Machining guidelines Chapter 3

Post-machining processes

3.1 Conditioning The process of conditioning is defined as treatment of dry polyamide products aiming at the fastest possible moisture accumulation. This can be necessary where parts made from polyamides must not change dimensions due to further water absorption, where parts are to be in permanent contact with water or submerged in water, or where specific material changes are should be brought about by water absorption.

Polyamide products are usually distinguished by the following moisture conditions:

  • dry (moisture content <0.2%)

  • humid (constant weight when stored in standard air at 23 °C/50% relative humidity)

  • wet (constant weight even after prolonged immersion in water)

For conditioning, besides the processes that use warm air and humidity, it is also possible to store in hot water (water temperature approx. 80 °C-100 °C max). In terms of effort this is the simplest method, but it does have some significant disadvantages. With thick-walled parts the water first diffuses in the surface regions and saturates them. However, after the conditioning period the lower layers still do not have the desired water content. After removing the parts from the water bath a portion of the absorbed water evaporates back into the air. The water content of the near-surface layers settles at the desired value, but the lower-lying levels fall below the intended value. It is therefore prudent to slightly extend the time in the water bath and then store the part in normal room air for several days of conditioning. After the loss of water from the near-surface layers this achieves virtually uniform water content over the cross section.

3.2 Important factors in the water absorption of polyamides The process of water absorption is generally only very slow and is influenced by various factors. The most important factors can be illustrated as follows:

3.2.1 Speed of water absorption Water or moisture is absorbed very slowly by polyamide until a state of equilibrium is achieved. Once equilibrium is reached moisture content can only be influenced by altering the environmental conditions, such as increasing the moisture content and/or increasing the temperature. The tendency of water molecules to diffuse into a solid increases significantly with increasing temperature. It therefore follows that, with increasing ambient temperature, less time is required to make a defined amount of water penetrate into a part made of polyamide. The size of the specific surface area (surface area per unit volume) is crucial. The larger the specific surface area of a part is, the larger the surface area available for attack by the water molecules and the resulting absorption speed is faster. It may be concluded from the above that for the practical use of polyamides short-term fluctuations in humidity in the environment have only a limited impact on the material properties, but that long-term fluctuations and possibly associated with high temperature can induce the changes in material properties previously described.

3.2.2 Water absorption in air Water absorption due to humidity is primarily determined by the relative humidity and not the air temperature. It should be noted that the process of water absorption due to humidity only takes place in the near-surface areas of thick-walled components and water absorption in the core of the part with the consequences described is normally not expected. (Fig.2)

3.2.3 Influence of the degree of crystallinity Since water is absorbed only by the amorphous proportions in the polyimide, the water intake also significantly depends on the degree of crystallinity. With increasing crystallinity the saturation concentration (depending on the chemical composition and the type of polyamide) as well as the absorption speed and the absorption capacity also increase. Cast polyamides have a higher degree of crystallinity compared to extruded polyamides. They absorb much less water and require significantly more time.

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