Can you vacuum form PETG with the film masking on?
In general the answer to this question is no. There are risks involved and is not usually recommended by any of the extruders of PETG to make a practice of vacuum forming with the film masking on.
However, PETG can be purchased with a vacuum formable film masking if specified. It is not the same masking that is normally used for general purpose applications, so those companies wishing to perform this kind of work, need to pay special attention to what they order. There may also be adjustments required on heating and cycle times to consider, so some experimentation should be done prior to production to ensure the best results.
To date, only one supplier that we have worked with provides this type of film masking, though we are told by some of our other suppliers that they have similar products in development.