DE4413158A1 - Removal of paint or plastics coating from large surfaces - Google Patents

Abstract

The appts. to remove a coating from large surfaces, such as paint or plastics and esp. PTFE, uses a TEA-CO2 laser with a high Fresnel count and a flexible opto-mechanical linkage arm (1,2,3,4,6) to guide the laser beam. The arm linkage assembly has at least two beam tubes (1,2) where one arm (1) has a linkage and rotary movement at the laser (5) while the other has a similar movement with the working head. The connection between them also gives a linkage and rotary movement between the tubes.

Description

The invention relates to a device for contactless Removing a layer of paint or plastic, for example Polytetrafluoroethylene from one component as well as for cleaning and Decontaminate one with paint or polytetrafluoroethylene coated surface by means of a pulsed laser beam using a TEA-CO₂ laser as a laser beam source, with the laser beam reflecting and focusing optical devices is performed.

The abbreviation TEA stands for transversely excited atmospheric pressure.

The paint systems currently used are characterized by high Efficiency and effective protection that  based on extreme adhesiveness and media resistance. Such However, properties increasingly lead to problems with Stripping paint coated with paint, for example PTFE Components. At certain time intervals, the paint is stripped of Components, for example in aircraft construction with the aim of Airplane, for example, for hairline cracks and the like to be able to examine. On top of that, too resistant coatings are subject to wear are. This makes it necessary to apply the coating at intervals to renew after first the previous one Coating including one underneath Adhesion promoter removed and the exposed surface of the Component for the application of a new coating has been ridden.

EP 0 391 113 A2 describes a process for stripping paint Workpieces, in particular fiber composite workpieces known. With this method for non-contact large-area Stripping paint layers, especially on The surface to be stripped is made of fiber composite materials by supplying laser radiation, preferably from one pulsed laser heated so quickly that the irradiated area evaporates faster than absorbed energy in deeper layers diffuses. Here, the light emissions of the outflowing Plasmas for regulating the paint stripping depth in particular Multi-layer systems used. Alternatively, this can also be done generated noise to regulate the paint stripping depth be exploited.

In another area of technology, namely in the area of Surface hardening of metals are known arrangements solve similar tasks, d. H. a relatively large stain generate with which the workpiece is covered completely can to the desired surface finishing to reach.

Set up arrangements corresponding to the state of the art for this, laser systems for cutting or welding tasks  are constructed. The beam cross section of such a laser is usually round and therefore not well suited to one Work area without gaps with as even as possible To cover energy density.

In addition, such lasers are designed so that they emit approximately Gausiform beam profile, which looks good lets focus. Such an intensity profile increases the Issues addressed with even coverage and is not to be set up with any of those in this context Claims compatible.

So far, the problem of stripping, cleaning or Decontamination usually so that using an integrator beam profile that was as homogeneous as possible was generated. Such Integrators consist of several on a curved Surface mounted flat mirrors together.

Among other things, a system of this type has the unpleasant one Property, only at a certain working distance, namely the image distance of the curved surface, the desired to produce homogeneous intensity distribution. The bridging Greater distances can then no longer be used with flat mirrors happen, but must be made via imaging optics become.

For the design of a paint stripping system

• a) optimized illumination by a homogeneous Laser spot,
• b) Compensation of distance fluctuations by large Depth of field,
• c) rapid precise positioning of the laser spot and economical working mode,
• d) beam guidance system for the transport of laser energy and
• e) safe and effective handling of gases and dusts

of importance.  

For an optimal work result it is necessary to use the laser beam so that

• f) the greatest possible removal rate (area per unit of time),
• g) removal as uniform as possible over the work surface arises and
• h) no structures remain in the removed material.

Accordingly, a laser spot is to be generated on the workpiece preferably has a rectangular geometry and a box-shaped one Shows intensity profile.

Known approaches to surface machining use faceted optics, so-called beam integrators, which split the incoming beam into many partial beams and overlay in such a way that a homogeneous beam profile in the image plane arises.

The fundamental problems of beam integrators exist in fol the following:

• 1) The superposition of many partial beams results in a low one Depth of field even if the incoming beam is a allows great depth of field.
• 2) Step through the many edges in the beam Diffraction structures, so-called "hot spots".
• 3) The beam coming from the laser must normally show circular symmetry.

The problem of diffraction structures is with the known Lö solutions are often mitigated by using the beam of a vibrating mirror while machining around its The desired position can tremble and the structures in this way diminish different. This solution cannot be used with pulsed laser systems insert the pulses in the microsecond range.

The invention has for its object a device introductory type that enables the To keep the spot area of a laser spot constant and a homogeneous intensity distribution over the laser spot ensure as well as the focal length of one of the focusing Change the mirror in the processing head so that the desired laser fluence, namely the required laser energy density is achieved.

This object is achieved in that a TEA-CO₂ laser high Fresnel number is provided, for which Guiding the laser beam is a flexible opto-mechanical joint arm is provided, which consists of at least two jet pipes exists, of which the one jet pipe is articulated and rotatable with the laser beam source and the other Jet pipe articulated and rotatable with one Machining head is connected and both beam pipes are articulated and are rotatably connected to each other.

In this way one arrives at a device with which it it is possible to keep the laser spot constant and the to achieve the desired laser fluence.

It is also recommended to keep the stain area constant to use adaptive optics whose radius of curvature is expediently adjustable via piezo setting elements. That leads to the further advantage that the focal length is the working distance adjusted and thus the stain area can be kept constant can.

To keep the energy on the workpiece constant the laser beam is dense over a larger area advantageously differently focused in two planes. Due to the different focusing of the laser beam in two Levels, the energy density of the workpiece is effective Laser beam over a larger area of this Workpiece area kept constant.  

To keep the fluence or the laser energy density constant the workpiece to be machined when using a Articulated arm always maintained the same optical beam length.

In a further embodiment of the invention, the intensity profile of the laser beam should be approximately box-shaped. Such one Intensity profile is optimal for stripping.

It is also recommended that the beam cross-section be rectangular or keep square.

Finally, the beam profile of the laser beam is usual with focusing optics on the surface to be treated map.

Due to the superimposition of very many fashions, such shows TEA-CO₂ laser high Fresnel number but just a satisfactory homogeneous beam profile. In addition, for example at stripping generally several laser pulses per Area element are required. With a suitable beam offset successive pulses can then be integrated Create effect.

A beam profile with a high proportion of transverse modes applies commonly called "bad" because of the additional fashions at one Smearing the beam profile on the time scale of the light frequency to lead. Fixed diffraction structures like those used up to now used systems occur, are therefore strongly suppressed.

The approximately rectangular cross section of such an arrangement together with the box-shaped intensity profile a particularly effective and economical way of working because here just stain can be placed next to stain and one complete coverage of the machining area is achieved becomes.

The invention is described below with reference to a drawing illustrated embodiment of the closer explained. It shows

Fig. 1 is a perspective view of an articulated arm,

Fig. 2 is a side view of the articulated arm according to Fig. 1 in the rest position,

Fig. 3 is a schematic representation of an inclined surface to be machined with adaptive optics,

Fig. 4 a focusing concave mirror with beam guide, the beam source is a laser of high Fresnel number, and

Fig. 5 different focus in two planes of the starting spot.

The articulated arm system shown in FIG. 1 consists of a first jet pipe 1 and a second jet pipe 2 , which are connected to one another via a central joint 3 and are articulated to a laser beam source 5 via a joint 4 near the laser and to a work head via a joint 6 close to the working head .

A first 90 ° mirror deflection unit 8 is connected via a fastening supply flange 9 with a rotating unit 10 to the laser beam source 5 . With the deflection unit 8 , a second 90 ° mirror deflection unit 12 is rotatably attached via a further rotary unit 11 , from which the jet pipe 1 is arranged via a further rotary unit 13 . At the upper end 14 of the jet pipe 1 , a third 90 ° play gel deflection unit is provided which is rotatably connected to a fourth 90 ° mirror deflection unit 17 via a further rotary unit 16 . The latter ( 17 ) is connected to the upper end 18 of the second jet pipe, which is rotatably connected to a fifth 90 ° mirror deflection unit 20 via a further rotating unit. The deflection unit 20 is rotatably connected via a rotating unit 21 to a sixth 90 ° mirror deflecting unit 22 , with which a laser processing head is provided via a rotating unit 23 and a flange 24 .

All deflection units are above the specified components freely rotatable with each other and articulated with each other connected so that with that shown in the drawing Articulated arm any movements within one spherical space can be exercised.

The laser beam 25 emerging from the laser 5 initially has the direction of the arrow 26 and is reflected in the deflection unit 8 by 90 ° and in the deflection unit 12 by a further 90 °, so that the laser beam in the beam tube 1 points in the direction of the arrow 27 . In the middle joint 3 , the laser beam is reflected in the deflection unit 15 by 90 ° and in the adjacent deflection unit 17 also by 90 °, so that it has the direction of arrow 28 in the beam pipe 2 . In the joint 6 , the laser beam is reflected at the deflection unit 20 by 90 ° and finally at the deflection unit 22 again by 90 °, so that it points in the direction of arrow 29 and enters the machining head in this direction.

In Fig. 3 is shown to be processed surface 30 against which a mirror is illustrated 31 having a variable curvature 32, wherein the curvature is formed so as to be processed surface 30 of the desired fluence, is therefore achieved the required laser energy density on the. The adaptive optics make it possible, regardless of the inclined position of the surface 30 to be machined, to represent the essentially parallel laser beam incident in the direction of the arrows 33 , 34 as a spot 37 or illuminated surface element 37 . This ensures that regardless of the inclined position of the surface 30 to be processed, the fluence of the stain 37 remains largely constant.

With reference to FIG. 4 is a schematic and simplified the Fokussie tion with a concave mirror 31 illustrates the case that the radiation comes from a laser of high Fresnel number. The laser beam falls on the curved surface 32 within the beams 33 , 34 which run essentially parallel to one another and is reflected by the latter in a region delimited by the beams 38 , 39 in the direction of the arrows 35 , 36 and forms in the region 40 a homogeneous intensity distribution of the reflected laser beam.

FIG. 5 shows how a square starting spot 41 can be changed to a rectangle 44 and a rectangle 45 by different focusing in the x-direction 42 and in the y-direction 43 , the areas of rectangle 44 and rectangle 45 are the same size. As a result, the intensity distribution can be kept constant over a longer distance in the z direction than in the case when the focusing in the y direction 43 would coincide with that in the x direction 42 .

Claims (8)

1. Device for the contactless removal of a layer of paint or plastic, for example polytetrafluoroethylene from a component and for cleaning and decontaminating a surface coated with paint or polytetrafluoroethylene by means of a pulsed laser beam using a TEA-CO₂ laser as a laser beam source, the laser beam being specular and focusing optical devices, characterized in that a TEA-CO₂ laser with a high Fresnel number is provided, a flexible opto-mechanical articulated arm ( 1 , 2 , 3 , 4 , 6 ) being provided for guiding the laser beam ( 25 ) which consists of at least two jet pipes (1, 2), one of which is a jet pipe (1) is articulated and rotatably articulated and rotationally movable with the laser beam source (5) and the other jet tube (2) with a machining head as well as both radiant tubes (1 , 2 ) are articulated and rotatably connected to each other. 2. Device according to claim 1, characterized in that adaptive optics ( 31 ) with variable radius of curvature ( 32 ) are provided for keeping the spot area constant. 3. Apparatus according to claim 2, characterized in that the radius of curvature ( 32 ) of the adaptive optics ( 31 ) is adjustable via piezo actuators. 4. Device according to one of claims 1 to 3, characterized in that to keep constant on the workpiece ( 30 ') effective energy density over a larger area of the laser beam ( 25 ) is differently focused in two planes. 5. Device according to one of the preceding claims, characterized in that the focal length of the focusing optics in two mutually perpendicular planes is different in size. 6. Device according to one of the preceding claims, characterized in that in order to keep the fluence or the laser energy density constant on the workpiece to be machined ( 30 '), essentially the same optical beam length is always maintained. 7. Device according to one of the preceding claims, characterized in that the intensity profile of the laser beam ( 25 ) is approximately box-shaped. 8. Device according to one of the preceding claims, characterized in that the beam cross section is rectangular or is square.