In-Die De-gating for Cast Magnesium is a First

Magnesium is making its debut in an automotive rear mirrormount support using tooling technology borrowed from the zinc and plasticsindustries.

The mirror mount support is being produced in a 200-tonhigh-pressure, hot-chamber die casting machine using in-die de-gating withtwo-stage technology.

"Magnesium lacks the plasticity to deform under load andwill shear upon reaching its elastic limit," says David Haener, engineeringmanager of Cast Products, Inc., Norridge, IL, the magnesium die builder."The mold design exploited this characteristic and allowed the runner tocleanly separate from the casting mid-cycle, thus eliminating the need for atrim die and the associated costs."

The magnesium component is used in an automatic-dimmingmirror developed by Gentexthat detects glare during night-time driving. An electrochromic gel issandwiched between two pieces of glass, each of which has been treated with atransparent, electrically conductive coating, and one with a reflector.

A forward-facingsensor recognizes low ambient-light levels and signals the mirror to look forglare. A rear-facing sensor detects glare from vehicles, sending voltage to themirror's gel in proportion to the amount of glare detected. The mirror dims inproportion to the glare and then clears when the glare is no longer detected.

The system requires a very stable support. "Plastics don'tmeet our vibration requirements," says Josh Owen, mechanical design engineer,Gentex. "Weight dictated that we use magnesium. Zinc is too heavy. Aluminumrequires new cavities after 150,000 shots."

Gentex, however, wanted the strength and weight benefits ofmagnesium with the cost-saving efficiencies of in-die de-gating technologies.

The two-stage ejection system mechanically separates thecasting from its gates and overflows during the ejection cycle. As a result, acomplicated two-cavity trim die is eliminated.

"Due to proprietary information, we aren't at liberty todiscuss (how the de-gating mechanism works)," says Haener. "But we can say a fullrunner drops from the machine, followed shortly thereafter by two flash-freemagnesium castings which are ready for vibratory finishing." High annual usagedictated construction of a two-cavity tool.

Elimination of the trim die cut about 20 percent from thetotal potential cost. Additional price-part savings are achieved as a result ofquicker turnarounds and labor savings. "No part trimming is required, onlynormal vibratory deburring and specified decorative finishing," says MichaelNovesky, project engineer for Chicago White Metal Casting, Inc., BensenvilleIL, which collaborated on the die design and produces the parts.

Importantly, adds Novesky, CWM  "was able to maintain the criticaldimensional accuracy of the ball-end tolerance of the part (held to ± 0.1 mm)for smooth adjustment of the rear view mirror." That indicates the potential of the no-trimprocess. It is unlikely the technology, however, would be suitable formagnesium parts of very large size. "Part design dictates whether a part can beauto-de-gated," says Owen. "I have parts being designed right now that will not beauto-de-gated due to design."

Ninety-nine percent of the zinc molds built by Cast Productsuse in-die de-gating. The Gentex part is the first time the de-gating process hasbeen used for magnesium in the United States. Gentex currently has five tools thatare auto-de-gating.

The high-tech mirror is giving Gentex a lot of life rightnow, even though other auto suppliers are sagging. Shares of the company'sstock jumped 16 percent in recent trading and are up 62 percent so far thisyear. "Despite what's been happening in North America,there's growing demand for our products," says Connie Hamblin, vicepresident of investor relations.

Chicago White Metal, Gentex and Cast Products recentlyreceived first place for the mount in the process category in an annual designcompetition held by the International Magnesium Association.

For information on potential use of the de-gating process,contact Chicago White Metal.