CAST-TO-SIZE EPOXY RESIN TOOLS FOR STAMPING SHEET METAL PANELS

Abstract: Very large tools can be cast-to-size by making a pattern (8, 10, 12) of the article to be formed by the tool, laying the pattern (8, 10, 12) up in a pattern box (14), applying a release coat (18) and coating the pattern (8, 10, 12) and box (14) with a tough, curable tool surface layer (26). A small amount of epoxy resin (32) filled with a thermally-conductive particulate filler is cast into the pattern box (14) before the surface layer (26) fully cures. Relatively large chunks (28) of thermally-­ conductive material are added to the box (14) which are covered with additional cast epoxy resin (32). The cure of the epoxy resin (32) is controlled to effect complete cure at room temperature within a few days but never to exceed a peak cure temperature above about 70°C.

Description: This invention relates to a novel method of making large, cast-to-size, epoxy resin tools which are rugged enough to stamp sheet steel. Background Cast polymeric material tools are well known for making tools to stamp sheet metal or to mould synthetic plastics materials. However, all cast polymeric materials such as nylons, polyesters,epoxy resins and polyurethanes shrink and/or change shape as they harden and cure. Therefore, it has not been possible to simply cast large tools to size. The problem is further aggravated by the fact that polymer shrinkage is not predictable at a particular location of a tool, particularly where the tool varies in cross section along its length. Furthermore, the uneven cure exotherm of thermosetting resins such as epoxy resins can cause a large tool to warp or lose strength. In the past, no satisfactory way was known to prevent or compensate for uneven exotherm in very large cast tool bodies. While it is relatively easy to compensate for polymer shrinkage in very small tools, it is a very serious problem when one wants to make large tools such as punch and die sets for stamping sheet steel automotive body panels. For example, epoxy resins based on aliphatic amine-cured diglycidyl ethers of bisphenol-A exhibit shrinkage of about 0.9% when unfilled, and from about 0.4 to 0.8% when filled with 20 weight percent of various fillers (see Plastics Materials and Processes, Schwartz and Goodman, Van Nostrand Reinhold Company, 1982, page 365). A tool two metres in length moulded from such an epoxy resin would therefore be expected to shrink as much as 8 to 18 mm. Accordingly, large moulds could not be cast to size directly from patterns. The shrinkage problem of large cast tools has been addressed in a number of ways. For example, US-A- 2,836,530 (Rees) discloses a method of tool making in which the surface of the mould for casting the tool is coated with a thin spacing layer of an easily removable material. A hardenable resin is cast into the mould and solidified. During solidification, the cast body shrinks. This shrinkage is compensated by withdrawing the hardened body from the mould and removing the spacing layer from the moulding surface. The body is then replaced in the mould and a layer of surfacing material the thickness of the removed spacing layer is added between the body and the mould. When this surfacing layer hardens, it compensates for the shrinkage of the precast large body. This method creates a number of problems. Because the tool surface is bonded to the body of the tool only after it has already set, there is always a danger of delamination. Moreover, this two-step moulding process is cumbersome an time-consuming. US-A- 3,239,590 (Trimble) considered as being the closet prior art, handles the shrinkage problem in a different way. A thin layer of wear-resistant plastics material is applied over the surface of the tool mould and is allowed to harden. A reinforcing layer of thermosetting plastics material and reinforcing binder is then built up behind the facing layer. Finally, a thin parting film is applied over the built-up and cured layers and a thermosetting plastics resin is poured over the film to fill the tool mould. This resin shrinks as it cures. After the poured resin has set, it and the parting film are removed from the mould. The film is stripped away and the shrunk cast backing is replaced in the mould over a layer of uncured thermosetting resin. This resin compensates for shrinkage in the backing and adhesively bonds the facing layer and backing together. An obvious disadvantage of this method is that it is time-consuming. Another problem is that the adhesive bond between the backing and facing would be the weakest point of the tooling and would be subject to fracture under the high pressure impact conditions of metal stamping. US-A- 4,423,094 (Dearlove et al) relates to a tough, durable, epoxy-novolac resin material for use in making metal stamping dies. While this material exhibits good characteristics for tooling, it requires a two-step cure. It must first be hardened at room temperature and then post-cured at an elevated temperature of about 150°C. It would be preferable to make dies that need no elevated temperature cure. It would also be preferable to mould tools from resins less expensive than epoxy-novolac resins. Accordingly, a new method of making large, cast-to-size epoxy tools for metal stamping and polymer moulding applications has been developed. More particularly, a method of making such tools directly from a full scale model of the article to be stamped or moulded has been invented. A method of making a large, cast-to-size, epoxy resin tool according to the present invention comprises the steps of providing a pattern of the article to be formed by the tool; orienting said pattern in a pattern box; applying a release coating to the surfaces of the pattern and the box which define the surfaces of the tool; partially filling said pattern box with a liquid epoxy resin containing at least 50 volume percent of thermally-conductive particulate filler before said surfacing compound fully cures; adding at least 30 volume percent of the tool volume of chunks of thermally-conductive material to said liquid epoxy resin; filling said box with additional liquid epoxy resin to cover said chunks; and fully curing said epoxy resin and said surfacing compound so that the internal temperature of the tool never exceeds about 70°C, the presence of the thermally-conductive filler and thermally-conductive chunks serving to mediate heat generated by the epoxy resin exotherm, so as to prevent run-away cure, shrink and shape change of the tool with respect to the pattern. Brief Summary In accordance with a preferred embodiment of the present invention, a large, cast-to-size tool suitable for stamping sheet metal can be made as follows. A model or actual article to be formed by the cast-to-size tool is provided. A parting agent is applied to the model surface and dams are constructed from clay or other suitable material to define the edges of the tool. A layer of hardenable material, preferably reinforced with a fabric or fibre layer is applied to the model surface to produce a reverse image, i.e. pattern, of the stamping tool surface. This layer is backed with a layer of syntactic foam, plaster or other filler sufficient to support the weight of the tooling epoxy resin. The pattern is enclosed in a jig and a pattern box is constructed around it. The form is oriented and supported in the box to locate the mould surface in a stamping position. A release coat is applied to the form and the interior box surfaces. A hardenable, abrasion-resistant surfacing layer is then applied over the release coat. Before this surface coating is completely cured, a liquid epoxy resin filled with at least 50 volume percent of small particles of metal is poured into the box. While the epoxy resin is still liquid, at least 30 volume percent of thermally conductive chunks of the same filled epoxy resin composition, rocks, metal, or other suitable reinforcing material, are added to the mould box. If necessary, a wire mesh screen can be positioned over the chunks to keep them from floating. The box is then filled with additional liquid epoxy resin to form the base of the tool. Thermocouples may be embedded in the liquid resin to monitor cure temperatures as well as anchors for mounting the finished tool to a press. The liquid epoxy resin is cured in the box at room temperature making sure that the internal temperature within the cast epoxy resin tool never exceeds about 70°C. Generally, complete cure takes from about 48 hours to one week depending on the size of the tool. It has been observed that the centre of the tool body seems to cure first. It is believed that this allows the still-liquid epoxy resin near the tool surface to reflow and compensate for any shrinkage that may occur as the body hardens from the centre outwards. A tool formed in this way can be used in a conventional metal press to stamp ordinary cold-rolled 0.9 mm thick sheet steel against a resilient block or in conjunction with a matched die. A matched die can be forms using the epoxy resin tool as a model as will be described hereinafter. Tools up to about 2.0 metres long, weighing over four thousand kilograms, have been made which show no measurable shrink or distortion compared to the pattern. While this brief summary relates to making metal-stamping tools, the method is equally applicable to the manufacture of tools for moulding thermoplastic or thermosetting polymers. Female tools for moulding plastics can generally be formed directly from a model of the article to be moulded. Detailed Description The invention and how it may be performed are hereinafter particularly described with reference to the accompanying drawings, in which: Figure 1 shows a clay buck for a full scale model of an automotive door panel from which the pattern for the punch of a matched punch and die set is taken. Figure 2 shows a section of the buck of Figure 1 after a clay dam has been constructed at the door edges. Figure 3 shows the door buck after the pattern for making an epoxy resin punch has been applied. Figure 4 shows the pattern fitted in a pattern box after an epoxy resin punch has been cast. Figure 5 shows the punch of Figure 4 after it has set. Figures 6 and 7 show a pattern box for moulding a die using the punch of Figure 5 as a pattern. Figure 8 shows the epoxy resin punch and die mounted in a sheet metal stamping press and a sheet metal part formed with them. Figure 9 is a diagrammatic view of epoxy resin moulds made in accordance with this invention for blow-moulding a bottle. Figure 10 shows an epoxy resin tool according to the present invention, mounted in an injection-moulding press for making a cup-shaped object. In accordance with a preferred embodiment of the invention, a matched set of epoxy resin tools for stamping an automotive door panel were made as follows. Figure 1 shows a clay buck 2 of a prototype automotive vehicle. Full size clay bucks are used in the automotive industry to make patterns for hammer-form dies used to make prototype automotive vehicle panels. A door panel 3 of clay buck 2 is used as a pattern. Referring to Figure 2, surface 4 of clay door panel 3 is sized to equal the size of the finished part less the thickness of the sheet metal used to make the part. Clay dam 6 is added to clay buck 2 to define the door edges 7. Figure 3 shows the dammed door panel after a pattern for casting an epoxy resin tool has been formed. The pattern is made by applying a very thin layer of parting agent 8 to the clay buck surface. Polyvinyl alcohol has been found to be a suitable parting agent which also serves to seal the clay to prevent it from mixing with the resin used to form the surface of the pattern. After parting agent 8 has dried, a fiberglass cloth impregnated with epoxy resin 10 is laid over the parting agent. The epoxy resin is gently worked with a roller or brush to make sure it conforms perfectly to the buck surface. After this layer has set, a syntactic polyurethane foam 12 is cast behind it to provide the epoxy resin-fibreglass layer with support. Referring to Figure 4, the pattern of Figure 3 is shown set in a wooden pattern box 14. The pattern is set in sand 16 or some other retaining medium, the top surface of which which defines flange 17 for the metal part to be stamped. The flange surface is provided with a smooth polymeric coating 18 to form a good surface for casting the epoxy resin. Box 14 shown in Figure 4 is made of wood, but another box material capable of retaining the pattern and the cast tool would be equally suitable. A very thin layer of parting agent (not shown) is applied to the surface of the pattern, the flange surfaces, and the box surfaces against which the tool will be cast. Polyvinyl alcohol is also suited to this application. Where very large tools are made, it is much preferred to mould-in the means for mounting the tools to a press. Figure 4 shows one of several steel pipes 20 with threaded internal diameters 21 at the ends which are later used to mount the tool to a press. Pipes 20 are inserted through box 14 and capped with bolts 22 to prevent leakage of epoxy resin. Different tools may require different surface characteristics to optimize wear and performance. Room temperature curable epoxy resins filled with silicon carbide powder or other abrasion-resistant fillers are suitable for sheet metal stamping tools. Another preferred surface material for stamping tools is UC-25, a room temperature curable, two-part polyurethane made by KONAP Chemical Co. This polyurethane seems to adapt well to stamping sheet metal. A surface layer 26 is applied to the pattern flange box surfaces. Because the layer is thin, generally not more than 15 mm thick, it conforms to the pattern without shrinkage. In order to get a good bond between surface layer 26 and epoxy backing resin 32, it is preferred to cast the body of the tool before surface layer 26 has fully cured. Accordingly, after the surface layer 26 is applied, pattern box 14 is filled about one-third of the way with a suitable epoxy resin 32, such as Magnolia Plastics, Inc. product nos. 6010, 6012, 6013, 6015, 6025 Magno-Ceram and Super-Ceram. At this point, steel rods 24 of the type used to reinforce concrete are positioned in the pattern box. These rods help reinforce very large tools but are not necessary in smaller tools. The box is then filled nearly to the top with chunks 28 of a thermally-conductive, high strength filler. The liquid epoxy resin already in the pattern box prevents the chunks from damaging the pattern or tool surface. It has been found that the filler chunks should comprise at least 30 volume percent and preferably about 70 volume percent of the entire tool in order to prevent shrinkage as the epoxy resin cures and to increase the compressive strength of the tool. It is also necessary that the filler chunks readily transmit heat generated by the epoxy resin exotherm. Dissipation of heat by the chunk filler prevents the occurrence of hot spots within the tool body which can cause it to shrink and warp. One very suitable chunk filler is large pieces of old epoxy tools made in accordance with this invention. Another suitable material is fieldstone rocks up to several centimeters in diameter. Rocks have high compressive strength and are sufficiently thermally conductive to moderate epoxy resin cure. Concrete chunks do not have adequate compressive strength for making sheet metal stamping tools but they could be adequate for lower compressive strength applications, such as in moulds for moulding plastics materials. Large pieces of scrap iron, steel, aluminium, copper, or ceramic would also be suitable. If the filler chunks have a lower specific gravity than the cast epoxy resin, a screen 30 can be placed over chunks 28 to keep them from floating. After all the chunks have been added, the pattern box is filled to the desired level with additional epoxy resin 32. The type of epoxy resin used to cast tools is not critical so long as it does not reach a temperature above about 70°C, and preferably below about 65°C, when it is allowed to cure at room temperature (about 23°C). Both during cure and end use of a tool, it should not be exposed to temperatures above the glass transition temperature Tg of the epoxy resin. At the glass transition temperature, the properties of an epoxy resin change from the resin being very hard and brittle to the resin being relatively soft and more flexible. Above the glass transition temperature, stress applied to a tool could cause permanent deformation therein. Exotherms above the glass transition temperature experienced during cure could cause shape changes which become permanent when the tool is fully hardened. As noted above, the incorporation of large chunks of thermally-conductive material greatly helps dissipate heat generated by the epoxy resin cure exotherm. It is also helpful to use as high a volume of filler chunks as possible to decrease the bulk of heat-generating epoxy resin present when the tool is cast. The preferred epoxy resin is a diglycidyl ether of bisphenol-A (DGEBA) resin such as the Epon series of DGEBA resins sold by Shell Chemical Co. The preferred epoxy resin hardeners are stoichiometric (rather than catalytic), aromatic, polyfunctional, primary amines such as dimethylaniline (DMA) or adducts thereof. The epoxy resin and curing agent are mixed in stoichiometric ratios (about 90 parts by volume epoxy resin to 10 parts by volume curing agent) immediately before they are used, to prevent premature hardening. The liquid epoxy resins should be filled with at least 30 volume percent and preferably 70 volume percent of a thermally-conductive particulate filler. As much filler as can be used without losing pourability of the epoxy resin is preferred. The use of two fillers is required by the practice of the present invention: first, large chunks (from a few millimetres to many centimetres across) of a thermally-conductive, high strength material that will bond well with the cast epoxy resin, and second, small particles (less than a few millimetres across) of thermally-conductive filler which are incorporated in the liquid epoxy resin before it is cast. This particulate filler adds strength to the epoxy resin, but it also mediates the exotherm of the epoxy resin cure along with the chunk filler. A preferred particulate filler is iron filings although other metal, ceramic, stone, glass, graphite, mineral fibre or other small particle fillers would also be suitable. Figure 5 shows the male tool or punch 34 of Figure 4 after hardening and removal from box 14. It could be mounted on a press as is and used in conjunction with a rubber pad to partially form a sheet metal door panel. Forming could then be completed by hammering the sheet metal around the tool edges with a soft mallet. However, the tools made by the subject method are also tough enough to be used as both members of a matched die set. Figure 6 shows the male tool 34 of Figure 5 mounted in pattern box 36 preparatory to making a complementary female tool 40. Tool 34 is fixed in the bottom of box 30 by bolts 22 inserted in the ends of steel pipes 20. A layer of wax 38 which has the same thickness as the metal to be stamped with the tool is laid on the surface of male tool 34. This allows the finished tools to work with one another without binding. A thin layer of parting agent (not shown) is applied to the wax 38 and the sides of box 36. Pipes 20' with threaded ends for mounting female tool 40 to the press are inserted through box 36 and capped with bolts 22'. An abrasion resistant surface coat 26' such as that used for the male tool is applied. Reinforcing rods 24' are then inserted in the box. Before surface coat 26' is fully cured, pattern box 36 is filled with epoxy resin 32' about one-third of the way up from the male tool. The box is then filled nearly to the top with chunks 28' of thermally conductive, high strength filler of the type described above. A screen 30' may be used to keep the chunks 28' from floating. Enough liquid epoxy resin 32' is added to cover the chunks and fill the box. Figure 7 shows a section of the box in perspective after the female tool has been cast using the wax coated male tool as a pattern. Figure 8 shows the matched male and female dies of Figures 6 and 7 mounted in a metal stamping press 41. Female tool 40 is fastened to binder ring 42 of top platen 52 by mounting bolts 46 in pipes 20'. Male tool 34 is fastened to binder ring 44 resting on base 48 on bottom platen 50 by mounting bolts 47. The tools were successfully used to stamp several 9 mm thick cold rolled steel door panels 54. Tools of this sort have been used to stamp many different sheet steel automotive exterior panels and structural parts. None of the tools has, to date, been run until failure. 209 engine cradles, a long, narrow part with a fairly deep draw, were stamped from 15.2 mm thick steel with no measurable tool wear or damage. Figure 9 shows moulds 100 and 102 made in accordance with this invention. The moulds shown are for blow-moulding a hollow bottle 106 in mould cavity 104. A thermoplastic parison for the bottle is inflated with air from blow pin 108 of manifold 110 whilst moulds 100 and 102 are closed. Ducts for carrying a liquid to regulate mould temperature can be incorporated in the moulds in the same manner that mounting pipes 20 were incorporated in the sheet metal stamping dies. The inclusion of a large volume of thermally-conductive filler in the epoxy resin and filler chunks in the tools makes them particularly useful in plastic moulding operations where mould temperatures may vary throughout each moulding cycle. In any moulding or stamping operation it is important to choose an epoxy resin with a higher glass transition temperature than the use temperature to prevent tool damage. Blow-moulds for making hollow automotive seat backs were made substantially by the method used to make the metal stamping tools described above. 150 seat backs were successfully moulded from glass-filled; modified polypropylene oxide resin, Noryl, sold by General Electric at about 330° C in the epoxy resin moulds. Figure 10 shows a clamp portion of a machine for injection-moulding a cup-shaped object with moulds made in accordance with this invention. Female moulds 60 may have an undercut base 62 which secures the mould in place in retainer plate 64. Plate 64 is shown with cooling lines 66 although these lines could be moulded into the epoxy resin moulds. Sprues 68 for filling the moulds would be formed at the time the female moulds 60 are cast. The polymeric material runs from the injection port 70 in mould base 72, through runners 73 then into sprues 68. Female moulds 60 have a generally cylindrical and an abrasion resistant surface 74. The moulds are shown filled with chunks 77 of broken-up tools of like composition. The chunks are embedded in an iron-filled epoxy resin 78. Male tools 80 also have an abrasion resistant surface layer 82 and a body of chunks 84 embedded in epoxy resin 86. Male moulds 80 have a base 88 by which they are secured in retainer plate 90. Annular orifices would be formed in the male moulds to accomodate ejector pins 92 carried on ejector plates 94. Mould base 96 is designed to accomodate the ejector mechanism. The epoxy resin used to make such tools is preferably filled with about 70 volume percent steel particles. This makes it easy to thermally cycle the tools as required in the injection moulding of thermoplastics. Seventy radiator shrouds were injection moulded in tools made from a material known as Super Ceram (sold by Magnolia plastics) in accordance with the invention. Some of the parts were made of polyethylene, some from acrylonitrile butadiene styrene and some from polycarbonate. Temperatures of about 320° C were experienced by the tools with no adverse side effects. The method of casting tools to size according to the present invention also has application in making vacuum-forming tools, compression-moulding tools, calendering rolls and to other synthetic plastics forming tools. In summary, the present invention comprises a new method of making very large, cast-to-size, epoxy resin tools. The method entails taking a pattern of a tool, laying the pattern up in a pattern box, applying a release coat and coating the pattern with a tough tool surface layer. A small amount of epoxy resin is cast into the box before the surface layer fully cures followed by the addition of chunks of thermally-conductive material. The box is filled with additional liquid epoxy resin to cover the chunks and form a flat backing for the tool. The cure of the poured epoxy resin is carefully controlled to effect complete cure within a few days but to never exceed a peak cure temperature above about 70°C.

Claims: A method of making a large, cast-to-size, epoxy resin tool (34) which includes the steps of providing a pattern (8,10,12) of the article to be formed by the tool; orienting said pattern (8,10,12) in a pattern box (14); and applying a release coating (18) to the surface of the pattern (8,10,12) and the box (14) which define the surfaces of the tool (34); characterised in that the method includes the steps of partially filling said pattern box (14) with a liquid epoxy resin (32) containing at least 50 volume percent of thermally-conductive particulate filler; adding at least 30 volume percent of the tool volume of chunks (28) of thermally-conductive material to said liquid epoxy resin (32); filling said box (14) with additional liquid epoxy resin (32) to cover said chunks (28); and fully curing said epoxy resin (32) so that the internal temperature of the tool (34) never exceeds about 70°C, the presence of the thermally-conductive filler and thermally-conductive chunks (28) serving to mediate heat generated by the epoxy resin exotherm, so as to prevent run-away cure, shrink and shape change of the tool (34) with respect to the pattern (8,10,12). A method of making a large, cast-to-size, matched set of epoxy resin tools (34,40) for forming sheet stock which includes the steps of producing a large, cast-to-size, epoxy resin first tool (34) according to Claim 1, orienting the fully-cured first tool (34) in a second pattern box (36); applying a removable layer (38) having the thickness of the sheet stock to be formed by the tool set (34,40) to the surfaces of the cured first tool (34) and the pattern box (36) that define the working surface of a second tool (40) of the set (34,40); and applying a release coating to the surfaces of the removable layer (38) and the second box (36) which define the surfaces of the second tool (40); characterised in that the method includes the steps of partially filling said second pattern box (36) with a liquid epoxy resin (32') containing at least 50 volume percent of thermally-conductive particulate filler adding at least 30 volume percent of chunks (28') of thermally-conductive material to said liquid epoxy resin (32'); filling said second box (36) with additional liquid epoxy resin (32') to cover said chunks (28'); and fully curing said epoxy resin (32') for said second tool (40) so that the internal temperature of the second tool never exceeds about 70°C, the presence of the thermally-conductive filler and thermally-conductive chunks (28') serving to mediate heat generated by the epoxy exotherm to prevent run-away cure, shrink and shape change of the second tool (40) of the tool set with respect to the first tool (34) of the tool set. A method of making a large, cast-to-size, epoxy resin tool (34) according to Claim 1, characterised in that the chunks (28) of thermally-conductive material are selected from the group consisting of chunks of tools of like composition, fieldstone or metal. A method of making a large, cast-to-size, epoxy resin tool (34) according to Claim 1, characterised in that thermally-conductive particulate filler consists essentially of metallic iron. A method of making a large, cast-to-size, epoxy resin tool (34) according to Claim 1, characterised in that reinforcing rods (20) are positioned in the pattern box (14) before additional liquid epoxy resin (32) is added to cover the chunks (28) of thermally-conductive material. A method of making a large, cast-to-size, epoxy resin tool (34) according to Claim 1, characterised in that chunks (28) of thermally-conductive material are selected from the group consisting of chunks of tools of like composition or fieldstone, and in that the thermally-conductive particulate filler consists essentially of metallic iron.

Inventors: MARTELL, WILLIAM ANTHONY, WEISS, LADISLAUS
Applicants: GENERAL MOTORS CORPORATION
Application year: 1985
Main classification: B29C39
IPC codes: B29C70, B21D37, B29C33, B29C39, B29L31, B29K63, B29K105