If I had to come up with a list of the most common issues our customers call us about, along with air bubbles, a sticky surface on their clear castings would be at the top. My first question to them is always: Are you using silicone?
In 95% of tacky surface issues, I can only remember a few instances when silicone wasn’t used either as the mold material or release agent. The problem seemed large enough to dig deeper and I found this issue to be more complicated than any one factor.
Why is this phenomenon more common with clear resins?
The properties of a polyurethane are greatly influenced by the types of isocyanates and polyols used to make it. Of the two types of isocyanates, aromatic and aliphatic, aromatics are the most common. In general, they are less costly and produce shorter gel times, while aliphatics are used when longer gel times or UV stability is necessary. If you are using a water clear resin, chances are it is an aliphatic system.
The chemistry of aliphatic urethanes is not necessarily incompatible with the chemistry of silicone; however, the more time it takes for an thermosetting material to crosslink and cure, the more chance it has to react with by-products of the silicone, particularly on the surface.
Is there a difference between tin or platinum cured silicone?
The type of silicone used, tin or platinum cured, is an important factor when looking at this problem. Isopropyl alcohol is a by-product of the chemical reaction in tin cured systems. The presence of alcohol on the surface of a mold reacts negatively with aliphatic urethanes, resulting in a semi-cured part with a sticky surface.
In the early days of my career at Hapco, we would generally recommend using platinum silicone vs. tin with our clear resins, but this rarely, if ever, solved the issue. After researching this problem in depth, the causes are not so straightforward. Like Hapco does with its urethanes and epoxies, manufacturers of silicones use a variety of additives to produce different physical properties. The quality or chemistry of raw components can, and does, have an effect on how well they work with aliphatic resins.
While it’s true that some platinum silicones worked better than others, post curing any silicone with heat can be the difference between a perfect part or a reject. Many of the platinum silicone users who called in regards to this issue didn’t know they had to post cure their molds. Even tin based silicones designed to work with aliphatic resins, like our Hapsil™ 360 for example, must be post cured to flash off any alcohol. In addition to flashing off negative by-products, preheating a mold to around 90F prior to casting is a good way to avoid shrink marks and suck backs, especially in larger parts.
Even though silicone molds are self releasing, many customers choose to use a mold release to extend their useable life. Using silicone based mold releases with aliphatic urethanes can exacerbate the problem even further. A non-silicone based release, like Hapco’s Grease-It™ Two is recommended.
What can be done to avoid this issue?
Some users have found that rubbing Vitamin C on the mold can help neutralize some of the negative by-products, although it hasn’t been researched sufficiently yet. The best advice I can give is:
1.) Always post cure your platinum or tin catalyzed silicone molds even if it will cure at room temperature.
2.) Always test a small amount of your desired casting resin with whatever silicone you plan on using.
There may not be a simple answer to every problem that casting clear resins in silicones presents, but I hope this article can at least give you a better understanding of some of the root causes. As they say, “knowing is half the battle.”