Patent Publication Number: US-2023133457-A1

Title: System and method for manufacturing three-dimensional structures

Description:
FIELD OF THE INVENTION 
     The invention relates to a system and method for manufacturing three-dimensional structures by filament deposition of build material paste. 
     BACKGROUND TO THE INVENTION 
     Additive manufacturing is currently widely used and various techniques exist. Additive manufacturing is a suitable technique for building a structure layer-by-layer and the manufactured structure can be employed in various applications. 
     Extrusion-based additive manufacturing methods have been employed for fabrication of three-dimensional structures. A build material (e.g. a viscous paste, a meltable polymer, a hydrogel, etc.) is extruded through a nozzle in the form of filaments. A certain arrangement of filaments can be obtained by relative movement of the nozzle with respect to a print bed during deposition by extrusion of the build material. During the material deposition, filaments of the build material are extruded from a nozzle and positioned relative to one another according to a predetermined pattern to provide a desired three-dimensional structure. The lay-down pattern is determined by the print path and has major impact on the shape and properties of the printed structure. The extrusion-based techniques can be used for printing three-dimensional structures. In this way, complex geometries and three-dimensional structures can be obtained which may be non-porous or which may be porous and comprise an interconnected network of internal pores which are accessible from the outside, and which may be required for some applications. 
     The existing systems and methods can be rather slow and inefficient to implement for mass production of three-dimensional objects such as porous structures. There is a need for improving the printing process of three-dimensional structures manufactured by an extrusion based printing process. Often, the printing process is rather slow, making it difficult to employ for printing various objects requiring a higher output while avoiding high costs involved. It is desired to obtain a system capable of increasing the output of printed  3 D structures in an efficient manner. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide for a method and a system that obviates at least one of the above mentioned drawbacks. 
     Additionally or alternatively, it is an object of the invention to improve the additive manufacturing process for three-dimensional structures. 
     Additionally or alternatively, it is an object of the invention to improve the efficiency of an extrusion based additive manufacturing process for fabricating three-dimensional structures. 
     Thereto, the invention provides for a system for manufacturing three-dimensional structures, the system comprising a plurality of printing stations for performing parallel printing in a confined space, wherein each printing station comprises a carrier, a deposition unit with at least one nozzle arranged for dispensing filaments of build material paste through an opening area thereof and a station controller configured to operate the deposition unit for deposition of filaments of a build material paste on the carrier in an interconnected arrangement in a plurality of stacked layers in order to form one or more three-dimensional structures, the at least one nozzle and the detachable carrier being relatively moveable with respect to each other, wherein the deposition unit is coupled to a reservoir unit configured to house the build material paste, wherein the reservoir unit includes at least one reservoir arranged outside of the confined space. 
     The paste reservoirs can be placed external to the working space or confined space of the printing station in which the three-dimensional structures are printed by extrusion of the build material into filaments and deposition of these filaments on the carrier. In this way, handling of the paste reservoir, for example disconnection thereof, can be carried out without requiring operations in the working space of the printing station. A more efficient, continuous and/or safer printing process can be obtained. The reservoirs can be easily replaceable without requiring access to the deposition head, and the risk to interaction with three-dimensional structures present on the carrier may be reduced to a minimum. This provides important advantages compared to placing the reservoir on or adjacent the deposition head of the printing station. The reservoir can be mounted on a remote location with respect to the deposition head. The build material paste (e.g. viscous paste) can be provided to the deposition head by means of tubing, or the like, providing a fluid communication between the reservoir and the deposition head. 
     The system can have multiple printing stations in an enclosed environment with changeable print reservoirs outside of the enclosed environment (e.g. outside of the housing). The reservoirs arranged outside of the enclosed environment can be easily replaceable, for example not requiring stopping of a printing process when the reservoir is replaced. 
     Optionally, the carrier is a detachable carrier which is removably arranged in the printing station. 
     Optionally, the confined space is enclosed by a housing. 
     Optionally, the at least one reservoir is arranged on an outer side of a housing enclosing the confined space. 
     Arranging the reservoir external to the housing of the printing stations and/or the housing of the system, can provide important advantages. 
     The reservoir mounted externally can facilitate replacement of the at least one reservoir. The reservoir can be better accessible for replacement, even during printing operations by the printing station. A replacement can be carried out while being protected from moving parts (e.g. at least one of a deposition unit and carrier of the printing station). 
     The system may have one or more housings. In some examples, the system has a system housing enclosing the plurality of printing stations. In some examples, each printing station has its own housing enclosing at least the carrier and the deposition unit. In some examples, the plurality of printing stations may be subdivided in groups of printing stations, each group of printing stations having a common housing enclosing at least the carriers and the deposition units of this group of printing stations. The housing may be fluid sealed enabling working with toxic or hazardous chemicals for which for instance gas extraction is required. 
     Optionally, the system is arranged such that each printing unit is individually accessible by means of an opening (e.g. panel or hatch or door or window). In this way, internal devices of the printing station can be accessed for instance by an operator (e.g. print nozzle replacement, resolving nozzle clogging, removal of parts that went wrong while printing, etc.) without the printing operations being carried out in the other stations being influenced by this. Furthermore, this can be done without affecting the supply and removal of carriers (e.g. trays) for the other printing stations. 
     Optionally, the at least one reservoir is detachably connectable to the housing by means of an attachment device. 
     It will be appreciated that different types of attachment devices can be employed. The attachment device may provide a quick attachment or coupling. In this way, fitting the reservoir(s) in the system can be facilitated. 
     Optionally, the housing includes a holder for the at least one reservoir, wherein the holder includes a coupling interface for removably coupling the at least one reservoir to the housing of the printing station, wherein the holder comprises a first interface for providing a fluid communication for build material paste between the at least one reservoir and the deposition unit, and a second interface for providing fluid pressure to the at least one reservoir. 
     The reservoir for holding the build material paste can be arranged outside the confined environment in which the printing is performed by extrusion of the build material paste. The reservoir can have a tubing (e.g. hose) providing a fluid connection (for transport of the build material paste) between the nozzle of the deposition unit and the paste reservoir. The second interface is arranged for providing pressure to the at least one reservoir, in particular to the inner volume of the at least one reservoir to cause extrusion of the build material paste towards the carrier. 
     Optionally, the at least one reservoir is replaceable and/or interchangeable. 
     The paste reservoir can be easily accessible for replacement, refilling, etc. The printing process can be significantly enhanced in this way. The reservoir can be replaced by the same reservoir (e.g. refilled) or by a different reservoir. 
     Optionally, the holder is arranged to provide a universal coupling enabling attachment of different types of reservoirs to the housing. 
     The coupling device may be provided by fast attachment means, a quick fastening device, a quick-connect device, a rapid connection unit, a coupling assembly, or the like. 
     Optionally, the at least one reservoir includes a communication unit configured to enable communicative coupling to one or more controllers of the system, wherein the communication unit is configured to communicate data indicative of an amount of build material paste inside the reservoir. 
     Optionally, the at least one reservoir includes one or more windows enabling a visual indication of the amount of build material paste inside the reservoir. The amount of build material paste inside the at least one reservoir may also be assessed by a sensor, which may be contained in the reservoir or positioned outside of the reservoir. 
     Optionally, the at least one reservoir includes at least one sensor for providing data indicative of the amount of build material paste inside the reservoir. Optionally, the at least one sensor is an internal sensor. 
     Optionally, each of the at least one nozzle is coupled to at least one reservoir for feeding build material paste. 
     Optionally, each of the at least one nozzle is coupled to at least two reservoirs for feeding build material paste. Advantageously, the continuity of printing processes can be improved in this way. 
     Optionally, at least one of the at least one nozzle is connected to an attachment device for at least two reservoirs which allows to switch from a first to another reservoir without interruption of a printing process. Nearly empty reservoirs can be replaced without interruption of a printing process. It may also be possible to switch from one build material paste to another build material paste without interruption of a printing process. 
     Optionally, the deposition unit includes at least a first nozzle and a second nozzle, wherein the first nozzle is coupled to a first reservoir for first feeding build material paste and the second nozzle is coupled to a second reservoir for second feeding build material paste. In some examples, the first and second build material paste are the same. It is also possible that the first and second build material paste are different. 
     Optionally, the first nozzle is further coupled to the second reservoir, and the second nozzle is further coupled to the first reservoir. 
     Optionally, the at least one reservoir is refillable. 
     Optionally, the system includes a positioning structure arranged for positioning the carrier within the printing station. 
     In some examples, the carrier includes a locking unit for ensuring that the carriers are positioned correctly within the printing station. The locking unit may for instance include one or more locking pins. 
     Optionally, the system includes an optical unit configured to check whether a carrier is placed in the printing station. The positioning however, can be performed using mechanical means. The printing station may for instance have a kinematic coupling. Optionally, the position of the carrier in the printing station is fixed. 
     Optionally, the system includes a plurality of integrated printing stations. 
     Optionally, the system is arranged to integrate two or more individual printing stations. 
     Optionally, the system includes a confined environment in which the three-dimensional structures are printed. 
     Optionally, the system includes several individual printing stations, at least a subset of the printing stations being based on micro extrusion technology. 
     Optionally, the system includes means for providing carriers to be printed on, for example a robotic system. In some examples, the robotic unit is arranged for enabling the provision and removal of carriers in the printing stations. The robotic unit can be configured to interact with the plurality of the printing stations of the system. 
     Optionally, the system includes an automated handling system within the confined environment formed by the system, the automated handling system being configured to provide carriers to each of the printing stations and to remove them from the printing stations, for example with three-dimensional structures printed thereon. 
     Optionally, the system includes a means for collecting carriers with printed three-dimensional structures (e.g. objects) for transport. 
     Optionally, the system includes a software program product configured to allow optimizing the output of the integrated printing stations. 
     Optionally, the system includes a confined environment, for example surrounded by a housing. 
     Optionally, the confined environment includes a physical shielding. This may provide for operator safety. 
     Optionally, the system includes a ventilation unit configured to ventilate/extract gas from the confined environment. 
     Optionally, the system includes a conditioning unit configured to condition the medium (e.g. air) inside the confined environment. In some examples, the temperature and/or humidity can be controlled. 
     Optionally, the system includes means for ventilation/extraction of gas, volatile substances and/or aerosols which may be released during printing in the printing stations. 
     Optionally, the system is configured to provide a modified gas atmosphere in at least regions of the confined environment. In this way, one or more of the printing stations are able to operate under a modified gas atmosphere (e.g. inert gas). 
     Optionally, the system is configured to provide controlled light conditions in the confined environment. One or more of the printing stations may thus be configured to operate under controlled light conditions. The printing stations may be configured to operate under light of a controlled wavelength, for example under UV or IR light or irradiation of any other desired wavelength One or more of the printing stations may be configured to operate under controlled light conditions for a desired period of time. 
     Optionally, each individual printing station is accessible from the outside of the confined environment of the system. 
     Optionally, the system is configured such that at least one of stopping of printing activities on a specific printing station, or carrier handling activities at a specific printing station, are carried out without requiring the printing activities on other printing stations or automated carrier handling by the robotic unit for the other printing stations to be interrupted. 
     Optionally, the printing stations of the system are configured to receive carriers for 3D printing of three-dimensional structure(s) thereon. The printing stations may include means enabling positioning of the carriers in an automated way. 
     Optionally, the printing stations of the system are configured to remove carriers with three-dimensional structure(s) printed thereon in an automated way. Thereby, the system may be provided to employ one single type of carriers, or to employ different types of carriers. Carriers may for example differ in size, geometric shape, height etc. 
     Optionally, the plurality of printing stations of the system are identical or different with respect to each other. 
     Optionally, the printing stations may have one of more printing heads. 
     Optionally, at least one reservoir providing a supply of build material paste for printing is placed on the outside of the confined environment. 
     Optionally, at least one reservoir providing a supply of build material paste for printing is exchangeable by means of a fast connection arrangement. 
     Optionally, the reservoir holding the build material paste is a paste cartridge. The cartridge may have connection means enabling a fast and easy attachment to the printing station or system. 
     Optionally, the robotic unit may be configured to provide carriers to the printing stations. The robotic unit may be configured to interact with the printing stations in order to arrange a pile or stack of carriers in a carrier holder. Additionally or alternatively, a rack or cart can be provided in which the empty carriers are placeable. Additionally or alternatively, a rack or cart can be provided in which carriers comprising printed three-dimensional structures are placeable. Additionally or alternatively, a conveyor belt may be employed for supplying carriers towards the robotic unit, and/or for transporting carriers carrying three-dimensional printed structures away from the robotic unit. The carriers supplied by the conveyor belt may for instance be empty carriers, such that they can be placed in the printing stations. 
     Optionally, the robotic unit may be an automated carrier handling system. 
     Optionally, the robotic unit includes at least one of an automated (computer controlled) translation and/or rotation systems such as robots, sledges, conveyor belts, plungers, rotating disks, etc. 
     Optionally, the robotic unit may be configured to fix the carriers during automated handling, e.g. by clamping or pin locking. 
     Optionally, fixation of carriers may be monitored during automated handling. 
     Optionally, the carrier may be a plate, tray or other object on which printing of three-dimensional structures is performed. 
     Optionally, the accelerations and vibrations occurring during transport of the carrier are controlled. For example, removing of the carrier with printed three-dimensional structure(s) thereon, may be carried out under controlled accelerations and vibrations of the robotic unit. This may allow transportation of parts with limited physical/vibrational stability. 
     Optionally, the robotic unit is configured to enable collection of the three-dimensional structures. Optionally, the robotic unit is part of a collection system. 
     Optionally, the robotic unit is placed inside the confined environment. 
     Optionally, the collection system includes carts or racks on which several carriers (e.g. trays, plates, substrates) are collectable. 
     Optionally, the collection system includes a box or container in which three-dimensional printed structures are collectable. 
     Optionally, the collection system includes a conveyor belt for transporting carriers towards and/or away from the system. 
     Optionally, each container is removable from the confined environment by unlocking a specific location (cf. collection docking station). 
     Optionally, each collection unit has a manual or automatic removal mechanism. 
     Optionally, the system includes a computer program product configured to be run on one or more controllers of the system. 
     Optionally, the computer program product is configured to control and monitor a supply of empty carriers. 
     Optionally, the computer program product is configured to control and monitoring a progress of a printing process on each of the individual printers. 
     Optionally, the computer program product is configured to operate the robotic unit such as to remove a particular carrier from the printing station after termination of the printing job on the printing station for the particular carrier. 
     Optionally, the computer program product is configured to operate the robotic unit such as to place the carrier and/or the printed three-dimensional structure(s) in a collection system. 
     Optionally, the computer program product is configured to follow up an overall status of print jobs involving multiple printing stations and multiple carriers. 
     Optionally, the computer program product is configured to estimate a time to finish a print job. 
     Optionally, the computer program product is configured to estimate a time for exchange of a material reservoir. 
     Optionally, the computer program product is configured to indicate when a collection unit for the carriers is full. 
     Optionally, the system is arranged for manufacturing three-dimensional structures. 
     Optionally, the system includes a plurality of printing stations in proximity to each other. In some examples, the plurality of printing stations are positioned next to each other. The printing stations may for example be arranged adjacent each other. 
     Optionally, the system includes several individual printing stations, at least a subset of the printing stations being based on micro extrusion technology. Micro extrusion is understood to comprise extrusion of a build material through an extrusion nozzle in the form of filaments. The build material may be a paste at room temperature. Optionally, viscosity of the build material is adjusted for 3D printing by means of temperature control (e.g. an increased temperature may be employed for lowering the viscosity). 
     According to an aspect, the invention provides for a method for manufacturing three-dimensional structures, the method including providing a plurality of printing stations for performing parallel printing in a confined space, wherein each printing station is provided with a carrier, a deposition unit with at least one nozzle arranged for dispensing filaments of a build material paste through an opening area thereof and a station controller configured to operate the deposition unit for deposition of filaments of a build material paste on the carrier in an interconnected arrangement in a plurality of stacked layers in order to form one or more three-dimensional structures, the at least one nozzle and the detachable carrier being relatively moveable with respect to each other, wherein the deposition unit is coupled to a reservoir unit configured to house the build material paste, wherein the reservoir unit is provided with at least one reservoir arranged outside of the confined space. 
     The at least one reservoir may be arranged external to the common enclosed housing of the printing stations. 
     Optionally, the system has multiple printing stations, wherein each station has one or more print heads for paste filament deposition. The system may include one or more housings allowing to control conditions or environment around the printing stations arranged in the housing. The system may be arranged to access one or more printing areas of the plurality of printing stations. 
     Optionally, each printing station is accessible via at least one access panel, hatch, door or window or the like. Optionally, the system is configured such that opening of a door linked to a housing containing one or more printing stations automatically stops printing by said one or more printing stations contained in the housing. 
     Optionally, each printing station is individually controllable. For example, print patterns to be followed, the printing speed, the used build material paste, the filament diameter, the filament deposition pattern, etc. can be adjusted for each of the printing stations, for example independently from each other for each of the printing stations. 
     Optionally, one or more build material paste reservoirs (e.g. containers) are arranged outside the housing (printing station(s) housing, or system housing). Optionally, the reservoirs are attached to the outside of the housing by means of quick fits. The reservoirs can be arranged to provide a supply of paste to the print head of the printing station(s). Such a quick fit can facilitate connection, to allow a quick exchange of the reservoirs as soon as they are empty. 
     Optionally, printing is performed on a removable carrier. The carrier may form a removable substrate on which three-dimensional structures are printable. 
     Optionally, each printing station contains a positioning structure arranged for positioning of the carrier in the printing station. In this way, it can be ensured that the carrier is always placed in a print station in the same way. The robotic unit can take care of moving the carrier to a storage system and/or transport system as soon as the carrier is full or the desired number of three-dimensional structures have been printed thereon. Optionally, the robotic unit is configured to always take the carrier from a same position and move it to the storage system and/or transport system. As the carrier is more accurately handled by the robotic unit, the carrier may be able to more accurately place it in the storage system and/or transport system. 
     According to an aspect, the invention provides for a use of the system according to the invention for manufacturing three-dimensional structures. 
     The system can have multiple printing stations in a confined space with changeable build material paste reservoirs outside of the confined space. According to another aspect, the system can have multiple printing stations in an enclosed environment (e.g. a housing) with changeable build material paste reservoirs outside of the housing. The reservoirs arranged outside of the confined space, advantageously the enclosed environment, can be easily replaceable, for example not requiring stopping of a printing process when the reservoir is replaced. 
     A detection system can be set up which is configured to detect when a reservoir needs to be replaced or replenished. Different types of detections are possible (e.g. optical detection, visual detection, etc.). 
     The system may guarantee an improved continuity in the printing operations of the plurality of printing stations. The printing stations can continue to work while adjustments are performed to one or more printing stations, e.g. when replacing or refilling a build material paste reservoir, adjusting control parameters of a printing station, etc. Additionally or alternatively, operations of individual printing stations can be halted while the other printing stations remain actively printing. 
     One printing station can be operated without the other printing stations being affected thereby. Hence, adjustments can be performed in one of the printing stations while the system can still continue to deliver output by means of the other printing stations. For example, if the carrier with printed three-dimensional structures is removed in one station, and/or if the build material pasta reservoir is replaced at one station, only that printing station is temporarily interrupted or is stopped and the other printing stations can continue performing printing operations. 
     In some examples, printing parameters of one printing station of the system can be changed (tuning, for example, flow rate) without affecting operations of the other printing stations. 
     It will be appreciated that the carrier can be embodied in various ways. The carrier can for instance be embodied as a plate, a tray, a print surface, a support, a substrate, a holder, or the like. In some examples, the carrier provides for a flat surface on which three-dimensional structures are printable. However, the carrier does not need to be flat. Other shapes are also envisaged. 
     According to an aspect, the invention provides for a system for manufacturing three-dimensional structures, the system comprising a plurality of printing stations and a robotic unit configured to interact with the plurality of printing stations, each of the plurality of printing stations being arranged to be accessible by the robotic unit, and wherein each printing station comprises a detachable carrier, a deposition unit with at least one nozzle arranged for dispensing filaments of a build material paste through an opening area thereof, and a printing station controller configured to operate the deposition unit for deposition of filaments of the build material paste on the detachable carrier in an interconnected arrangement in a plurality of stacked layers in order to form one or more three-dimensional structures, the at least one nozzle and the detachable carrier being relatively moveable with respect to each other, wherein the station controller of each of the printing stations is configured to control at least one deposition control parameter, wherein the robotic unit includes a handling device for handling detachable carriers, wherein the robotic unit is configured for providing, removing and/or replacing detachable carriers in the plurality of printing stations. Optionally, the system further includes a system controller configured to operate the robotic unit, and wherein the system controller is communicatively coupled to the plurality of printing stations for controlling at least an execution of printing tasks being performed on the plurality of printing stations. 
     Several printing stations or printers can be integrated in the system, the system further including an automatic robotic unit for applying, removing and/or replacing carriers (e.g. plate, tray or substrate) in the printing stations. The system can enable improved parallel printing at the plurality of printing stations. The printing operations can be better aligned with each other, provided by the individual control of the printing stations and/or the arrangement of the build material paste reservoirs. The printers arranged in the system may operate in a same way, i.e. be provided for extruding a filament of a build material paste from a nozzle, or may operate in a different way thereby combining different three-dimensional printing systems in the confined space. 
     The system controller can be configured to control the robotic unit and its interaction with different printing stations of the system. For example, when a printing station is ready printing the robotic unit can remove the carrier and/or the three-dimensional printed structures. Furthermore, the system controller can be arranged to control when the printing stations are to start the printing process, perform actions when the build material paste reservoirs are (almost) empty, etc. The station controller can be configured to control the local printing stations. 
     The robotic unit can be configured to determine a destination for the carrier removed from the printing station. The robotic unit does not necessarily has to put back a carrier with three-dimensional printed structures thereon on the same location (e.g. rack) from which the carrier was supplied to the printing station. This can, for example, depend on the optional post processing steps for the printed three-dimensional structures. 
     Different types of extrusion additive manufacturing arrangements can be employed, for example extrusion of a viscous paste into filaments, filament-fed extrusion, screw extrusion or a syringe extrusion. A combination of these technologies is also possible. 
     Optionally, the deposition unit includes more than two nozzles. In some examples, the deposition unit includes more than four nozzles, even more than six nozzles. By providing a larger number of nozzles (e.g. eight nozzles), the output of printed three-dimensional structures can be increased. Some of the nozzles can also be used for printing with different materials. 
     In syringe extruders material can be placed into a syringe and the printer can depress the plunger at a controlled rate to extrude filaments through a nozzle. The syringes may for example be filled with a viscous material. In some examples, additionally a heated jacket can be used for heating or cooling the syringe to adapt the viscosity of the build material paste or to melt the material (e.g. polymer filaments or granules) in situ to a desired degree before printing. Different types of syringe extrusion systems are possible. A pneumatic pressure can be applied to the plunger. Alternatively, the plunger can be depressed by means of a mechanical displacement for instance achieved by means of an electric motor. Mechanical displacement may allow for more direct control over the volumetric extrusion rate whereas in pneumatic printers, the extrusion rate may additionally depend on an interplay between needle geometry, material viscosity, pneumatic pressure, and obstruction by previously extruded filaments. Other alternative designs are also possible. 
     In screw extruders a material can be fed into a screw that is surrounded by a close-fitting sleeve, called a barrel. As the screw rotates, material can be forced through the nozzle at the end of the barrel. The rate of material extrusion from the nozzle can depend on the screw rotation speed. Screw extruders can accommodate materials in paste form, however, for example polymer granules can also be used. The screw extruder may contain a heating or cooling arrangement for the build material. 
     Filament-fed extruders may use reels of filaments fed into a heated melting chamber which is attached to a nozzle. The rate of material extrusion from the nozzle can depend on the rate at which the filament is fed off the reels into the melting chamber. Additive manufacturing software may control the extrusion rate based on the desired diameter of extruded filaments and the speed at which the nozzle is moving. 
     Various systems can be used for performing the extrusion based additive manufacturing method according to the invention. 
     It will be appreciated that the print head trajectory and velocity and/or acceleration is also regarded as print parameters which can be controlled by the system/method. 
     The system and method may be employed for manufacturing a three-dimensional porous structure, wherein the three-dimensional structure is formed having interconnected pores. The system and method may be employed for manufacturing a three-dimensional dense or massive structure, wherein the filaments are positioned in an adjacent manner and the three-dimensional structure is free of macro-pores between the filaments. 
     It will be appreciated that the three-dimensional structure can have filaments spaced apart, or it can be a dense structure with fibers adjacent to each other. When the filaments are adjacent to each other, the porosity can be provided by the filaments themselves. When the filaments are spaced from each other, the porosity can be mainly provided by the pores formed between the filaments. The filaments themselves can also be porous, having smaller pores. 
     According to an aspect, the invention relates to a computer implemented method for printing a three-dimensional structure. The computer implemented method may be configured to operate an additive manufacturing system to perform the steps of the printing method according to the invention. Optionally, the computer implemented method includes the steps of: receiving a model of a (porous) object to be manufactured, selecting one or more of the plurality of printing stations for printing the (porous) object, and defining a print path depending on desired characteristics of the (porous) object, using the received model of the object to be manufactured. The received model may for instance be a 3D representation of the object to be printed. 
     Optionally, a material extrusion additive manufacturing process is employed in which a build material, optionally a build material paste, is deposited continuously in a chosen arrangement. 
     It will be appreciated that the extruded filament may also be known in the art as a strut, fibre/fiber, rod, raster, extrudate, and other terms. 
     It will be appreciated that the term filament diameter may be understood as a characteristic length of a cross section of the filament being deposited. Other terms may also be used for this feature, such as for example, filament width, fiber diameter, filament size, strut width, etc. The filaments may have various cross section shapes. 
     It will be appreciated that the layer thickness can be seen as a layer height or slice thickness. It represents a z-increment when 3D printing the three-dimensional structure. 
     A wide range of materials can be used to compose the build material, with a vast range of properties. Examples are metals, composites, ceramics, polymers, natural materials, etc. Different materials may result in different mechanical properties. Hence, the print path may depend on the specific material used during deposition. 
     Examples of materials that can be used to compose a build material for extrusion based additive manufacturing process, include ceramic materials (e.g. alumina, zirconia, silica, silicon carbide, silicon nitride, etc.), composite materials (e.g. polymer ceramic composites), metals (RVS, titanium, copper, aluminum, silver, .etc.) zeolites, metal organic frameworks, carbon, graphene etc. Other materials suitable for extrusion based additive manufacturing are also envisaged, such as for instance polymer based materials. 
     It will be appreciated that the porosity may represent a pore (volume) fraction. The pore width or pore size in the three dimensional porous structure may define the porosity at a location or region of the porous structure. 
     Some embodiments may be implemented, for example, using a machine or tangible computer-readable medium or article which may store an instruction or a set of instructions that, if executed by a machine, may cause the machine to perform a method and/or operations in accordance with the embodiments. 
     It will be appreciated that any of the aspects, features and options described in view of the system apply equally to the method and the described device. It will also be clear that any one or more of the above aspects, features and options can be combined. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The invention will further be elucidated on the basis of exemplary embodiments which are represented in a drawing. The exemplary embodiments are given by way of non-limitative illustration. It is noted that the figures are only schematic representations of embodiments of the invention that are given by way of non-limiting example. 
       In the drawing: 
         FIG.  1    shows a schematic diagram of an embodiment of a system; 
         FIG.  2    shows a schematic diagram of an embodiment of a system; 
         FIG.  3    shows a schematic diagram of an embodiment of a system; 
         FIG.  4    shows a schematic diagram of an embodiment of a system; 
         FIGS.  5   a  and  5   b    show a schematic diagram of an embodiment of a system; 
         FIG.  6    shows a schematic diagram of an embodiment of a system; 
         FIG.  7    shows a schematic diagram of an embodiment of a system; 
         FIG.  8    shows a schematic diagram of an embodiment of a reservoir unit; 
         FIG.  9    shows a schematic diagram of a method for an extrusion process; and 
         FIG.  10    shows a schematic diagram of a three-dimensional structure. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    shows a schematic diagram of an embodiment of a system  1  for manufacturing three-dimensional structures, the system comprising a plurality of printing stations  3  for performing parallel printing in a confined space, for example a confined space enclosed by a housing (not shown). Each printing station  3  comprises a carrier  7 , a deposition unit  9  with at least one nozzle  11  arranged for dispensing filaments of build material paste through an opening area thereof, and a station controller configured to operate the deposition unit  9  for deposition of filaments of a build material paste on the carrier  7  in an interconnected arrangement in a plurality of stacked layers in order to form one or more three-dimensional structures. The at least one nozzle  11  and the detachable carrier  7  being relatively moveable with respect to each other. The deposition unit  9  is coupled to a reservoir unit  13  configured to house the build material paste, wherein the reservoir unit  13  includes at least one reservoir  15  arranged outside of the confined space. A tubing  17  may be arranged for providing fluid communication of build material paste between the at least one reservoir and the deposition unit. 
     The paste reservoirs  15  can be placed external to the working space of the printing station  3  in which the three-dimensional structures are printed by filament deposition on the carrier  7 . In this way, the paste reservoir  15  can be disconnected without requiring operations in the working space of the printing station  3 . A more efficient, continuous and/or safer printing process can be obtained. The reservoirs  15  can be easily replaceable without requiring access to the deposition head  9 . This provides important advantages compared to placing the reservoir  15  on or adjacent the deposition head  9  of the printing station  3 . The reservoir  15  can be mounted on a remote location with respect to the deposition head  9 . The build material paste (e.g. viscous paste) can be provided to the deposition head by means of tubing, or the like, providing a fluid communication between the reservoir  15  and the deposition head  9 . 
     The reservoir  15  mounted externally can facilitate replacement thereof. The reservoir  15  can be better accessible for replacement, even during printing operations by the printing station  3 . A replacement can be carried out while being protected from moving parts (e.g. at least one of a deposition unit and carrier of the printing station). The reservoir  15  for holding the build material paste can be arranged outside the confined environment in which the printing is performed by paste extrusion, the confined area may be defined at least partially by a housing (e.g. including doors). The paste reservoir  15  can be easily accessible for replacement, refilling, etc. The printing process can be significantly enhanced in this way. The reservoir can have a tubing (e.g. hose) providing a fluid connection (for transport of the build material paste) between the nozzle of the deposition unit  9  and the paste reservoir  15 . 
     A detection system can be set up which is configured to detect when a reservoir  15  needs to be replaced or replenished. Different types of detections are possible (e.g. optical detection, visual detection, etc.). 
     Each printing station may have a printing station housing (not shown). The housing can be formed by walls, a frame, a cage or the like. Combinations of housing elements are also possible. Instead of employing individual housing for each printing station, it is also possible to arrange a system housing (not shown). A combination of station housings and a system housing is also possible. The housing may define one or more confined areas with restricted access (for example by humans). 
       FIG.  2    shows a schematic diagram of an embodiment of a system. In the shown example, the deposition unit  9  and the reservoir unit  13  of a printing station  3  of the system  1  is shown. The reservoir unit  13  has a reservoir  15  arranged outside of the confined space formed by the printing station  3  or system  1 . In this example, each nozzle  11  of the deposition head  9  of the printing station  3  is coupled to a reservoir  15  for feeding build material paste. 
       FIG.  3    shows a schematic diagram of an embodiment of a system. In the shown example, the deposition unit  9  and the reservoir unit  13  of a printing station  3  of the system  1  is shown. The reservoir unit  13  has a reservoir  15  arranged outside of the confined space formed by the printing station  3  or system  1 . The confined space may for instance be formed by a housing or the like. In the shown embodiment, the deposition head  9  includes two nozzles  11 , namely a first nozzle  11   a , and a second nozzle  11   b  distanced from the first nozzle  11   a . The first nozzle  11   a  is provided with build material paste by means of tubing  17   a . The second nozzle  11   b  is provided with build material paste by means of tubing  17   b . The first nozzle  11   a  and second nozzle  11   b  are both in fluid connection with the two reservoirs  15   a ,  15   b . In this way, a redundant system can be obtained, wherein a first reservoir can be replaced while the other reservoir provides build material paste to the first nozzle  11   a  and/or the second nozzle  11   b.    
     In this example, the deposition unit  9  includes at least a first nozzle  11   a  and a second nozzle  11   b , wherein the first nozzle  11   a  is coupled to a first reservoir  15   a  for first feeding build material paste and the second nozzle  11   b  is coupled to a second reservoir  15   b  for second feeding build material paste, wherein the first nozzle  11   a  is further coupled to the second reservoir  15   b , and the second nozzle  11   b  is further coupled to the first reservoir  15   a . Optionally valves are arranged, such that fluid supply from the first and second reservoirs  15   a ,  15   b  can be selectively controlled. In some examples, the first nozzle  11   a  is to receive build material paste from the first reservoir  15   a  when the first reservoir  15   a  still holds sufficient build material paste. The first nozzle  11   a  can then receive build material paste from the second reservoir  15   b  when the first reservoir  15   b  is depleted (requiring refilling or replacement). Similarly, the same holds for the second nozzle  11   b  of the deposition unit  9 . It will be appreciated that a larger number of nozzles and/or reservoirs may also be arranged. 
       FIG.  4    shows a schematic diagram of an embodiment of a system. In the shown example, the deposition unit  9  and the reservoir unit  13  of a printing station  3  of the system  1  is shown. The reservoir unit  13  has a reservoir  15  arranged outside of the confined space. The nozzle  11  of the deposition unit  9  is coupled to two reservoirs  15   a ,  15   b  for feeding build material paste. Advantageously, when one of the first or second reservoirs  15   a ,  15   b  is replaced, the other reservoir  15   b ,  15   a  can still provide build material paste to the nozzle, improving the continuity of the printing process. 
       FIGS.  5   a  and  5   b    shows a schematic diagram of an embodiment of a system  1  respectively in a perspective view and a top view. The system  1  includes a plurality of printing stations  3  grouped together. In this shown example, twelve individual printing stations are integrated in the system  1 . Furthermore, the system has an optional robotic unit  5  with a handling device  5   a . The robotic unit  5  is operable in a confined area  20  defined by a system housing  10 . The robotic unit  5  can be configured to transport detachable carriers  7  from and to a collection system  21 . In this example, the collection system  21  includes a plurality of racks arranged for holding carriers  7 . 
     In this example, six printers are arranged on either side of a rail of the robotic unit on which the handling device can move for accessing the plurality the printing stations  3 . In this example, three printers are grouped in a housing. The housing has two extraction channels each. It will be appreciated that other arrangements are also envisaged. 
     The collecting system  21  may be provided for transporting or holding the carriers being removed from the printing stations  3 . In some examples, the collecting system  21  includes one or more racks with slots in which the carriers  7  can be placed by the robotic unit  5 . The robotic unit  5  can be configured to unload empty carriers in the printing stations and loading carriers with the one or more three-dimensional structures printed thereon in the collecting system (e.g. in a rack of the collecting system). 
     Multiple printing stations can be placed in the system housing. Each printing station can have one or more deposition units  9  with one or more nozzles  11  or printheads. Build material paste (e.g. viscous paste) can be fed to each nozzle  11  or printhead of the deposition unit of the printing station from one or more removable build material paste reservoirs, for example arranged externally with respect to the housing. The reservoirs can be removably arranged by means of quick couplings (enabling a quick and easy installation, removal and/or replacement of the reservoirs). 
     Optionally, an atmosphere is maintained in the system housing. Such a controlled atmosphere can also be obtained in optional individual housings of the printing stations. In some alternative examples, an open system is provided. Such an open system may for instance have a cage surrounding working areas (e.g. for security purposes). The atmosphere can be regulated in its entirety (e.g. within the system enclosure), or per print station (e.g. within each printing station individually). 
     Each printing station can have one or more doors, windows panels or hatches to access the station. The system may be configured such that when a door of an individual printing station is opened, operation of the individual printing station is paused, halted or stopped. The robotic unit may automatically stop when the door opens. 
     Printing of the filaments can be done on carriers formed by printing plates or printing tables. The plates can be placed or positioned in the printing station, using the positioning structure. 
     In some examples, all printing stations  3  are accessible by the robotic unit  5 . For this purpose the robotic unit  5  can be at least partially surrounded by the printing stations  3  (e.g. arranged centrally). The robotic unit  3  can be arranged to install and place empty carriers, and also removes the carriers with three-dimensional structures printed thereon. The carriers with three-dimensional structures printed thereon can be removed from the printing station and placed on a cart for removal. 
     In some examples, the system  1  further includes a positioning structure arranged for positioning the carrier within the printing station  3 . The positioning structure may be important to enable correct positioning of the carrier in the cart. If the handling by the robotic unit is not performed correctly, for instance due to bad positioning, the edge of the cart may be bumped unintentionally, which can disrupt the previously placed carriers and damage the three-dimensional structures printed thereon. 
     The positioning structure can effectively guarantee that the carrier is correctly positioned in the printing station. As a result, the robotic unit can carry the carrier correctly. 
     The positioning structure can be arranged to ensure correct positioning of the carrier  7  in the printing station  3 . For instance, when the print job is ready, the robotic unit  5  can transport the carrier  7  away from the printing station (e.g. to a collection system). The robotic unit  5  can more accurately handle and carry the carrier as the positioning structure ensures a more accurate positioning of the carrier inside the printing station. In some examples, sensors are provided to detect how and where carrier  7  is placed in printing stations. It can be better prevented that the carrier can be picked up in various different ways. Hence, collision with the collection system (e.g. having a cart) can be better prevented, without requiring advanced sensory systems. So instead of working purely on sensory data, mechanical positioning means are used that guarantee an improved positioning of the carriers in the printing stations of the system. However, additionally or alternatively, also sensory systems can be provided for handling and positioning of the carriers. In some examples, a combination of a number of sensors and one or more mechanical positioning structures are provided for enabling accurate positioning of the carriers. 
       FIG.  6    shows a schematic diagram of an embodiment of a system  1 . A part of the system  1  as shown in  FIG.  5    is illustrated in perspective view. In this example, the detachable carrier  7  are formed by trays which can be handled by the handling device  5   a  of the robotic unit  5 . The collection system  21  includes a plurality of slots  23  in which the carriers  7  can be positioned. In this example, each printing station has a deposition unit  9  with two nozzles  11 . However, a different number of nozzles can also be employed. It is also possible that the deposition unit  9  includes a deposition head with multiple nozzle openings arranged therein. It is also envisaged that the plurality of printing stations  3  have different deposition units  9 , for example with a different number of nozzles  11 . 
       FIG.  7    shows a schematic diagram of an embodiment of a system  1  in side view. In this example, the paste reservoirs  15  are placed external to the working space of the printing station  3  in which the three-dimensional structures are printed by filament deposition on the carrier  7 . In this way, the paste reservoir  15  can be disconnected without requiring operations in the working space of the printing station  3 . A more efficient, continuous and/or safer printing process can be obtained. The reservoirs  15  can be easily replaceable without requiring access to the deposition head  9 . This provides important advantages compared to placing the reservoir  25  on or adjacent the deposition head  9  of the printing station  3 . The reservoir  25  can be mounted on a remote location with respect to the deposition head  9 . The build material paste (e.g. viscous paste) can be provided to the deposition head by means of tubing, or the like, providing a fluid communication between the reservoir  15  and the deposition head  9 . 
     The reservoir  15  mounted externally can facilitate replacement thereof. The reservoir  15  can be better accessible for replacement, even during printing operations by the printing station  3 . A replacement can be carried out while being protected from moving parts (e.g. at least one of a deposition unit and carrier of the printing station). The reservoir  15  for holding the build material paste can be arranged outside the confined environment in which the printing is performed by paste extrusion, the confined area being defined at least partially by the housing  10  and doors  27 . The paste reservoir  15  can be easily accessible for replacement, refilling, etc. The printing process can be significantly enhanced in this way. The reservoir can have a tubing (e.g. hose) providing a fluid connection (for transport of the build material paste) between the nozzle of the deposition unit  9  and the paste reservoir  15 . 
     A detection system can be set up which is configured to detect when a reservoir  15  needs to be replaced or replenished. Different types of detections are possible (e.g. optical detection, visual detection, etc.). 
       FIG.  8    shows a schematic diagram of an embodiment of a reservoir unit  13 . The reservoir  15  are arranged on an outer side of the housing of the printing unit  3  or system  1 . The reservoirs  15  are detachably connectable to the housing by means of an attachment device  50 . In this example, the housing includes a holder  51  arranged for holding the reservoirs  15  in place. The holder  51  includes a coupling interface for removably coupling the at least one reservoir to the housing of the printing station  3  or the housing of the system  1 . The holder  51  comprises a first interface  53  for providing a fluid communication for build material paste between the at least one reservoir and the deposition unit, and a second interface for providing fluid pressure to the at least one reservoir (not shown). Advantageously, the at least one reservoir is replaceable and/or interchangeable. The holder  51  may be arranged to provide a universal coupling enabling attachment of different types of reservoirs  15  to the housing (e.g. reservoirs having other volumes, shapes or dimensions). 
       FIG.  9    shows a schematic diagram of a print path in an extrusion process for manufacturing a three-dimensional porous structure  1 . The print path illustrates how the filaments of the porous structure are deposited on the plurality of layers. The system is arranged to deposit interconnected filaments in a predetermined arrangement in a plurality of stacked layers. The filaments of the consecutive layers can be connected to one another to obtain a porous structure with interconnected pores. Furthermore, filaments of the consecutive layers can be angled with respect to each other. 
     In an extrusion process, a nozzle  101  is scanned along a print bed  103  depositing filaments following the shown print path  105 . It will be appreciated that it is also envisaged that the print bed  103  is moved instead of the nozzle  101  (kinematic inversion). A combination is also possible. In an alternative example, both the nozzle  101  and the print bed  103  can be moved during at least portions of the deposition process. 
     In  FIG.  9   a   , the print path  105  for the first layer on the print bed  103  is shown. In  FIG.  9   b   , the print path  105  of two layers are shown. In  FIG.  9   c   , the print path  105  is shown in which the fourth layer is being deposited. It will be appreciated that large variety of print path arrangement are possible for obtaining the arrangement of the interconnected filaments of the porous structure. 
     By altering the deposition pattern, the local mechanical properties of the three-dimensional structure can be locally changed, so that a different heat treatment for drying and/or calcination may be required. In this example, the porous structure being printed has a non-homogeneous filament-to-filament distance (interspacing). Homogenous interspacing is also possible. 
     Although this example illustrates extrusion printing of paste for forming a porous structure, it is also envisaged that the system can be employed for depositing non-porous three-dimensional structures, i.e. without pores between the filaments. 
       FIG.  10    shows a schematic diagram of an embodiment of a porous structure  110  obtained by depositing filaments  102  in a predetermined interconnected arrangement in a plurality of stacked layers  111  for forming a porous structure  110  with interconnected pores. In  FIG.  10   a   , a cross sectional side view of the porous structure  110  is shown. In  FIG.  10   b   , a cross sectional top view of the porous structure  110  is shown. 
     The porosity influences the stiffness or elastic modulus (cf. Young modulus), which is a measure of rate of change of stress over strain, defining how much a material deforms in response to a given force. Whether the filaments  102  are aligned or staggered also impacts the mechanical properties of the three-dimensional structure. For instance, a three-dimensional structure  110  with staggered filaments  102  may have a lower elastic modulus than a three-dimensional structure  110  with aligned filaments  102 . For example, for a aligned filament arrangement (as shown in this example), there can be a solid column from top to bottom of the three-dimensional structure, which exists because the filaments  102  intersect at similar positions. This solid column can strongly resist compression. In contrast, for a staggered filament arrangement, the filaments  2  may bend slightly and stress can be concentrated at hinge points. 
     Furthermore, the filament orientation can also influence the mechanical properties of the three-dimensional structure. For instance, a three-dimensional structure with a 0/90, 0/60/120 and 0/45/90/135 filament orientation may have different elastic moduli. It will be appreciated that other lay-down patterns are also envisaged, such as for example triangular, rectangular, hexagon, curved, zigzag patterns. These lay-down patterns can also influence the pore size. 
     The three-dimensional (porous) structure can be produced layer-by-layer in various ways. Although the embodiments in the figures show flat layers wherein all filaments are extruded for a single layer (with the nozzle at a constant height above the print bed) before the nozzle moves up by the layer thickness to begin printing the next layer, it is also envisaged that curved layers are printed by changing the distance between the nozzle and the print bed during the deposition of a single filament. By moving the nozzle away and closer to the print bed during said deposition, a curved shape can be obtained. 
     If printing of the three-dimensional structures on the carrier has been completed (e.g. print job completed or carrier/plate being full), the robotic unit can be operated to automatically remove the carrier with the printed three-dimensional structures and to place it in a receiving unit, such as a cart or holder e.g. for transportation. Each print station can be controlled individually. In some examples, different materials can be printed per printing station. Furthermore, it is possible to print different shapes per printing station. Also different quantities can be printed per printing station. The system can be configured to determine whether a correct number of three-dimension structures is printed on the carrier and then the carrier with printed three-dimensional structures (e.g. objects, parts, pieces, etc.) can be removed by the robot unit when the print job has been completed. 
     The system can be configured to control the robot unit taking into account printing operations in the plurality of the printing stations. The (sub)tasks for printing the three-dimensional structure(s) can be sent individually or directly to the printing station being selected for performing the printing of said three-dimensional structure(s). The system can be configured to enable an individual control for each printing station. This can be adjusted separately by an operator, if necessary, for example by means of a terminal. The individual control for the printing stations can for instance be arranged on an outside portion of the printing station or the system. In some examples, each printing station has an external terminal or interface for enabling individual control over the printing station. The printing process can therefore be easily adjusted for each of the printing stations. 
     In some examples, the system includes a global process control for controlling the handling of the carriers of the plurality of printing stations, and for providing starting signals to the printing stations for starting a print job. The printing station can then indicate when the carrier is full or when a printing (sub)task has been completed. Then, the robotic unit can be operated to get the carrier with the printed three-dimensional structures printed thereon. In some examples, the robotic unit can be configured to place a new carrier in the printing station. The robotic unit can then give a start signal to the printing station, and the printing station can be operated to start printing again with the next printing (sub)task. 
     The individual control provided for each of the printing stations makes it possible to tweak for minor printing deviations, for example by an operator. It is possible to better finetune the printing conditions of the plurality of printing stations with respect to each other. 
     For example, the paste employed in the different printing stations of the system can have slightly different viscosity. It is possible to compensate for such deviations by individual control of the printing stations of the system. In some examples, each printing station can be controlled individually, also from the outside of the confined environment (e.g. housing) of the system or the printing stations. In some examples, all individual printing stations can be placed in a confined environment of the system (cf. system housing). This can make it possible to work with toxic materials in a safe environment. 
     In some examples, the robotic unit is configured to deposit the printed three-dimension structures or the carrier with printed three-dimensional structures on a transport medium (in this case carts), so that they can be guided to a next process step (e.g. packaging and shipment or post-processing as may be needed for three-dimensional porous structures for catalysis). 
     It will be appreciated that various transport systems can be used for holding or carrying the carriers or printed three-dimensional structures. Some examples of transport systems are racks, carts, conveyor belt, etc. However, other arrangements are also possible. 
     It will be appreciated that the method may include computer implemented steps. All above mentioned steps can be computer implemented steps. Embodiments may comprise computer apparatus, wherein processes performed in computer apparatus. The invention also extends to computer programs, particularly computer programs on or in a carrier, adapted for putting the invention into practice. The program may be in the form of source or object code or in any other form suitable for use in the implementation of the processes according to the invention. The carrier may be any entity or device capable of carrying the program. For example, the carrier may comprise a storage medium, such as a ROM, for example a semiconductor ROM or hard disk. Further, the carrier may be a transmissible carrier such as an electrical or optical signal which may be conveyed via electrical or optical cable or by radio or other means, e.g. via the internet or cloud. 
     Some embodiments may be implemented, for example, using a machine or tangible computer-readable medium or article which may store an instruction or a set of instructions that, if executed by a machine, may cause the machine to perform a method and/or operations in accordance with the embodiments. 
     Various embodiments may be implemented using hardware elements, software elements, or a combination of both. Examples of hardware elements may include processors, microprocessors, circuits, application specific integrated circuits (ASIC), programmable logic devices (PLD), digital signal processors (DSP), field programmable gate array (FPGA), logic gates, registers, semiconductor device, microchips, chip sets, et cetera. Examples of software may include software components, programs, applications, computer programs, application programs, system programs, machine programs, operating system software, mobile apps, middleware, firmware, software modules, routines, subroutines, functions, computer implemented methods, procedures, software interfaces, application program interfaces (API), methods, instruction sets, computing code, computer code, et cetera. 
     Herein, the invention is described with reference to specific examples of embodiments of the invention. It will, however, be evident that various modifications, variations, alternatives and changes may be made therein, without departing from the essence of the invention. For the purpose of clarity and a concise description features are described herein as part of the same or separate embodiments, however, alternative embodiments having combinations of all or some of the features described in these separate embodiments are also envisaged and understood to fall within the framework of the invention as outlined by the claims. The specifications, figures and examples are, accordingly, to be regarded in an illustrative sense rather than in a restrictive sense. The invention is intended to embrace all alternatives, modifications and variations which fall within the spirit and scope of the appended claims. Further, many of the elements that are described are functional entities that may be implemented as discrete or distributed components or in conjunction with other components, in any suitable combination and location. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word ‘comprising’ does not exclude the presence of other features or steps than those listed in a claim. Furthermore, the words ‘a’ and ‘an’ shall not be construed as limited to ‘only one’, but instead are used to mean ‘at least one’, and do not exclude a plurality. The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to an advantage.