The base polymer (for example PETG, nylon, or PEEK) decides the part's fundamental nature: roughly how strong, how tough, how heat- and chemical-resistant it can be, and how hard it is to print. The form (next row) then modifies that base. Think of the material as the engine and the form as the tuning. Everything else in this guide is a measurable consequence of these two choices.

Neat is the pure polymer with no fillers. It is the toughest and most ductile version, the cheapest, and usually the easiest to print. CF blends in short carbon fibers: this is the stiffest and lightest form and holds its shape best under heat, but it pays for that by becoming noticeably more brittle, it is mildly electrically conductive, and it only comes in black. GF blends in short glass fibers: tougher and less brittle than CF, electrically insulating, and able to take color, at a small cost in stiffness. Both CF and GF are abrasive and chew through a brass nozzle, so they need a hardened steel one, and neither is safe for food, skin, or medical contact because the fibers can shed from the surface.

A quick bucket for both capability and difficulty. Standard materials are everyday and forgiving (PLA, PETG, ABS, ASA). Engineering materials are for functional parts with real mechanical, chemical, or thermal demands and ask more of the printer (PC, the nylons, PP, PVDF). High-Temp is the high-performance specialist class, from PPA up through PEEK, which generally needs a hardened hot end, very high temperatures, and often an actively heated chamber. Tier tracks price and print difficulty as much as raw performance.

A relative band for filament cost only, not the cost of the finished part. The full job cost climbs faster than the filament price alone, because the higher tiers also bring longer print times, mandatory drying, higher failure and scrap rates, hardened tooling wear, and more post-processing. A $$$$$ material can easily be ten times the delivered cost of a $ one once all of that is counted, which is why the step-down suggestions in each material's detail are worth taking when the part does not truly need the top grade.

Tensile strength is the most stress the material can take while being pulled apart, measured along the strong in-plane direction (1 MPa is about 145 psi). On the stress-strain diagram below it is the height of the peak: the highest force the part carries before it fails. Rough landmarks: PLA and PETG sit around 50 MPa, the nylons and PC run 55 to 80, PEEK reaches 90 to 110, and fiber-reinforced grades go higher still. Higher means it carries more pulling load before breaking. Read it together with elongation and impact, because a high number alone does not tell you whether it fails gracefully or shatters.

This is the honesty column for printed parts. A printed wall is built layer on layer, and the bond between layers is weaker than the solid plastic within a layer. This figure is the typical strength in the Z direction (across the layers) as a percentage of the in-plane number. 75 to 85 percent means strong layer adhesion; 55 to 70 percent means the layers are a real weak plane. The practical takeaway is in the diagram: orient the part so the main load runs along the layers, not across them, or a part that looks strong on paper can peel apart at a layer line well below its rated strength.

Modulus is stiffness, which is not the same as strength. On the stress-strain diagram it is the slope of the line at the start: a steep slope means the part barely moves under load and feels rigid, a shallow slope means it gives and flexes. (1000 MPa equals 1 GPa.) A part can be very stiff yet snap easily, or quite flexible yet nearly unbreakable, so stiffness has to be read next to strength and toughness. Fiber reinforcement raises modulus dramatically, which is exactly why CF and GF grades feel so rigid even though they are more brittle.

Where tensile pulls a part straight, flexural loading bends it, which is what most real brackets, arms, clips, and housings actually experience. Flexural strength is the most bending stress the outer surface survives before it yields or cracks. The diagram shows the setup: the part is supported at two ends and pushed in the middle. For a part loaded across its length, this number is often more relevant than the straight tensile figure.

Flexural modulus is stiffness measured in bending, and for printed parts it is usually the most useful stiffness number of all, because it directly predicts how far a beam or bracket deflects under a given load. A high flex modulus means the part barely sags; a low one means it visibly bows. Reinforcement raises it more than almost any other property, so a CF or GF grade is the first lever to pull when a part flexes too much under load.

Impact resistance is the will-it-shatter-or-take-the-hit number. The test machines a sharp V notch into a small bar, clamps it, and swings a weighted pendulum into it, measuring the energy absorbed before the bar fractures (see the diagram). The notch is deliberate: it concentrates stress at one point so the test measures the realistic worst case, how readily an existing crack or sharp corner will propagate. Low values (PLA around 2 to 4) mean brittle and prone to shattering; high values (PC, the nylons, PPSU) mean tough and forgiving. 'No break' means the bar bent without fracturing at all. Note that adding carbon fiber almost always lowers impact even as it raises stiffness, so stiff does not mean tough. Units differ by standard (ISO uses kJ/m squared, ASTM uses J/m), so only compare like with like.

Elongation at break is how much the material can stretch before it gives way, and it is the clearest single measure of ductile versus brittle behavior. On the stress-strain diagram it is the horizontal distance the curve travels before it ends. High elongation (the nylons, TPU, and PP at 50 to 500 percent or more) means the part bends, deforms, and gives plenty of warning before failing. Low elongation (PLA and the reinforced grades at 1 to 5 percent) means it fails suddenly with no warning. A ductile part tolerates overload, vibration, and impact far better than a brittle one of the same strength, which is why elongation is often the deciding property for parts that take abuse.

Heat Deflection Temperature is found by heating a supported bar under a fixed load until it bends a set amount (see the diagram). The light-load version uses 0.45 MPa (66 psi) and is the optimistic heat number, appropriate for parts that are unloaded or only lightly stressed when hot, such as a cover or a duct. It is always higher than the under-load figure, sometimes by 30 to 40 degrees, so do not use it for anything bearing real force at temperature.

The same heat-deflection test, but under a heavy 1.8 MPa (264 psi) load. This is the realistic hold-its-shape-under-working-stress number and the one to use for any load-bearing part that gets hot: a bracket in a hot car interior (70 to 80C), something in an engine bay (100C and up), or a fixture that sees a dishwasher or autoclave. It is always the lower of the two HDT values, so when in doubt, design to this one.

The glass transition is where the polymer's amorphous (disordered) regions change from a hard, glassy state to a soft, rubbery one. Whether Tg is the usable-temperature ceiling depends entirely on the polymer type, which the diagram shows. For amorphous plastics (PC, PEI/ULTEM, PSU, PPSU, PETG, ABS) there is nothing but amorphous material, so stiffness falls off near Tg and that sets the practical limit. For semi-crystalline plastics (PEEK, PPS, the nylons, PP, PVDF) the crystalline regions act like a skeleton that holds the part together well above Tg, all the way toward the much higher melting point, so the usable temperature sits far above Tg. This is why PEEK, with a Tg of only 143C, runs continuously at 250C, while ULTEM, amorphous with a much higher Tg of 217C, tops out around 170C. Negative Tg values (PP, PVDF, TPU) are normal and are not a temperature limit.

Continuous service temperature is the temperature the part can sit at for thousands of hours without meaningful loss of strength or stiffness. It is the most life-relevant thermal number for anything that lives hot, as opposed to a brief peak it could survive once. As the modulus-versus-temperature diagram shows, semi-crystalline materials carry this number far above their Tg, while amorphous materials land at or below theirs. For a part exposed to sustained heat, design to this figure with a margin, not to the more optimistic HDT or Tg.

Sunlight is harsh on most plastics. Poor performers (PLA, ABS, neat nylon) yellow, chalk, and grow brittle after months outdoors as UV breaks their chains. Good to excellent performers (ASA, PVDF, PEEK) hold their color and strength in direct sun for years. ASA is the affordable default for outdoor parts, which is why it is the recommended callout for marine and agricultural work where weather and UV are constant. A poor-UV material can still be used outdoors if it is painted or clear-coated to block sunlight, but unprotected it will degrade.

Written as Resists and Weak against, naming actual chemical families rather than vague ratings, because the difference between a part that lasts and one that fails is usually a specific solvent. A material under Weak against may dissolve, swell, soften, craze, or stress-crack on contact, sometimes within minutes. Two cautions: resistance almost always drops as temperature rises, so a chemical that is harmless at room temperature can attack the same part when hot; and concentration matters. For anything critical, verify the exact chemical, concentration, and temperature against the manufacturer's chemical compatibility chart.

Some polymers slowly absorb water from humid air or direct contact, and that absorbed water changes the finished part in service, not just during printing. Water works its way between the polymer chains and acts as a plasticizer: it lets the chains slide, so the part grows more flexible and a little tougher but loses stiffness and strength, and it physically swells, gaining size in every dimension. Nylon is the textbook case. A dry PA6 part left in humid conditions can take up 2 to 3 percent of its weight in water at equilibrium, and as it does, its stiffness can fall by roughly half while it grows enough to ruin a tight tolerance. Picture a nylon gear or a press-fit bushing machined and fitted bone-dry: after a few weeks at room humidity it swells, the gear mesh tightens or binds and the press fit shifts, even though nothing mechanical 'failed.' A precision part has to be dimensioned for its conditioned, in-service moisture state, not its dry state. A second, slower effect is hydrolysis, where hot water or steam chemically cuts the polymer chains and permanently weakens the part; PET, PLA, and PC are vulnerable to this at elevated temperature, which is why they degrade in repeated dishwasher or autoclave service. By contrast PP, PE, PVDF, PEEK, and the fluoropolymers barely absorb water and stay dimensionally stable in wet or humid service.

Whether the material is suitable for contact with food, skin, or tissue. Two things to keep separate: the resin and the printed part. Even when a base resin is offered in a food-grade or medical grade, a standard FDM part is generally not food-safe as printed, because the microscopic crevices between layers harbor bacteria that cannot be fully cleaned, and the nozzle, colorants, and additives can introduce contaminants. Reinforced CF and GF grades are never food, skin, or medical safe, because the fibers can shed from the surface. Genuine food-contact or medical use needs the right certified grade plus a sealing or post-process step (such as a food-safe epoxy coat) and validation, not just a 'food-grade' spool.

The sweet-spot uses where this material's particular balance of properties makes it the correct choice. Start here when matching a part to a material: if your part looks like one of these, this row is a strong candidate.

The conditions where this material will disappoint or fail, usually paired with a pointer to the material that handles that case better. Reading this row saves more bad parts than any other, because most material mistakes are not picking a weak material, they are picking a capable material for the one job it happens to be bad at.

The next material down in cost or difficulty that still does the job when this one is more than the part needs. A deliberate cost-control tool: if the part does not actually require this material's strength, heat, or chemical resistance, the step-down delivers the same result for less money and less printing hassle.