PP — Polypropylene
Polypropylene
PP (polypropylene) is a lightweight, economical and versatile semi-crystalline thermoplastic, valued for its excellent chemical resistance and fatigue performance. It is the reference for living hinges, housings and packaging parts in injection molding.
- Mold temperature: 20-60°C
- Melt temperature: 200-260°C
- Shrinkage: 1.0-2.5%
- Density: 0.90-0.91
- Manufacturer grades: Moplen HP500N (LyondellBasell), PP 56M10 (SABIC), Tatren HT2511 (MOL)
Processing
PP is not hygroscopic: no pre-drying is required, which simplifies processing and speeds up startup. Run the melt between 200 and 260 °C with a mold regulated between 20 and 60 °C; offset its high shrinkage (1.0 to 2.5 %) with careful holding and uniform cooling to limit sink marks and warpage on this semi-crystalline material.
Advantages
- No drying required (non-hygroscopic): fast startup
- Excellent chemical resistance to acids, bases and solvents
- Very good fatigue performance: integrated living hinges
- Low density (0.90 to 0.91): light and economical parts
Limitations
- High shrinkage (1.0 to 2.5 %) prone to warpage and sink marks
- UV- and oxidation-sensitive without stabilizer
- Low rigidity and limited thermal resistance unfilled
- Low surface adhesion: bonding and painting difficult without surface treatment
Common injection molding defects
- Sink marks — High semi-crystalline shrinkage: increase holding pressure and time, review rib thickness
- Warpage — Differential shrinkage: homogenize mold cooling and balance the fill
- Flow marks — Melt front too cold: raise melt temperature and injection speed
Typical applications
Food packaging, bumpers, living hinges, furniture