Polypropylene

Polypropylene is a semi-crystalline polyolefin and the standard closure resin. It has the highest service temperature of the commodity resins and is the only one that produces a true living hinge.

For a flip-top cap, a dispensing closure, or anything that has to survive steam sterilization, this is the material.

• Living hinge: A correctly designed and correctly gated PP hinge survives 100,000+ cycles. A badly gated one fails on the first flex. See molding notes.
• Sterilization: Autoclavable and steam sterilizable at 121°C. No other commodity resin here does this.
• Chemical resistance: Excellent across acids, bases, and most solvents.
• Fatigue resistance: High, which is what makes the hinge work.
• Cold impact: Homopolymer PP turns brittle below 0°C. If the product ships or stores cold, specify a copolymer grade.

Flip-top and screw closures. Dispensing and disc-top caps. Jars and thin-wall containers. Any sterilizable or hot-fill packaging. Cannabis child-resistant closures where the CR mechanism relies on material fatigue resistance.

Process: Injection molding.

Shrinkage: 1.0-2.5%.

Living hinge gating: The gate must be positioned so melt flows across the hinge, orienting polymer chains perpendicular to the hinge axis. If flow runs parallel to the hinge, the chains align the wrong way and the hinge splits on first actuation. This is the most common preventable failure in closure tooling.

Hinge land thickness: 0.25-0.40mm. Thicker will not fold cleanly. Thinner tears.

Draft: 1-2° minimum. Watch for weld lines forming around thread starts on multi-gated closures.