Prop: Transducer WallAuthor: Byron Brummer • Adapted from posts in the forums at www.rockyhorror.org.


Vacuforming

[Regarding the glass blocks that build up the back of the transducer wall] vacuforming is the best way to go IMHO. The blocks I made for Midnight Insanity’s wall (8″) and more recently Sins o’ the Flesh (7″) are both of 1/16″ thick vacuformed polycarbonate (lexan). A bit pricy (I think it was $200 in just plastic), but well worth it. This is Sin’s wall:

sinswall

They’re mounted in an “egg crate” grid of 1/8″ luan plywood, 2″ wide strips, that’s mounted in a 1×4 wood frame. It leaves about 1 inch between the back of the egg crate and the back of the wall. The egg crate is covered with white fabric stapled to the frame, leaving the entire inch for lighting. The entire back is covered in 1/8″ luan plywood. The result is extremely light, ridged/strong, and clearly shows the block detail from a great distance. Two people can easily pick the wall up, although it’s on two inline casters to easily move it around.

The “monitor” screen was also vacuformed…in a matter of speaking. I stapled a sheet of lexan to the frame it’s in and put the entire thing in the oven on blocks until it drooped down (normal prep for vacuforming), and then just took it out. No mold required and it stays “clear” that way since a mold would have to be extremely smooth to not cause optical issues.

Something to seriously consider when it comes to size… DOOR HEIGHT! More likely then not you’ll have to get your wall through a standard door somewhere and if you make it too accurate in size…it won’t fit. Measure your paths.

The wall was built to be “upgradable”, such that the sections (switch(es), monitor, magnets) all just screw into a frame made of 1×4. When/if I or someone ever gets around to making the magnets in 3d and working, it’s trivial to swap them in. The luan plywood backing doesn’t cover the feature sections. What you see in the photos above were not intended to be the final version…they’re just stand-ins.

The rivets were made by rounding off the end of a 1″ dowel on a 4″ wide belt sander, and then cutting the end off on a chop saw. Over, and over, and over. I used a 1″ forstner bit on a drill press to drill flat, 1/4″ deep, 1″ wide holes where all the rivets go and glued them in. They will never, ever break off. Ever.


Making the Buck

To make the buck I cut a square rod of wood to the dimensions of the “bumps”, approximately 1/2″ x 1/2″.

Bracing that stick vertically against a sacrificial board, I ran it over a 1/4″ round over bit on my router table. All four sides.

Then to the miter/chop saw, with a block clamped to it to repeat cuts quickly I chopped off that top 3/8″ making a tile.

Lather, rinse, repeat another 80 times… Once you have the router and chop saw setup it goes pretty quick.

Those tiles were arranged on a square board, inside a frame 1/2″ wide, 3/8″ thick, 1/4″ rounded over.

1/16″ inch holes drilled through each corner of all tiles to help the vacuum pull through.

buck1

buck3

buck2

Note the buck is about twice as thick/tall as the final blocks. The extra height is important to account for the plastic rounding at the bottom…the sides had to be straight/flat to fit.

Here’s a few more shots of a sample/failed block and my test “egg crate”. You can see how the edge of the block is cut flat with a bandsaw. Also…you can see what happens when I left the plastic in the oven a bit too long…

While not shown here, the blocks were attached to the egg crate (painted white) with white silicone caulking. I forget the specific type…it took a few tests to find something that stuck to polycarbonate (the vinyl kind didn’t hold well).

block1

block2

block3


Styrene vs. Polycarbonate

Styrene would definitely be both cheaper and easier to work with; it’s one of the easiest plastics to form. Polycarbonate by contrast is one of the most difficult materials by far to vacuum form. The disadvantage is strength; styrene is brittle and the thin spots from forming will be much more critical failure points.

In the design I used the grid and tiles work together like a honeycomb core panel. This transforms normal stress (bending) into sheer force absorbed by the “skin” of tiles. We can get away with an extremely weak core structure (2″ wide 1/8th in thick luan plywood is extremely weak) so long as our skin’s shear strength is high, which allows us to build it very light while still being incredibly strong.

Polycarbonate is incredibly strong, both normal and shear force, making it the ideal material for this use. But it’ll cost you about $1 – $2 / tile (which adds up, given how many tiles you need to make). And being hard to work with you’ll have more then a few bad pulls, especially on a home made forming rig.


Difficulties with polycarbonate

Moisture: This is a huge problem because polycarbonate absorbs moisture from the air (it’s “hygroscopic”) and when you heat it up to forming temperature that moisture turns to steam causing a ton of small bubbles, fogging it up and looking like ass. Bad. Real shops have expensive prepping machines to deal with this. My solution was to pre-bake it in the oven at 250 degrees for four hours. I made a little jig (out of wood…wood won’t burn at 250) to hold a half dozen 1 foot square sheets. I could only do small batches because I only had the one oven which I then used to heat the sheets to molding temperature.

Molding temperature: Poly has a very narrow working range. Too hot and it’ll droop too much/too fast and sag on your buck (see picture in previous post). Too cold and you’ll never get it to form on the buck far enough before it cools.

Speaking of cooling…since the working range it small it effectively cools off very quickly. You have to work fast.

It’s also a slow, stiff plastic. This means it takes more force (stronger vacuum) to form then something like styrene. You can get away with a 1-stage vacuum system such as just a Shop-Vac for styrene, but you’ll absolutely need a 2-stage system with a vacuum reserve (a large container holding a high, near absolute vacuum) to pull poly. The Shop-Vac pulls the sheet down and gets the bulk of the air out, before switching open the 2nd stage to do the hard press.

And the working temperature is hotter then most, making it much easier to burn yourself. The fact you have to move very quickly makes this even more risky. Make very sure you have your route very well planned and clear. No kids or pets allowed anywhere near you.


Home Rig

But that all said…you CAN pull polycarbonate very well with a ghetto home rig. It just takes a little practice and finesse. My own rig was my gas kitchen oven (electric is ideal, more even heating and less moisture), a Shop-Vac (stage 1), a 5 gallon air tank (vacuum reserve), and a venturi vacuum pump attached to my air compressor to both pre-evacuate the tank and running as part of the 2nd stage switch.

The reason I went with the venturi pump is because they are dirt cheap and I already owned an air compressor to power it. A real vacuum pump would work much better, I just couldn’t justify the cost. For comparison, here’s what Harbor Fright offers:

Air Vacuum Pump – $19.99: This is the venturi pump I used (modified to use standard air fittings and a 1-way check valve). It sucks ass and really takes a LOT of air compressor power behind it to power it effectively. The only thing it has going for it is the price…but even if it was free it wouldn’t be worth it. It did get the job done…eventually…but man it made it slow and difficult. :-/ Oh, and did I mention it screams like a cat being boiled alive? Ambulances have nothing on this beast.

2.5 CFM Vacuum Pump – $99.99: If I did it again…I’d at least buy this. The venturi pump just doesn’t work well without a HUGE air compressor behind it….mine is only 2hp/2gal.

3 CFM Two Stage Vacuum Pump – $159.99: This is what I would probably get if I did this again, especially at this sale price. You can’t have too much vacuum power when pulling polycarbonate!

And this is the air tank I used to hold my reserve vacuum: 5 Gallon Portable Air Tank – $29.99.

Can you tell I like Harbor Freight? 😉 I took out the fittings and set it up with a 3 way: One going to the vacuum pump (with 1-way check valve). Another going to a ball valve (to open/turn on the 2nd stage). And the other line running to a vacuum gauge to tell me when my pre-vacuum was “full” (read: really empty).

This tank worked out ok, but again I’d probably go bigger if I did it again. Probably at least one more of these (you can get away with a slower vacuum pump if you have a larger reserve vacuum chamber).

If you have access to a real vac former it’s a hell of a lot easier. Baking out the moisture still applies however (for poly).


Beefier Wall Blocks

I’d say be cautious about beefing up the structure as you can add a hell of a lot of weight in a hurry. A lot of pretty control panels built over the years have seen limited use just because they’re ultimately too heavy to be practical at many theaters.

That said every situation is different. Indecent Exposure’s panel was a grid of 3/4″ plywood, a full 12″ deep, heaviest thing I’ve ever seen. It lasted decades though, including many road shows and even fell out of the truck on the freeway and lived to do the show that night.

314_cross-ribIf I was going to consider a beefy wall I’d take a serious look at the newer acrylic blocks available: www.hy-lite.com

I’ve seen these in “tile” form as well; Solid 1/2″ thick, much lighter then a full real block, but still looks mostly the same. They’re very pretty.

The biggest obstacle for me with these are the limited sizes they come in; Almost exclusively 6″ or 8″. 6″ makes for a pretty dinky sized panel, while 8″ create a panel that won’t easily fit through a standard door. The vacuum blocks I made for Sins’ were just shy of 7″ (the structure is on 7″ centers). If I could find someone making an acrylic 7″, single wall, cross pattern tile/block I’d love to try making a panel with them. It’s entirely possible something like that is on the market already.

Another option would be to cast your own acrylic block tiles. Acrylic casting is pretty cheap and easy, the largest obstacle would be creating the master. I’ve thought about taking one of the commercial 8″ blocks and cutting out a 7″ square from it, then building up the less-critical edge. That smooth surface would transfer well to silicone for the mold, ensuring the acrylic castings are optically clear.

TMTOWTDI 😉

I think I’d only really recommend acrylic block if someone could find a supplier of the thin/solid type, in the correct pattern, in a 7″ size. Even then only for situations where an extremely heavy control panel wasn’t a issue, because between the weight of effectively a six foot square, 3/4″ sheet of solid acrylic and the beefed up support structure to hold that extra mass, it can’t help but weight quite a few times heavier then a vacuum poly solution.


Conclusion

Vacuum formed polycarbonate I’m still convinced is the way to go for most folks looking for an upgraded control panel. The look is 90% of what a 3D block would offer, with much less weight and much easier to accomplish. Building and using a home vacuum former is IMHO, easier and safer then working with all the chemicals required for casting, especially on the large scale required.

And once you’ve got the vacuum former you’re half way to making your own Storm Trooper armor. 😉

#Author: Byron Brummer#Prop: Transducer Wall

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