Reading a material datasheet
Open any filament's technical datasheet and you get a tidy column of numbers: tensile strength 50 MPa, modulus 3.5 GPa, elongation at break 6 %, HDT 55 °C. It looks authoritative, and it is — for a solid, flawless bar of that plastic, injection-moulded and pulled apart in a lab. Your printed part is none of those things. It's built from stacked beads with gaps between them, it's hollow inside, and it's stronger along the beads than across them. So the first skill isn't reading the numbers — it's knowing what they still tell you once the part is FDM instead of a lab coupon.
What each number actually means
Tensile strength (MPa) is how hard you can pull before the material tears. Tensile and flexural modulus (GPa) both measure stiffness, though they don't match exactly: flexural usually reads a little higher than tensile — how much the part resists bending under load. A high modulus feels solid and springs back; a low one flexes. Elongation at break (%) is how far the material stretches before it snaps, and it's your best cheap proxy for toughness: a brittle plastic breaks at a few percent, a tough one stretches to tens of percent and absorbs the blow. Impact strength measures the same instinct directly — energy soaked up in a sudden hit. HDT, the heat deflection temperature, is roughly where the part starts to soften and sag under load, and the closely related glass transition temperature is where the plastic stops being rigid and turns rubbery.
| Property | What it tells you | Units | For your part |
|---|---|---|---|
| Tensile strength | Load before it tears | MPa | How much force it survives |
| Tensile / flexural modulus | Stiffness — resistance to bending | GPa | Whether it flexes in use |
| Elongation at break | Ductility — stretch before snapping | % | Toughness proxy: brittle vs tough |
| Impact strength | Energy absorbed in a sudden hit | kJ/m² | Survives being dropped |
| HDT | Where it softens under load | °C | Heat it can take |
| Glass transition (Tg) | Where it turns rubbery | °C | The hard ceiling for heat |
Read them as relative, not absolute
Here's the caveat that saves you from bad decisions: every one of those numbers was measured on a solid, isotropic specimen. Your part is anisotropic and part-hollow, so its real strength depends on wall count, infill, and — heavily — orientation. That 50 MPa is not the strength of your bracket. Treat the datasheet the way you'd treat a spec sheet comparing two cars: use it to rank materials against each other, not to predict the exact number your part will hit. PETG's higher elongation than PLA is a reliable truth in your part too; the absolute figure is not.
Let the part pick the number
You don't need to weigh all six numbers equally. A part usually has one dominant demand, and that demand points at one number. A jig that must not deflect lives and dies on modulus. A clip that gets snapped on daily lives on elongation. A part in a hot car lives on HDT. Find the failure you're designing against, read the column that governs it, and let the rest be tie-breakers.
Once you can read a datasheet as a ranked comparison rather than a promise, the next step is untangling the three properties people most often confuse — that's Stiffness, strength and toughness.