Why all-electric injection machines.
That's not to say that superior energy efficiency isn't a strong part of the story. In some cases, we have even put electric screw drives on rebuilt hydraulic machines to capture some energy savings and the advantage of overlapping screw rotation with other machine functions. Such electric/hydraulic "hybrids" can be cost-effective.
But adding electric servos to a hydraulic machine still leaves you with a hydraulic machine. It may save some energy, run a little quieter, and cycle faster, but you still have all the baggage of hydraulics without the superior process capability of an all-electric machine. By the time you add servo drives on the pump and on the screw of a hydraulic machine, the result is probably close in cost to an all-electric press but still has the characteristics of a hydraulic machine.
What's more, a servo-driven pump might not respond the same as one with a fixed-speed motor, so you could end up with an energy-efficient machine that has more idiosyncrasies than a standard hydraulic press.
Rest of the story
One user of all-electric machines, a precision gear molder in the Northeast, runs his plant 24 hours a day, seven days a week, with the presses unattended for two shifts on weekdays and with only spot-check supervision on weekends. How is that possible? All-electric machines provide the accuracy and repeatability necessary for reliable unmanned molding.
This isn't an isolated case. All-electric machines have a window of process capability that is inherently much tighter than what can be achieved at comparable cost with hydraulic machinery. EMT enables improved process repeatability. It removes from the process several variables that require operators to monitor and adjust the machine (see chart at right). Tighter process capability translates into a variety of benefits, including reduced scrap, lower labor costs, and improved quality.
Better process capability
Electric drives do for molding machines what they've already done for machine tools and robots:
* Raise part quality for more good parts per shift.
* Improve repeatability without constant operator attention.
These benefits derive from the concept of "process capability," which is widely utilized in the metalworking industry but is in the early stages of acceptance in plastics molding. A process capability study determines whether a manufacturing operation is capable of producing parts within a specified tolerance range. Such studies are widely used to benchmark and grade the performance of machine tools. These studies establish machine capability before making parts, rather than inferring it from SPC data on parts already made. One graphic result of a process-capability study is a bell-shaped curve, familiar to anyone who has been exposed to SPC (see chart on facing page).
The Hunkar Class Factor standards (developed by Hunkar Laboratories) are the closest thing U.S. plastics processors have to these formalized tests for machine tools. Higher process capability is inherent in all-electric machine design. Even general-application electric machines have significantly better process capability than Hunkar Class 1, the highest of nine performance categories.
With a more capable process, you produce more good parts per day:
* Quick start-up and set-up are possible without oil preheating.
* Mold set-up parameters can be determined once, then used on re-runs with little or no adjustment. EMT reduces the role of "art" in molding.
* Less scrap is caused by process drift over time, which is common with hydraulic machines. Long-term repeatability reduces operator intervention, allows unattended production.
* Greater reliability and more production hours can be expected from a machine made of fewer parts.
* The "hidden" costs of hydraulic oil are eliminated (see below).
Hydraulic power wastes energy in converting electricity to mechanical motion. Hydraulic oil is also an environmental problem. There are molding plants in the U.S. that cannot afford to move, nor can they be sold, because of oil contamination of the soil. EMT eliminates oil from the workplace, along with spills, related slips and falls, hazardous-waste disposal, fire hazard, inventory cost, and oil mist that can contaminate molds and parts.
Hydraulic machines are maintenance intensive. The main source of production outages is hydraulic valves. EMT eliminates them, along with nuisances such as leaking hoses.
Set-up times are shorter, which helps make EMT better suited to just-in-time molding. Also, noise is lower for all-electric machines - less than 70 dbA, little more than an office copier.
If you're considering an electric machine or a hybrid, keep in mind that some of the energy savings in an all-electric press comes from the servo extruder drive, but all the performance improvement comes from precisely controlled electric drives for clamping, injection, and ejection.
David S. Bernardi is director of marketing and sales for Electron Technologies, a new unit of Cincinnati Milacron that is dedicated to all-electric molding machines.
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|Author:||Bernardi, David S.|
|Date:||Sep 1, 1998|
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