Energy Management in Plastics Processing - Part 7
Extrusion Blow moulding

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A series of energy efficiency worksheets by Dr. Robin Kent for the Carbon Trust to help the plastics industry reduce costs through efficient use of energy.

UK Government Environment and Energy Helpline 0800 585 794

Extrusion Blow Moulding

For blow moulding, the Specific Energy Consumption (the energy used to process a kg of polymer) varies from ‘typical’ values of 1.5 - 2.0 kWh/kg up to ‘high’ values of greater than 3.0 kWh/kg. If your factory SEC is greater than 2.0 kWh/kg there are some real savings to be made from energy efficiency and experience shows that energy savings of 5 to 10% can be made through simple low cost measures. For a company with a turnover of £5 million this means saving £10,000 to £20,000 per year for minimal expense. With rising energy prices and the Climate Change Levy, energy inefficient firms will be at a considerable commercial disadvantage.

Electricity use in a typical blow moulding factory

Machine

The major component of energy use is the extruder area which typically uses 40% of the total energy (see previous Worksheet on extrusion. As with other processes, energy efficient machines have lower long-term operating costs than standard machines will pay back any extra investment. 

The use of all-electric machines is an energy efficient option for blow moulding because these machines remove the energy losses at the electro-hydraulic interface and can reduce energy costs.

Whatever type of machine is used, good process parameter control gives efficient operation and can give huge savings.

Blow moulding machines use only small amounts of externally applied heat (most is generated mechanically) but heat transfer from barrel heaters can be maximised and evenly distributed by good seating to the barrel and the use of a conductive metal compounds. The energy used will be reduced and controlled by barrel insulation jackets - these also improve Health and Safety, reduce start-up times and generally have a pay-back of less than 1 year.

Parison weights are often up to 40% more than the weight of the final product. Any trimmed materials (tops and tails) can be recycled and recovered but the energy used is lost forever. Large tops and tails cost real money even if the material is recycled.

Regranulation should be done off-line (at night) to minimise energy costs, but first minimise tops and tail production - reduce and then recycle.

When a machine is not producing for a short time it is not practical to shut down the extruder but shutting down the hydraulic systems can give considerable energy savings.

Start-up procedures can be set to bring the energy demands online at the best possible time i.e. heaters until stabilised, hydraulics and finally the extruder drive. Similarly shutdown procedures can be developed to switch off the energy intensive areas of the machine.

Ancillaries

Parison forming must be complete before the outside surface chills and stops surface texture formation. The compressed air pressure for blowing should be just sufficient to form the parison before chilling but it can then be reduced to hold the parison against the mould surface.

Most of the heat put in during the melting stage must be removed before the product is released from the die. Product cooling time is about 50% of the cycle time and minimisation of the melt temperature will save energy in heating and cooling as well as reducing the cycle time.

The chiller system uses large amounts of energy and the process efficiency affects both time taken and energy used. Water has a better cooling efficiency than air and bubbles in the cooling water will decrease the efficiency of the cooling.

Cooling is most efficient with good contact between the parison and mould and this should be kept by the air feed during cooling.

Hydraulic systems for mould closing should be matched to the demand (blowing pressure x projected area) to reduce the energy needed and the hydraulic oil should be de-aerated on a regular basis to improve the efficiency of the hydraulic system. The hydraulic fluid should also be kept at a steady temperature to improve the process control and prolong the life of the oil.

General Tip

The real secret to reducing energy costs is not in the technical aspects of any process - it is in the management attitude. A desire to reduce costs through energy management and an effective implementation and monitoring programme will produce the results and the commercial benefits.

"Energy Management" Series.

The "Energy Management" series is designed to give plastics processors an insight into how to manage a valuable resource.

Download the complete series as an Adobe Acrobat file.

Last edited: 11/03/10  

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