This application is the National Stage of International Application No. PCT/NL2015/050843 filed Dec. 4, 2015, which claims the benefit of Netherlands Application No. NL 2013931, filed Dec. 5, 2014, the contents of which is incorporated by reference herein.
The invention relates to a method for manufacturing a printing bar unit for a printing system as well as to a printing bar unit, in which the printing bar unit is of the type that has a plurality of exchangeable printheads, of which each printhead has a plurality of inkjet nozzles. This makes it possible to exchange one or more of the printheads should one or more of the nozzles therein get out of order, thus not having to replace the entire printing bar unit. Such printing bar units can be used in single pass inkjet printing systems in which a substrate to be printed is moved in a direction x along a printhead unit which extends in a direction y over an entire width of the substrate. Such printing bar units can also be used in scanning type of inkjet printing systems in which a substrate to be printed is moved stepwise in a direction x along a printhead unit which may be smaller than a width of the substrate, and in which the printhead itself then can be moved in a direction y, perpendicular to a main substrate direction x, in order to be able to print an entire width of the substrate.
For single pass type printing of substrates, printing systems are known to comprise elongate lineheads with stationary rows of inkjet nozzles. For larger widths, such lineheads each comprise an elongate support bar that is equipped with a plurality of printheads, of which each printhead is replaceable and comprises a number of the nozzles. It is of great importance for the image quality that can be obtained with printing on a substrate, that each printhead is accurately positioned, both relative to printheads of their own linehead as well as relative to printheads of other lineheads. Dimensional stability of the nozzle locations in the printheads in the printing direction x as well as in a direction y perpendicular thereto is crucial. Another important aspect is that the support bar needs to have an expansion behaviour that is matched to the printheads, and any intermediate connection elements therebetween, during changes of temperature. This is important in order to prevent that, transitions between respective printheads become visible on a printed substrate if a pitch between two nozzles of two adjacent printheads becomes different from a pitch between two nozzles of a same printhead. This is also important because, in the case that each printhead is mounted to the support bar with two or more interspaced mounting positions in the y-direction, the support bar may start to curve in the case of such temperature changes. Moreover, considering two or more lineheads from the same system may have different temperatures but the nozzles on these lineheads have to stay aligned, it is often preferred that the support bar have a low thermal expansion coefficient and high thermal conduction. Furthermore, considering the linehead may span a large width, it is preferred that the support bar has a high e-modulus and is lightweight.
For example US 2013/0265363 shows a printing bar unit which comprises a T-shaped support bar which at both sides of a vertical portion is provided with four engagement recesses. Each recess can receive a complementary engagement projection of a printhead, substantially with a form fit. Each recess furthermore is provided on its opposite edges with tapped holes for mounting one of the printheads thereto. For this each printhead includes a printhead body that is able to eject ink from an array of inkjet nozzles, and a fixing member. The fixing member is pre-mounted to the printhead body with screws. A horizontal portion of the T-shaped base plate is provided with communication holes which are connectable to ink channels of the printheads. The communication holes connect to an ink supply tube. Inside the printheads the inkjet nozzles are each equipped with a controllable piezoelectric element.
A disadvantage herewith is that the printing bar unit is difficult and expensive to manufacture, particularly if the unit needs to span large printing widths, as for example may be the case when it is to be used as elongate linehead for single pass type of printing, in which the linehead needs to span the entire width of substrates to be printed. Furthermore it is disadvantageous that a high positioning accuracy of each of the individual printheads relative to the support bar is strongly dependent on the accuracy with which the support bar itself is manufactured and is strongly dependent on the rigidity of the support bar during use, for example when heating of the unit may occur.
GB-2,449,939 discloses a method for manufacturing a printhead support, in which an elongated support member is provided with connecting apertures that are positioned approximately where corresponding printhead alignment members are to be located. This support member gets placed on top of a jig in such a way that the connecting apertures get positioned around accurately located upwardly projecting spigots of the jig. With this the connecting apertures have a larger diameter than the corresponding spigots. Gaps between the spigots and the connecting apertures then get filled with a hardenable material. As soon as this material has hardened, the support member gets removed from the jig leaving mounting apertures behind where the spigots were located. Those mounting apertures then are destined for having printheads mounted thereto with their printhead alignment members.
A disadvantage with this is that the support member needs to be held securely in place on the jig not only during filling of the gaps with the hardenable material but also during hardening thereof. The slightest movement between the support member and the jig, immediately deteriorates the positioning accuracy of the to be formed mounting apertures. Another disadvantage is that for the hardening of the material, heating and/or curing is necessary, which may cause the support member and/or the jig to expand/deform, which then immediately may have a negative effect on the positioning accuracy of the to be formed mounting apertures. Yet another disadvantage is that measures need to be taken in order to prevent the hardenable material from sticking to the jig. Further it is noted that with this method the degree of positioning accuracy may leave to be desired, for example due to variations or deviations in shrinkage of the hardenable material during hardening. Also it is noted that the hardenable material needs to be from a specific type that is able to harden to a sufficient high and accurate degree, like Diamant Moglice, which makes it relative expensive.
There also needs to be access to insert the hardenable material whereas the geometry not always permits to have insertion holes and excess spill apertures. Finally it is noted that the filling of the gaps with the hardenable material is a relative difficult and time-consuming operation which is likely to contaminate the support member at locations around the connecting apertures.
The present invention aims to at least partly overcome the above mentioned disadvantages, or to provide a usable alternative. In particular it aims to provide an economic high precision manufacturing method for printing bar units as well as to provide printing bar units with which a higher accuracy of printing can be achieved without this having to incur high manufacturing costs.
This aim is achieved by a method for manufacturing a printing bar unit for a printing system according to the present invention. This method comprises the steps of providing a support bar having a plurality of primary mounting positions, of providing a plurality of exchangeable printheads, in which each printhead has a plurality of inkjet nozzles, and of releasably mounting the printheads to the support bar. According to the inventive thought the method is characterized in that, preceding the step of releasably mounting the printheads to the support bar, a plurality of reference organs are connected at the primary mounting positions to the support bar and during this connection or directly thereafter undergo an alignment finishing process for forming a plurality of accurately lined up secondary mounting positions. Only then in a subsequent step, the printheads are releasably mounted to those accurately lined up secondary mounting positions on the reference organs.
Thus it is advantageously possible to make the support bar and its primary mounting positions thereon with relative high thermal stability but also with yet relative inaccurate dimensional tolerances. The subsequent connection and alignment finishing process of the reference organs onto the support bar, is well able to lift those relative inaccurate dimensional tolerances of the primary mounting positions on the support bar to higher accuracy levels of the secondary mounting positions that are then formed by or on the reference organs. This makes it possible to even use lengths of standard profiles as support bars, making them relative cheap to manufacture. Also this makes it possible to drill relative large and/or inaccurate holes in the support bar for forming its primary mounting positions. The support bar can even be made from a material that is more rigid and/or more lightweight compared to the material of the reference organs. Furthermore the support bar can be made from a material that has a higher thermal stability (smaller coefficient of thermal expansion) and/or higher thermal conduction coefficient relative to the ones of the reference organs. The reference organs themselves can be formed by relative small elements compared to the support bar. This makes their influence on a deformation behaviour of the entire unit small. They may even be made of a material that is less thermally stable compared to the material of the support bar, that is to say a material that has a higher coefficient of thermal expansion. Further, the reference organs can be formed by elements that can easily undergo the required accurate alignment finishing process during and/or after connection to the support bar. Also any length differences or shape differences of the support bar, thus can easily be dealt with.
In a preferred embodiment the alignment finishing process of the method according to the invention comprises a machining operation, that is to say a controlled material-removal process, preferably with the aid of machine tools, in which the alignment-finishing of the reference organs takes place by means of an active removal of material of those reference organs in order to form the plurality of accurately lined up secondary mounting positions. In particular this machining operation then may comprise a face milling and/or grinding step of at least those parts/faces of the reference organs to which the printing heads are destined to get releasably mounted to. Such face milling and/or grinding are able to achieve a high degree of accuracy and are considered efficient, reliable and economic methods for performing said aimed alignment finishing process. The grinding preferably uses a grinding wheel and may include a process of polishing, for example one that starts with coarse abrasives and graduates to fine ones. Other types of machining or combinations thereof are however also possible, like drilling, reaming, planing or sawing. In the alternative, the alignment finishing process may also comprise a process of controlled material addition to at least those parts of the reference organs to which the printing heads are destined to get releasably mounted to.
In a preferred embodiment the alignment finishing process of the reference organs may be performed such that dimensional tolerances of the secondary mounting positions on the reference organs relative to each other become more accurate than dimensional tolerances of the primary mounting positions on the support bar relative to each other. With dimensional tolerance it is meant here the degree of accuracy with which the positions in x-, y- and/or z-directions of the primary and secondary mounting positions have come to lie relative to each other. For example this can be the accuracy of an aimed interspacing distance in a certain x-, y- and/or z-direction between two adjacent primary or secondary mounting positions. In particular the dimensional tolerances of the primary mounting positions on the support bar then may be larger than 0.1 mm, whereas the dimensional tolerances of the secondary mounting positions may become smaller than 0.1 mm, and more in particular even may become smaller than 0.02 mm. Thus relative inaccuracies of the support bar can be upgraded with a factor 10 for the entire unit.
The support bar and the reference organs can be made out of all kinds of materials. Advantageously it is now possible to make the support bar out of another material than the reference organs. In particular the support bar is made out of a material that is more rigid (higher e-modulus) and/or that is more lightweight and/or that has a lower coefficient of thermal expansion and/or that has a higher thermal conduction compared to the material of the reference organs.
In an embodiment the support bar may be made out of a ceramic material, for example SiC. This is a relative brittle material which is difficult to process, but which at the same time is relative rigid, lightweight and thermally stable, while having a high heat conduction coefficient. Other materials are also possible.
In an embodiment the reference organs may be made out of metal. This is a material that is relative easy to process, and which at the same time is still relative rigid. Other materials are also possible.
In another preferred embodiment the method may further be characterized in that, preceding the step of releasably mounting the printheads to the support bar, reference end blocks are connected to free ends of the support bar and undergo an alignment finishing process for forming reference positioning faces. Those reference positioning faces then amongst others are destined to be placed at complementary bearing points of the printing system. With this the same advantages go as described above for the reference organs, that is to say that any inaccuracies of the support bar's free ends can now easily be lifted to a higher level by the alignment finishing process of the end blocks. The accurately aligned/finished reference positioning faces on the end blocks make it possible to simply hang the entire unit in a printing system by means of free seating. Thus no pulling/pushing or momentum forces get exerted on the support bar. In cases of calamities, like blockings or accumulations of substrates underneath the unit, the support bar together with the print heads mounted thereto can move upwards out of its bearings and/or start to tilt. If desired the reference positioning faces can even be provided with suitable friction reduction or damping means. If desired, seats for the end blocks can also be made adjustable.
In a preferred embodiment the alignment finishing process of the end blocks may be performed such that a dimensional tolerance of the reference positioning faces of the reference end blocks relative to the secondary mounting positions on the reference organs becomes more accurate than a dimensional tolerance of the free ends of the support bar relative to the secondary mounting positions on the reference organs. With dimensional tolerance it is meant here the degree of accuracy with which the positions in x-, y- and/or z-directions of the reference positioning faces and the secondary mounting positions have come to lie relative to each other. For example this can be the accuracy of an aimed interspacing distance in a certain x-, y- and/or z-direction between one of the reference positioning faces and a respective one of the secondary mounting positions. In particular the dimensional tolerances of the free ends may be larger than 0.1 mm, whereas the dimensional tolerances of the reference positioning faces may become smaller than 0.1 mm, and more in particular even may become smaller than 0.02 mm.
Advantageously the alignment finishing processes of the reference end blocks and the alignment finishing process of the reference organs can get performed in a single simultaneous step. The support bar then can remain clamped and positioned in a suitable clamp while both the reference organs and end blocks get accurately positioned and lined up relative to each other during and/or after connection to the support bar. This not only saves time, but in the end, that is to say after the printing bar unit has been placed with its reference positioning faces at complementary bearing points of a printing system, also helps to improve the positioning accuracy of the secondary mounting positions relative to the rest of such a printing system.
In a first variant the alignment finishing process of the reference organs and/or end blocks may comprise a face milling and/or grinding of at least front faces of the reference organs and/or of the aimed reference positioning faces of the end blocks after they have been connected to the support bar. With this it is noted that the support bar itself then does not necessarily get face milled and/or grinded during this step, it may only be the reference organs and/or the end blocks that get to undergo the alignment finishing process.
In a second variant, which in particular may get performed preceding said machining operation like said face milling and/or grinding, the alignment finishing process of the reference organs and/or end blocks may comprise a varying of a thickness of a glue layer, possibly in combination with using one or more filling plates between the support bar and the reference organs and/or end blocks during their connection to the support bar. This then in particular can be obtained by using a glueing jig for accurately positioning the reference organs and/or end blocks to the support bar during the hardening of the glue.
The printheads can be mounted directly onto or against the secondary mounting positions of the reference organs. It is however also possible to mount intermediate adapter elements, like fixing members, to the secondary mounting positions of the reference organs, and to have the printheads releasably mounted, for example screwed or clamped, to those intermediate adapter elements.
Further advantageous embodiments are described herein.
The invention also relates to a printing bar unit, and to a printing system comprising one or more of such printing bar units.
The invention shall be explained in more detail below with reference to the accompanying drawings, in which:
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Some different inventive methods for manufacturing the units 2 shall now be explained below with reference to
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The possible use of the glueing jig in steps 2c and 4b shall now be explained in more detail with reference to
Besides the embodiments shown, numerous variants are possible. For example the materials, various dimensions and/or shapes of the distinctive components may differ. Instead of drilling holes in the support bar, it is also possible to already provide those holes in the support bar during manufacturing thereof. If for example the support bar is made out of ceramic material, then the holes can already be made therein while the ceramic material is still in its green phase. Despite the fact that such holes then are likely to be rather inaccurate because of shrinkages of the material during hardening, this is no problem, since according to the invention, the position accuracy of the printheads on the support bar can be greatly and easily improved during the subsequent connection and alignment finishing process of the reference organs. Instead of using a rectangular hollow beam as support bar, it is also possible to use a strip-shaped, T-shaped or L-shaped support bar or any other profile. This shall be dependent on the type of printheads that need to be mounted thereto and on the required rigidity. In the case of the hollow beam, the hollow inside the beam may be used for supplying fluids such as inks and/or steering signals, and/or gasses towards and from the respective printheads and their neighbourhood. Instead of glueing or otherwise connecting reference organs to the support bar that already comprise a through-going mounting opening therein, it is also possible to accurately drill such through-going mounting openings in the reference organs during the alignment finishing process. This then makes it possible to obtain through-going mounting openings with improved dimensional tolerances relative to the ones of the support bar with its first mounting positions. Instead of glueing, the organs and/or blocks can also be connected in other manners to the support bar, for example by clamping or screwing. It is also possible to obtain the support bar by means of a 3D-printing operation. In the alternative or in addition thereto it is also possible to perform a 3D-printing operation for making the reference organs on top of the support bar. Those 3D-printed reference organs then can be printed out of another material than the support bar, and those 3D-printed reference organs then in a subsequent step can undergo the alignment finishing operation according to the invention.
Thus according to the invention a manufacturing method and printing bar unit is obtained with which reference positioning faces and mounting positions for printheads can be optimally defined relative to each other, while being able to use all kinds of support bars, even ones which are rather inaccurate in their dimensions and which are difficult to directly machine such that they get more accurately defined. The invention advantageously can be used for both single pass and scanning types of printing systems, and for example can be used in the field of textile printing, décor printing, packaging printing, label printing, document printing above a flat track or above a curved track on which a continuous or discontinuous substrate is transported. When used in single pass printing systems, the printing bar unit according to the invention can advantageously form an elongate linehead, in particular one having a length of at least 1.0 meter which gets equipped with tens of printheads in line or staggered next to each other. Even at such long lengths, very high accuracies can be obtained for the positioning of the printheads. When used in scanning printing systems, the printing bar unit according to the present invention can advantageously also be formed with relative long support bars such that wider strokes can be made in one scanning movement of a shuttle to which the printing bar units are mounted.
Number | Date | Country | Kind |
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2013931 | Dec 2014 | NL | national |
Filing Document | Filing Date | Country | Kind |
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PCT/NL2015/050843 | 12/4/2015 | WO | 00 |
Publishing Document | Publishing Date | Country | Kind |
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WO2016/089211 | 6/9/2016 | WO | A |
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