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Molds and materials

• White latex This type of latex is a good general latex. It returns to its shape very well. It is pre vulcanized, so all you have to do is brush or spray it on and let it dry. You can spray this latex by diluting it with distilled water. I just used my airbrush to spray it. Make sure that you clean up your sprayer soon after using it. Latex will not destroy anything, but it can clog things up. This latex tends to be on the thin side, and it can take quite a few coats to get any thickness. I have found that I can make suitable sheets to be used in bladders by spraying it on my window and letting it dry. It is important to have talcum powder handy when using latex since after it is dry, it really likes to stick to itself. Latex does not stick well to anything else, and will peel off everything without a whole lot of trouble. Latex will stick to latex, so if you are planning to make a latex positive from a latex mold, use shelac as a mold release. There is also a clear variant of this latex, though I have not used it yet.
• Latex 74 This type of latex I did not like initially. It is smelly, it does not retain its shape as well as the white latex. Its saving grace though is that it is great for making molds since it goes on really thick and dries quickly. What would take maybe eight coats with white latex, you can do with one with this stuff. I also have seen it used for masks. From what I understand, you need to make a plaster mold that is a one part, or a multiple part that can be sealed so that no latex will leak out. from there, you fill this mold to the brim with latex 74 and let it sit for a couple of hours or until enough of the latex has coagulated on the surface of the plaster. Then you pour out the remaining latex (it is still good) and let the mold sit for a day or so until the latex has fully dried. This produces a nice thick wall of latex.
• Foam Latex Foam latex has turned out to be a tricky thing to work with. It also requires that you invest in some equipment. These being: a indoor thermometer, preferrably a barometer also, a oven thermometer, a gram scale or accurate liquid measures, perferrably an upright mixer but a hand-held will work. I have used Burman four part foam latex. I have heard of a three part that is a lot easier to use. When you get your kit for foam latex, and it says that you should make sure to take notes about conditions and mixing times, this is not a joke. Pay close attention to these things. Latex foam is tricky, because if you don't get things right, it will either not gell, or it will gell in the bowl before you even get to use it. Latex foam seems to be the very bes that you can get for prosthetic appliances or a skin for your animatronic. Its cell structure is exceedingly fine and is hard to see even if you stretch it and look closely. It can stretch further than polyurethane foam. All in all, I think that it is just better.

Silicone RTV This stuff is great! This is really only a molding material, but it is easy to use. RTV costs a lot, so when you use it, it is usually a good idea to get creative and do your best to keep the volume that you use of it down. RTV can deal with some pretty severe undercuts without problems. Nothing sticks to it except for more silicone, so you never need a mold release. You can actually do a one part mold of something such as a small bust with this stuff. Just make sure that you can get a good grip on it when you pull it out. One part molds are common with this stuff. Some varieties of silicone RTV can actually have some softer metals cast in them. When pouring RTV, which by the way stands for Room Temperature Vulcanizing, you should put it in a cup or some other type of container then poke a hole on the bottom edge. Put your item to be molded under it a couple of feet. When the silicone flows out, it will then become a very fine string, and any bubbles that were in it pop. If you have a budget, the you can put the mixed silicone in a vacuum chamber to pop all of the bubbles before you pour. When pouring, do not pour directly on top of the object, rather pour beside it, and let the silicone gradually flow over it. Silicone will pick up the finest of details. I understand that doctors sometimes use it to get impressions of individual cells! Make sure to mix this well. Any streaks that are not fully mixed will not cure. In an effort to minimize the silicone usage, you can do an impression coat of silicone and then after that dries, a thicker coat of silicone mixed with cab-o-sil. You can thicken any non water-based mixture with cab-o-sil. Make sure to wear a respirator since the individual particles of this stuff are the same size of cigarette smoke.

• Polyurethane rubber This is another good rubber to make a mold with. It is a little less expensive than RTV. You can get it with different shore A hardnesses (refer to the Burman catalog about shore A and shore D hardnesses) the lowest that I have found it is shore A 30 . This seems to be a little stiffer than the RTV that I used. It seems that this breaks down fairly fast in thin areas, but I have only used it once, so I can not be sure. Overall, it seems pretty nice to work with.
• Rigid Polyurethane foam This foam is pretty nice to prototype with. It can be milled and shaped if it is of a high enough density. You can buy this in 4x8 sheets, or a local sign shop or surfboard shop may have it in smaller quantities. I find that 10 lb. density is particularly nice for prototyping. You can also get this as an expanding foam. Be careful to not totally enclose this stuff since it can develop a good bit of pressure.
• Flexible polyurethane foam This is some neat stuff. If you are building an animatronic that will not be seen closely (at least 3 feet distance) this is a great solution. It has a fairly large cell structure with bubbles about the size of large grains of sand. You can vary the flexibility of this foam by adjusting the mix of part A and B. Latex sticks fairly well to this stuff if it is not stretched too much, otherwise, the latex will peel off. This stuff sticks to almost everything as it dries. You will need to cast it in RTV or possibly coat your mold in latex then pour the foam mixture. The latex will be the outer skin of your final piece . Since latex separates from most things, that opens the possibilities of what you can cast it in. Also be careful that you do not mix too much. It expands many times the original size of the ingredients,

Plaster is really hard to use, but it is the only thing that you can do foam latex with since it is porous. With plaster, make sure that you have no undercuts. If you do not, you will have a heck of a time getting your origional out of the plaster. Plaster does not stick to plaster. If you don't quite have enough plaster mixed to finish a pour, hurry up and mix a new batch. When you pour in this batch, try to mix it some with the already poured plaster. Cold water slows down the cure time. There are different types of plaster, but the differences are not all that great. I have used Pottery plaster and Ultracal. Pottery plaster is a white plaster that is fairly easy to break . Ultracal is the recommended type to use for foam latex because it hold up to heat pretty well. Ultracal is extremely hard. Make sure that you have your origional does not float up into this stuff. I had that happen, and I spent the better part of an hour chiseling away with a cold chisel and a mallet to free my origional. You want to let your molds cool as slowly as you can if you have baked them because they will spontaneously crack after repeated baking sessions. You might want to try soaking them in water a little before you use them again to do some more foam latex.

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