in Liquid Rubber & Plastic for Moldmaking & Casting

Sculpture & Foundries

Tips from the Mold Room

By Angus Macaulay

Coping with Porous Models
     Porous models, such as wood, stone, or terra-cotta, present some unique moldmaking challenges. Air from within a model can escape into the liquid rubber, causing defects in the mold. A model can also float or shift position within the liquid rubber. Liquid rubber can penetrate the pores of a model, mechanically locking the mold to the model.
     The following steps will help to overcome the challenges of porous models.
     Venting porous models from beneath is the first step (often overlooked) toward a successful mold. If a porous model is not vented, you risk air bubbles bleeding out and becoming trapped in the curing mold rubber. Air contained within a porous model requires an escape path. The air will always take the path of least resistance. Drill at least one hole no less than 1/8 inch in diameter through the base board directly beneath the model. The air in the model will readily travel through this path rather than through the surrounding liquid rubber, as long as the bottom of the model is not sealed. Sometimes we drill the hole right into the porous model.
     Fasten all models securely to a base board. This is particularly important for porous models. I like to seal the model to the base board with a bead of fast-drying caulk. Apply the caulk in a continuous, gasket-like fashion. This will prevent the mold rubber from flowing under the model and obstructing the predrilled vent holes. In the case of a low-density model such as wood, I would recommend driving two screws through the base board into the model. This will prevent the model from floating in the mold rubber. Although screws are always preferred, a high-density model such as stone can be fastened to the base board with a bead of caulk alone.
     Thoroughly seal a porous model to prevent the mold rubber from penetrating the pores. Petroleum jelly, lacquer, and shellac are all excellent sealers, but are not easily removed from the model after the moldmaking process. What do you do if you cannot alter the model's surface appearance? Poly PVA may be your solution.
     Poly PVA (Polyvinyl Alcohol Solution) is a water-soluble, film-forming coating often used as a barrier coat on rubber and resin molds. It is also an excellent sealer for porous models. Sealing with Poly PVA has become part of the standard procedure in the Polytek^® Mold Room for making molds of marble and other stone models. Some customers prefer the clear version, but I prefer the green version to be certain of complete coverage.
     The porous model is first coated with Poly PVA and allowed to dry. A solvent-based release agent, such as Pol-Ease^® 2300, can be used over the Poly PVA–coated model, and the moldmaking process can continue with little chance of staining. Avoid water-based release agents, as they will dissolve the Poly PVA coating. After the mold is completed, the Poly PVA sealer is easily removed with hot water and a soft brush.
The Best Mold Release for Silicones
     Do not use a silicone mold release with silicone mold rubbers. Although it is often done and often works just fine, more-dependable release is obtained when the surface to be covered is coated with a petroleum-based release agent. Our tests have shown that occasional sticking results when silicone releases, including Pol-Ease^® 2300, are used beneath both tin and platinum RTV silicone mold compounds.
     New petroleum-based Pol-Ease^® 2350 release agent is now available for use with TinSil^® and PlatSil^® silicones. Our tests have found it to be extremely effective for aiding both release of silicone molds from patterns and release of silicone from silicone in two-part poured molds. Pol-Ease^® 2350 is packaged in bulk liquid form. It can be applied by brush or sprayed with the Sure Shot Mini Atomizer shown in the accessory section of our Manual & Catalog.
Mold Release and Barrier Coat for PolyFoam™ Casting
     Pol-Ease^® PF mold release was formulated to extend the mold life of silicone molds used for high-production urethane foam casting. It is a fast-drying, non-silicone mold release. Pol-Ease^® PF is particularly effective when used in conjunction with a barrier coat.
     Barrier Coat PF is a fast-drying, water-based, paint-like coating which is applied to a Pol-Ease^® PF–treated silicone mold before casting. It is removed as an integral part of the foam casting as a primer coat. Pol-Ease^® PF and Barrier Coat PF are both packaged in bulk liquid form for use with your own sprayers. Barrier Coat PF is supplied as a neutral tinting base so you may create your own custom colors.
Extending Storage Life of Liquid Poly Urethane Materials Once Cans Are Opened
     We have all had unused liquids in opened containers "go bad" on the shelf. The usual problem is moisture contamination. You have been using the material off and on for several weeks and suddenly you find Part A has lumps in it or has formed a skin and must be replaced. Once opened, the unused portion of material has been exposed to moisture in the air.
     Many moldmaking and casting products will absorb or react with moisture. The more often the container is opened and the more material removed from the container, the greater the exposure to moisture and the sooner it will "go bad." The best way of avoiding this is to repackage the material into smaller, airtight containers full to the top when the original packaging is first opened for use. This is not always convenient, so Polytek^® is now offering an alternative.
     Poly Purge™ is a moisture-free, nonflammable, non-ozone-depleting gas in a can. A shot of Poly Purge™ should be dispensed into the opened material container prior to resealing. This will displace much of the moist air. By reducing the amount of moist air in the container, you will prolong the storage life of the unused liquid. You can also use Poly Purge™ for dusting keyboards and other delicate instruments.

     PlatSil^® 71-20 is a truly unique product. Most silicone mold rubbers have a 1:10 mix ratio, making it imperative to use a scale and to be accurate while weighing. PlatSil^® 71-20 can be mixed 1:1 by either weight or volume. A 1:1 mix ratio reduces the risk of off-ratio mixing. PlatSil^® 71-20 has a relatively low mixed viscosity (12,000 cps), so mixing is easy. Low initial mixed viscosity makes for rapid bubble release, often eliminating the need for vacuum degassing. If vacuuming is used, the system's low viscosity permits quick de-airing.
     PlatSil^® 71-20 has a 25-minute working time and cures to Shore A-20, demoldable within 4 hours, making it ideal when rapid results are desired—molding and casting can be done in the same day! Like other PlatSil^® products, 71-20 has a tough, knotty tear characteristic and exhibits virtually no shrinkage upon cure.
     A fast, soft rubber with super tear strength and high elongation is quite useful when casting complex objects with delicate projections, often encountered when replicating figurative sculpture. If repeated casting is to be done in a resin medium such as polyurethane or polyester, PlatSil^® 71-20 should be considered. Its mold life advantages will be most apparent.
     Call your Polytek^® representative to learn more or order a Trial Unit of PlatSil^® 71-20, or e-mail sales@polytek.com.

Can You Save 30% on Your Foundry Mold Rubber?
Don't Forget—Your Real Cost Is Cost per Cubic Inch!

     One factor that undoubtedly helps to determine which mold rubber to use is cost! Here's a quick review of some terminology and examples that you can use to calculate your real mold rubber costs. You may be surprised!
     Let's start by comparing two commonly used polyurethane mold rubbers, Poly 74-30 and Poly 72-40. Both of these rubbers are used for numerous poured and brushed applications, such as molding sculpture for bronze casting. A 5-gallon kit of Poly 72-40 weighs 49.5 pounds and costs $180. A 5-gallon kit of Poly 74-30 weighs 80 pounds and costs $308. This means the Poly 72-40 rubber costs $3.64 per pound ($180 ÷ 49.5) and the Poly 74-30 system costs $3.85 per pound ($308 ÷ 80). At first glance, one might think that Poly 72-40 is a less expensive mold rubber. Be careful!
     There is an important property that must be considered: the specific volume (or density) of each product. For Poly 72-40, the specific volume is 20 cubic inches per pound (in.3/lb.). This is how much volume is occupied by each pound of Poly 72-40. For Poly 74-30, the specific volume is 27.5 in.3/lb. So, each pound of Poly 74-30 occupies 37.5% more volume (space) than each pound of Poly 72-40 (27.5 ÷ 20 = 1.375 or 37.5% more).
     Since liquid mold rubber is used to fill a space, the real cost is cost per cubic inch, which is the price per pound divided by the specific volume. For Poly 74-30 the cost per cubic inch is $0.14. ($3.85 ÷ 27.5 = $0.14/in.3) and for Poly 72-40 it is $0.182 ($3.64 ÷ 20 = $0.182/in3). So, Poly 72-40 is actually 30% more expensive than Poly 74-30! The same calculation can be done for Poly 74-30 versus a standard polysulfide mold rubber with a specific volume of 20.5 in.3/lb. and a cost of approximately $4.50/lb. In this case, the Poly 74-30 is even more of a bargain—the polysulfide mold rubber is 56% more expensive!
     Poly 74 Series rubbers are the most economical, giving you the most volume for every pound of rubber.

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