Polypropylene is the plastic your kitchen containers, your car bumper clips and your living-hinge lids are made of. It is tough, chemically inert, and effectively unbreakable in bending fatigue. It is also, on a desktop printer, one of the hardest materials to get to stay on the plate — for the same chemical reason it resists everything else.
PETG is the default for functional parts precisely because it does none of the difficult things. So the honest framing is: print PETG unless the part needs something only polypropylene has.
What only polypropylene has
Living hinges that survive. This is the strongest argument. A PP hinge folds tens of thousands of times without failing; a PETG one cracks. If your design has an integral hinge, PP is not a preference, it is the material class the design assumes.
Chemical resistance. PP shrugs off acids, bases, solvents and oils that attack most printable plastics. Chemical storage, laboratory fixtures and anything that meets cleaning products belong here. PETG resists water and household chemicals well but is not in the same category.
Fatigue life generally. Clips, catches and snap fits that are opened daily last far longer in PP.
It is lighter than water. Its stored density band of 0.89 to 0.92 g/cm³ puts it below one, so a solid printed part floats — which for buoys, boat fittings and anything that might be dropped over a gunwale is a design property rather than a curiosity.
A kilogram goes a long way
That low density is a real economic point that gets lost behind the printing difficulty. A kilogram of 1.75 mm PP at 0.9 g/cm³ is 461.95 m; a kilogram of PETG at 1.27 is 327.36 m — 134.58 m more filament in the polypropylene spool, because PETG's 41.111% higher density costs it 29.134% of its length. A kilogram of PP therefore buys about forty per cent more filament than a kilogram of PETG, which is worth weighing against the higher price per kilogram before writing the material off as expensive. The stored figure is a range rather than a point, so it is worth redoing the sum at either end of it in the length and weight converter.
The same model printed in PP therefore weighs under three-quarters of what it weighs in PETG. PP retails at $40–90 per kilogram against PETG's $18–30, so it is still substantially the dearer material — just less so per part than the shelf price implies.
The adhesion problem, and the only real answer
Polypropylene is non-polar. There is nothing on its surface for an adhesive or a build sheet to bond to, which is exactly why it resists chemicals and exactly why it will not stay on your plate.
The standard solution is to give it a surface made of itself: polypropylene packing tape laid over the build plate. The part bonds to the tape, the tape holds to the plate, and removal is a matter of peeling. It works reliably and it is inelegant — the tape must go down without bubbles, it wears out, and a large part needs several overlapping strips laid carefully.
Nothing else works well. Glue sticks, hairsprays and specialist adhesives that hold everything else will not hold PP.
Shrinkage on top of that
PP's stored range is 1.0 to 2.5%, among the highest on this site, and the stored note adds a detail that matters more than the number: the contraction is crystallisation-driven, and PP parts keep moving for hours after the print finishes.
Two consequences. Dimensional accuracy is genuinely poor and the stored weakness list says so — do not design fits in polypropylene. And measuring a PP part straight off the plate tells you very little, because it is not finished shrinking; the shrinkage and tolerance method assumes a part that has stopped moving.
PETG, at 0.2 to 0.6%, is in an entirely different class here, and that alone decides most parts.
Where PETG simply wins
- Anything that needs to fit something. PP's dimensional behaviour rules it out.
- Anything you want to glue or paint. PP's weakness list names both; nothing sticks to it, including the finish you wanted.
- Anything printed on an open-frame machine. PP's stored enclosure requirement is required, driven by the shrinkage.
- Outdoor structural parts. Both rate fair-to-good outdoors, but PETG holds shape and PP does not.
- First attempts. PETG at difficulty 2 against PP at 5 is not a small gap.
One thing PP is better at that surprises people
Drying. Polypropylene is barely hygroscopic and the stored note calls it one of the few materials on this site where drying is genuinely optional. PETG absorbs moisture quickly and must be dried at 65 °C for 4 to 6 hours before a serious print.
So the material that is difficult in every other respect is the one you can leave on a shelf for a year and print straight from the bag. It is a small consolation, and it is real.
Designing a printed living hinge that actually works
Since the hinge is the main reason to attempt this material at all, it is worth saying how the geometry differs from a moulded one.
An injection-moulded PP hinge is a thin web perhaps a few tenths of a millimetre thick, and it gains its fatigue life partly from the way the polymer chains align as the melt flows through that narrow section. A printed hinge cannot reproduce that alignment, so it needs to be thicker and it needs its layers running the right way.
Print the hinge flat on the plate so the fold axis lies in the print plane and the bending happens along the extruded lines rather than across a layer boundary. A hinge printed standing up folds across the layer welds and will delaminate on the first few cycles regardless of material. Then exercise the hinge immediately after printing, before the part has finished contracting — folding it while it is fresh helps orient the polymer and noticeably improves its life.
The decision
Ask whether the part flexes repeatedly, meets aggressive chemicals, or must float. If none of those, print PETG and do not think about polypropylene again. If one of them, accept the tape, the enclosure and the dimensional uncertainty — because there is no substitute, and a PETG part in that role will fail in service rather than on the plate.