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Part 3: Reducing energy costs- Focus on injection molding

July 1, 2006

5 Min Read
Part 3: Reducing energy costs- Focus on injection molding

More than 90% of the energy costs in injection molding are due to electricity usage, but only 5%-10% of the energy consumed is actually input to the polymer-the other 90%-95% is used to operate the machine. Knowing that, large savings can be realized without affecting the product in any way.

Machines

The purchase cost of a molding machine will be less than the energy cost during its lifetime and, as discussed last month, it is important to consider the `whole life cost'' when buying new machines. New generation machines often have improved energy efficiency that can reduce product costs by over 3%.

Tip: Use `whole life costing'' for new machines.Tip: Ask machinery suppliers for information on additional equipment to reduce energy use.Getting the right machine for the job is vital and machine capacity should be matched to the product. Using large machines for small products is wasteful-large machines operating at partial load are less efficient than small machines operating at full load. Molding machines are most efficient near their design load, and total machine efficiency decreases as the operating conditions move away from the original design conditions.Tip: Using large machines for small products is inherently wasteful. Check that all jobs are processed on the appropriate machine. Tip: Total efficiency decreases as operating conditions move away from the design conditions.On conventional machines the hydraulic system needs peak power for a very limited time and the hydraulic system is overrated for most of the time. The use of accumulators for rapid hydraulic energy release can allow significant reductions in the size of the hydraulic system.Controlling the startup sequence of machines can reduce energy costs with few other effects. Attempting to start multiple machines at the same time will increase the Maximum Demand (MD) and the cost of energy.Tip: Plan and control the startup sequence to limit MD.Where barrel heating is required, heat transfer to the barrel is improved by pre-seating the heating element to the barrel by using flexible metal bearing compounds and by using insulated heater bands.Tip: Thermal efficiency can 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.Machines use energy even when idling and this can be anything from 52% and 97.5% of the full molding energy consumption. An idling machine is definitely not `free''. Idle periods of greater than 45 minutes may make it cheaper to switch off and restart.Tip: Switch off heaters and ancillaries between runs.Tip: Stop cooling water circulating through idle tooling.Tip: Stop supplying compressed air to idle machines.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.Preventive maintenance such as de-aeration of the oil system and maintenance of the controls will reduce energy costs.Tip: Monitor machine energy use to identify deterioration of the machine.Tip: Increased maintenance can lead to significant energy savings.All-electric moldingNew `all-electric'' machines can reduce energy use by between 30% and 60%, depending on the molding and the machine, and in some cases shorter cycle times are also possible to further reduce energy consumption and increase productivity. All-electric machines also eliminate the need for cooling the hydraulic system, along with the associated energy use.MoldsExcessive tool change times will waste energy even if the machine is idling. Rapid setup of tooling reduces wasted energy and improves productivity.Tip: Plan tool changes into production schedules and use rapid setup methods.Cooling time is often more than 50% of the total cycle time. Good cooling will reduce cycle times and energy usage - a double benefit. Cooling systems are often set and forgotten, happily wasting energy and your money.Tip: Check that cooling water is at the maximum temperature and minimum quality.Tip: Check that cooling water is efficiently treated and distributed.Tip: Air in the cooling system reduces cooling effectiveness. Degassed and pressurized systems can reduce cycle times and energy usage. Ancillaries and servicesIn a highly automated factory, the ancillary energy demand can be comparable to the main machine energy demand. The main opportunities are in minimizing the demand for utilities because the motors are generally small and run intermittently so it is not cost effective to retrofit more efficient motors.Tip: Specifying energy efficiency during ancillaries purchase will give rapid payback on any additional costs.Tip: Set handling systems to operate `on-demand''-link the controls to the machine operation.Granulation and scrap recovery uses large amounts of energy and can raise costs if carried out at the wrong time. Tip: Carry out granulation at night to reduce costs.Heat recovered from hydraulic systems and chiller units through heat exchangers can be used to provide free space heating for offices and other areas with short payback times.Tip: Look for opportunities to recover heat and reuse energy.Management controlsOptimizing machine settings reduces the energy needed for good product. One of the greatest wastes is `tweaking'' of machines by operators or setters to get their own personal settings.Tip: Optimized machine settings reduce energy use. Get machines set right, record the settings and don''t change them unless absolutely necessary.Tip: Use Statistical Process Control to control machine settings and operations.Tip: Develop and use effective startup, standby and close-down sheets to formalize machine settings and operations.The goalManagement is really at the heart of energy efficient molding because without good management, neither energy efficiency nor any other change in operating practices will be effective. Energy efficient injection molding is simply good molding practice. It is inexpensive and reduces all costs-not just energy costs.The author, Dr. Robin Kent ([email protected]), is managing director of Tangram Technology-specialist consulting engineers for energy management in plastics processing. You can visit Tangram online at www.tangram.co.uk.

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