Determining the exact cause of a component failure is the first step towards defining corrective action. ipolytech have extensive experience in polymer failure analysis of products manufactured from all major thermoplastic materials over all end user sectors.
The 4-gang socket was manufactured from two Nylon housing mouldings. The individual components were injection moulded, the wires and socket components assembled and, as the final stage of the assembly the two halves of the housing jointed by an ultrasonic weld.
The weld geometry consisted of a "tongue and groove" type joint, the tongue present on the lower half of the housing. To assist with focussing the ultrasound during welding and to produce a high stress a standard 30° energy director was moulded on the upper face of the "tongue".
Following initial batch approval, difficulties were encountered with inconsistent, low strength welds after products had been shipped to retailers.
Cause of Failure
Joint Design Unsuitable for the Choice of Material
The primary cause of failure is the joint design used and it’s applicability to the materials selected.
The use of an energy director is common practice in ultrasonic joint design. Using this geometry vibrations caused by the application of ultrasound are focused to a point. The point source also assists in increasing stress at the contact point. The design relies on the compressive vibrations generated by the ultrasound generating frictional heating of the polymer at the energy director tip. Molten polymer flows across the weld face as the joint is compressed, allowed to cool to freeze the weld face and form the bond.
This joint geometry works well for amorphous thermoplastics such as polycarbonate or ABS due to a broad melting range allowing for significant polymer flow. However, semicrystalline polymers such as the Nylon used in the socket have a limited melting range. Using the energy director design the tip melts as expected but when removed from the heat source rapidly freezes before it can flow over the entire weld face.
For semicrystalline polymers a far better joint deign is to use a shear of scarf joint.
Using this joint design a point contact is still maintained on the corner of the tongue. However, relative movement of the two weld faces is converted from compression to shear. As the heat is produced at the weld front molten polymer is smeared across the weld face. This joint design eliminates the need for polymer to flow across the weld face and hence produces a significantly stronger joint for rapid freezing semi crystalline polymers.
Ultrasonic Welding Technique Adopted is Inappropriate for the Material Used
A second factor contributing to failure in this instance is again related to the microsctructure of the polymer used. Semicrystalline polymers can be thought of as being relatively hard crystalline particles in a relatively soft, rubbery matrix. This rubber matrix produces high attenuation of the ultrasound.
For this reason semicrystalline polymers are most suited for use with “near field” ultrasonic welding techniques. In essence this means that the ultrasonic horn (the energy source) should be no more than 6 mm away from the weld surface. In this instance the distance was approximately 10 mm. The implication being that the weld face would see a relatively low proportion of the applied energy.
The "far field" technique employed in the socket assembly in question is far more suited to amorphous thermoplastic materials. Their glassy structure conducts ultrasound well with little attenuation. For such materials the ultrasonic horn can be placed some distance (up to 200 mm) from the weld face.
Nylon is a Hygroscopic Material
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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