A pool equipment maker’s ultrasonically welded nylon pump housings failed leak testing at more than 3%. Redesigning the energy director joint as a shear joint cut failures to 0.05% and raised burst pressure from 9.2 to 14.6 bar.
Key numbers
A pool equipment manufacturer welded the two halves of a nylon pump housing ultrasonically. The welds looked fine from the outside. More than 3% of housings failed the leak test, and the welding machine settings had been adjusted dozens of times without lasting improvement. Each failed housing had to be scrapped, because a failed weld cannot be reworked.
The situation
The housing ran under pressure in a wet environment, so leak-tightness was critical. Failures were expensive in scrap and in inspection time, and any escapes would have meant warranty claims and damaged reputation.
What the data showed
The team sectioned and examined failed welds and classified the failures by cause.
Incomplete welds were the largest cause, followed by flash in the joint, part misalignment and occasional cracked housings. The root cause lay in the joint design. The housing used an energy director joint, a small triangular ridge designed to melt and flow across the joint. That design works well for amorphous plastics, which soften gradually. Nylon is semi-crystalline and melts sharply: the energy director melted, then solidified again before it could flow across the full width of the joint. No machine setting could overcome that.
Misalignment made things worse. Without locating features, the halves could shift slightly before welding, leaving parts of the joint with too little contact.
What changed
- The joint redesigned as a shear joint with 0.3 mm interference, the joint type suited to semi-crystalline materials, giving a strong, hermetic weld as the walls melt together.
- Alignment features added so the halves located positively before welding.
- Weld parameters, including amplitude, pressure, weld time and collapse distance, validated with a designed experiment, building on the method shown in designed experiments for warped panels.
- Collapse distance monitored on every weld as the key process indicator.
Results
Leak failures fell from 3.1% to 0.05%. Burst pressure rose from 9.2 to 14.6 bar, a large increase in safety margin. Weld cycle time also fell, from 1.4 to 1.1 seconds, because the shear joint welded more efficiently.
Lessons for your operation
- Match the joint design to the material: energy directors for amorphous plastics, shear joints for semi-crystalline ones.
- When machine adjustments do not fix a welding problem, look at the joint design.
- Locating features before welding are as important as the weld itself.
- Collapse distance is often the best indicator of a consistent ultrasonic weld.
- Welding is decided on the CAD screen, long before the parts reach the welder.
