Last updated: 2026-09-05 · 5 min read

Both PA6 and PA66 are tough, abrasion-resistant engineering nylons that mold well and carry load for decades — and on a datasheet they can look interchangeable. They are not. The working summary: PA6 is the tougher, easier-processing, more moisture-hungry of the two; PA66 is the stiffer, more heat-resistant, more dimensionally precise one with a tighter processing window. Which one belongs in your part depends less on their datasheet midpoints than on moisture in service, temperature in service, and how tight your tolerances really are. Here is the comparison as it plays out on the molding floor.

Key facts

<>

  • PA66 melts hotter (its crystalline melting point sits tens of degrees above PA6’s), which is the root of most of its practical advantages — heat deflection, stiffness retention, solder-wave survivability in electronic assemblies.
  • PA6 absorbs more moisture and equilibrates slower; conditioned PA6 is tougher but softer and dimensionally larger than the dry-as-molded numbers suggest.
  • Both are hygroscopic and both must be dried before molding — hydrolysis during processing degrades either nylon the same way it degrades PC.
  • PA66’s narrow processing window rewards disciplined equipment; PA6 forgives wider variation and is often the safer choice on shared machines.
  • Glass filling changes both resins dramatically — and shifts the comparison toward PA66 for stiffness and heat, at the cost of anisotropic shrinkage and visible glass patterns on surfaces.

    • What is the actual difference between PA6 and PA66?

    PA6 (polycaprolactam). Polymerized from a single monomer type, PA6 crystallizes into a structure with a lower melting point and a wider processing window. It equilibrates at a higher moisture content in service, which plasticizes it: conditioned PA6 parts are notably tougher and more impact-resistant, and also softer, with dimensions that moved from the dry-molded state. Gear trains, rollers, wear pads and impact-prone housings are its home ground.
    PA66 (polyhexamethylene adipamide). Built from two monomer types in strict alternation, PA66 packs into a denser crystal structure with a higher melting point. It absorbs somewhat less moisture, retains stiffness at temperature, holds tighter tolerances and resists creep — at the price of a hotter, less forgiving melt and faster crystallization that can penalize surface finish. Electrical connectors, under-hood clips, thermal barriers and precision brackets are its home ground.

    The two definitions carry the whole comparison in miniature: PA6 buys application toughness with dimensional drift, and PA66 buys precision and heat with processing discipline. Neither is the better nylon. Each is the better nylon for a different failure mode — and the way to choose is to name the failure mode your program actually fears.

    How do their properties compare where it matters?

    Property axis PA6 tendency PA66 tendency
    Heat resistance Lower melting point; stiffness falls off earlier as temperature rises. Higher melting point and heat deflection; keeps modulus where PA6 softens.
    Toughness (conditioned) Higher equilibrium moisture plasticizes it — excellent impact behavior in service. Tough and capable, but drier in service; less built-in plasticization.
    Moisture uptake Higher; dimensions and properties shift more from dry-as-molded values. Lower; drifts less, which is why precision parts prefer it.
    Stiffness and creep Adequate; creeps more under sustained load, more so warm and wet. Better modulus retention and creep resistance, especially in heat.
    Processing window Wider; forgiving of machine variation and less-than-ideal control. Narrower; melts hotter, crystallizes faster, punishes marginal equipment.
    Surface and finish Often slightly glossier and more uniform, helped by slower crystallization. Fast crystallization can show flow marks and glass print on filled grades.

    One comparison note that saves programs from themselves: datasheet properties for both resins come in two states — dry-as-molded and conditioned — and the numbers differ enormously, particularly for impact strength and stiffness. A PA6 part specified from dry data will behave differently after a month of humid service; a PA66 bracket specified from conditioned data may not survive the dry, cold assembly line it passes through before ever reaching humidity. Insist on knowing which state every number on your comparison sheet represents, and read the datasheet accordingly — a searchable property database such as MatWeb carries both states for most grades.

    How does moisture change the choice?

    More than any other single factor. Both nylons drink water from the air, and everything about them shifts as they do: PA6 equilibrates at a higher moisture content and swells more, which means a tight interfit designed dry will bind in August and loosen in February; PA66 moves less but is not immune. The engineering responses are established and workable: condition test specimens the way the part will actually live, dimension critical fits at the midpoint of the moisture range rather than the dry extreme, and let the application’s humidity decide how much drift your tolerances can absorb. What the moisture data should also tell you is where the resin substitution question comes from: if the application is dimensionally critical and the environment is wet, the honest answer may be that neither unfilled nylon is the right family, and the conversation moves to acetal or to filled grades — decided before the deposit, not after a season of warranty returns.

    Which applications fit each resin?

  • PA6 fits — gears and wear surfaces that benefit from conditioning; impact-prone housings; parts whose tolerances have room for moisture drift; programs molded on shared, general-purpose equipment.
  • PA66 fits — under-hood and near-engine locations; connectors and electrical housings that must survive assembly heat; precision clips and brackets with tight, load-bearing fits; anything glass-filled for stiffness.
  • Either works — cable ties, fasteners, handles and general mechanical parts, where the choice is driven by availability, color program and supplier portfolio rather than performance.
  • Neither fits — long-term hot-water exposure without hydrolysis-resistant selection, clear optics, and applications where any moisture-driven dimensional drift breaks the design.

    • The check list ends where every resin conversation should: with the environment the part will actually live in. Written down — service temperature range, humidity, loads, assembly method — the PA6/PA66 decision usually makes itself. Held in someone’s head, it defaults to whichever resin the quote happened to price, which is not an engineering decision.

    What about glass filling?

    Add glass fiber and the comparison sharpens rather than blurs. Reinforcement multiplies the stiffness and heat advantages PA66 already holds, which is why glass-filled PA66 is the default structural nylon in automotive and electrical programs; PA6 filled grades exist and serve, but they compete from behind on modulus and heat. Reinforcement also multiplies both resins’ weaknesses: anisotropic shrinkage that warps flat parts, glass patterns that read on cosmetic surfaces, higher melt abrasiveness on tooling, and weld-line strength factors that a design must honor around every converging flow front. If your part is flat, cosmetic and reinforced all at once, expect the DFM conversation to be about fiber orientation and gate location before it is about anything else.

    What should T1 evidence include for nylon parts?

    1. Drying and moisture records. Lot number, dryer setpoints and a hopper moisture reading — nylons degrade wet, and the record protects both sides from a silent root cause.
    2. The process sheet as run. Measured melt and mold temperatures, hold profile and cycle — PA66 especially lives in a narrow window, and the as-run sheet is the only honest account of where the process sat.
    3. Dimensions reported against the drawing, with shrinkage stated. Nylons crystallize and shrink as they cool; the report should say what shrinkage the tool was built to, so later moisture-driven movement is interpreted against facts.
    4. Conditioning plan for test specimens. If mechanical evidence is part of acceptance, state whether specimens are dry or conditioned, and why that matches service.
    5. A written residual-risk note. What the trial could not test — long-term moisture effects, heat aging, creep under sustained load — named explicitly so T1 acceptance is not quietly stretched into a promise it was never making.

    The PA6-versus-PA66 debates I remember all ended the same way: the moment someone wrote down the service temperature and the humidity the part would actually see, the argument was over. The debates that never end are the ones where both sides argue from dry datasheet numbers about a part that will live wet. My rule is boring and effective — name the environment, name the tolerance, name the temperature, and the resin picks itself. Then the mold design conversation starts, and it is usually about shrinkage and weld lines, never about which nylon.

    — Arthur, Tooling Engineer at aCatMould

    Frequently asked questions

    Can I substitute PA66 where PA6 was specified?

    Not without a material review. PA66 shrinks differently, crystallizes faster and holds dimensions differently — a tool cut for PA6 will produce parts that measure differently in PA66, and the process window moves too. Substitutions after tooling owe a new DFM pass and usually a new T1.

    Which nylon is more dimensionally stable?

    PA66, in both directions that matter: it absorbs less moisture and its higher crystallinity gives it lower, more predictable shrinkage. For tight fits across humidity ranges, PA66 is the default — and acetal is the next conversation if even PA66 drifts too much.

    Do both nylons need drying before molding?

    Yes, both, in a desiccant dryer, to the moisture ceiling the grade’s datasheet states. Wet nylon hydrolyzes in the barrel the same way wet PC does — the part can look fine and lose toughness. Drying discipline is identical; the temperature and time windows differ by grade.

    Why does conditioned PA6 sometimes beat PA66 on impact strength?

    Because absorbed water plasticizes nylon. PA6 equilibrates at a higher moisture content, so in humid service it is effectively a tougher, softer material than its dry datasheet suggests. That is a feature for impact-prone parts and a liability for precision fits — the same moisture, read through two different applications.

    Is glass-filled PA6 or PA66 better for structural brackets?

    Filled PA66 is the usual answer for load-bearing brackets, especially warm ones, because reinforcement stacks on its existing stiffness and heat advantages. But the part geometry decides: flat, cosmetic reinforced parts fight warpage and glass print regardless of which nylon carries the fiber, and the gate plan becomes the real engineering work.

    How do I stop nylon parts from shrinking out of tolerance?

    Start before steel: mold shrinkage for the exact grade — filled or unfilled, and the direction dependence for filled grades — built into the tool, and a process that packs consistently. Post-molding, give parts time to equilibrate before final inspection where the drawing tolerates it, and inspect critical fits at a defined conditioning state rather than straight off the tool.

    Key takeaways

    1. PA6 trades toward service toughness, wider processing latitude and higher moisture uptake; PA66 trades toward heat, stiffness, precision and a narrower window.
    2. Moisture is the deciding variable in most programs — datasheet numbers mean different things dry and conditioned, and the service environment picks the state that counts.
    3. Both resins demand drying discipline; hydrolysis during molding degrades either nylon invisibly.
    4. Glass filling amplifies PA66’s structural advantages and both resins’ warpage and weld-line caveats — gate and fiber-orientation planning becomes the core DFM work.
    5. Write the environment down before choosing: temperature range, humidity, loads and fits decide the nylon; the quote should follow the decision, not make it.

    If your part lives warm, wet, loaded or precise — which is where most nylon programs live — send us the 3D model, the environment description and the drawing before the deposit. You will get the PA6/PA66 decision in writing with the datasheet states named, the shrinkage and gate plan for the grade, and the T1 evidence list, from a named tooling engineer, before any steel is ordered.

    Sources
    MatWeb — material property database for PA6 and PA66 grades, dry and conditioned states
    ASTM D638 — Standard Test Method for Tensile Properties of Plastics, ASTM International
    ISO 20457:2018 — Plastics moulded parts: Tolerances and acceptance conditions, International Organization for Standardization
    Autodesk Moldflow — injection molding simulation for shrinkage and fiber orientation, Autodesk

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