Making plaster slip molds keep details longer for long runs

[finnip]

Hey everyone I have a question I'm about to make a set of slip molds again to pour small statues, I have made the molds over and over,each time varying depending on how much detail is left the the castings,. Is their a way to strengthen the plaster molds so they can have a long run of castings made in them before they loose all of the castings details,,,,,I thought about adding glue to the plaster but this will reck the moisture absorbsion ??? What can I do any suggestions would be greatly appreciated

[neilestrick]

Hi @finnip, welcome to the forum! Someone with more plaster knowledge than me will chime in, but more info would help answer your question. What type of plaster are you using? Are you mixing it with the proper proportion of water? How many castings can you get from a mold before the detail suffers? What type of slip are you using?

[Jeff Longtin]

Hello Finnip,

The answers, to Neil's questions, would help us have a better picture of what's going on with your situation. Without that I'll throw out a few broad suggestions: I wouldn't suggest adding anything to the plaster itself. If anything you could try other kinds of plaster. Ceramical, for RAM presses, is absorbent and quite durable.  (More like cement than plaster.) It does absorb, just not as quickly, as pottery plaster. You would simply need to allow the slip to sit in the mold longer.

The other factor, which Neil touches upon, is the consistency of your slip. If your slip has too much deflocculant that will cause early deterioration of your molds. Are you monitoring your deflocculation  carefully?

The last aspect is design of your model. Can you fill in the detail such that the mold is more durable but allow for easy touch up after the piece comes out of the mold? Sometimes its more cost effective to have a mold with less detail, but can cast more pieces, than a mold with fine detail, which wears out quickly. 

[finnip]

Hey thanks for the response  I have been using casting plaster to make my molds with as little amount of water to make them together but as I start to use them with each pour you can see the details disappear, I have been using my own 'mined' clay I collect on my property sieved with no deflucuant which seems to work fine for a little bit,I have made these same little statue molds about 4 times over now and I suppose with all the seconds counted I would have made close to hundred of each ,,,that's why I thought this time I should ask someone with some knowledge of pottery and mold making what I can do to make a set of molds that I can keep using for years without wax casting and new mold making each time thanks in advance

 

[Chilly]

24 minutes ago, finnip said:

about 4 times over now and I suppose with all the seconds counted I would have made close to hundred of each 

 

Does that mean you've used each mould 4 times, or 100 times.

I recall that commercially, they replace moulds after 20-30 castings, as they always deteriorate.  They make a negative and positive masters, and re-mould from theose every x number of casts.

[Jeff Longtin]

6 hours ago, finnip said:

Hey thanks for the response  I have been using casting plaster to make my molds with as little amount of water

 

One question jumps to mind: are you using the proper amount of water or "as little as possible"? Most casting plasters, I think, use 35-40% water. 

Do you have access to "pottery plasters"? Casting plasters, from my knowledge, have hardeners which make them more durable than pottery plasters but also less absorbent. 

If my memory serves me correctly there are three kinds of "plaster"; pottery plaster - for casting ceramic objects, gypsum cement - harder than pottery plaster but still absorbent, for machine pressing of ceramic objects, and casting plaster - designed for the giftware industry for very hard plaster objects which take paint well. Does your source differentiate between the three?

Clay - without deflocculant you need to add more water than is necessary. Have you tried using a deflocculant? In theory your clay has no elements that are antagonistic to the molds but it could be your clay does have things in it that are not friendly to your plaster molds. Those unknowns could be causing early deterioration? 

[finnip]

Has anyone used and know if I can buy Herculite No. 2 Casting Plaster in Australia for not crazy prices??

[neilestrick]

On 4/16/2023 at 7:51 AM, finnip said:

with as little amount of water to make them

This is probably the biggest issue. Always use the specific amount of water that your particular type of plaster calls for. Plaster sets via a chemical reaction so it needs a certain amount of water for that to happen effectively.

[Jeff Longtin]

Another thought comes to mind...how long, and how aggressively, you mix the plaster also affects its strength.  (This is another reason why proper water amounts are important.)

My mix time for plaster is two minutes soak, two minutes mix, and one minute sit. If the plaster is still fluid I will continue mixing, for a minute or two, until I reach a "melted ice cream" consistency.

You know you're not mixing well enough if a slight layer of water appears on the surface of the setting plaster.