Method of model construction

[0001] The invention relates to a method for creating a prototype, as defined in the preamble to claim 1.

[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 m³ 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.

[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.

[0007] The invention is explained in further detail in the following with the aid of drawings, which show in:

[0008] FIG. 1A device for realizing the method;

[0009] FIGS. 2a-d A first variant of the method;

[0010] FIGS. 3a-d A second variant of the method;

[0011] FIG. 4A mixing head for realizing the method in a first operating position;

[0012] FIG. 5A mixing head for realizing the method in a second operating position;

[0013] FIG. 6A detailed view of the mixing head.

[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 interlacedfor 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] FIG. 6 shows a design option for these nozzles 170. The mixing chamber housing 105 is provided with two recesses 105A, 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 105B in the hardened nozzle tips 105C 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.