Filament types, read from composition up to properties

Most people pick filament by acronym: PLA for easy, PETG for tough, ABS for hot. That works about seven times out of ten and fails exactly when it matters, because the acronym never tells you why the material behaves the way it does.

This piece runs the other direction: start with what is actually in the spool, then derive what you read on the data sheet. Once that clicks, you can look at a grade you have never printed and predict most of its behaviour before you buy it.

What is in a spool

Filament is not a pure substance. It is a compound formulated to extrude, print and hold its shape. Four ingredients, in order of influence:

1. 01Base polymer — 80 to 99 % by weight. This is the material. It sets the heat ceiling, decides whether the grade is brittle or tough, and decides whether it absorbs water.
2. 02Additives — 0.5 to 5 %. Impact modifiers, flow aids, heat stabilisers, UV stabilisers, nucleating agents. They do not change what the polymer is; they change how hard it is to print.
3. 03Colourant — 0.5 to 3 %. Pigment or dye. It sounds cosmetic, but dark and metallic pigments change crystallisation rate, which changes both strength and shrinkage.
4. 04Filler or reinforcing fibre — 0 to 30 %. Mineral for a matte surface, chopped carbon or glass fibre for stiffness. This is the ingredient that moves the data sheet the most, and the one that eats nozzles.

Two numbers explain most of it

Before the families, two quantities are worth carrying around, because they predict behaviour better than any marketing line.

• —Glass transition temperature (Tg) is where an amorphous polymer goes from rigid to rubbery. Above Tg a part does not melt, but it starts to creep under load. The heat deflection figure on a data sheet always sits near Tg, a little below it for amorphous grades.
• —Crystallinity says whether the chains pack into ordered regions. Crystalline grades — nylon, and PLA to a small degree — are stronger, hold up better to solvents, and shrink more. Amorphous grades — PETG, ABS, PC — are more dimensionally stable and can be clear.

PLA — polylactic acid

PLA is a polyester made from lactic acid, which comes from fermented sugar rather than from oil. Its chains are straight and regular but crystallise slowly, so a printed part is close to fully amorphous. That is why PLA is stiff, accurate, and why it softens as soon as it passes a Tg of roughly 60 °C.

• —What it is good at — high modulus, the lowest shrinkage of the common FDM grades at around 0.3 %, and crisp surface detail.
• —The real weakness — elongation at break of about 4 %, so it fails without warning; and a heat ceiling low enough that a car parked in the sun destroys parts.
• —Common variants — mineral filled for a matte finish, flow modified for speed, carbon filled for stiffness.

PETG — a polyester with its crystals deliberately broken

Plain PET crystallises readily, which is why a drinks bottle is clear but still tough. The G in PETG is glycol: part of the glycol in the chain is swapped for a bulkier one, so the chains can no longer pack into order. The result is an amorphous polyester that does not crystallise as it cools, so it stays clear and does not shrink unevenly.

That is the origin of every PETG property you meet in the workshop: 18 % elongation before break instead of 4 %, good water resistance because polyester does not degrade quickly at ambient conditions, and strong layer adhesion — strong enough that it also bonds to smooth PEI hard enough to pull chips out of the plate.

ABS and ASA — three monomers, one job each

ABS is a copolymer of three monomers, and the easiest way to remember it is to give each monomer a job:

ASA keeps that recipe and replaces butadiene with an acrylate rubber. The butadiene segment carries double bonds in the chain, and those double bonds are exactly what ultraviolet light breaks, which is why outdoor ABS chalks and then surface cracks within months. Acrylate rubber is saturated — there is no double bond to break — so ASA holds its colour and its toughness in direct sun. That is the whole difference, and it explains why every other number on the two data sheets nearly matches.

The cost sits elsewhere: both shrink 0.7–0.8 %, twice PETG, so chamber heat is mandatory; and both emit styrene and ultrafine particles while printing, so ventilation is mandatory too.

TPU — thermoplastic rubber

TPU has no single Tg because it is not a single phase. Its chains alternate hard segments, which cluster into regions that act as physical crosslinks, with soft segments — polyester or polyether — that provide the stretch. The ratio between the two sets the Shore hardness.

• —More hard segment gives a firmer material that extrudes more easily and rebounds faster — that is TPU 95A.
• —More soft segment gives something that squashes like rubber and stretches several hundred per cent, but buckles in the extruder if you push it — that is TPU 85A.
• —Polyester or polyether soft segment decides the weakness: polyester resists oil and abrasion better, polyether resists water and mould better. For a part that sits in water, ask that question before you ask about Shore.

Nylon — polyamide, and the price of hydrogen bonding

Polyamides carry amide groups in the backbone, and those groups hydrogen bond to each other between chains. Hydrogen bonding is the source of everything good about nylon — very high toughness, abrasion resistance, a self-lubricating surface — and the source of the one problem that makes people give up: the same groups hydrogen bond to water just as happily.

A spool of PA-CF left in open humid air can absorb enough water to fail in an afternoon. It boils in the nozzle, pits the surface and cuts layer adhesion. This is not a material defect; it is known chemistry, and the fix is in the drying guide.

Polycarbonate — ring in the backbone, highest Tg in the common set

PC carries an aromatic ring in the main chain, which makes the chain stiff and hard to rotate. That is why its Tg reaches roughly 147 °C, well above every other common FDM material, and why it wants 265–300 °C at the nozzle — below 260 °C the layers never properly fuse and the part delaminates.

And this is where PC is most often misread. Everyone knows polycarbonate as bulletproof-glazing material, but the notched impact of printed PC is low: Polymaker publishes 4.1 kJ/m² for PolyLite PC — below PolyLite PLA. PC is notch sensitive: intact, it is extremely tough; with a cut already in it, the crack runs. The alloyed grades are where the tough numbers live — PolyMax PC at 21.3 kJ/m² and PC-ABS at 25.8, against 18.0 for PolyLite ABS. If your PC part has a sharp internal corner, that corner is where it breaks. The alloyed version is also easier to print — it gives up around 13 °C of heat deflection to bring nozzle temperature and difficulty back to ABS territory.

Resin — different chemistry entirely

Resin is not a thermoplastic. It is a liquid mixture of oligomers that form the backbone, reactive diluent monomers that set viscosity and join the reaction, a photoinitiator, and additives. Light at 405 nm triggers the initiator, and the reaction links everything into one crosslinked network.

The practical consequence: a resin part cannot be remelted or heat welded, and its properties depend on how far that reaction went. Under-curing does not show up immediately — it shows up weeks later, as parts that creep out of tolerance or turn brittle. Higher crosslink density means stiffer and more brittle; that is the whole difference between a model resin and a tough resin.

The summary table

There is no best material. There is only the material whose failure mode you can live with.

If you are standing in front of a specific choice, two pieces go straight at it: PLA vs PETG vs ABS for the common set, and the substitution guide for when the grade you want is out of stock. Full settings and test context sit on each Material Passport.