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HOW TO MAKE HOLLOW PRODUCTS BY
SLUSH CASTING PLASTIC RESINS


Hollow cast plastic resin allows lighter weight castings and saves material costs.


Cold cast bronze effect

By slush casting plastic resins you can:

• Make hollow, lighter weight products.

• Reduce the amount of expensive plastic resin that you use.

• Reduce the amount of expensive fillers that you use.

• Achieve many different “looks”, such as metal, wood, marble, granite, etc.

Wood resin effect
The "cold cast bronze" effect
 
The "wood resin" effect with some added stain

Slush casting with plastic resin is very similar to slush casting with plaster, so you may want to read the previous section on slush casting with plaster. The principal is the same - you build up thin layers of the casting material against the inside walls of the mold until you get the thickness you want. With plastic resin, two or three coats of resin are usually applied, one right after another. The finished casting is hollow and can be very lightweight. Or you can fill the interior with an inexpensive material like plaster to give the casting more weight.

You should use a rubber mold for slush casting plastic resins. The cured plastic resin will be hard and rigid, so you want to use a flexible mold. Be sure and use the proper release agent on the rubber mold so that the plastic resin does not stick to the mold. The companies that provide your plastic resin and/or the companies that provide your moldmaking material should be able to recommend the best release agents.

Unlike plaster, most plastic resins will not "stay put" against the walls of the mold or the previous coat of resin. As soon as you stop rotating the mold, the plastic resin will run off, leaving only a very thin coat behind. For this reason, a quick setting resin such as polyurethane plastic, or "5 minute epoxy", is normally used for slush casting. You must keep rotating the mold until the resin gels. As soon as the resin gels, you can prepare another batch of resin and give the mold another coat.

An advantage to using plastic resins is that you can put various types of decorative fillers in the first coat of resin. The first coat of resin will form the outside of the finished casting, and will normally be the only part of the casting that people see. You do not need to use filler in the second or third coats of resin, since no one would see the effect anyway. This can save a lot of money in materials, since some fillers, especially metal powders, are very expensive.

Bronze powder is one of the most popular types of filler used in this way. The finished castings are usually referred to as "cold cast bronze". To make cold cast bronze, mix your plastic resin, then add as much bronze powder to the resin as you can, while still keeping the mixture thin enough to flow. A consistency a little more liquid than honey is about right. Use this mixture of bronze powder in resin as your first slush cast in your mold. Tip, turn, and rotate the mold until all surfaces are coated with the bronze-filled resin, and keep rotating the mold until the resin gels and does not flow anymore.

Then m ake one or two more slush casts with plain unfilled resin inside the first slush cast.

When all the resin has cured, remove the casting from the mold. At this point the casting will look dull and lifeless. You won't be able to see the bronze because it is covered over by a thin layer of plastic. Take some very fine steel wool and rub over the entire surface of the casting. This removes a thin layer of plastic and reveals the bronze underneath. When you are satisfied with the look, dust off the casting completely, and apply a coat of spray sealer such as "Krylon" clear acrylic to protect and preserve the finish.

"Cultured marble" is also very popular, and the slush casting technique can enhance the "marbled" look greatly. The basic filler to make cultured marble is calcium carbonate, also known as "marble dust". Make two or three batches of cultured marble resin, but with a different color pigment in each batch, such as white, gray, and pink. Then put all the batches of resin into the mold at the same time to make the first slush cast. The colors will mix together and streak in unpredictable ways while you rotate the mold, similar to the coloring in real marble.

If you need a way to make many slush castings to produce some sort of commercial product, there are machines available called "rotational casting" machines. These machines work by automatically rotating the mold while the resin gels. Using a machine also allows you to use a plastic resin (such as polyester resin) that doesn't set quickly, but is less expensive than the resins that do set quickly.


Download a PDF source list

Includes 26 suppliers of all mold making and casting materials mentioned in ALL the demonstrations on this web site, including silicone rubber, polyurethane rubber, latex rubber, polyurethane plastic, epoxy resin, polyester resin, molding clay, foam board, release agents, pottery plaster, Hydrocal, metal powders, other plastic resin fillers, and gelatin molding compound. With company names, addresses, phone numbers, and web sites. Downloads to your computer as a PDF file.



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