About the sorption of water in nylon 6

As some local temperature is so high, to avoid the Nylon parts fragile, we're suggested to put the parts in the water for two days before shipment. Is Nylon easy to absorb water?

Nylon 6 must be dried when it is processed, then you obtain a product without humidity content. Its properties and dimensions will change according to temperature, relative humidity and time. If you maintain the nylon 6 product under standard conditions (50% rel. humidity & T=23ºC ) the humidity content may achieve about 3% at equilibrium. The dimensions may increase about 0,7%. This values will be bigger if relative humidity increases.

The mechanical properties will be lower when the nylon contains water, except the impact resistance that will be higher.

Moisture absorption in nylons is an ongoing process which is driven by the relative humidity of the atmosphere where the parts/materials are stored. When in a high RH environment polyamide resins will absorb moisture from the air IF the moisture content in the material at that time is less than the equilibrium moisture content for the resin for the given RH level. The reverse is true as well. Should the moisture content of the resin be more than it's equilibrium content value for the RH conditions where the resin/part is stored then the moisture content in the resin/part will decrease until it reaches the equilibrium content for those particular conditions. It is a common practice for processors to add moisture to parts molded from nylon resins when the ambient RH levels are low. This can be accomplished many ways but remember it is an ongoing process. As moisture levels increase strength & rigidity decrease while ductility/toughness increase as will dimensions & weight.

You can use a climate camera or simply water to make that the nylon absorbs water but you must be careful with the time. The nylon 6 could become softer if you put it to much time into the water. The dimensions of the product could be out of tolerances too. I suggest that you make tests with different conditions and times and then test the product (To check dimensions and properties as the hardness for example).