WO2016198291A1

WO2016198291A1 - A device for direct additive manufacturing by means of extrusion of metal powders and ceramic materials on a parallel kinematic table

Info

Publication number: WO2016198291A1
Application number: PCT/EP2016/062305
Authority: WO
Inventors: Matteo Strano; Massimiliano Annoni; Hermes GIBERTI
Original assignee: Politecnico Di Milano
Priority date: 2015-06-09
Filing date: 2016-05-31
Publication date: 2016-12-15

Abstract

The present invention describes an additive production system (1) based on extrusion and 3D deposition of a metal mixture (or advanced ceramic material) in powder and polymer binder. The system comprises a) a CNC injection unit (3) of material for metal or ceramic injection molding, which is fixed in space and provided with a nozzle (3d) for extrusion of a filament; and b) a parallel kinematic machine (4) with 5 degrees of freedom.

Description

A device for direct additive manufacturing by means of extrusion of metal powders and ceramic materials on a parallel kinematic table

The present invention relates to the field of additive manufacturing, which is also known as 3D printing, and in particular relates to a device for direct additive manufacturing of metal and ceramic objects.

Background of the invention

The "direct metal" additive manufacturing techniques, that is to say, capable of directly producing a metal object, can be classified as "Laminated manufacturing", "Powder-bed processes" and "Deposition processes" (Karunakaran KP, Bernard A, Suryakumar S, Dembinski L, Taillandier G. Rapid manufacturing of metallic objects. Rapid Prototype J 2012; 18:264-80).

The technique known as "Laminated manufacturing" is the simplest method of manufacturing additives, but requires the connection of a stack of planar sheets (Himmer T, Nakagawa T, Anzai M. Lamination of metal sheets. Comput Ind 1999;39:27-33). Connection methods are typically used, such as adhesive bonding, brazing, ultrasound welding, and diffusion welding, but all those methods require long processing times for each layer and this makes the technology suitable only for objects with relatively thick layers, and therefore unsuitable for products having small dimensions.

Another known technology involves "Powder-bed processes"; this relates to a layered manufacturing process, in which each layer is produced by means of a first spreading of a uniform thickness of powder inside a container and then by joining the particles which constitute the layer by means of the movement of a tool over a programmed 2D trajectory. This tool could be a laser beam (SLS, Selective Laser Sintering) (Roppenecker DB, Traeger MF, Gumprecht JDJ, Lueth TC. How to Design and Create a Cardan Shaft for a Single Port Robot by Selective Laser Sintering. Vol. 3 Des. Mater. Manuf. Parts A, B, C, ASME; 2012, p. 49), an electron beam (EB) (Gong X, Anderson T, Chou K. Review on Powder-Based Electron Beam Additive Manufacturing Technology. ASME/ISCIE 2012 Int. Symp. Flex. Autom., ASME; 2012, p. 507), an electric arc or simply a jet of polymer binder. The powder bed processes which are based on binders are referred to using the name 3D printing (Lipson H, Moon FC, Hai J, Paventi C. 3-D Printing the History of Mechanisms. J Mech Des 2005; 127: 1029). Among the powder bed technologies, 3D printing is the least expensive, but it is used in a very limited manner for metals and the relevant scientific literature is very scarce. Commercially, one relevant example is the Digital Metal^® technology. The electron beam melting (EBM) and selective laser melting (SLM) techniques (Murr LE, Quinones SA, Gaytan SM, Lopez MI, Rodela A, Martinez EY, et al. Microstructure and mechanical behavior of Ti-6AI-4V produced by rapid-layer manufacturing, for biomedical applications. J Mech Behav Biomed Mater 2009;2:20-32 are nowadays preferred to the sintering processes as a result of the superior mechanical properties of the components produced.

The depositing processes prevent the intrinsic disadvantages, the encumbrance and the complications connected with handling a powder bed. With regard to the powder bed technologies, an energy source a laser beam may be used (Armillotta A, Baraggi R, Fasoli S. SLM tooling for die casting with conformal cooling channels. Int J Adv Manuf Technol 2013;71 :573-83), an electron beam (Jamshidinia M, Kong F, Kovacevic R. Temperature Distribution and Fluid Flow Modeling of Electron Beam Melting^® (EBM). IMECE Vol. 7 Fluids Heat Transf. Parts A, B, C, D, ASME; 2012, p. 3089) or an electric arc (Anzalone GC, Wijnen B, Sanders PG, Pearce JM. A Low-Cost Open-Source Metal 3-D Printer. IEEE Access 2013; 1 :803- 10). In those cases, the powder is introduced via a coaxial nozzle with respect to the product, which increases progressively, layer by layer.

A less expensive depositing technology which is simpler and very popular is the so-called Fused Deposition Modeling (FDM). In this process, a filament is produced and deposited by causing it to extrude through a nozzle, to which the material is supplied by means of a pinion system. The FDM is the most popular deposition process for plastics objects, but cannot be used for metals and can be used only with difficulty for advanced ceramic materials. Intense research is still being developed in the entire world in order to increase the applicability of the additive production for extrusion by means of the development of new materials (Roberson D, Shemelya CM, MacDonald E, Wicker R. Expanding the applicability of FDM-type technologies through materials development. Rapid Prototype J 2015;21 : 137-43). Some authors have proposed the original FDM system for depositing a precursor filament of "green" ceramic material, that is to say, before sintering (Jafari MA, Mohammadi WH, Safari A, Danforth SC, Langrana N. A novel system for fused deposition of advanced multiple ceramics. Rapid Prototype J 2000;6: 161-75), or a filament with a mixture of metal and plastics material (Masood S., Song W. Development of new metal/polymer materials for rapid tooling using Fused deposition modelling. Mater Des 2004;25: 587-94). The conventional FDM machines can be used when the percentage of the ceramic component in the mixture is small with respect to the polymer component (Kalita SJ, Bose S, Hosick HL, Bandyopadhyay A. Development of controlled porosity polymer-ceramic composite scaffolds via fused deposition modeling. Mater Sci Eng C 2003;23:611-20). Therefore, this process suffers from some limitations: the advantage of using a precursor filament is compensated for by the problems encountered during the preparation and production thereof; during the extrusion, the instability under axial loading of the filament is a cause of frequent interruptions of the process, the counter-pressure encountered during the depositing limits the volumetric fraction of powder in the filament, reducing the possibility of success of the sintering of the constructed component. As a result of these problems, the original concept of FDM has been transformed by some authors, using as a starting material a ceramic clay or metal slurry (Kalita et al., ibid.) instead of a plastics filament and replacing the pinion system with a screw or piston injector (Bellini A, Shor L, Guceri SI. New developments in fused deposition modeling of ceramics. Rapid Prototype J 2005; 11 :214- 20). However, the use of a metal or ceramic slurry or feedstock further has some limitations, particularly in terms of the minimum diameter which can be used for the extrusion nozzle. In accordance with Poiseuille's Law, a reduction in the diameter of the nozzle will drastically reduce the flow and will require considerably greater pressures for extrusion. For very small diameters of the nozzle (less than 0.3 mm) and with viscous feedstock (with a low percentage of primary binder, typically less than 25%), it is also possible to encounter a complete blockage of the flow as a result of bridges which form between the particles. The extrusion head may have excessively high power, but this would increase the weight thereof and the inertia thereof and would make it typically difficult to control the movement in 2 axes of the extrusion head. A proposed alternative is to maintain the injection head fixed and to move only the deposition table. This alternative approach has been explored in the scientific literature and among commercially available technologies very rarely. One of the few well-known additive machines with a fixed head is the "Freeformer" of Arburg. However, the head does not deposit a filament, but instead constructs the component layer by layer by means of injection of extremely small droplets of plastics material, and not of metal or ceramic powders (see US8292610).

In that the movements are attributed to the head or to the table, in commercial additive machines, the degrees of freedom (also DOF below) of the system are from 3 to 5, variously distributed and configured. In scientific literature there are also descriptions of systems with 6 DOF. Optomec produces some printers having more than 3 DOF. The Freeformer from Arburg has 3 Cartesian axes, that is to say, serial translations, all assigned to the workpiece-carrying table. 3D Systems has printers whose platform moves along an axis z, from where it is then defined as the vertical axis of all the printers, while the head remains completely fixed. This is a result of the technologies used based on lasers, where movable mirrors inside the head allow the hardening of the material intended to be printed in the direction x-y (the two axes which are horizontal and orthogonal to the axis z). When a layer of material is solidified, the platform descends along the z axis and the process continues. Those mechanisms are described in US4929402, WO89/10254, US2003/0127436, US2006/0078638 and US2010/0288194 of 3D Systems. Stratasys adds movement in the plane of the head to the movement of the platform, still in the z direction, as described in US5340433 and US2010/0140849. In order to carry out those movements in the patents of Stratasys, reference is often made to a "gantry system", a serial structure which allows the movement of the head in the x-y direction as a result of guides which are arranged in a straddling manner (US2009/0314391 and US2013/0078073).

ExOne produces printers in which the platform always moves in the z direction while the head moves in the x-y direction; furthermore, there is present a component (coating device) which is used to spread the powder on the platform. This is necessary for the binder-jetting technology; the coating device can move in a horizontal direction, releasing material while the head during its movement x-y injects the binder. US2013/0004607 describes in greater detail the movement mechanism. The platform is moved by means of a screw type shaft which is caused to rotate by an electric motor and a nut. The head and the coating device are moved by means of three linear guides and two electric motors. Two guides are moved by the same motor and are positioned so as to support the third one in a state orthogonal to the two guides. The kinematic structure of the Arcam printers (of the EBM type) is similar to that of the printers from 3D Systems. In this case, the electron beam which is suitably redirected in the head, which is fixed, strikes the material on the platform which is movable in the z direction.

In printers for private use, mainly hobby type printers, it is also possible to find kinematic solutions which are different from those previously described for industrial printers. Therefore, there are always no more than 3 translational degrees of freedom which move the head, including some with parallel kinematics. For example, DeltaMaker proposes a kinematic solution with 3 parallel axes but the DOF are assigned to the head and not the platform. Those low-cost printers all use the conventional FDM technology described in US5340433.

Until now, technologies based on lasers or EBM are not compatible with the production of low- cost printers.

A typical limitation of the conventional additive production processes with 3 controlled axes results from the outer surface of the objects which is stepped or irregular. This "stepped effect" is currently inevitably connected with the principle of the layered production (Li J Bin, Xie ZG, Zhang XH, Zeng QG, Liu HJ. Study of Metal Powder Extrusion and Accumulating Rapid Prototyping. Key Eng Mater 2010;443:81-6). For the purpose of reducing this effect, the deposited portion can be orientated in an optimum manner (Armillotta A. Assessment of surface quality on textured FDM prototypes. Rapid Prototype J 2006; 12:35-41; Armillotta A, Cavallaro M, Minnella S, 2013. A tool for computer-aided orientation selection in additive manufacturing processes. High Value Manufacturing : Advanced Research in Virtual and Rapid Prototyping : Proceedings of the 6^th Int. Conf. on Advanced Research in Virtual and Rapid Prototyping, Leiria, Portugal, 1-5 October, CRC Press, 2013). Machines for additive production with 5 and 6 axes (DOF) are configured in the deposition processes so as to modify the orientation of the workpiece during the deposition (Thrimurthulu K, Pandey PM, Venkata Reddy N. Optimum part deposition orientation in fused deposition modeling. Int J Mach Tools Manuf 2004;44:585-94; Milewski J., Lewis G., Thoma D., Keel G., Nemec R., Reinert R. Directed light fabrication of a solid metal hemisphere using 5-axis powder deposition. J Mater Process Technol 1998;75: 165-72). Another advantage, when a kinematic system with more than three axes is used, is that the structures for supporting the workpiece, which are typical of the additive systems based on extrusion (N. Turner B, Strong R, A. Gold S. A review of melt extrusion additive manufacturing processes: I. Process design and modeling. Rapid Prototype J 2014;20: 192-204), can be reduced or avoided, at least for relatively simple forms. This allows a reduction in the deposition times.

Some examples are known of parallel kinematic systems with the objective of reducing the production costs (Zhang J. Adaptive Slicing for a Multi-Axis Laser Aided Manufacturing Process. J Mech Des 2004; 126:254). Another example is provided in the article by Song X, Pan Y, Chen Y. Development of a Low-Cost Parallel Kinematic Machine for Multidirectional Additive Manufacturing. J Manuf Sci Eng 2014; 137:021005, which describes a system with 6 DOF, in which the degrees of freedom (axes) are distributed between the workpiece-carrying platform and the formation head. It is known that parallel kinematic machines, with respect to the most widespread serial architectures, can also ensure precision, repeatability, rigidity and speed during positioning. In general, the manipulators with a parallel kinematic system are not suitable for those applications which require a large working area, while they represent the best selection when a high degree of precision is required for relatively small working areas, that is to say, less than approximately 400 mm as a characteristic dimension.

Despite the most recent advances in the configuration and construction of devices for additive production, or alternatively 3D printing, there are still various problems which have not yet been solved satisfactorily. Above all, a significant aspect is still the problem of producing a device for direct additive production of metal or advanced ceramic objects which is inexpensive, at the same time ensuring the typical dimensional tolerances of the mechanical industry of medium precision.

Furthermore, a significant aspect is still the problem, during direct additive production of metal or advanced ceramic objects, involving the so-called "stepped effect" which does not allow a high level of surface finish of the object manufactured.

Finally, a highly significant problem common to all the additive technologies dedicated to metal objects is the slowness of the construction process for layers of the object.

Summary of invention It has now been found that the problems present in the prior art are solved by the present invention.

The class of technology which can allow costs to be kept low and the deposition time to be kept short is certainly that of FDM technology, that is to say, based on the extrusion of a continuous filament, but on condition that the deposition times of the support structures are reduced and that it is possible to extrude, at high extrusion speed, very thin filaments, so as not to lose precision during manufacture.

In particular, it has been found that the problems are solved by using as the extrusion system (or group) a high-powered machine of the same type as those used industrially for injection- molding of plastics materials and PIM (Powder Injection Molding) materials. However, that machine has to be provided with a suitable extrusion nozzle, which is suitably configured for the additive process and which has inevitably to be held fixed in space, thereby conferring all the degrees of freedom on the workpiece-carrying table. It can be seen from the prior art set out above that there do not currently exist, to the knowledge of the inventors, additive machines which simultaneously have the following two features: 1) they use an extrusion head for a continuous filament from powders or granules; 2) they do not move the processing head. Those two features are indispensable for being able to extrude, with ease, repeatability and controllability, materials (feedstock) which are constituted by metal or ceramic powders having a high percentage of metal, that is to say, having a low percentage of binder, that is to say, highly viscous. Those materials are generally produced and marketed in the form of granules or powders. It has further been found that a movement of the workpiece-carrying table, which ensures speed and precision of the movements and at the same time allows a reduction or elimination of the need for supports and reduction or elimination of the stepped effect, can be obtained by moving the workpiece-carrying table (also referred to as the deposition table) by means of a parallel kinematic system with 5 axes of movement (degrees of freedom - DOF).

To the knowledge of the inventors, a typical extrusion head of the PIM technology has not ever been used for the purposes of metal additive production and has not ever been used in combination with a parallel kinematic system with 5 degrees of freedom.

Therefore, the present invention relates to a device for the metal or advanced ceramic direct additive manufacturing as defined in the independent claim.

The present invention further relates to a process for the direct additive manufacturing of objects of metal or of advanced ceramics as defined in the independent claim.

Additional embodiments of the invention are as defined in the dependent claims.

These objects and other objects and the relevant advantages will also be described in detail with reference to Figures, in which :

Figure 1 is a general view of the device of the present invention, in which there are illustrated the basic elements of the CNC (Computer Numerical Control) injection unit (3), the parallel kinematic machine (4), rigid links (5) and the elements necessary for the support structure (2). Auxiliary elements are not shown. Figure 2 shows an exemplary embodiment of the injection group (3) shown in Figure 1.

Figure 3 shows an exemplary embodiment of the parallel kinematic machine, involving the detail of the linear Delta section.

Figure 4 shows an exemplary embodiment of the parallel kinematic machine, involving the detail of the spherical agile wrist section. Figure 5 shows two exemplary embodiments of the nozzle in the injection group (or extrusion).

Figure 6 shows an exemplary embodiment of the calculated working area. Detailed description of the invention

The present invention relates to a device for the metal or advanced ceramic direct additive manufacturing comprising: a. an injection system (or unit) of feedstock for powder injection molding, which is fixed in space and provided with a nozzle for extrusion of a filament, said injection system is numerically controlled by a Computer Numerical Control (CNC); b. a parallel kinematic machine with 5 degrees of freedom (or axes of movement).

In an embodiment of the invention, in the device, the injection group comprises a loading system of the starting material and an extrusion head and a nozzle. In an embodiment of the invention, the injection group comprises a plasticizing device, preferably a piston, and an extrusion device, preferably a piston. In a preferred embodiment, the plasticizing piston is oriented to an axis inclined with respect to the vertical direction of the extrusion system, the inclination is between 30° and 60°, preferably 45°. In another preferred embodiment, the injection piston is vertical and the plasticizing piston is placed in an axis inclined with respect to the vertical direction of the extrusion system, the inclination is between 30° and 60°, preferably 45°.

In a preferred embodiment, the diameter of the piston of extrusion is variable as a function of the diameter of the extruded filament which it is desirable to obtain, preferably between 5 and 25 mm, more preferably 14 mm. In an embodiment, the extrusion piston can apply a pressure up to 200 MPa.

In a preferred embodiment, the nozzle of the extrusion group has a minimum diameter of 0.1 mm. In various embodiments, the nozzle has a convergent-parallel profile or a step-convergent profile.

According to the present invention, the parallel kinematic machine comprises a first platform with three translational degrees of freedom along the x, y and z axes and a second platform placed in series with the first platform with two rotational degrees of freedom according to two roll and pitch nautical angles, the second platform having a workpiece-carrying table.

Typically, for the machine the 5 axes of movement are imparted and controlled by a parallel kinematic robot. In an embodiment, the first platform comprises three rigid links connected to a mobile platform on one side and a linear guide on the other side, wherein each of the three rigid links is provided with a motor and each rigid link comprises a parallelogram system, capable of reproducing a PUS (Prismatic-Universal-Spherical) kinematic chain. Typically, the second platform comprises a spherical agile wrist.

Advantageously, the work area of the device is overdimensioned, in particular thanks to application of genetic algorithm optimization techniques.

In a preferred embodiment, the device described herein is used with conventional CAD/CAM techniques. For example, it is connected to a data source representative of the 3D object to be manufactured. In an embodiment of the invention, the device is further connected to an apparatus suitable for receiving the data and converting the data into layer data which are processed for forming layer data used by the apparatus for controlling the elements a) and b) described above and manufacturing the 3D object.

In an embodiment of the invention, the device described herein may also be mounted on board with a milling mandrel so as to process the material which has just been deposited.

The present invention further relates to a process for the direct additive manufacturing of metals or of advanced ceramics using the device as disclosed herein, comprising: a. gravity feeding a powdered mixture of metal or advanced ceramic material and a polymeric binder into an extruder; b. 3D deposition of a metallic or ceramic "green" from said extruder; c. partial debinding of the deposited material; d. sintering to approximately the density of the solid material. Preferably, the mixture has high viscosity and low polymer content.

With reference to Figure 1, a direct additive manufacturing device (1) comprises, in the basic elements thereof, a support structure (2a, 2b, 2c), shown here in some of the elements thereof. In this exemplary embodiment, the support structure (2a, 2b, 2c) acts as a support for the basic elements of the device. There is provided an injection system (or unit) (3) which comprises a loading unit, which is illustrated in Figure 1 with a hopper (3a), which is positioned to supply the feedstock which will form the object manufactured by the device (1) in communication with a plasticizing unit (3b), which is in communication with an extrusion group (3c), which terminates with a nozzle (3d). The injection system (3) is placed in a fixed position above a parallel kinematic machine (also referred to as "manipulator") and also referred to below as "machine" (4) for short, which will be described in greater detail in the following Figures 3 and 4. The machine (4) is a parallel kinematic manipulator with 5 movement axes (degrees of freedom) and substantially comprises a first platform (4a) with three translational degrees of freedom along the axes x, y and z and a second platform (4b) with two rotational degrees of freedom in accordance with two nautical roll and pitch angles (considering a sequence xyz), the second platform carrying a workpiece-carrying table (4c).

The parallel kinematic table (4a) is in a relationship owing to rigid links (5) with the vertical elements (2a) of the support structure, which also act as a guide for the movement of the links (5) along the axis Z.

The extrusion system (or unit) is a modified injection-molding unit, wherein the plasticizing function is carried out by a first injection device, preferably a piston or screw, and the injection function is carried out by a second device, preferably a piston or a screw. This configuration allows a continuous and stable extrusion process, a great extrusion force, also allowing rapid extrusion of a highly viscous fluid with a very small diameter of the discharge filament, for example, from approximately 0.1 to approximately 0.9 mm.

The movements of both pistons must be synchronized. According to the invention, the position of the two pistons is numerically controlled in a closed loop, thereby allowing control of the speed of extrusion in a precise manner. The material which is deposited for extrusion on the work plane allows the direct production of metal "greens" or advanced ceramic products, which have to be subsequently sintered. The selection concerns the use of commercial precursors as the first material (containing metal or ceramic material), which are intended for PIM processes (Powder Injection Molding). There are very few examples of use of the MIM technique in literature relating to additive manufacturing (Bellini A, Shor L, Guceri SI. New developments in fused deposition modeling of ceramics. Rapid Prototype J 2005; 11 :214-20), wherein the problem of stepping is further not encountered and therefore not solved, and there is a limitation to the manufacture of very simple objects.

While conventional MIM moulded products have an inherent limitation on their maximum feasible size and weight, the present invention is better suited to large sized products, since there is no need of moulds and since the working space of the PKM work-table can be designed as large as desired. In an embodiment of the invention, the work space is a parallelepiped with a square base (220 mm the side length) and with 400 mm height. In an embodiment of the invention, the deposition speed can be as fast as 50 mm/s.

In accordance with the present invention, the term "advanced (or special) ceramic materials" is intended to be understood to be ceramic materials which are obtained for the most part from products of synthesis characterized by new functions (dielectricity, ferroelectricity, superconductivity, biocompatibility, etc.), and by properties (thermal resistance, hardness, chemical stability) greater than those of conventional ceramic materials which are obtained for the most part from natural substances. Another innovative aspect of the present invention is the assignment of all five degrees of freedom to the work table (object carrier), by means of a robot (understood to be a machine) with parallel kinematics.

Advantageously, the device of the present invention allows a high level of skill during the relative positioning of the product with respect to a powerful and heavy (25 kg) extrusion nozzle, which would be very difficult to move with speed and precision.

The extrusion unit

The extrusion unit (3) is shown in Figure 2. That group comprises a supply system (31) which is placed in connection with a plasticizing cylinder (34). The cylinder contains a plasticizing piston (32) which is substantially fluid-tight in the cylinder (32). The plasticizing cylinder is in communication with an extrusion cylinder (35), which contains an extrusion piston (33), which is substantially fluid-tight in the cylinder (35). The extrusion cylinder (35) terminates in a nozzle (39).

The extrusion unit is provided with an actuation system for the pistons (32, 33). The actuation system is preferably independently provided for each piston and can be actuated in an independent and coordinated manner. By way of example, the actuation system may be an electric motor, for example, of the "brushless" type. Figure 2 shows in an exemplary manner two motors (36) and (37) for the pistons (33) and (32), respectively.

The extrusion system is advantageously provided with heating elements (38) which are positioned in a suitable manner to ensure the fluidity of the material (41) to be extruded. Unlike in conventional FDM, the starting material (also called feedstock) is not in the form of a filament but instead in the form of granules or powder, supplied by gravitational force via the supply unit (31), which is typically a hopper.

A material plasticizing system (32), which has an axis inclined with respect to the vertical direction, supplies heated feedstock (41) to the injection cylinder (35). The inclination of the axis of the plasticizing system (32) with respect to the vertical axis of the injection system (35) is typically between approximately 30° and approximately 60°, preferably 45°.

An injection piston (33) having a vertical axis moves and pressurizes the feedstock (41) which is heated by the elements (38) and which is already plasticized (that is to say, with the polymer portion in the liquid phase), causing it to flow through the injection nozzle (39). The diameter of the piston (33) which acts on the feedstock is variable and in accordance with the diameter of the extruded filament which it is desirable to obtain. In an exemplary embodiment of the invention, the extrusion cylinder has a diameter between approximately 5 and 25 mm, preferably approximately 14 mm. The injection pressure of the heated material, before the nozzle, may be fixed at a very high level, up to 200 MPa in accordance with the dimensions of the motor and the diameter of the piston. In an exemplary embodiment of the invention, the pressure range applied varies from approximately 10 to approximately 200 MPa, typically between approximately 15 and approximately 45 MPa. For example, with the device of the present invention, it is possible to deposit a quantity of material which varies from approximately 4000 to approximately 40,000 mm³, in accordance with the dimensions of the piston and the chamber. That quantity can be established in the configuration phase. For example, the maximum quantity of material which can be deposited during a single travel of the piston is equal to approximately 9000 mm³ with a diameter of the piston equal to 14 mm. In this case, if there is extruded a discharge filament having a diameter of 0.6 mm, it is possible to deposit a filament having an approximate length of 32,000 mm before recharging the injection cylinder. In a preferred embodiment of the invention, the nozzle is provided with a closure member (not shown in Figure 2) which can interrupt the depositing at any time. Preferably, the closure member is controlled by computer. Once the injection travel is completed, the piston (33) is withdrawn and the screw very rapidly fills the cylinder again with new material ready to be extruded. The extrusion system according to the invention allows a high level of control, minute by minute, of the extrusion speed and allows the injection of highly viscous material at an extrusion temperature between approximately 100°C and 230°C, preferably between approximately 150°C and 200°C. The extrusion temperature is established in accordance with the starting feedstock material used for the extrusion. The system allows the extrusion of a filament with a minimum diameter of 0.1 mm.

The extrusion nozzle

The prior art of the FDM technologies has until now paid very little attention to the configuration of the extrusion nozzle, though a fundamental component is involved.

It has been found that the configuration of the nozzle influences the effective diameter of the filament extruded (after the swelling, that is to say, the inevitable increase of the filament being discharged from the nozzle), the speed profile of the material inside a given transverse section of the material, the stability and speed and direction of the material being discharged, In the device according to the present invention, as a result of the high pressure (in the range approximately from 10 to 200 MPa) necessary for the extrusion, the configuration of the nozzle is even more important than in conventional FDM.

It has been found, see Figure 5A, that a nozzle with a convergent-parallel profile generates a stable flow having a regular geometry, stabilizes the direction of the viscous flow being discharged and is suitable for medium diameters of the extruded filament, giving rise to a small swelling. With a proper design of the diameter and the length of the parallel region of the nozzle, a balanced compromise between pressure drops, regularity of the plastic flow and swelling, can be found.

It has also been found, see Figure 5B, that a nozzle having a step-convergent profile can be used as a variant of the type described above, only with a smaller minimum diameter of the calibrated portion and a smaller axial length of the calibrated portion, which allows smaller pressure drops with respect to the preceding type.

Feedstock materials

With the device of the present invention, it is possible to use as the supply materials commercially available materials which are intended for the MIM process (Metal Injection Molding) and the CIM process (Ceramic Injection Molding), which are generally indicated as PIM materials (Powder Injection Molding). The feedstock is composed of a finely powdered metal or ceramic fraction which is mixed with a polymer fraction, the binder, or wax. The binder has to be removed after the depositing by means of a de-binding operation. The binder may be hydrosoluble or be dissolved using solvents. The residual portion of binder is removed during the final production phase, involving the sintering of the material.

The device according to the present invention provides materials with mechanical properties which are comparable with those of the products obtained by means of MIM, a process which requires a die, unlike the present invention, however.

The supply materials which are commercially available may include by way of example of the metal or ceramic material, respectively:

Hardness of product

> 320 HV > 1200 HV

extruded and sintered

The modulus of elasticity of both materials, after extrusion and sintering, is similar (approximately 200 GPa), the ceramic material is approximately four times harder than stainless steel.

The function of the percentage of binder Since the typical granulometry of the powder of the basic materials is similar and since they are practically unaffected by the temperature and pressure range of extrusion, the mechanism of the extrusion process is influenced only by the rheology of the binder.

With regard to the applications of the additive production, the percentage of binder is a very important parameter: high values facilitate the extrusion process and deposition by way of a lower viscosity of the feedstock. The adhesion of the filament deposited on the preceding layers is brought about because the filaments are extruded approximately at the same temperature of the extrusion head, and the cooling process thereof in air is slow, with respect to the extrusion speed. The higher the content of binder, the easier is the adhesion of the various layers during deposition. A high percentage of binder further makes the filament more flexible during the deposition and makes it easier to obtain small radii of curvature thereof, that is to say, intricate deposition trajectories.

Vice versa, the success of the sintering operation and the volumetric contraction of the material being sintered are influenced in a negative manner by an increase in the content of the binder in the feedstock. The compromise to be achieved between ease of extrusion, ease of adhesion during deposition and ease of de-binding and sintering is such that the percentage of binder has to be accurately selected for each type of binder and basic material.

The above-mentioned optimization operations are within the normal competences of the person skilled in the art. According to the present invention, the binder is contained in the starting mixture typically between approximately 15% and approximately 50%, preferably between 20% and 35% by volume.

After the de-binding operation, the workpiece is subjected to a reduction of mass greater than approximately one half of the content by mass of binder, in a range between approximately 3% and approximately 12%. The density of the extruded filaments, during the de-binding, is subjected to a slight increase, of a few percentage points.

After sintering, the materials are quasi completely densified to the nominal density of the homogeneous material (for example, relative density: 98.5%). The relative density is not equal to 100% as a result of residual micro-porosity. It is very important to emphasize that relative density values which are very similar can be obtained when the feedstock MIM, instead of being extruded in a filament, is injected and compacted inside a die with a configuration which is conventional in MIM dies.

The robot table (parallel kinematic machine) Unlike what has been done in the extremely widespread scientific and industrial architectures, used in the field of additive manufacture, the present invention provides for a different way of moving the workpiece-carrying table with five degrees of freedom with parallel kinematics, maintaining the extruder in a fixed position. This configuration allows greater flexibility to be obtained in terms of generating the extrusion travel, that is to say, a high level of adaptability, high speed and precision of positioning.

The present invention further solves the problem of stepping or at least, allows a substantial reduction thereof.

In applications of additive production, the direction in which the material is extruded, it is preferable that it is orthogonal to the surface of the workpiece on which it is deposited, as in the present invention in which it is possible to vary the inclination of the table and to better follow the deposition on the inclined surfaces of the workpiece. Furthermore, the 5 axes make it possible to deposit the material by reducing and in some cases eliminating the need for supports, which are generally used for projecting structures. In fact, by inclining the table in a suitable manner, it is possible to avoid depositing the material in a projecting manner when it is still warm. The workpiece-carrying table according to the present invention is based on a hybrid structure which is characterized by two parallel kinematic mechanisms (PKM) which are positioned in series. As shown in Figures 3 and 4, respectively, the first mechanism has three translational degrees of freedom while the second mechanism is a manipulator having two rotational degrees of freedom. The workpiece-carrying table is mounted on the second platform. The hybrid kinematic solution selected allows the achievement of good precision and a sufficiently great working space. The platform has 5 DOF, three DOF of positioning along the axes x, y and z and two rotational DOF which are represented by the first two nautical angles, rolling and pitching, considering a sequence XYZ. The five DOF have been selected to facilitate the deposition process of the feedstock and to allow a 3D deposition. The two manipulators are capable of working in a coordinated manner independently of each other.

Kinematics

Figure 3 shows a diagram of the first parallel kinematic machine which constitutes the 3D printer which is referred to as linear Delta. This system is known, see, for example, Milewski J., Lewis G., Thoma D., Keel G., Nemec R., Reinert R. Directed light fabrication of a solid metal hemisphere using 5-axis powder deposition. J Mater Process Technol 1998;75: 165-72. This architecture is provided with three rigid links (61a, b, c), also referred to as "arms", which are each connected to the movable platform at one side and to a linear guide (62a, b, c) at the other side. The drive is supplied by three electric motors (63) which are connected to the three linear transmission units. Each rigid connection (link) is constituted by a parallelogram type system which is capable of reproducing a PUS (Prismatic-Universal-Spherical) kinematic chain.

This solution ensures that the movable platform is always parallel with a given reference plane, for example, the ground. The second portion of the kinematics, which is mounted in series on the linear Delta platform, is based on a parallel kinematic manipulator with two rotational kinematic degrees of freedom, called the spherical agile wrist, of which an exemplary embodiment is shown in Figure 4. Devices of this type are known, see, for example, Zhang J. Adaptive Slicing for a Multi-Axis Laser Aided Manufacturing Process. J Mech Des 2004; 126:254. In Figures 3 and 4, the vectorial illustrations are purely illustrative without any limiting character. The device of the present invention is mainly intended for objects of small/medium dimensions, therefore it is important to deposit with great precision, owing to the small dimensions of the extruded filament, in a working area of approximately 400 mm². In addition to a small working space, other important aspects of the PKM are: poor range of mobility of the articulations and the risk of auto-collision between the links. For all the reasons set out above, there is still a low commercial circulation of those robot architectures with respect to serial robots. As a result of the non-linear transmission of movements and forces from the space of the joints to the work space, the effectiveness levels obtained with parallel machines are difficult to standardize. It is therefore necessary to configure them specifically for each application. By the hypothesis, for example, that it is desirable to have a useful square working area of the TCP (Tool Center Point) of 220 mm per side, for the kinematic constraints of the system it is necessary to over- dimension the effective area. The reason for this is the fact that, when the work table has to be inclined, in order to ensure that the distance between the extruder and the position in which the material is deposited is kept constant, the linear Delta has to compensate for the movements as a result of the inclination of the two rotational degrees of freedom of the platform. The effective form of the work space has been obtained by using a genetic algorithm for constrained optimization which is carried out in MATLAB, of which an exemplary result is shown in Figure 6, where the work space of the manipulator (Manipulator Workspace) is indicated by the outer line and the desired work space (Desired Workspace) is indicated by the internal square.

The system is capable of moving the TCP in a working cube, with a typical dimension of the side between approximately 200 and approximately 250 mm, with the complete orientation of the parallel kinematic machine carrying the workpiece-carrying table. The spherical wrist can rotate the workpiece-carrying table in a range of ±60°. The three linear guides have been used not only as an actuation system but also as a PKM robot frame. There are further possible alternative solutions, in which the linear guides can be shorter and connected to a carrier structure for the robot.

Claims

1. A device for the metal or advanced ceramic direct additive manufacturing comprising: a. an injection unit of material for injection molding, which is fixed in the space and provided with a nozzle for extrusion of a filament said injection system is numerically controlled by a Computer Numerical Control (CNC); b. a parallel kinematic machine with 5 degrees of freedom.

2. The device according to claim 1, wherein said injection unit comprises a loading system of the feedstock material and an injection molding head with a nozzle.

3. The device according to claim 2, wherein said injection unit comprises a plasticizing piston or screw conveyor, and an extrusion piston or screw conveyor.

4. The device according to claim 3, wherein said injection unit comprises a plasticizing piston and an extrusion piston, and said plasticizing piston and extrusion are numerically controlled in a closed loop.

5. The device according to any one of claims 2-4, wherein said plasticizing piston is placed on an axis inclined with respect to the vertical direction of said loading system, said inclination is comprised between 30° and 60°, preferably 45°.

6. The device according to claim 5, wherein said extrusion piston is vertical and said plasticizing piston is placed on an axis inclined with respect to the vertical direction of said loading system, said inclination is comprised between 30° and 60°, preferably 45°.

7. The device according to claim 6, wherein the diameter of said piston of extrusion is variable as a function of the diameter of the extruded filament to be obtained, preferably between 5 and 25 mm, more preferably 14 mm.

8. The device according to claim 7, wherein said extrusion piston exercises a pressure up to 200 MPa.

9. The device according to any one of claims 1-8, the nozzle of said extrusion group has a minimum diameter of 0.1 mm.

10. The device according to any one of claims 1-9, wherein said nozzle has a convergent- parallel or a step-convergent profile.

11. The device according to any one of claims 1-10, wherein said plasticizing and/or extrusion system are provided with heating elements.

12. The device according to any one of claims 1-11, wherein a shutter is placed on said nozzle.

13. The device according to claim 12, wherein said shutter is computer-controlled.

14. The device according to any one of claims 1-14, wherein said parallel kinematic machine comprises a first platform with three translational degrees of freedom along x, y and z and a second platform placed in series with the first platform with two rotational degrees of freedom according to two roll and pitch nautical angles, said second platform bearing a workpiece-table.

15. The device according to claim 14, wherein for said machine the 5 axes of movement are operated and controlled by a parallel kinematic robot.

16. The device according to claim 15, wherein said robot comprises a manipulator for each of said first and second platform, which work independently from each other or in a coordinated manner.

17. The device according to claim 16, wherein said first platform comprises three rigid links connected to a mobile platform on one side and a linear guide on the other side, wherein each of the three rigid links is provided with a motor and each rigid link comprises a parallelogram system, capable of reproducing a PUS (Prismatic-Universal-Spherical) kinematic chain.

18. The device according to claim 17, wherein said second platform comprises a spherical agile wrist.

19. The device according to any one of claims 1-18, wherein the work area is overestimated, in particular with genetic algorithm computations.

20. The device according to claim 19, wherein the work area is of about 400 mm².

21. The device according to any one of claims 1-20, wherein said device is connected to data source representative of the 3D object to be manufactured.

22. The device according to claim 21, wherein said device is further connected to an apparatus suitable for receiving said data and converting said data into layer data which are elaborated for forming layer data used by said apparatus for controlling the elements a) and b) described in claim 1 and manufacturing said 3D object.

23. The device according to any one of claims 1-22, which is provided on board with a milling mandrel.

24. A process for the direct additive manufacturing of metals or of advanced ceramics using the device as disclosed in claims 1-23 comprising : a. gravity feeding a powdery mixture of metal or advanced ceramic and a polymeric binder into an injector; b. 3D deposition of a metallic or ceramic "green" from said injector; c. partial debinding of said deposited material; d. sintering to approximately the density of the solid material.

25. The process according to claim 24, wherein said mixture has high viscosity and low polymer content.

26. The process according to any one of claims 24-25 wherein the extrusion temperature is comprised between 150° and 230°C.

27. The process according to any one of claims 25-26, wherein said polymeric binder is contained in said mixture in a percentage comprised between 15% and 50% in volume, preferably between 20% and 25% in volume.