EP0257425A2

EP0257425A2 - Casting

Info

Publication number: EP0257425A2
Application number: EP87111549A
Authority: EP
Inventors: Norman Brown; David Rastall
Original assignee: Metal Castings (Worcester) Ltd
Current assignee: Metal Castings (Worcester) Ltd
Priority date: 1986-08-19
Filing date: 1987-08-10
Publication date: 1988-03-02
Also published as: GB2193914B; GB8620153D0; GB2193914A; EP0257425A3

Abstract

In the diecasting of an aluminium alloy, there is used a core (14) of the same alloy. The core is formed by diecasting and is discarded after a single use.

Description

The present invention concerns casting of bodies having shapes which are not simple shapes, cores being used to achieve undercut formations in the castings. By a core, we mean an element or assembly of elements which is disposed in the cavity in which the casting is to be formed, the core being formed separately from the body which defines that cavity and the core defining a part of the surface of the casting but being separated from the casting after the latter has solidified, for example, after the casting has been removed from the cavity which is formed.
According to a first aspect of the present invention, there is provided a method of casting wherein there is provided a body defining a cavity, a metal core is placed in the cavity, the cavity is charged with molten metal, the molten metal cools to form a casting containing the core, the casting is separated from the body and the core is removed from the casting and wherein the core is formed at least partly of a metal having a composition substantially the same as that of the casting.
By reference to the metal of which the core is formed having a composition which is substantially the same as the composition of the casting, we mean that, in a case where the core is formed entirely of that metal, the composition of the core is either identical with that of the casting or is sufficiently similar for the core and the casting to be remelted together in a bulk supply of metal for making further castings. It is common for a number of imperfect castings to be produced during the production of a large number of castings which are intended to be identical one with another. Imperfect castings may be produced at the beginning of a run, when a die in which in all of the castings are to be produced and associated ducts through which the molten metal is supplied are below the normal working temperature. It is desirable to remelt such imperfect castings so that the metal of which they are formed is cast once more. If the casting contains a core having substantially the same composition as the casting itself, it is unnecessary to remove the core from an imperfect casting, before re-melting the casting.
The core may be an assembly comprising a plurality of components. Preferably, the core or each component thereof is formed by diecasting. Even in a case where the core is removed from an acceptable casting, the core may be re-melted after a single use. When all of the cores are re- melted after use, it is unnecessary to carry out any maintenance of the cores and no disadvantage arises from any damage caused to the cores, during removal from the castings. Thus, the shape of the core may be changed during removal of the core from the casting. The shape of the core may be changed by removal of material and/or deforming the material of the core.
The core preferably has a wall thickness over at least a majority of the area of the wall which is not substantially less than, and is more preferably substantially greater than, the wall thickness of a part of the casting which engages the core. This ensures that the core does not receive from the molten metal sufficient heat to melt the core. It will be understood that a core of particularly low mass may become fused to the casting, unless arrangements are made to extract heat from the core whilst it is contact with the molten metal introduced into the cavity.
The method of the invention has been devised for use in the diecasting of light metals and alloys thereof. These metals include aluminium, magnesium and zinc.
According to a second aspect of the invention, there is provided a method of diecasting wherein a light metal core is mounted on or in an open die, the die is closed with the core inside, liquid metal, preferably having a composition which is substantially the same as that of the metal of which the core is formed, is admitted to the die cavity, the die is opened after the liquid metal has solidified to form a casting, the casting is removed from the die and the core is removed from the casting.
The method is useful in pressure diecasting, when the molten metal in the die is subjected to a pressure in excess of 10,000 psi (690 bar), although can be applied where the die is subjected to less than 10,000 psi(690 bar), for example dwon to 8000 psi (552 bar) or even less.
An example of a method in accordance with the invention will now be described, with reference to the accompanying drawing, wherein there is represented diagrammatically a cross-section through a two-part die, the parts of which are separated from each other, a casting which has been formed in the die and a core in the casting.
The die illustrated in the drawing comprises first and second parts 10 and I I respectively which collectively define a die-cavity. For use, the die parts are mounted in a die casting machine which may be of known construction and the die part I is provided with a gated runner system 12 through which molten metal can be driven by means not shown from a bulk supply into the die-cavity, when the die is closed. The means for melting and storing the bulk supply of molten metal and for driving portions of metal from that bulk supply into the die cavity form no part of the present invention and are therefore not described herein or shown in the drawings.
The die part 10 is generally of hollow, and in the example described generally cylindrical form and is open at one end only. The die part I I carries a core 13 which may be permanently fixed in the part I or removably mounted therein. When the die is closed, the core 13 extends along the interior of the die part 10 to the closed end thereof. When the die is prepared for use, an annular core 14 is mounted on the core 13 to lie within the die cavity, when the die is closed. The core 14 is so positioned on the core 13 that the core 14 is spaced from the closed end of the die part 10 and is also spaced from the die part II, when the die is closed. When molten metal is injected into the die cavity, it flows around the core 14 and this core therefore establishes an undercut formation in the casting. Once the casting has solidified, the die is opened and the casting is then ejected from the die part 10. After the casting has been ejected, the core 14 is removed from the casting.
In a case where the core 13 is fixed with respect to the die part II, the core 13 is withdrawn from the core 14 when the die is opened or when the casting is ejected. Alternatively, if the core 13 is removably mounted in the die part II, the core 13 may remain in the core 14, when the die is opened. In the latter case, the die part 13 may be withdrawn from the core 14 after the casting has been ejected from the die part 10 but in preparation for removal of the core 14 from the casting .
The core 14 is of generally annular shape and preferably comprises at least four arcuate components which are assembled together prior to being mounted on the core 13. These components may have integral formations which are in mutual engagement to maintain the assembled relation of the core parts until the core is to be removed from the casting. In this case, these formations may be deformed or removed, prior to removal of the core from the casting. Alternately, one or more additional elements may be used to maintain the core parts in assembled relation with one another. For example, there may be provided a circumferential band 15. The parts of the core 14 may be assembled with one another on the core 13 or prior to mounting of the core 14 on the core 13. The core 14 may be assembled on the core 13 prior to mounting of the core 13 in the die part II.
Typically, the casting 16 is formed of an aluminium alloy. The components of the core 13 are formed of the same alloy. These components may be formed by diecasting, by extrusion or by machining from suitable stock. The core 14 may be non-circular and can be asymmetrical, without inconvenience. The die parts 10 and I and the core 13 are formed of steel. Each of these is used repeatedly in the making of further castings.
If desired core 13 could be made of aluminium alloy or any other suitable material, and where this is not fixed with respect to die part I l, may, like core 13, be disposable after a single casting.
The arrangement of the core 14 may be such that one component thereof is readily moved relative to the other components in a radially inwards direction, once the core 13 had been removed from the core 14. In this case, the core 14 may be removed from the casting without substantial damage to the components of the core 14. Alternatively, the shape of one or more components of the core 14 may be modified during the removal procedure, by example by machining some material away from the core 14 or by deforming that core. The components of the core 14 are preferably re- melted, after a single use, so that new core components are used for each casting.
For some simple castings however, the core 14 may be easily extracted from the casting, thereby enabling some re-use of the core 14, or at least some portions of the core 14, to give economic advantage.
The core 14 does not contribute greatly to the combined weight of the casting and core and therefor does not increase the difficulty of handling the casting. The core 14 and the casting have substantially the same co-efficient of thermal expansion and so shrinkage of the casting onto the core during cooling, is at least reduced.
It will be appreciated that the temperature difference between the core 14 and the cast metal helps prevent fusion of the core to the casting.
The molten metal is preferably subjected to pressure whilst in the die cavity. A pressure in excess of 50 psi (3.45 bar) is used preferably about 8000 psi (552 bar) or above. More preferably, a pressure in excess of 10,000 psi (690 bar) is attained.
The features disclosed in the foregoing description, or the accompanyi ng drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately or any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

I. A method of casting wherein there is provided a body (10,11) defining a cavity, a metal core (13,14) is placed in the cavity, the cavity is charged with molten metal, the molten metal is cooled to form a casting (16) containing the core, the casting is separated from the body and the core is removed from the casting, characterised in that the core is formed at least partly (14) of a metal having a composition substantially the same as that of the casting (16).
2. A method of diecasting wherein a light metal core (14) is mounted on or in an opened die (10,11), the die is closed with the core inside, liquid metal is admitted to the die cavity, the die is opened after the liquid metal has solidified to form a casting, the casting is removed from the die and the core is removed from the casting.
3. A method according to Claim 1 or Claim 2 wherein the core comprises a plurality of components (13,14) assembled together.
4. A method according to any preceding Claim wherein the core (14) is formed by diecasting.
5. A method according to any preceding Claim wherein each core (14) is melted after a single use.
6. A method according to any preceding Claim wherein the molten metal is subjected in the cavity to a pressure of at least 50 psi (3.45 bar).
7. A method according to Claim 6 wherein the molten metal is subjected in the cavity to a pressure in excess of 10,000 psi (690 bar).
8. A method according to any preceding Claim wherein the core (14) has a wall thickness over at least a majority of the area of the wall not substantially less than that of the part of the casting which engages the core.
9. A method according to Claim 8 wherein the core has a wall thickness over at least a majority of the area of the wall, which is substantially greater than that of the part of the casting which engages the core.
10. A method according to any preceding Claim wherein the shape of the core is changed by removal of material and/or by deformation during removal of the core from the casting.
11. A method according to any preceding Claim wherein the core is other than circular.
12. Any novel feature or novel combination of features disclosed herein or in the accompanying drawing.