Polypropylene is the second most produced plastic on earth and it is in your kitchen right now: the tub the margarine came in, the flip-top lid on the shampoo, the fibres in the doormat. It is a polyolefin — a chain of nothing but carbon and hydrogen, with no polar groups anywhere along it. That chemical blankness is the source of both its great virtues and its one enormous printing problem.
Nothing sticks to it, and that includes the bed
Adhesion needs surface energy: something on one surface that wants to interact with something on the other. Polypropylene offers nothing to interact with, which is why it resists staining, why glue slides off it, why paint peels from it, and why manufacturers flame-treat or corona-treat PP parts before printing labels on them.
On a printer this means the usual adhesion toolkit is useless. PEI does not hold it. Glue stick does not hold it. Hairspray does not hold it. What holds polypropylene is polypropylene: a layer of PP packing tape laid across the build plate, or a purpose-made PP build sheet. Like sticks to like, and nothing else does.
That is not a workaround, it is the standard method, and it works well. Lay the tape without bubbles, print onto it, and accept that the tape is a consumable.
The largest dimensional correction on this site
PP is semi-crystalline and crystallises enthusiastically as it cools, so it contracts 1.0% to 2.5% — the widest and largest shrinkage range of any material here. At the top of that range a nominal 100 mm feature comes off the plate at 97.5 mm; to land on 100 you would model it at 102.56 mm, a correction of 2.56 mm. 2.56 mm is more than most people's entire tolerance budget, which is why polypropylene parts tend to be designed with clearance rather than compensated into fit. The shrinkage compensation calculator will give you the correction, but on PP the more useful question is whether the feature has to be dimensional at all.
Worse, the crystallisation continues after the print stops. A polypropylene part measured straight off the plate and measured again the next morning will have moved, so the sensible workflow is to let a part sit for a day before you machine, drill or fit it to anything.
Nozzle 220 to 250 °C, bed 85 to 110 °C, enclosure required — the enclosure is fighting the shrinkage, not the temperature.
What all that buys
Chemical resistance that is close to total. Acids, alkalis, alcohols, most solvents, salt, bleach: polypropylene ignores them at room temperature. A printed funnel, chemical tray, battery-acid catch pot or darkroom tank in PP is a genuinely serviceable piece of laboratory equipment, and very little else you can print comes close.
Fatigue life nothing else matches. This is polypropylene's signature property. A thin PP web bent back and forth survives an extraordinary number of cycles — it is the reason every flip-top bottle cap in the world is moulded in it. Printed living hinges work, provided the hinge is thin, oriented so the extrusion runs along the hinge line rather than across it, and flexed a few dozen times immediately after printing to align the polymer chains. That last step feels superstitious and is not: the first flexes cold-draw the material and measurably improve its life.
It floats. At 0.89 to 0.92 g/cm³ it is the lightest filament in common use, less dense than water. A kilogram of 1.75 mm PP is about 461.95 m — far more filament per spool than anything else on the shelf, which offsets a price of $40 to $90 per kilogram more than you would guess. Priced per metre rather than per kilogram, PP stops looking like the expensive choice — and per metre is the honest unit whenever the densities being compared differ this much. The length and weight converter reports metres per kilogram directly, so that is one field rather than a calculation.
It barely absorbs water. Drying is genuinely optional here, which is true of almost nothing else on this site. A PP spool left open for a year prints the same as a fresh one.
What polypropylene is genuinely bad at
- Bed adhesion on anything except PP. See above; there is no clever settings fix.
- Dimensional accuracy. Between the shrinkage magnitude, its range, and the post-print drift, precision work is out of reach.
- Gluing and painting. The same inertness. Mechanical fastening and heat welding are the honest joining methods, and PP welds beautifully to itself with a hot-air gun.
- Stiffness and strength. Tensile of 20 to 35 MPa, and it is flexible rather than rigid. PP parts feel slightly waxy and yielding.
- Layer adhesion, which is mediocre — the same crystallisation that drives shrinkage works against interlayer bonding.
- Sunlight, unless stabilised. Unstabilised PP chalks and embrittles outdoors faster than most people expect.
Where it is the only sensible answer
Living hinges, snap-lid boxes and flexure mechanisms. Chemical and laboratory containers. Parts that will be in contact with cleaning products or fuels. Fatigue-loaded clips that would fail in every other filament. Replacement parts for existing polypropylene items, where matching the material also means the weld will hold.
If the requirement is chemical resistance but not fatigue life, PA12 is far easier to print and gets most of the way there. Polypropylene is worth its difficulty specifically when a part must bend, repeatedly, forever.