Casting silicone in a plaster mould previously used to cast latex query

I'm fairly sure this was covered on the old board, but obviously I can't look it up at present so I need to pick expert brains:

Recently I made a two-piece plaster mould from a plasticine (sulphur-free NewPlast) master. I have made a number of latex skins (with polyurethane foam fillings) using this mould. As the results are not as good as I had hoped, I feel I need to experiment with casting in silicone instead. Am I correct in supposing that a mould which has been used for casting latex cannot subsequently be used to cast silicone, because some substance in the latex will leach into the silicone and inhibit its curing? Or have I simply dreamt that up?

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Well, I'm no expert, but I know this one - it depends on what kind of silicone. A platinum-cure silicone won't cure properly - they're extremely picky about what they come into contact with. But a tin-cure silicone won't have a problem. 

Dragonskin and ecoflex are platinum-cure and won;t work, but GI-1000 shouldn't have a problem. This is assuming it's a puppet mold and you need an SPFX silicone - if it's a prop and you can use a moldmaking silicone I believe most of them are tin-cure (might want to make sure though). 

Thanks for that, Strider!

It's a puppet head, so it looks like I'll have problems as they mostly seem to be platinum-cure. Ah well, no matter! I have a number of other heads to make, two of which I've sculpted in NewPlast, but I've not yet even made the moulds yet.

Having said that, Smooth-On Mold Max 27T is tin-catalysed, and there's a stockist in my town... Cheers!

I am not sure that the latex will leave much of a residue that would affect the platinum silicone. If you have some, just brush a little on the mold surface and that will tell you whether it affects it or not. Another approach would be to seal the plaster surface. Take a small stiff brush and some soapy water and scrub out the inside of the plaster mold. Rinse with water and then let it dry. You could put it in an oven at about 100 degrees for 6 or 8 hours and that would insure it is thoroughly dry. After the plaster is dry, brush on a couple of coats of shellac which will completely seal the plaster and anything that may be left in it from the latex. Depending on the size and detail level of the mold, you might lose a little crispness of detail from the shellac, but it might be worth a try.

Thanks for the tips, John! I don't currently have any casting silicone rubber (I do have mouldmaking silicone, though), I'm just gathering information at the moment and your post is definitely going on my List of Useful Info About Silicone Rubber.

As an aside, do you or anyone else know if the type of translucent silicone rubber used in prosthetics and for puppets is similar in texture, elasticity, 'feel' etc. (i.e. pretty much all characteristics except colour) to the grey silicone rubber used in mouldmaking?

Finally, the datasheets for the T20 silicone rubber I have (for mouldmaking) don't say whether it's tin- or platinum-catalysed. However, the same datasheet for the catalyst states that among the decomposition products are... oxides of tin! Do I take this to mean that it is tin-catalysed? There is no mention anywhere of platinum or compounds thereof.

I attempted to cast Platsil in a mould previously used for foam latex, and it stayed liquid forever.  But it's not completely clear cut - that mould had also been made over a sculpt in plasticine that contained suphur.  Either the clay residue, or the latex residue, could have stopped the platsil curing.    After that I tested the silicone on everything, to make sure the plasticines, release agents, mould materials, and anything else in the room would not stop it curing, until I found out what was safe.  Certainly, direct contact with dried liquid latex, foam latex, or plasticines with sulphur all stopped the platsil dead in its tracks, but I didn't test it on plaster with just the residue from latex or just from plasticine alone.  It's such fussy stuff I wouldn't risk it anyway.

Apart from colour and translucency, silicones used for casting puppets or makeup pieces are generally softer than most mouldmaking silicone rubbers.  The figure that matters is the Shore Hardness.  Platsil Gel 10 and Dragon Skin have a Shore A hardness of 10, which is softer than 20 or 30.  (There are also even softer ones, like Smooth-On's Ecoflex 00-30.  The 00 means it is a different scale, and I found it hard to figure out if the numbers go in the same direction, or in reverse order - is a harddness of 00-5 harder or softer than 00-30?)  I think I have also seen a Shore A hardness of less than 10, which would make it softer than 10.   But mouldmaking rubbers tend to be Shore A 20,  30, or even firmer, which is not as suitable for puppetmaking.  My guess is that the T20 is a shore hardness of 20, which is harder than I would want to use.

There is, or was, a tin cure silicon made for prosthetics and puppets in the UK, called Alien Breed.  I just went to the link to it at Animation Toolkit, and the page is gone, and now they seem to have a "silicone skin for stop motion" instead:

http://www.animationtoolkit.co.uk/stop-motion-silicone-skin-500g-1kg/

Unfortunately they say they can't ship outside the UK, so I haven't been able to try it.

It doesn't say it's tin cured, but it doesn't have the 1:1 mixing ratio typical of platinum cure silicones for prosthetics, so I'm pretty sure it is.  If I had to cast silicone in a plaster mould I have already used for latex, that is the sort of stuff I'd be looking for.

Brilliant, thanks very much, Nick! I was only curious about moulding rubber as I have a fair bit surplus to requirements, so I was thinking of doing experimental casts in my dodgy mould in the hope that its characterisitics are closely analogous to those of prosthetics silicone (which I can't currently afford, my finances being extremely dodgy at the moment). However, it is now very clear that this is not the case so there is little point my doing such tests.

I shall ditch the current mould (which I'm only using for experiments anyway as it's so compromised by bubbles etc.), so any subsequent moulds I make shall not have latex cast in them, they'll be used directly for silicone. Fortunately, the clay I like sculpting with is sulphur-free.

I shall probably get some PlatSil, as it's sold in a shop not far from me so I won't have to pay extortionate amounts for shipping!

Anyone have a good source of Somafoma, Dragonskin and GM Hot Foam Latex?

I've cast both tin and platinum silicones as well as foam latex and straight latex in the same plaster molds and all have been successful, it doesn't matter what you cast in a plaster mold as long as it's clean and you lube it each time with a little vaseline or whatever you use for a mold release



MartindeMadrid said:

Anyone have a good source of Somafoma, Dragonskin and GM Hot Foam Latex?

These are the usual places for various casting/puppet skin supplies:

http://www.burmanfoam.com

http://www.monstermakers.com

http://www.gmfoam.com

http://smooth-on.com

You should be able to get the Dragonskin and GM foam from some of those - not sure on the soma-foma. Ok, a quick check turns it up at Smooth-On: http://www.smooth-on.com/a117/Higher-Density-Silicone-Foam;-New-Som...

** edit

Ok, it looks like GM no longer sells through their website, but they have a list of distributors, some of which have websites: http://www.gmfoam.com/distributors.html

That list could also prove important for other supplies. 

Hi Martin,

GM foam latex is now sold by Burman, who bought the business from Gil Mosco.  It is virtually identical to Burman's own formula, except that I have found it a tiny bit more forgiving when it comes to mixing time.   http://www.burmanindustries.com/estore/search.php?mode=search&p...

I see they now also have their own prosthetics silicone, Cine Skin, with a shore A hardness of 5.  That's a little softer than A-10, but I think not as soft as 00-30.  The ratio of 100 to 10 - I think that must be parts A and B - make me think it may be a tin cure silicone, which is not as fussy as platinum cure silicones.  But it doesn't really say.

Soma Foama can be bought from the Compleat Sculptor in NY.  I can't post a direct link to a particular product, they all come up under the same URL:  http://www.sculpt.com/

Go to their online catalog, then to Special Effects, and you should see Soma Foama.  Ron can probably confirm this, I think that would be where he got it.

They have Smooth-On's softer Ecoflex 00-30 platinum silicone, but I'm not seeing Dragon Skin there.  I have bought some pigments and Psycho Paint base from them and there was no problem about shipping overseas.

You can get Dragon skin directly from Smooth-On I believe.  So I guess you could get the Soma Foama at the same time.    

http://www.smooth-on.com/Silicone-Rubber-an/c2_1115_1129/index.html

I see you are online so I will post this now.

Many thanks, as always, Nick, for taking the time to not only answer, but explain.

StopmoNick said:

Hi Martin,

GM foam latex is now sold by Burman, who bought the business from Gil Mosco.  It is virtually identical to Burman's own formula, except that I have found it a tiny bit more forgiving when it comes to mixing time.   http://www.burmanindustries.com/estore/search.php?mode=search&p...

I see they now also have their own prosthetics silicone, Cine Skin, with a shore A hardness of 5.  That's a little softer than A-10, but I think not as soft as 00-30.  The ratio of 100 to 10 - I think that must be parts A and B - make me think it may be a tin cure silicone, which is not as fussy as platinum cure silicones.  But it doesn't really say.

Soma Foama can be bought from the Compleat Sculptor in NY.  I can't post a direct link to a particular product, they all come up under the same URL:  http://www.sculpt.com/

Go to their online catalog, then to Special Effects, and you should see Soma Foama.  Ron can probably confirm this, I think that would be where he got it.

They have Smooth-On's softer Ecoflex 00-30 platinum silicone, but I'm not seeing Dragon Skin there.  I have bought some pigments and Psycho Paint base from them and there was no problem about shipping overseas.

You can get Dragon skin directly from Smooth-On I believe.  So I guess you could get the Soma Foama at the same time.    

http://www.smooth-on.com/Silicone-Rubber-an/c2_1115_1129/index.html

I see you are online so I will post this now.

Mike,  many thanks.  You, like Nick, Anthony, Isomer, and, well, everyone else on the board are always so very generous with your hard-won knowledge.  I love this place!

Strider said:



MartindeMadrid said:

Anyone have a good source of Somafoma, Dragonskin and GM Hot Foam Latex?

These are the usual places for various casting/puppet skin supplies:

http://www.burmanfoam.com

http://www.monstermakers.com

http://www.gmfoam.com

http://smooth-on.com

You should be able to get the Dragonskin and GM foam from some of those - not sure on the soma-foma. Ok, a quick check turns it up at Smooth-On: http://www.smooth-on.com/a117/Higher-Density-Silicone-Foam;-New-Som...

** edit

Ok, it looks like GM no longer sells through their website, but they have a list of distributors, some of which have websites: http://www.gmfoam.com/distributors.html

That list could also prove important for other supplies. 

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