The term “viscosity” refers to the thickness or flowability of a liquid. Viscosity numbers range from 1 (water) to millions of centipoise (cP) or pascal seconds (Pa.s), 1cP = 0.001 Pa.s. Refer to our viscosity comparison chart here.
Urethane and epoxy resins with viscosities ranging from <100cP to 1,000cP are ideal for most generic casting applications. They de-air very well on their own and flow easily into closed molds, whether mixed and poured by hand or dispensed using meter-mixing equipment. However, there are many specialty materials, such as, Hapco’s Steralloy™, Filterbond™ and Hapflex™ resins that are formulated for highly-engineered applications, and because of their unique chemistries, they have a thicker viscosity than other products, making them a bit trickier to process.
When mixing and pouring by hand, Hapco always recommends vacuum degassing the mixed resin prior to pouring. With viscous materials, it can be helpful to add a few drops of a surfactant, such as Hapco’s Anti-Air™ product, which reduces surface tension and allows the resin to degas more easily. However, vacuum degassing alone does not always alleviate air bubbles due to cavitation of the material as it flows through the mold. It may also be necessary to cure your parts under pressure using a pressure-pot or molding chamber, like Hapco’s unique X-Series Molding Chambers.
When using meter-mix dispensing, Hapco recommends designing a mold that fills from the bottom up. A general rule in this case is to design the mold so that the output opening(s) equals 2-4 times that of the input. In simple terms, if you have a 0.50” diameter input, your out-put should equal 1”-2” in diameter. This enables a “pressure drop,” which minimizes any back-pressure build-up caused by shooting a viscous material into a closed mold.
When dealing with complex mold geometry, it may be beneficial to use a two-step degassing process. After initially degassing the resin mix, fill the molds and place them under vacuum again for an additional few minutes. This not only helps to release trapped air caused by material cavitation, but it will also “pull” the viscous material into the cavity to ensure a complete fill, especially if your mold has thin walls or complex geometry. While degassing the molds, the material inside will not swell up as it did during the initial degassing step, however, it may continue to “boil” somewhat. Therefore, it is advisable to fabricate a small “chimney” around the top of your mold to prevent material from spilling out. You can do this easily with wax, putty, or a simple strip of packaging/duct tape wrapped around the top of the mold. After secondary degassing you may find the need to top off the molds to ensure they are filled to proper height, in which case you should be able to do so without the need for further degassing
Other suggestions for thinning higher viscosity materials are as follows: Pre-heat the resin to 80° – 110°F. It is really only necessary to pre-heat the thicker component which is typically the Part A for most materials. As a general rule, for every 10° you heat the material above room temperature, the material viscosity is cut in half. Bear in mind though, that heat will also cause the material to gel faster, thereby reducing your overall work time. In lieu of pre-heating the resin, you can pre-heat the molds instead. This will maintain work time for mixing, and still thin the resin viscosity as it flows into the warm molds. Another suggestion would be to add a small amount of solvent, such as, isopropyl alcohol or acetone into the resin mix. Solvents will cut the viscosity without impacting curing or material properties in most cases, as they will flash off quickly once the material starts its exothermic reaction.
The bottom line is that you will need to incorporate the proper equipment and techniques into your process in order accommodate using viscous materials. Water-thin materials require very little in the way of specialized equipment and they certainly make things easier. However, limiting your material offerings can also limit your opportunities for getting more of those “high-dollar” projects. My advice for expanding your business opportunities is to think “outside of the mold-box,” and have enough flexibility in your process to take on those jobs that nobody else wants!