US20010002617A1

US20010002617A1 - Apparatus and methods for die casting

Info

Publication number: US20010002617A1
Application number: US09/738,447
Authority: US
Inventors: Donald Schofield; Mark Ring; John Lennon; Walter Gustafson
Original assignee: United Technologies Corp
Current assignee: Raytheon Technologies Corp
Priority date: 1998-12-23
Filing date: 2000-12-15
Publication date: 2001-06-07

Abstract

An apparatus and method for die casting includes a multi-part, reusable die having at least two die sections defining a die cavity for receiving molten material to be cast. The die sections are moveable relative to one another. Each die section defines a portion of the die cavity and the die sections define the die cavity when positioned in engagement with one another. A melting unit of the apparatus melts the material, and a shot sleeve receives molten material from the melting unit and is in fluid communication with the die. A plunger unit urges material from the sleeve into the die. The apparatus further includes a reduced pressure means, e.g., a vacuum unit, coupled to at least the die for providing a reduced pressure to at least the die cavity. A sealing mechanism of the apparatus is coupled to the die. The sealing mechanism engages the die sections to define an evacuation cavity, including at least the die cavity, when the die sections are brought into close proximity with one another, thereby enabling evacuation of the die while the die sections are in spaced relation and more thoroughly and completely evacuating the die cavity.

Description

This application claims the benefit of U.S. Provisional Application No. 60/113,570, filed on Dec. 23, 1998. [0001]

FIELD OF THE INVENTION

The present invention relates to apparatus and methods for casting, and more particularly to such apparatus and methods for die casting.

BACKGROUND OF THE INVENTION

In a typical die casting apparatus, two or more die sections are brought into engagement to define a die cavity having a shape corresponding to the part to be cast. Each die section or platen includes cooperating surfaces which meet at a parting line. Each portion may also include a groove, extending around the portion of the die cavity which intersects the parting line, for receiving a thin sealing gasket which seals the die cavity from the surrounding atmosphere when the die sections are brought into engagement with one another. Vacuum machinery may be coupled to the die, for purposes of providing a reduced pressure environment in the die cavity.

Associated die casting apparatus melts the material to be cast, and the molten material is transferred into an injection cylinder or shot sleeve that is coupled to the die. The molten material is subsequently forced into the die and solidifies to form the part. See, e.g., U.S. Pat. Nos. 2,932,865 to Bauer, 3,533,464 to Parlance et al. and 3,646,990 to Cross.

In the case of parts which have relatively thin cross sections, for example gas turbine engine components such as blades and vanes, the die cavity (when the die sections are brought together) is defined by die surfaces that are very close together, e.g., some components have radii on the order to a few mils, and thin minimum sectional thicknesses. Moreover, a given die may include several, e.g., up to 12, cavities which may not each have separate connections to the vacuum source. These thin die cavity sections are difficult to evacuate completely of air and liquids such as water vapor, even where the die is heated. A die cavity evacuated of all gases and liquids is essential to well-formed, high quality die cast parts.

Accordingly, the vacuum must applied for a long time to ensure thorough evacuation of all sections of the cavity, or each “sub-cavity” must include a separate connection to the vacuum source. The first case adversely affects cycle time (the time between successive castings). Since shorter cycle times are critical to economically producing parts, any delay in the process adversely affects the economics of the process. Despite efforts to remove all gasses/liquids from the die cavity, it is not uncommon for some gasses and/or liquids to remain in the die cavity. The second case adds complexity and associated cost, and also additional locations at which leakage into the die can occur with resulting defects. In either case, such defects result in a part having a substantially reduced strength compared to a part without defects.

While embedded, e.g., non-surface connected, porosity can in most cases be eliminated by processes such as hot isostatic pressing (HIP), generally a greater degree of porosity in a part requires a longer HIP cycle time, higher temperature, higher pressure or some combination of these criteria, which may enable undesirable grain growth. Any remaining defects result in a part having a substantially reduced strength and fatigue resistance compared to a part without such defects. The additional processing also adds significantly to the time and cost of producing parts.

Any reduction in cycle time enhances the cost effectiveness of producing die cast parts, since more parts can be produced per unit of time.

Resultant wasted materials and time associated with discarded parts can be substantial, as a new part must be produced. In addition, the necessary additional investment of time and materials necessary to repair the defects, if repair is possible, can also be substantial.

It is an object of the present invention to overcome the drawbacks and disadvantages of such prior art apparatus and methods for die casting.

SUMMARY OF THE INVENTION

The present invention is directed to an apparatus and method for die casting in which the die cavity is evacuated.

According to the present invention, an apparatus is disclosed for die casting in which molten material is injected under pressure into a multi-part, reusable die. The apparatus includes a die with at least two die sections defining a die cavity for receiving molten material to be cast. The die sections are preferably supportably moveable relative to one another, and each die section defines a portion of the die cavity so that the die sections define the cavity when positioned in engagement with one another. A melting unit of the apparatus melts the material, and a shot sleeve is coupled to the die for receiving molten material from the melting unit. The apparatus also includes a plunger unit in sealing and moveable engagement with the shot sleeve, for urging material from the shot sleeve into the die. A vacuum unit is attached to at least the die for providing a reduced pressure in at least the die cavity. A sealing mechanism of the apparatus is coupled to the die, and engages the die sections to define an evacuation cavity including at least the die cavity when the die sections are brought into close proximity with one another. The present invention enables evacuation of the die while the die sections are spaced slightly apart, and thus the die cavity can be more quickly and thoroughly evacuated.

Another aspect of the present invention is directed to a method of die casting in an apparatus having a die including at least two cooperating die sections which define a die cavity and are moveable relative to one another. Each die section has a mating surface positioned in opposing relationship to one another. The apparatus further includes a melting unit for melting the material, and a shot sleeve, which receives molten material from the melting unit and is coupled to the die. A plunger in the sleeve urges material from the sleeve into the die. The apparatus also includes vacuum unit coupled to at least the die for providing a reduced pressure in at least the die cavity. The method includes the steps of providing a sealing member on a mating surface, the sealing member extending around at least the die cavity; moving the die sections into close proximity such that the sealing member engages the other mating surface and the mating surfaces are in close proximity but spaced apart; evacuating the die cavity while the die sections are in close proximity but spaced apart; moving the die sections such that the mating surfaces engage one another; and injecting molten material into the die cavity.

One advantage of the present invention is that the die sections remain spaced apart, i.e., not fully closed together, and thus the die cavity remains substantially open during evacuation of the vacuum chamber. The die cavity is therefore more readily and quickly evacuated. The gasses and/or liquids which might otherwise be trapped in thin portions of the cavity during and even after the evacuation of a fully closed die cavity and resulting in the defects noted above, are evacuated. Accordingly, many of the longer cycle times and defects in parts associated with prior art die casting apparatus and methods are substantially eliminated.

Another advantage of the present invention is a lack of reliance on a thin, non-expandable seal between the die sections to keep gasses and liquids out of the die cavity during casting. Accordingly, the defects in parts caused by such gasses and/or liquids in prior art molding apparatus and method are substantially eliminated.

Other advantages of the present invention will become apparent in view of the following detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a die casting apparatus used in accordance with the present invention, including a die composed of two die sections. [0017]
FIG. 2 is a view illustrating the two die sections and a sealing member in accordance with the present invention. [0018]
FIG. 3 is a view of a die section and the sealing member. [0019]
FIG. 4 is a sectional view of the sealing member. [0020]
FIGS. 5 and 6 are views of the die section and sealing member in accordance with the present invention in respective first or evacuating and second or sealed positions. [0021]
FIG. 7 is a view of a second embodiment of the sealing member in accordance with the present invention. [0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to FIG. 1, the apparatus of the present invention is indicated generally by the [0023] reference numeral 10. We prefer to melt the material in an induction skull remelting or melting (ISR) unit 12 of the apparatus 10, for example in a unit of the type manufactured by Consarc Corporation of Rancocas, N.J. The unit 12 includes a crucible surrounded by an induction coil coupled to a power source 13. The fingers include passages for the circulation of cooling water from and to a water source (not shown), to prevent melting of the fingers. The field generated by the coil heats and melts material located in the crucible. Numerous other apparatus may be used to melt the material with equal effect, e.g., VIM, electron beam melting. The melting unit 12 may be located in a vacuum environment, e.g., a vacuum chamber (not shown in FIG. 1).
In order to transfer molten material from the [0024] melting unit 12 to a shot sleeve 16 of the apparatus, the unit is mounted for translation as indicated by the arrow 14, and also for pivotal movement (not shown) about a pouring axis. Other apparatus for manipulating the melting unit to transfer molten material, e.g., hand ladling or automatic ladling, may be employed with equal effect. The shot sleeve may be located in a vacuum, environment (not shown in FIG. 1), together with or separately from the melting unit. Molten material is poured from the melting unit 12 through a pour hole 18 of the shot sleeve 16.
The [0025] shot sleeve 16 is in turn coupled to a multipart, reusable die 20, which defines a die cavity 22. A sufficient amount of molten material is poured into the shot sleeve to fill the die cavity, which may include cavities for more than one part.
The illustrated die [0026] 20 includes two sections, 20 a, 20 b, (but may include more sections) which cooperate to define the die cavity 22, for example in the form of a compressor blade or vane for a gas turbine engine. In such a case, the die cavity includes one or more thin sections which can be difficult to evacuate. The die 20 is also preferably coupled directly to a vacuum source 24, to enable evacuation of the die prior to injection of the molten metal. The die may also be located in a vacuum chamber (also not shown). One section of the two sections 20 a, 20 b of the die is typically fixed, while the other part is movable relative to the one section, for example by a hydraulic assembly (not shown). The die preferably includes ejector pins (e.g., 26, 26 in FIG. 2) to facilitate ejecting solidified material from the die.
An injection device, such as a [0027] plunger 28 cooperates with the shot sleeve 16 and hydraulics or other suitable assembly (not shown) drive the plunger in the direction of arrow 30, to move the plunger between the position illustrated by the solid lines and the position 28′ indicated generally by dashed lines, and thereby inject the molten material from the sleeve 16 into the die cavity 22.
A sealing mechanism of the present invention and its operation is illustrated further in FIGS. [0028] 2-6. In FIGS. 2 and 3, a die section 20 a includes a channel 32 which extends around the die cavity 22. While the illustrated channel 32 is rectangular, other shapes, e.g., oval or rectangle with more rounded corners, may also be employed. A sealing member 34 is positioned in the channel 32 (the sizes of the channel 32 and sealing member 34 are exaggerated in FIG. 3). The sealing member is resilient, to enable sealing when the die sections are brought into close proximity and/or engagement with one another. In the preferred embodiment, the sealing member is composed of a fluoroelastomer such as Viton (R) from E.I. Du Pont deNemours & Co. of Wilmington, Del., although other materials may be employed with equal effect. The sealing member is preferably also tubular and therefore inflatable, and is connected to a source of pressurized gas 36 (FIG. 2), e.g., argon or air and preferably a non-reactive gas. Viton (R) is useful in operating conditions up to about 400°F. Where the die is heated to higher temperatures, other materials may be employed. The sealing member is preferably also inflatable (as indicated by the dashed lines in FIG. 4, to more positively seal the die cavity from the surrounding atmosphere and enable evacuation of the die cavity while the die sections are separated, which in turn enables faster and more thorough evacuation of the die cavity, particularly any thin sections, e.g., corresponding to thin airfoil sections.
The [0029] other die section 20 b includes a shroud 38 which cooperates with the sealing member 34, and defines a vacuum chamber when the shroud engages the sealing member (FIG. 5). While the illustrated embodiment utilizes a shroud with the die section 20 b, other arrangements are possible, e.g., part of the mating surface 21 b of the die portion 20 b may engage the sealing member 34 directly (FIG. 7), provided that such contact occurs prior to the mating surfaces 21 a, 21 b of the dies engaging to close the die cavity. In addition, the mating surfaces may define another channel (not show) for receiving an additional, conventional seal such as an “o-ring” seal, as described above.
In operation, material to be melted is placed in the [0030] melting unit 12, and the power source 13 is activated and melts the material. Preferably during this time, the die sections are moved (FIG. 5) so that the shroud 38 engages the sealing member 24, while the mating surfaces are in close proximity but spaced apart slightly from one another, e.g., up to an inch or so. The vacuum unit 24 is operated to evacuate the die cavity and the rest of the volume defined by the shroud 38, sealing member 24 and associated portions of the die sections 20 a, 20 b. The die sections are then moved to bring together the mating surfaces of the die sections to close the die cavity, and molten material is transferred from the melting unit 12 into the shot sleeve 16 and injected using the plunger from the position indicated by 28 to the position indicated by the dashed lines at 28′. After the material solidifies in the die cavity 22, the die portions are moved out of engagement, and the ejector pins 26 are activated to remove the part from the die cavity. The entire operation may be controlled by a controller (not shown), to automate the process.
One advantage of the present invention is that the die sections remain spaced apart during evacuation, i.e., not fully closed together, and thus the die cavity remains substantially open during evacuation of the vacuum chamber. As a result, the cavity is evacuated more quickly and in particular the gasses and/or liquids which might otherwise be trapped during and after the evacuation of a fully closed die cavity, which results in the defects noted above, are evacuated. This advantage is particularly great where parts having thin minimum sectional thicknesses, e.g., airfoils, are to be cast. Accordingly, many of the defects and longer cycle times associated with prior art die casting apparatus and methods are substantially eliminated. [0031]
Another advantage of the present invention is a lack of reliance on the thin, non-expandable seal between the die sections to keep gasses and liquids out of the die cavity during casting. Accordingly, the defects in parts caused by such gasses and/or liquids in prior art molding apparatus and method are substantially eliminated. [0032]
Where the additional, conventional seal is utilized, the die cavity is even more effectively sealed against leakage and the undesirable effects of such leakage. [0033]
The present invention also substantially reduces the volume which is evacuated, compared to evacuating a chamber which surrounds the entire die structure, thereby minimizing the time needed to evacuate the die cavity. [0034]
In sum, the present invention enables the more rapid, high quality die casting of materials in which the die cavity is evacuated, since the cavity can remain open during evacuation to ensure the quick, thorough evacuation of all parts of the die. [0035]
From the foregoing, an apparatus and method for die casting is disclosed in some detail. It will be recognized by those skilled in the pertinent art that numerous changes and modifications may be made without departing from the spirit and scope of the invention. Accordingly, the invention has been described by way of illustration rather than by limitation. [0036]

Claims

What is claimed is:

1. An apparatus for die casting in which molten material is injected under pressure into a multi-part, reusable die, comprising:

a die including at least two die sections defining a die cavity for receiving molten material to be cast, the die sections being supportably moveable relative to one another, each die section defines a portion of the die cavity and the die sections define the mold cavity when positioned in engagement with one another;

a melting unit for melting the material;

a shot sleeve in fluid communication with the die for receiving molten material from the melting unit;

a plunger unit in sealing and moveable engagement with the shot sleeve for urging material from the sleeve into the die;

a reduced pressure means coupled to at least the die for providing a reduced pressure to at least the die cavity; and

a sealing mechanism coupled to the die, the sealing mechanism engaging the die sections to define an evacuation cavity including at least the die cavity when the die sections are brought into close proximity with one another, thereby enabling evacuation of at least the die cavity while the die sections are in spaced relation.

2. The apparatus of

claim 1

, further comprising:

means for controllably supporting and moving the mold sections toward and away from one another; and

a controller for controlling movement of the supporting and moving means and the operation of the reduced pressure means, so that the die sections are moved into and supported in the spaced apart relationship while the reduced pressure means is operated to reduce the pressure within the die cavity prior to moving the die sections into engagement.

3. The apparatus of

claim 1

, wherein at least one of the die sections defines a sealing channel, the sealing mechanism including a tubular sealing member coupled to the channel, whereby the sealing member engages the channel and the other die section when the die sections are positioned in the spaced apart relationship.

4. The apparatus of

claim 3

, wherein the other die section includes a sealing shroud, the shroud engaging the sealing member.

5. The apparatus of

claim 3

, wherein the tubular sealing member is hollow and resilient, and further comprising a source of compressed gas for selectively inflating the sealing member.

6. The apparatus of

claim 3

, wherein the sealing member is composed of a fluoroelastomer.

7. The apparatus of

claim 1

, further comprising a source of compressed gas coupled to the sealing mechanism, wherein the sealing mechanism includes an inflatable, resilient sealing member.

8. The apparatus of

claim 7

, wherein the sealing member is inflatable up to a thickness of about 1 inch.

9. The apparatus of

claim 1

, wherein the sealing mechanism enables evacuation of the die cavity when the die sections are separated by less than about 1 inch.

10. A method of die casting in an apparatus having a die composed of at least two cooperating die sections defining a die cavity and being moveable relative to one another, each die section having a mating surface positioned in opposing relationship to one another, a melting unit for melting the material, a shot sleeve for receiving molten material from the melting unit and in fluid communication with the die, a plunger unit urging material from the sleeve into the die, and vacuum unit coupled to at least the die for providing a reduced pressure to at least the die cavity, comprising the steps of:

providing a sealing member on a mating surface, the sealing member extending around at least the die cavity;

moving the die sections into close proximity such that the sealing member engages the other mating surface and the mating surfaces are in close proximity but spaced relation;

evacuating the die cavity while the die sections are in close proximity but spaced relationship;

moving the die sections such that the mating surfaces engage one another to define the die cavity; and

injecting molten material into the die cavity.

11. The method of

claim 10

, further comprising the step of:

controlling movement of the supporting and moving means and the operation of the reduced pressure means, so that the die sections are moved into and supported in the spaced apart relationship while the reduced pressure means is operated to reduce the pressure within the die cavity prior to moving the die sections into engagement.

12. The method of

claim 10

, wherein the tubular sealing member is hollow and resilient, and further comprising the step of selectively inflating the sealing member.

13. The method of

claim 10

, wherein during the step of evacuating the sealing mechanism enables evacuation of the die cavity when the die sections are separated by less than about one inch.

14. The method of

claim 10

, further comprising the step of:

ejecting the part from the die cavity.

15. A method of die casting with a reduced cycle time in an apparatus having a die composed of at least two cooperating die sections defining a die cavity and being moveable relative to one another, each die section having a mating surface positioned in opposing relationship to one another, a melting unit for melting the material, a shot sleeve for receiving molten material from the melting unit and in fluid communication with the die, a plunger unit urging material from the sleeve into the die, and vacuum unit coupled to at least the die for providing a reduced pressure to at least the die cavity, comprising the steps of:

evacuating the die cavity while the die sections are in close proximity but spaced relationship; and

moving the die sections such that the mating surfaces engage one another.

16. The method of

claim 15

, further comprising the step of:

injecting molten material from the sleeve into the die cavity.

17. The method of

claim 10

, wherein the sealing member is hollow and resilient; and further comprising the step of inflating the sealing member prior to the step of moving the die sections into close proximity.