Enabling pyrometry in absorber-free laser transmission welding through pulsed irradiation

Abstract

Laser transmission welding has become an established joining technique in series production of plastic components. Besides its unique process related properties, it also offers several monitoring methods to ensure a constant welding result. In most cases, pyrometry is the method of choice which contactless detects the heat emitted from the interface of the joining partners. Considering the transmission properties of polymers and the optical components between the workpiece and pyrometer, only a small fraction of the thermal process emission is able to penetrate to the detector of the pyrometer. Typically, in classic laser transmission welding, the thermal emission is measured in the spectral range 1.1–2.5 μm limited by the laser wavelength on the one side and the absorption capability of polymers/optical elements in the optical path on the other side. In absorber-free laser transmission welding, laser sources in the range of 1.6–2 μm are used in order to exploit the intrinsic absorption of thermoplastics. With the laser emitting in the sensitivity range of the pyrometer, optical filters have to be used to isolate the thermal radiation from the laser radiation which, however, attenuates the already weak thermal signature even further. An approach that does not require any optical filters is presented in this paper. The concept is operating the laser in a pulsed mode which enables detection of thermal emissions between two consecutive pulses without being overlaid by the laser radiation. Though laser radiation is not delivered continuously, the result demonstrates that it is still possible to obtain a nearly homogeneous seam when choosing an appropriate pulse regime. It is also shown how the detected signal can be utilized to adjust the focal position, which is an important but time-consuming aspect in absorber-free laser welding.

Optical Coherence Tomography for 3D Weld Seam Localization in Absorber-Free Laser Transmission Welding

“…Absorber-free laser transmission welding possesses several advantages (i.e., contactless input of energy, high precision, no adhesives and no particle formation) which enable the fulfilment of the aforementioned demands [1][2][3]. In contrast to conventional transparent-absorbent welding, where one partner contains a laser absorbent additive [2,4,5], or an additional absorbing material is placed in the joining zone [6][7][8][9], the samples analyzed in this work are made of identical polymers and were joined directly using absorber-free laser transmission welding [10][11][12][13][14][15][16]. Figure 1 shows the process principle of absorber-free laser transmission welding.…”

“…If the seam extends too far vertically to the upper surface, a visible and tactile bulge occurs. An overly short seam insufficiently penetrating both joining partners causes instability and inadequate tightness [1,10,12]. Nevertheless, horizontal alignment is also important, since cells or drugs can temporarily linger and lump in remaining gaps if a fluidic channel is not sealed exactly at its edge.…”

“…Nevertheless, horizontal alignment is also important, since cells or drugs can temporarily linger and lump in remaining gaps if a fluidic channel is not sealed exactly at its edge. This creates an unacceptable risk to the patient using such a device and requires process control [12,18]. Figure 1.…”

Modelling and thermal simulation of absorber-free quasi-simultaneous laser welding of transparent plastics

“…One promising approach is laser transmission welding. The temporal and spatial defined and contactless energy input leads to a precise welding process at low thermal and mechanical stress [2,3]. In classic laser transmission welding, beam sources are used which emit radiation in the transparent wavelength range of most polymers (λ = 800-1100 nm).…”

“…The electromagnetic irradiation energy is converted into heat which leads to melting of both joining partners in the joining zone. A welding seam is then generated as soon as the melt solidifies [3,4]. One important factor for the achievement of a high and reproducible welding seam quality is the amount of absorber material.…”