WO2015091485A1

WO2015091485A1 - Laser printing system

Info

Publication number: WO2015091485A1
Application number: PCT/EP2014/077967
Authority: WO
Inventors: Thomas Mattes; Stefan Paternoster; Gerd Cantzler; Jochen Philippi; Stephan Gronenborn; Gero Heusler; Holger Moench; Ralf Conrads
Original assignee: Eos Gmbh Electro Optical Systems
Priority date: 2013-12-17
Filing date: 2014-12-16
Publication date: 2015-06-25
Also published as: EP3079912A1; US20160279707A1; EP3705300A1; US20160311230A1; JP6585597B2; US10723139B2; EP3079912B1; US11858204B2; US10518555B2; MX2016007805A; JP6810199B2; BR112016013879A2; CN105829113A; CN105829113B; RU2016123826A; KR20160099568A; JP6310560B2; RU2656205C1; JP2017503683A; ES2799123T3

Abstract

The invention describes a laser printing system (100) for illuminating an object moving relative to a laser module of the laser printing system (100) in a working plane (180), the laser module comprising at least two laser arrays of semiconductor lasers and at least one optical element, wherein the optical element is adapted to image laser light emitted by the laser arrays, such that laser light of semiconductor lasers of one laser array is imaged to one pixel in the working plane of the laser printing system, and wherein the laser printing system is a 3D printing system for additive manufacturing and wherein two, three, four or a multitude of laser modules (201, 202) are provided, which are arranged in columns (cl, c2) perpendicular to a direction of movement (250) of the object in the working plane (180), and wherein the columns are staggered with respect to each other such that a first laser module (201) of a first column of laser modules (cl) is adapted to illuminate a first area (yl) of the object and a second laser module (202) of a second column (c2) of laser modules is adapted to illuminate a second area (y2) of the object, wherein the first area (yl) is adjacent to the second area (y2) such that continuous illumination of the object is enabled.

Description

Laser printing system

FIELD OF THE INVENTION

The invention relates to a laser printing system and a method of laser printing, namely in the field of 3D printing by means of lasers for additive manufacturing, for example, used for rapid prototyping. The invention does not refer to 2D printing such as printing of documents.

BACKGROUND OF THE INVENTION

Selective-laser melting machines consist of a single high-power laser and a scanner to scan the laser over the area to be illuminated. In such machines the laser and the scanner are arranged outside of a process chamber and the laserlight can pass through an entrance window into the process chamber that contains a building area. To increase the processing speed, it is desirable to have a printing head with several independent channels i.e. an addressable array of lasers covering a significant part of the area. Preferably, the printing head covers the full width of the area to be printed with one addressable laser source per pixel, so that the print head needs to be moved only in one direction. Reliability and service costs of such addressable arrays may be an issue.

US 2005/0151828 Al discloses a device for xerographic laser printing. The xerographic printing system has a laser printbar imager assembly including a plurality of micro- optic light emitting arrays. The micro-optic light emitting array includes a plurality of vertical cavity surface emitting lasers, where each vertical cavity surface emitting laser is focused with a micro-optic element.

SUMMARY OF THE INVENTION

It is thus an object of the present invention to provide an improved laser printing system and a corresponding method of laser printing. According to a first aspect a laser printing system for illuminating an object moving relative to a laser module of the laser printing system in a working plane is provided. The laser module comprises at least two laser arrays of semiconductor lasers and at least one optical element. The optical element is adapted to image laser light emitted by the laser arrays, such that laser light of semiconductor lasers of one laser array is imaged to one pixel in a working plane of the laser printing system.

Preferably, an area element of the pixel is illuminated by means of at least two semiconductor lasers.

Known laser printing systems either use single high power lasers or arrays of lasers. In case of high power lasers, for example, a single edge emitting semiconductor laser may be used whereas in case of laser arrays Vertical Cavity Surface Emitting Lasers (VCSELs) are preferably used. VCSEL arrays can be easily manufactured in wafer based processes but usually emit less power than edge emitting semiconductor lasers. The optical systems of these known laser printing systems project or focus the light emitting layer of each semiconductor laser to the working plane.

In contrast to this approach a preferred embodiment of the present invention proposes to image at least two laser arrays to two pixels in the working plane by means of an optical element. The image of the laser arrays does not comprise sharp images of the light emitting layers of the semiconductor lasers. Light emitted by means of at least two lasers of one of the laser arrays illuminate each area element of the pixel such that there is no area element which is only illuminated by means of one single semiconductor laser. Preferably, three, four or a multitude of semiconductor lasers of one laser array illuminate one area element of a pixel at the same time. It may even be that two laser arrays are imaged to the same pixel at the same time. More intensity can thus be provided to the working plane by using a multitude of semiconductor lasers per area element of a pixel. A diffuse image of a multitude of semiconductor lasers of the arrays form the pixels in the working plane. The laser printing system may be more reliable due to the relatively low contribution of each single semiconductor laser to the illumination or energy input to an object in the working plane by means of optical energy. A malfunction of a single semiconductor laser of a laser array does consequently not cause a mal- function of the laser printing system.

The laser module may move relative to the laser printing system (scanning) and/or the object may move relative to the laser printing system. The object may be a powder layer which can be sintered by means of the laser printing system. It may be preferred that only the object is moved. The laser printing system may be enabled to illuminate the full width of the object moving perpendicular to the width of the object by means of one, two, three, four or more laser modules. The semiconductor lasers may be edge emitting semiconductor lasers but VCSEL arrays may be preferred due to lower costs.

The optical element may be arranged in a way that an object plane of the optical element with respect to the working plane does not coincide with a plane of the semiconductor lasers such that the cones of laser light emitted by adjacent semiconductor lasers overlap in the object plane. The plane of the semiconductor lasers of the laser arrays is defined by means of the light emitting layers of the semiconductor lasers. The light emitting layers comprise the optical cavity of the semiconductor lasers comprising the active layer and the corresponding resonator mirrors. The optical element may be a single imaging lens or a more complex imaging optic defining an object plane relative to the working plane. The arrangement of the object plane with respect to the light emitting layers of the semiconductor lasers of the laser arrays may cause a diffuse overlapping image of the light emitting layers in the working plane. The energy distribution in the working plane may thus be more homogeneous in comparison to a projection of each light emitting layer of the semiconductor layers to the working plane. Furthermore, the optical element may be as simple as one projection lens per laser module but more complex combinations of lenses may be used in order to increase the distance between the working plane and the laser modules. No micro-lens arrays may be needed in order to pro- vide a sharp projection of each light emitting layer.

The laser module or laser modules of the laser printing system preferably comprises three, four or a multitude of laser arrays. A single laser array may be imaged to one pixel in the working plane. The pixels may be adjacent to each other such that a part of the emitted optical power of one laser array overlaps with the optical power emitted by another laser array. It may even be that two three or more laser arrays may be mapped to the same pixel in the working plane. The optical element may comprise an array of micro-optical elements which may image, for example, laser light of two, for example, adjacent arrays of the laser module to one pixel in the working plane. Two or more arrays may in this case be imaged to one pixel. Alternatively or in addition it may be that laser light emitted by different laser arrays may illu- minate the same part of a surface of an object at different times. The latter means that light of a first array may illuminate a defined surface of the object at a time ti and light of a second array may illuminate the defined surface of the object at a time t₂ later than ti wherein the object was moved relative to the laser module(s). Furthermore, the printing system may comprise laser modules with different working planes. The latter may be done by placing laser modules at different heights relative to a reference surface and/or by providing different optical elements. Different working planes may be advantageous for three dimensional printing. Alternatively or in addition it may be that the laser module(s) can be moved with respect to the reference sur- face being parallel to the working planes being always at a defined distance with respect to the laser modules.

The laser arrays of the laser module or laser modules may be arranged in columns perpendicular to a direction of movement of the object in the working plane. The columns may be staggered or cascaded with respect to each other such that a first laser array of a first column of laser arrays is adapted to illuminate a first area of the object and a second laser array of a second column of laser arrays is adapted to illuminate a second area of the object, wherein the first area is adjacent to the second area such that continuous illumination of the object is enabled. The images of the laser arrays may partly overlap as discussed above.

The laser arrays may be rectangular with the long side of the rectangle being arranged parallel to a direction of movement of the object in the working plane. This arrangement allows higher total powers per pixel by providing more semiconductor lasers per pixel, without reducing the resolution in the lateral direction perpendicular to the direction of movement of the object.

According to the invention, the laser printing system comprises two, three, four or a multitude of laser modules. Using a multitude of laser modules may enable a larger printing area. Furthermore, complex optical elements may be avoided by using, for example, one imaging lens per laser module.

Further, the laser modules are arranged in columns perpendicular to a direction of movement of the object in the working plane. The columns are staggered or cascaded with respect to each other such that a first laser module of a first column of laser modules is adapted to illuminate a first area of the object and a second laser module of a second column of laser modules is adapted to illuminate a second area of the object, wherein the first area is adjacent to the second area such that continuous illumination of the object is enabled.

The number of columns of laser modules may be arranged in a way that the dis- tance between laser modules in one column of laser modules is minimized. The module diameter and the width of the image of the arrays may determine the number of columns needed in order to enable an area covering illumination of the object by means of the laser modules. The bigger the diameter of the module in relation to the width of the image of the array arrangement the more columns may be needed.

The laser arrays of each laser module may be arranged in an elongated arrangement with the long side of the elongated arrangement being arranged perpendicular to the di- rection of movement of the object in the working plane. Each laser module may comprise, for example two, three or more columns of laser arrays perpendicular to the direction of movement of the object in the working plane. The number of arrays per column may surpass the number of columns. This arrangement may enable a homogeneous illumination of the object by means of a relatively simple driving scheme of the single arrays especially if more than one laser module is comprised by the laser printing system. Each area element of the object may in this case only be illuminated by one dedicated laser array wherein adjacent laser arrays illuminate adjacent pixels. The velocity of movement of the object in the working plane may be adapted in order to define the total energy per area element of the object.

The laser printing system may comprise two, three, four or a multitude of laser modules wherein the laser arrays of each laser module are arranged in an elongated arrangement in order to enable a broad workspace (printing width perpendicular to the direction of movement of the object) of the laser printing system.

The laser arrays of each laser module may alternatively be arranged in an elongated arrangement with the long side of the elongated arrangement being arranged slanted or rotated with respect to the direction being perpendicular to the direction of movement of the object in the working plane. A defined slanting angle or rotation of the elongated arrangement of laser modules around their centers may enable integrated intensity profiles with smooth slopes, which may also overlap with the adjacent pixels, to improve the homogeneity of the total intensity distribution, especially if the pixels are slightly misaligned with respect to each other. The latter reduces the alignment effort of the laser arrays and thus the manufacturing costs of the laser modules and the laser printing system. The misalignment may in extreme cases be compensated by an additional calibration run of the laser printing system in which the velocity of movement of the object in relation to the energy input per time and area element of a calibration object is determined.

Alternatively, two, three or more laser arrays of the same laser module or different laser modules may be arranged to illuminate the same area element of the object. The laser arrays may be arranged to subsequently illuminate the area element. The energy input per time to an area element of the object in the working plane may be increased. This may enable higher velocities of the object and thus higher throughput of the laser printing system. Furthermore, the tolerance regarding misalignment of laser arrays and malfunctions of single semiconductor lasers may be improved. The driving schemes of different arrays may be adapted based on calibration runs with calibration objects as described above.

The optical element of the laser modules may be arranged to demagnify the image of the laser arrays in the working plane. The demagnification may enable a smaller pixel size and higher energy densities. Each laser array may further comprise a micro-lens array being part of the optical element, the micro-lens array may be arranged to lower the divergence of the laser light emitted by the semiconductor lasers. The reduction of the divergence may be used to find a compromise between an overlap of the laser light emitted by the semiconductor lasers in the object plane and the size of the single pixel. Furthermore, the distance between the laser array and the working plane may be adapted by means of the micro-lens array and/or the optical element (imaging optic) may be simplified.

The density of laser arrays may change in dependence of an area of the object to be illuminated by means of the laser printing system. The latter may enable higher power densities at defined parts of the object. Alternatively or in addition the density of the semiconductor lasers within the arrays may be adapted such that, for example, less or more intensity may be provided at the rim of the pixels. Furthermore, the shape of the arrays may be tailored in order to improve the homogeneity and/or to create a defined intensity distribution in the work- ing plane. The arrays may, for example, have a diamond, triangle, round, elliptical, trapezoid or parallelogram shape.

The laser printing system comprises at least a first and a second laser module arranged next to each other. Each laser module may comprise at least two laser arrays, wherein at least one of the two laser arrays of the first or the second laser module is arranged as an overlap laser light source such that in operation the same area element in the working plane can be illuminated by the overlap laser light source and a laser array of the laser module arranged next to the laser module comprising the overlap laser light source.

The overlap laser light source is arranged to compensate potential misalignments of the laser modules which may result in unintended illumination gaps on the object in the working plane. Therefore the overlap may be partial.

The laser arrays may illuminate each one pixel in the working plane. The laser array which is arranged as overlap laser light source may be arranged to illuminate the same pixel or a part of the same pixel as a laser array of the neighboring laser module. This means that both laser arrays can illuminate the same area element in the working plane at the same moment in time. Alternatively, the overlap laser light source may be arranged to illuminate the same area element as a laser array of the neighboring laser module but later or earlier in time. The light of the overlap laser light source may, for example, illuminate one area element of an object in the working plane at a time tl and the laser array of the neighboring laser module may illuminate the same area element at a time t2 later than tl because of the movement of the object relative to the laser modules. The relative movement may be caused by a movement of the object, a movement of the laser modules or movement of the object and the laser modules. The total intensity which is provided to a defined area element of the object has to be adapted such that essentially the same energy is provided per area element as in the case of perfectly aligned laser modules which wouldn't need an overlap laser light source. The energy which is provided per area element has to be adapted such that defects in the object are avoided. Only the overlap laser light source or the laser array of the neighboring laser module may be used if there is a perfect match between the illuminated areas. Alternatively, both may be used with adapted intensity (e.g. 50% intensity) wherein the adapted intensity may be adapted to the relative velocity of the object with respect to the laser module. The adaption of the provided laser light may be important if there is no perfect match between the illuminated area elements (e.g. only half overlap due to misalignment) in order to avoid that too much or too little energy is provided.

The technical measures as described in dependent claims 2 to 13 and the corresponding description may be combined with the overlap laser light source as described above.

A total energy which is provided to at least one defined area element in the working plane may be such that essentially the same energy is provided per area element as in the case of aligned laser modules without an overlap laser light source.

In addition, a total energy which is provided to at least one defined area element in the working plane may be such that essentially the same energy is provided per area element as in the case without a time offset t2 -tl between the illumination of the at least one defined area element by the laser array and the corresponding overlap laser light source.

The adapted intensity of a laser array and/or a corresponding overlap laser light source may be such that an energy loss in the time between the illumination by the laser array and the illumination by the corresponding overlap light source of a defined area element in the working plane that is illuminated by the laser array at a time tl and by the overlap laser light source at a time t2 or vice versa is compensated. The adapted intensity of a laser array and/or a corresponding overlap laser light source may be selected depending on a building material used for 3D printing.

In a laser system which is not claimed comprising an overlap laser light source, laser light sources as single lasers may be used instead of laser arrays as described above. The technical measures as described in dependent claims 2 to 15 and the corresponding description may be combined with the overlap laser light source in the laser system comprising single lasers (instead of laser arrays) if applicable.

One pixel may be illuminated by a multitude of semiconductor lasers of a laser array at the same time and the total number of semiconductor lasers may be such that failure of less than a predetermined number of the semiconductor lasers reduces the output power of the laser array only within a predetermined tolerance value. This avoids that the requirements with respect to the working life of the semiconductor lasers is unnecessarily increased.

A laser module may be configured to illuminate at least 2, more preferably 4, 8, 16, 32, 64 or more pixels using a single optical element associated with the laser module.

The optical element associated with a laser module may have an outer contour obtained from a circular or rotationally symmetrical contour which is truncated on two opposing sides and wherein the opposing sides are aligned with respect to each other along an axis which is preferably oriented in a direction perpendicular to the direction of movement. By means of this a compact design of an illumination unit comprising a plurality of modules which are staggered in the direction of movement can be achieved.

A control device may be provided which controls the semiconductor lasers individually or the laser array in such a manner that a semiconductor laser or a laser array which is not used for illuminating is used for providing heat to the working plane.

The semiconductor laser or a laser array which is not used for illuminating may be operated with a lower power than a semiconductor laser or a laser array which is used for illuminating.

At least two semiconductor lasers of one laser array or at least two sub-groups of semiconductor lasers of one laser array may be individually addressable such that an output power of the laser array is controllable by switching off one or more semiconductor lasers or one or more sub-groups of semiconductor lasers. This allows to perform various functions with the respective laser array, such as to use a laser array for heating without melting or sintering of the building material or to provide a required intensity in the case of overlap laser light sources.

A plurality of semiconductor lasers forming an array may be arranged such that an outer contour of the array has a substantially polygonal, preferably a substantially hexagonal shape. With such a design the intensity distribution of the array is substantially free from sharp edges.

According to a still further aspect, a 3D-printing system includes a process chamber comprising a support for layers of a material, wherein the laser modules are arranged in the process chamber and wherein preferably a protective device is arranged at a side of the laser modules which faces the support.

The protective device may be formed of at least one plate that is transparent for the laser light, preferably at least one glass plate. The protective device protects the optical elements and light sources and keeps the laser modules free from vapors and condensates.

A temperature control device may be provided that controls the temperature of at least the surface of the protective device oriented towards the support.

The temperature control device may be configured to heat the protective device such that thermal radiation from material in the working plane to the protective device is substantially prevented and the heat transport into the illumination unit ist prevented as good as possible.

The 3D-laser printing system may be configured to solidify a material layer by layer at locations corresponding to the cross section of an article to be formed in each layer using the laser modules.

The material may be a powder.

The laser modules form an illumination unit and the illumination unit may be configured to move across the working plane.

One laser array can be comprised of a single semiconductor laser but may include at least two seminconductor lasers.

The semiconductor lasers may comprise VCSELs (Vertical Cavity Surface Emitting Lasers) and/or VECSELs (Vertical External Cavity Surface Emitting Lasers).

According to a further aspect of the present invention a method of laser printing is provided. The method comprises the steps of:

moving an object in a working plane relative to a laser module;

emitting laser light by means of a laser module comprising at least two laser arrays of semiconductor lasers and at least one optical element; and imaging the laser light emitted by the laser arrays by means of the optical element, such that laser light of semiconductor lasers of one laser array is imaged to one pixel in a working plane. Preferably, an area element of the pixel is illuminated by means of at least two semiconductor lasers.

The method may enable a more homogeneous intensity distribution in the working plane.

The method may comprise a further step of moving the laser module(s) perpendicular to a reference plane being parallel to the working plane. The movement perpendicular to the reference plane enables different working planes which are parallel to each other.

According to a further aspect, the laser printing system used in the method is a 3D printing system for additive manufacturing and two, three, four or a multitude of laser modules are used which are arranged in columns perpendicular to a direction of movement of the object in the working plane, and wherein the columns are staggered with respect to each other such that a first laser module of a first column of laser modules is adapted to illuminate a first area of the object and a second laser module of a second column of laser modules is adapted to illuminate a second area of the object, wherein the first area is adjacent to the second area such that continuous illumination of the object is enabled.

A powder material which transforms under the influence of radiation emitted by the semiconductor lasers, e.g. melts or sinters, may be used in the method.

The method may further include a step of moving an illumination unit across the working area.

It shall be understood that the laser printing system of claim 1 and the method of claim 33 have similar and/or identical embodiments, in particular, as defined in the dependent claims.

It shall be understood that a preferred embodiment of the invention can also be any combination of the dependent claims with the respective independent claim. In particular, the method can be performed with the laser printing system according to any of claims 1 to 32.

Further advantageous embodiments are defined below.

BRIEF DESCRIPTION OF THE DRAWINGS These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

The invention will now be described, by way of example, based on embodiments with reference to the accompanying drawings.

In the drawings:

Fig. 1 shows a principal sketch of a first laser printing system.

Fig. 2 shows a section of the first laser printing system.

Fig. 3 shows a principal sketch of a section of a second laser printing system.

Fig. 4 shows a principal sketch of an arrangement of laser arrays in a laser mod- ule of the laser printing system.

Fig. 5 shows a principal sketch of a first arrangement of laser modules of the laser printing system.

Fig. 6 shows a principal sketch of a second arrangement of laser modules of the laser printing system.

Fig. 7 shows an integrated intensity profile with every second pixel off in the arrangement of laser modules shown in Fig. 6.

Fig. 8 shows an integrated intensity profile with an arbitrary pattern of on/off switched pixels in the arrangement of laser modules shown in Fig. 6.

Fig. 9 shows a principal sketch of method steps of a method of laser printing. Fig. 10 shows a principal sketch of a third arrangement of laser modules of the laser printing system.

Fig. 1 1 shows a principal sketch of a 3D printing system for additive manufacturing.

Fig. 12 shows a principal sketch of a top view of the 3D printing system for ad- ditive manufacturing.

Fig. 13 shows a principal sketch of the first arrangement of laser modules and the respectively associated printing areas in the working plane.

Fig.14 shows a principal sketch of an embodiment of an optical element associated with a laser module.

Fig. 15 shows a principal sketch of an alternative arrangement of laser light sources in an array of laser light sources.

Fig. 16a shows a principal sketch of an arrangement of laser light sources in an array and an associated integrated intensity profile of the array. Fig. 16b shows an arrangement of laser arrays according Fig. 16a in a laser module as depicted in Fig. 4 with a pattern of on/off switched pixels and an associated integrated intensity profile.

Fig. 17a shows a principal sketch of an arrangement of the laser light sources in an array similar to Fig. 15 and an associated integrated intensity profile of the array.

Fig. 17b shows an arrangement of laser arrays according to Fig. 17a in a laser module as depicted in Fig. 4 with a pattern of on/off switched pixels and an associated integrated intensity profile.

In the Figures, like numbers refer to like objects throughout. Objects in the Figures are not necessarily drawn to scale.

DETAILED DESCRIPTION OF EMBODIMENTS

Various embodiments of the invention will now be described by means of the Figures.

Fig. 1 shows a principal sketch of a first laser printing system 100. The laser printing system 100 comprises two laser arrays 1 10 with semiconductor lasers 115 and an optical element 170. The semiconductor lasers 1 15 are VCSELs which are provided on a semiconductor chip. In this case all VCSELs 115 of one array 110 are provided on one chip. The opti- cal element 170 is an imaging lens with a focal distance f. The arrays 110 have a width D perpendicular to the plane of the drawings which is diffusely imaged to a working plane 180 by means of the imaging lens. The width d of the diffuse image of each array 110 with the width D in the working plane 180 defines the width of a pixel in the working plane 180. The width of the pixels d is smaller than the width D of the respective array. The images of the arrays are thus demagnified. The distance b between the working plane 180 and the imaging lens or optical element 170 is bigger than the focal length f of the imaging lens. The optical element 170 or imaging lens defines together with the working plane 180 an object plane 150 in distance g bigger than the focal length of the imaging lens. The light emitting surfaces of the VCSELs 1 15 are not arranged in the object plane but behind the object plane in a distance such that no sharp projection of the light emitting surfaces of the VCSEL 115 is provided. The distance a between the light emitting layers of the VCSELs 115 and the object plane is chosen in a way that the laser light of at least two VCSEL 115 of one laser array 1 10 simultaneously illuminate an area element of a pixel. Figure 2 shows the arrangement of a divergence angle of laser light emitted by one VCSEL 115 in relation to the object plane 150 in more detail. The divergence angle of the VCSELs 115 is given by an angle a as shown in Figure 2 and defines the cone of laser light emitted by the single VCSEL 115. The VCSELs 115 in the laser array 1 10 do have a distance p with respect to each other (pitch). The relation between pitch p and distance a has to fulfill the condition: a > p*(tanoc )^_1

Laser light emitted by the VCSELs 1 15 of the laser array 110 overlap in the ob- ject plane 150 such that each area of the same size as the laser array 110 in the object plane 150 is illuminated by means of at least two VCSEL 1 15. Each area element of the pixel defined by the pixel size d is consequently also illuminated via the imaging lens by means of at least two VCSELs 1 15 of the respective laser array 110. The VCSELs of each laser array are driven in parallel and thus emit laser light at the same time. The size of the pixel is given by d=M*D, wherein the magnification M is given by M=b/g.

The image of the laser array 110 in the working plane 180 is diffuse in order to increase the homogeneity of the energy input to the object in the working plane 180 and improve the reliability with respect to malfunctions of single VCSEL.

The total distance between the laser arrays 110 of the laser module and the working plane 180 may be increased by means of a micro-lens array 175 which may be combined with the laser array 1 10 as shown in Fig. 3. The micro-lens array 175 may be arranged between the laser array 110 and the object plane 150 in order to decrease the divergence angle a of each VCSEL 1 15. The distance a and therefore the total distance to the working plane 150 has to be increased in order to fulfill the condition a > p*(tana )^"' if the pitch of the

VCSELs 115 remains the same. In an improvement of the condition discussed with respect to Fig. 2 may be achieved by taking into account an active diameter v of the VCSELs 115 in case of VCSELs 1 15 with circular aperture. The active diameter v corresponds to the diameter of the light emitting area of the active layer. The relation between active diameter v, pitch p and distance a has in this improved embodiment to fulfill the condition: a > (p-v)*(2tanoc )^_I .

Fig. 4 shows a principal sketch of an arrangement of laser arrays 110 in a laser module of the laser printing system 100. The laser or VCSEL arrays 110 are not quadratic but rectangular, with the long side of the rectangle being arranged in the direction of the movement of the object (see Fig. 5). This allows higher total powers per pixel, without reducing the resolution in the lateral direction. The VCSEL arrays 110 are further arranged in two columns which are slightly shifted with respect to each other (cascaded or staggered arrangement). This enables a defined overlap with respect to the illumination of area elements of the object if the object moves perpendicular to the direction of the columns of VCSELs.

Fig. 5 shows a principal sketch of a first arrangement of laser modules of the laser printing system 100. The laser modules comprise staggered or cascaded arrangements of laser arrays 1 10 as shown in Fig. 4 and an optical element 170. The optical element 170 images all laser arrays 1 10 of the respective laser modules to the working plane 180 of the laser printing system 100. The optical element 170 defines the total size Y of the laser module wherein the width of the arrangement of laser arrays 110 of the respective laser module defines the printing width y of one laser module. The laser modules are arranged in columns parallel to each other wherein each column is shifted such that a continuous area can be illuminated in the working plane 180 if the object moves in direction 250 relative to the laser modules. The printing area can thus be adapted to the size of the object in the working plane independent on the size Y and printing width y of the single laser module. The number of columns needed in order to continuously illuminate an object moving in the working plane 180 depends on the size Y and printing width y of the laser modules. The laser modules within one column are separated by at least by a distance Y such that at least N=Y/y columns are needed. The cascaded optical elements 170 may be fabricated as a single piece e.g. by glass molding. Alternatively, a lens array may be assembled from individual lenses by active or passive alignment. Fig. 6 shows a principal sketch of a second arrangement of laser modules of the laser printing system. The arrangement is quite similar to the arrangement discussed with respect to Fig. 5. The laser arrays 110 of the laser modules are slanted (rotated around their center) with respect to a direction perpendicular to the direction of movement 250 of the object relative to the laser modules. This enables integrated intensity profiles with smooth slopes as shown in Figs. 7 and 8, which may also overlap with the adjacent pixels, to improve the homogeneity of the total intensity distribution, especially if the pixels are slightly misaligned with respect to each other.

Fig. 7 shows an integrated intensity profile in a direction 610 perpendicular to the direction of movement 250 of the object relative to the laser modules with every second pixel off in the arrangement of laser modules shown in Fig. 6. The pixel profile is almost triangular, with large slopes that overlap with the adjacent pixels. Fig. 8 shows an integrated intensity profile with an arbitrary pattern of on/off switched pixel in the arrangement of laser modules shown in Fig. 6. The numbers "1" and "0" indicate which of the adjacent laser arrays 1 10 are switched on or off. The integrated intensity profile shows the overlap of two or more neighboring pixels in the working plane 180.

Fig. 9 shows a principal sketch of method steps of a method of laser printing. The shown sequence of steps does not necessarily imply the same sequence during execution of the method. Method steps may be executed in different order or in parallel. In step 910 the object like a sheet of paper is moved in the working plane of the laser printing system relative to the laser module. In step 920 laser light is emitted by means of the laser module comprising at least two laser arrays of semiconductor lasers and at least one optical element. In step 930 the laser light emitted by the laser arrays is imaged, such that laser light of semiconductor lasers of one laser array is imaged to one pixel in the working plane and an area element of the pixel is illuminated by means of at least two semiconductor lasers. The object may be moved and at the same time laser light of the laser arrays may be emitted and imaged to the working plane.

When using individually addressable lasers or laser arrays, the maximum speed in the 3D printing process can be obtained when along a line all individual pixels can be writ- ten at the same time, i.e. by a separate laser or laser array per pixel. Typical line widths in a laser printing system or machine are in the order of 30 cm or more. On the other hand, the size or printing width of a laser module of individually addressable lasers or laser arrays is limited to a few cm. These laser modules correspond usually to one micro-channel cooler on which the laser modules are arranged.

It is therefore necessary to use a number of laser modules and corresponding micro-channel coolers and to stack them together to a complete laser printing module or print- ing head. Alignment tolerances between neighboring micro-channel coolers with laser modules may result in a gap in the working plane 180 to which no or not sufficient laser light can be provided. In worst case such a gap leads to defects with respect to processing of the object as printed sheets of inferior quality or in the parts produced by means of a 3D printer / additive manufacturing machine.

In view of the typical size of a laser light source 1 16 of 100 μηι and the fact that several alignment tolerances add up together, the problem of a gap is a severe issue. Even with tight tolerances in each individual step of assembling the laser printing system, the overall tolerance chain can lead to significant deviations of 30 μηι or more.

It may be advantageous in this respect not only to provide overlapping intensity distributions but to use additional laser light sources 116 at the edge of each laser module. Said laser light sources 116 are so called overlap laser light sources 117 which are arranged such that the light of these overlap laser light sources 1 17 overlaps with light of laser light sources 1 16 of neighboring laser module. This means that the pitch between neighboring laser modules is smaller than the total printing width of the laser module by at least the width of one laser light source 116 (e.g. 100 μηι).

If the maximum tolerance from the mechanical / optical alignment of neighboring laser modules is smaller than the width of one laser light source 116, it is sufficient to have - by design - an overlap of one laser light source 116 in order to avoid gaps in the working plane to which no laser light can be provided. Anyhow, it may alternatively be possible to pro- vide more than one overlap laser light sources 117 if the maximum tolerance from the mechanical / optical alignment of neighboring laser modules is bigger than the width of one laser light source 116. It may in this case be possible to use the overlap laser light sources 117 in accordance with the width of the gap between neighboring laser modules. The laser printing system may in this case be calibrated such that the overlap laser light sources 117 fill the unintended gap between the laser modules. Depending on the gaps and the width of one laser light source

1 16 it may be that one, two, three or even more of the overlap laser light sources 117 are used in order to enable a continuous, i.e. seamless illumination of the working plane. Fig. 10 shows an embodiment of such an arrangement with overlap laser light sources 117 which are arranged in an overlapping arrangement of neighboring laser modules which are laser sub-modules 120 in order to compensate potential misalignment of laser sub- modules 120 with respect to each other. The overlap laser light sources 117 are indicated by a line pattern.

The printing width of neighboring laser sub-modules 120 overlap by a complete laser light source 1 16 or more explicit overlap laser light source 1 17. A laser light source 116 may comprise different as the previous embodiments only a single laser or in accordance with the previous embodiments a laser array such as laser arrays 110. The single lasers may com- prise optical elements like micro-lenses. In case of laser arrays micro-lens arrays may be comprised. The arrangement of the laser sub-modules 120 is similar to the arrangement as shown in Fig. 5. The laser modules shown in Fig. 5 are arranged such that each laser array 110 illuminates a dedicated pixel or area element in the working plane 180. The laser sub-modules 120 as shown in Fig. 10 are arranged such that in case of no alignment errors during assembly the overlap laser light sources 117 are adapted such that they can illuminate the same area element in the working plane 180 as a laser light source 116 of a neighboring laser sub-module 120.

Figs. 11 and 12 show schematically an embodiment of a 3D-laser printing system for additive manufacturing. Referring to Fig. 11, the 3D-laser printing system includes a process chamber 300 with a support 400 for carrying building material and a three-dimensional article 500 to be built thereon. On the support 400 a building platform 450 may be provided which serves as a removable base for removing the three-dimensional article 500 after the building process is finished. It shall be noted that the building platform 450 may also be omitted. A boundary structure 470, such as vertical walls, may be arranged around the support 400 to confine layers of the building material on the support 400. The boundary structure may be arranged as a removable frame, which may include a vertically movable base which is removably attached to the support 400, similarly to the building platform 450. As illustrated in Fig. 12, a building area 480 may be defined by the boundary structure 470. The building area 480 may have a rectangular contour as shown in Fig. 12 or any other contour such as but not limited to a square-shaped or a circular contour.

Above the support 400, an illumination unit 700 is arranged. Preferably, the illumination unit 700 is movable across the building area 480 in a direction depicted by the arrow in Fig. 12 which is the direction of movement 250 in this embodiment. The illumination unit 700 may be configured to be moved back in an opposite direction. It may be switched on or switched off during the back movement.

The support 400 is movable up and down relative to the illumination unit in a vertical direction, i.e. in a direction perpendicular to the direction of movement 250 of the il- lumination unit 700. The support 400 is controlled in such a manner that an uppermost layer of the building material forms the working area 180.

The 3D-laser printing system further includes a control system 800 for controlling various functions of the 3D-printing system. A recoating device (not shown) may be provided to apply layers of building material onto the building platform 450 or the support 400 or the movable base of a removable frame (not shown). Furthermore, one or more separate heating device(s) (not shown) may be provided that may be used to heat an applied layer of building material to a process temperature and/or to control the temperature of the building material in the boundary structure 470, if necessary.

The building material preferably is a powder material that is configured to trans- form under the influence of the laser light emitted by the laser light sources into a coherent mass. The transformation may include, for example, melting or sintering and subsequent solidification and/or polymerization in the melt. Preferably, the building material is a plastic powder, for example a thermoplastic powder. Examples of such plastic powders are PA 12 (polyamide 12) or other polyamides, polyaryletheretherketone, such as PEEK or other polyetherketones. The powder may also be a powder from a metal or a metal alloy with or without a plastic or metal binder, or a ceramic or composite or other kind of powder. Generally, all powder materials that have the ability to transform into a coherent mass under the influence of the laser light emitted by the semiconductor lasers can be used. The building material may also be a pastelike material including a powder and an amount of liquid. Typical medium grain sizes of the powder lie between 10 μιη or even less and 100 μηι, measured using laser diffraction according to ISO 13320-1.

Typical wave length of the laser light sources are preferably 980 or 808 run in conjunction with absorbers (laser light absorbing additives to the powder material), e.g. but not limited to Carbon Black, suitable to enable a sufficient absorption of the chosen wave length. In principle any wavelength is possible as long as a suitable absorber material can be added to the powder material. Typical layer thicknesses of the powder layers may range between about 10 μιη and about 300 μπι, in particular for plastic powders, and about 1 μιη up to about 100 μιη, in particular for metal powders. The illumination unit 700 will be described more in detail with reference to Fig, 1 1 to 13. Fig. 13 shows an arrangement of laser modules similar to that of Fig. 5 with the difference that more than two columns and the demagnified image produced by the laser modules with the optical elements in the working plane 180 are shown. Fig. 13 shall not be considered as a perspective view but only as a schematic sketch depicting the arrangement of modules and the corresponding demagnified images. As schematically depicted in Fig. 13, the illumination unit 700 includes a plurality of laser modules 200 arranged in columns perpendicular to the direction of movement 250. Like in Figs.5 and 6, the columns of the laser modules are staggered with respect to each other such that a first laser module 200i of a first column cl of laser modules is adapted to illuminate a first area y 1 of the powder in the working plane 180. The second module 200₂ of a second column c2 of laser modules is adapted to illuminate a second area y2 of the powder in the working plane 180, wherein the first area yl is adjacent to the second area y2 such that continuous, i.e. seamless illumination of the object is enabled. By means of this, the illuminated areas yl, y2 in the working plane 180 form a contiguous area in the direction perpendicular to the direction of movement. As further depicted in Fig. 13, laser modules that are staggered in the direction of movement 250 form cascades. A first cascade kl is fomed by the first laser modules 200_1; 200₂, 200_n of the columns. A second cascade k2 ist formed by the second laser modules 2011 , 201₂, 201„ of the columns and so on. The number of cascades is such that the sum of the individual printing widths y in a direction perpendicular to the direction of movement 250 covers the width of the building area 480. For different 3D-laser printing systems having different building areas, the number of cascades can be easily adapted to cover the different widths of the respective building areas 480. In a typical example of a 3D- laser printing system for additive manufacturing using VCSEL as semiconductor lasers, one array may have several hundreds of semiconductor lasers, for example VCSELs, one module may include 2 x 16 = 32 arrays, one cascade may include 9 modules and the illumination unit may include several of those cascades, for example 3. This typically allows to illuminate a building area 480 of about 84 mm. Other building areas can be achieved by selecting appropriate numbers of modules per cascade and of cascades. As described above, one single optical element 170 is associated with one module and one module is preferably used to illuminate 16, 32 or 64 pixel in the working plane.

Referring again to Fig. 11 , since the illumination unit 700 is arranged within the process chamber, it is exposed to the ambient conditions that exist in the process chamber 300, such as the average temperature, temperature gradients, vapors, gas flows, such as inert gas flows, dust, splashes of molten material which could emerge from the building area, monomers emerging from the transformation process of the building material and moving around in the process chamber etc. A distance between the outermost optical element of the illumination unit 700 that is facing towards the building area and the working plane 180 may be in the range between about 5 mm to about 50mm. This arrangement of the illumination unit 700 is different from the known laser-melting or laser-sintering machines. To protect the illumination unit 700, a protective device 750 is arranged on a side of the illumination device 700 facing the support 400. The protective device 750 may be realized by at least one plate that is transparent for the laser light. The transparent plate may be integrally formed with the illumination device 700. In particular, the protective device 750 may be a glass plate. Moreover, the protective device 750 may be a single piece protecting all modules of the illumination unit 700 or may be composed of a plurality of pieces, one for each module. A distance between the outermost surface of the protective device and the working area may be only several millimeters, for example, about 5 mm. More generally, if a specific demagnification of n: 1 is intended, a distance between the laser emitting portion of the semiconductor lasers and the outermost optical element (in the optical path) may be essentially about n times the distance between the outermost optical element and the working area 180.

Preferably, a temperature control device (not shown) is associated with the protective device 750. The temperature control device may be realized in the form of a number of (i.e. one or more) heating elements. Preferably, the heating elements are arranged on the transparent plate, in particular only on such positions, where effectively no laser light is transmitted or where no laser light is intended to be transmitted. More preferably, the heating elements are provided on a side of the protective device 750 that faces away from the support 400, i.e. that faces towards the laser light sources of the illumination unit 700. This facilitates cleaning of the protective device and reduces abrasive wear of the heating elements. The heating elements may be in the form of heat conductive paths. In particular, the heating elements may be vapor- deposited or provided in the transparent plate during manufacturing of the protective device. In a further modification, the protective device 750 may include an assembly of two or more glass plates with vacuum or gas in-between the plates for thermal isolation. With such a design, a heat flow into the interior of the illumination unit 700 can be reduced or even prevented. In the case of an assembly of plates, the heating device may be provided at an inner side of one plate facing towards an adjacent plate, in particular of the outermost plate facing towards its adjacent plate. The temperature control device controls the temperature of the protective device 750 in such a manner that the temperature is adjusted to a specific temperature preferably in a range between around a few (preferably 10 at most, more preferred 5 at most and most preferred 3 at most) Kelvin below the process temperature to a few (preferably 10 at most, more preferred 5 at most and most preferred 3 at most) Kelvin above the process temperature. Due to the energy consumption and limited efficiency of the semiconductor lasers, the illumination unit 700 is cooled and preferably held at a temperature that can be considerably lower than the process temperature of the transformation process of the building material, depending on the building material used. Hence, heat loss by thermal radiation from the layers of building mate- rial to the illumination unit 700 is reduced or prevented. Moreover, the forming of condensates at the surface of the protection device 750 can be reduced or avoided. Those condensates would reduce the transparency of the glass plate/laser window/ protective device and therefore would reduce the disturbance and/or the amount of absorbed laser light energy at the surface of the powder material. As a consequence, the quality of the three-dimensional articles to be built would be decreased. The temperature control device therefore ensures good quality of the three-dimensional articles to be built.

The presence of the protective device 750 requires the image distance b, i.e. the distance between the optical element 170 and the working plane 180 (see Fig. 1), to be a certain minimum image distance. Due to the necessary demagnification, the object distance g, i.e. the distance between the object plane 150 and the optical element 170, is relatively high. The divergence angle a of each VCSEL 115 results in the fact that the beam path of VCSEL-arrays of adjacent modules cross each other which renders a module-wise imaging onto the object plane 150 difficult. To avoid this, the illumination unit 700 includes micro lens arrays 175 as depicted in Fig. 3 for each module.

Preferably, the laser arrays 110 of the modules 200 are arranged as depicted in

Fig. 14. In a further preferred embodiment, an optical element 170 associated with such an arrangement of the laser arrays 110 has a contour obtained from a circular or rotationally symmetrical contour, which is truncated on opposing sides and wherein the opposing sides 1 of the optical element 170 are aligned with respect to each other along an axis which is preferably orientated in a direction perpendicular to the direction of movement 250. More precisely, in the case of the arrangement of the laser arrays as in Fig. 14, the optical element 170 has the contour of a modified rectangle with two opposing circular segment-shaped short sides s that connect the parallel long sides 1. This takes into account that a circular optical element would not be fully illuminated with the rectangular arrangement of the laser arrays as depicted in Fig. 14. Hence, the portions of a circular optical element that are not fully illuminated can be omitted. By means of the shape of the optical element 170, the size of a module in the direction of movement 250 can be reduced. As a result thereof, the size of the entire illumination unit 700 in the direction of movement 250 can be reduced. This has the advantage that a line oriented in the direction of movement can be illuminated within a reduced time which enhances the productivity of the whole 3D-printing system. Also, neighboring pixels at the border between one module 2001 and a neighboring module 200₂ of one cascade kl and/or of one module 200_n of one cascade kl and a neighboring module 2011 of a neighboring cascade k2 can be illumi- nated with reduced time offset. This also increases the quality of the three-dimensional article.

The arrangement of the VCSELs in the laser array 110 defines the intensity profile. If the arrangement is substantially rectangular, i.e. the VCSELs are arranged in the array in rows and columns, the integrated intensity profile 600 of the array is substantially rectangular, i.e. the integrated intensity profile has a so-called "flat top" profile as depicted in Fig. 16a. In a module according to Fig. 4, where several arrays 1 10 are switched on and several arrays are switched off, the integrated intensity of the module in a direction 610 perpendicular to the direction of movement 250 is as shown in Fig. 16b, i.e. has sharp edges (in the case that the object plane 150 is coincident with the active area of the semiconductor lasers).

It may be desirable to have an integrated intensity profile without sharp edges. This can be achieved by an arrangement according to Fig. 15, wherein the VCSELs in one array 110 are positioned in rows and columns and wherein the outer contour of the array is substantially polygonal, in particular, substantially hexagonal. The individual VCSELs are positioned at grid points that are staggered from one column to the next column, wherein the columns are oriented perpendicular to the direction of movement 250. Preferably, the outer con- tour of the array has a hexagonal shape with two opposing parallel sides p which extend perpendicular to the direction of movement 250.

As depicted in Fig. 17a, the integrated intensity profile 600 of a laser array with a substantially hexagonal shape as shown in Fig. 15, has rounded edges and is similar to a Gaussian intensity distribution. For a laser module with switched on/off arrays, the integrated intensity profile 600 along a direction 610 comprises rounded transitions as depicted in Fig.

17b. Hence, deviations from an average value of intensity are smaller.

With the illumination unit 700, one pixel in the working area is illuminated by a multitude of semiconductor lasers of a laser array 1 10 at the same time. The total number of semiconductor lasers may be selected such that failure of less than a predetermined number of the semiconductor lasers reduces the output power of the laser array 110 only within a predetermined tolerance value. As a result thereof, the requirements with respect to the working life of the individual VCSELs may not be unusually high.

The individual VCSELs of a laser array may be grouped in sub-groups with respect to their addressability by control signals. A sub-group may include at least two VCSELs. At least two sub-groups of VCSEL of one laser array may be individually addressable such that an output power , i.e. an intensity, of the laser array 110 is controllable by switching off one or more sub-groups of VCSEL. Also, an embodiment may be provided where the semiconductor lasers of one laser array are individually addressable so that an output power of the laser array may be controlled by switching on/off individual semiconductor lasers.

In a further embodiment, the semiconductor lasers or the laser arrays of the illumination unit 700 can be further controlled such that a semiconductor laser or a laser array which is not used for illuminating can optionally be used for providing heat to the building material in the working plane 180. To accomplish this, a control device is provided which controls the semiconductor lasers individually or the laser arrays in such a manner that the semiconductor lasers or a laser array which is not used for illuminating emits less intensity as required for transforming the building material so as to only heat the building material in the working plane. This heating can be used in addition to the separate heating device described above or as an exclusive heating system that pre-heats the building material to a working temperature.

The illumination unit 700 may include overlap light sources 117 as explained with reference to Fig. 10. The overlap light sources 117 are preferably provided at the border between one module of one column to a neighboring module of a neighboring column, for ex- ample module 2001 of column cl and module 200₂ of column c2 in Fig. 13 and/or from one module in one cascade to a neighboring module in a neighboring cascade, for example module 200_n in cascade kl and module 2011 in cascade k2 in Fig. 13. The overlap light source 117 balances the energy loss resulting from a time offset of adjacent pixels perpendicular to the direction of movement 250 due to the staggered arrangement of a module and/or due to the cascaded arrangement of the modules.

The overlap light sources 117 can be controlled in such a manner that energy losses due to time offset and/or energy losses or energy excesses due to misalignment of VCSELs or arrays can be compensated. Hence, the sum of energy that is provided to the work- ing area by overlap light sources 117 can be adjusted to be the energy necessary for illuminating in the case of time offset zero and/or perfectly aligned VCSELs or arrays. The energy provided by the overlapping VCSELs or arrays can be selected depending on the type of building material. Influencing factors may be the heat conductivity of the powder bed, the heat conduc- tivity of the melt or the sintered mass, the particle size, etc.

In a further modification, the semiconductor lasers of the illumination unit are realized by VECSELs (Vertical External Cavity Surface Emitting Laser).

The 3D-printing system described above is operated as follows. Layers of the building material are successively deposited onto the support 400 or the building platform 450 or a previously illuminated layer such that the new layer of building material forms the working plane 180. Then, the illumination unit 700 moves across the building area 480 in the direction of movement 250 and selectively illuminates the building material in the working area 180 at positions corresponding to the cross-section of the three-dimensional article in the respective layer. After one layer has been illuminated, the support is moved downward such that the new layer can form the working area 180.

While the invention has been illustrated and described in detail in the drawings and the foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive.

From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known in the art and which may be used instead of or in addition to features already described herein.

Variations to the disclosed embodiments can be understood and effected by those skilled in the art, from a study of the drawings, the disclosure and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality of elements or steps. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Any reference signs in the claims should not be construed as limiting the scope thereof. List of reference numerals:

100 laser printing system

1 10 laser array

1 15 semiconductor laser

116 laser light source

1 17 overlap laser light source

120 laser sub-module

150 object plane

170 optical element

175 micro-lens array

180 working plane

200, 200i, 200₂,200_n

201 i, 201_2; 201n laser modules250 direction of movement

300 process chamber

400 support

450 building platform

470 boundary structure

480 working area

500 three-dimensional article

600 integrated intensity

610 direction perpendicular to direction of movement

700 illumination unit

750 protective device

800 control unit

910 method step of the object

920 method step of emitting laser light

930 method step of imaging the laser light

Claims

1. A laser printing system (100) for illuminating an object moving relative to a laser module of the laser printing system (100) in a working plane (180), the laser module comprising at least two laser arrays (110) of semiconductor lasers (115) and at least one optical element (170), wherein the optical element (170) is adapted to image laser light emitted by the laser arrays (110), such that laser light of semiconductor lasers (115) of one laser array (110) is imaged to one pixel in the working plane (180) of the laser printing system (100), and

wherein the laser printing system is a 3D printing system for additive manufacturing and

wherein two, three, four or a multitude of laser modules are provided, which are arranged in columns perpendicular to a direction of movement (250) of the object in the working plane (180), and

wherein the columns are staggered with respect to each other such that a first laser module of a first column of laser modules is adapted to illuminate a first area of the object and a second laser module of a second column of laser modules is adapted to illuminate a second area of the object, wherein the first area is adjacent to the second area such that continuous illumination of the object is enabled.

2. The laser printing system (100) according to claim 1, wherein an area element of the pixel is illuminated by means of at least two semiconductor lasers (115).

3. The laser printing system (100) according to claim 1 or 2, wherein the optical element (170) is arranged in a way that an object plane (150) of the optical element (170) with respect to the working plane (180) does not coincide with a plane of the semiconductor lasers (115) such that the cones of laser light emitted by adjacent semiconductor lasers (115) overlap in the object plane (150).

4. The laser printing system (100) according to one of claims 1 to 3, wherein the laser module comprises three, four or a multitude of laser arrays (1 10).

5. The laser printing system (100) according to one of claims 1 to 3, wherein the optical element (170) comprises one lens being adapted to image the laser light of the laser arrays

(110) to the working plane (180).

6. The laser printing system (100) according to one of claims 1 to 3, wherein the optical element (170) is adapted such that images of the laser arrays (110) overlap in the working plane (180).

7. The laser printing system (100) according to one of claims 1 to 3, wherein the laser arrays (110) of the laser module are arranged in columns perpendicular to a direction of movement (250) of the object in the working plane (180), the columns are staggered with respect to each other such that a first laser array ( 110) of a first column of laser arrays ( 110) is adapted to illuminate a first area of the object and a second laser array (110) of a second column of laser arrays (1 10) is adapted to illuminate a second area of the object, wherein the first area is adjacent to the second area such that continuous illumination of the object is enabled.

8. The laser printing system (100) according to one of claims 1 to 3, wherein the laser arrays (110) are rectangular with the long side of the rectangle being arranged parallel to a direction of movement (250) of the object in the working plane (180).

9. The laser printing system (100) according to one of claims 1 to 3, wherein the number of columns of laser modules is arranged in a way such that the distance between laser modules in one column of laser modules is minimized.

10. The laser printing system (100) according to one of claims 1 to 3, wherein the laser arrays (110) of each laser module are arranged in an elongated arrangement with the long side of the elongated arrangement being arranged perpendicular to the direction of movement (250) of the object in the object plane (180).

11. The laser printing system (100) according to one of claims 1 to 3, wherein the laser arrays (1 10) of each laser module are arranged in an elongated arrangement with the long side of the elongated arrangement being arranged slanted to the direction being perpendicular to the direction of movement (250) of the object in the working plane (180).

12. The laser printing system (100) according to one of claims 1 to 3, wherein the optical element (170) is arranged to demagnify the image of the laser arrays (110) in the working plane (180).

13. The laser printing system (100) according to one of claims 1 to 3, wherein each laser array (1 10) comprises a micro-lens array (175), the micro-lens array being arranged to lower the divergence of the laser light emitted by the semiconductor lasers (115).

14. The laser printing system (100) according to one of claims 1 to 13, comprising at least a first and a second laser module arranged next to each other, each laser module comprising at least two laser arrays (110), wherein at least one of the two laser arrays (110) of the first or the second laser module is arranged as an overlap laser light source (117) such that in operation at least one defined area element in the working plane (180) can be illuminated by the overlap laser light source (117) and a separate laser array (110) of the laser module arranged next to the laser module comprising the overlap laser light source (117).

15. The laser printing system (100) according to claim 14, wherein the total energy which is provided to the at least one defined area element of the object is such that essentially the same energy is provided per area element as in the case of aligned laser modules without an overlap laser light source (1 17).

16. The laser printing system (100) according to one of claims 1 to 15, wherein one pixel is illuminated by a multitude of semiconductor lasers of a laser array at the same time and wherein the total number of semiconductor lasers is such that failure of less than a

predetermined number of the semiconductor lasers reduces the output power of the laser array only within a predetermined tolerance value.

17. The laser printing system (100) according to one of claims 1 to 16, wherein a laser module is configured to illuminate at least 2, more preferably 4, 8, 16, 32, 64 or more pixels using a single optical element associated with the laser module.

18. The laser printing system (100) according to claim 7, wherein the optical element (170) associated with a laser module has an outer contour obtained from a circular or rotationally symmetrical contour which is truncated on two opposing sides and wherein the opposing sides are aligned with respect to each other along an axis which is preferably oriented in a direction perpendicular to the direction of movement (250).

19. The laser printing system (100) according to one of claims 1 to 18, wherein a control device is provided which controls the semiconductor lasers (115) individually or the laser array in such a manner that a semiconductor laser (1 15) or a laser array which is not used for illuminating is used for providing heat to the working plane (180).

20. The laser printing system (100) according to claim 19, wherein the semiconductor laser (115) or a laser array which is not used for illuminating is operated with a lower power than a semiconductor laser (115) or a laser array which is used for illuminating.

21. The laser printing system (100) according to one of claims 1 to 20, wherein at least two semiconductor lasers (115) of one laser array (1 10) or at least two sub-groups of semiconductor lasers of one laser array (110) are individually addressable such that an output power of the laser array (1 10) is controllable by switching off one or more semiconductor lasers (115) or the sub-groups of semiconductor lasers.

22. The laser printing system (100) according to one of claims 1 to 21, wherein a plurality of semiconductor lasers (115) form an array (1 10) and are arranged such that the outer contour of the array (1 10) has a substantially polygonal, preferably a substantially hexagonal shape.

23. The laser printing system (100) according to one of claims 1 to 22, further comprising a process chamber (300) comprising a support for layers of a material, wherein the laser modules are arranged in the process chamber and wherein preferably a protective device (750) is arranged at a side of the laser modules which faces the support.

24. The laser printing system (100) according to claim 23, wherein the protective device is formed of at least one plate that is transparent for the laser light, preferably a glass plate.

25. The laser printing system (100) according to claim 23 or 24, wherein a temperature control device is provided that controls the temperature of at least the surface of the protective device oriented towards the support.

26. The laser printing system (100) according to claim 25, wherein the temperature control device is configured to heat the protective device such that thermal radiation from material in the working plane to the protective device is substantially prevented.

27. The laser printing system (100) according to one of claims 1 to 26, wherein the laser printing system (100) is configured to solidify a material layer by layer at locations corresponding to the cross section of an article to be formed in each layer using the laser modules.

28. The laser printing system (100) according to claim 27, wherein the material is a powder.

29. The laser printing system (100) according to one of claims 1 to 28, wherein the laser modules form an illumination unit (700) and wherein the illumination unit is configured to move across the working plane.

30. The laser printing system according to one of claims 1 to 29, wherein one laser array includes at least two seminconductor lasers.

31. The laser printing system according to one of claims 1 to 30, wherein the

semiconductor lasers are VCSEL (Vertical Cavity Surface Emitting Lasers).

32. The laser printing system according to one of claims 1 to 30, wherein the

semiconductor lasers are VECSEL (Vertical External Cavity Surface Emitting Laser).

33. Method of laser printing, the method comprising the steps of

moving an object in a working plane (180) relative to a laser module;

emitting laser light by means of the laser module comprising at least two laser arrays (1 10) of semiconductor lasers (1 15) and at least one optical element (170); and imaging the laser light emitted by the laser arrays (110) by means of the optical element (170), such that laser light of semiconductor lasers (115) of one laser array (1 10) is imaged to one pixel in the working plane (180);

wherein the laser printing system (100) is a 3D printing system for additive manufacturing and wherein two, three, four or a multitude of laser modules are used which are arranged in columns perpendicular to a direction of movement (250) of the object in the working plane (180),

and wherein the columns are staggered with respect to each other such that a first laser module of a first column of laser modules is adapted to illuminate a first area of the object and a second laser module of a second column of laser modules is adapted to illuminate a second area of the object, wherein the first area is adjacent to the second area such that continuous illumination of the object is enabled.