Why Most Laser Welding Doesn't Work on Complex 3D or Highly Filled Plastics and What ATA Does to Fix That

The Issue with the Transparent Layer

Traditional laser transmission welding works on a simple principle: one part needs to be laser-transparent, and the other needs to absorb it. The laser goes through the top layer and is absorbed at the interface, generating heat that melts the two surfaces together.

The minimum transmission needed in the top layer is usually more than 20%. If you go below that level, the laser energy will be absorbed by the top surface instead of the joint. You don't get a weld; you get damage to the surface.

There are several factors that push transmission below that level. PPS and PEEK are two examples of high-performance plastics with crystalline structures that block light. Additives like glass fibers, flame retardants, or carbon black make transmission much worse. Dark pigments and even some lighter colors that match the design absorb more than clear materials. Thick wall sections can also make materials impenetrable to the laser.

What Engineers Do Instead

Engineers usually turn to one of three methods when laser transmission welding doesn't work, but each has its own set of pros and cons:

Ultrasonic or vibration welding is quick, but it leaves behind particles that can be harmful. When you put electronics or sensors in plastic cases, the mechanical vibrations put stress on sensitive internal parts, which can be a problem.

Infrared or hot plate welding can be used on any color or material, but the process is time-consuming. The cost of tooling is high for each new part shape, and heat spreads beyond the weld zone, which could damage internal parts.

Adhesive bonding is flexible, but it's hard to make sure it's of good quality. The bondline can’t be seen, the cure times are long, and a lot of structural adhesives have chemicals that can't be used in medical or food contact applications. Furthermore, long-term adhesive degradation due to aging causing a serious issue.

None of these methods can match the accuracy, cleanliness, and speed of laser welding.

How Absorbing-to-Absorbing Welding Works

The solution removes the optical requirement completely. Instead of passing the laser through one part, both surfaces are heated directly and separately, then brought together while still molten.

There are four steps in the process. First, the laser heats both joining surfaces at the same time while keeping the parts apart. In about 1.5 seconds, the parts move into contact. The molten surfaces come together with a controlled joining force, the clamping force remains the same while the surfaces cool.

The idea is similar to hot plate welding, but the execution is quite different. Laser scanning can heat specific areas on complex 3D surfaces instead of being limited to flat interfaces. There are no custom heated platens and no slow thermal soaking.

Materials Approved for ATA Welding 

Our testing at LPKF has shown that absorbing-to-absorbing laser welding works on almost all thermoplastics, for example PBT, POM-C, PPS, PA6, PA6.6, PP, ABS, LCP, PPA, and PK. This includes a number of materials that were simply not accessible to laser welding before, notably PPS and high-fill-content grades of PPA.

One important factor is the amount of carbon black. Tests on PA6.6 with carbon black content between 0.05% and 1.00% showed that the process is feasible for a wide spectrum of absorption ranges.

What Else ATA Makes Possible

The transparent layer requirement wasn't just blocking dark materials; it also prevented color-matched components. Two parts in the same color, including white, can now be joined. White-to-white laser welding becomes possible by adding a suitable NIR absorber.

Parts with complicated shapes can also be made. The beam path distorts when the top layer has 3D features in the weld zone. Direct surface heating doesn't change the path of the beam. And filled materials like glass fiber reinforcement, flame retardants, and

conductivity additives are no longer a problem as ATA doesn't require transmission through the top layer.

An Unexpected Benefit: Better Quality Control

In traditional laser transmission welding, thermal inspection can only see the heat signature of the top surface. The real joint interface is hidden under it.

This is changed by ATA. Before they come together, both joining surfaces are exposed and melted. Thermographic systems like LPKF’s best-in-class TherMoPro can take a thermal picture of each surface on its own, giving you a direct view of the weld zone for the first time.

This matters for quality-critical applications. You're not inferring weld quality from indirect measurements. You're measuring it directly, on every part.

Industry Applications

ATA becomes important in three specific areas:

In the automotive industry, flame retardants or glass fiber fill are often used in PPS connectors, sensor housings, and parts that handle fluids. Integrated electronics could be damaged by heat during current processes like hot plate or infrared welding. ATA heats only the weld rib leaving the rest of the part unaffected.

Medical devices increasingly use PEEK in light colors. It can be sterilized and is safe for the body, but it was very challenging to laser weld it before. ATA makes it possible to weld these parts together without using glue or slow thermal bonding methods.

Being able to make housing assemblies with complex shapes that match the color of consumer products is a big plus. Designers have more freedom without giving up quality or looks.

The Bottom Line

Laser welding has been the preferred plastic joining method for decades for applications with high optical and performance requirements, but those requirements excluded a lot of industrial materials.

ATA removes those restrictions with the same laser precision, same cleanliness and same flexibility.

If you've been told your parts can't be laser welded, it's worth reconsidering.