Laser & UV plastic welding for injection-moulded assemblies
Laser transmission welding for moulded assemblies
Laser transmission welding is a non-contact method of permanently joining two injection-moulded thermoplastic parts at a buried interface — no adhesive, no solvent, no mechanical fastener. One part is transparent to the laser beam; the other contains an absorber, typically carbon black, that converts laser energy into heat precisely at the joint. The surfaces melt and fuse as the beam moves along the weld path, producing a hermetic, flash-free bond with no external marking.
Nordmould coordinates this as a secondary operation through its partner toolroom network. It is particularly suited to assemblies where particulate contamination or vibration stress would rule out contact-based joining.
How laser transmission welding works
A near-infrared (NIR) laser operating at 800–1,100 nm is directed at the assembly from above. The upper part — transparent or translucent to the laser wavelength — passes the beam through to the absorbing lower part, where it converts to heat. That heat melts a narrow zone at the interface on both surfaces; the two melt pools merge under light clamping pressure and solidify into a continuous polymer bond on cooling.
Four parameters govern the process: 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 |
Scan strategy selection depends on part geometry, required cycle time, and joint precision requirements.
UV welding vs laser welding — the terminology
The terms "UV welding" and "laser welding" are sometimes used interchangeably in sales literature. The correct distinction:
- 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 liquid adhesive cured by UV exposure. Not a true weld; bonds via adhesive rather than polymer fusion.
- UV laser ablation / marking (355 nm) — surface treatment or marking, not joining.
Nordmould's offering is NIR laser transmission welding. The term "UV/laser welding" in the page title follows common buyer search language.
Compatible materials
The process requires one part to transmit the laser and one to absorb it. Compatible base materials for the transmissive layer:
| 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 significantly |
| PA (nylon) | Generally compatible; natural, unfilled grades perform best |
| PMMA | High optical clarity; very good transmission |
| POM | Acceptable; semi-crystalline structure reduces transmission slightly |
| PET / PBT | Compatible in many grades; confirm additive package at DFM |
Materials with heavy glass fibre loading, opaque flame-retardant packages, or high crystallinity in thick sections may not transmit reliably. Both parts must also have compatible melt temperatures. Joining dissimilar polymer families (e.g. PP to PC) requires process development and is not always feasible without an interfacial compatibiliser. Material compatibility is confirmed at DFM review for every assembly project.
Joint design for laser welding
Joint design matters as much as laser parameters. Key rules:
- Clamping contact 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.
- Transmissive wall thickness. Keep to 0.5–4 mm. Beyond 4 mm, laser energy attenuates and weld uniformity drops.
- Weld-line width. A minimum of 1–2 mm gives sufficient joint area for structural applications.
- Weld plane geometry. Complex three-dimensional weld interfaces require profiled clamping fixtures, which adds tooling cost. Flat or gently contoured planes are preferred.
- Entry surface finish. The transmissive layer must be free from thick textures or cloudiness at the laser entry face; standard matte SPI finishes are acceptable.
Nordmould's free DFM review covers joint geometry for assemblies that include a welding operation.
Laser welding vs 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: no vibration (safe for PCBs, sensors, delicate internal components), no flash or particulate contamination, and reliable hermetic welds on complex two-part housings. Ultrasonic welding is the first choice for simpler parts at high volume where cycle cost dominates. Nordmould recommends the right method after reviewing the assembly.
Applications
- Medical device housings — sterilisable hermetic enclosures where particulate contamination must be zero
- Automotive sensor covers — radar, LiDAR, and camera housings requiring pressure-tight, vibration-resistant welds
- 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
Laser welding is coordinated as a quoted secondary operation alongside the core moulding and tooling work, so 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 coordinates this process through its partner toolroom network 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 network 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; material compatibility is confirmed 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 0.5–3 mm in production depending on beam diameter and scan strategy. 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 suits delicate electronics, hermetic enclosures, optical assemblies, and parts where vibration stress would damage internal components. Nordmould quotes both methods and recommends the right 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 attenuate the laser and reduce weld uniformity. Suitability for your specific wall section and part geometry is confirmed 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.