When in a liquid form, resin is susceptible to air bubbles like any other liquid. The lower the viscosity the harder it is for air bubbles to escape. When liquid resin cures, these air bubbles can be trapped at the surface or embedded inside the hardened plastic. This causes us problems when our goal is to have a nice looking prop.
To reduce these air bubbles we can use a variety of techniques at various stages of casting.
Before combining part A and B of a resin, each component can be vacuum degassed to draw air bubbles out. This is especially useful in low viscosity resins, and is normally not needed for thinner resin.
It is also possible to degas the mixed resin so long as it is not a fast setting resin (vacuuming the material will agitate the material, generating heat and causing the cure to accelerate).
Avoid introducing air into the resin by stirring slowly, and by keeping the mixing stick inside of the resin until finished stirring.
When combining parts A and B pour the lower viscosity component into the higher viscosity component. This is because the thicker part is less likely to splash and introduce air into the other part.
With the exception of fast setting resin, a mixed cup of regular resin can be placed in a outer container of warm water. With the mixing stick resting inside the resin, air bubbles will be repelled from the warm water in the outside vessel, and will be attracted to the stick. They will rise to the surface at the stick.
Pre-heated molds can have the same effect, repelling air bubbles from the mold surface. Pre-heated molds will also encourage a more uniform curing profile, since the resin will cure from both the mold surface as well as from the center of the casting (where the resin will exotherm).
Use pressure to shrink air bubbles.
Place the resin filled mold into a pressure chamber and fill with air until it reaches 40-60 psi. Needed pressure will depend on viscosity of the resin. Never pressure cast a mold if the rubber was not degassed, or the mold will be damaged by the air bubbles in the rubber collapsing.
Mold must be kept under pressure until resin has set to at least a thick gel (otherwise air bubbles will simply expand back to their original size).
Molds can be knocked, tapped, or placed onto a vibrating surface to shake air bubbles from trapped positions to the top of the mold. Most useful for open top molds where air bubbles can escape.
In certain circumstances injecting resin into a mold can have the effect of forcing some air out to make room for incoming resin. This can be achieved through various devices or machines which inject resin. Multi part molds can also be designed to force excess resin out when two halves are pressed together.
When molding a part, think about the flow of air as you design the mold. Including vents in the mold wherever there is an undercut will help air to escape as resin fills the mold.