RTV Silicone Rubber

The molds described later are made from RTV (Room Temperature Vulcanizing) silicone rubber. Two types are covered: one-part silicone used mainly as a sealant and two-part silicone used for encapsulating and mold-making. Both make acceptable molds. A look at the advantages and disadvantages of each is necessary before considering which to use.

Silicone sealant comes in 1 to 3 ounce squeeze tubes and 10.3 ounce cartridges which fit into standard caulking guns. It requires no curing agent. Curing begins immediately upon exposure to air. It typically "skins" within 5 minutes and completely cures within 24 hours. It has several advantages:

• Toughness: A mold made from this can withstand much abuse without tear or wear.
• Availability: It is an "off-the-shelf" item found in most hardware and automotive parts stores.

Cost: The price varies considerably depending on where you buy it and in what quantity, but can be anywhere from $0.50 to $2.00 per ounce. The molds described later use only a thin layer of sealant with a plaster-of-Paris backing, resulting in an inexpensive mold.

There are some disadvantages:

• Toughness: Its very toughness makes it difficult to work with once it has cured. Cutting it to add a vent for instance requires very sharp tools and precise work.
• Workability: In its uncured state, it does not pour, so is much more difficult to work into the small details of your model. It is by design an "adhesive" and tends to stick to everything, including the tools you are using.

There are many manufacturers who make a wide variety of silicone sealants. When choosing one there are two things to look for. First it must be 100% silicone, NOT "siliconized" caulk or acrylic latex caulk with silicone. Secondly you must find its operating temperature range, printed on the package. It must be able to withstand temperatures of around 400 degrees F constant and 450 intermittent. Sealants available in auto parts stores, made for auto gaskets, withstand anywhere from 500 F constant and 600 F intermittent to 650 F and 700 F!

Two-part RTV is a pourable silicone rubber which is packaged in one pound or larger containers. It is enough for an average of ten individual molds for 15mm figures. The two parts, a resin (uncured rubber) and a catalyst (hardener) must be mixed together in the correct proportions for curing to take place. It has several advantages:

• Ease of use: As it is pourable, many steps in the moldmaking process are eliminated. The liquid flows around rather than having to be squeezed onto the model.
• Workability: The cured mold is not as, tough as a sealant one. Vents are easily added later with a sharp knife.

It also has several disadvantages:

• Cost: At about $25 per pound (retail) it is an expensive item if you are just beginning. It cannot be purchased in smaller quantities than one pound and you may have trouble buying just one pound if you try to buy from an industrial supplier.
• Availability: Actually, it is available from a variety of retail and mail order stores, now. In fact, I recently purchased some from a local hobby shop at a miniature figure show. Check out the References and Product Sources section.

Two-part RTV comes in several types distinguished mainly by hardness and temperature resistance. The softest, with a durometer of 45, has the best "creep", meaning it flows and fills the best and readily releases air bubbles entrapped while mixing. It makes the most flexible mold but also has the lowest temperature resistance at 400 F so a shorter mold life from tearing and burn-out can be expected. In general, the harder RTVs have greater temperature resistance but the viscosity also increases giving greater durability and burn-out resistance but less flexibility and some loss in reproducing fine details. They cannot accommodate deep undercuts as well as the softer type. Air bubbles also are not released as well. Your specific casting needs will determine which is best for you.

Examples of two-part RTV products suitable for reproducing fine details but having a lower heat tolerance (~ 400 F) is Dow Corning 3110 or General Electric RTV11. Examples of products in the 500+ F range, and intended for casting low melting point alloys, would be Dow Corning 3120 and GE RTV60. Both Dow Corning and GE sections in the Thomas' Register catalog volumes describe their silicone products in detail with recommended uses. The instruction booklet that comes with the Alumilite Super Casting Kit (the booklet is available for $3.95 retail), has information about all of Dow Corning's Silicone Rubber, also.

(The Model) , (The Two Part Mold) , (Table of Contents)