plaster of paris cracked?

Hii my plaster of paris cracked. i want to turn a 250x500 cylinder of plaster of paris on a lathe for a positive for slipcasting. its mounted onto the faceplate with a pine plank in the middle to help with the support of the plaster we made that last week thursday and all was fine but this morning it had a big crack in the length of it. what might cause it 

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RobertW3275 months ago

your piece of timber has absorbed the water from the plaster and expended causing the plaster to crack.

iceng RobertW3275 months ago

+1

Very astute analysis.. -.-. . --. -.

seandogue5 months ago

As Downunder35m suggested, for slipcasting, wouldn't it make more sense to use wood for this piece? I'm not sure why one would want to spend the effort to machine paster of paris when it's highly likely to fail once the casting is removed.

seandogue5 months ago

idk, at a guess...

1) it was not thoroughly dried before machining

2) mechanical stresses from machining triggered a fracture

3) thermal stressed during machining caused a fracture.

4) mechanical stresses from mounting (separate from machining stresses) the item caused it to fracture.

brinleypritchard (author)  seandogue5 months ago
oki that makes sence but we havent machined it yet only by hand shaped it when it was set properly.

2+
But even more important is the fact that the plaster won't be cured within a day or two - unless your measurements are in mm and not cm or even inches.
Mounting it is also a big problem as the vibrations will be passed onto the plaster and it won't handle that together with compression forces.
The only really working way that I know of is to include some metal rod with a plate into the cast or to have the cast made into a short piece of aluminium tubing that is used to hold it all in the lathe.
Same for the other end if it is a long piece.

Might be a stupid question but:
Why not machine something easier to use and then make a negative mold from it?
Even wood that you cover with a thin layer of varnish when done will be better than turning plaster....

+1

rickharris5 months ago

Assuming you need a 250 x 250 tube or cylinder - Buy one made of aluminium or plastic, google will be your friend there.

Turning plaster isn't likely to end well - by its nature it is very fragile.

You could try incorporating some binding material into the plaster, glassfibre strands, horse hair, straw, nylon strands, cotton strands etc. This will enhance the structural strength although not by a huge amount.