Why does PETG stick too well (or not at all) to my bed?

PETG fails at the bed in both directions. On smooth PEI it can bond hard enough to tear the sheet. On a greasy sheet, with the nozzle too high, or in a draft, the same plastic lifts. Use a powder-coated or textured plate, and use glue as a release film when the plate is the one PETG welds to. Bed adhesion, plates, PETG printing, and warping are the general pages. Drying is separate: a wet spool strings and pops, it does not explain a torn PEI sheet.

The plate, from the manufacturers

Prusa's PETG article says a powder-coated TXT or satin sheet is highly recommended, and that printing on smooth PEI might damage the sheet. The instruction is explicit: do not print on the smooth PEI sheet, because the adhesion may be too strong. If you use smooth PEI anyway, apply glue stick first. Their profile on that page is 230 °C (446 °F) then 240 °C (464 °F), and a bed of 85 °C (185 °F) then 90 °C (194 °F). Prusament PETG lists 240–260 °C (464–500 °F) and 70–90 °C (158–194 °F). PolyLite PETG lists 220–250 °C (428–482 °F) and 70–90 °C (158–194 °F). Creality PETG lists the same 220–250 °C (428–482 °F) and 70–90 °C (158–194 °F). PETG is the material hub.

Prusa's warping article says the same damage in one line: bed adhesion might be too high with PETG or PC and might result in a damaged PEI surface. A thin glue stick on the cold sheet is both extra grip and a separation layer. Wipe grease off with 90% or higher IPA before you decide the plastic will not stick.

Bambu's glue guide says textured PEI generally needs no glue, but PETG can bond so strongly that the model is hard to remove. Glue on that plate reduces adhesion so you are not prying the part off. Smooth PEI is the other way around: glue is required for everything except PLA, to avoid damaging the plate. The discontinued Cool Plate is only suitable for PLA. Bambu says other filaments such as PETG and ABS may bulge and stick. Cool Plate SuperTack needs no glue.

The threads

r/FixMyPrint 12xa9z5 is a bad top surface. The score-72 comment says add top layers because the infill is showing through. That is not a bed problem.

r/FixMyPrint 117fwds is a warp. The title's settings are PETG at 240 °C (464 °F) and a 70 °C (158 °F) bed. The score-12 comment says cool the part evenly and kill drafts. The score-10 comment says that person never cools PETG. Prusa splits the same choice: fan on for detail and less stringing, fan off when the part has to be as tough as it can be, with the first layers still uncooled so they do not deform.

The bank title for r/BambuLab 11oor5u asks why a Cool Plate errors on PETG when the plate text mentions PETG. Bambu's current glue page says that Cool Plate is discontinued and only suitable for PLA. No comment excerpt was attached for that thread, or for r/FixMyPrint 10339n7 and r/ender3 1033yjm.

Questions