Ultrasonic welding
- Permanent, sealed joint
- No extra fasteners or cure time
- Parts cannot be opened without damage
Secondary operation
Ultrasonic Welding of Molded Plastic Parts: Joint Design and Resin Fit
Ultrasonic welding uses high frequency vibration to melt a small joint between two molded parts. The joint fuses in seconds with no adhesive, screws, or cure time.
How it works
Friction heat at the joint, not the whole part
Ultrasonic welding is a common way to close housings and join molded halves.
Ultrasonic welding comes down to the energy director, resin fit, and what to send with the RFQ. Ultrasonic welding is a secondary operation on a molding program. Assembly features cover snaps and bosses. Laser welding and solvent bonding are different joints.
A horn presses the parts together and vibrates at ultrasonic frequency. Friction at a small feature called an energy director melts a thin layer of resin, which flows and solidifies into a weld once the vibration stops.
Roles
Not sure which joint
Ultrasonic welding fuses the joint. Snaps stay on assembly features. Laser welding needs a clear top. Solvent bonding softens the surface.
| Page | Use it for |
|---|---|
| Ultrasonic welding (this page) | Energy director, resin fit, and the RFQ notes |
| Secondary operations | Welding scoped with the molding program |
| Assembly features | Snaps, bosses, and other molded joints |
| Laser welding | Joining through a clear top part |
| Solvent bonding | Joining by softening the surface |
| All processes | The rest of the process index |
Fit check
What welds well and what needs a closer look
Qualitative guidance. Confirm the joint design before the tool is built.
| Factor | Tends to weld well | Needs a closer look |
|---|---|---|
| Resin family | Amorphous resins such as ABS and polycarbonate | Semi-crystalline resins such as nylon and acetal |
| Resin pairing | Same resin on both parts | Dissimilar resins |
| Joint | Energy director or shear joint designed in | Flat faces with no joint feature |
| Moisture | Dried, stable parts | Hygroscopic resins that absorb water |
Design notes
Design the joint into the molded parts
Weld quality is decided when the parts are designed, not at the welder.
Add an energy director or shear joint on the mating edge
Keep the horn contact face flat and close to the joint
Leave space around the joint for flash, or add a flash trap
Keep fragile features and electronics away from the joint area
Side by side
Ultrasonic welding versus snap fits
Both avoid adhesives. Serviceability usually decides.
Snap fits
- Can be designed to open for service
- No welding step after molding
- Seal needs a gasket or overmold
Continue exploring
Related guides
Sibling processes and supporting references.
All processes
All specialty processes and secondary operations.
All processesPad printing
Ink transferred by a silicone pad onto curved and recessed surfaces.
Pad printingHot stamping
Heated die and foil for metallic or solid color marks.
Hot stampingSecondary operations
Common operations done after molding.
Secondary operationsUltrasonic welding vs adhesive
Choosing a joint for molded halves.
Ultrasonic welding vs adhesiveAssembly features
Locators, snaps, and joints designed into the part.
Assembly featuresFAQ
Common questions
Can ultrasonic welding make a watertight joint?
It can, when the joint is designed for sealing and the resins weld well. State sealing requirements up front so the joint is designed for them.
What should a welded assembly RFQ include?
CAD for both parts, the resin for each, and any sealing or strength requirement.
What is an energy director?
A small ridge molded into the joint. The horn vibrates there, so the heat stays at that ridge instead of melting the whole part.
Do both parts have to be the same resin?
The same family welds more readily. Name both resins on the RFQ. Dissimilar resins need a joint review before steel is cut.
How is this different from adhesive?
Adhesive adds a bond line and a cure step. Ultrasonic welding fuses the plastic at the joint. If the resins will not weld, review another joint.
How is this different from laser welding?
Laser welding joins through a clear top part. Ultrasonic welding needs the horn on the joint. The part design is not the same.
Next step
Get DFM feedback on your design
This step can be scoped with the molding RFQ. Upload CAD for an instant quote.