Three stages of laser welding quality control

Different products and customers have different quality requirements. Most products have high requirements for welding strength and appearance, and some products have airtightness requirements. This article introduces the three stages of quality control of laser welding, focusing on welding distance measurement and temperature measurement.

Before welding

Plastic parts purchased externally or produced by ourselves need the following inspections:

Size allowable laser transmittance melting characteristics, need to be inspected by special instruments or third-party professional inspection. In most cases, the samples are new proofing, parts from different batches.

During welding

The following measurements can be performed based on the equipment functions:

The test results are fed back to the welding process, which can be divided into open-loop control system and closed-loop control system. The welding quality of open-loop control can be observed and analyzed. Closed-loop control, through real-time feedback control of data, such as only when welding reaches the set welding depth.

After welding

To ensure the quality of welding, the next process can be inspected. According to the functional requirements of the product, the following general tests are carried out:

Pressure or vacuum sealing test machine Tensile and compression test Weld seam image analysis In addition, destructive tests should be randomly sampled regularly to inspect the welding strength of mass production.

Measurement of welding distance (collapse displacement)

During synchronous laser welding, a circle of welds softens or liquidizes at the same time, and part of the molten glue is squeezed and connected by applying pressure. In this process, the upper part moves downward, and the welding distance/collapse distance is generally 0.1-0.4mm.

As can be seen from the figure, the typical laser welding collapse displacement curve. First, there is a small rise due to the thermal expansion of the plastic. When the plastic is completely melted, the liquid melt will be squeezed out of the weld, the plastic at the weld will be further melted, and the welding depth will continue to increase. If the laser is turned off, the collapse will not stop immediately. During welding, the melt cools and solidifies, and a small hysteresis displacement is observed.

The collapse displacement can compensate for the deformation of the plastic part and the assembly gap, ensuring that the weld is completely gap-free within a week, improving the welding strength and sealing.

Non-contact temperature measurement

Measure the thermal radiation emitted by the surface. The higher the temperature of an object, the more radiation it emits and the shorter the wavelength. After calibrating the standard temperature radiation source, an accurate temperature value can be obtained.

If calibration is not performed, the temperature measurement value will be low.

More interestingly, the wavelength of infrared light generated during plastic welding is between 100-400掳C.

When laser welding plastics, since the surface of the welded part is not the weld, but between two parts, it has a greater impact on the amount of heat radiation. Long-wave infrared radiation is absorbed by the upper part and cannot reach the upper surface. Therefore, temperature measurement can only use short wavelengths. Semiconductor photodiodes are generally used for temperature measurement.

The heat radiation area cannot reach the absolute temperature because it is absorbed by the upper layer, and only a relative temperature signal can be obtained. This signal can be compared with the welding signal of the same part OKAY part.

To obtain the absolute temperature value, the pyrometer must be specially calibrated for the same plastic part and thickness part. In fact, this calibration is not performed, only the relative temperature signal is compared and analyzed.