The present invention relates to a mould for the injection moulding of metals such as magnesium and its alloys, and to an injection moulding process using such a mould.
More specifically, the latter includes a body in which at least one cavity is formed, reproducing the shape of the piece to be moulded, and at least one duct for feeding the molten metal into the cavity.
In the prior art, once the molten metal is injected, it solidifies in the feeding ducts as well as in the mould cavity. The ratio of the mass of the moulded piece to that of the total mass of solidified metal is typically around 0.5. This means that around half the material is wasted in each moulding cycle and must be recycled in special processes which are fairly complex and of not insignificant cost.
US-A-5 772 933 discloses a mold for plastics and aluminium diecasting having the features disclosed in the preamble of claim 1 which follows.
The object of the present invention is to counter the aforesaid disadvantages of the prior art.
This object is achieved according to the invention by providing a mould, and an associated moulding process having the characteristics claimed specifically in the subsequent Claims.
Advantages and characteristics of the present invention will become apparent from the detailed description which follows, with reference to the appended drawings, provided purely by way of non-limitative example, in which:
A mould for the injection moulding of metals, such as magnesium and its alloys, has (see Figures 1-3) a body 10 which includes a first die 12 and a second die 14, arranged facing each other so as to form a cavity 16 reproducing the shape of a piece to be moulded. The body 10 of the mould also includes, in association with the die 12, a plurality of plates 18, 20, 22 which directly or indirectly define a plurality of ducts for introducing the molten metal into the cavity 16 and have a common initial portion 24 and separate end portions 26.
An external plate, indicated 18, supports an injection feed socket 28 having a first duct 30 formed therein which forms part of the common initial portion 24 of the feeding ducts. A distribution plate, indicated 20, has a second duct 32 formed in it, arranged in series with and substantially orthogonal to the first duct 30. An injector-support plate, indicated 22, carries a plurality of injectors 34 which define respective ducts running substantially orthogonally from the second duct 32 and constituting the separate portion 26 of each duct.
The distribution plate 20 is arranged in a cavity 36 defined by the external plate 18, by the injector-support plate 22 and by spacer bars 38, the height of which is greater than that of the distributor plate 20 and which extend between facing peripheral portions of the external plate 18 and of the injector-support plate 22. The plates 18, 20 and 22 and the bars 38 are fixed together by screws 40, while additional screws 42 enable the injection-support plate 22 to be fixed to the die 12. Ventilation holes 41 are formed in the bars 38 in order to prevent condensation from forming in the cavity 36. Spacer elements 44 and positioning elements 46 are arranged between the distribution plate 20 and the external plate 18 and the injector-support plate 22.
The feed socket 28, the distribution plate 20 and the injectors 34 have heating means, in particular electrical resistors 48, operable to keep the metal liquid in the respective portions 24, 26 of the feeding channels. Electrical resistors 48 are also arranged in some of the spacer elements 44.
The separate end portions 26 of the feed ducts also have respective selective interception means (see Figures 4 and 5) which include a stopper 50 able to slide transverse the flow direction of the metal. Each stopper 50 is mounted (see Figure 2) on the free end of a rod 52 of a piston 54 slidable in a cylinder 56 of a respective actuator member 57, which may be hydraulic or pneumatic, for example.
The cross sections of the portions 58 of the feed ducts downstream of the stoppers 50 become steadily larger (see figure 4) towards their ends opening into the mould cavity 16.
The mould also has means for monitoring and adjusting the temperature, in particular thermocouples 60 which are associated with the injector-support plate 22, the die 12 and the resistors 48, and are connected to a central control unit 62 which is able to conveniently adjust the heat generated by the various resistors 48 on the basis of information on the temperature from the thermocouples 60, in such a way that an operating temperature corresponding to the design value is achieved across all points of the mould. In order to achieve better temperature regulation, it has proved to be advantageous to fit each injector 34 (see Figure 2) with two separate resistors 48a, 48b, the first 48a being positioned between the plates 20, 22 and the second 48b being positioned between the plate 22 and the die 12.
In order to carry out an injection cycle in the mould described above, a molten metal-feeding nozzle 66, known per se, is applied (see figure 1) to the opening 64 of the feed socket 28, with the shape of the opening 64 preventing any penetration of air, loss of molten metal or stoppage of the latter.
The molten metal, which is at a temperature of around 620-700°C, thus flows into the first duct 30, enters into the second duct 32 and from there separates into the ducts 26 formed in the various injectors 34, thereby being delivered into the cavity 16 until it is entirely full. This procedure is completed extremely quickly (it takes, for example, around 20-40 ms to inject 1,500 g of material) and under extremely high pressure, which can go as high as 800 bar. Thanks to the structure of the mould, the extremely high reaction forces generated by such pressure are mainly discharged onto the bars 38 and onto the external plate 18 and the injector-support plate 22, thus protecting the components most closely involved in the injection flow, which are undoubtedly more delicate.
The central control unit 62 controls the power output of the various resistors 48 in order to ensure that the metal is in a molten state at each point of the ducts, that is at a temperature over 600°C. At the same time, the average temperature of the mould, and that of the dies 12, 14 in particular, must be considerably lower (around 250°C) and that of the more sensitive parts, such as the control unit 62 and the actuator members 57, must be even lower (around 40°C).
In order to be able to obtain such a differentiated temperature pattern, various arrangements are used. In particular, thanks to the air in the cavity 36, the distributor plate 20, where the most heat is produced, is substantially thermally-insulated from the other plates, to which it is connected only by small thermal bridges constituted by the spacer elements 44 and the positioning elements 46, as well as by the injectors 34 and the feed socket 28.
The cylinders 56 of the actuator members 57 are mounted externally of the body 10 of the mould (see figure 3), thereby increasing the possibility of external heat dissipation, and have internal circuits (not shown in the drawings) for the circulation of a cooling fluid. Similar circuits could also be arranged in other components, such as the external plate 18 and the injection-support plate 22.
The wiring box 62 is also arranged to project from the body 10 of the mould in order to increase the possibility of heat dissipating outwards, and has insulating panels 68 interposed between it and the body and a perforated bar 70, which both reduce the heat flowing from the mould.
In addition, appropriate coupling means, such as the positioning elements 46 (see Figure 1) of the distribution plate 20 in respect of the injector-support plate 22, and the interstices 72 (see Figure 4) left in the area where the stopper 50 is mounted on the rod 52, enable the various elements to be kept correctly aligned, despite the effect of thermal expansion.
To return to the description of the injection moulding process, once the molten metal has filled the cavity 16, solidifying substantially instantly, the stoppers 50 are pushed by their respective actuator members 57 into positions whereby they intercept the ducts 26. The liquid metal which is in the portion of the ducts 26 upstream of the stoppers 50 is thus separated from the solidified metal in the downstream portions 58. It is advantageous if the stoppers 50 are force-fitted to the surfaces forming their seat, in order to avoid any air seeping into the metal.
Once the ducts have been blocked, the dies 12, 14 are opened and the moulded piece is extracted by a conventional method. When it is removed, the piece takes with it the fragments of metal which solidified in the downstream portions 58 of the ducts 26. These fragments - which do not hinder removal of the piece, thanks to their tapered shape - constitute the only portion of material which is wasted or must be recycled. The metal which remained in a liquid state in the upstream portion of the ducts will form the head of the flow of material to be injected for the next operating cycle.
In any case, the waste fragments make up only a tiny fraction - less than 5% - of the material injected into the cavity 16, while with prior art techniques the mass of the material wasted or to be recycled is about equal to that of the material injected into the cavity 16.
Naturally, the principle of the invention remaining the same, manufacturing details and embodiments may vary widely from those described purely by way of non-limitative example, without thereby departing from the scope of the invention. In particular, a mould structure of the type described above could be provided not just for one but for both the dies forming the mould cavity.