Last updated: 2026-09-01 · 5 min read

Nylon vs Polycarbonate: Which Resin Should You Specify for a Structural Molded Part?

Nylon (polyamide, PA) and polycarbonate (PC) are both engineering-grade thermoplastics used for structural injection-molded parts — but they solve different problems. Nylon offers superior fatigue resistance, chemical resistance, and wear performance; polycarbonate offers superior impact strength, optical clarity, and dimensional stability under static load. The selection depends on which failure mode is dominant in the application: cyclic fatigue and chemical exposure point to nylon; impact loading and transparency point to polycarbonate.

Cavity — The hollow space inside the mold that forms the part shape. Multi-cavity molds produce multiple parts per shot. Cavity count determines production output but also mold cost and complexity. Each cavity must be balanced for uniform fill. See also related guide.

Key Facts

  • Nylon absorbs moisture (1.5–3.0 % at equilibrium in 50 % RH). The absorbed water acts as a plasticizer — it increases toughness and elongation but reduces tensile strength and stiffness. A nylon part tested dry-as-molded will have different mechanical properties than the same part after 48 hours in a humid environment.
  • Polycarbonate is amorphous (no crystalline melting point) and transparent in its natural state. It has the highest impact strength of any commodity engineering resin — notched Izod of 600–900 J/m — but it is susceptible to stress cracking when exposed to certain solvents (ketones, esters, chlorinated hydrocarbons).
  • Nylon is semi-crystalline (melting point 220–265 °C depending on grade). It is opaque or translucent in natural form and cannot be made optically clear. The crystalline structure gives nylon excellent fatigue resistance (10⁶+ flex cycles without failure) and chemical resistance to oils, fuels, and most organic solvents.

    • Cavity — The hollow space inside the mold that forms the part shape. Multi-cavity molds produce multiple parts per shot. Cavity count determines production output but also mold cost and complexity. Each cavity must be balanced for uniform fill. The related guide covers adjacent topics.

    Property Comparison

    Property Nylon 6/6 (dry) Polycarbonate
    Tensile strength 70–85 MPa 55–70 MPa
    Flexural modulus 2.8–3.2 GPa 2.3–2.5 GPa
    Notched Izod impact 50–80 J/m 600–900 J/m
    Heat deflection (1.8 MPa) 70–90 °C 130–140 °C
    Moisture absorption (24 h) 1.5–3.0 % 0.1–0.2 %
    Mold shrinkage 1.0–2.0 % 0.5–0.7 %
    Fatigue endurance limit excellent (10⁷ cycles) moderate
    Optical clarity opaque transparent (88–90 % transmission)

    When Nylon Is the Correct Choice

  • Parts subject to repeated flexing or cyclic loading — snap-fits, living hinges, clips, gear teeth, and latch mechanisms. Nylon’s semi-crystalline structure provides fatigue resistance that amorphous resins (PC, ABS, PMMA) cannot match. A nylon snap-fit survives 10,000+ engagement cycles; a PC snap-fit fails at 500–1,000 cycles due to stress-crazing.
  • Parts exposed to oils, fuels, greases, and organic solvents — nylon is resistant to hydrocarbons, making it the standard material for automotive under-hood components (fuel-line clips, air-intake manifolds, oil-filter housings). PC dissolves or crazes in the presence of ketones, esters, and chlorinated solvents.
  • Wear surfaces and bearing applications — nylon has a low coefficient of friction (0.2–0.4) and good abrasion resistance. Unfilled nylon is used for bushings, rollers, and wear pads. Glass-filled nylon (GF30) is used for gears and cams that must operate without external lubrication.
  • High-temperature under-hood applications (with heat-stabilized grades) — heat-stabilized nylon 6/6 (with 0.5–1 % copper-iodide or Hindered Amine Light Stabilizer) maintains mechanical properties at 120–150 °C continuous use. Glass-filled grades (PA66-GF30) extend the heat-deflection temperature to 250 °C at 1.8 MPa.

    • When Polycarbonate Is the Correct Choice

    Polycarbonate is specified when the primary requirement is impact resistance, optical clarity, or dimensional stability under static load in a non-aggressive chemical environment:

  • Impact-critical components — safety housings, protective covers, helmets, riot shields, and automotive headlamp lenses. PC’s notched Izod of 600–900 J/m is 8–15× higher than nylon’s 50–80 J/m. A polycarbonate housing can absorb a 2-meter drop onto concrete without cracking; a nylon housing of the same wall thickness may fracture at the gate or rib intersection.
  • Transparent or translucent parts — headlamp lenses, indicator covers, light pipes, and display windows. PC transmits 88–90 % of visible light (compared to PMMA at 92 %). Nylon is opaque and cannot be used for optical applications.
  • Dimensional stability under static load — PC’s low moisture absorption (0.1–0.2 %) means dimensions do not change with humidity. Nylon absorbs 1.5–3.0 % moisture, which causes dimensional growth of 0.3–0.5 % — significant for parts with tight assembly tolerances. A nylon gear that fits at 23 °C / 50 % RH may bind at 35 °C / 90 % RH due to moisture-induced swelling.
  • Structural housings with flame-retardancy requirements — several PC grades achieve V-0 rating at 1.5 mm without halogenated additives, preserving impact strength and color stability. Nylon typically requires halogenated or phosphorus-based flame-retardant additives to the same rating, which reduces mechanical properties.

    • Frequently Asked Questions

    Can nylon and polycarbonate be blended?

    Not directly — nylon and PC are immiscible (they do not mix at the molecular level). A blend without a compatibilizer produces a weak, phase-separated material. Compatibilized nylon-PC blends exist (e.g., for automotive body panels that need both impact resistance and chemical resistance), but they are specialty grades with specific processing requirements. For most applications, the engineer selects one or the other rather than blending.

    Does moisture absorption affect nylon’s mold design?

    Yes. Nylon shrinks 1.0–2.0 % during molding (partially due to crystallization, partially due to thermal contraction). After molding, the part absorbs moisture from the atmosphere and grows 0.3–0.5 % in each dimension. The mold cavity must be sized for the equilibrium-conditioned part (the dimensions after moisture absorption), not the dry-as-molded dimensions. A mold designed for dry nylon produces parts that are undersized at equilibrium humidity.

    Which resin is easier to mold?

    PC is easier to mold in most respects — it is amorphous (no crystallization shrinkage variation), has low moisture sensitivity during processing (0.02 % max before molding, achieved by 4 hours at 120 °C), and has lower mold shrinkage (0.5–0.7 %). Nylon requires careful drying (0.1 % max moisture before molding, 4–6 hours at 80 °C under vacuum or desiccant), higher mold temperature (80–120 °C to control crystallization), and attention to gate vestige (nylon tends to string at the gate during mold opening).

    Key Takeaways

    1. The nylon-vs-PC decision starts with the failure mode. If the part fails by cyclic fatigue, chemical attack, or wear — specify nylon. If the part fails by impact, requires transparency, or must hold tight dimensions in variable humidity — specify polycarbonate.
    2. Nylon’s moisture absorption (1.5–3.0 %) is the most overlooked design factor. It changes dimensions, reduces stiffness, and increases toughness — all at the same time. The part must be designed for equilibrium-conditioned properties, not dry-as-molded properties.
    3. PC’s chemical sensitivity is the most common field-failure cause. A PC housing that survives impact testing in the lab may crack in the field when exposed to a cleaning solvent, a lubricant, or an adhesive used during assembly. Chemical compatibility must be verified for every substance the part contacts in service.

    A quotation that presents both nylon and polycarbonate as options — with the mechanical properties, the moisture sensitivity, and the chemical compatibility documented for each — gives the engineer the data to make an informed selection. A quotation that specifies one resin without the comparison has made an assumption about the operating environment that may not be valid.

    Sources
    DSM Engineering Materials — nylon data sheets
    Covestro — polycarbonate design guide
    Plastics Technology — resin selection

    Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet