Abstract:
With a model for creating a prototype (rapid prototyping), two components (A, B) are mixed inside a mixer ( 12 ) of a traversing application head and the resulting model-building mass is ejected from the nozzle ( 14 ) of the application head. As a result, the prototype is constructed in layers. The two components react with each other and increase in volume, so that the layers of the prototype can be relatively large, approximately 1-5 cm (FIG. 1).

Description:
TECHNICAL FIELD  
         [0001]    The invention relates to a method for creating a prototype, as defined in the preamble to claim 1.  
         PRIOR ART  
         [0002]    A plurality of so-called rapid prototyping methods of this type is known, all of which use a synthetic material for constructing a prototype in layers (e.g. see magazine “INDUSTRIEANZEIGER” [Industry Advertising Journal] 47-48/97, pp 52-64).  
           [0003]    As a result of constantly decreasing product cycles, the rapid prototyping method increasingly gains in importance. The layer-type construction generally occurs fully automatic, wherein the control signals are gained from a 3-D CAD data set. Methods used so far are relatively slow and can be used only for small prototypes. Constructing prototypes with a volume in the order of magnitude starting at 1 m 3  is practically impossible. As a result, large-volume prototypes such as automobile prototypes on a scale of 1:1 must still be constructed primarily with mechanical methods and with a correspondingly high expenditure in materials and costs.  
         SUMMARY OF THE INVENTION  
         [0004]    Starting with the prior art, it is the object of the invention to create a prototyping method, which permits the construction of large-volume prototypes, which are created at least essentially automatic.  
           [0005]    This object is solved with a method having the features as defined in claim  1 .  
           [0006]    The prototype created with a method according to the invention is constructed in layers, using a synthetic material that increases in volume immediately prior to or following the application, or if applied with an application head, for example a PU (polyurethane) high-resistance foam. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0007]    The invention is explained in further detail in the following with the aid of drawings, which show in:  
         [0008]    [0008]FIG. 1A device for realizing the method;  
         [0009]    [0009]FIGS. 2 a - d  A first variant of the method;  
         [0010]    [0010]FIGS. 3 a - d  A second variant of the method;  
         [0011]    [0011]FIG. 4A mixing head for realizing the method in a first operating position;  
         [0012]    [0012]FIG. 5A mixing head for realizing the method in a second operating position;  
         [0013]    [0013]FIG. 6A detailed view of the mixing head. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0014]    [0014]FIG. 1 shows a device for realizing the method according to the invention. The device is provided with an application head attached to an overhead gantry  20 . The application head  10  consists of a mixer  12  with a nozzle  14 . A suitable mixer/nozzle unit is described in the following with reference to FIGS.  4  to  6 .  
         [0015]    Two components A and B, respectively located inside tanks  30 ,  40 , are initially conveyed with the two low-pressure conveying systems  36 ,  46  to the two high-pressure pumps  32 ,  42 . The high-pressure pumps  32 ,  42 , which are driven with the aid of two servomotors  34 ,  44  via electromagnetic linear units and which operate based on the double-action piston-pump principle, pump the components with a system pressure of 100-200 bar to the mixer  12 . The delivery pressure level is detected with the aid of pressure sensors and the values transmitted to the central computer. The pulsation in the connected lines is smoothed by means of nitrogen bubbles.  
         [0016]    The components are mixed inside the mixer  12  and exit through the nozzle  14 . The components A and B react chemically, which leads to a volume increase of the substance. The application head traverses all three spatial directions, so that a prototype is built up in layers. The foam formed with the two components A and B achieves its final volume so rapidly and develops such a high starting rigidity that for each passage of the application head a new layer can be deposited on the previously created foam layer. The thickness of each foam layer in this case is approximately between 1 and 5 cm. A central computer controls the complete system, wherein the control signals are generated from a CAD data set. It is important that the mixing head movement on its spatial curve is synchronized with the pumping capacity of the high-pressure pumps  32 ,  42 . For an exact control of the respective amounts to be pumped and thus also the mixing ratio of the two high-pressure pumps  32 ,  42 , it is recommended that the pumps be operated with servomotors  34 ,  44 . The characteristics of the foam can also be changed during the prototype production through an exact control of the mixing ratio.  
         [0017]    For example, the following operational parameters are possible:  
         [0018]    MDI (isocyanate) serves as component A and a polyol mixture with additives serves as component B. For example, the following formulation can be used:  
                                                                           Product name/Manufacturer   Weight shares                                        Component A                Desmodur VL/Bayer   134.50           Component B           Desmophen 250 U/Bayer   9.00           Desmophen 550 U/Bayer   35.50           EW-Pol 1100/III./Henkel   10.00           castor oil/Graf   20.00           DAMP Fyrol 6/Akzo   5.00           APPO Fyrol 51/Akzo   5.00           Exolit TP 622/Hoechst   12.00           Martinal ON-920/Martinswerke   26.00           Additive DT/Bayer   2.00           Dabco/Goldschmidt   6.00           Fomrez UL 1/Witco NRC.   0.80           Tegostab B 8408/Goldschmidt   1.20           Topanol O/ICI   2.00           Hostaflamm RP 602/Hoechst   6.00           Filler material: titanium dioxide (optional)   5-15           Filler material: zinc oxide (optional)   5-15                      
 
         [0019]    The actual chemical reaction then occurs in the mixer  12  with a potlife of approximately 10 seconds.  
         [0020]    For one particularly advantageous embodiment, the arrangement comprises a measuring system, which controls the form of the already produced prototype part. Through feedback to the central computer, deviations from the desired value can thus be corrected during subsequent passages, for example by changing the mixing ratio for components A and B or by changing the speed of the application head.  
         [0021]    In most application cases, the outside dimensions of a prototype, produced exclusively with the above-described method, are not exact enough. In addition, a smooth, hard surface that can be lacquered is frequently required, which cannot be produced with the foaming method. For that reason, two methods are suggested for the finishing work on the prototype.  
         [0022]    With the first method (see FIG. 2), the basic prototype is initially finished as undersized model (Figures a), b)). In a second step, a synthetic material that is generally a two-component plastic is deposited on the basic prototype with a second application head  50 . As a rule, this second application head  50  must operate with three linear and two rotational axes. Since it is probably difficult to deposit the synthetic material in such a way that an exact outside dimension results, it is recommended that the synthetic material be applied with excess dimensions and be cut down to the desired dimensions, following the curing of the synthetic material. A cutter head  60  used for this can also be CNC controlled and generally must operate with 5 axes. All operations can conceivably be realized with the same overhead gantry, wherein only the operating heads are replaced.  
         [0023]    With a second method (see FIG. 3), the basic prototype is first produced as oversized model. Subsequently, this basic prototype is cut down to the desired dimensions with a cutter head  60 . To obtain a hard surface, a two-component synthetic material is then deposited with a spraying head  70 .  
         [0024]    It is furthermore suggested that an intermediate layer be inserted between the layers of the prototype, for example an aluminum sheet or a foil. Intermediate layers of this type can be smooth, perforated or perforated and interlaced—for example a rib mesh. Individual sheet metal sheets/foils of this type are placed onto the top layer following each “passage” of the application head. This can be done by hand or by means of a robot. Intermediate layers of this type have the advantage that they can absorb tensile forces and thus can stabilize the prototype. Whether and how many such intermediate layers are necessary or desirable depends among other things on the size of the prototype and the material selection.  
         [0025]    A suitable application head is described in the following:  
         [0026]    A suitable mixing head is shown in the cross-sectional representation in FIG. 4. Mixer  12  and nozzle  14  in this case form a single structural. A mixing-chamber housing  105  has a cylindrical mixing chamber  100 , which is open toward the bottom and thus forms the nozzle  14 . The piston  150  is positioned so as to be axially displaceable inside the mixing chamber housing  105 . The hydraulic piston  160  that is positioned inside the hydraulic cylinder  120  effects the axial displacement of the piston  150 . The hydraulic piston  160  is rigidly connected to the hydraulic rod  165 , which caries the first ball bearing  168  on its lower end. The splined shaft  155  is held on the inner raceway for the ball bearing  168 , so that the hydraulic rod  165  and the splined shaft  155  are axially coupled, but are not connected with respect to the rotation around axis A-A. The splined shaft  155  penetrates the coupling element  140 . As a result, the coupling element  140  and the splined shaft  155  are connected for their rotational movement. The piston  150  that is arranged inside the mixing chamber  100  is attached to the end of the splined shaft  155 . The above-mentioned coupling element  140  is connected via the second ball bearing  145  to the bearing flange  110 , which in turn is rigidly flanged to the mixing chamber housing  105 . The coupling element  140  has an essentially symmetrical design with respect to the axis A-A and carries the gear rim  142  on the outside. The motor  135  can be used to drive the gear rim  142  and thus also the coupling element  140 . The gear wheel  136  is arranged on the shaft of motor  135 , which in turn is connected by means of the toothed belt  137  to the gear rim  142 . The motor  135  is flanged via the console  130  to the lantern  115  or the bearing flange  110 .  
         [0027]    The stirring rod  152 , which extends parallel to axis A-A inside the mixing chamber  100 , is arranged on the coupling element  140  and extends through the piston  150 .  
         [0028]    The two nozzles  170  are arranged inside the mixing chamber housing  105 . The viscous liquids to be mixed are pushed through these nozzles into the mixing chamber  100 . The two nozzles  170  are shown only schematically in FIGS. 4 and 5.  
         [0029]    [0029]FIG. 6 shows a design option for these nozzles  170 . The mixing chamber housing  105  is provided with two recesses  105 A, into which respectively one nozzle body  171  is inserted (shown is only one nozzle body  171  herein). Inside of the nozzle body  171 , the externally built-up high pressure (see above) is adjusted by means of an injection piston  172  and the liquid is pushed through the nozzle body opening  173  from the nozzle body  172 . Arrangements of this type are known in the technical field and will not be described further herein. The actual nozzle openings in this case are the exit bores  105 B in the hardened nozzle tips  105 C in mixing-chamber housing  105 . The nozzle bodies can also conceivably be extended up to the mixing chamber, so that the front of the nozzle body forms a component of the side wall of the mixing chamber. In that case, the nozzle-body opening and the exit bore in the nozzle tip would be identical.  
         [0030]    The principal mode of operation for the mixer is described in the following:  
         [0031]    The first operating position of the device is shown in FIG. 4. In that case, the piston  150  is located above the nozzle openings for nozzles  170 . In this position, the liquids to be mixed are injected through the nozzles  170  into the mixing chamber  100 . The injecting occurs normally under high pressure, meaning with injection pressures above 100 bar. During the injection operation, the coupling element  140  and thus also the piston  150  and the stirring rod  152  are rotated with the aid of motor  135 . Even highly viscous liquids can be mixed as a result of the rotation of piston  150  and the stirring rod  152 . The mixed-together liquids exit at the lower end of mixing chamber  100 .  
         [0032]    Following the completion of the mixing operation, the supply of the two liquids through the nozzles  170  is shut down. Subsequently, the piston  150  is pushed axially downward inside the mixing chamber  100  (see FIG. 5) through pressure applied by the hydraulic piston  160 . The remaining residues are thus pushed out of the mixing chamber  100  and the mixing chamber  100  is cleaned. At the same time, the stirring rod  152  is scraped off and thus cleaned. If the production is to be restarted, then piston  150  is pulled back to the position shown in FIG. 1 and the cycle can restart.  
         [0033]    The automatic cleaning function described herein ensures that the mixer/nozzle unit can be cleaned easily during each interruption in the production, for example for inserting an intermediate layer (see above), which is extremely important with quick-hardening PU foam, such as the one used for this example.