PP. Polypropylene.
Living hinges.
Chemical-resistant.
Polypropylene (PP) — semi-crystalline thermoplastic. Lightweight, chemical-resistant, fatigue-resistant. Famous for living hinges. Cost-effective for many applications.
Key principles.
Semi-crystalline
Density 0.9 g/cm³ — lightest engineering thermoplastic. Floats on water.
UTS 30-35 MPa
Moderate strength. Excellent fatigue (living hinges last millions of cycles).
Excellent
Resists most chemicals. Limited only by strong oxidizers, hot aromatics.
Food contact
21 CFR 177.1520. USP Class VI grades for medical.
Unique property
PP's exceptional fatigue resistance enables thin flexing hinges (caps, cases).
Cheap
~$1.50-2.50/kg. Among cheapest engineering plastics.
FAQ
PP vs HDPE?
PP: stiffer, higher service temp (100°C vs 80°C), better living hinges. HDPE: tougher impact, better chemical resistance.
Living hinge design?
Hinge thickness 0.3-0.5 mm, length 1-2 mm. Properly designed PP hinges last 1M+ cycles.
UV stability?
Standard PP poor outdoor (1-2 years). UV-stabilized 5-10 year. Carbon-black 20+ year.
Welding PP?
Hot air, ultrasonic, vibration welding all work. PP welds itself well.
Machining PP?
Soft, flexible, gummy. Sharp tools, slow speeds. Bar stock available.
Major applications?
Bottle caps, automotive bumpers, food containers, medical syringes, ropes.