SYSTEM FOR JOINING THERMOPLASTIC WORKPIECES BY LASER TRANSMISSION WELDING

Abstract

A system for joining at least two workpieces of thermoplastic material by laser transmission welding, the system comprising at least one device for generating laser radiation and an imaging optics having a first optical axis, wherein laser radiation emitted by the device for generating laser radiation is guided into a joining zone and wherein the imaging optics is configured for optical imaging from a joining plane in the joining zone to a detection plane of a radiation detecting device. According to an aspect, a deflection mirror is arranged at a coupling point on a second optical axis running parallel to the first optical axis for deflecting the laser radiation from an entrance axis enclosing a non-zero angle, preferably a right angle, with the second optical axis into a direction along the second optical axis.

Claims

1. A system for joining at least two workpieces of thermoplastic material by laser transmission welding, the system comprising at least one device for generating laser radiation and an imaging optics having a first optical axis, wherein laser radiation emitted by the device for generating laser radiation is guided into a joining zone, and wherein the imaging optics is configured for optical imaging from a joining plane in the joining zone to a detection plane of a radiation detecting device, wherein a deflection mirror is provided at a coupling point on a second optical axis, running parallel to the first optical axis, for deflecting the laser radiation from an entrance axis enclosing a nonzero angle, preferably a right angle, with the second optical axis, into a direction along the second optical axis, wherein the distance between the coupling point and the joining plane with respect to the second optical axis is smaller than the distance from the detection plane to the joining plane with respect to the first optical axis, wherein the diameter of the deflection mirror is adapted to a cross-sectional shape and cross-sectional size of the laser radiation and, when projecting the deflection mirror onto a projection plane perpendicular to the first optical axis, the projection area of the deflection mirror in the projection plane is arranged completely within a minimum aperture in the beam path of the imaging optics and only partially covers the minimum aperture.

2. The system according to claim 1, wherein the projection of the deflection mirror covers at most two thirds of the minimum aperture of the imaging optics.

3. The system according to claim 1, wherein the first optical axis and the second optical axis coincide to form a common optical axis.

4. The system according to claim 1, wherein at least one wavelength selective element is arranged in a region along the first optical axis between the detection plane and the coupling point.

5. The system according to claim 4, wherein the wavelength selective element is formed as a short pass filter, long pass filter, band pass filter or notch filter.

6. The system according to claim 1, wherein the deflection mirror is held by a deflection mirror mount, wherein the deflection mirror mount comprises at least one holder base and a holder element, the holder base having a free space which is bound or enclosed by the holder base at least partially circumferentially about the first optical axis and a projection of the free space onto a projection plane perpendicular to the first optical axis completely encloses a minimum aperture of the imaging optics, wherein at least one brace extends from the holder base in a straight line or curved into the free space and connects the holder base to the holder element, and wherein the holder element is configured to hold the deflection mirror.

7. The system according to claim 1, wherein the radiation detecting device is configured to detect electromagnetic radiation in the near infrared region of the electromagnetic spectrum.

8. The system according to claim 1, wherein the radiation detecting device is configured to detect electromagnetic radiation in a detection range in a subrange of the electromagnetic spectrum comprising in a central region the wavelength of the laser radiation.

9. The system according to claim 1, wherein the radiation detecting device is configured to detect electromagnetic radiation in a detection range in a subrange of the electromagnetic spectrum that comprises the wavelength of the laser radiation only in a peripheral region or not at all.

10. The system according to claim 1, wherein the radiation detecting device comprises at least one photodiode, in particular an indium-gallium-arsenide photodiode.

11. The system according to claim 2, wherein the projection of the deflection mirror covers not more than half of the minimum aperture of the imaging optics.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] FIG. 1 a schematic overview of an embodiment of a system according to an aspect of the present disclosure for joining at least two workpieces of thermoplastic material by laser transmission welding; and

[0035] FIG. 2 an exemplary deflection mirror mount together with deflection mirror for a system according to an aspect of the present disclosure for joining at least two workpieces of thermoplastic material by laser transmission welding in three-dimensional isometric representation.

DETAILED DESCRIPTION OF EMBODIMENTS

[0036] FIG. 1 shows a schematic overview of an embodiment of a system according to aspects of the invention for joining at least two workpieces made of thermoplastic material by means of laser transmission welding in the form of a process head 1 with a (process head) housing 2. Except for the feed for laser radiation 5, indicated in the form of an optical waveguide 4 with a fiber connector 3 at one end of the optical waveguide 4 leading to the housing 2 of the process head 1, all other elements of the system are arranged inside the housing 2. In the representation according to FIG. 1, the fiber connector 3 is connected laterally to the housing and has a beam expansion (not shown in further detail) for laser radiation 5 guided to the process head 1. The beam path of the laser radiation 5 entering from the fiber connector 3 into the interior of the housing 2 of the process head 1 is indicated in FIG. 1 by means of dashed lines as schematic outer beam boundaries of the laser radiation 5 perpendicular to the direction of propagation of the laser radiation 5 in accordance with the principles of ray optics or geometrical optics. After entering the housing 2, the laser radiation 5 initially propagates along the entrance axis 6.

[0037] A device for generating laser radiation, i.e. a laser radiation source or ‘laser’ for short, which can in principle also part be of a system according to an aspect of the present disclosure for joining at least two workpieces of thermoplastic material by laser transmission welding, is not shown in the representation of an exemplary embodiment according to FIG. 1, in particular because the ‘laser’ may be arranged outside the process head 1. As a result, the process head 1 can be made much more compact. Moreover, the process head 1 can be movable to some extent at least along one spatial axis and, if necessary, can additionally be rotatable and/or pivotable at least about one (further) spatial axis. As a ‘laser’ in the exemplary embodiment shown in FIG. 1, a continuous wave fiber laser with a maximum output power of 200 W, a wavelength of 1940 nm and a diffraction coefficient M.sup.2<1.1 may be provided.

[0038] At a beam splitter 7, a portion of the laser radiation 5 (about 1% to 2% of the power density of the laser radiation 5 incident on the beam splitter 7) is deflected perpendicular to the entrance axis 6 and directed to a device for laser power measurement 8. The device for laser power measurement 8 may comprise a photodiode configured for detecting the laser radiation and one or more filters for attenuating the laser radiation directed to the device for laser power measurement 8 (not shown in detail in FIG. 1) so as not to overload or even damage the photodiode. The device for laser power measurement 8 allow monitoring of the power of the ‘laser’ during operation and are thus also important for controlling the output power of the ‘laser’.

[0039] The main portion of the laser radiation 5 passes the beam splitter 7 and propagates further along the entrance axis 6 until the laser radiation 5 hits the deflection mirror 9. The deflection mirror 9 deflects the laser radiation 5 incident along the entrance axis 6 by 90°. The laser radiation 5 then propagates along the optical axis 15 in the direction of two optically transparent workpieces 11 and 12 to be joined. The optical axis 15 and the entrance axis 6 are aligned perpendicular to each other and intersect at the coupling point 24. The coupling point 24 is located in the center of the mirror surface of the deflection mirror 9. The laser radiation 5 is focused by a first lens 10. In the embodiment according to FIG. 1, the focal plane of the laser beam 5 coincides with the joining plane 13 at the boundary between the two adjacent workpieces 11 and 12 to be joined. The schematic representation in FIG. 1 follows the principles of beam or ray optics, whereby the focus is represented in a highly simplified manner as being point-shaped. Actually, the focus of the laser radiation 5 has a non-vanishing or non-zero extension in the focal plane/joint plane 13, i.e. it is a area of finite size, cf. the concept of the Gaussian beam as a more realistic approximation of the “actual” focus of a bundle of electromagnetic radiation.

[0040] On the one hand and primarily, the first lens 10 serves to focus the laser radiation 5. On the other hand and secondarily, however, the first lens 10 is also part of the imaging optics 10, 21, essentially consisting of the aforementioned first lens 10 and the second lens 21. The first lens 10 is thereby mounted in a frame 26 being part of the housing 2 of the process head 1 and is thus also a housing window. The imaging optics 10, 21 image the joining plane 13 to a detection plane 22 of a radiation detecting device indicated in the illustration in FIG. 1 as a photodiode 23. The photodiode 23 is configured as an InGaAs photodiode with one pixel. The emphasis of the detection range of the photodiode 23 is in the near infrared at wavelengths between 1500 nm and 2500 nm. The wavelength of the laser radiation 5 then lies exactly in the center of the detection range of the photodiode 23. To shield the photodiode 23 for example from parts of the laser radiation 5 reflected/scattered at optical interfaces of the first lens 10 or the workpieces 11, 12, a notch filter 20 is arranged as a wavelength-selective element in the beam path of the imaging optics 10, 21, which (almost) completely suppresses (“filters”) electromagnetic radiation with the wavelength corresponding to the laser radiation 5 in the range of 1940±50 nm, but transmits electromagnetic radiation with all other wavelengths in the detection range of the photodiode 23 (almost) unhindered.

[0041] The optical axis 15, along which the laser radiation propagates after deflection at the deflection mirror 9 in the direction of the workpieces 11, 12 to be joined, is at the same time the optical axis of the imaging optics 10, 21. A distinction between a first optical axis associated with the imaging optics 10, 21 and a second optical axis associated with the laser radiation 5 after deflection at the deflection mirror 9 is not necessary in the example shown in FIG. 1, since the axes in this case coincide anyway to form an optical axis 15. The irradiation of the workpieces 11, 12 to be joined by laser radiation 5 during the heating of the workpieces 11, 12 to be joined caused at least by partial absorption of the laser radiation 5 in a vicinity of the focus of the laser radiation 5 leads to an emission of thermal radiation. A part of this thermal radiation 14 can pass through the imaging optics 10, 21 to the detection plane 22 of the photodiode 23. The beam path of the thermal radiation 14 between the joining plane 23 and the detection plane 22 and thus the beam path of the imaging optics 10, 21 is indicated in FIG. 1 by solid lines as a symbolic outer beam boundary perpendicular to the direction of propagation of the thermal radiation 14 in accordance with the principles of ray optics or geometrical optics. Actually, similar to the focus of the laser radiation 5, there is no point-shaped emission point of the thermal radiation, but a spatial area of finite size, whereby a cross-section through this area along the joining plane/focal plane 13 completely encloses at least the (actual, finitely sized) focus 27 of the laser radiation 5. Thermal radiation 14 which reaches the first lens 10 is initially collimated thereby. In the opposite direction, the collimated laser radiation 5 guided to the first lens 10 is focused, as already mentioned. Between the coupling point 24 and the joining plane 13, the beam paths of laser radiation 5 and thermal radiation 14 run coaxially, wherein the laser radiation 5 and the thermal radiation 14 each propagate along the optical axis 15, but in opposite directions. The beam cross-section of the laser radiation 5 is considerably smaller (less than half as large) than the beam cross-section of the thermal radiation 14. However, the deflection mirror 9 for the laser radiation 5 cuts a central hole in the further beam path of the thermal radiation 14, because the deflection mirror 9 together with the holder element 18 for the deflection mirror 9 as part of the deflection mirror mount 16 is located on the optical axis 15 in the center of the beam path of the thermal radiation 14 or, respectively of the imaging optics 10, 21 and consequently makes it impossible for part of the thermal radiation 14 to reach the detection plane 22. The braces 19 as connection between the holder element 18 and the holder base 17 of the deflection mirror mount 16 lead to further small ‘losses’ of thermal radiation 14, i.e., thermal radiation 14 not reaching the detection plane 22.

[0042] Such a deflecting mirror mount 16 together with deflecting mirror 9 is exemplarily shown on its own in a three-dimensional isometric view in FIG. 2. FIG. 1 shows a cross-section through such a deflecting mirror mount 16 as part of the process head 1. The deflection mirror mount 16 comprises a holder base 17, a holder element 18 and four braces (struts) 19 connecting the holder base 17 and the holder element 18 (of the braces 19, only three are shown in FIG. 2, a fourth brace 19 is almost completely hidden by the holder element 18). The holder base 18 is flat cuboidal in shape with an extension 31 to form a pedestal on one of the two flat, wide cuboidal sides. The holder base 18 has a central free space (a clearance, a recess, an opening) 25 with a circular cross-section, which is bound or enclosed by the holder base 18 completely circumferentially around the center of the circle of the cross-section of the free space. The free space 25 extends completely through the holder base 17 parallel to the shortest sides of the holder base 17. The holder element 18 is arranged in the center of the free space 25 or concentrically to the edge of the free space 25. The holder element 18 is connected to the holder base 17 by four braces 19 extending straight from the edge into the center of the free space 25. The braces 19 are arranged equidistantly along the edge of the free space 25 (at an angle of 90° to each other). Due to the holder element 18 and additionally, although to a much lesser extent, due to the braces 19, a not inconsiderable part of the free space 25 is blocked and in this sense is no longer “free”. The holder element 18 is formed by a cylinder cut obliquely at an angle of 45° on one side, this side thus providing an angle of 45° with the cylinder symmetry axis 28. The deflection mirror 9 is fixed to this obliquely cut side of the holder element 18. The deflection mirror 9, in particular the mirror surface 29, is elliptical (the obliquely cut side of the holder element 18 is also elliptical). The deflection mirror 9 thereby fits exactly on the obliquely cut side of the holder element 18. The center of the deflection mirror 9 coincides with the cylinder symmetry axis 28 of the holder element 17, which in turn runs exactly through the center of the free space 25 (circle center of the cross section of the free space 25). If the deflection mirror mount 16 together with the deflection mirror 9, as shown in FIG. 1, is arranged as part of a system according to aspects of the invention or a process head 1, then the axis of cylindrical symmetry 28 of the holder element 17 coincides with the optical axis 15 and the center 30 of the mirror surface 29 of the elliptical deflection mirror 9, which is also the coupling point 24, lies on the optical axis, 15. The elliptical form of the deflection mirror 9 causes that, in an arrangement of the deflection mirror 9 as shown in FIG. 1, when the mirror surface 29 of the deflection mirror 9 is projected onto a projection plane perpendicular to the entrance axis 6 as well as onto a projection plane perpendicular to the optical axis 15, the respective projection of the mirror surface 29 is circular.

LIST OF REFERENCE SIGNS

[0043] 1 Process head [0044] 2 Housing [0045] 3 Fiber connector [0046] 4 optical waveguide [0047] 5 Beam path laser radiation [0048] 6 Entrance axis [0049] 7 Beam splitter [0050] 8 Device for laser power measurement [0051] 9 Deflection mirror [0052] 10 First lens [0053] 11 First workpiece [0054] 12 Second workpiece [0055] 13 Joining plane [0056] 14 Radiation path thermal radiation [0057] 15 Optical axis [0058] 16 Deflection mirror mount [0059] 17 Holder base [0060] 18 Holder element [0061] 19 Brace [0062] 20 Notch filter (wavelength selective element) [0063] 21 Second lens [0064] 22 Detection plane [0065] 23 Photodiode (radiation detecting device) [0066] 24 Coupling point [0067] 25 Free space [0068] 26 Frame [0069] 27 Focus of laser radiation [0070] 28 Symmetry axis of holder element (cylinder symmetry axis) [0071] 29 Mirror surface of deflection mirror [0072] 30 Center of mirror surface of deflection mirror [0073] 31 Extension/Socket of holder base