What is the welding strength of plastic laser welding?

For different products and customers, quality requirements may be different. Most products have welding strength and appearance requirements, as well as some airtightness requirements. This article first introduces the three stages of plastic laser welding quality control, and then focuses on plastic laser welding distance measurement and temperature measurement.

Plastic parts purchased from outside or made by yourself need to be inspected as follows:

鈽匘imensional tolerance;

鈽匧aser color transmittance;

鈽匨elt characteristics;

Use special instruments for inspection or submit to a professional third party for inspection. In most cases, new sample parts and parts from different batches are sampled for inspection.

Depending on the capabilities of the device, the following measurements can be made:

鈽匨easure laser power;

鈽匨easure the current and voltage of the laser module;

鈽匨easure temperature;

鈽匨easure welding distance;

Depending on whether the measurement results are fed back to the welding process, it can be divided into an open-loop control system and a closed-loop control system. In open-loop control, the data can check and analyze the welding quality. In closed-loop control, the data can be fed back in real time to control the welding process, such as stopping welding only after reaching the set welding depth.

After welding is completed, it can be inspected in the next process to ensure the welding quality. According to the functional requirements of the product, common tests are as follows:

鈽匬ressure or vacuum sealing test.

鈽匨echanical tension/compression test.

鈽匴eld cross-section image analysis.

In addition, the welding strength in mass production should be randomly and regularly extracted for destructive testing.

Measure the welding distance (collapse displacement).

During synchronous and quasi-synchronous laser welding, a circle of the weld is softened or liquid at the same time, and part of the molten glue is squeezed out and connected by the applied clamping force. During this process, the upper part moves downward, and the welding distance/collapse distance is usually 0.1-0.4mm.

The typical laser welding collapse displacement curve is shown in the figure above. At first, it will detect a small rise due to the thermal expansion of the plastic. Once the plastic is completely melted, the liquid molten material will squeeze out of the weld, and the plastic under the weld will be further melted, and the welding depth will increase. When the laser is turned off, the collapse will not stop immediately. The molten material at the weld will cool and solidify, and then a small and lagging displacement will be observed.

The collapse displacement can compensate for the deformation and assembly gap of plastic parts, ensure that the weld is complete and gap-free all around, and improve strength and sealing.

Non-contact temperature measurement.

A pyrometer measures the thermal radiation of a surface. The hotter the object, the more thermal radiation there is and the shorter the wavelength. If the pyrometer is calibrated with a radiation source of standard temperature, an accurate temperature value can be obtained.

If it is not calibrated, the temperature measurement will be underestimated.

More interestingly, in the temperature range of 100-400鈩? the infrared wavelength of plastic welding heat radiation is about a few microns.

In laser plastic welding, the weld is not on the upper surface, but between the two parts, so the heat radiation is affected by the upper part. The long-wavelength infrared radiation is absorbed by the upper part and cannot reach the upper surface. Therefore, temperature measurement can only use short wavelengths. The commonly used temperature detection element is a semiconductor photodiode.

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

If you want to obtain absolute temperature values, the pyrometer must be specially calibrated for parts of the same type of plastic and the same thickness. In actual work, no such calibration is done, only a comparative analysis of the relative temperature signals.