OOGOO Silicone Casting or Molding

Learn to use commonly available and low cost materials to make a firm but flexible, soft but strong, skin like sculpture, mold, or cast object.

"Oogoo" is 100% silicone RTV caulking, tricked into forming like bread dough and then hardening slowly and completely, by adding corn starch. It can be pushed into a mold to cast a flexible, soft, yet firm reproduction of an original object, or it can accept an object to form a flexible mold for the rapid reproduction of a hard object. The original object or the mold can be 3D printed, or can be produced from an existing physical object. It can also just be molded by hand into any imaginable shape, customized by the artist as they wish.

Chemistry

100% Silicon caulking is cured via moisture which normally occurs on contact with air, after being applied in thin layers. e.g. around the bathtub. However, in thicker applications, the moisture from the air can't penetrate to the inner layers of the material, resulting in a thin solid shell with a gooey, unset center. It also tends to harden (on the outside) very quickly, which wouldn't give the user time to form it correctly, and it will not harden anywhere that it's container (mold) prevents contact with the air. The hack is to mix it about half and half with cornstarch. The fine particles of the starch soak up a bit of moisture naturally and then trigger the setup of the silicone throughout the well mixed area. This also causes the moisture to seep into the silicone slowly, and prevents the setup from happening quite as fast, thus allowing the artist time to work the material.

The time it takes to set can be controlled to some degree by changing the ratio of the components. More cornstarch will cause it to setup faster. Less will cause it to setup slower. Sources quote 3 to 5 minutes for 50:50, and 30 to 45 minutes with 25:75 (1/4 cornstarch to 3/4 caulking). My experience has been that it sets up when it wants to.

To control the amounts, measure out the cornstarch first, comparing it by volume to the caulking you expect to use before you squeeze that out. Then use up all the decanted cornstarch when mixing.

Comparison to Commercial Silicone 2 Part

Compared to 2 part silicone casting chemicals, oogoo is nearly the same in it's material properties with few disadvantages: It is not quite as strong, as it lose a bit of internal adhesion due to the cornstarch. It is not available in as wide a range of setting times, firmness, and color as the commercial versions. There do appear to be some advantages (other than price) however; because the two parts are kneaded together like bread dough rather than being mixed as the liquids must be, fewer air bubbles are trapped and there is no need (or ability) to vacuum treat the setup to remove them.

Colors

Colors can be added by including just a few drops of linseed based oil paints. Color should be added to the silicone and mixed well before the cornstarch is added to ensure contiguous color. However, some righteous riots of color can be created by adding drops of different colors at the end of the kneading process. You can also just paint the object before it finishes setting.

Cautions / Safety

Odor/Ventilation while setting: One serious (but no too serious) concern about oogoo is that RTV type silicones set by a chemical reaction which releases acetic acid. That sounds really bad, but it's not. Acetic acid has another name: Vinegar. That's all it is. But if it isn't allowed to dissipate, the vinegar smell can get /really/ strong! So be sure to do this in a space with good ventilation. It doesn't need to be outside, but open a window if you can, or at a minimum have a small fan blowing the air past, but not on, your work. And while it's setting up, don't sniff it.

Gloves while setting: You can use your hands, or you can wear rubber, latex, or nitrile gloves. If you have very sensitive skin, or are overly worried about vinegar, that's probably best. I just use my hands because I'm stupid I like to feel what I'm doing. You should wear gloves because in addition to the vinegar, there could be anything in that caulking; it's not mfgred for food or skin safety.

Skin contact: The final safety of the finished product is another potential issue. There are food safe / food grade commercial silicone two part mixes. Hardware store caulking is not designed for that, although I have seen some advertised for kitchen counters. Having said that, the finished item will be relatively solid, and I have never noticed any weeping or off-gassing, so there should be no transfer onto you after it sets. I would strongly recommend using a covering of some sort if your object will be used against the skin or inserted anywhere; not only to protect yourself, but to protect your artwork from picking up and holding odors, pathogens, or whatever else it might pick up from you. Protect yourself, protect a friend.

Preperation

The biggest issue with using oogoo is the mess. Cornstarch is a fine powder which will go EVERYWHERE if you sneeze or drop a book next to it. The caulking is sticky in a way that sort of defies description. However, with care, the combination is lovely; the sticky cancels the dusty and the result is just like a nice firm bread dough. In fact, if you've ever made bread, you have an advantage in this class.

Kneading: The best way I've found, is to put a thin layer of cornstarch down on the bottom of a a bowl, plate, or pan. Then push out a winding bead of the caulking in a single layer on top of the starch. You don't want a big glump of it. Next, add a solid layer of corn starch about half as thick as the bead. Now, carefully press and (slowly) pat the starch down into the caulk. Pay careful attention to the feeling of the surface you are touching, and the instant you feel anything sticking, move to the side or re-cover that area with starch. When you have flattened it out to about half it's former thickness, cover your fingers in cornstarch and carefully peel up and fold the material in half. Add a bit more cornstarch anywhere it looks sticky, then again press / pat it down. Alternate folding from each side, top, bottom, left, right, until the entire mass is consistent and not too sticky to the touch. You may find that you need to add more cornstarch than you intended to get it to a point where it is manageable.

Mixing: Another way to do it is to start with a glump and a dump of each material in a cup and then mix that with a stick. This may lead to more air pockets which can show up to deform your final mold or casting. It's typically not an issue if you are hand forming an object.

Mixing then kneading: And you can combine those methods: Start mixing, then move to kneading to try to work out the bubbles.

Use

Now you can move the material into a cup or other container if you are going to use it to mold an object, or you can press it into a mold, or you can just form it with your hands. You can also form it around an object by hand but you will need to keep it from sagging and hold it still for as much as 15 or 20 minutes to get a good solid mold. This can be a challenge, but it is possible. Try using more corestarch for a faster setting time. Using some container to hold the mold while it sets is valuable because then after you remove the mold, and remove the original object from it that you are molding, you can put the mold back in the container and it will support the sides and keep the mold in shape when you are casting the next part. If removing the original or cast parts is difficult, you can cut down the side of the mold to provide a way to peel the mold back and release the object.

MOLDING: If used as a mold for solid objects, you can massively increase the speed of duplication of object over 3D printing. E.g. you can print one object, then made an oogoo mold of it, then cast more objects quickly. You can make multiple molds to cast multiple objects at once, instead of grinding out one item at a time on a printer. You can also just use a purchased mold to make any shape available. Chocolate molds are excellent as they are strong enough to hold the shape well.

CASTING: If you make a solid plaster or other mold of an object, you can then cast a final object from oogoo. The applications are endless, from pot holders, to "rubber" feet, to robotic pneumatically actuated tentacles. ^1,
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SCULPTING: The oogoo can be worked by hand and formed into any shape that pleases you.

OOGOO Silicone Casting / Molding