There are different techniques used to achieve a metallic finish with a resin. "Cold-Casting" is a term used to describe the process of mixing metal powder with a resin and applying the mixture into a mold. The finished casting gives the appearance and weight of solid metal. The metal cold-cast process is faster and much less expensive when compared with foundry casting of molten metal (lost wax process). Different metal powders such as Bronze, Brass, Aluminum, Nickel Silver, and Copper can be used depending on the desired effect.
To make a metal cold-casting, metal powder is mixed into the resin until the mixture is thick and creamy. The mixture is then "slush-cast" (poured into a mold and rolled around) or brushed onto the mold surface (gel coat) until the resin cures. The gel coat is then back-filled with straight resin, resin mixed with metal powder, resin mixed with lead shot (for weight) or Foam-iT!® 5 rigid foam.
Our example below will detail making a cold-casting using bronze powder (also referred to as "bonded bronze") by first applying a "gel coat" layer and then adding more resin on top of the gel coat (back filling).
- Bronze Powder: Recommended metal powder mesh size for mixing with resin is: -325 to -225. Note: Metallic powders are available from Smooth-On and its distributors.
- Casting Resin: Smooth-Cast™ 325 ColorMatch™ works best because it is neutral clear amber and readily accepts metal powders and/or color pigments, and also has no odor.
- Liquid Pigment: Adding SO-Strong™ Black tint to the resin/bronze powder mixture will give the final casting added definition and dimension. Pro Tip: experiment with different colors for different effects.
- Rubber Mold
- Mold Release Agent: Use only a release agent specifically made for mold making and casting such as Mann Ease Release™ 200 (available from Smooth-On and its distributors).
- Measuring and Mixing Containers
- Mixing Paddles
- Disposable ("chip") Paint Brushes
Amount of resin, bronze powder and pigment required will vary depending on the desired effect. The mixing ratio can be as low as 1A : 1B : 1 Metal Powder. Using more metal powder gives the finished casting greater weight thereby simulating the weight of real metal castings (i.e. 1A : 1B: 3 Metal Powder). However, the addition of steel or lead beads to the "back-fill" resin can also be used to add weight at a much lower cost than the metal powder. For this example, we will use the following:
1A : 1B : 1 Bronze Powder
Liquid Pigment: 1 drop per 2 oz. / 56 grams resin
Cold Casting Steps
- Apply release agent to rubber mold. To prevent resin mixture from sticking to rubber mold, thoroughly spray mold release agent over entire mold surface. Brush into all surface detail and follow with second light mist coating of release agent and let dry for 15 minutes.
- Dispense equal amounts of Part A, Part B and Metal Powder by volume into separate measuring containers (example: 1 cup Part A, 1 cup Part B, and 1 cup metal powder).
- Add color pigment to Part B and mix thoroughly. How Much Pigment? Add 1 drop SO-Strong tint per 2 ounces (56 grams) of resin used (4 oz total resin = 2 drops of tint to be used).
- Combine resin Parts A and B in a clean mixing container. Mix well for 30 seconds.
- Sift in Bronze Powder to the resin and mix all components for 30 seconds.
- Pour mixture into mold cavity and brush mixture over entire mold cavity. The objective is to coat mold surface thoroughly until mixture begins to set up or "gel". Continuously brushing mixture up onto all vertical surfaces until mixture begins to thicken will ensure a uniform coating. Do not brush too aggressively, as you will disturb the release agent coat. Allow gel coat to cure 15 minutes.
- Back-fill with resin, resin mixed with metal powder, or Foam-iT! 5. Pro Tip: Adding steel or lead beads to this mix gives the finished piece the weight necessary to simulate the feel of a real solid bronze casting. Fill almost to top, leaving 3/8" (1 cm) headspace. Let this mixture gel and cap remaining headspace with Smooth-Cast 325 resin pigmented with dark brown pigment or Smooth-Cast 325 resin and bronze powder to closely match the color of gel coat.
- Entire casting should be thoroughly cured before demolding. Remember: the resin/bronze mixture or filled resin will take longer to harden than unfilled resin. Cure time depends on size of casting, mold configuration, amount of fillers used, etc. Generally 60 minutes is a sufficient amount of time. Smaller pieces may take longer. Applying mild heat will accelerate cure time. Let cool to room temperature.
- Demold the casting from mold.
- For best results, first wipe down the entire surface of the model with a towel soaked in mineral spirits. The mineral spirits will soften the plastic slightly and make it easier to bring out the metal shine when buffing with steel wool.
- Lightly abrade casting with medium/fine steel wool until the desired metal shine and luster is attained.
- Highlighting surface with black shoe polish will give casting added depth and dimension.
- Patina effect can be achieved with patina paints available through Sur Fin Chemicals: Tel. (323) 262-8108 (Super Antique 40, Berde, Rusty Brass, and Easy Brown).
- To prevent oxidation of the metal, spray finished casting with two coats of clear semi-gloss spray (acrylic works well) available at most hardware/DIY stores.