In markets such as automobile industry or medical technology a constant process monitoring is often required. What functionalities and which process parameters should be monitored and qualified in production is verified in the evaluation process.

During the evaluation process of a welding technique, components are tested of weldability by our application engineers. At first the overall impression of the welding partners is checked due to color, evenness of the welding zone, a correct fit, general dimensions, fiber distribution, etc.

In some cases additional, detailed investigations of materials or components are necessary.

In-Process

• Spectroscopy

  Testing the transparency of plastics during the evaluation process with the working laser has been proofed as good in most cases. But since each additive that is mixed into the plastics changes optical behaviors of the base material, material transparency may change also with wavelength. So, critical components should be examined over a bigger wavelength range. A spectroscopy measurement determines the transparency of the material in dependent of the wavelength. Thus, the most suitable laser source can be selected. The measurement curve represents a polycarbonate filled with additives. The transparency degrades constantly from 800 nm to 1100 nm. Thus, a laser source that radiates at 808 nm is recommended.

  But if this material is used as the absorbent welding partner, transparency should be low and lasers with wavelength of 980 nm or 1070 nm are a better choice.

• Optical penetration depth

  The determined absorption of the spectroscopic measurement is a value representing the entire thickness of the sample and is therefore not necessarily an adequate measure of the weldability. For this, the optical penetration depth is used which is a measure of the energy distribution in the material that is generated by conversion laser energy into heat. It is determined experimentally with spectroscopy investigations on thin sections.

• Contact in the joining plane

  The principal of laser penetration welding is based on contact between the joining partners in the welding zone. Otherwise, effective heat conduction cannot be ensured. The grade of contact can be visualized using pressure paper that changes color intensity depending on pressure.

• Post-Process

  During evaluation the product monitoring methods are qualified that will be used in production.

At this stage of the project the look of the welding seam is investigated.

• Destructive Tests

  • Fracture appearance

    Independently of the destroying method, the welding seam may be visually qualified by fracture appearance. In a good weld material is merged from one joining partner into the other. Fractures from both materials are visible in the broken welding seam.

  • Thin section

    This investigation is more time consuming, but as result the heating zone is visualized as a lens at one location exemplary. The lens of the thin section is highlighted by a clever light setting under a microscope.

• Post – process – non-destructive tests

  Non-destructive tests are in general more cost intensive, but visualize defects in the welding zone.

  • Ultrasonic

    Frequencies between 0.5–10 MHz are set for sample testing. Defects may be detected depending on setting angle, material and sample thickness. The technique is based on material thickness measurements. Defects in the material reflect the sonic wave earlier than if the entire thickness of the sample is penetrated.

  • X-ray

    X-rays penetrate the sample and produce a 2-D image that visualizes cavities, defects, flow lines and cracks. However, a direct spatial assignment is not possible.

  Computer tomography

  Computed tomography is a sectional imaging technique based on X-rays that visualizes a sample in 3-D mode. Cavities, defects, flow lines and cracks can be assigned locally.