Thermoplastic
Polypropylene
Cheap, chemically inert, and the only common plastic with a real living hinge.
Published values for unfilled grades at 23 °C unless stated. Confirm against the supplier datasheet for the grade you specify.
What it is
Polypropylene is a semi-crystalline polyolefin. It is the lightest common molding plastic, floats in water, absorbs essentially no moisture, and resists almost every acid, base, and solvent a product will meet in normal use.
Its defining property is fatigue resistance. A correctly designed PP hinge survives millions of cycles, which is why every flip-top closure and every hinged instrument case is molded from it in one piece.
Where it is used
- Living-hinge closures, cases, and battery doors
- Chemical containers, labware, and fluid handling
- Automotive under-bonnet ducting and battery trays
- Appliance and packaging components
- Medical disposables and autoclavable trays
Processing notes
No drying required in most cases, which removes a whole step from the cell.
Melt 200–250 °C, mold 20–60 °C. Cooler tools reduce cycle but increase shrinkage variation.
Shrinkage is high and direction-dependent, 1.3–1.8 %, and continues for up to 24 hours after ejection. Measure parts the next day, not off the press.
Available through
Design notes
Living hinges must be gated so the flow crosses the hinge, and the hinge web should be 0.25–0.40 mm thick with a generous radius on both sides. Flex the hinge immediately after ejection to orient the polymer.
Wall 1.0–3.0 mm. PP flows well, so thin walls are achievable, but high shrinkage makes thick sections sink badly.
Nothing bonds to PP without surface treatment. Design mechanical joints, not adhesive ones.
Grades and variants
- Homopolymer, stiffer and clearer, lower impact at low temperature
- Copolymer, better impact, the usual choice for hinges and cold service
- Talc or glass filled (10–40 %), much stiffer, no longer suitable for hinges
- Random copolymer, improved clarity for containers
- Medical grade, gamma-stable
Alternatives
If Polypropylene is not the right answer
Specifying this material?
Send your CAD and target volumes. We come back with a price, a lead time, and any material or design notes that would reduce either.