Extrusion is not only a final forming process for products but it is also an intermediate process for many other processes such as injection molding, blow molding and blown film. The cost-effective operation of extrusion screws is therefore essential for a high proportion of the plastics processing industry.
The initial cost of energy-efficient extruders may be higher but it will give rapid returns on the small extra investment. Options such as high-efficiency motors and variable-speed drives have excellent payback as both new and replacement purchases.
Whatever the age of the machine, it is essential to get the right extruder for the job and the screw diameter and design should be checked to make sure they are right for the polymer and product. Extruders run most efficiently (not only in energy terms) when operating at the design conditions for the motor and screw. The extruder should be set to run at the maximum design speed, as this is usually the most efficient setting. The screw speed should only be reduced from the maximum if there is difficulty producing good product.
- Using large extruders for small profiles wastes energy and costs money.
- Total efficiency (including energy efficiency) is best when operating at the design conditions.
- Set the extruder to run at its most efficient speed (usually the maximum design speed) and control the screw speed to give an extrusion rate as close to the maximum as possible and still produce good product.
- Size and control the electric motor to match the torque needed by the screw.
Optimizing the extruder speed in this way maximizes the heat from mechanical work and minimizes the amount of additional electrical energy needed to heat the plastic. Provided the downstream equipment does not limit the output, the energy consumption of an extruder per kilogram of processed polymer can be decreased by nearly 50% by doubling the rotational speed of the extruder.
Good extrusion efficiency demands that the polymer is kept at the optimum processing temperature while at the same time is prevented from overheating. The processing window is sometimes quite small and overheating from shearing can occur unless accurate temperature control is achieved. Accurate temperature control will not only help to produce good product but will also minimize energy costs.
- Check extruder controls to make sure the heating and cooling are working efficiently together and not competing with one another.
- Barrel insulation reduces energy costs and reduces temperature fluctuations. It has a payback of around 12 months and also reduces health and safety concerns.
- As with injection molding machines, standby operation of extruders uses significant amounts of energy in utilities through barrel heaters, cooling water and vacuum calibrators (if used).
- Find the minimum standby settings and establish setting sheets so operators always leave machines in this condition when not producing.
- Turn off barrel heaters and cooling fans between runs when the time between runs is sufficient and after barrels have been purged.
The energy use in an extruder is a measure of the condition of the machine and can be used as a sensitive diagnostic tool. Increasing energy consumption from an established benchmark is an early warning of deterioration of the machine condition and the need for maintenance of the machine. Setting up a Total Productive Maintenance (TPM) program involves some small additional effort and costs but will lead to significant energy savings in processing and will also keep machines and systems in the best condition.
Implementing a total productive maintenance (TPM) program involves effort and cost but will lead to significant energy savings.
Energy-efficient extrusion is easy and it will improve profits and save money but only if you take the action necessary to reduce costs.
This is part four of our six-part series of practical advice that processors can use to slash their energy bills. Author Dr. Robin Kent is managing director of Tangram Technology – specialist consulting engineers for energy management in plastics processing.