By far the biggest source of air bubbles is the process of mixing the resin. Careful mixing of pure resins can yield good results, but you will want to fill most castings, and all powdered materials are full of air. You can subject most freshly-mixed resins to a vacuum to eliminate air from the mix, placing the material in a bell jar and evacuating the air to 29 hg (inches of mercury) or higher. Realize that the mix will rise like bread under vacuum, easily tripling its initial volume, so don’t overfill the container or you will overflow it. Urethanes present a problem due to their fast cure time. Even with a fast vacuum system, they will aready be setting by the time you begin to pour. So the use of mixing paddles which minimize air entrainment is important, as is the sizing of a batch in relation to the mixer and mixing vessel. Pouring degassed material into a well-designed mold will eliminate 95% of small air bubbles.
Finally: you are down to that air which was not displaced as the mold filled, or was introduced into the material by turbulence as you poured. This is particularly noticeable in materials with high surface tension, as they can skate right over detailed mold surfaces, trapping air bubbles that will cling in place. This problem is most significant with highly detailed mold surfaces such as fine hair textures or sharp protruding points.
There are two solutions to this: The first is mold rotation and/or agitation. By sloshing, banging, vibrating and, where possible, sticking your gloved hand or a brush in there and physically displacing the bubbles, you can get the air to rise out of the gates or vents of a well designed mold, if the resin isn’t so heavily filled that air won’t rise through it. The other solution is to apply pressure, and there are 2 ways to do that. For slow-setting materials, you can craft a vacuum chamber, big enough to contain your average mold and hook it up to your pump. Do not forget to pre-vacuum your resin before filling the mold, or nearly all your resin will boil out of the mold and down the sides when under vacuum. Be aware that some resins have volatile components that vaporize under vacuum and will never fully stop bubbling, so even with pre-vacuumed material, you may have to design your mold with a reservoir feature above the gate to contain any resin that boils out and funnel it back into the gate as the resin settles back down.
What will happen under vacuum is this: any air bubbles entrapped in undercuts or mixed into the material will expand hugely. The bubbles will become so buoyant that they will rise and burst while most of the air in undercut areas will bleed off as the bubbles expand beyond the undercut area entrapping them. You still have air bubbles in your casting at this point; in fact, the undercut entrapments are still the same size as they were before. However, the remaining air is at nearly zero air pressure. It is crucial in vacuum casting that you release the vacuum while the resin is still liquid. This slams 14 pounds per square inch (atmospheric pressure) against the resin and squeezes the remaining air bubbles either into solution or to a microscopic size.
This technique does not work well with urethane resins because there is inadequate time to evacuate a chamber big enough to hold an average mold. For urethanes, you have to skip the vacuum part and cut straight to the pressure. You want to pressure-cast urethanes at 50 to 80 pounds per square inch. This is too bad, really, because it is far easier to build something to hold 14 pounds psi OUT than to build something that holds 80 pounds psi IN. (Pressure vessels can be extremely dangerous, so don’t try building one unless you really know what you’re doing.) You can use an autoclave (one with no heating system) and a big enough compressor to bring up the pressure in the autoclave within 20 seconds or so. For smaller castings, some shops use painter’s pressure pots. To do pressure casting, you fill the mold, pop it into the chamber, lock it, slam the pressure on and hold it there until the resin has cured. This technique should eliminate 98.5% of all air-caused flaws in cast resin parts.
©2000 Christopher Pardell, with contributions from Dan Spector and Andrew Werby