GB2123335A - Moulds of rigid plastics foam - Google Patents

Abstract

In one embodiment (Figure 3) a self-skinning polyurethane foaming mixture is poured into a mould comprising a tapered dam (2) with a shaped panel (1). When set the mixture forms a pattern (3) with a hard, smooth surface, which can be used to cast a metal die, or to produce a sand mould for casting. A multi-part jig can also be made using such moulded plastics parts. <IMAGE>

Description

SPECIFICATION Moulds of rigid plastics foam This invention relates to the use of a mould or moulds of rigid plastics foam. The invention includes i) a method of constructing a press tool; ii) a method of making a pattern; and iii) a jig made of moulded rigid plastics foam. The invention includes the construction of dies for a sheet metal press.

The manufacture of a press tool for pressing out sheet metal panels, for example vehicle body panels, has conventionally commenced with the making of two wooden patterns for casting matching top and bottom metal dies. Pattern making for press tools is a skilled and time-consuming process in which the wooden patterns are cut, shaped and finished according to drawings produced from a prototype panel. The pattern making process normally includes enlarging certain dimensions of the pattern relative to those of the required panel to allow for shrinkage in the casting of the dies. The completed patterns are used for casting dies which are then machined to provide a smooth surface finish.The whole process must be carried out very carefully and expertly to ensure that the spacing between the top and bottom dies when mounted in their operative position in a press is uniform over the whole area of the panel.

It is one object of this invention to provide an improved method of making a pattern for casting which is both more rapid and less expensive than the known method.

According to a first aspect of this invention, a method of constructing a press tool forforming a metal article, for example a sheet metal panel, comprises building a mould in which at least part of the mould wall is formed by a master article, moulding a rigid plastics foam pattern in the mould, and using the pattern to cast a metal die for mounting in a press.

According to a second aspect of this invention, a method of making a pattern for casting a metal article comprises making a mould corresponding in shape to the shape of the outer surface of the article to be cast, and pouring a liquid foaming plastics mixture into the mould to form a rigid positive foam impression of the article for use in producing a sand mould for casting. The plastics mixture is preferably a twin-component mixture.

A pattern for casting a matching die correspond ing to the other side of the article or panel may be moulded by building a second mould in which the same master article or panel is inverted with respect to its position in the first mould. Advantageously, the first and second moulds may be built as a single structure comprising firstly the master article or panel and secondly a sleeve attached to the periphery of the article or panel and extending on both sides of it. Thus, to mould the first pattern for casting an upper die, the mould is placed with a first side of the article or panel facing upwardly, and to mould the second pattern for casting a lower die, the mould is placed the other way up with the other side of the article or panel facing upwardly.

To allow for shrinkage of dies during casting the master article or panel may be slightly enlarged beyond the dimensions of the required press-formed article or panel.

The preferred moulding material is a two-part liquid polyurethane foaming mixture for which the foaming reaction does not rely on moisture in the atmosphere. Such a mixture foams as it is poured into the mould and, being liquid, seeks out and fills cavities in the mould. The mixture forms a substantially unbroken liquid layer adjacent smooth mould surfaces to form a hard, smooth skin when the foam hardens. A pattern formed in this way is substantially rigid and is an accurate, finished pattern for casting.

By using both side of a master panel of the required thickness for moulding upper and lower patterns, the dies are automatically cast with closely matched surfaces which will have the required uniform spacing when used as a pair to press form sheet metal of substantially the same thickness as that of the master panel.

By relying on a moulding technique to transfer the shape of the master panel to the patterns, the skilled and time-consuming process of fashioning wooden patterns from drawings is avoided, thereby reducing the total time and hence the cost involved in constructing a press tool. Transportation of large patterns is considerably easier than with wooden patterns due to the very much lighter weight of foam patterns.

According to a third aspect of the invention, the second of a matched pair of patterns may be moulded by removing the first pattern from its mould, placing the first pattern with its shaped surface uppermost, and building a dam around the sides of the first pattern to form a mould cavity for the second pattern. The shaped surface of the first pattern is then covered with a layer of a uniform thickness corresponding to the required thickness of the press-formed panel and the foaming mixture is poured into the cavity. The second pattern so formed is matched to the first pattern due to the uniform thickness of the layer. This method is suitable for casting dies for single or double acting presses, marqueting and air pins on draw dies being allowed for when the dam is shaped and attached to the master prototype panel.The layer of uniform thickness maybe of wax, or may be a sheet or film of thin plastics material.

The invention also relates to the manufacture and use of a jig for assembling a structure from a plurality of component pieces, for accurate assembly of for example a vehicle bodyshell or a portion thereof, from pressed sheet metal panels.

In the design and development stages of a vehicle bodyshell several so called pre-build or preproduction bodyshells are generally produced for testing purposes and for setting up an assembly line for eventual mass production. Conventionally, once a bodyshell shape has been designed, bodyshell sections are assembled from a set of handmade panels, and these sections are used to construct a jig on which subsequent pre-production body sections can be assembled, the purpose of the jig being to hold press-formed panels in a predetermined relationship whilst they are welded together. Such jigs generally comprise several bolted together wood or metal members each shaped to engage surface features, for example edges, recesses or apertures, in the panels so that each panel can be positioned accurately relative to an adjacent panel or panels.

When a set of panels has been placed on the jig, they are welded together by spot-welding.

The construction of such jigs is a time-consuming and therefore expensive process which may have to be carried out whenever changes are made to the bodyshell shape. In the development and preproduction stages particularly, time is at a premium, and any means whereby this process can be speeded up is of advantage.

Thus according to another aspect of this invention there is provided a jig for assembling a structure from a plurality of component pieces, wherein the jig comprises a body of rigid plastics foam material shaped to correspond to the shape of the structure by virtue of having been formed in a mould comprising a pre-assembled structure of the required shape.

Taking the example of a vehicle body section, the jig may be constructed by pre-assembling a section of the required shape from handmade body panels, attaching additional panels where necessary to form a container whose inner surfaces include the bodypanels, and then pouring a liquid foaming mixture into the container. When the mixture has hardened, the preassembled section is removed, leaving a light, rigid body or jig of plastics foam material on which individual panels can be laid in the required mutual orientation.

This process can be completed in a comparatively very short time, and is therefore particularly suited to a situation in which apparatus for assembly of a number of identical structures has to be provided quickly and at a relatively low cost. In the field of vehicle bodyshell manufacture, this aspect of the invention has application in the construction not of only of mock-up or pre-production bodyshells, but also in mass production, and in large and small scale assembly of replacement body sections for repair work. However, this aspect of the invention may have applications outside this field.

The invention will now be described by way of example with reference to the drawings in which: Figure l is a cross-section through a master panel; Figure 2 is a cross-section of the master panel mounted in a sleeve to form a dual mould; Figure 3 is a cross-section of the mould with its first half filled with polyurethane foam material; Figure 4 is a cross-section of the rigid foam pattern after removal from the mould; Figure 5 is a cross-section of the mould inverted with respect to the position shown in Figure 3, and with its second half filled with polyurethane foam material to form a second rigid foam pattern; Figure 6 is a cross-section through the two patterns placed face to face to illustrate the uniform spacing between the surfaces corresponding to the original master panel;; Figure 7 is a perspective view of three panels to be assembled into a body section; Figure 8 is a perspective view of a 'master' body section constructed from the panels shown in Figure 7; Figure 9 is a perspective view of a mould including the body section of Figure 8; Figure 10 is a perspective view of a moulded jig with new panels laid on its outer suraces; Figure 11 is a diagrammatic section of another dual mould; Figure 12 is similarto Figure 11, showing the inclusion of core boxes; Figure 13 is similar to Figure 12, showing afirst stage of foaming; and Figure 14 shows a set of patterns produced from the mould of Figure 11.

As part of the development work for sheet metal panels, for example vehicle body panels, one or more prototypes of each panel are made by hand from a sheet metal blank of the same thickness as the required finished product. One method in accordance with this invention begins by taking such a prototype panel and enlarging it slightly to allow for shrinkage when a die corresponding to the panel is cast. (Wooden patterns for casting are made larger than the actual article to be cast for the same reason). Enlarging the prototype panel may be carried out by cutting it and inserting narrow strips of metal, or by stretching, or by other methods. In the description which follows, the enlarged panel is referred to as the 'master' panel.

To produce patterns for a press tool, the master panel 1 (Figure 1) is secured inside a tapered sheet metal sleeve or dam 2 (Figure 2), the dam 2 following the outline of the panel with tack welds holding it in place. The join between the panel 1 and the dam 2 may be sealed with plastics seam welder material. It will be noted that the dam 2 extends on both sides of the panel to form a dual mould having moulding cavities for making impressions of both sides of the panel.

Referring to Figure 3, a first pattern is moulded by coating the walls of the mould with a release agent and pouring a liquid twin-component polyurethane foaming mixture into the mould. A suitable polyurethane mixture is ISOFOAM RM 500. This makes a rigid, self-skinning plastics foam. The mixture foams in the mould, filling any cavities and hardens to form a very light and rigid mass having a hard smooth unbroken skin corresponding exactly to the shape of the mould, so that in appropriate cases the skin can have a glass hard finish. This mass is removed from the mould and forms a pattern 3 (Figure 4) which is suitable for making sand impressions as in the normal casting process.

Depending on the depth of the mass of material required for the pattern, it may be necessary to cool it during foaming so that it hardens uniformly. Thus air may be fan-blown against the outside of the dam 2. Or a group of tubes blanked off at their lower ends (and made of aluminium for example) may be placed so that the mixture foams around them and heat is let out by way of the group of tubes. A mass of say 8" in depth can be moulded in this manner. Another alternative, suitable for a deep pattern, is to pan the foam mixture in several successive operations, to form laminations, allowing each to harden before the next is poured. The laminations bond readily to one another. Such laminations may for instance each be 3" thick.

Having removed the first pattern 3, the mould is then inverted, Figure 5, release agent is applied to the inner surfaces of the other mould casting, and again the liquid polyurethane foaming mixture is poured into the mould. Thus a second, matching pattern 4 is obtained which has a surface profile corresponding to the other side of the master panel.

Figure 6 shows both patterns 3 and 4 placed face to face with a vertical spacing equal to the thickness of the required panel. The moulding method described above results in the perpendicular spacing at any given point on the surface of one of the patterns to be substantially constant over the whole surface area. With accurate casting of dies using these patterns and minimal finishing of the operative surfaces of the dies, the same property is exhibited by the two dies when mounted in a press and driven to their operative position.

The method can be used with shallow or deep draw panels, with a substantial time saving (particu larly in the case of a deep draw panel) over the conventional method.

If required, moulding of a large pattern may be carried out section by section, i.e. only one portion of the mould cavity need by filled at one time, the neighbouring portion being filled after the previously poured portion of material has hardened. The surface quality of the pattern in substantially unaffected by this step by step procedure.

Relatively large dies are generally hollowed out and ribbed on the back face to reduce their weight and the quantity of metal used in casting. In moulding a plastics foam pattern for such a die, a core may be suspended from the top edges of the dam 2 (see Figure 2) so that when the foaming mixture 3 fills the mould cavity (Figure 3) a recess is formed in the rear (upper) face of the pattern, corresponding to the recess required in the cast die.

The core may be a hollow box or cup-shaped sheet of metal suspended on bars resting on the dam 2.

Referring now to Figure 7, the starting point for making a jig in accordance with one aspect of the invention is preferably a set of 'master' panels, in this example three car body panels 10, 12 and 14.

These three panels are spot-welded together along their edges 16A, 16B and 18A, 18B to form a master body section 20, Figure 8. At this stage, electromag nets 22 may be placed face down on the inner surfaces of the body section 20, the connecting leads (not shown) being held clear of the body section 20 by for example a mesh steel sheet placed on the side edges 24. The electromagnets may be energised to hold them in position.

Referring to Figure 9, a false end is attached to the 'master' body section 20 by welding panels 26, 28 and 30 on the edges 32,34 and 36, so forming an open container 38. The container 38 is then filled with a polyurethane liquid foaming mixture, as previously described, which sets to form a rigid block having a hard, smooth outer skin conforming exactly to the inner surfaces of the 'master' body section. The nature of the foam can be modified if required by attaching a lid (not shown) to lugs 40 on the container edges.

Removal of the container 38 reveals a rigid foam jig or 'buck' 42 (Figure 10) on which individual panels 10A, 12A and 14A can be laid in the required orientation for welding together along their edges.

Elongate recesses may be provided or moulded in the jig corresponding to the welding lines, to prevent the welding operation affecting the plastics material.

Such recesses may be lined with a heat resistant material, for example steel. it is also possible to rivet the panels together. The electromagnets 22 embedded in jig 42 during moulding have their operative surfaces exposed on the outer surface of the jig so as to hold the panels 10A, 12A and 14A in position during moulding, by energising the electromagnets.

If the panels are to be spot welded along the seams 16 and 18, voids may be cut in the jig 42 to provide access to the inner surfaces of the panels. Such voids may be machined out or may be formed during moulding by placing core blocks in the container 38 prior to pouring the forming mixture.

Depending on the shape of the final structure to be assembled, the jig may be moulded on the inner surface of the pre-assembled structure as described above, or alternatively on the outer surfaces. In the latter case a wall of additional panels is built around the structure to form a suitably sized container for moulding.

As an alternative to electromagnets for holding panels in the required position, projecting spigots may be embedded in the jig for engaging corresponding holes or recesses in the component panels, or in some cases it may be sufficient to rely on the presence of well defined surface features for positive location.

In this manner a polyurethane mould can be made either taking the outside, the inside, a corner, or whatever shape is required, and repeating it exactly where multiples of panels are having to be put together and providing underthe area each panel is to occupy at least one electromagnet that can be switched on and off, for example by a foot switch, and this method can thus be used to assemble various panels. Once the panels are all assembled the operator switches off the magnets and takes the assembly away from the jig.

Referring to Figures 11 to 14, there is seen in Figure 11 a master panel 50 with a surrounding, tapered inner, dam 51, attached to it; the taper permits removal of one pattern part in the form of a punch, which will be described below. Also shown is an outer dam 52, attached to the panel and inner dam. The outer dam includes a base 53 with a protrusion 54 for moulding a bead (if required) on a blank holder, a tapered inner wall portion 55 and tapered upper and lower outer wall portions 56, 57, each terminating in portions 58 for incorporation of lugs on a blank holder.

Figure 12 shows the placement of hollow tapered core boxes 59, 60, 61 and 62, whose purpose is to reduce the volume of the patterns formed in the mould by providing voids, and thus to reduce the weight of castings produced from the patterns.

These core boxes are supported from the walls of the dams by suitable bars, not shown.

Figure 13 shows a first stage of moulding, with the foam material poured into the upper mould, formed by the panel 50, the dam 51, and the upper part of the dam 52. As the material enters the moulding spaces, it expands and rises and seeks out and fills all the cavities of the mould, and as it sets it forms a hard, smooth skin, with a finish like polished wood, or like glass, according to the material used. When a pattern set has been formed in the upper mould, the moulding structure is inverted, and pouring repeated at the other face of the panel 50, to fill the space between the lower core box 60 and the panel 50.

Figure 14 shows the pattern set produced by these operations. It is ready for casting. The pattern portion 61 constitutes a punch, the pattern portion 62, one part of a blank holder and the pattern part 63 the other part of the blank holder and a die or dies.

The pattern set includes a bead-forming protrusion and a bead-forming depression as shown. The spacing 64 (exaggerated) between the portions 61 and 62 on the one hand and the portion 63 on the other, will be that used in the stamping of the metal parts, and will correspond to the thickness of the master panel 50 seen in Figure 11 when the press is operated.

At the Figure 14 stage, the parts of the pattern set can be clamped together and excess material machined off to provide a parallel top and bottom to the cast die set. This can be done for example by high speed planing or routing machines.

In Figure 14 voids 65 produced by the core boxes referred to above can also be seen.

This method of making die sets is suitable for various kinds of such sets and various presses.

Considerable economy is achieved in that the parts of the die set are made as one and the clearance between them is that of the material used to construct them. Thus upper and lower parts of a die set will match without the need for excessive hand work and machining. The usual manufacture of a die set, by making each pattern individually, and then after casting, to machine match them and hand finish them. This takes much time. The present method avoids the need for such matching, which is in effect done initially when the pattern is made. In the usual practice beads are sometimes not added to blank holders at the casting stage, but are only added later by machining. As disclosed above, the method of the invention avoids the need for that.

Claims (8)

1. A method of constructing a press tool for forming a metal article, for example a sheet metal panel, comprising: making a mould in which at least part of the mould wall is formed by a master article, for example a master prototype sheet metal panel; forming a rigid plastics foam pattern in the mould; and using the pattern to cast a metal die.

2. A method of making a pattern for casting a metal article comprising: making a mould matching in shape the shape of an outer surface of the article to be cast; and pouring a liquid plastics foam material into the mould to form a rigid positive foam impression of the metal article for use in producing a sand mould for casting.

3. A method according to claim 1 or claim 2 wherein the plastics foam material is a twincomponent self-skinning polyurethane which does not rely on moisture for the foaming reaction.

4. A method according to claim 1 wherein a second pattern for casting a matching die corresponding to the other side of the article is moulded by making a second mould and placing therein the same or a like master article inverted with respect to its position in the first mould, and then there moulding the second rigid plastics foam pattern in the second mould.

5. A method according to claim 4 wherein the first and second moulds are parts of a single structure comprising the master article and a sleeve about the article, attached to its periphery, and extending on both sides of it.

6. A method according to claim 5 wherein the sleeve tapers away from both sides of the article.

7. A method according to claim 5 or claim 6 wherein the single structure is disposed with a first side of the article facing upwardly, the first pattern is moulded thereon within the first sleeve part, and then the single structure is disposed with the other side of the article facing upwardly and the second pattern is moulded thereon within the second sleeve part.

8. A method according to any preceding claim wherein at least one core box is placed in a mould, to reduce the volume of the rigid plastics foam pattern, so as to reduce the weight of a casting or castings produced therefrom.