DE102006008776B4 - Method for joining at least two joining partners consisting of thermoplastic material by means of laser radiation - Google Patents

Abstract

Method for joining at least two joining partners (1, 2) each consisting of thermoplastic material by means of laser radiation (4), in which the laser radiation (4) is focused by at least one joining partner (1) into a volume area, the areas of both joining partners (1, 2 ), which enter into a mutual material connection by means of a material fusion induced by the laser radiation (4), characterized in that the wavelength of the laser radiation (4) is chosen such that the laser radiation (4) passes through the thermoplastic material of both joining partners (1, 2) is partially absorbed, and
the laser beam (4) provides an annular beam cross-section (4 ') at least in the region of the surface of the at least one joining partner (1, 2), so that the laser beam has no or low radiation intensity in the beam center region, and
the annular beam cross-section (4 ') of the laser beam (4) is focused in a point-like manner into the volume region in which both joining partners (1, 2) touch.

Description

Technical area

The invention relates to a method for joining at least two joining partners, each consisting of thermoplastic material, by means of laser radiation, with a laser emitting the laser radiation and at least one element focusing the laser radiation onto a volume area, in which both joining partners have at least sections a common joining plane and both joining partners in the way of a material fusion induced by the laser radiation enter into a mutual substance connection.

State of the art

Plastic welding with laser radiation is a widely used and increasingly well-established joining technique for the proper, precise joining of at least two joining partners consisting of thermoplastic plastic materials. Probably the most important and at the same time most common way of producing a connection between two plastic components with laser radiation is so-called transmission welding, in which laser light penetrates a joining partner transparent to the laser wavelength and is absorbed by a non-transparent joining partner. The thermoplastic material absorbing the laser radiation melts and at the same time the transparent joining partner is also locally melted by the heat occurring in the melting region, as a result of which both components form a material connection or welded joint.

Even in the production of articles on an industrial scale, such as housing parts for electrical switches or similar electrical components, which should have an acceptable, uniform as possible overall optical appearance, the laser beam welding has already successfully introduced into mass production. So goes out of the WO 95/02869 A1 a directed thereto manufacturing process, in which two of thermoplastic workpieces existing material parts are joined together by means of transmission welding. As in the precautions described above, it is the case in each case a workpiece part through which the laser beam passes into the joint joining region from a material with the highest possible transmission coefficient for the laser wavelength to choose, whereas the other workpiece part has the highest possible absorption based on the laser wavelength.

In the case of laser beam transmission welding of at least two joining partners, a main challenge is thus to provide suitably selected pigmentations in the respective joining partners, so that the laser radiation passes through one of the two joining partners without loss and is absorbed by the other joining partner. In the simplest case, this takes place in a transparent-black combination by admixing soot particles in the absorbent joining partner. However, the use of soot particles is neither practicable nor desirable in those cases where the plastic materials have color tints or are made of both sides of transparent plastic material. In these cases, to generate sufficient absorption of the laser wavelength within the respective plastic material, either laser light-absorbing intermediate layers or organic absorbers can be used, which are to be mixed into the joining partner to be absorbed. Such absorbers are usually applied by an additional process step to the surface of the respective joining partner and also influence the color effect of the joining partners, especially in the region of the joining plane, in which both joining partners come into physical contact. In addition, such absorbers are very expensive and in some cases thermally unstable, so they are not suitable for a classic plastic manufacturing process.

With the hitherto customary procedure when carrying out laser welding processes on joining partners made of thermoplastic material, the optical properties of the plastics are adapted to the particular laser beam wavelength used by suitable addition of additives. Since such absorbers usually have a natural color in the visible spectral range, it has therefore until now been unavoidable to have to compromise between the technical design choice and the color scheme of the joining product to be manufactured.

It is therefore desirable to adopt measures or modifications to existing laser beam welding processes, with which it is possible to add at least two joining partners consisting of thermoplastic materials without the use of absorber materials influencing their individual color impression by means of laser beam welding methods. So it should be possible in particular to have two joining partners from optically identically appearing thermoplastic material by means of laser beam welding without absorber-related color deviations, especially in the joining area to obtain.

From the DE 28 21 883 B1 is to take a device for material processing, for example. For the purpose of drilling, welding or punching, in which a laser beam by means of an axicon is expanded annularly and the annular laser beam is directed to the surface of a workpiece to be machined. By acting in the workpiece heat accumulation in the interior, enclosed by the annular laser beam area large welded joints can be produced.

Of the DE 100 04 538 A1 a laser transmission method by plastics for the purpose of mutual welding can be removed, in which a joining partner is as transparent as possible and the other joining partner has the highest possible absorption capacity. In order to avoid that disturbing surface softening occurs, a cooling agent is supplied to the surface of a joining partner.

In the JP 2005 161 620 A a laser welding method for transparent workpieces is described in which a plurality of individual laser beams is directed in a ring-shaped arrangement through a workpiece on a common joining plane of the two workpieces, in which the plurality of laser beams are superimposed.

Presentation of the invention

The invention has for its object to add a method for joining at least two each consisting of thermoplastic material joining partners by means of laser radiation is focused in the laser radiation through at least one joining partner in a volume range, the areas of both joining partners comprises, in the way of a laser radiation Induced material fusion enter into a reciprocal material compound, further develop such that a material fusion in the joining region of both joining partners without color impairment and in particular without the use of the laser wavelength absorbing absorbers should be possible. Furthermore, it is necessary to create the possibility of two joining partners, which consist of optically the same for the laser beam wavelength thermoplastic materials, by means of laser beam welding together.

The solution of the problem underlying the invention is specified in claim 1. The concept of the invention advantageously further features are the subject of the dependent claims and the further description with reference to the exemplary embodiments.

The idea underlying the invention, while avoiding the use of certain incorporated into the joining partners absorber materials assumes that the thermoplastic material of the joining partners is taken as given and is searched on the basis of the optical properties of the thermoplastics for suitable laser beam wavelengths in which the respective plastic material has a finitely large absorption coefficient, through which a suitably selected entry of laser beam energy for the melting process sufficient light absorption occurs. In principle, it does not matter whether or not the thermoplastic plastic materials contain absorption-enhancing pigments or additives. An essential aspect on which the measure according to the invention is based is the requirement that the energy density deposited by the laser beam within the respective joining partner in regions in which no material melting is desired be selected correspondingly low, whereas in the joining plane, along both Joining partners are spatially brought together as close as possible, as high as possible energy density is generated by the joining partners is melted in a localized volume range and enter into an intimate connection with each other after appropriate cooling. In order to ensure this, the laser beam provides, according to the solution, an annular beam cross-section at least in the area of the surface of the at least one joining partner, so that the laser beam has no or low radiation intensity in the beam center region, whereas in the region of the annular beam cross-section of the laser beam an increased radiation intensity prevails However, it is not capable of producing sufficient material heating or heating for a melting process. Along the beam path propagating in the interior of the at least one joining partner, the annular beam cross-section is spot-focused on those in the joining plane located volume region in which a maximum energy density or a marked increase in light intensity to produce a molten material is desirable.

Since at least two joining partners to be joined together are arranged one above the other in order to carry out the laser beam welding in the so-called transmission technique, the laser beam required for the welding process must be passed through at least one of the two joining partners. Due to the coordination of the laser wavelength to a correspondingly selected absorption capacity of the respective joining partner to be irradiated, material heating in the area of the joining partner along the beam passage is unavoidable. The wavelength of the laser radiation is preferably to be selected as a function of the optical absorption capacity of the respective joining partner, so that the degree of absorption is between 5% and 40%, particularly preferably between 10% and 20%. By a suitable choice of the laser beam shape and the associated light intensity distribution along the However, in the surface and volume regions of the irradiated joining partner, in which no material melts are to occur, the absorption of heat by absorption should be just so small that heat energy dissipation through the thermal conductivity inherent in the material can take place into surrounding material regions before material degradation occurs due to softening or melting ,

A laser beam form in the sense of the sense represents an at least partially in the beam cross-section ring-shaped laser beam, which tapers conically in the beam direction and is focused in a point-like volume range. In particular, on the surface of a joining partner, via which such a laser beam is coupled into one of the two joining partners, the beam absorption takes place along annularly formed laser beam cross-sectional planes, in which an absorption-related material heating inevitably takes place. However, heat dissipation takes place in such a beam shape both radially outward, in adjacent to the illuminated ring shape material areas as well as in the direction radially inward, unlit material areas. The heat transport directed radially on both sides within a beam cross-sectional plane can ensure effective cooling of the annularly illuminated material region within the joining partner, so that material softening in these areas can be avoided.

Due to the ring-shaped laser beam which tapers conically towards a point-shaped volume region in the direction of the joining plane, the laser radiation intensity in the region of the joining plane is significantly increased with respect to the radiation intensity at the joining component surface. Moreover, due to the radiation concentration on the point-like volume region, heat removal in the above sense is considerably restricted. resulting in a significant heat accumulation in the focus area, which leads to the local melting of the plastic material together with the residual absorption of the thermoplastic material.

For the technical realization of the approach according to the solution for joining the at least two joining material consisting of thermoplastic material by means of laser radiation, an optical element focusing the laser radiation onto a point-shaped volume area is required, the point-shaped volume area lying in a common joining plane between the two joining partners. To ensure an interaction between the laser radiation and the joining partners in the sense of radiation absorption, a suitable choice of the laser wavelength is to be made in such a way that the laser radiation is partially absorbed when it passes through the respective thermoplastic material of at least one joining partner. Depending on the nature of the thermoplastic materials to be joined, wavelengths are typically to be selected in the range between 400 nm and 2000 nm. Due to the almost arbitrary availability of laser radiation sources, with which the entire spectral range specified above can be covered, individual wavelength adjustments can be made to material-specific absorption bands.

Thus, in addition to the laser beam optically focussing element at least one further optically the laser beam influencing element is provided, through which the laser beam assumes a geometric beam shape and / or a radiation intensity distribution along at least one beam cross-sectional plane, in which the laser beam a central laser beam area and at least one of the central laser beam area peripherally at least partially surrounding laser beam region, wherein the central laser beam region has a lower laser beam intensity than in the peripheral at least partially surrounding laser beam region. Particularly preferred for such an optical element is a rotationally symmetric pyramidal prism, d. H. a so-called axicon prism, through which a laser beam with homogeneous light intensity distribution along its beam cross section is converted into an annular beam cross-sectional shape. Of course, instead of the rotationally symmetric pyramidal prism preferably used above, all alternative optically diffractive or refractive elements are usable, such as lenses, prisms, optical gratings, holographic optics, roof prisms, which can be combined with a symmetrical beam deflector.

In a simplest embodiment, the laser light source used is a diode laser to which an optical fiber connects for further beam guidance, from which a Gaussian light beam usually emerges, ie a light beam with symmetrical intensity increase in the beam center, which in the further beam path by means of a strong focusing lens with high numerical aperture Focusing or focusing a focal point. For purposes of light beam concentration exclusively in a focus area located in the joining plane between two joining partners to be joined together and to avoid excessive absorption-induced material heating, in particular on the material surface and the material region located between the material surface and the joining plane, the focused laser beam with an optical element becomes geometrically such divided, so that the acted upon by the laser beam joint partner is penetrated by a laser beam, whose beam center or beam center has no or only low light output, as a result of which the surface of the joining partner, as described above, is not permanently impaired by heat-induced influence. As already mentioned, all the refractive or diffractive optical elements are suitable for effecting such a beam transformation, the simplest possibility being an axicon prism or a roof prism in combination with a focusing lens and a symmetrical beam deflector, through which the laser beam is first split and then focused again in the appropriate joining partner is directed. The laser element dividing the optical element can be arranged either in the beam path before or after the focusing lens or the laser beam focusing element. Further details may be taken from the description with reference to the following embodiments.

In addition, it has proven to be advantageous to separate the joint partners to be provided at the beginning of the laser beam process along their joint joining plane separated by a small intermediate gap, so that at the thereby forming optical interfaces a significant increase in light intensity due to occurring multiple reflections of the laser radiation at both Interfaces results. The prevailing in this area heat accumulation is able to bring the adjacent joining partners most effective melt, so that the required for the formation of the melt minimum energy input can be reduced, so that by the measure of the gap formation in particular the heat input and thus the thermal load in the can not be reduced to the joining compound contributing material areas of at least one joining partner. The physical union of the joining partners spaced apart by a small intermediate gap along a joining plane takes place through the thermal expansion of the thermoplastic materials by way of the melting process, whereby ultimately the contact of both joining partners and thus the desired welded connection can be produced.

Another possibility for the least possible thermal load of at least one joining partner in the surface region and along the beam region in which no material melt phenomena should occur, represents the pulsed supply of laser energy, in the focus of the laser beam due to a sufficiently high light intensity and existing in this area Heat accumulation is ensured an intended material melting, whereas in the remaining beam range more efficient heat dissipation can be done in not illuminated by the laser beam material areas.

Also, temperature-dependent increases in the absorption of the thermoplastic materials contribute to improving the efficiency of the laser radiation energy deposition in the region of the joining plane with increasing temperatures.

A further advantageous alternative provides an active cooling of the surface of at least one joining partner, via which the laser beam is coupled into the joining partner for the purpose of the welding process. In this case, at least on that upper side over which the laser beam is coupled into the joining partner, a contact pressure tool that is transparent to the laser radiation is applied over the entire area, which has an active cooling device. One possible embodiment of a cooling device is water channels integrated into the pressing tool, through which appropriate cooling water can be passed.

Brief description of the invention

The invention will now be described by way of example without limitation of the general inventive idea by means of embodiments with reference to the drawings. Show it:

1 Schematic representation of a focusing optics for increasing the light intensity in the joining plane of two joining partners while reducing the light intensity at the surface of a joining partner,

2 alternative representation of a focusing optics compared to 1 , such as

3 in representation for cooling the surfaces of two to be joined by laser beam welding joining partner.

Ways to carry out the invention, industrial usability

In 1 Let's assume that the joining partners 1 and 2 along the joining plane 3 touch and with the help of a laser welding process with each other to form a material connection should be available. The joining partners 1 . 2 each consist of similar thermoplastic material, with identical optical properties.

A laser source, not shown, emits a laser beam 4 , which usually has a Gaussian beam profile and also has a wavelength which corresponds to the absorption properties of the thermoplastic material of the joining partners 1 and 2 is adjusted such that the absorption coefficient for the selected wavelength is preferably between 5 and 40 percent, particularly preferably between 10 and 20 percent.

The laser beam 4 meets in the embodiment according to 1 to an optical element influencing the beam shape of the laser beam 5 formed in the form of an axicon prism. The rotationally symmetrical axicon prism 5 is capable of producing the laser beam of uniform cross-section 4 in a laser beam 4 ' to be converted with an annular beam cross section. The ring-shaped laser beam 4 ' includes an inner, non-illuminated area peripherally bounded by an annular beam area in which a much higher light intensity prevails. In the beam direction the axicon prism 5 Following is an optically focusing element in the form of a focusing lens 6 provided that the annular laser beam 4 ' focused. In order to achieve the greatest possible splitting and thus a large-scale distribution on the surface of the joining partner 1 deposited light power, it is an axicon prism 5 to choose with the largest possible pyramidal angle. In order to further through the focusing lens 5 focused annular and conically tapered laser beam on one in the joining plane 3 located focal point or focus area 7 is in the beam direction of the focusing lens 6 a beam reflector 8th in the in 1 illustrated form provided.

With the help of in 1 The optical components shown, the annular laser beam enters with a ring-shaped jet area in the joining partner 1 and focuses in the focal point 7 in such a way that, as explained above, a build-up of heat in this area leads to a local melting of the material, which ultimately results in a substance connection between the two joining partners 1 and 2 results.

The in the area of the surface of the joining partner 1 However, heat deposited by radiation absorption can cause the joining partner 1 neither to bring to the surface nor in surfaces near areas melt, especially since the heat deposited there in adjacent, not acted upon by the laser beam material areas is derived. However, this is due to the high laser beam intensity increase in the focus area 7 due to additional heat accumulation effects and beyond occurring temperature-dependent absorption peaks not the case.

An alternative arrangement of optical elements containing the laser beam 4 can transform in a suitable way, is in 2 shown. Here passes a gaussian beam profile exhibiting laser beam 4 a focus lens 6 and then strikes a prism-like beam splitter 9 with beam reflector integrally combined in the beam direction 10 , This arrangement also leads to a laser beam conically tapered in the beam direction and having an annular beam cross-section, which, as it were, follows the exemplary embodiment 1 the surface of the joining partner 1 happened without Materialerweichungs- or melting phenomena and in the joining plane 3 into a focal point or area 7 is focused in which the thermoplastic materials of the joining partners 1 . 2 be melted locally.

In 3 are two joining partners 1 . 2 shown in a highly schematic manner, along the joining plane 3 include a small gap S. The coupled into the gap S, in the focus area 7 concentrated laser beam 4 experiences between the directly opposite interfaces of the joining partners 1 . 2 Multiple reflections, resulting in an increased heat accumulation effect and the materials of the joining partners 1 . 2 in the focusing area 7 be melted. To avoid surface overheating at the entry surfaces of the joining partners 1 . 2 are pressing tools 11 . 12 provided, on the one hand support the joining process after formation of the melt, on the other hand, the surfaces of the joining partners 1 . 2 to cool. The pressing tools 11 . 12 are from one for the laser radiation 4 made of transmitting material and have cooling channels 13 on, can be funded by the cooling liquid. With regard to the laser beam training, reference is made to the exemplary embodiments 1 and 2 referenced in the embodiment according to 3 by suggesting only a focusing lens 6 is shown very highly schematic.

LIST OF REFERENCE NUMBERS

1, 2

joining partner

3

joining plane

4

laser beam

4 '

annular laser beam

5

axicon

6

focusing lens

7

focus area

8th

beam reflector

9

beamsplitter

10

beam reflector

11, 12

Anpresswerkzeuge

13

cooling channels

Claims (6)

Method for joining at least two joining partners, each consisting of thermoplastic material ( 1 . 2 ) by means of laser radiation ( 4 ), in which the laser radiation ( 4 ) by at least one joining partner ( 1 ) is focused in a volume range, the areas of both joint partners ( 1 . 2 ), which by way of a laser radiation ( 4 ) induced material fusion enter into a mutual substance compound, characterized in that the wavelength of the laser radiation ( 4 ) is selected such that the laser radiation ( 4 ) when passing through the thermoplastic material of both joining partners ( 1 . 2 ) is partially absorbed, and that the laser beam ( 4 ) at least in the region of the surface of the at least one joining partner ( 1 . 2 ) an annular beam cross section ( 4 ' ), so that the laser beam in the beam center region has no or a low radiation intensity, and that the annular beam cross-section ( 4 ' ) of the laser beam ( 4 ) in the volume range in which both joining partners ( 1 . 2 ), is focused point-like. A method according to claim 1, characterized in that the thermoplastic material of the respective joining partner ( 1 . 2 ) absorbs the laser radiation with an absorption factor A, for which applies: 5% ≤ A ≤ 40%, preferably 10% ≤ A ≤ 20%. A method according to claim 1 or 2, characterized in that the laser radiation intensity in the at least partially peripherally surrounding laser beam region is selected such that at least in the region of the surface of the at least one joining partner ( 1 ) no material degradation takes place by way of material softening. Method according to one of claims 1 to 3, characterized in that in which both areas of the joining partner ( 1 . 2 ), the joining partners include a gap (S), and in that the laser beam focused in the volume region encompassing the two regions of the joining partners is located between the interfaces of the joining partners that are located directly opposite the gap (S) 1 . 2 is reflected several times, causing the materials of the joining partners 1 . 2 be locally brought to the melt by means of a heat accumulation effect. A method according to claim 4, characterized in that after formation of the melt both joint partners ( 1 . 2 ) are pressed together. A method according to claim 4 or 5, characterized in that the compression of both joining partners ( 1 . 2 ) by means of cooled, on the surfaces of both joining partners ( 1 . 2 ) auflegbare, pressing tools ( 11 . 12 ) is carried out.

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