Electronic injection molding machines get supersized

Compared to hydraulic presses, all-electric injection-molding machines use less energy. Engineers replace hydraulic lines, fittings, and oil coolers with electric wires, motors, and drives. The result is a cleaner design with distinct environmental benefits. But there's more to the story. Better repeatability means better parts, and more of them.

Whether these benefits offset the higher cost of electric technology is for the molders to decide. But that old argument is giving way to engineering innovations. Engineers continue to increase clamp forces as larger servo motors become available. And while they're waiting for bigger motors, they invent new ways to get more out of existing ones. Two examples: Batavia, OH-based Cincinnati Milacron's two-stage injection unit. It provides up to a fourfold increase in effective shot capacity on its all-electric Powerline series molding machines. Another machine builder, UBE Machinery Inc. (Ann Arbor, MI), synchronizes two 55-kW ac servo motors to achieve high shot capacity on its 950-ton all-electric. For design engineers, higher shot capacity and clamp force means added flexibility to simplify designs by using larger, more complex molded components.

Cincinnati Milacron and its German subsidiary Ferromatik Milacron (Malterdingen) reported in February record sales of 400+ all-electric injection-molding machines in 1998, a 50% increase over 1997. "The industry is really just beginning the changeover from hydraulic power to electric that has already occurred in both machine tools and robotics" says Barr Klaus, technical director for Milacron's Elektron Technologies business unit.

Milacron's decision to create the Elektron Technologies business unit and develop electric molding technology led to the introduction of the Powerline series last year. The new two-stage injection unit is breaking the shot-capacity "sound barrier" that has limited the growth and capability of Milacron's high-tonnage, all-electric injection-molding machines.

Big shots. Reciprocating-screw injection units--which have a lot of advantages in the high-power environment of hydraulics--are limited in an all-electric design. As shot weights increase, electromechanical drives become large and impractical. "It hasn't been cost-practical to go beyond a 76-oz reciprocating-screw unit for our 725-ton machine," says Klaus, "which is arguably small for this size machine. Breaking through that ceiling with a new two-stage all-electric injection unit not only ends the capacity limit, but also vastly improves the versatility of the large molding machines," says Klaus. The design extends the range for minimum shot size down to 2 to 3% of barrel capacity, eliminates guesswork about sizing an injection unit, and allows a range of exotic extrusion capabilities. The two-stage high-capacity, high-pressure injection unit gives Milacron's Powerline Series a booster shot of capacity, versatility, and melt quality.

A complete departure from old thinking, the two-stage unit frees the injection barrel design from satisfying plasticizing requirements. An independent shooting chamber allows use of a smaller-diameter injection barrel and longer injection stroke for a given volume, making it easier to generate high injection rates, pressures, and volumes with smaller electromechanical drives.

To improve the mechanical efficiency of both the inject and eject axes, Elektron engineers replaced ball screws with roller screws from SKF Motion Technologies (Bethlehem, PA). They also replaced the belt-driven ball screw on the clamp axis with a double rack-and-pinion gearbox from PIV (Frankfurt, Germany). The design helps the machine achieve mid-60-db noise levels, according to Klaus.

Introduced in 150-, 110-, and 80-oz capacities, the new injection units bring economical large-shot capability to Milacron's Powerline series all-electric machines, along with melt quality, compounding, and venting advantages for free-standing extruders. Key to advancing the development of large all-electric IMMs, the new 150-oz/23,000-psi unit quadruples the effective shot capacity of the 725-ton electric machine with a conventional reciprocating-screw unit. Other capacities and pressures to 30,000 psi are available for machines down to 440 tons.

Dual-motor drives. Until recently, power limitations in ac-servo motor technology prevented plastic machinery manufacturers from applying an all-electric design to the large-tonnage market. But then one manufacturer decided to synchronize a pair of motors to get the needed power. UBE Machinery Inc. is set to deliver the first 950-ton electric machine this month. A joint development between UBE Industries Ltd. (Tokyo), UBE Machinery Inc.'s parent company, and Niigata Engineering Co., Ltd. (Niigata, Japan), the 950-ton unit is reportedly the world's largest injection molding machine using only ac servo motors to achieve both clamp and injection motion.

The machine uses two 35-kW ac-servo motors to drive the clamp unit, and two 55-kW ac-servo motors from Mitsubishi (Vernon Hills, IL) to drive a conventional-style reciprocating-screw injection unit. In both cases, a belt from each motor connects to a single drive gear. Each motor amplifier is synchronized with a controller using a proprietary control algorithm. "We chose ac servo technology because it's more common in industrial applications and has lower maintenance than dc," explains UBE's Marketing and Sales Manager Taku Tawarada.

The new toggle-style electric molding machines use ball screws from NSK (Bloomingdale, IL), and precision controlled ac-servo drives to get greater consistency throughout a product's mold cycle. In fact, the machines achieve positioning to within 0.1 mm, according to Tawarada. "The design eliminates machine variability," he adds. "When the mold is mounted back on the machine, the machine's pre-programmed menus eliminate any fine-tuning of the mold recipe for a quality part."

The two companies have big plans for this series of electric toggle-style plastic injection molding machines, and will have 720- and 1,100-ton machines later this year. Machines up to 2,500 tons are in place for when even larger ac-servo motors become available.

Unlike hydraulic power units, which must keep the pumps running all the time, servo motors reduce electrical consumption by an estimated 60 to 80%.

To speak with a company representative, call (800) 828-6344 x011 and enter the Product Code below.

Roller screws from SKF: Product Code 4699

Gearboxes from PIV: Product Code 4700

Ball screws from NSK: Product Code 4701

Ac servos from Mitsubishi: Product Code 4702