WO2000001497A1

WO2000001497A1 - Method for removing a plastic material adhering to a surface and device for carrying out said method

Info

Publication number: WO2000001497A1
Application number: PCT/EP1999/004618
Authority: WO
Inventors: Gerhard Müller; Georg Bostanjoglo
Original assignee: Laser- Und Medizin-Technologie Ggmbh Berlin
Priority date: 1998-07-04
Filing date: 1999-07-02
Publication date: 2000-01-13
Also published as: AU4905899A; DE19830072A1

Abstract

The invention relates to a method and a device for removing a plastic material adhering to a surface and to a device for carrying out said method. According to the method the surface is exposed to at least one laser pulse having a duration ti≤200 ns. For ionization of the adhering plastic material the surface is exposed to an irradiation of at least 0.02 J/cm2.

Description

Process for removing plastic material adhering to a surface and device for carrying out the process

The invention relates to a method for removing plastic material adhering to a surface, in which the surface is irradiated with at least one laser light pulse, and to a device for carrying out the method with at least one laser for generating laser light pulses.

The document US-A 5 373 140 discloses such a method, which is used for cleaning molds in which molded parts are produced from plastic material. When a molded part is removed from a casting mold, particles or thin layers of the casting material generally adhere to the surfaces of the casting mold. In the method mentioned, they are removed by irradiation with pulsed laser light. The light striking the surface is absorbed by the material residues typically made of synthetic rubber or similar thermoplastic plastics. The amounts of light radiated in this known method are so high that the material residues are heated by absorption of the radiation and then evaporate or, due to explosive evaporation, heated solvent inclusions are blasted off the surface. It is a disadvantage in doing so that in this known method a relatively high heat input into the upper surface also results in proper casting. In the evaporation process, the organic plastic material can become charred. The resulting carbonized fragments absorb the incident laser radiation to an increased extent and lead to local overheating, which can lead to the burning of residual constituents of the char products into the surface of the casting mold.

Modern molds and injection molds for the production of molded parts from thermoplastic are usually provided on their surfaces with a thin and hard non-stick coating, which typically consists of titanium alloys. As a result of the high process temperatures in the production of the plastic molded parts, titanium dioxide forms on the surface of such coatings, which forms a thin, crystalline surface film either as anatase or rutile. This reduces the effectiveness of the non-stick coating, but cannot be removed by the method known from US Pat. No. 5,373,140 together with the adhering plastic without damaging the coating layer underneath.

The object of the invention is therefore to develop a method of the type mentioned at the outset and an apparatus for carrying out the method in such a way that thermal damage to the irradiated surface is avoided.

The object is achieved in the method of the type mentioned at the outset in that the laser light pulse has a pulse duration t _j 200 200 ns and in that the surface for ionizing the adhering plastic material has a

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Exposure to at least 0.02 J / cm is exposed.

The object is further achieved by a device for carrying out the method according to the invention, which has at least one laser for generating laser light pulses and in which means for generating an irradiation of at least 0.02 J / cm on the surface by means of laser light pulses with a pulse duration t _j <200 ns are provided.

According to the invention, the surface with the plastic material adhering to it is irradiated by the laser light pulse over a period of at most 200 ns. The laser must have such a high power that the surface is exposed to radiation of at least 0.02 J / cm. Because of this extremely short selected radiation duration with a correspondingly high pulse energy is achieved that electrons are released from the plastic material due to absorption of the radiation, so that the irradiated areas of the plastic material are ionized and spontaneously flake off from the surface due to electrostatic repulsion. Depending on the material, irradiation with only one or a few laser light pulses is sufficient to achieve the desired cleaning effect. As a rule, a pulse repetition rate of the laser in the range up to a maximum of 20 Hz is used. The contaminated surface is per laser light pulse

9 exposed to a material-dependent radiation of at least 0.02 J / cm and at most 60 J / cm ² . Under these boundary conditions according to the invention, the irradiated plastic material will not essentially evaporate, but will be ionized as a result of a photo effect, ie the release of electrons bound in the material by light, and spontaneously blasted off from the surface to be cleaned by the electrostatic repulsion generated thereby. Thermal excitations of the plastic material as a result of the radiation, that is to say molecular vibrations, do not reach the amplitude required for the material to evaporate due to the short pulse duration. The irradiated surface of the mold does not experience a harmful temperature increase.

By using the method according to the invention for cleaning contaminated surfaces, the evaporation of the adhering plastic material is essentially avoided. This prevents charring and subsequent burning of material residues, which means that the surface can be cleaned quickly and without affecting its properties. The method according to the invention is preferably used for cleaning the inner surfaces of molds or injection molds in which plastic molded parts are produced. Since the method can be used directly at the place of use of the molds, it is not necessary to first uninstall the treated surfaces of a casting mold and then to adapt them to the required production parameters, for example a certain process temperature, after cleaning.

The process according to the invention has the further advantage that the formation of toxic gases is significantly reduced by the evaporation of plastic materials.

In preferred exemplary embodiments of the method according to the invention, surfaces are cleaned which consist of a metal, a plastic or a mineral see material are made. In particular, the method according to the invention is also suitable for cleaning surfaces which are coated with a coating layer, for example a titanium alloy. These are cleaned without damage, while an anatase or rutile film is removed.

In a preferred embodiment of the invention, the pulse duration t _{j of} the laser light pulses is less than 100 ns, in particular between five and twenty ns. This extremely short exposure time ensures that the energy required for ionization is only implemented on the surface for the removal of plastic material.

Laser light with a wavelength between 200 and 3000 nm is advantageously used. With waves longer than 3000 nm, the probability of ionization decreases, so that undesired thermal ablation processes become more important. When using laser light with a wavelength of less than 200 nm, on the other hand, the deflection of the laser light onto the surface to be cleaned can only be accomplished with undesirable loss of intensity due to the absorption properties of common optical components.

In a further exemplary embodiment of the invention, the surface is exposed to a radiation of more than 2 J / cm per laser light pulse. To this

In this way, the number of laser light pulses with which each surface section must be irradiated in order to achieve complete cleaning can be kept low.

Preferably, the radiation to which the surface is exposed is maintained at a value less than 60 J / cm. This largely avoids thermal detachment processes.

In a development of the method according to the invention, the plastic particles detached by radiation are sucked off. This prevents such particles from collecting in possibly inaccessible sections of a casting mold and thereby making time-consuming post-cleaning necessary.

In a further advantageous embodiment of the invention, a surface section with a certain extent is irradiated for each light pulse and the the entire surface is gradually cleaned in a grid pattern. Surface sections irradiated by successive light pulses can also overlap, ensuring that all surface sections are irradiated with the same frequency. Such a method forms the basis for a fully automatic cleaning of soiled molds and injection molds, for example with the aid of a robot or robot arm.

To generate the required energy and power of the laser light pulses, an optical amplifier connected downstream of the laser is preferably used, at the output of which laser light pulses with a pulse duration t _j of less than 200 ns and a pulse energy of at least 100 and at most 3000 mJ, preferably however, leak more than 1000 mJ. The generation of such high pulse energies by the optical amplifier has the advantage that the laser light beam can then be expanded, so that with each laser light pulse a comparatively large surface section of the invention

9 required high radiation of at least 0.02J / cm can be exposed. The area irradiated with a laser light pulse varies depending on the spatial shape of the surface to be cleaned. Depending on the radius of curvature or structures formed on the surface, such as grooves or webs, the irradiated area can be between 5 mm and 5 cm. This ensures that one per pulse

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Energy of at least 0.02 J / crrr strikes the surface with which the desired effect, namely spontaneous ionization of the plastic particles without disadvantageous heating of the surface, is achieved.

The use of an optical amplifier connected downstream of the laser has the further advantage that the amplified laser beam has a very homogeneous intensity profile.

In a preferred embodiment of the device according to the invention, the laser is a neodymium: yttrium aluminum garnet (Nd: YAG) laser, which is Q-switched to generate light pulses with a duration of at least 5 and at most 20 ns. In this embodiment, an Nd: YAG amplifier is used as the downstream optical amplifier. This solid-state laser system operating at a wavelength of 1064 nm has the advantage over other intensive laser light sources such as excimer lasers that it can be operated with particularly little maintenance and can be manufactured in very compact and also portable embodiments. Nd: YAG laser and amplifier rods with a diameter of up to 25 mm are now also available. This has the advantage that the saturation of the amplifier is reduced while maintaining the pulse energy gain.

To expand the laser light beam to a predetermined extent transversely to its direction of propagation, in a further exemplary embodiment a beam shaping assembly formed from suitably arranged optical lenses is provided. This can be designed in particular as a zoom lens, as a result of which a radiation value adapted to the respective application conditions can be set in a particularly simple manner.

According to the invention, in a further exemplary embodiment, the sensitive optical systems of the device according to the invention are protected from mechanical damage by the explosively flaking material particles by means of a protective unit provided between the beam shaping assembly and the surface to be cleaned. The protective unit has an aperture facing the surface with an opening for the passage of the laser beam and a protective glass facing the beam shaping assembly, which extends transversely to the direction of propagation of the laser beam approximately over the extent of the beam shaping assembly. The protective glass can be anti-reflective with an appropriate coating in order to keep the reflection losses as low as possible. The cover serves to protect the protective glass, since this could quickly degrade due to the impact of detached particles.

Further advantages and features of the invention will become apparent in the following description of a preferred embodiment of the invention with reference to the drawing. In it show:

Fig. 1 shows a portable cleaning device in a schematic, open side view

Fig. 2 is a longitudinal sectional view of a protective unit used in the embodiment of FIG. 1.

FIG. 1 shows a greatly simplified, open side view of a portable cleaning device 10, in the housing 12 of which a Q-switched Nd: YAG laser 14 is fastened. For the sake of simplicity, these, like the other components of the cleaning device 10, are only shown as blocks in FIG. 1. The Nd: YAG laser 14 is connected via a supply cable 1 6 to an outside of the housing arranged and not shown here supply unit for electrical power and cooling water connected. In an alternative embodiment with lower performance requirements for minor post-cleaning work, an air-cooled laser can also be used.

A laser beam 18, characterized by cross hatching and emerging from the laser 14, is directed via two plane mirrors 20 and 22 to the input of an Nd: YAG amplifier. The folding of the beam path by 180 ° within the cleaning device 10, which is effected with the aid of the mirrors 20 and 22, has the advantage that its housing can be made compact. The Nd: YAG amplifier 24 is connected via a supply cable 26 to a supply unit, also not shown, for supply with cooling water and electrical power. For example, an Nd: YAG amplifier rod is used which has a larger diameter than the laser rod of the Nd: YAG laser 14. This enables an intense laser beam with a large diameter to be generated. For this purpose, the mirrors 20 and 22 can also be designed such that they expand the beam 18 emerging from the Nd: YAG laser 14 into a parallel light beam with a larger diameter. The essential shaping of the beam cross section, however, takes place in a beam shaping unit 28 connected downstream of the Nd: YAG amplifier 24. This is designed as a zoom lens and enables the beam diameter to be continuously adjusted over a wide range. Alternatively, the beam shaping unit can be implemented as a lens or lens systems, which are selected for the preparation of a particular processing task and inserted into the beam path.

After leaving the beam shaping unit 28, the laser beam 18 enters a protective unit 30, which protects the upstream optical components as well as the laser 14 and the amplifier 25 from removed particles and environmental influences. The protection unit 30 is described in detail below with reference to FIG. 2. The end of the cleaning device 10 towards the contaminated surface is formed by an end piece 32. This can be removed and replaced in order to facilitate use on differently shaped surfaces on which the laser beam 18 strikes after leaving the end piece 32.

The cleaning device 10 with the end piece 21 can be placed directly on the surface to be cleaned. Operation at a distance from the surface is basically also possible. FIG. 2 shows a longitudinal sectional view of the protective unit 30. This connects the beam shaping unit 28 with its figure 2 to the right and the end of the device. The laser beam enters the protective unit through an anti-reflective protective glass 34 and then passes through an aperture 36. The space delimited by the protective glass 32 and the aperture 36 is flushed with air through an opening 38 let into the top of the housing 12. The underpressure required for sucking in ambient air through the opening 38 is produced by a suction pump (not shown), and also the plastic particles entering the end piece 32 from the surface to be cleaned in the direction of the screen through an opening 40 and a suction pipe 42 from the cleaning device 10 leads out and, together with any vapors that arise, leads as exhaust air to a filter (also not shown). The end piece is designed so that there is a sufficiently strong draft of air to extract the ablated particles that the proportion of laser radiation which - apart from the radiation used for cleaning the surface - emerges from the housing 1 2 to protect the operating personnel is as small as possible and at the same time an observation of the processing location is made possible. The diaphragm 36 and the air flow symbolized in FIG. 2 by an arrow 44 branched at the base largely prevent detached particles from striking the protective glass 34. A cleaning system shown in FIGS. 1 and 2 can be used to generate beam diameters of more than 5 mm on surfaces.

The cleaning device 1 0 can be manufactured in different versions according to the respective requirements. For example, the housing can be provided with connections for a shoulder, pelvic or other belt, so that the device can be carried comfortably when used. Fastening options for a tripod or a robot arm can also be provided. Finally, the cleaning device 10 can also be made mobile.

Parts of the structure that provide stability can be made from carbon-fiber composite materials or from titanium. Instead of or in addition to being attached to a housing 12, as shown in FIG. 1, the components of the cleaning device can also be fitted into a tube made of carbon-fiber composite material. This further increases stability against abrupt movements or shocks. The application of the method according to the invention is not limited to the removal of thermoplastic materials such as synthetic rubber from press or injection molds. Rather, coloring or other dirt layers from metals, plastics or mineral substrates can also be removed.

Claims

claims

1 . A method of removing plastic material adhering to a surface, in which the surface is irradiated with at least one laser light pulse, characterized in that the laser light pulse has a pulse duration t _j 200 200 ns and that the surface is closed r I onisie r ngdesanha te

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Plastic material is exposed to radiation of at least 0.02 J / cm.

2. The method according to claim 1, characterized in that the pulse duration t _{j of} the laser light pulse is less than 100 ns.

3. The method according to any one of the preceding claims, characterized in that the pulse duration t _{j of} the laser light pulse is between 5 and 20 ns.

4. The method according to any one of the preceding claims, characterized in that the surface is exposed to an irradiation of more than 0.2 J / cm per laser light pulse.

5. The method according to any one of the preceding claims, characterized in that the surface is exposed to an irradiation of more than 2 J / cm per laser light pulse.

6. The method according to any one of the preceding claims, characterized in that the surface is exposed to an irradiation of less than 60 J / cm per laser light pulse.

7. The method according to any one of the preceding claims, characterized in that the wavelength of the substantially monochromatic laser light pulse is between 200 and 3000 nm.

8. The method according to any one of the preceding claims, characterized in that the surface is the inner surface of a die or injection mold.

9. The method according to claim 8, characterized in that the surface is made of a metal, a plastic or a mineral material.

10. The method according to any one of the preceding claims, characterized in that the surface is coated with a titanium alloy.

1 1. Method according to one of the preceding claims, characterized in that detached plastic particles are suctioned off.

1 2. The method according to any one of the preceding claims, characterized in that with each laser light pulse in each case a surface section is irradiated with a predetermined extent and that the entire surface is gradually cleaned in a grid-like manner.

1 3. The method according to claim 1 2, characterized in that overlapping surface sections irradiated by successive laser light pulses overlap.

14. The method according to any one of the preceding claims, characterized in that the surface is irradiated with laser light pulses with a pulse repetition rate of less than 20 Hz.

1 5. Device (1 0) for performing the method according to one of claims 1 to 14, with at least one laser (14) for generating laser light pulses, characterized by means (14, 24, 28) for generating an irradiation of at least 0.02 J / cm on the surface due to laser light pulses with a pulse duration t _j <200ns.

1 6. Apparatus according to claim 1 5, characterized by an optical amplifier (24) connected downstream of the laser, which amplifies the laser light pulses to an energy of at least 100 and at most 3000 mJ, preferably more than 1000 mJ.

1 7. Apparatus according to claim 1 6, characterized in that the laser (1 4) is a Q-switched neodymium: yttrium aluminum garnet (Nd: YAG) laser for generating laser light pulses with a pulse duration t _j of at least 5 and at most 20 ns , and that the downstream optical amplifier (24) is an Nd: YAG amplifier.

1 8. Apparatus according to claim 1 7, characterized in that the Nd: YAG amplifier (24) contains a cylindrical Nd: YAG amplifier rod, the diameter of which is larger than that of an Nd: YAG laser (14) used: YAG laser rods.

1 9. Device according to one of claims 1 5 to 1 8, characterized by a beam shaping assembly (28) formed from optical lenses.

20. The apparatus according to claim 1 9, characterized in that the beam shaping assembly (28) is a zoom lens.

21. Apparatus according to claim 19 or 20, characterized by a protective unit (30) provided between the beam shaping assembly (28) and the surface with a diaphragm (36) facing the surface, which has an opening for the passage of the laser beam (1 8), and one of the beam shaping assembly (28) facing protective glass (34) which extends transversely to the direction of propagation of the laser beam (1 8) approximately over the extent of the beam shaping assembly (28).

22. The device according to one of claims 1 7 to 21, characterized in that the laser (14), the amplifier stage (24), the beam shaping assembly (28) and the protective unit (30) are arranged in a common housing (1 2), which has an exit opening for the laser beam (1 8).

23. The device according to claim 22, characterized in that within the housing (12) at least between the laser (14) and the amplifier stage (24) two mirrors (20, 22) are arranged to form a folded beam path.

24. The device according to claim 22 or 23, characterized in that the housing outlet opening is surrounded by a hollow body (30, 32) which extends in the direction of propagation of the laser light and is open in this direction and which has at least one lateral opening (38, 40) for Has connection of a suction device.