All-electric injection presses are still a new concept for most molders. The market is flooded with claims and counterclaims. It's the new millennium. Should you cling to your hydraulic oil, familiar and warm, buy one of the new hybrids, or take the plunge into 100 percent electric-powered injection molding?
Suppliers selling hydraulic and hybrid presses raise some questions about all-electrics. Here is a sampling:
Do all-electrics suffer? Standard types of all-electric presses can't match the injection speed of super-pumped hydraulics powered by big accumulators, but the electric advocates say only a handful of molding jobs need to run at Mach 5.
* Barr Klaus of Milacron: ``I will give you this: For high- speed, thin-wall injection molding, electric machines don't have that capacity today.'' So accumulator machines that can unleash a speed of say 100 milliseconds, or one-tenth of a second, win that battle. ``When you get into longer fill times, like three-tenths of a second, four-tenths of a second or five-tenths of a second, which 90 percent of the packaging industry doesn't fill faster than that ... you'll find that electric machines typically outperform hydraulic machines, even equipped with an accumulator.''
* Taku Tawarada at Ube conceded all-electric presses have trouble matching the speed of ``souped-up accumulator'' presses from, say, Netstal. But Ube's electrics are 70-80 percent faster than the company's standard hydraulics. A high-speed option makes it much faster.
* Robert Weinmann of Netstal said Netstal is exploring an alternative type of drive for its prototype, standard all-electric press. The reason? To get injection speeds of at least 400 millimeters a second.
* William Ball of Niigata: Do electrics have slower injection? A standard all-electric can inject at 300mm per second. ``It depends on the setup. We have a machine with standard injection speeds, and we have high-performance machines which incorporate larger servomotors and inject at higher rates. I mean, there's almost no limit to how fast you can inject if you want to do it and spend the money.'' Niigata points out that bigger accumulators and hydraulic pumps cost money, too.
* Jerry Boggs of Sumitomo: Our all-electrics run at 200mm and 300mm, depending on the size and model. Injection velocities up to 500mm a second are available as an option on smaller models. The real issue comes for larger-sized screws, those 3 inches in diameter and larger. ``That's when combining hydraulics [and electric power] starts to make some sense.''
* Mike Urquhart of Husky: It doesn't make much difference on very small shot sizes. ``When you need the higher stroke distance, that's when it makes a difference.''
* Jerry Johnson of JSW: ``The big important feature is compound motion. Because we have four individual servomotors, each one of those are closed-loop controlled. So they can run independently or simultaneously.''
* Tim Glassburn of Toshiba: ``When you say `go from zero to 200 mm a second,' it's right now,'' Glassburn said of the electrics, snapping his fingers. ``It doesn't have to wait for the pressure to build up. It doesn't have to wait for the valves to shut.''
* William Ball of Niigata: ``To date, if you had to look at an area that probably would need a good hard look at, it's shot capacity. Especially I would think on the medium-sized machines - by medium-sized I mean, say 200 through 400 tons. Here again, it's a function of servomotors.'' On the bigger machines, it's not as much of a problem because you can go to two servomotors. The focus in Japan has been on all-electric presses for precision parts such as connectors, with small shot sizes. ``But as this thing evolves, and we move more into the general-purpose molding area, we will have to address that issue of shot size. And we're doing it.''
* Jerry Johnson of JSW: This used to be an issue during the pioneering electric days, ``because the servomotors weren't big enough to handle the barrel size similar to a hydraulic machine. But today it's not an issue because, from our 245-ton machine all the way up to the 720-ton machine, we offer two separate injection units, and one equals that of a hydraulic machine [for shot capacity].''
Pack and hold
During longer pack and hold periods, electric servomotors can overheat, as the rotary motor stops spinning or spins very slowly. That can trip thermal overloads. Hydraulic power can hold all day long if you want. (Supporters of all-electric technology cite improvements in motor cooling and heat deflection, which they say have improved the situation.)
Also, all-electric presses measure pack and hold in a new way - by torque - instead of by pressure.
* Robert Schad of Husky: ``It's much easier to do it hydraulically because the hydraulics kind of conform to the material, while on the electric side, you need very extensive software. You have the motors going at very slow speeds, and that's not good on variable-speed drives.''
* Barr Klaus of Milacron: There are some parts with really thick walls that require very long holding times. ``I'm talking about fringe, fringe, fringe applications that don't come up very often. This is not 2 percent of what's out there, it's less than that.''
* Taku Tawarada of Ube: ``It is true that the torque peak on the motor can happen at the pack and hol d stage. But the design concept is to accommodate that torque peak.'' The servomotor has to be large enough to handle it. As to using torque to control pack and hold, it's not as direct a form of measurement as pressure, but the machine takes that into account.
* Tim Glassburn of Toshiba: ``If it's holding, and nothing's moving, then you will build up heat. Our motors are designed to dissipate the heat through the motor design.''
Wear of ball screws or roller screws
These elements spin at high speeds cycle after cycle. How long will they last?
* Tim Glassburn of Toshiba: ``We've been running ball screws on our machine tools for years and years. They're lubricated. They take a special type of grease so there's no contamination.'' A small replaceable cartridge lubricates the ball screw regularly, after several thousand shots. ``It's not a lot of grease, like an old toggle system.''
* William Ball of Niigata: ``Mechanically, everything wears. There's no question about that. We've been asked to predict life expectancies of our ball screws. And the information that comes back from Japan, based on quite a bit of field experience there, is 10 to 15 years.'' Hydraulic parts also wear out. ``People don't break tie bars or split platens or tear up screw barrels. That doesn't happen often, on anybody's machine. It's the hydraulics.''