Ultrasonic Welding Joint Design — Energy Directors and Shear Joints

Almost every ultrasonic welding problem we are asked about is a joint design problem being attacked with machine settings. Get the joint right and the weld is easy on any decent machine. Get it wrong and no amount of amplitude will rescue it.

The energy director

A flat face butted against another flat face does not weld well — the energy spreads across the whole area and nothing gets hot enough. An energy director is a small triangular ridge moulded onto one half of the joint. It concentrates all the ultrasonic energy into its apex, which melts first and then flows out to fill the joint.

Feature	Dimension	Why
Height	0.3 to 0.7 mm (0.01–0.04 in)	Enough melt volume to fill the joint without excessive collapse
Base width	No more than 20–25% of wall thickness	Wider than that and the energy is no longer concentrated
Included angle — amorphous	90° (ABS, PS, PC, acrylic)	Amorphous resins soften over a range; a broader apex suits them
Included angle — semi-crystalline	60° (nylon, PP, PE, acetal)	Sharp melting point needs a finer point to concentrate energy harder

The most common design error. An energy director drawn to look neat rather than to dimension — too wide, too shallow, or omitted entirely on a butt joint. It is a moulding-tool change to fix, which is why it must be settled before the tool is cut.

The four joint types

Joint	Best for	Minimum wall	Notes
Butt with energy director	General purpose, easiest to mould	Standard	Simplest and cheapest. No self-location, so tooling has to hold alignment. Flash visible.
Step joint	Mechanical strength with better cosmetics	1.6 mm (0.062 in)	Self-locating, stronger than butt, tolerant of moulding variation. Hides flash on one side.
Tongue and groove	Hermetic seals, medical devices	2.3 mm (0.09 in)	Greatest strength potential. Hides flash inside and out. Hardest to mould — needs tight tolerances.
Shear joint	Hermetic seals in semi-crystalline resins	Thinner walls acceptable	Parts telescope together as the edge melts, so the melt never sees air. The answer for nylon and PP.

Shear joint dimensions

Weld depth should be 1.25 to 1.5 times the wall thickness to approach parent-material strength. The interference between the two halves is what does the work — too little and it will not seal, too much and you overload the machine and mark the parts.

Near field or far field

This is about where the horn touches the part relative to the joint.

Near field — horn contact within 6 mm (0.25 in) of the joint. Shorter weld times, lower energy, more consistent. Design for this whenever you can.
Far field — horn contact further than 6 mm from the joint. The energy has to travel through the part to reach the weld, so you need higher amplitude and longer weld times. Workable in rigid amorphous resins; difficult to impossible in soft semi-crystalline ones.

If the joint is far field and the resin is polypropylene, expect trouble. PP is soft and absorbs vibration rather than transmitting it. That combination is the single most common reason an ultrasonic welding trial fails — and it is a reason to look at spin welding instead.

Other design rules that matter

Minimum wall 1.5 mm. Below about 1.5 mm (0.06 in) the wall behaves like a spring and absorbs the energy instead of passing it to the joint.
Radius every internal corner. A sharp 90° internal corner is a stress concentration, and under ultrasonic vibration it will crack.
Support the joint from below. The fixture must back up the weld line. An unsupported joint flexes and the energy goes into movement rather than melt.
Keep it flat and level. Any twist between horn face and joint gives you an uneven weld across the part.
Size limit. A single continuous weld is practical up to roughly a 250–300 mm square footprint, material depending. Beyond that, expect multiple horns or a different process.
Design a flash trap if cosmetics matter. Molten material has to go somewhere — give it a designed void rather than letting it squeeze out where it shows.

Do this before the mould tool is cut

All of the above is cheap to change in CAD and expensive to change in steel. Send the part model and we will have the joint reviewed by application engineers who weld for a living — before the tool exists, not after the first shots fail.

Joint design:
where welds are won or lost.