IllustrationUltrasonic welding joins two plastic parts by vibrating one against the other at an ultrasonic frequency, often 20 or 40 kHz, under pressure. A small molded feature at the joint melts and fuses, usually in under a second, with no screws or glue. For a sealed housing, the joint type, resin pair and horn distance decide the result.
An ultrasonic weld is fast, needs no screws or glue, and can seal. It also locks the battery and board inside for good, and the vibration passes through every part in the housing. This guide covers joint choice, resins, the electronics inside, and when another closure fits better.
Figures come from equipment makers, a resin supplier and a welding institute. Confirm each one with your molder and weld supplier.
What is ultrasonic welding, and how does it join plastic parts?
A welder has a power supply, a converter, a booster, a horn and a fixture for the lower part. TWI lists 15, 20, 30, 35, 40 and 70 kHz as typical frequencies. Dukane describes its horns vibrating at 20 or 40 kHz (both as of November 2026).
The horn presses on the top part and vibrates. Friction at the joint makes heat, the plastic there melts and flows, and the vibration stops. The press then holds the parts while the melt cools. Dukane says most parts weld in less than one second, and that a higher force during the hold can improve joint strength and hermetic sealing (Dukane, as of November 2026).
TWI lists the limits: joints often need special design details, similar materials work best, tooling costs can be high, and vibrations can damage electrical components, although higher frequency equipment can reduce that problem (TWI, as of November 2026).
Energy director or shear joint: which one should you use?
The joint is molded into both parts, so choose it before the mold is cut. Dukane says good joints start with a small initial contact area (as of November 2026).
| Joint | How it works | Seal | Best fit |
|---|---|---|---|
| Butt joint with an energy director | A small ridge on one part melts first and flows across the joint. | Dukane calls a simple energy director a strength-only bond. | Suits amorphous resins such as ABS and polycarbonate. Not the best choice for semi-crystalline resins. |
| Step joint | A step locates the parts. It can carry an energy director or shear interference. | Controls outside flash only, so Dukane says seal quality will vary. | Strength-only bonds where nothing else aligns the parts. |
| Tongue and groove with an energy director | A tongue sits in a groove, with an energy director. | Dukane calls it the easiest and most robust option for sealed parts. It needs thicker walls. | Works well with amorphous resins. |
| Shear joint | A molded interference fit. The walls melt as they slide together. | Dukane calls it the most effective choice when a sealed assembly is needed. | Semi-crystalline resins such as nylon and PPS. Needs tight molding tolerances. |
Sources: Dukane joint design article, Dukane Guide to Ultrasonic Plastics Assembly and TWI joint design FAQ, all as of November 2026.
Energy director size
Dukane's Guide to Ultrasonic Plastics Assembly (part no. 403-536-02, August 2011) gives two shapes. For most amorphous resins it uses a 90 degree apex, so the height is half the base, with heights from 0.127 to 0.762 mm. For polycarbonate, acrylics and semi-crystalline resins it uses a 60 degree equilateral ridge, with a base from 0.254 to 1.27 mm. The base should be 20% to 25% of the wall thickness (as of November 2026).
Solvay's bulletin for its specialty resins asks for an energy director height of at least 0.4 mm on semi-crystalline parts for more strength. It also suggests 0.25 mm of clearance between the assembled parts to trap flash (version 2.3, as of November 2026).
Shear joint interference
Dukane's guide ties shear interference to part size, with a minimum wall of 1.8 mm (0.075 in) and a minimum lead-in of 0.5 mm (0.020 in).
| Part diameter | Interference per side | Part dimension tolerance |
|---|---|---|
| Under 0.75 in (18 mm) | 0.2 to 0.3 mm (0.008 to 0.012 in) | ±0.025 mm (±0.001 in) |
| 0.75 to 1.50 in (18 to 35 mm) | 0.3 to 0.4 mm (0.012 to 0.016 in) | ±0.050 mm (±0.002 in) |
| Over 1.50 in (35 mm) | 0.4 to 0.5 mm (0.016 to 0.020 in) | ±0.075 mm (±0.003 in) |
Source: Dukane Guide to Ultrasonic Plastics Assembly, Table 3-1, as of November 2026.
Two more limits apply. Dukane says semi-crystalline parts with a shear joint should be no larger than 3.5 in (88.9 mm) in diameter, while amorphous parts may be larger. Solvay recommends a joint depth of 1.25 times the wall thickness when the joint must be as strong as the wall. Dukane and Solvay both call for rigid wall support in the fixture, and TWI explains why: the telescoping action pushes the walls outward and can remove the interference (all as of November 2026).
Which resins suit ultrasonic welding plastic housings?
The two halves must be chemically compatible. Dukane says like resins weld to themselves, such as ABS to ABS. Dissimilar resins may weld only when their melt temperatures are close and their molecular structure is alike, and each semi-crystalline resin is compatible only with itself. Polyethylene and polypropylene look alike but will not weld to each other. Dukane also advises using the same supplier and the same resin grade on both halves (as of November 2026).
Dukane says semi-crystalline resins absorb vibration, so less energy reaches the joint, and that hermetic seals are typically easier with amorphous resins (as of November 2026).
Additives and handling change the result as well:
- Moisture. Dukane says hygroscopic resins such as nylon should be welded right after molding, or stored sealed with a desiccant, or dried before welding. Moist parts foam at the joint, which makes a hermetic seal hard (as of November 2026).
- Fillers. Solvay says fillers up to 40% can improve weldability, but near 50% there may be too little resin at the joint for a reliable hermetic seal (as of November 2026).
- Other additives. Dukane lists mold release, lubricants, plasticizers, flame retardants, regrind and pigments as factors that affect the weld. Test the exact grade and color you will ship.
How far can the joint sit from the horn?
The vibration has to travel through the top part to reach the joint. Dukane calls a horn-to-joint distance of 6 mm (1/4 in) or less near field, and anything farther far field. Far field needs higher amplitude, longer weld times and higher pressure, and Dukane generally advises it only for amorphous resins (as of November 2026). Solvay puts the line at 6 to 7 mm and says near-field joints work much better with semi-crystalline resins.
Dukane adds part rules that matter on a small housing (as of November 2026):
- Keep the joint on one plane, parallel to the surface the horn touches, so the energy travels the same distance all the way round.
- Put a generous radius on corners. Sharp corners can crack or melt under vibration.
- Expect little or no weld directly under holes, voids and bends. Port holes and button openings near the joint need a check.
- Tabs can break off. Dukane suggests a radius at the root, thicker tabs, or 40 kHz equipment.
Will ultrasonic welding damage the electronics and battery inside?
It can. Emerson's Branson brochure says ultrasonic welding is well suited to parts with delicate internal electronic components (© 2023, as of November 2026). TWI lists damage to electrical components among the disadvantages and says higher frequency equipment can reduce it. Test welds on real boards settle it for your device.
Dukane describes a wearable medical device with internal electronics where it used 40 kHz to protect sensitive internal components (Dukane case study, as of November 2026). Its guide lists the causes of internal damage and the fixes:
- Too much amplitude or energy: lower the amplitude, the pressure or the weld time.
- Too long an exposure to ultrasound: shorten the weld.
- Components mounted too close to the joint: mount them properly, isolate them from the housing, move them away from high-energy areas, or damp the energy locally.
None of the sources we read gives a rule for welding around a lithium cell. Keep the cell away from the joint, record its condition before and after test welds, and run the full functional test on welded samples next to unwelded boards from the same lot.
If the parts inside cannot take the vibration, Emerson describes two other routes. It says laser welding lets preassembled parts be joined without affecting internal components, and that its thermal welding, which includes heat staking, is gentle and will not damage fragile components (as of November 2026).
Can an ultrasonic weld make the enclosure waterproof?
TWI says an ultrasonic weld is not waterproof by default, and that watertight or airtight joints are possible with the correct design, wall thickness, materials, frequency and power (as of November 2026). That means a sealing joint (shear or tongue and groove), dry resin, flat and parallel mating faces, and stable weld settings. Dukane notes the shear joint is sensitive to changing molding conditions, so stable molding is part of the seal.
The weld only closes the main seam. Buttons, charging contacts, speaker and microphone openings each need their own seal. A water rating belongs to the whole device after testing. For what IP67 and IP68 tests cover, see IP67 vs IP68 test conditions. For a tracker housing taken from rating to weld joint choice and leak checks on samples, see IP67 vs IP68 tracker enclosure design.
Ultrasonic weld, screws, snap fits or adhesive: which closure fits?
The weld is permanent. Decide how the device will be serviced and whether the battery must ever come out before you choose it. The table is engineering reasoning from the sources above. It describes no specific project.
| Closure | Opens for service? | Seal | What it needs |
|---|---|---|---|
| Ultrasonic weld | No. The housing has to be cut or broken open. | Possible with a sealing joint, then proven by test. | A molded joint, a horn and a fixture. TWI notes tooling costs can be high. |
| Screws with a gasket | Yes, as often as the screws and bosses allow. | Comes from the gasket and even clamping. | Screw bosses, a gasket, more parts and longer assembly. |
| Snap fits | Depends on the design. Some open with a tool, some are one-way. | Needs a gasket or another sealing feature. | Hooks and undercuts in the mold. |
| Adhesive | Usually no. | Depends on the bead, the surface prep and the cure. | Dispensing, cure time and a bond check on both resins. |
| Laser welding | No. | Possible with the right joint and resins. | One part must let the laser pass through, and the other must absorb it, per Emerson. |
A weld needs a horn and fixture made for the part. One option is to close the first units with screws or snap fits and switch to welding once the housing design is stable. For hook types and service access, see snap fit joint design.
What should you ask your hardware team?
Bring this list to the enclosure review before the mold is released.
- Which resin and grade is each half made from, and do both come from the same supplier?
- Which joint type did you pick, and why does it fit that resin?
- Is the joint within about 6 mm of the surface the horn touches?
- How are the parts dried and stored between molding and welding?
- Which internal parts sit closest to the joint, and how are they isolated?
- How is the seal checked, on how many samples, and after which aging or drop steps?
- How will a unit be serviced, or its battery replaced, once the housing is welded shut?
Joint design and closure choice sit inside the mechanical work. For how that work is organized, see engineering.
What is an energy director in ultrasonic welding?
It is a small ridge, usually triangular, molded on one joint face. It concentrates the vibration so the ridge melts first and flows across the joint. Dukane says a simple energy director is usually 0.25 to 0.75 mm tall and gives a strength-only bond (Dukane, as of November 2026).
Can you ultrasonic weld two different plastics?
Sometimes. Dukane says dissimilar resins need close melt temperatures and a like molecular structure, and each semi-crystalline resin welds only to itself. Polyethylene and polypropylene will not weld to each other (Dukane guide, as of November 2026). Ask both resin suppliers before you commit.
Is an ultrasonic weld waterproof?
Not by default. TWI says watertight or airtight joints are possible with the correct design, wall thickness, materials, frequency and power (TWI, as of November 2026). The housing earns a water rating only when the whole device passes the test.
How can Duobay help with a welded enclosure?
Duobay develops custom devices with a Shenzhen engineering team that covers mechanical, electrical, firmware, app and test work. The enclosure, joint and closure method are part of that mechanical work, and we deliver samples and small batches. Source files are assigned to you at every paid gate. Initial calls are free. Tell us about your device.
If you want a welded or screwed housing checked against the parts and seal your device needs, tell us about your device.
Sources. All pages accessed November 6, 2026. Guides are revised, and their values are starting points, so confirm current figures with your molder, resin supplier and weld supplier.
- TWI, What is ultrasonic welding? (link)
- TWI, Which joint design is most suitable for semicrystalline materials? (link)
- Dukane, Joint Design for Ultrasonic Plastic Bonding, by David Cermak (link)
- Dukane, Ultrasonic Plastic Welding process page (link)
- Dukane, Guide to Ultrasonic Plastics Assembly, part no. 403-536-02 (August 2011) (link)
- Dukane, servo ultrasonic welding for a glucose monitoring device (link)
- Solvay Specialty Polymers, Ultrasonic Welding technical bulletin, version 2.3 (revised November 2013) (link)
- Emerson, Branson Joining Technologies for Plastics brochure (© 2023) (link)
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