Ask someone to compare two plastics and weight is usually the first thing they think of. Density does tell you that – but it also tells you your cost per part, whether a component floats or sinks during finishing, and how a filled or reinforced grade will actually perform compared to its unfilled base resin.
If you’re pricing a part by resin cost per kilogram, density is the number that turns that price into cost per part. Get it wrong and your cost model is wrong before you’ve moulded a single unit.
What Density Actually Tells You
Density is mass per unit volume — how much a given amount of plastic weighs. Three practical things follow from that:
- Part weight. For the same mould cavity, a denser material makes a heavier part. That matters for shipping cost, handled weight, and anything with a weight spec.
- Cost per part, not just cost per kilogram. Resin is priced by weight, but your part is a fixed volume. A cheaper resin that’s also denser can end up costing more per part than a pricier, lighter one.
- Process behaviour. Density determines whether a component floats or sinks in a wash tank, and it shifts noticeably once you add glass fibre or mineral filler — a “PC” and a glass-filled PC are not the same material to plan around.
Density by Material
Reading the Chart
Materials cluster into three practical bands:
- Lightweight (under 1.05 g/cm³): PP and PE — these are the only common plastics that float in water, which matters for parts that get washed or transported wet.
- Mid-range (1.05–1.30 g/cm³): most engineering plastics used in enclosures, housings and mechanical parts sit here — ABS, nylon, polycarbonate, acrylic.
- Dense (over 1.30 g/cm³): PVC, acetal (POM) and PTFE. Glass-filled grades of lighter resins can push into this range too, which is worth checking before you assume a filled and unfilled version of the same material weigh about the same.
One correction worth flagging directly: a commonly circulated reference figure puts PTFE density around 1.06 g/cm³. That’s wrong – it’s roughly half the real figure. Verified fluoropolymer data puts virgin PTFE at 2.10–2.20 g/cm³, making it by far the densest material on this chart.
Common Questions
Does higher density mean stronger? No. Density is about mass and volume, not mechanical properties. A dense, brittle material and a light, tough one can serve completely different jobs — density tells you weight and cost, not strength.
Why does a glass-filled grade weigh more than the base resin? Glass fibre is denser than most plastic resins, so adding it raises the compound’s overall density, sometimes significantly — a 30% glass-filled nylon can be noticeably heavier than unfilled nylon at the same wall thickness.
How much does density actually change my per-part cost? More than most people expect. Two resins priced similarly per kilogram can differ 20–30% in density, which shifts cost per part by roughly the same margin once you’re filling the same cavity volume.
For expert guidance on choosing the right plastic for your needs, contact Plastinternational today!
