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Best Silicone for Slip Casting Master Molds?

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Trying to figure out what the best silicone will be for casting master molds for slip casting.  I'm looking at purchasing some product from smooth-on since it looks like they have a good product list, but there are a lot of options and I'm wondering what others have used in the past.  I'm looking at either Mold Max 30/40 which is a tin cure silicone and recommends vacuum degassing, or a Mold Star 30 which is a platinum cure silicone but more expensive.

The Mold Max series recommends vacuum degassing, but does anyone know if its really necessary?  

I'm also curious about the best durometer to use.  I've seen people use 40 durometer stuff for master molds, but is 30 acceptable?

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Hi @JF_Potterit seems to be 15 hours since you posted your question, and no-one has answered.

Most of the members here are studio or small scale production, or hobby potters.  

I've done a tiny (very tiny) amount of slip-casting and mould-making.   And although I've read many books on the subject, I've not had the need to make masters, and know nothing about silicone.


I'm sorry, but I suspect I'm not alone here, and that your question is way above most of our knowledge.

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Hi JF!

Thanks to your post, I've enjoyed some reading 'bout silicone moulding products.

I'm sticking with wheel thrown clay, however...

You might find some good reading; no doubt you've exhausted what's available from Smooth-On website - their FAQs, "World Famous" technical page, etc. Try "smooth on forum" search string -> resinaddict.com, taxidermy.net, sculpture.net, hobbytalk.com, modelmakers.org ...and here...

Searched for 'smooth-on' in All Content (ceramicartsdaily.org)

Rubber Slipcast Mold Masters - Need Help - Studio Operations and Making Work - Ceramic Arts Daily Community

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I've used a couple of Smooth On products for building plaster press molds.  One thing I've learned is to plan to use the entire batch you buy.  Once opened, there's a shelf life as described in the product descriptions.  Very complicated to calculate volume needed for some projects.  Since I'm only building a 2 dimensional form, as opposed to a 3 dimensional model, I've gone to multiple layers of latex.

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Hey JF,

There are two types of molding rubber: silicone and urethane.  I have used urethane extensively. There is a local source for Polytek so that's the brand I have most experience with. 74-55 is a great rubber medium for block and case molds.  (55 has some flexibility but keeps its shape. 30 is more flexible.) The advantage with urethanes is that you don't need to de-air. You get some bubbles but not enough to affect the molding surface. Jeweler friends use silicone but that's only because they pour hot wax into the molds and silicone stands up well to the heat. (Or so they tell me.) Plus urethane is not as expensive.

In the 80's Donald Frith wrote a book about mold making, "Mold Making for Ceramics". It's a very good book. In it he mentions/recommends Smooth-On. Upon reading this I called Smooth-On and they were kind enough to send me some samples. (I told them I was just starting out as a mold maker.) They're urethane products are similar to Polytek, but Polytek has a local rep, so I have stuck with them over the years. 

Shelf life - the manufacturers usually recommend a shelf life of six months to one year.  While that's ideal I have had urethane left over, from a project, that has sat on the shelf for several years.  Old urethane seems to capture air bubbles more easily, and it has a nasty smell, but it has worked for simple forms. (Forms with smooth surfaces without detail) A trick is to put leftover urethane into smaller containers with little room for air. fyi- I only use old rubber for my projects. I buy new when I start a new project for a client.

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