Lost Wax

When doing investment brass, what is the typical amount of shrinkage people see between the master and the finished casting?

I'm planning a socket that has to have a reasonable fit to the timber and leather components.

Wayne
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If you are casting brass into an investment mold using a lost-wax method, the size of the casting should be virtually the same as your model. Normally you'll see shrinkage only in either the molds or the wax models from them. Some mold materials shrink, as do injection waxes. But invested wax models burned out and then cast with brass should not have perceptible shrinkage (unless you're needing extremely precise fittings-depending on the alloy, there will be some shrinkage just due to the metal cooling/contracting, but we're talking hundredth's of an inch or less). I couldn't imagine your leather or wood articulations being that precise in any case as both materials swell and contract with varying heat/moisture levels. I certainly wouldn't be concerned.

I've done hundreds of lost wax castings in clay moulds - generally rather small castings of jewellery-size but also larger objects: see http://web.comhem.se/vikingbronze/sunchariot.htm.
Generally the shrinkage from master to finished object seem to fall within the range of 2.8 - 4.2 %, but I have a few times experienced as modest shrinkage as 1.8 %, and a few times even up to 5 %.

I've made a few handfuls of smaller castings by the ceramic shell method, a method that should minimize shrinkage as the ceramic mould material already is sintered before making the mould. Surprisingly those castings have show a very similar pattern, with shrinkages between 2.8 - 4.2 %.

When planning the pole for the Trundholm Sun Chariot model, 44 centimeters long, I estimated the length shrinkage to be 4 %. In the end it showed to be just 3.8 %. Which I presume is rather good for such a long and slender object.

Wow! What a wide range of answers.

Every metal and every alloy of metal(s) has a standard shrinkage rate. The rate is well documented in the pattern making industry. In that industry they use measuring devices that have the shrinkage built into the device.

EXAMPLE: If the shrinkage rate for metal ‘C’ is 2.5%, then the rulers will have that 2.5% built into ever inch, and fraction of an inch. You say that would make for a bunch of rulers? You are correct. A large pattern making shop that makes patterns for many metals will have an entire wall of rulers, all labeled for the metal or alloy to be used.

The statement made by several responders that allude to “various” shrinkage rates for one alloy is just plain bunk. If you would like to verify this, go to the Copper Development Association website and read the data on just a few alloys.

My two cents,

Fredrick the Köhler
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Yes but not all metals of the same type are exactly the same. If you bother with all the scientific gumph you also need to allow for different grades of metal different compositions of alloys in them.

It is rare rather than the rule to find two copper alloys from different companies that are exactly the same. You can depend on silver being pretty much correct but never brass or copper alloys.

Add to that a lot of metals we get do not come direct from a mining refinery but from different refineries around the world. That is why metals from different refineries need to be analyzed before precision casting can be done. Rule of thumb applies more often than mathematical formulae.

You never know with some suppliers how much scrap they add to increase the volume and cut the cost. With scrap you never know the exact composition because it may have been through any number of casting processes.

So yes the purist formulae do apply sometimes but in the real world that is not always the case. You either analyze each suppliers batch or make a percentage allowance of up to 1 percent for the difference. If you find a supplier who has exactly the same batch every time remember to send him Xmas and birthday cards and be prompt with payments. LOL.

Lothar
Foundry owner.
Who casts all metals from Gold to Iron.
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If you can obtain a copy of the alloy [and I will not buy from any seller who will not provide me with a certified copy as I often cast items that may be exposed to foods] then you can find all you need to know about ANY copper based alloy in the materials provided by the Copper Development Association. I have used their books for thirty years now and have NEVER
found an alloy that was not listed.

As to materials not in the copper based area, I am not as knowledgeable as I seldom cast them.

Fredrick
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You'll have to keep in mind that there isn't just the shrinking rate of the alloy you'll have to be aware of, but also the shrinking rate of the mould material in the cases when you use materials that have to be fired before casting - such as ceramic materials.

When I work with moulds of clay/sand/grog/organics - the early medieval recepy - each mixture tend to be a bit different from the last one I made. Which explains the rather wide range of shrinkage I told about earlier.

The ceramic shell method is designed for hi-tech use where you need a controllable shrinking ratio of the moulds, but when I mix the material for home casting I don't pay much attention on estimating shrinking. Which means that I usually end up with slightly different mixes here too.