PETG forms in a narrower window than most operators expect, and the window is defined by sheet temperature rather than oven setting. A press set to 160 °C can deliver 130 °C to a 5 mm sheet on one side and 150 °C on the other, and the part that comes out has one glossy flank and one hazy flank.
The band, by thickness
- 2 mm — 120–135 °C sheet temperature, 20–30 s in a 2 kW/m² oven
- 3 mm — 130–145 °C, 40–55 s
- 4 mm — 135–150 °C, 60–80 s
- 6 mm — 145–160 °C, 110–140 s
Above roughly 165 °C PETG starts to crystallise and the surface turns milky, which no amount of polishing removes. Below 115 °C the sheet will form but the walls thin unevenly and the part springs back when it leaves the mould.
The sag test beats the pyrometer
A 500 × 500 mm frame with the sheet clamped and heated will sag 15–25 mm when it reaches forming temperature. Watch the sag, not the display. A pyrometer reads the surface, and the surface is always hotter than the core on a radiant oven; the sag reflects the whole thickness.
Dry it, or accept the bubbles
PETG absorbs moisture, and 0.2% is enough to produce a fine bubble field on a 4 mm part. Dry at 65 °C for four hours per 3 mm of thickness before forming, and keep the sheet in a sealed bag between drying and the oven. This is the single most common cause of a rejected first article, and it is invisible in the finished part until it is backlit.
Cooling, where the last three mistakes live
- Demoulding above 60 °C leaves shrink marks that appear a day later
- Air-cooling a 4 mm part without a fixture lets the walls bow outward by 1–2 mm
- Stacking warm parts flattens the detail on the one below and leaves a ghost impression
Form, trim, then anneal at 65 °C for one hour per 3 mm if the part will see heat in service. For the material comparison behind the choice, PETG against acrylic and ABS and curved panel forming are the natural next reads.
Mould temperature is the other half of the process. A cold mould below 40 °C freezes the surface while the core is still moving, which produces the hazy flank that gets blamed on the oven. Hold the mould at 55–65 °C for a 3–4 mm part and the surface stays clear, with the part releasing cleanly and less shrink.
Draw ratio is what tells you whether the sheet is thick enough before you form it. A part 150 mm deep formed from a 500 mm sheet is at a 3.3:1 draw, and the wall at the base will thin to roughly two-thirds of the starting gauge. Size the sheet from the draw, not from the blank size, or the first article will fail on wall thickness at the deepest point.
Vent the mould before the first trial. Trapped air between the sheet and a deep cavity produces a gloss mark that looks like a material defect and is actually a venting problem. Two 0.5 mm vents at the deepest point remove it, and they cost less than the sheet you will otherwise scrap proving the point.
Before you build a thermoforming trial, send the part drawing and target wall to oversales@rpplexiglass.com; we will confirm the grade and the sheet size that yields it.
Shops new to the material usually ask these three things first.
Frequently Asked Questions
Q: What temperature should PETG be thermoformed at?
A: Sheet temperature of 120–135 °C at 2 mm rising to 145–160 °C at 6 mm. Above about 165 °C the surface crystallises and turns milky.
Q: Why do bubbles appear in thermoformed PETG?
A: Moisture. Dry the sheet at 65 °C for four hours per 3 mm of thickness and keep it sealed until it enters the oven.
Q: Should thermoformed PETG be annealed?
A: Yes if the part sees heat in service: 65 °C for one hour per 3 mm of wall thickness prevents later shrinkage and bowing.