GB2183516A - Pressure diecasting cores - Google Patents

Abstract

In pressure diecasting, a core material is used which while withstanding the casting pressure, can be disintegrated by chemical, mechanical or thermal means in order to remove it from a cavity in the casting. The material can be a chemical cement, which can be moulded in the plastic state, sets rigid in die and will be disintegrated by heating to an elevated temperature. An example shows a composite core comprising a permanent core body (16) and disposable core parts (18, 20) formed therearound and arranged to define, within die parts (10, 12) a casting cavity (22) including an undercut (24). <IMAGE>

Description

SPECIFICATION Pressure diecasting cores This invention relates two pressure diecasting, and in particu la r to cores for pressu re diecasting .

Cores used in pressure diecasting have to be capable of withstanding considerable pressures and also the shock wave produced by molten metal being forced into the mould. Forthis reason cores are generally made from metal having a melting point higherthan that ofthe metal being cast. This is suitable when the cores can be withdrawn intact once the mould is opened, but ifthe core is an awkward shape,forexample having an undercut, or ifthe entrance lies on a parting plane of the mould, then it may be impossibleto use a rigid core.

Conventional sand cores as used in gravity fed casting are not sufficiently robust to withstand the conditions experienced in pressure diecasting.

According to a first aspect of the present invention, there is provided a method of pressure diecasting, wherein at least part of a mould core is made from a disintegrable material.

According to a second aspect of the present invention, there is provided a core for pressure diecasting, the core being made from a chemical cement (as hereinafter defined) which is capable of withstanding the injection pressures experienced in the pressure diecasting, e.g. of aluminium or magnesium, without disintegrating, but which does disintegrate on reaching an elevated temperature somewhat lowerthanthe melting point of the metal being cast.

The term 'chemical cement' means a material which can be formed or moulded in a plastic state, but which sets rigid on contact with air. Asuitable material is the material sold by DSM of Holland underthetradenameKupron.Thismaterial issold as a material from which moulds themselves can be constructed. Itis probablethatthecomposition of this particular cement may require some modification to ensure that it disintegrates at about 600"C.

The invention is particularly concerned with the pressure diecasting of aluminium and magnesium which have (relatively) low melting points of 660"C and 651"C respectively.

In the casting process, the solidification timefor the metal being cast will be around one ortwo minutes. Oversuch a shorttime period, little heat will be conducted into the body ofthe core itself.

However once casting is completed, the temperature of the core will rise, eitherwhilstthe casting is left in the mould, or after the casting has been removed from the mould and placed in a special oven where the core temperature can be raised to a temperature somewhat below the melting point of the metal which has been cast. The composition ofthe cement can be such that, at this elevated temperature, the core will startto disintegrate, and can be removed from the casting.

The composition of the cement should be adjusted orcontrolledtomaketherigidityofthecement suitablytemperature dependent, and this may be trueforthe material Kupron aswell as for other chemical cements.

The invention will now be further described, by way of example, with reference to the accompanying drawing which shows a section through a mould with a core in accordance with the invention.

The drawing shows a mould with two parts 10 and 12 which meet on a parting line 14. A permanent core 16 of metal projects into the internal core cavity and has mounted on it two disposable core sections 18 and 20. Molten metal to be cast is forced into the mould under pressure through a conventional gating system to fill the remaining cavity 22. The metal then solidifies.

Once solidification is complete and the product has cooled, the permanent mould is withdrawn leaving the disposable core sections 18 and 20 in the casting. The mould halves 10 and 12 are opened, and as a final step the disposable cores are removed. The method of removal ofthe disposable cores will depend on their composition, but may be accomplished by chemical, mechanical orthermal means.

The disposable core sectionswill normally be mounted on the permanent core before the core is positioned in the mould. It is importantthatthejoint between the two core types is adequate to prevent any penetration of molten metal between them. One way of accomplishing this would beto makethe disposable core so that it surrounds all the external faces ofthe permanent core so that the molten metal does not see any permanent/disposable core interface.

It will be noted that the mould cavity shown in the drawing has an undercut region 24. It would not be possible to form th is u ndercut by casti ng if a permanent core alone were to be used, as itwould be impossibleto withdrawthe core. The use of disposable core sections as shown however allows undercut regions two be cast and thus allowsthe technique of pressure diecasting to be extended to shapes which previously could not be formed in this way.

Although the drawing shows disposable core sections being used in conjunction with a permanent core, it is also possible within the scope ofthis invention, to use disposable cores alone, with no permanent core at all.

1. A method of pressu re diecasting, wherein at least part of a mould core is made from a disintegrable material.

2. A method as claimed in Claim 1,forthe pressure diecasting of aluminium, magnesium and their alloys.

3. Acorefor pressure diecasting, the core being made from a chemical cement (as hereinbefore defined) which is capable of withstanding the injection pressures experienced in the pressure diecasting, e.g. of aluminium or magnesium, without disintegrating, but which does disintegrate on reaching an elevated temperature somewhat lowerthanthemeltingpointofthemetal being cast.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Pressure diecasting cores This invention relates two pressure diecasting, and in particu la r to cores for pressu re diecasting . Cores used in pressure diecasting have to be capable of withstanding considerable pressures and also the shock wave produced by molten metal being forced into the mould. Forthis reason cores are generally made from metal having a melting point higherthan that ofthe metal being cast. This is suitable when the cores can be withdrawn intact once the mould is opened, but ifthe core is an awkward shape,forexample having an undercut, or ifthe entrance lies on a parting plane of the mould, then it may be impossibleto use a rigid core. Conventional sand cores as used in gravity fed casting are not sufficiently robust to withstand the conditions experienced in pressure diecasting. According to a first aspect of the present invention, there is provided a method of pressure diecasting, wherein at least part of a mould core is made from a disintegrable material. According to a second aspect of the present invention, there is provided a core for pressure diecasting, the core being made from a chemical cement (as hereinafter defined) which is capable of withstanding the injection pressures experienced in the pressure diecasting, e.g. of aluminium or magnesium, without disintegrating, but which does disintegrate on reaching an elevated temperature somewhat lowerthanthe melting point of the metal being cast. The term 'chemical cement' means a material which can be formed or moulded in a plastic state, but which sets rigid on contact with air. Asuitable material is the material sold by DSM of Holland underthetradenameKupron.Thismaterial issold as a material from which moulds themselves can be constructed. Itis probablethatthecomposition of this particular cement may require some modification to ensure that it disintegrates at about 600"C. The invention is particularly concerned with the pressure diecasting of aluminium and magnesium which have (relatively) low melting points of 660"C and 651"C respectively. In the casting process, the solidification timefor the metal being cast will be around one ortwo minutes. Oversuch a shorttime period, little heat will be conducted into the body ofthe core itself. However once casting is completed, the temperature of the core will rise, eitherwhilstthe casting is left in the mould, or after the casting has been removed from the mould and placed in a special oven where the core temperature can be raised to a temperature somewhat below the melting point of the metal which has been cast. The composition ofthe cement can be such that, at this elevated temperature, the core will startto disintegrate, and can be removed from the casting. The composition of the cement should be adjusted orcontrolledtomaketherigidityofthecement suitablytemperature dependent, and this may be trueforthe material Kupron aswell as for other chemical cements. The invention will now be further described, by way of example, with reference to the accompanying drawing which shows a section through a mould with a core in accordance with the invention. The drawing shows a mould with two parts 10 and 12 which meet on a parting line 14. A permanent core 16 of metal projects into the internal core cavity and has mounted on it two disposable core sections 18 and 20. Molten metal to be cast is forced into the mould under pressure through a conventional gating system to fill the remaining cavity 22. The metal then solidifies. Once solidification is complete and the product has cooled, the permanent mould is withdrawn leaving the disposable core sections 18 and 20 in the casting. The mould halves 10 and 12 are opened, and as a final step the disposable cores are removed. The method of removal ofthe disposable cores will depend on their composition, but may be accomplished by chemical, mechanical orthermal means. The disposable core sectionswill normally be mounted on the permanent core before the core is positioned in the mould. It is importantthatthejoint between the two core types is adequate to prevent any penetration of molten metal between them. One way of accomplishing this would beto makethe disposable core so that it surrounds all the external faces ofthe permanent core so that the molten metal does not see any permanent/disposable core interface. It will be noted that the mould cavity shown in the drawing has an undercut region 24. It would not be possible to form th is u ndercut by casti ng if a permanent core alone were to be used, as itwould be impossibleto withdrawthe core. The use of disposable core sections as shown however allows undercut regions two be cast and thus allowsthe technique of pressure diecasting to be extended to shapes which previously could not be formed in this way. Although the drawing shows disposable core sections being used in conjunction with a permanent core, it is also possible within the scope ofthis invention, to use disposable cores alone, with no permanent core at all. CLAIMS

1. A method of pressu re diecasting, wherein at least part of a mould core is made from a disintegrable material.

2. A method as claimed in Claim 1,forthe pressure diecasting of aluminium, magnesium and their alloys.

3. Acorefor pressure diecasting, the core being made from a chemical cement (as hereinbefore defined) which is capable of withstanding the injection pressures experienced in the pressure diecasting, e.g. of aluminium or magnesium, without disintegrating, but which does disintegrate on reaching an elevated temperature somewhat lowerthanthemeltingpointofthemetal being cast.

4. Amethod of pressurediecasting,substantially as herein described with reference to the accompanying drawing.

5. A core for pressure diecasting, substantially as herein described, with reference to the accompanying drawing.