- Information about technological processes
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Metal Injection Molding
- The "lost wax casting" process
dates back far-off times and was used by peoples and cultures
from all over the world: from the Chinese to the ancient Egyptians,
from the Aztecs to the Greeks, so to manufacture working tools
and artistic sculptures. As significantly reported by a number
of valuable Roman bronzes in the Capitolium Temple.
- Therefore, this process enables to reduce
production times considerably, to advantage of costs. The possibility
to create quite complex geometrical shapes makes this process
the most suitable production system mainly when an item needs
having particular design characteristics, such as connections,
intersections, ribs, grooves and slots which cannot be obtained
by other manufacturing processes; also troubles which might arise
when assembling different parts produced by standard machining
can be avoided. Furthermore, a higher flexibility can be granted
when using every special material that improves quality of products,
but whose machinability is difficult, sometimes impossible. In
particular, lost wax casting is of the utmost
importance when handling many metal alloys of stainless or alloyed
steel, nickel, and so on, aiming at getting a better quality performance,
without defects or unevenness, without any wastes of costly materials.
Thanks to the features of this process, an original model can
be slightly changed or modified, when necessary; consequently,
experimental parts can be manufactured to be tested before starting
series-production of a definitive workpiece.
- The MIM (Metal Injection Moulding)
process was developed in the early '90s in the United States,
mainly for use in the military sector. In the following years,
when the first difficulties were overcome, this method has been
spreading more and more, both because of the interest shown by
a growing number of companies that developed the process, and
because of an increased number of sectors where pieces made by
the MIM process can be used.
- At present, companies operating in the aerospace,
military, precision mechanics, medical, optic, photographic, clockwork
and weapon sectors are interested in MIM technology, and are using
Main goals of the MIM process are to realize average-small pieces
with a complex shape, in average-big lots, in high-resistance
materials, using as much automated working cycles as possible.
Moulding is got by ordinary injection moulding processes, at a
high-speed production rate and minimum faultiness.
The so-moulded items undergo de-waxing (de-binding) and sintering
steps, so reaching their final size without further handling.
A final machining, generally limited and roboted, consists of
piece tumbling and polishing. The main steps of the whole process
are described hereafter.
- Raw material (nickel alloys, stainless steel,
and so on) is mixed with the so-called binder (made of waxy, jelly-like
plastic materials) in a well-defined ratio to obtain a well-blended
compound needed in the following steps. Such a mixing originates
FEEDSTOCK, which (See INVESTMENT
CASTING ) buys as a certified ready compound.
As for plastic, feedstock is used as a moulding material for the
so-called GREEN BODY, i.e. the wished workpiece having a bigger
size and resistance due to the binder only.
Then, the binder is removed making care that no piece failures
or deformation occur. This difficult step is completed after creating
a BROWN BODY, i.e. a workpiece having the necessary residual binder
to keep its own consistency.
Finally, the workpiece is finished by removing the whole residual
binder and vacuum sintering, which enables to obtain the final
mechanical, geometrical characteristics desired.
Tumbling and polishing give pieces brilliance as well as the lowest
roughness. Further thermal treatments can also be foreseen.
More information: INVESTMENT