Ways to improve resin casting: pressure and vacuum chambers

Using a pressure chamber dramatically improved my resin casting.

Getting into resin casting can be a difficult thing, with so many details to consider (what silcone and resin to use, how to create effective vents, etc.), that it is easy to get overwhelmed. I found one of the trickiest elements was determining how pressure and vacuum chambers can be used to improve the process. Both are used to reduce bubbles in the final product, but since they work in very different ways, each is suitable for a different aspect of the process. A vacuum chamber is used to physically remove bubbles from a material (and all the air from the chamber itself) by creating a vacuum, while a pressure chamber shrinks any bubbles present in the material via pressure. Because the mixing process of silicone introduces air bubbles into it, a vacuum is applied to the mixture before using it (pouring it) to create a mold. This is possible due to the slow curing time of most silicone (~5 hrs). Resin, however, solidifies much more quickly (~15 min.), making a vacuum not a viable way for removing the bubbles in resin before injecting it into the mold. Instead, the mold is placed in a pressure chamber after the resin is added, to shrink any bubble to a negligible size. Although using both strategies are not absolutely necessary for creating molds and using them for casting, from experience, it dramatically improves the final product. I currently use both, and they allowed me to nicely cast the true-scale Space Marine that I recently built. In this post, I wanted to talk about both vacuum chambers and pressure chambers, and how they were important for dramatically improving my resin casting abilities.

Degassing silicone for a better mold
Since most silicones come as two parts that need to be mixed together before using, you will always introduce air bubbles into the silicone during that mixing process. By pouring the silicone high above the containment frame for the mold, you allow the silicone to trickle into the frame via a narrow stream, popping a lot of the larger bubbles. Although this helps significantly, it often not enough to remove all of them. The standard way to solve this issue is to apply a vacuum to the mixed silicone for a few minutes before pouring it. When a vacuum is applied to the silicone, it expands dramatically as the air trapped within it begins to escape, before it ultimately collapses back in on itself. After this “degassing” process, the mixture is ready to pour like normal (high pour) and let solidify, creating the mold. It is worth emphasizing that you want to degas before pouring and not the other way around. If you degas after pouring, the silicone does not settle properly after it collapses, creating a lackluster mold (it can also bubble over the frame, causing other issues).

After mixing the two part silicone, I put the mixture in a vacuum chamber to degass it (I). This degassed silicone is removed from the vacuum chamber (II) and then poured into the containment reservoir from high above to ensure that no bubbles are introduced during this step (III). Finally, the silicone is allowed to cure in a pressure chamber at 40 psi (not pictured).

If you degas the silicone only after it is poured into the containment frame, it does not come out very well (Mold material: Smooth-On Mold Max 14MV).

To degas silicone, I am currently using a 1.5 gallon vacuum chamber and a 3.6 CFM 110V 1/4HP single stage vacuum pump (ultimate vacuum 0.8Pa), both from BACO.

If you are going to cast your resin under pressure (which I strongly recommend and will talk about below), it is important to allow your mold to cure under pressure as well. This acts as one final failsafe to remove any other bubbles that may have been introduced while pouring the mold. This is done by simply putting the just degassed and poured mold directly into a pressure chamber for the duration of the curing time.

When resin casting at 40 psi, if the silicone was not degassed to create the mold (top), any small bubbles present in it will get filled in. This does not occur if the silicone used to create the mold was degassed before it was poured (bottom).

Why you should use a pressure chamber, and how to create one
A vacuum chamber is not suitable for removing bubbles from two part resins because they cure too rapidly (many between 10-15 minutes). Instead, pressure (about 30-40 psi) is used to shrink any bubbles present to a negligible size after it is added to the mold. This process is as simple as putting your resin-injected mold inside the a specialized pressure chamber and increasing the pressure (via an air compressor) to between 30-40 psi, and letting it solidify under pressure. Although I have seen some suggest that you should use slower curing resins (~30 min) when using a pressure chamber to ensure it does not start to solidify before the pressure has been increased adequately, I use Smooth-Cast 300 resin (10-15 min curing time) routinely in my pressure chamber and it works excellently.

Using a pressure chamber is simple, just put the resin-filled molds directly inside and increase the psi to 40, until the resin cures.

While there are commercially available pressure chambers for casting (C.A. Technologies; Smooth-On), I opted to create my own (significantly cheaper). To create my pressure chamber, I modifed a 2-1/2 gallon Air Pressure Paint Tank. The process is a pretty simple one, requiring you to plug up a few of the unnecessary ports (a ¼” brass cap to the air regulator and a ⅜” for the L joint), and add a 1/4" female plug so that you can attach a compressor. I made a few additional adjustments, like adding a ball valve regulator to more easily adjust the airflow into the chamber (Link to a nice video talking about the modifications). With these modification done, the chamber is ready to use.

How I created a pressure chamber with a 2-1/2 gallon Air Pressure Paint Tank. 

It is worth noting that if you intend to use a pressure chamber to cast with, it can exacerbate small imperfections present in your silicone molds. Any microbubbles that might not have been noticeable before will get filled in with resin. I had this problem with my first mold for the true-scale Space Marine. The silicone mold had tiny bubbles in it that were not noticeable when not casting under pressure, but when increasing it to 40 psi, resin was pushed into these tiny bubbles, creating additional resin flash all over the model. This problem can largely be prevented if you degas your silicone before pouring the molds (talked about above).

My true-scale Space Marine cast with the help of both a vacuum and pressure chamber.

Conclusion
By degassing the silicone that I use when creating a mold, and casting under pressure, I have been able to achieve substantially higher quality resin casts, which has saved me a substantial amount of time cleaning up each cast. While the addition of a pressure and a vacuum chamber (along with their respective pumps) is quite an investment, if you are serious about creating high quality resin casts, they are invaluable. In the long run, I think it pays off saving innumerable hobby hours. I hope the results speak for themselves! Any feedback, questions, or comments are welcome!

-Adam Wier