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November 26, 1999

3 Min Read
Magnesium TXM free to grow with new hot runner

Hot runner manufacturer Ju-Oh, working together with JSW, has commercialized a viable solution to low material shot yield, which has limited the potential growth of magnesium TXM in Asian markets. Until now, it was not uncommon for more than 60 percent of the shot in high-speed, thin-wall consumer electronics and IT parts to be wasted in sprues, runners, and flash, leading to high part costs (see June 1999 IMMC article). In August, however, Ju-Oh introduced its new TXM hot runner and hot runner control system, designated 614-MM (Figure 1). The system has already been found to increase material yield rates by 70 percent or more. It is also said to reduce molding cycle times by 30 percent.

The key to the system's success is Ju-Oh’s field-proven electromagnetic induction heating technology, which exploits the superior thermal conductivity of magnesium.

Hot On/Off Nozzles
The system incorporates a hot on/off injection nozzle contacting the mold and hot runner manifold. An electromagnetic induction heating coil wound around the nozzle quickly applies heat to the magnesium slurry. Coils are also in the manifold drop nozzles.

The coil creates a high-frequency current, thereby generating a magnetic flux in the coil’s center. Heating begins at the nozzle surface. The coil generates heat, but only becomes slightly warm, allowing it to speed high energy into the system without causing problems like broken wires. Cooling is equally fast, because only the nozzle needs to be cooled, not the coil.

After the required shot volume passes through the nozzle, the electric charge stops and the nozzle rapidly cools down. The slurry in front of the nozzle rapidly cools, arresting flow. When the hot nozzle heats up for the next shot, the solidified magnesium slug in front of the nozzle quickly returns to a flowable state. The thin, disk-shaped slugs that may form inside the nozzle can be retrieved and reground for reuse.

The magnesium slurry can expose the manifold and nozzle to temperatures of 630C or more, depending on the grade of alloy used. Plastic melt generally doesn’t exceed 200C. To withstand higher magnesium TXM slurry temperatures, JSW has developed a nickel-free, high-heat-resistant steel alloy containing high-heat materials like cobalt, tungsten, and molybdenum.

Additional Benefits
Ju-Oh’s system reduces the volume of molded parts, so cycle times can be reduced. In trials running a direct-gated A4 notebook PC case (Figure 2), cycle time was decreased from 40 seconds to 25 seconds. The amount of mold release (lubricant) needed to remove the sprues and runners also can be reduced, since sprues and runners have been eliminated.

In addition, the projected area of the part becomes somewhat greater since the shot volume is reduced using hot runners. Subsequently, the mold clamping pressure can be reduced. The A4 case shown was formerly run in a 450-ton press. With hot runner tooling it was molded on a 220 tonner. Hot runners also reduce the amount of air to be evacuated from the mold, an important factor when molding magnesium.

Ju-Oh sells its 614-MM system on its own in cooperation with JSW. A two-cavity TXM hot runner mold from Ju-Oh costs about 6.5 million yen (about $62,000), which includes the controller.

A 614-MM single-cavity system was sold to Toyota Group (Nagoya, Japan). Toyota has already ordered three more. A half-dozen or so other TXM molders have been testing the units over the past few months.

Diecasters of thin-wall magnesium alloy parts threatened by the first rush of TXM may have felt themselves to be in a cool, calm, and collected position, competitively speaking, because of the higher cost of magnesium TXM parts. But JSW and Ju-Oh have just turned up the heat.

Contact information
Ju-Oh Inc.
Kanagawa, Japan
Itsuo Shibata
Phone: +81 (463) 24-3223
Fax: +81 (463) 24-5566
E-mail: [email protected]

Japan Steel Works Ltd.
Tokyo, Japan
Kazuo Kitamura
Phone: +81 (3) 3501-6164
Fax: +81 (3) 3595-4615
Web: www.jsw.co.jp

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