Which plastics can be ultrasonically welded?

Many amorphous and semi-crystalline thermoplastics can be ultrasonically welded, but suitability depends on the exact grades, stiffness, additives, moisture, geometry and access for the horn and fixture. Use the equipment supplier's guidance and trials with the proposed parts.

What is an energy director and why does it matter?

An energy director is a moulded ridge that concentrates initial contact and helps initiate melting. It is one joint option, not a requirement for every ultrasonic joint: a shear joint uses controlled interference instead.

What are typical energy director dimensions?

Set the base width, height, included angle, collapse allowance and flash space together for the chosen material and joint. Supplier design templates are starting points for an application review, not universal dimensions. Confirm the final geometry through moulding and welding trials.

How strong is an ultrasonic weld compared with the parent material?

No universal percentage of parent-material strength applies. Agree a load direction, test method, acceptance limit and conditioning or ageing requirements, then test representative assemblies. Leak-tightness and strength are separate requirements and need their own validation.

Can dissimilar plastics be ultrasonically welded?

Some dissimilar grades can be joined, but chemical compatibility and overlapping processing temperatures alone do not prove weldability. Confirm the exact pair with the welding supplier and test the joint. Mechanical ultrasonic staking is a different method and can retain a dissimilar component without fusion at that interface.

Ultrasonic Welding of Plastic Parts: Design Guide for Moulded Assemblies

Updated: 2026-10-23

Ultrasonic welding is a permanent joining option for moulded thermoplastic assemblies. Joint geometry, material grade, horn access and support must be considered before the mould is finalised. Strong or leak-tight joints are design and qualification targets, not automatic outcomes of the process.

How does ultrasonic welding work?

A sonotrode transmits high-frequency vibration into supported parts, heating the joint region. Force is maintained while the joint solidifies. Frequency, amplitude, force, weld time and hold time are selected for the application; loading and unloading also contribute to the production cycle. For an equipment example, Branson's GSX platform offers different frequencies and control modes. Its specifications do not establish a cycle time for your assembly.

Why must the joint be designed into the moulded part?

The joint controls where melting starts and where displaced material goes. An energy director concentrates contact at a ridge; a shear joint instead uses controlled interference. Plan alignment, support, collapse allowance and flash containment together. Dukane's joint-design templates distinguish energy-director and shear designs and require application-specific dimensions. Adding these features later may require tool changes. Dukane shear-joint template.

What joint designs are used?

Butt joint with energy director: a moulded ridge concentrates initial contact. Set its dimensions and the surrounding wall for the selected grade, equipment and acceptance requirements.
Step joint: a recess can conceal the weld line and accommodate displaced melt. Check both internal and external flash; a hidden line does not establish a sealed joint.
Tongue-and-groove: alignment and space for displaced melt can be designed together. Confirm clearances, collapse and fill of the joint by trials; the groove does not guarantee complete flash containment or leak-tightness.
Shear joint: controlled interference melts as the parts move together. Review rigidity, support and dimensions with the welding supplier, particularly for semi-crystalline grades. This is a joint option, not a universal maximum-strength solution.
Staking or spot welding: staking reshapes a moulded boss to retain another component mechanically; spot welding joins suitable local areas. Neither is interchangeable with a continuous sealing weld.

Decide where displaced melt may flow and whether it could affect appearance, function or cleanliness. Specify an allowable weld bead and, where needed, a recess to collect excess material. Verify containment and any seal with representative assemblies.

How should materials be screened for welding?

Start with the exact commercial grades on both sides, including fillers, colour and additives. Dukane's plastics-assembly guide treats material selection together with joint design and processing. A family-level rating such as “excellent” cannot approve your assembly.

For ABS, PC, PC/ABS, PS and PMMA, review rigidity, vibration transmission and any cosmetic damage. For PP, PA and POM, assess the joint geometry and process window with the supplier; one energy-director shape or energy setting does not suit every grade. For moisture-sensitive grades, specify the condition of the moulded parts at welding, not only pellet drying. Soft TPE and filled grades require their own feasibility review. Similar polymer names do not prove that a dissimilar pair will fuse.

Use the materials guide to shortlist grades, then qualify the proposed combination with actual parts and the required strength or leak tests.

Which moulding and handling controls matter?

Joint surfaces: agree allowable release agents, contamination and cleaning with the welding supplier; keep unqualified coatings away from the interface.
Moisture: follow the exact grade's drying requirements before moulding and define storage, conditioning and moisture limits before welding.
Geometry: inspect agreed ridge or interference dimensions, moulding flash and cavity wear against drawing limits.
Support: locate and support the assembly so the horn can transmit vibration consistently without unwanted movement or damage.
Nearby features: review inserts, coatings and decoration for their effect on access and vibration. An insert near a joint is not automatically prohibited; qualify the actual arrangement.

How is weld quality controlled?

Define acceptance first: load direction and minimum force for strength, or pressure, medium and permitted leak rate for sealing. Use representative assemblies for destructive and functional tests, including conditioning and ageing where relevant. Establish process limits from those results, with an agreed sampling plan and response to excursions. Logged energy, time, force and collapse help monitor consistency; an in-window cycle does not independently prove joint strength or leak-tightness. See Dukane's process-validation discussion. Combine process records with the agreed product tests and moulding quality controls.

When should you choose ultrasonic welding?

Consider ultrasonic welding where a permanent joint is appropriate and the material, geometry and access can be validated. For serviceable assemblies compare snap-fits or screws; for other material combinations or large parts assess alternative joining methods with the process supplier. Compare the complete tooling, handling, inspection and production cost at the planned quantity.

For a project enquiry, identify who will develop the joint, supply the horn and fixture, perform trials and approve the test plan. Confirm any proposed welding operation, documentation and responsibilities in the written offer.

Related guides

Preparing a welded assembly? Send the drawings and acceptance requirements, including the proposed grades, quantity, strength or leak test and any open process questions.