Mythbusters plastic mold trick

[patooyee]

If anyone has ever seen the epidsode of Mythbusters where they build Buster II from scratch, you'll remember the vacuum mold machine they used to create the molds out of plastic to make Buster's flesh out of. Basically what they did is carved the body pieces out of foam and placed the pieces on a table inside of a vacuum chamber that had a sheet of clear plastic at the top. They then heated the plastic until it was pliable, lowered it down onto the molds, and applied vacuum to the underside of the plastic to form it perfectly over the foam "decoys."

I would like to do something similar for a buggy hood, mianly because I want something unique, but also because no factory hood is easily workable for my buggy. Carving the foam decoy would be easy, but creating a vacuum chamber would probably be fairly difficult. (Vacuum source would just be a shop vac.) It would also be difficult to heat the plastic evently so that it would form right.

I was thinking that it may be possible to do something similar with a heat gun and some clear plastic stuff. (Forgot the actual name for it right now, but you can get it at Lowe's.)

Anyway, what are your ideas? Heat gun plausible? Vacuum chamber plausible? If so, how woudl I heat the sheet of plastic effectively.

J. J.

[Scout Dude]

Mike (login name on here) made a vacuum thing for doing wood laminate. See if he can help you.

[sn0border88]

why not make the foam mould and take it to somewhere that does it. I doubt they would charge much.

[sceep]

mold it then lay glass.

[RickyRetardo]

X2 on making the mold and laying fiberglass. In the long run, it might be cheapest way to go as fiberglass is pretty easy work with and to fix, or if it's completely destroyed you could make a new one in no time.

[CrashKnott]

It's called thermo-forming, and it's not as easy as they made it look. Plastic is some particular stuff when it comes to melt temp, versus burnt plastic.

Scout Dude's got some good advice though...make it out of whatever you can, then try and find somebody who does thermo-forming. Keep in mind, the size of the "bed" of the thermo-forming machine will likely dictate how big your hood can be (usually beds are 36" x 36")

A more feasible solution would be to carve it out of foam or whatever, then lay fiberglass over it. You can either use that fiberglass piece to make your hood, or as the negative to pour some acrylic or 2 part something into for your hood...all depends on what you're trying to do and how much effort/$ you wanna put in.

Good luck!

-I seem to remember someone making buggy hoods out of clear acrylic kinda recently.

[jpmassey]

We did this exact same thing for a race car body a few years back. I built molds out of foam (spray can stuff) and wood, layed fiberglass on top, and then smoothed it out with bondo.
The trick is to heat the plastic to the right temp, too little it doesnt form, and too much it turns liquid. I ended up building a big oven, basically a big metal box made from an old AC system, and adding several 220v oven coils inside. It allowed the temp to be held constant and even.
The plastic was clamped in a wooden frame, and when ready, was simply removed from the oven and pulled down over the mold. We had drilled hundreds of holes in the mold to pull a vacum with, but it turned out to be much more effective just smoothing it out by hand.

[Triaged]

x3

Read some of the how-to articles here
http://www.fiberlay.com/

[guidolyons]

J.J.-
I assume you would be using the vacuum mold to make the mold, then lay fiberglass for the actual hood.

I think it would be fairly difficult to make a vacuum mold and get a sheet of the plastic large enough to make a mold for a buggy hood, probably cost prohibitive for a the few times that you would use it.

Years ago in college I took a plastic manufacturing class and we had a small 2'x2' vacuum mold. It had a large heating element similar to a heater or an electric stove, held the edges of the plastic sheet firmly you ran the heating element until the plastic got soft, then lowered it over the mold, the lower half had a bunch of small holes like an air hockey table to draw the air out.
Yes a shop vac would work, the challenge would be heating the sheet evenly, getting it into place, drawing the vacuum evenly, all before it began to harden again. I honestly don't think you could heat a section at a time, it would have to be done at once. Also, the plastic sheet (Polystyrene IIRC) sheet hardened fairly quickly and was pretty brittle once it hardened. Not all plastics can be used for vacuum forming, and fiberglass resin will disslove polystyrene.

I doubt that a vacuum bag like woodworkers use for laminates would work, because of the heat invovled.

Like snoboarder said, make a "decoy" form you like and check around and see if you can get a few molds made from it from a plastics place that has a large vacuum mold. Maybe somewhere that does boats?

Or go ahead and make a mold from fiberglass, then use the fiberglass mold to make your hood.

[paulkeith]

its already been said, but fiberglass man. it will still be easy to get a unique shape, and is relatively cheap if you buy bulk scrap glass (check ebay, they sell it by the pound). get a block of foam, cut/hotwire/sand to the shape you want it, and lay the glass. pull/scrape the foam out, sand/bondo the top, prime, and paint if you want to. done and done.

[soilantgreen]

One of my customers is a large wholesale supplier of acrylic pan faces for interior lighted signs. (You know, those signs that have the lights on the inside that, at night, light up so you can still read what the sign says?) He builds such big faces that he has invented his own giant ovens for making them. A large sign shop like that could make what you want, BUT fiberglass would be so much easier.

[ZukIzzy]

Fiber glass is easier but I have built several airpalne canopies from plexi and it is doable. Your oven can be plywood or even a large cardboard box for a one time thing heated by a couple heat guns or electric shop heaters and vacume from a shop vac is fine. You have to build a plug or mold and then work out the vacume. Pool table felt is your freind . You cannot heat form Lexan in a home shop. It will haze and "craze" ( thousands of small cracks) when you heat it exposed to air. I have tried and could not get it to work.

wayne

[upnover]

I have done some reverse molds using Rhino Linings. It's pretty fast to do due to the fast set up time and in the end its less prone to cracks and breakage than fiberglass.

Wayne

[LT1SCOUT]

Originally Posted by jpmassey

im pretty sure that thing was deemed a giant fire hazard, taken out back, and shot.

[kiyul4130]

Originally Posted by patooyee

If anyone has ever seen the epidsode of Mythbusters where they build Buster II from scratch, you'll remember the vacuum mold machine they used to create the molds out of plastic to make Buster's flesh out of.
J. J.

did you watch that episode yesterday? cause i know i did.

[patooyee]

Yes, but I think it was a rerun, right?

J. J.

[heep86]

jj, if you still know anyone at auburn, then find someone in industrial design. when i was there we had 2 vaccume formers. one would accomodate 4 x8 sheets of polystyrene. the students should have after hours access to it. i dont think the heat gun will work, you need to heat the whole sheet at once. give me a call and i'll tell you more about the process

[ThatKid]

Vacu-Forming is expensive and hard.

Fiberglass would be a shit ton easier, and cheaper.

[nissancrawler]

I think fiberglass would be much better for repair, too. If it cracks, file it out, fill it with some kitty hair and resin, and off you go again. If you want to get really lightweight, you could use microbeads, too.