FormRub 35 Mold Rubber by Polytek
FormRub 35 Liquid Rubber is the softest option (Shore A35) in the FormRub Series. This two-part, polyurethane rubber has high elongation and tear strength.
Porous models, such as wood, plaster, stone, pottery, or masonry must be sealed then coated with a release agent. Multiple coats of paste wax dried and buffed will seal most surfaces. Potters soap can be used as a sealer for plaster. Lacquer, paint, PVA, and Pol-Ease 2350 Release Agent also work well as sealers for many surfaces. Alternatively, PolyCoat, a sealer and semi-permanent release agent, can be used on most porous or non-porous models. Porous models must be vented from beneath to prevent trapped air from forming bubbles in the rubber.
Models made of sulfur-containing modeling clay (e.g. Roma Plastilina) should be sealed with shellac [CAUTION: When shellac is used as the sealer, it must be thoroughly coated with release agent because polyurethane rubbers bond tenaciously to shellac. In fact, uncoated shellac may be used to bond polyurethanes to certain surfaces.]
Non-porous models (e.g. metals, plasticine, wax, glazed ceramics, fiberglass and polyurethanes) and sealed, porous models should be coated with a release agent such as Pol-Ease 2300 Release Agent or PolyCoat.
Mixing and Curing
Before use, be sure that Parts A and B are at room temperature and that all tools are ready. Surface and air temperature should be above 60°F during application and for the entire curing period.
Read product labels to ascertain correct mix ratio and determine if pre-mixing of Part A or Part B component is required. Weigh Part B into a clean metal or plastic mixing container and then weigh the appropriate amount of Part A into the same container. Mix thoroughly. Hand mixing with a Poly Paddle is best to avoid mixing air into the rubber. While mixing, scrape the sides and bottom several times to ensure thorough mixing. Pour the rubber as soon after mixing as possible for best flow and air bubble release. Vaccum degassing helps to provide bubble-free molds, but is usually not necessary.
Allow rubber to cure at room temperature, 77°F (25°C). Carefully demold after approximately 16 hours. Final cure properties are obtained in about seven days, but molds may be used with care after curing for 24-48 hours. Heat accelerates the cure -- low temperatures slow the cure. Avoid curing in areas where the temperature is below 60°F (15°C).
Both Parts A and B react with atmospheric moisture and, therefore, should be resealed or used up as soon as possible after opening. Before resealing, Poly Purge, a heavier-than-air, dry gas, can be sprayed into open containers to displace moist air and extend storage life. For 55-gallon drums of Parts A and B, affix Drierite cartridges on the small bung during dispensing to protect product from moist air entering the drum.
Softening the Rubber
Add Poly 74/75 Part C Softener to FormRub for a lower viscosity mix and a softer cured rubber. When using Part C, cure time is longer and there is some loss of strength in the rubber and increased tendency to shrink after repeated castings. Determine the quantity of Part C required through experimentation.
Accelerating the Cure
Cure time can be shortened with the addition of an Accelerator, such as Poyl 74/75 Part X, or by adding heat. Exercise caution when using Part X since the rapid onset of gelling may trap air bubbles on or near the surface of the model. Heat also accelerates the cure. It is recommended not to exceed 140°F (60°C).
Using the Mold
Typically, no release agent is necessary when casting plaster or wax in FormRub molds. For casting plaster, sponge, dip or spray the mold with Pol-Ease. Mold rinse and then pour plaster on the wet mold to reduce air bubbles in the plaster and aid release. For casting resin, first spray the mold with Pol-Ease 2300 Release Agent or PolyCoat. For casting concrete, use a form release, such as Pol-Ease 2650 or 2601. Avoid solvent-containing releases, since they can cause mold distortion (i.e. shrinkage or swelling).
After repeated casting with certain resins, plaster and concrete molds may shrink slightly since these materials extract oils from the mold. The proper selection of release agent and/or barrier coat can minimize this effect. If shrinkage becomes evident, a light application of Pol-Ease Mold Dressing can help to restore the mold to its original dimensions.
FormRub molds can last many years if stored undistorted on a flat, non-porous surface in a cool, dry location out of direct sunlight. If occasional outdoor use is required, add 0.5% UV additive to the total mix weight to reduce the characteristic surface degradation caused by sunlight. Never store FormRub molds outside as UV exposure will eventually degrade the rubber.
Wipe tools clean before the rubber cures. Denatured ethanol is a good cleaning solvent, but is highly flammable and must be handled with caution. Coat work surfaces with wax, Pol-Ease 2300 Release Agent or PolyCoat so cured rubber can be removed.See More