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Hybrid or all-electric: can either one pay off for you?

Energy savings aren't the only benefit of a hybrid or all-electric molding machine. Speed, precision, quietness, and cleanliness may be more important to most users. Here is another view of these issues, which were explored in April's PLASTICS TECHNOLOGY.

The electric screw-drive option has been available on injection molding machines for many years, offering a well-recognized potential for energy savings. Battenfeld began supplying machines with two-speed electric screw motors in Germany back in the 1960s. Servo-electric screw drives have been available since the beginning of the Eighties and have been supplied in North America since 1994.

Yet the electric screw-drive option has remained less popular than conventional hydraulic screw drives because of the additional cost (up to 15%) for an electric screw drive. If you consider the average power cost in North America, the increased investment for the servo-electric screw drive, and typical return-on-investment (ROI) goals, the servo-motor option often makes no sense if based on energy savings alone. Leaving energy aside, however, hybrid machines can still have their place in modern injection molding operations.

The case for hybrids

Molders have selected servo-electric screw drives for several reasons, but the most important motivation has been the ability for simultaneous operation of the extruder and other machine functions to shorten the overall cycle time.

In applications where very high recovery rates are required, a servo-electric screw drive allows the standard oil reservoir size to be retained instead of increased, due to the installed hydraulic power. The hybrid approach means that oil residence time will be increased and oil-heat build-up will be reduced, resulting in less wear and tear on hydraulic-system components and a higher level of oil cleanliness.

Servo-electric motors with power ratings suitable for screw drives can also enhance a machine's ability to achieve precise control of screw speed.

What about servo pump drives?

Replacement of standard AC motors with AC servo motors to drive the hydraulic pump of an injection machine is another hybrid electric/hydraulic solution offered by several machine manufacturers.

In this case, the pump servo motor will slow down to an idle speed when no machine motions are taking place - such as during the time between the end of screw recovery and the start of clamp opening. Actual energy savings will depend on the length of the machine idle time relative to the overall cycle time. Justification of the required investment for a servo-motor pump drive may vary from case to case, but under average conditions it will seldom result in a short payback period.

On the other hand, the servo-electric pump drive can reduce the machine's noise level substantially, which is certainly a desirable feature for the equipment user.

All-electric for peak precision

While hybrid machines retain conventional hydraulics for injection and clamping, all-electric machines perform all movements through servo-electric motors. All-electrics thereby achieve a level of precision and repeatability required for extremely close-tolerance parts. Compared to hydraulic and hybrid-electric machines, the major benefit of all-electric machines is their process capability. The most stringent tolerance levels - and some commercial molding tolerances are now measured in microns - may be difficult to reach with either open- or closed-loop controlled hydraulic injection on a hybrid machine.

In addition to their precision-molding capability, all-electric machines offer the greatest reductions in energy usage and noise levels of any type of molding machine. And due to the absence of hydraulic fluids, electric machines are environmentally friendly and clean-room compatible.

Even though early predictions of sweeping hydraulic-machine replacement have not materialized, the number of all-electric machines sold continues to increase at a moderate pace. The cost of servo motors requires that all-electric machines be sold at a premium, and major cost reductions are not expected soon. Time will tell which markets are receptive to paying such a premium in order to benefit from the very high molding precision available with all-electric machines.
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Copyright 1998, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Title Annotation:molding machines
Author:Meyer, Wolfgang
Publication:Plastics Technology
Date:Jun 1, 1998
Words:640
Previous Article:An unusual match: nylon & PVC can go together.
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