Method and printing system for printing a three-dimensional structure, in particular an optical component

A method and printing system for printing a three-dimensional structure, in particular an optical component, by depositing droplets of printing ink side by side and one above the other in several consecutive depositing steps by means of a print head. In each depositing step a plurality of droplets is ejected simultaneously by a plurality of ejection nozzles of the print head. The print head is moved relative to the deposited droplets in a moving step performed between at least two consecutive depositing steps in such a manner that the droplets deposited in the same position in the at least two consecutive depositing steps are ejected at least partly from two different ejection nozzles.

BACKGROUND

The present invention relates to a method for printing a three-dimensional structure, in particular an optical component, by depositing droplets of printing ink side by side and one above the other in several consecutive depositing steps by means of a print head, wherein in each depositing step a plurality of droplets is ejected simultaneously by a plurality of ejection nozzles of the print head.

The print head comprising the plurality of ejection nozzles and is moved relative to a substrate on which the droplets are deposited step by step. The movement of the print head as well as the ejection of droplets from certain ejection nozzles have to be controlled in such a manner that the three-dimensional structure is built up.

Prior art document EP 2 846 982 A1 describes in detail how the print head is controlled and how the movement/ejecting can be realized by using so-called intensity image determining the shape of the three-dimensional structure.

The intensity image comprises a two-dimensional pattern of different color intensities. The pattern consists of different pixels, wherein each pixel represents a certain position in the three-dimensional structure to be printed. In particular, each pixel represents a certain position of a two-dimensional projection of the three-dimensional structure onto a flat base plane. On the one hand, the distribution of the intensity in the intensity image represents the shape of the three-dimensional structure as the intensity in each pixel is a value for the height of the three-dimensional structure at the corresponding position. On the other hand, the height of the later printed real three-dimensional structure in a certain position depends on the number of droplets of printing ink and accordingly to the amount of printing material deposited in this position.

The intensity image is transferred to an inkjet printer. The print head of the inkjet printer processes the intensity image in such a manner that the print head moves in several subsequent steps in such a manner that ejection nozzles of the print head are respectively positioned accordingly to pixels in the intensity image and deposit in parallel certain amounts of printing material in the individual positions in each single step. The three-dimensional structure is thereby built up step by step until the amount of printing material deposited in each position correspond to the color intensity in the pixels of the intensity image. The droplets are deposited side by side and one above the other in order to generate the desired three-dimensional shape. After deposition of the droplets, adjacent deposited droplets merge which each other and are subsequently cured by UV-light.

Practice has shown that there are always some deviations between the different ejection nozzles in one print head as they do not precisely eject the same amount of printing ink. In particular, there is always at least one inaccurately working nozzle ejecting significantly less amount of printing ink with each droplet due to clogging of the nozzle by cured printing ink or by contamination with e.g. foreign particles and impurities. The resulting deviations sum up with every new layer of printing ink (usually there are thousands of layers stacked above each other) to inequalities and non-uniformities in the printed three-dimensional structures. Usually, these inequalities and non-uniformities are so small that no visible and disturbing influences occur. However, when printing three-dimensional structure serving as optical components, like lenses and in particular ophthalmic lenses, even the finest small inequalities and non-uniformities lead to serious optical defects disturbing the optical beam path. In particular, these inequalities and non-uniformities generate unwanted diffractive phenomena. The problem is that the locations of inaccurate working nozzles in the print head are usually not known and additionally change over time due to clogging.

SUMMARY

It is therefore an object of the present invention to provide a method and a printing system for printing three-dimensional structures, in particular optical components, without inequalities and non-uniformities arising from deviations in the ejecting characteristics between different ejection nozzles, so that diffractive effects in the printed three-dimensional structure can securely be avoided.

The object of the present invention is achieved with a method for printing a three-dimensional structure, in particular an optical component, by depositing droplets of printing ink side by side and one above the other in several consecutive depositing steps by means of a print head, wherein in each depositing step a plurality of droplets is ejected simultaneously by a plurality of ejection nozzles of the print head and wherein the print head is moved relative to the deposited droplets in a moving step performed between at least two consecutive depositing steps in such a manner that the droplets deposited in the same position in the at least two consecutive depositing steps are ejected at least partly from two different ejection nozzles.

It is herewith advantageously possible to ensure that all droplets deposited in one single position or pixel of the three-dimensional structure do not solely originate from one single ejection nozzle because if this ejection nozzle works inaccurately, the above mentioned unwanted inequalities and non-uniformities occur. In contrast, each position or pixel in the printed three-dimensional structure receives droplets from more than one ejection nozzle according to the present invention, so that potential deviations resulting from few inaccurate working ejection nozzles are compensated for and averaged out, even if the location of inaccurately working nozzles changes due to clogging and declogging. The underlying idea is that in case the print head comprises a plurality of accurate ejection nozzles and e.g. one inaccurate ejection nozzle, there are no inequalities and non-uniformities in the printed three-dimensional structure, at all, if the e.g. one inaccurate ejection nozzle deposits a more or less constant number of droplets in almost each position or pixel of the three-dimensional structure. With other words: It must only be ensured that the e.g. inaccurately working ejection nozzle does not deposit all or most droplets in one single location or pixel of the three-dimensional structure but that the droplets of inaccurately working ejection nozzle are spread over an area of the three-dimensional structure being as large as possible during the whole printing process. Consequently, the method according to the present invention substantially increases printing accuracy and provides printing of three-dimensional structures which can serve as optical components, like lenses and in particular ophthalmic lenses, due to their improved quality.

The printing ink comprises preferably transparent or trans-lucent printing ink. Preferably, the printing ink comprises an UV curable liquid monomer becoming a polymer if being cured. Preferably, the droplets are deposited onto a substrate. The substrate can be a part of the printed structure or a support plate for supporting the deposited droplets only during the printing process. Movement of the print head relative to the deposited droplets is preferably obtained by actively driving the print head, while the substrate on which the droplets are deposited preferably stands still, or by moving the substrate on which the droplets are deposited, while the print head preferably stands still. It is also conceivable that both the print head as well as the substrate are moved actively. However, the wording “moving the print head relative to the deposited droplets” does not necessarily means in the sense of the present invention that the print head is actually moved because alternatively the substrate on which the droplets are deposited can e.g. solely be moved to obtain the relative movement between the print head and the deposited droplets. This can be done also in any of the following preferred embodiments. The printing data are provided to the printer be means of an intensity image. The intensity image preferably comprises a two-dimensional pattern of different grey or colour intensities. The pattern consists of different pixels, wherein each pixel represents a certain position in the three-dimensional structure to be printed. In particular, each pixel represents a certain position of a two-dimensional projection of the three-dimensional structure onto a flat base plane. The distribution of the intensity in the intensity image represents the shape of the three-dimensional structure to be printed as the intensity in each pixel is a value for the height of the three-dimensional structure at the corresponding position. The height of the printed three-dimensional structure in a certain position depends on the number/size of droplets of printing ink and accordingly to the amount of printing material deposited in this position. The print head deposits printing ink in dependency of the intensity image, so that a three-dimensional structure is printed having the shape of the software based-virtual design given by the intensity image.

According to a preferred embodiment of the present inventions, the print head is moved relative to the deposited droplets according to a predefined moving scheme. Preferably, the moving scheme is being defined at the beginning of the actual printing process, prior to the first relative movement of the print head and/or the first deposition of droplets. In this way, it is advantageously possible to choose the moving scheme such that defects caused by malfunctioning ejection nozzles are distributed across the three-dimensional structure such that in particular optically visible defects are avoided. In particular, a moving scheme is chosen such that diffractive phenomena and aberration effects are minimized or entirely avoided. To this end, it is necessary to choose a moving scheme that ensures an ideally uniform distribution of defects. More precisely, let a z-axis be defined by the direction of flight of the deposited droplets and an x-y-plane be the plane perpendicular to this z-axis below the first printed layer. Then the projection of the location of all defects of the printed three-dimensional structure into the x-y-plane preferably ideally yields a uniform distribution of points. Through a most uniform distribution, aberration effects are advantageously minimized. By distributing the defects caused by malfunctioning ejection nozzles as uniformly as possible over the three-dimensional structure, aberration effects caused by clusters of defects or alignments of defects are avoided or at least reduced.

In a preferred embodiment of the present invention, the predefined moving scheme is determined prior to printing depending on the geometry of the three-dimensional structure to be printed and depending on the dimensions of the print head. Preferably, the predefined moving scheme takes into account the ration of the width of the print head, i.e. the number of ejection nozzles available and the width of the three-dimensional structure to be printed. In particular, the predefined moving scheme depends on the number of redundant ejection nozzles, i.e. the difference between the width of the print head and the width of the three-dimensional structure to be printed, preferably in pixels. The number of redundant ejection nozzles usually differs from layer to layer. In particular, when printing a convex lens, the number of redundant nozzles increases with each layer. Preferably, the change of redundant ejection nozzles with layer number is taken into account in the definition of the predefined moving scheme.

In a preferred embodiment, the predefined moving scheme is determined prior to printing depending on the accuracy required of the three-dimensional structure to be printed. In particular, accuracy comprises the difference between intended and printed geometry as well as the difference between intended and printed optical properties. In particular, the predefined moving scheme is defined such that the printed structure satisfies the required accuracy.

According to a preferred embodiment, predefining the moving scheme comprises choosing a compensation set of compensating nozzles and choosing a step sequence of moving distances X and moving directions φ by which the print head is moved relative to the deposited droplets during a sequence of moving steps. In particular, the moving scheme comprises a set of moving distances and moving directions ((X1, φ1), (X2, φ2), . . . ) for each moving step. The moving distance X specifies the distance by which the print head is moved relative to the deposited droplets during each moving step. The moving distance may be specified in pixels or in multiples of the distance D between two adjacent ejection nozzles. The moving direction φ specifies the direction in the x-y-plane by which the print head is moved relative to the deposited droplets during each moving step. For example, the moving direction may be defined as the polar angle as measured from one corner of the print head.

In a preferred embodiment, the number of compensating nozzles used in a depositing step is chosen depending on the width of the layer to be printed in the depositing step. Preferably, the number of compensating nozzles is equal or smaller than the difference between the width of the print head and the width of the layer to be printed, both specified in pixels, i.e. in multiples of the distance D between two adjacent nozzles. The compensating nozzles are thus not strictly necessary to print the current layer. They can be used to replace malfunctioning nozzles. In particular, the compensating nozzles can be moved in the active printing region in a moving step in order to deposit droplets of printing ink on the three-dimensional structure to be printed.

According to a preferred embodiment of the present invention, the number of compensating nozzles in the compensation set is chosen depending on the number of layers to be printed.

Preferably, the number of compensating nozzles is chosen such that a most uniform distribution of defects is achieved. The number of compensating nozzles is preferably larger the more layers have to be printed in order to provide a maximal compensation space and thus a distribution that is as uniform as possible.

In a preferred embodiment of the present invention, the number of compensating nozzles differs for at least two depositing steps. In particular, the number of compensating nozzles is adjusted according to the layer to be printed in the current depositing step. If e.g. a subsequent layer has a smaller width than a previous layer, the number of compensating nozzles used in the printing of the subsequent layer may be decreased as compared to the number of nozzles used in the printing of the previous layer. In particular in printing convex structures, e.g. convex optical lenses, the compensation space can thus be increased over the printing process and printing accuracy increased. In particular, the accuracy in a region of small width may be maximized by using only a section of the print head with properly working ejection nozzles.

According to a preferred embodiment of the present invention, the step sequence is chosen such that each of the compensating nozzles deposits droplets of printing ink at each position at most once, i.e. during at most one depositing step. Here, position refers to the projection of the location of the deposited droplet into the x-y-plane. In this way, an accumulation of errors in one position is advantageously avoided. Still, this may lead to unwanted aberration effects. E.g. assuming a single malfunctioning nozzle, by moving the print head by a single nozzle in each moving step, a defect is translated across the structure leaving a line of defects when projected into the x-y-plane. Multiple such lines cause unwanted aberration effects. Therefore, in an alternative preferred embodiment, the step sequence is chosen such that each of the compensating nozzles deposits droplets of printing ink at each position at most once in a fixed interval. Here, the interval refers to a number of layers or depositing steps.

According to a preferred embodiment of the present invention, a step sub-sequence is defined for each layer such that every position occurs only once in the step sequence comprising all step sub-sequences and all steps in the step sequence are randomly shuffled using a random generator. Defining the step sub-sequence comprises predefining each possible step location for each layer, e.g. the first layer has 10 possible step locations, the second layer has 11 possible step locations, the third layer has 12 possible step locations etc. Combining all step sub-sequences yields a set of possible locations that can be used during the printing process. Preferably, every location is unique and does not repeat. From this the step sequence is obtained through a random shuffling of the set of possible locations obtained from combining all step sub-sequences. Preferably, the random shuffling is carried out through a random generator that randomly shuffles each step. In a preferred embodiment, the step sequence randomly generated in this way satisfies a certain set of constraints. The constraints comprise e.g. a minimal step size and a maximum randomization position for the first few layers.

According to a preferred embodiment of the present invention, the moving distance X differs during at least two moving steps. In this way, a regular pattern of defects, as e.g. the line described above, is advantageously avoided. Preferably, the moving distance X differs during at least two consecutive moving steps between which a depositing step is carried out.

In a preferred embodiment of the present invention, the moving direction φ differs during at least two moving steps. Preferably, the moving direction φ differs during at least two consecutive moving steps between which a depositing step is carried out.

According to a preferred embodiment of the present invention, the print head is moved relative to the deposited droplets during at least one moving step for a moving distance X being smaller than a nozzle distance D between two adjacent nozzles during the moving step. E.g. the moving distance X is half the nozzle distance D. Through this microstepping, an accumulation of defects is advantageously reduced.

Preferably, the print head is randomly moved relative to the deposited droplets in the moving step. It is herewith advantageously possible that droplets ejected by the inaccurately working ejection nozzle(s) are almost evenly distributed over a least a section of or the entire three-dimensional structure, as in each depositing step the print head is moved randomly, and therefore averaged out due to stochastical effects. A particular advantage is that the location of the inaccurately working ejection nozzle(s) in the print head is of no importance and therefore has not to be known when performing the inventive method. To make this approach work, the horizontal extension of the print head is preferably substantially greater than the horizontal extension of the three-dimensional structure to be printed, so that even if the print head is moved randomly, there is always an overlap between ejection nozzles and three-dimensional structure to be printed. Moving the print head randomly can be achieved by providing a controller of the print head with a random generator or by providing the controller with a (e.g. pre-defined and pre-stored) moving scheme which is based on randomly generated parameters initially provided by a random generator at some previous moment, e.g. during designing or manufacturing of the printer.

According to preferred embodiment of the present invention, the print head is moved relative to the deposited droplets during the moving step for a moving distance X being smaller than a nozzle distance D between two adjacent nozzles during the moving step. It is herewith advantageously possible that the inventive method does not only work in a given pixel matrix depending on the distances between adjacent nozzles but also with intermediate sizes and higher resolution. The method is performed in such a manner that in a subsequent depositing step printing ink from more than one ejection nozzle is deposited in the location of a formerly deposited droplet. In this way, the negative influence of a single or few inaccurately working ejection nozzles on the quality of the whole three-dimensional structure can be further reduced. Preferably, the print head is moved relative to the deposited droplets during the moving step in such a manner that the moving distance X is less or equal than one half of the nozzle distance D and preferably less or equal than one quarter of the nozzle distance D.

Furthermore, it is conceivable that the print head is moved relative to the deposited droplets during the moving step for a moving distance X and wherein the moving distance X is modified after each moving step. The moving distance X can e.g. be randomly modified within a predefined interval after each moving and/or depositing step.

According to another preferred embodiment of the present invention, a moving direction φ of the relative movement between the print head and the deposited droplets is changed between two subsequent moving steps between whom at least one depositing step is performed. The moving direction φ can be changed about any angle between 0 and 360 degrees between two sequencing moving steps. It is e.g. conceivable to turn the moving direction φ about 180 degrees, so that the print head is moved in opposite directions in two sequencing moving steps. Preferably, the moving direction φ is modified after each moving step. The moving direction φ can e.g. be randomly modified after each moving and/or depositing step. In each case, the moving direction φ preferably remains parallel to the substrate and/or perpendicular to the ejecting/flying direction of the droplets.

If the print is randomly moved relative to the deposited droplets according to a predefined moving scheme, wherein the moving scheme is initially generated using a random generator, the predefined moving scheme preferably determines moving distance X and/or moving direction φ for each individual moving step.

As known from the prior art, the deposited droplets are at least partly cured after each step of depositing droplets. The printing ink of the deposited droplets are either fully cured after each depositing step or only partly cured. In the second case, a final curing step is performed after finishing the three-dimensional structure.

According to another preferred embodiment of the present invention, the movement of the print head relative to the deposited droplets in the moving step is realized in that the print head and/or the substrate vibrates. It is conceivable that the print head and/or the system continuously vibrates (or oscillates) during moving and depositing steps, so that the relative movement is automatically obtained between subsequent depositing steps. In this way, also a random and not predefined movement of the print head between two subsequent depositing steps can be achieved because the exact moving distance between to subsequent depositing steps cannot be predicted due to the high frequency of the vibrations. The vibration of the print head and/or the substrate is stimulated by means of a vibration generator. Preferably, the vibration generator stimulates the print head and/or the substrates continuously, so that the moving steps and the depositing steps completely, mainly or partially overlap in time with each other. The print head and/or the substrate are stimulated by a vibration generator to high frequency vibrations, preferably with a frequency of more than 1 megahertz, particularly preferably more than 100 megahertz and most particularly preferably more than 1 gigahertz.

Alternatively, the print head and/or the substrate vibrates only during moving steps. However, preferably the print head and/or the substrate vibrates during moving, depositing and/or curing steps.

Preferably, stimulating the print head and/or the substrate to high frequency vibrations means that the print head and/or the substrate periodically and translationally oscillates in at least one longitudinal oscillating axis. Particularly preferably, the print head and/or the substrate is stimulated to high frequency vibrations along two longitudinal oscillating axis perpendicular to each other. The one or two longitudinal axis are arranged parallel to a main plane of the substrate.

In an alternative embodiment of the present invention, the print head and/or substrate is stimulated to high frequency vibrations additionally (to the vibrations along the one or two longitudinal axis parallel to the main plane) or solely around a rotational axis perpendicular to the main plane of the substrate. That means a rotational oscillation of the print head and/or the substrate is stimulated. The rotational axis can e.g. be located in the center or at the border of the substrate, the print head or the three-dimensional structure to be printed.

The object of the present invention is also achieved with a printing system for printing a three-dimensional structure, in particular an optical component, by performing the inventive method, wherein the printing system comprises a print head for depositing droplets of printing ink side by side and one above the other in several consecutive depositing steps, wherein the print head is movable relative to the deposited droplets in a moving step performed between at least two consecutive depositing steps and wherein the print head comprises a plurality of ejection nozzles for ejecting a plurality of droplets simultaneously, wherein the printing system is configured in such a manner that the print head is moved in the moving step in such a manner that the droplets deposited in the same position in the at least two consecutive depositing steps are ejected at least partly from two different ejection nozzles.

Analogously to the inventive method, the printing system according to the present invention advantageously provides a substantially increased printing accuracy and therefore provides printing of three-dimensional structures which can serve as optical components, like lenses and in particular ophthalmic lenses, due to their improved quality. The printing system comprises in particular a print head and a controller for controlling movement of the print head relative to the deposited droplets and/or a substrate on which the droplets are deposited. The controller can be implemented into the print head or realized as a separate unit. Again, movement of the print head relative to the deposited droplets is preferably obtained by actively driving the print head, while the substrate on which the droplets are deposited preferably stands still, or by moving the substrate on which the droplets are deposited, while the print head preferably stands still. It is also conceivable that both the print head as well as the substrate are moved actively. However, the wording “moving the print head relative to the deposited droplets” does not necessarily means in the sense of the present invention that the print head is actually moved because alternatively the substrate on which the droplets are deposited can e.g. solely be moved to obtain the relative movement between the print head and the deposited droplets. The printing system preferably comprises at least one drive unit for actively moving the print head and/or the substrate, wherein the drive unit is controlled by the controller. Furthermore, the printing head comprises in particular a random generator for randomly moving the printing head relative to the deposited droplets in the moving step. Alternatively, the printing system and in particular the controller comprises a storage for storing a predefined moving scheme which is based on randomly generated parameters initially provided by a random generator at some previous moment, e.g. during designing or manufacturing of the printer.

Preferably, the printing system is configured in such a manner that the print head is moved relative to the deposited droplets for a moving distance X during the moving step and wherein the moving distance X differs for at least two moving steps.

Preferably, the printing system is configured in such a manner that a moving direction φ of the relative movement between the print head and the deposited droplets differs for at least two moving steps.

Preferably, the printing system is configured in such a manner that the print head is moved relative to the deposited droplets for a moving distance X during the moving step and wherein the moving distance X is modified after each moving step.

Preferably, the printing system is configured in such a manner that a moving direction φ of the relative movement between the print head and the deposited droplets is changed between two subsequent moving steps between whom at least one depositing step is performed.

According to a preferred embodiment of the present invention, the printing system comprises a vibration generator for stimulating the print head and/or the substrate to high frequency vibrations to obtain relative movement between the print head and the deposited droplets in the moving step. It is conceivable that the print head and/or the system continuously vibrates (or oscillates), so that the relative movement is automatically obtained between subsequent depositing steps.

The vibration generator is configured such that the print head and/or the substrate translationally oscillates along one longitudinal axis parallel to the main plane of the substrate or along two longitudinal axes respectively parallel to the main plane and perpendicular to each other. Furthermore, the vibration generator is configured such that the print head and/or substrate is stimulated to a rotational oscillation around a rotational axis perpendicular to the main plane. The rotational axis can e.g. be located in the center or at the border of the substrate, the print head or the three-dimensional structure to be printed.

DETAILED DESCRIPTION

InFIG.1, a method and a printing system1for printing a three-dimensional structure2are illustrated. In the present example, the three-dimensional structure2comprises an optical component and in particular an ophthalmic lens.

The printing system1comprises a print head3equipped with a plurality of ejection nozzles4. The ejection nozzles4are arranged in parallel on the lower side of the print head3. Each ejection nozzle4is in fluid connection with a reservoir of printing ink (not shown) and comprises piezoelectric crystals to eject a droplet6of printing ink from the print head towards a substrate5. The printing system1can therefore also referred to as DOD (droplets-on-demand) inkjet printer. In each depositing step10, a volley of several droplets6are ejected in parallel and simultaneously towards the substrate5, so that a layer of deposited droplets6arranged side by side onto the substrate5is generated. With each following depositing step10, a further layer of deposited droplets6are provided onto the former layer of deposited droplets6.

After deposition of the droplets6, adjacent deposited droplets6merge at least partially which each other (the deposited droplets6are therefore illustrated only schematically by dashed lines) and are subsequently cured in a curing step11by UV-light emitted by LED's (light emitting diodes)8of the print head3. The printing ink comprises a transparent or translucent printing ink, preferably an UV curable liquid monomer becoming a polymer if being cured. The depositing steps10and the curing steps11are repeated subsequently until a desired three-dimensional structure2is built up.

In order to deposit droplets6in certain positions onto the substrate5, the ejection nozzle4are individually controllable by a controller (not shown) of the printing system1. The horizontal extension of the print head1is substantially greater than the horizontal extension of the three-dimensional structure2to be printed, so that a movement of the print head3relative to the substrate2is not necessary to build up the three-dimensional structure2in the present example. The print head3typically comprises 1.000 to 5.000 ejection nozzles4arranged in parallel. The print head3and the substrate5are movable relative to each other. In the present example, movement of the print head4relative to the substrate5is obtained either by actively driving the print head4or by actively driving the substrate5respectively by corresponding drive units (not shown).

The print head and in particular the individual ejection nozzles4are controlled by the controller in dependency of an intensity image (not shown). The intensity image comprises a two-dimensional pattern of different color intensities. The pattern consists of different pixels, wherein each pixel represents a certain position in the three-dimensional structure2to be printed. In particular, each pixel represents a certain position of a two-dimensional projection of the three-dimensional structure2onto the substrate5. The intensity of the color in each pixel of the intensity image represents the height of the three-dimensional structure2at the corresponding position and therefore the number of droplets6to be deposited in this position by the corresponding ejection nozzles4. The controller now controls the plurality of printing nozzles4in such a manner that the number of droplets6deposited in each position on the substrate5corresponds to the intensity of the intensity image after all depositing steps10have been subsequently performed. The three-dimensional structure2is thereby built up step by step until the amount of printing material deposited in each position correspond to the color intensity in the pixels of the intensity image. In this manner, the droplets6are deposited side by side and one above the other in order to generate the desired three-dimensional structure2. As mentioned above, curing steps11are performed optionally between two subsequent depositing steps10in order to partially cure the deposited droplets6and to avoid that the deposited droplets6completely deliquesce after deposition.

In practice, the ejection characteristics of the ejection nozzles4are affected by clogging of printing ink and contamination with e.g. foreign particles and impurities. For this reasons, it happens from time to time that one or few ejection nozzles4of the print head3eject(s) less amount of printing ink in each depositing step10. Ejection nozzles4with a suchlike ejection characteristic are hereinafter referred to as inaccurately working ejection nozzles4′. The other ejection nozzles4are hereinafter referred to as accurately working ejection nozzles4. As clogged ejection nozzles4sometimes becomes open again (declogging) and accurate working ejection nozzles4getting clogged due to unpredictable circumstances, the locations of the inaccurate working ejection nozzles4′ inside the print head3changes and cannot be determined or considered during printing.

The resulting deviations of the ejection characteristics between accurately working ejection nozzles4and inaccurately working ejection nozzles4′ in the same print head3lead to inequalities and non-uniformities in the printed three-dimensional structure2. Usually, these inequalities and non-uniformities are so small that no visible and disturbing influences occur. However, in the present example, the three-dimensional structure2comprises an ophthalmic lens, wherein even the finest small inequalities and non-uniformities lead to serious optical defects disturbing the optical beam path when using the ophthalmic lens. In particular, these inequalities and non-uniformities generate unwanted diffractive phenomena.

In order to avoid these inequalities and non-uniformities in the printed three-dimensional structure2, although the print head3comprises accurately working ejection nozzles4and inaccurately working ejection nozzles4′, a relative movement9between the print head3and the substrate5is accomplished in a moving step12always performed between two subsequent depositing steps10. Consequently, the print head3and/or the substrate5is moved by the drive unit, even if a movement of the print head3relative to the substrate5between the depositing steps10is not required to build up the three-dimensional structure2due to the larger horizontal extension of the print head1compared to the horizontal extension of the three-dimensional structure2to be printed.

The relative movement9is only performed to ensure that all droplets6deposited in one single position on the substrate5or pixel of the three-dimensional structure2do not solely originates from one single ejection nozzle4because if this ejection nozzle4is an inaccurate working ejection nozzle4′, the above mentioned unwanted inequalities and non-uniformities occur at this position. The relative movement9provides that droplets6of inaccurately working ejection nozzles4′ are spread over an area of the three-dimensional structure2being as large as possible during the whole printing process, so that their negative influence on the lens quality is averaged out due to stochastical effects. After each depositing step10, the print head3is moved for a moving distance X and along a moving direction φ parallel to the horizontal plane7of the substrate5.

Preferably, the moving distance X and the moving direction φ of the relative movement9changes after each moving step12in order to avoid any regularity when distributing the droplets6of the inaccurately working ejection nozzle4′ over the entire three-dimensional structure2. The relative movement9and particularly the moving distance X and the moving direction φ is randomly changed to achieve that droplets6ejected by the inaccurately working ejection nozzles4′ are almost evenly distributed over at least a section of the three-dimensional structure2. In order to provide this random movement, the controller of the print head3comprises a random generator or a storage for storing a predefined moving scheme which is based on randomly generated parameters initially provided by a random generator at some previous moment, e.g. during designing or manufacturing of the printing system1. The moving direction φ can be changed about any angle between 0 and 360 degrees between two sequencing moving steps12. It is e.g. conceivable to turn the moving direction φ about 180 degrees, so that the print head is moved in opposite directions in two sequencing moving steps12. The moving distance X can be smaller than a nozzle distance D between two adjacent ejection nozzles4during the moving step12, e.g. one half of the nozzle distance D or one quarter of the nozzle distance D. The moving distance X can be changed in an interval between one quarter of the nozzle distance D to one nozzle distance D with steps of one quarter nozzle distance D. In this case, each location of the three-dimensional structure2obtains printing ink in a certain depositing step10originating from more than one ejection nozzle4.

Preferred values of a changing moving direction φ between the two angles 0 and 180 degrees (plus and minus) and moving distance X in an interval between one quarter of the nozzle distance D to one nozzle distance D with steps of one quarter nozzle distance D is: Xi=−D/2, −D/4, +D/4, +D/2. It is conceivable that the moving distance X randomly switches between these four X-values.

In an alternative embodiment of the present invention explained with reference toFIG.1, the movement of the print head3relative to the substrate5in the moving step12is realized in that the print head3vibrates with high frequency. In this case, the drive unit comprises a vibration generator continuously stimulating the vibration of the print head3to translational and/or rotational oscillations. In this way, also a random and not predefined movement of the print head3is achieved because the exact moving distance between to subsequent depositing steps10cannot be predicted due to the high frequency of the print head vibrations. In this embodiment, the moving steps and the depositing steps completely overlap in time with each other because the print head3is moved due to the stimulated vibrations also during depositing droplets6in the depositing step10.

InFIG.2, different steps of the method according to the exemplary embodiment of the present invention explained with reference toFIG.1are shown. As described above, the method comprises the depositing step10of ejecting a plurality of droplets6simultaneously and in parallel towards the substrate5, followed by an optional curing step11to at least partly curing printing ink of the deposited droplets6and a final moving step12for providing a relative movement9between the print head3and the substrate5in order to minimize the influence of inaccurately working ejection nozzles4′. The depositing step10, curing step11and the moving step12are repeated in order to build up the desired three-dimensional structure2step-by-step (also referred to as layer-by-layer) unit the desired three-dimensional structure2is finished. Finally, a final curing step13is performed optionally.

InFIG.3a printing method according to an exemplary embodiment of the present invention is illustrated. The printing method according to the present invention comprises depositing droplets6of printing ink side by side such that a three-dimensional structure2is built up layer by layer. The droplets of printing ink are deposited by a print head3comprising a number of ejection nozzles4. The width spanned by the print head, i.e. the number of ejection nozzles4times the distance D between two adjacent ejection nozzles, exceeds the width of the three-dimensional structure2to be printed. The additional ejection nozzles4, or only part of them, can advantageously be used during the printing process to compensate for potentially malfunctioning ejection nozzles4′. More specifically, the printing scheme is set up such that the print head3is being moved between at least two depositing steps10in a moving step12such that the droplets6deposited in at least one position stem from two different ejection nozzles4. Preferably, a moving scheme is determined at the beginning of the printing process. The moving scheme preferably comprises selecting a sequence of moving steps12and a number of compensating nozzles4″. The sequence of moving steps12comprises in particular a set of moving distances X with moving directions φ. E.g. the sequence of moving steps12can be written as ((X1, φ1), (X2, φ2), (X3, φ3), . . . ), where X1is the distance the print head3is moved relative to the deposited droplets in the direction φ1during the first moving step12, X2is the distance the print head3is moved relative to the deposited droplets in the direction φ2during the second moving step12, etc. Here, the moving distance is e.g. given in units of the distance D between two adjacent ejection nozzles4and the moving direction φ as the polar angle measured from one corner of the print head3. In a preferred embodiment, the moving direction φ is the same for all moving steps, e.g. 0°, i.e. perpendicular to the printing direction. The moving direction may take on either of the two values 0° and 180°, i.e. the moving back and forth perpendicular to the printing direction. Preferably, the moving distance X is an integer multiple of the distance D between two adjacent ejection nozzles4. In an alternative preferred embodiment, the moving distance X is smaller than the distance D. E.g. the moving distance may be one half or one quarter of the distance D between two adjacent ejection nozzles4. A moving step12may be carried out after each depositing step10. Alternatively, it is conceivable that a moving step12is carried out after every other depositing step10or in any other interval. Optionally, curing steps11are carried out after at least one depositing step10. It is conceivable that a curing step11is carried out after each depositing step10. The number of compensation nozzles4″ may be kept fixed during the entire printing process or may vary after any desired and beneficial number of moving steps12. E.g. the number of compensation nozzles4″ may vary depending on the width of the three-dimensional structure2: with decreasing width of the three-dimensional structure the number of compensation nozzles4″ may be increased. For a convex optical component, for example, the width decreases with increasing height of the structure. In this case, the number of compensation nozzles4″ can be advantageously increased with the number of layers printed. In this way, the likelihood of using malfunctioning ejection nozzles4′ is further decreased. In a preferred embodiment, the number of ejection nozzles4that can be used as compensating nozzles4″ is large enough to use a section of function ejection nozzles4for the printing of the small-width layers of the three-dimensional structure2to be printed. The upper part ofFIG.3illustrates a moving scheme according to a preferred embodiment of the present invention. On the y-axis the number of layers14is being depicted. The width of the three-dimensional structure2in units of the distance D between adjacent ejection nozzles4, i.e. in number of pixels15, is plotted on the x-axis. In the exemplary moving scheme illustrated here, it is assumed that the print head3comprises a single malfunctioning ejection nozzle4′. The number of malfunctioning ejection nozzles4′ is assumed to remain constant over the printing process for the sake of simplicity and ease of illustration. The defect17caused by the malfunctioning ejection nozzle4′ is shown as a point in the diagram. The defect17comprises a droplet of reduced volume, a missing droplet or any other defect caused by a malfunctioning ejection nozzle4′ such as a clogged or partially clogged ejection nozzle. As can be seen from the diagram, the moving scheme consists of a sequence of moving steps12. During each moving step, the print head3is moved in a moving direction φ perpendicular to the printing direction3relative to the deposited droplets6. It is insignificant whether the relative movement consists in a movement of the print head3or a movement of the printing plate on which the droplets6are being deposited, as only the relative movement is of significance here. The print head3is moved by a moving distance X, which in this example is an integer multiple of the distance D, i.e. the moving distance X is larger than D. The moving distance X is kept constant for a fixed number of moving steps12. In particular, the moving distance X remains the same until the number of compensating nozzles4″ has moved over the entire width of the three-dimensional structure2. The next moving step12moves the print head3to the starting position and the series of moving steps12is repeated. As can be seen from the diagram, in this way, the defect17caused by the single malfunctioning ejection nozzle4′ is propagated across the full width of the three-dimensional structure2. In this way, a certain degree of averaging is achieved. Whereas such a moving scheme is sufficient to average out volumetric differences caused by malfunctioning ejection nozzles4′, it is insufficient to average out optical defects caused by malfunctioning ejection nozzles4′. In particular, the defects17form lines in the three-dimensional structure2which act as interference grid for light passing through the structure2. The slope of these lines depends on the moving distance X. A moving distance X larger than the distance D results in a slope below 45°, a moving distance X smaller than the distance D, as achieved through microstepping, results in a slope larger than 45°. Hence, through the choice of moving distance, the form of the interference grid and hence the resulting interference pattern is determined. Preferably, the moving distance X is chosen such that the interference pattern occurs in non-functional regions of the three-dimensional structure2or is moved to angles that are invisible for a potential user of the three-dimensional structure2. A corresponding interference pattern is depicted in the lower part ofFIG.3. The lower part ofFIG.3depicts the interference pattern caused by a single laser beam passing through an optical structure2printed with the moving scheme defined above. As the slit width varies with the viewing angle, i.e. with the incident angle of the laser light, the interference pattern depends on the viewing angle as well. The corresponding three-dimensional structure2exhibits defects in the form of blurry bands that vary with the viewing angle. Thus, even though a certain improvement in accuracy is achieved, it is insufficient for most optical applications, i.e. in cases that the three-dimensional structure2to be printed comprises an optical component of higher accuracy.

InFIG.4a printing method according to an alternative exemplary embodiment of the present invention is illustrated. The exemplary embodiment illustrated inFIG.4differs from the exemplary embodiment illustrated inFIG.3in the moving scheme and hence in the achieved accuracy of the printed three-dimensional structure2. The moving scheme is again illustrated in the upper part of theFIG.4. The diagram plots the number of layers14over the width in number of pixels15of the three-dimensional structure2. Again, it is assumed that the number of malfunctioning nozzles4′ remains constant over the printing process and that only a single ejection nozzle4is malfunctioning. This is for illustrative purposes only. In this exemplary embodiment, the moving scheme comprises a moving direction φ perpendicular to the printing direction for all moving steps12. The moving distance X, however, is no longer constant, but varies according to a predefined scheme. The scheme is selected such that the defects17caused by the malfunctioning ejection nozzle4′ is most favorably distributed over the three-dimensional structure2. Preferably, the defects17are distributed as uniformly as possible over the three-dimensional2. One way of achieving such a distribution is by selecting step sub-sequences for each layer. E.g. the first layer may have ten possible step locations, the second layer may have eleven possible step locations, the third layer may have twelve possible step locations etc. From the set of step locations obtained from combining all step sub-sequences a set of all possible step locations is obtained. The set is chosen such that each location is unique and never repeats. Preferably, the steps contained in this set are shuffled randomly, e.g. through a random generator, yielding the step sequence. In particular, the randomization is carried out under certain constraints, e.g. a minimal step size and/or a maximal randomization position, in particular for the first few layers. In this way, patterns and accumulation of defects are advantageously avoided. This reduces in particular the extent of optical effects caused by the defects17, e.g. defect lines are advantageously avoided. In particular, the interference pattern caused by the defects17is improved as compared to the interference pattern caused by the moving scheme according to the exemplary embodiment ofFIG.3. This can be deduced from the lower part ofFIG.4. The lower part ofFIG.4shows the interference pattern caused by a single beam of laser light passing through a three-dimensional structure2at a fixed angle. The visibility and orientation dependence of the defects17is significantly reduced.

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