For three years in a row I’ve brought my homemade prop and costume exhibit to the Bay Area Maker Faire. I try to make it bigger and better each year, so in an attempt to outdo myself yet again, I’ve decided I need to build a life-size replica of this handsome devil:
This is ED-209, the killer bad-guy robot from the original Robocop movie. If you’re just now tuning in, be sure to look at the previous articles in the series, where I explain deciding on the scale and knocking together some of the prototype parts, building fiberglass molds to replicate large parts, and making silicone rubber molds to cast small detail parts in urethane resin.
The whole build is coming together at a good pace and I’m still on track to (barely) get this guy together in time for the gates to open at the Maker Faire next month. There’s not a moment to spare, so let’s move on to…
Thorsson’s Insane Project #209, Part 4: Vacforming
Vacforming, also called “vacuforming,” “vacuum-forming,” or “thermoforming,” is a process in which a sheet of plastic is placed in a frame, heated until it’s stretchy and pliable, then forced over a forming buck (often called a “plug,” “mold,” or “pattern”) until it cools. Once it has cooled and become rigid it will stay in the same shape as the buck even after it’s been removed. If you’ve watched the video above you should have a pretty good idea of how it goes, but here’s a few more details.
The wide variety of plastics that can be formed in this manner make it an amazingly versatile fabrication method. Vacformed parts can be found everywhere; unless you live under a rock in a dark corner of an all-organic isolationist nature preserve, you come into contact with them every day. They include everything from car dashboards to the thin styrene lids you see on disposable coffee cups. Even gun turret windows on World War II aircraft were often made using this method. The forming bucks can be made of wood or metal or molded gypsum products, and the equipment needed can be bought or just as easily homemade. In short, vacforming is a valuable skill to add to your repertoire.
The main disadvantage to vacforming is that it’s not suitable for making parts that have “undercuts” that will lock the plug into the formed part. The best way to describe this is graphically, so let’s look at a pair of examples.
First, here’s an ideal shape for vacforming:
It’s a fairly shallow piece that has no undercuts or negative angles that the sheet of plastic will wrap around and lock onto. Once the plastic has cooled, the forming buck will practically fall out of the bottom.
Here’s a tougher shape for vacforming:
This is one of the ammo boxes that will be attached to the gun pods. It’s a bit on the tall side and the sides are nearly vertical, so there will be webbing where the plastic tries to stretch around the corners, but there are no undercuts and with a bit of coaxing the formed part should slide off of the forming buck without breaking. So it’ll work.
Here’s a terrible shape for vacforming:
This piece has all sorts of high details close together with deep valleys in between them. The plastic is almost certain to form webbing when it creases and stretches across these shapes. The worst part is the other side though:
Once the plastic sheet has cooled around this shape, it’ll lock into place and something will have to break in order to remove the forming buck from the formed part. I could mitigate this by tweaking the design a bit, filling in the undercuts and making the raised details as separate castings, but in this case I’ll just skip all that trouble and make a silicone mold to cast the part in resin.
Once you’ve designed the forming bucks, it’s time to cook some plastic. In my workshop, I’ve got a homemade oven specifically designed for this application. You can see it hanging from the rafters in many of the photos:
The “oven” is essentially a series of nichrome wire heating elements that run on 220V in a double-walled steel box with hooks on each end to catch the the frame that holds the plastic. It’s not especially pretty, but it gets the job done.
To use it, I just clamp a sheet of plastic into the frame, hang the frame from the oven, plug it in, and wait.
While the plastic is cooking, I lay out the forming bucks I’ll be using on the table:
Once the plastic is warm and stretchy, the oven is turned off, the vacuum is turned on, and the sheet is pulled down over the table:
Once the plastic sheet has cooled, it’s just a question of quickly trimming the waste plastic off of the formed parts, gluing them together, and adding some detail parts.
The same process is going to be used for all sorts of parts. The most visible will probably be the rails that stick out of the back of the legs. Since they’re going to be viewed from all sorts of angles, I won’t be able to get away with just making the outsides. So once I made up a set of forming bucks for the inside and outside, it was time to pull a mess of parts:
There are all sorts of different ways to attach thin plastic parts together. Since these pieces were pulled in sheet styrene, I can get good, strong joints by using a solvent cement. In essence, the solvent dissolves a bit of the plastic and the parts are pressed together. When the solvents flash off, the solids are left behind and the parts are effectively welded together.
Of course, the first step is to trim the edges, line up the parts, and tape them in place so they don’t slide around:
In order to facilitate solvent welding, the parts need to be clamped securely. When I was younger and dumber I used to invent all sorts of amazing apparatuses with sticks and strings and weights and pulleys to apply pressure in the hard-to-reach places on the inside of parts like this. It wasn’t until one night a few years ago, when I got stuck holding some armor parts with my gloved fingers while waiting for solvent to dry, that my friend Trevor looked over and asked, “Why don’t you just hold it with magnets?”
The man is clearly a genius.
Now I keep a large stock of rare-earth magnets on hand for this exact purpose. It’s as though the gods gave lodestone to ancient man just so we could one day have an easier way to hold together our costume armor parts while the glue dries.
With everything clamped securely in place, I use a syringe bottle to drip a bit of solvent along the top of the seam.
Capillary action draws the solvent into the seam and then it’s time to wait for the plastic to dissolve a bit. While it’s still a bit gooey, I slide the magnets back and forth along the seam to squish the parts together a bit more:
I usually take it as a good sign when a bead of squishy plastic squeezes out and stays out.
After a few minutes of drying time, the two parts have become one and the seams are essentially as strong as any other area.
So that’s a few more notes about vacforming. Here’s a few more photos of work in progress:
As always, I’m uploading photos almost daily to the MAKE Flickr pool, so if you’d like to see the latest pics of what I’ve got together, be sure to check it out.
With only 33 days until I have to load the finished prop onto a truck, there’s more to come (provided my head doesn’t explode in the meantime).
Stay tuned…
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