Optimized recursive foundry tooling fabrication method

#The present invention relates generally to metal casting methods, and more particularly to a method for efficiently producing a metal casting mold for a complex part by recursively identifying the cores for the casting and the molds for making the cores defining the complex part once a parting surface for the part is defined.

#In metal casting discrete mechanical parts using sand molds, patterns are used in fabrication of the molds to ensure that the resulting cast parts have the correct geometry, or can be readily finished to the correct geometry. The pattern is a model of a part to form a mold cavity substantially defining the part shape, but is not simply a facsimile of the part because additional shapes (sprues, runners, gates, etc) are used to form channels for inserting molten metal, or shape modifications to provide taper (draft) on some surfaces of the part to facilitate withdrawal of the part from the mold. The principal molding material conventionally used in foundries is silica sand, which, when mixed with water and a binder (e.g. clay), can be formed to a complex geometry which retains its shape while being filled with metal and allowed to cool. The mold is usually destroyed when the casting is removed and must be recreated using the pattern for each cast part to be produced.

#Mold design and fabrication are especially difficult if the cast part has sufficiently complex geometry or when the parting surface is defined such that the pattern cannot be withdrawn easily from the mold. In order to accommodate complex geometries by means of conventional casting methods, the pattern maker uses cores and loose pieces to ensure that those parts of the cavity which should be filled are filled. In standard practice, molds are often made up of two halves. The pattern is also made up of two parts mounted on the two sides of a board which represents the dividing (parting) surface (which may be more complex than a single plane) between the two mold halves. The two mold halves are formed by packing sand around each side of the pattern board, and subsequently combined to form the cavity left when the pattern is removed. The mold must therefore be made such that the pattern can be withdrawn from the mold. If the mold is made in two halves, each part of the pattern must be removable from the corresponding mold half. In order to define the casting pattern, the pattern maker modifies the pattern around the complex features of the part (to render it removable from the mold) using extra pieces of mold-like material, called cores or loose pieces, for filling extraneous spaces around the correct cavity shape for the cast part. The cores are generally made from bonded silica sand, and the molds used to make the cores, called core boxes, are permanent molds, usually made of wood or hardened epoxy.

#The invention will be more clearly understood from the following detailed description of representative embodiments thereof read in conjunction with the accompanying drawings wherein:

#FIGS. 1a-e show in a comprehensive fashion the foundry casting mold fabrication method of the invention with relation to a representative three dimensional part and associated parting surface, cores, core boxes and rigging which are defined in the practice of the invention;

#FIG. 2 is a block diagram of the steps of the method for constructing a mold pattern according to the invention;

#Referring now to the drawings, FIGs 1a-e show in comprehensive fashion an overview of the rapid foundry tooling system and fabrication method of the invention with reference to a representative complex three dimensional part 10 intended to be cast. In accordance with a principle feature of the invention, a plurality of shape features (in selected sizes and locations), including bosses, disks, slots, shafts, blends and other simple shapes are used to define the structure of part 10. Therefore, part 10 may be defined by cylinder 11, disk 12, boss 13, (half) cylinder 14, block 15 and slot 16. The structure of part 10 is first defined based on data representing size and shape of each consitituent feature entered into a CAD system. The structure of part 10, having been defined in terms of CAD system data, may then be displayed in any representative view on the CAD system display. Once the structure of part 10 is defined as just described, an initial parting surface 17 for casting part 10 is then selected, and the associated sprues, runners, gates, risers, cores, core boxes and mold are then iteratively generated in order to optimize the design of the resultant pattern, pattern board and mold. For example, with reference to FIG. 1b showing part 10 from below, the initial parting surface 17 indicated in FIG. 1a suggests volumes 18 and 19 of specified shapes as requiring cores in corresponding shapes and locations in a pattern for part 10. However, with reference to FIG. 1c, identification of volumes 18 and 19 in FIG. 1b indicate an appropriate new parting surface 17' (with parting surface offsets) which eliminate the necessity of cores for volumes 18,19. The automatic identification and generation of an appropriate offset parting surface 17', or other parting surface which results in minimum quantity and complexity of cores, is a critical feature of the invention. Once the offset parting line is generated, the invention specifies location of the appropriate rigging (sprues 20, gates 21, runners 22 and risers 23) for casting part 10 such as illustrated in FIG. 1d. Once the rigging for part 10 is specified, pattern board 25 (FIG. 1e) and mold configuration are automatically generated.

#Referring now to FIG. 2, shown therein is a block diagram of the method steps for constructing a mold pattern according to the invention. The listing of a representative computer program useful in executing the algorithm for constructing the mold pattern, including identification of required cores, in the practice of the method of the invention, and used in demonstration of the invention, is presented in Appendix A hereto. As suggested in FIG. 2, and with reference to the computer listing in Appendix A, the geometry of the part to be cast is first identified and defined in terms of CAD) system data, and an appropriate parting surface for optimum orientation of the part within the mold is generated as at 26. (See Computer Graphics Handbook Geometry and Mathematics, by Michael E. Morrison, Industrial Press Inc. (1990), the entire teachings of which are incorporated by reference, particularly Part 10, "Transformations".) Geometry of the part to be cast may be defined in terms of any suitable CAD data system as would occur to the skilled artisan guided by these teachings, the software used to define the geometry of parts in demonstration of the invention being SHAPES (Release 1.5, XOX, Inc., Minneapolis MN (1995)), and is incorporated by reference herein.

#Two solids, called mold blanks, which represent the volume of the mold on the upper (cope) side and lower (drag) side of the parting surface, are generated as at 27. Core requirements 28 for the mold blank defined with respect to the parting surface are then identified. Each positive feature of the part is swept to the parting surface and the part is subtracted from the projection, and any remaining volume is added to the core. Negative features are swept away from the parting surface to the top or bottom of the mold and subtracted from the projection, the remainder is intersected with the part; and any remaining volume is added to the core. Once core requirements, if any, are identified for the selected parting surface, optional new parting surfaces are successively generated at each combination of two or more vertices defining a unique new plane through the part. The optimum parting surface is selected by considering the number and complexity of the core requirements and the number of surfaces of the part to be drafted for each parting surface so generated and considered.