What Happens When a Rib Lands on the Weld Line
Structural parts carry ribs for stiffness, which is fine in itself. The trouble is where the rib sits: once it lands directly below or behind the weld line, the energy path has been rewritten.
Three things happen at once
One: the vibration is diverted. A rib is a stiffer path and the wave travels along it preferentially. With the horn pressing above, part of the energy runs down the rib into the body of the part and less arrives at the interface. The section over the rib fails to fuse while the sections either side are fine.
Two: the force is spread unevenly. The rib raises local stiffness, so at the same weld force that section barely deflects while the more flexible regions either side sink first. Real pressure along the joint then varies, and some of it seats while some keeps a gap.
Three: the rib root suffers. The junction of rib and wall is already a stress concentration and often carries a moulding sink mark. Weld force and vibration add to it, and whitening or cracking appears there — at some distance from the weld, which is why welding is rarely suspected.
What to change
First choice: move the rib away from under the weld. Leave a gap so the wall carrying the joint stays uniform. Done on the drawing, this costs nothing.
Where it cannot move: stop the rib short of the weld face. Terminate it some distance below, leaving a section of uniform wall in between. Some energy is still diverted, but stiffness near the interface becomes even.
Radius the rib root. Worth doing regardless of welding, and more so with it.
Support the rib section in the fixture. Where the structure demands a rib, back it up in the nest so support stiffness along the joint stays roughly uniform.
Not only ribs
The same reasoning applies to any feature that changes local stiffness: bosses, snap fits, pads, abrupt wall transitions and metal inserts. All of them make energy and pressure uneven along the joint.
The check is simple: walk the joint and look at what is behind it. Every discontinuity in stiffness is a candidate for an unfused section or a crack later.
If the tool is already cut
When the tool cannot be changed, a few things still help.
Add support in the fixture so both stiff and flexible regions are backed up. Switch to energy or depth mode so the machine adapts to the real condition instead of running a fixed time. Move the horn contact to a point the rib does not influence. Where necessary, accept that the joint's strength is set by its weakest section and find another load path in the structure, such as a screw or a staked point.
These are remedies with limited reach. What actually saves money is walking the joint on the drawing.
The design-review checklist and the dimensional system for eight joint types are on the downloads page. Fixture support, energy and depth modes are on the ultrasonic welding machine pages.
Send drawings showing rib positions to 1427498429@qq.com, or call +86 769 8202 9510 / +86 137 6001 0932.