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 ± 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.
Tweet This
3 
