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Hydrolytic Stability

Revised 03/07/2008 .



Nylon is a hygroscopic material.  Over time it will abosorb in excess of 3% of its mass of water from the atmosphere.  The rate of moisture absorption is time, relative humidity and temperature dependant.

In the initial production trials for the 4 gang socket components were produced within two days of injection moulding.  Therefore the mouldings can be considered to be in their "dry as moulded" condition.

However, in subsequent production, mouldings were produced in a mass volume production and used over a period of months.  During this time they would have abosorbed a significant amount of moisture.

Absorbed moisture during ultrasonic welding has a number of effects:

  • It reduced the modulus of the material, hence increases attenuation and damping of the ultrasonic waves.

  • Due to the weld temperatures it vapourises at the weld front and produces a weak, foamed weld.

  • Nylon is highly susceptible to hydrolytic degradation.  The presence of moisture and the weld temperatures can induce localised degradation of the weld leading to a brittle weld structure.

For these reasons it is important that Nylon is either moulded soon (within approx. 1 day) of moulding, or that components are stored in a sealed desiccated bags or dried prior to welding.

Other materials susceptible to such atmospheric moisture effects include polycarbonate (PC), polyethylene terephthalate (PET) and polybutylene terephthalate (PBT)

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