CPT International 02/2021
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As aresult, the melt does not have to<br />
be overheated as much (> 630 °C) and it<br />
is possible to manufacture components<br />
with smaller wall thicknesses. However,<br />
due tothe temperature to which the<br />
casting chamber is permanently subjected,<br />
the maximum possible casting pressure<br />
is limited and is only at amaximum<br />
of 350 bar, which severely restricts the<br />
component size. In die-casting, the<br />
gating system and the overflows must<br />
be adapted to the respective component<br />
geometry. This leads to relatively<br />
high material consumption. For<br />
example, in cold-chamber die-casting,<br />
the component‘s share of the shot<br />
weight is often in the range of 40 to<br />
60 %for compact, thick-walled components.<br />
In the case of flat, thin-walled<br />
components, as little as 30 % is<br />
sometimes achieved. Figure 1shows a<br />
comparison of the casting processes in<br />
terms of shot weight and energy<br />
consumption.<br />
Both cold and hot chamber processes<br />
have high-energy losses due to the<br />
necessary overheating of the melt and<br />
require environmentally harmful reactive<br />
gases (mainly R134a: 1300 times the<br />
global warming potential of CO 2<br />
or SO 2<br />
:<br />
toxic, corrosive) to prevent the melt surface<br />
from oxidizing.<br />
Thixomolding<br />
Another manufacturing technology for<br />
magnesium castings is Thixomolding.<br />
Semi-solid metal molding is amolding<br />
process inwhich amold isfilled with<br />
partially molten metal in which solid<br />
particles are homogeneously dispersed<br />
in the melt. The thixotropic properties<br />
(e.g. gel liquefies when stirred, thickens<br />
when at rest) of metal alloys in the<br />
semi-solid state and semi-solid casting<br />
were invented by Prof. Merton C. Flemings<br />
and his colleagues in the 1970s at<br />
MIT (Massachusetts Institute of Technology)<br />
in the USA. The machine technology<br />
is comparable to plastics injection<br />
molding.<br />
The process is characterized by good<br />
control of the melt temperature, alow<br />
melt volume, short residence times and<br />
intensive shearing of the melt. The melt<br />
can be processed in the semi-solid<br />
range. At the Bavarian State Research<br />
Institute Neue Materialien Fürth GmbH,<br />
Dr. Andreas Lohmüller and his team<br />
were able to demonstrate advantages<br />
over conventional die casting processes,<br />
such as better mechanical properties of<br />
the castings with lower porosity. Sofar,<br />
however, the maximum possible solid<br />
phase content has been limited to<br />
Figure 5: Processing temperature ofthe MAXImolding process compared to common series<br />
processes for the alloy AZ91 (Dr. Lohmüller, NMF GmbH).<br />
about 30 to 40 %. The shot weights are<br />
limited (maximum 3to4kg), so that<br />
large thick-walled components could<br />
not be produced so far. Simple upscaling<br />
does not seem to be possible at<br />
present. For material savings, it can be<br />
assumed that optimized gating systems<br />
similar to today‘s hot sprue systems,<br />
Thixomolding with direct injection as<br />
well as compact components are possible.<br />
The low temperature favors hot<br />
runner systems.<br />
Currently, there are only afew<br />
manufacturers of Thixomolding injection<br />
molding machines and the machines<br />
are very complex. They require<br />
intensive and very expensive servicing,<br />
as many complex and moving parts<br />
come into direct contact with molten<br />
magnesium (Figure 2).<br />
Dr. Frank Czerwinski, author of the<br />
book „Magnesium Injection Molding“<br />
published by Springer Verlag, wrote<br />
that the production of non-dendritic<br />
structures in the melt is also possible<br />
under precise heat addition and precise<br />
temperature control. This is done<br />
without shearing the melt, which is<br />
done in an extruder of aThixomolding<br />
machine. This extruder is also the most<br />
expensive machine part, as it is made<br />
of aspecial steel alloy suitable for operation<br />
at high temperature and pressure.<br />
However, the screw with the<br />
non-return valve installed at the front<br />
is notoriously prone to leakage and<br />
inconsistent operation. Maintenance of<br />
these units, which is required after<br />
only 500,000 cycles, requires the use of<br />
special removal tools and acleaning<br />
process that uses an environmentally<br />
harmful hydrochloric or phosphoric<br />
acid. This process, while an advance in<br />
the production of high quality parts, is<br />
definitely complex and costly to maintain.<br />
Disadvantages of the extruder in a<br />
Thixomolding machine:<br />
> The technology was adopted from<br />
plastic injection molding and thus is not<br />
robust enough for metal alloy injection<br />
molding at high operating temperatures,<br />
> requires exotic materials to withstand<br />
magnesium (bushings of Stellite<br />
12 or special steel No. 1.2888),<br />
> the cylinder wall thickness cannot be<br />
increased arbitrarily because external<br />
heat from heating zones does not penetrate<br />
into the interior,<br />
> has acomplex heating and conveying<br />
distance of over 2.5 m,<br />
> uses ascrew that has not been heated<br />
to advance material along the melt<br />
path,<br />
> inorder to stop, the screw has to be<br />
removed for acid cleaning (two additional<br />
installations necessary to maintain<br />
the main machine),<br />
> replacing the screw is very expensive.<br />
More than 100,000 US-Dollar can<br />
be expected,<br />
> maintenance intensive,<br />
> operation requires alot of training<br />
and experience.<br />
CASTING PLANT &TECHNOLOGY 2/2<strong>02</strong>1 19
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