I've been using kapton as the build surface but I accidently gouged it after my Z-axis home switch got bumped out of position. 200mm sheets with adhesive aren't cheap so I started looking for alternatives. It occurred to me that window tinting film is cheap, easy to find, and pretty durable. It also has an adhesive. Surprisingly, I didn't find any references to tinting film on the reprap wiki or forum.
Car windows can get pretty hot parked in the sun, but I wasn't sure the material would handle 100+ celsius. A little research on Google tells me the stuff is vinyl but that doesn't help much. Kind of like calling Bigfoot a 'critter' - Might be true, but ain't really saying much.
3M seems to be the big player and their site did have an MSDS. Their film is PET with a metal oxide coating. Score! - PET has a melting point around 250C.
I called a local tinting retailer and they gave me enough scraps to easily cover 8 or 10 build plates, way more than I need. I got samples of two different thicknesses.
My first attempt was just to clean the glass well, peal and stick. What looks like water droplets are actually air bubbles. But, my first test prints were very positive. ABS stuck as well as it did to kapton and separated easily as the bed cooled.
My next attempt, after googling a bit, was to clean the class and then spray with a mild soap solution. Then peal the film and lay it onto the wet glass. Squeegee the water out. Much better. But when I heated it, I got some large bubbles. I assume trapped water is boiling. Perhaps I should lay it in the sun for a few hours/days before heating it.....
I finally broke down and replaced the Gen3 electronics with a RAMPS board and after working through some issues, it finally seems to be working.
I usually print in ABS so I also invested in a Prusa PCB heat bed from Ultimachine. There isn't much information out there on mounting the thing and most of the pictures I saw had people mounting them on a piece of plywood. This is probably perfectly safe but there's something that bothers me about mounting something designed to get hot on top of something flammable.
I had a piece of 12" smooth Hardiboard. This is siding material made from cement infused fiber. It's very flat, very hard and heat resistant. As a test, I held the torch on a scrap for a while to see what the result would be. It scorched the topside but barely penetrated at all.
It's a little heavier than I'd like, but the combination of the hardiboard and PCB is actually lighter than the old acrylic and removable MDF that I was using before.
The downside is this stuff is damn hard to cut and throws off a lot of dust. I milled a couple pockets for the thermistor and the wires so the bed would lay flat. I also cut a few notches to remove material and lighten it further. It seems to be working well.
The big things I wanted out of this build were a stepper driven extruder, 5D firmware, a Makergear hot-end and PLA printing.
Next steps involve a better build platform, debugging the z axis sticking and a bunch of testing.