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Why PC Sheet Bubbles in Laser Cutting: Pre-Drying Is the Key

Wuxi Cremage Technology Co., Ltd ·

The short version

If the cut edge of laser-cut PC (polycarbonate) sheet bubbles, the laser settings are not to blame and neither is the sheet quality: PC absorbs a small amount of moisture from the air, laser cutting is a thermal process, and that moisture vaporises instantly in the cut, leaving bubbles sealed into the molten edge. The remedy in one sentence: pre-dry the sheet before cutting; conditioning afterwards is recommended.

1. The mechanism: slight moisture uptake plus a thermal cut

PC sheets absorb moisture from the air. Harmless in service — but laser cutting heats the cut far beyond the boiling point of water, so moisture held in the sheet vaporises and escapes through the molten edge as bubbles. Thermoforming practice lists the same failure mode with the same diagnosis — “bubbles in the sheet: cause moisture, remedy pre-drying”. The rule is identical here: moisture in, bubbles out.

2. How to pre-dry

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

Operating points:

  • Circulating air oven, set to 120–125 °C;
  • Remove the standard protective masking film first — it is not suitable for thermal loads (a heat-resistant grade is available on request);
  • Hang the sheets or lay them in a frame with 20–30 mm spacing so air circulates freely;
  • Once dried and cooled to room temperature, process within a maximum of 10 hours, or re-dry; best practice is to keep the sheet in the oven until the laser is ready;
  • Aluminium packaging straight from production protects against moisture and can avoid pre-drying if the material is processed shortly after delivery.

3. Two more things to watch

  • Sheets over 2 mm thick develop brownish cut edges — inherent to laser cutting thick PC, unrelated to drying; where appearance matters, switch to sawing or milling.
  • Ventilation: hydrocarbon-based gases form during laser cutting, so ensure good extraction in the work area.

4. Acceptance criteria

  • Bubble-free edge = pre-drying was adequate. Still bubbling? Check oven temperature and time against the table, and confirm the sheet went to the laser within 10 hours of drying.
  • Brown edge = thickness-related (>2 mm), not fixable by drying — choose a cold cutting process (sawing, milling) instead.
  • One layout note: the first heating above the glass transition temperature (approx. 145 °C) causes a one-off shrinkage of 3–6 %, so leave that allowance when nesting parts.

FAQ

Q: My cut edge bubbles — did I receive a bad batch?

No. PC always absorbs some moisture; any batch stored in a humid environment will bubble if laser-cut without pre-drying. Dry it per the table above and the problem disappears.

Q: The sheet was dried yesterday — is it still “dry” today?

After drying and cooling to room temperature, process within a maximum of 10 hours (depending on ambient conditions); beyond that, re-dry. Overnight normally exceeds this window.

Q: Can the brownish edge be removed by drying or by tuning the laser?

No. Laser cutting sheets over 2 mm leads to brownish colouring at the edges — it is inherent to thermally cutting thick PC. For appearance-critical parts, saw or mill instead.

Closing

Laser cutting excels at complex contours, and pre-drying is the switch that controls edge quality. Wuxi Cremage Technology applies a strict “dry first, cut second” discipline in PC and acrylic fabrication, and offers cold cutting routes — sawing and milling — for thick or appearance-critical parts, with accuracies down to ±0.01 mm. See our industrial plastic machined parts page. Further reading: PC sheet CNC machining parameters, chemical resistance and stress cracking of PC and cold bending and cold folding of PC.

References

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