This is how to make a very long lasting, non-shrinking mold, suitable for casting many materials.
Plaster, cement, resin, and foam for a few. It's not very good for casting latex though, as the latex tends to run off the sides of the silicone.
Definitions and abbreviations;
Matrix mold: a mold with a seperate inside layer, supported by a hard shell.
Mother mold: the hard shell that gives support to a matrix mold.
Dividing wall: a (usually) clay barrier, used to create a flat edge where the mold halves meet.
The better the wall, the better the seam will be on the final casting.
Positive: Pos. the original sculpt or item you are molding.
Negative: Neg, the mold created from a positive.
Casting: what you remove from the mold, the end product.
Registration or Keys: lining up two or more sections. Can be pegs, bumps, notches, anything that forces the parts to align only one way.
Plaster of Paris: PoP
Release: thin coating of something that prevents the Pos. from adhereing to the Neg. etc...
Tube Silicone: 100% silicone caulking. Only 100% silicone!
First you must eliminate any major undercuts in the positive.
Just use clay to fill any areas that would lock the mold onto the original.
Make a "fill neck", in this case (a skull) the fill neck would be where the actual neck would be.
This is just a rod of clay that when removed, will leave a hole in the bottom of your mold, later to be used for filling.
Make it at least 2 inches in diameter to as large as you want.
You may need to fill in under the jaw, behind teeth, etc...
After preparing the pos. coat it with release, soap will do.
Let it dry, you can speed it up with a hair drier, but don't melt the clay.
Cover the entire pos. except end of fill neck, with silicone.
I'd suggest 1/2 inch thick, as tube silicone isn't the strongest.
Take your time, be careful not to entrap any air between the pos. and the silicone.
The surface of the silicone need not be perfectly smooth, in fact a few bumps will help align the silicone inside the mother.
Helpful tip: Mist the silicone with water when you have it all covered.
One part (tube) silicone is moisture cure.
*Advanced tip* Stir in a few drops of water to speed up the cure, don't stir in any air though.
After the silicone has fully cured, lay up a dividing wall. Make the wall about three inches high.
With the pos. sitting on the fill neck, put a wall of clay dividing the skull ear to ear, all the way down the fill neck.
Add registration keys to the wall.
These are simply pieces of clay that will create notches in the side of the mother mold to allow the halves to fit together positively, in perfect alignment.
Once more, watch out for undercuts.
Coat the whole thing with a thin layer of soap as a release, again be careful with the clay.
Apply the PoP to the side with the registration keys. This will be either the face or the back of the skull.
Make the PoP mother mold at least an inch thick. Let the PoP dry.
Remove the dividing wall and notches clay. In this case it is not vital to have a perfect seam, as this seam is only between the mother halves and will not effct the casting.
Apply a good coat of Vaseline to the edge of the mother where the wall was, also up onto the outer surface of the first half. Not so thick that it leaves gobs, but you need a layer on the surface to allow you to seperate the halves.
Check to ensure you have no undercuts with the registration notches.
Make the second half of the mother the same way as the first, being careful to get the PoP into the reg. notches, and to be fairly close to the same thickness.
After the two halves have cured, dig out the fill neck clay, and carefully pull the mother mold halves from the silicone matrix mold.
Carefully cut the silicone, to remove the original. Make the cut in the back of the skull, up from the fill neck only as far as you need to get the skull out.
clean the inside of the silicone, re-assemble the mold, duct tape the mother together.
Apply a layer of release to the inside of the mold, and add your casting material!
Some things I may not have answered...
Soap release is just a thin layer of dried soapy water, applied as a wet froth. Nearly any soap will work.
Number of casts you can get out of this kind of mold, Thousands. Really. This is a very durable mold.
This mold uses the mother to hold the silicone in place, the one short seam is 90 degrees from the mothermold seam, so it is held in place very well.
The general rule with molds is; Hard pos. - Flexible neg.(supported by a removable mother mold.)
Hard Neg. - Flexible Pos. (Latex masks)
Hard neg. will sometimes work with a hard pos. but that's not for beginners, and if everything is flexible, nothing will hold it's shape.
PoP is quite absorbent, and Poly Foam is very adhesive... I wouldn't trust soap for PoP/Foam. Try paste wax. Yep, Turtle wax for your car! Buff it into the plaster until you get a nice smooth slippery layer.
Another helpful hint: Great stuff is a moisture cure one part Poly-Foam.
Lightly mist a layer of foam with water before adding another layer, you can get a thicker amount of foam to cure that way.
Now that you know how to make a professional silicone mold, I must ask that you use this new skill only for good, not for evil.
Please, don't recast things that others own the rights to. You wouldn't like someone stealing your creations.
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Putrid, it sounds like you've got it!
Vlad makes a very good point about cost effectiveness versus durability versus ease of use versus... The list goes on.
I would do a similar mold process for the paper-Mache skulls. (In fact, I have!)
A latex detail mold with a hard mother backing for support.
Latex isn't as pliable as silicone, and is therefor better suited for a clay or paper-mache press.
I wouldn't think any release is needed. I've used this mold for cement, UC30, FRG, Pop, Sculptamold (basically that's like paper-mache) and clay.
Thanks for the info Lurks in the shadows. Looks like silicone is the way I'll go.
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The cold winter air lets me know I'm still alive. I feel it as I breath in and see it as it leaves me. It's the point in-between I'm not sure about.....
Very In-depth...I'm going to try this with papier mache pulp and with Great Stuff. I've found that working with Great Stuff can be more hassle than it's worth. Thanks Lurks!
Hi sis, the reason I'm using a mold, is because I won't have to worry about the mache sticking to an original, I already have molds made, and pressing the mache' in seems to be much easier than applying it on. This way, the outer layer of the mache' has the ultimate detail, whereas if you apply mache' to the outside of an original, it loses detail with each additional layer you apply. No need for a release agent between mache' and latex. And yes, you could use other types of molds, but then you might very well need a release agent. The problem then becomes trying to get the release agent off of the mache' so you can paint it.
Putrid, At the moment I'm experimenting with unboiled. I seem to get way too much shrinkage with the boiled. I'm probably just applying it too thickly. But there again, I said this is only the start of the experiment.
Vlad, is the toilet paper doing well? What type of glue mix are you using? Can't you get the stuff done any faster? JK LOL Looking forward to seeing your results.
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The cold winter air lets me know I'm still alive. I feel it as I breath in and see it as it leaves me. It's the point in-between I'm not sure about.....
sisvicki,
Thanks!
You wouldn't happen to be a yooper, would you?
(I just happen to be a peninsulanian myself. L.P.)
Pennywise,
If I may answer your question to Vlad. I hope I'm not stepping on any toes...
With silicone, you usually don't need any release. But hey, it won't hurt.
With paper-mache and silicone, I don't foresee any difficulties. Except possibly some distortion caused by pressing the mache in too hard, just pay attention to the mold and you'll have no problems.
Silicone (100% that is!) won't stick to much, and vise-versa.
But as always, do a small test to see if you are about to waste your efforts and money!
Especially when using materials for "non-standard" purposes.
Linear Mode
