US20080053637A1

US20080053637A1 - Method and apparatus for manufacturing aluminum die-cast product

Info

Publication number: US20080053637A1
Application number: US11/709,208
Authority: US
Inventors: Masafumi Hamasaki; Kazuhiko Inoguchi; Masaya Hatanaka; Shinji Saho
Original assignee: Mitsubishi Heavy Industries Ltd
Current assignee: Mitsubishi Heavy Industries Ltd
Priority date: 2006-08-31
Filing date: 2007-02-22
Publication date: 2008-03-06
Also published as: JP2008055473A; CN101134235A; CA2599108A1

Abstract

A method and an apparatus for manufacturing an aluminum die-cast product are provided, wherein an occurrence of shrinkage cavity is eliminated or reduced to a very small amount, and a sound and high-quality aluminum die-cast product having no casting defect can be produced. The method for manufacturing an aluminum die-cast product by performing die casting while a part, in which a casting defect tends to occur due to solidification shrinkage of a melt, which has been pressure-filled in a mold, during a solidification process, is pressurized with a local pressurization pin includes the steps of allowing the local pressurization pin to follow the movement of a solidification interface based on a solidification interface movement speed, which is calculated in advance, of the melt and keeping pushing the local pressurization pin in until the solidification is completed.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a method and an apparatus for manufacturing an aluminum die-cast product suitable for producing pressure vessels and the like required to have airtightness.
This application is based on Japanese Patent Application No. 2006-236216, the content of which is incorporated herein by reference.
2. Description of Related Art
In general, with respect to an aluminum die-cast product, it is known that a solidification shrinkage space centering a portion, in which the solidification speed of a melt filled in a mold is the smallest, occurs resulting from the solidification shrinkage during die casting. In an aluminum alloy, about 6% of volume shrinkage occurs during solidification. When the resulting shrinkage space is included in the inside of a product, a shrinkage cavity occurs in a concentrated manner, and this results in a casting defect. The shrinkage cavity tends to extend over a wide range as the thickness of the casting portion is increased and the mold temperature is increased (temperature gradient is small).
When the product is a pressure vessel, the above-described shrinkage cavity, which is a casting defect, causes pressure leakage, and is one of an important factors leading to reduction of the yield of product.
A local pressurization method of pressurizing partly a part in which a shrinkage cavity tends to occur, while a melt having been filled is in a semi-solid state just before solidification is known as a technology for reducing occurrences of shrinkage cavities during casting (refer to, for example, Japanese Unexamined Patent Application, Publication No. 59-156560 and Japanese Unexamined Patent Application, Publication No. 8-155622).
In the technology shown in Japanese Unexamined Patent Application, Publication No. 8-155622, a local pressurization pin is pushed in with an oil hydraulic cylinder just before solidification, the oil hydraulic cylinder is operated until the measured squeeze pressure reaches a predetermined range of the set squeeze pressure while the pressure of the local pressurization pin is detected, and the stroke value of the local pressurization pin is controlled so as to reduce or eliminate the shrinkage cavity.
However, the technology shown in Japanese Unexamined Patent Application, Publication No. 8-155622 is no more than that the local pressurization pin is merely pushed in at the time set with a timer while the pressure is detected, and it is not said that the local pressurization pin is controlled while the solidification speed and the solidification timing of the melt having been filled in the mold are grasped accurately.
In order to reliably close a solidification shrinkage space which is an opening resulting from the solidification shrinkage, the local pressurization pin must be pressure-controlled while the timing is adjusted in accordance with the solidification. However, in the known local pressurization method, the local pressurization pin control is not performed while the solidification speed is grasped accurately. Therefore, a probability of success in closing the solidification shrinkage space is low, and elimination of shrinkage cavity has not yet been reached.
As described above, the die-cast pressure vessel is under circumstances where the yield has not reached a satisfactory region against the pressure leakage resulting from the shrinkage cavity.

BRIEF SUMMARY OF THE INVENTION

The present invention has been made in consideration of such circumstances. Accordingly, it is an object of the present invention to provide a method and an apparatus for manufacturing an aluminum die-cast product, wherein an occurrence of shrinkage cavity is eliminated or reduced to a very small amount, and a sound and high-quality aluminum die-cast product having no casting defect can be produced by controlling the push-in operation of a local pressurization pin.
In order to solve the above-described problems, the method and the apparatus for manufacturing an aluminum die-cast product according to the present invention adopt the following means.
A method for manufacturing an aluminum die-cast product according to an aspect of the present invention is a method for manufacturing an aluminum die-cast product by performing die casting while a part in which a casting defect tends to occur due to solidification shrinkage of a melt, which has been pressure-filled in a mold, during a solidification process is pressurized with a local pressurization pin, the method comprising the steps of allowing the above-described local pressurization pin to follow the movement of a solidification interface based on a solidification interface movement speed, which is calculated in advance, of the above-described melt and keeping pushing the local pressurization pin in until the solidification is completed.
In the method for manufacturing an aluminum die-cast product according to an aspect of the present invention, the local pressurization pin is allowed to follow the movement of a solidification interface based on a solidification interface movement speed (depth (mm) from mold surface to last solidification portion/solidification completion time (s) of last solidification portion), which is calculated in advance, of the melt, and the local pressurization pin is kept being pushed in until the solidification is completed. Therefore, the local pressurization pin can be pushed in while the timing is adjusted in accordance with the solidification of the melt. That is, the solidification interface movement speed of a portion, which must be locally pressurized, is grasped accurately by an analysis in advance, and the local pressurization pin is allowed to follow the movement of a solidification interface based thereon and is kept being pushed in until the solidification is completed, so that a solidification shrinkage space (resulting in a shrinkage cavity) can reliably be closed. Therefore, an occurrence of shrinkage cavity can be eliminated or reduced to a very small amount, and a sound aluminum die-cast product having no casting defect can be produced. Consequently, the yield in the die casting can be improved significantly, and a high-quality aluminum die-cast product can be produced at a low cost.
The method for manufacturing an aluminum die-cast product according to an aspect of the present invention may have a configuration in which the pressurization with the above-described local pressurization pin is started 0.4 to 1.4 seconds after the filling of the above-described melt is completed.
According to this configuration, since the pressurization with the local pressurization pin is started 0.4 to 1.4 seconds after the filling of the melt is completed, the local pressurization pin is allowed to follow the movement of a solidification interface and can reliably be kept being pushed in until the solidification is completed. That is, if the start of pressurization with the local pressurization pin is too early, since solidification is not started and the resistance to pushing-in is small, the push-in speed becomes too large. Consequently, the local pressurization pin is pushed in up to a stroke end, and it becomes difficult to keep pushing the local pressurization pin in until the solidification is completed. Therefore, when the pressurization with the local pressurization pin is started within about 1 second from just after the start of solidification (0.4 seconds after the filling is completed), the local pressurization pin is not pushed in up to the stroke end, and the local pressurization pin can reliably be kept being pushed in until the solidification is completed. In this manner, an occurrence of shrinkage cavity can be eliminated or reduced to a very small amount. If 1.4 seconds have elapsed from the completion of filling, solidification of the part in which the local pressurization pin is pushed proceeds and it becomes difficult to keep pushing the local pressurization pin in until the solidification is completed.
The start of solidification of the melt having been filled is almost the same regardless of the size and shape of the die-cast product. Consequently, the start time of pressurization with the local pressurization pin can be set as described above, independently of the type of aluminum die-cast product.
The method for manufacturing an aluminum die-cast product according to an aspect of the present invention may have a configuration in which the average push-in speed V of the above-described local pressurization pin is specified to be 1.2 mm/s<V<2.63 mm/s.
According to this configuration, since the average push-in speed V of the local pressurization pin is specified to be 1.2 mm/s<V<2.63 mm/s, the local pressurization pin is allowed to follow the movement of a solidification interface and can reliably be kept being pushed in until the solidification is completed. In addition, pushing-in can reliably be performed up to the position suitable for adequately crushing the shrinkage cavity. Here, when the thickness of the part to be locally pressurized is assumed to be about 20 mm, since the solidification completion time of the last solidification portion is 3.8 seconds, the solidification interface movement speed is 2.63 mm/s. When the local pressurization pin is pushed in while the above-described speed is taken as an upper limit of the pushing-in, the local pressurization pin can reliably be pushed in up to the position suitable for adequately crushing the shrinkage cavity. If the push-in speed becomes 1.2 mm/s or less, the resistance to pushing-in becomes excessive, and the local pressurization pin cannot be pushed in. As the depth of pushing-in of the local pressurization pin is decreased, the crush of shrinkage cavity tends to become inadequate. Therefore, it is an important factor for reliably closing the solidification shrinkage space (resulting in a shrinkage cavity) to control the push-in speed of the local pressurization pin and ensure an adequate depth of pushing-in. That is, since there is a difference in solidification time from the completion of filling of the melt between the first solidification completion portion (a portion in contact with a mold) and the last solidification portion, no effect is exerted even when the local pressurization pin is pushed in instantaneously within this period. It is an important factor for reliably eliminating or reducing an occurrence of shrinkage cavity to push in up to an adequate depth over a few seconds at the above-described push-in speed.
The method for manufacturing an aluminum die-cast product according to an aspect of the present invention may have a configuration in which the above-described aluminum die-cast product is a pressure vessel.
According to this configuration, the aluminum die-cast product is a pressure vessel. Therefore, a pressure vessel having no casting defect (shrinkage cavity), which becomes a factor of pressure leakage, can be produced by die casting. Consequently, the yield in the die casting against the pressure leakage can be improved significantly, and a high-quality pressure vessel can be produced at a low cost.
The method for manufacturing an aluminum die-cast product according to an aspect of the present invention may have a configuration in which the above-described pressure vessel is a housing of a compressor.
According to this configuration, since the pressure vessel is a housing of a compressor, a highly airtight housing, which does not cause leakage of refrigerant gas sealed in the inside, can be produced by eliminating the occurrence of shrinkage cavity or reducing to a very small amount. In particular, with respect to the compressor housing, in many cases, a thick-walled boss portion of the housing is worked by cutting after casting. Even when the outer surface is clean, if a shrinkage cavity, which is a casting defect, is present in the inside, it is exposed at a cutting surface so as to become a factor of the leakage of refrigerant gas. According to the above-described configuration, the occurrence of shrinkage cavity can be eliminated or reduced to a very small amount and, thereby, a die-cast method most suitable for the compressor housing can be provided.
An apparatus for manufacturing an aluminum die-cast product according to an aspect of the present invention is an apparatus for manufacturing an aluminum die-cast product by performing die casting while a part in which a casting defect tends to occur due to solidification shrinkage of a melt, which has been pressure-filled in a mold, during a solidification process is pressurized with a local pressurization pin, the apparatus including the local pressurization pin disposed in accordance with the above-described part, in which the casting defect tends to occur, so as to locally pressurize the part, a push-in device for pushing the above-described local pressurization pin in toward the above-described part, and a control portion for controlling the above-described push-in device in such a way that the above-described local pressurization pin is allowed to follow the movement of a solidification interface based on a solidification interface movement speed, which is calculated in advance, of the above-described melt and the local pressurization pin is kept being pushed in until the solidification is completed.
According to the apparatus for manufacturing an aluminum die-cast product according to an aspect of the present invention, since the control portion for controlling the push-in device to push in the local pressurization pin in such a way that the local pressurization pin is allowed to follow the movement of a solidification interface based on the solidification interface movement speed, which is calculated in advance, of the above-described melt and the local pressurization pin is kept being pushed in until the solidification is completed is included, the local pressurization pin can be pushed in while the timing is adjusted in accordance with the solidification of the melt by this control portion. That is, the solidification interface movement speed of a portion, which must be locally pressurized, is grasped accurately by the analysis in advance, and the local pressurization pin is allowed to follow the movement of a solidification interface based thereon and can be kept being pushed in until the solidification is completed, so that a solidification shrinkage space resulting in a shrinkage cavity can reliably be closed. Therefore, an occurrence of shrinkage cavity resulting from the solidification shrinkage can be eliminated or reduced to a very small amount, and a sound die-cast product having no casting defect can be produced. In addition, the yield in the die casting can be improved significantly, and a high-quality die-cast product can be produced at a low cost.
The apparatus for manufacturing an aluminum die-cast product according to an aspect of the present invention may have a configuration in which an overflow portion in accordance with the above-described part, in which the casting defect tends to occur, is disposed in the above-described mold, the above-described local pressurization pin is attached in such a way that the local pressurization pin can be pushed in the above-described overflow portion, and the above-described overflow portion is locally pressurized with the above-described local pressurization pin.
According to this configuration, since the overflow portion is disposed in the mold, the local pressurization pin is attached so as to locally pressurize the overflow portion, an occurrence of a trace of pressurization due to the local pressurization on the die-cast product side can be prevented. Therefore, constraints to the part to be locally pressurized are eliminated, and a higher-quality die-cast product having no trace of pressurization can be produced.
According to the method and the apparatus for manufacturing an aluminum die-cast product of the present invention, since the local pressurization pin is allowed to follow the movement of a solidification interface of the melt having been filled in the mold and can be kept being pushed in until the solidification is completed, so that the solidification shrinkage space resulting in a shrinkage cavity can reliably be closed. Therefore, an occurrence of shrinkage cavity resulting from the solidification shrinkage can be eliminated or reduced to a very small amount, and a sound aluminum die-cast product having no casting defect can be produced. In addition, the yield in the die casting can be improved significantly, and a high-quality aluminum die-cast product can be produced at a low cost.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a sectional view of an aluminum die-cast product (compressor housing) produced by a method and an apparatus for manufacturing an aluminum die-cast product according to an embodiment of the present invention.

FIG. 2 is a partial schematic configuration diagram of an apparatus for manufacturing an aluminum die-cast product according to an embodiment of the present invention.

FIG. 3 is an explanatory diagram of a solidification closed loop formation status of a locally pressurized portion, predicted by an analysis in the method for manufacturing an aluminum die-cast product according to an embodiment of the present invention.

FIG. 4 is an explanatory diagram of the quality check results of local pressurization prototypes by a method for manufacturing an aluminum die-cast product according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment according to the present invention will be described below with reference to FIG. 1 to FIG. 4.
FIG. 1 shows an example of an aluminum die-cast product produced by a method and an apparatus for manufacturing an aluminum die-cast product according to the present embodiment. Here, a housing 1 to be used for a compressor is shown as an example of the aluminum die-cast product.
The housing 1 is formed from an aluminum alloy ADC12 (JIS). A compression mechanism is incorporated in the inside of the housing 1. A refrigeration gas sealed in a refrigeration cycle is suctioned into the housing 1, and this is compressed and discharged from the housing 1 to the outside, so that a function as a pressure vessel is exerted.
The above-described housing 1 takes the shape of a cup having a thin-walled cylinder portion 1A, and boss portions 1B having a thickness a few times larger than the thickness of the cylinder portion 1A are integrally die-cast at a plurality of positions on the periphery of the cylinder portion 1A. The thickness of the boss portion 1B is specified to be 20 mm at the maximum. The boss portion 1B is a portion where a screw hole, a through hole, or the like is formed by cutting work after die casting. If a solidification shrinkage space (resulting in a shrinkage cavity) occurs resulting from solidification shrinkage in the inside of the boss 1B during die casting, the space is exposed at a cutting surface by the cutting work. Since this becomes a cause of the leakage of refrigeration gas, elimination of an occurrence of shrinkage cavity during die casting is inevitable in order to produce the housing 1 which does not cause pressure leakage nor gas leakage.
FIG. 2 shows a part of a die-cast manufacturing apparatus 10 for producing an aluminum die-cast housing 1 in which shrinkage cavity that is a casting defect does not occur.
A mold 11 is mold-clamped with a mold clamping apparatus, although not shown in the drawing, and just after the mold clamping, the inside of the cavity 11A is evacuated with a vacuum pump. With respect to the mold 11, under the general aluminum die-cast condition, the surface temperature of the cavity 11A is controlled at 250° C.±50° C. A melt 12 is injected and filled into the cavity 11A of the mold 11 through a gate, a plunger, and the like, although not shown in the drawing. The melt temperature is 680° C.±50° C. under the general aluminum die-cast condition. In general, the injection of the melt 12 is performed by a two-stage motion of the plunger. In a low speed region, a rod pressure and a head pressure are set at constant pressures, and the plunger is moved slowly. Switching to a high speed is performed aiming at the time when the melt 12 passes the gate. In a high speed region, the plunger is accelerated sharply, and the filling of the melt 12 into the cavity 11A is completed in about 0.1 seconds. In general, the speed of this high-speed injection is 1.0 m/s or more.
The mold 11 includes an overflow portion 11B connected to the cavity 11. This overflow portion 11B is disposed in accordance with the part in which a shrinkage cavity that is a casting defect tends to occur during casting. That is, in the above-described housing 1, overflow portions are disposed at a plurality of positions (merely one position is shown in FIG. 2) in accordance with the bosses 1B (thickness 20 mm) having a thickness larger than the thickness of the cylinder portion 1A. A local pressurization apparatus 13, which can independently pressurize the melt 12 filled in the above-described individual overflow portions 11B, is attached outside the mold 11.
This local pressurization apparatus 13 is provided with a pressurization cylinder 14 to be attached to the mold 11 and a local pressurization pin 16, which is connected to a piston 15 of the pressurization cylinder 14 and which is moved by a predetermined range of stroke. The local pressurization pin 16 is disposed in such a way as to be pushed in the overflow portion 11B. In the present embodiment, a maximum push-in stroke L thereof is set at, for example, 18 mm, and the pin diameter is set at, for example, 9 mm.
The pressurization cylinder 14 is to push the local pressurization pin 16 into the overflow portion 11B. As is publicly known, oil hydraulic circuits 20A and 20B are connected to the pressurization cylinder 14 in such a way that one chamber 14A is supplied with oil from an oil pressure supply source, although not shown in the drawing, through a directional control valve 17, a solenoid valve 18, and a throttling valve 19, and the oil is discharged from the other chamber 14B through the directional control valve 17. The oil hydraulic circuit 20B is provided with a flowmeter 21 for monitoring the stroke of the local pressurization pin 16.
The directional control valve 17, the solenoid valve 18, the throttling valve 19, and the flowmeter 21 disposed in the oil hydraulic circuits 20A and 20B of the above-described pressurization cylinder 14 are controlled and monitored by a control unit 22.
In order to push the local pressurization pin 16 into the overflow portion 11B while the timing is adjusted in accordance with the solidification of the melt 12 filled in the cavity 11A of the mold 11, the control unit 22 is configured to be able to control the movement of the local pressurization pin 16 by using the following two points as parameters.
(1) The start time ts of the local pressurization pin 16 is determined by setting a timer for the time of opening or closing of the solenoid valve 18.
(2) The push-in speed V of the local pressurization pin 16 is controlled by adjusting a flow rate of the pressurizing oil through changing of the valve opening of the throttling valve 19.
In order to reliably close the solidification shrinkage space and eliminate an occurrence of shrinkage cavity defect by locally pressurizing the part, in which a shrinkage cavity that is a casting defect tends to occur, with the above-described local pressurization apparatus 13, it is necessary that the position of occurrence of the defect is a position suitable for crushing by the pushing-in of the local pressurization pin 16 and that the movement of the local pressurization pin 16 is controlled appropriately while the timing is adjusted in accordance with the solidification of the melt 12.
FIG. 3 shows a solidification closed loop formation status of a locally pressurized portion, predicted by an analysis.
A solidification closed loop formation status while the local pressurization pin is in the standby state was predicted. It was predicted that a solidification closed loop occurred just below the local pressurization pin 16 disposed at the maximum thick-walled place (thickness 20 mm), at which a solidification closed loop was expected to occur, and it was determined that the defect was able to be closed by appropriately controlling the local pressurization pin 16. Furthermore, there was a difference of 3.6 seconds in solidification time, which was predicted by the analysis, from the completion of filling of the melt between the first solidification completion portion (a portion in contact with the mold 11) and the last solidification portion. Therefore, it was estimated that no effect was exerted even when the local pressurization pin 16 was pushed in instantaneously within this period, and the local pressurization pin 16 had to be pushed in up to an adequate depth over a few seconds. As also shown in FIG. 3, in the present embodiment, solidification of the melt 12 is allowed to start 0.2 seconds after the filling is completed, and an elapsed time until the solidification is completed is 3.8 seconds.
According to the above-described analysis, it was found that the solidification shrinkage space (resulting in a shrinkage cavity) was able to be closed reliably by grasping accurately the solidification interface movement speed of a portion, which had to be locally pressurized, in advance, allowing the local pressurization pin 16 to follow the movement of a solidification interface based thereon, and keeping pushing the local pressurization pin 16 in until the solidification was completed. The solidification interface movement speed of the melt 12 can be calculated based on “depth (mm) from mold surface to last solidification portion/solidification completion time (s) of last solidification portion”.
According to this finding, local pressurization products were prototyped by using two parameters of the above-described (1) the start time ts of the local pressurization pin 16 and (2) the push-in speed V of the local pressurization pin 16, and the quality thereof was checked.
FIG. 4 shows the quality check results of local pressurization prototypes. The shape (circle, triangle, and rectangle) of the plotted point showing each experimental condition indicates the X-ray evaluation result, the pattern (solid, dots, and empty) of the plotted point indicates the specific gravity, and the number added to the plotted point indicates the average push-in speed V of the local pressurization pin 16.
As a result, mostly, an improvement is recognized as compared with a known core pin method (a core pin is inserted by partial pressurization into a boss portion 1B in the die casting so as to mold a pilot hole by die casting). However, as the push-in depth of the local pressurization pin 16 is decreased, crushing of defect becomes inadequate. As the local pressurization pin is started earlier and the push-in speed V is increased, the local pressurization pin 16 tends to be pushed in up to a stroke end. However, in some cases, minor defects are observed. Therefore, it is believed to be desirable that these conditions are avoided in order to ensure the quality in mass production stably.
The specific gravity in the known core pin method was about 2.67 g/cm³, whereas the specific gravity of each prototype was 2.71 g/cm³or more. Therefore, an improvement was recognized.
In particular, with respect to all prototypes within the range indicated by a thick solid line shown in FIG. 4, no defect was recognized in the X-ray evaluation and, in addition, it was recognized that high quality exhibiting the specific gravity of 2.73 g/cm³or more was obtained.
That is, the start time of the local pressurization pin 16 is specified to be 0.4 to 1.4 seconds after the filling of the melt 12 is completed. It is desirable that the pressurization with the local pressurization pin 16 is started 0.4 to 1.4 after the filling of the melt 12 is completed, and the pushing-in of the local pressurization pin 16 is continued until the solidification of the melt 12 is completed (here, 3.8 seconds after the filling is completed). If the start of pressurization is too early, since solidification is not started and the resistance to pushing-in of the melt 12 is small, the push-in speed becomes too large. Consequently, the local pressurization pin 16 is pushed in up to a stroke end, and it becomes difficult to keep pushing the local pressurization pin 16 in until the solidification is completed. On the other hand, if the start of pressurization with the local pressurization pin 16 is too late, since solidification proceeds, the push-in depth of the local pressurization pin 16 is decreased, and crushing of defect may become inadequate. When the pressurization with the local pressurization pin 16 is started within about 1 second just after the filling of the melt 12 is completed and solidification is started (0.4 seconds after the filling is completed), the local pressurization pin 16 is not pushed in up to the stroke end, the local pressurization pin 16 can reliably be kept being pushed in until the solidification is completed, and the local pressurization pin 16 can be pushed in up to an adequate depth.
It is desirable that the average push-in speed V of the local pressurization pin 16 is specified to be within the range of 1.2 mm/s<V<2.63 mm/s. That is, when the average push-in speed V of the local pressurization pin 16 is specified to be 1.2 mm/s<V<2.63 mm/s, the local pressurization pin 16 is allowed to follow the movement of a solidification interface of the melt 12 and can reliably be kept being pushed in until the solidification is completed. In addition, pushing-in can reliably be performed to the position suitable for adequately crushing the shrinkage cavity.
Here, when the thickness of the boss portion 1B to be locally pressurized is assumed to be about 20 mm, the solidification completion time of the last solidification portion is 3.8 seconds (refer to FIG. 3). Therefore, the solidification interface movement speed is 10 mm/3.8 s, that is, 2.63 mm/s. When the local pressurization pin 16 is pushed in while this is taken as an upper limit push-in speed V, the local pressurization pin 16 can reliably be pushed in up to the position suitable for adequately crushing the shrinkage cavity. In FIG. 4, there are conditions, under which no defect is recognized in the X-ray evaluation and high quality exhibiting the specific gravity of 2.73 g/cm³or more is obtained, outside the range indicated by the thick solid line. However, if the average push-in speed V of the local pressurization pin 16 exceeds 2.63 mm/s, the push-in speed of the local pressurization pin 16 is too large relative to the solidification interface movement speed and, thereby, the local pressurization pin 16 may be pushed in up to the stroke end before a shrinkage cavity occurs. Therefore, it is desirable that these conditions are avoided as much as possible in order to ensure high quality in mass production stably.
If the push-in speed becomes 1.2 mm/s or less, the solidification proceeds and the resistance to pushing-in becomes excessive, so that the local pressurization pin 16 may not be pushed in up to an adequate depth. Therefore, it is also desirable that these conditions are avoided as much as possible in order to ensure high quality in mass production stably. If the push-in depth of the local pressurization pin 16 is decreased, crushing of defect tends to become inadequate. Consequently, it is an important factor for reliably closing the solidification shrinkage space (resulting in a shrinkage cavity) to control the push-in speed of the local pressurization pin 16 and ensure an adequate depth of pushing-in.
According to the present embodiment as described above, the following operation and effect are exerted.
According to the present embodiment, the part, in which a casting defect (shrinkage cavity) tends to occur due to solidification shrinkage of the melt 12 pressure-filled in the mold 11 during a solidification process, that is, the boss portion 1B of the compressor housing 1, can be die-cast by pressurizing with the local pressurization pin 16.
At this time, since the local pressurization pin 16 is allowed to follow the movement of a solidification interface based on the solidification interface movement speed, which is calculated in advance, of the above-described melt and the local pressurization pin 16 is kept being pushed in until the solidification is completed, the local pressurization pin 16 can be pushed in while the timing is adjusted in accordance with the solidification of the melt 12. Consequently, the solidification shrinkage space (resulting in a shrinkage cavity) can reliably be closed and an occurrence of shrinkage cavity can be suppressed in the part in which a shrinkage cavity tends to occur.
Therefore, the occurrence of shrinkage cavity resulting from the solidification shrinkage can be eliminated or reduced to a very small amount, and a sound housing 1 having no shrinkage cavity defect can be produced. In addition, the yield in the die casting can be thereby improved significantly, and a high-quality housing 1 can be provided at a low cost. Particularly in the compressor housing 1 to be used as a pressure vessel, in many cases, a screw hole or a through hole is formed in the thick-walled boss portion 1B, in which a shrinkage cavity tends to occur, by cutting work after casting. Therefore, even when an outer surface is clean, if a shrinkage cavity is present in the inside, the space is exposed at a cutting surface and becomes a cause of the leakage of refrigeration gas. However, since the occurrence of shrinkage cavity can be eliminated or reduced to a very small amount, as described above, a highly airtight housing not causing leakage of refrigerant can be produced.
Since the pressurization with the local pressurization pin 16 is started 0.4 to 1.4 seconds after the filling of the melt is completed, the local pressurization pin 16 is allowed to follow the movement of a solidification interface and can reliably be kept being pushed in until the solidification is completed. Therefore, it is avoided that the start of pressurization with the local pressurization pin 16 is too early, the local pressurization pin 16 is pushed in up to a stroke end, and it becomes difficult to keep pushing the local pressurization pin 16 in until the solidification is completed. Furthermore, it is avoided that the start of pressurization with the local pressurization pin 16 is too late, the push-in depth of the local pressurization pin 16 is decreased, and the shrinkage cavity defect cannot be crushed adequately. Consequently, a shrinkage cavity can reliably be eliminated or reduced to a very small amount.
Since the average push-in speed V of the local pressurization pin 16 is specified to be 1.2 mm/s<V<2.63 mm/s, the local pressurization pin 16 is allowed to follow the movement of a solidification interface of the melt 12 and can reliably be kept being pushed in until the solidification is completed. In addition, pushing-in can reliably be performed up to the position suitable for adequately crushing the shrinkage cavity. Therefore, the local pressurization pin 16 is allowed to follow the movement of a solidification interface and can be pushed in up to an adequate depth over a few seconds at an appropriate speed. Consequently, the solidification shrinkage space (resulting in a shrinkage cavity) can reliably be closed, so that elimination or reduction of occurrence of shrinkage cavity can reliably be realized.
In the local pressurization of a part, in which a casting defect (shrinkage cavity) tends to occur, since the overflow portion 11B is disposed in the mold 11 and the overflow portion 11B is pressurized with the local pressurization pin 16, an occurrence of a trace of pressurization due to the local pressurization on the die-cast product, that is, the housing 1, side can be prevented. Therefore, constraints to the part to be locally pressurized are eliminated, and a higher-quality die-cast product having no trace of pressurization can be produced.
In the above-described embodiment, the local pressurization apparatus 10 having the configuration, in which the local pressurization pin 16 is pushed in with a pressurization cylinder 14 by using the oil pressure, is explained. The local pressurization apparatus 10 is not limited to such an oil hydraulic type. It may be an electric local pressurization apparatus in which an electric servo motor is used, and the rotational drive thereof is converted to a linear motion by a ball screw shaft so as to push in the local pressurization pin 16.
The hosing 1 of the compressor is shown as an example of aluminum die-cast products, but not limited to this. As a matter of course, the present invention may be applied to various aluminum die-cast products, e.g., aluminum alloy cylinders, crankcases, transmission cases, and the like.

Claims

1. A method for manufacturing an aluminum die-cast product by performing die casting while a part in which a casting defect tends to occur due to solidification shrinkage of a melt, which has been pressure-filled in a mold, during a solidification process is pressurized with a local pressurization pin, the method comprising the steps of:

allowing the local pressurization pin to follow the movement of a solidification interface based on a solidification interface movement speed, which is calculated in advance, of the melt; and

keeping pushing the local pressurization pin in until the solidification is completed.

2. The method for manufacturing an aluminum die-cast product according to claim 1, wherein the pressurization with the local pressurization pin is started 0.4 to 1.4 seconds after the filling of the melt is completed.

3. The method for manufacturing an aluminum die-cast product according to claim 1, wherein the average push-in speed V of the local pressurization pin is specified to be 1.2 mm/s<V<2.63 mm/s.

4. The method for manufacturing an aluminum die-cast product according to claim 1, wherein the aluminum die-cast product is a pressure vessel.

5. The method for manufacturing an aluminum die-cast product according to claim 4, wherein the pressure vessel is a housing of a compressor.

6. An apparatus for manufacturing an aluminum die-cast product by performing die casting while a part, in which a casting defect tends to occur due to solidification shrinkage of a melt, which has been pressure-filled in a mold, during a solidification process, is pressurized with a local pressurization pin, the apparatus comprising:

the local pressurization pin disposed in accordance with the part, in which the casting defect tends to occur, so as to locally pressurize the part;

a push-in device for pushing the local pressurization pin in toward the part; and

a control portion for controlling the push-in device in such a way that the local pressurization pin is allowed to follow the movement of a solidification interface based on a solidification interface movement speed, which is calculated in advance, of the melt and the local pressurization pin is kept being pushed in until the solidification is completed.

7. The apparatus for manufacturing an aluminum die-cast product according to claim 6, wherein:

an overflow portion in accordance with the part, in which the casting defect tends to occur, is disposed in the mold;

the local pressurization pin is attached in such a way that the local pressurization pin can be pushed in the overflow portion; and

the overflow portion is locally pressurized with the local pressurization pin.