Analysis of short wavelength infrared radiation during laser welding of plastics

Abstract

In this paper, a new measurement system and a new approach in calculation for infrared (IR) radiation investigation in quasi-simultaneous transmission laser welding of plastics are presented. The measurement system is based on a MW/SWIR (medium-wave/short-wave IR) camera and optical filters narrowing the spectral region to SWIR. The measured signals contain radiation from the melted zone in between the semitransparent and absorbing polymers, as well as radiation from the surface and interior of the semitransparent polymer. The new calculation approach was developed to distinguish between these signals. It is based on simplification of the process to two places with two temperatures (surface and molten interface) and knowledge of the spectral optical properties of the material, filters, and camera response. The results of measurement and calculation for three different optical filters and polyoxymethylene samples with two thicknesses are shown and discussed. Good agreement is obtained for the calculation variant using normal transmissivity of the semitransparent polymer.

Measurement of core temperature through semi-transparent polyamide 6 using scanner-integrated pyrometer in laser welding

“…This is caused by the heat conduction from the joining zone. With increasing sheet thickness, the maximum temperature is reached later, caused by the increased distance between the joining area and the upper surface, as also reported by Polster et al [26] and Martan et al [3].…”

“…This is based on the spectral emittance for the given temperatures during welding, since the maximum of the heat radiation is emitted in the mid-wavelength infrared region and thus far away from the aforementioned spectral range. On top of that, the heat radiation emitted from the joining zone is attenuated due to absorption in the upper polymer [3]. Pyrometers with photodiodes, which are sensitive in the before mentioned range are to recommend, since these systems are operating at a high sampling rate and are cheap and easy to use.…”

“…Hereby, the filtering of the heat radiation in the upper polymer is taken into account. The temperature signal is then measured during quasi-simultaneous laser transmission welding, where a high reproducibility of the welds is given, since the energy input can be adjusted precisely [3]. Afterwards, the measured signal is compared with temperature courses calculated in a thermal process simulation, in order to check if the measured temperature can be assigned to a spatial domain and finally to check the accuracy of the calibration for the herein used injection-molded polyamide 6 with the brand name ''Ultramid B3s".…”

An integrated methodology for estimating the interface temperature and effects of powder diameters in laser transmission welding process

“…The method of thermal imaging monitoring and simulation has been investigated by numerous researchers to get an accurate temperature in the joint. For example, Martan et al [10] established a new measurement system with a medium-wave/short-wave IR camera for infrared radiation investigation in the process of LTW. However, the temperature measured by these methods is lower than the actual temperature.…”

“…This is because the emitted heat radiation for the given temperature during welding has a spectral emittance in the mid-wavelength infrared region [11]. The absorption of transparent thermoplastic restricts the heat radiation from the interface to the surroundings, and the tested surface temperature is lower than the actual temperature in the joint [10]. As published by Goyal et al [12], the temperature tested directly by the thermal imaging camera is below 85°C, the melting of thermoplastics cannot occur at this time.…”