Method for monitoring a manufacturing process of a 3D printer comprising a magnetically levitated print bed

Abstract

In various embodiments a method for quality management of a printing process of a 3D object (402) comprising a plurality of layers (404, 406) is provided. The method comprises a magnetically levitated print bed (408) onto which print material is deposited to manufacture a 3D object, the method comprising determining a weight of the print material that has been deposited onto the print bed so far from the force acting on the levitated print bed and comparing the calculated weight to the measured weight in a continuous fashion or at discrete time intervals. The method can be used to determine whether at least one section of the layer of the printed 3D object meets expected quality standards based on the comparison.

Description

[0001] METHOD FOR MONITORING A MANUFACTURING PROCESS OF A 3D PRINTER COMPRISING A MAGNETICALLY LEVITATED PRINT BED

[0002] CROSS-REFERENCE TO RELATED APPLICATIONS

[0003] The present application claims the benefit of priority of European Patent Application No. 23195423.1 , filed September s, 2023, the content of which is hereby incorporated by reference it its entirety for all purposes.

[0004] FIELD OF THE INVENTION

[0005] The present invention relates to the field of additive manufacturing and, in particular, to a method for monitoring a manufacturing process of a 3D printer comprising a magnetically levitated print bed.

[0006] BACKGROUND OF THE INVENTION

[0007] Additive manufacturing (AM), more popularly known as 3D printing, refers to a group of technologies which enable manufacturing of objects or physical parts of any kind through the addition of materials, characteristically layer by layer, on a print bed, which may be also referred to as the build platform. This process is inherently different to classical machining which works via subtraction of a material block by drilling, milling etc.

[0008] A 3D printer typically creates a 3D object by an additive process. A 3D model is firstly designed using Computer Aided Design (CAD) software, and then sliced into layers by a slicing software (software engine). The object is then built, layer by layer, by moving the extruder nozzle of the 3D printer in a coordinated motion in the plane comprising the layer (e.g. x- and y-directions). Once the layer is finished, the extruder nozzle (or the plane containing the finished layer) is moved vertically (e.g. z-direction) in order to begin the extrusion process of the subsequent layer. One by one, successive layers of the print material are created until the object is fully built from bottom up (seen from the perspective of the 3D printer). The actions and the three- dimensional movements of the extruder nozzle which determine where the print material is extruded are specified and controlled by a computer numerical control (CNC) programming language. Typically, G-code is used for that purpose in consumer as well as in industrial 3D printers.

[0009] It goes without saying that in order to obtain a 3D printed object with desired mechanical properties, each printed layer has to be manufactured according to the design. That is, both the outermost portion of a layer and its infill pattern have to be printed with high precision according to the design pattern. Based on design pattern of the 3D object to be printed, the 3D printer software is able determine how much print material should be extruded in a given layer and also in a given section of the layer. Variations in the output of the extruder nozzle lead to defects in the printed 3D object which may remain undetected. For example, if more or less print material is extruded in a certain region of the infill as compared to the actual design (cases correspond to over-extrusion and under-extrusion, respectively), such a variation may not be visible from the outside e.g. once the finalized product is inspected. In case of under-extrusion, the thickness or diameter of the extruded material is too small and in an extreme case gaps between adjacent extruded sections may be visible, when the layer is inspected. In case of over-extrusion, more material is extruded by the 3D printer than expected, e.g. according to software calculations. When an over- or under-extruded portion is arranged at the outer shell of a layer, it may affect the dimension / outer appearance of the manufactured object.

[0010] Deviations in the amount of extruded material, especially in the infill pattern, are unwanted, since they lead to mechanically stronger or weaker portions within the 3D printed project and may thus alter its mechanical properties. Variations in the amount of extruded material may be caused by a multitude of factors such as incorrect nozzle height, incorrect print temperature, and presence of dust and dirt in the extrusion mass which may lead to partial clogging or blockage of the nozzle. When the temperature of the extruder is too low, for example, the print material may not melt entirely and starts to adhere to the inner surface of the extruder nozzle. Taking into account the inherent inaccuracies of thermostats of up to 10% in the worst case, it may be seen that avoidance or at least reliable detection of over- and under-extrusion is a rather challenging task which is of great importance in 3D printing. The interaction between the magnetic fields of the mover and the stator module levitates the print platform above the stator unit. By carefully controlling and changing the strength and orientation of the magnetic fields, stable levitation and frictionless movement of the print platform above the stator module can be achieved.

[0011] To achieve stability and precise control, the levitation mechanism is equipped with a sophisticated control system. The control system monitors the position and orientation of the platform using magnetic fild sensors, for example, placed strategically across the structure. These sensors provide continuous feedback on the platform's position, allowing the control system to make real-time adjustments.

[0012] Based on feedback from the magnetic field sensors (e.g. Hall sensors), the control system of the magnetic levitation system can adjust the electric current flowing through the coils in the stator module. By modulating the strength and distribution of the magnetic fields, the control system can counteract external disturbances which may cause changes in position and / or orientation of the print platform, thus ensuring stability and controlled movement of the print platform throughout the entire manufacturing process. The levitation mechanism, in conjunction with the control system, offers the ability to precisely manipulate the print platform in various directions and axes. By dynamically adjusting the magnetic fields, the print platform can be moved horizontally, to a lesser extent vertically, or even rotated.

[0013] SUMMARY

[0014] The present invention provides a method which may be used to detect deficiencies, such as over- and under-extrusion during the 3D printing process, during a 3D printing process performed on an additive manufacturing machine with a levitated print bed. As outlined above, based on the design of the layers of the 3D object, the volume of the printed material which is extruded in a given layer or in a given section thereof can be determined by the 3D printer software. The volume of a given layer or of a section thereof and usually corresponds to the combined volume of all of its filaments, i.e. curves and lines making up the layer or the considered section thereof. Using the diameter of the extrusion nozzle of the 3D printer and the density of the print material used for printing is known, the weight of a given printed layer or a section thereof may be calculated. According to the present invention, the weight of the 3D object being printed is determined during its manufacture by a 3D printer comprising a magnetically levitated print bed. The magnetic levitation system is configured to determine a weight of the print platform, onto which the print material is being deposited, from the magnetic force that needs to be generated to maintain the print bed in its levitated state above the stator of the magnetic levitation system. In doing so, the weight of individual layers and / or sections of individual layers can be compared to the theoretical weight obtained from calculations which the printed 3D structure should have until the considered point in time of the manufacturing process. A deviation of the measured weight from the calculated weight, if present, may indicate a deficiency of the manufacturing process, such as over- and under-extrusion, taking place during the 3D printing process. The extruded print material may be, for example, any polymer suitable for 3D printing.

[0015] According to various embodiments, a method is provided for monitoring a manufacturing process of a 3D printer, wherein the 3D printer comprises a levitated print bed onto which print material is deposited to manufacture a 3D object, the method comprising determining a parameter indicative of a force acting on the levitated print bed to maintain its levitation, determining a weight of the print material that has been deposited onto the print bed so far from the force acting on the levitated print bed, and comparing the determined weight of the print material that has been deposited onto the print bed so far to a calculated weight of the print material should have been deposited onto the print bed so far. For the purpose of the present method, a 3D printer having magnetically levitated print bed is used. The printing means of the 3D printer according to various embodiments may correspond to ordinary printing means known from prior art, with the 3D object being manufactured by printing layer upon layer. By measuring, for example in a continuous manner, the weight of any section of the print material extruded onto the printing platform, the weight of the corresponding section of the print material and thus the 3D object may be determined and compared to the corresponding calculated value. As the printing process progresses, the weight of the 3D object being manufactured on the printing platform should gradually increase. The method according to the invention is configured to detect deviations of the increase of weight during the 3D printing process from an expected / projected increase obtained from calculations. Thus, the method according to various embodiments is able to detect over- and under-extrusion, for example, directly when it occurs, which manifests itself in a decrease or increase, respectively, of the measured weight of a section of a layer as compared to the corresponding calculated weight thereof. It should be understood that the weight of the print material extruded onto the printing platform of course encompasses print material printed / extruded onto a layer already present on the print bed of the 3D printer. A 3D object printed by a 3D printer as referred to herein may refer to any real-life object which may be manufactured by means of a 3D printer, e.g. a 3D object from the field of architecture, playthings, industrial design (automotive, aerospace, military, engineering etc.), medical industries, biotech (e.g. human tissue replacement), fashion, food, and many other fields.

[0016] The weight of at least a section of the layer may be calculated based on a path-length and a cross-section of the extruded print material contained in that section and the density of the print material. From the path-length of the extruded print material and cross-section of the printed material its volume may be calculated. The cross-sectional area of the printed material may be determined from the diameter of the used extruder nozzle and taking into account further parameters such as the travel speed of the extruder nozzle. The volume of the considered section of extruded material may be approximated, for example, by a cylinder or a flattened cylinder extending along the path of the extruding nozzle. Once the volume of the section of the layer has been calculated, its theoretical weight can be calculated by multiplying it with the density of the used print material. The density of the print material may be considered at the print temperature at which the print material is extruded from the nozzle or at a temperature which the print material is expected to have when it begins to form a bond with the layer provided underneath which may be approximated by the print temperature.

[0017] According to further embodiments of the method, the print bed may be a magnetically levitated print bed. According to further embodiments of the method, the 3D printer may comprise an extruder which is movable independent of the levitated print bed along a vertical axis perpendicular to the levitated print bed.

[0018] According to further embodiments of the method, the levitated print may comprise permanent magnets and may be controlled by an electromagnetic field generated by coils of a magnetic levitation platform.

[0019] According to further embodiments of the method, the weight of the print material that has been deposited onto the print bed so far may be calculated from the force acting on the print bed divided by the acceleration due to gravity. For this purpose, the magnitude of the magnetic force which needs to be generated by the stator module of the magnetic levitation system to keep the print bed levitated at a predetermined distance above the surface of the stator module may be used as a reference value and compared to the magnitude of the magnetic force which needs to be generated by the stator module of the magnetic levitation system to keep the print bed including print material which has been deposited thereon levitated at that distance above that surface. From the surplus in the magnetic force that needs to be generated to keep the heavier print bed in levitation, the surplus weight of the print bed, i.e. the print material, can be determined.

[0020] According to further embodiments of the method, the weight of the print material that has been deposited onto the print bed may be determined several times during the manufacturing process. In particular, a weight value may be acquired at a point in time corresponding to the beginning of the 3D printing of a portion of a layer or entire layer to be monitored. A further weight value may be acquired at a further point in time corresponding to the end of the printing of the portion of a layer or the entire layer to be monitored. Using this scheme, the weight of any section of a layer may be determined. Measuring of the weight of the print material extruded or printed on the print bed may be also performed in predetermined time intervals or at predetermined points in time, for example at points in time at which homogenous, symmetric and / or particularly challenging sections have been printed.

[0021] According to further embodiments of the method, the weight of the print material that has been deposited onto the print bed may be determined continuously during the manufacturing process. Thus, the method according to this embodiment may provides an online or “live” monitoring of the printing process by continuously acquiring weight values of the 3D object during printing over time. By monitoring the evolution of the weight of the 3D object being printed and comparing it to the calculated evolution of the weight thereof a precise quality management may be implemented. In particular, this method enables pinpointing deviations between measured and calculated weight values to specific points and / or sections of the printed object. If errors in (a) certain section(s) of the printed 3D object repeat throughout various prints, the design of the 3D object may be evaluated and possibly optimized to obtain a design which is easier to manufacture and has a reduced probability generating errors during its manufacture. For the purpose of the present invention, continuous measurement may include quasi-continuous measurement of the weight of the levitated print bed which is performed at least several times a second, such a every 500 ms, every 250 ms, every 100 ms or even more often. In effect, the sampling rate at which the weight measurements are acquired may be determined by the resolution of the force measurement from which the weight values for the levitated print bed are obtained.

[0022] Overall, when the weight is monitored continuously, a pause in the printing process, e.g. after finishing a layer of the 3D object, generates a plateau in the corresponding graph and may be easily detected by an algorithm. Therefore, a pause after the manufacture of a layer may be used to generate a marker in a corresponding weight vs. time graph such that portions of a graph in the weight vs. time diagram may be easily allocated to their corresponding layers.

[0023] In addition, the extent or the path length over which the deviation spans may be determined based on the weight measurements. Based on this information, it may be decided whether the weight deviation leads to tolerable “defects” in the finalized 3D printed object or not. For example, defects in the form of over- and / or under-filling may be tolerated when their density along the printing path and / or their density within a certain area of the layer is below a predefined threshold.

[0024] According to further embodiments, wherein at least one parameter of the printing process is adjusted when the deviation between the calculated weight and the measured weight of the print material that has been deposited onto the print bed exceeds a first predetermined threshold for a predefined number of times.

[0025] In general, the magnitude of the deviation between the measured and calculated weight of a given section of a layer (or an entire layer) provides information about the severity of the under- or over-extrusion. For example, the variance of the measured weight may be also used as a quality parameter. The first predetermined threshold defines the maximum magnitude of over- or under-extrusion that will be tolerable for the considered section of a layer. For example, if under- or over-extrusion with a certain magnitude (for example, 5% more weight than expected / calculated) is detected to occur in more than a certain number of layers of the 3D object and / or in more than a certain number of consecutive layers of the 3D object, at least one parameter of the 3D printer may be adjusted to counteract over- or under-extrusion. The adjusting of the at least one parameter may be performed in a closed feedback loop, e.g. implemented by means of a PI D controller. The adjusting of the at least one parameter aims at identifying optimal process parameters (nozzle height, print temperature, movement speed of the extruder nozzle etc.) in order to shift the printing process as close as possible back towards the optimal printing process which is free of over- and under-extrusion.

[0026] In a further embodiment, if, after a certain number of adjustment attempts of the printing process parameters or after a certain time during which printing process parameters have been adjusted the 3D printing state cannot be brought within optimal parameters, the method may include terminating the 3D printing process.

[0027] According to further embodiments, the method may include aborting the manufacturing process, when the deviation between the calculated weight and the measured weight of the print material that has been deposited onto the print bed exceeds a second predetermined threshold. The second predetermined threshold defines the magnitude of over- or underextrusion (for example 10%) that will not be tolerable during the printing process and will lead to termination of the 3D printing process. Such a termination criterion may be useful when printing 3D objects which are used for implantation into a patient, for example. The termination criterion may prevent printing of prosthetic parts which do not satisfy requirements relating to mechanical properties and may be at risk of breaking due to a weak spot with the consequence of the need for a further surgery to replace the broken prosthetic part. In analogy to the first predetermined criterion, the second predetermined criterion may be coupled with a frequentness condition. That is, the 3D printing process may terminate when the deviation between the calculated weight value and the measured weight value exceeds the second predetermined threshold for more than a predefined number of times or for a longer time period than a predefined threshold period.

[0028] According to further embodiments of the method, the weight of the print material that has been deposited onto the print bed may be obtained from a calibration curve obtained beforehand relating reference weights of the print bed (including reference weights placed thereon) to corresponding forces acting on the levitated print bed. The force acting on the print bed may be primarily the gravitational force which is dependent on the mass of the print bed. Assuming an equilibrium state which keeps the print bed in a levitated state, the gravitational force acting on the print bed corresponds to the counteracting magnetic force acting on the print bed to keep it in a levitated state.

[0029] A method for forming a 3D object by an additive manufacturing machine featuring levitated print beds and a corresponding additive manufacturing machine which may be generally used in the context of the present invention may be found in the international application PCT / EP2022 / 083548.

[0030] BRIEF DESCRIPTION OF DRAWINGS FIG. 1 shows an exemplary design of a layer of a 3D object which may be used in the process of 3D printing.

[0031] FIG. 2 shows an exemplary diagram which shows a weight vs. time graph which may be used to monitor a 3D printing process according to the method of the present invention.

[0032] FIG. 3 shows an exemplary table which may be used for quality management according to the present invention.

[0033] FIG. 4 shows a sketch of an exemplary 3D object during its manufacture on a levitated print bed.

[0034] FIG. 5 shows a diagram illustrating different scenarios of weight progression during printing of a 3D object.

[0035] FIG. 6 shows an exemplary force-to-weight curve.

[0036] DETAILED DESCRIPTION

[0037] The following description of the embodiments is merely illustrative in nature and is in no way intended to limit the invention, its application or uses. Additionally, the invention may be practiced according to the claims without some or all of the illustrative information.

[0038] FIG. 1 shows a visual representation of an exemplary design of layer 100 of a 3D object to be printed. As can be seen, layer 100 is quadratic in shape and includes three outer shell layers 102, wherein the outermost shell describes a slice of the actual outer surface of the 3D object once it has been printed. Layer 100 further includes infill pattern 104 consisting of lines arranged at right angles with respect to one another. The thickness of the lines is dependent on the diameter of the used extruder nozzle. In FIG.1 , a dotted rectangle denotes exemplary section 106 of layer 100 to which the method according to various embodiments may be applied. That is, during printing, the weight of section 106 may be determined by measuring or tracking the weight between the start point and the end point of the path of the extruding nozzle corresponding to section 106. It may be seen that the start point and the end point correspond to the points at which the marked T-shaped section 106 of infill pattern 104 intersects the dotted rectangle. In general, an exemplary section 106 may include any portion of layer 100 comprising any infill pattern 104. In particular, section 106 may include the whole layer 100, i.e. its outer shell layers 102 and its infill pattern 104.

[0039] In FIG. 2 exemplary diagram 200 is shown with the x-axis 202 denoting time in minutes and the y-axis 204 denoting weight in grams. In diagram 200, graph 206 is shown which describes a recorded progression of the weight of the printed 3D object during its manufacture on the levitated print bed. Graph 206 may be seen to be overall continuous in nature, with linearly increasing portions located between plateaus 208 which are highlighted by dashed circles (only every second plateau being denoted with a reference sign). Graph 206 relates to a scenario in which the 3D printing process is paused after completion of each layer of the 3D object. During each pause the weight of the 3D object is not increased any further and gives rise to plateaus 208 which may be advantageously used as markers which delimit subsequent layers from one another. Due to the fact that the linear increasing portions between plateaus 208 have the same slope, graph 206 indicates that the 3D printing process of every examined layer has been performed at the same printing rate (extruded print mass per time). In addition, since the slope is uniform in each linear section of graph 206, the printing process has taken place at a constant print material output rate, indicating absence of over- and under-fill (except for the case of a global offset of the entire graph 206). By comparing the weights corresponding to each of the plateaus 208, the weight of each layer which has been printed can be determined. Comparison of those weight values obtained from directly measuring the 3D object during its manufacture with calculated / expected theoretical values provides reliable information about possible defects in the respective layer, most prominently over- and underfills. Graph 206 is a continuous line and implies that it has been generated by continuously monitoring the weight of the 3D object being printed on the printing platform vs time. Here, the advantage of the use of a magnetic levitation system becomes evident: It enables continuous tracking of the weight of the object that is being printed on a levitated print bed. However, the method described herein may be just as well performed by sampling the weight of the printed 3D object at discrete points, for example in given time intervals. The weight sampling interval may be chosen longer for slow 3D printing speed, e.g. 3 s, and shorter for high 3D printing speed, e.g. 1 s.

[0040] If the measured weight values, e.g. the weight values corresponding to plateaus 208 in diagram 200, correspond to theoretically predicated weight values, then it may be deduced that every printed layer comprises the predetermined amount of print material. Therefore, with a very high probability the lines of print material making up the printed layer are free of over- and underfill. In addition, the uniformity of the slope for a given layer may be checked to verify that statement. If the weight value at any of the plateaus does not correspond to the theoretically predicated weight value, then it may be deduced that more or less print material has been deposited in the respective layer and therefore with a very high probability that layer is affected by under- and / or overfill. In order to further investigate the location at which a defect is suspected, graph 206 may be zoomed into and compared to the calculated evolution of the weight of the printed 3D object at a finer time scale for better spatial localization of the point or region within the layer at which the measured weight curve starts to deviate from the calculated weight curve. It is noted that in case of a layer affected by over and / or under-extrusion, the absolute weight at every following plateau 208 will not correspond to the calculated weight (due to the offset caused by the overfill or underfill). In such cases the difference of weights measured at plateaus 208 may be used to determine the actual weight of a plateau as measured by the weighing device.

[0041] An exemplary implementation of a quality management scheme according to the present invention is shown in FIG. 3. Table 300 has an exemplary format in which the results of the comparison between measured weight and calculated weight may be evaluated. It is noted that while the smallest weight unit considered in Table 300 is a layer, the considered weight unit may be chosen smaller and relate to portions of layers. Column 302 of table 300 contains the layer number, in the present example 1-10. Column 304 contains the actual weight, i.e. the measured weight of each of the examined layers as obtained from the analysis of at east one operational parameter of the magnetic levitation system. Column 306 contains the planned weight, i.e. the projected weight as obtained from calculations of each of the examined layers. Column 308 lists, for each layer, the relative deviation in per cent between the measured weight and the calculated weight. In column 310 the classification of the determined deviation as listed in column 308 is given, i.e. whether the determined deviation is acceptable or not. Every detected deviation is classified based on a comparison of its magnitude to a user defined deviation acceptance threshold 316 which in the present example is set to 2%. Thus, every deviation with a magnitude larger than 2% is classified as not acceptable in the fifth column. In column 312 of table 300 the most probable cause for the deviation is given. Here, based on the sign (+ or -) of the determined deviation it may be judged whether the detected deviation is correlated with an under-extrusion or over extrusion. It should be understood that table 300 shows only one simple example how the quality management according to the method disclosed herein may be implemented.

[0042] In the following, a typical exemplary embodiment of the method according to the present invention is described. While it is assumes that individual layers are considered, any other weight unit may considered comprising, for example, multiple layers or portions of a layer. At the outset of the method, the number of filaments comprised by each layer of the 3D object may be calculated. This calculation may be performed by the 3D printer software or by special software installed on a computer based on the architecture / layout of the layers. Based on the determined number of filaments and further taking into account the extruder nozzle diameter, the volume of material to be deposited in each layer may be calculated. In a last calculation step, an expected weight of each of the layers maybe determined from the density of the material used for manufacturing each of the layers. The actual printing process is performed such that the weight of the 3D object being printed may be determined during the printing process. This is achieved by means of a magnetically levitated print bed configured such that the weight of material extruded thereon may be determined. The weight may be recorded continuously during the printing process or at predefined time intervals, e.g. every second, every two seconds, every three seconds, or every five seconds. Under normal circumstances, the recorded weight of the 3D object during printing should increase linearly as the print material is deposited continuously (see FIG. 2). In case the rate at which the weight increases during the printing process drops drastically or vanishes, this means that the extruder nozzle has stopped depositing material and may be interpreted as clogging of the nozzle. As already described above, it may be desirable to make the extrusion by the 3D printer pause for a predetermined amount of time after completing each of the layers of the 3D object. Since the weight will not change during those pauses, they may be used as gauge points for attributing (the usually linear) portions of the weight vs. time graph to layers. Alternatively, the weight may be recorded during those periods of unchanging weight to determine the weight of the respective layer. Quality management may be performed based on a comparison of the measured weight to the predetermined / calculated weight. If the error is within a predefined threshold, quality assurance is considered to be passed. Otherwise, the user may be informed that the 3D printed object is affected by under-extrusion or over-extrusion. If it is determined that under- or over-extrusion has occurred more than a certain number of times and / or has been present for a collective period of time which is longer than a predefined time and / or was detected in more than a certain number layers, the flow of the print polymer may be automatically adjusted to compensate for the deviation in flow by adjusting at least one parameter of the printing process.

[0043] A sketch of an exemplary 3D object during its manufacture on a levitated print bed is shown in FIG. 4. In sketch 400, the 3D printed object 402 is positioned on a magnetically levitated print bed 408 which is levitated above a stator 10 of a magnetic levitation system. The 3D printed object 402 comprises a number of consecutively printed layers, starting with a first layer 406 and ending with a last layer 404 to be extruded during the additive manufacturing process. In this example, each layer has the same weight, namely 2 grams. The combined weight of the 3D printed object 402 and of the magnetically levitated print bed 408 results in force Fm, which is counteracted by a electromagnetic force Femgenerated within the stator 410 of the magnetic levitation system in such a way that the distance between magnetically levitated print bed 408 and stator 410 remains constant throughout the printing process. That is, the control loop within the magnetic levitation system tries to maintain an equilibrium state in which the electromagnetic force Fem acting on the levitated print bed 408 corresponds to the gravitational force Fmacting on the print bed including the 3D printed object. Such an equilibrium of ferees keeps the levitated print bed 10 at a constant distance above the surface of the stator 410. Since the weight of the levitated print bed 408 is known and constant, the weight of the 3D printed object 402 may be determined at any time during the manufacturing process in that manner. Since the movement of the levitated print bed 408 above the surface of the stator 410 is frictionless, the force acting on the levitated print bed 408 (including 3D printed object 402) corresponds to the gravitational force.

[0044] FIG.5 shows exemplary progressions of the weight of a 3D printed object from which various error types may be derived. Graph 500 shows possible weight evolution of several 3D printed objects consisting of multiple layers, where the x-axis indicates the layer number and the y- axis indicates the determined weight. The first weight curve 502 represents the ideal weight profile of a 3D object being printed where the weight increases by 2 grams for each printed layer. A tolerated deviation from the ideal weight is indicated by the gray shaded area around the first weight curve 502. The second weight curve 504 represents a printing process in which the third layer is affected by overflow, resulting in a weight increment of more than 2 grams for the fourth layer. The third weight curve 506 represents a printing process affected by underflow, resulting in a weight increment of less than 2 grams for the fourth layer. The fourth weight curve 508 represents a printing process affected by detachment and loss of the 3D printed object from the levitated print bed, resulting in a drop of measured weight to zero. It may be further seen that the loss event took place after the fifth layer has been finished. 3D object weight deviations which are larger than a predefined threshold and / or happen more often than a predefined number of times result in termination of the printing process, depicted in graph 500 as termination events 510.

[0045] In the graph 600 of FIG. 6, in which the x-axis indicates a force in arbitrary units and the y-axis indicates a weight, a force-to-weight calibration curve 602 is shown which may be obtained in a calibration process before the actual 3D printing process by placing reference weights on the print bed and determining the corresponding forces acting on the levitated print bed. The calibration curve 602 may correspond to an extrapolation of the individual sampled data points. The calibration curve 602 may be used during the 3D printing process to convert the electromagnetic force Fem generated within the stator 410 of the magnetic levitation system into a weight of the levitated print bed including the 3D print object thereon.

Claims

Claims1. Method for monitoring a manufacturing process of a 3D printer, wherein the 3D printer comprises a levitated print bed onto which print material is deposited to manufacture a 3D object, the method comprising: determining a parameter indicative of a force acting on the levitated print bed to maintain its levitation; determining a weight of the print material that has been deposited onto the print bed so far from the force acting on the levitated print bed; comparing the determined weight of the print material that has been deposited onto the print bed so far to a calculated weight of the print material should have been deposited onto the print bed so far.

2. Method of claim 1 , wherein the print bed is a magnetically levitated print bed.

3. Method of claim 1 or 2, wherein the 3D printer comprises an extruder which is movable independent of the levitated print bed along a vertical axis perpendicular to the levitated print bed.

4. Method of any one of claims 1 to 3, wherein the levitated print bed comprises permanent magnets and is controlled by an electromagnetic field generated by coils of a magnetic levitation platform.

5. Method of any one of claims 1 to 4, wherein the weight of the print material that has been deposited onto the print bed so far is calculated from the force acting on the print bed divided by the acceleration due to gravity.

6. Method of any one of claims 1 to 5, wherein the weight of the print material that has been deposited onto the print bed is determined several times during the manufacturing process.

7. Method of any one of claims 1 to 6, wherein the weight of the print material that has been deposited onto the print bed is determined continuously during the manufacturing process.

8. Method of any one of claims 1 to 8, further comprising: adjusting at least one parameter of the printing process when the deviation between the calculated weight and the measured weight of the print material that has been deposited onto the print bed exceeds a first predetermined threshold.

9. Method of claim 8, wherein at least one parameter of the printing process is adjusted when the deviation between the calculated weight and the measured weight of the print material that has been deposited onto the print bed exceeds a first predetermined threshold for a predefined number of times.

10. Method of any one of claims 1 to 9, further comprising: aborting the manufacturing process, when the deviation between the calculated weight and the measured weight of the print material that has been deposited onto the print bed exceeds a second predetermined threshold.

11. Method of any one of claims 1 to 10, wherein weight of the print material that has been deposited onto the print bed is obtained from a calibration curve obtained beforehand relating reference weights to corresponding forces acting on the levitated print bed.

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