Plastic workpiece and process for producing it

The invention relates to a workpiece, in particular a housing for an electrical switch, according to the preamble of patent claim 1 and a method for welding workpiece parts according to the preamble of patent claim 7.

Plastic housings for electrical switches are used to hold the electrical components, such as fixed contacts, switch contacts, etc., and, if necessary, other components. The housings generally consist of several housing parts with a generally complex spatial shape. The housing parts are assembled during assembly of the switch, a fixed connection between the housing parts often having to be established. This connection must be adapted to the contour of the housing parts and can therefore have a three-dimensional design.

It is known, particularly if the housings are to be hermetically sealed to the outside, to connect the housing parts to one another by means of ultrasonic welding. The disadvantage here is that due to the ultrasonic vibrations, solder connections to the electrical components and the electrical components themselves can be destroyed, which ultimately makes the switch unusable. There is therefore a need for a method for welding the housing parts of electrical switches that does not impair the electrical components.

From DE-OS 36 21 030 it is known to weld plastic films together by the action of laser beams. For this purpose, the plastic foils are laid flat on top of one another. A focused laser beam then passes through the foils, the foils being heated in the joining zone in such a way that they get into a molten state so that they are connected to one another in the joining zone after cooling.

Foils are thin parts, so that heating is possible when the laser beams pass through the entire joining zone. This usually results in an acceptable weld between the foils. In the case of thicker workpieces, such as in the case of electrical switch housings, the known joining method, however, only allows the workpiece to be heated superficially, so that a usable welded connection between two workpiece parts cannot be achieved. In particular, for structural reasons in the case of housing parts of electrical switches, the joining zone is often arranged on the inside of the housing. Such an internal weld connection is obviously not producible with the known joining process.

Furthermore, EP-A2-0 159 169 discloses a method for welding plastic plates by means of laser beams. In this method, a second plate is arranged on a first plate, which is formed by using an additive in the plastic to absorb the laser beam. A black dye, predominantly carbon black, is used as an additive. The plastic of the second plate contains no additives, so that the second plate is largely transparent to the laser beam. A laser beam then acts on the second plate, the laser beam penetrating the second plate and being absorbed in the first plate, so that the adjoining contact surfaces of the two plates melt and combine with one another during the subsequent cooling.

A disadvantage of this process is that the second plate must not contain any additive and is in an undyed, milky-white state, while the first plate is colored with a black dye. The workpieces produced in this way consequently consist of parts with very different colors, so that the overall visual impression is impaired. Particularly in the case of housings for electrical switches, however, it is desired that the entire housing have a homogeneous visual impression, particularly with regard to the color. In addition, the plates shown in the publication are flat structures that are welded together on a flat surface. Housing parts, especially those for electrical switches o. Like., However, usually have complicated geometries, the joining zone of the parts to be connected must follow this geometry and is therefore not limited to one plane. The publication does not provide any suggestion for the production of complex, spatially extending joining zones. The known method is therefore not suitable for welding housing parts, in particular for electrical switches.

Finally, EP-A1-0 483 569 discloses a device for welding a plastic lid to a plastic mug used for holding food by means of laser beams. The lid is placed on a flange-like edge of the plastic cup and is made transparent to the laser beams in this area. Between the lid and the cup there is a layer on the flange-like edge which is absorbent for the laser beams. The energy of the laser beams is converted into heat in this absorbing layer, so that the plastic partially melts in areas of the lid and the cup, the lid and the flange-like edge welding together.

The details of the transparent area and the absorbent layer remain open, so that the disadvantages already described for EP-A2-0 159 169 also occur here. In particular, it should be emphasized that although a plastic cup is a three-dimensional workpiece, the weld seam between the cover and the cup here again runs in a flat or flat edge region. This publication, therefore, does not provide any suggestion for the design and manufacture of a complex workpiece, in particular a spatially extending joining zone.

Further publications, which show details of workpieces made of welded workpiece parts, are described below. What these publications have in common is that their disclosure regarding the use of a workpiece part that is transparent and absorbent for laser beams does not go beyond the disclosure of EP-A1-0 483 569.

GB-A-1 051 397 relates to the welding of two semiconductor wafers by means of optical radiation, the two semiconductor wafers being made of different materials with different absorption coefficients for the radiation.

The optical radiation first penetrates the material with a lower absorption coefficient and melts the two materials at the connection point, so that welding occurs there. The two materials can be pressed together during welding.

US-A-5 279 693 describes a method for welding a Velcro fastener made of thermoplastic material to a thermoplastic web by means of laser beams. The Velcro fastener located between two webs has a lower softening point than the webs, so that the lower energy of the laser beam after crossing a web is still sufficient to heat the Velcro fastener.

FR-A-1 506 163 relates to a method for welding thermoplastic layer materials by means of laser beams. At the junction there is an additional layer consisting of dye, so that there is an increased absorption of the laser beams. As a result, the area adjacent to this layer is heated by thermal conduction, with which the welded connection is produced.

US-A-3 477 194 shows a product packaging consisting of cardboard and thermoplastic cover materials. The cardboard is printed with a radiation-absorbing, for example carbon-containing, ink at the points to be welded. The cardboard is then covered with a first thermoplastic film, the goods are inserted and then covered with a second thermoplastic film.

Irradiated infrared radiation, which is generated, for example, by a laser, is absorbed by the paint and heats the foils, so that the foils are welded together.

DE-U-9 015 782 shows a machine for closing a can with a lid made of sheet metal. The edge of the can is connected to the edge of the lid via a circumferential weld seam on a rabbet-like web. The weld seam is created using a laser beam.

A method for welding thermoplastic materials by means of pressure and heat is known from WO 89/10832. In this method, a tool, which has a transparent area for supplying infrared radiation, is placed under pressure at the point to be welded. A film placed on the materials or located between the materials is designed to absorb radiation, so that the heat generated there heats the materials by heat conduction for the welding.

EP-A1-0 288 884 shows various configurations for a butt weld seam in the laser welding of metal sheets or plastic sheets. In these configurations, the flanks for the butt weld are profiled in a tongue and groove manner with different cross sections.

Finally, US-A-3 769 117 discloses welding a tube head to a plastic tube body by means of laser beams. The tube is rotated during welding, with a roller simultaneously exerting a pressure on the weld.

Starting from EP-A1-0 483 569, the object of the invention is to design a workpiece consisting of a plurality of workpiece parts in such a way that the workpiece parts can be connected to one another by means of laser beams, in particular on an internal, three-dimensionally extending joining zone, and a method for the production to specify such a workpiece.

This object is achieved in a generic workpiece by the characterizing features of claim 1 and in a generic method by the characterizing features of claim 7.

The invention is based on the idea that the laser beams penetrate the first workpiece part closest to the laser beam source essentially unhindered and are largely absorbed in the second workpiece part, thereby melting the two workpiece parts at the joining zone. For this purpose, at least in the areas touched by the laser beams, the first workpiece part is largely transmissive for the laser beams and the second workpiece part is largely absorbent by a corresponding proportion of additives. For this purpose, the first workpiece part has a transmission coefficient that is high compared to the second workpiece part and a low absorption coefficient, ie the first workpiece part has a larger transmission coefficient than the second workpiece part and the second workpiece part has a larger absorption coefficient than the first workpiece part. However, since both workpiece parts contain additives, these are opaque to light rays in the area visible to the human eye, which advantageously results in a substantially homogeneous optical impression. After welding, the individual workpiece parts are essentially no longer distinguishable from the human eye.

Further embodiments of the invention are the subject of the dependent claims.

In particular, the second workpiece part can be designed to be absorbent on the surface in such a way that the laser beams only penetrate a small distance into the second workpiece part. As a result, good welding results are achieved even with small laser powers. The transmission and absorption coefficient for the two workpiece parts, which determines the degree of transmission and absorption, can be set by means of additives in the plastic, for example dye pigments. It has proven to be favorable if the first workpiece part is set to a transmission T greater than 60% and absorption A less than 30%, and the second workpiece part is set to an absorption A greater than 90% and in particular a negligibly low transmission T.

The joining zone can, in particular in the case of workpieces of complex design, run three-dimensionally in space and also inside the workpiece, so that the workpiece parts can be optimally adapted to the respective intended use. In order to achieve a good seal of the interior of the workpiece, the adjoining walls of the workpiece parts can be designed to overlap, a sectoral welding in the joining zone by means of laser beams at the critical areas, such as corners, bushings or the like, being sufficient. The workpiece parts to be connected to one another can have an overlapping joint in a wide variety of configurations in the joining zone.

The quality of the joining zone can be increased by applying pressure to the joining zone until it cools down. The pressure is preferably applied adjacent to the focus of the laser beams, so that the penetration of the laser beams into the workpiece is not hindered. However, it is also conceivable to use transparent clamping means for the laser beam, so that the pressure can also be applied directly in the area of ​​action of the laser beams. The pressure effect is preferably tracked by the movement of the laser beams along the joining zone. This movement of the laser beams can also be carried out in a spatially programmable manner in a simple manner, for example by using a robot or multi-axis handling device. Sectoral welds in particular at individual points on the workpiece can thus be produced in a simple manner. A further optimization of the manufacturing process can be achieved by regulating the operating parameters of the laser beam source in accordance with the measured process parameters, such as pressure and temperature, which prevail in the joining zone.

The advantages achieved by the invention are, in particular, that a high-quality weld which can be overlapping or blunt and in particular can run at least partially inside the workpiece, even in the case of workpieces with large wall thicknesses, when welding by means of laser beams. In contrast to conventional ultrasonic welding, the seam design can be freely adapted to the design requirements, ie it can be arranged anywhere in the housing. The course of the weld seam can be produced by a programmable beam guidance of the laser beams or movement of the workpiece, so that a flexible production of different workpieces is possible. The workpiece parts are only welded by heating them, additional additives are in any case not necessary for the actual welding. Rejects are avoided, particularly in the manufacture of electrical switches, since there is no fear of destruction of the electrical components.

Embodiments of the invention are shown in the drawings and are described in more detail below. Show it<dl id="dl0001" compact="compact"><dt>Fig. 1</dt><dd>schematically a device for welding a housing for an electrical switch,</dd><dt>Fig. 2</dt><dd>a longitudinal section through a housing,</dd><dt>Fig. 3</dt><dd>2 shows an enlarged detail according to detail X from FIG. 1,</dd><dt>Fig. 4</dt><dd>a longitudinal section through part of a housing in a further embodiment,</dd><dt>Fig. 5</dt><dd>2 shows a longitudinal section through part of a housing in yet another embodiment,</dd><dt>Fig. 6</dt><dd>a longitudinal section through the wall of a housing in another embodiment,</dd><dt>Fig. 7</dt><dd>a longitudinal section through the wall of a housing in yet another embodiment,</dd><dt>Fig. 8</dt><dd>a longitudinal section through the wall of a housing in yet another embodiment,</dd><dt>Fig. 9</dt><dd>a cross section through a housing with a complex spatial configuration and</dd><dt>Fig. 10</dt><dd>a section along the line 10-10 of Fig. 9th</dd></dl>

1 schematically shows a device for welding workpiece parts by means of a laser beam in accordance with the method according to the invention. This device consists of a laser beam source 1, for example an Nd: YAG laser, the light exit opening of which is connected via a flexible light guide 2 to an optical system 3, so that the laser beam source 1 can be positioned anywhere in the room. The optics 3 are arranged above a workpiece holder 4, which can be moved by means of drives by a control, for example as indicated in FIG. 1 by means of the directional arrows 5. Of course, the optics 3 can also be designed to be movable.

The workpiece parts to be welded to one another, namely the parts for a housing 6, are expediently arranged on the workpiece holder 4, the housing 6 in the present case being the housing for an electrical switch. The housing 6 consists of two housing parts, namely a cup-shaped housing base 7, which forms the second workpiece part, and a housing cover 8, which represents the first workpiece part. Both the housing socket 7 and the housing cover 8 are made of plastic, preferably a thermoplastic. 2, the housing base 7 has a circumferential shoulder 9, on which the housing cover 8 is placed. At paragraph 9, the housing cover 8 is welded to the housing base 7 by means of laser beams along a continuously circumferential joining zone 10, so that the interior of the housing 6 of the electrical switch is hermetically sealed, in particular is therefore protected against the ingress of dust.

As can further be seen from FIG. 1, the housing cover 8 and the housing base 7 are heated along the joining zone 10 by means of laser beams 11, which are emitted by the optics 3 onto the housing 6, in such a way that the housing base 7 and the housing cover 8 are in the joining zone 10 get into a molten state. This creates a connection between the housing base 7 and the housing cover 8 in the joining zone 10, which solidifies during the subsequent cooling and forms a weld seam. Thereafter, if necessary, thermal annealing can follow in the joining zone 10 in order to reduce welding stresses. By a corresponding movement of the workpiece holder 4 and / or the optics 3 or the laser beam source 1, an all-round elongate joining zone 10 is achieved on the housing 6. If it is sufficient, the housing 6 can also be welded only at individual points, so that a continuous but a section-wise, sectoral joining zone is obtained. Advantageously, lower welding stresses are introduced during the sectoral welding, which may make subsequent annealing superfluous.

It is immediately apparent that a correspondingly movable, three-dimensionally arranged joining zone 10 can also be produced in the housing 6 by corresponding movement of the laser beam source 1 or the optics 3 and the workpiece parts 7, 8 relative to one another. For this purpose, for example, the workpiece holder 4 is designed as a multi-axis handling device that can be moved in three dimensions according to the arrows 5. A robot can also be used for this, or the optics 3 can be moved accordingly. The desired spatial movement of the workpiece holder 4 is programmable, so that the corresponding spatially configured joining zone 10 is then generated during the welding. Advantageously, as a result, it is possible to produce joining zones 10 of complex design in a manner that is easy to handle and flexible, as occurs in particular in housings 6 for electrical switches.

As can be seen particularly in FIG. 2, the joining zone 10 lies in the interior of the housing 6 by design, the joining zone 10 having an overlapping joint due to the shoulder 9. According to a further embodiment, which can be seen in FIG. 4, the housing cover 8 and the housing base 7 are formed with a butt-overlapping joint 17. In yet another embodiment according to FIG. 5 the housing base 7 and the housing cover 8 have a wedge-shaped overlapping joint 18. In these further embodiments as well, the joining zone 10 is at least partially inside the housing 6.

In yet another embodiment, a further shoulder 19 which overlaps like a shoulder is shown in FIG. The workpiece parts 7, 8 each have a shoulder 20, 20 'projecting from the actual wall, the joining zone 10 being arranged at the transition between the projecting steps 20, 20'. The laser beams 11 act approximately perpendicularly on the projecting shoulder 20 on the side opposite the projecting shoulder 20 '. Still further training for shoulder-shaped joints can be seen in FIGS. 7 and 8. 7, the workpiece part 7 is provided with a step-shaped receptacle into which the wall of the workpiece part 8 is inserted. Finally, in FIG. 8, the workpiece part 7 is provided with a step-like receptacle into which the corresponding wall of the workpiece part 8 is inserted according to a shoulder-like joint 22.

Finally, FIG. 10 also shows a groove 23 which overlaps in a groove-like manner, with which, in particular, a good sealing effect in the manner of a labyrinth seal against the ingress of dust into the interior of the housing 6 is achieved. For this purpose, a groove 24 is formed in the wall of the workpiece part 8, in which a corresponding spring 25 engages on the wall of the workpiece part 7. The laser beams 11 strike the surface of the workpiece part 8 approximately perpendicularly in the region of the groove 24, which creates a joining zone 10 between the tongue 25 and the groove 24 on the side facing this surface of the workpiece part 8.

FIG. 9 shows a section through a somewhat more complex workpiece part 6, in which the walls 26 have a three-dimensional spatial contour which is adapted to the respective intended use. Such a complex housing 6 is used, for example, in an electrical switch for electric hand tools and has an internal cavity 27, delimited by the walls 26 of the workpiece part 8, which serves to accommodate the functional parts of the electrical switch. In order to achieve good dust-tightness for the cavity 27, there is a groove 24 in the wall 26, on which a tongue 25 of the further workpiece part 7 engages, as has already been explained with reference to FIG. 10.

As can also be seen in FIG. 9, there are sectoral joining zones 28 in the groove 24, so that the two workpiece parts 7, 8 are welded to one another there at the connecting surfaces lying against one another. The sectoral joining zones 28 have a hook-shaped or zigzag-shaped course, which is generated by a correspondingly programmable, three-dimensional spatial relative movement between the laser beams 11 and the workpiece parts 7, 8. The joining zones 28 are not applied continuously, but only in individual sectors. This is primarily the area of ​​the corners of the walls 26 or of turning points in which the weld seam has an abrupt change in direction. In addition, sectoral joining zones can also be arranged in areas on which bushings o. Like., For example, for shafts, tappets, etc., from the outside in the cavity 27 of the housing 6. Such areas are particularly critical for tightness and are securely sealed by the sectoral joining zones 28.

Only straight-line surfaces run between the sectors of the joining zones 28, which can be sealed with less effort. It is sufficient here for the non-welded overlap between the grooves 24 and the spring 25 on the walls 26 of the workpiece parts 7, 8, with which they act as sealing grooves. To this end, snap connections 29 or the like can also be used. be attached between the sectors of the joining zones 28, with the aid of which the connecting surfaces on the walls 26 of the two workpiece parts 7, 8 are pressed onto one another in a sealing manner in these regions. Since only sectoral joining zones 28 are arranged in such a case, time is advantageously obtained when welding the workpiece parts 7, 8.

In order to achieve sufficient heating of the joining zone 10, the housing base 7 and the housing cover 8 have different properties, at least in partial areas, for the spectrum of the laser beams 11. Thus, the transmission coefficient for the laser beams 11 in the housing cover 8 is greater than that in the housing base 7 while, conversely, the absorption coefficient in the housing base 7 is larger than in the housing cover 8. The two workpiece parts consisting of the housing base 7 and the housing cover 8 and the optics 3 of the laser beam source 1 are now positioned relative to one another such that the laser beams 11 strike the housing cover 8 in a first coupling zone 12 on the side facing away from the joining zone 10, as shown in FIG. 3 can be seen. The first workpiece part, namely the housing cover 8, is at least partially transmissive for the spectrum of the laser beams 11 due to its large transmission coefficient in the range from the first coupling zone 12 to the joining zone 10. As a result, at least some of the laser beams 11 can penetrate the housing cover 8. This part of the laser beams 11 then penetrates a second coupling zone 13, which is shown in FIG. 3rd is indicated schematically in the second workpiece part, namely the housing base 7. As can be seen, the first and second coupling zones 12, 13 lie essentially on the straight line formed by the laser beams 11. In particular, this straight line is approximately perpendicular to the coupling zones 12, 13.

The housing base 7 is at least partially absorbent in the area of ​​the joining zone 10 on the second coupling zone 13 for the spectrum of the laser beams 11 due to its large absorption coefficient, so that at least a part of these laser beams 11 incident on the second coupling zone 13 in an absorption zone 14 the housing base 7 warmed up. It may be sufficient if the housing base 7 is designed to be absorbent on the surface of the second coupling zone 13 or the wavelength of the laser beams 11 used is chosen accordingly such that the laser beams 11 penetrate the housing base 7 only a short distance, preferably a few μm, whereby the absorption zone 14 is also only formed superficially in the housing base 7, as indicated in FIG. 3. Due to heat conduction from the absorption zone 14 into the adjacent housing cover 8, the latter is also heated in a heat conduction zone 15. As a result of the heating in the absorption zone 14 and the heat conduction zone 15, the plastic becomes molten, so that after cooling, the joint solidified joining zone 10 forms in these two zones 14, 15.

As already mentioned, the housing 6 consists of a plastic. The use of styrene-acrylonitrile copolymer, polyamide or the like has proven to be particularly suitable for the housing of an electrical switch. To adjust the partial transparency of the housing cover 8 and the partial absorption of the housing base 7 for the spectrum of the laser beams 11, that is to say the transmission and absorption coefficient, additives used in the plastic, such as color pigments, glass fibers or the like, are used as additives. The corresponding setting is made by a different proportion of the additives in the two workpiece parts 7, 8, the proportions being chosen so that the reflectivity of the two workpiece parts 7, 8 is essentially the same for the spectrum of the visible light rays. In particular, it has been found to be advantageous to achieve the high absorption coefficient for the housing base 7 by means of black dye pigments. Corresponding tests have shown that it is advantageous to pigment the partially absorbent plastic of the housing base 7 by 1% to 2% dyes and to reduce the pigmentation of the plastic of the partially transmitting housing cover 8 for the plastic types mentioned. The two workpiece parts 7, 8 are then essentially opaque for the light spectrum visible to the human eye and colored approximately uniformly black. The two workpiece parts 7, 8 in the housing 6 then advantageously offer an optically homogeneous impression, so that the different setting of the respective plastic is imperceptible to the eye.

Experiments using an Nd: YAG laser of 10 W power, which generates laser beams with a wavelength of approx. 1.06 µm, which is particularly suitable for the types of plastic mentioned, and a feed speed of the housing 6 relative to the optics 3 of 3 m / min have shown that good results can be achieved for the welding by observing the following parameters. The housing cover 8 should have a transmission T of greater than 60% and an absorption A of less than 30% and possibly a reflectivity R of less than 20% for the spectrum of Nd: YAG laser beams. The housing base 7 should furthermore have an absorption A of greater than 90% and in particular a negligible transmission T and possibly a reflectivity R of less than 10%. The percentages are each related to the laser beams 11 incident in the coupling zone 12, 13. The following applies in each individual case<maths id="math0001" num=""><math display="block"><mrow><mtext>T + A + R = 100%.</mtext></mrow></math><img file="EP0751865B2_D0001.tif" /></maths>

It should also be emphasized that the portion of the laser beams 11 reflected at the respective coupling zones 12, 13 makes little contribution to the heating in the joining zone 10 and should therefore be kept as small as possible by appropriate adjustment of the plastic.

In order to improve the quality of the welding, a pressure effect in the area of ​​the joining zone 10 can be carried out by the laser beams 11 during or after the heating and melting of the joining zone 10. 1 shows a pressure roller 16 which acts on the top of the housing cover 8 in the region of the joining zone 10. This pressure roller 16 tracks the laser beams 11 in accordance with the feed movement of the workpiece holder 4 according to the directional arrow 5 '. The pressure of the pressure roller 16 thus acts until the joining zone 10 cools, thus preventing the melt from escaping from the joining zone 10 and eliminating the risk of void formation in the joining zone 10. It can be seen from FIG. 1 that this pressure action takes place adjacent to the focus of the laser beams 11, so that the laser beams 11 are undisturbed in the coupling zone 12 on the housing cover 8. Of course, instead of a pressure roller 16, hydraulic, pneumatic or the like hold-down devices, not known per se, can also be used, which can be arranged outside the guidance of the laser beams 11 due to the freely orientable beam guidance. If these hold-downs consist of a material that is largely transparent to the spectrum of laser beams 11, these hold-downs can also be located in the guidance of the laser beams.

The quality of the welding can be further influenced positively by continuously regulating the operating parameters of the laser beam source 1 as a function of the process parameters prevailing in the joining zone 10. These process parameters are temperature, pressure and. Like .. They are measured continuously and, in the event of deviations from the target values, the operating parameters of the laser beam source 1 are changed accordingly until the desired target values ​​are reached again.

The invention is not restricted to the exemplary embodiments described and illustrated. Thus, the invention can be used not only in the manufacture of electrical switches, but can also be used on any workpieces, housings or the like made of plastic, for example for household appliances, packaging, etc.