Abstract:
This invention relates to processes and systems of rapid prototyping and production. Its features includes flexible material deposition along tangential directions of surfaces of a part to be made, thereby eliminating stair-shape surface due to uniform horizontal layer deposition, increasing width of material deposition to increase build up rate, applying the principles of traditional forming/joining processes, such as casting, fusion welding, plastic extrusion and injection molding in the fabrication process so that various industrial materials can be processed, applying comparatively low cost heating sources, such as induction heating and arc-heating. Additional features include varying width and size of material deposition in accordance with geometry to be formed and applying a differential molding means for improved shape formation and surface finishing.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS AND OTHER DOCUMENTS 
       [0001]    This application claims the benefit of prior U.S. provisional application No. 62/085,509, filed 29 Nov. 2014, the contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates to systems and methods of rapid prototyping and production (or called additive manufacturing or 3D printing). Especially, this invention relates to fabricating 3D prototypes, articles, components and molds at improved surface finish and increased speed. 
         [0003]    Existing major rapid prototyping (also known as additive manufacturing or 3D printing) techniques include methods such as SLM (Selective Laser Melting) for making metal parts (for examples, EOS M400, referring to http://www.eos.info/systems_solutions/metal/systems_equipment/eos_m_400, 3D Systems SPro 250, see http://production3dprinters.com/sites/production3dprinters.com/files/downloads/sPro-125-250-SLM-Direct-Metal.pdf, or Renishaw AM250, referring to http://www.renishaw.com/en/am250-laser-melting-machine--15253)), and SLA ((Stereolithography) (for example, 3D Systems ProJet HD 7000, referring to http://printin3d.com/sites/printin3d.com/files/downloads/ProJet-6000-7000-USEN.pdf), FDM (Fused Deposition Modeling) (e.g. Stratasys FDM 900m, referring to http://www.fortus.com/Products/Fortus-900mc.aspx) and 3DP (3D Printing) (jetting binders to powder bed layer-by-layer) for making plastic parts. 
         [0004]    In general, these existing rapid prototyping methods apply a layer-by-layer construction methodology. Materials are dispensed in horizontal layers and within each layer joined by point scanning. Material build-up by horizontal layers, regardless of the 3D shape to be built, creates inevitable layered (stairs-like) surface feature, resulting in poor surface finish. Material joining by point scanning is basically “scanning a 3D body by one tiny point”, resulting in slow build-up rate. Combined operation of layer dispensing and point-scanning joining slows down the process further.  FIG. 1  illustrates an example 3D part.  FIG. 2  illustrates the fabrication of this example 3D part by the existing methodology.  FIG. 2( a )  shows the blade portion and  FIG. 2( b )  shows the cross-sectional view. Dotted lines  201  indicate the grid structure of horizontal layers and solid curves  203  indicate trajectories of point scanning. Stairs-like surface features at  214  and  212  are inevitable. 
         [0005]    When using the SLM technique to make a mold for plastic injection molding, the surface finish can be about 40 um Ra and a machining tolerance of 200˜500 um is generally required, which makes post machining cost significant. There are studies on post polishing using laser beams. (Referring to Lamikiz et al., “Laser polishing of parts built up by selective laser sintering”; International Journal of Machine Tools &amp; Manufacture 47 (2007) 2040-2050). In order to improve forming speed, a so called “skin-core strategy” was developed, which uses a laser of small focal spot to scan edges of patterns in each layer and a larger focal spot to scan the interior. (Referring to (1) K. Wissenbach, “Fantasia Project Shows Selective Laser Melting Can Produce Complex Components Quickly and Cost Effectively”, http://www.ineffableisland.com/2010/05/fantasia-project-shows-selective-laser.html?showComment=1318241730096; (2) C. Hinke, “Direct, Mould-less Production Systems”, http://www.production-research.de/_C12577F20052BDC7.nsf/html/de_040d66b2c812b739c1257829005207de.html). But these methods also increase equipment costs. 
         [0006]    In the FDM technique, U.S. Pat. No. 5,121,329, which is incorporated herein for this current application by reference, describes methods of moving a material dispensing head along curved trajectories to produce curved surfaces or frames and of dispensing materials of variable thickness by changing material feed rate (referring to FIG. 10 and FIG. 12 of that patent). However, because the FDM method uses a fixed orifice size to dispense material, the effect of speed Increasing is likely to be limited. In another FDM related technique, U.S. Pat. No. 8,221,669, which is incorporated herein for this current application by reference, describes the use of ribbon (non-cylindrical) filament as material, in contrast to the cylindrical filament used in most current commercial systems, in order to reduce the so called “response time”, that is, the delay time from the start or stop of the feeding mechanism to the actual flow rate change at the tip of the extrusion tip of the liquefier. But it should be noted that faster material deposition is not the purpose nor mentioned in this patent. 
         [0007]    There are other methods developed or under development for making metal objects. 
         [0008]    For example, applying the FDM technique to make metal parts has been attempted. U.S. Pat. No. 7,942,987, which is incorporated herein for this current application by reference, describes a method of heating a metal alloy to a temperature between a solidus temperature and a liquidus temperature to obtain a semi-solid metal alloy with enough viscosity so that it can be extruded. However, the “point scanning” and “layer by layer” issues are not addressed in this approach. 
         [0009]    Another approach is called Laser Deposition Technology (LDT) or Laser Engineered Net Shape (LENS). Metal powder is injected into a focused beam of a high-power laser under tightly controlled atmospheric conditions. The focused laser beam melts the surface of the target material and generates a small molten pool of base material. Powder delivered into this same spot is absorbed into the melt pool, thus generating a deposit. By moving the laser beam and the deposition relative to the target material, 3D shapes can be built up. A description of the process can be found from http://www.rpm-innovations.com/laser_deposition_technology and related technical details can be seen in U.S. Pat. No. 4,323,756 and No. 5,043,548, which are incorporated herein by reference. A very similar method, except using wire metal instead of powder, was described in U.S. Pat. No. 5,578,227, which is incorporated herein by reference. In general, these approaches are basically still a “point scanning” based approach. Further, surfaces of built-up parts are usually rough. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    The basic principle of the Flexible 3D Freeform technique is to dispense a solidifiable material in a fluid state from a dispensing head onto a base member to build up the material, which solidifies under preset ambient conditions, in a basically continuous fashion according to a predetermined relative movement sequence between the dispensing head and the base member. Based on this principle, this invention comprises a feature of dispensing the solidifiable material along the tangential directions of the surface of the 3D article to be fabricated so that the layered surface feature in existing rapid prototyping processes is eliminated and the surface finish is improved. This invention also comprises a feature of enlarged width of dispensed materials and a corresponding new 3D forming procedure so that fabrication speed can be increased significantly. For these purposes, this invention includes a motion mechanism of multiple degrees of freedom to provide the required relative movement sequence between the dispensing head and the base member. Further, this invention includes a feature of adjusting the width, thickness and flow speed of the dispensed material according to needs from local geometry of a 3D article during the dispensing process. The dispensing head dispenses material in a few basic shapes including ribbon (band), wire and dot. When the geometry of the 3D object to be fabricated allows, wide, ribbon-shaped material can be dispensed so that building speed can be increased. Dispensed materials of wire- and dot-shapes can be applied to build up fine and complex features. Still further, this invention includes a differential molding means, which applies a solid or fluid means in contact with selected positions on dispensed material while it is solidifying, to restrict the flow of and to shape the solidifying material. By this means, good surface finish can be obtained. Curvature in the direction along the width of a ribbon-shaped dispensed material can also be made by this means. 
         [0011]    A variation of this invention is to dispense a joinable material in particulate form and simultaneously apply a joining means to the material dispensed at the target area such that the dispensed particulate material starts to join into an integral material. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  depicts an example of 3D article. 
           [0013]      FIG. 2  illustrates the layer-by-layer construction methodology used in the prior arts. 
           [0014]      FIG. 3  explains the basic concept of this invention using the 3D object depicted in  FIG. 1  as an example. 
           [0015]      FIG. 4  illustrates the spatial relationship between a material dispensing head of this invention and an example 3D surface. 
           [0016]      FIG. 5  illustrates two examples of mechanisms to provide the required relative movements between the material dispensing head and the base in this invention. 
           [0017]      FIG. 6  depicts two methods to form a 3D thin-shell surface from joining ribbon-shaped materials together by this invention 
           [0018]      FIG. 7( a )  illustrates an example embodiment of a material dispensing system for metal in this invention; (b)-(c) illustrates its cross-sectional views from side. 
           [0019]      FIG. 8  illustrates another example of the material supply unit in cross-sectional view, using an induction heating means to melt a metal wire, and operation of the differential molding means in this invention. 
           [0020]      FIG. 9  illustrates an example method of adjusting cross-sectional size of dispensed material during dispensing by adjusting dispensing head orientation in this invention. 
           [0021]      FIG. 10  illustrates another example method of adjusting cross-sectional size of dispensed material during dispensing by adjusting dispensing head orientation in this invention. 
           [0022]      FIG. 11  illustrates an example approach of adjusting cross-sectional size of dispensed material during dispensing by adjusting dimension of the exit of the dispensing head in this invention. 
           [0023]      FIG. 12  illustrates another example approach of adjusting cross-sectional size of dispensed material during dispensing by adjusting dimension of the exit of the dispensing head in this invention. 
           [0024]      FIG. 13  illustrates forming of an arbitrarily twisted ribbon shape by relative motions between the base and the dispensing head in this invention. 
           [0025]      FIG. 14  illustrates examples of operation of the differential molding means in this invention. 
           [0026]      FIG. 15  illustrates an example design of the differential molding means based on material deformation in this invention. 
           [0027]      FIG. 16  illustrates another example design of the differential molding means based on non-deformable shapes in this invention. 
           [0028]      FIG. 17  illustrates an example design of the differential molding means for controlling dimension of width of dispensed material in this invention. 
           [0029]      FIG. 18  illustrates another example design of the differential molding means based on fluid flow and pressure in this invention. 
           [0030]      FIG. 19  illustrates an example design of mounting and retracting mechanism of the differential molds system in this invention. 
           [0031]      FIG. 20  illustrates still another example approach of adjusting cross-sectional size of dispensed material during dispensing by adjusting dimension of the exit of the dispensing head in this invention. 
           [0032]      FIG. 21  illustrates still another example approach of adjusting cross-sectional size of dispensed material during dispensing by adjusting dimension of the exit of the dispensing head in this invention. 
           [0033]      FIG. 22  illustrates an example approach of using an arc-based heating unit for auxiliary and localized heating in this invention. 
           [0034]      FIG. 23  illustrates an example process of making a 3D article by this invention. 
           [0035]      FIG. 24  illustrates an example of making a mold core by the flexible mold surface forming technique in this invention. 
           [0036]      FIG. 25  illustrates another example of making a mold core by the flexible mold surface forming technique in this invention. 
           [0037]      FIG. 26  illustrates an example of making a mold cavity by this invention. 
           [0038]      FIG. 27  illustrates still another example approach of adjusting cross-sectional size of dispensed material during dispensing by adjusting dimension of the exit of the dispensing head in this invention. 
           [0039]      FIG. 28  illustrates still another example approach of adjusting cross-sectional size of dispensed material during dispensing by adjusting dimension of the exit of the dispensing head in this invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0040]      FIG. 3  explains the basic concept of the invention using the 3D object depicted in  FIG. 1  as an example. When making the hub  102 , materials can be dispensed in horizontal layers and built up vertically, as shown by  310  of  FIG. 3( b ) , similar to  212  of  FIG. 1( b ) , because the inner wall of the hub is vertical. However, when making the outer wall, the preferred material dispensing direction is along the tangential direction of the wall, so that stair-like features can be eliminated and good surface finish can be obtained ( 312 ). Especially, when making the blades  103 , dispensing materials along the tangential direction of the curved blade geometry can improve surface finish very significantly ( FIG. 3( b )   314 ). In  FIG. 3 ( a ) , dotted-lines  301  depict the grid structure using this Flexible 3D Freeform technique. The grid structure has 3 layers stacked together and each layer is distributed along the tangential direction of the curved blade geometry. Materials dispensed according to the grid structure are shown as  318   a ,  318   b  and  318   c.    
         [0041]    Moving the dispensing head to trace arbitrary 3D surfaces relative to the base member requires a mechanism of multiple degrees of freedoms. In general, if the exit of the material dispensing head is just a single orifice, close to a spot or a point in geometry, then a motion means of 3 degree-of-freedom (DOF) is enough to allow a section of an arbitrary 3D surface to be traced by a point spot. However, if dispensing of wide band-shaped material is preferred, then a motion means of at least 4 DOF is preferred.  FIG. 4( a )  illustrates the situation. The exit of the material dispensing head  403  has a long (width) edge  405   a  and a short edge  405   b . In order to dispense material at maximal width, the dispensing head must move and trace the 3D surface  110  in a direction perpendicular to the edge  405   a . In order to accommodate the change of surface curvature perpendicular to the motion direction (which is x-direction in  FIG. 4 ), one rotational DOF (A-axis, which rotates about x-direction) is needed. That is, the system has 3 independent linear DOFs (x, y, z) and one rotary DOF (A). A more preferred arrangement applies 3 independent linear DOFs (x, y, z) and 2 rotary DOF (A and B, which rotates about y-direction). This is illustrated in  FIG. 4( b ) . The addition of B DOF allows the dispensing head to always maintain a fixed angle with respect to the target area. A still more preferred arrangement is to add a third rotary DOF C, which rotates about z-direction. This allows the dispensing head to move in basically any direction with maximal dispensing width, or with any reduced dispensing width, which will be further described in later texts. 
         [0042]    There are many options of mechanisms to provide the required relative movements between the material dispensing head and the base.  FIG. 5( a )  illustrates one example, which is basically in a robotic arm configuration  520 . The rotations at 3 joints, J 1 , J 2  and J 3 , can place the end  521  of the robotic arm to basically any position (x, y, z) relative the base area  501 , whereas A. B and C 3 axes can rotate the material dispensing system  401  to any orientation.  FIG. 5( b )  illustrates another example, which is basically a 5-DOF (5-axis) mechanism comprising 3 linear DOFs (X, Y, Z) and 2 rotational DOFs (A about X-axis and B about Y-axis). The base area  501 , carrying the base and the 3D article to be fabricated on the base, is attached to the A-axis rotary stage. The material dispensing system is attached to the Z-stage. The dispensing head can rotate about the Z-axis, forming the 6th DOF (C-axis). Other configurations and variations can be made by people skilled in the arts based on the teaching of this current document. 
         [0043]    Further, this invention includes the feature of adjusting width and thickness of the dispensed material according to needs from local geometric characteristics of a 3D article during the dispensing process. The dispensing head dispenses material in a few basic shapes including ribbon (band), wire and dot. When the geometry of the 3D object to be fabricated allows, wide, ribbon-shaped material can be dispensed so that building speed can be increased. If geometric features to be built are small and fine, then the width and thickness of the dispensed material can be reduced to dispense wires and dots accommodate the situation. 
         [0044]    For example,  FIG. 6( a )  depicts a 3D thin-shell surface comprising 5 ribbon-shaped materials joined together ( 368   a - e ).  350  indicates the cross-sectional curve of an ideal smooth 3D curved surface. The curvature on one side ( 350   a ) is larger than that on the other side ( 350   b ). When this surface is fabricated using the current invention, the side with smaller curvature can be made by dispensing and joining wider ribbon materials, such as  368   d  and  368   e , whereas the side with larger curvature requires ribbon materials of smaller width, such as  368   a  and  368   b . Wider ribbons correspond to faster build rate, which speeds up the whole fabrication process. Accordingly, this feature allows maximization of material dispensing rate based on local geometry. 
         [0045]    There are at least 2 methods to adjust width and thickness of dispensed materials. The first method adjusts the orientation of the dispensing head together with the material flow rate to change size of cross-sections of dispensed materials. The second method applies a dispensing head with an adjustable exit. Details will be described in later texts. 
         [0046]    Still further, this invention includes a differential molding means, which applies a solid or fluid means in contact with selected positions on dispensed material while it is solidifying, to restrict the flow of and to shape the solidifying material. By this means, good surface finish can be obtained. A further variation of this means can create curvature in the direction along the width of a ribbon-shaped dispensed material, in order to match curvatures of local geometries. For example, in the 3D thin-shell surface depicted in  FIG. 6( a ) , if the ribbon-shaped dispensed materials have rectangular cross-section, as illustrated as  3681 , then the fabricated surface will not completely match the ideal cross-sectional curve  350 . However, as illustrated in  FIG. 6( b ) , if the dispensed ribbon-shaped material can be made to have curvature along its width direction, then every band ( 367   a - e ) can be made to match the local curvature of the ideal cross-sectional curve  350 , and the fabricated object will have 3D curvatures closely matching the ideal smooth 3D curved surface. 
         [0047]    Differential molding by solid means applies a small solid surface (called “differential mold”) in contact with selected portions of the dispensed material while it is solidifying, to restrict the flow of and to shape the solidifying material. This solid means further comprises mechanisms to change curvature in the width direction of the dispensed solidifying material. Differential molding by fluid means applies different pressures generated from fluid flows to achieve similar effects. Details are to be described in later texts. 
         [0048]    Combining a motion mechanism capable of multiple-DOF flexible 3D paths, a material dispensing head that can adjust the size of dispensed material, and a differential molding means that can shape curvature of the dispensed material in the width direction, fast fabrication of 3D curved surface can be realized. As illustrated in  FIG. 3( b ) , each of the ribbon-shaped structures depicted by different tones of gray ( 318   a ,  318   b ,  318   c , and  318   d ) represent one band of ribbon-shaped material dispensed by the dispensing head in a single pass. For example, the light gray band structure  318   c  is formed by a dispensing motion that basically sweeps a curve PQ along the direction of arrow  390 . This maximizes material deposition rate. 
         [0049]    Accordingly, this invention includes the following procedure of fabricating a 3D article: 
         [0050]    (1) Analyze the geometric shape and the requirements (such as surface finish, material forming direction or strength requirements etc.) of different portions of the 3D article and make a process plan. This process plan divides the geometric data of the 3D article into a number of zones and sets an order of sequence of fabrication of these zones. 
         [0051]    (2) Based on the process plan, disassemble the geometric data of the 3D article and generate a collection of Component Geometries that can be recombined to form the shape of the 3D article. The so called Component Geometries include geometric volumes of bands (ribbons), wires (lines) and dots (points). Further identify and select zones that require good surface finish and then disassemble and convert these selected zones into collections of band and wire Component Geometries. 
         [0052]    (3) For each Component Geometry generated in step (2), determine a corresponding set of forming parameters (including thickness, width, curvature and dispensing trajectory). 
         [0053]    (4) For each set of forming parameters generated in step (3), determine a set of process parameters, including material feed rate, ambient conditions (such as temperature), material dispensing rate, dispensing head speed etc. 
         [0054]    (5) Following the sequence determined in the process plan, dispensing material to form each Component Geometry. The forming method for each Component Geometry is to move the material dispensing system (including the dispensing head and the differential molding means) along the dispensing trajectory, relatively, and dispense solidifiable material according to the corresponding set of forming parameters. 
         [0000]    By forming each Component Geometry in sequence, all the Component Geometries combine to form the 3D article. 
         [0055]    Under proper situations, the dispensing head capable of adjusting exit size and the differential molding means for curvature formation do not need to be applied together. For example, the 3D thin-shell curved surface of  FIG. 6( a )  can be fabricated without using the differential molding means for curvature formation. 
         [0056]    In the broadest sense, the solidifiable material of this invention includes any material that has a fluid state and can be converted into a solid state under specific environmental conditions. 
         [0057]    For example, many polymers exhibit the property. Most thermoplastics exhibit fluid state at elevated temperatures and solidify at lower temperatures. Typical examples include Nylon, PMMA and polystyrene (PS) such as ABS etc. Wax is another material that exhibits the property depending on temperature. Wax is another material that exhibits the solidifiable property based on temperature. 
         [0058]    Polymer materials can be dispensed by extruding, which corresponds to plastics extrusion processes in traditional bulk plastic processes, or by jetting (Injection from fluid state), which corresponds to injection molding. 
         [0059]    Photo-curable polymers, such as photoresist and liquid photo-lithographical polymer used in the SLA process, can also be used. In this case, the environmental condition that solidifies the polymers is mainly exposure of light, especially UV light. 
         [0060]    The solidifiable materials can be metals, which exhibit the solidifiable property based on temperature. In analogy to traditional bulk processes, such as casting, continuous casting and fusion welding, molten metal can be dispensed from a dispensing head, such as a tundish with a nozzle, and then be allowed to cool down and solidify. Molten metals, especially those of lower melting points, can also be dispensed by injection, in analogy to metal injections applied in die casting. Another method to dispense molten metal is to shoot metal droplets. 
         [0061]    Another form of solidifiable materials includes pastes, that is, the mixture of fine solid particles and a liquid state carrier. For example, in the prior art of the process of metal injection molding, metal particles are mixed with a fluid carrier made of polymer and wax at an elevated temperature. This mixture is then injected into a mold and cooled down and becomes a solid. The wax and the polymer are then removed in a melting and burning process, leaving a green part composed of metal powder, which is then sintered. In the current invention, a similar mixture of metal powder with wax and polymer in fluid state at elevated temperature can be used as the solidifiable material and temperature can be used as the main control of environmental condition. Solid parts can be dispensed by extrusion or by injection and be built up in the manner described previously. The same processes of wax/polymer removal and sintering can then be applied to make the final solid metal parts. 
         [0062]    Another example of paste includes ceramic slurry, which is basically a mixture of fine ceramic powder with water and binders. The mixture can be dispensed and built up in the manner described previously. Depending on the fluidity and viscosity of the mixture, the material can be dispensed either by jetting (injection), if the fluidity is high and viscosity is low, or by extrusion, if the fluidity is low and the viscosity is high. Simultaneously with the dispensing, a flow of heated air can be applied to the dispensed material at the target zone to quicken the drying process. A green part can thus be formed. The green part can then be sintered into a solid ceramic part. 
         [0063]    Still another example of paste is concrete. 
         [0064]    Another form of solidifiable material is glass. It can be extruded and temperature can be used as the main environmental control to solidify it. 
         [0065]    A variation of this invention is to dispense a joinable material in particulate form and simultaneously apply a joining means to the material dispensed at the target area such that the dispensed particulate material starts to join into an integral material. For example, metal or plastic powders can be used as the particulate joinable materials and a localized heating, which can be provided by a laser or an electric arc, can be used as the joining means. The dispensed powder is partially or fully melted by the localized heating and then, when out of the localized heat, cools down and re-solidifies into an integral part. For another example, a separate localized dispensing of binder material to the dispensed powder can be used as the joining means. Various binder materials used in processes such metal powder injection molding or ceramic powder molding can be used to join metal and ceramic powders. 
         [0066]    A system of this invention includes a material dispensing head, a motion mechanism of multiple-DOF, which can generate flexible 3D paths between the dispensing head and the base, a material supply system that can control material feed rate and an ambient condition control system. It can further include a differential molding means that can shape curvature of the dispensed material in the width direction. The processing method includes the process plan mentioned previously, the process of disassembling geometric data, the process of generating the forming parameters and processing parameters, as well as the procedure of controlling the operation of the material dispensing head and the differential molding means, and the procedure of controlling the multi-DOF motion mechanism. Examples of embodiment of the invention are further described below. 
       Example 1 
     Molten Metal as Solidifiable Material, 3D Freeform by a Casting Based Means 
     1.1 the Material Dispensing System 
       [0067]      FIG. 7( a )  illustrates an example embodiment of a material dispensing system for metal, which includes a material supply unit  402 , a dispensing head  403  and an optional differential molding means  420 .  FIG. 7( b )-( c )  illustrates its cross-sectional views from side. The material supply unit  402 , which includes a heating means (such as induction heating or electric arc heating) and a material feeding means (a metal wire powder can be used), sends the material into the dispensing head  403 . The dispensing head includes a material cell  404  with an exit  405  at the lower end. A heating means outside of the material cell keeps the metal in molten state  480 . A material not reactive to the molten metal is preferred for making the material cell. For example, aluminum oxide, graphite or other high temperature ceramics can be used to contain molten steel. Ceramics can also be used for metals of high melting points, such as titanium, cobalt, chromium and nickel alloys. Steel cells can be used to contain molten copper, aluminum can be used for molten tin; whereas molten aluminum can use titanium, titanium coated with boron nitride, or steel coated with nitralloy. The molten metal can flow out of the exit under the effect of gravity. Alternatively, a gas pressure from the top of the material cell can push the molten metal out of the exit. The actual size of the material cell and the exit depends on size of the 3D article to make, dimensional tolerance and materials. 
         [0068]      FIG. 8  illustrates a different example of the material supply unit  402  in cross-sectional view, wherein an Induction heating means  495  melts a metal wire  470 . The solid metal wire  470  is fed from rear end and can push and retract the molten metal  480  at the front like a piston. The material supply unit, the dispensing head  403 , and the heating means are packed inside a casing  497  and an inert gas  499 , such as argon or CO 2 , blows through the casing to cover the whole system including the material dispensing and solidification area to prevent oxidation of metal. 
         [0069]    As the molten metal flows out of the exit, it forms a droplet attached to the outside of the exit due to surface tension. By touching the droplet with the base (or solidified material on the base), the temperature of the droplet decreases due to thermal conduction and solidification starts.  FIG. 7  and  FIG. 8( b )  depict the initial stage of the material dispensing process, the metal droplet touches a metal plate  431 , which is attached to the base (metal frame  430 ) and is used as a starting point of the forming process. The base  430  together with the starting metal plate  431  then act as a heat sink and the metal droplet starts to solidify from the contact position  450  at the starting metal plate. The ambient condition control unit (not shown) controls the temperatures of the base and the starting metal plate within a proper range and the heating means around the material dispensing head  403  sets the temperature of the molten metal at an elevated, proper range. When the base with the starting metal plate moves along the direction indicated by arrow  490 , the metal can be dispensed, formed and solidified in a manner similar to continuous casting. Metal  453  close to the exit of the dispensing head is in molten state. A little away from the exit, there is a short solidification zone  452  wherein the molten metal solidifies. Further away from the exit and the solidification zone, solidified metal forms a band-shaped solid  451 , which extends to the initial solidification point  450 . Because of the effect of surface tension, as long as the temperatures and the speeds of motion and material supply are properly controlled, molten metal will flow out of the exit and follow the solidification path without dripping down. In the situation shown in  FIG. 8( a ) , molten metal is dispensed onto the surface of previously dispensed and solidified metal  514 , which also becomes a part of the heat sink. 
       1.2 Motion System 
       [0070]    A motion system with multiple degrees of freedom is used to provide relative motion between the material dispensing head and a base member. This part has been described in previous sections related to  FIG. 4  and  FIG. 5 . For example, as shown in  FIG. 13 , the base member (metal frame  430 ) is attached to a structure  610  on the base  501  of a motion system. By relative motions between the base and the dispensing head  403 , a surface of arbitrarily twisted shape  380  can be made. 
       1.3 Systems and Methods for Adjusting Cross-Sectional Size of Dispensed Material 
       [0071]    The first preferred method of adjusting cross-sectional size of dispensed material during dispensing is to adjust the angle of the dispensing head relative to the track of dispensing motion and adjust material flow rate. As depicted in  FIG. 9 , changing the angle  702  of the leading edge  405   a  of the exit of the dispensing head relative to the motion direction of dispensing  701 , with matched adjustment of material flow rate, band-shaped materials of the same thickness but different widths can be dispensed, as illustrates at  710   a ,  710   b  and  710   c . Depicted in  FIG. 10 , reorient the dispensing head and changing the angle  703  of the leading edge  405   a  relative to the vertical direction, with matched adjustment of material flow rate, vertical wall-shaped materials of the same thickness but different heights can be dispensed, as illustrated at  711   a ,  711   b  and  711   c.    
         [0072]    The second preferred method of adjusting cross-sectional size of dispensed material during dispensing is to use a gating mechanism to adjust the dimension of the exit of the dispensing head.  FIG. 11  depicts one example design of the material dispensing head with adjustable exit size. The material-containing cell  404  in this design includes 3 main parts: a U-shaped main body  404 A 1 , a side-slab  404 A 3  that inserts into and slides in the main body and an exit lip  404 A 4  that slides over the exit  405 . (Restraining structures and bearings of the sliding mechanisms are not shown.) Pulling the slide slab along arrow  901   a  increases the length of the exit  405  and moving the exit lip along arrow  902   a  opens the exit to the maximal size, as illustrated in  FIG. 11( a ) . Pushing the slide slab along arrow  901   b  decreases the length of the exit  405  and moving the exit lip along arrow  902   a  reduces the exit to the minimal size, as illustrated in  FIG. 11( b ) . 
         [0073]      FIG. 12( a )  depicts, in exploded view, another example design of the material dispensing head with adjustable exit size. The material-containing cell  404  in this design includes 4 main parts: a main body  404 B 1 , a side-slab  404 B 3 , a cover  404 B 2  that covers the main cell space  404 B 10  and the side-slab, and an exit lip  404 B 4  that slides over the exit  405 . The side lab is basically restrained on 5 surfaces by the cover, the main body, two restraining structures  404 B 13  and  404 B 14  of the main body and the tip edge of the exit lip at  404 B 40 . Therefore, the side slab can slide linearly along arrow  903 . Restraining structures for the lip  404 B 4  can be of a similar design and is not shown. Similar to the design of  FIG. 11 , the exit  405  is also opened and closed by the sliding motions of the slide slab  404 B 3  and the lip  404 B 4 . When the slide slab is pushed to the right along arrow  903  and the lip is pushed down along arrow  904 , the exit is closed down, as shown in  FIG. 12( b ) . Near the exit  405 , the tip edges of the main body, the side slab and the lip are shaped into wedge-shapes, as depicted at  404 B 11 ,  404 B 31  and  404 B 41 . This way, the exit opening  405  can always be kept at the lowest position of the assembly of the dispensing head. The possibility of mechanical interference or collision between parts of the dispensing head and the solidified workpiece can be minimized. 
         [0074]    The designs illustrated in  FIG. 11  and  FIG. 12  basically apply the principle of adjusting the area of the exit  405  from two different directions (specifically, perpendicular directions) by two independently adjusted gating members. This way allows the exit opening to be smoothly adjusted between a large maximal size and a very small minimal size. 
         [0075]      FIG. 27  depicts another example design of the material dispensing head with adjustable exit size. Compared to  FIG. 11 , this design uses multiple side-slabs stacked together  404 F 3 , without an exit lip. By moving and positioning each side-slab ( 404 F 31 ,  404 F 32  or  404 F 33 ) independently, the size of the exit  405  can be adjusted discretely in thickness direction but continuously in width direction. 
         [0076]      FIG. 28  depicts another example design of the material dispensing head. It uses a single side-slab  404 G 3  that has a recess feature  404 G 20  on its leading edge. When the slab is positioned to fully-closed position, this recess feature and the inside wall of the main body  404 A 1  form a nozzle structure with the cell space  404 G 10  at back and the orifice  405 G at the exit face, for dispensing materials in wire- and filament-shapes. 
         [0077]    The contacting surfaces between the main body and the side-slab (or the lip) need to have two functions: bearing function for sliding motion and sealing function for preventing liquid metal from leaking out. The bearing material can include graphite, ceramics such as aluminum oxide, silicon nitride, silicon carbide. Cast iron, brass, Nitralloy and Zerodur can also be used if the solidifiable material used is a non-ferrous metal. 
         [0078]    For non-wetting sliding surfaces, surface tension of the molten metal will basically prevent itself from seeping into the sliding interfaces. Further, the material feeding involves a pressure only slightly above normal atmospheric pressure. Therefore, leaking is generally not a major concern. 
       1.4 Differential Molding Means 
       [0079]    The material dispensing system can further include a differential molding means, which applies a small solid means (called differential mold) in contact with selected positions on dispensed material while it is solidifying, to restrict the flow of and to shape the solidifying material to obtain desired cross-sectional shape. By this means, good surface finish can be obtained. Curvature in the direction along the width of a ribbon-shaped dispensed material can also be made by this means. 
         [0080]    As depicted in  FIG. 7 , by placing a small solid surface  420  underneath the solidification zone  452 , this solid surface together with the leading edge of the exit  405   a  forms restrictions on both sides of the dispensed material so that the dispensed material solidifies into a band (ribbon) shape. The differential mold  420  can also be applied to the top side of the solidification zone to make the top surface of the dispensed material smooth.  FIG. 8( a )  illustrates this situation. Part of the surface of the differential mold  420  touches previously solidified material at  451 , a small cavity  601  is formed by the remaining part of the differential mold surface together with restricting surfaces of previously dispensed and solidified materials at  451  and  514 . The dispensing head dispenses material to full this cavity. The newly dispensed material solidifies. Then the differential mold and the dispensing head move to the right to begin the next dispensing step.  FIG. 8( b )  illustrates the situation when two differential molds are applied to opposite sides of dispensed material. Upper differential mold  420   a  and lower differential mold  420   b  together with previously solidified material  451  form a small cavity for receiving dispensed molten metal  452 . In principle, in order to have good bonding between newly dispensed material and previously solidified material, a small portion of the previously dispensed material needs to be re-melted and then re-solidified together with the newly dispensed material, as indicated by  452 R. By moving the upper and the lower differential molds together with the dispensing head, long, band-shaped solid can be formed. 
         [0081]    In general, the longitudinal direction of the differential mold is parallel to the solidification front of the melt and perpendicular to the track of the dispensing. Therefore, when a flat surface is used as the differential mold, a solid band (or ribbon) can be formed and the surface of the band in its transverse (width) direction is flat. This is illustrated in  FIG. 14( a ) . Curved band  381  is formed by moving a differential mold from location  420   h , together with the operating dispensing head  403 , to location  420   i . The track of dispensing is indicated by the dashed curve  788 , which is generated by the multi-DOF motion mechanism. In the transverse (width) direction, indicated by dash lines  784 , the surface of the band is flat (straight line). Such bands can still be joined to approximate a 3D curved surface, as shown in  FIG. 6( a ) . 
         [0082]    If the differential mold is made to be able to change its curvature along its length direction, then band-shaped geometry having curvature in its traverse (width) direction can be formed. As illustrated in  FIG. 14( b ) , curved band  382  is formed by moving a differential mold from location  420   j , together with the operating dispensing head  403 , to location  420   k . The track of dispensing is indicated by the dashed curve  789 . In the transverse (width) direction, as illustrated, initially the differential mold is made to curve upward  420 J so the surface has a positive curvature (or bending upward) as indicated by dash lines  784   j . In the later stage of dispensing, the differential mold is made to curve downward  420   k  so the surface has a negative curvature (or bending downward) as indicated by dash lines  784   k . As a result, combining this curvature capable differential mold with the multi-DOF motion mechanism, band-shaped geometry with curvatures in both directions (along the dispensing track as well as its transverse (width) direction) can be formed. 3D surfaces of almost arbitrary curvature can be formed by joining bands with variable curvature in both directions, as illustrated in  FIG. 6( b ) . In  FIGS. 14( a ) and ( b ) , the differential molds are depicted on top of the dispensed material stripes ( 381 ,  382 ). They can also be under the dispensed material or on both the top and the underside of the dispensed material, depending on situations and needs, as described in previous paragraphs. 
         [0083]    It should be noted that the so called “minute size” of the differential mold is measured relative to the size of the object to be fabricated, rather than by an absolute standard. 
         [0084]    The differential mold that can change curvature along its longitudinal direction can be constructed by at least two approaches. The first approach applies a deformable member and an actuation means that changes the curvature of the member.  FIG. 15  depicts one example by this method, which mainly comprises two bendable foils. These two deformable foils can be the two parts ( 420 A 1   a .  420 A 1   b ) of a single U-shaped foil  420 A 1 , as shown in  FIG. 15( a ) . Two handles ( 420 A 2 ,  420 A 3 ) are attached to the separate ends of the two foils and are connected at a pivot  420 A 4 . When a force opens the handles, as illustrated in  FIG. 15( b )  at  1501 , both foils bends inward and become concave. When a force closes the handles, as illustrated in  FIG. 15( c )  at  1502 , both foils bend outward and become convex. The curvature of the foil surfaces can be controlled by adjusting the extent of opening (or closing) of the handles. The open and close of the handles can be actuated by a suitable mechanism such as two co-axial worm gears of opposite spiral directions (not shown). This differential mold can be applied to the top surface of dispensed material by using the lower foil  420 A 1   b , or to the underside surface of the dispensed material by using the upper foil  420 A 1   a.    
         [0085]    The second approach uses a curved, non-deformable member and makes use of different portions on the member, each portion having different curvature, to make contact with dispensed material to meet the required curvature.  FIG. 16  illustrates one example of this approach, which Includes a rotatable curved rod. The curved rod  420 B 1  is attached to a shaft  420 B 2 , which is rotatable about axis  1600 . For convenience of description, axis  1600  is oriented as parallel to the x-direction. The rod has a curved section that deviates away from axis  1600  toward one direction indicated by a marker  420 B 4 , with the largest deviation at  420 B 3 . When a dispensed material passes over the upper surface of the curved section of the rod in transverse direction, i.e. along y-direction, then depending on the orientation of the rod with respect to the rotational axis  1600 , the contacting surface between the rod and the underside of the material will have different curvatures. 
         [0086]    For example, in  FIG. 16( a ) , the marker  420 B 4  points toward z-direction, indicating that the peak point of the curve  420 B 3  also points toward z-direction. Thus, the curved section of the rod acts as a differential mold of convex surface to the underside of the material and the curvature is equal to the curvature of the rod&#39;s curved section along x-direction. If the rod rotates so that the marker  420 B 4  points 90 degree away from z-direction, as shown in  FIG. 16 ( b ) , then the peak of the curved section  420 B 3  points toward y-direction. That is, the curved rod now basically lies flat on a plane parallel to the x-y plane. Its top surface is basically also flat with respect to z-direction. As a result, the rod acts as a differential mold of flat surface to the underside of the material. If the rod rotates so that the marker  420 B 4  points 180 degree away from z-direction, as shown in  FIG. 16 ( c ) , then the peak of the curved section  420 B 3  points toward −z direction. The rod acts as a differential mold with a concave surface to the underside of the material. Note that when the angle between the marker  420 B 4  and the +z direction is 0 or 180 degree, the rod has maximal curvature in the z-direction, convex or concave. 
         [0087]      FIG. 16( d )  depicts a general case when the rod rotates to an angle θ with respect to z-direction. To a dispensed material passing over the upper surface of the curved section of the rod in transverse direction, i.e. along y-direction, the curvature it experienced can be obtained from the projection of the rod curve  1602  onto the x-z plane  1605 , as shown as curve  1604 . The angle of rotation θ thus controls the curvature of the differential mold. The rotation can be actuated through a suitable mechanism such as a rack and pinion system (not shown). This differential mold can be applied to the top surface or to under-surface of the dispensed material. 
         [0088]    In order to control dimension of width of the dispensed material, the differential mold can include edge shaping features.  FIGS. 17( a ), ( b ) and ( c )  illustrate a few examples of differential molds having a small perpendicular edge ( 4201 ,  420 A 21  or  420 B 21 ) relative to the differential molding surface. This small vertical edge  4201  can restrict the material  452  in the width direction during solidification, when the main differential mold surface is on the top of or under the material. When forming thin structures of an object, such as the blades of  FIG. 1 , the edges can be shaped and trimmed as a final step after the main portion of a blade is formed almost to the final dimension. The edges can then be formed with the help of an edge shaping mold that shapes only the edges but not the main surfaces of the blade.  FIG. 17( d )-( e )  illustrates one such example of edge shaping differential mold. The end  420 C 1  has a step structure that can be applied to the underside of edge of a structure, while the opposite end  420 C 2  has a similar but upside down step structure, which can be applied to the topside of edge of a structure. By rotating the device about axis  1601 , the two opposite ends can be selected as needed.  FIG. 17( f )-( g )  illustrates another example of edge shaping differential mold. This example has a short post  420 D 1  eccentrically attached to a shaft of larger diameter  420 D 2 . The joining area  420 D 21  forms edge shaping surfaces. Rotation of the shaft  420 D 2  can place the short post at bottom, as shown in  FIG. 17( f ) , or at top, as shown in  FIG. 17( g ) , or at any other angle relative to the center axis  1602 . 
         [0089]    In general, materials used for making the material cell  404  can also be used to make the differential mold, especially those made of non-deformable members. For examples, ceramics, carbon and their composites can be used to make the curved bars of  FIGS. 16 and 17 . Ceramics can be used for metals of high melting points, such as steels, titanium, cobalt, chromium and nickel alloys. For differential molds based on deformable members, graphite sheets and metals can be used. Steels can be used for molten copper. Steels coated with Nitralloy can be sued for aluminum alloys. Aluminum can be used with tin. For handling molten steels, except graphite, metals of melting point higher than steel may also be used, such as refractory metal tungsten and molybdenum. These metals do form alloys with iron at elevated temperature. To avoid this, the metal surface can be coated with a thin layer of alumina by plasma spray technique, so that the alumina layer shields the metal base from direct contact with the molten steel. In metal coating industry, alumina coated refractory metal foils as thin as 0.01 mm have been used as “alumina coated boats” as evaporation sources. (For example, see products of Megatech of Cannock. Staffordshire, England, http://www.megatechlimited.co.uk/29-alumina-coated-boats). Such a thin foil can also provide the deformability required for curvature adjustment. 
         [0090]    The differential mold can also be based on the principle of fluid flow and pressure. The basic concept is to apply multiple channels of gas jets over the dispensed molten metal in the solidification zone and shape the surface of the molten metal by adjusting the flow speeds and pressures of different channels. An example system is depicted in perspective view in  FIG. 18( a )  and in cross-sectional view (sectioned along the width direction of the dispensed material) in  FIGS. 18( b ) and ( c ) . The system includes a bundle of small diameter tubing  420 E 1 . The exhaust end of the tubing bundle forms an array of gas outlets  420 E 2 , which is to be placed over the top of the dispensed molten materials in the solidification zone. The Inlets end of the tubing connects to a manifold  420 E 3 , which is supplied with an inert gas from inlet piping  420 E 4 . In the manifold, each tube in the bundle is connected to a different controllable flow restricting device. For examples, tube E 1   a  is connected to restricting device E 3   a , and tube E 1   b  to device E 3   b  etc. The flow restricting device can be controlled through a mechanical or electro-mechanical mechanism, such as a piezo-electric actuator, so that the flow rate in the corresponding tube can be adjusted. When the array of gas outlets  420 E 2  is placed slightly off the top surface of the dispensed molten materials  452  in the solidification zone, the gas flow pushes the metal surface and forms a small gap. A higher flow rate in a tube will result in a higher pressure, and a larger gap, between the corresponding tube outlet and the molten metal surface and a larger gap. A lower flow rate will have a reverse effect. By adjusting different flow restricting devices, different flow rates in different tubes can be generated and different pressures can be provided over different parts of the molten metal surface, thereby shaping its cross-sectional profile. For example, in  FIG. 18( b ) , flow rates in the central tubes, such as E 1   c , are higher than those in side tubes, E 1   a  and E 1   e , resulting in higher pressure in the middle of the molten metal E 2   c . Thus, the metal surface is pushed down, forming a concave shape.  FIG. 18( c )  illustrates an example of reversed situation, wherein a convex profile is formed by supplying higher flow rate in side tubes, D 2   a  and D 2   e , than in central tubes. Argon, CO 2  or other inert gas can be used. One advantage of this gas flow system is that the molten metal does not touch the solid part of the differential mold. Therefore, the tubing can be made from various kinds of metals. 
         [0091]    When the size of dispensed molten metal is small, the effect of surface tension could surpass the effect of gravity and could cause problem in material dispensing and in the performance of the differential molds. In this case, the solutions include applying a pressure at the upstream of the molten metal (by a gas pressure or by a piston effect such as the one shown in  FIG. 12 ) to “squeeze” the molten metal out and using upper and lower and even edge shaping differential molds simultaneously to confine the molten metal as it solidifies. 
         [0092]      FIG. 19  depicts an example design of the mounting and retracting mechanism of the differential molds system, with respect to the dispensing head  403 , which moves toward +x direction (right) when it dispenses materials. The upper differential mold  420   a  and the lower differential mold  420   b  are connected to two arms ( 422   a ,  422   b ) respectively. Arm  422   a  is mounted to a base  424  at axis  4291 . Arm  422   a  can rotate about axis  4291  and move the upper differential mold to operation position at  420   a  or to resting position at  420   ar . Similarly, arm  422   b  can rotate about axis  4292  and move the lower differential mold to operation position at  420   b  or to resting position at  420   br . Thus, the differential molds can be engaged or disengaged according to need. As described previously, the differential molds move together with the dispensing head relative to the dispensed material. In the case when the dispensing head needs to rotate about z-direction, the orientation of the differential molds can be adjusted accordingly by rotating the base  424  about z-direction, for example, along a track  4295  (rotary mechanism not shown). 
         [0093]    The ambient condition control unit includes means for controlling the temperature of the atmosphere surrounding the material dispensing area, such as using an air conditioner or a fan, if necessary. It can also include means for controlling the temperature of the base or members onto which the dispensed material attaches. Such examples include passing cooling or heated fluids through internal passages in the base to control its temperature. Electric heat or heated air or inert gas can also be used over the material dispensing area or over the whole workpiece and its base. Whenever needed, the temperature of the differential molds can also be controlled by similar means. These means for controlling temperatures of the ambient, the base and the differential molds apply to different solidifiable materials, not limited to molten metal. 
       Example 2 
     Plastics (Polymer Material) as Solidifiable Material, 3D Freeform by an Extrusion-Based Means 
     2.1 the Material Dispensing System 
       [0094]    The material dispensing system is similar to the system of  FIG. 8 . Metals, such as aluminum, copper or steel, can be used for the material supply unit  402  and the dispensing head  403 . The heating means can heat up the metal and then the metal can heat up the plastic material. Inert gas protection is generally not needed. 
       2.2 Means for Changing the Width of Dispensed Materials 
       [0095]    The means of changing the width of dispensed materials by using a dispensing head of adjustable exit size and the corresponding basic mechanisms, as depicted in  FIGS. 11 and 12 , also apply to plastic materials. However, due to differences in properties between plastics and metals, the internal shapes of the material supplying unit and the dispensing head are different. 
         [0096]    In the case of metals, for example in  FIG. 12( a ) , the flow speed of the molten metal  480  along the supply duct  402 B 1  in the material supply unit  402 B could decelerate at entering the material cell  404 B 10 , which has a cross-sectional area larger than that of the supply duct. But as long as the flow volume rate is kept constant along the flow path, the material cell can still be kept fully flooded and material dispensing rate can be maintained. That is, a single solid wire  470  acting as a piston to its melt in a supply duct of fixed cross-sectional area can provide various volume flow rates, by varying feed rate, to satisfy the need of dispensing of materials of different sizes (widths). 
         [0097]    However, in the case of extrusion of polymers, melt must be accelerated steadily and there should be no dead spots (stagnation zones) along the flow path, according to know-how from traditional bulk extrusion processes (see W. Michaeli. Extrusion Dies for Plastics and Rubber, 2nd ed., Hanser, Munich, 1992, p. 190, which is incorporated herein for this current invention by reference). Therefore, shapes of internal duct, cell space and gating members as well as gating mechanism should be designed to have (1) continuous lines without steps or jumps and (2) always decreasing cross-sectional areas along the flow path, even between parts having relative movements. Two example designs are described below. 
         [0098]      FIG. 20  depicts a first preferred system of the material supply unit and the dispensing head for polymer dispensing by extrusion.  FIG. 20( a )  shows cross-sectional views and  FIG. 20( b )  shows an exploded view. A wire of solid material  470  is fed into the material supply unit  402 D via a duct  402 D 1  and is heated to become melt  480 . The dispensing head includes a main body  404 D 1 , a cover part  404 D 2  and a side-slab  404 D 3  for adjusting the size of the exit  405 . The melt flows through the cell space (melt chamber)  404 D 10  and exits at exit  405 . The cell space is confined at left by the side-slab, which can rotate about an axis  2004  ( 2004   a ) to open up (e.g. position  404 D 3   a ) or close down (e.g. position  404 D 3   c ) the exit. The side-slab includes a shaft structure  404 D 31  that sliding fits into a bore  404 D 11  on the main body to allow such rotation. The cover part  404 D 2  has a raised structure  404 D 21  that, when the system is assembled, forms the space for the cell space and the space for motion of the side-slab. The leading edge of the raised structure at  2005  touches the top edge of the side-slab at  2006 , forming a contacting line and a mechanical seal so that the melt will be confined in the cell space and will not flow to the back of the side-slab. This contacting line  2004  is made to locate right on the rotation axis  2004   a  of the side-slab mechanism, so that rotation of the side-slab does not change its location nor affect the sealing. This way, the flow lines of the melt are always continuous without steps or jumps regardless of angle of the side-slab. Further, the cross-section of the cell space is made to be always decreasing by decreasing cell gap sizes along the flow path, as illustrated in sectional views of A-A ( 2001 ), B-B ( 2002 ) and C-C ( 2003 ). The thickness of the side slab, which moves inside the gap, also varies along the flow direction accordingly. 
         [0099]      FIG. 21  depicts another preferred system of the material supply unit and the dispensing head for polymer dispensing by extrusion, which can dispense large amount of material whenever needed.  FIG. 21( a )  shows an exploded view of the material supply unit  402 C (showing a cross-section without showing its opposing halt) and the dispensing head  404 C.  FIG. 21( b )  illustrates the system assembled together. The dispensing head includes 2 parts,  404 C 1  and  404 C 2 . Part  404 C 2  can slide relative to  404 C 1  to open and close the exit  405 . Part  404 C 2  includes a curved feature  404 C 21  as one internal surface of the material cell  404 C 10 , so that the cell has a shrinking cross-section along the flow path. In the material supply unit, the material duct  402 C 1  can take multiple solid wires ( 470   a - d ) aligned in an array, as shown in  FIG. 21( b ) . When part  404 C 2  opens to maximal position, the top of the curved feature  404 C 22  aligns to the edge of the lower end of the material duct  402 C 11 , such that the curvature is generally continuous. In addition, the curves at  404 C 22  and at  402 C 1  are made to approach the top surface of the dispensing head at  404 C 23  in a near asymptotic fashion so that when part  404 C 2  closes to a smaller exit size, as shown in  FIG. 20( b ) , the flow direction of the melt does not change abruptly. As a result, the system has a combined internal shape of always decreasing cross-sectional area along the flow path and the dispensing head can still change its exit size. 
         [0100]    Solid wires ( 470   a - d ) can be fed into the material supply unit by using a set of rollers or gears ( 510   a - d ,  511   a ). These rollers can be controlled independently so that material feed rate can be controlled to match dispensing volume rate. When the exit is opened to the maximal size, all solid wires can be fed at the same time to supply the required large volume flow rate. When the exit is closed down to the smallest size, only one solid wire needs to be fed to supply the minimal dispensing rate. 
         [0101]    By using adjustable exit described above, this invention can change material deposition rate and width as required by local geometry. The design of stacked multiple gating members of  FIG. 27  and design of the recess feature and nozzle structure of  FIG. 28  can also be applied. When the exit is closed down to minimal size, the system becomes basically similar to the FDM technique. In addition, the methods of changing the width of dispensed materials by adjusting the orientation of the dispensing head as depicted in  FIGS. 9 and 10  apply to plastics as well. The design and operation of the differential molds, as depicted in  FIGS. 14-19 , also apply to plastic materials. 
         [0102]    One issue in forming and shaping of polymer material is the so called die swell effect, which involves non-linear scale change when the mechanical boundary conditions around a polymer melt change. When the melt leaves the exit of the dispensing head, the material will expand and will not maintain the cross-sectional shape of the exit. To correct such dimensional change, the exit shape can be designed in anticipation of the die swell effect by referring to experiences and data form traditional polymer extrusion process, for example in the reference book by W. Michaeli,  Extrusion Dies for Plastics and Rubber , New York: Oxford University Press, or by using polymer processing simulation software. 
       Example 3 
     Molten Metal as Solidifiable Material, 3D Freeform by a Casting-Based Means, with Auxiliary on-Spot Heating 
       [0103]    If the temperature of the base material, which the solidifiable material to be dispensed onto, is too low or its heat sink is too large, the molten metal from the dispensing head may not be able to heat up the previously solidified material within a short time. In this case, the newly dispensed metal could solidify prematurely without good bonding to the base material. To resolve this issue, an auxiliary heating can be applied on spot, i.e. at the targeted dispensing location, to locally preheat the base material. The auxiliary heating source should be able to deliver concentrated heat in a relatively short time. Such a heating source can be constructed based on the principle of a few industrial fusion welding systems, such as gas tungsten-arc welding, plasma-arc welding, or laser welding.  FIG. 22  depicts an example system of this invention with an auxiliary arc-based heating unit.  801  indicates an auxiliary heating unit based on the principle of plasma-arc welding torch.  802  is a tungsten electrode connected as negative electrode. Plasma gas  899  flows through an inner passage  804 , whereas shielding gas flows through an outside passage  803 . The housing of the dispensing head  403  is connected as positive electrode. As a result, the arc forms mainly in the space  810  between the tungsten electrode and the front end of the dispensing head housing  403 . However, high temperature plasma gas can still jet down to reach the target area  514   a , rather similar to the case of non-transferred plasma welding scenario. Because plasma arc could generate very high temperature at the core of the plasma gas, a high temperature material, such as graphite, should be used at the outside of the dispensing head housing as the positive electrode. Proper regulation and control small current pulses can generate just enough concentrate heat to heat up the target area. This arrangement allows the arc heating unit to be placed at a distance away from the workpiece (target area) with enough space for the dispensing head tip and the differential mold  420   a.    
       Example 4 
     Plastics (Polymer Material) as Solidifiable Material, 3D Freeform by an Injection-Based Means 
       [0104]    Polymers in fluid state can be dispensed by methods other than extrusion. For example, the principle of a plastic injection molding machine, more specifically the injection-molding screw mechanism, can be applied to convert solid polymer pellets into melt. Such a mechanism is described in, for example, E. Lokensgard,  Industrial Plastics: Theory and Applications,  5th ed., Delmar, Clifton Park, N.Y. 2010, p. 155-159, which is herein incorporated into this invention by reference. 
       Example 5 
     Molten Metal as Solidifiable Material, 3D Freeform by an Injection-Based Means 
       [0105]    Molten metals, especially those of lower melting points, can also be dispensed by injection, in analogy to metal injections applied in die casting. Another method to dispense molten metal is to shoot metal droplets. M. Orme and R. F. Smith, “Enhanced Aluminum Properties by Means of Precise Droplet Deposition”,  Journal of Manufacturing Science and Engineering , August 2000, vol. 122, p. 484-493 describes such a system for shooting aluminum droplets in details, which is incorporated to this current invention by reference. 
       Example 6 
     Making a 3D Article 
       [0106]      FIG. 23  illustrates the process of making a 3D article, using the 3D part shown in  FIG. 1  as an example. For simplicity and clarity, it is assumed that the base  501   a  and the part do not move, whereas the material dispensing system  401  moves. From  FIG. 23( a )  to  FIG. 23( c ) , the material dispensing system dispenses materials on the base to make the interior of the hub. From  FIG. 23( d )  to  FIG. 23( e ) , the external surface of the hub is made. In  FIG. 23( f ) , the roots  603  of the blades are made. This step is similar to the process of  FIG. 10 . The roots serve as starting structure for blade making in the next step. In  FIG. 23( g ) , blades are made. Material dispensing starts from the roots  603  and moves away from the hub. Depending on material and thickness, dispensed suspending component geometries could have enough stiffness to maintain their shapes without the need of external supports. Whenever external supports are needed, they can be pre-fabricated by the same process before the suspending portions of the part are made. For example,  610  indicates an external support frame attached to the base  501   a  and the fixture frame  430  on the base, together with short support posts  605  connected to it. When the edge portion  601  of the blade is formed, material is dispensed to pass by and join with the short posts, so as to be supported. When the internal bands are formed, such as  602 , they can use adjacent previously dispensed and solidified portion as support. External supports can also be formed under the suspending structure, as what is usually done in the existing FDM process. 
       Example 7 
     Making Mold Inserts, Especially Seamless Mold Inserts with Conformal Cooling Passages 
       [0107]    By applying the material dispensing process of this invention, a new method of making 3D article, especially metal molds, featuring combined additive and subtractive processes can be devised. The so called subtractive process is producing shapes by removing materials from a stock. The so called additive process is adding materials by this current flexible 3D freeform method. This new method is especially suitable for making seamless mold inserts with conformal cooling passages. This method can be called “flexible mold surface forming technique”. 
         [0108]      FIG. 24  illustrates the first example of making a mold insert by the flexible mold surface forming technique. A mold core  2401  is first produced by machining, as shown in cross-sectional view in  FIG. 24( a ) . Its surface profile  2402  is close to but slightly smaller than the final mold profile. A system of connected ditches (recess structure)  2403  is then made into the surface by machining. A set of holes ( 2404   a - d ) are then drilled to connect to two ends of the ditch system.  2410  illustrates a portion of the mold core surface  2402  and the ditch  2403  in enlarged perspective view. In the next step, as illustrated in  FIG. 24( b ) , the material dispensing process of this invention is applied to cover the mold core surface with a layer of material.  2411  shows a local scenario where the dispensed and solidified material ( 2408 , dashed lines) covers the original mold core surface  2402  as well as the ditch  2403 . The dispensed material  2408  becomes the new mold surface, covering all ditches. Therefore, the ditch system now becomes internal, conformal passages for cooling fluid. In the completed mold core, cooling fluid enters the core from inlet  2405  and flows into the covered ditch system through holes  2404   a - b . The cooling fluid then flows in the covered ditch system  2403  and circulates through the core right underneath the mold surface and then, through holes  2404   c - d , out of the outlet at  2406 . 
         [0109]      FIG. 25  illustrates the second example of making a mold insert by the flexible mold surface forming technique. The mold core  2401  is shown in cross-sectional view.  2413  depicts part of the mold core surface and the surface cooling duct (ditch) in enlarged perspective view. In comparison with  FIG. 24 , the surface of the mold core  2402  is machined to a dimension very close to the final mold surface, leaving only allowance for final surface polishing. The spiral ditch structure around the core surface includes an additional recessed step feature  2403   a  along both banks of the ditch  2403 . In the step of material dispensing, the solidifiable material is dispensed over the recess feature  2403   a  to cover up the spiral ditch but not the core surface  2402 . In this way, much less material is needed and faster processing can be achieved. When the ditch is wide, in order to prevent dispensed material from falling into the ditch, especially in the case when materials in particulate form are dispensed, a lower differential mold  420   b  can be applied under the dispensing head  403  and positioned between the two banks of the ditch at the level of the recessed step. The differential mold can thus block the opening of the ditch under the dispensing head so that the dispensed material flows to the recessed step but not into the ditch. This is illustrated in  FIG. 25 ( b ) . The lower differential mold  420   b  can be a separate device  420  or can be a part of the dispensing head, as depicted in  FIG. 25 ( b ) . This method can also be applied to the case of  FIG. 24 . 
       Example 8 
     Making Seamless Mold Cavity 
       [0110]    This invention can also be applied to make seamless mold cavity.  FIG. 26( a )  depicts a cross-sectional view of a mold insert  2601  with the mold cavity  2602  and cooling passages  2603 . Such geometry usually requires a slow process of die sinking using electric discharge machining (EDM) to make.  FIG. 26( b )  illustrates a cross-sectional view of such a geometry made by using the current invention. The process starts with a starting block  2610 , which can be made by machining. Then a shell of the inner surface  2611  is made by using the current invention.  FIG. 26( c )-( e )  depicts the process of making the inner shell  2611 . After the inner shell is made, additional layers of materials  2612  are added to the exterior as reinforcement. Internal cooling passages  2403   a  can also be formed by leaving grooves and then covering them during the buildup of the layers of materials  2612 . 
         [0111]    Various fillers for tool steels can be used as the solidifiable materials for making mold inserts and information can found from publications such as  Tool Steel Filler Metal Characteristics TIC Welding  from http://www.stood.ind.com/Catalogs/FISC/05catpg394.pdf, and  Welding - Tool - Steel:Difficult but Rewarding Task. Solutions with Effective, Practical Advice  from http://www.welding-advisers.com/Welding-Tool-Steel.html, both documents are incorporated herein for this current invention by reference. 
         [0112]    In general, a mold is a tool. Other tools, such as cutting tools or cutting tool holders, with complex Internal cooling passages can also be made by the similar methods described in examples 7 and 8.