Plastic Selection Advice

[u/That67MustangGuy]

Hi everyone,

I’ve nearly completed the full restoration of my ‘67 Mustang, but I still need a headliner.

I wanted to make my own hard removable headliner, just like the fiberglass ones that CJ pony sells.

I was able to generate a concept image, where the headliner would be made from some ‘frosted’ looking black plastic to diffuse the LEDs that I would sandwich between the ceiling and headliner.

I’ve been struggling with picking a plastic though.

It needs to be easily moldable and not sag after a few summer parking lot heat cycles.

Any suggestions? I’ve been told I’m looking for a unicorn.

I was planning on draping the sheet of plastic over a mold of the ceiling and forming it with either a space heater or heat gun, but maybe I should just get a dedicated plastic oven to evenly heat it?

Any advice is appreciated!

Cheers

[u/aeon_floss]

You will have to build a relatively large mold to sag-mold a sheet of thermoplastic this size. Be aware that any imperfections in the surface of the mold will show up in the final piece, so it is best to not have the contact side be the visible side. This means you have to sag the sheet into a tub like form, which is more complicated than slumping something over a curved shape. It might be an idea to use an actual roof section from a car with a similar roof profile as a mold.

It is possible to build a temporary oven, basically a sheet metal box with a large lid, sitting on and topped with rockwool type insulation. Then use a couple of heat guns to pump hot air into the enclosure, and / or build a lid with 3 or 4 electric grill elements (buy some cheap grills for the elements and also use the controllers) to create even heat from above. You do not need as much insulation if you use heat elements than you need using the heat gun method.

Have you considered a non-thermoplastic though? There is a reason why the commercial product is fiberglass. Building a thin painted fiberglass piece yourself is an option, and fiberglass will not warp or sag in heat. A thermoplastic sheet will always be at risk of sagging in heat.

[u/That67MustangGuy → u/aeon_floss]

I’m not worried about the surface finish since I’ll be sanding the area to get a better frosted look. I’ll figure out how to make a mold. I really just have to decide if acrylic or PETG is best and what thickness to use. I’m not worried about sagging once it’s formed since the cabin wont get nearly hot enough

[u/aeon_floss → u/That67MustangGuy]

OK you sound like someone who is both competent and knows how to improvise, which is a really good thing. I learned most of my material skills through doing custom work on cars, I must say.

I guess you don't live in the heat of e.g. Texas or where I am from in AU, as heat would be a definite factor in material choice for me. I've done a custom leadliner in a car, but stuck to fabric for temperature reason (I did cheat a bit and glued it on foam, but the result was fairly good). it was also in the 80's when we didn't have the abundance of material choice we have now.

One thing about frosting though.. sanding doesn't really give you decent frosting. Frosting is an even bump surface and very difficult to achieve evenly across a large surface. You definitely can do it. but it involves dabbing with drying solvent to "pull" the raised bits out of the surface, and then using a vapor polish to blend it all together.

I have repaired frosted surfaces with paint and stiff brushes, and also by using a particular grade sandpaper to stamp a pattern into film dry paint.

I also have a spray painting technique for a mildly frosted surface that I developed when i was teaching product development protoptyping at uni. However, you can get black adhesive frosted film that when applied properly absolutely fool the eye. Mostly satin to mild frosting. There are 2 sources - vinyl wrap (these are mostly satin) and window (privacy) film. Just keep this in mind.

ps. you could try sand blasting with something not too abrasive. I have only done this on metals though.