The process of latex setting up is called vulcanization.
Vulcanization describes the process of heating the latex and setting it at a high temperature so that all the molecules of latex unite and form a single, continuous surface.
When the latex is heated between 100°F -120°F (38°C - 48°C), for a period of 8 to 12 hours, the process of vulcanization becomes complete, yielding a high-quality mold. Plaster and concrete can be poured with absolute confidence.
Time also promotes vulcanization. A mold can be left at room temperature, kept constant for five or six days and will vulcanize by itself. Still, heating is actually the best way to cure, or vulcanize, a latex mold.
Below is a list of some of the problems that beginning mold makers have had with latex. None require more than a bit of attention to avoid.
DELAMINATION: Sometimes a mold maker will put the beginning one or two coats of latex on his or her mold and leave it for four or five days before finishing the mold making process. Unfortunately, after that time, when they try to apply the next coat, it peels off. This occurs because vulcanization has already taken place and the first coats are completely set. As a result, you have delamination.
Be sure that all coats are within 36 hours of the previous coat.
BUBBLING: Sometimes, for a wide variety of reasons, air bubbles rise in the latex when it is applied. These bubbles remain trapped either between layers of latex, or between the latex and the model. These can be eliminated by using an air hose (or a drinking straw) and gently massaging an air stream across them until they open and are eliminated.
Most bubbles can be avoided by gently using crisscross patterns when applying the latex.
BLISTERING: When the sealant, or shellac, is not fully dried before the latex is applied, the solvents that are still present and evaporating from the shellac become trapped in the latex. Like trapped air, they force the latex away from the surface and create blisters.
Eliminate the blisters with a syringe or needle. Simply burst them and fill in the space with the next coat of latex.
To avoid this problem, leave the shellacked model exposed to the air long enough for all the solvents to evaporate. Usually this only requires that the model be left 24 hours.
PUDDLING: Latex is applied as a thick liquid. While brushing it onto the mold it will drip and form puddles at the base or in areas where it can gather. Using simple short strokes, continuously brush it back across the model.
Puddling is not a serious problem. Even the puddles will ultimately vulcanize,or cure,over time.
STICKING: While demolding latex, it sometimes has a tendency to stick to itself. Simply dusting both sides of the mold with talcum powder will prevent sticking.
BACK-UP MOLD: Without something to help separate the back-up mold, usually made of polyester resin, from the latex, a heat bond can build between them, causing the latex to stick. This problem is avoided by sealing the surface of the latex with a special high temperature resistant wax, or HX-Barrier Coat. This creates a wall between the fiberglass and the latex.
It is important to completely separate the latex from the heat and chemicals of the setting fiberglass resin.
CONCRETE: When working with concrete,the mold life can be extended by using a mixture of castor oil and alcohol as a lubricant and separator. Mix one part of castor oil to nine parts of alcohol by volume. Brush a light coat of this mixture onto the inside of the mold. Not only does this help lengthen the life of the mold, but it also lubricates the mold and assists the concrete in filling every detail. The lubrication will also allow for an easier removal of the cast piece from the latex.
Avoid ultraviolet light, like sunlight, when working with latex. Over time, sunlight will degrade the latex.