Holding Consistency Across a Multi-Cavity Tool

Same machine, same settings, and out of an eight-cavity tool some parts weld well and others do not — always the same two cavities. This is not a welding problem.

Cavity spread is the same order as the interference

Shear joint interference is typically a few tenths of a millimetre, and director height is under a millimetre. If cavity-to-cavity variation reaches the same order, parts from different cavities carry different real interference — some high, some low, some already a clearance.

Only one parameter set can be run. It suits the middle cavities and not necessarily the ones at the ends, and the result is the same few cavities failing.

Measure first, decide after

The correct order is find out what the cavity spread actually is, then fix the joint dimension.

The method is plain: sample every cavity, measure the critical dimension on the weld face, and look at the distribution. If the spread is a significant share of the interference, the ideal value on the drawing does not exist in production and the design has to leave absorption margin.

If the answer is "never measured", measure. Drawing the ideal value and rescuing it at the welding station is the most expensive route.

Leaving margin in the design

Choose a joint form that absorbs tolerance. A shear joint relies on side-wall interference and is insensitive to height variation; a butt-style energy director turns height variation directly into differences in fused volume. On large parts and many cavities, the shear joint is markedly more stable.

Set interference from the low cavity. Setting it from the average leaves the low cavities without enough friction. Setting it from the low end gives the high cavities more flash, but everything fuses. Flash can be caught in a trap; an unfused joint cannot be recovered.

Treat the weld-face dimension as critical. Include it in tool acceptance and routine sampling, rather than measuring only the envelope and assembly dimensions.

What to do in production

Use relative depth mode. Absolute depth references the machine origin, so incoming height variation transfers straight into weld depth; relative depth starts from each part's actual contact point and absorbs some of it. With a wide cavity spread the difference is obvious.

Use energy mode. Energy mode watches how much energy goes in and stretches or shortens the cycle as incoming material varies, which is steadier than time mode.

Trace by cavity. Where parts carry a cavity number, tie the weld data to it and the problem cavity is visible immediately. Where they do not, add it to the tool — a small investment with a large return.

The tool's half

If the spread genuinely exceeds what the joint can absorb, the tool needs work rather than the welder. Common causes are uneven cooling, gate position differences and insert wear. Rescuing that at the welding station means paying for it indefinitely.

Tolerance absorption by joint form and interference bands by part size are in the joint design guide on the downloads page. Relative depth mode and multi-dimensional monitoring are on the ultrasonic welding machine pages.

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