Joint design is where ultrasonic welding is won or lost

Most of the ultrasonic welding problems that land on our bench are not machine problems. Somebody has a good 20 kHz machine, a properly tuned stack, a clean horn, and the weld still comes out weak or patchy. Nine times out of ten the answer is in the part, not the machine.

So before you spend on wattage you do not need, look at the joint.

What the joint actually has to do

Ultrasonic welding works by friction. The horn vibrates against the top part 15,000 or 20,000 times a second, that vibration travels down to the joint line, and the rubbing at the interface generates heat. The plastic melts, the parts are pressed together, the melt cools, and you have a joint.

That only works cleanly if the energy has one obvious place to go. Give it a large flat contact area and the heat spreads thin — you end up cooking the part, marking the surface, and still not getting a proper melt at the joint. Give it a small, deliberate concentration point and it melts there in a fraction of a second.

That concentration point is the whole game. Every joint design below is a different way of creating one.

The energy director

This is the workhorse. A small triangular bead runs around the joint on one half of the part, and the flat face of the other half lands on it.

Rough dimensions that work for most parts under about 25 mm across:

• Height: 0.3 to 0.6 mm. Under 0.25 mm and the moulder cannot hold it reliably. Over about 0.8 mm and you get flash you have to hide.
• Included angle: 60 to 90 degrees. Sharper melts faster but is harder to mould and fills less gap.
• Volume: this is the part people skip. The bead has to contain roughly enough material to fill the gap it creates. If your bead is 0.5 mm tall, the melt from it has to fill 0.5 mm of travel. Too little material and the parts bottom out before the joint is full.
• Position: set it back 0.5 mm or so from the outside edge so flash squeezes inward where nobody sees it.

Put the director on whichever half is easier to mould and easier to keep clean. On a two-shot enclosure that is usually the base.

The shear joint

When you need a seal that holds pressure or keeps liquid in, the shear joint is the better answer. Instead of landing a flat face on a bead, one part has a slight interference fit into the other. Welding pushes the top part down past the wall of the bottom part and the two melt along a vertical face.

The numbers that matter:

• Interference: 0.2 to 0.4 mm total for parts up to about 18 mm. Bigger parts need more, but the interference should stay proportional or the machine has to work far too hard.
• Lead-in: a 30 degree chamfer, 0.5 to 0.8 mm deep, so the parts locate before the ultrasonics fire.
• Weld depth: 1.25 times the wall thickness is a good starting point. Deeper is stronger up to a point, then you are just adding cycle time.
• Sidewall support: the outer wall has to be held by the fixture, or it will simply flex outward instead of welding.

Shear joints are excellent on ABS, polycarbonate and acrylic. They are much fussier on polypropylene and polyethylene, because those materials want to slide and cool rather than fuse along a wall.

Step and tongue-and-groove joints

A step joint is a butt joint with a small ledge, so the parts self-align and the flash has somewhere to go. It welds nearly as well as a plain energy director joint and it hides the joint line, which matters on anything a customer looks at.

Tongue-and-groove is the strongest of the common designs and the most expensive to mould. The tongue sits in a groove with clearance on both sides, so flash is trapped completely, alignment is automatic, and the weld area is large. If you are making a part that has to pass a pressure test and look clean, this is what you want. Expect the mould to cost more and the tolerances to be tighter.

Wall thickness, ribs and the things that steal your energy

Ultrasonic energy has to travel from the horn face to the joint. Anything in the way absorbs some of it.

• Wall thickness: below about 1.5 mm the wall struggles to carry the vibration, especially in semi-crystalline materials. Thin lids on tall boxes are a common failure.
• Ribs and bosses near the joint: these split the energy path. Keep them at least 3 mm clear of the weld line where you can.
• Sharp internal corners: stress concentrates there and the part cracks instead of welding. Radius them.
• Snap fits and screw bosses inside the part: vibration reaches them too. If they are thin, they break. We have seen plenty of parts where the weld was perfect and an internal clip had snapped off inside.
• Draft angle: a heavily drafted wall changes the interference along its length. On shear joints, keep draft on the welding face as low as the moulder will accept.

Near-field and far-field

If the joint is within about 6 mm of the horn face, that is near-field welding. Beyond that, far-field.

Amorphous plastics such as ABS and PC transmit vibration well and can be welded far-field without much trouble. Semi-crystalline plastics — PP, PE, nylon, acetal — damp the vibration as it travels, so a joint 40 mm from the horn face may simply never get hot enough. For those materials, design so the horn can sit close to the joint, or accept that you will need more amplitude and a horn shaped to reach in.

This is also why a tall PP box is harder to weld than a flat PP tray, even though the joint looks identical on the drawing.

The mistake we see most

A flat butt joint with no energy director, on a part that has already been tooled.

By the time it reaches us, the mould is cut and the customer wants a machine that will make it work. More wattage helps a little. It never fixes it properly. The weld stays inconsistent from part to part because it depends on how flat that particular moulding came out, and mouldings are never that flat.

If the mould is still on the drawing board, add the director. It is a small change and it will save you the cost of the machine several times over. If the mould is already cut, a shear-style modification to the core is sometimes possible, and sometimes a custom horn with a shaped face can concentrate the energy where the part will not. Send us photos and we will tell you honestly which of those you are looking at.

Before you commit to a mould

Weld a sample. We do ultrasonic welding job work for exactly this reason — you send the parts, we set up a horn and fixture, and you get to hold the welded assembly and try to break it before anybody spends money on tooling or a machine.

A morning of sample welding has saved more of our customers more money than any other single thing we do.

Questions we get asked