Polyurethane resin opens the doors to creating different types of castings. The process is simple – just mix the components, de-air and pour in the mold. Special colored dyes can also be added.
Polyurethane resin is the material of choice for making figurines, giftware, point-of-purchase displays, rapid prototypes, special effects and sculpture reproductions. Some formulations are suitable for jewelry casting, encapsulation and even for life casts. This material is very easy to use and can be cast in both rigid and flexible molds.
The Mixing
All polyurethane casting resins are two-part systems that have to be mixed one-to-one by volume. Measure out equal parts (by volume) of the base and catalyst in two separate containers. Combine them in a third container while scraping the sides and bottoms of both containers to get all the materials out. A thorough mixing for a minute or so is essential; again, scraping the sides of the container to ensure a proper mix.A low exothermic reaction will start once the materials are mixed.
Resins tend to capture air during the mixing process which will show up as unbecoming air bubbles on the surface of the casting. To get bubble-free casts, it is advisable to use a pressure pot that will compress the trapped air to microscopic size. Once the cast has cured to a hard plastic, the air will not have a chance to spring back to its original size!
The Pouring and Curing
The mold has to be prepared beforehand for the casting. Coat the internal surfaces of the mold with an appropriate release agent and allow it to dry; only silicone molds will not require a release agent.
Pour the mixed resin material into the mold and allow it to sit undisturbed. Most resins have a working time of 10 to 20 minutes and will start to gel soon. However, the casting should be allowed to set for 30 to 60 minutes (follow manufacturer instructions). To test if the casting has cured, use a tooth pick to carefully touch a corner of the surface to see if it has hardened. Do not use fingers as this will leave finger prints in case the resin is still tacky. Once set, the casting can be demolded and allowed to cure further.
The Finishing
Regular paints and dyes will not work with polyurethane castings. Special urethane dyes are available. Put a drop or two of the concentrated dye in the resin and mix thoroughly to get a uniform color. Alternatively, mixing with a few stirs will create a lovely marbling effect. Dipping a tooth pick in the dye and drawing or swirling it in the resin after it has been poured in the mold will also give a striking effect.
EnvironMolds is the much-loved art store that offers everything you need for resin casting – the Casting Materials tab has a variety of options of polyurethane resins, the Art Supplies tab offers the dyes and the Studio Accessories tab features mixing containers, spatulas, gloves, pressure pot and more.