Abstract:
A method for casting a plastic die including constructing an open ended container having a floor configured to the desired contour shape of the die. A plurality of elongated cores having tapered and arcuate peripheral edges are spaced within the container and secured in place by traversing members. A cavity is formed in the container therebetween for the injection of hardenable material.

Description:
FIELD OF THE INVENTION 
     The invention relates to an improved casting process to form a die having a multidirectional surface and in particular for a die for massive items such as aircraft wings. In particular, the invention provides a method of producing plastic dies for tooling suitable for use in forming sheet metal, aluminum or other metals, including a technique such as stretch forming, hydroforming, stamping etc. 
     BACKGROUND OF THE INVENTION 
     In the aeronautics industry, for example, the manufacturing of exterior surface portions of the wings and fuselage may included dimensional portions having lengths over 30 feet. As a result, the manufacturing process involves a number of expensive tooling operations. In addition, the massive dimensions needed in the preparation of a master die model for these exterior surface portions can be cumbersome and heavy. It is necessary therefore, that the master die model be easily movable or transportable to the manufacturing facility of the airplanes. It is also necessary to provide a master model die that is cost effective and eliminates some of the expensive tooling operations to produce the die. It is also necessary to produce a model die that is rigid yet relatively lightweight and easily transportable to the manufacturing point. 
     SUMMARY OF THE INVENTION 
     The improved process for making a plastic die according to the present invention includes constructing a wood lattice framework having a top surface in the desired contour shape. The lattice framework is built up on the sides of the wooden framework so that the contoured shape top surface forms the floor of the boxed-in model. The lateral rods function to secure wooden cores within the boxed-in model. A plurality of tapered wooden cores having arcuate corners are inserted into the wood model. The wooden cores may have different shapes and lengths to accommodate the various stress levels of the manufactured portion. Lateral rods are fed through apertures in the upper walls wooden cores to hold the wooden cores at predetermined levels above the contour floor. The cavity within the boxed-in model surrounding the wooden cores is filled with a resin material or other appropriate hardenable material. The exterior of the wood cores may be previously coated before insertion into the boxed-in model with a wax-type material so that the cores do not adhere to the resin and may be easily removed from the boxed-in model once the resin has solidified. A honeycombed surface is then provided by the removal of the cores after the resin has solidified, and the honeycomb surface forms a bottom surface for the die. The sides and the lattice framework can then be removed from the die so that the desired contour surface is exposed. The contour surface is finally machined to finish the contour to the precise die form. 
     It is the intent of this invention to provide an improved method of casting a plastic die for large structures such as those required in the aeronautics industry. 
     It is another object of this invention to provide a cost effective method of casting a plastic die for such structures. 
     It is further an object of this invention to provide a method of casting a plastic die that provides a relatively lightweight, yet strong die. 
     It is also an object of this invention to further improve the process and the resultant die as disclosed in U.S. Pat. No. 5,802,696 issued to the applicant by strengthening the overall finished die. 
     Other objects, advantages and applications of the present invention will become apparent to those skilled in the art when the following description of the best mode contemplated for practicing the invention is read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The description herein makes reference to the accompanying drawings wherein like reference numerals refer to like parts throughout the several views, and wherein: 
     FIG. 1 is a perspective view of the first stage of the die producing structure embodying the invention; 
     FIG. 2 is a perspective view showing a typical core used according to the present invention; 
     FIG. 3 is a view of sectional cores during the coating process; 
     FIG. 4 is a partial view of the placement of the sectional cores within the die producing structure; 
     FIG. 5 is a schematic view of plastic material being ejected into the die producing structure between the sectional core; 
     FIG. 6 is a perspective view of the solidified plastic die having a honeycomb surface within the die producing structure; 
     FIG. 7 is a partial view of the solidified plastic die during the machining process of the honeycomb surface; 
     FIG. 8 is a perspective view of the machining process of the contour surface of the plastic casting die; 
     FIG. 9 is a perspective view of the plastic casted die having drilled holes at intersections of the honeycomb surface; 
     FIG. 10 is a perspective view of a finished die constructed according the present invention; and 
     FIG. 11 is a sectional view of the finished die taken along lines  11 — 11  in FIG.  10 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The FIGS. 1-9 illustrate the steps for producing and casting a plastic die using a die producing structure  100  that is easily assembled and that provides a lightweight mold or die that is easily transportable. As seen in FIG. 1, a lattice framework  10  is initially constructed. The lattice structure  10  forms a base of the die producing structure  100  and has a bottom planar surface  12  and a top surface  14 . The top surface  14  is formed to essentially duplicate the contour surface of the desired shape for the final die. A series of vertical extensions  16  extend from the bottom surface  12  to the top surface  14 . The vertical extensions  16  have varying lengths to conform with the contoured top surface  14 . The lattice framework  10  is then enclosed or boxed in at all four sides with upwardly extending walls  18  such that the sides of the lattice framework  10  are entirely enclosed and the contoured top surface  14  now defines the floor of the die producing structure  100 . The die producing structure  100  has an open ended upper portion  20 . 
     A plurality of hollow wooden cores  30  are used for placement within the die producing structure  100  to define a honeycomb for injection of the hardenable material  38 . As shown in FIG. 2, each wooden core  30  is shaped into an elongated and tapered hollow box for placement within the die producing structure  100  in a vertical position. The wooden cores  30  are tapered such that the smaller tapered end  34  is inserted into the die producing structure  100 . The taper of the vertical walls  31  of the core  30  should be at least a 2° angle (α). A taper of 2° can result, for example, in a core having a length (L) of 6.0 foot and dimensions of 11.0 inches for each side wall A 1  and B 1  at open end  31  to have dimensions of 9.0 inches for each side wall A 2  and B 2  at the bottom closed end  34 . If a side wall A 1  or B 1  is 8.0 inches the side wall A 2  or B 2  will be 6.0 inches at end  34  of core  30 . Each core  30  is further shaped to eliminate angular corners along the peripheral edges. Instead, any angular corner  33  is modified to include an arcuate surface having at least a 1.0 inch radius, (R 1  and R 2 ). The arcuate surface R 2  of a corner at the tapered end  34  of the core may have a different radius than the arcuate surface R 1  of the top end of the core. The tapered walls and the arcuate corners of the outer peripheral edges help to eliminate a vacuum being created under the cores  30  when the cores  30  are being removed from the die producing structure  100 . In the prior art, the cores included elongated, rectangular hollow boxes. When a row of wooden cores were removed from the die in the prior art, the non-tapered vertical walls caused a vacuum between the bottom surface of the wooden core and the core surface which made the disengagement of the wood core  30  from the die producing structure  100  difficult. By changing the shape of the cores as described supra, the cores can be easily removed from the die producing structure  100 . More importantly, the tapered and rounded cores result in more resin located at the top surface  14  of the die producing structure to provide an overall stronger and more durable die. 
     As shown in FIGS. 3 and 4, each core  30  has through apertures  32  on opposing sides of the core  30  so that traverse rods  28  may be received through apertures  32  to extend through core  30 . Therefore, when the cores  30  are positioned within the die producing structure  100  a row of cores  30  may be arranged so that the apertures  32  are in linear alignment. The through apertures  32  are spaced at a predetermined height position above the bottom closed end  34  of the core  30 . Feeding the traverse rods  28  through the wooden cores  30  maintain the cores  30  in a stationary position. The bottom  34  of each core  30  is also maintained at predetermined heights above the contour floor  14  of the die producing structure  100 . The heights maintained above the contour floor  14  may vary depending upon the stress level that will be experienced upon a certain section of the die. 
     As shown in FIG. 3, each core  30  before being inserted into the die producing structure  100  is preferably partially coated with a microcrystalline material  36 , such as wax, to prevent the hardenable material  38  that is later injected into the die producing structure  100  from adhering to the exterior surfaces  40  of the cores  30 . As shown, the core  30  is not entirely coated with the micro crystalline material  36 . The core  30  is coated only below the through apertures  32 . 
     In addition, a selected number of the cores  30  may also include a preformed lower extension  42  added to the bottom  34  of the core  30  and shaped to conform to the variance of the contour floor  14  at a prescribed location of the core  30  within the die producing structure  100 . The lower extensions  42  on the selected cores are preferable made of a heat resistant material so that the hardenable material  38  does not penetrate the extensions  42  when injected into structure  100 . The lower extensions  42  added to the bottom  34  of a selected number of cores  30  allow the space between the bottom of the core  30  and the contour floor  14  of the die producing structure  100  to maintain the predetermined distance. The distance between the floor  14  and the cores  30  are determined by manufacturing requirements and can vary preferably from approximately four to six inches. 
     As shown in FIG. 4 the wooden cores  30  are placed within the die producing structure  100  such that the through apertures  32  on the cores  30  are positioned in alignment so that the traverse rods  28  may be received therethrough. The traverse rods  28  extend beyond the opposing upwardly extending walls  18 , therefore a row of wooden cores  30  are held at predetermined distances from the contour floor of the die producing structure  100  and held in position by the traversed rods  28 . The wooden cores  30  although preferably aligned so that the through apertures  34  therein form a line for receiving the traverse rods, the cores  30  need not be evenly spaced between each other, but should be spaced according to the stress levels that the resultant die wall will experience. For example, if a portion of a die for an aircraft wing is subject to high stress levels during the manufacture of the wing, less cores would be placed in that region so that more of the resin or other appropriate hardenable material will occupy that space. Further, the cores may be produced to have different shapes and lengths of vertical side walls to better accommodate the varying shapes of the die producing structure. 
     Once the die producing structure  100  is filled with the spaced wooden cores, a supply of resin, such as plastic, or other appropriate hardenable material  38  is injected into the cavity  44  of the die producing structure  100  formed around the cores  30 , as shown in FIG.  5 . The hardenable material  38  is allowed to solidify by conventional methods. Once the resin or hardenable material  38  has solidified, the traverse rods  28  are pulled from the wooden cores  30 . The wooden cores  30  can then be easily removed from the die producing structure  100  along with the longitudinal members  22 . The upwardly extending walls  18  of the die producing structure  100  are also removed so that the casted die  46  may be removed from the lattice framework  10 . The resultant die  46  provides a honeycomb back surface  48  and a contour front surface  50  that coincides with the desired contour of the die. The contour front surface  50  will have a thickness according to the stress levels that will be experienced at that portion of the die as a result of the spacing of the cores  30  from the floor  14  in the die producing structure  100 . This thickness will provide stability to the resultant die  46 . 
     The honeycomb surface  48  is then machined to remove jagged edges and burrs as shown in FIG.  7 . The contour front surface  50  is also machined by a five axis NC milling machine  51  that is computer  52  controlled to the precise measurements and contour of the die required. After the final die  46  is machined, apertures  54  are drilled at the cross sections of the honeycomb surface  48  of the die mold  46 . A solid base structure  56  may then be secured onto the honeycomb surface  48  via screws or bolts  58  drilled into the aperture  54  at the cross sections. Only a portion of a solid base structure  56  needs to be attached to the honeycomb surface in order to provide a sliding surface  56  for the base that does not damage the honeycomb surface  48 . 
     FIG. 10 shows a typical finished die  46  with the contour front surface  50  exposed for use. FIG. 11 shows a sectional view of FIG. 10 to show typical shapes of honeycomb configuration formed as a result of the cores  30 . The honeycomb walls  61  are tapered as the walls  61  approach contour surface  50 . The outer peripheral edges and corners  33  have an arcuate surface of at least a 1.0 inch radius. It is apparent from FIG. 11 that the cores  30  can be different shapes to provide honeycomb interiors  60  in the die  46  to accommodate the curvature of the contour front surface  50  as well as the stress levels of the die  46  at certain areas. It also can be seen that the number of cores  30  to manufacture a die will vary depending on the size of the fixed die. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiments but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, which scope is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures as is permitted under the law. It is understood that manufacturing requirements may include modification to the contour front surface  50  after machining. An example of such a requirement would be apertures for dowel or bolt locations in the contour surface  50 .