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Source for liquid thermosets: silicones, moldmaking urethanes, casting plastics, polyester resins, epoxy.
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Clear Polyster Casting Resin

GENERAL DESCRIPTION: EP4101 (AKA Castolite AP & Castolite AC) is a room temperature cured, air-inhibited, rigid, water-clear casting resin. The liquid color is pale turquoise to clear, however when properly cured, EP4101 is crystal clear with excellent sparkle. EP4101 is widely used to make clear paperweights, jewelry, crystal balls, or to encapsulate dried biological or botanical specimens. When used with fiberglass fabric, vandal resistant windows or resin-stained glass can be made. Eager Plastics' extensive line of transparent dyes are available for a wide variety of effects.

EFFECTS: Interesting marble-like appearances can be obtained by adding marble dust to the resin prior to casting.

MIX RATIO: The gel time and cure time are both dependent on three parameters: (1) the percentage of MEK-P catalyst used; (2) the temperatures of all ingredients and the surrounding environment; (3) the mass concentration of the resin, i.e., a solid-cast shape vs. thin film. A good starting point, at 72°F, is 1-1/4 % catalyst by weight. This is equivalent to 10 drops catalyst per ounce of resin, or 1 teaspoon catalyst per pint of resin. This level will result in a 15 min pot life and a 24-hour cure. As temperatures rise above 72°F, decrease catalyst, and vice-versa. Polyester resins will not cure below 60°F. Experiment until the correct ratio is determined for your specific application. In general, mix what you can use in 10 minutes. Remember to always wear eye-protection when working with polyester resin and catalyst.

Typically castings should be poured 1/4"-1/2" thick maximum. Castings poured thicker than this are subject to overheating due to the exotherm. Results may be cracked, browned castings.

RELEASE AGENTS: If using Eager Polymers’ P-Series silicone or polyethylene molds, no release agent is needed for casting polyesters. If casting into any other materials (urethane, metal, gel-coated fiberglass, epoxy, etc.), it is best to first layer a few coats of release wax and buff them smooth. Then brush a thin coat of PVA and allow it to completely dry before casting.

When curing, polyester resins often give off heat. For this reason, EP4101 should be cast in layers no greater than 3/4" to avoid overheating of the casting. Overheating can cause browning and cracking of the cured casting. If large castings are desired, our line of water-clear urethane casting plastics can be poured up to 18" thick.

The resultant room temperature cured polyester system has these properties:

Specific Gravity: 1.11
Maximum Casting Thickness: ½ to ¾”
Viscosity: 300 cps
Hardness (Durometer): 85 D or 40 Barcol
Tensile Strength: 10,000 psi
Elongation: 10%
Shrinkage: 2% linear
Styrene Content: 45-50%
Maximum Use Temperature: 165°F

EP4101 is available in the following sizes:
  • Quart Kit (2-1/4 lb)
  • Gallon Kit (9 lb)
  • 5 Gallon Kit (45 lb) + Hazardous Shipping Fee
  • *55 Gallon Drum (500 lb) + Shipping

    *Manufacturer's recommended shelf life is 3 months. However with the help of proper handling and storage material can last up to 6 months. *Quart, Gallon and 5 Gallon Kits include the catalyst. With drums, order catalyst separately

    Other Eager Polymers Web Pages of interest on this topic include:
    What is the difference between clear urethanes and clear polyesters?
    How To sheet on making crystal clear castings.
    How do I remove the shrinkage wrinkles on my polyester casting?

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