Energy Management in Plastics Processing - Part 4
Injection 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.

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Injection Moulding

Over 90% of the energy costs in injection moulding are accounted for by electricity. This makes electricity purchasing very important for moulders and costs can be significantly reduced by good purchasing and operational controls. Only 5 to 10% of the total energy used in the process is actually input to the polymer, the other 90 to 95% is used simply to operate the machine and large savings can be made.

Moulding machines

As with most machines, the initial cost of a moulder will be less than the cost of energy used during its lifetime. The energy cost will be even more for machines that are not energy efficient. Although it may cost more initially, energy efficiency will save money in the long term, a factor that is becoming more important in markets where customers expect decreasing prices through the lifetime of a product.

  • Tip - Use ‘whole life costing’ for new machines and include the energy costs.
  • Tip - Contact machinery suppliers for information on additional equipment to reduce energy consumption.
  • Tip - New generation machines often have improved energy efficiency and can reduce product costs by over 3%.

Getting the right machine for the job is vital and the machine should be closely matched to the product.

  • Tip - Using large machines for small products is inherently wasteful. Are all jobs on the appropriate machines?
  • Tip - Total efficiency decreases as the operating conditions move further away from the design conditions.

Electric motors account for 60% of the electricity used in moulders and the moulding cycle causes intermittent, variable loads with Power Factor values in the region of 0.7. PF correction equipment can increase the PF to greater than 0.95 with a payback of less than one year.

  • Tip - Improving the PF is cost effective and simple with excellent payback.
  • Tip - Motors are most efficient near the design load. Oversized motors at part load are less efficient than small motors at full load. Check all motor sizes.

Controlling the start-up sequence of machines can reduce energy costs with no other effect. Starting multiple machines at the same time will increase the Maximum Demand and the energy cost.

  • Tip - Fit a warning device to the MD meter to sound when the load approaches the allowable limit.
  • Tip - Plan and control the start-up sequence.

Machines use energy even when idling, the amount varies with the machine to but can range from 52% up to 97.5% of the full moulding consumption. An idling machine is not ‘free’.

  • Tip - Idle periods of between 20 to 45 minutes may make it cheaper to switch off and restart.
  • Tip - Are barrel heaters and cooling fans left on between runs?
  • Tip - Is cooling water circulating through idle tooling?
  • Tip - Is compressed air supplied to idle machines?

‘All-electric’ machines are an energy efficient moulding solution and can both reduce energy use and make computer control easier and more direct. On conventional machines the hydraulic systems provide peak power for a very short time and the hydraulic system is overrated for most of the time.

  • Tip - The use of accumulators for rapid hydraulic energy release can significantly reduce the hydraulic system size.

Heat transfer to the barrel is improved by pre-seating the heating element to the barrel and by using flexible metal bearing compounds.

  • Tip - Thermal efficiency can also be improved by barrel insulation. This has a rapid payback (generally under one year) and improves other areas such as Health and Safety and fluctuations due to air currents.

Preventative maintenance such as de-aeration of the oil system and maintenance of the controls will reduce energy costs.

  • Tip - Monitor the energy use to identify deterioration of the machine.
  • Tip - Increased maintenance can lead to significant energy savings.

Moulds

Product cooling time is generally more than 50% of the cycle time. Efficient cooling can greatly reduce cycle times and energy usage - a double benefit.

  • Tip - Is cooling water at the maximum temperature and minimum quality, how efficiently is it treated and distributed?
  • Tip - Air in the cooling system reduces the cooling effectiveness. Degassed and pressurised systems can reduce cycle times and energy usage.

Excessive tool change times will waste energy if the machine is idling. Rapid set up of tooling reduces energy and improves overall factory effectiveness.

  • Tip - Are tool changes planned into production schedules? Are they quick?

Ancillaries and services

Ancillaries use energy in electric motors and consumption of utilities. For highly automated production the total ancillary energy demand can be comparable to the machine energy demand. The main opportunities are minimising the demand for utilities. Motors are generally small and run intermittently and it is often not cost effective to retrofit more efficient motors or controls.

  • Tip - Specifying energy efficiency during design of handling and ancillaries will give rapid payback on any additional costs involved.
  • Tip - Check that handling systems can be set to operate ‘on-demand’ only.
  • Tip - Match utilities to the demand.

Granulation and scrap recovery uses large amounts of energy and can will raise energy bills considerably.

  • Tip - Carry out granulation at night.

Heat recovered from hydraulic systems and chiller units through heat exchangers can be used to provide space heating for offices and other areas with pay back times of 6 months.

  • Tip - Look for opportunities to recover heat and reuse energy.

Management controls

Tweaking of machines by operators causes more lost time and energy than almost any other cause.

  • Tip - Optimising the machine settings reduces the electrical energy needed. Get machines set right, record the settings and do not change them unless absolutely necessary.
  • Tip - Use Statistical Process Control to control machine settings and performance.

The goal

Management is really at the heart of energy efficiency, without good management, neither energy efficiency nor any other change in operating practices will be effective. Energy efficient injection moulding is simply good moulding practice. It is inexpensive and reduces all costs – not just energy costs. Start working on energy efficiency today and reduce your costs to become a world class energy efficient moulder.

"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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