Mold Making

Lesson 3: RTV and Alginate Molds

Mold Overview

Molds can be used to create multiple casting of one original piece. There are several types of molds. Most industry made jewelry molds are vulcanized rubber molds. Rubber is put under both heat and pressure until it hardens, yet retains its flexibility. This process helps increase the life of the mold. Vulcanized rubber molds last a long time. However, these molds are unsuitable for molding anything that cannot take heat or pressure.

For molding items that are heat or pressure sensitive, RTV (room temperature vulcanizing) molds are the best option. RTV molds can be either silicone or polyurethane plastic. Depending on the mold material, these also have a long life, and require less equipment to produce. Alginate and brushable latex can also be used to create molds.  

Standard molds are comprised of 2 halves. The halves fit into each other snugly with out movement. Brass “buttons” can be used to register the halves, or tabs cut into the mold. These are called “closed” or 2-part molds.

Depending on the original, you can also make an “open” or 1-part mold. These molds are easier to produce since they don’t require cutting or separating the mold, but can only be used on pieces that don’t have a lot of undercutting.

In order to create multiples of a piece, the mold is injected with molten wax, creating a wax version of the piece. Multiple waxes are created, and then each wax is cast.

A wax injector is used to shoot the wax into the mold. Simply pouring the wax into the mold will not get a full impression of the piece. The wax cools too quickly for that. By using compressed air to get force behind the wax as it comes out of the injector, you can be assured of complete waxes.

Open molds can be filled by pouring wax into them.

Both molds can be used for casting other materials than wax.

Alginate Molds:

• Mix the alginate according to box instructions. Add the water to the powder. It should remind you of pudding when correctly mixed.
• Using cold water extends the setting time, using hot water shortens the setting time.
• Spread the alginate over the object to be molded.
• The alginate will thicken as it cures- usually 5-15 minutes. It will begin to separate from the object as it cures.
• Peel the alginate off of the object.
• Your alginate mold is ready to use.
• Alginate molds have a short life. They are best for life casting and taking molds of anything with fine detail. Alginate degrades over time and with use and the mold is best used within a few days of being made. If you are planning to make a lot of waxes from the alginate, you may want to take multiple molds.

RTV Molds:

• Read the instructions for the compounds. RTV Molds are usually comprised of a Part A and a Part B (activator or catalyst). You need to check to see whether the molds need to be vacuumed during mixing and what the mixing ratio is.
• Make sure that your mold frame is clean, and spray with mold release.
• For 2-part molds, use U-frame molds. For 1-part (or open back) molds, you can use any container.
• If making 2-part mold, attach sprue and funnel to original.
• Spray original with mold release.
• Arrange original in the mold frame. Check that original is placed so that it is only touching the frame at the designated opening (for 2-part) or attached firmly to bottom of mold frame (1-part)
• Mix mold compound. Wear gloves. Measure very carefully and stir completely and thoroughly.
• Pour compound smoothly, slowly, and evenly into frame.
• Let mold cure.
• Once cured, remove mold from frame. Thin flexible spatulas can assist in this.
• If a 2 part mold, separate and cut apart.
• Trim any excess from the mold.
• Your RTV mold is ready to use!

Mold Rubber:

• Silicone rubbers come in two types- platinum and tin cure. Check the mix ratios and whether they are by weight or volume before buying. Also check on strength and cure time. Some silicone rubbers require a vacuum chamber as part of the mixing process.
• Polyurethane rubbers are generally stiffer than silicone rubbers and usually less expensive. 
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