The master model for the foot claw was fashioned out of an oil-based clay. Once the basic claw shape was made (gray clay) an extension was added to the rear where the claw will meet the costume fabric (brown clay). The colors of clay generally have no associated meaning or properties, although in this case, the brown clay is softer. The firm gray clay assumes a very consistent, smooth finish just by using the fingers to rub and shape it. It also helps to warm the clay by working with it in smaller chunks. The model is built actual size, and as finished as possible.
|Plaster molds don't work
Until recently, the only mold making I was involved in was plaster or plaster-based investing, such as in lost-wax casting. I have used plaster molds to cast clay slip (liquefied clay), as well as plaster and wax models. I already tried a smaller test model, and this failed when the epoxy glued itself to the plaster. I figured that the mold-release that I was testing just wasn't designed for use in plaster molds (instructions indicate that it is for use in rubber or latex molds with epoxy or polyester resin castings.) To create a plaster mold, the clay claw is imbedded about halfway into the plaster.
Holes can best be scooped out of the plaster while it is in the process of setting. If you try to do this after the plaster sets, plan on using power tools. The depressions will be filled with plaster during the second stage, creating an alignment pin-and-socket arrangement. The model and first layer of plaster are coated with Vaseline to allow them to separate once the second half of the mold sets.
Plaster was scooped out at the thick end of the claw, then clay was used to create an extension from the claw to the edge of the mold box. This becomes the fill opening to the mold. After the plaster sets, the mold is forced open and the clay model removed. The entire surface of the plaster mold and the claw shaped depression are coated with Vaseline to seal it and prevent the resin from sticking or leaking at the mold seam (in theory, anyway.)
If plaster molds worked as well as I imagined, it would not have leaked epoxy resin all over my drop cloth. Eventually the leak slowed down as the epoxy thickened. As you can see in the image to the right, Vaseline is not proof against epoxy sticking to the plaster any more than the mold release was.
The clay master set into plaster. After a coating the plaster with Vaseline, a second layer of plaster is poured over the top.
Epoxy leaks out of plaster molds
Not only did the epoxy leak, it also sticks, causing the first cast claw to break.
|Rubber molds do work
For the next attempt, I tried using two-part rubber mold compound. Instead of doing this like the plaster mold, in two parts, the claw was suspended directly into a box filled with the rubber mixture. Once the rubber had set overnight, the box was peeled away (sort of.) and a razor blade used to cut the rubber down one side. This allows me to break the seal between the rubber and resin when pulling the casting out. Once the clay model was removed from the rubber, I'm ready to start casting claws.
All but one of the claws was cast using 2-part marine epoxy. The pale yellow appearance of the epoxy is completely concealed by the powdered charcoal used to tint the mix. The second claw I cast in polyester resin. After 10+ hours in the mold, the surface of the resin was still tacky, although the portion exposed to the air was completely cured. Clear spray-on gloss polyester seals this surface and creates a nice finish. The epoxy did not have this problem, and I have been able to cast two claws a day, allowing as little as 8-10 hours between pouring and removal from the mold.
|Clay model suspended in two-part
A bucket of claws, plus one in the mold.
|Finishing the casting
By only using a little powdered charcoal, the thin edge of the claw has a slight translucency. Since each claw is cast from a new mixture, each claw varies. Some claws have slightly translucent tips, while others appear fully opaque. The spring will be used to attach the claw to the steel frame of the foot, or to copper tubing in the "hands".
Surface imperfections and irregularities are not removed, partially because actual claws would not be flawless, and second, because it won't matter for the overall (10 foot rule*) costume appearance (or I'm lazy - take your pick.) One other consideration is that the claws are going to be very exposed, and subject to scuffing and scratching, just like a new pair of shoes. Even if they start perfect, they will not remain so. Vaseline residue form the casting gives a semi-gloss finish to the claws, and does not require the final clear coat.
A finished epoxy claw
* 10 foot rule - Costume Judging is generally done from a distance of 10 foot or more. Costume details that cannot be discerned at this distance generally would not impact on any judging decision. In craftsmanship judging, details count, but so should consistency.)
This process with clay and a rubber mold was repeated for the hand claws, although the original model was slimmed down to make a slightly smaller version.
The frame for the claw is made from 3/8" copper tubing. The spring embedded into the epoxy claws will just fit inside the tube, and a screw or pop rivet will prevent it from easily pulling out. Patterned after the feet, the hands have three claws on top, and two "fixed" claws below.
Soldering steel washers to either side of a tubing section creates a finger joint. A copper rod is placed in the opening of the washers, then soldered to the base tube (or another finger joint.) Using pop rivets, springs are then attached to the hinged joint, pulling the finger closed. When creating a multiple-jointed finger, small tubing is soldered to the back of each joint to guide the cable. In this case, only one joint will be made, and only on the top three fingers. A length of copper tubing is soldered to the base of the hand, to extend up the forearm to the elbow. 1/4" tubing is soldered to the arm to run cables to each finger. Cable thimbles are attached to the end where I can pull each one with a finger, while holding the grip attached to the arm section.
Side view showing spring attachment and the cable guide tubing.
A bicycle hand grip and the finger cable guides.
The hand claw master.
The base frame for the hand.
Hinges are made by soldering washers to the finger joints.
Tubing to guide the cables is attached to the arm.
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