GB2279025A - Casting models having separable forming structure portions - Google Patents

Abstract

In producing a forming tool such as an injection moulding tool, a casting model 21 is made by making a forming structure portion 22, e.g. by stereolithography or laminated object manufacture, and connecting it in a separable manner to a bolster portion 23, e.g. made of rigid foam. A casting mould is formed by ramming moulding sand 49 against the model 21. The bolster portion 23 (re-usable) is removed from the casting mould before the other portion 22 (re-usable or exchangeable). A tool blank is cast in the mould and at least its forming structure (cavity-defining surfaces) is machined to the required dimensions, e.g. by electric discharge machining. <IMAGE>

Description

CASTING MODEL This invention relates to casting models for use in the production of forming tools, in particular (but not solely) injection moulding tools such as are used to define one or more surfaces of a cavity for moulding a part made of plastics material, e. g. a trim or body part to be used in a motor vehicle.

In one aspect the invention further develops the casting processes and tool making procedures which are the subject of our co-pending applications GB-A-2262251 and EP-A-O 546 226 ("Casting Process") and GB-A-2262466 and EP-A-O 546 225 ("Tool Making"), all filed on 12 December 1991. The disclosure of each said application is hereby incorporated in the present specification.

Our application GB-A-2262251 proposes the fabrication of forming tools using a casting process in which the forming tool shape is achieved by casting metal, usually zinc alloy, into a casting mould.

Figure 1 of the accompanying drawings shows a cross-section through an injection moulding cavity 1 defined between respective forming structures 2, 3 on upper and lower casting models 6, 7 which can be used in the fabrication of injection moulding tools for producing an elongate part made of plastics material, e. g. a trim or body part to be used in a motor vehicle.

As seen in Figures 2 and 3, the forming structure 2, 3 are supported by relatively large bolsters 8, 9 respectively. The forming structures 2, 3 have respective cavity-defining surfaces 4, 5 and sheet faces 13, 14.

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. The casting model must have sufficient rigidity so as to maintain the desired shape through the compacting process and provide a casting to the required shape and dimensions. This is particularly important if the casting is to be used as a forming tool, such as an injection moulding tool, as the accuracy of the casting will determine the amount of subsequent machining needed to bring the casting to the required tool dimensions.

To provide a casting model with the required physical characteristics, a master model, fabricated and shaped using wood and plastics resin, has been used as the casting model. As such, the master model can be provided with the desired physical characteristics, but the wood and plastics resin materials are relatively difficult to work and relatively expensive to produce.

Hence, a casting model of complex shape can take weeks or even months to fabricate. 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 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 one-piece casting model, and forming a mould of moulding said 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.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.

A further major disadvantage is that the low density foam model tends to become distorted and compressed 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.

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. However, the coating acts as a rigid shell and does not always provide sufficient rigidity to prevent collapse of the foam 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.

It is also possible to machine a casting model from a body of rigid foam material, for example, by NC machining using NC data derived from CAD data representative of the part to be produced. These rigid foam models can be removed from the sand mould prior to casting. However, machining is usually done with a radius tip cutter and in producing a one piece casting model 11, such as that shown in Figure 4 of the accompanying drawings, it is not possible to machine re-entrant corners 12 without a corner radius because the minimum radius is determined by the radius of the cutter used. Hence, if re-entrant corners without a radius are required these corners need to be finished by hand with attendant increases in costs.

During removal of the casting model from the mould, great care must be taken to avoid disturbing the compacted sand/resin mixture, in order to minimise defects in the casting. This can be quite difficult to achieve in practice because, as can be seen from Figures 1 and 2, that part of the model carrying the cavity defining surfaces 4,5 is relatively small compared to the bolsters 8, 9, requiring, in effect, the model to be removed 'blind' from the mould. Avoiding damage to the mixture is particularly important for a casting to be used as a forming tool, as damage to the sand/resin mixture will result in surplus material on the casting which must be removed by subsequent machining, with attendant increases in cost.

What is desired is a casting model which substantially alleviates the above problems.

In one aspect the present invention provides a casting model for use in making a casting mould for casting a forming tool which comprises a bolster supporting a forming structure, the casting model comprising a bolster portion and a forming structure portion, the said portions being connected together so as to be separable.

The invention also provides a method of making such a casting model.

In another aspect the invention provides a method of making a forming tool using such a casting model, comprising the steps of (a) forming a casting mould against the casting model; (b) removing the casting model from the casting mould; (c) casting a metal in the casting mould, thereby producing a forming tool blank comprising a bolster supporting a forming structure; and (d) machining at least the forming structure to a predetermined shape.

The bolster portion can be a re-usable portion, since the bolster of a forming tool is not normally subject to design changes. The material of this part can be selected primarily for its durability and/or rigidity; such material may comprise wood, rigid foam, plaster, resin, aluminium, or a combination of two or more of these.

The forming structure portion is changeable, corresponding to a portion of the casting whose shape is dictated by the part to be formed or which is likely to be subject to design changes, whether during the development of the part or at some time in the future.

The physical volume of this portion will usually be small in relation to the total volume of the tool, so that it will be economical (both in time and materials) to form this portion of the model by a more sophisticated process, such as stereolithography (SL), or laminated object manufacture (LOM) or any other equivalent process.

The changeable portion (which may consist of a number of separate pieces) is preferably of rigid and/or durable material . It may be formed by NC machining of rigid foam. The shape of this portion (or each of its constituent pieces) can be selected to have no machining of re-entrant corners which are exposed in the finished model.

In particular the forming structure portion may correspond to the cavity-defining surface and shut faces of an injection mould tool.

The use of a separate exchangeable portion can reduce the time needed to produce the casting model and the amount of material used.

The bolster portion is relatively easy to produce, and conventional machining techniques can be used to make it. It may be made up from a number of modular pieces.

The changeable (forming structure) portion can be of a complex shape, requiring NC machining in which NC cutter paths have to be defined. There is no need to define cutter paths for the bolster portion.

The changeable portion does not need to be as durable as the bolster portion, since the working life of the changeable portion is dictated by the part to be produced, whereas the bolster portion may be made reusable in the production of other tools.

Since the model is to be removed from the mould before casting, there is none of the production of toxic gases which occurs when a destructible model is used as in the prior art. By making the portions of the casting model mutually separable, removal of the model from the mould can be facilitated. The bolster portion, which is relatively bulky but of a comparatively simple shape, can be removed relatively easily from the mould first, leaving the smaller changeable portion, which can then be more easily manipulated to release it from the moulding material without causing damage.

The portions may be connected by pegs and/or adhesive material, for example.

The changeable portion may be built up layer by layer using CAD data, e. g. using the SL or LOM techniques mentioned above.

In SL systems, such as those described in US-A-4 575 330, a plastic part is automatically built up by successively printing cross-sections of photopolymer (such as liquid plastic) on top of each other until all of the thin printed layers are joined together to form a whole part. Photocurable polymers change from liquid to solid in the presence of light, and their photospeed with ultra-violet light is fast enough to make them practical model building materials. The material that is not polymerised when a part is made is still usable and remains in liquid form as successive parts are made. An ultra-violet laser generates a small intense spot of light which is moved across the liquid surface with a galvanometer mirror X-Y scanner driven by data derived from the CAD data.

In the LOM technique, rolled material such as paper is indexed onto a work surface, and a laser under the control of solid model CAD data then describes the corresponding horizontal cross-section of the part to be produced. When the laser has defined that section, another layer of material is indexed onto the surface, and the next section is cut by the laser. Each cut section is superposed on the preceding section, with interposed adhesive, and in this manner the part is built up layer by layer until a three-dimensional model of the part described by the CAD data is produced.

Alternatively the adhesive may comprise a heat-sensitive coating on the paper material.

Since LOM utilises solid based sheet materials to create the model, the speed of cutting depends on the peripheral length of each cross section of the part to be defined rather than "infilling" the total surface area of each layer, as would be required in SL. Hence LOM is particularly applicable for producing models which have relatively large solid volume or relatively thick wall sections. As the model is built up layer by layer it is surrounded by surplus material which is not required as part of the finished model and is therefore sliced into cubes by the defining laser. This cube formation facilitates removal of the surplys material in small sections to be discarded as waste once the fabrication process is completed.

In the context of the present invention, LOM, if used in a novel way, has the advantage that it can, from the same CAD data, produce a model of the part to be injection moulded and models of the essential portions of the moulding tools.

The invention will be described further, by way of example only, with reference to the accompanying drawings, in which Figure 1 is a fragmentary schematic cross-section through a cavity defined between co-operating upper and lower casting models for use in the fabrication of an injection moulding tool; Figure 2 is a sectioned schematic perspective view of the upper model; Figure 3 is a sectioned schematic perspective view of the lower model; Figure 4 is a schematic cross-section through a one-piece casting model; Figure 5 is a schematic section through a casting model comprising four separable portions; Figure 6 is a schematic section through a changeable forming structure portion of the casting model of Figure 5, in a two-piece embodiment; Figure 7 is a schematic section through a changeable portion for a casting model corresponding to the tool of Figure 2; ; Figure 8 is a schematic cross-section through an assembly produced by laminated object manufacture (LOM) in accordance with the present invention; Figure 9 shows the assembly of Figure 8 after it has been separated into composite parts; Figure 10 is a schematic section through an assembly produced by LOM and consisting of four separable layers; Figure 11 is a schematic section through an item produced by stereolithography (SL); Figure 12 is a schematic section through a casting frame containing moulding sand compacted around the casting model of Figure 5; and Figures 13 and 14 are views similar to Figure 12 and show the successive steps involved in removing the casting model from the moulding sand in the casting frame.

The casting model 21 shown schematically in Figure 5 is to be used in making a mould for casting a moulding tool blank which is to be machined (e.g. by electric discharge machining) to produce one half of an injection tool.

The casting model 21 comprises a changeable forming structure portion 22 which is releasably connected to a bolster portion 23 by pegs 24, for example, which may be plain or threaded. The changeable portion 22 is shown located in a recess 16 in the bolster portion 23 but equally could be located on the upper surface of the model 21.

The bolster portion 23 is made up of modular pieces 17, each of which is machined from a body of rigid plastic 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 Geigy under specification No. XB 5120, which has been blocked up by hand to an approximate shape by building up layers of the foam material in order to minimise the amount of machining needed. Such foams are available in a number of grades having, typically, a density in the range of about 0. 25 3 to 1. 1 g/cm3 and a compressive strength greater than about 4 N/mm2.

A settable flowable material may be applied to the exposed parts of the machined surfaces of the bolster portion 23, 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 and finish. A suitable material is plaster of Paris, a curable resin (e.g. an epoxy or can body resin), or paint. A plaster or resin coating may, additionally, be provided with a varnish or paint coating to improve surface finish and aid release from the compacted sand/resin mixture.

The changeable portion 22 can be made in the same way. Since the shape of the surface 26 which is to be exposed in the mould is relatively complex, NC machining will normally be used. In order to avoid re-entrant corners needing hand-finishing (as explained above), the portion 22 may be made up of two (or more) separate pieces 18 (Figure 6) which are fixed together (e.g. by pins 19), each having no re-entrant corners. The recesses receiving the pegs 19, 24 can be machined by any suitable technique to the required accuracy to locate the various parts.

Figure 7 shows a changeable forming structure portion 27 for fitting to a bolster portion (not shown) to make a casting model for use in the production of the upper part of an injection moulding tool.

Figure 8 shows an assembly 29 incorporating the changeable forming structure portion 27 produced by LOM (the layers being parallel to the bottom surface 31).

This assembly comprises a model 32 of the part to be produced by injection moulding, the portion 27 which is separated from the model 32 by a cut surface 33 defined by lines of weakening in each of the layers transversed by the surface 33 (which is to constitute the changeable portion of one casting model), and a portion 22 which is separated from the part model 32 by a cut surface 37 and which is to constitute the changeable portion of the other casting model 21. The assembly 29 is separated (Figure 9) along the cut surfaces 33, 37 to provide the constituent parts.

Figure 10 shows an assembly 44 which facilitates the separation of the portions 22, 27 and the part model 32. In this assembly, the adhesion between the adjacent layers of the laminate material is interrupted along three planes 46, so that the assembly can be readily divided into four strata 47 which can be separated and then joined back together again after the various parts have been separated along the cut surfaces 33 and 37, to provide the part model 32 and portions 22, 27 shown in Figure 9. Alternatively, the CAD data controlling the LOM process can be arranged so that each of the strata 47 can be produced as individual solid models. Each stratum can then be broken down by fracturing along the surfaces 33,37 as appropriate, and the various segments thus produced can be joined together to provide the model 23 and the portions 22 and 27.

If each changeable portion (22 or 27) was being made in a conventional LOM process, it would be surrounded by excess material provided with a lattice of lines of weakness in the layers so that the material could be broken away easily to leave the required portions.

Thus, two separate LOM processes would have to be used to make the two portions 22 and 27 with the attendant increases in machining time. Similarly, if the part model 32 was being made in a conventional LOM process, the surrounding material (22, 27) would be provided with a lattice of lines of weakness. In contrast, in the present process no such lattice is provided in the portions 22, 27, which are thus made simultaneously by a single LOM process. However, if a part model 32 is not required, the corresponding material can be provided with a lattice of lines of weakness to facilitate separation of this material from the portions 22, 27.

Hence, in the LOM process described above, what is normally regarded as the waste laminate material around the required solid model is actually used to provide the forming structure portions of the casting model.

However, it should be realised that 'conventional' LOM models may also be used to produce the forming structure portions of the casting models for the upper and lower halves of the injection moulding tool.

As an alternative to LOM, one or both of the changeable portions 22, 27 could be produced by stereolithography (SL). The portion itself is defined in the resin directly, and there is no exterior waste to be removed. However, in order to save time, as shown in Figure Ili only the essential surface layer 41 of the portion needs to be defined in solid resin, the interior surface of this layer being supported on a resin lattice 42. This lattice may subsequently be drilled out and replaced by a filler of solid resin or plaster, to provide the changeable portion which can be fixed to the bolster portion of the casting model.

The items produced by LOM or SL can be coated e. g.

with metal, and/or surface treated, e. g. fine sanded to improve surface finish and, therefore, casting definition, before or after they are connected to the bolster portions.

Figures 12 to 14 show the steps involved in producing a casting mould using the casting model 21 shown in Figure 5. First the model 21 is placed in a casting frame 48, and then moulding sand 49 of a flowable consistency, e. g. a slurry or a sand/resin mixture, is rammed against the casting model 21, preferably using a high density ramming technique, in order to form a mould body conforming to the surface of the model. Since the model 21 is relatively rigid, there is substantially no deformation of its surface, even when high density ramming, such as by using pneumatic ramming tools operating at a pressure of about 0. 55 MPa, is employed.

The casting frame 48 is then closed by a lid 51 and inverted and the bolster portion 23 is carefully removed from the surrounding mould body 52 (Figure 13). Removal of the bolster portion 13 is comparatively easy, since it is of a relatively simple shape.

Finally, the changeable portion 22 is carefully removed, manipulation of this portion being facilitated by the pegs 24 which project upwards from it. If the portion22 is made up of separate pieces 18 (Figure 6), they may also be removed in sequence.

The resulting mould body 52 (Figure 14) is subsequently used, in a conventional casting process, typically using steel, aluminium or zinc alloy to produce a casting which can be used in a tool making process, such as that described in our patent application GB-A-2 262 466, in which the casting is subjected to electric discharge machining (EDM).

It is to be noted that items similar to the items 22, 27 produced by LOM or SL could be metal coated, e. g.

by spraying, so that they could be used as EDM electrodes and thus serve a dual purpose and further reduce costs. If the casting is sufficiently accurate, the electrode produced in this way could be used to provide the limited machining required of the casting and could obviate the need to produce separate electrodes for the EDM process, which are typically fabricated from expensive graphite material.

It will be realised from the above description that the use of a multi-part casting model can provide significant advantage over known techniques.

For example, in the development of a motor vehicle a large number of differently shaped injection moulded parts are required, such as trim parts. Traditionally these may be produced by making master models of each part, e.g. wood and resin models, and then making trial and production quality moulding tools using, for each tool, conventional NC machining techniques; a separate model and tool being required for each part. With the present invention the injection moulding tools may be produced using casting techniques as described in our co-pending application GB-A-2 262 251. However, instead of making a unitary casting model for each part, modular casting models may be used. The various vehicle parts required may be grouped for size and a bolster produced consisting of a number of interlockable modular parts so that a range of forming structures can share common bolster modular pieces.Hence, the number of individual bolsters to produce a given number of vehicle parts can be substantially reduced.

Additionally, the cost of producing models by LOM or SL is directly related to the time required respectively to define either the laminate shapes or to solidify the photopolymer, usually by using a computer controlled laser. For SL the time required to define a part is actually dependent on the volume of the part required.

It will be appreciated, therefore, that to use either LOM or SL processes to provide both the bolster and forming structure portions of the casting model would not be economic, in view of the definition time required. However, by making the bolster and forming structure portions as co-operating but separable structures, either of these techniques can be advantageously be used to define the latter portions.

For example, an automobile 'A' pillar moulding may typically be about 60 cm in length and about 10 cm in width but may have a profile depth of less than 1 cm.

With a unitary casting model, each half of the injection moulding tool used to define the mould cavity to mould the part may have a bolster in excess of 1 metre in length, about 60 cm in width, 50 cm in depth. As will be appreciated, such large structures cannot in practice be made easily using LOM or SL techniques. However, the bolster can be made easily by machining relatively rigid foam, and the forming structure can quickly and economically be made direct from CAD data, using either LOM or SL. The two portions can readily be fixed together to provide the casting model, and as the casting sand is rammed down around the casting model in the casting process there is no requirement to positively hold the forming structure to the bolster.

There is only a requirement to locate the forming structure relative to the bolster such as by the pegs previously described. Additionally, if the bolster is made as a number of interlocking pieces, a modular design can be adopted for the bolster, which is of relatively simple shape. Hence, if the bolster for the above-mentioned 'A' pillar is made up of three pieces, e. g. a central piece and two end pieces, the central piece could be replaced by a different central piece of shorter length to enable the end pieces to be used in a bolster for a casting model for a shorter injection moulded part, such as a 'B' pillar shroud for a smaller class vehicle. The bolster pieces can therefore be used to form a number of casting models and can be retained for use in fabricating another trim part, the only portion of the casting model respective to any trim part being the forming structure corresponding to the shape of the trim part. As described above, the forming structure can be produced by LOM or SL, enabling designers also to view a solid model of the part which can subsequently be used in the actual fabrication of the part.

Claims (29)

1. A casting model for use in making a casting mould for casting a forming tool which comprises a bolster supporting a forming structure, the casting model comprising a bolster portion and a forming structure portion, the said portions being connected together so as to be separable.

2. A casting model as claimed in claim 1, in which the bolster portion is removable from the casting mould before the forming structure portion.

3. A casting model as claimed in claim 1 or 2, in which the said portions are connected by pegs fitted in bores in at least one of the said portions.

4. A casting model as claimed in any preceding claim, in which the forming structure portion extends into a recess in the bolster portion.

5. A casting model as claimed in any preceding claim, in which at least one of the said portions is made up of separate pieces.

6. A casting model as claimed in claim 5, in which the said pieces are connected together so as to be separable.

7. A casting model as claimed in any preceding claim, in which the forming structure portion comprises a multiplicity of superimposed layers of material bonded together.

8. A casting model as claimed in claim 7, in which the superimposed layers are successively cured layers of plastics material directly adherent to one another.

9. A casting model as claimed in claim 7, in which the superimposed layers are bonded together by interposed adhesive.

10. A casting model as claimed in any preceding claim, in which the forming structure portion has a surface coating.

11. A casting model as claimed in claim 10, in which the surface coating comprises a metal layer.

12. A casting model as claimed in any preceding claim, in which the bolster portion comprises at least one rigid foam body.

13. A casting model as claimed in claim 12, in which the rigid foam body has a machined surface covered by a solid coating blocking pores in the machined surface.

14. A method of making a forming tool using a casting model according to any preceding claim, comprising the steps of: (a)forming a casting mould against the casting model; (b) removing the casting model from the casting mould; (c) casting a metal in the casting mould, thereby producing a forming tool blank comprising a bolster supporting a forming structure; and (d) machining at least the forming structure to a predetermined shape.

15. A method as claimed in claim 14, in which the removal step (b) comprises removing the bolster portion of the casting model from the casting mould before removing the forming structure portion from the casting model.

16. A method as claimed in claim 14 or 15, in which the machining step (d) comprises electric discharge machining.

17. A method as claimed in any of claims 14 to 16, in which the metal used in the casting step (c) is a zinc alloy.

18. A method as claimed in any of claims 14 to 17, in which the forming tool is an injection moulding tool.

19. A method of making a casting model according to any of claims 1 to 13, comprising the steps of: (i) making a bolster portion; (ii) making a forming structure portion; and (iii) connecting the said portions together so that they are separable.

20. A method as claimed in claim 19, in which at least part of the bolster portion is made by machining a rigid foam body and applying to the machined surface a flowable material which sets to form a solid coating blocking pores in the machined surface.

21. A method as claimed in claim 19 or 20, in which at least part of the forming structure portion is made by a process comprising successively superimposing a multiplicity of layers of material and bonding the layers together.

22. A method as claimed in claim 21, in which the said process is stereolithography.

23. A method as claimed in claim 21, in which the said process is laminated object manufacture.

24. A method as claimed in claim 23, in which the said process produces an assembly comprising the forming structure portion, a second forming structure portion for making a second casting model corresponding to a second forming tool cooperating with the first forming tool, and an intermediate portion which lies between the first and second forming structure portions and corresponds to the part to be produced by the forming tools.

25. A method as claimed in claim 24, in which the intermediate portion is separated from the forming structure portions and constitutes a model of the part to be produced by the forming tools.

26. A method as claimed in any of claims 23 to 25, in which the said process produces an assembly comprising a plurality of strata which are separable from one another so as to facilitate separation of the forming structure portion(s) from the assembly.

27. A casting model substantially as described with reference to any of the embodiments illustrated in Figures 5 to 11 of the accompanying drawings.

28. A method of making a forming tool, substantially as described with reference to Figures 5 to 14 of the accompanying drawings.

29. A method of making a casting model, substantially as described with reference to Figure 5 and either Figure 8 or Figure 11 of the accompanying drawings.