The G in PETG is a chemistry note that turned an industrial packaging resin into the most useful functional filament on a desktop machine. Plain polyethylene terephthalate is a regular, tidy molecule, and regular tidy molecules crystallise as they cool. Crystallisation is what makes a PET bottle preform go from clear to hazy, and on a printer it is what makes a part shrink unpredictably for hours after the job finishes. Glycol modification replaces some of the ethylene glycol in the chain with a bulkier ring-shaped diol, cyclohexanedimethanol. The chain can no longer pack neatly, so it cannot crystallise, so it stays amorphous, clear and dimensionally calm.
Almost every practical property of PETG follows from that one substitution, including two that surprise people.
The temperature window is wide, but the reason to move inside it is layer strength
Nozzle from 230 to 250 °C, bed 70 to 85 °C. Unlike PLA, where the argument for the low end is real, PETG rewards the top of its range. An amorphous polymer bonds between layers by chain diffusion across the interface — the freshly extruded bead has to stay soft long enough for molecules to tangle into the layer below. Hotter melt, longer diffusion time, stronger part. Push to the bottom of the range and you get a part that looks fine and splits along a layer line under a load it should have shrugged off.
The same logic runs through cooling, and it is the setting most people get wrong because they arrive from PLA. Full-blast part cooling freezes each bead before diffusion has happened. Half fan speed is a sane starting point, with the fan lifted for bridges and dropped again immediately after. If your PETG parts are delaminating, look at the fan before you look at anything else.
Stringing is usually water, not retraction
PETG's reputation for cobwebs sends people straight to retraction settings, where they lose an evening. The material is hygroscopic, and absorbed water flashes to steam in the melt zone, which both drives ooze and leaves the surface pitted. Dry it at 65 °C for four to six hours and re-run the same print before touching a single retraction value; a large fraction of PETG stringing complaints end there.
What is left after drying is genuinely a tuning job, and it runs opposite to PLA again. PETG is soft and tacky at the extruder, so long fast retractions grind the filament flat against the drive gear and cause a jam three hours later. Short retraction, moderate retraction speed, and a small Z-hop to keep the nozzle from dragging.
The plate problem runs backwards
Almost every filament has an adhesion problem in the direction of not sticking. PETG has the opposite one: it bonds chemically to smooth PEI hard enough to tear chunks of the sheet out with the part. The damage is permanent and the plate is not cheap.
- Textured PEI is the simple answer. The reduced contact area brings the bond down to something a part can be popped off.
- Glue stick on a smooth plate is a release agent here, not an adhesive. That inversion catches people out — the same product that rescues an ABS first layer is what stops PETG welding itself down.
- Do not over-squish the first layer. A PETG first layer wants slightly more Z gap than PLA. Squashed thin, it increases contact area exactly where you least want it.
What PETG is genuinely bad at
Overhangs and bridges. It droops. The melt is viscous and slow to set, and unsupported spans sag where PLA would hold a clean line. Design around it or accept the finish.
Fine detail. Small features, sharp text and miniature work come out soft and blobby. People buy PETG for figurines because it is tough, and then wonder why the faces look melted. Print detail work in PLA and keep PETG for the bracket that holds the display.
Sustained load. It is more ductile than PLA, which reads as "stronger", but a PETG part under a constant load will slowly deform rather than hold its shape. Shelf brackets carrying weight for months are a genuine weak case.
Glue. PETG resists the solvents that would otherwise weld it, so most household adhesives make a joint that peels. Roughen the surfaces and use epoxy, or design a mechanical joint and skip the adhesive question.
Outdoors, in water, and the food-contact question
UV resistance is decent — not ASA's, but far beyond PLA's. A PETG garden fitting will chalk slowly over years rather than crack over months, and its low water absorption in service means a part that lives in the rain does not swell.
Food contact deserves a straight answer rather than a marketing one. PETG resin is used throughout food packaging and many grades carry food-contact compliance from the pellet maker. That says nothing about your printed part. Layer lines are a corrugated surface that bacteria colonise and no domestic wash reaches, most nozzles are brass rather than food-grade, and colourants and processing aids in a filament are not the same as those in a packaging grade. If a part will touch food repeatedly, treat it as a mould for something else, or coat it, or buy the part.
What it costs, against PLA, for the same model
PETG is denser than PLA, so a given model costs more in PETG even at an identical price per kilogram. Take a model that needs 100 g of PLA at 1.24 g/cm³: at PETG's 1.27 g/cm³ the same volume is 2.4% heavier, and 100 g of 1.75 mm PETG is 32.74 m of filament against PLA's 33.53 m — a difference of 0.79 m, which you can push either way in the length and weight converter. Across a full spool it is the difference between 327.36 m and 335.28 m.
A worked job to put beside that. An outdoor bracket consumes 120 g and runs for 5 hours on a machine drawing 110 W. Price the spool at $24 per kilogram, take the US average electricity rate, charge machine wear at $0.35 an hour and allow 5% for failures, and the four terms come out as:
- Filament — $2.88. The largest single term, and the only one most people count.
- Electricity — $0.10, for 0.55 kWh. Rounding error, even on a five-hour job.
- Machine time — $1.75. Larger than the electricity by a factor of seventeen.
- Failure allowance — $0.25.
Total $4.98. Filament is nearly three-fifths of that, which is unusually high — on most materials profiled here machine time competes with it. Raise the machine rate in the print cost calculator and you will watch the ordering swap over.
Where it earns its place
Outdoor brackets, hose and irrigation fittings, protective covers, printer parts that sit near a warm bed, and anything that will be dropped. It is the default answer whenever someone describes a part that has a job to do and lives below about 70 °C — which is most parts most people print after their first month.
If the part needs to survive a parked car in summer, that is the ABS and ASA conversation instead.