Custom Car Bubble Top

One of my bucket list projects has always been to build a reproduction of Ed Roth's magnificent Mysterion show car. To make a short story long, he built it in 1962, showed it for a couple years, sold it to fellow custom car guy Ray Farhner in about 1966. Ray showed it for a year or so but the car was so poorly built the frame kept breaking just hauling on a trailer to shows. It was mercifully destroyed in 1967, never to be seen again. Revell made a model kit of the car and Mattel made a Hot Wheels version. It is undoubtedly the least documented, most famous car of the golden era of car customizing. Above is the magazine cover that made me fall in love with the car and the second photo is of the reproduction I built, finishing it in 2016. I even wrote a book about the car and my project!

This instructable is on how a bubble top is made for one of these show cars.

Step 1: Designing the Top

Of course the first step is to design the top. There is very little information in the literature on the car. Fortunately Revell produed a plastic model kit of the car. Under close scrutiny, the model is not very accurate in key details but it is a very good reference for scaling major parts of the car. It is an accurate 1/25th scale. The top base outline is very accurate from my investigation so I used it to draw that shape. I did scrounge some very useful photos of the real car. The photo above shows how I was able to get a very accurate profile design by extracting a couple key dimensions from the model kit, transfer those to the photograph, then scale the rest of the critical dimensions.

Step 2:

Using my full size design, I built the pattern for the top shown in the first two photos above. It was built from common plywood and 2x4s from the lumber store. This is all that is needed to make the top! The bottom outer rim of the top is cut into a 3/4" plywood sheet. Then a strong 2x4 cage is build over that hole. I stress strong; it must be able to absorb a ton or so pressure and not deform. All the plywood edges that touch plastic were rounded over with a 1/2" radius router and sanded smooth. This is all that was required to yield an optically clear transition.

With this pattern, I had to find a company that could use it to literally blow the bubble top. I found such a company in Los Angeles that specializes in blowing plastic bubbles for scientific uses, for advertising signs, etc. They are only about an hour and half drive from my home so I threw my pattern in my pickup and headed down there.

The manufacturing process is really pretty simple if you have the proper tools. All you need is an oven to heat a 6' square sheet of Plexiglass to 350F and a 20 ton press that can accommodate said 6' square sheet. I jest of course. You have to find a company that specializes in this product. To make the top, the company heated a sheet of 1/4" thick Plexiglass in one of their several dedicated ovens. The plastic softens to the perfect rubbery consistency in a narrow temperature range of 240F to 255F. Too cool and the bubble won't blow, too hot and the plastic is damaged. They then manually took the sheet out of the oven which was now like a sheet of rubber. They moved fairly quickly because there is a window of about a minute before the plastic cools below the workable temperature. The plastic was placed on the bed of a big hydraulic press and they lowered the press which has a 1" thick steel plate which in turn presses down on my pattern, sandwiching the plastic between the pattern and the press bed. The final step is to literally blow the bubble in the soft plastic with compressed air until it reaches the proper tallness, then wait a few second for it to solidify into the final shape. I had two tops blown just in case I damaged one in building my project. I needed both it turned out. Be sue to ask the operator to blow the part quickly. If done too slow, the plastic is cooling and becoming more rigid and that induces stresses in the stretched plastic that will cause headaches in using the final product.

Note the bed of the press is white. This is a cloth covering that serves to protect the soft plastic but more importantly it spreads out the air stream that is injected under the plastic to blow it up. Without this diffusion layer, the air jet can prematurely cool the first spot of plastic it encounters and distort the top.

That's it. I threw the mold and two top bubbles in my pickup and headed home.

Step 3:

First photo above shows the two tops on my shop floor. They came out perfect. I then had to modify the top to fit the rim of the car I was building. The second photo show how I cut the bottom rim to shape to fit the fiberglass body rim. An air or electric cutoff saw with a thin, grit blade does a great job. The problem is any cutting method leaves a rough edge which has deep scratches. These scratches if not removed will initiate cracks that will ruin the plastic bubble. Plexiglass is very strong IF it doesn't have any defects on its edges. It is used as windows in jet aircraft as testimony to its strength. There is a very simple trick to sealing the edges of Plexiglass to remove these stress risers. I first removed the deepest scratches by sanding the edge with 100grit sandpaper. Then here is the trick; just play the flame of a propane torch along the edge. The plastic instantly melts to a polished surface eliminating the problem. Practice on some scrap plastic because applying too much heat will distort the bubble and ruin the plastic. You will be amazed how little time is needed for the flame to perfect the edge. The flame itself does the work, it only has to contact the plastic for a fraction of a second. You can move so fast, there is no danger of damaging the plastic.

WARNING: DO NOT use ANY solvents on the plastic after it is blown. There are internal stresses induced into the plastic as it is stretched to shape. The cooler the plastic while stretching, the more stress induced. These internal stresses are released if the plastic is contacted by solvents (lacquer thinner, polyester fiberglass resin, etc.) and the surface will craze with a maze of micro-cracks. This phenomenon does not happen in flat sheets of Plexiglass which are passivated with no internal stresses.

The final three photos show my top mounted in its car rim, then a photo of it tinted with catalyzed urethane clear paint to achieve the look of the original. Luckily the solvents used in the paint did not attack the plastic. I was lucky. The top was glued to the rim using urethane adhesive from the home center in the final photo. The top has a layer of masking tape to protect it during car construction. Use a compound leverage glue gun on this stuff since a standard cheapie glue gun doesn't have the power to squeeze this viscous compound out of the tube.

The rubber rim shown in the photos was a half-round 1 1/8" shape I found on the internet. It was glued to the top with Super-Glue which welded it permanently.

Step 4:

Comments

[Youtube knight16]

Wow that's great!

[Dumbphone]

Fantastic job on the bubble tops. This is a cool tutorial. Thanks for sharing.

[OldMoparGuy]

Most cities have a big sign company that will often have a vacuum forming machine. Vac forming machines already have heating coils in the top, allowing the correct heating to be easily seen when the plastic sags down the correct amount. You don't need to use fabric or blow cold air on the part that may be alot easier to deal with the end product. The issues with polycarbonate (Lexan) or acrylic (Plexiglass or Perspex) plastic are true, no matter what technique is employed. The secret to reduce brittleness is tempering by reheating it in an oven, at a lower temperature, for a few hours. Makes all of the difference in the world! I have used this on dozens of 12" x 30" polycarb and other plastics, that I added holes and trimming after. Flame smoothing the edges is a good trick for stress relieving on all edges.

[spectre_man]

I'm totally inexperienced with this but curious--and a few poeple who've commented here might be able to clarify, in addition to the original poster. If you have a positive and negative mold, why can't the plastic just be pressed into shape? And how does the blowing work--does it just seal around the edge and then "inflate" the canopy to the shape of a (negative) mold? It seems like the application of compressed air would cool the canopy, making any shape-forming difficult if the molds and pressure hadn't completed the job by themselves.

[Willys36]

The problem with positive molds is that the surface must be absolutely perfect to achieve the required optical quality. A 'blown' bubble achieves the necessary perfect optics automatically. The cold air impingement could be problematic but as I mention in the Instructable, the bed of the tool is covered with felt which serves to stop the jet effect and spread out the air so it uniformly fills the bubble with no differential cooling.

[BigAndRed]

You don't need the 2ton press to make this. Used to make helicopter screens back in the 80's with similar technique, compound curves in perspex. Hughes 300 and 500 and Bell Jet-ranger. Pitts special biplane and Corby Starlet home built aircraft as well.
Heat the perspex in the oven and remove when rubbery, then its stretched by 4 people each holding a corner.
Sandwich the heated perspex between the male and female parts of the 2 part wooden frame. Clamp the frame with spring clamps and turn on the compressed air.
When the plastic has cooled it was trimmed and edged with a thin layer of fiberglass woven cloth for strength.
Technology hasn't changed much since then.

[Willys36]

That is definitely a got t make a top but requires a very accurate male and female mold for an optically pure result. My tops didn't have a mold the shape of the final bubble. All it had was the profile hole of the base of the bubble in a flat sheet of plywood. It needed the press to hold that plane onto the hot plastic then air pressure is blown under the plastic which literally blows a bubble.

[Willys36]

OOPS! Just re-read your comment and see that you did actually blow your bubble, just had a 2-part wood profile pattern. There are guys in the hot rodding community who do it that way but I have only had this experience with the commercial press guys. I still like using the press since the clamping method takes considerably more time from the oven to the air compressor and you only have in the range of a minute or two before the plastic cools too much to blow. You want to blow the bubble with the plastic as hot as possible so there aren't extra internal stresses introduced by stretching cool plastic that wants to be set. You obviously had a system where you got your bubble blown quickly avoiding the induced stresses. The last one I had blown with the press method was not as good as the first two. The guys doing the work were obviously new to the job and took forever blowing the bubble. I was watching and could tell near the end that the plastic was beginning to set while stretching. Sure enough, when I was assembling it into my fiberglass car body, everywhere the fiberglass resin touched the plastic it immediately crazed.

[starphire]

Thank you for teaching me that plastic domes can be made with pressurized air, not just vacuum forming. I always wondered how they did that with thick sheets of plexiglass. Also, I am amazed that the super glue didn't fog the edge of your dome! Usually methacrylate fumes turn clear acrylic powdery white in minutes.

[Willys36]

I am absolutely sure the glue would have done a number on the plastic if it got onto a visible spot. I took precautions; first placed the rubber exactly where it was to be on the top then outlined it with blue painters tape on both edges; used the heavy-bodied glue so it wouldn't run; ran a thin bead of glue down the exact center of the 1 1/8" wide rubber section so the glue would squish to the edge but no further. I had to end up making 3 tops before I got my final one so had lots of practice! The glue was never a problem.

[ragtoptruck1]

Grew up loving Ed Roth's work and had several of his "T" shirts....I was in Reno NV last year and made a stop at the auto museum.... http://www.automuseum.org/ ...just to see his cars on display there. Anyone with a love for cars should see the museum. Glad to see someone else has the same love for his work, great job....thanks. I do hope you made the frame for the car a bit heavier

[Willys36]

I built a 2x4 steel tubing frame. The chrome side rails are just bolt-on decorations, no structural contribution.

[stever_sl]

I'm powerfully curious - what did it cost to have the 2 clear tops made?

[Willys36]

I ended up having 3 made. The first two were $446 each. The third one was done several months later and they charged over $700 for it. I should have negotiated better on that one!

[Andrews-Design]

Fantastic work! Ed Roth was a true visionary in the hot rod community! I'm partial to the Mega Cycle myself

[Willys36]

Thanx. Yes every gearhead kid has that one car that hit him square in the psyche and ruined him from ever having a normal life again!

[Toxictom]

Be sure to enter this in the plastics contest.

[Willys36]

should be there. That's why I posted it!!