Casting process

Abstract: A casting model is made by machining a rigid foam body 51, 52 and applying a flowable material fig 4 which sets to form a solid coating blocking the pores of the machined surface. A casting mould is formed against the coated surface fig 5 and the casting model is then removed before casting.

Description: This invention relates to a casting process for use in the fabrication of injection moulding tools. Forming tools, such as injection moulding tools , are usually fabricated by making a positive master model from wood and resin materials which conforms to the shape of the article to be manufactured by the forming tool. For the fabrication of an injection moulding tool, for example to produce plastic parts such as a bumper of an automobile, the master model is copied into a metal tool blank using pantographic machining techniques to provide one part of the tool. The positive master model is then provided with a cover layer of a suitable material, typically glass reinforced plastic, which when removed from the master model, provides a "negative" model of the article for manufacture. The negative model will therefore have a surface which has a shape corresponding to that of the article for manufacture. The shape of this surface is then copied into a second metal tool blank also using pantographic machining techniques cooperating part of the injection moulding to provide the tool. The tool blanks are usually of aluminium alloy or steel depending upon the number of articles to be produced by the injection moulding tool. As will be appreciated from the above process, as co-operating parts of the forming tool are machined from the tool blanks by NC machining, with possibly some finish machining using electrical discharge machining (EDM) techniques, the machining time for large forming tools, such as a tool for injection moulding an automotive vehicle impact bumper, can be in the region of two to three months. To this must be added the time taken to produce the wood and resin master model which, as the materials are difficult to work, can take, typically, six to twelve weeks to manufacture. The manufacturing cost of forming tools for relatively large and complex shapes is therefore, extremely high and the forming tool can take, in total, nine months to produce if this conventional fabrication process is adopted. In order to avoid this complicated and time consuming process, it has, therefore, been considered by the applicant to fabricate forming tools using a casting process in which the forming tool shape is achieved by casting metal, usually aluminium alloy, into a casting mould. Casting moulds for use in fabricating cast metal shapes are usually produced by forming a slurry of sand or a sand/resin mixture around a casting model. The casting model is placed within a frame and the mixture is compacted into the space between the frame and model and allowed to set. For large forming tools the sand mixture casting mould must have sufficient inherent strength to support the weight of the cast material, which can be in excess of 10t. The mixture is therefore compacted around the casting model using high density ramming techniques. In view of the compaction forces required, the casting model must have a sufficient rigidity so as to maintain the desired shape through the compaction process and provide a casting to the required shape and dimensions. Furthermore, the metal, when cast into the casting mould, may contain casting defects, such as air bubbles, weakening the cast structure. If such defects cannot be rectified the cast is usually scrapped and the casting process repeated. It is desirable, therefore, to produce a casting model which can be used to produce several moulds, if necessary. This is particularly advantageous when the desired shape is relatively large and has a relatively complex profile, such as, for example, automotive or 'white goods' parts and body panels. If the casting model is not re-usable, the whole process must be repeated. To provide a casting model with the required physical characteristics, the master model, fabricated and shaped using wood and plastics resin is usually used as the casting model. As such, the master model can be provided with the desired physical characteristics, but, as stated above, the wood and plastics resin materials are relatively difficult to work and relatively expensive to produce. Furthermore, in the automotive industry, design changes are frequently required for body and trim parts in view of the complexity of part shape and the interaction with adjacent parts. Such design changes in the final shape can be relatively slow to implement when the wood and plastics resin master model is used as the casting model. It has also been considered to make a casting mould by shaping a body of low density plastics foam ("STYROFOAM" trademark) to form a casting model, and forming a mould of moulding sand against the shaped surface of the foam model. However, as the plastics foam used is a relatively open cellular structure, the moulding sand sets into the surface cavities in the foam material, binding the casting model quite firmly to the sand mould. The low density foam casting model must therefore be left in place and allowed to burn out during the casting of the hot metal. This has the severe disadvantage that toxic fumes, including cyanide gas, are produced as a result of the burn out step, which is not desirable from the view point of environment protection. Furthermore, because the foam material is burnt out of the sand mould as the hot metal is poured in, contraflow of the molten metal and burn off products can cause impurities or voids in the cast metal. If the flow of the hot metal into the sand mould is too slow the metal can form localised relatively cool areas of skin and, therefore, not all of the foam material of the casting model may be burnt off, leaving impurities in the casting. If the flow of hot metal is relatively fast, as would be required to reliably ensure that all of the foam material is burnt off for large casting models, waves can be produced in the flowing material, causing it to flow over as it sets, leading to the formation of voids in the cast material. In practice, the formation of these impurities or voids is a limiting factor in the size of low density foam casting model which can be used. Also, as will be appreciated, the moulding sand when compacted about the foam casting model, will flow into and fill the cellular cavities of the foam material which extend to the surface of the casting model. The surface of the sand mould at the sand/foam interface is therefore of relatively coarse texture and this surface is exposed when the foam of the casting model is burnt off. In view of the above, castings produced by the use of such low density foam casting models have a relatively poor surface finish which can require substantial localised 'dressing' and general surface machining before being suitable for use in product manufacture. Additionally, if the actual casting is found to include impurities, air bubbles, or similar defects, casuing weak points in the cast structure, the casting process has to be repeated. However, as the casting model has been destroyed by the actual casting process, a further casting model must be produced, with the attendant delays and additional costs to the overall process. A further major disadvantage is that the low density foam model tends to deform when the moulding sand is rammed against it. The degree of deformation is non-uniform and can be relatively difficult to predict. If the shape is for use in manufacturing a high precision mould, additional tolerances must be allowed for in the cast shape, with the surplus material, which typically can be as much as 20 mm in thickness, being removed by a suitable machining technique. For 'male' shapes the deformation of the casting model can be critical as there may be insufficient material in the actual cast shape, i.e. the casting produced may be undersized, necessitating a repeat of the entire casting process, including the reproduction of the foam casting model. The relative ease with which low density plastics foam can be deformed means that the material is unsuitable for use when the sand mould is relatively large and is therefore produced by high density ramming techniques in order to provide sufficient inherent strength to support the cast material. In practice, therefore, low density foam casting models are limited to the production of relatively small items, typically having dimensions not exceeding 300 mm, such as automobile engine water pump housings. In order to improve the deformation resistance of low density foam casting models it has also been considered to provide the foam material with a surface coating of relatively rigid material which may be applied, for example, by spraying. JP-A-60 255 233 discloses such a process, in which a foamed polystyrene block is machined to produce an expendable pattern which is rendered temporarily re-usable by applying curing agent which forms a surface hardened layer. The pattern is finally used as an ordinary expendable pattern. However, the coating does not provide sufficient rigidity for larger foam casting models as the foam material tends to collapse beneath the coating. Reinforcing structures, made of wood for example, may be used for incorporation into the foam material to minimise deformation during the compaction process. However, the composite wood/foam casting model is relatively difficult to produce and does not provide uniform rigidity. GB-A-2 123 332 discloses a process in which a self-skinning polyurethane foaming mixture (ISOFOAM EM 500) is poured into a mould. The mixture sets to form a pattern with a hard smooth skin which is used directly to produce a sand mould. Low density plastics foams are considered unsuitable for use in casting processes using relatively heavy metal alloys, such as zinc alloy, which, for large castings, would require inherently strong sand moulds to support the cast material both during actual casting and during cooling down of the cast material. In particular, for zinc alloy the cooling down is usually performed under controlled conditions and over a relatively large period of time to ensure a high quality casting, so inherent strength in the sand mould is of paramount importance. Additionally, as the low density foam is a relatively soft cellular material, it is difficult to machine with reasonable accuracy and surface finish by conventional rotary cutter machining techniques, for example by NC milling. The foam material tends to distort in front of the machine cutter, causing the material to tear away instead of cutting away cleanly, leaving an irregular finished surface with surface cavities. These difficulties in machining low density foam, coupled with the unpredictable compaction during the subsequent casting mould production process, mean that NC data representative of the article to be manufactured would not be used in the production of a low density foam casting model. Such low density models are usually made by cutting and shaping by hand, using templates to achieve the final shape. However, as will be appreciated, the accuracy achievable is severely restricted. The above factors can add significantly to the overall cost of the casting process. Furthermore, because the NC data is not used in the production of the low density foam casting model, the data cannot be verified until a subsequent and more critical stage of the overall production process, for example, the machining of expensive graphite electrodes for use in electrical discharge machining (EDM) of the cast material. A forming tool production process using zinc alloy and EDM is described in our co-pending application No. EP-A-0 546 225 filed simultaneously with the present application and entitled "Tool Making". The present invention provides a method of producing by a casting process an injection moulding tool for producing an article by injection moulding, the method comprising: (a) making a foam casting model; (b) forming a casting mould against the casting model; (c) removing the casting model from the casting mould; (d) casting material into the casting mould; and (e) separating the cast material from the casting mould; wherein the foam casting model is produced by (i) machining a foam body having a density of at least 0.25 g/cm 3 and compressive strength of at least 4 N/mm 2 enabling the foam body to be machined, using NC data derived from design data for the article for production, so as to form a machined surface of a predetermined shape on the foam body substantially corresponding to at least a part of the shape of the article for production and (ii) applying to the machined surface a flowable material which sets to form a solid coating blocking pores in the machined surface. The invention will be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a flow chart of a preferred procedure for casting a tool blank which is to be machined to make an injection moulding tool; and Figures 2 to 6 are diagrammatic cross-sections illustrating various successive stages in the making of a casting mould for casting a pair of male and female injection moulding tool blanks. Referring to Figure 1, by computer aided design (CAD) techniques, numerical control (NC) data is derived at step 10 for a casting model designed on the basis of a part to be made using an injection moulding tool via the design of the tool or of the tool blank. At step 12 a body of rigid plastics foam which has high density and, having due regard to the production process, a virtually non-deformable characteristic, such as polyurethane resin foam available from CIBA GEICY under specification No. XB 5120, which has been blocked up by hand to an approximate model shape, by building up layers of the foam material, so as to minimise subsequent machining, is subjected to NC machining in accordance with the data from step 10, in order to form a casting model with a machined surface for a predetermined shape. Because of the inherent characteristic of the foam material, machining work is substantially easier, compared with wood or resin materials. Such foams are used as a light weight modelling material and are available in a number of grades having, typically, a density of at least 0.25 g/cm 3 , preferrably in the range of about 0.25 to 1.1 gm/cm 3 and a compressive strength of at least 4 N/mm2, preferrably 4 to 30 N/mm 2 . At step 14 a settable flowable material is applied to the machined surface of the model, e.g. by spraying. The flowable material sets to form a solid coating which blocks the pores of the foam body which extend to the machined surface; if necessary, the coating may be smoothed, e.g. by sanding, to the desired profile. A suitable material is plaster of Paris, a curable resin (e.g. an epoxy or car body resin), or paint. A plaster or resin coating may, additionally, be provided with a varnish or paint coating to aid release from the sand mould. The coating is supported by the rigid foam body and can therefore be thin and need not be self-supporting. At step 16 moulding sand of a flowable consistency, e.g. a slurry or a sand/resin mixture, is rammed against the coated surface of the casting model in a frame, preferably using a high density ramming technique, in order to form a mould body conforming to the coated surface. Since the model is rigid, there is substantially no deformation of the coated surface, even when high density ramming, such as by using pneumatic ramming tools operating at a pressure of about 80 p.s.i. (0.55 Mpa), is employed. The casting model is therefore particularly suited to the production of sand moulds for casting relatively large shapes, and in particular, for the casting of articles in heavy alloys, such as zinc alloy. At step 18 the casting model is removed from the mould body. Removal is facilitated by the smooth coating, which parts easily from the mould body and does not tend to carry away grains of sand with it. Consequently, the surface of the mould body is comparatively smooth and produces a good surface finish on the casting. Also, since the coating is firmly anchored in the pores of the foam, the model can be re-used to reproduce the mould body, should the casting be defective. Additionally, since the model is removed from the mould body before a forming tool blank is cast in the mould, no toxic fumes are generated during casting. The tool blank is then, optionally, machined to the desired shape, preferably by electrical discharge machining (EDM) using an electrode which is machined using NC data from step 10. The resulting injection moulding tool (for example) may be a female cavity tool used in conjunction with a male core tool to define an injection moulding cavity for making the article in question. The female cavity tool has a generally concave moulding surface and generally plane so-called shut faces, which cooperate with corresponding shut faces on the male core tool to close the injection mould. Figures 2 to 6 illustrate successive stages in the making of a casting mould for casting a female cavity tool blank and a male core tool blank which are subsequently to be machined to form an injection moulding tool. As described above, first of all, to produce casting models, a female blank block 51 and a male blank block 52 are built up from layers of rigid high density plastics foam (Figure 2). In Figure 3 the blocks are subjected to the NC machining (using a cutting tool 53) to form female and male blank casting models 54,55 (step 12 above). The blocks are machined so as to leave excess material (e.g. 1 to 5 mm) over the surfaces corresponding to the forming and shut faces of the injection moulding tools. In Figure 4 settable material is applied to the models 54,55 by spray heads 56 (step 14 above). In Figure 5 the coated models 54,55 are placed in a casting frame 57, and green sand 58 (mixed with a resin binder) is added and is compacted to a high density by a pneumatic rammer 59 (step 16 above). The casting frame 57 is then turned over and the models 54,55 are carefully removed from the compacted sand 58 (step 18 above), leaving a female tool blank casting cavity 60 and a male tool blank casting cavity 61, into which cavities a molten metal (Zn alloy) is to be cast from a ladle 62. The machining of the casting model using the NC data is particularly advantageous when the foam model is to be used in a forming tool production process using EDM, as the NC data (part data), by its use in machining the casting model, can be used to verify the NC data for the production of the EDM electrode. The use of NC machining also means that an allowance can easily be made to accommodate the contraction that the cast metal is subject to, after casting and on cooling. The high density rigid foam material can be machined relatively accurately and, as it does not suffer from compaction during step 16, it can be machined almost exactly to the dimensions of the artice to be formed. Hence the NC machining datum modification can be reduced to the absolute minimum in step 12. Even for large casting models, such as for use in the production of a vehicle impact bumper, it is possible that the casting made by using the rigid foam can be accurate to within 1 mm, so the NC machining datum adjustment can be reduced to this figure (in practice this can be achieved very simply by using the same NC data as the part shape but changing the diameter of the cutting tool used). As explained above, when normal low density casting foams are used, at least 20 mm is, typically, required as surplus cast material to compensate for compaction, even for the relatively small articles for which the low density casting models can be used. Furthermore, as the rigid foam can be accurately machined, the shut faces of the injection moulding tool can be accurately defined in the foam casting model. For injection moulding tools a 30 mm shut face width is normally the accepted minimum and this can be achieved using the rigid foam. (For "Styrofoam" casting models, the shut faces have not normally been defined and hence, if EDM had been proposed to be used on the subsequent cast shape, the entire mating surfaces of the tool would have required machining. Alternatively, further NC data would have been required to define the shut faces by NC machining.) Minimising the width of the shut faces is particularly important for EDM as the machining rate is determined by the volume of material to be removed. Therefore, the rigid foam not only permits the use of a foam casting model for relatively large moulds but also enables the advantages of subsequent EDM to be maximised as not only is there a substantial reduction in the depth of material to be removed in the casting (1 mm as compared with 20 mm) but there is also a reduction in the surface area of the material to be machined. The ability to machine a relatively rigid foam in the above manner thus enables an injection moulding tool to be cast in such a way that the area on the tool to be finished by a subsequent machining process is reduced to a minimum, such as the machining of a shut face frame of the injection moulding tool and the finish machining of any cavity and core slide faces. The use of a relatively rigid foam master model, as described above, is particularly advantageous in the production of injection moulding tools for use in the automotive industry, where a trial development tool of a desired shape is required prior to a production tool of the shape. The rigid foam casting model, being retained after the first casting process, i.e. to produce the development tool, can be used to produce the production tool. Additionally, if minor design changes are required between the development and production tools, these can be accommodated relatively easily in view of the easy working of the foam material (in comparison with wood or resin structures), the use of the NC data to machine the casting model, and the provision of a plaster, curable resin or paint coating.

Claims: A method of producing by a casting process an injection moulding tool for producing an article by injection moulding, the method comprising: (a) making a foam casting model (54;55); (b) forming a casting mould (58) against the casting model (54;55); (c) removing the casting model (54;55) from the casting mould (58); (d) casting material into the casting mould (58); and (e) separating the cast material from the casting mould (58); wherein the foam casting model (54;55) is produced by (i) machining a foam body (51;52) having a density of at least 0.25 g/cm 3 and compressive strength of at least 4 N/mm 2 enabling the foam body to be machined, using NC data derived from design data for the article for production, so as to form a machined surface of a predetermined shape on the foam body substantially corresponding to at least a part of the shape of the article for production and (ii) applying to the machined surface a flowable material which sets to form a solid coating blocking pores in the machined surface. A method as claimed in claim 1, further comprising smoothing the coating before step (b). A method as claimed in claim 1 or 2, in which the flowable material is plaster of Paris or a resin. A method as claimed in any of claims 1 to 3, further comprising painting or varnishing the coating before step (b). A method as claimed in claim 1, in which the flowable material is paint. A method as claimed in any preceding claim, in which the casting mould comprises a mixture of sand and resin. A method as claimed in any preceding claim, in which the casting mould is formed using high density ramming. A method as claimed in any preceding claim, in which the foam body has a density of 0.25 to 1.1 g/cm 3 . A method as claimed in any preceding claim, in which the foam body has a compressive strength of 4 to 30 N/mm 2 . A method as claimed in any preceding claim, in which the foam body comprises a polyurethane resin based foam material. A method as claimed in any preceding claim, further comprising machining the cast material by electrical discharge machining (EDM). A method as claimed in any preceding claim, in which the cast material comprises zinc alloy, aluminium alloy, or steel.

Inventors: PUDDEPHATT KEVIN JAMES, PUDDEPHATT, KEVIN JAMES
Applicants: NISSAN EUROP TECH CENTRE, NISSAN EUROPEAN TECHNOLOGY CENTRE LIMITED
Application year: 1991
Main classification: B22C7
IPC codes: B22C7