PMC™-746 was developed to make molds for casting gypsum plasters. This product is well suited for use as a rubber case mold – especially large case molds where extra rigidity is required. Shore hardness is 60A. Because of its durability and moisture resistant properties PMC™-746 is also used by zoos and museums for a variety of mold-making, display and exhibit applications. It features a convenient mix ratio (2:1 by weight or volume), and contains no mercury.
Other applications include making plaster block molds, reproducing ornamental plaster (architectural restoration), pre-cast concrete molds, casting waxes, Smooth-On rigid polyurethanes and epoxies and also for making a variety of special effects for movies and theatre.
Preparation - Materials should be stored and used in a warm environment (73°F/23°C). They also have a limited shelf life and should be used as soon as possible. Wear safety glasses, long sleeves and rubber gloves to minimize contamination risk.
Some Materials Must Be Sealed - To prevent adhesion between the rubber and model surface, models made of porous materials (gypsum plasters, concrete, wood, stone, etc.) must be sealed prior to applying a release agent. SuperSeal™ or One Step™ (available from Smooth-On) are fast drying sealers suitable for sealing porous surfaces without interfering with surface detail. A high quality spray shellac is suitable for sealing modeling clays that contain sulfur or moisture (water based). Thermoplastics (polystyrene) must also be sealed with shellac or PVA.
In all cases, the sealing agent should be applied and allowed to completely dry prior to applying a release agent.
Non-Porous Surfaces - Metal, glass, hard plastics, sulfur free clays, etc. require only a release agent.
Applying A Release Agent - A release agent is necessary to facilitate demolding when casting into or over most surfaces. Use a release agent made specifically for mold making (Universal™ Mold Release available from Smooth-On). A liberal coat of release agent should be applied onto all surfaces that will contact the rubber.
IMPORTANT: To ensure thorough coverage, lightly brush the release agent with a soft brush over all surfaces of the model. Follow with a light mist coating and let the release agent dry for 30 minutes.
Because no two applications are quite the same, a small test application to determine suitability for your project is recommended if performance of this material is in question.
IMPORTANT: Shelf life of product is drastically reduced after opening. Immediately replacing the lids on both containers after dispensing product will prolong the shelf life of the unused product. XTEND-IT™ Dry Gas Blanket (available from Smooth-On) will significantly prolong the shelf life of unused liquid urethane products.
MEASURING & MIXING...
Liquid urethanes are moisture sensitive and will absorb atmospheric moisture. Mixing tools and containers should be clean and made of metal, glass or plastic. Materials should be stored and used in a warm environment (73°F/23°C).
IMPORTANT: Pre Mix the Part B before using. After dispensing two Parts A and one Part B into mixing container, mix thoroughly for at least 3 minutes making sure that you scrape the sides and bottom of the mixing container several times.
If Mixing Large Quantities (16 lbs./7 kgs. or more) at one time, use a mechanical mixer (i.e. Squirrel Mixer or equal) for 3 minutes followed by careful hand mixing for one minute as directed above. Then, pour entire quantity into a new, clean mixing container and do it all over again. Although this product is formulated to minimize air bubbles in your the cured rubber, vacuum degassing will further reduce entrapped air. A pressure casting technique using a pressure chamber can yield totally bubble free molds. Contact Smooth-On or your distributor for further information about vacuum degassing or pressure casting.
Pouring - For best results, pour your mixture in a single spot at the lowest point of the containment field. Let the rubber seek its level up and over the model. A uniform flow will help minimize entrapped air. The liquid rubber should level off at least 1/2” (1.3 cm) over the highest point of the model surface.
Curing - Allow rubber to cure overnight (at least 16 hours) at room temperature (73°F/23°C) before demolding. Cure time can be reduced with mild heat or by adding Smooth-On “Kick-It™” Cure Accelerator. Do not cure rubber where temperature is less than 65°F/18°C.
Post Curing - After rubber has cured at room temperature, heating the rubber to 150°F (65°C) for 4 to 8 hours will increase physical properties and performance.
Using The Mold - If using as a mold material, a release agent should be applied to the mold before each casting. The type of release agent to use depends on the material being cast. The proper release agent for wax, liquid rubber or thermosetting materials (i.e. Smooth-On liquid plastics) is a spray release made specifically for mold making (available from Smooth-On or your distributor). Prior to casting gypsum plaster materials, sponge the mold with a soap solution for better plaster flow and easy release. In & Out™ II Water Based Release Concentrate (available from Smooth-On) is recommended for releasing abrasive materials like concrete.
Performance & Storage - Fully cured rubber is tough, durable and will perform if properly used and stored. The physical life of the rubber depends on how you use it.