Paul,
Your right, it's just the surface in which some of the sand is infused,

in

most cases.

However with sand castings the liquid metal can draw off loose silica or

air

from the surface of the mould, causing flaws, small bubbles and even

voids

in the casting. The thinner the casting and the further the metal needs

to

travel the more likely this is to happen, as the metal cools as it

travels

through the sand.

Even when the metal appears to have no flaws,when it's machined you will
notice( in thin castings) that the further the parts of the castings is

from

the source of the inlet pour - the more the metal is likely to machine

less

well as the source area. This is because the metal is not as dense, the
further it is from the source.

I noticed when I did the bellhousing the front wasn't as dense as the

back

engine flange area.
The next time I will pour from the front, as this is where most of the
machining will be done.

These problems disappear with pressure castings.
George (down under)

To make it machine properly it needs to be heat treated. This is a low
cost process here in So Cal. All my sand castings are heat treated
before I machine on them.

Paul Lamar

Paul,
I'm hearing you Matey.
I will try that next time.
George (down under)

 
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