Laser welding is a modern technology used to create unbreakable joints in materials (usually plastics). It is a form of welding that uses heat from a laser beam and pressure(from a downward force pushing the parts together) to create very strong joints. Compared with traditional methods that weld plastics, has several advantages including the absence of any additional material, low operating costs and it is very efficient. In industrial practice, what is known as “penetration laser welding” is used, which is recommended for joining plastics with different (laser) absorption properties.
A major advantage of this method is easier automation, this method is regularly used in many different industries. Together with its increased usage, there is also an increased need to check the quality of the weld and the welding process itself. For this reason, many companies, particularly in the automotive industry, have successfully used the ThermoInspector system developed by the Czech company Workswell s.r.o.
Workswell s.r.o. is a Central European business and manufacturing company with its main headquarters in the heart of the European Union. The company’s primary product is to provide contact-free temperature measurement solutions. Clients of Worskwell s.r.o. include, in addition to the car makers BMW, Škoda, Renault and Kia, companies such as ČEZ, RWE, Bosch, Honeywell, Tesco, Heineken, Kofola, IKEA, Net4Gas, Semikron, ArcelorMittal and hundreds of various small, medium and large industrial enterprises. Regular clients include prestigious foreign universities.
The industrial use of laser welding thermoplastics is very often used to weld two different types of plastic parts one permeable thermoplastic that the laser can pass through and one other thermoplastic that absorbs the laser heating it up and melting it to the permeable layer. The amount of laser beam absorption by the thermoplastic can be modified to vary the weld strength.
There are two thermoplastics with different absorption properties meaning the way that the laser affects them differs. When laser welding plastic one thermoplastic material (which allows the laser to pass through) is placed on top of another thermoplastic with different absorption properties. This second layer fully absorbs the lasers energy, heating it up and melting it which creates a strong bond between the two layers. The laser beam is directed at the areas that you desire to be welded and joins the components in that place. As the bottom surface of the top layer and top surface of the bottom layer are in direct contact with one another as the bottom layer heats it melts bot materials creating an extremely strong weld.
The welding regime is determined by how the laser beam heats welded components. It is possible to select:
contour mode – The beam repeatedly creates the trajectory of the contours of the future weld.
quasi-simultaneous mode – when the laser beam is routed to the welding spot with fast-moving mirrors.
- simultaneous mode – multiple diodes are used, and the area is heated all at once.
Laser welding control with the ThermoInspector system
The Workswell ThermoInspector is a temperature inspection system used for monitoring, analysing and evaluating the welding of plastics and metals, it can test input-output inspection, quality, temperature and other more during the whole manufacturing process. It can continuously measure and evaluate the thermal field on the surface of the measured item and consequently check its thermal characteristics, maximum and minimum temperatures to evaluate the dispersion of the temperature along the thermal cut and check how fast the material heats up in the stated areas and many other statistic indexes that are essential during series production. The system contains a thermocamera, an evaluating control unit and a touch panel for the visualization of inspection outputs. The ThermoInspector features an input – output card, the inspection results can be connected to the control system or PLC.
User interface of Worskwell Thermoinspector system
The principle of evaluating temperatures on the weld
Laser welding requires a special approach in both temperature measurement and temperature evaluation. The weld cannot be evaluated solely based on the measured temperature, but only by setting the temperature limits requirements. If the conditions aren’t exactly correct then the user is exposed to the risk that several products will be eliminated unnecessarily, or a number of errors will be ignored.
Based on our extensive experience with the measurement of temperatures during welding, the ThermoInspector evaluates the quality of weld using three methods:
- point two-phase evaluation – each pixel from the area of interest is evaluated (along the weld) and the time curve of the increase in temperature is analysed during the welding along with the time curve of the decrease in temperature after welding. This series of curves is compared with the standard and according to set limits, then the welding is evaluated to ensure that it was done correctly.
- envelope experimental method – by analysing the settings of up to ten “perfect” products created by the welding processes, the so-called “envelope” of correct values is created. The welding temperatures in these “perfect” pieces with their maximum and minimum temperatures set the welding temperature for all the other pieces. If these temperatures are not correct during manufacturing and a temperature “outside the envelope” is detected, the product is detected as defective.
- fast point measurement – the round area of interest (ROI) is divided into a matrix of individual pixels or groups of pixels and during testing, the number of pixels that have higher or lower temperature than the stated range is measured. If the product does not fulfil the criteria, it is excluded as NOK.
As can be seen from the individual evaluating procedures, before the application of a thermo-vision system into actual operation, tests were run to determine what temperatures are acceptable. As a result, these tests primarily state the extent to which we comply with the standard.
Extensive experience with the Workswell ThermoInspector during laser welding shows that it is not possible to exactly state what will be a high-quality and what will be a low-quality weld solely on the basis of single measured temperatures (or the thermal profile). The temperature limits must be stated by measuring the weld temperature from multiple trial products. The ThermoInspector is prepared for this necessary step. Integrating a ThermoInspector through either Workswell or from their partners, the whole system can be implemented within a matter of minutes onto the manufacturing line to collect the data on the ideal temperatures during manufacturing, a suitable evaluation method and the temperature limits. These tests are an integral part of the offer for the whole system. Without these tests it wouldn’t be possible to ensure a reliable and problem-free system that runs on a 24/7 regime.
The ThermoInspector system is a tested and true system that is used when inspecting welding in companies such as, Pierburg, TRW automotive, FTE automotive which produce components for the automotive industry. Due to the fast and exact analysis of temperatures (for measurement as well as complex statistical processing), these companies can easily determine erroneously produced pieces and can immediately exclude them from distribution. This means that they significantly reduce reclaim costs and resolve problems in production.