WO1995028244A1

WO1995028244A1 - A device for casting precision castings

Info

Publication number: WO1995028244A1
Application number: PCT/FI1995/000219
Authority: WO
Inventors: Matti HYVÄRINEN
Original assignee: Hyvaerinen Matti
Priority date: 1994-04-15
Filing date: 1995-04-13
Publication date: 1995-10-26
Also published as: AU2260495A; FI97955C; FI941743A; FI97955B; FI941743A0

Abstract

A device for casting precision castings, the said device incorporating a chamber (20), a mould (12), and equipment (15) for generating a pressure difference between the interior and exterior of the mould. The mould is situated completely inside the chamber, excluding a pouring hole (34) on the upper surface of the mould, through which hole the metal (18) to be cast is introduced into the mould. Between the upper surface of the mould and the cover (22) of the chamber (20) is placed a seal (30), such as a sharp-edged ring gasket, penetrating into the upper surface of the mould. The mould seal and the seal (24) of the chamber cover can be sealed simultaneously by the same movement that closes the chamber cover.

Description

A DEVICE FOR CASTING PRECISION CASTINGS

The object of the invention is a device for casting precision castings, the said device incorporating a chamber and a mould, which is at least partly inside the chamber, and equipment for generating a pressure difference between the interior and exterior of the mould.

The mould used in casting precision castings such as jewellery, items relating to dental technology, or other similar products is usually made of a plaster composition. The mould is made by casting it over a model made of wax which is placed in a perforated metallic mould cylinder. Once the plaster composition has hardened, the wax is melted away, thus leaving a cavity in the mould for the metal to be cast. Before casting the mould is heated to the desired casting temperature, after which the molten metal is poured into the mould. When the metal has solidified, the disposable plaster mould is removed from around the casting.

The above lost wax method is an extremely precise and economical way of making many precision products. In small series it is also considerably more economical than another precision casting method, that is, chill casting. The lost wax method is, however, practicable only when all the casting stages can be made to function as desired, because the method is not without problems.

For example, in casting jewellery several problems occur, one of which is the incomplete filling of the mould cavities. Among other things, it is obvious that if the design of a piece of jewellery is complex, an air pocket remaining in a mould cavity may prevent the penetration of molten metal into it, thus rendering the casting unsuccessful. Since a wax model of a piece of jewellery is disposable, unsuccessful casting may cause irreparable damage. Attempts to eliminate the problem have included increasing the filling pressure by means of centrifugal force and reducing the pressure outside the mould in the chamber, in which case the difference in pressure between the interior and exterior of the mould facilitates the filling of the mould. This pressure difference can be generated, for example, by placing the mould in a vacuum chamber. The molten metal will then penetrate well into every cavity of the mould during casting.

The metal to be cast can also be melted in the vacuum chamber. In this case, there is no air in the mould to prevent the filling of the mould when the molten metal is poured into the mould. The casting result is excellent, but the equipment required for vacuum casting is structurally extremely complex and expensive.

Another way of utilising negative pressure in casting is to place the casting mould only partially in the vacuum chamber. The upper surface of the mould and the cast hole in it are in contact with outside air, but the rest of the mould is situated inside the chamber, under at least a partial vacuum. The plaster mould thus forms an intermediate piece between the outside air and the vacuum.

Since plaster is a porous material, the pressure difference between outside air and the vacuum chamber tends to level out through the pores of the mould. A sufficient vacuum is, therefore, maintained by means of a vacuum pump connected to the chamber. Thus there is slight air flow through the plaster mould, away from the metal being cast, which facilitates the filling up of the mould cavities and improves the casting result.

A known vacuum chamber solution relating to the above method is such that the plaster is cast in a specially made mould cylinder equipped with sealing joints. Once the plaster has hardened, the special cylinder, which is open-ended, is placed in an opening made for the cylinder in the vacuum chamber. Sealing is carried out by means of annular rubber and asbestos seals placed between the cylinder's sealing joints and the chamber.

The problem with this construction is that the rubber seal heats up during casting. During casting, the temperature of the mould is about 400°C to 600°C, which a rubber or plastic seal cannot withstand. Hard asbestos or similar seals, on the other hand, do not seal properly, because it is difficult to make the sealing surfaces sufficiently flat and smooth. For these reasons the sealing of the vacuum chamber deteriorates rapidly and the casting may fail because of it.

Another disadvantage of known devices is that the whole upper surface of the plaster mould is under atmospheric pressure. In this area of the mould there is no clearly detectable air-flow away from the metal being cast, and it has been found that in this case the upper part of the plaster mould is not filled completely. Therefore, the construction does not function very well, even though expensive and complex mould and sealing solutions are applied.

The aim of the present invention is to eliminate the last- mentioned problems and to provide a new device which does not have the above disadvantages. It is characteristic of the device for casting precision castings relating to the invention

- that the mould is situated for the most part or completely inside the chamber, excluding the pouring hole on the upper surface of the mould, through which the metal to be cast is introduced into the mould,

- that a mould seal is placed between the upper surface of the mould and the chamber cover or a part connected to it, and that the mould seal and the seal of the chamber cover can be sealed more or less simultaneously, either by the same movement that closes the chamber cover, or by a separate movement subsequent to closing the cover.

By means of the invention, the vacuum chamber of the device for casting precision castings can be sealed substantially more effectively and securely. The sealing points of the mould seal and the cover seal are sealed at the same time as the cover is pressed into place. The surface of the plaster mould under atmospheric pressure is also essentially smaller. As a result, the filling of the mould improves and successful casting is ensured.

The invention is described in the following by way of examples, with reference to the appended drawings, in which

Figure 1 shows a vertical section of a vacuum chamber solution for a device for casting precision castings in accordance with the prior art.

Figure 2 shows a vertical section of a vacuum chamber solution for a device for casting precision castings in accordance with the invention.

Figure 3 corresponds to figure 2 and shows a second embodiment of the vacuum chamber solution.

Figures 4 and 5 show the different stages of closing the vacuum chamber of figure 2.

Figure 1 shows a known device solution for casting precision castings in accordance with the prior art, where the plaster mould 12 is cast in a specially made mould cylinder 11 equipped with sealing joints 13. After the plaster mould 12 has hardened, the mould cylinder 11 has been placed in the opening on top of the vacuum chamber 10. A ring seal 14, including both an asbestos and a rubber seal, is placed between the sealing joints 13 of the cylinder 11 and the vacuum chamber 10. In the side wall of the vacuum chamber 10 an opening and a connection point 15 for the vacuum pump hose are formed. Negative pressure is generated in the chamber by means of a vacuum pump. The pump is not shown in figure 1.

The mould cylinder 11 is open at both ends and openings 16 have been formed in its side walls. A slight air flow takes place through these openings through the porous plaster mould, away from the metal 18 being cast. This facilitates the filling of the cavities 17 of the plaster mould 12 and improves the casting result.

When casting jewellery or other precision castings, the molten metal 18 is poured into the pouring hole 34 formed on the upper surface of the plaster mould 12, from where the metal 18 being cast spreads into all the cavities 17 of the mould. However, the uppermost cavities 17 of the mould present a problem. This is due to the fact that the entire upper surface of the plaster mould 12 is in contact with the outside air. Because the flow of air through the plaster is very slow, this means that a sufficient pressure difference fails to be generated in the upper part of the plaster mould 12. Therefore, the uppermost cavities of the mould 12 remain unfilled, as shown in figure 1.

Figure 2 shows a device for casting precision castings according to the invention, which uses a simple, straight mould cylinder 21, inside which the plaster mould 12 has been cast. No special sealing joints need to be formed in the cylinder 21, because the mould cylinder 21 is completely inside the vacuum chamber 20.

Sealing between the outside air and the vacuum chamber 20 has been arranged by means of a single annular sharp-edged ring gasket 30 or several sharp-edged ring gaskets placed within each other. The sharp-edged ring gaskets 30 are formed on the bottom surface of the sealing joint 23 so that the knife-like edge of the sharp-edged ring gasket 30 penetrates into the upper surface of the plaster mould 12 when the sealing joint 23 and the cover connected to it by means of screws 33 are mounted in their place. An O-ring seal 24 is placed between the vacuum chamber 20 and its cover 22, and the cover 22 is tightened into place by means of quick-locking clasps 29.

In the centre of the vacuum chamber 20 cover 22 an opening is formed for the replaceable sealing joint 23. The space between the cover 22 and the sealing joint 23 is sealed by means of an annular sheet gasket 26 which may, for example, be an O-ring seal. If necessary, the sealing joint 23 can be replaced by another of a different size depending on the size of the mould, by removing the screws 33.

In figure 2, thermal insulation material 25 which is, for example, fireproof fabric, is attached to the bottom surface of the sealing joint 23. The purpose of the thermal insulation material 25 is to prevent heat from being transmitted to the cover 22 of the chamber and at the same time to increase the tightness of the sharp-edged ring gasket. By adjusting the thickness of the thermal insulation material, the depth of penetration of the sharp edge of the sharp-edged ring gasket 30 into the plaster mould 12 can also be regulated. If there are several sharp-edged ring gaskets 30 within each other, the thermal insulation material 25 may be omitted. In this case, sealing is already achieved by shallower penetration of the sharp-edged ring gasket into the surface of the plaster mould 12. Excessively deep penetration might break the surface structure of the plaster mould.

A sealing structure incorporating at least one dhsrp-edged ring gasket and, where necessary, thermal insulation material, forms efficient thermal insulation, preventing the overheating of the chamber 20 cover 22 and its seals 26 and 24, as only very little heat passes through the thin sharp- edged ring gaskets 30. For these reasons, the seals of the structure relating to the invention are practically non- wearing. In fact, their service life is essentially longer than in the known solution shown in figure 1. In this case even the chamber itself does not heat up to any significant degree.

In figure 2, the mould cylinder 21 is supported on the opposite side of the cover 22 by an intermediate base 27, under which are placed three pressure springs 28. Uniform compression is achieved by means of the three springs 28, even if the bottom of the mould 21 is not completely even or symmetrical. The total compressive force of the springs 28 should be so high that the sharp-edged ring gasket 30 will penetrate sufficiently into the upper surface of the plaster mould 12. If necessary, the compression of the springs 28 can be adjusted, for example, by means of spacers placed on the bottom of the chamber 20. The number of springs may obviously vary as necessary. In some cases, a single spring placed in the middle of the mould 21 may suffice.

The essential aspect of the device solution shown in figure 2 for casting precision castings is that the sealing points of the sharp-edged ring gasket 30 and the seal 24 of the cover 22 are sealed in the same movement, when the cover 22 is pressed into place by means of the quick-locking clasps 29.

Figure 3 corresponds to figure 2 and shows a second embodiment of the casting device relating to the invention. The difference is that the intermediate base 27 is pressed against the cover 22 by means of a pneumatic cylinder 31 and piston 32 instead of by means of springs. In this solution, only the joint between the cover 22 and the vacuum chamber 20 at the location of seal 24 is sealed when the cover 22 is closed by means of the quick-locking clasps 29. The joint of the sharp-edged ring gasket 30 is sealed until the intermediate base 27 is pressed against the cover 22 by means of the piston 32.

In the device solutions shown in both figure 2 and figure 3 for casting precision castings, the cover 22 remains well in place due to the effect of atmospheric pressure, as soon as sufficient negative pressure has been generated in the chamber 20. In the solution of figure 3, the intermediate base 27 may be pressed against the mould 12 by means of the movable piston 32 or a corresponding lever or screw mechanism.

Figure 4 shows the vacuum chamber of the device for casting precision castings relating to figure 2 before it is closed. Figure 4 shows that a mould cylinder 21 is placed on the intermediate base 27 in the chamber 20, inside which cylinder the plaster mould 12 is cast. However, the intermediate base 27 supported by springs 28 remains so high that the cover 22 resting on the plaster mould 12 remains slightly open. The size of this opening is denoted by reference letter (d). At this stage, the annular sharp-edged ring gasket 30 of the sealing joint 23 connected to the cover 22 is pressed against the upper surface of the plaster mould 12 by the weight of the cover 22.

In figure 5, the cover 22 has been closed by means of the quick-locking clasps 29. When the cover 22 was closed it was pressed downwards by the amount of the opening shown above, that is, by the distance (d), against the O-ring seal 24. Correspondingly, the intermediate base 27 has at the same time also been pressed down by distance (d) by the mould cylinder 21 situated in between. The springs 28 situated under the intermediate base 27 and resisting this movement then press the plaster mould 12 on the intermediate base 27 against the sharp-edged ring gasket 30. As a result, the knife-like edge of the annular sharp-edged ring gasket 30 of the sealing joint 23 penetrates into the upper surface of the plaster mould 12, thus sealing this space.

The essential aspect of the closing movement of the chamber 20 cover 22 of the device for casting precision castings shown in figures 4 and 5 is that, in addition to closing the cover 22, the same movement also seals the chamber 20 on top of the plaster mould 12 around the filling hole, by means of the sharp-edged ring gasket 30.

It is obvious to a person skilled in the art that the different embodiments of the invention may vary within the scope of the claims presented below.

Claims

1. A device for casting precision castings, the said device incorporating a chamber (20) and a mould (12), which is at least partly inside the chamber, and equipment (15) for generating a pressure difference between the interior and exterior of the mould, c h a r a c t e r i z e d in that

- that the mould (12) is situated for the most part or completely inside the chamber (20), excluding the pouring hole (34) on the upper surface of the mould, through which the metal (18) to be cast is introduced into the mould,

- that a mould seal (30) is placed between the upper surface of the mould (12) and the chamber (20) cover (22) or a part (23) connected to it, - and that the mould seal (30) and the seal (24) of the chamber (20) cover (22) can be sealed more or less simultaneously, either by the same movement that closes the chamber cover (22), or by a separate movement subsequent to closing the cover.

2. A device as claimed in claim 1, c h a r a c t e r i z e d in that the seal (30) of the mould (12) can be sealed by the same movement as the seal (24) of the cover, so that the mould (12) is spring-loaded inside the chamber against the cover (22) or its part (23).

3. A device as claimed in claim 1, c h a r a c t e r i z e d in that the seal (30) of the mould (12) can be sealed by a separate movement subsequent to sealing the cover (22) seal (24), so that a means of pushing (32) is incorporated in the mould for pressing the mould inside the chamber against the cover (22) or its part (23).

4. A device as claimed in claim 1, 2 or 3, c h a r a c t e r i z e d in that the mould (12) and the mould seal (30) can be moved with respect to each other by locating a movable intermediate base (27) against the mould on the opposite side of the cover (22).

5. A device as claimed in claim 4, c h a r a c t e r i z e d in that the intermediate base (27) is spring-loaded against the mould (12) by means of one or more springs (28).

6. A device as claimed in claim 4, c h a r a c t e r i z e d in that the intermediate base (27) is pressed against the mould (12) by means of a movable piston (32) or a lever or screw mechanism.

7. A device as claimed in any of the claims 1 to 6, c h a r a c t e r i z e d in that the mould (12) seal (30) consists of at least one sharp-edged ring gasket which penetrates into the surface of the mould thus forming a sealing which makes it possible to generate a pressure difference between the interior and exterior of the mould.

8. A device as claimed in claim 7, c h a r a c t e r i z e d in that a layer (25) is placed in connection with the annular sharp-edged ring gasket (30) against the mould (12), by means of which layer the depth of penetration of the sharp-edged ring gasket can be regulated and which at the same time increases the tightness of the seal.

9. A device as claimed in claim 8, c h a r a c t e r i z e d in that the layer (25) situated against the mould (12) in connection with the sharp-edged ring gasket (30) is thermal insulation material.

10. A device as claimed in claim 7, 8 or 9, c h a r a c t e r i z e d in that the mould (12) and the sharp-edged ring gasket (30) can be moved with respect to each other so that the sharp-edged ring gasket incorporated directly in the cover (22) or its part (23) can be pressed against the mould or vice versa.