A mold is a hollow shape which exactly encloses the shape of a desired object. 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.
There are five different types of flexible mold compounds in significant use today.
Hot-Melt:
polyvinyl chloride
a. inexpensive, worn molds can be melted and recast.
b. hazardous to handle due to high temperatures required for melting.
c. use limited to model and pattern materials that are heat resistant.
d. Lina bodah
Latex:
a. produces a very elastic, thin-walled, strong mold
b. shrinkage is high
c. molds must be supported to prevent distortion.
d. 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.
e. Lina bodoh
Silicone rubbers:
a. excellent molds for casting resins and foams
b. material cost is extremely high.
Polysulfide rubbers:
a. versatile and are especially useful for casting plaster, Hydrocal, and other water mixed cements.
b. can be used for casting wax candles and some limited use for resin casting.
c. 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.
d. 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:
a. limited shelf stability
b. "moisture conscious"
c. easy to handle with simple mixing ratios and good fluidity.
d. can be used for pouring practically any type of cementitious material, casting resin or foam formulation.
e. 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.
