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How to remove/shatter single-use plaster mold

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Hi all,

I have an unusual case where I need to only do a single slip cast, of a full figure with lots of unavoidable undercuts. I plan to create the mold using a lost wax process. I don't plan on preserving the mold after casting.

My question is how difficult it will be to break and remove the plaster from the clay once the slip has set, and if you have any tips for achieving a weaker plaster mix to make things easier.


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I'm unsure that you can produce a plaster slip-casting mould with a lost-wax process. Wouldn't melting the wax contaminate the plaster surface?

A highly speculative, but possibly relevant thread, relying on the thermal degradation of the plaster.

@tkw954 Did you have any success?

PS More details on the size and shape of your master would be helpful.


Edited by PeterH
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@PeterH That thread is brilliant thank you!!

Like @tkw954 I was intending to 3D print the master, but using a wax-based low temperature filament. However using water- or acid-soluble filament sounds better, and could hopefully leave minimal contamination.

The master would be a roughly 30cm tall full human figure with limbs and potentially separate fingers (the worst possible for casting essentially). I  aim to get only 80% of the way with casting, cracks and damage are okay because I intend to thoroughly rework with more clay by hand. Also, I plan to use a wire armature, which means the mold will need to be at least two parts, but that's a separate challenge..

In the interest of keeping the clay workable, I don't want to subject it to any heat, so I'm considering adding sawdust or structural weakness to the plaster to assist in removal.

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I'm not convinced that the shrinkage of the clay against the undercuts will not harm the casting, but ...

Recycling of plaster of Paris
Plaster of Paris can be produced by thermal treatment of recycled gypsum powder at a temperature of 180°C for 2 h. The results of this project show that POP could be recycled repeatedly with the same procedure without altering the required setting time and working characteristics of recycled POP powder for prosthetics and orthotics, and even improving the compressive strength of casts. Thus, recycling POP could preserve the environment and reduce pollution. It seems that recycling POP could reduce the cost of importing new POP. However, further study is needed to compare the costs of importing versus recycling POP.

... remembering they were recycling dry plaster.
... and grinding it to a powder.

In your case the firing cycle would have to be slow enough at the right temperatures to successfully candle the clay. Also you would need to dry out the plaster.

PS Faint memory that maybe freshly made pieces can be candled faster/hotter than partly-dry ones.
From https://community.ceramicartsdaily.org/topic/29727-candling-temperature-drying-greenware-in-kiln/?do=findComment&comment=227839
What I certainly don't understand is a point I heard Ian Gregory make. That he could fire (without candling) pots a few hours after he made them, but otherwise has to wait about a week to do so.  A tentative guess is that a half-dry pot could have a dry (and thermally insulating) outer layer that lets the outer layer to become "too hot" before evaporative cooling kicks in, while a fully damp pot supports an evaporatively cooled interface  moving gradually from the outer surface inwards.


Edited by PeterH
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NASA seem to have had similar problems creating wind-tunnel models, there might be some interesting thoughts in:
Improved ceramic slip casting technique https://ntrs.nasa.gov/citations/19940015904
... and the associated patent

Not least the idea of using a shell mould (i.e. constant wall thickness).

PS They seem to apply (paint?) the shell on a wax positive.

If you prefer to cast it then a previous reference might be relevant
3D Printing a Mold for a Slipcasting Mold

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