WEB EXCLUSIVE:Next-generation Mg molding machines

At IPF 2002 (Nov. 9-13, Tokyo, Japan) both the Japan Steel Works Ltd. (Tokyo, Japan) and Sodick Plustech Co. Ltd. (Yokohama, Japan) displayed their latest-model machines for injection molding Mg alloys. Both Sodick’s redesigned first-generation Mg-Plus and JSW’s all-new third-generation JLM-MG II are engineered to improve molding accuracy and good parts yield.

JSW calls its JLM-MG II the “Mg Rocket Shot” for good reason. The 280-metric-ton model at IPF is capable of reaching injection speeds up to 6.5 m/sec. That’s meters per second, folks, not millimeters. Faster rates mean thinner parts, shorter fill times, and improved yield.

New, extremely responsive piston cartridge valving provides high-speed acceleration and braking. Closed loop injection velocity control profiling ensures repeatability.

Also, both the barrel and the screw are redesigned for better thermal conductivity and shot size control. JSW has developed a proprietary iron-based alloy with low creep resistance for its barrel construction that allows Mg molders to keep material in the barrel for a longer period of time. The new screw permits flood feeding for improved stability.

The clamping unit also has been improved. Larger-diameter tiebars are among the new features added to improve rigidity and platen parallelism when operating at high speeds. The 280-tonner’s clamp reportedly can operate at 350 tons, if necessary.

Even the hydraulic system is new. It now has a doubled variable displacement pumping system for faster mold open/close action and cycle phase operation. Overall, energy consumption for the new 280-tonner is about 15 percent lower than it was for the second-generation 220-ton press. Meanwhile, it only costs about 10 percent more. A 650-ton Mg Rocket Shot will be taking off this spring.

Two Stages, Two Cylinders
The basic operating principle of Sodick’s Mg-Plus hybrid remains the same: The material molded also acts as the injection unit’s plunger. A servodriven screw screws itself into the back of a solid magnesium alloy rod and an accumulator-assisted, hydraulically powered ram pushes the bar stock, like a plunger, through heating zones that condition the material. The molten front end of the rod expands during injection, sealing the system to prevent backflow and ensure shot size stability (see November 2002 IMM, pp. 64-65).

All that’s still true, but something new has been added.

On an 80-metric-ton model at IPF 2002, Sodick displayed a newer version of its Mg molding concept. Borrowing from its Tuparl series of hybrid presses for molding thermoplastics, Sodick has split the two stages of its injection process—material conditioning and injection—into two cylinders to improve molding accuracy and reduce cycle times. The rod-loading magazine and self-sealing material heating cylinder is now piggy-backed on top of a second high-speed and high-pressure plunger injection unit.

In addition to its patent-pending, self-sealing material heating cylinder design, Sodick has applied for patents on two new proprietary devices: a wear-free shutoff valve to prevent backflow between the two cylinders and a cooled, mechanical seal on the injection plunger that prevents oxygen from getting in, obviating the need for using inert gases like nitrogen.

Sodick officials are confident that the improvements they have made to their Mg-Plus Series will provide 95 percent-plus good parts yield. Mg-Plus presses are available in 40-, 80-, 180-, and 250-ton sizes.