The answer is yes — with a level of conditioning that depends on which nylon grade is in the design and how carefully the process is managed. Nylon is not a uniformly behaved substrate. PA6 and PA66 bond well to TPU through genuine chemical adhesion. PA12 requires additional effort. All polyamide grades introduce a moisture variable that ABS and polycarbonate do not — and ignoring it is the most common source of nylon overmolding failures in production environments where the process was validated under laboratory conditions that do not reflect the factory floor.
The Adhesion Chemistry
TPU’s urethane linkages are polar. Nylon’s amide groups are also polar. At processing temperature, these groups interact at the interface through hydrogen bonding — urethane NH groups forming bonds with carbonyl groups in the polyamide backbone. This interaction is genuine chemical adhesion, not surface-level mechanical interlocking, and it produces bond strength that under optimized conditions exceeds the cohesive strength of the TPU itself.
The strength of this interaction depends on how many amide groups are available at the PA surface. PA6 repeats an amide group every 6 carbons; PA66 every 6.5 carbons on average; PA12 every 12 carbons. The longer the carbon chain, the fewer the amide groups at the surface, and the weaker the urethane-amide interaction. This is why PA12 is the most difficult nylon grade for TPU overmolding without surface preparation.
Nylon Grade Compatibility Summary
PA6 (Nylon 6): Compatible with TPU without primers under standard overmolding conditions. High amide group density at the surface supports the urethane-amide interaction. Pre-drying required; mold temperature 60–80°C. Cohesive failure achievable on unfilled grades.
PA66 (Nylon 6/6): Similar to PA6 in TPU compatibility. Slightly higher crystallinity than PA6 may require mold temperatures toward the upper end of the 60–80°C range. Compatible without primers on unfilled grades.
PA6/10, PA6/12: Intermediate carbon chain lengths between PA66 and PA12. Adhesion is generally adequate but lower than PA6 and PA66. Validate specifically for the grade in use.
PA12 (Nylon 12): Difficult substrate for TPU without intervention. Long carbon chain reduces amide group density, significantly limiting the urethane-amide interaction. TPU adhesion on PA12 typically produces adhesive failure at low peel loads without primers or mechanical interlocks. Silane-based coupling agents applied to the PA12 surface before overmolding, or mechanical interlock features in the substrate design, are required for structural bond strength.
Glass-fiber-reinforced nylon: Surface chemistry varies with fiber orientation and fiber content at the surface. Adhesion is generally lower and more variable than on unfilled grades. Both chemical and mechanical approaches are needed for reliable bond strength on glass-filled PA.
Managing the Moisture Variable
The most important process variable for TPU adhesion to nylon is not one that can be adjusted on the injection molding machine — it is the moisture content of the PA substrate at the time of overmolding.
Dry-as-molded PA6 has higher surface energy (40–44 mN/m) than moisture-conditioned PA6 (below 38 mN/m). TPU adhesion correlates with surface energy, so parts overmolded immediately after PA molding produce stronger bonds than parts that have conditioned at ambient humidity for hours or days before overmolding.
In insert molding operations where the PA substrate is molded at one location and overmolded at another, or where PA inserts are stored in open bins between operations, production bond strength is typically lower and more variable than development results obtained on freshly dried substrates.
Protocol for managing moisture in production:
– Dry PA inserts at 80°C for two to four hours before overmolding
– Vacuum-seal dried inserts in moisture-barrier packaging if storage before overmolding is unavoidable
– In two-shot molding, maximize cycle rate consistency to minimize substrate exposure time between stations
– Establish a maximum time limit between PA drying and overmolding, and test bond strength at the limit as part of process validation
For a moisture management protocol review for your specific production environment, Email Us.
TPU Grade Selection for Nylon
Ether vs. ester base chemistry. Ether-based TPU is the correct specification for any nylon application where the assembly will see moisture, humidity, or aqueous cleaning agents in service. PA substrates already introduce a moisture-rich interface environment; ester-based TPU’s hydrolytic susceptibility creates a compounding risk. Ester-based grades are appropriate only for dry, interior PA applications where initial bond strength is the priority and service environment moisture is definitively excluded.
Shore hardness. Softer grades (Shore 60A–80A) conform more readily to the PA surface during injection, increasing contact area and bond strength. Harder grades require tighter process control and are appropriate for structural zones subject to sustained mechanical loading.
Processing temperature window. TPU must process within the 190–240°C range. PA6 and PA66 process at 230–280°C, and the melt temperature differential between substrate and overmold material requires attention in two-shot tooling to prevent substrate degradation from the TPU barrel temperature at the interface.
Process Requirements for TPU on Nylon
Pre-dry both materials. PA at 80°C for two to four hours; TPU at 80–100°C for two to four hours. Process both within 30 minutes of drying completion.
Mold temperature. Maintain 60–80°C minimum. Below this threshold, the interface solidifies before adequate urethane-amide interaction develops. PA substrates benefit from mold temperatures toward the upper end of this range.
Gate placement. Direct TPU flow across the PA bond surface rather than along it. Flow parallel to the interface generates weld lines at the bond zone; flow perpendicular distributes contact pressure evenly.
Mechanical interlocks. For PA12 and glass-filled grades, mechanical interlock features in the PA substrate — through-holes, undercuts, channels — supplement chemical adhesion and provide redundant retention.
Incure’s adhesive and coating formulations support difficult nylon overmolding and adhesive bonding applications, including primer systems for PA12 and glass-filled grades that extend achievable bond strength beyond standard TPU overmolding chemistry. For technical support specific to your PA grade and application, Contact Our Team.
Visit www.incurelab.com for more information.