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5 Frequently Asked Molding and Casting Questions

December 27, 2016

Here are 5 questions that we get asked on a weekly basis. As always, if you have any questions, you can contact us at [email protected]

  • How do pick what type of material will be best to make my mold?

The type of material you will make your mold from depends on what you will be casting into the mold. Are you replicating a garden gnome and casting in concrete? Are you making chess pieces and casting in a urethane resin? Are you making candles and working with wax? The end product is what determines the molding material to use.

For concrete, Plaster of Paris, and Hydrocal, liquid latex is what you would use for a brushable mold. Our 75A urethane rubber would be used if you wanted to make a pourable block mold. The reason for this is the abrasion resistance of the latex and and urethane rubber. These materials are able to withstand repeated castings of coarse materials.

If you are going to be casting with an epoxy resin, urethane resin or wax, you will want a material like silicone mold making rubber that has a good heat resistance as all of those casting materials are warm to hot while curing.

See All Epoxy Resin Products

The other general rule of selecting a molding material is the hard/soft dynamic between the casting material and mold. If you are going to be casting a hard part (most common), you want the mold itself to be soft and flexible. That way, the mold will give and flex for the part to come out. If you want your piece to be cast from a soft material, like a silicone, you would want the mold to be more rigid and the part will flex to come out of the mold. It is extremely difficult to remove a hard cast from a hard mold or a soft part from a soft mold. There are some exceptions to this rule, mostly when working with fiberglass and resin, but it is a good rule of thumb.

  • What is the shelf life of my product?

All of products have a use by date printed on their label. This is usually six months from the packaging date. That is how long we expect the product to reliably last. However, some products may last longer on the shelf than others.

128 Silicone with pourable or brushable catalyst The white silicone base will last at least six months if stored in a closed container in a room temperature (68-72 F) environment. Sometimes it can last up to year, if it isn’t opened and closed too often. By 18 months it is usually too viscous to work well. The catalyst has a similar shelf life, always good up to six months, sometimes lasting up to a year.

125 Silicone, simple 1:1 mix ratio– This silicone will last on the shelf, UNOPENED, for six months. However, once you open this product and expose the “A” side to air, it needs to be used within sixty days. This is because the catalyst is in the “A” side and it will start to cure slowly once opened.

Latex–  If stored in a closed container in a room temperature environment, the mold making liquid latex will last at least six months, sometimes 12 to 18 months. However, if latex gets too cold, it will be ruined. Do not store the latex in environment where it will get below 50 F. So unheated garages and sheds during the winter and fall months are not a good idea. If your latex looks chunky, like cottage cheese, it has gotten too cold and needs to be thrown out.

Urethane Casting Resins– If stored in an air tight container, it will last six months. Urethane is very moisture sensitive before it is are mixed and cured, so using it on a rainy or humid day, using paper or wood mixing utensils and/or containers can cause moisture contamination that leads to bubbles in the urethane. You can make sure your casting resin lasts by using a dry gas nitrogen blanket (like X-Tend it) to displace the air in the bottle after every use.


See All Urethane Casting Resins


Epoxy Resins– Epoxy resins and hardeners can last for years on the shelf. We label it with a six month use by date, but it doesn’t really go bad. Sometimes the resin side will freeze (at about 50 F) and become crystallized and cloudy looking, but it just needs to be warmed up to return to its former clarity with no impact on the resin’s strength and durability. The 21 hardener will darken over time, turning more yellow. This is purely a cosmetic issue which doesn’t affect the strength or durability of the final product.

With all of these products, if it is past the use by date on the label, a small test batch will tell you if the product is viable.

  • How should I store my molds when not in use?

Molds should be stored in a room temperature (68-72 F) environment, out of direct sunlight, in an air tight bag or container. If possible, it always best to store the mold with a cast piece in it. Glove molds should be stored with their mother molds in place. Latex molds are especially sensitive to heat and should never be stored anywhere where the temperature could reach over 72 F.

  • How long will a mold last?

Production life ( the amount of casts you will get from a mold) is hard to predict. Things that affect production life include:

Mold release will prolong the life of your mold, especially if used every time you do a cast.

Detailed and/or complex molds don’t last as long as simple molds.

Molds stored properly will last longer than molds just tossed on a shelf.

The materials used in the molds will also contribute to production life. Generally the hotter the casting material becomes while curing, the harder it is on the mold. Also more abrasive materials will wear the mold out faster.

Library life (how long a properly stored mold will last when not in use) is easier to predict and are as follows:

Tin catalyst silicone molds (like our 125 or 128 silicones) last about 5-7 years when stored properly

Platinum catalyst silicone molds (like our Food Grade silicone or silicone putty) last 20+ years when stored properly.

Latex molds last 10-20 years when stored properly.

Urethane rubber molds (like our 75A urethane rubber) last 20+ years when stored properly.

  • Can I color or paint the urethane casting resin?

Yes, our urethane casting resin can be colored. We carry white, black, red, yellow, and blue colorant for the casting resin. Any colorant for use with urethane will work with our casting resins. People frequently use powdered pigments and dyes in the casting resin. Always mix up a small test batch to make sure you are getting the color and result you want.

The casting resin can also be painted. For best results, dust or submerge your part in talc for 12 to 24 hours, and then wash with a liquid dish soap (like Dawn) with cold water and pat dry. Do not sand after the talc and washing. Some people also like to use a primer such as Plasti-Kote, a sandable primer that works well and can be found at most auto supply stores. Acrylic paints work very well. Other types of paints should be tested first. For a spray paint, Krylon Fusion works great.

Hopefully this information has been helpful to you.

From all of us at AeroMarine Products, have a lovely holiday!



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