Material selection
HDPE vs Polypropylene
HDPE and Polypropylene are the two cheapest common molding plastics, both chemically inert, both unfilled at well under a dollar a kilo, and both frequently proposed as substitutes for each other on a cost-driven part. They are not interchangeable: one holds a living hinge for millions of cycles, the other survives an impact at freezing that would crack the other. The numbers below are the same published datasheet values used on each material's own page, set side by side.
| HDPE | Polypropylene | |
|---|---|---|
| Density | 0.95 g/cm³ | 0.90 g/cm³ |
| Tensile strength | 22–31 MPa | 30–35 MPa |
| Flexural modulus | 1.0 GPa | 1.3 GPa |
| Heat deflection (0.45 MPa) | 75 °C | 100 °C |
| Mold shrinkage | 1.5–3.0 % | 1.3–1.8 % |
| Water absorption | <0.01 % | <0.02 % |
| Relative cost | Very low | Very low |
Published values for unfilled grades at 23 °C unless stated, taken from each material's own datasheet page. Confirm against the supplier datasheet for the grade you specify.
What HDPE is
HDPE is a linear polyethylene with high crystallinity, soft compared with engineering plastics but extremely tough, especially below freezing, where most rigid plastics turn brittle. The surface is waxy and low friction, which makes it the material of choice for bearings, chutes, and wear strips, and it is inert enough for potable water and food contact. Shrinkage is the highest of the common molding plastics and highly anisotropic, 1.5 to 3.0 percent, so tolerances tighter than about ±0.3 percent of dimension are not realistic without a dedicated tooling trial. No drying is needed before molding.
What Polypropylene is
Polypropylene is a semi-crystalline polyolefin, the lightest common molding plastic, and its defining property is fatigue resistance: a correctly designed PP hinge survives millions of open-close cycles, which is why every flip-top closure and hinged instrument case is molded from it in one piece, a design freedom HDPE does not offer. It carries a materially higher heat deflection temperature than HDPE, 100 °C against 75 °C, and a stiffer flexural modulus, 1.3 GPa against 1.0 GPa. Shrinkage is still high and direction-dependent, 1.3 to 1.8 percent, and continues for up to 24 hours after ejection; parts should be measured the next day, not off the press. No drying is required in most cases either.
HDPE against Polypropylene
| HDPE | Polypropylene | |
|---|---|---|
| Heat deflection | 75 °C, the lower of the two | 100 °C, clears most under-bonnet and hot-water contact requirements HDPE cannot |
| Stiffness | 1.0 GPa flexural modulus, softer | 1.3 GPa, noticeably stiffer at the same wall thickness |
| Low-temperature impact | Best of the two: stays tough well below freezing, the reason it is specified for cold-service and outdoor parts | Weaker at low temperature; a copolymer grade closes some of the gap but does not match HDPE below freezing |
| Fatigue / living hinge | Not a hinge material: no living-hinge grade or design guidance exists for it | The only common molding plastic with a genuine living hinge, millions of cycles when correctly gated and designed |
| Friction and wear | Waxy, low-friction surface, the default choice for bearings, chutes, and wear strips | No particular wear advantage; not specified for sliding or bearing surfaces |
| Chemical resistance | Excellent against acids, bases, and solvents; the reference material for aggressive-fluid containers | Equally excellent against the same range, with the added benefit of gamma-stable medical grades for sterilised parts |
| Shrinkage | Highest of the common molding plastics, 1.5–3.0 %, harder to hold tight tolerance | Still high but tighter and more predictable, 1.3–1.8 % |
| Relative cost | Very low | Very low, essentially tied |
Which one, for which part
- Any part with a living hinge, flip-top closures, hinged cases, battery doors, is Polypropylene's alone: HDPE has no equivalent hinge design and will crack or take a permanent set instead of flexing.
- Cold-service or outdoor parts that see sustained sub-freezing temperature favour HDPE: it stays tough where PP and most other common plastics turn brittle.
- Bearings, wear strips, chutes, and low-load sliding surfaces are HDPE's case: its waxy, low-friction surface is the reason it is specified for these at all.
- Parts that need to clear 90–100 °C, hot-fluid handling, under-bonnet ducting, autoclavable medical trays, need Polypropylene: HDPE's 75 °C heat deflection temperature is not enough margin.
- Chemical containers and fluid handling where either resin's resistance is adequate: the tie-breaker is usually stiffness and heat, which point to PP, or low temperature and wear, which point to HDPE, not chemical resistance itself.
Our status
Trumould molds both HDPE and Polypropylene in-house on the same injection molding lines, and both are also available through our CNC machining capability for prototypes and low-volume parts. Send the part, the temperature range and cycle-life requirement it has to meet, and target volume, and the quote states which resin is recommended and why, not just a price for whichever one you asked about first.
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