A MOULDING DEVICE

Abstract: The present invention relates to a moulding device comprising a moulding block (1) having a moulding cavity (2) therein. A chamber (5) is connected to the cavity (2) in which a piston (4) slides. There is a closing means (6) provided between the chamber (5) and the cavity (2) which is moveable from a first position in which the mouth of the chamber (5) is sealed to allow the chamber (5) to be charged with a molten substance whilst the piston (4) is retracted, to one or more further positions which allow the molten substance to be injected directly into the cavity (2) when the piston (4) is activated.

Description: A MOULDING DEVICE The present invention relates to moulding devices, in particular, high pressure die casting machines and is particularly, although not exclusively, concerned with the production of metal castings having low porosity. There are a number of high pressure die casting devices/machines in use but many produce cast articles which have a high degree of porosity. If the level of porosity is allowed to rise above certain limits, the pores can adversely affect the properties of the component so as to lead to the failure or deterioration of the cast article during use (most porosity remains internal; only if the die castings are machined is the porosity usually exposed) . Therefore, in any high pressure die casting device, an object will be to reduce the porosity levels as close to zero as possible. A high pressure die casting (hpdc) machine which is used extensively is the "cold chamber" hpdc machine which manufactures aluminium alloys. This machine operates by transferring molten metal from a shot sleeve into a die cavity by means of a high velocity piston or plunger. The molten metal is forced along a series of channels or a runner system and through a fixed, narrow gate or opening into the die cavity. The liquid metal is then effectively sprayed through the gate to produce a first coating over the surface of the die cavity and then the remainder of the liquid metal is introduced into the cavity to complete the cast article. The first coating of liquid metal commonly produces a very fine grain surface layer having a very smooth surface finish. However, the cold chamber hpdc process suffers from two major disadvantages. First, because the molten metal has to flow through a runner system of channels, its temperature will fall by the time it reaches the narrow gate and consequently, it will freeze around the narrow gate which reduces the pressure which can be transmitted effectively from the runner and gate onto the metal in the die cavity. The reduction in pressure transmission will produce die castings which are notoriously porous and may, therefore, not be heat treatable for fear of blistering. Furthermore, subsequent machining operations will expose the porosity which causes a high rejection rate. Secondly, the cold chamber hpdc process is commonly only about 33% efficient because approximately 50% of the cast metal (i.e in the runner and gate sections) needs to be removed from each casting for remelting. Moreover, since there is an additional casting scrap rate of 5-15% the efficiency of the cold chamber hpdc process is rarely greater than 25% in material utilisation and considerably less than 20% in energy utilisation. Several solutions have been proposed to reduce porosity. One such solution involves evacuating the die set prior to casting with a view to reducing gas entrapment. However, the casting still freezes or solidifies remote from the point of application of pressure and therefore, the solidifying casting cannot be fed from the reservoir of metal in the runner and wad. Hence, contraction cavities arise in the casting. A further solution proposed was to purye the die set with oxygen or another suitable gas which would combine spontaneously with the liquid metal to remove gas from the die set. However, contraction cavities are still formed. The mould is sprayed with a lubricant prior to the casting which evaporates on contact with the hot metal so that gases are still present in the mould. A slightly different approach was to apply enhanced pressure on the wad by a smaller secondary piston but porosity still exists in the casting due to remote application of the pressure and freezing off at the gate. In order to attempt to limit the amount of porosity, another approach to high pressure die casting has been devised which is more accurately described as a "squeeze" casting. Squeeze casting is the term used to denote processes in which liquid metal is solidified under the action of a high external pressure. Two different types of squeeze casting technology have evolved based upon different approaches to metal metering and metal movement and also upon the manner in which the pressure is applied to the metal in the mould. These two processes have been given the names "direct" and "indirect" squeeze casting. In direct squeeze casting, the die set is a split mould consisting of a lower female cavity and an upper male punch. Sufficient pressure is applied to the punch, which moves to compress the liquid/solid mixture during freezing to suppress the appearance of either gas porosity or shrinkage porosity in the casting. Direct squeeze casting is thus a hybrid process combining gravity die casting with closed die forging. In indirect squeeze casting, liquid metal is injected into a closed die cavity by a small diameter piston, by which mechanism the pressure is also applied during freezing. Squeeze pressures are limited by the size of the piston and, for large area castings, some thin sections of the casting may freeze off locally and prevent the transmission of pressure to remoter regions thus allowing porosity to form. The current art of indirect squeeze casting uses vertical injection of liquid metal into the die set which has either a vertical or horizontal opening. It is to be appreciated that squeeze casting can produce much lower levels of porosity than high pressure die casting and therefore, a combination of both types of casting would be desirable. In hpdc (usually horizontal machines) the wad and runners are the only parts which are pressurised to a maximum extent because the gate freezes or solidifies and then the metal in the die freezes under low pressure. In indirect squeeze casting (vertical or horizontal machines) , the same is also true but to a lesser extent because the gates are wide open and are of fixed geometry. According to the present invention there is provided a moulding device comprising a moulding block defining a moulding cavity, a chamber connected to the cavity in which a piston is slidable, characterised in that a closing means is provided between the chamber and the cavity, the closing means being moveable from a first position, in which the mouth of the chamber is sealed to allow the chamber to be charged with a molten substance whilst the piston is in a retracted position, to one or more further positions which allow the molten substance to be injected directly into the cavity when the piston is activated. An advantage of the present invention is that because the piston acts directly on the molten substance in the mould rather than via a runner system and narrow gate, porosity levels will be reduced as there will be little or no freezing of the molten substance at the gate into the block. Preferably, the closing means is moveable to a second position spaced at a small distance from the mouth of the chamber to define a narrow orifice such that a fine spray or jet or film of molten substance is forced through the orifice and is deposited on the inner surface of the cavity when the piston is activated. Whether the molten substance is introduced in spray, jet or film form will depend on the substance used. The initial spraying of molten substance produces a good surface quality in the moulded article. Preferably, the closing means can be moved into a third retracted position which allows a less restricted entry of molten substance into the cavity. Preferably, the closing means slides through a wall of the cavity. Preferably, the closing means slides through a wall of the cavity opposite to the mouth of the chamber into the cavity. By locating the closing means opposite to the mouth into the cavity it is possible to have a symmetrical arrangement which will allow a regular spray pattern to be produced when the closing means is only partially retracted. Preferably, when the closing means is fully retracted it forms part of the inner surface of the cavity. Preferably, the closing means is a pillar which is slidable in a sleeve. Preferably, the device is for use in high-pressure die casting or indirect squeeze casting of metals. The present invention further provides a method of moulding an article comprising the steps of closing the mouth of a moulding block, charging a chamber connected to the mouth of the moulding block with a molten substance, opening the mouth to directly inject the molten substance by moving a piston slidable in the chamber. Preferably, the mouth to the block can be partially opened to define a narrow orifice such that when the piston is operated, a fine spray or jet or film of the molten substance is forced through the orifice and deposited on the inner surface of cavity. Preferably, the mouth to the block can be opened further to allow less restricted entry of the molten substance into the cavity. A preferred embodiment of the present invention will now be described in detail, by way of example only, with reference to the accompanying drawings, of which: Figure 1 depicts a multi-cavity moulding block in perspective; Figure 2 is a side view of the block in Figure 1 connected to a piston/closing means arrangement in a closed position according to a first preferred embodiment of the present invention; Figure 3 is a side view of the arrangement in Figure 2 partially open; Figure 4 is a side view of the arrangement in Figure 2 fully open; Figure 5 depicts a casting produced by the moulding block in Figure 1; Figure 6 depicts the moulding block used to produce a typical wheel centre which can be cast using the moulding device of the present invention; Figures 7 and 8 show a second preferred embodiment of the moulding device comprising the moulding block in Figure 6; Figure 9 depicts a simple square plate (single cavity) moulding block in perspective; Figures 10 and 11 show a third preferred embodiment of the moulding device comprising the moulding block in Figure 9. Figure 1 shows a moulding block 1 used for die casting of metals. The block 1 has a moulding cavity 2 which comprises a central region 3a and four leg cavities 3b. The central region 3a will form what is known as the "wad" or "biscuit" and the cavities 3b will form four "legs" or "coupons". Clearly, the "coupons" could be in any shape depending on end requirements. Figure 2 is a side view of the block 1 when connected to a piston 4 which is slidable in a chamber 5. In Figure 2 a pillar 6 which is slidable in a sleeve 8 seals the central region 3a and the piston 4 is fully retracted. Chamber 5 will be charged with molten metal in this position. Figure 3 shows the arrangement when the pillar 6 has been partially withdrawn to produce an annular gap or gate 7 through which the molten metal can pass. The piston 4 will be pushed towards the central region 3a and a fine spray of molten metal will coat the inner surface of the four leg cavities 3b. Figure 4 shows the pillar 6 fully retracted such that it clears the region 3a forming a wall of the region 3a. The piston 4 is pushed to its extreme position to inject all the molten metal into the region 3a and leg cavities 3b. Figure 5 depicts a casting produced by the moulding block in Figure 1. There is a central "wad" 9a, four "legs" 9b which are generally rectangular in shape and each of which is connected to the central wad 9a by a substantially triangular runner 9c. Figure 6 depicts a moulding block for a typical wheel centre which can be produced in accordance with the moulding device of the present invention Like reference numerals represent like features to those in Figures 1 to 5. Figures 7 and 8 show the configuration of a suitable moulding device used to produce such a wheel centre. In Figure 7, the piston 4 is fully retracted and the pillar is in the position which seals the moulding cavity 2 from the chamber 5. In this embodiment, the pillar comprises a fixed inner cylinder 6a and a slidable outer cylinder 6b. When outer cylinder 6b is withdrawn, a narrow gap or gate 7 is produced through which the molten metal can pass. This type of geometry is useful in moulding wheel centres, for example, so that excess metal in the centre can be avoided. Figure 9 depicts a simple square plate single cavity moulding block. Figures 10 and 11 depict the moulding device which could comprise such a moulding block before and after actuation of the pillar 6 and piston 4. In use, liquid alloy in sufficient quantity to fill the moulding cavity 2 comprising a central region 3a and several leg cavities 3b is poured, pumped or syphoned into the shot sleeve 5. The piston 4 is retracted and the pillar 6 is in a position to close off the sleeve 5 from central region 3a. As piston 4 is moved forwards, pillar 6 is moved to a position which creates a gap or gate 7 through which the liquid alloy is injected at high velocity into the moulding cavity 2. This first phase of injection coats the moulding cavity 2 with a skin of alloy which solidifies with a smooth surface finish and which possesses a fine grain size. The pillar 6 is then withdrawn to a further position and further liquid alloy is injected through the wider gate until the moulding cavity 2 is full of alloy. Pressure is maintained on the central wad 3a formed in the central region by piston 4 effectively squeezing the cast alloy until it has solidified. In order to minimise the porosity in the casting, it is desirable to apply a gradient of temperature from the periphery of the moulding cavity 2 (i.e the tips of leg cavities 3b) to its central region 3a. The central region 3a will then be hottest so as to remain liquid for the longest time to allow alloy to be fed to the extremeties of the casting as the alloy progressively solidifies. It has been found that wheel centres made in this way are of extremely high quality and have negligible porosity. The sleeve 5 could be positioned between a number of small die cavities. The pillar 6 region would then produce a wad of waste metal between the castings but the runner system would be decreased. It is preferable if the pillar 6 is machined along its length to create grooves through which gas can be evacuated from the moulding cavity 2. When the pillar 6 is in a closed position (e.g Figure 2) the grooves would emerge into the central region 3a and allow air or another gas to be extracted. When the pillar 6 is partially retracted (e.g Figure 3) the grooves would be enclosed by a tool steel sleeve 8 which would prevent access of liquid metal from central region 3a into the vacuum system. Clearly, the fully retracted position will determine the thickness of the final casting. Furthermore, the pillar need not be a solid cylindrical construction as depicted in Figures 7 and 8. Typically, the pillar will be made of tool steel or iron-nickel or an iron-nickel-cobalt alloy of low expansion coefficient. The front face can be coated with thin layers of, for example, Al_0 3 /TiN. Usually the pillar will slide in a tool steel sleeve 8. Additionally, copper alloy bushings or rings could be provided between the pillar 6 and sleeve 8 to conduct heat away and to limit the separation between the pillar and sleeve. Alternatively, other high temperature materials such as sialons or other ceramics could be used to make the pillar. Although aluminium alloys have been discussed, it is intended that the present invention will use magnesium and other alloys as well as solid/liquid slurries (rheocasting and thixocasting) and particulate metal matrix composites. Clearly, the moulding device of the present invention will overcome the problems of partial freezing of the molten metal prior to entering the moulding cavity and will benefit from the effects of squeeze casting as the piston pressure acts directly on the liquid in the cavity rather than via a long runner system. As mentioned earlier, it is envisaged that the moulding device of the present invention could be used both in die casting arrangements ("cold chamber" and "hot chamber") and in injection moulding arrangements with minor design modifications to suit end requirements.

Claims: CLAIMS : 1. A moulding device comprising a moulding block (1) defining a moulding cavity (2) therein, a chamber (5) connected to the cavity (2) in which a piston (4) is slidable, characterised in that a closing means (6) is provided between the chamber (5) and the cavity (2) , the closing means being moveable from a first position, in which the mouth of the chamber (5) is sealed to allow the chamber to be charged with a molten substance whilst the piston (4) is in a retracted position, to one or more further positions which allow the molten substance to be injected directly into the cavity (2) when the piston (4) is activated. 2. A device as claimed in Claim 1 characterised in that the closing means (6) is moveable to a second position spaced at a small distance from the mouth of the chamber (5) to define a narrow orifice (7) such that a fine spray or jet or film of molten substance is forced through the orifice and is deposited on the inner surface of the cavity (2) when the piston is activated. 3. A device as claimed in Claim 1 or Claim 2 characterised in that the closing means (6) can be moved into a third retracted position which allows a less restricted entry of molten substance into the cavity (2) . 4. A device as claimed in any preceding claim characterised in that the closing means (6) slides through a wall of the cavity (2) . 5. A device as claimed in Claim 4 characterised in that the closing means (6) slides through a wall of the cavity (2) opposite to the mouth of the chamber (5) into the cavity (2) . 6. A device as claimed in Claim 4 or Claim 5 characterised in that when the closing means (6) is fully retracted it forms part of the inner surface of the cavity (2) . 7. A device as claimed in any preceding claim characterised in that the closing means (6) is a pillar which is slidable in a sleeve (8) . 8. A device as claimed in any preceding claim for use in high-pressure die casting or indirect squeeze casting of metals. 9. A moulding device substantially as herein desribed and as illustrated in the accompanying drawings. 10. A method of moulding an article comprising the steps of closing the mouth of a moulding block (1) , charging a chamber (5) connected to the mouth of the moulding block (1) with a molten substance, opening the mouth to the block (1) to directly inject the molten substance by moving a piston (4) slidable in the chamber (5) . 11. A method as claimed in Claim 10 characterised in that the mouth to the block (1) can be partially opened to define a narrow orifice (7) such that when the piston (4) is operated, a fine spray or jet or film of the molten substance is forced through the orifice (7) and deposited on the inner surface of cavity (2) . 12. A method as claimed in Claim 10 or Claim 11 characterised in that the mouth to the block can be opened further to allow less restricted entry of the molten substance into the cavity (2) . 13. A method of moulding an article substantially as herein described with reference to the accompanying drawings.

Inventors: CHADWICK GEOFFREY A, CHADWICK, GEOFFREY, A.
Applicants: CHADWICK GEOFFREY A, HI TEC METALS R & D LTD, HI-TEC METALS R&D LTD.
Application year: 1992
Main classification: B22D17
IPC codes: B22D17