Magnesium is making its debut in an automotive rear mirror mount support using tooling technology borrowed from the zinc and plastics industries.
The mirror mount support is being produced in a 200-ton high-pressure, hot-chamber die casting machine using in-die de-gating with two-stage technology.
"Magnesium lacks the plasticity to deform under load and will shear upon reaching its elastic limit," says David Haener, engineering manager of Cast Products, Inc., Norridge, IL, the magnesium die builder. "The mold design exploited this characteristic and allowed the runner to cleanly separate from the casting mid-cycle, thus eliminating the need for a trim die and the associated costs."
The magnesium component is used in an automatic-dimming mirror developed by Gentex that detects glare during night-time driving. An electrochromic gel is sandwiched between two pieces of glass, each of which has been treated with a transparent, electrically conductive coating, and one with a reflector.
A forward-facing sensor recognizes low ambient-light levels and signals the mirror to look for glare. A rear-facing sensor detects glare from vehicles, sending voltage to the mirror's gel in proportion to the amount of glare detected. The mirror dims in proportion to the glare and then clears when the glare is no longer detected.
The system requires a very stable support. "Plastics don't meet our vibration requirements," says Josh Owen, mechanical design engineer, Gentex. "Weight dictated that we use magnesium. Zinc is too heavy. Aluminum requires new cavities after 150,000 shots."
Gentex, however, wanted the strength and weight benefits of magnesium with the cost-saving efficiencies of in-die de-gating technologies.
The two-stage ejection system mechanically separates the casting from its gates and overflows during the ejection cycle. As a result, a complicated two-cavity trim die is eliminated.
"Due to proprietary information, we aren't at liberty to discuss (how the de-gating mechanism works)," says Haener. "But we can say a full runner drops from the machine, followed shortly thereafter by two flash-free magnesium castings which are ready for vibratory finishing." High annual usage dictated construction of a two-cavity tool.
Elimination of the trim die cut about 20 percent from the total potential cost. Additional price-part savings are achieved as a result of quicker turnarounds and labor savings. "No part trimming is required, only normal vibratory deburring and specified decorative finishing," says Michael Novesky, project engineer for Chicago White Metal Casting, Inc., Bensenville IL, which collaborated on the die design and produces the parts.
Importantly, adds Novesky, CWM "was able to maintain the critical dimensional accuracy of the ball-end tolerance of the part (held to Â± 0.1 mm) for smooth adjustment of the rear view mirror." That indicates the potential of the no-trim process. It is unlikely the technology, however, would be suitable for magnesium parts of very large size. "Part design dictates whether a part can be auto-de-gated," says Owen. "I have parts being designed right now that will not be auto-de-gated due to design."
Ninety-nine percent of the zinc molds built by Cast Products use in-die de-gating. The Gentex part is the first time the de-gating process has been used for magnesium in the United States. Gentex currently has five tools that are auto-de-gating.
The high-tech mirror is giving Gentex a lot of life right now, even though other auto suppliers are sagging. Shares of the company's stock jumped 16 percent in recent trading and are up 62 percent so far this year. "Despite what's been happening in North America, there's growing demand for our products," says Connie Hamblin, vice president of investor relations.
Chicago White Metal, Gentex and Cast Products recently received first place for the mount in the process category in an annual design competition held by the International Magnesium Association.
For information on potential use of the de-gating process, contact Chicago White Metal.