UV-curing resin: precise LED irradiation for perfect bar printing
Ein Report of AIMIS-FYT|esa
Over the project
As part of an additive manufacturing process in which a UV-curing resin is used to print rods and rod structures, the irradiance of the LED module is to be measured. A total of nine LEDs from Nichia, which are positioned around the nozzle (see Fig. 1 left), serve as the UV radiation source for curing the extruded resin. Three LEDs form a group (1, 2, 3) in which the distance a and angle α are the same (see Fig. 2). Each LED is also provided with a lens. This sets a defined beam angle (FOV) (see Fig. 1 right). Six of the LEDs have a beam angle of 30° and irradiate the area of the nozzle outlet and a part along the printed rod. The other three LEDs have a beam angle of 10° and focus directly on the outlet area of the nozzle.
Picture 1: Arrangement of LEDs around the nozzle and FOV of the lenses
Picture 2: Geometric arrangement of the LEDs around the nozzle
Measurement execution
In order to know which irradiance E_e in the wavelength range of 365 nm actually arrives at the nozzle outlet and along the printed rod for the given LED configuration, the mobile measuring device X1-1-RCH-116-4 from Gigahertz-Optik is provided as a loan device for the measurement. The device consists of the optometer X1-1 and the detector RCH-116-4.
For the measurements, the UV detector is placed below the nozzle (see Fig. 3). The irradiance is measured at different distances b from the nozzle outlet and at several UV intensities. At each measuring point, the LEDs are switched on for two seconds. After these two seconds, the last measured value displayed on the optometer is recorded. The pause between two measurements is 5 seconds.
Figure 3: Measurement using a radiometer
Measurement results:
Fig. 4 shows the irradiance E_e as a function of the UV intensity for different distances b from the nozzle outlet.Each curve corresponds to a specific distance b. It is easy to see that the curves initially rise relatively steeply at low UV intensity and flatten out or are saturated as the UV intensity increases.
At high UV intensities, the electrical power of the LEDs is converted directly into power loss or heat output, as temperature measurements have confirmed.
Figure 4: Irradiance as a function of the UV intensity
In Fig. 5, the irradiance is also shown as a function of the nozzle distance b for different UV intensities. Due to the arrangement of the LEDs, the maximum is at a distance of b=20 mm behind the nozzle (see top curve in Fig. 4).
Figure 5: Irradiance as a function of the distance b to the nozzle outlet
During the measurement tests, it was found that the irradiance drops sharply at high UV intensities in the first few seconds after the LEDs are switched on (several 100 mW/cm^2).
The reason for this is the rapid heat development of the LEDs, which leads to power loss.
To investigate the effect more closely, additional measurements are carried out in which the irradiance is measured as a function of time.
For this, the LEDs are switched on for 20 seconds each at different UV intensities. The measurements are carried out for a distance b=20 mm.
Figure 6: Irradiance as a function of time
The heating of the LEDs is also clearly visible in the print results. In Fig. 7, ten bars were printed with the same settings. In the upper image, the bars were printed immediately one after the other. The irradiance was no longer sufficient for sufficient curing of the last bars. In the lower image, a pause of 60 seconds was observed between each bar so that the LEDs could cool down sufficiently.
Figure 7: Top: No pause between the printing of two bars; Bottom: 60 second pause
Conclustion:
Using the radiometer from Gigahertz-Optik, the irradiance of the LED configuration could be precisely measured and the power loss of the LEDs at high UV intensities could be shown as a function of time. The next step is to further improve the cooling of the LEDs.
At a glance:
- Measuring the irradiance of the LED module, which is required for printing rods and trusses
- Irradiance is measured at different distances b from the nozzle exit and at several UV intensities
- Measurement of the irradiance as a function of time