A mold is a hollow shape which exactly encloses the shape of a desiredobject. The object is usually created by pouring a liquid into the mold and allowing it to solidify: typical liquids include molten metal or plastic, plaster of Paris, epoxy resin. Molds generally divide into two classes: solid or flexible.
Types
There are five different types of flexible mold compounds in significant use today.
Hot-Melt
An example of hot-melt material is polyvinyl chloride. These are inexpensive, and worn molds can be melted and recast. It can be hazardous to handle due to high temperatures required for melting. The use of these materials are limited to model and pattern materials that are heat resistant.
Latex
Latex produces a very elastic, thin-walled, strong mold but the shrinkage is high. Latex 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, so making the mold is slow and time consuming.
Silicone rubbers
Silicone molds are excellent molds for casting resins and foams but the material cost is extremely high.
Polysulfide rubbers
Polysulfide rubbers are versatile and are especially useful for casting plaster, Hydrocal, and other water mixed cements. It can be used for casting wax candles and some limited use for resin casting. It is 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. it 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
Polyurethane compounds have limited shelf stability, are "moisture conscious" and can be used for pouring practically any type of cementitious material, casting resin or foam formulation. It is easy to handle with simple mixing ratios and good fluidity. 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.
