Nylon PA6 vs PA66: A Buyer's Guide

PA6 and PA66 sit side by side in nearly every polyamide portfolio, share an HS classification under heading 3908, and on a casual datasheet read look close enough to swap. They are not. The two differ in melt point by roughly 40 degC, in moisture uptake by a couple of percentage points that move dimensions and stiffness, and in how each behaves once a 30 percent glass charge and a humid service life enter the picture. Getting the choice wrong shows up as warped connectors, brackets that creep at temperature, or a part that passed dry-as-moulded testing and failed in the field.

The chemistry sets up everything that follows. PA6 is polymerised from a single monomer, caprolactam, giving a polymer with one amide group per repeat and a single, relatively broad melting transition. PA66 is built from two monomers, hexamethylenediamine and adipic acid, producing a more symmetrical, more densely hydrogen-bonded chain that crystallises tighter and melts higher. That structural difference -- one building block versus two -- is the root cause of the melt point, stiffness, and moisture gaps a buyer has to manage. For a wider view of where both sit against POM, PBT and PC, see our engineering plastics comparison.

Melt point, heat resistance and processing#

The headline number is melt temperature. PA66 melts at roughly 255-265 degC against 215-225 degC for PA6 -- a 40-45 degC gap that translates directly into short-term heat resistance and the ability to survive reflow and wave soldering. That is why PA66 owns the hot corners: under-bonnet automotive, electrical parts near solder reflow, and anything seeing brief excursions above 200 degC. PA6's lower melt point is a processing advantage, not just a limitation -- a wider, more forgiving window, lower energy to melt, and better flow into thin or complex tooling.

PA66 crystallises fast and over a narrow range, which shortens cycle times but tightens the moulding window and makes gate and cooling design less forgiving. Both grades must be dried hard -- to roughly 0.2 percent residual moisture or below -- before moulding, because water in the melt drives hydrolytic chain scission, dropping molecular weight and leaving splay and brittle parts. This is non-negotiable; nylon is the textbook hygroscopic resin and a wet pellet ruins a good grade. See our note on moisture and desiccants for hygroscopic resins for drying practice.

Stiffness, toughness and the moisture trade#

Unfilled and dry, PA66 is the stiffer, stronger, higher-HDT polymer; PA6 is the tougher, more impact-resistant one with better fatigue behaviour and a smoother surface. But neither lives dry. Both are hygroscopic, and absorbed water acts as a plasticiser: it lowers modulus and tensile strength, raises impact toughness and elongation, and swells the part. The practical consequence is dimensional -- a tight-tolerance PA6 part can grow noticeably as it equilibrates to ambient humidity, which is precisely why dimension- and electrically-critical components lean toward PA66, whose lower uptake means smaller dimensional swing.

A nylon datasheet without a stated conditioning state is half a number -- dry-as-moulded stiffness is a figure your part will never actually see in service.

This is the single most common procurement error in polyamide: comparing one supplier's dry-as-moulded (DAM) value against another's conditioned (50 percent RH or water-saturated) value. DAM strength and stiffness always read higher because the polymer hasn't yet softened. Conditioned values reflect real service. Demand that the test state appears explicitly on the certificate -- DAM versus conditioned -- and compare like with like. Our guide to reading a polymer COA covers what else belongs on that document, from melt-flow data per ISO 1133 to lot traceability.

Glass-filled grades: where the real engineering happens#

Unfilled nylon is a minority of the engineering tonnage. The structural workhorses are glass-filled -- typically 30 percent, with 15, 35, 40 and 50 percent grades all common -- and that is where PA6 versus PA66 gets decided in automotive and electrical. Glass fibre transforms the mechanics: a 30 percent GF grade roughly triples unfilled tensile strength, sharply raises modulus, and pushes HDT close to the melt-influenced ceiling. Crucially, glass also blunts the moisture penalty, because the load path now runs largely through the fibre network rather than the moisture-sensitive matrix. For the broader filler picture across resins, see glass-filled grades explained.

The split in practice: 30-50 percent GF PA66 is the default for engine-bay brackets, throttle bodies, charge-air components and connectors that see continuous duty above 120 degC or reflow soldering. 30 percent GF PA6 is highly capable wherever peak temperature is moderate and you want easier moulding, a better surface finish for visible parts, or a lower landed cost. Electrical and electronic parts often favour PA66 for its higher heat and lower moisture-driven dimensional drift, frequently with flame-retardant packages targeting a UL94 V-0 rating -- talk to the OmniaStrata engineering plastics desk when a flame class is in spec.

When each wins -- and the commercial read#

Strip it to a decision rule. Choose PA66 when the part sees high or sustained heat, reflow soldering, tight dimensional tolerance under humidity, or maximum stiffness in a glass-filled structural role. Choose PA6 when you want toughness and impact resistance, an easier and wider processing window, superior surface and colour, film or fibre applications, or simply a keener price for a part whose thermal envelope is moderate. Most of the polyamide market is one of these two calls made well.

• —Heat and soldering: PA66 -- higher melt point and HDT win every time near 200 degC and above.
• —Toughness, films, monofilament: PA6 -- better impact, broader processing window, cleaner finish.
• —Tight tolerance / electrical: PA66 -- lower moisture uptake means less dimensional swing.
• —Cost-sensitive moderate-heat parts: PA6 -- typically the value option with ample performance.
• —Glass-filled structural, high heat: 30-50% GF PA66 -- the automotive and connector default.

On price, treat both as feedstock-driven and cyclical rather than fixed: PA66's adipic acid and HMDA chain has historically been the tighter, more volatile supply story, while PA6's caprolactam route is generally more broadly sourced -- so the PA66 premium widens and narrows with intermediate availability rather than holding a constant spread. Frame any quoted differential as indicative, not a locked figure, and confirm at the time of enquiry. The practical takeaway for a buyer: specify the conditioning state, the glass loading, and the actual service temperature on your enquiry, not just "nylon" -- that single discipline avoids most field failures and lets the desk match grade to part precisely. Send your specification and we'll line up qualified PA6 and PA66 origins against it.