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Bracket component showing supporting webs and radiusing of
sharp internal corners. Using one feeder position, dimentions
A<B, B<C .
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Like
any other production method investment casting has a few simple
design rules which allow its advantages to be succesfully
realised.Often the foundry can suggest slight modifications
to conventional designs which will make the casting both easier
and cheaper to produce, ensuring maximum soundness and in
no way impairing its function.Keep the following points in
mind :
Don't Design for the machine Shop !
Adjust your thinking. Flat surfaces and right angles are the
natural results of the machining but they can be difficult
to cast.So are they really necessary ?Curved and blended contours
not only are prefer for casting, but they result in sound
components.
Similarly, Don't ask for machine-Cut tolerances on dimensions
which don't warrant them.Specifying over-close tolerances
on non-functional surfaces is unnecessary and uneconomic.Point
out the really critical dimensions and let the foundry concentrate
upon achieving them.
Helping the foundry Man :
It is INTRICAST's job to advice upon the best casting design
but you can help by appreciating the importance of correct
feeding of metal to the casting.
During solidification, thin sections naturally tend to cool
first and as they solidify the metal contracts.
At this stage the sections should be able to draw further
liquid metal from areas which are still molten:but if none
is available , shrinkage, porosity or cavities will result.
Suggestions for Redesigning :
Ideally, when :
Feeders, should be attached to the thickest sections, with
thin sections furthest from point of entry of metal.
Contours should be as smooth as possible , and abrupt changes
in thickness of sections should be avoided to assist smooth
flow of metal.
Sharp internal corners must be avoided as they form points
of stress concentration. Introduce a small radius.
The minimum number of sections should be brought together
at one point to avoid 'hot spots' in the solidifying casting.
Unsupported features will tend to distort owing to strains
set up by differential rates of cooling.Tie-bars can be added
to hold certain dimensions more rigid,and are removed during
fettling.
Finally, place in-gates on surfaces which will subsequently
be machined,if possible, rather than where a smooth as-cast
finish is required.
The time to decide upon casting design is while the component
is still on the drawing-board.For better to design for costing
rather than attempt to modify a part intended for production
by other methods.
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