Method and device for welding thermoplastic synthetic materials using laser light

The invention relates to a method for welding Thermoplastic materials with laser light, wherein the laser light In a welding region of a workpiece or several Adjacent workpieces of thermoplastic material . Furthermore, the invention relates A device for welding thermoplastics With laser light according to the method, with a laser as Light source for the laser light and with an optic and / or An optical fiber for directing the laser light into a laser beam Welding area of ​​a workpiece or several parts Adjacent workpieces made of thermoplastic.

The welding of thermoplastics is done Usually by contacting heating elements to the Thermoplastics. By heat transfer from the Heating elements on the thermoplastics are these Heated and plasticized, for example a compound Of two workpieces made of thermoplastic Is. There is a disadvantage in that the thermoplastic Plastics at the interface to the heating elements Temperature, ie, is largely plasticized are. The plasticization of the plastic is actually Where, for example, the connection of two workpieces is required Should be carried out. In the contact area to the heating elements There is a risk that, upon removal of the heating elements Of the workpieces the still plastic workpieces to be damaged. Accordingly, it is used in many applications It is necessary first to cool the heating elements and only then From the workpieces. However, the During welding. A further disadvantage of welding thermoplastics Plastics using heating elements is that For the formation of geometrically complicated designed Weld seams also correspondingly complicated designed heating elements Particularly in the case of small series production Or individual production is hardly profitable.

In a process known from DE 42 25 679 A1, Laser light of a CO₂Laser with A wavelength of 10.6 μm is used. The laser light is replaced A heating element and heats the thermoplastic directly on. Laser light with a wavelength of 10.6 μm But not appreciably into the depth of a workpiece Thermoplastic, even if the plastic in the Wavelength range of the visible light is transparent. The Laser light with the wavelength of 10.6 μm is instead reduced to A few tenths of a millimeter of workpiece depth is completely absorbed and in Heat. That is, when using a heating element The heat falls in the region of that of the welded joint Typically opposite surface of the laser facing the laser Work piece. Since the laser light is contactless on the thermoplastic Plastic, there is no danger A damage of the workpiece when removing a heating element. After welding, however, the workpiece has to be still Cooled, so that its partial heating by the heating Particularly plastic and therefore sensitive surface Is damaged. This is short cycle times during welding As well as the heat transport from the surface of the Workpieces up to the area of ​​the actual welding In Furthermore, in the known method, That the devices required for its implementation are very much Expensive. CO₂Lasers are already complex in themselves. Add to this Is that currently no suitable optical fiber for laser light With a wavelength of 10.6 μm are known. Accordingly, The laser light with this wavelength with complicated mirror-arm arms Or the like to the respective workpiece become.

German patent document DE 38 13 570 A1 discloses a method for welding Of thermoplastics with laser light It is known to select the wavelength of the laser light so that A complete absorption of the laser light in the respective ones Workpieces. Concrete wavelengths for the laser light But are not indicated.

It is known from DE 44 32 081 A1, in a process for the Welding of thermoplastics with laser light, Laser light with a wavelength of 0.50 to 10.90 μm use. The wavelength range should be from 0.70 to 3 μm Particularly preferred. For this preferred wavelength range As an example, only an Nd: YAG laser is used , Whose laser light is known to have a wavelength of 1.06 μm. Laser light with this wavelength is produced by Known thermoplastics without additives To small proportions. To a good implementation of the Laser light in heat, therefore, must be one of the two Workpieces at the wavelength of 1.06 μm non-transparent , Or it must be arranged behind the workpieces Black surface. At the respective interface To the transparent plastic is converted the Laser light in heat. If it is an additional black The disadvantages are the same as for a Otherwise heated heating element. For a combination of Workpiece made of thermoplastic without additives, Which is transparent to light with a wavelength of 1.06 μm, And a work piece made of a thermoplastic material, Additives at this wavelength are not transparent Is obtained when the two workpieces are connected in a planar manner Although a concentration of the heat conversion to the area of ​​the Actual welding, welding of two workpieces On impact is not possible. Furthermore Are characterized by the necessity of a workpiece at the Relevant wavelength non-transparent thermoplastic Plastic, there are strong restrictions on the Applicability of the known method.

It is known from DE 25 44 371 to produce very thin films Thermoplastic of about 15 μm in thickness Of laser light with a wavelength of 10.6 μm flat With each other. For this purpose, the laser light is amplified by the Films made of thermoplastic plastic on one Support. This is at the short absorption length Of laser light of this wavelength in thermoplastic Plastics is only possible with the small thickness of the films. Of the Underlay is controlled by the laser light to a temperature of Heated to approximately 1900 ° C., in which it irradiates a radiation in the Spectral range of 1.7 to 3.4 μm. This radiation Is said to be made by the plastic, which is polyethylene Then sufficiently absorbed, although the Absorption length of light in plastic at shorter wavelengths Is greater than that of the original laser light Wavelength of 10.6 μm. So is actually only a very Limited part of the irradiated light energy for a Heating of the relevant region of the films. Farther Due to the high temperature of the support rod Welding the films to a contact with it in a costly manner be avoided. In such an arrangement, under no A concentration of the thermal radiation of a Hot support rod in the spectral range of 1.7 to 3.4 μm To a limited welding area, since the voltage measured by the Original laser light heated areas of the support rod Than spot light sources emitting in all free spatial directions To be effective.

DE 296 21 859 U1 discloses a diode-pumped laser, Whose crystal has a Tm: YAG crystal, a Tm, Ho: YAG crystal, A Tm: YLF crystal or a Tm, Ho: YLF crystal, and Which has a wavelength of the radiated laser light of approximately 2 μm having. The laser light can enter into a water-poor or Water-free optical waveguide, in particular into a quartz-quartz fiber Can be coupled in. Quartz-quartz fiber have contributed Wavelengths in the range of 2 μm have such a large transmittance coefficient On that a guide of the laser light with Low attenuation even over longer fiber lengths of 1 to 3 m is possible.

The object of the invention is to provide a method for the Welding of thermoplastics of the beginning Described, which is the problem-free and fast Welding also thicker plastic workpieces Formation of permanent weld seams, in particular of impact seams, Is possible. Furthermore, an apparatus for carrying out the invention is described Of the procedure.

In the method of the initially described type, the In accordance with the invention, Wave length of 1.8 to 2.2 μm, Ie, about 2 μm. Laser light in this wavelength range is used by many thermoplastics Plastics directly, ie independently of any Fillers or additives, over an absorption length of Some mm. That is, one with laser light of this Wavelength irradiated workpiece is heated up in depth, Ie, where appropriate, where a weld seam is formed should. Thus, plastics such as polyamides, polyacetals, Polyesters, polyethylenes, polypropylenes, polycarbonates, polyolephins, Which are often not to be glued, easily Permanently weld together. Especially good experiences Are present with polypropylenes. Also welding different ones Thermoplastics is feasible. A Overview About the current use Plastics gives the following table in which the specification "Appearance" refers to the visible light range And colored "colored" to dyed dyes Variants of the respective plastic. The added Colorants behave in the new process, the Intrinsic absorption of Thermoplastics of Laser light with a wavelength of approx. 2 μm is used, neutral. For example, plastics can also be used with the new process In the wavelength range of the visible region Light are transparent or translucent. Dyes and others However, additives may be added to the plastics for other reasons May be added. The handling of the laser light with a wavelength Of about 2 μm is problem-free. It can be used in water or warm water Quartz quartz fibers without too much damping Over larger fiber lengths. In this wavelength range Is a focussing of the laser light on a movable one Point or a stroke or even the non-focusing Projection of the laser light to a line with usual optics So that very fine welding seams are also formed can. This is possible down to a width of 0.1 mm. Preferably, the novel process comprises the two steps of: Workpieces with a width of 0.7 To 2 mm. However, weld seams are also readily available With larger widths possible. Thickness of the two Workpieces are typically between 50 μm and 5 mm, The center of gravity being 0.2 to 2 mm thick films. When the workpieces are welded to each other, their impact is equal Material thickness exactly the width of the weld. The sensible Material thicknesses for welding on impact start at 0.5 Mm. In comparison to the absorption length of laser light with the Wavelength of 2 μm in thermoplastics of each According to material approx. 1 to 10 mm are the sensible material thicknesses In the region of the weld seam in the same order of magnitude Or are smaller up to about one order of magnitude Although a lower power yield of the laser light in the Compensates for a particularly homogeneous distribution of the heating The depth of the area detected by the laser light is obtained. <tables><table><tgroup cols="2"><tbody><row><entry align="center">plastic</entry><entry align="center">Appearance</entry></row><row><entry align="center">PS</entry><entry align="center">Transparent or colored</entry></row><row><entry align="center">PMMA</entry><entry align="center">Transparent or colored</entry></row><row><entry align="center">PC</entry><entry align="center">Transparent or colored</entry></row><row><entry align="center">PE</entry><entry align="center">Natural or colored</entry></row><row><entry align="center">PA</entry><entry align="center">Natural or colored</entry></row><row><entry align="center">POM</entry><entry align="center">Natural or colored</entry></row><row><entry align="center">SAN</entry><entry align="center">Natural or colored</entry></row><row><entry align="center">SECTION</entry><entry align="center">Natural or colored</entry></row><row><entry align="center">PP</entry><entry align="center">Natural or colored</entry></row><row><entry align="center">SB</entry><entry align="center">Natural or colored</entry></row><row><entry align="center">PVC</entry><entry align="center">Natural or colored</entry></row><row><entry align="center">PET</entry><entry align="center">Natural or colored</entry></row><row><entry align="center">PSU</entry><entry align="center">Natural or colored</entry></row><row><entry align="center">PBT</entry><entry align="center">Natural or colored</entry></row><row><entry align="center">TPE</entry><entry align="center">Natural or colored</entry></row></tbody></tgroup></table></tables>

The new process allows two plastic workpieces Are overlappingly welded together, the laser light Transverse to the workpieces. Typically warms The laser light illuminates the workpieces at the contact area , Whereby an attenuation by absorption in the region of the Rear work piece by a rear of this workpiece Can be compensated for. The reflector Itself is uncritical, since, for its part, it is not by the Laser light is heated. It can even be used during the Welding.

With the new method, it is also possible, in particular, to add two Workpieces of plastic welded together. The laser light is defined in a defined, typically Very narrow area parallel to the plane of the impact On both workpieces. The opposing narrow sides Of the workpieces are transmitted through the laser light Whole depth of impact. Again, here again A reflector attenuates the laser light by absorption In the direction of the impact depth. In the possibility, Both thinner films, as well as thicker workpieces To be welded is a particular advantage Of the new process as compared to the prior art.

The workpieces can be welded in the area of ​​a Desired welding connection between holding elements , At least one of which pass through the laser light Is allowed. The other holding element can be the already mentioned reflector be. Both holding elements can be cooled during welding , So that the effective heating of the workpieces occurs An inside area. This is at the flat Welding of, for example, plastic films. At the Forming a bump that extends over the entire bump depth The cooling of the holding elements must not be so Can be operated in such a way that the workpieces close to the Are no longer plasticized.

However, the new process can also be carried out completely without contact The workpieces being welded in the region of a Desired welding connection.

In the new method, the laser light can be provided with an optical fiber To the workpiece or the workpieces from the Thermoplastic resin. The weld When connecting two workpieces can be a very Complicated geometric design, as long as the Can be guided in its direction. This can But with simple, widely used positioning devices Without problems.

However, the laser light can also have a larger linear shape Area of ​​a desired weld to the workpieces To the thermoplastic. This is For example with an optical fiber plate, on whose A plurality of lasers are arranged one narrow side, while the Opposite narrow side for attachment to one or both Of the workpieces. In the light guide plate, Laser light held by total reflection, and the light intensity Of the laser light is across the width of the plate Homogenized.

The device of the type described in the opening paragraph is according to the invention Characterized in that the laser illuminates the laser light by means of a laser beam A wavelength of 1.8 to 2.2 μm. Such lasers Are known, for example, from DE-GM 296 21 859.6. These Described lasers with a wavelength of approx The implementation of the process is particularly well suited. At the same time Are, in particular, diode-pumped Tm: YAG-Tm, Ho: YAG-, Tm: YLF- and Tm, Ho: YLF-lasers. These solid state lasers Are already comparatively inexpensive. Furthermore A suitable diode laser can also be used, Which emits the laser light with the wavelength of 2 μm. The Laser light with a wavelength of 2 μm can also be produced with a low Effort and focus. Further, For the new device in comparison to a Corresponding device from the prior art with a Hd: YAG laser is a laser with an order of magnitude Lower light output is sufficient since its laser light Surprisingly effective and concentrated the plastic in the Relevant areas of the workpiece or the workpieces Heating. Nevertheless, despite the lower light output Already within short exposure times of the laser light with the Wavelength of approx. 2 μm a continuous heating of the thermoplastic Plastic, resulting in high welding speeds At the same time great quality of the resulting Weld seams allowed. In the preferred embodiment of FIG New device, the laser is a continuously emitting one Line laser

The invention is described in the following with reference to exemplary embodiments Explained and described. FIG.<dl tsize="6"><dt>FIG</dt><dd>The basic structure for carrying out the new Method,</dd><dt>FIG</dt><dd>A first embodiment of the novel method,</dd><dt>FIG</dt><dd>A modification of the embodiment of the new method According to FIG. 2,</dd><dt>FIG</dt><dd>A second embodiment of the novel method,</dd><dt>FIG</dt><dd>A modification of the embodiment of the novel Method according to FIG. 4,</dd><dt>FIG</dt><dd>A second view of the modification of the second embodiment Embodiment of the new method according to FIGS</dd><dt>FIG</dt><dd>Details of a device for a further Modification of the embodiment of the method according to 5 and 6.</dd></dl>

FIG. 1 shows a laser 1, which is a diode-pumped laser diode Tm: YAG solid-state laser, the laser light 2 with Of a wavelength of 2 μm. The laser light 2 is irradiated with Of a lens 3 into an optical fiber 4. The light guide Is a water-free quartz-quartz fiber. The light guide 4 has only a small attenuation for the laser light 2, also With larger fiber lengths. The light from the optical fiber 4 again Emitting laser light 2 is converted by an optical system 5 and Then meets a workpiece 6 made of a thermoplastic material Plastic, for example of a polyolefin. The laser light 2 is welded in a welding region 19 of the workpiece 6 via a Absorbed length of a few mm, ie heat converted.

The heating of a film-like workpiece 7 and of a film-like workpiece 7 lying behind it Further foil-like workpiece 8, Thermoplastic plastic, is used in the arrangement According to FIG. 2, for the flat connection of the workpieces 7 and 8 A weld 9 is used which extends in the heated welding area 19. The welding of the workpieces takes place 7 and 8 are non-contact, ie the workpieces 7 and 8 lie In the region of the weld seam 9 is completely free and at a distance from the Optics 5.

In the embodiment according to FIG . Instead, the light guide 4 lies directly on one Quartz glass plate 10, which comprises a first holding element 11 for the A pair of the workpieces 7 and 8 to be connected to each other Education. On the back of the workpieces 7 and 8 is a Second holding element 12 in the form of a reflector 13 is provided. The laser light 2 emerging from the optical fiber 4 passes through The quartz glass plate 10 is passed through, and is then removed step by step from The workpieces 7 and 8, the workpieces 7 and 8 being absorbed 8 in the welding region 19, ie in the region of the desired welding region Weld 9. The light beam applied to the reflector 13 The rest of the laser light 2 is incident on the workpieces 8 And 7, resulting in uneven distribution Of the heating of the workpieces 7 and 8 via the welding region 19 Due to the progressive absorption at least partly Is compensated. Plasticizing of plastic of workpieces 7 and 8 on the outer surfaces moves in FIG Of the same order of magnitude as the plastication in the area of Mutual arrangement of the workpieces 7 and 8 It, the holding elements 11 and 12 after the execution of the weld seam 9 to move apart very quickly, without the welded connection Again. It is also possible to use the Holding elements 11 and 12 to permanently cool, since the heating Of the workpieces 7 and 8 in the contact region with respect to the holding elements 11 and 12 is unnecessary for the formation of the weld 9.

Fig. 4 shows another embodiment of the welding of Fig Two workpieces 7 and 8 with the laser light 2. The workpieces 7 And 8 of thermoplastic resin are abutted Respectively. The laser light 2 from the optics 5 is thus applied to the laser beam Workpieces 7 and 8, in that both workpieces 7 and 8 are moved over The total depth of the impact 14 and thus the desired depth Weld 9. Up to a material thickness of Workpieces 7 and 8 of a few mm, this is the wavelength Of the laser light 2 of 2 μm.

4, the workpieces 7 and 8 in the region of the Weld seam 9 are not guided or held, according to FIG. 5 a rear support by means of a reflector 13 Is provided. On the entrance side Of the laser light, a quartz glass plate 15 is arranged whose Narrow side facing the holding element 12. The quartz glass plate 15 serves as the light guide 4 for the laser light 2, which is shown here Directly from lasers 1 configured as diode lasers 16 on the Opposite the reflector 13, of the quartz glass plate 15 Into the quartz glass plate 15. The laser light 2 Is guided through total reflection in the quartz glass plate 15, With its intensity distribution over the width of the quartz glass plate 15 is homogenized at the same time, and then meets On the workpieces 7 and 8 in the region of the impact 14, ie in FIG The welding region 19 or the desired weld seam 9.

FIG. 6 shows the arrangement according to FIG. 5 from FIG. 5 Right angle and leaves the majority Of the diode lasers 16.

The contact surface of the quartz plate 15 is shown in FIGS. 5 and 6 On the workpieces 7 and 8. FIG. 7 sketches the case of FIG More complicated course of the desired weld, Opposite the diode laser 16, of the quartz plate 15 has a complicated curve-shaped course. The use of a quartz plate 15 as the light guide is always Then useful when comparatively wide welds are applied And when a plurality of like weld seams Are to be executed. Alternating geometries of the Weld seams and particularly narrow welds are provided with the Arrangement according to FIGS. 1-4.