Hybrid presses for a hybrid process

Husky has carried all that it has learned about TXM—a hybrid diecasting/injection molding process for molding thixotropic metals like magnesium—over to its hybrid servoelectric/hydromechanical Hylectric injection molding machine platform. Husky revealed at the Third International TXM Magnesium Conference (May 22-24, Toronto, ON) that it expects to take orders this fall for 2003 deliveries, following the completion of Beta-site testing and validation protocols. Husky's Hylectric TXM machines will be available in sizes ranging from 120 to 1000 metric tons and reportedly will be price competitive with TXM presses from JSW Plastics Machinery Inc. (Elk Grove Village, IL).

Husky was licensed by Thixomat Inc. (Ann Arbor, MI) to manufacture and service TXM machines in North America and Europe in 1998 (see "Husky Licensed to Manufacture Thixomolding Systems," April 1998 IMM, pp. 112-113). The Japan Steel Works Ltd. (Tokyo, Japan) was licensed by Thixomat in 1991 to build TXM presses in Asia. Both OEMs compete for global market share, each with their proprietary machine designs.

More than 200 TXM presses are in commercial operation with Thixomat's 40 licensees. Less than a handful are Husky machines. These were built around its now-defunct G Series press platform. Husky began accepting orders in 1999, but marketing reportedly slid through the cracks shortly thereafter, largely because of the introduction of its Hylectric Series.

Officials of Husky's Thixosystems Div. are confident that the technical improvements they have made to their new Hylectric TXM presses will more than make up for any lost time. They are equally confident that machines bearing their mark will further advance the acceptance of magnesium TXM in the design and molding communities.

Two-piece Barrel Design
The hydromechanical clamp end of Husky's TXM Hylectric looks pretty much like its plastics molding counterparts. Husky sources say Hylectric clamps can run molds at lower tonnage requirements than any other machines—30 percent lower—and that they have the largest tiebar spacing in their class. They add that they can run 20 percent faster than other comparable machines.

The TXM Hylectrics are equipped with Husky's proprietary Reflex platens. Reflex platens are designed to provide uniform force distribution, minimizing deflection, mold wear, and, of particular interest to TXM molders, flash. Also, a proprietary spigot nozzle is on the end of the machine nozzle. Since no carriage cylinder force is applied directly to the mold in the two-piece barrel design, there reportedly is no magnesium blowback. The result is predictable good-parts yield with minimal downtime, according to Husky engineers.

The ultrahigh-speed, reciprocating screw shooter on the TXM Hylectric is a brand-new design. Like the injection units on Hylectrics for thermoplastics, a servoelectric screw drive is standard, but that's where the similarity ends. TXM barrel temperatures are up around 600C. And shots of the thermally conductive, quick-setting magnesium slurries are injected at speeds from 2 to 5 m/sec (16 ft/sec).

The new inline Hylectric injector is built more like one you would find on a diecasting press. It's much more robust. The old unit was more like a plastics shooter. It was supported back at the cold end. Shock and load were distributed through the barrel, which had to be beefy enough to take it. The nozzle end of the barrel is supported in a cradle, which is clamped directly onto the stationary platen.

Heating of the material is done in the first stage of the unit, which has a thinner barrel for better thermal conductivity. The second-stage barrel at the nozzle end is built thicker and stronger. A heavy casting running along either side of the barrel and butted up against the stationary platen is sized to withstand 1.5 times the force generated by melt injection to reduce shock and vibration. By separating melt preparation from injection and isolating force distribution, the unit is supported only where it needs to be.

Too Fast to Profile
Husky presently is using Inconel in its barrels. The two-stage approach may open up the possibility of using less expensive materials of construction. Electrical current going into each zone on the barrel is constantly monitored. One to three heating elements are used, depending on the zone. Husky has developed a new, top-secret type of heating element for its new TXM injection unit, one created to deliver the heat without burning out.

TXM injection is too fast to profile. Magnesium is incompressible and flows like water, Husky technicians say. As soon as the runner sets up, you're done. Its TXM Hylectrics are equipped with a dedicated black-box injection unit controller capable of achieving 250-ms scan times.

Conventional position sensors used in plastics molding can't keep up with injection speeds as fast as 6 m/sec. Instead, Husky prefers using a transducer that has been used in CNC applications. It is directly connected to the centerline of the injection piston and can deliver accurate readings at speeds of up to 20 m/sec. Vertical piston accumulators now are used to assist injection.

TXM Secondary Systems
Husky's Internet-capable PC-based Polaris machine control system is standard. Pierre Pinet, Thixosystems product manager, says it is conceivable that Husky could integrate the control for all of the downstream equipment in a TXM cell into the powerful Polaris controllers. He says Husky intends to take the same approach integrating TXM secondary systems as it does integrating plastics molding systems.

Husky has switched from using one of its side-entry servo robots to a dual-arm, top-entry design. One arm is programmed for part removal while the other dispenses lubricant (mold release) after every shot. The activities of each are choreographed to reduce cycle time. An integrated air curtain draws out the mist and fumes. Husky also has integrated an automatic die lubricant mixing system, parts conveyors with integrated cooling fans, and an Argon gas management system with automatic switchover.

Among a number of other specially designed auxiliaries, the company has developed systems for quick and easy screw removal and servicing to minimize downtime. These include a barrel stand with its own integrated temperature controller, a screw removal cart, and a screw-cleaning apparatus. A number of different screw sizes are presently available for each model of injection unit. Husky works with its network of strategic partners to outsource other secondaries such as hot oil mold temperature controllers—one each for the core, cavity, and sprue.

Work on hot sprues is the first step Husky is taking toward developing its own complete hot runner systems. Hot sprues can eliminate the cone-shaped sprues typically found in TXM cold runner molding, in some applications saving as much as 40 percent of shot weight, reducing cycle times by 35 percent, and opening the processing window a little wider. Husky's TXM hot sprues presently use traditional mineral-insulated bands. They will be manufactured at Husky's hot runner facility in Vermont.

Though Husky builds molds, it specialized in PET preform tools and has no intention of building TXM molds. Instead, it plans to develop strategic partnerships with a network of global TXM moldmakers, lending them its expertise in thin-wall moldmaking to build hybrid molds for this hybrid process that will run on its hybrid presses.

Editor's note: TXM is IMM's abbreviation for the process of molding a thixotropic metal slurry usually referred to as "Thixomolding" or "Thixomolded," which are registered trademarks of Thixomat Inc.