The final step in laser plastic welding A comprehensive welding defect detection process based on optical coherence tomography (OCT) technology

In laser plastic welding processes, welding itself is not the ultimate goal. What truly determines whether a product can operate reliably for five, ten years, or even longer is whether its post-weld quality has been thoroughly and comprehensively verified. As plastic welding becomes widely adopted in high-reliability applications such as automotive thermal management systems, energy storage systems, and liquid cooling circuits in AI data centers, the industry is reaching a consensus:

Welds that lack comprehensive structural-level inspection are, by definition, manufactured with inherent uncertainties.

It is precisely in this context that OCT (Optical Coherence Tomography) technology has become the indispensable final component of the closed-loop laser plastic welding process.

1. Why is comprehensive post-weld inspection mandatory for laser plastic welding?

1. Black plastic welded with completely invisible internal structure

In current laser plastic welding processes, a combination of materials with upper-layer transmission and lower-layer absorption is widely employed, where the lower-layer material typically consists of black or dark-colored engineering plastics.

After welding is complete:

· Good appearance = reliable internal welding

· The actual fusion zone of the weld, interface continuity, and presence of voids cannot be determined visually.

· Traditional visual inspection methods are almost completely ineffective in this scenario.

· Upper transparent plastic layer

· Lower layer: Absorbent plastic

· Laser transmission occurs, forming a fusion zone at the interface.

· Emphasis: The melt zone is enclosed by the material and not visible externally

2. Fit-fit assembly makes "virtual soldering" more concealed

In numerous pipeline joints and sleeve-type assemblies, these issues inherently exist prior to welding:

· Axial/Radial interference fit

· Initial Mechanical Seal

This leads to a highly misleading phenomenon:

Even when welding energy is insufficient or molecular fusion has not yet fully occurred, leakage may still occur in the short term.

· Pipeline/Connection Structure

· Demonstrates mechanical interference fit

· Note: No true melting zone has formed.

· Compare "appears well-sealed vs. actually not properly welded"

3. Welding defects often exhibit "slow failure".

In laser plastic welding, the true hazard is not necessarily immediate leakage, but rather:

· Local hyperthermia → Molecular chain degradation

· The melting zone is discontinuous.

· Microscopic voids/bubble residues

These defects occurred within a short period of time:

· Difficult to detect through functional testing

· However, under prolonged thermal cycling, pressure cycling, and medium erosion, it is highly prone to evolve into a failure source.

4 In critical systems, welded components are already considered "safe components".

In the following systems, the failure consequences of plastic welded components are unacceptable:

·  Automotive Cooling and Thermal Management System

·  Energy Storage Liquid Cooling Circuit

· High-density cooling system for AI data centers using AI-based technologies

 Post-welding comprehensive inspection is not a "quality upgrade," but rather a requirement for system safety.

II. Why are existing post-weld inspection methods insufficient?

III. Why has OCT become the optimal solution for laser plastic welding?

With application upgrades, the requirements for welding inspection are undergoing fundamental changes:

OCT technology meets the following requirements simultaneously:

· without prejudice

· Micrometer-level resolution

· high-velocity scanning

· Easy to integrate into production lines

�� This is not a substitute for air tightness testing; rather, it marks the first time we have "seen welding itself."

IV. Why is OCT "perfectly suited" for laser plastic welding?

1 Utilizes the optical properties of laser welding materials

Laser transmission welding inherently requires the following:

· The upper material exhibits transmittance in the near-infrared region.

And OCT works precisely in this way:

· The near-infrared band has a wavelength close to that of laser welding.

· The incident OCT light penetrates the upper layer of material.

· Reflections occur at interfaces of varying depths

· Image reconstruction is performed based on the returned optical wave information.

2. Engineering Understanding of OCT

OCT can be understood as follows:

It is the "optical version of B-ultrasound," but with resolution improved to the micrometer level.

It is highly sensitive to the following locations:

· Material Interface

· Region of abrupt refractive index change

· Melt boundary

These are precisely the areas where laser plastic welding defects are most concentrated.

V. Types of Welding Defects That OCT Can Detect Fully

✅ False soldering / Insufficient soldering

· The welding interface is clearly visible.

· No continuous melting zone

✅ The molten zone is discontinuous

· Weld seam interruption

· Unstable energy or pressure leads to...

✅ Gaps/bubbles at the welding fusion surface

· High-reflection signal

· Often associated with material moisture content and rapid temperature increase

✅ Abnormal geometric consistency of the weld seam

· Uneven depth

· Excentric welding

· Assembly errors are visually apparent.

VI. Conclusion: From "being weldable" to "welding correctly"

Laserswelding technology has reached a sufficiently mature stage, with the primary challenge now shifting to verifiability. OCT technology is not designed merely for "higher-level visualization"; rather, it addresses these issues for the first time under mass-production conditions.

· Has this part really been welded properly?

· Is the weld seam continuous and stable?

· Does this welded component have the capability for long-term service?

Only when welding under effective process control combined with 100% online inspection forms a true closed loop does laser plastic welding truly enter the era of high-reliability manufacturing.