Method for polishing workpieces by treating them with energetic radiation

Abstract

The present invention relates to a method for polishing workpieces (1), in particular made of glass or plastic, by treating them with energetic radiation (2), in particular laser radiation. First, a workpiece (1) is provided which comprises a first surface (11) and a second surface (12) positioned opposite thereto. Next, the first surface (11) is polished by exposing it to energetic radiation (21) having first treatment parameters and the second surface (12) is polished by exposing it to energetic radiation (22) having second treatment parameters. The method is characterized in that the first treatment parameters for polishing the first surface and the second treatment parameters for polishing the second surface are selected such that, due to the exposure of the second surface (12) to energetic radiation (22) having second treatment parameters, the second surface (12) is subjected to greater thermal stress than the first surface (11), due to its exposure to energetic radiation (21) having the first treatment parameters.

Description

[0001] Patent application:

[0002] Process for polishing workpieces by processing with energetic radiation

[0003] Applicant:

[0004] Fraunhofer Society for the Promotion of Applied Research

[0005] The application relates to a method for polishing workpieces, in particular made of glass or plastic, by processing with energetic radiation, in particular laser radiation, comprising the following method steps: providing a workpiece which has a first surface and a second surface lying opposite it, polishing the first surface by irradiation with energetic radiation with first processing parameters and polishing the second surface by irradiation with energetic radiation with second processing parameters.

[0006] The smoothing of surfaces of high-precision optical components and surfaces of thin glass and thermoplastics is now largely achieved through mechanical processing methods such as grinding and polishing. It should be noted that the surfaces of optical elements, in particular, require the highest precision in terms of surface quality, such as surface roughness, and dimensional accuracy. In conventional grinding, material is mechanically removed from the surface using a defined relative movement between the tool and the workpiece and a defined grinding pressure. In conventional machining, the desired shape of the workpiece is usually created by grinding in a first processing step. However, this removal process creates a large number of microcracks in the area of ​​the surface and in the area just below the surface (also known as "sub-surface damage" SSD).In brittle materials such as glass and thermoplastics, this promotes the formation and propagation of cracks, as well as the formation and propagation of fractures.

[0007] To correct and completely eliminate microcracks on and just below the surface, conventional surface processing involves multi-stage grinding cycles followed by multi-stage polishing cycles using increasingly finer tools and abrasives or polishing agents. Depending on the surface to be smoothed, this method can achieve surface roughnesses on the order of Ra = 1 nm. However, this conventional process is correspondingly time-consuming and results in wear and tear on tools and consumables. Furthermore, abrasives and polishing agents can become embedded in the surface and reduce surface quality due to absorption and scattering losses. In addition, material is removed from the surface in a statistically random manner during grinding, which can lead to inaccuracies and longer processing times.

[0008] Methods for polishing surfaces, particularly the surfaces of optical components, using energetic radiation, such as laser radiation, are already known in the art. These methods enable the complete removal of microcracks in the surface area and in the area just below the surface within a single polishing step, thus simplifying or replacing the conventional processing process consisting of multi-stage grinding cycles followed by multi-stage polishing cycles.

[0009] Polishing surfaces with energetic radiation is based on the absorption of energetic radiation in a thin surface layer of the workpiece, achieving temperatures close to the surface just below the specific vaporization temperature of the respective material. The viscosity of the material is reduced by heating, resulting in a smoothing of the surface due to surface tension. This results in a fundamentally different operating principle compared to conventional polishing processes. Instead of physically removing material through mechanical processing, the surface is smoothed by remelting, i.e., a material redistribution.

[0010] For example, WO 2012 / 119761 A1 describes a method for manufacturing optical elements by processing with energetic radiation. In this process, quartz glass blanks are first treated with coarse material removal and then polished with energetic radiation. Surface parameters are then compared with a target value. If the surface parameters lie outside a defined tolerance range, further steps of fine material removal using energetic radiation are performed until the surface parameters are within the defined tolerance range. For this purpose, the surface of the workpiece to be processed is scanned with a radiation source in parallel paths, for example, in a meandering pattern.

[0011] While the technologies previously used to polish surfaces with energetic radiation offer advantages over conventional mechanical processing methods such as grinding and polishing, the thermal influence on the workpiece inherent in polishing with energetic radiation causes distortion of the workpiece geometry. This proves particularly disadvantageous in the manufacture of thin components, such as optical elements. In practice, the distortion caused by polishing with energetic radiation often has to be subsequently corrected by conventional mechanical polishing. To avoid causing further microcracks, this polishing step must be very fine, which, depending on the severity of the distortion, can lead to very long post-processing times.Due to the post-processing often required to correct distortion caused by polishing with energetic radiation, the fundamental practical advantages of polishing with energetic radiation in terms of minimizing processing steps and processing time are partly offset.

[0012] The object of the present invention is therefore to provide an efficient method for smoothing and polishing thin glass and thermoplastics by processing with energetic radiation, which causes fewer deviations in the workpiece geometry when smoothing or polishing the workpieces with energetic radiation. In particular, a method is to be specified with which very smooth and highly precise surfaces of optical elements or other thin workpieces made of glass or thermoplastics can be manufactured.

[0013] This object is achieved by the method according to claim 1. Advantageous embodiments of the method are the subject of the dependent claims or can be derived from the following description and the exemplary embodiments. According to the invention, this object is achieved by a method for polishing workpieces, in particular made of glass or plastic, by processing with energetic radiation, in particular laser radiation. For this purpose, a workpiece is provided which has a first surface and, opposite it, a second surface. The first surface is polished by irradiation with energetic radiation using first processing parameters, and the second surface is polished by irradiation with energetic radiation using second processing parameters.The method is characterized in that the first processing parameters for polishing the first surface and the second processing parameters for polishing the second surface are selected such that by irradiating the second surface with energetic radiation with second processing parameters, the second surface is subjected to a higher thermal stress than the first surface by irradiating it with energetic radiation with the first processing parameters, in order to thereby reduce a distortion of the workpiece.

[0014] Such a process did not appear to be effective for reducing distortions in laser-polished workpieces until now, since when machining both sides of a workpiece, it was expected that machining the first surface and the second surface with identical machining parameters would be preferable for reducing distortions.

[0015] Surprisingly, however, the inventors of the present invention discovered that by machining the second surface with second machining parameters that induce higher thermal stress in the workpiece than is the case with machining the first surface with the first machining parameters, a lower distortion of the workpiece can be achieved compared to machining the workpiece on one side or compared to machining the first and second surfaces of the workpiece with identical machining parameters. This contradicts the common assumption that machining the first and second surfaces with identical parameters should compensate for distortion in the workpiece geometry.The thermal stress generated in the workpiece when machining the second surface with the second processing parameters is increased compared to machining the first surface with the first processing parameters by appropriately selecting the parameters to reduce the workpiece's distortion. To achieve this, either individual processing parameters or several processing parameters can be adjusted, whereby the total thermal stress generated when machining the second surface must be higher than when machining the first surface.

[0016] When thermal stress is mentioned in connection with the method according to the invention, what is meant is, strictly speaking, thermally induced mechanical stress.

[0017] Compared to conventional polishing with energetic radiation, this has the technical advantage, among other things, that complex mechanical regrinding after machining a workpiece with energetic radiation to compensate for any distortion is no longer necessary. This technical advantage is reflected in a reduction in processing time and costs. Furthermore, no foreign substances such as chemicals and polishing agents need to be applied to the surfaces of the workpiece, thus preventing contamination and the resulting loss of surface quality. Furthermore, occupational safety is increased, as avoiding mechanical regrinding prevents the release of fine material dust, which should be avoided by the operator for health reasons.

[0018] In addition, the general advantages of laser polishing compared to conventional mechanical polishing methods are to be mentioned, such as a high surface rate, a high degree of geometric freedom and an achievable micro-roughness of less than 1 nm. For quartz glass, a surface rate of up to 0.6 cm 2 / s and for N-BK7 an area rate of up to 4 cm 2 / s. Laser polishing also enables the selective polishing of workpieces with varying degrees of complexity. When polishing with energetic radiation, the energetic radiation is directed, preferably vertically, onto one of the workpiece's surfaces. The energetic radiation is absorbed within a thin boundary layer close to the surface, causing the material to heat up. In quartz glass, for example, this boundary layer is on the order of several micrometers. The temperature increase reduces the viscosity of the material, and the material begins to flow. Surface roughness is smoothed by surface tension without any material removal.

[0019] White light interferometry (WLI), atomic force microscopy (AFM), laser scanning microscopy or profilometry are suitable for determining surface roughness.

[0020] White light interferometry (WLI), laser interferometry, laser scanning microscopy or profilometry are suitable for determining the form deviations in the workpiece geometry.

[0021] To determine form deviations in the workpiece geometry, the so-called peak-to-valley distance (PV distance) is measured in white light interferometry. This distance is the distance between the highest and lowest points on the surface being measured. The peak-to-valley distance can be measured according to the standards DIN EN ISO 101 10-5 and ISO 14999-4.

[0022] Particularly in the case of thin workpieces, processing with energetic radiation leads to considerable distortion, which is why the method according to the invention unfolds its advantageous effect particularly with thin workpieces. Accordingly, the advantages of the method according to the invention are particularly evident in workpieces that have a first and a second surface that are considerably larger in relation to the other outer surfaces of the workpiece. The distance between the first and the second surface corresponds to the thickness of the workpiece. This particularly refers to workpieces with a thickness in the range of 0.5 mm to 10 mm. Workpieces made of materials such as glass, in particular quartz glass, borosilicate crown glass (e.g. N-BK7) and heavy flint glass (e.g. N-SF6) as well as thermoplastics such as polycarbonate (PC), polymethyl methacrylate (PMMA) and polyamides (PA) are suitable for the method according to the invention.It should be noted that thermoplastic materials do not necessarily have to be transparent.

[0023] Furthermore, the workpieces include, for example, imaging optics, aspherical lenses or freeform lenses.

[0024] In general, the first and second surfaces of the workpiece can be prefabricated by cutting, drawing, forming, grinding, injection molding, laser ablation or additive manufacturing processes.

[0025] The energetic radiation is preferably laser radiation that is absorbed near the surface. It is particularly preferably infrared laser radiation from a CO2 laser. For example, a laser can be operated in continuous, pulsed, or modulated mode, and a laser power between 1 W and 4000 W, preferably between 100 W and 2000 W, can be used. If a laser is used in pulsed mode, for example, the laser can be operated with a pulse duration of 1 ps to 1000 ps and a repetition rate of 1000 Hz to 150,000 Hz. Infrared radiation, microwave radiation, or electron beams are also suitable as energetic radiation.

[0026] The surface section to be machined can be irradiated across its entire surface by expanding a laser beam with suitable optics, such as lens systems, or it can be scanned by at least one energetic beam. The beam can be guided over the surface section to be machined using mirrors and / or machine axes. Multiple beams of energetic radiation can also be used for simultaneous or consecutive machining of the surface, for example, to achieve a more uniform temperature application to the workpiece or a higher surface rate. Either consecutive machining of the first surface and then the second surface can be performed, or simultaneous machining of the first and second surfaces.

[0027] Furthermore, it is possible to perform the process for polishing workpieces under a protective gas atmosphere. Argon, helium, carbon dioxide, and nitrogen are particularly suitable for this. This prevents contamination from burning into the surface of the workpiece during processing with energetic radiation and achieves a lower surface roughness.

[0028] In a technically advantageous embodiment, the first and second processing parameters are selected from the group comprising process temperature, interaction time of the energetic radiation with the respective surface, scanning speed, beam diameter, track pitch and combinations thereof.

[0029] The process temperature is the maximum temperature of the workpiece within the interaction zone of the energetic radiation with the workpiece. The interaction zone represents the area in which the energetic radiation interacts with a surface during irradiation. Process temperatures range between 300 °C and 2500 °C for glass and between 100 °C and 500 °C for plastics.

[0030] The interaction time is the time during which the energetic radiation interacts with the material. The interaction time ranges between 0.5 s and 10 s for glass and 1 s and 1000 s for plastics. Interaction times over 60 s are particularly relevant for extremely rough surfaces (surface roughness Ra > 3 pm), such as those of additively manufactured components.

[0031] The beam diameter of the energetic radiation can be in the range of 100 pm to 25 mm, preferably in the range of 1 mm to 10 mm.

[0032] The track pitch is the distance between the individual processing tracks on which a surface is treated with energetic radiation. The track pitch is in the range of 1 pm to 2000 pm, preferably in the range of 5 pm to 200 pm.

[0033] In a technically advantageous embodiment, the workpiece cools down between the machining of the first and second surfaces during consecutive machining of the first and then the second surface.

[0034] In particular, the workpiece cools to a temperature below the material-specific glass transition temperature, or melting temperature for materials with a fixed melting temperature. It is preferred that the workpiece has the same initial temperature at the start of processing the second surface as at the start of processing the first side, thereby achieving a reproducible polishing result.

[0035] In a technically advantageous embodiment, the processing of the second surface with energetic radiation using the second processing parameters produces a higher temperature gradient in the workpiece than the processing of the first surface with energetic radiation using the first processing parameters.

[0036] Due to the higher temperature gradient between the temperature at the second surface and the internal temperature of the workpiece, which is caused by processing the second surface with energetic radiation using the second processing parameters in the workpiece, the second surface is subjected to a higher thermal stress than the first surface. This results in the surprising technical effect that the distortion of the workpiece is reduced during the inventive processing of the workpiece with energetic radiation.

[0037] In a technically advantageous embodiment, the process temperature during processing the second surface is higher than the process temperature during processing the first surface. Depending on the material of the workpiece, the process temperature is in the range of 100°C to 2500°C, preferably in the range between the material-specific glass transition temperature, or melting temperature for materials with a fixed melting temperature, and the material-specific evaporation temperature of the workpiece.

[0038] The choice of process temperature for processing the first and second surfaces depends heavily on the material. For N-BKT glass, for example, a temperature 0.1% to 30% higher in degrees Celsius is required for processing the second surface than for processing the first surface.

[0039] As a result, the workpiece is subjected to a higher thermal stress when machining the second surface and a higher temperature gradient is generated in the workpiece, so that the technical advantage is achieved that the distortion of the workpiece is lower when machining the workpiece with energetic radiation than with conventional laser polishing processes.

[0040] In a technically advantageous embodiment, the interaction time of the energetic radiation when processing the second surface is higher than the interaction time when processing the first surface.

[0041] The choice of interaction time for machining the first and second surfaces is highly material-dependent. For machining the second surface, an interaction time 1% to 500% longer than for machining the first surface should be selected.

[0042] As a result, the energetic radiation interacts with the second surface for a longer time when irradiating the second surface than when irradiating the first surface. Thus, the workpiece is subjected to an overall higher thermal stress when machining the second surface due to the longer interaction time.

[0043] In a technically advantageous embodiment, the workpiece is preheated before irradiation with energetic radiation. During consecutive processing of the first and then the second surface, the preheating temperature is selected to be lower before irradiating the second surface with second processing parameters than before irradiating the first surface with first processing parameters. The preheating temperature is selected material-specifically in the range between 25°C and 1300°C. Preheating achieves the technical advantage that the development of thermal stresses in the workpiece can be regulated when processing a surface with energetic radiation. Higher preheating temperatures lead to lower thermal stresses in the workpiece.To reduce the workpiece's distortion, the preheating temperature is therefore selected to be lower when irradiating the second surface with second processing parameters than when irradiating the first surface with first processing parameters. Irradiating the second surface with the second processing parameters creates a higher temperature gradient in the workpiece and exposes the workpiece to a higher thermal stress, which reduces the workpiece's distortion.

[0044] In a technically advantageous embodiment, a temperature of the first and / or second surface is monitored and in particular controlled during processing, wherein in particular the process temperature of the first and / or second surface is measured.

[0045] The respective temperature can be measured at one or more fixed positions relative to the interaction zone or at one or more variable positions on the first and / or second surface. These measurements can be performed using a point, line, or area thermal camera, or a pyrometer.

[0046] For example, the thermal radiation in the immediate interaction zone of the energetic radiation on the surface of the workpiece can be measured, and the temperature can be determined from this. For this purpose, a pyrometer can be permanently connected to the energetic radiation source. Using suitable control technology, the temperature measured in this way can be used, for example, to adjust the power of the radiation source. Alternatively, or in addition, the scanning speed can be adjusted instead of the power of the radiation source to achieve the desired process temperature.

[0047] This enables precise process control. Furthermore, the temperature can be maintained within a specific tolerance range between a maximum and minimum temperature, for example, for heat-sensitive materials or workpieces with localized, particularly sensitive surface areas, or for complex workpiece geometries where different thermal conduction properties exist at different positions.

[0048] In a technically advantageous embodiment, the first and second processing parameters are adjusted such that a constant process temperature or a specified temperature profile is achieved on the first and / or second surface. For this purpose, a pyrometer can be used to determine the temperature, and via a suitable control technology, the temperature thus measured can be used, for example, to adjust the first and second processing parameters such that a constant process temperature or a specified temperature profile is achieved.

[0049] In a technically advantageous embodiment, during the processing of the first or second surface, the energetic radiation for irradiation is directed onto the respective surface and guided over the respective surface at a relative speed, so that the energetic radiation is formed into a quasi-line, wherein the respective surface is polished, in particular by repeatedly passing over the respective surface with energetic radiation formed into the quasi-line. To form the quasi-line, the energetic radiation is moved back and forth along a line, wherein the length of the line is at least as long as the extent of the surface to be polished.

[0050] By superimposing a translationally oscillating movement component and a feed movement component, a meandering or zigzag-shaped relative movement of the energetic radiation relative to the workpiece is achieved. The translationally oscillating movement component has a scanning speed in the range of 100 mm / s to 10,000 mm / s, preferably in a range of 2,000 mm / s to 10,000 mm / s, and the feed movement component has a feed speed in the range of 0.1 mm / s to 50 mm / s, preferably in a range of 1 mm / s to 10 mm / s. When using such high scanning speeds, the energetic radiation generates a temperature field which, for materials with low thermal conductivities, such as glass, exhibits approximately no or at most very slight temperature variation in the scanning direction.This is referred to as a quasiline, which is created by the energetic radiation oscillating translationally at high speed on the workpiece and which is moved across the surface at the feed rate. When the surface is irradiated with energetic radiation in this way, it is also referred to as a quasiline machining strategy.

[0051] As a rule, the quasi-line processing strategy is chosen, particularly for polishing glasses, but also thermoplastics, using energetic radiation.

[0052] The surface of the workpiece is polished by passing over the respective surface once. It is also possible to polish the respective surface by passing over the respective surface multiple times with the energetic radiation. The number of passes can be between 1 and 20. The number of passes depends, among other things, on the scanning speed used. Machining with a single pass is particularly preferred. At high scanning speeds, more passes tend to be required, with each individual pass partially reducing the surface roughness and healing or melting the microcracks in the area of ​​the surface and in the area just below the surface.

[0053] Since the energy input can be lower when using multiple individual passes than when using a single pass, the overall energy input is more homogeneous. Thus, passing over a workpiece surface multiple times offers the technical advantage of achieving a lower surface roughness in a more gentle manner than with a single pass.

[0054] After each complete passage over the respective surface, a process pause between 0 s and 10 min, preferably between 0 s and 30 s, can be carried out.

[0055] The process pause between the individual passes on a surface, for example, achieves the technical advantage that the workpiece can cool down during the pause and thus overheating of the surface due to preheating from previous passes can be avoided, thus preventing unwanted material evaporation.

[0056] In a technically advantageous embodiment, the number of passes is between 1 and 20, preferably between 1 and 5, wherein the number of passes when machining the second surface is higher, preferably 1 to 2 passes higher, than when machining the first surface.

[0057] As a result, the energetic radiation interacts with the second surface for a longer time overall when irradiating the second surface than when irradiating the first surface. Thus, the workpiece is subjected to a higher overall thermal stress when machining the second surface due to the longer overall interaction time.

[0058] Typically, a higher number of passes is selected for machining thermoplastics than for glass, and the other machining parameters are adjusted accordingly for the specific material. This allows for smoother machining of the workpiece and reduces distortion, to which plastics are generally more susceptible.

[0059] In a technically particularly advantageous embodiment, the first surface is machined with one pass and the second surface is machined with several passes, wherein when machining the second surface during the first pass the second machining parameters are identical to the first machining parameters for machining the first surface.

[0060] After one or after a specific number of passes, the workpiece can be examined and, in particular, the surface quality of the first or second or both surfaces can be determined. Furthermore, the distortion of the workpiece can be measured after one or after a specific number of passes, and the machining parameters can be adjusted based on these measurement results. For example, one or both surfaces can be optically measured and compared with a target surface contour and target surface roughness. If such measurements reveal deviations outside a specified tolerance range, a further pass is performed. During a further pass, any remaining waviness or other deviations from the target surface contour or target surface roughness can be reduced. This allows the process to be flexibly adapted to the respective workpiece.

[0061] In particular, when machining the second surface, 1 to 5 additional passes are performed. The process temperature corresponds to 10% to 90% of the process temperature during the first pass and / or the interaction time corresponds to 10% to 90% of the interaction time during the first pass.

[0062] In addition, it has also proven advantageous to select the process temperature and / or the interaction time gradually lower with each subsequent pass over the second surface when machining the second surface.

[0063] After one or a specific number of passes, the workpiece can be examined and, in particular, the surface quality of the first or second or both surfaces can be determined. Furthermore, the distortion of the workpiece can be measured after one or a specific number of passes, and the processing parameters can be adjusted step by step with each subsequent pass based on these measurement results. In particular, the process temperature and / or the interaction time can be gradually reduced with each subsequent pass of the second surface. This step-by-step processing process can be continued until the measurement results are within a desired tolerance range.

[0064] This enables particularly controlled and material-friendly machining of the workpiece. Furthermore, the process can be flexibly adapted to the workpiece.

[0065] In a technically advantageous embodiment, during the processing of the first or second surface, the energetic radiation for irradiation is directed onto the respective surface and guided over the respective surface at a relative speed such that the energetic radiation is formed into a quasi-tophat distribution.

[0066] The machining strategy in which the energetic radiation is shaped into a quasi-tophat distribution is characterized by the fact that, unlike the quasi-line machining strategy, the feed movement component of the scanning speed is also very high, so that the entire surface of the workpiece to be machined is scanned with the energetic radiation in a very short time, typically in less than one second. After each scan cycle, the surface area to be machined is immediately scanned again, so that multiple scans take place per second. Preferably, the orientation of the scan paths is rotated by an angle, e.g., 90°. The aim of this machining strategy is to achieve the most homogeneous temperature field possible across the entire surface area to be machined. For this purpose, the scanning speed can be varied within the surface area to be machined and / or during a scan cycle.This makes it possible, for example, to compensate for the increased heat dissipation at the edge of the surface area to be processed. The scanning process, consisting of a large number of scan cycles, is repeated for a specified time corresponding to the interaction time.

[0067] While in the quasi-line machining strategy a surface is machined comparatively slowly in the feed direction by energetic radiation within a comparatively low number of passes, in the quasi-top-hat machining strategy a surface is machined at a comparatively significantly higher speed in the feed direction with a very high number of scanning cycles with energetic radiation.

[0068] The advantages of the quasi-tophat processing strategy are particularly evident in thermoplastics, which are processed at a lower process temperature than glass.

[0069] The proposed method is explained again below using exemplary embodiments in conjunction with the drawings. Figure 1 shows a schematic representation of the processing method according to a preferred embodiment of the invention.

[0070] Fig. 2 is a schematic representation of the processing of the workpiece using energetic radiation in quasi-line distribution.

[0071] Fig. 3 is a schematic representation of the machining of the workpiece using energetic radiation in quasi-tophat distribution.

[0072] Fig. 1 shows a workpiece 1 whose first surface 11 and second surface 12 are being polished by means of energetic radiation 2. In a first step, the first surface 11 is polished by irradiation with energetic radiation 21 using first processing parameters. The workpiece 1', which has already been polished on one side, is further processed in a subsequent step, and the second surface 12 is polished with energetic radiation 22 using second processing parameters. If the surface parameters of interest lie outside a defined tolerance range after the first surface 11 and second surface 12 have been processed once, further polishing steps using energetic radiation are carried out until the surface parameters of both surfaces (11, 12) lie within the desired tolerance range and the workpiece 1' has been polished to the desired extent.

[0073] Fig. 2 shows the movement of the energetic radiation 2 across the workpiece 1 in the quasi-line machining strategy. The energetic radiation 21 with first machining parameters is directed onto the first surface 11. In an enlarged view, Fig. 2 further shows how the energetic radiation 21 with first machining parameters is guided over a partial area 111 of the first surface. To form a quasi-line 21a, the energetic radiation 21 with first machining parameters is moved back and forth along a line 21a and, in addition, the entire line 21a is guided over the partial area 111 of the first surface. This movement is achieved by superimposing a translationally oscillating movement component v Scan y and a feed motion component v scan xgenerated. Analogously, the second surface 12 can be polished with second machining parameters in the quasi-line machining strategy, even if this is not explicitly shown in Fig. 2.

[0074] Fig. 3 shows the movement of the energetic radiation 2 over the workpiece 1 in the quasi-tophat machining strategy. The energetic radiation 21 with first machining parameters is directed onto the first surface 11. In an enlarged view, Fig. 3 further shows how the energetic radiation 21 with first machining parameters is scanned at the scanning speed v Scan is guided over a partial area 1 1 1 of the first surface. After a scan cycle, the partial area 1 1 1 of the first surface is immediately scanned again, whereby the orientation of the scan paths is rotated by an angle.

[0075] Analogously, the second surface 12 can be polished with second machining parameters in the quasi-tophat machining strategy, even if this is not explicitly shown in Fig. 3.

[0076] Although rectangular workpieces are depicted in the figures for the sake of simplicity, this should not be considered a limitation. The method according to the invention is also suitable for other shapes, such as round ones.

[0077] In the following, two concrete application examples for the method according to the invention will be presented.

[0078] The method according to the invention can be used to polish a thin workpiece 1 made of N-BK7 glass using laser radiation. A quasi-line scanning strategy is selected for this purpose. The workpiece 1 has a thickness of 5 mm.

[0079] To machine the first surface 11, a laser power of 700 W, a beam diameter of 20 mm, and a feed rate of 8 mm / s are selected, and machining is performed within a single pass. Two passes are performed to machine the second surface 12. During the first pass, a laser power of 700 W, a beam diameter of 20 mm, and a feed rate of 8 mm / s are selected. During the second pass to machine the second surface 12, an adjusted laser power of 350 W is selected, while the beam diameter and feed rate are maintained as in the first pass.

[0080] Subsequently, the distortion of the workpiece 1" is determined using white light interferometry via the peak-to-valley distance. With this selection of machining parameters, a peak-to-valley distance of 9 pm is measured on the second surface 12, which corresponds to a geometric deviation from the planar target geometry, i.e., the distortion of the surface.

[0081] In a comparative measurement on an N-BK7 reference workpiece 1, whose first surface 11 and second surface 12 are machined consecutively with identical machining parameters, a significantly higher distortion is observed. To enable a comparison, the machining parameters selected are those used in the inventive application of the method for machining the first surface 11 of the N-BK7 workpiece 1 to be compared.

[0082] Specifically, first the first surface 11 and then the second surface 12 of the reference workpiece 1 are machined with a laser power of 700 W, a beam diameter of 20 mm, and a feed rate of 8 mm / s using a quasi-line machining strategy. This results in a peak-to-valley distance of 35 pm measured on the second surface 12 using white light interferometry, which corresponds to the geometric deviation from the planar target geometry, i.e., the surface distortion.

[0083] In a comparative measurement on another N-BK7 reference workpiece 1, in which only the first surface 11 is machined with a laser power of 700 W, a beam diameter of 20 mm, and a feed rate of 8 mm / s using a quasi-line scanning strategy, a peak-to-valley distance of 45 pm, measured using white light interferometry, is obtained. Accordingly, machining of the reference workpiece 1' on only one side results in significantly higher distortion than machining of an N-BK7 workpiece 1" on both sides with adapted process parameters.

[0084] Furthermore, the method according to the invention can be used for polishing a thin workpiece 1 made of the thermoplastic polymethyl methacrylate (PMMA) using laser radiation. For this purpose, a quasi-line machining strategy with temperature control that keeps the process temperature constant is selected as the scanning strategy. The workpiece 1 has a thickness of 3 mm.

[0085] To machine the first surface 11, a process temperature of 225°C and a feed rate of 2 mm / s are selected, and the machining is performed within a single pass. To machine the second surface 12, a process temperature of 225°C and a feed rate of 2 mm / s are selected for the first pass. For the second pass of the second surface 12, a process temperature of 180°C and a feed rate of 2 mm / s are selected.

[0086] Subsequently, the distortion of the workpiece 1" is determined using white light interferometry via the peak-to-valley distance. With this selection of machining parameters, a peak-to-valley distance of 8.7 pm is measured on the first surface 11 and a peak-to-valley distance of 9.4 pm is measured on the second surface 12, which corresponds to a geometric deviation from the planar target geometry.

[0087] In a comparison measurement on a PMMA reference workpiece 1, which is also 3 mm thick and whose first surface 11 and second surface 12 are machined one after the other using identical machining parameters, a significantly higher distortion is obtained. In order to enable a comparison, the machining parameters selected are those used in the inventive application of the method for machining the first surface 11 of the PMMA workpiece to be compared. Specifically, first the first surface 11 and then the second surface 12 of the reference workpiece are machined with one pass, a process temperature of 225°C and a feed rate of 2 mm / s with a quasi-line scanning strategy and temperature control that keeps the process temperature constant. This leads to a distortion of 21.7 pm on the first surface 11 and 23.8 pm on the second surface 12, measured by means of white light interferometry.

[0088] There is therefore a considerable difference in the extent of the shape distortion depending on the selection of the processing parameters for the respective processing of the first 1 1 and second surface 12. With the previously mentioned selection of processing parameters, a shape distortion that is 2.5 times lower can be achieved for this workpiece 1 ” compared to the reference workpiece 1 ” with identical processing parameters when processing the first 1 1 and second surface 12. White light interferometry measurements on comparable PMMA workpieces 1 with a thickness of 3 mm, which have not been processed with energetic radiation, result in a distortion of 2.3 pm on the first surface 1 1 and a distortion of 3.1 pm on the second surface 12.

Claims

Claims 1. A method for polishing workpieces (1), in particular made of glass or plastic, by processing with energetic radiation (2), in particular laser radiation, comprising the method steps: - Providing a workpiece (1) having a first surface (11) and opposite a second surface (12); - polishing the first surface (11) by irradiation with energetic radiation (21) with first processing parameters; - polishing the second surface (12) by irradiation with energetic radiation (22) with second processing parameters; characterized in that the first processing parameters for polishing the first surface and the second processing parameters for polishing the second surface are selected such that by irradiating the second surface (12) with energetic radiation (22) with second processing parameters, the second surface (12) is subjected to a higher thermal stress than the first surface (11) by irradiation with energetic radiation (21) with the first processing parameters, in order to thereby reduce a distortion of the workpiece.

2. Method according to the preceding claim, characterized in that the first and second processing parameters are selected from the group comprising process temperature, interaction time of the energetic radiation with the respective surface (11, 12), scanning speed, beam diameter, track pitch and combinations thereof.

3. Method according to one of the preceding claims, characterized in that the first and the second surface are machined consecutively and the workpiece (1 ') cools down between the machining of the first (1 1) and the second surface (12).

4. Method according to one of the preceding claims, characterized in that the processing of the second surface (12) with energetic radiation (22) with the second processing parameters produces a higher temperature gradient in the workpiece (1 ', 1 ") than the processing of the first surface (1 1) with energetic radiation (21) with the first processing parameters.

5. Method according to one of the preceding claims, characterized in that the process temperature when machining the second surface (12) is higher than the process temperature when machining the first surface (11).

6. Method according to one of the preceding claims, characterized in that the interaction time of the energetic radiation (21, 22) when processing the second surface (12) is higher than the interaction time when processing the first surface (11).

7. Method according to one of the preceding claims, characterized in that the workpiece (1, 1') is preheated before irradiation with energetic radiation (2) and the first and the second surface are processed consecutively, wherein the preheating temperature before irradiation of the second surface (12) is selected to be lower than before irradiation of the first surface (11).

8. Method according to one of the preceding claims, characterized in that during processing a temperature of the first (1 1) and / or second surface (12) is monitored and in particular controlled, wherein in particular the process temperature of the first (1 1) and / or second surface (12) is measured.

9. The method according to claim 8, characterized in that the first and second processing parameters are adapted such that a constant process temperature or a fixed temperature profile is produced on the first (11) and / or second surface (12).

10. Method according to one of the preceding claims, characterized in that during the machining of the first (11) or second surface (12) the energetic radiation (21, 22) is directed onto the respective surface (11, 12) for irradiation and is guided over the respective surface at a relative speed, so that the energetic radiation (21, 22) is formed into a quasi-line, wherein the respective surface (11, 12) is polished, in particular by repeatedly passing over the respective surface (11, 12) with energetic radiation (21, 22) formed into the quasi-line 1 1. Method according to claim 10, characterized in that the number of passes is between 1 and 20, preferably between 1 and 5, wherein the number of passes when machining the second surface (12) is higher, preferably 1 to 2 passes higher, than when machining the first surface (1 1 ).

12. The method according to claim 1 1, characterized in that the processing of the first surface (1 1) is carried out with one pass and the processing of the second surface (12) is carried out with several passes, wherein when processing the second surface (12) in the first pass the second processing parameters are identical to the first processing parameters for processing the first surface (1 1).

13. The method according to claim 12, characterized in that when machining the second surface (12) 1 to 5 further passes are carried out and the process temperature corresponds to 10% to 90% of the process temperature during the first pass and / or the interaction time corresponds to 10% to 90% of the interaction time during the first pass.

14. The method according to claim 12, characterized in that when machining the second surface (12), the process temperature and / or the interaction time is selected to be gradually lower with each further pass over the second surface (12).

15. Method according to one of claims 1 to 9, characterized in that during the processing of the first (1 1) or second surface (12) the energetic radiation (21, 22) is directed for irradiation onto the respective surface (1 1, 12) and is guided at a relative speed over the respective surface, so that the energetic radiation (21, 22) is formed into a quasi-tophat distribution.

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