In the latest excerpt from The Advanced Art of Stop-Motion Animation, Ken A. Priebe continues his lesson on building puppets, focusing on silicone and plastic casting.
Foam latex has long been a popular material for creating the outer skin or entire body of a stop-motion puppet over its armature. The basic elements that are needed for a foam latex puppet are the sculpt (the official term for the original clay sculpture of the puppet) and a plaster mold formed around the sculpt. The foam latex goes inside the mold along with the armature and becomes a cast replica of the sculpt. Foam latex comes as a series of about five different liquid agents that are mixed together to gel into a soft, spongy material. Once it is cured by baking it in a convection oven, it basically behaves like upholstery foam, which springs back into shape even after touching it. Although foam latex is stinky and tricky to mix properly, it is relatively easy to repair and very lightweight, which are both important qualities for a stop-motion puppet.
Another material that has become very popular in recent years for puppet fabricators is silicone. Silicone is easier to mix than foam latex (only two agents are needed, instead of five) and can be used to create both puppets and molds. The appearance of silicone skin for a puppet has a different quality than foam latex; it can potentially be smoother and more translucent. Although foam latex can be cast and painted to appear very smooth, it will often have a certain degree of texture to it. The smoothness and attention to detail from the original sculpt that silicone provides gives a great deal of creative freedom for different character designs. The other advantage that silicone has is that it will only stick to itself, so it can be worked with alongside a variety of other materials. At the same time, there are some materials that do not react well with silicone, including latex and any sulfur-based modeling clays.
When working with silicone or any other chemicals, it’s important to understand what you are dealing with. By law, safety sheets must be provided and sent along with any chemicals you purchase or order, and it’s important to read them carefully before cracking the chemicals open. Safety precautions and guidelines will typically tell you to protect your skin, lungs, and eyes from the material, so rubber/latex gloves, masks, and safety glasses are very important to have on hand. The instructions will also tell you how long you can work with the product before it starts changing. The time span for working with silicone before it sets (and is no longer in a pourable liquid form) is referred to as its pot life; its cure time is how long it takes to cure fully. Pot life is usually only a few minutes, and cure time is typically 24 to 48 hours. Silicone comes in at least two liquid parts that need to be mixed together: the inert silicone base and an additional 10% of an activating agent that causes it to solidify. There are also softening or retarding agents you can buy that can be mixed in to change the silicone’s consistency or slow down its pot life, but these are only necessary if you want additional control over it. Once these different agents are one uniform color after mixing (in a cup or bucket, with any kind of stirring tool), you know it’s ready to start using before it sets.
Another important thing to understand about silicone is there are some types that are better for molding (creating a negative impression of a sculpt) and others better for casting (creating the replica of the sculpt that comes out of the mold). Within these two categories, there will be different brands and varieties, each with their own characteristics. Molding silicone is generally much denser and harder, so if you use it for puppet skin over the armature, you won’t be able to bend it easily. It also typically comes in specific colors like pink, purple, orange, blue, or green, which are not very convenient base colors for a puppet. Certain brands of molding silicone include Mold Max, RTV, and many others. Most types of silicone that work well for molding are of the tin-cure variety, but others that are platinum-cure can be used as well.
For casting a silicone puppet, the best silicone products are of the special effects variety. These are mostly platinum-cure, much softer, and will easily bend and flex over an armature like real skin. Casting silicone typically becomes transparent or milky translucent when it cures, and you want to make sure the activator that mixes with the base says it’s translucent on the bottle. For this reason, if you want a specific color for your puppet, an acrylic tint must be added to the base while mixing. This step has traditionally been important because silicone does not respond well to being painted afterward. In more recent years, however, there have been new developments made in certain silicone products that enable them to be painted. In those cases, tinting may not be necessary. All the same, tinting in the actual mixing process creates one less step later and ensures a smooth surface in the color you want, and you do not need to worry about hiding brush strokes in applying paint. For skin tones, you can alternatively use just a few drops of oil-based foundation from your local drug store’s make-up department. Some popular brands of casting silicones include DragonSkin, Plastil, and EcoFlex. There is also a product by Smooth-On called Soma-Foma, a silicone foam that bonds well with other brands and can help to create extremely lightweight puppets. (It also has an extremely fast pot life, about 30 seconds, so it must be worked with very quickly before concealing it into a mold for curing.)
Many different kinds of silicone, both for molding and casting, can be browsed and acquired through Smooth-On (http://www.smooth-on.com), the Compleat Sculptor (http://www.sculpt.com), Sculpture Supply Canada (http://www.sculpturesupply.com), and other special effects/sculpture service companies that sell these kinds of products, depending on your country of residence.
Casting a Silicone Puppet
The general rule for molding and casting, because the mold needs to come apart, is that if your cast is meant to be soft and flexible (like foam latex or silicone), its mold should be created in a hard material (like plaster). If your cast is meant to be a hard material that does not bend (like plastic or resin), its mold should be created in a soft material (like silicone). There are some methods for using silicone in both molding and casting simultaneously, which are mentioned later in this chapter, but for the time being, let’s say you need to cast flexible silicone around the armature of your puppet using a plaster mold. A basic overview of this process, using pictures from the creation of Charlie’s hands from Ava, is provided below. (All photos in these molding/casting sections are courtesy of Bronwen Kyffin and Melanie Vachon.)
The first step is to create the sculpt, which can be of the entire puppet or just part of it (Figure 3.58). When creating a sculpt for eventual casting in silicone, it is important to use a modeling clay that is not sulfur based. Silicone is generally sensitive to sulfur materials, which can cause it not to cure properly. Some brands of clay that can be used include Chavant NSP and Prima Plastalina. Before creating a mold for the sculpt, it should be lacquered with Krylon Crystal Clear spray to create a barrier that keeps it from getting damaged or reacting with any other substances to which you’ll be subjecting it.
The sculpt is surrounded completely by a bed of a different kind of clay, commonly a brand called Klean Klay (http://www.kleanklay.com), which is not sulfur based and is ideal for the mold-making process (Figure 3.59). Klean Klay is not recommended for creating the actual sculpt, but it works well for making molds. The clay bed is continually smoothed out and wrapped around the sculpt with a palette sculpting tool so that it is airtight and lines up exactly through the midpoint in a perfectly straight line around its perimeter. The point where the clay bed touches the sculpt is where the seam will be located when both halves of the mold are put together and pulled apart. In many cases, this is at the exact equator of the sculpt, but depending on the design of what you are molding, it is useful to find a point on the sculpt where the seam will not be very visible to the camera. In the clay bed, registration points called keys are created using marbles or any other round or square objects, which will help both halves of the eventual mold to fit together (Figure 3.60). It’s best to plan ahead when figuring out how you plan to get the silicone inside the mold. It’s possible to just pour it into the open molds, but you might prefer to have a hole for pouring it in while the mold is clamped shut. In this case, it’s a good idea to sculpt in a pry point for sticking in a tool to help pull it apart later. You also may want some exit channels built in so you know that the silicone is flowing through the entire cavity all the way to the highest point (like the fingertips for a full body cast). The idea behind the mold itself is to make it as perfectly smooth and functional as possible, think ahead to how you will fill it and get it apart, and think of it as two puzzle pieces that fit together around your sculpt.
Next, the sculpt in its clay bed needs a wall built around it in order to hold the shape of the first half of the plaster mold (Figure 3.61). The wall can be built from almost anything, such as slabs of clay or, in this case, large-sized Duplo or Lego blocks. The smooth, square shape created by the blocks provides a nice surface for the mold to be built against. To avoid leakage and air pockets, the corners of the Lego fortress can be patched with more Klean Klay.
Once the wall is complete, plaster (Ultracal or Hydrocal 30, not art-store plaster of paris varieties) is mixed and poured in to create the first half of the mold (Figure 3.62). After this hardens, the Lego wall is taken away, the clay bed removed, and all clay remnants washed away with water. The sculpt remains inside the plaster mold, which is coated with Vaseline to aid in eventually prying it apart from the next mold half. The process with the Lego wall can now be repeated as another layer of plaster is poured on top of the first mold half inside. When this is complete, there will be a two-part hard plaster mold for the original sculpt (Figure 3.63), which can now be removed. As an alternative to plaster, you can also create molds for a silicone puppet from a two-part plastic or resin. This would involve mixing and pouring it over the sculpt in a manner similar to this method.
Now, it’s time to get the mold ready to cast some silicone! First, the armature is laid inside the mold, and it needs a way to stay suspended right in the middle because everything around it will be eventually filled in with sticky, gooey liquid silicone. There are a few ways to do this, which may vary depending on the design of what you are casting. It also depends on how you plan to get the silicone into the mold. For Charlie’s hands, the armature was held in place by a lump of Klean Klay placed within the entrance hole where the silicone was poured in. Both halves of the mold should be clamped shut as tightly as possible to avoid leakage and to ensure that the mold will be filled up properly (Figure 3.64). As the mixed silicone batch is poured into the entrance hole, the mold cavity surrounding the armature, which is being suspended upside-down, fills up. When pouring the silicone into or onto any space, it’s important to avoid the creation of air bubbles as it collects and starts to cure. Bubbles can create unwanted warping in the surface of your puppet. To alleviate bubbles, it helps to first tap the container (with the silicone inside it) firmly onto a flat surface after mixing it; this forces the bubbles out. When applying it or pouring it out, you can also drizzle the silicone in a thin, high pour, raising it slowly about 1 foot above the mold (Figure 3.65). These methods will work well if you’re doing this on your own at home, and if you have the means or space, following this process in a specially built vacuum chamber will help to suck any air bubbles out of the silicone as you work with it.
Another method for getting the silicone into the mold is to leave both halves of the mold open, mix a batch of silicone, and start by pouring a thin layer directly into the mold cavity. This is essentially skinning the impression of the sculpt in the mold and creating the outer layer of the cast, so it’s important to get into every nook and cranny of the mold and avoid trapping any air bubbles inside. After skinning the mold, more silicone is poured into the mold to fill it up, and the armature is rested on top of the setting silicone in exactly the right suspended position. Within a few minutes, depending on the pot life of the silicone, the first half will begin to cure. Meanwhile, the second half of the mold is skin-coated and filled in with more silicone. Then, the first half of the mold with the armature in it can be flipped over, pressed onto the second mold, clamped shut, and left to cure completely. All of the silicone will bond with itself inside to create the bulk of whatever you are casting.
Once the silicone is cured, the mold is pried open to reveal the cast of the original sculpt, with its armature living inside and totally flexible for animation (Figure 3.66)! The next step is to trim off any extra flash of thin silicone that may have spilled over the mold cavity. This can be done with fine scissors to trim it right up to the edge of the seam. To help conceal the seam a bit more, it may help to cut a groove into the edge and patch it up with a thin layer of silicone. During this setting time before it completely cures, continual patching, smoothing, or texture stamping can still be worked into the surface, depending on the design and look you are after.
In another example, for Ava’s neck, an additional piece of silicone is cast to form a shape similar to a shirt dickie to go under her clothes (Figure 3.67). The shape itself is sculpted in clay, and the first part is molded upside-down so that the second half can be molded around it right-side-up. The result is a thin, flexible neck piece that has been shaped to fit around the armature over her shoulders.
Making a Silicone Mold
As I mentioned before, in most cases, a hard mold is required to create a soft cast, but it is also possible to create a silicone mold (using molding silicone) for a silicone puppet (using casting silicone). If it is a two-part mold similar to the plaster ones I just described, the steps for creating it, like using the Lego wall around the clay bed and pouring over it, are basically the same. The big difference in using this method is that you must apply onto your mold and sculpt a universal release agent so that the silicone will not bond to itself and can easily be pulled apart. If you miss a spot, it will bond as one piece because silicone only sticks to itself. If you are casting silicone within silicone, this is very important.
The advantage to a flexible silicone mold is that it is more forgiving than a plaster mold when it comes to undercuts. An undercut is any surface on the sculpt (and resulting cast) where the mold can easily get locked in underneath and cause damage to the cast when pulling it apart. Undercuts on the sculpt can be avoided by always creating shapes that are angled the right way toward it, but if they do occur, a flexible silicone mold is a bit easier to twist around it when releasing the cast. Creating a two-part mold that fits together and comes apart like the plaster mold can be done easily in silicone, and then it can be filled with any material that will create a hard cast. Within a soft silicone mold, you can cast duplicate hard copies of props, toys, and any hard parts of a puppet (Figure 3.68), such as accessories or entire heads. Silicone molds can be filled with resins, plastics, and even melted clay for clay puppets.
For Ava, the heads for both puppets were cast in plastic in a silicone mold. A different approach was taken in creating these molds, which were created as one piece in a bucket instead of a two-part mold created in two halves. The sculpt for the head is first done in sulfur-free plasticine clay, and then attached to a post to hold it in place. For a head or any other object like this, it’s a good idea to build the initial sculpt around a ball of foil attached to the post. This will help cut down on the weight and help keep it from slipping off when it’s suspended in the mold later.
The next step is to cover the sculpt in a thin skin of molding silicone to create the detailed impression that will be inside the mold. The silicone is drizzled over the top of the sculpt and pushed around with a brush to get inside every nook and cranny, without bubbles. Rather than brushing the silicone on, which creates more bubbles, it’s more like pushing it around and into the cracks, without worrying about washing the brush, either. As the silicone drips and collects at the bottom, it can be lifted back onto the top and essentially pushed around to coat it completely. The idea is to skin the entire sculpt in a layer of silicone without bubbles (Figures 3.69 to 3.71).
The next step is to suspend the skinned sculpt upside-down in a bucket (or in a plastic cup for a smaller-sized head). The post to which the head is attached can be attached to a wooden plank that rests over the top of the bucket or suspended in some other way to hold it there. Another solution, to alleviate the possibility of the head falling off the post and into the bucket, is to fill the bottom of the bucket with a layer of silicone and let it cure. This would act as a cushion to rest the head on while it’s suspended. Once that is all in place, the rest of the space inside the bucket is filled to the brim with more silicone and left to cure overnight. Bubbles are not as much of an issue in this filler space for the mold itself because they will rise to the surface anyway. The post to which the head or object is attached, suspending it into the surrounding mold, will also serve as an entrance channel for pouring the plastic in during the casting process. As another part of this process, it is important to note on the bucket itself (and later on the resulting mold) the positions of the front, backs and sides of the head inside (Figure 3.72).
Once the silicone is cured, it will be more firm on the surface and much less tacky. The mold can then be pulled out of the bucket or cup, and it will have taken on the same shape of the inside, much like a Popsicle (Figure 3.73). The next step is to get the head out! To accomplish this, an X-Acto blade is used to cut through the silicone as strata-cut layers in a zigzag pattern down a side of the mold (Figure 3.74). The cut is essentially acting as the seam for the mold itself. Therefore, it’s important to place this opening near the back of the object being cast, or somewhere the camera won’t necessarily notice it, in the event of cutting into the sculpt. The zigzag pattern also serves the purpose of registration keys so that the mold will always go back together in the same place. Cutting a straight seam line will cause it to slide around and could create inconsistencies in the resulting cast. Once the incision is complete, the flexible mold can be pried open to remove the sculpt, leaving a negative impression of the sculpt inside and a spout for pouring in the casting material (Figure 3.75). All remnants of clay that stray inside must be washed away completely with rubbing alcohol and water to avoid any bits of it getting stuck in anything else. Now, it should be ready to fill with plastic for casting!
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.
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).
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.
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)!
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.
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