Vacuumm forming, or thermoforming, consists on heating a plastic sheet in an oven and then form it against a mold, replicating a pressure chamber with the help of a vacuumm cleaner. This is the best/cheapest way of forming plastic sheets without industrial machinery.
The key thing here is choosing the right material. We had two options: a clear plastic sheet applying spray black tint on it after forming, or an already tinted material. The allready tinted material, guaranties an uniform tint all over the surface of the visor, while the spray black tint is more like an artistic work and the result is not an industrial quality result. The thinnest, allready black tinted material, we found was a 3mm thick Acrylic. Knowing acrylic has a low stretch ratio, and the minimum thickness avaiable was 3mm we thought we would need at least a 500x700mm sheet to guaranty the complet form. This is a pretty big size if compared with a standar kitchen oven, so we decided to build one with the scale needed.
To form a 3mm acrylic sheet the temperature recommended is 170-190ªC (the wide range is just because depends on the color of the sheet and the type of oven), while to form a clear PETG sheet only 120-140ªC are needed. If we respect this temperatures, we can have the material inside the oven as long as we want without degrading it. Acrylic is an "elastomer" material, this means that when heated after forming it slowly gets back to the original flat form of the sheet. However, PetG is a "plastomer", what means that heat will only exponentaly stretch more the material. That makes PETG a lot easier material to form than the acrylic. Anyway, we decided to try both.
After few acrylic sheets wasted, we came to the conclusion that, with acrylic, wasnt an ensured process. We could correctly form 1 of every 3 tryies. With the same temp 190ªC and same vacumm pressure, we hardly managed to form one succesfull visor. Once we completely formed a black tinted acrylic visor we realize that the inner surface of the material (the surface that touches the mold) is fully covered of small little dots and imperfections. At this moment we knew, that forming the visor was not going to be as easy as we thought at first.
A polycarbonate sheet we had in the workshop, gived the same small dots and bubbles problem so we came to the conclusion that the material of the male mold was making some kind of reaction or exppeling gas when instantly heated by the plastic. Then started the mold madness: 4 different molds with 4 different materials.
GelCoat mold:
GelCoat is the material applied on the first layers of a boat. Its a slightly elastic material, impact resistant and hydrophobic. Few layer of gelcoat + a polyurethane resin interior. Gelcoat is hardly sandable, but with patience you can achieve a perfectly shinny finish.
Cons: not suitable for thermoforming process. The plastic sticks completely to the surface when forming, making it impossible to demold without breaking the visor. On top of that, the heated plastic creates blisters all over the gelcoat surface sanding ( making sanding y after each form) and gives the same small dots and bubbles problem on the visor resultant.
High temperature Epoxy:
This high temperature epoxy is a mix of resing and small pasticles of metal and is suitable for thermoforming jobs. Its pretty hard to find on the internet. Making a plaster of paris mold, we made an exact copy of the gelcoat mold, this time using this HTEpoxy and glass fiber on the inside. The result is easy to sand and polish, but, has a huge problem.
Cons: After slightly sanding, the surface comes to porous. A very little holes/dots that we suspect would mark the plastic when formed. The plastic does not particularly stick to the mold, but for geometry reasons of the visor, we werent able to demold the formed piece out without breaking it.
High Temperature Silicone:
After having problems demolding this particular visor we thought that a elastic silicone mold would be a solution. Plus, the silicone is an hydrophobic material and would discard the posibility of moisture on the mold expeling gas in contact with the hot plastic. The silicone mold has a removable glass fiber interior, making even easier the demolding process.
Once we had the High Temperature Silicone mold we continued with the first forming with PETG. The demolding system worked like a charm, and the PETG formed almost instantly to the desired shape. But there was on big problemm to solve: te same small dots/bubbles al across the inner surface of the visor. With the HTSilicone mold, the moisture or expelt gas was discarded as the origin of the imperfections. We tried to pre-heat the mold too, but didnt solve the problem neither. With all the possibilities of heat reaction discarded, we reach to the conclusion that the extrem clean workplace was the last possibility to try. Bingo!
We completely covered the silicone mold with duct tape. An easy removable duct tape that would allow us to retire it in the last moment before the forming, and with it all the possible small dust particles. We covered with new aluminium paper the inside of the the oven too. This way, everything would be as clean as possible.
The last, but not least, cleaning key is to preserve the protection film of the PETG untill the moment before forming. You should chek with a small sample if the film could be peeled out after heating the material to forming temp. This was our case, the PETG came wrapped with a protection film that degrades at higher temparture than the PETG itself. So you can heat the sheet, and then, beforme forming, remove it in a second and finally proceed to form.
This is the ultimate way to form, as close as posible to perfection, PETG sheets. Following every step, we would be able to avoid bubbles or small dots on the inner surface. Result of a huge trial/error labor.