For different products and customers, there may be some differences in quality requirements. Most products have welding strength and appearance requirements, and some have air tightness requirements. In this article, we first introduce the three stages suitable for laser welding quality control, and then focus on the two methods of "welding distance measurement" and "temperature measurement".

Before welding (Pre-Process)
Plastic parts purchased from outside or made by yourself need to be inspected as follows:

Dimensional tolerance
Laser color transmittance
Melting properties
Special instruments need to be used for inspection or submitted to a professional third party for inspection. In most cases, spot inspections are conducted on newly sampled parts as well as parts from different batches.

Welding (In-Process)
Depending on the device capabilities, the following measurements are possible:

Laser power measurement
Laser module current and voltage measurements
temperature measurement
Welding distance measurement
Depending on whether the measurement results are fed back into the welding process, they can be divided into open-loop control systems and closed-loop control systems. Open loop control, data can be viewed and analyzed for welding quality. Closed-loop control, data can be fed back in real time to control the welding process, for example, welding will only stop when the set welding depth is reached.

After welding (Post-Process)
After the welding is completed, it can be inspected in the next process to ensure the welding quality. Depending on the functional requirements of the product, common tests are as follows:

Pressure or vacuum tightness test
Mechanical pull/pressure test
Image analysis of weld profiles
In addition, destructive testing should be conducted randomly and regularly to verify the welding strength in mass production.

Welding distance (collapse displacement) measurement
In the synchronous and quasi-synchronous laser welding process, a circle of welding seam is softened or liquid at the same time, and part of the molten glue is extruded through the applied clamping force and the connection is achieved. During this process, the upper part moves downward, and the welding distance/collapse distance is usually 0.1-0.4mm.
 
The typical collapse displacement curve of laser welding is shown in the figure above. A small rise is detected first, which is due to thermal expansion of the plastic. Once the plastic is fully melted, the liquid molten material will be squeezed out of the weld, and the plastic below the weld will melt further, increasing the welding depth. When the laser is turned off, the collapse does not stop immediately. As the molten material cools and solidifies at the weld, a small, hysteretic displacement is observed.

The collapse displacement can compensate for the deformation and assembly gaps of plastic parts, ensuring that the weld seam is complete and gap-free, and improves strength and sealing.

Non-contact temperature measurement
Pyrometers measure the thermal radiation emitted from a surface. The hotter the object, the more thermal radiation it emits and the shorter its wavelength. If the pyrometer is calibrated on a radiation source at a standard temperature, accurate temperature values can be obtained.

Without calibration, temperature measurements will be low.

   
What is more interesting is that in the temperature range of 100-400°C, the infrared wavelength of plastic welding heat radiation is about a few microns.

Since in laser plastic welding, the weld is not on the surface of the upper part, but between the two parts, the thermal radiation emitted is affected by the upper part. Infrared radiation with longer wavelengths is absorbed by the upper part and cannot reach the surface of the upper part. Therefore, only short wavelengths can be used for temperature measurement. A commonly used temperature detection element is a semiconductor photodiode.

Since the heat radiation part is absorbed by the upper parts, the absolute temperature cannot be obtained, only a relative temperature signal can be obtained. This signal can be compared with the signal of a welded OKAY part of the same part.

If absolute temperature values are to be obtained, the pyrometer must be specifically calibrated for the same type of plastic and the same thickness of the part. In actual work, such calibration is not done, only relative temperature signals are compared and analyzed.