| Casting processes - Lost Foam
The process is suitable
for series production of aluminium castings up to about 20kg; iron (Grey,
ductile and malleable) to about 50kg and copper based alloy castings.
Because of the possibility of carbon pickup, the process is unsuitable for
most low and medium carbon steel castings but austenitic manganese steel and
other high carbon steel castings can be made satisfactorily.
Brief description
Pre-forms of the parts
to be cast are moulded in expanded polystyrene (or other expandable
polymers) using aluminium tooling. Gluing EPS mouldings together can form
complex shapes. The pre-forms are assembled into a cluster around a sprue
then coated with a refractory paint. The cluster is invested in dry sand in
a simple moulding box and the sand compacted by vibration. Metal is poured,
vaporising the EPS pre-form and replacing it to form the casting.
Special
characteristics of lost foam casting
- High production
rates are possible
- High dimensional
accuracy is achieved
- No cores are needed
- Complex shapes can
be cast
- Machining can be
eliminated
- Minimum shot blast
and grinding is needed
- All sand is
re-usable
- Environmentally
good
- Expensive tooling
restricts the process to long run castings
- Specialised casting
equipment is needed but the capital cost if lower than for other high
volume processes such as green sand
- New technology must
be learned
- Long lead times are
needed to develop new castings
Critical assessment
The strong patents
that covered the process and the need for users to take licences from the
patent holder handicapped the early development of Lost Foam Casting. Only
when the patents expired around 1980 was free development possible.
Several major foundry
groups, mainly large automotive companies, invested large resources in the
process. They were looking for a way of escaping from the limitations of
split-mould casting technology and of avoiding the ever-increasing capital
investment needed for modern automatic green sand moulding or gravity die
techniques.
For the right
application, the process offers significant advantages over all other
casting methods, and its use is steadily growing as the most suitable
applicants are found. However, it seems unlikely to achieve more than a
specialist position, not unlike shell moulding, in the range of casting
processes available.
The best applicants
are those where the precision of the process enables machining to be
eliminated, such as valve parts and pipe fittings, or where complex shapes
not possible by other casting processes can be made using the ability to
glue preforms together.
Courtesy of the
Institute of Cast Metals Engineers.
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