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
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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