally, the barrel adjuster does have an effect on the melt, which is what we would expect, but nowhere is the effect as large as the area variation. It’s best to measure the melt temperature to really understand what’s going on.

 

Rule 11 doesn’t apply to dies and dies, because there’s no screw turning there.

That’s why outside temperature changes are more effective there. However, these changes are from the inside out and are not uniform unless homogenized in a stationary mixer, which is an effective tool for melt temperature changes as well as for mixing.

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