How to Calculate Silicone Weight for Mold Box Castings

Creating custom silicone molds requires accurate volume subtraction. A standard block mold consists of a container box containing a core object (the master model) that is submerged in liquid silicone rubber. To avoid wasting expensive silicone, you must calculate the volume of the outer mold box and subtract the volume of the object inside it.

Silicone Volume (ml) = Mold Box Volume (ml) - Master Model Volume (ml)

Once the volume in milliliters is determined, you must scale it by the specific gravity (density) of the silicone rubber preset. Liquid silicone is heavy; typical systems have a specific gravity of 1.18 to 1.34 grams per milliliter depending on the fillers used. This means 1 liter of liquid silicone weighs up to 1.34 kilograms.

Platinum Cure vs. Tin Cure Silicone

When formulating molds, makers choose between two chemical curing systems:

• Platinum-Cure Silicone: Standard for food-grade, skin-safe, and high-durability applications. It offers virtually zero shrinkage and exceptional chemical resistance, making it ideal for resin casting molds. However, platinum silicone is sensitive to cure inhibition from materials like sulfur, clay, and latex.
• Tin-Cure Silicone: Typically mixed in a 100:10 ratio or 10:1 ratio. It is cheaper and resistant to cure inhibition, making it suitable for casting plaster, concrete, and wax. However, tin-cured molds shrink slightly over time and degrade faster.

Minimizing Overfill Waste

Liquid silicone does not expand or contract significantly during the curing cross-link process. A waste factor of 10% is recommended to account for the residue left on the walls of mixing cups and stirring sticks. Measuring Part A and Part B by weight on a digital scale is the only reliable way to guarantee standard cure speeds and structural elasticity.