Nylon vs polypropylene properties sit at opposite ends of the thermoplastics market: one is an engineering resin priced for performance, the other a commodity resin priced for volume. Parts that fail in service usually fail because the selection was made on a single datasheet number, most often tensile strength, without weighing moisture, heat, chemical exposure and fatigue. This guide compares the two polymers on the properties that decide part life, and shows how to write the decision into an RFQ.
Nylon vs polypropylene properties: the numbers that decide
| Property | PA6 nylon (unfilled) | PP homopolymer/copolymer |
|---|---|---|
| Density | 1.13-1.15 g/cm³ | 0.90-0.91 g/cm³ |
| Tensile strength | 70-85 MPa | 25-40 MPa |
| Melting point | 220 °C (PA66: 255 °C) | 160-170 °C |
| Continuous service temperature | 100-120 °C | 60-100 °C |
| Moulding shrinkage | 0.8-1.5% | 1.5-2.5% |
| Water absorption, 24 h | 1.5-2.5%, equilibrium higher | Below 0.03% |
Glass fibre closes part of the gap: GF30 PA6 reaches roughly 130-170 MPa against 70-90 MPa for glass-filled PP, but it also raises shrinkage anisotropy and abrasive wear on the barrel and screw.
Moisture and dimensional stability
PA6 moisture absorption is the property that most often surprises buyers switching from PP. Nylon takes up water along its polar amide groups, gaining weight and, more importantly, dimension: parts grow, stiffness drops and tight fits drift out of tolerance in humid service. PP is essentially hydrophobic, holding dimensions and dielectric properties in wet environments. The mitigation for nylon is not avoidance but discipline: dry the resin below 0.2% before moulding and condition or re-inspect parts that must hold tight tolerances.
Heat, wear and fatigue
Nylon keeps useful strength and stiffness well past the temperature where PP softens, which is why it dominates under-bonnet and power tool applications. Nylon wear resistance and self-lubricating behaviour make it the default for gears, bushings and slides, with PV limits quoted per grade. PP answers with fatigue: a correctly designed pp living hinge survives hundreds of thousands of cycles without cracking, and PP copolymer grades carry high impact at low temperature, a combination nylon does not match at similar cost.
Chemical exposure and cost
PP is the safer choice against acids, bases and aqueous media, tolerating everything from detergent to dilute alkali. Nylon handles oils, fuels and many solvents well but is attacked by strong acids and phenols, and it swells in polar solvents. On price, nylon typically costs one-and-a-half to three times PP per kilogram, and the gap widens with glass filling, so the resin decision carries real programme weight.
Processing and applications
The two resins impose different shop-floor disciplines. PP moulds at 200-250 °C straight from the bag with short cycles and wide windows; nylon moulds at 230-300 °C but must be dried at 80-100 °C for four to six hours to below 0.2% moisture, or the melt hydrolyses and parts come out brittle and splay-marked. Nylon's higher shrinkage anisotropy also demands balanced cooling and gating, while PP needs attention to weld lines and its larger overall shrinkage. In practice the split is simple: nylon goes where load, heat and wear justify the price, such as gears, clips, fasteners and under-bonnet parts; PP goes where volume, chemical exposure, living hinges and low moisture drift justify its cost, such as containers, housings, packaging and household products.
Writing the choice into an RFQ
State the service environment before stating the material: peak and continuous temperature, chemical contact, humidity, cycle life and the two or three dimensions that genuinely matter. Ask the supplier to propose the grade, the drying regime and the shrinkage compensation in the tool plan, and to justify the choice against the plastic materials hub, the PA6 vs PA66 comparison and the industrial polypropylene specification guide, or shortlist suppliers via the verified manufacturer directory.
