This tutorial is for all the serious mould makers and explains in high detail all you need to know to create advanced silicone rubber moulds for various applications.
This in-depth tutorial explains the entire silicone rubber moulding process with simple to follow step by step instructions to make a mould of an apple.
Silicone Moulding Rubber supplied by Crafty Corner is a two part tin cure silicone rubber elastomer cross-linking at room temperature by a poly-condensation reaction. The resulting product is flexible, elastic and gives excellent tear resistance.
Silicone Moulding Rubber is ideally suited for detailed reproduction.
Moulds made from Silicone Moulding Rubber are used in the manufacture of pieces in polyester resin, plaster, cement, polyurethane, candle making, wax and soap.
It has special applications in:
Collected or ordered from our offices in Randburg, Johannesburg, Gauteng.
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082 855 2236
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Silicone rubber is an ideal material for making moulds of lifecastings and other objects used in sculpture, special effects and taxidermy.
As with liquid latex, it yields a light, flexible, detailed mould, but has the added advantages of longer life, resistance to chemicals and decomposition. It is the recommended material for making long-lasting moulds. A silicone mould also can be made in less time than a latex mould, particularly if “fast” catalysts are used.
Among silicone's few disadvantages is that it is more expensive than latex, and not quite as elastic or tear resistant. The most common silicone compounds used for mould making are RTV or "Room Temperature Vulcanizing" silicones that are mixed in two parts (a base and a catalyst) to induce curing. The silicone mixture is poured or spread over a prepared model or specimen, then reinforced with gauze or other reinforcing material between layers for increased strength and tear resistance.After the silicone mould is cured a shell mould is often constructed to provide rigidity for the rubber after it is demoulded. The shell mould can consist of fiberglass, plaster or urethane. The shell mould is often referred to as a “mother mould.”
When making moulds in a studio where such equipment is available, de-airing in a vacuum chamber is recommended to remove trapped air bubbles. However, when working on larger moulds, vacuuming can be avoided by painting the silicone over the model. The brush will tend to remove the surface air.
Safety Note: RTV silicone rubber compounds are relatively safe and odour free when properly used; however, the curing agents or catalysts may be toxic if ingested and are irritants to eyes and bare skin. Tinbased silicone cannot be used against the skin. Certain platinum-based silicones are deemed skin safe.
There are two common classes of RTV
1.) Tin catalyzed or “condensation cure” silicones which require moisture to cure and;
2.) Platinum catalyzed or “addition cure” silicones.
Silicones in the first group are the less expensive and easier to use. They are typically of low viscosity (easily poured) and are not inhibited by many materials. In contrast, platinum cure silicones (often called “elastomers”) are inhibited by many naturally occurring materials, including sulfur, tin, and amines. This makes them unsuitable for certain clays, which contain sulfur or latex gloves. However, platinum-cure silicones have the greatest chemical, microbial, and temperature resistance. LifeRite, the skinsafe silicone, is a platinum cured silicone.
Silicones in the tin group are often used for low-volume plaster casting; condensation cure silicones are usually foolproof and typically have a 100:2 to 100:5 mixing ratio by weight and once mixed, cure against most mould making media. It uses the condensation in the air and gives off alcohol as a by-product. Since there is evaporation in the form of the alcohol, there will be shrinkage of the same amount – approximately 0.2-0.5%.
Silicone is stored in the original unopened packaging at a temperature between -5 - + 30°C, the product may be stored almost indefinite.
Catalyst should be stored for no longer than 6 months as the product starts to crystallise due to active ingredients. More catalyst can be ordered from us in the event that you have exceeded the self life.
Thinners or "diluetants" are available for thinning or decreasing the viscosity of some RTV silicones. However, they are expensive and do not seem very effective (one must add a large amount of diluetant to achieve a small effect on viscosity, and thorough mixing is difficult and time consuming). Diluents also weaken the cured silicone. Therefore you should purchase silicone in the desired viscosity rather than using diluetants.
Mix both of the two components well before use.
Common mix ration of 100 parts of silicone moulding rubber add too 2 parts catalyst, unless specified by manufacturer or agent.
Mix the 2 components thoroughly by hand using a flat blade to ensure that minimal air bubbles are introduced.
Working time is 25-40 minutes depending on ambient conditions, such as temperature and humidity.
The best curing conditions are at 23-25 °C at a relative humidity of 50%. Higher levels will reduce the pot life and increase the curing rate and lower levels will increase the pot life and decrease the curing rate.
At 23°C the de-mould time will be 4-5 hours, but this could vary depending on humidity levels.
Because of settling during storage, always stir the silicone in the original container before pouring into a mixing cup. Stir steadily with a circular motion rather beating it or using an up-and-down motion, to avoid trapping air bubbles, scraping the bottom of the can to loosen any settled material. Then let the silicone rest for a few minutes or more to allow air bubbles to rise to the surface (this small resting time will not cause any re-settling of the silicone itself).
Tip: Tongue depressors or craft sticks make good, cheap stirrers for small batches, while paint stirrers make good stirrers for larger batches.
Next, pour the desired amount of silicone into a separate mixing container, such as a plastic cup or wax-free paper cup. Select mixing containers with relatively straight bottoms and sides, and little or no inner lip to make stirring easier. Use a gram scale to weigh out the amounts for the proper manufacturer's mix ratios. Add the catalyst to the base and mix thoroughly scraping the sides and bottom of the container to get a thorough mix.
After measuring the proper amount of catalyst into a separate mixing container, mix in the catalyst with the base. If you are not in a hurry, it is best to use the recommended dose (or even slightly less) of the standard catalyst. One need not be extremely precise in measuring the amount of catalyst (with practice an "eyeballed" measurement is normally sufficient). One can be off by 10- or 20-percent without much effect in the cure or final product (a little more will speed the cure, a little less will slow the cure). Cure time is also somewhat affected by temperature and humidity (heating accelerates the cure). Despite the latitude in the amount of catalyst used, there are minimum and maximum limits. An excessive amount of catalyst may not allow enough work time and can result in a brittle mould; too little may cause incomplete or uneven curing.
One has at least 20-minutes of available "work time" time with most standard catalysts before the silicone starts to cure or "set up." Stir well for at least two- or three-minutes, and scrape all parts of the container to achieve a thorough mix. However, avoid overly vigorous motions that can introduce air bubbles. After mixing, one may let the silicone rest a few minutes to allow air bubbles rise to the top (which should be broken before pouring), or one can use a deairing vacuum chamber if available. After mixing, it is best to re-pour the silicone into a third container, to avoid using any of the poorly mixed silicone that often exists at the bottom and sides of the container.
Using special fast catalysts the cure times can be reduced substantially, sometimes to an hour or less.
If a vacuum chamber is available, use it to remove trapped air from the mixture before pouring. When subject to a vacuum, the silicone mixture should well up as air pockets rise and burst. As soon as the material settles down, proceed to the pouring/application procedures described below. If a fast catalyst was used, you must work quickly to avoid having the silicone cure before it is applied.
One of the main concerns when applying silicone to a subject is to avoid trapping air bubbles on the surface. Besides choosing a low viscosity free-flowing) type of silicone, there are several techniques that are helpful in this regard.
Using any of these methods, you may need to manually push the silicone around a bit to encourage even coverage (using a small tool such as a craft stick), and/or repeatedly pull it from the deeper to the shallower sections (where it tends to pool). As the silicone cures it will begin to "stay put."
If you find there is not enough silicone mixed to cover the model to a good depth, it is better to spread it in a thin layer over the entire model, and then apply a second batch over it, rather than to cover only part of the model and then fill in the second batch (the latter method tends to leave small seams between the pours). To reinforce the mould (and help avoid tearing), one may embed gauze, open-weave cloth, or nylon screening between layers, as described below.
After the silicone has started to cure, but while it is still tacky, one may gently apply strips of gauze or cheesecloth to increase the strength of the mould, especially if two layers are being laid down. This strengthens the mould against tearing, a common silicone mould problem. Be sure not to push the gauze through to the model surface. After the gauze is applied, apply another layer of silicone to thoroughly cover the gauze layer. Typically the finished mould should be at least 3-4 mm thick even in the thinnest sections. One may also wish to only reinforce the edges of the mould (especially any thin edges), which are more prone to tearing than the main body of the mould. When selecting gauze, be sure to get the non-elastic type (the “stretch” kind commonly sold today tends to bunch up). If you cannot find the old fashioned conforming gauze in rolls, an alternative are the gauze bandage pads that can be unfolded to a square sheet, and cut as desired.
After the prescribed cure time has lapsed and the outer surface of the mould feels firm and dry, the mould is usually ready for removal or “demoulding.” However, if the mould has any deep areas, or time is not of the essence, it is best to wait a bit longer to ensure that all areas of the mould are fully cured. When removing the mould, gently peel up all the edges first, and then the middle section. If any portion seems too soft, immediately stop pulling and replace any lifted sections, allowing more time to cure further.
Regardless of whether any gauze reinforcement is applied, most moulds will need some type of rigid supporting structure (sometimes called a "mother mould", “backing,” or “jacket”) to ensure that the original mould keeps its shape during storage and casting. Small, uncomplicated moulds, or ones that can be poured so that the backside is level, may not need such support. However, in other cases a mothermould is recommended.
The mother-mould may be made of expandable foam, casting plaster, urethane plastic, or fiberglass resin. Before pouring the mother mould material, make sure the original mould is fully cured, lifted off the model, and then replaced back onto it (otherwise it will be difficult to lift the original mould off the subject).
The cheapest and fastest way to create a mother-mould is with a plaster jacket. The plaster may be simply poured over the mould (with a retaining walls created to contain it if necessary), or applied in "plaster bandages"(strips of burlap or open-weave cloth soaked in plaster slip). However, plaster jackets are heavy, prone to breakage, and have no flexibility (which may be needed on some moulds).
After removing the mould, be sure to clean it and the specimen before storing either. If you used Vaseline or other oil-based release agents, or silicone without a release agent, the residue or discoloration can often be removed or reduced by washing the specimen with kerosene or other petroleum solvents. Avoid getting solvents on your skin, and always use plenty of ventilation.
Be sure to test an unimportant specimen first to be sure the solvent does not dissolve the matrix or otherwise worsen the situation.
Rigid casts may be made from silicone moulds using common casting materials such as Plaster-of-Paris, Hydrocal, Forton MG, plastics, resins, epoxies, cement, or other materials. Many silicones can even withstand low-temperature metal casting (check manufacturer’s product literature for temperature ranges). Normally a release agent is not needed for plaster casting, but for urethane and resin casts, or complicated moulds with severe undercuts or many crevasses, a release agent such as thinned vaseline (dilute with parrafin) and or barrier coat (such as a varnish or paint applied to the mould before casting) is advisable. Depending on the nature of the mould, one may need to build a frame around it to retain any overflow casting compound. Mix and pour the casting material according to manufacturer's instructions.
After the cast has hardened completely, slowly peel the mould from the cast. Work around all the edges before pulling up the middle sections. If the cast is deep, the remaining volume may be filled with expandable foam if desired, the same way it may be used to fill the volume of deep moulds.
Colouring silicones can be a challenge. But with a little knowledge and some patience you can master it. On the surface (externally), the trick is to use the correct silicone-based paints.
To colour silicones internally, you can add just about any pigment or dye to tin silicone as tin silicone is very forgiving. It will cure under just about any circumstances. The easiest pigments to use for translucent silicone are actual artists’ oil paints pigments (but not water-based oils). These oil paints can be purchased at any art store.
But you can paint silicone by using a silicone base as other paint types will not stick to the surface and easily rub off. You can create your own paints using a translucent silicone as a base and simply adding pigment or dye. If your piece was made out of translucent silicone just mix up a small batch of same, thin it with any solvent, add your pigment then paint.
When mixing your own paints, recommended proportions vary. Add pigment to achieve desired colour, but do not over do it. Typically, usage is from slight traces to 4% by weight.
The most effective and controllable way of painting the surface of silicone is with an airbrush. You must dilute your silicone paint with an airbrush solvent made for silicone.