VACUUM PRESSURE PROCESS AND APPARATUS FOR HIGH PRESSURE DIE CASTING

Abstract

 A vacuum pressure process for high pressure die casting in a mould comprising a fixed part (1) and at least a movable part (2) defining inbetween a mould cavity (3) and a shot sleeve (6) for feeding the mould cavity (3) with molten metal: The vacuum process comprises the steps of producing a first reduction of the mold cavity pressure to a first vacuum stage, once the molten metal to be casted is inside the shot sleeve (6) and producing a second reduction of the mold cavity pressure from the first vacuum stage to a second vacuum stage lower than the first vacuum stage, after the first vacuum stage is established and before the filling of the mould cavity (3) begins. The second pressure stage is maintained at least until the mould cavity has been filled.

Description

TECHNICAL FIELD

[0001] The present invention relates to vacuum die casting methods and vacuum die casting apparatuses and especially high pressure die casting (HPDC) technology for producing thin-wall components with high performance, high dimensional accuracy, high production efficiency and considerable economic benefits for automotive and other industries.

STATE OF THE ART

[0002] It is known in the art a vacuum system which removes the air in a unique stage from the mould with a direct line from the vacuum system to the mould cavity. This system is represented in figure 1. The main disadvantage of this system is the short time to remove all volume of air in the shot sleeve and in the mould cavity. If the vacuum system is applied too early, the prefillings, presolidfications or the break of the melt front happen reducing significantly the HPDC casting quality. However, if the vacuum is not applied so premature, the remaining time is too short to evacuate all the gas in the cavity, making the system not so efficient and the HPDC quality not so good.

[0003] It is also known in the art a vacuum system which removes the air in two stages. In the first stage it evacuates the air from the shot sleeve through a wide cross-section line and in the second stage from the mould to achieve a desired pressure in the mould cavity. This system is represented in figure 2. The main disadvantage of the second system is the low soundness of wide cross-section line in the shot sleeve. The aluminium closes the wide cross-section hole in the shot sleeve when the aluminium completely fills its volume.

[0004] JP2002-224807 A discloses a conventional High Pressure Die Casting process for preventing the air entrapment porosity from leaking into the cavity of the die out of the backside of the plunger tip.

[0005] EP 0 051 310 A1 discloses a process which is known in the industry under the name Vacural^©. The melt is aspirated into the casting chamber by vacuum produced in the mould and precisely controlled. The vacuum system is produced by a buffer vessel which the vacuum in is produced by a pump. This kind of vacuum system provide valves, filters and pressure measuring equipment to ensure the proper vacuum level.

[0006] DE 19645 104 A1 describes a technology which permits to obtain a higher vacuum in the mould cavity thanks to two different vacuum buffer vessels, without the need to connect the vessels. The vacuum is generated with a dearetion valve on the mould.

[0007] US 2009/0050289 A1 describes a technology comprising at least two vacuum phases in the total casting cycle. The first vacuum phase is made in the casting chamber (shoot-sleeve plus pouring area) with large cross section and higher exhaust capacity. The second vacuum phase is made through the casting cavity.

[0008] EP 2 058 065 B1 describes a vacuum die casting method carrying out the casting with evacuation of a casting cavity, in which a molten metal is poured from a molten metal inlet of a plunger sleeve, followed by forming a vacuum chamber surrounding the inlet and an open end of the plunger sleeve that is on the opposite side of the die, and an evacuation of the vacuum chamber and the cavity starts before an operation of a plunger tip starts. Thus, it is necessary to seal also the pouring area. The target of the present invention is to solve the problems regarding the seal performance, pressure differential, and the stability of the degree of vacuum in a short time.

[0009] EP 1 970 145 A2 describes a method for venting molds without mechanically moving parts on a die casting mold, and neither vacuum pumps nor a vacuum tank are required. The mold cavity to be filled is only released when it has been vented and the casting chamber is closed. The vacuum is provided by a ventury device and thus is not possible to achieve vacuums below 200 mbar.

DESCRIPTION OF THE INVENTION

[0010] The purpose of vacuum system of the invention in the high gravity die casting (HPDC) process is to evacuate a given volume of air from the mould cavity and the shot sleeve, thus avoiding air porosity and other defects as laminations and cold flakes in the HPDC castings. The invention relates to a novel vacuum die casting preferably for Al, Mg, Zn alloys, Pb or brass in order to improve the quality of the components.

[0011] The present invention could also be applicable to any kind of die casting process without shot sleeve, such as gravity die casting processes or low pressure process.

[0012] A first aspect of the invention relates to a vacuum pressure process for high pressure casting (in cold or hot chamber) in a mould comprising a fixed part and at least a movable part (with or without moving cores) defining inbetween a mould cavity and a shot sleeve for feeding the mould cavity with molten metal.

[0013] The process comprising the steps of:

• producing a reduction of the mold cavity pressure to a first vacuum stage, once the molten metal to be casted is inside the shot sleeve. Preferably when a molten metal feeding inlet is open
• producing a reduction of the mold cavity pressure to a second vacuum stage lower than the first vacuum stage, after the first vacuum stage is established and before the filling of the mould cavity begins,
• maintaining the second pressure stage at least until the mould cavity has been filled with the molten metal.

[0014] The initial pressure of the mold cavity can be the atmospheric pressure.

[0015] The process is able to extract the gases in the mould by the application of a sequence of the vacuum process in two stages during the first injection phase of the HPDC process which includes the adjustment of both the first and second vacuum levels and also their activation optimized trigger points taking into account the piston stroke during the metal injection.

[0016] In some embodiments the process comprises the steps of:

• connecting a first vacuum producing device to the cavity of the mould, at an opposite position of the shot sleeve, the first vacuum producing device being able to reduce the mold cavity pressure to the first vacuum level,
• connecting a second vacuum producing device pump to the cavity of the mould, at an opposite position of the shot sleeve, the second vacuum producing device being able to reduce the mold cavity pressure from the first vacuum stage to the second vacuum stage lower than the first vacuum stage,
• pouring a molten metal through an inlet of the shot sleeve,
• begin the pushing movement of a plunger piston inside the shot sleeve,
• activate the first vacuum producing device
• once the inlet of the shot sleeve has been closed by the piston movement the first vacuum stage is provided in the mould cavity by the first vacuum producing device.
• after the first vacuum stage is established and before the piston begins the filling of the mould cavity the second vacuum stage is provided in the mould cavity by the second vacuum producing device, this second vacuum stage being maintained until the mould cavity has been filled.

[0017] In some embodiments the first vacuum stage reduces the pressure in the mold cavity to a pressure between 975 mbars and 200 mbars and preferably to a pressure between 900 mbars and 450 mbars. The second vacuum stage reduces the pressure in the mold cavity to a pressure between 200 mbars and 0,1 mbar, preferably to a pressure between 60 mbars and 1 mbars, and more preferably between 60 and 20 mbar.

[0018] The first stage of pressure can comprise at least an additional stage reducing the pressure to an intermediate pressure value. The second stage of pressure can comprise at least an additional stage reducing the pressure to an intermediate pressure value.

[0019] In some embodiments the process comprises a cleaning system connected to the cavity of the mould, at an opposite position of the shot sleeve, when the mould is open, after the demoulding has finished.

[0020] A second aspect of the invention relates to a vacuum apparatus for high pressure casting comprising

• a mould with a fixed part and at least a movable part (with or without moving cores) defining inbetween a mould cavity,
• a shot sleeve communicated with the mould cavity for feeding the mould cavity with molten metal, the shot sleeve comprising an inlet for pouring a molten metal into the shot sleeve, and a plunger piston for pushing the molten metal inside the mould cavity.

[0021] The apparatus further comprising

• a first vacuum producing device connected to the mould cavity, at an opposite position of the shot sleeve, for reducing the mould cavity pressure to a first vacuum stage,
• a second vacuum producing device connected to the mould cavity, at an opposite position of the shot sleeve, for reducing the mould cavity pressure to a second vacuum stage lower than the first vacuum stage,

[0022] In some preferred embodiment the first vacuum producing device comprises a vacuum ejector.

[0023] In alternative embodiments the first vacuum producing device comprises a vacuum pump and a vessel.

[0024] In some embodiments the second vacuum producing device comprises a vacuum pump and a vessel.

[0025] In some embodiments the first vacuum producing device and the second vacuum producing device are connected to a manifold.

[0026] Usually, the fluids and particles are filtered before the vessels and vacuum ejectors. However, the filter clogs, reducing the vacuum pressure capacity of the system. To solve this problem the apparatus of the invention can further comprise a bag cleaning system (item 18 of Figure 8) connected through a conduct to the cavity of the mould.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] To complete the description and in order to provide for a better understanding of the invention, a set of drawings is provided. Said drawings form an integral part of the description and illustrate embodiments of the invention, which should not be interpreted as restricting the scope of the invention, but just as examples of how the invention can be carried out. The drawings comprise the following figures:

Figure 1 is a cross-sectional view of a die-casting machine including a vacuum apparatus known in the state of art.

Figure 2 is a cross-sectional view of another die-casting machine including a vacuum apparatus known in the state of art.

Figure 3 is a cross-sectional view of a die-casting machine including a vacuum apparatus connected to the cavity of the mould, according to one embodiment of the invention.

Figure 4 is a cross-sectional view of a die-casting machine including a vacuum apparatus connected to the cavity of the mould, according to another embodiment of the invention.

Figures 5A, 5B and 5C are schematic cross-sectional views of the apparatus of the invention showing different phases of the injection process.

Figure 6 is a graphic to shows the evolution of the vacuum pressure in the time depending on the plunger position according to processes of the state of art shown in figures 1 and 2. The axis of abscissa is time and that of ordinate is pressure (left) and the piston stroke (right).

Figure 7 is a graphic to shows the evolution of the vacuum pressure in the time depending on the piston position according to the present invention. The axis of abscissa is time and that of ordinate is pressure (left) and the piston stroke (right).

Figure 8 is a cross-sectional view of a die-casting machine including the vacuum apparatus of the embodiment shown in figure 3, including a cleaning system connected to the mould cavity.

Figure 9 is a cross-sectional view of a die-casting machine including the vacuum apparatus of the embodiment shown in figure 4, including a cleaning system connected to the mould cavity.

Figure 10 illustrates the vacuum apparatus connected to the cavity of the mould, without manifold, according to one embodiment of the invention.

Figure 11 shows a typical vacuum ejector.

DESCRIPTION OF WAYS OF CARRYING OUT THE INVENTION

[0028] Figure 1 is a cross-sectional view of a die-casting machine including a vacuum apparatus known in the state of art. The vacuum system removes the air in a unique stage from the mould with a vacuum pump 15 and a vessel 16 connected directly to the mould cavity 3 by the conduct 9.

[0029] Figure 2 is a cross-sectional view of another die-casting machine including a vacuum apparatus known in the state of art. The vacuum system removes the air in two stages. In a first stage form the shot sleeve 6 through a wide cross-section connected to the line 17 and in a second stage through a connection 17' to the mould cavity 3. The vacuum system comprises a vacuum pump 15 and two vessel 16, 16' connected in parallel to the vacuum pump and conduct 9.

[0030] Figure 3 shows a basic embodiment of a die-casting machine including the vacuum system 8 of the invention connected to the mould cavity 3 of the mould. The mould comprises a fixed part 1 and movable parts 2. The casting is obtained by solidification of molten metal alloy in the mould cavity 3. A piston 4, which moves and presses a mass of molten metal 5 into the mould cavity 3 as a result of its linear displacement through the shot sleeve 6. The shot sleeve 6 is provided with a filing opening 7 through which the molten metal 5 is poured before the beginning of the injection process.

[0031] In the embodiment shown in figure 3 the vacuum system 8 is formed by a first vacuum producing device comprising a vacuum ejector 11 and a second vacuum producing device comprising of a second vacuum pump 15 with a second vessel 16. Vacuum, which is controlled by the valves 13 and 14, is provided by the second vessel 16 and the ejector vacuum 11 to the manifold 12. Then, vacuum continues over the conduit 9 up to the mould cavity 3 through a deaeration valve 10, which is mounted in the mould.

[0032] The vacuum ejector works on the Venturi principle. As can be seen in figure 11 a compressed air is supplied through connection (A) to ejectors. It flows through the venturi nozzle (B). The air is accelerated and decompressed during this process and a vacuum is created. Air is drawn in this way through the vacuum connection (D). The aspirated air and the compressed air escape through the silencer (C).

[0033] In the embodiment shown in figure 4 the vacuum system 8 is formed by a first vacuum producing device comprising of a first vacuum pump 15' with a first vessel 16' and a second vacuum producing device comprising of a second vacuum pump 15 with a second vessel 16. Vacuum, which is controlled by the valves 13 and 14, is provided by the first vessel 16' and the second vessel 16 to the manifold 12. Then, vacuum continues over the conduit 9 up to the mould cavity 3 through a deaeration valve 10, which is mounted in the mould.

[0034] In a further embodiment, both vacuum stages are regulated by measuring the vacuum level achieved and/or air flow. The fist vacuum stage is set from the minimum to a maximum deaeration via a regulator valve in the air supply. In the second vacuum stage the vacuum level in the vessel is regulated by means of a valve in charge of maintain a desired vacuum level in it. Therefore, in the first vacuum stage a first depression is achieved and with the second vacuum stage a next and final depression is obtained.

[0035] As previously described, the vacuum process comprises two stages. The method will be explained in detail with references to the figures 5A, 5B and 5C.

[0036] Figure 5A shows the precise moment in which the metal in form of a molten mass 5 has been just poured, through the filling opening 7, into the shot sleeve 6, the injection permission signal is activated, and the first injection phase is about to happen. Once the filling opening 7 of the shot sleeve 6 has been closed by the movement of piston 4, the first vacuum stage is provided by the first vacuum producing device (vacuum ejector 11 or first vessel 16' and pump 15'). The gas, steam and fumes inside the mould are evacuated by the deareation valve 10 and through the vacuum system 8 they are expelled to the atmosphere. The control of the first vacuum stage is done by means of the valve 14 which is activated by the position of the piston 4 or by a delay from the injection permission signal. The first vacuum stage ends when the second vacuum stage is activated.

[0037] Figure 5B shows the precise moment in which the piston 4 is being push froward and the second vacuum stage is activated. This step happens during the first injection phase and before the mould filling. Thereby, the rest of the mould cavity 3 is effectively evacuated, and the maximum vacuum is achieved without a dramatic change in the pressure conditions reducing a sucking effect on the metal front. This second vacuum stage is controlled by means of the valve 13, which is connected to the second vessel 16, and is activated by the position of the piston 4 or by a delay from the activation of the first vacuum stage. At the same instant, the first vacuum producing device (vacuum ejector 11 or first vessel 16' and pump 15') is closed via the valve 14.

[0038] Figure 5C shows the end of the second vacuum stage in which the mould cavity 3 has been filled, the deareation valve 10 is closed and the second vacuum producing device (15,16), in accordance with the second vacuum phase, is disconnected via the valve 13.

[0039] Figure 6 is a graph showing an example of change of the pressure in the internal cavity, (the axis of abscissa being time and that of ordinate the internal absolute pressure) of the conventional systems disclosed in figures 1 and 2. Curves C1 and C2 shown the effects of the vacuum die casting method. The discontinuous plotted curve represents the position of the piston 4 in the shot sleeve 6 and shows that the vacuum level is achieved before the highspeed injection starts. Both systems are able to produce the vacuum pressure curve during the injection phases, reducing the internal pressure in only one step. The main difference between them is the moment of the application of the vacuum system. In this example, the evacuation curve C1 represents the vacuum system of figure 2 and the evacuation curve C2 represents the vacuum system of figure 1As shown by the evacuation curve C1, using the vacuum system of figure 2, the start timing of evacuation can be set earlier than the vacuum system of figure 1. That is to say, the evacuation can be started when the piston 4 is positioned in the initial position (T0) that represent the instant in which the filling opening 7 of the shot sleeve is closed. As shown by the evacuation curve C2, in the vacuum system of figure 1 the evacuation starts after the piston 4 passes through the filling opening 7, the evacuation starts at the time T2, which is later than the time T1, and the pressure is higher than that of the evacuation curve C1 to avoid metal front disturbances. Both systems are going from the beginning of their application from the 1 atm to the minimum pressure achievable thanks to the vacuum system. The vacuum pressure curves of both system occurs in the piston low-speed injection phase.

[0040] Figure 7 may represent the effect of the vacuum method of the invention in the die casting process. In this example, the first vacuum stage starts at T3 after T0 that represent the instant in which the filling opening 7 of the shot sleeve is closed. A certain time afterwards and before the beginning of the mould filling T5, the second vacuum stage is activated at T4 and remains until at least the end of the mould filling T6. Thus, the system of the invention is able to produce this vacuum pressure curve during the first (plunger movement) and second (mold cavity filling) injection phases. The vacuum pressure curves are made in the shot sleeve low-speed injection phase but in two stages. The first stage is made thought the mould cavity and when the piston 4 of the shot sleeve 6 closes the filling opening 7 allowing cavity and shot sleeve dirtiness extraction, and the second stage is made as closer as possible to the commutation point (T5) to achieve the minimum pressure between the T5 and T6 (when the mould is filling with the molten metal). This intermediate pressure during the first vacuum stage allows to reduce and optimize the air volume inside of the mould cavity 3 before the filling of the mould without creating turbulences in the aluminium melt and breaking the melt front. When the piston 4 is closer to the commutation point a second and very low pressure is achievable to minimize the air content in the mould.

[0041] In an advantageous embodiment, instead of first and second vacuum stages, more than two vacuum stages can be used. As a result, a smooth change in the vacuum and air flow is obtained in the metal front without generate any alteration on it.

[0042] Alternatively, to the connection of the vacuum ejector 11 and the vessel 16 to the manifold 12, the vacuum ejector 11 can be in an independent system connected to the conduit 9 as it is shown in figure 10 In that case, the first gas, steam and fumes extracted from the inside of the mould are expelled directly to the atmosphere without going through the rest of the vacuum system

[0043] In some embodiments the apparatus also comprises at least 3 sensors installed in the vacuum system to control the 3 main variables of the vacuum die casting process. They are the vacuum pressure, the air flow and the humidity of the air flow. The variables include limits in order to control the variables and guarantee the correct procedure set. The alarms include pollution information, if the vacuum line is operative or by the contrary is blocked. In an example, an alarm may be created if the variables are out of the range establish. Furthermore, the readings may be monitored and registered in a base data to analyse and carry out statistical analysis.

[0044] Figures 8 and 9 show a bag cleaning system to reduce pressure drop and to ensure a good cleanliness level of the circuits and filters. The cleaning system comprises vessel 18 connected to the conduit 9 via a valve 19 and manifold 12. A vessel between 2 liters and 50 liters connected to a 4-6 bars line is opened when the mould is opened in order to clean the system.

[0045] In this text, the terms first, second, third, etc. have been used herein to describe several devices, elements or parameters, it will be understood that the devices, elements or parameters should not be limited by these terms since the terms are only used to distinguish one device, element or parameter from another. For example, the first device could as well be named second device, and the second device could be named first device without departing from the scope of this disclosure.

[0046] In this text, the term "comprises" and its derivations (such as "comprising", etc.) should not be understood in an excluding sense, that is, these terms should not be interpreted as excluding the possibility that what is described and defined may include further elements, steps, etc.

[0047] On the other hand, the invention is obviously not limited to the specific embodiment(s) described herein, but also encompasses any variations that may be considered by any person skilled in the art (for example, as regards the choice of materials, dimensions, components, configuration, etc.), within the general scope of the invention as defined in the claims.

Claims

1. Vacuum pressure process for high pressure die casting in a mould comprising a fixed part (1) and at least a movable part (2) defining inbetween a mould cavity (3) and a shot sleeve (6) for feeding the mould cavity (3) with molten metal, the process being characterized by comprising the steps of

- producing a first reduction of the mold cavity pressure to a first vacuum stage, once the molten metal to be casted is inside the shot sleeve (6),

- producing a second reduction of the mold cavity pressure from the first vacuum stage to a second vacuum stage lower than the first vacuum stage, after the first vacuum stage is established and before the filling of the mould cavity (3) begins,

- maintaining the second pressure stage at least until the mould cavity has been filled.

2. Vacuum pressure process according to claim 1 wherein, the process comprises the steps of:

- connecting a first vacuum producing device to the mould cavity (3), at an opposite position of the shot sleeve (6), the first vacuum producing device being able to reduce the mold cavity pressure to the first vacuum stage,

- connecting a second vacuum producing device to the mould cavity (3), at an opposite position of the shot sleeve (6), the second vacuum producing device being able to reduce the mold cavity pressure to the second vacuum stage lower than the first vacuum stage,

- pouring a molten metal through the filling opening (7) of the shot sleeve (6),

- begin the pushing movement of a piston (4) inside the shot sleeve (6),

- once the filling opening (7) of the shot sleeve has been closed by the movement of the piston (4) the first vacuum stage is provided in the mould cavity by the first vacuum producing device.

- after the first vacuum stage is established and before the piston (4) begins the filling of the mould cavity (3) the second vacuum stage is provided in the mould cavity (3) by the second vacuum producing device, this second vacuum stage being maintained until the mould cavity (3) has been filled.

3. Vacuum pressure process according to claim 1 or 2 wherein the first vacuum stage reduces the pressure in the mold cavity to a pressure between 975 mbars and 200 mbars and the second vacuum stage reduces the pressure in the mold cavity to a pressure between 200 mbars and 0.1 mbars

4. Vacuum pressure process according to any of previous claims, wherein the first stage of pressure comprises at least an additional stage reducing the pressure to an intermediate pressure value.

5. Vacuum pressure process according to any of previous claims, wherein the second stage of pressure comprises at least an additional stage reducing the pressure to an intermediate pressure value.

6. Vacuum pressure process according to any of previous claims further comprising a cleaning system (18) connected to the cavity of the mould, at an opposite position of the shot sleeve (6), when the mould is open, after the demoulding has finished.

7. Vacuum pressure process according to any of previous claims wherein the molten metal is an alloy of Al, Mg, Zn, Pb or brass.

8. Vacuum apparatus for high pressure die casting comprising

- a mould with a fixed part (1) and at least a movable par (2) defining inbetween a mould cavity (3)

- a shot sleeve (6) communicated with the mould cavity (3) for feeding the mould cavity (3) with molten metal, the shot sleeve (6) comprising a filling opening (7) for pouring a molten metal (5) into the shot sleeve (6), and a piston (4) for pushing the molten metal (5) inside the mould cavity (3),

characterized by comprising

- a first vacuum producing device connected through a conduct (9) to the cavity of the mould, at an opposite position of the shot sleeve (6), for reducing the mold cavity pressure to a first vacuum stage,

- a second vacuum producing device connected through a conduct (9) to the cavity of the mould (3), at an opposite position of the shot sleeve (6), for reducing the mold cavity pressure to a second vacuum stage lower than the first vacuum stage,

9. Vacuum apparatus according to claim 8 wherein the first vacuum stage reduces the pressure in the mold cavity to a pressure between 975 mbars and 200 mbars and the second vacuum stage reduces the pressure in the mold cavity to a pressure between 200 mbars and 0.1 mbars.

10. Vacuum apparatus according to claim 8 or 9, wherein the first vacuum producing device comprises a vacuum ejector (11).

11. Vacuum apparatus according to claim 8 or 9, wherein the first vacuum producing device comprises a first vacuum pump (15') and a first vessel (16').

12. Vacuum apparatus according to any of claims 8 to 11, wherein the second vacuum producing device comprises a second vacuum pump (15) and a second vessel (16).

13. Vacuum apparatus according to any of claims 8 to 12, wherein the first vacuum producing device and the second vacuum producing device are connected to a manifold (12).

14. Vacuum apparatus according to any of claims 8 to 13 further comprising a bag cleaning system (18) connected through the conduct (9) to the mould cavity (3), at an opposite position of the shot sleeve (6).

15. Vacuum apparatus according to any of claims 8 to 14 wherein the molten metal is an alloy of Al, Mg, Zn, Pb or brass.

Drawing