A Helpful Guide to Casting Clear Resins
Achieving clear, bubble-free parts using clear epoxy or urethane is not impossible, but as anyone with experience will tell you, it’s not without its challenges either. The task can be even more daunting if the part possesses complex detail or undercuts, however with the right combination of materials, equipment, and expertise, attaining water-clear castings without excessive rejects is possible.
Ultraclear 480. The sample on the right was cast at 70 PSI while the sample on the left was cast at ambient pressure.
Vacuum degassing and/or pressure casting are perhaps the most popular if not the most efficient methods employed to create clear, void-free and bubble-free castings. Additional time and energy are required, and rejects are still possible.
Pulling a vacuum on liquid resin will remove air. Pressurizing will squeeze it down to invisible sizes while the part cures.
Heating resin and vibrating the mold is another method of choice for casting clear or any thermoset resin. This procedure helps relieve surface tension and allows air bubbles to more easily escape while filling the mold. Ultimately, the combination of heat and vibration can yield better results, but it is not a failsafe.
A mold is secured to a vibrating table to help air move to the surface while filling.
Regardless of the process, cast and mold material must be compatible for water-clear casts. For instance, some mold materials and release agents are not compatible with aliphatic urethanes(clears).
A medical component cast in an silicone mold that wasn’t post cured. This was left with a tacky surface and many tiny bubbles which were exacerbated due to not being pressure cast.
Choosing the correct release agent and applying it correctly is important to avoid tackiness and surface defects. Silicone-based release agents tend to react poorly with clear resins, causing cure-inhibition and other defects. This is why many molders will opt for a silicone mold to avoid the releasing process altogether, but it brings its own set of challenges to casting clear resins.
Grease-IT 2 is an example of a PVA release agent.
The only release agent that can be considered a fail-safe is Polyvinyl Alcohol(PVA). This one part liquid, which can be sprayed or brushed on, dries to form a non-reactive film over the part.
RTV Silicone rubber, be it tin or platinum-based, are most often the choice of liquid molders because of their self-releasing properties and flexibility. The major issue with casting clear resins in silicone molds is the fact that the surface of the part can be tacky or uncured upon de-molding. This phenomenon, typically referred to as cure inhibition, is a major challenge with very limited solutions. Post-curing the silicone mold before use is essential in flashing off some of the natural oils and acids on the surface. Those substances are the major reason why many clear resins have trouble fully curing. Unfortunately, post-curing is not always possible when molds are exceptionally large.
Polishing the finished piece is almost always necessary, especially when considering that upon de-molding most parts have parting lines, gates, and vents that require removal. This can be achieved with a benchtop buffing machine or done by hand. Either method will require a polishing compound. This can add a considerable amount of time and energy depending on the size and complexity of the piece.
Ultimately, success when casting crystal clear resins is best achieved when the process (pressurizing, vibrating, etc.) and materials come together to provide the best outcome.
Ultraclear™ is Hapco’s series of water clear casting resins. They are a 1:1 ratio by weight and volume and very low viscosity to make mixing and pouring easy. They are also 100% mercury free unlike most clear resins on the market.
Hapsil™ 360 is Hapco’s RTV silicone rubber that was designed to be compatible with aliphatic casting resins and not inhibit the cure.
The X-Series Molding Chambers allow the user to control the pressure and temperature of parts during the curing process. They come in 4 sizes.
This article is helpful for anyone who is getting ready to do resin casting.