Ultrasonic Welding Design Guide for Plastic Parts

Ultrasonic welding joins two plastic parts by converting high-frequency mechanical vibration into heat at a designed contact feature. There is no consumable, no adhesive, no cure time — which is why it dominates the assembly of high-volume plastic housings.

Figure 1. Ultrasonic Welding Design Guide for Plastic Parts.

The catch: the weld quality is determined almost entirely by the joint geometry, which is fixed long before the welder is set up. Design the joint properly and the process window is wide and forgiving. Design it badly and you will spend the life of the product chasing a weld that will not hold.

1. How the Process Works

1. The two parts are clamped together under a controlled force.
2. The sonotrode (horn) contacts one part and vibrates at typically 20 kHz (or 15/30/40 kHz for specific sizes), with an amplitude of roughly 10–60 µm.
3. Vibration is concentrated at a small designed feature — the energy director — where the two parts first touch.
4. Friction and viscoelastic heating melt the energy director within milliseconds.
5. Pressure is maintained while the melt flows across the joint interface and the parts seat against each other.
6. Vibration stops and the melt solidifies, forming a weld.

The energy director is the key: it concentrates the vibration energy into a small volume so that melting happens at the joint, not in the bulk of the part.

2. Material Selection for Ultrasonic Welding

Dissimilar materials: ultrasonic welding of different polymers only works if they are chemically compatible. The reliable pairs are the ABS/PC/PS/SAN family, and blends within the same family. PP will not weld to ABS. If you need to join dissimilar materials, plan for a mechanical joint (snap fit, screw) or an adhesive.

3. Joint Designs

### 3.1 Butt joint with an energy director (most common)

• Triangular energy director, 0.3–0.5 mm high, with a 60°–90° included angle (90° is the most common).
• The director sits on one part; the other part has a flat mating surface.
• The joint lands on a flat land of at least 1.5× the director width.
• Best for: ABS, PS, PC/ABS, general housings.

### 3.2 Step joint

• Adds a vertical wall alongside the weld, which self-aligns the parts and shields the flash.
• Prevents the melted material from flowing onto a cosmetic surface.
• Recommended whenever the joint line is visible.

### 3.3 Tongue-and-groove joint

• The tongue centers the parts and provides a longer flow path.
• Groove width slightly larger than the tongue to allow for melt flow.
• Good for parts that need high alignment accuracy.

### 3.4 Shear joint

• A vertical interference of 0.2–0.4 mm on a stepped wall.
• The vibration creates a shear weld along the vertical interface.
• Required for PP, PE and other low-loss materials that will not weld with an energy director alone.
• Needs a rigid surrounding wall to support the joint and prevent deflection.

### 3.5 Staking and spot welding

• Staking: a molded boss is passed through a hole in the other part and ultrasonically melted over to form a rivet head.
• Spot welding: a horn tip plunges into the part and creates a local weld, without any special joint feature — useful for large surfaces where the parts are already clamped together.

4. Geometry Values Worth Putting on the Drawing

Two rules that are almost always violated:

1. Keep the joint line clear of cosmetic surfaces. Where the two parts meet, melt is displaced and a flash line forms. Design a step joint or a flash trap if the joint is visible.
2. Support the part under the welding horn. A part that flexes absorbs the vibration energy instead of delivering it to the joint, and the weld fails intermittently — the hardest defect to diagnose.

5. Process Settings That Matter

Prefer collapse (distance) control to time control. A weld controlled by how far the parts collapse together is far more repeatable than one controlled by a timer, because it directly measures the outcome rather than the input.

6. Common Defects and Fixes

Figure 2. Quick reference: the key checks and reference values from this guide.

7. Design Checklist

• [ ] Energy director specified: height, angle, base width, on one part only
• [ ] Joint land width ≥ 1.5 mm and clear of cosmetic surfaces
• [ ] Step joint or flash trap where the joint line is visible
• [ ] Wall thickness at the joint ≥ 2.0 mm
• [ ] Rigid support beneath the joint (thick wall or rib), so energy reaches the interface
• [ ] Alignment features with adequate clearance for assembly
• [ ] Material confirmed as ultrasonic-weldable; both parts compatible
• [ ] PP/PE joints designed as shear joints, not energy-director butt joints
• [ ] Weld process controlled by collapse distance, not by time
• [ ] Weld strength verified by a pull or burst test on production parts

Conclusion

Ultrasonic welding is one of the most economical assembly methods available for plastic parts — one operation, no consumables, no cure, fully automatable. But it is unforgiving of a joint that was drawn without thinking about where the energy will go. Specify the energy director, give the joint enough wall and support, and keep the melt away from the visible face.

If you are designing an assembly that will be ultrasonically welded, our DFMA team will review the joint geometry and propose the welding method together with the rest of the assembly design.