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    Questions about constructing multipart silicone mold.

    Will addition cure (platinum based) silicone stick well to a fully cured layer of the same silicone? Or must the first layer still be tacky when applying more silicone?

    I have a very delicate sculpture, made of Super Sculpy, and I am afraid of baking it because it may crack. So I am considering making a mold over the soft clay. First brushing the whole sculpture with a layer of silicone, and then adding clay shims (is there any kind of clay, or other way, to make the shims adhere to the initial silicone layer?) to make silicone mold sections over it, and cutting with a knife through the initial silicone layer as each mold section gets done. The reason I wish go about it this way, is that if the clay shims had been added directly onto the uncured sculpture its details would be damaged. The first layer of silicone will protect the sculpture.

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    The only thing silicon sticks to is silicon, so you should have no problem there...not sure what you mean by clay shims....are they the register points?
    A picture would be helpful in helping you. Soft clay moulding can be tricky at best....could you not use a heat gun to harden the clay at least a little?


    Mah ' Crub

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    Yes, I mean clay shims for register points, and setting dividing walls for the mold sections. Regular multipart mold-making technique.
    Sorry, I don't have digital camera yet.

    If I can get a heat gun, that might good idea. Or perhaps if I only leave the sculpture for a few minutes in the oven.

    I am a little scared of covering the soft sculpture with a first layer of silicone. Because the clay I have for making mold shims doesn't automatically stick to cured silicone, and it may therefore be difficult to make the shims stay in place while applying the more silicone. Perhaps there is some sticky silicone-based clay that can be used? Another problem is that the first layer of silicone should remain clean from surface contamination after the shim is removed, so that it will stick well to the next section of silicone.

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    Kilh is offline Kaleidoscopical and Immediate Learning Humanoid Level 4 Gladiator: Meridiani
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    Normal waterbased pottery clay or WED clay will stick to the silicone good enough. And you can just clean it up with water. If you have enough lying around you can also just use sculpey for the walls. Done that, works like a charm.

    Anyway, I really wouldnt recommend molding unbaked sculpey if it's a finished, detailed piece. No matter how soft your brush for the silicone is - it will deform the surface to some degree. After that your clay walls will deform the sculpey, no matter how careful you are. If you really really (-really-) have to mold it unbaked, do it in one pour (a solid block of silicone) and cut the mold open afterwards. Depending on the model and negative space the weight of the silicone could do some damage as well, but as long as you carefully fill the mold from the bottom, the risk is pretty low.

    And please really consider baking it. Actually I cant imagine any situation where molding a finished sculpey sculpt unbaked would yield any benefits. If you're afraid of cracking, almost any kind of sculpey damage can be fixed with superglue and epoxy/polyester putty in a matter of minutes. A deformed mold with a completely destroyed master sculpt (you'd destroy it in the demolding process) on the other hand will not be fixed that quick. Baking the outer 2-3 millimeters should leave it solid enough for molding and handling, but I hardly see a reason to risk anything...
    I also wouldnt recommend trying the heatgun idea. Even if you have a lot of experience with heatgunning sculpey, your fear of baking it can probably not be solved by basically torching it.

    So, my 2 cents in a nutshell: Carefully bake it, use superglue if necessary, mould it, done.

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    Good advice from Kilh...so that's two of us who advise against soft clay molds...and he is right, unless you have some experience with the heat gun you could torch it and ruin it, so a short bake at say 225 for maybe 10 min would work best ,{ depending of course how thick your clay is , but you imply that it's frail and so most likely thin,} Ok ....now, I don't use clay registers, I like to use Marbles...yes, like the ones you played with as a lad. So to do this, when you pour your silicon mould first layer, make sure it is on a level surface, and then,{ assuming you have made a box to contain your piece and the silicone,} I would hot melt glue my marbles to a wooden dowel, and suspend them only halfway into the top of the freshly poured first layer. This will capture them in the first layer and by letting to upper half protroud, when you pour your second layer,{ over the cured first layer,} you will have great register points....keeping in mind that you have to pre - plan for a way to suspend the dowels and their marbles into your wet silicone....I make a frame over my box and hot melt glue the dowels to that, making sure the marbles sink aprox. halfway into the silicone.....this is of course only one way of several billion to make register points.....

    Mah ' Crub

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    I have never thought of marbles before. Good idea. They could also be pressed halfway into the clay shims, which will save the effort of sculpting keys.

    Kilh, I will really consider baking it. Meanwhile I will experiment with placing waterbase and Super Sculpey clay shims over an old silicone mold I have, to see if they stay in place.

    As to applying silicone with a brush onto soft clay, that's my least worry. The silicone is freeflowing (without thickener) and a synthetic brush is very soft. I have done it before onto Roma Plastilina (condensation silicone). And rubber molds are often made over soft clays like Chavant and Roma.
    And I think the risk of deforming the Sculpey with the clay walls also depends on how thick the initial silicone layer is made (say 1/4 inch would be sufficient) and carefully shaping the clay walls to near perfect fit before applying them.

    In either case, I will start from the bottom and build the mold sections upwards, so the weight will not drag the sculpture down. It has too much negative space to be cast in an solid block of silicone (that would be too difficult, or impossible, to cut).

    By the way, will the sculpture really be exactly the same after baking? Will it not look slightly thinner (shrinkage) after "oils" in the Sculpey have evaporated from the heat?

    Edit: clay shims = clay walls (to avoid any misunderstanding)

    Last edited by Jorganus; July 13th, 2011 at 05:09 AM.
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    if you bake the sculpey on a really low heat in the oven (50-70) and leave it for any time from 20 mins to 2 hours(depending how hard you want it cooked) and leave it just to cool down after in the oven, until the oven is cool, it should be hard enough to the touch, and you shouldnt have any worry about cracking. Just dont keep opening the oven door as thats when the cracks come through good luck...how ever you decide to do it

    Earhole Bongsplat
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    Bongsplat, yes, low heat, and letting it cool off slowly sounds very sensible! I will certainly do that... if I decide to put it in the oven.

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    Jorganus, in the past 4 years I havent seen any noticeable shrinkage when it comes to baking sculpey. As far as my understanding goes the oil also doesnt evaporate, it reacts with the vinyl particles.

    As far as baking goes. No matter what you're baking, ignore the instructions on the box. I think the question about proper baking has been answered in a very detailed manner before, you might want to check the search function. Basically 3-12 hours on a low temperature (105 C for me) + half of the time at a lower temperature + 4-5 hours to cool down in the closed oven.

    Also I wouldnt compare unbaked sculpey to plasteline clay. Chavant has a pretty strong surface if you apply a layer of clear coat or dull spray.

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    I just read on the Sculpey webpage that their clays shrink "less than 1%".

    As far as the problem of putting clay walls onto silicone, it can't be avoided anyhow, as it must be dealt with when making the hard shell mold parts over the finished rubber mold.

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    Actually I'm still a bit confused why you want the clay walls stick to the silicone. Usually you'd want to support the walls with a cardboard/clay/foam/stuffed hamster structure from below or something. Again - some pictures would really help helping you

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    Quote Originally Posted by Kilh View Post
    Actually I'm still a bit confused why you want the clay walls stick to the silicone...
    Yea, this thread is a challenge in attempting clear language description!

    I have not done this work before. But I have the idea that if the clay wall is not close onto the silicone, silicone will trickle under it, onto the next section and ruin the sharp division between the mold sections. Hmm... maybe it doesn't matter after all, since I have to cut the initial silicone layer anyway as I go along.

    But if I make the mold section directly onto the sculpture, without an intial layer of silicone, then it is important that the clay walls fit exactly. Otherwise the mold sections will not become perfect. And you can't cut through excess silicone that has trickled underneath a clay wall once two mold sections are together, then it's too late; and doing it before would damage the sculpture surface, and the next mold section would show the damage.

    This was a bit difficult to put into words! Sorry if I confused you. We'd better let it rest here perhaps.

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