Thermal imaging camera control of laser welding of plastic

ThermoInspector - termogram laser welding

There are 3 possibilities of how to weld plastic using laser beam:

  • Spot welding (both plastic or metal welding)
  • Circumferential welding (both plastic or metal welding)
  • Contour welding – welding head moves towards the object or vice versa. This defines the characteristic motion curve. (typical for plastic welding). Normally fibre laser or solid-state laser is used.

Furthermore, contour welding will be described more precisely:

LED laser, used for contour welding, has standard wavelength of 1μm. This wavelength in near infrared is ideal to let laser go through the material which is transparent for this wavelength (clear plastic). Then, heat is excited on the opaque (for this wavelength) plastic layer (black plastic) in interface of those two plastics which leads to welding them together. Generally, temperature which is needed to start the melting of the opaque plastic is about 190°C. The heat itself would not be enough to cause the welding which is why there is need of pushing the two plastic materials together either by gravity force or pressure machine.

This principle is also described in the picture below:

Figure 1: Laser welding principle

Figure 1: Laser welding principle

Power of LED laser light can vary from 25 – 50 W to 200 – 300 W. Its biggest advantage beside an ancient CO2 laser is that LED laser operates at 1μm and its energy can be absorbed by plastic much better than using CO2 laser which operates at 10μm. Another advantage is the price of LED laser, which is lower than for CO2 laser.

ThermoInspector - termogram laser welding

Welding head operates typically from 25 to 30 cm from the object and as the laser beam is relatively small, laser dispersion vary from 100μm to 0.5mm. When controlling the welding, it leads to requirement to use a camera of wide FOV with capability to focus on as small distance as possible.

Sometimes, the size of the laser generated pulse can be set to very small amount, which leads to the fact that the heating moment is very short (e.g. 100ms) and it requires a 30Hz camera to be able to control this action at least. More often, welding takes more time (from 2 to 10s).

When choosing the thermal camera to control this action, it is important to consider its capability to have as big density of pixels on the laser beam as possible also.

ThermoInspector - termogram laser welding

There are two possibilities of welding propagation. Either robot is moving towards the welding contour with one beam or laser has more than one beam. When using more beams simultaneously in different places of the contour it is faster. Also, the heat transfer heats up surroundings of the welding spot up to 0.5cm which accelerate the welding.

When welding two plastic part together surface temperature of the one above (clear plastic) is controlled by a thermal camera. This temperature is much lower than the one of opaque plastic and depends on the thickness of the material. For thin materials it can normally be from 80°C to 90°C and for thick materials (3 – 5cm) the temperature varies from 40°C to 50°C. It is why there is need of short delay between the laser beam radiation and thermal control of the propagated temperature of the plastic surface.

How does Workswell ThermoInspector control works?

There are two possibilities of using ThermoInspector control during laser welding depending on the welding process.

Workswell ThermoInspector laser welding
  • When laser beam begins to weld, the trigger in ThermoInspector is set. Then there is a defined delay to the beginning of the temperature measurement due to the thermal propagation from the weld spot to the object surface, which takes few milliseconds.
  • When using simultaneous welding, it is impossible to put the thermal camera close enough because of the mechanical obstacles during the welding process. Principle of the temperature measurement must be slightly different. The trigger in ThermoInspector is set to the moment when weld head leaves from the direct view on the welding contour. Any delay in no needed in this case because the time necessary to move the machine corresponds to the time of temperature propagation (about 0.5s – 1s). The temperature can be measured immediately after the trigger is set.

The region of interest in this case is a selected region from the scene of thermal image, which represent the area in which are the most important information to control the welding process. This ROI is always selected near the welding spot (generally 20px from the middle of sport) as shown on the picture below.

Figure 2: Definition of ROI scheme

Figure 2: Definition of ROI scheme

The ThermoInspector assembly is unique not only for the temperature measurement, but especially for quality control and its possibility to work in real time.

The importance of thermal camera measurement is not only in temperature measurement itself. Position of minimal and maximal temperature in time graph is much more important than their absolute values. To say that the weld should stable, both minimal and maximal temperatures must fit the tolerance band which has to be set before the measurement.

Tolerance band is made from many graphs of temperature measurement in time of the same product, in the same place (same condition). Due to the measured temperature, an envelope is set.

Figure 3: Determination of tolerance band

Figure 3: Determination of tolerance band

Tolerance band cannot be set too narrow, because of the different materials that can be used for different types of welding. Every material is characteristic, and temperatures of welding will be slightly different. If the band was too narrow, it would give lots of false negative results. On the other hand, if it was too wide, lots of the results would be recognized as false positive.

Even using the thermography control of welding process is very effective, it is not possible to express with absolute certainty if the product is of high quality or not, the only possibility to do that is using the blasting test. Quality of the weld is affected by many factors such as laser power, force of pressing two layers together, time of welding etc.

Main advantages of Workswell ThermoInspector

ThermoInspector - termogram laser welding
  • Repeatability
  • Possibility to detect the place where the excitation of the heat is highest
  • One specific wavelength measurement – higher speed and precision
  • Possibility of using more thermal camera simultaneously to accelerate the detection
  • Weld slice localization:
    • Thermal profiles evaluation – along the welded contour there are obtained about 20 – 30 temperature profiles (shown in red on the figure below). The maximum values are searched in each of them. They also have to be checked whether each of them has at least a minimum number of pixels of a certain temperature value, thus defining a surface with a certain temperature.
Figure 4: Thermal profiles along the weld contour

Figure 4: Thermal profiles along the weld contour

    • Another approach to the same problem – the contour is surrounded by an annulus in the case of circular contours. Annulus is divided into many temperature profiles (see figure below), which are evaluated as in the previous approach.
Figure 5: Thermal profiles in the annulus

Figure 5: Thermal profiles in the annulus

Positioning of the thermal camera

ThermoInspector - termogram laser welding
  • Spot welding: 45° from the laser beam axis
  • Circumferential welding: welding head axis is connected to the thermal camera and object is rotating (creating circumferential welding)
  • Contour welding: thermal camera is fixed to the object and covers the entire contour area

It is important to have camera with wide FOV and small resolution. Typical configuration of thermal camera from Workswell company is shown below:

  • Workswell WIC 640, focal length 19 mm, FOV 32° x 26°
  • Workswell WIC 336, focal length 19 mm, FOV 17° x 13°

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