Is annealing required in injection moulded part?

Annealing helps in releasing stresses frozen in during injection moulding of highly crystalline materials like PA6. This generally increases values for mechanical properties. If annealing is done in water, PA6 absorbs some of it and gets plasticized. This increases flexibility of the component, and thereby impact strength also. However, this is a reversible process, i.e., this water may get lost on exposure to high ambient temperatures, over a period of time. This may reduce component flexibility but stress relaxation would anyway have happened. For PA, annealing is done at around 80 deg C.


As a cost reduction strategy, injection moulders sometime avoid annealing and optimize cycle time to minimum for higher production rate, which leads to the possibility of high residual internal stresses in the part. Solution is to ask your supplier for a specific PA grade which has minimal tendency for stress or may be ask you moulder to re-optimize injection mould pressure and temperature conditions. This would minimize stress but not eliminate it.

The need for post-mould annealing is also dependent on your application and end-use environment. Moisture uptake also plays a key role in stress relief over time (i.e. function of time/temp/humidity). Assuming your part is for a dimension critical application; and using unfilled & unmodified PA6, without knowing gating position (or type) relative to section thickness, conditioning under high humidity for 2-3 days may be sufficient. However, the high variation in wall thickness is not ideal, and not conducive to reducing moulded-in stress unless you also have well designed flow and gating, and ideal mould temperature control.

Just a cautionary note on annealing at elevated temps (and assume it's in the order of 120-150C), then the type of stabilizer system comes into play and the annealing environment/medium becomes more important. In Nylon 6 and 66 R&D work I've used high temperature silicone oils to do both annealing and reversion analysis (2-3hrs), and closed oven with nitrogen purge at 80-90C over 1-2 days, for determination of degree of relaxation in test specimens. Using elevated temps for extended periods in air plus high humidity can alter PA6 mwt and leach out low mwt stabilizers. If very critical dimensions are to be maintained then a mineral filled or short glass fibre/glass bead filled PA6 might be considered, which would be less prone to all these issues and perform better with part thickness variation.