Plaster Of Paris

Dotfuscator

how long does it take to set?
i've poured it into a small container with not too much water with a small object in it which i want to remove when the plaster sets.
how long should i wait before removing the object?

Monkey

Doesn't sound like you know how to mix it. You can't decide how much water to use as you are suggesting - it just has to be the right ratio of water to plaster otherwise it will crumble.

Put some water in a bowl. Add plaster until the plaster stops sinking and starts to form "Islands". You can leave it in this state for as long as you like it won't set until you mix it - by hand is the best way. Mix it until there are no lumps and it will begin to set. The time it takes it depends how much you are using but its better to be patient with it. It gives off heat while its hardening - I'd leave it about an hour and then make sure its cold and dry to touch.

Dotfuscator

you're right, i've no idea how much water to put in. well the first attempt didn't go very well. i'll try what you suggest next time, thanks.

i put an object in the plaster but want to remove it after so that there'll be a hole in the plaster. is there any way to do this without having the object get stuck in it?

coolwings

Do you mean making a mould like this?


The way is get a box, Grease the bottom and sides with cooking oil or vaseline.

Half fill the box with plaster of paris, grease the sinker and press down until half sunk in the plaster.
Then when it is half set (15 - 30 minutes), push 4 nails into each corner pointy-end down, until halfway into the plaster.
When all is hard, grease the top of the sinker again, and also the surface of the plaster that it sits in. Do not let any grease get onto the nails.
Next fill the box up to the top with plaster of paris. Leave to set.

When hard either disassemble the box or remove the cube of hardened plaster carefully.
use a knife inserted into the "grease-line" to prise the top off the bottom of your mould.

Next remove the sinker carefully, and using a sharp tool scoop out the funnel shape filler sprue-hole. Half of it in the top and the other half in the base of the mould.
Also cut a fine line out to the other side to allow air to bleed out as the hot lead, resin or whatever goes in through the funnel.

This should work for you. But you should know that plaster is a crumbly brittle mould material. I changed over to fibreglass and silicone a long time ago due to moulds cracking after only one or two uses. I get my casting glass and silicone and 6mm chopped fibreglass at www.greenhobbymodel.com and previously I used to get the plaster at the local pharmacy who were very helpful in getting me started in casting.

A partial "fix" for the flaky quality of the plaster is to add fibres in some form. Wrapping cloth around the insides of the box before pouring the plaster in, will produce a tougher outside to your mould later on. Stirring in some of the 6mm chopped fibreglass strands into the wet plaster adds a lot of strength, far more than you would think. Stirring the 6mm strands in is well worth doing after you have done your first "dummy run" experiments and are working on your first proper mould.

Hope this helps.

Dotfuscator

thanks coolwings for the instructions, i'll try and follow them next time i make one. have tried a few dummy runs at this stage, mostly disasters. one worked well. did find it to be very crumbly but it's only for a little experiment that i'm doin so it should be fine.

can't figure out why to use nails though?

coolwings

They align the top side of the mould precisely onto the bottom side.
Locating pins in other words.

sunny2004

are you trying to make a mould to make a copy of something ??

Dotfuscator

no i'm tryin to make an outline of an object and just leave the outline in the plaster, that's all. i won't be usin it to copy anything