MANUFACTURE OF DIES FOR PRESSURE CASTING

Abstract: The problem of leakage of fluid material from the mould cavity (36) in a pressure forming process between the faces of separate die-parts (29 and 31) is conventionally overcome by careful machining of the die parts. The invention provides a cheaper solution in which separable die parts are formed by casting a metallic slurry against a pattern (11) of the article to be pressure formed. Preferably a second cast die-part (31) is cast against a previously formed, first die part (29).

Description: , ^ , MANUFACTURE OF DIES FOR PRESSURE CASTING TECHNICAL FIELD This invention relates to methods of forming from a fluid substance an article within a cavity and more particularly, but not exclusively, is concerned with methods of injection moulding of polymeric substances and methods of pressure die-casting metals. Various methods of forming an article within a cavity from a fluid substance are known. Amongst such methods are methods of casting metals such as pressure die- casting and methods of moulding polymeric materials such as blow moulding and injection moulding, in which the fluid substance is urged to flow into and fill the cavity by the application of a fluid pressure differential across the fluid, which differential is distinct from that which arises from any static head of pressure there may be in the fluid. Such methods are referred to hereinafter as methods of pressure forming. Generally, the pressure differential in methods of pressure forming is provided by applying a large positive pressure behind the fluid flowing into the cavity, although it is possible also to provide the pressure differential by evacuating the cavity to a greater or lesser extent. For pressure forming, it is usual to form the cavity from a die composed of two separable parts in order that the article formed within the cavity should be removable following the forming operation without destroying the die defining the cavity. The die- parts meet and contact one another along a parting line which traces a path around the wall of the cavity. In pressure forming, close contact of the die-parts one with another along the parting line is essential if the fluid substance is not to leak from the cavity. The need for closeness of contact between the die- parts increases as higher pressure differentials are employed. Accordingly, it has hitherto been customary to manufacture die-parts for high pressure forming methods by careful machining of ferrous, or other high melting point alloy, stock within very fine tolerances. The resulting die-parts can be used without detriment in long production runs of pressure formed articles. However, the machining operations needed to manufacture the die-parts are expensive and, consequently, articles produced in short production runs by such dies are -also expensive. It is widely known that substances change their dimensions firstly on cooling and secondly on trans¬ forming from a liquid to a solid. This is evident when metals are cast conventionally into moulds. Metals which are used commercially on a large scale are not pure substances but rather are solutions of amounts of various solute elements within a solvent substance. When these metals freeze, they throw out primary solid of a composition which is somewhat different from that of the liquid, and generally the primary solid comprises dendritic particles. The metals freeze over a freezing range of temper¬ atures between their liquidus and their solidus. During freezing, as the temperature descends through the freezing range, the dentritic particles of primary solid interact with one another at the same time as the metal is changing its dimensions as a result of the drop in temperature, and this tends to produce defects such as porosity and segregation in the resulting casting. It is generally believed that these phenomena, unless rigorously controlled, are responsible for weaknesses in the casting and give rise to uncertainty in the dimensions thereof, so rendering castings unsuitable for immediate use as articles whose strength and precise dimensions, surface detail and texture are of crucial importance. DISCLOSURE OF INVENTION According to the present invention, these is provided a method of pressure forming an article within a cavity formed between first and second die-parts, the die-parts having been formed, and the shape of the cavity having been determined, by casting a slurry (as hereinafter defined) against a pattern of the said ar-ticle. In this specification, a "slurry" means a homogeneous or substantially homogeneous metallic mixture whose continuous phase is liquid and whose dispersed phase is solid, the temperature of the slurry being one at which the liquid and the solid can co-exist in equilibrium. The slurry can be, for example, a eutectic alloy, in which case the dispersed phase comprises two or more solid phases. More typically, however, the dispersed phase comprises a single primary solid phase which appears over at least the upper part of the freezing range of the alloy. Usually, the freezing range is from 60° to 100°C and the primary solid is dendritic. The substance which forms the slurry is acted upon when it is in its freezing range (normally by stirring) in order to maintain the dispersed phase as a multitude of discrete particles rather than as a massive solid deposit around the sides of the vessel in which the slurry is contained. Typically, the slurries referred - JREX£- OMPI /., WIPO - , - H - to herein are about 0? by weight solid so that, at the moment of their casting, 5055 of their latent heat has already been evolved and 50? of their freezing contraction has already taken place. For accurate forming of the die cavity, the pattern should be of a metal whose coefficient of thermal expansion in the relevant temperature range is similar to that of the alloy being cast. Preferably, the pattern is heated prior to casting the slurry against it. It is considered that preheating the pattern helps to reduce the extent of dimensional change which the pattern undergoes during casting while it is in contact with the slurry, and so assists to.provide a close fit between the pattern and the casting. Also, such preheating should reduce the rate of cooling of the slurry when it first contacts the pattern and so should assist in main- taining the slurry fluid long enough for it to fill the cavity. For similar reasons, the first die-part should be heated prior to casting the second die-part against it. Preferably, the second die-part is cast against the first die-part. The metal from which the die-parts are formed can be an alloy of aluminium. In such a case, the pattern against which the die-parts are cast can, for example, be of aluminium or of brass . Unexpectedly, it has been found that the present method is satisfactory for use in short experimental production runs. The applicants had previously supposed that die-parts, if made in this way, would deteriorate as soon as they became subject to the conditions of temperature and pressure prevailing in a pressure forming method to such an extent that it would not be worthwhile to produce such die-parts. Instead, the die-parts have shown themselves capable of retaining sufficient integrity to remain serviceable after a large number of articles have successively been pressure formed within them. Although the die-parts formed in the present method may not have such a long life as the conventional die-parts, they are generally much cheaper to produce. They are expected to be particularly useful in design experiments, pre-production runs and short produc- tion runs where small numbers of pressure formed articles are " required. Even on long production runs they may be useful as they can be cheaply replaced at regular intervals. Aluminium die halves formed in the present method been found suitable for use as cavities in the pressure die-casting of "Mazak" (Registered Trade Mark) zinc alloy in which the alloy is injected into the cavity at a pressure typically around -4 5 x 10 Kg/m , and for the injection moulding of k nylon at pressures typically around 133 x 10 Kg/m . An advantage of the method of the present invention is that cooling channels, and even a complex network of such channels, can be set into the die-part during the casting process. Networks of cooling channels are feasible which it might not be possible readily to provide by the conventional machining processes. The provision of precisely located cooling channels is of increasing importance in the injection moulding industry . The invention will now be further described with reference to the following Examples.. Example 1 The steps involved in forming a die for use in a method according to the present invention will now be described by way of example, reference being made to the accompanying drawings, in which: Figure 1 is a view, in perspective, of an aluminium pattern for a door handle to be formed in nylon by an injection moulding method according to the present invention; Figure 2 is a vertical section of a pair of moulding boxes prepared for use in one stage of a method according to the present invention; Figure 3 and H are vertical sections similar to that of Figure 2, of further stages in the method; and Figure 5 is a vertical section of a pair of die halves which have been cast in the moulding boxes, showing schematically how they can be used in an injection moulding process to produce a nylon handle correspon- ding to the aluminium pattern. In Figure 1 there is shown an aluminium pattern 11 for a design of door handle. Marked on the pattern 11 is a parting line 12 which corresponds to the edges of the projection of the pattern 11 onto a plane Z being the plane in which lies the base 13 of the pattern. The parting line 12 traces out a continuous loop around the circumference of the pattern 11. While there are no re-entrant surfaces on the pattern 12, the method is applicable to patterns having such re-entrant surfaces, use being made of cores in known manner, whenever it is necessary to do so. Turning now to Figure 2, a moulding box 1*. was filled with moulding sand 15 and the pattern 11 was placed on the top surface 16 of this sand. The pattern 11 was fixed against movement relative to the sand 15 by screws projecting downwardly into the sand 15 from holes tapped in the pattern 11. These screws are not shown in the drawing in order that the drawing should suffer no loss of clarity. Sand was banked up around the pattern 11 until its surface corresponded closely with the parting line 12. Four conical depressions (not shown) were made in the sand on the parting line. (These generate conical projections on one die half which themselves generate conical depressions in the other die half on subsequent casting of it against the first die half, thereby to locate the die halves accurately against one another). A proprietary mould coating was.applied to the exposed area of the pattern 11 above the sand 15 and to the top surface 16 of the sand in the area of sand surrounding the pattern to provide these areas with a coating of a rough, pimpled texture. The pattern and coated area of sand were heated. An upper mould box 17 was filled with a quantity 18 of moulding sand and a cavity 19 was formed within the sand 18. The cavity 19 was sufficiently large to provide a clearance all around the pattern 11 with the upper box 17 in its working position above the box 1*1. Two runners 20 were cut in the sand to link •_ the cavity 19 to the top surface 21 of the sand 18. A quantity of a commercial casting alloy of aluminium was melted in a crucible. During such heating, the upper box 17 was placed upon the lower box 1 and suitable channels (not shown) formed from moulding sand were arranged above the upper box 17 to provide a means for guiding the metal to be cast into the runners 20 and for providing during casting a etallostatic head of pressure sufficient to ensure that the cavity 19 is properly filled during casting. When the aluminium alloy in the crucible was fully molten it was transferred to a ladle in which it was allowed to cool. During such cooling, the alloy was agitated by stirring and vibrating it and its physical appearance and viscosity closely monitored. As the alloy cooled through its liquidus, particles of primary solid began to form. The stirring to which the alloy was continuously subject substantially prevented the establishment of temperature gradients across the alloy in the ladle and resulted in the formation of a slurry of the alloy, within which the proportion of solid material was dependent upon the temperature of the alloy. The slurry was cast against the exposed surface of the pattern 11 in the cavity 19. The air adjacent the pattern 11 was displaced by the incoming alloy, and its escape from the cavity 19 was assisted by the rough texture of the mould coating. The escape of the air prevented the formation on the casting of defects caused by entrapped air. The alloy cast within the cavity 19 was allowed to cool, and it and the pattern 11 were subsequently removed from the moulding boxes. The shape of the casting was found to conform accurately to the shape of that part of the pattern 11 which was contacted by the slurry during the casting operation and the pattern fitted snugly within the corresponding recess in the casting. The surface smoothness of the casting depends upon the texture of the mould coating. When fixing the roughness of the coating, a balance is struck between the need to assist escape of air and the need for a smooth casting surface. The casting was used as a heat sink (otherwise called a "chill") .in the next stage of the process, as shown in Figure 3. Figure 3 shows the casting produced as described above employed as a heat sink 22 and set within a quantity 23 of moulding sand within the lower moulding box 14. The pattern 11 was placed within the recess of corresponding shape in the heat sink 22 and, if desired, secured therein by the use of screws projecting through the heat sink 22. Again, the screws are not shown for reasons of clarity. A cylinder 2*1 of mild steel having a ridged cylindrical surface was placed on the top surface 25 of the heat sink. A sprue for the subsequent injection moulding process was later machined within this steel cylinder 2 ." The exposed surfaces of the pattern 11 and the top surface 25 of the heat sink 22 were given a coating of the proprietary mould coating of control¬ led roughness,as above. As above, the coated areas were heated, along with the cylinder 2h . A number of cooling tubes 28 of a copper alloy were arranged within the upper moulding box 17 in position such that, when the box 17 was filled with moulding sand and the desired cavity 27 was formed in the sand, the cooling tubes 28 spanned the cavity 27. As before, runners 20 were cut in the sand 26 to link the cavity 27 with the upper surface of the -gυ EAr ft OMPI sand . The upper box 17 was placed in its working position above the lower box 1*1 and aluminium slurry was cast into the cavity 27, as above, to form the first die half. The cast metal was allowed to cool. On removal from the moulding boxes, the first die half was found to conform closely in shape to the area of the pattern 11 above the heat sink 22 and to the top surface 25 of the heat sink. The pattern was a snug fit over the corresponding area of the first die half. In the next stage of the process, as shown in Figure ■ -., the first die half 29 carrying the pattern 11 was located within a quantity 30 of moulding sand within the lower moulding box 1*J. In the same way as the first die half was cast, a second die half provided with cooling tubes 32 was cast against the pattern 11 and the top surface 33 of the first die half 29. The casting of the second die half against the first die half helps to provide a closeness of fit of the two die-parts against one another that is of a high order. The first die half 29 and the second die half 31 prepared as above were used as an injection moulding die (otherwise called "tooling") in an injection moulding process as shown in Figure 5- Preparation of the die halves for use as tooling in such a process involved machining in the cylinder 2k a channel 33 for use as a sprue, and forming in one of the die faces 33 and 3 1 *! a runner 35 linking the die cavity 36 with the sprue 33. The back faces of the die halves were machined parallel to facilitate their mounting in an injection moulding machine. The dies were then mounted in the injection moulding machine, the cooling channels provided by the tubes 28 and 32 connected to the cooling system thereof and the sprue 33 connected to a supply of synthetic polymeric material under pressure. The die halves were then brought into contact and the polymeric material injected into the die cavity through the sprue 33 and runner 35 while maintaining a flow of cooling fluid through the cooling channels of the die halves. The die halves were subsequently separated to free the moulding from within the cavity. The die halves were examined for damage and the moulding inspected. No defects were found so another moulding was pressure formed within the cavity. 1000 mouldings were formed successively in a single experimental run. The last few mouldings to be formed were satisfactory and the ultimate life of the dies used for injection moulding has yet to be ascertained. Example 2 The procedure of Example 1 above was followed in order to produce a pair of die-halves similar to those of Example 1. The die halves were mounted in a machine for pressure die-casting of zinc alloys. In a single experimental run, the die halves were used to produce successively 200 castings of Mazak alloy. Although the last few mouldings to be produced showed some evidence of deterioration of the die halves, all the mouldings were nevertheless satisfactory. Proper escape of air from cavities in which slurries are cast can be assisted by placing the cavities under partial vacuum. Preferably, and as in the above Examples, a mould coating is also employed, but with a sufficiently high vacuum a mould coating may no longer be required. As there is considerable inconvenience associated with working with high Vacuum it appears likely that mould coatings will normally be employed, either with or without vacuum assistance during pouring. With certain patterns it may not be necessary to cast a heat sink. Thus, with a simple shape it may be sufficient to provide an aluminium block of simple shape to which the pattern can be secured, or to machine a suitable heat sink. The mould coating used in the above Examples comprised a suspension in water of finely divided zirconium oxide, aluminium oxide and colloidal graphite with a small amount of sodium silicate as a binding medium. Other mould coatings can be used, provided that, at the working temperature, they are stable and compatible with the pattern and the slurry. Cooling tubes within the die-parts can be provided as a single length or as several separate lengths of cooling tube. The arrangement of cooling tubes can be chosen with a view to establishing a specific pattern of cooling within the cavity, for example, to ensure that the article being pressure-formed solidifies directionally. While reference is made above to die halves and to first and second die-parts it is to be understood that the invention is applicable to pressure forming methods in which the cavity is defined by more than two die-parts .

Claims: CLAIMS 1. A method of pressure forming an article comprising the steps of: casting a first slurry (as hereinbefore defined) against a pattern of a first portion of the said article to form a first cast die-part and determine the shape of a first cavity therein; casting the first or a second slurry against a pattern of a second portion of the said article to form a second cast die-part and determine the shape of a second cavity therein; and pressure forming the said article between said first and second die-parts in a mould cavity which comprises said first and second cavities. 2- A method according to claim 1 in which said first cast die-part " and said second cast die-part are formed and arranged so that they are in contact with one another along a parting line during said step of pressure forming. 3. A method according to claim 2 wherein the step of casting to determine the shape of said second cavity is effected against said first cast die-part. 4. A method according to any one of the preceding claims wherein at least one of the pattern of the first portion and the pattern of the second portion is heated prior to casting against it. 5. A method according to any one of the preceding claims wherein 505. by weight of the first slurry and, if present, the second slurry, is solid. 6. A method according to any one of the preceding claims wherein the first and second die-parts are - Hi - cast from aluminium or an alloy of aluminium. 7. A method according to any one of the preceding claims wherein at least one of the steps of casting the first die-part and casting the second die-part includes the step of forming a cooling channel within the die-part being cast. 8. A method according to any one of the preceding claims wherein the mould cavity is subjected to a partial vacuum during said steps of slurry casting. 9. A method according to any one of the preceding claims, being a method of pressure die casting of a metal. 10. A method according to claim 9, wherein said metal is an alloy of zinc. 11. A method according to any one of claims 1 to 7 being a method of injection moulding of a plastics material. 12. A die part or pressure formed article formed by practice of a method according to any one of the preceding claims.

Inventors: WHEELDON J
Applicants: SELLY OAK DIECASTING LTD, WHEELDON J
Application year: 1980
Main classification: B22D17
IPC codes: B22D17, B29C45