2 STAGE
INJECTION MOLDING

Overcoming the limits of all-electric injection molding

by Gerald Jones

The need for sophisticated plastic injection molding systems is an ever-growing one. Production of DVD, audio and data storage devices, electronic transaction cards, precision medical devices, telecommunications, and information processing equipment is among the fastest growing markets in the world. In an industry dominated by hydraulic systems, the demands for more precise, energy-efficient molding technology have pushed "all-electric" systems into a position to deliver a much needed technological lift. The development of a two-stage injection unit by Cincinnati Milacron is the single advancement that makes "all-electric" a viable complete system alternative to hydraulic IMMs.

ELECTRIC AND HYDRAULIC SYSTEMS AT A GLANCE

Today's reciprocating screw design was developed specifically to lower the cost of hydraulic machines. The screw design allows for dual use in both the extrusion phase for melting plastic, and to drive the injection plunger as it pushes a "shot" of molten plastic into a mold. However, the screw's back and forth motion constantly changes its effective length and therefore compromises its ability to homogenize plastic each time the screw is stroked.

All-electric injection molding machine from Cincinnati Milacron

A reciprocating screw can't stroke more than 4-5 times its diameter. Anything more increases recovery time and produces poor melt quality. Designers compensate for the loss of effective length with larger diameter screws to increase throughput. As injection volumes increase, the diameter of the screw in a reciprocating unit must be scaled up to a point where it is no longer cost effective to drive them forward with available electromechanical technology.

A standard hydraulic IMM cannot physically brake its reciprocating screw when the melt is transferred from the fill in order to be packed. It is the compression of the molten plastic (melt pressure) that actually stops the screw. Molten plastic is elastic, and can actually push the screw rearward. To overcome this tendency to bounce the screw rearward, molders often increase the pressure in the packing stage, and thus overfill the mold. This can cause flash within flash-prone molds. To deal with this, hydraulic machines compensate with clamp tonnage. Since the mold is overfilled and overpressured, there is no real process control. Although it influences the accuracy of the system, the reciprocating screw dramatically cuts costs in hydraulic IMMs by driving the extrusion and injection components from a single hydraulic pump. The reciprocating screw has its advantages; however, without process control and sustained repeatability, reliable automated production capability is not easily achieved.

Energy savings are a part of electric molding technology. Placing electric servos on the extruders of rebuilt hydraulic machines to take advantage of energy savings and the "continuous screw rotation" of the screw design can be cost effective. Adding servos to a hydraulic machine doesn't change that it is still a hydraulic machine. Energy and noise reduction may be achieved, but the drawbacks of hydraulic oil and lack of repeatable process control still remain. All-electric IMMs tend to be more expensive than hydraulic, yet by the time a servodrive is added on the hydraulic pump and extruder, it may be close to the same price. It is also possible that a servodriven pump won't be able to respond the same as one with a fixed-speed motor. The end result of combining all-electric and hydraulic could conceivably be an energy-efficient machine with more idiosyncrasies than a conventional hydraulic.

THE INNOVATION

Hydraulic machines can share a drive for the extrusion motor and injection cylinder, using the extrusion screw as an injection plunger to lower the machine cost. This is why the reciprocating screw design is more popular.

The two-stage unit is a departure from traditional thinking. It frees the design of the injection barrel from the task of plasticizing. 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 and more precise electromechanical drives. It takes less power to generate injection pressure with a small diameter screw, while lengthening the stroke to get the required volume.

Two-stage injection molding system

The two-stage injection unit can shoot its full volume, unlike reciprocating units that are usually sized twice as large as their maximum shot capacity to make up for loss of effective length (back and forth motion). Cincinnati Milacron's all-electric IMMs use a single-screw extruder to melt the plastic and meter it into the injection barrel through a port in the shooting cylinder.

With extrusion as a separate function, plasticizing rates can be sized to exact requirements, with none of the process compromises from a reciprocating screw. Injection control is far more precise, because there are no valves that have to seat before the shot can be delivered into the mold.

THE LIGHTER SIDE
OF ACCURACY

All-electric IMMs have electric motors that drive the injection piston. The accuracy and repeatability they are capable of is impressive. Cincinnati Milacron's Elektron Technologies division manufactures the Powerline series of IMMs, which incorporate digital absolute encoders on the motor shaft. These encoders have 64,218 increments of resolution per rotation. The two-position system on the two-stage injection unit is capable of resolving increments of 12 microns of linear motion.

STRESS-FREE
MOLDING

When dealing with optical molds, fill precision is critical to maintaining the optical quality of the part. Since all-electric IMMs do not have to overpack the mold to prevent bounce-back, they avoid overpacking that "molds in" stress which degrades the optical clarity of the part.

Ferromatik Milacron, based in Germany, developed what it has called the Disco line of DVD and CD molding machines. Two 50-ton media molding presses were provided to GE Plastic's Optical Media Development Center (OMDC) for study by GE and other stakeholders in the OMDC, and replicators doing proof-of-process testing.

"The technical staff at the OMDC will employ the servoelectric Disco machines to evaluate and develop new DVD manufacturing processes that can deliver productivity enhancements for our customers," said John O'Sullivan, OMDC program leader. "For example, we'll be exploring clamp positioning control for coining, which is thought to be critical to molding a flat, stress-free DVD. With simple control settings that offer repeatability to 1/100th of a millimeter, the all-electric machine will help because it eliminates mechanical settings and other variables that are inherent to coining in a hydraulic machine environment."

The Disco's injection unit is tailored for polycarbonate processing. The compact machine requires 3.3m of floor space, and is suitable for class 1000 cleanroom use. Hydraulic machines have certain intrinsic qualities that prohibit cleanroom use, primarily involving hydraulic oil.

For more information, contact Thomas L. Jarrold, Milacron Incorporated/Plastic Technologies Group, 4165 Halfacre Rd, Batavia, OH 45103. 513-536-2428. http://www.milacron.com Circle 628.