Abstract:
In the case of a method and an apparatus, three-dimensional objects ( 16 ) can be produced from a solidifiable material by the sequential discharging of discontinuous drops ( 15 ). To this end, the solidifiable material is plasticized in the fluid phase and is introduced into a material store ( 12 ) having at least one discharging unit ( 13 ) which can be clocked. From there, the material is discharged in a dropwise manner by means of a discharging unit ( 13 ) in the direction of an object carrier ( 14 ) for an object ( 16 ), wherein the object carrier ( 14 ) and an outlet opening can be moved at a relative spacing in relation to one another in space in order to influence the drop shape. The creation of the drops is supported by changing the relative spacing in an alternating manner in opposite directions during the discharging of the drops from the discharging unit ( 13 ) and during the application of the drops to the three-dimensional object ( 16 ) during the production of the object.

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims the priority from German patent application 10 2010 044 268.2, filed on 02 Sep. 2010, the disclosure content of which is hereby incorporated by reference in its entirety. 
       TECHNICAL FIELD 
       [0002]    The invention relates to a method and an apparatus for producing a three-dimensional object from a solidifiable material according to the preamble of claims  1  and  6 . 
       RELATED ART 
       [0003]    In plastics parts manufacture, injection molding or extrusion are used to produce parts in large batch sizes and production runs. The advantage of plastics injection molding in particular resides in the highly accurate production of complex part geometries, the functional versatility of the injection molding method optimally satisfying the requirement for an inexpensive and economically viable production of plastics parts. 
         [0004]    On the other hand, there is ever greater demand for plastics parts which are produced in very small numbers of units, or even only as single units, up to small batch sizes, such as for example samples which must be provided very rapidly and with properties which are similar to those of injection-molded parts. Manufacturing processes for producing such parts are known widely as “prototyping”. Such parts are in most cases produced by generating a geometry on the basis of 3D data. These geometries are produced in a wide variety of forms using appropriate means such as fusing powder layers by inputting heat, for example by means of a laser, generative systems such as printing processes, the powder particles being bound in different ways, or indeed using “melt strand” methods. 
         [0005]    EP 1 886 793 A1, on which the precharacterising clause of claim  1  is based, discloses an apparatus in which a plasticizing unit known in principle in injection molding technology is coupled to a pressurisable material storage means for the fluid phase of a material. In order to produce an object on an object carrier, said material is discharged via a discharge opening of a discharge unit in the form of discontinuous droplets. Due to the adhesive forces of the material, an elevated pressure and melting temperatures for the material are necessary for this purpose, especially since the droplets are intended to have a size of 0.01 to 0.5 mm 3 . Said apparatus already comprises control means for the object carrier for movement in the x, y and also the z directions relative to the discharge unit. When setting up the apparatus, the spacing between the discharge unit and object carrier is selected such that the droplets can form a free flying droplet over their trajectory. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    On the basis of this prior art, the problem underlying the present invention is that of providing a method and an apparatus for producing a three-dimensional object using solidifiable materials which promote droplet generation. 
         [0007]    Said problem is solved by a method having the features of claim  1  and by an apparatus having the features of claim  6 . 
         [0008]    It has been demonstrated in practice that only to a limited extent does the spacing between discharge unit and object carrier or the three-dimensional object to be created thereon affect the ability of a free flying droplet to form. While this is indeed helpful, droplet formation and also the shape of the resultant object may, however, be improved in that the relative spacing between object carrier or three-dimensional object, on the one hand, and outlet opening, on the other hand, is varied alternately in opposite directions on discharge of the droplets and on application of the droplets. For instance, to produce discontinuous droplets, droplet detachment may preferably be influenced in that, after impingement of the droplet on the object, detachment is accelerated by increasing the relative spacing. By an acceleration opposite to the direction of gravity, it is likewise preferably possible to influence the distribution of the droplet on the object such that cavities are better filled and the droplet is flattened. It is likewise advantageous if, prior to triggering of the next droplet, the discharge unit briefly comes into contact with the droplet in order, so to speak, to press down or geometrically stabilize the latter, so contributing to even application. 
         [0009]    With suitable control means, the apparatus may thus be used so efficiently that, even with minuscule droplets, it is possible to produce a high-quality three-dimensional object, such as a prototype, from the 3D data. The stated interrelationships or spacings are to be taken into account in order to obtain the corresponding structures and geometries. 
         [0010]    Further advantages are revealed by the subclaims and the following description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The invention is explained in greater detail below with reference to an exemplary embodiment illustrated in the Figures, in which: 
           [0012]      FIG. 1  is a schematic three-dimensional representation of object carrier and discharge unit, 
           [0013]      FIGS. 2   a - 2   f  show a schematic sequence during application of a droplet onto the object carrier. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0014]    The invention will now be explained in greater detail by way of example with reference to the appended drawings. However, the exemplary embodiments are merely examples, and are not intended to restrict the inventive concept to a specific arrangement. Before the invention is described in detail, it should be pointed out that it is not limited to the particular components of the apparatus and the particular method steps, since these components and methods may vary. The terms used herein are merely intended to describe particular embodiments and are not used in a limiting manner. In addition, where the description or the claims uses the singular or indefinite article, this also covers a plurality of said elements, providing that the overall context does not unambiguously indicate otherwise. 
         [0015]      FIG. 1  shows an apparatus for producing a three-dimensional object  16  from a solidifiable material which is either in the fluid phase from the outset or may be liquefied. The three-dimensional object is built up by sequential application of discontinuous droplets  15  from a cyclable discharge unit. The object  16  is created by the droplets  15  layer by layer on an object carrier  14 . By using control means  18  with the assistance of associated actuators (not shown in the drawings), the object carrier  14  may be displaced in the x, y and z directions. The control means additionally drive the discharge unit  13 . In this manner, it is inter alia also possible to influence the relative spacing s ( FIGS. 2   c ,  2   f ) between the object carrier  14  or three-dimensional object  16 , on the one hand, and the outlet opening  20  of the discharge unit  13 , on the other hand, relative to one another in order to influence droplet shape. In other words, the object carrier  14  or object may be moved towards or away from the outlet opening and/or the outlet opening  20  may be moved towards or away from the object carrier, in order to vary the relative spacing. 
         [0016]    The discharge unit  13  is part of a plasticizing unit  11  which is known per se in principle from injection molding technology, which plasticizing unit at the same time also comprises the pressurisable material storage means  12  for introducing the fluid phase into the material storage means. The pressure on the fluid phase in the material storage means  12  generates the discontinuous droplets  15  in directly coupled manner. This is described in greater detail in EP 1 886 793 A1. 
         [0017]    The solidifiable material is a plasticized material, such as for example silicone, or a plastic sable material such as a plastics material or also pulverulent materials, the essential feature being that the solidifiable material is either in the fluid phase from the outset or may be liquefied. The material may also be a material which can be melted reversibly when exposed to heat and is thus recyclable. It is thus conceivable for the material to be converted into the fluid phase by melting and to resolidify after dispensing, or for it to be provided in the fluid phase and to require solidification after dispensing, for example by energy input, or for it to comprise volatile constituents, the volatilization of which brings about curing. Any other materials may be used, provided that said materials can be plasticized by the apparatus and above all can be discharged by the at least one discharge unit  13 . 
         [0018]    In the fluid phase, the solidifiable material exhibits a so-called frontal laminar flow. Frontal laminar flow inter alia involves the melt depositing itself on the wall. This becomes clearest from an examination of knowledge arising from injection molding technology. When filling a simple, rectangular mound channel, the melt is injected via a so-called gate and begins to propagate circularly from this point with continuous flow fronts until it fills the entire width of the cavity. Some time thereafter, the region between the inlet and flow front may be considered to be virtually formed. Particular flow conditions, the “frontal laminar flow”, prevail at the flow front itself, as the flow lines in this region have the appearance of a source when observed relative to a system of coordinates which moves together with the front. The melt flows between two rapidly solidified layers of melt located close to the surfaces of the cavity, the melt progressing towards the flow front at higher velocities in the middle of the cavity. Shortly before the melt reaches the flow front, its velocity component in the direction of flow declines, and it flows obliquely to the wall until it lies against the wall. 
         [0019]    On the one hand, due to its laminar nature, frontal laminar flow is advantageous for producing ‘oriented’ droplets  15  on the object carrier  14 , while on the other hand precisely those problems which complicate implementation using devices and materials known from injection molding technology also arise here in particular with the formation of small droplets. Wall adhesion means that the melts may only be converted with difficulty into droplets with desired small volumes, preferably in the range of less than or equal to 1 mm 3 , and with desired flight velocity while on the other hand an appropriately high viscosity of the material is precisely of significance for the formation of a suitable droplet shape of a discontinuous droplet. 
         [0020]    This also differentiates the materials used from the prior art waxes. Thanks to their viscosity, waxes may be dispensed using normal thermal printing or inkjet methods, i.e. by purely kinematic, pressure-less acceleration without a pressure difference of the molten droplet. The materials used here differ therefrom not least in that their viscosity number is higher by one or more powers of ten. The dynamic viscosity number of the solidifiable material is accordingly between 100 and 10000 [Pa·s], the solidifiable material preferably being a plastics material or resin which is conventional in injection molding technology. This makes it necessary to carry out processing from a pressurisable material storage means  12 , since pressures of more than 10 to 100 MPa (100 to 1000 bar) are in any event necessary, in particular if small discharge openings  20  are used in order to achieve small droplet volumes. 
         [0021]    The desired volume of the droplet  15  is preferably in particular in the range from 0.01 to 0.5 mm 3 , preferably in the range from 0.05 to 0.3 mm 3  and particularly preferably in the range of around 0.1 mm 3 . The diameter of the discharge opening  20  is in particular less than or equal to 1 mm, preferably around 0.1 mm. At a wholly conventional injection velocity of 100 [cm/s], which delivers the melt through a so-called pin gate 0.1 [mm] in diameter, a value of 10,000 m/s is obtained for volume flow by area. This leads to a frontal laminar flow for the fluid phase with flow velocities of up to 10,000 m/s. 
         [0022]    With its discharge unit  13 , the apparatus has the task of discharging high-viscosity fluid materials, such as molten plastics materials, in minuscule quantities of down to a few micrograms from a material storage means  12  which is under elevated pressure and possibly a high melting temperature. The minuscule quantities/droplets  15  of the material are discharged in discrete individual portions, the size of which may be influenced by the apparatus. The discharged portions have such a high kinetic energy that they are capable of overcoming adhesive forces, detaching themselves from the apparatus and forming droplets  15  in order to build up the three-dimensional object  16  on the object carrier  14 . 
         [0023]    As these materials are liquid but of high-viscosity with high adhesive force and low weight, the kinetic energy is transferred by means of a pressure difference between the material storage means  12  and the “flight space” for the droplets  15 . Portioning is effected by means of a cyclically operated aperture which is for example provided with a nozzle needle or the like as closure means. As a result of the required dimensions of the portions and also of the viscosity characteristics, pressures in the range from 100 MPa (1000 bar) and above, closure apertures of smaller than 0.1 mm and furthermore closure times of less than 0.001 s are normally necessary. Since the materials are mainly plastics materials, melt temperatures of up to 450° C. prevail in the material storage means. 
         [0024]    The discontinuous droplets  15  must be able to form under these conditions while, on the other hand, relatively stringent quality requirements apply to the three-dimensional object  16  to be produced. Control means  18  therefore alternately influence as required in opposite directions the velocity and/or direction with which at least one of the elements comprising object carrier  14  or three-dimensional object  16 , on the one hand, and outlet opening  20 , on the other hand, are moved relative to one another with accompanying variation of the relative spacing s. Said influence is above all exerted on discharge of the droplets from the discharge unit  13  and on application of the droplets  15  onto the three-dimensional object  16 . 
         [0025]      FIGS. 2   a  to  2   f  show the associated method sequence. In order to apply the discontinuous droplets  15 , the solidifiable material is plasticized in the plasticizing unit  11  and discharged in droplet form from the material storage means  12  therein with the cyclable discharge unit  13 . According to  FIG. 2   a , the droplets  15  are discharged from the outlet opening  20  towards the object carrier  14 . Some droplets, from which the outline of the object  16  may be recognized, are already present on the object carrier  14 . The object carrier  14  or three-dimensional object  16 , on the one hand, and the outlet opening  20 , on the other hand, are movable in space relative to one another with accompanying variation of the relative spacing thereof in order to influence droplet shape. In comparison with  FIG. 2   a , in  FIG. 2   b  the droplet  15  has already emerged from the discharge unit  13  and is now in contact with the object carrier  14  or the three-dimensional object  16  which has already been built up thereon. As may be seen, the droplet  15  is at this moment connected both to the discharge unit  13  and to the object carrier  14 . The spacing between discharge unit and object carrier here roughly corresponds to the length I of the droplet on discharge. 
         [0026]    If, in the next step, the relative spacing s according to  FIG. 2   c  is increased in this case by a vertical downwards movement of the object carrier  14  (alternatively, the discharge unit  13  could also be moved upwards), this promotes detachment of the droplet  15  from the discharge unit  13 , which at the same times assists in allowing the next droplet to be discharged in a timely manner. 
         [0027]    After detachment of the droplet  15 , according to  FIG. 2   d  the object carrier  14  is moved vertically upwards in the direction of the arrow opposite to the direction of gravity. This acceleration opposite to the direction of gravity promotes the distribution of the droplet  15  on the object carrier  14  or the three-dimensional object  16  such that cavities are better filled and the droplet  15  is flattened. 
         [0028]    If that were not enough, in the next step according to  FIG. 2   e  the relative spacing s is reduced to the relative spacing s′, down to the height of application of a layer, i.e. reduced until the discharge unit  13  is in contact with the droplet located on the three-dimensional object  16 . This brief contact prior to triggering of the next droplet by the discharge unit on the three-dimensional object  16  leads to additional geometric stabilization and thus to even application. After this contact, the object carrier  14  and/or the discharge unit  13  can be displaced relative to one another in the direction of the horizontal arrow, before the next droplet is triggered according to  FIG. 2   f , before which the relative spacing s is increased again by, in the exemplary embodiment, a vertical movement of the object carrier. 
         [0029]    The alternating movement of the object carrier  14  or three-dimensional object  16 , on the one hand, and the discharge unit  13 , on the other hand, with accompanying variation of the relative spacing s between said elements in a substantially vertical direction, thus in or opposite to the direction of gravity, distinctly increases the quality of the object  16  to be produced. On the one hand, the droplet  15  is cleanly detached and, on the other hand, the applied droplet  15  is clearly solidified on the object. 
         [0030]    It goes without saying that the present description may be subjected to the most varied variations, changes and adaptations which are of the nature of equivalents to the appended claims.