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Remove the Masking Before Heating: PC Sheet Protective Film Rules

Wuxi Cremage Technology Co., Ltd ·

The short version

The masking film supplied on PC sheet is transport protection, not process protection: the standard polyethylene (PE) film cannot take thermal loads and must be removed before drying, hot line bending and thermoforming. Heated with the film on, it softens and bonds to the sheet, peels off incompletely and leaves adhesive marks — for optical surfaces, that is a scrapped part. Where the process genuinely requires forming with film in place, specially manufactured heat-resistant unprinted film must be ordered.

1. Why you must not heat with the film on

PC sheets are supplied with a polyethylene-based masking film to protect the smooth surfaces during transport and processing; the standard protective masking film is not suitable for exposure to thermal loads or thermoforming, and should be removed before processes such as drying, hot line bending and/or thermoforming.

The reason is straightforward: PE is a commodity plastic that does not tolerate high temperature, and the heating zone of a bending or forming process runs far above what it can take (thermoforming heats the sheet to 170–205 °C). Under heat the film softens and bonds to the PC surface; it then will not peel off cleanly and leaves adhesive marks, turning a polished face into scrap. The damage is irreversible and costs the whole sheet. The film’s protection ends where heating begins — inside any heating process it is a contaminant.

2. When to strip it

Strip the film at three points in time: before drying, before hot line bending, before thermoforming. Whenever the sheet goes into an oven or onto a heated station, the film comes off first.

1. Before drying (pre-drying). PC sheet absorbs moisture from the air, so drying before forming is recommended; insufficient pre-drying causes bubbles during heating, which spoils the optical quality of the finished part. Drying parameters:

Sheet thickness (mm)Drying time at 125 °C (hours)
11.5
24
37
412
518
622
830

Set a circulating air oven to 120–125 °C; hang the sheets or lay them in a frame, with the film removed and sheets spaced 20–30 mm apart so air can circulate freely. Keep pre-dried sheets in the drying oven until processing; once cooled to room temperature they should be processed within a maximum of ten hours (depending on ambient conditions) unless re-dried. Pre-drying before laser cutting follows the same logic — see our article on pre-drying before laser cutting.

2. Before line bending and thermoforming. Thermoforming requires the sheet to be heated evenly and in a controlled process to 170–205 °C, with two-sided IR heating recommended (faster, more uniform, shorter cycles). Note also the one-off shrinkage on first heating above the glass transition temperature of 145 °C: up to about 6% for sheets up to 3 mm and up to about 3% for thicker sheets — allow for it when cutting blanks.

3. When forming with film is genuinely required. Specially manufactured, unprinted grades of film allow thermoforming with the masking left on — but these are available on request, not standard supply.

3. After stripping: the bare surface needs care

Once the film is off, the surface is fully exposed — and the stages that follow involve the most manual contact and tooling:

  • Cleaning: before forming, clean the sheet thoroughly with an anti-static detergent or ionised compressed air to prevent embedded dust and other surface defects on the finished part;
  • Handling: wear gloves, hold the sheet by its edges, and avoid sliding it across benches or tools;
  • Turnaround: move the stripped sheet to the machine promptly; do not let bare sheets stack and drag on trolleys;
  • Static: a dried sheet attracts dust more readily, so prefer ionised air for cleaning.

For detergent choice and wiping technique, see our PC sheet cleaning guide.

A thermoformed PC windscreen-type part: optical surface quality is decided by stripping and cleaning before heating (Image source: Exolon® technical manual)

FAQ

Q: The temperature is not that high — can I oven-dry with the film on?

No. The standard film is unsuitable for exposure to thermal loads, and that includes drying at 120–125 °C. Strip first, dry second.

Q: Is there a film that can stay on during bending?

Yes. Specially manufactured unprinted heat-resistant film allows thermoforming with the masking left on; it must be ordered from the sheet supplier, and its approved processes and temperature range confirmed.

Q: The bare sheet picked up light scratches after stripping — is it still usable?

It depends on location and application: light scratches outside optical areas usually do not affect structural parts, while optical windows should be inspected immediately after stripping. The real fix is to shorten the bare-sheet exposure time and handle to the anti-scratch, anti-dust rules above.

What Cremage can do for you

Wuxi Cremage Technology runs line bending equipment alongside precision PC/acrylic machining. For equipment guards and custom PC parts, film stripping, drying and bending are executed to the rules above — no film-on heating, no adhesive-mark scrap; typical products can be seen under industrial plastic parts. When you send us drawings, film stripping and pre-drying are written into the process card so the finished faces come out clean and mark-free.

References

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