Using Silicone Caulk as a Mold Material

16 years ago

I was reading the back posts and saw that some of you have been using silicone caulk for making molds. I have been working with a variety of silicones for some time and thought I'd post some information that might be helpful.

There are three basic types of silicone. The first two are two-part silicones which must be measured and mixed just prior to use.

1) Platinum cure (addition) silicones are extremely pure and can produce extremely accurate molds that last indefinately. They are used for medical and theatrical prosthetics, medical equipment, cookware, candy molds, and baby bottle nipples. Unfortunately, these are also the touchiest of the silicones to work with and their cure can be hindered by moisture, sulfur, latex, tin, loud cursing and bad hairstyles.

2) Tin cure (condensation) silicones are commonly used as a mold making material for art and industry. They are not approved for long term contact with the skin, or for cookware or other food contact, though some are used in making seals for potable water supplies. It is also long lasting and makes accurate molds with a life of several years. They are still very stable and safe compounds for a wide variety of applications. They will cure underwater and actually require some moisture in their chemical process. In fact, there is very little that will inhibit the cure of a condensation silicone.

3) The remaining class is one-part, self curing silicones like caulking and aquarium sealant. They are a variant of tin cure and are sold in air-tight tubes. These are further divided into two subclasses based on their catalyst:

a) Acetoxy - these are the typical ones you will find at your home centers etc. They have a strong vinegar (acetic acid) odor while curing.

b) Oxime - are referred to as odorless cure silicones and can be found in some building supply stores but are usually more expensive.

The biggest problems with using silicone caulking for molds is that it is rather thick and easily traps air, and that it will not cure properly in very thick applications. These can both be remedied in the same way.

The reason they won't cure in thick layers is that they require the moisture in air to cure. Thick layers develop an air-tight skin, effectively resealing the caulking underneath. In applications more than 1/4 inch thick, you can often come back days later and the underside will still be soft (if it is against a non-water bearing material like plasticine clay) or will cure from both sides and have a pocket of goo in the middle.

The way to solve this problem is to get some moisture throughout the silicone. You can't just mix in water, it won't mix well and you'll end up with a mess. The two products I have found that work best are glycerine (available at any pharmacy or in the health and beauty department of most chain stores) and acrylic (not oil) artists or craft paint.

Start with clear 100% silicone caulk, like GE I or GE II, or DAP 100% Silicone Caulk. Squeeze out the amount you will need into a plastic cup that is large enough to give you stirring room. For each ounce of caulk, add four or five drops of glycerine and a drop of acrylic paint. Use a wooden craft stick to stir until you get a uniform color trying to avoid trapping any more air than necessary.

You can use just the acrylic paint, but I like adding the glycerine because it improves the cure and also reduces the adhesive qualities of the silicone, making it easier to remove from the model. You could also do this with just the glycerine, but it is more difficult to tell when you have obtained a uniform blend.

Don't overdo it with either the acrylic or the glycerine as more than a few drops per ounce will result in a weaker end product. You cannot thin caulking with these materials without sacrificing a lot of the good characteristics of the silicone.

Once you get a uniform color, you have from 15 minutes to an hour before the product begins to thicken, depending on the temperature and humidity. Working in a cool dry environment will extend your application window. If you are outdoors in southern Florida, in August, work fast.

Adequate cure for handling should take under two hours and it will cure evenly throughout, rather than from the surface inward. Again, heat and humidity will speed things up.

I like to brush on a thin layer first, getting out all the air bubbles and making sure you have good contact everywhere. Then a thicker coat can be spatulated on. Usually about 1/4 inch works for palm sized items, but you can go up to an inch for very large works.

Once the silicone has cured, you can make a support shell out of plaster bandages applied a couple of layers thick. This "mother" mold will hold the flexible silicone in place during casting. Again, the bigger the mold, the thicker the mother.

Though it is probably not necessary, I like to give my newly made molds overnight to finish curing before I start casting in them.

If you feel you need to thin silicone caulking, xylene is the solvent of choice, but work outdoors and protect yourself from the vapors, they can cause health problems, so read the label. Mineral spirits will also work, but weaken the material and leave it with a greasy feel. Mineral spirits will also slow the cure from hours to possibly days.

Also, volatile solvents will result in shrinkage of the finished mold in proportion to the amount of solvent added to the silicone.

I personally like dry mold releases rather than greasy ones. For porous materials like plaster, I like to buff in several coats of Johnson's paste wax, leaving only a micro thin layer on the surface and then allowing it to completely haze over before making the mold.

For non-pourous surfaces, like glass, I take Ivory dish soap and mix it 1:3 with distilled water. Use a soft artist's brush and swish the sudsy mix over the surface of the model while drying with a blow drier in the other hand. The result should be a thin soap film that is relatively dry to the touch.

Petroleum jelly (Vasoline) will also work, but is messy to clean up. I don't like PAM or other cooking sprays.

Silicone sprays make great mold releases for everything EXCEPT silicone. They tend to become part of the mold and may actually increase adhesion.

Also avoid petroleum based lubricating sprays. They are messy and make it difficult to brush the silicone over the surface evenly.

For really difficult releases, there is a material called PVA (polyvinyl alcohol) sold in craft stores that sell acrylic casting kits used for making clear paperweights and such. Brush it on, let it dry, and then apply a thin film of paste wax. If that won't release it, nothing will.

PVA is soluable in water, so don't try to use the soap film for the secondary release.

When making a cast in a silicone mold, a release often isn't necessary as not much sticks to silicone, except more silicone. If you feel you must use a release anyway, silicone spray lube, soap, paste wax, or PVA will all work.

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