Pruning Trees. And how to remove investment plaster from castings.
by John
The investment plaster is is what the mold is made from when parts are cast in bronze and aluminium.
It is incredibly fine dust until mixed with water, and these need to be in precisely measured weights.
The mixture is poured into the casting cylinder in which the parts are attached to a wax or PLA “tree”. (see yesterday’s post for a photo of a tree).
After casting, much of the investment plaster is blown out of the cylinder when the still hot cylinder and its contents are plunged into cold water. The steam which forms acts like a steam cleaner.
But the parts are still covered with a tenacious layer of investment plaster. More can be scraped off with a screwdriver, wire brush, pressure hose etc. etc. but there remains a lot of plaster in the nooks and crannies, joins and holes.
And it is a fair pain to get it all out.
Yesterday’s castings looked like this after I had sawed the 34 pieces off the trees.
and after more sawing and use of a thin disk abrasive wheel…
Then most of the unwanted bronze branches were machined and sanded off…
It happened serendipitously. I placed the parts in a gemstone tumbler with sharp stainless steel shot, to take off the sharp edges. And hallelujah! The remnants of the investment powder were also removed. The curved arms in the photo are stainless steel and brass, bent around a mandrel. I have not yet decided which to use.
In future I will use the gemstone tumbler at an earlier stage, to get rid of the investment as soon as possible.
And here is another stage of the pruning of the trees…..
johnsmachines 
Regarding your wondering if you’ll “void the warranty”, I love one of the mottoes embraced by the “Makers”: “If you can’t hack it, it’s not yours”.
Your casting notes are enlightening, and will surely be helpful when I finally get around to trying it. Many thanks.
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Just a thought John, assuming the investment plaster is predominately calcium based as in P of Paris.
It may be worth dropping a scrap bit in dilute hydrochloric or sulphuric acid.
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I will try that John, but I think that the “plaster” is silica based.
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Just did a quick search and the source of all miss-information says it is mostly a gypsum plaster and acid doesn’t seem to do a lot. Suggests the physical removal and try seeping in Sodium Bi-carbonate. So, a case of suck it and see??
But as you discovered tumbling in the gem polisher works with minimal effort on your part provided the bits fit into the drum. OR get a bigger boat, er, drum.
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I am using Goldstar Omega Plus investment powder, the main active ingredient is Cristobalite which is SiO2. I call it “plaster” for convenience, but that word is misleading, because it suggests gypsum. My bad.
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