Laser & UV plastic welding for injection-moulded assemblies
What is laser and UV plastic welding, and why does it matter for moulded parts?
Laser transmission welding is a non-contact method of permanently joining two injection-moulded thermoplastic parts at a buried interface, with no adhesive, no solvent, and no mechanical fastener. One part is transparent to the laser beam; the other contains an absorber — typically carbon black — that converts the laser energy into heat precisely at the joint. The surfaces melt and fuse together as the beam moves along the weld path. The result is a hermetic, flash-free joint with no external marking. Nordmould offers this as a secondary operation, handled through its partner toolroom network, for assemblies where cleanliness, precision, or part integrity rules out contact-based joining.
How does laser transmission welding work?
A near-infrared (NIR) laser, typically operating at 800–1,100 nm, is directed at the assembly from above. The upper part — optically transparent or translucent to the laser wavelength — passes the beam through to the lower part, which contains an absorber that converts the light to heat. The heat melts a narrow zone at the interface on both surfaces, and the two melt pools merge under light clamping pressure. On cooling, the joint solidifies to form a continuous polymer bond.
The process is controlled by four main parameters: laser power, scan speed, beam diameter, and clamping pressure. These are set per material combination and part geometry during process development. Three scan strategies are common in production:
| Scan strategy | How it works | Best for |
|---|---|---|
| Contour welding | Laser traces the weld path in a single pass | Irregular joint shapes, medium volumes |
| Quasi-simultaneous welding | Laser scans the full path rapidly and repeatedly until the melt is uniform | Short cycle time, small precision parts |
| Radial / mask welding | Flood illumination through a shaped mask | Simple round or symmetric joints in high volume |
Nordmould selects the scan strategy based on part geometry, required cycle time, and the precision needed at the joint.
What about UV welding — is that the same thing?
In common usage, "UV welding" and "laser welding" are sometimes used interchangeably in plastic-joining sales literature, which causes confusion. The correct distinction is:
- Laser transmission welding (NIR, 800–1,100 nm) — the dominant industrial method for moulded thermoplastic assemblies. Heat is generated at the buried interface by laser absorption.
- UV-cure bonding (365–405 nm) — a separate process where a liquid adhesive is cured by UV exposure. It is not a true weld; it bonds via adhesive rather than polymer fusion.
- UV laser ablation / marking (355 nm) — surface treatment or marking, not joining.
Nordmould's offering is laser transmission welding. The term "UV/laser welding" used in the page title follows common buyer search language but refers specifically to the NIR transmission welding process as the primary joining route for moulded assemblies.
Which materials are compatible?
The process requires one part to transmit the laser and one to absorb it. Compatible base materials for the transmissive layer include:
| Material | Transmittance notes |
|---|---|
| PC (polycarbonate) | High natural transmittance to NIR; excellent base choice |
| PP (polypropylene) | Good transmittance; carbon-black absorber needed in lower layer |
| ABS | Moderate transmittance; colour and additives affect performance |
| PA (nylon) | Generally compatible; natural grades perform better |
| PMMA | High optical clarity; very good transmission |
| POM | Acceptable; semi-crystalline structure reduces transmission slightly |
| PET / PBT | Compatible in many grades |
Materials with heavy glass fibre loading, opaque flame-retardant packages, or high crystallinity (e.g. PEEK in thick sections) may not transmit reliably. Material compatibility is confirmed as part of Nordmould's DFM review for every assembly project.
Both parts must have compatible melt temperatures and similar surface energies. Joining dissimilar polymer types (e.g. PP to PC) requires process development and is not always feasible without an interfacial compatibiliser.
How should the joint be designed for laser welding?
Good joint design determines weld quality as much as the laser parameters. The key rules:
- Contact pressure is essential — the two surfaces must be in intimate contact along the full weld path before the laser fires. A gap of even 0.1 mm reduces weld strength significantly.
- Wall thickness of the transmissive layer — keep to 0.5–4 mm. Beyond 4 mm, laser energy is attenuated and weld uniformity drops.
- Weld-line width — a minimum of 1–2 mm gives sufficient joint area for structural applications.
- Flat or gently contoured weld planes — complex three-dimensional weld interfaces require profiled clamping fixtures, which adds tooling cost.
- Surface finish — the transmissive layer must be free from thick textures or cloudiness at the laser entry face; matte SPI finishes are generally acceptable.
Nordmould's free DFM review covers joint geometry for any assembly that includes a downstream welding operation.
How does laser welding compare with other plastic joining methods?
| Method | Vibration/stress on part | Flash/particulate | Hermetic seal | Part size range | Capital cost |
|---|---|---|---|---|---|
| Laser transmission welding | None (non-contact) | None | Yes (reliable) | Small to medium | High |
| Ultrasonic welding | Yes (vibration) | Minimal if well-designed | Yes (possible) | Small to medium | Moderate |
| Hot-plate welding | Low | Some flash possible | Yes | Medium to large | Moderate |
| Vibration welding | Yes (linear vibration) | Some flash | Yes | Medium to large | Moderate–high |
| Adhesive bonding | None | None | Depends on adhesive | Any | Low |
Laser welding's main advantages for moulded assemblies are: no vibration (safe for PCBs, sensors, and delicate internal components), no flash or particulate contamination, and a hermetic weld achievable on complex two-part housings. Ultrasonic welding remains the first-choice route for simpler parts at high volume where equipment cost per cycle is the dominant factor. Nordmould recommends the right method after reviewing the assembly.
What applications is this process suited to?
- Medical device housings — sterilisable hermetic enclosures where particulate contamination must be zero
- Automotive sensor covers — radar, LiDAR, and camera housings where the weld must be pressure-tight and vibration-resistant
- Electronic enclosures — two-shell housings with PCBs or batteries inside where ultrasonic vibration is a risk
- Fluid containers and microfluidics — channels and reservoirs requiring leak-proof bonding between moulded layers
- Consumer products — wearables, white goods subassemblies, and optical assemblies where external surface quality is paramount
Nordmould handles laser welding as a quoted secondary operation, coordinated with the core injection moulding and tooling work so that joint geometry is reviewed before the tool is cut.
Frequently asked questions
What is laser transmission welding of plastics? Laser transmission welding passes a near-infrared laser beam through a transparent upper part and into an absorbing lower part. The absorbed energy melts both surfaces at the interface, which fuse together on cooling to form a hermetic, flash-free weld. Nordmould uses this process to join moulded assemblies cleanly and precisely.
Is this process UV welding or laser welding? The correct term is laser transmission welding, using a near-infrared (NIR) laser at around 800–1,100 nm. It is sometimes loosely called UV welding in commercial contexts, but UV (355 nm) is a separate, less common process. Nordmould's partner toolroom uses NIR laser transmission welding as the primary non-contact joining method for moulded plastic parts.
Which plastics can be laser welded? Compatible materials include ABS, PC, PP, PA (nylon), PMMA, POM, PET, and PBT. One part must transmit the laser and the other must absorb it — typically achieved by adding carbon black to the lower layer. Highly crystalline resins or those with glass fibre or flame-retardant additives may weld poorly; Nordmould confirms compatibility at DFM review.
What joint strengths and tolerances are achievable? Laser-welded joints can reach the tensile strength of the parent material when geometry and process parameters are optimised. Weld-line width is typically 0.3–2 mm depending on beam focus and clamping. Positional accuracy is controlled to within the fixtured part tolerance, often ±0.1 mm or better for small assemblies.
How does laser welding compare to ultrasonic welding? Ultrasonic welding is faster per cycle and lower in equipment cost, making it efficient for small, simple parts in high volume. Laser welding excels for delicate electronics, hermetic enclosures, optical assemblies, and parts where vibration stress would damage internal components. Nordmould quotes both methods and recommends the best fit for your geometry and volume.
What wall thickness and part size is suitable? The transmissive upper layer is typically 0.5–4 mm thick; thicker sections reduce transmitted laser energy and weld strength. Nordmould confirms suitability for your specific wall section and part geometry as part of the free DFM review.
Does laser welding leave visible marks on the outside of the part? Because the heat is generated at the buried interface, the external surfaces are not directly affected. The weld is internal and flash-free, which is why the process is preferred for optical, medical, and consumer-facing assemblies where surface quality matters.
What applications is laser welding used for? Typical applications include medical device housings, automotive sensor covers, electronic enclosures, fluid containers requiring hermetic seals, and consumer product assemblies. Nordmould handles these as secondary operations alongside the core moulding and tooling work.