Achieving optically clear parts using thermoset liquid molding is not impossible. But as anyone with experience will tell you it’s not a slam dunk either, especially if the cast possesses complex detail. So unlike the days of old, today’s materials and methods allow manufacturers and model-makers alike to attain water-clear parts without excessive rejects. It just takes the right combination of process and resins.
Vacuum degassing and 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.
Heating resin and vibrating the mold is another method of choice for casting clears (or any thermoset resin). This procedure helps relieve surface tension and allows air bubbles to more easily escape while filling the mold. Ultimately, heating/vibrating can yield better results, but it is not a failsafe.
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).
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.
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, often 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 bench top 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 process (this includes the equipment) 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.