in Liquid Rubber & Plastic for Moldmaking & Casting

Sculpture & Foundries

Casting Material: Friend or Foe
Different casting materials can be more or less damaging to a urethane mold. Plaster is probably the "friendliest" casting material. Our customers routinely cast hundreds of plaster castings in well-constructed Polytek® urethane molds. Concrete would be next on the list. With proper care, a Polytek® urethane mold can easily produce over 100 castings in concrete. Resins are the most damaging to rubber molds. Whether you cast polyester, urethane, or epoxy resins, the heat and chemical attack can rapidly degrade urethane rubber molds. Under good conditions, 50 resin castings or more can be achieved in Polytek® urethane. If you are not getting at least this many castings from your molds, read on and see if you are doing everything that you can to prolong the life of your molds.

Use the Right Rubber
Choosing the correct mold rubber is the first step in maximizing your mold life. All of Polytek®'s urethane mold rubbers are top-quality products, but some may be better choices than others for your specific application. The hardness of the rubber is the main consideration. If you were making a mold of an ornate Corinthian capital, Poly 74-30 would be an excellent choice, while Poly 75-80 would be much too hard. Conversely, if you were making a formliner for a highway sound wall, Poly 75-80 may be ideal, while Poly 74-30 would be far too soft. As a general rule of thumb, choose the hardest rubber for your application that will still allow for easy demolding of your castings. There may be exceptions to this rule, so don't hesitate to contact the Polytek® Technical Staff and, as always, start with a small-scale test.

Design for Easy Demolding
The size and shape of your castings and your mold design also have an impact on the life of your mold. Large resin castings will produce a higher exotherm than smaller castings of the same resin. Prolonged exposure to high exotherm temperatures can cause degradation in any rubber mold. The shape of a casting effects the rubber in a mechanical manner. If a mold must be stretched a great deal to remove each casting, it will fail sooner than a mold with fewer undercuts. Although you may have no control over the size or shape of a given model, you do have complete control over the design of the mold. Construct your mold with demolding in mind. Remember, the easier the castings come out of the mold, the longer the mold will last.

Handle with Care
Many urethane molds fail through handling long before they would have failed from casting degradation. Even a well-constructed mold can be torn if mishandled. Learn the shape of the object being cast before demolding. Separate the mold from the casting all the way around the edges first. Next, separate the easiest sections. Finish the process by working the rubber gently away from the most complicated areas, applying pressure in the direction that causes the least physical stress to the mold. If you don't manhandle your molds, they will serve you well for many castings.

Select the Best Release Agent
Use of the proper release agent will prolong the working life of your mold, with the exception of plaster casting, where none is required. In concrete casting, form releases can sometimes be more damaging than the casting material. Field experience has shown that overexposure to commonly used petroleum-based form releases can cause swelling or shrinkage in rubber molds. Water-based systems are far better for your molds. If the concrete castings will not be stained or painted, Pol-Ease® 2601 is an excellent form release. In applications requiring staining or painting, consult your current supplier of form release for a water-based recommendation. The use of a good release agent is critical in resin casting. Pol-Ease® 2300 and Pol-Ease® 2450 are excellent choices for resins of all types. Don't assume all release agents work the same. Release agents can make or break you when it comes to mold longevity.

Demold A.S.A.P.
Residence time is often overlooked, but is just as important as any other factor. Take your castings out of the mold as soon as possible. If a casting can be removed in 2 hours, don't leave it in the mold overnight. The less time the casting material spends in the mold, the less time it has to chemically attack the rubber.
Mold making is an inves™ent of both time and materials. Follow these guidelines to protect your inves™ent. Choose the proper mold rubber. Design the mold for easy demolding. Handle the mold with care. Use an effective mold release.

Quick Solutions
Preventing Plaster Buildup
Are you are getting plaster buildup on your urethane molds? First, wash away any residue with water, detergent, and a scrub brush. Then, when casting, allow the plaster to cure longer in the mold before demolding. This may seem obvious, but it solves most plaster buildup problems.

Preparing Water Clay
What is the best way to prepare a water-clay model? Pol-Ease® 2350 should be sprayed or brushed over the leather-hard model. Allow the release agent to dry for 30 to 60 minutes before applying mold rubber. The film left behind acts as both a release agent and a barrier between the wet clay and the curing mold rubber. We have found this to be the most effective surface preparation for both urethane and silicone mold rubbers.

     Dozens of gargoyles have been cast in Polytek®'s 3-foot x 3-foot Rotocasting Machine, pictured above with the production mold shown on the machine. Rotational casting allows perfect surfaces on all sides of complex parts, using fast-setting resins such as Polytek®'s Poly 15 Series Liquid Plastics or gypsum-based materials such as Forton MG.
     Angus Macaulay, Polytek®'s moldmaking and casting expert, says, "There is no easier way to get perfect surfaces on complex parts. Air entrapment that would require complex venting with a stationary casting is not even an issue with rotocasting."
     Rotational casting is a method that produces a hollow, seamless part. A resin or other liquid that solidifies or sets quickly is poured into a mold, which is then rotated on two axes to evenly distribute the casting medium on the mold surface as it is thickening. Size is not a problem. Polytek® has machines that will hold molds up to 6 feet x 6 feet. If a solid part is needed, the hollow rotocast pieces can be filled with low-cost foams, cements, or resins.
     The gargoyle shown above measures 20 inches wingtip to wingtip and is 18 inches tall. Depending on amount of mineral filler used, the castings weigh between 5 and 10 pounds. The mold is of PlatSil® 71-20, with a shell of Poly 15-6 and Poly Fiber.
     For more information, a customer service technician at Polytek® will be happy to assist.

The first easy-to-use, 1:1-mix (by weight or volume), soft mold rubber was introduced by Polytek® 15 years ago. It is still used by nearly all major foundries and sculptors, many of whom began using it back in 1984 when it was first introduced.
     Sculptor and stonecarver Larry Terrafranca recently wrote us about his work at the Broadmoor Hotel, the grand old five-star resort in Colorado Springs. His letter (at right) speaks for itself. Larry has been using Poly 74-30 for many years and says, "It performs with the reliability and consistency I've learned to  expect." For an elaborate fountain at the Broadmoor, Poly 74-30 was used to make molds of some of the original figures. These molds were used to produce plaster positives to "point out" Larry's limestone carvings. He thickened the rubber and applied it by brush to the original figure in the fountain. He said, "Working with Poly 74-30 in April, outdoors, in the bowl of the fountain certainly presented cold, damp, poor conditions—and the molds came out just fine."
     Larry grew up in Washington, DC, and was inspired by the many sculptures around the beautiful city. He holds a master's degree in sculpting from the University of New Mexico and apprenticed at the National Cathedral in Washington, DC. Larry can be reached at 4550 Dooley Way, Colorado Springs, CO 80911; (719) 634-7711.
     Poly 74-30 has many user-friendly advantages: no weighing, low viscosity, easy mixing, easy pouring, can be thickened with Cab-O-Sil® to any thickness desired for brush-on application, very tolerant of poor surface conditions and off ratio mixing, and low cost.
     Poly 74-30's specific gravity is about 1.0. Thus 1 pound of Poly 74-30 yields 27 cubic inches, more than almost all other mold rubbers. If one calculates the cost per cubic inch of rubber, Poly 74-30 is the low-cost leader.

Dear Mr. LeCompte:
     I met you years ago at the Corcoran School of Art in Washington, DC, at the Johnston figure competition. I guess you were just beginning your company, Polytek®. At the time we compared notes. I traded my sculpted clay figure for a quart of your product. Some years later I came to NJ to take your casting seminar. I've used your product over the years. My latest project in Colorado is at the Broadmoor Hotel. I'm doing a total recarving of an old Italian fountain at the entry of this 5-star institution. I used 74-30 to make plaster casts of critical parts. It performed with the reliability I've learned to expect from this product. The plaster positives I created from the molds were used to "point out" pieces in Kansas limestone. Photos enclosed. If there is an opportunity to describe my project in your catalog, I would be honored to be included. Thanks again for creating such a useful product. Also, please thank your staff, whom I have called on occasion for advice. They have always been most helpful, particularly Angus.

Sincerely,
Larry Terrafranca

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