3D printer

Abstract

The invention specifies a 3D printer (1), which has a print head (6) for extruding a transparent ink strand (8), and has a transparent printing plate (10) for depositing the extruded ink strand (8). The 3D printer (1) also has an imaging apparatus (20), which provides a pattern formed at least from straight strips (22), and an optical recording device (14), which is designed to record an image (30) of the pattern and also of the deposited ink strand (8) during the designated printing operation by the 3D printer (1) from a bottom side (18) of the printing plate (10). The 3D printer (1) also has a controller (12), which is designed to derive at least one variable, which is characteristic of the printing operation, from the image (30) recorded by means of the recording device (14).

Description

[0001] Description

[0002] 3D printing device

[0003] The invention relates to a 3D printing device.

[0004] 3D printing devices were originally used for prototype production, or at best for small-scale production. The primary goal was to avoid cost-intensive processes such as injection molding, which require expensive injection molding tools. The simplest form of such "rapid prototyping" processes is so-called "fused deposition modeling" (FDM), in which thermoplastic material is usually melted and deposited in strands, either in webs or layers, using an extruder nozzle on a usually heatable plate, usually in a heated room. With suitable process control, a three-dimensional construct with a resolution in the sub-millimeter range can be produced by fusing the strands together.The coordinates are usually provided to the 3D printing device based on CAD models, which are converted into layered paths, known as G-codes, using slicer software. Since the development of this "bottom-up" concept, numerous other 3D printing processes have been developed. These sometimes also utilize other material classes, such as photo-crosslinkable resins, which are then solidified into a component using stereolithography. The use of such manufacturing methods is also increasingly being pursued for the production of larger series than small batches, as well as for individual components. Due to the gradual progress in manufacturing, these manufacturing methods are also referred to as "additive manufacturing." Intensive research is also being conducted in the field of regenerative medicine into the adaptation of additive manufacturing processes to artificially produce tissue or even organs.In so-called biofabrication, living cells are printed together with biocompatible materials to form three-dimensional constructs with defined pore sizes. To enable cell survival and tissue maturation, hydrogel-forming materials are used in particular, which are primarily intended to mimic the natural extracellular matrix of native tissue. For example, biomaterials based on collagen, gelatin, fibrin, hyaluronic acid, alginate, and silk proteins, as well as synthetic polymers such as PEGDA or POx, and various blends and chemically modified variants thereof, are used. The FDM process is often used in biofabrication because, in contrast to, for example, the inkjet process, which is also used for cell printing, it allows for processing a broader spectrum of materials and 3D constructs can be produced in a relatively short time.

[0005] For processing, however, the materials must first be brought into a liquid, malleable state - e.g., melt, hydrogel, or precursor solution - in order to then retain a defined 3D shape after a phase transition. The flow properties (or rheological properties) of the materials to be processed are of significant importance, both for processing and for the shape retention of the product. The main characteristic is usually the so-called viscosity. This describes the relationship between shear stress and shear rate. The shear stress is the stress that must be applied to deform a liquid at a certain shear velocity or shear rate. "Low viscosity" therefore means that low stresses must be applied to deform a fluid. These are then low-viscosity fluids, whereas an increasing (i.e.increasing) viscosity (“highly viscous”) describes an increase in “thickness” or “viscosity”. If there is a linear relationship between shear stress and shear rate, the viscosity is independent of the shear rate and is therefore constant - this is referred to as Newtonian flow behavior, which applies to water and many oils, for example. More complex systems, such as plastic melts, high-molecular-weight and concentrated solutions or multi-phase systems such as suspensions, on the other hand, often do not exhibit Newtonian behavior, since their viscosity is frequently dependent on the shear rate or other factors such as time-dependent structural changes in the material. Polymer melts and highly concentrated solutions usually exhibit shear-thinning behavior, i.e. their viscosity decreases with increasing shear rate.Physical hydrogels can be destabilized under high shear stress and converted into a liquid state, which allows them to be processed in an extrusion printing process. Such hydrogels or their precursors (i.e., aqueous solutions), which are loaded with cells and can be converted into a gel immediately after printing (by self-assembly or targeted cross-linking using, for example, ions or irradiation), are referred to below as "bioinks." The printing resolution depends on the dimensional stability of the deposited strand, which ideally should spread as little as possible, i.e., flow apart. How far (or strong) the strand can spread depends, in turn, on the relationship between (temporal) throughput, viscosity, and the rate of gelation.The difficulty in process optimization in bioprinting therefore lies in systematically recording the interplay of the parameters involved and optimizing them in a feedback loop. For this purpose, the time-resolved flow behavior of the printed strand must be recorded and documented, quantified, and ideally modeled based on parameters (pressure, flow rate, deposition rate, temperature, viscosity, etc.). However, determining throughput is often difficult to determine solely based on process parameters (pressure, temperature, etc.), especially for relatively low-viscosity materials and / or due to nonlinear flow behavior (e.g., structural viscosity, etc.).

[0006] The invention is based on the object of providing an improved 3D printing device. This object is achieved according to the invention by a 3D printing device having the features of claim 1. Furthermore, this object is achieved according to the invention by a method having the features of claim 11. Further advantageous and partly inventive developments of the invention are set forth in the subclaims and the following description.

[0007] The 3D printing device according to the invention has a print head used to extrude an at least initially transparent ink strand. Furthermore, the 3D printing device has a transparent printing plate for depositing the extruded ink strand. Furthermore, the 3D printing device has an imaging device that provides a pattern formed at least from rectilinear, preferably at least partially parallel, stripes (or lines), as well as an optical detection device configured to capture an image of the pattern and the deposited ink strand from an underside of the printing plate during normal printing operation of the 3D printing device. Furthermore, the 3D printing device has a control device configured to derive at least one variable characteristic of the printing operation from the image captured by the detection device.

[0008] At least initially transparent is understood here and below to mean in particular that the ink strand, in particular the ink used, is transparent after extrusion at least for a certain time (e.g. up to 5 or 10 seconds), in particular until solidification (due to crosslinking and / or crystallization) begins or has progressed to a certain degree, and can then optionally also become opaque.

[0009] The method according to the invention serves to operate a 3D printing device, in particular the one described above. A transparent ink strand is extruded by means of a print head (in particular the one mentioned above) of the 3D printing device and deposited on a transparent printing plate (in particular the one mentioned above). Furthermore, an imaging device (in particular the one mentioned above) is provided, which provides a pattern formed at least from rectilinear, optionally also at least partially parallel, stripes. Furthermore, an image of the pattern and the deposited ink strand is captured from an underside of the printing plate, and at least one variable characteristic of the printing operation is derived from the captured image.

[0010] Preferably, the control unit of the 3D printing device is configured to perform the method described above, preferably automatically, optionally in interaction with an operator of the 3D printing device. Conversely, in optional variants of the method according to the invention, the steps described below as being performed (or to be performed) by the control unit are performed accordingly. Thus, the 3D printing device and the method equally have the features and advantages described here and below.

[0011] According to the invention, optical features of the deposited ink strand are detected, and the aforementioned characteristic variable is inferred from these features. This is advantageous in that features can be observed that are a manifestation (sequence) of selected process parameters (e.g., throughput, pressure, feed rate, or the like). However, such detection of the aforementioned characteristic variable is decoupled from the aforementioned process parameters. The invention is based on the consideration that the image of the pattern detected from below the printing plate is influenced by the transparent ink strand. This influence, in turn, can be used to infer the dimensions of the ink strand and thus process parameters, in particular their values.

[0012] In an optional variant, the control unit is set up to use the characteristic variable to regulate or control the printing process.

[0013] The transparent (printer) ink is preferably a bioink, i.e., a material that is biologically compatible and particularly suitable and intended for use in or on the human body, e.g., as a tissue substitute, implant, or the like. Such bioinks or biomaterials are preferably based on collagen, gelatin, fibrin, hyaluronic acid, alginate, and / or silk proteins, but additionally or alternatively also on synthetic polymers such as PEGDA or POx, as well as various blends and chemically modified variants thereof.

[0014] In a practical embodiment, the imaging device is formed by a plate printed with the pattern. Optionally, this plate can also be transparent and backlit, so that the pattern printed on it is projected as a kind of shadow on the printing plate and the ink stream.

[0015] In an alternative embodiment, the imaging device is formed by a screen on which the pattern is displayed.

[0016] However, in both the case of the printed plate and the screen, preferably no projection or the like is performed onto the printing plate and the ink stream. Rather, the pattern is viewed by the detection device through the printing plate and the transparent ink stream, i.e., captured.

[0017] Advantageously, the imaging device, in particular the plate, is coupled to the print head and is in particular moved therewith.

[0018] In a preferred embodiment, the control unit is configured to use an optical distortion - e.g. a local twisting, deformation and / or change in size - of the pattern, in particular caused by the ink strand, to infer a (preferably local) width, a curvature and / or a height of the deposited ink strand in the image. This is based on the consideration that a transparent ink strand deposited on the printing plate acts comparable to a (in particular flat-convex) cylindrical lens due to its spreading which occurs - in particular under conventional conditions - and thus also locally distorts, in particular twists, the pattern viewed through the ink strand. The width of the ink strand can, for example,can be determined relatively easily by determining the start and end of the deformation of an individual stripe of the pattern, provided that the angular position of the stripe relative to the direction of travel (in particular the printing direction) of the ink stream is known. The curvature and / or height of the ink stream can be determined, for example, using optical equations. In particular, the path of rays through the ink stream, which acts similarly to a cylindrical lens, and through the (at least approximately) plane-parallel printing plate is simulated using formulas from so-called matrix optics, i.e. an optical transfer function (or “lens equation”) is created in matrix form. Based on this transfer function, for example, and depending on knowledge of the various parameters such as the curvature of the lens, etc.in particular the image size (image height, in particular as the distance of the "outermost" image point to the optical axis), the object size (object height, in particular as the distance of the "outermost" object point to the optical axis) or even the curvature of the lens can be determined. Because the pattern is captured both with and without imaging through the strand, the respective angle of the pattern (in the simplest case, a single stripe) relative to the printing direction can be easily read. The relationship between object size and image size can be deduced from the angular ratio (more precisely: the ratio of the two tangents to each other). Since the image size can be determined (in particular on an image sensor of the capture device), the object size can also be deduced.This allows the curvature of the cylindrical lens formed by the ink strand to be determined using the optical transfer function, particularly with knowledge of the respective refractive indices of the ink strand, the printing plate, and the intermediate medium (usually air). From this curvature, the optically detectable width of the ink strand, and the assumption of a parabolic strand profile, the height of the ink strand at its apex and its cross-sectional area can then be determined.

[0019] Particularly preferably, the stripes of the pattern are set at an angle to a main printing direction (i.e. the direction in which the ink strand runs in a straight line without slant, e.g. also referred to as the 0° direction). In a particularly expedient embodiment, the stripes of the pattern are set at an angle of at least approximately 45° (i.e. with a deviation of e.g. up to + / - 5°) to the main printing direction. With such an slant, a distortion of the pattern or its stripes becomes particularly apparent compared to a vertical orientation. With stripes running parallel to the main printing direction, there would be a risk that no stripe would be visible through an ink strand or that a stripe would be arranged in such a way relative to the ink strand that a distortion would be difficult or impossible to detect.With an oblique alignment, however, the probability is particularly high that at least one streak will cover the ink strand. To create a particularly reliable design here, it is also conceivable for the pattern to be formed from intersecting streaks. A further development of intersecting streaks can, for example, be squares that are spaced apart from one another or directly adjacent to one another and in particular have an edge length of 1 to 5 cm, preferably up to 2 cm. Optionally, the streaks or just some of the streaks in a partial area of ​​the pattern can be arranged in a star or radial pattern around a common center. For example, these star-shaped streaks are aligned with an angular offset of 5 to 15 degrees to one another. The latter simplifies the detection of distortion or twisting of the streaks in any printing direction.

[0020] Optionally, the stripes can also be interrupted (dotted and / or dashed) and vary in width. This can facilitate the assignment of the optical image (i.e., the picture) of a specific stripe to its "origin" on the imaging device.

[0021] According to a further expedient embodiment, the control unit is configured to infer a throughput and / or a printing speed (which each form the aforementioned characteristic variable) from the (in each case particularly local) width, curvature and / or height of the deposited ink strand. In particular, the control unit is configured to determine the (local) cross-sectional area of ​​the ink strand based on at least one of the aforementioned features, in particular from their combination (at least width and curvature), and to use this to infer its volume and volume throughput. The local cross-section is recognized as representing the local throughput. Thus, using a comparatively simple optical observation setup, the (volume) throughput can be determined "inline", i.e., during the process, and can be used, if necessary, to control or regulate the process.With known printing speed and cross-sectional area (calculated from curvature and strand width), the throughput follows as a process-relevant parameter.

[0022] In an advantageous embodiment, the control unit is also configured to infer the spreading behavior of the ink based on a temporal progression of the width, curvature, and / or height of the deposited ink strand. In particular, the development of the ink strand is observed in several chronologically successive images, and from this, a conclusion is drawn about the spreading behavior, i.e., how strongly the ink jet continues to spread after deposit (in particular transverse to its printing direction or longitudinal extent). Alternatively, a predefined "pattern," i.e., in particular, several predefined "detection points" or "measurement points" along the deposited strand, can be viewed in a single image. The respective measurement points are recognized as being assigned to different points in time after the strand has been deposited, so that the spreading behavior can also be determined in a single image.The spreading behavior, in turn, provides information about process parameters, especially their influence on the 3D printed image (which is influenced by the spreading behavior). For example, the degree of pre-gelation or similar can be adjusted if the spreading behavior is outside a target range (too strong or too weak).

[0023] In a preferred embodiment, the control unit is configured, in a calibration mode, to detect a plurality of calibration strands arranged on the printing plate or a carrier to be placed thereon and having a known cross-sectional geometry, and to use this to compensate for a measurement deviation. Here and in the following, "measurement deviation" is understood not only to mean a deviation from an "actual" measurement, for example the optical determination of the width of the ink strand, but also from a calculation of the curvature and / or height of the ink strand based on optical calculations. In particular, for the known dimensions of the calibration strands, it is particularly easy to check whether the detection of the width or the calculation leads to results that are at least within a predetermined tolerance range.

[0024] Optionally, this calibration mode can be used as a standalone mode in which only the calibration strands are measured. Alternatively, the calibration mode can also be used in parallel with the intended processing, for example, by arranging the calibration strands in a non-printing area on the printing plate on which the ink strand is intended to be deposited. In particular, this can also be used to compare the deposited ink strand with the calibration strands. In the latter case, it is less a calibration per se and more a type of interpolation method or nearest neighbor method, in which the geometry of the currently viewed ink strand is compared with the calibration strands.

[0025] The calibration strands are preferably made of a transparent material, in particular with a refractive index comparable to the (bio)ink and / or a strand width and / or cross-sectional area comparable to the expected strand widths or cross-sectional areas. In particular, the calibration strands are made of one or different liquid silicone rubber (types). "Comparable" here means in particular that the refractive index must lie within a range with a predetermined tolerance (e.g., + / - 10%) around the value of the corresponding ink size. With regard to the strand widths or cross-sectional areas, various calibration strands with widths from 1 mm to 10 mm and variations in the strand heights from 0.1 mm to 2 mm are preferably used.

[0026] In an optional embodiment, the 3D printing device also has a radar sensor which is configured to detect the width, curvature and / or height of the deposited ink strand at least locally (i.e. in particular in the manner of a sectional image) during normal printing operation of the 3D printing device and to feed it to the control unit. The control unit is configured, for example, to use this local information as a characteristic variable when determining the throughput and / or printing speed. In this case, the data (dimensions) detected and transmitted by the radar sensor are used in particular to support or verify the dimensions detected and determined by the optical detection device. The radar sensor is designed, for example, as a UWB radar sensor (“ultra-wide-band”).Such a sensor has a comparatively high spatial resolution and is also capable of "seeing through" certain structures (materials). The radar sensor can optionally be directed at the printing plate from below, just like the optical detection device, and thus must pass through it. Alternatively, the radar sensor is conveniently positioned above the printing plate to allow direct measurement of the deposited ink stream.

[0027] The advantage of the optical detection device (i.e., optical detection itself) is fundamentally that its field of view is usually larger than that of the radar sensor, allowing for a larger area of ​​detection and analysis at a glance. In comparison, the radar sensor can usually only detect a comparatively narrow area (e.g., approximately linear). Nevertheless, the 3D printing device with the radar sensor for (at least locally) detecting the width, curvature, and / or height, i.e., in particular, the strand geometry, optionally also represents a standalone invention independent of the presence of the optical detection device. In other words, the 3D printing device in this case can comprise the print head, the print plate (optionally transparent to radar) for depositing the extruded ink strand, as well as the radar sensor and control unit described above.The imaging device and the optical detection device can also be omitted in this embodiment. In this variant, the control unit is particularly configured to infer the volumetric flow rate from the ink jet's width, curvature, and / or height, determined by the radar sensor.

[0028] Here and in the following, "characteristic" means in particular that the quantity to be determined contains quantitative information about the value of the respective corresponding process or material parameter (e.g., throughput or spreading behavior), so that this information can be clearly derived from the quantity. However, the quantity to be determined can also be a quantity that is directly or indirectly proportional to the process or material parameter, or that has a non-linear relationship with it, for example, a logarithmic, exponential, or polynomial (i.e., quadratic, cubic, etc.).

[0029] The conjunction “and / or” is to be understood here and in the following in particular in such a way that the features linked by means of this conjunction can be formed both together and as alternatives to one another.

[0030] Below, one embodiment(s) of the invention is explained in more detail with reference to a drawing. In the drawings:

[0031] Fig. 1 shows a schematic side view of a 3D printing device,

[0032] Fig. 2 shows an image of a part of the 3D printing device captured by an optical detection device,

[0033] Fig. 3 shows a schematic side view of an optical model for an evaluation of the image, and

[0034] Fig. 4 in view according to Fig. 2 an alternative embodiment of the 3D printing device.

[0035] Corresponding parts are always provided with the same reference symbols in all figures.

[0036] Fig. 1 shows a 3D printing device 1. This device has a gantry frame 2 from which a printing unit 4 is suspended. The printing unit 4 has, in a manner not shown, a reservoir for the material to be printed, in the present embodiment a "bio-ink" for remodeling body tissue. Furthermore, the printing unit 4 has a print head, illustrated here by a print nozzle 6, through which an ink strand 8 is extruded. For depositing the ink strand 8, the 3D printing device 1 also has a print plate 10. This is transparent, e.g. made of glass, alternatively also of polystyrene. The printing unit 4 also has a drive (not shown in detail), by means of which at least the print nozzle 6 (but usually the entire printing unit 4) can be moved in a plane parallel to the print plate 10.To control a printing process, the 3D printing device 1 has a control unit 12, which in the present embodiment is integrated into the printing unit 4. Alternatively, the control unit 12 can also be formed by a computer that is connected to the printing unit 4 for data transmission purposes.

[0037] To monitor a printing process, the 3D printing device 1 further comprises an optical detection device, specifically a camera 14. This is connected to the control unit 12 via data transmission technology. The camera 14 is installed such that, in the present embodiment to save installation space, it is arranged below a plane of the printing plate 10 and its field of view is aligned by means of a mirror 16 from an underside 18 onto the printing plate 10—more precisely, through it. Furthermore, the 3D printing device 1 comprises an imaging device 20 that provides a pattern of rectilinear, in the present embodiment also parallel, stripes 22 (see Fig. 2), which in turn can be viewed (i.e., detected) through the printing plate 10 by means of the camera 14. Specifically, the imaging device 20 is formed by a plate 24 on which the stripes 22 are printed. This plate 24 is coupled to the printing unit 4.The printing nozzle 6 extends through the plate 24. The plate 24 is thus moved along with the printing nozzle 6. In an alternative embodiment, the camera 14 can also be positioned such that it "looks" through the printing plate 10 without a mirror, ie, with its optical axis aligned vertically.

[0038] The control unit 12 is configured to carry out a method for operating the 3D printing device 1, which is described in more detail below. For this purpose, the control unit 12 uses the camera 14 to continuously capture images of the printing plate 10, the ink strand 8 deposited thereon, and thus also of the stripes 22 during a printing process, i.e. while the ink strand 8 is being deposited. Since the deposited ink strand 8 varies in its width, exit cross-section (i.e. when exiting the print nozzle 6) and also its spreading behavior, i.e. how strongly the ink strand 8 spreads transversely across the printing plate 10, depending on process settings such as throughput and printing speed, the stripes are distorted differently by the respective ink strand 8. An image 30 captured by the camera 14 is shown as an example in Fig. 2.

[0039] The procedure of the control unit 12 is based on the assumption that the deposited and spread ink strand 8 behaves comparable to a cylindrical lens. This is indicated by the fact that the stripes 22 are rotated at least in a central longitudinal region of the ink strand 8. The control unit 12 determines the current (and local) width a of the ink strand 8. This can be done, for example, based on the optically detectable (longitudinally running) outer edges of the ink strand 8. Furthermore, the control unit 12 determines a curvature of the ink strand 8 using an optical calculation method described in more detail below as an example. Here, the control unit 12 uses the rotation of the stripes 22, which in the present embodiment run at 45 degrees to a main printing direction 32, and in the present case thus also 45 degrees to the orientation of the ink strand 8, as the basis.For this purpose, the control unit 12 determines an angle 6 for the alignment of the stripes 22 (specifically, exactly one stripe 22) against the main printing direction 32 without deflection by the ink line 8 and an angle G' for the alignment of a stripe 22 imaged by the ink line 8 and thus deflected (i.e., distorted, in particular twisted).

[0040] Fig. 3 provides an example of how the optical calculation method is carried out. This is specifically a paraxial approximation using matrix optics. In Fig. 3, the ink line 8 is shown in section and forms a cylindrical lens that rests on the printing plate 10. For the beam path starting from an object point, here specifically a point that is part of a strip 22 and is arranged at a distance G ("object size") from the optical axis O, the following equation for the exit vector h results from the assumption that the beam path (here exemplary for a first beam S1) initially runs parallel to the optical axis O up to the ink line 8. out on the underside 18 of the pressure plate 10: where h in the entrance vector of the beam path for the beam S1 at the ink jet 8, whose vector components represent the distance G and the angle in radians to the optical axis O,

[0041] Ti is the translation matrix between the object and the entry into the ink line 8,

[0042] Bi the refraction matrix upon entering the ink strand 8,

[0043] 7 the translation matrix in the ink line 8 to the printing plate 10,

[0044] B2 the refraction matrix when passing into the printing plate 10,

[0045] T3 the translation matrix in the pressure plate 10, and

[0046] B3 describe the refraction matrix upon exiting the printing plate 10.

[0047] Analogous to h in give the components of the vector h out also the distance and the angle in radians to the optical axis of the corresponding ray at the exit point, specifically at the “end point” of this ray used for equation (1).

[0048] This can be written out as: where n gthe refractive index of the printing plate 10 (here assumed to be glass), n the refractive index of the ink strand 8, g the distance from the object to the underside 18 of the printing plate 10 (“object width”), h the local height (or thickness) of the ink strand 8 at the entry point,

[0049] D describes the thickness of the printing plate 10, and p the curvature of the ink strand 8.

[0050] In an analogous way, the exit vector h out can also be determined for the second beam S2. From this, the image distance Z? and the image size B can then advantageously be determined from the intersection point of the two beams S1 and S2.

[0051] From the experimentally accessible ratio < 3> the known object size G and known object distance g and the calculable image size B, the curvature p can be calculated. Based on this and the optically detectable width a of the ink tube 8, the maximum height / / ' can be determined: h(%) = H(1 - ( 2x / a) 2 ) (4)

[0052] The curvature p is obtained by differentiating equation (4) twice and calculating the absolute value to where by rearranging the maximum height / / ' can be determined, since the curvature ? can be determined from equation (3).

[0053] Based on the height / / 'and the curvature p, the cross-sectional area A of the ink line 8 (and thus also the throughput if the printing speed is known) can now be determined.

[0054] In addition, the cross-sectional area A can also be calculated from equation (4) by the

[0055] Knowing height / / 'and width v, determine: a

[0056] A = fahx) ■ dx (6)

[0057] 2

[0058] This in turn results in:

[0059] A = ^ a - H (7)

[0060] The product of the cross-sectional area A and the printing speed, in turn, results in the (volume) throughput of printing ink (in the present embodiment, an alginate-based bio-ink). Throughput is a quantity that is difficult to determine from other process data, particularly control variables such as the pressure acting on the ink.

[0061] Basically, the two (or further rays passing through the ink strand 8) S1 and S2 provide their intersection point "B", i.e., the image size B. Nevertheless, to solve equation (2) it is also necessary to specify the (local) strand height h in each case, to know it more precisely. Since the x-coordinate is uniquely determined (calculable) from the geometry and choice of the starting angle of the respective beam S1 or S2 against the optical axis, the corresponding local height h can be determined using a value for the maximum height / / and the width a. Likewise, the curvature p contained in equation (2) is given by equation (5). Thus, the maximum height / / is the only free parameter of equation (2). The height / / will now be varied (iterated) until the calculated ratio of object size d / and image size B corresponds to the experimentally accessible tan 0 / tan g', so equation (3) is satisfied.

[0062] The choice of a value for the object size 6 n equation (2) is basically arbitrary, as long as the value lies in the interval 0 < G < a / 2, preferably in the interval a / 10 < G < a / 4. The first beam S1 is preferably selected with the starting angle 0 against the optical axis (parallel to the optical axis) (see Fig. 3). A suitable starting angle for the second beam S2 against the optical axis results from the ratio of the selected object size ( / ) to the object distance g. The following orders of magnitude can usually be assumed: the object distance g is typically 25 mm (in particular + / - 5 mm), the width a of the ink strand 8 in the range 0.1 to 1 mm and the maximum height / / 'below the width a. Based on these orders of magnitude, it can be seen that the use of the paraxial approximation described above is justified.

[0063] Optionally, the control unit 12 also determines a temporal development of the width of the ink strand 8, optionally also its curvature and / or height, based on several consecutive images 30. From this, it can be deduced how strongly and for how long the ink tends to spread, or how strongly the ink strand 8 spreads under given process parameters. This information is important for better assessing and / or controlling the possibilities for the three-dimensional construction of structures.

[0064] In order to be able to teach and / or check the optical detection of the dimensions and the calculation of the curvature, etc., of the ink strand 8, several, specifically two, calibration strands 34 are arranged on the printing plate 10. These are made of a material with a known optical refractive index (specifically, a refractive index approximating the refractive index of the ink), in the present embodiment a liquid silicone rubber. In the present embodiment, one of the two calibration strands 34 is deposited with a liquid silicone rubber with low spreading and thus a narrow width, and the other with a higher throughput but also with a liquid silicone rubber with greater spreading and thus a greater width. The width, curvature, and height of the calibration strands 34 are known. This makes it possible to check whether the calculation of the control unit 12 is correct.Alternatively, the control unit can also approximate the dimensions of the ink strand 8 by comparing and interpolating between the known calibration strands 34. The calibration strands 34 are cross-linked ("cured") and can thus be reused together with the printing plate 10. Fig. 4 shows an image of an alternative imaging device 20. The pattern formed by the stripes 22 is designed differently here. The stripes 22 form a plurality of squares 40 with an edge length of 3 cm. As a further exemplary embodiment - shown here in one of the squares 40 - a plurality of stripes 22 are arranged in a star-like or radial manner relative to one another. In the present exemplary embodiment, the rays 22 are arranged rotated by 15° relative to one another. Using such patterns, any printing directions for the ink strand 8 can also be set and still be assessed with comparatively high reliability according to the scheme described above.

[0065] The subject matter of the invention is not limited to the exemplary embodiment described above. Rather, further embodiments of the invention can be derived by those skilled in the art from the above description.

[0066] List of reference symbols

[0067] 1 3D printing device

[0068] 2 portal frames

[0069] 4 printing unit

[0070] 6 pressure nozzle

[0071] 8 ink strands

[0072] 10 printing plate

[0073] 12 Control unit

[0074] 14 Camera

[0075] 16 mirrors

[0076] 18 Bottom

[0077] 20 imaging device

[0078] 22 stripes

[0079] 24 plate

[0080] 30 images

[0081] 32 Main pressure direction 34 Calibration line

[0082] 40 square a width

[0083] 9, 6' angle

[0084] G Distance

[0085] 0 Pressure plate g Distance h Height

[0086] H maximum height

[0087] D Thickness

[0088] B Distance

[0089] S1 beam

[0090] S2 beam

Claims

Claims 1. 3D printing device (1 ), comprising - a print head (6) for extruding a transparent ink strand (8), - a transparent printing plate (10) for depositing the extruded ink strand (8), - an imaging device (20) providing a pattern formed at least from rectilinear stripes (22), - an optical detection device (14) which is designed to detect an image (30) of the pattern and of the deposited ink strand (8) from an underside (18) of the printing plate (10) during the intended printing operation of the 3D printing device (1), and - a control device (12) which is designed to derive at least one variable characteristic of the printing operation from the image (30) captured by the capture device (14).

2. 3D printing device (1) according to claim 1, wherein the imaging device (20) is formed by a plate (24) printed with the pattern.

3. 3D printing device (1) according to claim 1, wherein the imaging device (20) is formed by a screen on which the pattern is displayed.

4. 3D printing device (1) according to one of claims 1 to 3, wherein the control device (12) is configured to infer a width, a curvature and / or a height of the deposited ink strand (8) based on an optical distortion of the pattern in the image (30).

5. 3D printing device (1) according to one of claims 1 to 4, wherein the stripes (22) of the pattern are set at an angle of at least approximately 45° to a main printing direction (32).

6. 3D printing device (1) according to claim 4 or 5, wherein the control device (12) is configured to infer a throughput and / or a printing speed as a characteristic variable from the width as well as the curvature and / or the height of the deposited ink strand (8).

7. 3D printing device (1) according to claim 5 or 6, wherein the control device (12) is configured to infer a spreading behavior of an ink used for the ink strand (8) based on a temporal progression of the width, the curvature and / or the height of the deposited ink strand (8).

8. 3D printing device (1) according to one of claims 1 to 7, wherein the control device (12) is configured to detect, in a calibration mode, a plurality of calibration strands (34) which are arranged on the printing plate (10) or a carrier to be placed thereon and which have a known cross-sectional geometry, and to use them to compensate for measurement deviations.

9. 3D printing device (1) according to claim 8, wherein the calibration strands (34) are formed from a material with a comparable refractive index and / or comparable viscosity as the ink.

10. 3D printing device (1) according to one of claims 1 to 9, comprising a radar sensor which is configured to detect at least locally the width, curvature and / or height of the deposited ink strand (8) during the intended printing operation of the 3D printing device (1) and to supply it to the control device (12) as a characteristic variable for determining the throughput and / or the printing speed. 11 . Method for operating a 3D printing device (1 ), wherein according to the method - a transparent ink strand (8) is extruded by means of a print head (6) of the 3D printing device (1) and deposited on a transparent printing plate (10), - an imaging device (20) providing a pattern formed at least from rectilinear stripes (22) is provided, - an image (30) of the pattern and of the deposited ink strand (8) is captured from an underside (18) of the printing plate (10), and - at least one variable characteristic of the printing operation is derived from the captured image (30).

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