When the Joint Is Not on One Plane

A flat joint is straightforward: shape the horn to match and press. The difficulty comes when the joint itself rises and falls in three dimensions — the parting line of a curved housing, a lid with an inclined face, or sections of joint at different heights.

Two constraints

One: the horn must touch every face at once. A horn is a rigid body with a fixed face profile. Where the joint undulates, the working face has to follow it as a curve or a set of steps, and fit closely — a tenth of a millimetre out and the pressure on that section changes a great deal.

That demands horn machining accuracy matched to the part's moulding accuracy. Where moulding deviation exceeds the profile tolerance of the horn, even a perfect horn will not seat.

Two: amplitude is not uniform across an irregular face. This is the deeper constraint. The more irregular the end face, the harder it is to keep every point moving equally, because the raised and recessed regions sit at different distances from the node.

The result is high regions seating firmly with higher amplitude and low regions worse on both counts, so fusion varies markedly along one joint.

What to do

First see whether the joint can be flattened. The cheapest route. Where the styling allows, put the parting line on one plane or reduce the undulation. Moving it on the drawing costs far less than a complex horn later.

Small undulation: a contoured horn. Within a few millimetres of height difference, one curved horn can cover it, with the amplitude distribution calculated during design and slots added where flexural modes need suppressing.

Large undulation: weld in sections. Split the joint and weld each section with a flat or near-flat horn. The cycle lengthens but each section is controllable. Design deliberate overlaps so the junctions do not become weak points.

Multiple horns in zones. Several horns each cover a section, working inside their comfortable range, simultaneously or in sequence. The machine is more complex, but on large parts of complex shape it is the only stable route.

Stepped joints specifically

Where the joint is several planes at different heights rather than a continuous curve, another option exists: let the horn sections float. Each section presses independently on its face, with elastic elements compensating the height difference, so pressure is close to equal. The cost is complexity and more maintenance points.

The other approach is to let the part suit the process: bring the joint sections to a common height in the part design, so the structure compensates rather than the horn.

Verify the amplitude distribution

Whichever route is chosen, verify the horn once made: place thermal paper or thin weldable film under the face, run a weld into air, and read the depth of the mark. Deeper means higher amplitude, and the distribution is visible at a glance.

This matters especially on complex profiles, where calculation and reality diverge more than on flat horns. Do it when accepting a new horn and during routine maintenance.

Horn design, the relation between gain and profile, and amplitude uniformity on large horns are in the horn selection and design guide on the downloads page. Multi-head configurations and maximum horn sizes are on the ultrasonic welding machine pages.

Send drawings showing the three-dimensional path of the joint to 1427498429@qq.com, or call +86 769 8202 9510 / +86 137 6001 0932.