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PP

Polipropileno

PP·Polyolefins·Semi-crystalline

Polypropylene (PP) is the world's second-most-produced plastic, and the most versatile commodity thermoplastic in injection molding. It is lightweight (density 0.9–0.91, floats on water), chemically resistant, food-grade certified without BPA, and unique among commodities for its ability to survive thousands of bending cycles without breaking — the foundation of the integrated-hinge caps on shampoo bottles and toolboxes.

But being semi-crystalline gives it its own personality: it shrinks 1–3% (vs 0.4–0.7% for amorphous ABS), warps easily if the part isn't well designed, and demands higher mold temperatures than many people expect. Here we have compiled the reference ranges from the PDS, plus the questions that come up over and over on the shop floor: how to control warping, when to choose homopolymer vs copolymer, why it almost never needs drying, and when to switch to HDPE.

Share your experience in the comments — ranges vary by manufacturer and grade, and collective discussion is what gets us out of trouble on the floor.

The ranges shown in these data tables were compiled by the MVPS team from various parameter sheets and literature, integrating the lower and upper limits for each material type.

This information must be carefully reviewed when developing injection molding processes. Final ranges and processing tolerances are the responsibility of the engineer in charge.

These ranges are not recommended for developing specific process tolerances. MVPS always recommends requesting and consulting the supplier's data sheet.

General Properties

Chemical StructureSemi-crystalline
Specific Gravity (Density)0.95:1
L/D Ratio20 – 30:1
Compression Ratio1.5 – 3:1
Tonnage Factor3.86 – 5.41kN/cm²
Thermal Diffusivity0.1362mm²/s
Max Shear Rate100,0001/s
Shrinkage1 – 3%
Regrind50%
Heat Deflection (HDT) @ 1.82 MPa49°C
Glass Transition (Tg) @ 10°C/min-21°C
Vicat Softening @ 50N75°C

Drying

Drying Temperature60 – 80°C
Drying Time1 – 2h
Recommended Moisture0.5%
Recommended Dryer TypeAir
Dew Point-40°C

Temperatures

Melt220 – 279°C
Nozzle229 – 302°C
Front221 – 291°C
Middle210 – 271°C
Rear199 – 249°C
Demolding38 – 91°C
Mold (Cooling)21 – 79°C
Feed Throat10 – 49°C

Processing

Back Pressure6.9 – 13.8bar
Screw Speed80 – 150RPM
Injection SpeedHigh
Barrel Occupancy30 – 70%
Injection Pressure600 – 1,800Pbar
Holding Pressure150 – 1,440Pbar
Cushion6.4 – 12.7mm

Mold

Runner Diameter3.94 – 9.14mm
Gate Diameter1.02 – 1.52mm
Gate Area0.81 – 1.82mm²
Wall Thickness0.64 – 3.81mm

Venting

Depth (Vent Depth)0.0102 – 0.0203mm
Land (Vent Land)0.508 – 1.02mm
Width (Vent / Clearance)3.05 – 7.62mm
Relief (Relief Channel)0.127 – 0.2032mm

Frequently asked questions

Polypropylene is a semi-crystalline thermoplastic formed by polymerizing propylene (a monomer from natural gas). Two main families exist: homopolymer (PP-H, propylene only — stiffer, better gloss, more impact-brittle) and copolymer (PP-C, propylene + ethylene — better impact especially at low temperature, slightly less stiff). Typical density is 0.9–0.91 g/cm³, the lowest of all commodities.
Yes. PP is FDA and EFSA food-grade approved, contains no BPA or phthalates, and is one of the safest plastics for food packaging. It is used in yogurt tubs, reusable containers, caps, baby bottles and microwave utensils. Even so, ask your supplier for a lot-specific certificate if your application is regulated (medical, infant) because additives and colorants may not be certified.
Yes. PP is recycling code #5, 100% recyclable, and increasingly accepted by municipal programs (unlike ABS #7 which is minority-accepted). The PDS recommends up to 25% regrind while maintaining virgin-equivalent properties. Multiple reprocessing cycles degrade the polymer chain progressively — expect impact loss and more yellowing after the 3rd or 4th cycle.
PP has a higher service temperature (handles up to ~120°C vs ~80°C for HDPE), is stiffer and has better gloss. HDPE is more impact resistant, especially at low temperature, and better for wet barrier applications. Rule of thumb: for living hinges (integrated-hinge caps) always PP; for drums, jugs and rugged industrial packaging, HDPE; for autoclavable parts or hot-liquid contact, PP.
PP is one of the least hygroscopic polymers — it absorbs less than 0.02% of ambient moisture, well below the critical threshold for surface defects. That's why the PDS marks it as 'Air' instead of 'Desiccant'. Exception: filled grades (glass fiber, talc) may need light drying at 80–95°C for 2–4 h because the filler absorbs moisture. If you see silver streaks, suspect the filler before the base PP.
Warping in PP is almost always from differential shrinkage: the material shrinks 1.5–2% in the flow direction and only 1.0–1.5% perpendicular, creating internal stress. Solutions: (1) uniform wall — avoid thickness variations greater than 20%, (2) balanced cooling on both sides of the mold, (3) adequate holding pressure to compensate shrinkage, (4) generous cooling time (PP crystallizes slowly). If geometry forces uneven walls, consider mineral-filled PP (talc) which reduces differential shrinkage.
Depends on the application: homopolymer for stiff parts with good gloss at room temperature (rigid caps, containers, accessories). Copolymer (random or impact/block) when the part sees impacts, low temperature, or needs a perfect living hinge (block copolymers handle hinge fatigue tests better). For freezer packaging, always copolymer.
PP wins on: cost (~30% cheaper), chemical resistance, food-grade without extra certification, living hinge, weight. ABS wins on: dimensional rigidity, surface finish, ease of painting and bonding, UV stability (in stabilized grades). For a paintable cosmetic housing → ABS. For a food container, bin, or functional cap → PP. For toys with fine detail → ABS. For outdoor furniture → UV-stabilized PP or ASA.
PP is very UV-sensitive — the C–H bonds on tertiary carbons are easy to photo-oxidize. Without stabilization, a sun-exposed part loses 50% of its impact in just months. The solution is HALS (Hindered Amine Light Stabilizers) added in outdoor grades, typically as a masterbatch at 0.1–0.5%. For garden furniture, agricultural containers, or any outdoor application, always ask the supplier for a UV-stabilized grade.
The PDS marks 27–66°C for the mold. Hotter (50–65°C) = better gloss, higher crystallinity (more stiffness), lower residual stress, but longer cycle. Cooler (30–40°C) = short cycle and high throughput, but higher warping risk and worse surface. For commodity packaging aim for 30–40°C. For technical or cosmetic parts (with good surface quality), 50–60°C. If the PP is glass-filled, raise to 60–80°C to avoid visible surface fibers.

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