1. Melting temperature of your wax -
Urethane Rubber - if your using wax that melts at between 130 F / C and 200 F / C, professional candle makers will choose a urethane rubber like PMC-744.
Advantages; easy to use and economical. Disadvantages; urethane rubber is adhesive and will stick to most original models and some casting waxes. Using sealing agents and release agents in conjunction with urethane rubber is unavoidable.
Silicone Rubber - if you are using wax that melts above 200 F / C, silicone rubber is your best choice.
Advantages; Silicone rubber resists higher temperatures vs. urethane rubber and will last longer in production. Also, silicone rubber does not stick to many surfaces so sealing agents and release agents are often not required. Disadvantages; Silicone rubbers tend to be more expensive than urethane rubbers. Also, silicone rubbers that offer high tear resistance require special processing equipment in order to successfully use the material. Specifically, an accurate gram scale for weighing components and vacuum degassing equipment to de-air the liquid rubber are required.
The exception to this is ‘OOMOO' silicone rubber which is low cost and does not require special equipment to process. The trade off is that OOMOO rubber does not have tremendous tear strength and is suitable for relatively simple shapes.
2. Model Configuration
Models that have simple shapes and limited undercuts can be reproduced using a soft to medium hardness (Shore 20A - 60A) urethane or silicone rubber. Highly complicated shapes with severe undercuts require a soft, stretchy silicone rubber such as ‘Dragon Skin' silicone. Dragon Skin is incredibly strong and stretchy and will last a long time in the production of wax castings.
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