Metal Cold Casting
There are different techniques used to achieve a metallic finish with a urethane resin. "Cold-Casting" is a term used to describe the process of mixing urethane resin with a metal powder and pouring the mixture into a mold to obtain a casting that gives the appearance of solid metal. Different metal powders such as bronze, brass, aluminum, nickel-silver and copper can be used depending on the effect desired. The metal cold-cast process is faster and much less expensive when compared with foundry casting of molten metal (lost wax process).
Overview - To make a cold-casting, metal powder is mixed into the resin, until the mixture is thick and creamy. The mixture is then "slush-cast" (poured in to a mold and rolled around or brushed in to keep the mold face coated, until the mixture sets). The remaining cavity is then filled with plain resin, resin mixed with a different filler to give the casting body and dimensional stability, or foam.
Specifics - Cold Cast Bronze - Our example will detail making a cold-casting using bronze powder (also referred to as "bonded bronze").
Bronze Powder - Recommended metal powder mesh size for mixing with resin is 325.
Metallic powders are available from most Smooth-On distributors. Call our technical help line to find your nearest distributor.
Casting Resin - Smooth-On manufactures a variety of urethane resins that can be used for metal cold-casting. Smooth-Cast 325 Colormatch is recommended because it is a neutral color and readily accepts metal powders and/or pigments. SC 325 is also inexpensive, has extremely low viscosity (pours like water), and sets up quickly.
Liquid Pigment - Adding a dark pigment (black or dark brown) to the resin/bronze powder mixture will give the final casting added definition and dimension.
Also required: Rubber Mold; Mold Release Agent; measuring and mixing containers; mixing paddles; Gram scale or triple beam balance for weighing components.
Amount of resin, bronze powder and pigment required will vary depending on the desired effect. Most customers will experiment by varying the amounts of resin, metal powder and pigment used in combination to attain a desired effect. For this example, we will use the following:
Part B of Resin: 100 Parts Bronze Powder: 900 Parts
Part A of Resin: 100 Parts Liquid Pigment: 10 Parts
1. Apply Release Agent to Rubber Mold** - To prevent resin mixture from sticking to rubber mold, thoroughly spray mold release agent (Smooth-On Universal Mold Release) over entire mold surface. Brush into all surface detail and follow with second light mist coating of release agent and let dry for 20 minutes.
2. Mix Metal Powder and Pigment with Part B of Resin - To allow ample mixing time, mix metal powder and pigment thoroughly with Part B of SC 325 (Blue Label) prior to adding Part A (Yellow Label). Dispense 100 grams of Part B into clean mixing container. Add 900 grams of bronze powder and 10 grams of black pigment to Part B and mix thoroughly. Mixture will be very thick.
3. Add 100 grams of Part A - to the Part B/bronze powder/pigment mixture and mix thoroughly.
4. Pour Mixture - into the mold cavity. Gently roll mixture (slush cast) around mold cavity, being careful not to let a substantial amount settle to the bottom of cavity. The objective is to coat mold surface uniformly with mixture until it sets up or gels.
5. You now have a Hollow Casting - that can be back-filled with resin, resin mixed with different filler (such as URE-FIL from Smooth-On) or foam. (Tip: Adding steel or lead beads to the back fill resin gives the finished piece weight necessary to simulate the feel of a real solid bronze casting.)
6. 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 30 to 40 minutes is a sufficient amount of time. Applying mild heat will accelerate cure time. Let cool to room temperature.
7. Demold the casting from mold - Lightly abrade casting with medium/fine steel wool until the desired metal shine and luster is attained. Note: Patina effect can be achieved with liquid patinas.
TIP: To prevent oxidation of the metal, spray finished casting with two coats of clear gloss spray (acrylic works well).
Other techniques used to achieve metallic finish include:
A) Metal Powder/Shellac "Paint-On" - A casting is made with unfilled resin. After demolding the casting, release agent (if any) is removed from the castings surface with solvent. Next, metal powder is mixed with shellac to a brushable consistency and brushed onto resin casting.
B) Metallic Resin Paint - After removing unfilled resin casting from mold, release agent (if any) is removed.
* Recommended metal powder mesh size for mixing with resin is: -325 to -225. Metallic powders are available from Smooth-On and its distributors.
**Use only a release agent specifically made for mold
making and casting such as Universal