DESCRIPTION: TinSil® 70 Series rubbers consist of a liquid Part B base and Part A accelerator, which after mixing at the proper ratio by weight, cure at room temperature to flexible, high tear strength, RTV (room temperature vulcanizing) silicone rubbers. All TinSil® 70 Series mold rubbers are tin-catalyzed, condensation-cure systems. They are ideal for molds where easy release or high temperature resistance is needed. They are recommended for evaluation as mold materials for polyester, epoxy and polyurethane resins and waxes. TinSil® 70-60 is useful with low melting metals.

TinSil® 70-25 cures to Shore A-25 hardness within 24-48 hours. TinSil® 70-30 is slightly tougher and firmer than TinSil® 70-25. TinSil® 70-10 and 70-20 are softer for more delicate castings. TinSil® 70-60 is the hardest, highest heat resistant rubber for casting low melting metals.

WHY SILICONES: Silicone rubber is used most often for casting liquid plastic resins, such as Poly Urethane, epoxy or polyester because the resins or the barrier coats used with them do not require release agent. Thus, plastic parts from silicone molds are usually ready for finishing with no washing of release or surface imperfections due to release agents.

Silicone molds also withstand the high temperatures (above 250^oF) of some polyester or acrylic resins or low melting metals better than any other rubber.

Thus, silicones' non-stick quality makes short run resin casting easy, but for high production of many parts per mold, release agents are often used to prolong mold life. If this is the case, some Poly Urethane rubber molds may perform nearly as well at half the price. If your mold costs are high, it may pay for you to review the possibilities of a lower cost rubber with a Polytek representative.

Silicone rubber molds are almost never used for cement and plaster casting and are only needed at foundries with the stickiest of foundry waxes.

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MODEL PREPARATION: Porous models such as wood or plaster should be sealed to prevent penetration of the rubber into the pores of the material. Wax, petroleum jelly, lacquer, paint and most other coatings are suitable sealers. The sealed or non-porous model and other materials that will contact the rubber should be sprayed or coated with a very light coat of release such as a 10% by weight solution of petroleum jelly warmed and dissolved in mineral spirits. Silicone-based release agents (such as Pol-Ease® 2300) are not to be used on surfaces which will contact the liquid rubber since inhibition and/or adhesion may occur. Porous models should be vented from beneath to prevent trapped air from causing bubbles in the rubber. Liquid silicone rubbers usually will bond to cured silicone rubbers unless a parting agent is used. In every case where there is any question about the compatibility between the rubber and the prepared model surface, a test cure should be made on an identical surface to determine that complete curing and good release are obtained. The cure of silicone mold compound may be inhibited by some surfaces, especially oil based clays containing sulfur. A test cure on questionable surfaces is highly recommended.

Molds may be reinforced with open mesh nylon or dacron cloth which stretches. The fabric should not be too close to the mold surface or the weave of the cloth may show through to the face of the mold.

USING THE MOLD: No release agent is necessary for casting most materials in TinSil® or PlatSil® molds, but for longer mold life with epoxy, polyurethane or polyester resins, a barrier coat or release agent is recommended. TinSil® molds can be stored for a year or more, but as with most tin-catalyzed silicones, molds may eventually deteriorate and lose their elasticity. Molds made with excess catalyst may be subject to degradation on aging faster than silicone rubbers cured with less catalyst. PlatSil® 71 & 73 Series rubbers will last many years without deterioration.

Note that the tin-cured silicones release alcohol during cure, which must be allowed to evaporate prior to casting polyurethanes, as alcohol will inhibit the surface cure of polyurethanes. Exposure of 24 hours in a warm location, exposed to air, is often adequate, but the mold can be baked for four hours at 212oF (100oC) to speed alcohol evaporation.

SAFETY AND CLEANUP: The Material Safety Data Sheets should be read before use. TinSil® 70 and PlatSil® 71 & 73 Series rubbers are safe to use if directions are followed. Avoid contact with skin and eyes as irritation may develop. Use only with adequate ventilation as respiratory irritation, headache and nausea could be caused by overexposure to vapors. Best method of cleanup is by wiping with disposable paper towels. If solvents must be used, xylene, toluene, trichloroethane, naptha and denatured alcohol are suitable, but should be handled with respect for vapor and flammability hazards.

Thickening for Brush On: All TinSil® 70 Series rubbers can be thickened with Cab-O-Sil® for brushing on a skin mold. As an alternative to Cab-O-Sil®, a liquid additive, TinThix, may be mixed into the Part B, base, at levels up to 5%, which yields a non-sag viscosity. A similar product, PlatThix, is available for use with PlatSil® RTV's.

COLORS FOR SILICONE RUBBERS: Sometimes artists color in oil can be used to tint silicones if tested to your satisfaction in trial mixes. Colors dispersed in silicone fluids can be obtained from Dispersion Technology (732) 364-4488.

THINNING AND SOFTENING WITH SILICONE FLUID: Low viscosity 100 cs. Silicone Fluid can be sparingly added to the mixed rubber to thin the mix with some loss of strength and hardness as well. More than 10% fluid addition may exude from the cured rubber. A 5% addition to PlatSil® 71-30 will reduce hardness to approximately Shore A-22. A 10% addition to TinSil® 70-15 will reduce hardness to approximately 10-12.

BARRIER COATS: A barrier coat is a fast drying lacquer type primer sprayed into a silicone mold prior to pouring plastic. When dry the liquid plastic cures against the barrier coat which comes out of the mold on the plastic resulting in a pre-primed part and longer mold life.

PHYSICAL PROPERTIES & PACKAGING

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