5 different types of flexible mold compounds in significant use today.
• inexpensive, worn molds can be melted and recast.
• hazardous to handle due to high temperatures required for melting.
• use limited to model and pattern materials that are heat resistant.
• produces a very elastic, thin-walled, strong mold
• shrinkage is high
• molds must be supported to prevent distortion.
• 10 to 20 brush coats must be applied for adequate thickness, with time allowed to "dry" in between each coat, making the mold is slow and time consuming.
• excellent molds for casting resins and foams
• material cost is extremely high.
• versatile and are especially useful for casting plaster, Hydrocal, and other water mixed cements.
• can be used for casting wax candles and some limited use for resin casting.
• easy to handle, non-adhesive and can be poured over clay or practically any type of model or pattern, using very simple release agents such as soap or Vaseline.
• can be used to take impressions directly from a waterclay or plastelene model, so that permanent master models can then be cast in plaster or Hydrocal.
Polyurethane flexible mold compounds
• limited shelf stability
• "moisture conscious"
• easy to handle with simple mixing ratios and good fluidity.
• can be used for pouring practically any type of cementitious material, casting resin or foam formulation.
• Cost of material is far below that of the silicones and appreciably lower than polysulfides.
From the standpoint of general utility and economy, the polyurethanes surpass all other types.