So you want to make a plastic casting of an object or sculpture, but don’t know where to begin?
Semi-rigid urethane plastics (such as Smooth-Cast 57D) can be used to make hollow castings. This method of pouring molds is a highly efficient use of material, and is ideal for creating highly durable and lightweight castings.
Have a mold ready to begin? Below are easy to follow, step-by-step, instructions for making hollow castings using Smooth-On urethane plastic.
Step One - Release Agent
For easy demolding, we highly recommend using a release agent on the working surface of your mold. While urethane plastic will not bond to the surface of a silicone mold (such as Dragon Skin), using a release agent (such as Mann’s Ease Release 200) will help protect the working surface and extend the life of your mold. This is particularly important for molds intended for use over a long period of time.
For best results, spray release agent lightly and evenly over the working surface of your mold. Then use a soft bristle brush to spread the release agent over the surface and into tight corners.
Allow release agent to dry for 30 minutes.
Be careful not to apply too much release agent, or your casting may not cure.
Step Two - Mixing Your Urethane Plastic
Smooth-Cast 57D is mixed 1A:1B by volume. As the saying goes, measure twice, mix once! Always make sure you measure out parts A and B in separate, clean and dry containers.
Note: Always mix your Smooth-Cast products in a well-ventilated area. These products give off fumes which may be harmful in large quantities.
At this stage, SO-Strong tints can be added to Part B if you would like your castings to have a base tone straight from the mold. Mix your tint thoroughly with Part B to ensure a smooth and even color.
Pour both parts A and B into a mixing container and stir thoroughly.
Step Three - Rotate the Mold
Smooth-Cast 57D has a working time, or Pot Life, of about 3 minutes at room temperature (73°F / 23°C), so make sure your mold is close at hand while mixing your urethane plastic.
Note: Excess heat or high humidity may cause your plastic to cure faster, or not cure at all. Always make sure to read instructions provided with your Smooth-On products.
Pour the mixed Smooth-Cast into your silicone rubber mold and rotate for about 3 minutes. Try to make sure plastic coats the inside of the mold evenly, and pay special attention to raised edges where the casting will tend to be thinnest.
Because of Smooth-Cast 57D’s short pot life, mixing smaller batches is more manageable, and will make it easier to create a casting with an even thickness on all sides. For thicker castings, multiple pours of Smooth-Cast may be required.
Allow the urethane plastic to partially cure for about 8 minutes at room temperature for about 8 minutes before pouring in additional layers. It will usually take 3 layers to reach a thickness of ¼”.
Step Four - Allow Casting to Set
Smooth-Cast 57D will turn from a liquid to a solid about 3 minutes after mixing at room temperature, but it will need 30 minutes to fully cure. Castings which have thinner walls may take a bit longer to fully cure. Set your mold aside for now, and allow for the urethane plastic to harden.
Note: Smooth-Cast 57D will give off some heat during the curing process. Thick or solid castings may give off enough heat to irritate skin. In some cases protective gear may be required.
Step Five - Demolding
Has it been 30 minutes? Congratulations! Your hollow casting can now easily be pulled from your silicone rubber mold.
Your durable plastic casting will contain all the detail of the original object or sculpture. It can also be sanded, primed and painted as desired.
Step Six - Backfilling and Assembling (Optional)
For additional stability and support, hollow castings can be backfilled with rigid foam (such as Foam-It!). Foam cores will add support to a casting without adding a great deal of weight.
Smooth-Cast 57D can be bonded with other urethane plastic pieces using adhesives. Assemble multiple castings together into larger, more complicated, sculptures.
When in doubt, we always recommend creating smaller test pieces before committing large amounts of material to a full sized casting. Testing variations, such as thickness of casting or tints, may save you a time and money in the long run.
Also always remember to wear proper protective gear when working with all Smooth-On products. Protective equipment such as gloves, goggles and breathing protection may be necessary in some situations.
Additional information about Smooth-Cast 57D can be found here:
For any questions about mold making, castings or Smooth-On product visit our FAQ