It was August 2007 and I was just beginning to incorporate electronics in costumes-
Predator costumer Carl Toti contacted me via the Hunter's Lair, wanting to know if it was possible to add animatronics to a replica Predator backpack and cannon he was creating. I believed it could be done so we began collaborating on the project and we've been friends ever since.
Carl is an extremely talented sculptor and an absolute perfectionist. He didn't just want to make a replica backpack and cannon- he wanted them to be as accurate and faithful to the original movie items as possible- which proved to be a tall order. The Predator backpack and cannon are pretty complex movie props and there really wasn't a lot of documentation available concerning how the original props were made, let alone any really good close up photos of the original props. Carl would spend nearly five years on his quest for Predator nirvana, gathering bits of information as it became available, constantly re sculpting to make it more accurate to the original. But there were always areas of his sculpt that were questionable because he could never find photos of the original backpack and cannon taken from the right angles that would show him what he needed to see. Then..... pay dirt!
Fast forward a few years. Carl got a lucky break- two individuals happened to be in the right place at the right time. Art Andrews (he runs The Replica Prop Forum- better known as The RPF, as well as The Hunter's Lair and The Dented Helmet) and friend George Frangadakis were able to obtain multiple photos of the original props and supply them to Carl. The photos revealed an enormous amount of detail he hadn't seen before and while it would require an extensive overhaul of his sculpt, it would be worth the effort.
When I received the backpack I couldn't believe it- it really was one of the most impressive prop replicas I had ever seen. The photos just don't do it justice. Here's how Carl created the backpack- Tools and Materials
Clay sculpting tools
Home built roto-caster
2 gallons of Smooth-On Plasti-Paste II for the mother mold http://www.smooth-on.com/Urethane-Plastic-a/c5_1120_1346/index.html
1 gallon of Smooth-On Smooth-Cast 300 casting resin http://www.smooth-on.com/Urethane-Plastic-a/c5_1120_1209/index.html
"The sculpt started with a dense foam pull of the torso armor which had to be sculpted and cast first (naturally). I then constructed a foam core and hot glue core skeleton which clay sticks to very nicely. I used Chavant medium NSP clay which comes in 10 lb. blocks which I melt in my crock pot. The physical property of the clay is such that it is easy to work with when warm, and feathers nice. As it cools, it becomes harder, which makes it ideal for machine-like parts because you can actually carve lines and grooves into it, etc.
I populated the sculpt with "greeblies" (a term in the prop building world for small detail items) I obtained from model tank kits, as well as having a lot of them custom printed from Scott Andrew's 3D printer
to exactly match the ones I couldn't find, as seen in the reference photos. Above all, the backpack and cannon had to be screen accurate. I molded the backpack in place on my mannequin with brush-on silicone, followed by a Plasti-Paste mother mold on top of that.
If that wasn't enough, I then had to figure out how to build a roto-caster large enough to cast the darn thing!"
Chavant NSP is a sulphur free clay- if your clay has sulphur in it and it comes into contact with a silicone mold material the silicone mold will not cure. So once the sculpt is finished you have to mold it. For large items like this backpack a mother mold is the way to go as a large box mold would be impractical- a box type mold would use an enormous amount of silicone molding compound and it would be extremely heavy. A mother mold is a type of mold that has a silicone layer surrounded by a rigid backing (often called a jacket)- sometimes the backing is fiberglass or plaster but in this case Plasti-Paste was used as a large plaster backed mold can be really heavy.
To make the mold first parting lines are created on the sculpt using card stock or thin wood pieces- this helps divide the mold into multiple sections that can be bolted together. Next silicone molding compound is brushed onto the sculpt- it begins with a thin layer followed by a heavier layer (a thickening agent is added to the silicone.) The thin layer is brushed on first so it will capture all of the tiny details in the surface of the sculpt. The thin layer will allow air that is trapped to escape in the form of bubbles- if you brush on a thick layer the air can't escape and you can end up with a lot of surface imperfections in your cast part. While the silicone is curing small chunks of silicone from old molds are stuck onto the surface, forming keys. These keys will help hold the silicone in place against the rigid backing so everything lines up just right when you go to cast resin in the mold.
Once the silicone has cured a rigid backing material is applied to it. After the backing has cured additional sections of the sculpt are molded using the same process. When molding the additional sections a mold release agent (in this case Vaseline) needs to be applied to the mold parting lines as silicone will stick to itself. Once all the sections have been molded holes are drilled through the rigid backing so bolts can be used to hold all of the mold sections together. Next a hole is cut into the mold so the resin can be poured in- the backpack casting uses approximately 96 oz of casting resin. After this is done the mold can be taken apart and the sculpt is removed from the mold and the mold is ready for casting in the roto-caster.
The roto-caster was needed because there is no way to slush cast it- the mold is simply too big and too heavy to try and hold in your hands- the mold ready to cast weighs over 50 pounds. The roto-caster spins the mold in a frame-within-a-frame assembly so once you pour the resin in you spin the mold and you get a nice even resin wall thickness in your finished casting. The backpack and cover for the med kit area were done using separate molds. The backpack is entirely hollow to reduce weight and allow room for electronics as well as a future screen accurate med kit.