I know aluminium contracts a bit after casting. Is there any way to minimise this.
What is the metal with the lowest contraction that can be cast by an amateur? I'm having a bit of trouble finding contraction percentages e.g. for cast iron - is this figure the same as the coefficient of linear expansion? (Or does that only apply when the metal is solid).

The "shrink factor" is something that is endemic to any cast material. If varies for each and every alloy and is something that also, to some extent, depend on the mould materials being used.

Liquid metal and solidified metal, depending on how it is "solidified" will not have the same volume.. Annealed material will not have the same dimensions as hardened and tempered. Simply put, metallurgy ain't that simple nor can it be simplified to that level.

Badically, you can not minimize the shrinkage - you can only account/compensate for it in your tooling and/or moulds when you size/construct them.

When you start to do post casting heat treating, you're into a whole new realm of shrinkage/distortion to learn and/or deal with.

...What is the metal with the lowest contraction that can be cast by an amateur?...

An answer to this specific question is; Type metal. That is the metal that was cast into letter shaped molds for typesetting. Type metals are low melting point alloys with all sorts of horrible things like lead, cadmium, bismuth in them. Some formulations shrink on cooling and keep shrinking but some will shrink and then undergo a phase change and expand again making them a neat fit in the letter mold.

I am not in my office right now, but I do believe that the shrikage linear factor is 1/8" for every 12".
I will confirm tomorrow.

Aluminum is sometimes said to shrink 5/32" per foot, sometimes they say 3/16" per foot. Either will work just fine for home casting.
Shrink scales are commercially available, sometimes you even see them on eBay. They're nice because you make your pattern just like you would normally since the dimensions of the scale are already "stretched" to compenate for shrink.
You can make your own scale though, I was taught how to do that when I was studying under my grandfather, a master pattern maker.

for Aluminum I use 3/16" shrink for patterns under 18", over that I use 5/32"--and I use 5/32" for brass/bronze for all sizes.

http://www.freemansupply.com/StarrettSteelShrin.htm

:cool:

Hi,

I manufacture rotational mould tools that are cast from aluminium. the tools must be 3% bigger than the required plastic component to get the required dimensions. When making the patter for the ali casting i always allow 1% on top of that. If you make your patterns 1% bigger than the required ali casting you should get the correct dimensions. 1% contraction is standard with lm25 aluminium.

Hi,

I manufacture rotational mould tools that are cast from aluminium. the tools must be 3% bigger than the required plastic component to get the required dimensions. When making the patter for the ali casting i always allow 1% on top of that. If you make your patterns 1% bigger than the required ali casting you should get the correct dimensions. 1% contraction is standard with lm25 aluminium.

That would be just a little under 1/8" per foot, for sand casting in aluminum using the typical sand casting alloys, you'd likely come up a bit short on castings over 6 or 8 inches... (15.2cm or 203.2cm to you)

Whats the adjustment for varying wall thickness (say 3/16" vs 3/8"?

Pres

Whats the adjustment for varying wall thickness (say 3/16" vs 3/8"?

Pres

that's so small of a dimension you can pretty much ignore the shrink.

if you're going to do much pattern making, it's wise to buy a shrink scale for the metal you're working in.

if you use CAD, most programs will allow you to "grow" parts, you can do that then dimension the result.

For a rotational mould we always make the wall thickness 10mm. so it isn't a variable ive had to deal with..

Most of the pattern makers i know work on 1% contraction for 10mm Thick Ali.

For a rotational mould we always make the wall thickness 10mm. so it isn't a variable ive had to deal with..

Most of the pattern makers i know work on 1% contraction for 10mm Thick Ali.

ya well, y'all drive on the wrong side of the road too. :)

spin casting is highly specialized, someone asking for shrink info is probably not heading that route.

Trust me guys, use 5/32" per foot and you'll never go wrong (it's 1.3% for you metric-heads). Get a shrink scale and you wont be sorry.

http://www.freemansupply.com/StarrettSteelShrin.htm


Bill Monroe
Foundry Owner, Patternmaker since 1969.

Shrink is pretty much going to remain constant as in so much per inch (mm). The "so much" factor will change from material to material. HOwever, as-cast versus solution and aged material MAY differ as a result of the grain structures generated via the subsequent heat treatment the parts can/will see. This is why you need to know EXACTLY what the heat treat plans are/will be for the part you're making.

You can kluge things and say that 3/16" material shrinkage will be so little that it is inconsequential to that of something 3/8" thick BUT, BUT, realistically, if you stack up 20 or 30 3/16's together the shrinkg amount gets SUBSTANTIAL. We got burned by doing that exact form of SWAGGING and never forgot the ugly episode!!!!

In any event, it is pretty safe to say that a 3/16" section will shrink ROUGHLY 1/2 that of a 3/8" section in the as-cast condition.....