Through-Transmission Laser Welding
Key Takeaways
- One part transmits, the other absorbs. The beam passes through the upper component and deposits its energy at the interface.
- Heat is generated only where it is wanted, so the process adds no vibration and no particulate.
- Fit-up decides weld quality once absorption is adequate — a gap cannot be closed with more power.
- Carbon black is the traditional absorber, which is why so many laser-welded assemblies are dark.
Through-Transmission Laser Welding
Through-transmission laser welding is a joining process in which a near-infrared beam passes through a transmissive upper component and is absorbed at the surface of a lower component, generating heat precisely at the joint interface so that both faces melt and fuse under clamping pressure. It is the dominant laser welding configuration for thermoplastics, and the reason the technique is viable at all on materials that would otherwise be damaged by surface heating.
The value of the arrangement is that energy is deposited inside the assembly
rather than on its outside. Nothing on the visible surface is heated appreciably, so cosmetic
faces survive intact, and no mechanical energy is applied to the assembly at all —
which is why the method suits enclosures containing electronics, sensors or fluid paths that
ultrasonic or vibration welding would disturb.
What Each Half of the Joint Must Do
| Transmissive part | Absorbing part | |
|---|---|---|
| Optical requirement | Pass the beam to the interface with minimal loss | Absorb the beam at or very near its surface |
| What defeats it | Wall thickness, pigment loading, glass filler, crystallinity, moulded-in weld lines | Insufficient absorber loading, or absorber that is dispersed too deeply |
| Usual approach | Natural or lightly pigmented resin; amorphous polymers transmit better than semi-crystalline | Carbon black, or a dual-purpose additive where colour matters |
| Most common oversight | Assuming a clear-looking part transmits well — visible clarity and near-infrared transmission are different properties | Assuming loading tuned for marking contrast will also serve the weld |
Both parts should normally be the same polymer, or polymers of demonstrated compatibility.
Dissimilar materials with different melt temperatures and viscosities rarely produce a sound
weld regardless of the optical arrangement.
Joint Design and Fit-Up
Once absorption is adequate, weld quality is a mechanical question. This is where most
programmes actually succeed or fail:
- Intimate contact is mandatory. The melt has to bridge the interface, and
a gap gives it nothing to bridge to. Adding energy across a gap burns material rather than
welding it. - Gap tolerance is tighter than for ultrasonic or vibration welding. Part
warpage and moulding tolerance have to be controlled to the joint specifically, not merely
to the overall part dimensions. - Design a locating joint. A step, tongue-and-groove or shear joint
positions the parts and concentrates melt where it is wanted, instead of relying on flat
faces meeting perfectly. - Clamping must be uniform along the weld path. Uneven pressure produces a
weld that is strong in some places and absent in others. - The clamping tool sits in the beam path. Glass or acrylic pressure plates
must be kept clean and unscratched, because their condition directly changes delivered
energy. - Keep the weld path away from gates and moulded weld lines in the
transmissive part, which scatter the beam locally and produce a repeatable weak spot.
Welding Strategies
| Strategy | How it works | Best suited to |
|---|---|---|
| Contour welding | The beam traces the joint path once, melting progressively | Large or complex two-dimensional paths; flexible, lower tooling cost |
| Quasi-simultaneous | The beam scans the whole path repeatedly at high speed, so the entire joint melts together | Assemblies needing joint collapse and a hermetic seal; tolerates minor gaps better |
| Simultaneous | Fixed diode arrays or a masked source heat the entire joint at once | High volume, fixed geometry; shortest cycle time, highest tooling cost |
| Radial and mask welding | Rotational or masked exposure | Cylindrical parts, or very fine feature definition |
Diagnosing Weld Defects
| Symptom | Likely cause | Correction |
|---|---|---|
| Weak weld with correct absorber loading | Fit-up, not optics | Check clamping pressure and joint gap before raising power. |
| Strength varies along the joint line | Transmission varying through the upper part | Measure transmission across the transmissive component; wall thickness and pigment dispersion both show here. |
| Voids or burning at the interface | Excess energy, or dwell too long at a point | Reduce power or raise traverse speed. Voids are trapped decomposition gas. |
| A repeatable weak spot at one location | Gate mark or moulded weld line scattering the beam | Relocate the gate, or route the weld path away from the feature. |
| Welds degrade over a production run | Clamping glass fouling or scratching | Clean and inspect the clamp on a schedule — it is an optical component. |
| Passes assembly test, leaks in service | Qualified on initial strength only | Add thermal cycling and chemical exposure to the qualification, and examine failure mode rather than load alone. |
| Cannot weld a light-coloured assembly | Carbon black is the only absorber considered | Specify a dual-purpose or near-infrared-absorbing additive that is not visually black. |
Related Terms and Reading
- Laser plastic welding
- Dual-purpose laser additives
- Laser welding of plastics
- Carbon black selection for through-transmission welding
Applying this in production
The Sabreen Group provides independent engineering support for through-transmission weld development, material pairing and joint qualification. If you are specifying a process, qualifying a material or troubleshooting a production problem, our engineering services team can help. Contact us to discuss your application.
Frequently Asked Questions
Why must one part transmit and the other absorb?
So that heat is generated at the joint interface rather than on the outside of the assembly. The beam passes through the upper component without depositing appreciable energy, is absorbed at the surface of the lower one, and the resulting heat melts both faces together under clamping pressure. That is what allows cosmetic surfaces and internal components to survive a process that is welding a few millimetres away.
Does a clear part necessarily transmit the beam well?
No, and the assumption causes a lot of wasted development. Visible clarity and near-infrared transmission are different properties: a part can look water-clear and still attenuate at 1064 nm, and a semi-crystalline polymer scatters the beam through its crystalline regions regardless of how it looks. Measure transmission at the working wavelength rather than judging by eye.
Why is the weld weak when absorber loading is correct?
Usually fit-up. The faces must be in intimate contact under clamping pressure because the melt has to bridge the interface; where there is a gap, extra energy burns the material rather than closing it. Laser welding tolerates gaps less well than ultrasonic or vibration welding, so joint tolerance and part warpage have to be controlled specifically at the weld.
Must a laser-welded assembly be black?
Not any longer. Carbon black is the traditional absorber and remains the cheapest, which is why so many laser-welded assemblies are dark, but dual-purpose and near-infrared-absorbing additives now provide absorption without being visually black. That allows light, coloured and in some systems near-clear welded assemblies, at a higher additive cost.
Which welding strategy should be chosen?
Contour welding suits large or complex paths with modest tooling cost; quasi-simultaneous scanning melts the whole joint together, which allows joint collapse and tolerates minor gaps better, making it the usual choice for hermetic seals; simultaneous welding gives the shortest cycle for high-volume fixed geometry at the highest tooling cost. Gap tolerance and volume normally decide it.
