I tried making my first 2-part mold for the first time, and made the rookie mistake of not making a key for the mold to pry open on the outside. Also, I used plaster of paris, so the mold inside the mouth broke off. For mass production, should I use a silicone mold?

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No, you need to use dental stone, also called Ultracal or Crystal R in the UK. This is a very hard type of plaster and is ideal for making these moulds. Plaster of Paris is completely unsuitable. Also be sure to put a piece of wire into the mouth when casting the mould to strengthen the piece. And use some fibre reinforcement such as glass fibres to help strengthen to overall mould - although I sometimes omit this as moulds for silicone do not undergo the stress of going into a hot oven.

I also sometimes forget to put in a pry space, so once I have got the mould halves apart have to scrape one out.

Use soft soap as a release agent for the finished mould.

Thanks!

Bear in mind the Genetral principle of using a hard mould to cast something soft, and vice versa.

If by mass production you really mean lots, then a plastic resin cast will be more durable than dental stone, and of course more expensive and less forgiving of mistakes. I find the dental stone casts adequate for my needs.

Well, I only need about an army's worth of faces. However, since I sculpt in polymer clay, I can make multiple moulds.

Agree with Simon - a harder plaster like Ultracal 30 or Hydrostone will help.  But the bit of mould that extends into the inside of the mouth does tend to be a weak point, so (also as Simon said) I usually place a loop of wire in the mouth to reinforce the plaster.  You can see that in this head making video, from around 1:20.  The wire will embed in the plaster, and give a little more strength to the piece going into the mouth:    https://www.youtube.com/watch?v=5fWdZnQRzB0&t=143s    That is Ultracal 30 plaster, a greyish colour, being used for the mould.  It is made in the US.

(There is also a wire in the eyes, so there will be little bits of wire sticking out of the mould to put the eye beads on when casting.)

I have glued the broken mouth piece back on once or twice - held long enough to make the head cast, but came off while I removed the cast from the mould.  Depends, if the mouth is nearly closed there isn't much surface area to put the glue on,  if it is open wider you have more surface so there is a better chance of getting it to stick.

If you are casting in silicone, a hard fibreglass mould is a good option.  Either polyester resin (cheaper, but smelly) with chopped strand mat or fibreglass cloth, or epoxy resin with fibreglass cloth reinforcing will give you a strong light mould.  (The binders in the chopped strand loosen up when put in polyester resin or plaster,  as you want them to, but they don't in epoxy resin, that's why it has to be the cloth if you are using epoxy.)

If you are casting in latex, plaster is better.

If you are casting hard heads then a silicone mould is the way to go.  But for soft flexible heads, use a hard mould.

Thanks, Nick! I appreciate the wisdom. I will order Ultracal. I think as for my material I can get away with using normal latex, as my mouths probably will be thin enough to allow it to bend.

If you are casting the skin in liquid latex, be sure to have a look at some of Richard Swensson's puppet making tutorials.  Look for Bluworm on Youtube.  He does a mix of buildup with cushion foam (like the body I made for removable heads), and cast sections of latex skin to cover areas that need the detail and precise shapes, which gives great results and animates well.    It is a fairly inexpensive approach, as well.  

You probably know this already - unlike silicone or foam latex, liquid latex does not make a solid cast.  You pour it in the mould and let it stand a while, then pour off the excess, and dry it to end up with a thin skin.  Some of the water in it is absorbed by the plaster mould, leaving a thicker layer that sticks to it, and that is what dries to make a skin. 

Is there an inexpensive silicone or some way to make foam latex without having to buy an oven for?

The best silicone is platinum based and is not cheap. It is not compatible with tin based silicone, and reacts badly to sulphur and latex, so cannot be used in the same mould.  But it is good stuff. I use Platsil Gel 00, but there's Dragonskin, Ecoflex and others out there.

The only way to make a sort of latex foam without using an oven (and never use your kitchen oven for cooking latex!) is to make a skin of liquid latex inside the mould then pour some flex foam polyurethane foam into the mould. Smooth-on make suitable foams. I have had some success with this method.

Bear in mind that although the materials are expensive, you are using quite small quantities at a time.

I might bite the bullet and buy all the foam latex stuff. Actually, what about those ovens they make for little girls to make cookies with?

For foam latex, you need scales, a mixer, and an oven, as well as the foam latex kit.  I use it for full body monsters because it is much lighter in weight than silicone, easier to paint, and because I have had the set-up for doing it for many years.  It has two disadvantages:   It can form little surface wrinkles when you compress it, especially as the puppet gets older.   Not a problem for scaley wrinkly monsters, but a bad look for a puppet of a pretty girl or a child.  (They have smooth faces, so there are no creases to guide where the latex folds, and when you close a mouth that was sculpted open, you can get a little horizontal line coming out from each side of the mouth that does not look like any wrinkles we get naturally.)  And it does deteriorate over time, pretty good for 3 years or so but likely to start drying out and cracking eventually.  

If you aren't set up for doing foam latex, silicone starts to look good.  It doesn't need a mixer or an oven.  Scales are still a good idea to get the right amounts of Part A and B, but you can do it by volume.  The disadvantages of silicone are:  It is heavy, and more resistant to bending, so for a bulky character that can be a problem.  But for just a head, it's not a big deal.  And it can be tricky sometimes, and is more difficult to get a good paint job, though there are plenty of experts on Youtube who can show you how to get fantastic results.  And it has advantages - keeps forever, though it can wear out from a lot of handling, and the surface does not form un-natural wrinkles.  It also catches the light differently, it's a bit translucent so if you backlight it you get a bit of a glow.  

Liquid latex is the cheapest option, and a good place to start as a beginner.  I made many masks, props, and hand puppets from hollow latex for years, because foam latex could not be bought here.

Here is a quick look at the process of moulding and casting a foam latex puppet - this creature is bulky and would not have worked as well in silicone.  It shows what is involved.  But a combination of sheet foam buildup, with some sections of cast latex skin like Richard Svensson (Bluworm youtube channel) does, could have produced a very good result.  

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