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energy management | Strategy

ing motor speed control for pump volume control – it is more effective to reduce the speed of the motor than to reduce the output volume of the pump. Retrofi tted VSDs can also reduce the energy used by this type of machine but again this is typically not a suitable solution if they have small motors or short cycle times.

Variable speed motor/variable volume pump This type of system (so called P/Q technology) uses a combination of VSD and pump technology to optimise both the motor speed and the pump volume to deliver the oil required in the most effi cient combination of motor and pump speeds. As with other technologies the speed of the pump is

greatly reduced in the hold and cooling phases. When the volume demand at low pressure is high the system will move the swash plate to generate more volume at low pressure rather than increase the motor speed as this is a more effi cient option. Machines using this technology have reported energy use of 15-45% less than standard hydraulic machines. The cost is typically around 12% more than standard hydraulic machines.

Servo motor/fi xed volume pump The use of servo motors to drive pumps is a relatively recent innovation. However, the recent reduction in servo motor prices means it is now an option offered by many manufacturers. Typically applied to toggle clamp machines, the servo motor is used as a variable speed drive for a fi xed volume pump (piston or gear pumps are both used). This type of system can be set up to slow down or stop if no oil is required and because it is a

About the author

Dr Robin Kent is an independent exert specialising in plastics processing technology and business management. Founder and managing director of plastics engineering consultancy fi rm Tangram Technology ( uk), he is also the author of Energy Management in Plastics Processing: Strategies, Targets, Techniques and Tools, which has just been published in its second edition. This practical workbook takes readers through the

complete set of options for saving energy at the plastics processing level and across the entire plant. It explores benchmarking and site surveys, understanding energy supplies and bills, measuring and managing energy usage and carbon footprinting. This revised and expanded edition is available now from the Plastics Information Direct bookstore:

34 INJECTION WORLD | September 2013

servo motor can actually reverse direction. Machines using this technology have reported energy use of 40-60% less than standard hydraulic machines but the cost is only around 5% more than standard hydraulic toggle machines due to the simple nature and low cost of gear pumps. It is a promising technology for new machines or retrofi t option.

All-electric drive All-electric IMMs use servo motors to directly drive the machine motions and are highly energy effi cient, although the numerous motors adds to the machine cost. Very early models were limited in shot capacity for a given clamp force, limited in injection rate, nozzle touch force and ejector force and used energy during holding. However, these issues have largely been resolved by developments such as pulsing motors on/ off to reduce over-heating or by using electromagnetic braking to prevent the screw from backing off during pack/hold time without needing force from motor.

Hybrid drive technology Some manufacturers use hybrid technologies combin- ing both electric and hydraulic operation for specifi c applications. This allows moulders to benefi t from the advantages of both electric and hydraulic operations. Hybrid machines may not achieve all of the benefi ts of fully all-electric machines. Hybrids come in a range of options and most use a

servo motor (as for all-electric IMMs) to control the screw drives and use hydraulics for other machine movements such as clamping and holding and some- times for very high injection rates. The hydraulic systems employed tend to be energy-effi cient and use either variable speed drives or variable volume pumps to achieve energy savings.

The next steps Improving the energy effi ciency of an existing site or IMM can be as simple as running the existing equip- ment harder to minimise the effect of the base load and this is certainly recommended if the orders are there. There are many other actions that can be taken in

the area of services and machines but one of the most effective actions in the long-term is to ensure that the IMMs are as energy effi cient as possible by choosing the right technology for the IMM motor. The average IMM will be in service for over 15 years and the cost of the energy used will be far greater than the capital cost of the IMM. Getting the right motor is an impor- tant decision for the future and selecting an IMM should be based on more than the simple initial purchase price.

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