Process and apparatus for pressure die-casting

Abstract

 A process and an apparatus for the die-casting of pressure-cast metal parts according to which a metered quantity of the molten metal in a furnace (20) is fed, under pressure, into the cavity (75 - Fig. 8) of a die assembly (70, 76, 80 - Fig. 8) by means of an injection device (19) operated under controlled pressure and speed conditions; the molten metal is supplied at a pressure ranging between 50 and 350 Atm (4,9 - 34,3 MPa), and under laminar flow conditions by means of a thermally controlled injection device comprising zones thermally regulated at different temperatures for maintaining a condition of axial alignment between the chamber (21) and the plunger (22) of the injection device (19).

Description

[0001] The present invention relates to the pressure die-casting of metals, in particular light metals or white metals, such as for example aluminium, magnesium or other metals and their alloys, and more particularly refers to a process and an apparatus for the pressure die-casting of metal-alloy wheels or pressure-cast pieces in general, by means of which injection of the molten metal into the cavity of a die may be performed under controlled temperature, pressure and speed conditions in order to produce high-quality pressure-cast pieces.

[0002] In the production of pressure-casting, for example alloy wheels, it is necessary to ensure that the cast piece has optimum resistance and mechanical isotropy characteristics, i.e. is defect-free, has no air inclusions and is able to withstand the high stresses to which this piece is normally subjected during use; in certain cases, such as for example for alloy wheels, it is generally required that there should be a good surface finishing and good pressure sealing properties on the rim of the tyre.

[0003] In the production of pressure-cast pieces designed to have optimum mechanical properties, such as for example alloy wheels, currently use is made exclusively of low-pressure die-casting processes, according to which the molten metal is fed into the cavity of the die by means of compressed air, at a pressure of the order of about 1 Atm (101 KPa).

[0004] The use of low-pressure die-casting techniques, as compared to high-pressure die-casting processes normally performed with horizontal-type machines which use pressures of the order of 500-800 Atm (49 - 78,4 MPa) or more, has in practice proved necessary in order to avoid air-inclusion phenomena and the formation of blisters which generally tend to weaken the structure of the piece, making it fragile and more prone to breakage under high stresses.

[0005] Although, with the use of low pressure, pieces having good mechanical properties and an acceptable surface finish are obtained, the low pressure technique nevertheless involves an extremely long operating cycle and low productivity. Moreover, the low pressure requires that the die be formed with metal flow channels and passages having a large cross-sectional area in order to ensure that the metal in the molten state, when it comes into contact with the cold walls of the die, does not solidify rapidly, blocking said passages and channels totally or partially and preventing total filling of the die cavity. With low pressure it is therefore necessary to increase considerably the cross-sectional areas of the ducts for flowing of the molten metal, thereby increasing the dimensions of the die, the mass of material and the weight of the cast piece. Moreover, the surface finish of a piece produced at low pressure is of a fairly poor quality and, in the case of alloy wheels, requires additional finishing operations at the point where the material is fed into the die.

[0006] Moreover, the pieces obtained by means of low pressure have a low mechanical isotropy such that they must undergo additional heat treatment which is particulary delicate in that they must reach temperatures in the region of or substantially close to the melting point of the metal.

[0007] In the production of pressure-cast metal pieces it has therefore been attempted to use high-pressure injection systems; systems of this kind are described for example in US-A-4,850,420, US-A-4,583,579, DE-A-3,636,936, DE-A-3,013,226 and DE-A-3,505,554.

[0008] According to these known systems the molten metal is fed horizontally into the cavity of the die, exerting a very high pressure, equal to or greater than 500-800 Atm (49 - 78,4 MPa), and with high speeds, ranging between 40 and 120 m/s, obtaining cast pieces with a high surface finish, in extremely fast cycle-times. However, high pressure in the production of pressure-cast pieces intended to withstand high mechanical stresses has proved to be unsuitable owing to the extreme fragility of the pieces themselves as a result of the phenomenon of air-inclusion produced by the high speed and the turbulent movement of the molten metal which is injected.

[0009] In high-pressure die-casting operations, with machines of the horizontal type, there is also the problem of inadequate thermal control of the process and the machine itself, in that the device injecting the molten metal is subject to extreme and uncontrollable mechanical deformations which result in misalignment between the molten metal collection chamber and the plunger of the said injection device. Typical arrangements of the injection device and molten metal in horizontal pressure die-casting machines are described in the prior documents cited above. In particular, the patent US-A-4,583,579 proposes monitoring the expansions of the chamber and the plunger of the injection device in order to calculate the instant when the plunger is to be retracted, depending on the temperatures measured following each injection; this patent also refers generally to the use of means for cooling the plunger and a part of the injection chamber in an entirely uncontrolled manner. This document does not deal with and therefore does not propose any useful solution to the basic problem of pressure die-casting, which consists in obtaining conditions for the injection of molten metal which are controlled and suitable for the production of pressure-cast pieces with optimum mechanical properties and surface finish.

[0010] The use of pressure die-casting machines of the vertical type is illustrated in GB-A-489,944, DE-A-1,156,205 and in JP-A 01 178,359; however, these documents also refer to pressure die-casting machines of the high-pressure type which have the drawbacks indicated above. In particular the patents GB-A-489,944 and JP-A 01 178,359 relate to a machine with an injection chamber of the "open" type in which the molten metal must be poured from above, causing the well-known phenomena of oxidation of the molten metal owing to its long exposure to air, prior to injection. Therefore, in the production of pressure-cast pieces, in particular alloy wheels or pieces for which optimum mechanical properties are required, high-pressure die-casting, using machines of the horizontal and vertical type, has been entirely abandoned.

[0011] Moreover, the efficiency of the operating cycle is extremely limited. The patent DE-A-1,156,205 which defines the technological background closest to that of the present invention, as defined in the pre-characterizing part of Claim 1, illustrates a vertical pressure die-casting machine of the "closed" chamber type in which the injection device is directly connected, via a duct, to a furnace supplying the molten metal and in which the injection device comprises a vertical injection chamber and a plunger movable inside said chamber, between a retracted position, where it uncovers a lateral feed opening for the molten metal, and an advanced position where the molten metal is injected into the cavity of a die. However, this document also does not deal with and solve the basic problem of control of the pressure die-casting process for the production of high-quality metal pieces.

[0012] Despite the fact that many ideas have been proposed or suggested for the use of horizontal or vertical type machines in the production of pressure-cast metal pieces, nowadays in practice use must still be made of low-pressure die-casting, along with the limitations and drawbacks which this technology involves in terms of the excessive weight and dimensions of the cast pieces, long production times and parts which are often unacceptable from a quality point of view for many applications.

[0013] The object of the invention is to provide a process and an apparatus for the vertical pressure die-casting of metal parts by means of which it is possible to operate at relatively high pressures, compared to low-pressure die-casting, which are nevertheless considerably lower than those normally used for the production of pieces cast using high-pressure techniques, ensuring at the same time fast production cycles and the realization of cast pieces with a high surface quality and good mechanical properties, which cannot be obtained using other known technologies.

[0014] A further object of the present invention is to provide a process and an apparatus, as defined above, by means of which it is possible to reduce the phenomenon of air inclusion and the formation of oxides in the molten metal, both during feeding of the metal into the injection device and during injection into the die, obtaining cast pieces with a high degree of mechanical isotropy, as a result of which, in certain cases, it is possible to reduce substantially the resistant cross-sectional areas and hence the flow passages for the metals inside the die, achieving in this way cast pieces which are structurally strong, have thin walls and a comparatively very low weight, in extremely short operating cycles; for example, in the case of alloy wheels it is possible to reduce the weight and the production cycle about 3 or 4 times, compared to a wheel normally produced under low pressure, obtaining wheels with high mechanical and crystallographic properties of the alloy used, as well as a superior finish.

[0015] Yet another object of the invention is to provide an apparatus for the production of alloy wheels according to the process mentioned above, substantially devoid of deformation phenomena affecting the parts intended to come into contact with the high-temperature molten metal, thus ensuring an operational and processing efficiency and reliability which cannot be achieved with the other known techniques.

[0016] A further object of the invention is to provide an apparatus for pressure die-casting which uses in combination an injection device and a die particularly suitable for implementation of a medium-pressure die-casting process, according to the invention, as well as the production of wheels of a different type, without having to change the entire production die; for the purposes of the present invention "medium pressure" is understood as meaning a pressure for the injection of molten metal into the die ranging between 50 and 350 Atm (4,9 - 34,3 MPa).

[0017] The above objects can be achieved by means of a process and an apparatus for the production of metal-alloy pieces, in particular alloy wheels, according to the features of the independent claims 1 and 4.

[0018] In particular, the present invention relates to a process for the production of pressure-cast metal pieces , according to which a metered quantity of molten metal is fed into an injection device comprising a sleeve defining a vertical injection chamber and a plunger movable inside said chamber between a retracted position, where it uncovers a lateral opening for entry of the molten metal, and an advanced position, where the molten metal is injected into the cavity of a die assembly, characterized by:

• providing an axially aligned arrangement of the sleeve and the injection plunger, while maintaining under different temperature controlled conditions the zones of the plunger and the part of the chamber of the injection sleeve located in the vicinity of the pressure-casting die assembly, which come into contact with the molten metal;
• detecting the temperatures of the plunger and the injection sleeve in the region of the said temperature controlled zones; and
• injecting the molten metal into the cavity of the die assembly upon reaching of a predetermined temperature difference between said temperature controlled zones of the sleeve and the injection plunger, at a pressure ranging between 50 and 350 Atm (4,9 - 34,3 MPa), maintaining laminar flow conditions of the molten metal while it is injected and flows into the cavity of the die assembly, ensuring the required axially aligned condition between sleeve and plunger of the injection device.

[0019] These and other objects, features and advantages of the present invention will emerge more clearly from the description, together with the accompanying drawings, in which:

Fig. 1

is a general view of the apparatus with some sectioned parts;

Fig. 2

is a side view of the apparatus according to Figure 1;

Fig. 3

is a longitudinal sectional view, on a larger scale, of a first embodiment of the device for injecting molten metal into the shaping die;

Fig. 4

is an enlarged detail of the duct connecting the furnace to the molten-metal injection device;

Fig. 5

is a view along the line 5-5 of Figure 4;

Fig. 6

is a longitudinal sectional view of a second embodiment of the injection device;

Fig. 7

is a cross-sectional view along the line 7-7 of Figure 6;

Fig. 8

is an enlarged detail of the apparatus according to Figure 1, with the die in the closed condition;

Fig. 9

is a view similar to that of the preceding figure with the die in the open condition.

Fig. 10

is a general diagram of the system for temperature-regulation of the injection device;

Fig. 11

is a general diagram of the system for controlling the pressures and speeds of injection of the molten metal into the die during pressure die-casting.

[0020] As shown in Figures 1 and 2, the apparatus comprises a base 10 fixed to the floor opposite a pit 11, from which there emerge four side columns 12 held at the top by cross-beams 35.

[0021] A first movable platen 13 supporting the lower part 14 of a die and an upper platen 15 supporting the upper part 16 of the die are movable between a position close to one another, where the die is closed, and a position far from one another, where the die is open, so as to allow respectively the injection of molten metal into the die and the removal of the cast piece, in the manner illustrated below.

[0022] The raising and lowering movement of the lower platen 13 is obtained in a manner known per se by means of a pair of hydraulic cylinders provided in the bottom part of the apparatus, not shown; correspondingly, the movement of the upper platen 15 is obtained by means of a pair of hydraulic cylinders 17. In Figure 2, finally, 18 denotes an auxiliary cylinder supported centrally by the upper platen so as to perform closing and opening operations of the die.

[0023] The apparatus comprises, moreover, a device 19 for injecting molten metal, contained in a furnace having a pressurised pot 20, provided movably on one side of the apparatus itself.

[0024] More precisely, the device 19 for injecting molten metal comprises a cylindrical sleeve 21 defining an injection chamber arranged vertically and centrally with respect to the base 10. Inside the sleeve 21 there slides a plunger 22 connected via a long rod 23 to a hydraulic cylinder 23'.

[0025] As shown in Figure 3, the bottom part 21A of the sleeve 21, on one side thereof, has a radial inlet connector 24, downwardly slanted, which may be connected to the duct 25 of the furnace 20, as described further below with reference to Figures 4 and 5 of the accompanying drawings.

[0026] The apparatus according to Figures 1 and 2, on the side opposite to the furnace 20, comprises a service table 26 which may be used both for unloading the cast pieces and for replacing the die. The table 26 is slidably supported on a frame 27 and is operated so as to move forwards or backwards by means of a cylinder 28; the support frame 27, in turn, slides along horizontal guides 29, as schematically shown.

[0027] In Figure 1, finally, 30 denotes a tiltable chute for unloading the deadhead 31 which is conveyed away by means of a conveyor 32; the deadhead 31 is removed from the plunger 22 of the injection device, by a lateral pusher 46.

[0028] In Figure 2 the reference number 33 denotes a locking member for the upper platen 15, positioned on one side of the cross-members 35 for locking the upper platen 15 in the closed condition of the die during the metal injection phase.

[0029] The furnace 20 containing the molten metal is of the pot type, i.e. designed to contain a given quantity 36 of a molten metal alloy which is constantly maintained at the melting temperature by means of a suitable heating system, not shown.

[0030] The furnace 20 is of the pressurised type; the air under pressure is fed inside the furnace by a duct 37 connected to a pressurised-air source 38 by a filter 39, a solenoid valve 40 and a pressure regulator 41.

[0031] Moreover, in Figure 1, 42 denotes a probe which penetrates inside the furnace so as to provide a signal indicating the height of the molten metal bath 36 and, consequently, an indication of the quantity of metal fed into the injection sleeve 21.

[0032] The furnace 20 is supported in a movable manner, for example by means of a system of articulated bars 43 provided on a carriage 44 equipped with wheels sliding on rails; the furnace may be operated so as to be raised and lowered by means of a jack 45 or move on the rails by means of a hydraulic cylinder 45A. The movement of raising and/or sliding of the furnace 20 is used to separate the duct 25 supplying the molten metal from the inlet connector 24 of the injection device 19, so as to allow cleaning or prevent the latter from heating excessively, causing dangerous uncontrolled expansion of the injection sleeve 21 with respect to the thrusting plunger 22.

[0033] Moreover 47 in Figure 1 indicates, in the form of a solenoid valve symbol, the system for connecting the cylinder 23' to a pressurised-oil source 48. The solenoid valves 40, 47, the probe 42 and suitable devices for controlling pressurisation of the furnace 20 and the temperature of the sleeve 21 and the injection plunger 22, as explained below, are appropriately connected, by means of suitable interface circuits, to a programmable control unit 49 having a microprocessor which governs the entire production cycle.

[0034] A first embodiment of the device 19 for injecting molten metal is shown in the enlarged detail of Figure 3; the device 19 comprises a cylindrical sleeve 21 which is suitably temperature-regulated by circulation of a diathermal fluid inside an insulating jacket 51 surrounding the sleeve itself. In particular, in the example of Figure 3, the sleeve 21 has two independent temperature-regulation zones, i.e. designed to be heated and/or cooled during the production cycle, as explained below with reference to Figure 10, of which a first zone close to or in the region of the shaping die is heated and/or cooled by circulating a heating and/or cooling fluid along a first helical channel 50, while the second temperature-regulated zone, located in the region of the lateral inlet opening for the molten metal, is heated and/or cooled by circulation of a heating and/or cooling fluid along a second helical channel 50A. The sleeve 21 terminates at the top in a large annular flange 52, intended to come into contact with the die, which may in turn be heated and/or cooled by heat conduction or in another manner. The flange 52 forms a cavity 53 which opens out towards the chamber 54 of the sleeve 21, which cavity, as shown in Figure 6, is suitably shaped with a conical or diverging side wall, so as to adapt itself to a corresponding part 70 of the die, with which it defines ducts 74 for distribution of the molten metal which must be fed into the cavity of the die.

[0035] Correspondingly, the bottom part 21A of the injection sleeve and its inlet connector 24 may also be temperature-regulated in the same way as the top part of the sleeve 21.

[0036] The temperatures, and consequently the expansion of the two parts of the sleeve 21, are detected by suitable heat sensors indicated schematically by 55 for the first upper zone 50, 50B of the injection sleeve 21 and by 55A for the lower part 21A.

[0037] Similarly, the temperatures and the expansion of the plunger 22 are detected and controlled by a heat sensor 56 which penetrates into the body towards the tip of the plunger itself.

[0038] According to the present invention, in order to obtain pressure-cast pieces with optimum mechanical properties and a good surface finish, it is advisable to inject the molten metal into the die, under moderately high pressures and under laminar-flow conditions and temperature controlled conditions. Therefore, in addition to temperature-regulation of the injection sleeve 21, from tests and experiments carried out, it has been found that, in addition to temperature-regulation of the injection sleeve it is necessary to perform adequate cooling of the injection plunger, so that the surfaces intended to come into contact with the molten metal of the plunger 22 and that part of the sleeve 21 which delimits the injection chamber close to the die, at the moment of injection, have a temperature difference Dt of predetermined value, i.e. such as to ensure correct axial alignment between plunger 22 and injection sleeve 21 and adequate tolerances as regards the gap between the said plunger and the internal wall of the injection chamber.

[0039] Therefore the plunger 22 is continuously cooled by the circulation of water or other suitable fluid supplied by a separate cooling circuit. In this connection, according to the example shown in Figure 3, the plunger 22 has an internal cavity 57 inside which water or other cooling fluid supplied by a central duct 58 which extends longitudinally inside the hollow rod 23, coaxially with a duct 59 for recirculation of the cooling fluid, is continuously made to circulate.

[0040] The rod 23 of the plunger is provided moreover with a pneumatic cleaning system for cleaning the molten-metal inlet connector 24, this being performed by means of a strong jet of compressed air.

[0041] More precisely, as shown in Figure 3, the connector 24 for introducing the molten metal into the injection chamber 54 is arranged on one side of the injection sleeve 21, in the vicinity of the bottom, and is suitably downwardly slanted condition.

[0042] Therefore, the rod 23 of the plunger 22 on the same side as the inlet 24, in a position underneath the plunger 22, has a blowing nozzle 60 which can be connected to a compressed-air source at 60A under the control of a logic processing unit 49; as explained further below, at the end of the forward movement of the plunger 22, when the nozzle 60 is aligned with the inlet connector 24, it is possible to generate a strong jet of air which cleans the connector 24, ejecting the molten metal residue adhering to its walls.

[0043] Figures 4 and 5 show the system for connecting the duct 25 of the furnace to the connector 24 for introducing the molten metal into the injection chamber 54.

[0044] As shown, the duct 25 comprises a tubular element 63 which extends inside the furnace 20 nearly as far as the bottom; the tube 63 is surrounded by a collar 61 made of ceramic or other heat-insulating material designed to withstand high temperatures and is kept constantly heated at the metal melting temperature by means of a heating means 64.

[0045] The duct 25 terminates in a semi-spherical nozzle 65 which sealingly fits into a corresponding semi-spherical seat at the front end of the inlet connector 24. Reference 66 denotes moreover locking members for locking the duct 25 to the connector 24, which act against a flange 67 at the front end of the connector itself. The locking members 66 are connected to respective actuators 68 so as to be moved towards and away from the flange 67 or to rotate through 90°, as schematically shown in Figure 5, so as to allow engagement and disengagement of the supply duct 25 with respect to the inlet connector 24.

[0046] A rotating closing member 69, schematically shown in Figure 5, serves to close the duct 25 and/or prevent depressurisation of the furnace when it is separated from the injection device 19.

[0047] With reference now to Figures 6 and 7 we shall now describe a second improved embodiment of the device 19 for injecting molten metal into the die, in accordance with the process according to the invention.

[0048] Unlike the other systems and the other high-pressure die-casting apparatus, the apparatus operating in accordance with the present invention has proved to be suitable for the production of pressure-cast pieces both of small and large dimensions, obtaining metal castings of 20 or 30 kilograms which in some cases have reached and exceeded 40 kg, with molten-metal contact times of the order of several minutes, at temperatures equal to or slightly less than those for melting of the metal.

[0049] In an attempt to obtain the most suitable conditions for injection of the molten metal into the mould, guarantee better results and ensure the required axial alignment between injection sleeve 21 and plunger 22, not only has it proved useful to perform intense varied cooling of the piston 22 and the injection sleeve 21 in the zones intended to remain in contact with the molten metal for a long time, but also to perform injection of the molten metal into the die, upon reaching a predetermined temperature difference Dt, between the plunger head and that part of the injection chamber close to and/or intended to come into contact with the die or part supporting the latter, by preventing jets of molten metal being sprayed from venting holes of the die assembly.

[0050] During the tests carried out with an apparatus according to the invention, it has also proved useful, not only to define the dimensions of the injection device so as to obtain rapid and efficient removal of the heat, but also obtain easy and rapid replacement of the worn parts of the injection device which come into direct contact with the molten metal.

[0051] Therefore, as shown in Figures 6 and 7, the cylindrical sleeve 21 is made removable from the external jacket 51 and has two different temperature-regulation zones, defined by the ducts 50, 50A for circulation of the heating and/or cooling fluid, formed directly in the wall of the sleeve. Furthermore, the duct 50 for temperature-regulation of the first zone of the sleeve 21 close to the die continues into a duct 50B which extends peripherally in the flange 52 intended to come into direct contact with the said die; in Figure 6 the same reference numbers have been used to indicate parts similar or equivalent to those of Figure 3.

[0052] Correspondingly, the plunger 22 has a removable head 90 of comparatively limited thickness so as to improve the exchange of heat between the external surface of the head 90 intended to come into contact with the molten metal in the injection chamber 54, and the cooling fluid which is made to circulate inside the internal chamber 57 of the plunger 22. The head 90 is fixed removably to the body 22 of the plunger and correspondingly the body of the plunger 22 is fixed removably to the rod 23 by means of a set of locking screws 91 and 92.

[0053] Reliable and efficient regulation of the temperature of the plunger may be obtained by providing a cylindrical core 93 inside the chamber 57, provided peripherally with helical fins 94 defining a continuous channel for circulation of the fluid, which is connected to a spiral channel 95 formed between the contact surfaces of the core 93 and the head 90 of the injection plunger. The central duct 58 supplying the fluid extends through the internal core 93, emerging centrally inside the spiral duct 95, while the discharge hole 59 opens out inside the chamber 57 in a position underneath the core 93 and at the point where the helical duct defined by the peripheral fins 94 emerges; in this way the cooling fluid is initially brought into contact with the head 90, cooling it rapidly, and subsequently cools the cylindrical body of the said plunger which thus retains a cylindrical configuration, like the injection chamber 54.

[0054] On the basis of the tests carried out, it has been noted that good results are obtained by maintaining a temperature difference between the upper part of the sleeve which defines the injection chamber directed towards the die, and the head 90 of the injection plunger 22, ranging between 10° and 80°C, preferably between 30° and 60°C, for temperature values of the piston between 180° and 280°C and for temperatures of the upper part of the injection sleeve between 220° and 320°C; obviously these values, experimented on aluminium alloys, can vary from case to case depending on the melting temperature of the metal used, the dimensions and the materials used for the injection device as well as other process parameters.

[0055] With reference now to Figures 8 and 9 we shall describe the features of the die assembly with modular elements, which can be used in combination with the apparatus according to the present invention.

[0056] In Figures 8 and 9 the same reference numbers as those of the preceding figures have been used to indicate similar or equivalent parts.

[0057] As shown, the die assembly comprises a bottom die part 14 removably fixed to the lower movable surface 13 of the apparatus, and a top die part 16 removably fixed to the upper movable surface 15, as shown.

[0058] The bottom die part 14 consists of a bottom die 70 removably fixed, for example by means of bolts (not shown) to a bottom die-carrier 71 supported by the lower movable surface 13 of the apparatus. The bottom die 70 of the die assembly is provided with a peripheral casting ring 72 formed with a circular seat open downwards, into which the circular flange 52 of the sleeve 21 fits.

[0059] Between the die 70 and the casting ring 72 there are formed injection channels 74 which, on one side, open into the cavity 75 of the die assembly defined by the bottom die 70 and by the top die 76, while, on the other side, they open downwards and towards a metal distribution chamber defined by the cavity 53 at the top end of the injection sleeve, between the opposing and similarly shaped surfaces of the aforementioned cavity 53 and the bottom die 70.

[0060] Correspondingly, the top die part 16 comprises the top die 76 removably fixed, for example by means of bolts or other disengageable connecting means (not shown), to an upper die-holder 77 connected via spacers 78 to a base plate 79 which in turn is removably fixed to the upper movable surface 15.

[0061] The top die part further comprises a series of movable side-blocks 80 which are arranged circumferentially around the top die so as to define together with the latter the shaped neck of the wheel onto which a normal tyre is sealingly fitted.

[0062] The side-blocks 80, as shown in Figure 7, can be opened radially so as to allow extraction of the wheel 81 when the die assembly is opened; therefore, the side-blocks 80 are connected, via bars 82, to a movable carriage 83 provided between the upper die-holder 77 and the base plate 79. The individual thrusting bars 82 are arranged inclined and diverging downwards, being guided in their sliding travel by respective guide tubes 84.

[0063] The carriage 83 is connected to the cylinder 18, the rod of which is extended by an extractor 85 suitable guided in a sliding manner through the top die 76 and the respective die-holder 77. The extractor 85 terminates in a shaped head 86 defining the hub of the cast wheel. The die assembly, finally, is completed by the appropriate ducts for circulation of a cooling fluid.

[0064] With reference to Figure 10, we shall now describe the system for temperature-regulation of the sleeve and the plunger of the injection device.

[0065] As can be seen from said figure, the first temperature-regulation circuit 50, 50B of the injection sleeve 51 is connected to an oil-type temperature regulator 96 comprising electric heating means 97 for heating the oil, and cooling means 98 which can be actuated independently of one another, with times and temperatures programmed and controlled by means of the central processing unit 49.

[0066] More precisely, the diathermic oil cooling means inside the temperature regulator 96 is in the form of a coil for the circulation of a cooling fluid, for example water at 12°C, connected to a cooling fluid source via a solenoid valve 100 controlled by the processing unit 49, and a first cooler 101, with one or more stages, which in turn is air-cooled.

[0067] In a substantially corresponding manner, the second circuit 50A for temperature regulation of the injection sleeve 51 is connected to an oil-type temperature regulator 102 comprising heating means 103 and cooling means 104 in the form of a coil connected to the fluid source 99, via a solenoid valve 105 and the cooler 101 mentioned above. Although the cooling water is at the same temperature for both the temperature regulators 96 and 102 it is evident that the oil contained in them can be maintained at different temperatures by operating the respective heating means, or in some other way, automatically under the control of the processing unit 49 by means of which it is possible to program and monitor on a display the values and temperature histograms of the various temperature-regulated zones.

[0068] In a manner substantially similar to the two temperature-regulation circuits 50 and 50A of the injection sleeve 51, the cooling circuit for the plunger 22 is connected to the water source 99 via a solenoid-valve 106 controlled by the processing unit 49, and an air cooler 107.

[0069] As mentioned above, the electronic processing unit 49 comprises memory means which can be programmed with the various operating data and parameters of the machine according to the process of the present invention; in particular it is programmed with the temperatures of the two temperature-regulated zones of the injection sleeve 51, with the cooling temperature of the head of the plunger 22 as well as with the value Dt of the temperature difference between the first temperature-regulated zone 50, 50B of the sleeve 51, in the region of the zone close to or in contact with the pressure-casting die, which values may be read from a special display of the processing unit 49.

[0070] Finally, with reference to Figure 11, we shall describe the system for controlling the displacement in time of the piston and the pressure of the oil supplied to the hydraulic cylinder 23' operating the injection device 19 which, indirectly, via special calculation algorithms provided in a memory of the electronic processing unit 49, are able to supply corresponding values relating to the speed and pressure injection of the molten metal into the die, to which a laminar flow state of the metal itself corresponds.

[0071] As shown, a linear displacement transducer 108 is connected to an arm 108' fixed to the rod 23 of the piston of the hydraulic cylinder 23'.

[0072] The displacement S1 detected by the transducer 107 via an interface 109 or in another way is converted into an electrical signal U1 supplied to the inlet of a regulating circuit 110 forming part of the electronic processing unit 49, by means of which it is possible to regulate and convert, via a special calculation algorithm stored in the logic circuit 111 for operation of the machine, the displacement in time of the rod 23 into data indicating the displacement speed of the plunger 22 of the injection device and consequently the maximum injection speed of the molten metal calculated at the point where the ducts supplying and distributing the molten metal into the die cavity have the smaller cross-section.

[0073] In turn the single-acting cylinder 23' is connected to a pressurised-oil source 48 via a solenoid valve of the proportional type, in turn operationally connected to the logic control circuit 111.

[0074] Finally, the pressure P of the oil supplied to the hydraulic cylinder 23', via an interface 112 is converted into a signal U2 supplied to a second data inlet of the regulating circuit 110 which, via another calculation algorithm of the logic circuit 111, is converted into a pressure signal P corresponding to the value of the specific pressure at which the molten metal is injected into the die.

[0075] Operation of the pressure die-casting apparatus and the process according to the invention are described hereinbelow.

[0076] The molten metal 36 inside the furnace 20 is kept at the desired melting temperature by means of the heating system with which the furnace is provided. It is assumed moreover that the die assembly is open with the two surfaces 13 and 15 completely raised in the condition shown in Figure 7 and that the duct 25 supplying the molten metal of the furnace is separated from the inlet connector 24 of the injection chamber. The plunger 22 is moreover lowered in the condition shown in Figure 1.

[0077] Under these conditions, at the start of each working cycle, on the basis of the working program stored in the permanent memory of the electronic processing unit 49, the solenoid valves 100, 105 and the electrical resistances 97 and 103 of the two temperature regulators 99 and 102 are activated so as to bring the diathermic oil to a suitable temperature for preheating of the temperature-regulated zones 50 and 50A, for example to temperatures close to that for melting of the metal, or slightly lower, the values of which have been prestored in the processing unit 49. These temperatures are detected by the heat sensors or thermocouples 45 and 55A and communicated to the logic control unit 49 which compares them with the prestored reference temperatures, so as to avoid sudden jumps in temperature and predispose the said sleeve 21 in the required condition of axial alignment with the plunger 22. At the same time, the plunger 22 is brought to the required temperature which is detected by the heat sensor 21A and communicated to the logic control unit 49 which compares it with the corresponding prestored reference temperature. In the meantime the furnace 20 has been pressurised to a predetermined pressure value which will be kept constant, so as to allow, at each working cycle, exact metering of the quantity of molten metal which from time to time will be supplied into the injection sleeve 21.

[0078] After closing and preheating the die assembly, bringing it into the condition shown in Figure 8 and after moving up the furnace 20, locking its duct 25 to the inlet connector 24 of the injection chamber, under the control of the processing unit 49 a quantity of pressurised air is supplied into the furnace, sufficient to cause the molten metal inside the duct 25 to rise and partially fill the injection chamber 54, until a predetermined level is reached. In the meantime the cooling systems of the temperature regulators 96 and 102 are activated.

[0079] As soon as the heat sensors 55, 55A and 56 supply the processing unit 49 with the values of the temperatures detected, indicating the degree of expansion of the sleeve 21 and the plunger 22, and as soon as the difference Dt in temperature detected between the plunger 22, in particular that of its head, and the temperature of the injection sleeve part in contact with the die assembly, has reached a predetermined value, equal to a reference value stored in the processing unit 49, the latter supplies a consent signal for commanding the forward movement of the plunger 22 which is moved a short distance upwards until it closes the inlet connector 24, in view of the fact that the necessary conditions of axial alignment between sleeve 21 and plunger 22 have been achieved.

[0080] At this point the furnace 20 is moved away and its supply duct 25 separated from the connector 24, following disengagement of the locking members 66 which previously had been operated so as to maintain a sealed connection between the semi-spherical nozzle 65 of the duct 25 and the corresponding semi-spherical cavity at the end of the connector 24. At the same time the closing member 69 of the furnace duct is brought into the closed condition, after causing the descent of the molten metal left inside the duct 25 into the furnace.

[0081] After separation of the furnace from the injection device, the plunger 22 is made to rise again until the air blowing nozzle 60 coincides with the duct of the connector 24, supplying a strong jet of air in order to clean any molten-metal residue from the connector 24 and thus prevent the said connector from becoming accidentally blocked.

[0082] The movement of the plunger 22 continues upwards, with the sleeve 21 and the plunger 22 always being maintained under temperature controlled conditions, until the distribution chamber 53, into which the injection chamber 54 opens, is completely filled.

[0083] During this phase, the position and the speed of displacement of the plunger 22 are detected by the linear transducer 107. Continuing the upward movement of the plunger 22 under the control of the processing unit 49, the metal material in the molten state is consequently forced to flow along the channels of the bottom die part, supplying the latter at different predetermined injection points, for example along the peripheral edge of the die cavity, at a predetermined pressure, for example ranging between 50 and 350 Atm (4,9 - 34,3 MPa), and at a speed which are controlled by the control system shown in Figure 11. In this way, conditions of laminar flow of the molten metal are obtained, while the latter is fed into the die cavity; in this way, dangerous turbulence phenomena and trapping of air in the jet are avoided, which, otherwise, could result in the formation of blisters and a greater degree of fragility.

[0084] Therefore, during designing of the die assembly, it is necessary to calculate the appropriate flow cross-sections in accordance with the metal throughput and the pressure exerted on the molten metal, such that the injection speed measured at the point where the metal is introduced into the die cavity is in a laminar condition with a laminar speed of the flow equal to or less than 12 metres per second.

[0085] When injection of the metal into the die has been completed and after a sufficient amount of time has lapsed to allow consolidation thereof, the die assembly may be raised and opened, as shown in Figure 7, separating then the deadhead 31 which is dislodged from the plunger and made to fall along the inclined channel 30 onto the conveyor 32. At the same time, the die assembly, as it opens, causes the side-blocks to move apart, freeing the cast piece which thus falls onto the table 26 positioned underneath.

[0086] When the operating cycle has been completed, it may be resumed for the production of a new pressure-cast piece in the manner described above, while at the same time cooling and varied temperature-regulation of the plunger and the sleeve of the injection device are continued.

Claims

1. Process for the production of metal pressure die-castings, according to which a metered quantity of molten metal (36) is fed into an injection device (19) comprising a sleeve (21) defining a vertical injection chamber, and a plunger (22) movable inside said chamber between a retracted position, where it uncovers a lateral opening (24) for entry of the molten metal (36), and an advanced position, where the molten metal (36) is injected into the cavity of a die assembly, characterized by:

- providing an axially aligned arrangement of the sleeve (21) and the injection plunger (22), while maintaining under different temperature controlled conditions the zones (90, 50, 50B) of the plunger (22) and the part of the chamber of the injection sleeve (21) located in the vicinity of the pressure-casting die assembly, which come into contact with the molten metal (36);

- detecting the temperatures of the plunger (22) and the injection sleeve (21) in the region of the said controlled temperature zones (90, 50, 50B); and,

- injecting the molten metal (36) into the cavity of the die assembly upon reaching of a predetermined temperature difference (Dt) between said temperature controlled zones (90, 50, 50B) of the sleeve (21) and the injection plunger (22), at a pressure ranging between 50 and 350 Atm (4,9 - 34,3 MPa), maintaining laminar flow conditions of the molten metal (36) while it is injected and flows into the cavity of the die assembly, ensuring the required axially aligned condition between sleeve (21) and plunger (22) of the injection device (19).

2. Process according to Claim 1, characterized in that the molten metal (36) is supplied at peripheral injection points (74) and/or in the central part of the die cavity.

3. Process according to the preceding claims, characterized in that the peripheral injection points (74) are positioned along an external edge of the outline of the wheel defined by the die cavity.

4. Apparatus for the pressure die-casting of metal castings, comprising:

- a pot furnace (20) of the pressurised type for containing molten metal (36);

- a horizontally arranged die assembly (70, 76, 80), said die assembly (70, 76, 80) having a bottom die part (70) and a top die part (76) movable relative to one another between an open condition, and a closed condition in which said die assembly parts (70, 76, 80) define a shaping cavity (75) corresponding to the piece to be produced, said die assembly (70, 76, 80) having channels (74) for supplying the molten metal (36) which open into the aforementioned cavity (75);

- an injection device (19) for injecting the molten metal (36) comprising a cylindrical sleeve (21) having a lateral opening (24) for feeding metal (36), which can be connected to the furnace (20) by a supply duct (25), and a plunger (22) axially sliding in the aforementioned cylindrical sleeve (21), the plunger (22) being connected to an operating actuator (23');

- and pumping means (42) for supplying a metered quantity of molten metal (36) from the furnace (20) to the sleeve (21) of the injection device (19);
   characterized in that it comprises at least a first heat sensor (55) and at least a second heat sensor (21A) for detecting the temperatures of that parts of the sleeve (21) and the plunger (22) of the injection device (19), which are intended to remain in contact with the molten metal (36);
   said first (55) and second sensor (21A) being operationally connected to an electronic control unit (49) of the programmable type;
   detecting means (107, 112, 110) for detecting the speed of displacement of the injection plunger (22) and the pressure on the metal inside the chamber of the injection sleeve (21);
   said control unit (49) being programmed with data relating to reference pressures, speeds and temperatures so as to command said injection device (19) to supply the molten metal (36) into the cavity (75) of the die assembly, upon reaching of a predetermined temperature difference detected by said first and second sensor (55, 21A), at said reference speed and pressure suitable for maintaining conditions of laminar flow of the molten metal (36) while it is injected and flows into the cavity (75) of the die assembly.

5. Apparatus according to Claim 4, characterized in that the chamber (54) of said injection sleeve (21) terminates in an enlarged cavity (53), said enlarged cavity (53) defining with a corresponding opposing surface of the bottom die part (70), a distribution chamber for the flow of molten metal (36) into the cavity (75) of the die assembly.

6. Apparatus according to Claim 4, characterized in that the supply duct (25) of the furnace (20) terminates in a semi-spherical nozzle (65) which sealingly fits into a corresponding semi-spherical seat at the front end of a connector (24) for introducing the molten metal (36) into the sleeve (21) of the injection device (19), and in that disengageable connecting means (66) are provided to sealingly connect said semi-spherical nozzle (65) to the semi-spherical seat of the inlet connector (24).

7. Apparatus according to Claim 4, characterized in that said sleeve (21) of the injection device (19) comprises an inlet connector (24) for the molten metal (36), radially and downwardly oriented, and in that pneumatic cleaning means (60, 60A) are provided for cleaning the aforementioned connector (24).

8. Apparatus according to Claim 4, characterized in that said pneumatic cleaning means comprise a blowing nozzle (60) connected to a compressed-air source, in an underlying position and sliding together with said plunger (22) of the injection device (19).

9. Apparatus according to Claim 4, characterized in that it comprises means (96, 102) for circulating a heating and/or cooling fluid for keeping the sleeve (21) of the injection device (19) at a thermostatic temperature different from the thermostatic temperature of the plunger (22) of the said injection device (19).

10. Apparatus according to Claim 4, characterized in that it comprises means (107, 109) for detecting the displacement of the injection plunger (22) and means for detecting the pressure of the oil supplied to the hydraulic cylinder (23') actuating the injection device (19), and in that said electronic control unit comprises a memory programmed with calculation algorithms for providing data indicating the speed of injection and flow of the molten metal into the die assembly, and the pressure exerted on the molten metal of said injection device.

11. Apparatus according to Claim 4, in which the die assembly comprises a bottom die part (70) and a top die part (76), further characterized in that it comprises a first movable platen (13) supporting the bottom die part (14), a second movable platen (15) supporting the top die part (16), and actuating means for moving said surfaces (13, 15) with the respective parts (14, 16) of the die assembly, between a raised position where the die assembly (14, 16) is open, and a lowered position where the die assembly (14, 16) is closed and connected to the aforementioned injection device (19).

12. Apparatus according to Claim 11, characterized in that said bottom die part (14) and said top die part (16) each comprise a shaping part (70, 76) defining the die cavity (75), removably fixed to a support part (71, 77).

13. Apparatus according to Claim 12, characterized in that the bottom part (14) of the die assembly further comprises an annular element (72) arranged peripherally around said forming die (70), so as to define peripheral channels (74) for injecting molten metal (36) into the cavity (75) of the die assembly, respectively comprising a seat (72) for sealingly seating the top end of the sleeve (21) of the injection device in a chamber (53) for distribution of the metal flow.

14. Apparatus according to Claim 11, characterized in that it comprises a series of radially movable side-blocks (80) which peripherally surround the die (70) of the top die part (16).

15. Apparatus according to Claim 14, characterized in that said movable side-blocks (80) are connected to diverging bars (82) sliding in guide collars (84) and supported by a carriage (83) axially slideable with respect to said top die part (16).

Drawing