Joints on Curved Surfaces: Is the Amplitude Still Uniform After Contouring?

On automotive interiors, appliance fascias, handles and shaped containers, the joint often does not lie in one plane but follows a curve in space. The horn has to be contoured to match, and with that comes the amplitude distribution problem.

What contouring changes

A horn is a resonant body designed around a half wavelength, with maximum amplitude at the face and a nodal plane part way along. Once the face is cut to a curve, points on it no longer sit at the same distance from the node, and amplitude stops being uniform: raised and recessed regions can differ by a considerable proportion.

The result is that some sections of a curved joint melt first and others have not started. Sections already melted keep being pressed and tend to mark or overheat, while the later ones remain under-fused. No parameter setting satisfies both ends at once.

What design can do

Limit the depth of contour. The greater the height variation, the harder the amplitude distribution is to control. Keep the variation across the joint surface within a limited range at the design stage, and split a joint with large variation into two separately welded sections where necessary.

Analyse the amplitude distribution. A complex contoured horn needs modal and amplitude analysis before manufacture and measurement at multiple points on the face afterwards, confirming the distribution is within tolerance. This step cannot be skipped — trying parameters by experience afterwards never recovers what getting the horn right would have given.

Compensate with the energy director. Make the director taller where amplitude is lower, so it contacts first and melts more, offsetting the difference. This is compensation on the tool side, used alongside optimisation on the horn side.

Consider welding in passes. Divide the curved joint into sections and weld each with a relatively flat small horn. Cycle time is traded for consistency, and on difficult curves it is often the only workable route.

The fixture gets harder too

A fixture for a curved part must follow the contour without adding constraint. Those requirements pull against each other: the closer the fit, the more likely the part is forced into the fixture's shape, leaving residual stress and distortion after release.

The usual approach is a primary locating surface that follows the contour, slight clearance elsewhere, and support only where the weld force has to be resisted. Locating points must agree with the moulding datums rather than each being defined separately.

Curved parts also slip under weld force, because the force has a component tangential to the surface that pushes the part along. The fixture needs positive stops, or the force direction should be brought as close to perpendicular to the joint as possible.

What the machine has to provide

Curved parts demand more of descent parallelism and rigidity: slight rocking during descent is magnified into local loss of contact on a curve. Frame rigidity, guide accuracy and stability through the descent show up directly in yield on these parts.

Use energy or depth mode. Seating on a curved part varies more than on a flat one to begin with, and time mode compensates for none of it. Energy mode delivers the same energy to every part, depth mode controls the amount of fusion, and either gives much better consistency.

Assess feasibility early

Curved joints vary enormously in difficulty: a shallow curve and a genuine space curve are two different orders of problem. Assessing from the 3D model works; assessing from a parameter table does not. The conclusion may be that it welds, that it welds in sections, or that another joining method is advisable, and the earlier that is known the better.

Horn design, amplitude distribution and contoured joints are on the downloads page. Energy mode, depth mode and frame rigidity are on the ultrasonic welding machine pages.

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