Standard Procedures for Making a Silicone Mold

With silicone being a popular and useful mold making material that delivers variety of benefits, artists should be aware of the basics when working with this rubber. Following are some tips on the same.

There are varied options of materials when it comes to making molds. While any malleable material like clay, wax, alginate or resin can be used to create a negative of the master model, rubbers have emerged as the material of choice, especially for professional mold makers. Then again, while there is a choice of polyurethane, latex and thermoset mold rubbers, silicone is preferred for many a reason.

To start with, mold making silicone is easy to use. The material has sufficient working time and cures fairly quickly to deliver durable, tear resistant and heat resistant molds. In fact, the molds will last for years and can be used to make any number of castings.The molds do not shrink much either. A variety of materials – like wax, plaster, gypsum, resins and even low-temperature melt metal alloys – can be cast in a silicone mold. Demolding is also easy as silicone does not stick to anything except itself.

Basic Preparation

Silicone rubber can be used to make molds of anything from figurines, statues, architectural pieces and picture frames to soaps, candles, toys, jewelry and more. The applications also include conventional prototype tooling and stereolithography.

The silicone has to be mixed with the catalyst in the specified ratio prior to use. Both the base and catalyst should first be stirred in their own containers before weighing the required amount of silicone base into a clean mixing container. A useful tip is to tilt the container and roll the material all the way around the sidewall, leaving about two inches from the top. This may sound unnecessary at first, but the coating will work to keep the catalyst from getting absorbed into the container, thus giving a better mix.

After this, the required amount of catalyst can be weighed into the container. Mix the two together by stirring with a stiff, flat-ended metal spatula until a uniform color is obtained. It is better to keep scraping the container walls and bottom to insure a thorough mix.

While artists usually vacuum degas the silicone rubber mix, it is generally not needed for most applications as a uniform flow into the mold box is enough to minimize entrapping of air. In case it is being deaired, the mixing container should be filled only to one-third of its depth to allow sufficient room for expansion during the deaeration.

Additionally, it is noted that low temperature and humidity increases both the work and pot life of silicone rubbers. Some artists even opt to refrigerate the base material before using it in hot environments.

Finally, the catalyst container should be closed tightly after use as exposure to air for an extended period can cause a film or crust to form on the catalyst and this hydrolyzed material will lead to improper curing.

MoldRite 25 Silicone is a popular material for general silicone mold making. The quality of impressions captured by these silicone molds is exceptionally outstanding.

Effect of Temperature on Mold Making and Casting

Temperature can play havoc with the curing of molds and casts by delaying/accelerating the process or not allowing the material to cure at all. Knowledge of the properties will ensure better results.

It is not just about the technique or dexterity of the artist; the quality of the mold or cast is affected by many other characteristics as well. Temperature plays a starring role here – ranging from the temperature and humidity of the surroundings to even the warmth/coldness of the material and model/mold.

This is why it is always recommended that all rubber and resin compounds should be stored at room temperature (72°F/23°C). For instance, if a material is stored at elevated temperatures, like in a hot garage or in direct sunlight, both the shelf life and pot time will get reduced drastically. In contrast, if latex rubber freezes, it becomes unusable and has to be thrown away. Other frozen materials can still be used after they are brought back to room temperature.
Let us take a look at how the temperature factors can affect the making of molds and casts:

Cold – A cold environment will usually prolong both the working time and cure time of most materials like epoxy resins, urethane rubbers and platinum cure silicones. The evaporation process of latex rubber also gets delayed in colder temperatures. In case the environment is too cold, some of these materials may even fail to cure at all. However, tin-cured silicones are not as dramatically affected by colder temperatures.

Heat – The opposite is also true as higher temperatures are known to accelerate the cure time and most materials tend to cure much more quickly when it is hot. In fact, many artists deliberately apply heat to hasten the curing process. However, tin-cured silicone rubbers are again an exception as they are not as affected by heat and cannot be ‘heat cured’.

Humidity – Rubbers are best used in a low humidity environment. Higher humidity tends to accelerate the curing in tin-cured silicone rubbers while it has the opposite effect of slowing the evaporation and curing process in latex rubbers. Making molds or casts of urethane rubbers, plastics or foams in humid environments can cause bubbles or foaming in the material. However, platinum-cured silicones and epoxy materials are not affected by humidity.

In addition to this, if the model that is being used to make the mold happens to be too cold, the curing of the mold will take unnecessarily longer to cure and vice versa. Therefore, the model/mold should be brought to room temperature prior to use. When making a mold of a frozen model, it will start condensing once the mold material is applied which will in turn delay the curing. This is why it is better to use accelerated silicones as they will not be affected by the moisture.

Finally, good quality liquid latex rubber, silicone or epoxy materials can be easily sourced from EnvironMolds at at the most reasonable prices.