Method of producing cast-to-size tools

Abstract: A process for producing multipiece complementary tooling castings is disclosed in which hollow internal surfaces are cast in a mould (30) having a soft, low strength sand core piece (36) to define such hollow portions and external surfaces are cast against hard, resin-bonded sand mould surfaces (34). The castings are removed from their respective moulds while still warm and nested and pressed together to bring their working surfaces into conformance with each other. The result of this practice is a set of tool pieces that require little machining and display high accuracy in their as-cast condition.

Description: This invention pertains to the making of cast tools such as metal stamping dies and plastic moulding dies. More particularly, this invention relates to a mould making and casting practice by which such dies can be cast very close to their final working surface dimensions so as to reduce or eliminate the machining of the die surfaces. Many manufactured products are made by drawing sheet metal or moulding a synthetic resinous material between a closely matched set of die surfaces. In order to carry out such metal drawing or moulding operations, it is necessary to provide suitable tooling in the form of draw dies or mould sections. The part-shaping surface of such tools must be carefully made to precisely shape the workpiece to be drawn or moulded into the three-dimensional configuration of the desired part. Such tools have been used for many years. They are usually hard, massive bodies that are used in combination of two or more tool details (or simply details) in a powerful press. The press operates to separate the dies for the insertion of, for example a sheet metal blank. The press then closes and the blank is pushed by a convex tool (a male punch) into a concave cavity (a female punch). The flow of the metal is frequently controlled by a binder ring that operates in cooperation with the punches. Such tools are typically made by casting a metal alloy of suitable hardness and durability into a configuration that duplicates as closely as possible the shape of the articles to be formed. Each of these castings may weigh hundreds or thousands of kilograms. Despite improvements in the casting of such tools, it has almost always been necessary to machine the cast metal forming surface of the tool to provide the desired surface shape and dimensions. The machining is required because the casting always undergoes shrinkage and distortion as it solidifies and cools to ambient temperature. Certain alloy compositions are known to undergo minimal shrinkage during the casting process. An example of such an alloy is Kirksite, a zinc-based alloy containing aluminium and copper of approximately ternary eutectic composition. However, even such low shrinkage, castable compositions still undergo some distortion from the intended configuration during the casting process and on cooling to room temperature. It is known to design and construct patterns of the casting that are oversized to allow for shrinkage of the molten metal on solidification and cooling. It is also known to provide relatively hard mould surfaces to resist misshapen moulds when the hot metal is poured. These practices have provided improvements in the accuracy of large castings, but have not solved the problem of producing a cast-to-size stamping die or mould tool. The problem is further complicated by the fact that such tools comprise combinations of details that are intended to mate. However, each tool detail deforms in the casting process to exacerbate the problem. For example, many sets of sheet metal drawing dies comprise at least three members; for example, a male punch, a female punch and a binder ring. In general, the male and female punches have mating surfaces over and against which the sheet metal is stretched and drawn into the configuration of the desired part. The binder ring is shaped to fit about the male punch and to engage the sheet metal and control its flow as it is pushed into the female cavity. In the case of the automobile industry, for example, scores of body parts such as door panels, bumpers, bonnets, roofs, boot lids, body pillars and brackets are formed by sheet metal drawing or stamping operations, and each part requires a set of tools. The cost of such tools greatly increases when each member of the set must be cast and then machined to a final configuration. Such sheet metal drawing operations, plastic moulding operations and the like would benefit greatly from a practice which would produce all pieces of a die set to substantially their final configuration in the as-cast condition. A method in accordance with the present invention is characterised by the features specified in the characterising portion of claim 1. It is an object of this invention to provide a method that combines both a sand mould preparation and casting practice by which a set of unmachined tool castings, for example, a male punch, a female punch and a binder ring, are all brought into very close conformation with their desired shapes and dimensions. In other words, the complementary material-shaping surfaces of the two punches and the binder ring are cast such that when the castings reach room temperature, the working surfaces are substantially in their desired finished conditions. It is a further object of this invention to provide a method of preparing moulds particularly suitable to the casting of the binder ring and, when necessary, the female punch so as to minimize distortion of these cored parts upon casting. It is a further object of this invention to provide a complementary practice of casting and handling the castings of such tool details that they interact with each other in the final shaping of each piece. For purposes of illustration, the practice of the present invention will be described in an embodiment using a zinc-based alloy. The preferred alloy is one that has measurable shrinkage characteristics but is more durable than Kirksite. It consists essentially by weight of about 10 percent copper, 4 percent aluminium, 0.04 percent magnesium, 0.04 percent iron and the balance, except for incidental impurities, substantially all zinc. The practice of the present invention will always involve the preparation of at least two, usually three and sometimes up to four large tool detail castings. Typically, such castings comprise a male punch with a convex working surface, a female punch with a concave working surface, and a binder ring with a hollow interior. It is known that in most instances the male punch casting is easiest to cast to size. Its working surface does not require a core to define its shape. The shrinkage of the metal at the working surface is entirely in an inward direction and unimpeded. Therefore, so long as the male punch pattern is prepared in a manner that precisely accounts for metal shrinkage upon solidification and cooling and a suitably hard mould surface is employed, the configuration of the cast punch will be accurate. The principal tool details that are difficult to cast to size are binder rings (and certain cored female punches) because they have hollow portions or concavities and must shrink against a core or like mould surfaces. In accordance with the mould preparation aspect of the present invention, there is provided hard, unyielding resin-bonded sand mould surfaces for the male punch, at least non-cored surfaces of the female punch and the outer periphery of the binder ring. However, there is employed a softer oil-bonded green sand for mould surfaces defining the internal periphery of the binder ring and internal surfaces of the female punch that are defined by large or deep core bodies. This combination of outer hard mould surface for the binder ring and more compressible internal mould surfaces permits the binder ring to shrink in a predictable way without excessive deformation. Further details of this mould preparation aspect of the present invention will be described in more detail below. In accordance with a particularly critical aspect of the present invention, the casting of such a three-piece die set is timed so that despite substantial differences in their mass and cooling rate, they can be removed from their respective moulds (an operation sometimes called "shakeout") while still relatively hot and plastic or pliable. Preferably, the casting of the three pieces is coordinated so that they can be shaken out from the mould and nested and pressed together with their working faces in their operative positions under suitably high pressure so that as the cast tools cool to ambient temperature, the faces conform to each other in their desired respective final configurations. When the punches and binder rings have been cast of the above alloy, it is preferred, for example, to shake out the male punch (with its higher as-cast accuracy) at the lowest temperature (for example, 120°C) and the female punch and binder ring at higher temperatures (for example, 175°C and 220°C, respectively). Thus, the male punch working face serves as the master tool surface against which the other cast surfaces conform as they cool. This mould preparation has been used with casting nesting practice for tooling pieces, each weighing one or more tonnes. The practice has uniformly resulted in the preparation of multi-piece tool sets in which the punch and binder ring surfaces display a remarkably close fit, typically within one millimeter of the finish dimensions, in the as-cast condition. By casting the several pieces with such accuracy, final machining of the working surfaces is eliminated or greatly reduced. Only hand finishing is required, thereby reducing preparation time and labour cost. The present invention will now be described with reference to the accompanying drawings, in which:- Figure 1 is a perspective view partly in section of the drag portion of a foundry mould for casting a male punch in accordance with the present invention; Figure 2 is a perspective view partly broken away and in section of the drag portion of a foundry mould for casting a binder ring; Figure 3 is a perspective view partly broken away and in section of the drag portion of a foundry mould for casting the female punch; Figure 4 is an exploded perspective view of a generically representative male punch, binder ring and female punch as in their press open position; and Figure 5 is a perspective view of the three die pieces in their nested or press closed forming position. The practice of the present invention will be illustrated using the above-described durable alloy consisting essentially by weight of 10 to 11 percent copper, 3 to 4 percent aluminium, 0.035 to 0.055 percent magnesium, 0.06 percent maximum iron and the balance zinc, except for incidental impurities. The specific composition used consisted essentially by weight of 10 percent copper, 4 percent aluminum, 0.04 percent magnesium, 0.04 percent iron and the balance zinc. This alloy is very useful because of its hardness and strength and because it can be cast at temperatures below 600°C. Tooling for prototype steel parts and for up to several thousand production pieces can readily be prepared by this practice using this alloy. Each step in the practice of the present method is carried out carefully so as to achieve the best accuracy possible in the surface dimensions of each finished tool detail. The preparation of foundry patterns is a known and skilled art. In the practice of the present invention, a dimensionally correct pattern for each of the male punch, the female punch and the binder ring is used at the start. A number of different materials are suitable for accurate and durable patterns, such as wood, high density expanded polystyrene foam and epoxy composites. Each can be machined and finished to the desired dimensions. As with all foundry patterns, the dimensions of the finished pattern are expanded to allow for volumetric contraction of the specific cast metal as it solidifies and cools to room temperature. The shrinkage allowance for a specific alloy to be cast is known or can readily be determined through simple casting trials. This data is then applied when machining a pattern. In the manufacture of tools from the zinc-based alloy described above, it has been recognized that high density (0.32 grams/cm³) expanded polystyrene foam will give a dimensionally accurate pattern that does not deform under mould-making conditions. In this example, the patterns are used to make three sets of cope and drag mould parts for the three tools to be produced. Generally in the casting of these large tool pieces, the drag mould portion contains the working face and body of the cast tool as well as gates and runners needed to convey the molten alloy into the casting cavity. A balanced or symmetrical runner/gating system that introduces metal uniformly into the drag mould cavity to minimize thermal gradients and stresses upon cooling is desirable and necessary for a dimensionally accurate casting. The cope mould generally contains only risers for the casting. Reference will now be had to the drawing figures. Figure 1 illustrates the drag portion of a mould which has been formed to make the male punch. For purposes of simplification, this Figure 1 is partly broken away and in section and illustrates only the casting cavity. It does not illustrate the gating and runner system which foundrymen well know how to provide in the mould. Also, for purposes of simplification in the drawing, the tooling parts depicted herein are intended to generically represent any tool. However, they are not drawn with the rounded and curved working surfaces that are normal in such tools. Male punch drag mould 10 is made with resin-bonded, fine grained (for example, AFS grain size No.101) zircon (zirconium silicate) sand lining layer 12 that constitutes the cavity-defining surfaces 16 which shape the working face of the cast tool. This lining 12 is backed by a resin-bonded silica sand (for example, AFS grain size No.48) body 14. This resin-bonded zircon sand facing/silica sand backing mould 10 is necessary to provide resistance to the pressure of the molten metal as it flows into it and to retain the desired shape of the male punch. The cast punch, of course, is formed in the cavity 18 defined by surfaces 16. Bottom surface 16' of the cavity 18 will define the portion of the punch (shown at 50 in Figure 4) that first engages the sheet metal that it draws in cooperation with the female punch and the binder ring. The top of the punch will be defined by the cope portion of the mould, not shown. It is preferred to use zircon sand in the resin-bonded surfaces of the moulds because it is available in the form of fine rounded grains (for example, AFS grain size No.101) that provide a smooth, accurate mould surface that is replicated in the casting surface. Furthermore, zircon sand has a relatively low coefficient of linear thermal expansion, 0.0054°C⁻¹ at room temperature, and relatively high thermal conductivity, 32 x 10⁻⁴cal/cm/°C/sec at 0°C. These properties contribute to suitable solidification of the cast metal and accuracy of the cast surface. Other sands having similar properties and characteristics, for example, chromite sand [(Fe,Mg)O · (Cr,Al,Fe)₂O₃], may be substituted for zircon. Silica sand is preferred for backing up the zircon surface layer and for coring applications as will be described. Silica is inexpensive and abundant. Its somewhat higher coefficient of linear expansion, 0.032°C⁻¹, and lower thermal conductivity, 20 x 10⁻⁴ cal/cm/°C/sec, are suitable in these applications. The moulding sand -- both the zircon adjacent to the pattern surface and the silica away from the pattern -- was bonded with Ashland Linocure (oil modified alkyd resin) air set resin. One hundred parts of zircon sand was mixed with 1.0 parts of resin, and 100 parts of silica sand was mixed with 1.5 parts of resin. The sands were packed about the pattern in a suitable flask and the resin allowed to polymerize. The pattern was then removed and the mould readied for pouring. In this illustration, the male punch is the easiest casting to form with high dimensional accuracy on its working surface. Because of its shape, all of the shrinkage tends to be in an inward direction, and there is no distortion around any core piece or other section of the mould which would contribute to a distortion or change in dimension of this punch. The combination of the predictable shrink zinc alloy and the hard mould surface with the zircon sand face layer usually makes it possible to cast a male punch that is accurate to within one millimeter of the final dimensions on its working surface. This high degree of dimensional accuracy on the male punch allows its use as a master to which the female punch and binder ring are made to conform, as will be seen later in the description of the process. Figure 2 illustrates (in perspective, partly in section and broken away) a drag mould 20 for forming the binder ring (seen at 52 in Figure 4). The preparation of this mould is one of the critical aspects in the practice of the present invention. This drag mould 20 is made as follows. The binder ring pattern (not shown) was positioned on a moulding plate within a flask wall. Placed in the centre of the ring pattern was a polystyrene foam plug of low density. Moulding sand (coated with Ashland Linocure as described) was then packed around the outside and top of the ring pattern, first to form the facing layer 22 of the resin-bonded zircon sand and then the body portion 24 of the resin-bonded silica sand. The resin hardened upon standing in air after the sand was packed in place. At this stage, the binder ring drag mould 20 consisted of body section 24 and cavity liner section 22. After the resin had cured, the partly-completed mould was rolled over and the polystyrene plug was removed. Then, the area within the pattern (not shown in Figure 2) was filled in with a differently prepared silica sand to make the central core portion 26 of mould 20. The polystyrene plug had temporarily occupied the space that ultimately becomes core 26. Here, the silica sand was bonded with a suitable oil binder to produce a pliable green sand which was packed to shape the internal walls 53 of the binder ring 52. This silica sand was mixed, for example, with Petro Bond binder, produced by Baroid Drilling Fluids of Houston, Texas, to form a low-strength moulding composition. One hundred eighty parts of washed silica sand of 140 AFS grain fineness number, 9 parts of Petro Bond, 3.5 parts of Petro Bond oil, and 0.1 part of P-1 catalyst were mixed and used. This low strength oil-bonded sand area for central core portion 26 allows the zinc-based alloy to shrink unimpededly, thereby producing a binder ring casting with no distortion. If this central core portion 26 of the binder ring drag mould is formed of a high strength, resin-bonded moulding sand, the binder ring is distorted upon solidification and shrinkage because it cannot shrink freely and predictably in the inward direction. Where this core portion 26 is large, generally larger than about 0.5 m x 0.5 m, it is preferred to use loose, unbonded silica sand in the centre region (27 in Figure 2) of the core portion 26 with the oil-bonded sand around the periphery 28 of the core portion 26. Core periphery 28 and facing layer 22 define between them the drag mould casting cavity 29 for binder ring 52 (Figure 4). Figure 3 shows a perspective view of a drag mould 30 for the female punch (54 in Figure 4). Again, this is a perspective view of the mould partly in section and broken away. The cavity portion 32 of the mould is configured for the casting of a female punch 54 adapted to receive the male punch 50 to be cast in mould 10. Mould 30 is made entirely of resin-bonded sand. Outer portion 34 is made entirely of resin-bonded silica sand. Core portion 36 defines the working face 60 of the female punch and, therefore, surface portion 38 of core 36 is made up of a resin-bonded zircon sand. The bottom portion 40 of mould 30, of course, defines the upper working surface of the female punch and includes a ring portion 42 moulded in which is intended to form the draw bead portion 56 in the female mould. As stated above, it is usually intended in accordance with the practice of the present invention that the drag mould for the female punch 54 be made entirely of a resin-bonded sand. However, where the depression of the cavity 32 exceeds about 100 millimeters, it is preferred to make the inner portion of core 36 from a softer oil-bonded sand like the core portion 26 in the binder ring mould 20. This allows for more uniform and predictable shrinkage of the female punch casting as it solidifies and cools to ambient temperature. When the drag and cope portions for the male punch, the female punch and the draw ring have been completed, it is necessary to plan the timing of the casting of these separate tool details. In accordance with the practice of the present invention, it is preferred to carefully schedule the pouring of these castings which may require one, two or more days so as to achieve a suitable mould shakeout temperature for each tool detail. The strategy is as follows. The male punch usually exhibits the highest dimensional accuracy as cast. Therefore, it is poured and removed from its mould first so that it will be the hardest of the warm castings and can serve as a master model for the conformation of the working surfaces of the other casting details. In contrast, a binder ring generally exhibits the largest dimensional variation and hence is shaken out of its mould last to permit it, while still warm, to conform to the female and male punches during the nesting stage of the process which will be described below. Thus, it is contemplated that the respective castings will be removed or shaken out of their moulds while the castings are still well above ambient temperature. Figure 4 shows the three pieces in an exploded view. Male punch 50 is in the upper punch, binder ring 52 is intermediate, and female punch 54 is lowermost. Obviously, provision must be made, which is not shown in the drawing, for attachment to these large castings so that they can be handled with a crane or other suitable lifting means. The upper surface of female punch 54 is now seen to contain the draw bead ring 56 and a shelf 58 on which the binder ring 52 can rest when the tools are in their closed position. A specific example of an actual casting and shakeout practice for a three-part automobile pillar support draw die will better illustrate this aspect of the process. Each tool detail was cast of the above-specified zinc alloy. The male punch weighed 620 kilograms, the female punch weighed 2070 kilograms and the binder ring weighed 1580 kilograms. The pouring schedule and shakeout times were as follows: Day 1: 10:30 a.m. - Pour entire male punch at 535°C. 10:35 a.m. - Cast female punch in two pours. The first pour at 538°C fills the mould to just below the parting line and the second at 566°C fills the risers. The metal in the mould was at 399°C at the start of the second pour. Day 2: 12:30 p.m. - Cast the binder ring in two pours with the metal at 527°C and 583°C, respectively. The metal in the mould was at 399°C at the start of the second pour. Day 3: Shakeout the male and female punches at 175°C to 205°C. Start to shakeout the binder ring at 260°C by removing the cope to cool the casting to the desired nesting temperature of 220°C. Nest the two punches and binder ring and place about 3000 kilograms (three tons) on the casting assembly. The temperatures of the three details on nesting were: male punch, 120°C; female punch, 175°C; and binder ring, 220°C. Thus, the castings, while still warm, are nested together. The binder ring is placed on shelf 58, and male punch 50 is inserted into the female cavity through binder ring 52. The tools are shown in their nesting position in Figure 5. In addition to the weight of the respective pieces acting against each other in their nested position, an additional compressive stress is applied in the form of 3000 kilograms of additional weight (giving a compressive stress of 70 to 100 kPa). The additional weight is indicated by the arrow in Figure 5. The castings are then cooled to ambient temperature in the nested arrangement. The compressive stress imposed on this assembly causes the binder ring 52 and the female punch 54 to conform to the most accurate casting, the male punch 50. After this assembly had been cooled to room temperature, the external weight was removed. The castings were processed into finished tools. The working surfaces of the three separate tool details were examined and measured to be sure that they conformed accurately to the desired surface dimensions and configuration. In the actual set of dies which was cast and nested as described above, a measurement using small clay balls of various sizes placed between the castings revealed that there was a 0.1 to 0.15 millimeter fit between the casting details. This is a remarkably close fit for three very large tool details in an as-cast condition. An as-cast dimensional accuracy of one millimeter or better frequently means that cast tool details can be used "as is" or with moderate hand finishing of the working surface. Expensive machining set-up and operation is neither required nor desirable for such minor surface corrections. Thus, the mould-making process which includes using a soft, yieldable core structure where interior shrinkage of the casting is to occur and hard mould surfaces on external surfaces of the casting is found to provide a significant improvement in the as-cast accuracy of the tool pieces. Furthermore, the nesting of the cast tool pieces while they are still warm enough to be plastic or pliable further shapes and conforms the separate pieces and increases markedly the accuracy of their surface on cooling. This nesting step must be practiced on the as-cast pieces while they are still cooling. It is not as effective to permit the pieces to cool to room temperature and then reheat them for such a nesting operation because the cooling and reheating introduces distortions in the cast tool details. Although this process was developed and tailored using the specified zinc-based die alloy, it is applicable to the casting of other tooling materials including iron-base, copper-base and aluminium-base as well as other zinc-base alloys. While the specific pouring, casting shakeout and nesting temperatures will vary for each alloy, the pattern and mould preparation and gating and runner system design are similar to those that have been described above.

Claims: A method of casting mating metal tool members comprising two punches having engaging surfaces for the shaping of compliant material, characterised by separately casting each punch (50,54) in a resin-bonded sand mould (10,30) in which the cavity-defining face (16,38) is shaped by a pattern representing the respective shaping surface with accurate allowance for shrinkage of the metal to be cast and such cavity-defining face is formed of a fine particulate sand, removing each cast punch from its mould after full solidification thereof with at least one of the castings at an above-ambient temperature at which its material-shaping surface is plastically formable, and placing the castings together and pressing their intended engaging surfaces together to plastically shape them into conformance with a predetermined configuration. A method as claimed in claim 1, in which the material-shaping surface of one of the punches (50) is more accurate as-cast than the second (54) and that punch (50) is removed from its mould (10) and allowed to come to a lower temperature at which it presents a harder shaping surface and is used as the master tool against which the shaping face of the second punch is conformed during the pressing step. A method of casting mating metal tool members comprising a male punch (50), a female punch (54) and a binder ring (52) having engaging surfaces for the shaping of compliant material, characterised by separately casting each punch in a resin-bonded sand mould (10,30) in which the cavity-defining face (16,38) is shaped by a pattern representing the respective shaping surface with accurate allowance for shrinkage of the metal to be cast and such cavity-defining face is formed of a fine particulate sand, casting the binder ring (52) in a like resin-bonded sand mould (20) that defines the outer periphery of the ring and with a compliant green sand inner core (26) that defines the inner periphery (53) of the binder ring, removing each cast punch and the cast binder ring from their respective moulds after full solidification thereof at an above-ambient temperature at which they are plastically formable, and placing the castings together and pressing their intended engaging surfaces together to plastically shape them into conformance so as to reduce the need for finish machining. A method as claimed in claim 3, in which the material-shaping surface of the male punch (50) is more accurate as-cast than the female punch (54) or the binder ring (52) and the male punch is removed from its mould (10) and allowed to come to a lower temperature at which it presents a harder shaping surface and is used as the master tool against which the shaping face of the female punch and the binder ring are conformed during the pressing step. A method as claimed in claim 3, in which the placing step comprises placing the castings together with the male punch (50) at the lower above-ambient temperature and the female punch (54) and binder ring (52) at respectively successively higher temperatures, and the pressing step comprises pressing the castings together with a stress greater than their respective weights, the cooler, harder male punch serving as a master tool against which the engaging surfaces of the other castings conform as they cool. A method as claimed in anyone of claims 3 to 5, in which the fine particulate sand of the cavity-defining faces (16,38) has a higher thermal conductivity and a lower coefficient of thermal expansion than silica sand; and in which the inner core (26) of the mould (20) for the binder ring (52) is a compliant oil-bonded green sand-containing inner core.

Inventors: RYNTZ EDWARD FRANK JR, RYNTZ, EDWARD FRANK, JR.
Applicants: GEN MOTORS CORP, GENERAL MOTORS CORPORATION
Application year: 1993
Main classification: B22D17
IPC codes: B22C9, B21D37, B22D21, B22D29, B22D23, B22D31, B23P15