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Strategy | energy management

As the IMM motor uses more than 69% of the power input to the IMM, it is essential to get the right motor arrangement for the job. There are now a range of motor options (as shown in Figure 5) but whatever the motor type, the fi rst essential is to get the right size of motor and IMM for the task and to make sure the IMM is appropriate for the polymer and product.

It is important to remember that IMMs run most

effi ciently (and not only in energy terms) when operat- ing at the design conditions. As far as is possible, the IMM should be set to run at its maximum design speed, as this is usually the most effi cient speed for heating and for production effi ciency. Progress in controlling the main motors of IMMs has

been rapid and diverse over the last 20 years and purchasers of IMMs are now faced with a bewildering choice of motor types. IMMs were traditionally based on hydraulics and the main motor was a hydraulic pump. The introduction of all-electric and ‘hybrid’ IMMs allowed greatly reduced energy use (as can be seen in Figure 3) but hydraulic machines are not, by any means, dead and new control and pump systems have been developed over recent years to improve their energy effi ciency considerably.

Select the right drive

Hydraulic IMMs are the traditional approach and most of the installed IMMs are based on hydraulic power systems. The new developments in this area are mainly concerned with slowing the motor down when hydraulic oil is not needed to avoid wasting energy.

Figure 4: Production costs versus production rate

Fixed speed motor/fi xed volume pump Until relatively recently, the main source of power was a fi xed speed hydraulic pump, but these are now relatively rare because of control improvements and rising energy costs. The amount of hydraulic fl uid needed during the injection moulding cycle varies continuously but despite this varying demand, the conventional fi xed displace- ment, vane or gear type of hydraulic pump will be operating at a fi xed speed.

This fi xed speed is designed to deliver the required hydraulic fl uid for the maximum transient demand (generally during injection). At times other than that of maximum demand the excess hydraulic fl uid will be diverted back to the sump via a relief valve and the pump’s energy use will be largely wasted. This means that the pump motor is delivering excess hydraulic fl uid for all but the maximum demand period and using excess energy for all but the maximum demand period. This system has a low effi ciency, particularly during the hold and cooling phases, and much of the energy input is wasted during these phases.

Existing IMMs of this type can be considered for retrofi tted VSDs. However, if they have small motors or run short cycle times a retrofi t solution may not be appropriate because the opportunities to slow the motor down will be limited.

Fixed speed motor/variable volume pump This type of system was common from some manufac- turers and is still available, although it is rapidly being superseded by newer and more effi cient technology. It uses variable volume pumps to provide improved control of the amount of oil generated.

Figure 5: IMM motor and drive options

While much more energy-effi cient than fi xed volume pumps, these machines are still not fully energy-effi - cient. However, effi ciency can be increased by substitut-

September 2013 | INJECTION WORLD 33

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