My halloween costume this year is a somewhat obscure Rick and Morty reference from season one. Here's a clip for the uninitiated (spoiler alert??): https://www.youtube.com/watch?v=gF-i9ypRDXQ . I wanted to make the horrifying mechanical underwear.
I decided to use foamed PVC sheet (usually sold as Sintra or Komatex) because it's flexible enough for a wearable but sturdy enough to mount hardware to. It's also really cheap and I thought i'd be able to heat form it reasonable well.
I have precious little experience pattern making so I took the brute force approach and cast my underwear region in plaster. I apologize for this picture.
After a disastrous attempt with torn bedsheets and plaster of paris I bought some plaster cloth from the art store. That stuff works great. Just wrapped a pair of briefs in packing tape, stuck my legs through two produce bags and went for it. It dried before I'd finished watching Christine.
I cut off the plaster panties with sheet metal shears and hot glued them back together once the plaster felt dry, adding some cardboard struts through the middle for rigidity. Next I cut a VERY rough shape out of pvc, clamped it to the form and starting waving the heat gun over it.
I fully expected to ruin the first attempt but it turned out solidly Good Enough. What worked best was heating small sections at a time and holding it against the mold with my bare hand since it wanted to stick to my gloves that had a rubberized grip. It's still formable when the temp is just under the pain threshold and that way it also didn't pick up any glove texture. When it gets too hot the surface will start getting kind of rough (this may also be the point it begins emitting toxic gasses?), but it's formable before then. Also don't put bare clamps on it like it did because when it softens they'll sink right in.
The underwear in the show had a second layer so after I trimmed the first one down and made sure it fit I put it back on the mold and formed a second layer right on top. Here it is after I'd cut out the interior voids, trimmed the outline and drew in the LEDs for reference.
I primed both pieces when they were clamped together so I could use the top piece to mask off areas that needed to take glue later.
Meanwhile I'd been obsessing over spike design.
I agonized over the dimensions for far too long and eventually settled on this design, printed in ABS. The head is hollow to decrease the mass getting thrown back and forth and I embedded an aluminum tube through the arm for support and mounting. Probably could have saved some more weight in the arms but costumes are made of small concessions and I was worried about finishing on time.
The halves were glued with Weld-On 16 solvent glue and enthusiastically clamped. It says it's for acrylic but works great with ABS.
The aluminum rod through the middle gave me a solid-enough thing to cut threads into to mount the arms to the servo horns. I didn't want to risk mounting directly to the ABS especially when the whole thing was gonna be whipped back and forth repeatedly.
The dried arms were sanded and brushed with an ABS/acetone mixture for strength and to soften any high spots.
Ugh, definitely should have made the arms thinner and longer but it was too late to change anything.
Notes on Rub 'N Buff
The internet told me that Rub 'n Buff was the deal for putting a metal finish on prop pieces and my own experiment with metallic spray paint looked terrible so I went the Rub 'n Buff route. The internet further told me to prime and sand your pieces really well to get a nice looking finish. After doing a bunch of tests with the materials I was using I'll add:
-Start with a really smooth surface. Like really smooth. I wet sanded down to 1000 grit which looked pretty good but any spots i missed really stood out once the Rub 'n Buff was applied.
-Apply thin layers. I got the best results using my finger and a decent amount of pressure to really smear it out. A little covers a large area and any thick chunky patches will quickly dry and don't buff out. Sure you can tell yourself they look like heat scale on hot rolled steel and they sort of do but they mostly look like clumpy wax.
-Buff it with a real buffing cloth. Where I had applied a little too much the palm of my hand was good to shine it up and knock off a little of the texture, but a microfiber cloth did a much better job where I'd applied a thin even coat.
-It dries really fast so do small sections at a time.
Looks pretty okay.
I masked off the area where the top feature would get glued to the base. Rub 'n Buff is a wax and would doubtless not work well with glue.
You can see where I got lazy with sanding or messed up the application. It's pretty hard to get a consistent surface with this stuff over a large area. Still, nice and shiny. To get the darker surface on the base I mixed black and silver. The top feature and spikes are pure silver.
I had some really bright surface mount LEDs I used for the lighting. I didn't want to metal-ize the disks of lighting acrylic I painstakingly cut out so I glued them in after. I had this idea that I would use the big light as a reflectance based button for triggering the spikes, hence the light sensor in the center, but I never had time to try and implement it.
With the cover glued on and the lights partially blinding you it actually looks pretty good.
Servo mounting was kinda last minute. I cut some brackets out of PVC and printed out some standoffs to bolt the assembly to the side.
There's an outer bracket which supports and contains the arm as it rotates (the servo horn isn't actually screwed onto the spline) and an inner bracket mounted to the servo through its mounting holes. A bolt passes through both brackets and a standoff (white) which clamps the whole assembly pretty securely to the side. It's not pretty but it looks decent enough from the front.
I feel like I owe someone an apology for this. Is there an engineering version of Catholic guilt? I'd wanted to use an ATTiny2313 to control the servos and have a few buttons for different modes but at 9:30 on Halloween night I just loaded a sketch onto an Arduino and gaffer taped it to a LiPo pack. There's a button to start and stop the servos. You turn it on by plugging it in.
That whole mess is taped to a piece of webbing hot glued to the inside of the inner PVC piece. There's a plastic clasp for quickly detaching the whole thing in case the LiPo starts shooting fire. Note that I don't endorse strapping these batteries to your body in this fashion.
It's actually really comfortable to wear and at least from the front looks pretty close to the show.