Plastic Casting
Plastic for casting starts as a two-part liquid that is essentially mixed together and allowed to harden inside the mold. Silicone molds are friendly for plastic casts because of their non-porous nature. Plastic is moisture sensitive, so if it is cast in anything porous, it will not set properly, and it may take longer in humid climates. As always, wear protective clothing like an apron, gloves, and safety glasses when using this stuff.

[Figure 3.76] Bottles of two-part plastic, with cups marked for the amounts to mix together.

When you have your plastic ready to mix together, the empty silicone mold should also be standing by and resting back in the same kind of bucket or cup in which it was initially created. The two parts of whatever amount of plastic you use should be mixed together equally (1:1) by volume rather than weight. Each part can simply be poured into a separate cup, with a mark drawn on with a Sharpie to find a common point to pour into. Using patterned Dixie cups helps for finding this common point on two identical cups (Figure 3.76).

[Figure 3.77] Each part of the plastic is poured into a separate cup.

After reading all safety instructions and having everything you need ready to go, both bottles of plastic are shaken before opening. The typical pot life for most two-part plastics is about 4 minutes, with a cure time of about 30 minutes. In each cup, equal amounts of parts A and B are poured separately up to the designated marker (Figure 3.77). Part B is then poured into part A and scooped out with a stirring tool to make sure all of it gets out of the cup. Both parts are then stirred together for whatever length of time is indicated by the instructions (Figure 3.78). The next step is to simply pour the liquid plastic into the mold’s entrance hole so that it can fill the empty cavity all the way to the top (Figure 3.79). A little bit of plastic is left inside the cup so that it can be watched while it hardens and act as a barometer for letting you know when the plastic inside the mold is ready to come out.

[Figure 3.78] The two parts are combined and stirred together.

[Figure 3.79] The mixed plastic is poured into the mold.

At this point, at least two different approaches can typically be taken with the plastic inside. If the plastic is simply left to harden while the mold sits still, the plastic will harden as a solid object inside. One thing to keep in mind in this case is that air bubbles in the plastic will off-gas upward as it cures, so it may not be uncommon to find a huge bubble at the top of the cast when releasing it, which would need some patching up with more plastic. The other method, referred to as slush casting, involves keeping the mold rotating constantly and slowly while the plastic is curing inside (Figure 3.80). This slushes and guides the plastic to the outer wall of the cast, leaving the inside hollow. A hollow head or other such object will allow for an attachment piece to be placed inside for affixing to a body armature, like a K&S tube, which can be attached inside the head with a bit more plastic to root it in there. A hollow head also cuts down on the weight, which is always important. While slushing the plastic around, it is still a good idea to know where the front and back of the head are inside the mold and to spend more time on the front of the head, where the face is. If any part of the cast needs to be patched, it’s better to have this happen in the back, where it might not have as much screen time. It is best to plug the entrance hole with some Klean Klay or a cork, and move the mold around over the top of a garbage can in case any plastic leaks out. (The sample shown here in Figures 3.79 and 3.80 are for a smaller head that fit inside a large plastic cup, whereas the same process would have been done in a larger bucket for Charlie’s head.)

When the cure time has passed and the remaining plastic inside the cup has turned white and hard, the mold can be taken out and pried open, and then release into the world a plastic cast replica of the sculpt (Figure 3.81)!

[Figure 3.80] The mold is rotated slowly over a garbage can to slush the plastic around inside.

[Figure 3.81] The sculpt and final plastic cast for Charlie’s head.

Ken A. Priebe has a BFA from University of Michigan and a classical animation certificate from Vancouver Institute of Media Arts (VanArts). He teaches stop-motion animation courses at VanArts and the Academy of Art University Cybercampus and has worked as a 2D animator on several games and short films for Thunderbean Animation, Bigfott Studios, and his own independent projects. Ken has participated as a speaker and volunteer for the Vancouver ACM SIGGRAPH Chapter and is founder of the Breath of Life Animation Festival, an annual outreach event of animation workshops for children and their families. He is also a filmmaker, writer, puppeteer, animation historian, and author of the book The Art of Stop-Motion Animation. Ken lives near Vancouver, BC, with his graphic-artist wife Janet and their two children, Ariel and Xander.