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Should I use polypropylene or HDPE?
Use polypropylene for most molded parts: it is stiffer, handles more heat, melting near 329 °F (165 °C) versus 266 °F (130 °C) for HDPE, and it is the standard material for living hinges. Use HDPE when parts must take impact below freezing, need low friction and abrasion resistance, or are machined wear strips, guides and cutting boards.
Polypropylene (PP)
High-density polyethylene (HDPE)
By the numbers
Priced on the same representative part: housing, injection molding, 10,000 pcs.
Tensile strengthMPa
PP
33
HDPE
26
Densityg/cm³
PP
0.905
HDPE
0.95
Max service temp°C
PP
100
HDPE
80
Raw material cost$/kg
PP
1.5
HDPE
1.5
Which should you pick?
Pick PP when
- The part is a living-hinge cap or closure
- It sees dishwasher or hot-fill temperatures
- Thin-wall containers, housings and trays need stiffness
- Lowest density matters: about 0.9 g/cm³
Pick HDPE when
- Parts take impact below 32 °F (0 °C)
- The part is a wear strip, conveyor guide or slide
- It is a cutting board or machined food-processing part
- Tanks and containers should flex rather than crack
Key differences
| PP | HDPE | |
|---|---|---|
| Living hinges | Standard; survive hundreds of thousands of flexes | Not used |
| Cold impact | Homopolymer turns brittle near 32 °F (0 °C); use impact copolymer | Tough below freezing |
| Molding shrink | 1.0–2.5% | 1.5–4% |
| Friction and abrasion | Moderate | Low friction; good abrasion resistance |
| Machined parts | Rarely machined | Machines easily from sheet and rod |
Questions
Is HDPE or polypropylene more chemical resistant?
Both resist water, acids, bases and most solvents at room temperature, and neither bonds or paints easily. PP keeps its properties to higher temperatures; HDPE stays tougher in the cold. Check the specific chemical and temperature on the resin supplier's chart.
Are polypropylene and HDPE food safe?
Many grades of both comply with FDA 21 CFR 177.1520, and NSF/ANSI 51 listings are common for food equipment. Compliance depends on the grade and colorant, so get the resin supplier's letter.
Which is better for a living hinge?
Polypropylene. Designed correctly, a PP hinge web of 0.008–0.015 in (0.2–0.4 mm) flexes hundreds of thousands of times. HDPE hinges fail much sooner, so it is not used for them.
Try both on your part.
The builder prices the same geometry in every suitable process and material, with DFM for each.