HK1070325B - Laser welding method for structured plastics - Google Patents

Description

The present invention relates to a laser jointing process for joining different plastic or plastic workpieces with other materials, where the upper workpiece facing the laser source is made of a material transparent to the laser beam and the second workpiece is made of a material absorbent to the laser beam, so that the adjacent contact surfaces of the two workpieces melt and join together during subsequent pressure cooling.

The method is known, for example, from EP-A1-997 261 where a curtain-like laser beam is directed at the workpieces, the area not to be heated being covered by a mask. The known method combines structured and unstructured workpieces. The two workpieces are to be whitened only at the contact surfaces, so that a corresponding mask is used.

Laser welding of plastics requires a controllable thermal energy dosage, the determining factor for which is the energy density and irradiation duration, which determine the speed of the heating process and the maximum temperature of the plastic.

The surface of the welding surface is welded with the desired welding structures corresponding to the workpiece, using the well-known mask welding principle mentioned above. The desired surface is welded with a curtain-like laser beam, which forms a line on the weld plane, whereby the energy dosage is controlled by the laser power and the grid speed. The spatial distribution of the heat energy is determined by the mask, because in a type of radiation the energy density along the direction of propagation of the light remains almost unchanged.

The use of the mask is sometimes disadvantageous in the known method, since it requires adjustment and, as a result, the throughput of the component is low.

The present invention is therefore intended to propose a method of assembling structured workpieces by means of high-throughput lasers using the process of through-hole welding mentioned at the outset.

This task is solved according to the invention by the method with the characteristics of the main claim.

The optics of the laser beam are focused in such a way that the energy density is maximized in the area of the contact surfaces by means of an optical system according to the present invention. Depending on the application, the optics are selected by selecting the lenses in such a way that the cross-section of the area becomes larger or smaller. By this setting, the material of the second workpiece melts only at the contact surfaces, although the surfaces as well as the deeper contact surfaces are irradiated by the laser beam. This causes the workpieces to be melted and connected only in the area of contact of the largest surfaces.

It is preferable to use a laser beam with a beam belt with a high energy density, since this allows a high energy density only in a small area according to the above specifications.

The laser beam is generated by a cylindrical lens. The linear laser beam is imaged by a cylindrical convergent lens only on the focal plane of the lens. This means that the energy density of the laser beam along the direction of propagation depends on the distance to the laser source. The maximum energy density is in the beam tail. The geometry of this beam tail (height h and width b) depends on the optical aperture and focal length of the lens.

In the case of a structured workpiece, the welding joint shape is determined only by the flat structure on the focal plane if the height difference between the structures is significantly greater than the radius thickness.

The following illustrations of the invention are given in conjunction with the accompanying drawings: Figure 1a basic outline for welding two plastic parts;Figure 2an enlarged representation of a beam waist;Figure 3the basic design with a standard diode laser; andFigure 4the basic design with the laser beam fed through a light conductor.

Figure 1 shows a processed laser beam 1 hitting a cylinder lens 2 in enlarged form. The cylinder lens 2 transforms the laser beam 1 into a laser beam 3 which varies in the direction of propagation in the energy density. This causes the laser beam not to have the same width or diameter in the longitudinal direction, with the areas with high energy density being characterized by a figure with the areas with lower energy density having smaller widths or widths. In the example, the laser beam 3 is shaped so that it emits a laser beam 4 in width. The laser beam 3 is enlarged in Figure 2 in the area of the beam 4 in width.

Figure 1 also shows a first laser-transparent workpiece 5 and a second workpiece 6 below it which is not transparent to laser radiation. The figure shows that workpiece 5 is drawn in a transparent way for illustration purposes, but this does not mean that this workpiece cannot also be coloured. Workpiece 6 has a structure with 7 lower surface areas 8 which are not to be softened by the laser beam.The high energy density area around beam 4 is in the area of the contact surfaces 7 and the formation of the laser beam 3 makes the energy density in the area of the lower surface areas 8 insufficient to melt them. This only results in a softening in the area of the contact surfaces, so that in this area the workpieces 5, 6 are connected in the two known ways.The essential point is that the workpieces 5, 6 and 7 are arranged with their contact surface 7 in the radius 4 range.

Figure 3 shows a schematic representation of a standard diode laser 9 that brings a cone-shaped laser beam 1 to the cylinder lens 2. As described in the context of Figure 1, the cylinder lens 2 transforms the laser beam 1 into the laser beam 3, which is then moved over the area of the surface 10 to be welded, without regard to the lower areas of the surface 8. It is noted that the area of the surface 10 to be welded does not have to coincide with the entire surface of the workpieces. It is also clear that it ultimately depends on the relative motion between the workpieces 5, 6 and the laser beam 3. In the embodiment example, the workpieces 5, 6 are moved.

In the example shown in Figure 4, the laser beam 12 emitted from a light conductor 11 in the shape of a cone is directed through a spherical converged lens 13 to the cylinder lens 2.

In the example, a linear laser beam and the workpieces are moved relative to each other, and a laser point can be created in place of the laser line, which must then be guided accordingly for surface irradiation.

Claims (2)

1. A laser joining method for connecting different workpieces (5, 6) made of plastic or for connecting plastic to different materials, the upper workpiece (5), facing the laser source, consisting of a material which is transparent for the laser beam (3) and the second workpiece (6) consisting of a material which is absorbent for the laser beam (3), so that the mutually adjacent contact surfaces (7) of the two workpieces (5, 6) melt and bond to one another during the subsequent cooling under pressure, at least the second workpiece (6) having a structured surface with lower-lying surface regions (8), facing the other workpiece (5), and the laser beam (3) and the components being moved in relation to one another, characterised in that the laser beam (3) is focused by an optical system in such a way that the energy density is at a maximum in the region of the contact surfaces (7) and the melting of the material of the second workpiece (6) takes place only at the contact surfaces (7), the contact surfaces (7) and the lower-lying surface regions (8) being irradiated by the laser beam and the workpieces (5, 6) being melted and connected to one another only in the region of the contact surfaces (7).
2. The method as claimed in claim 1, characterised in that the height (h) and the width (b) of the beam constriction (4) produced by the focusing is set in accordance with the structure of the workpieces (5, 6) to be welded.