GB2160454A

GB2160454A - Improvements in the manufacture of castings

Info

Publication number: GB2160454A
Application number: GB08415740A
Authority: GB
Inventors: Terence William Davis
Original assignee: Individual
Current assignee: Individual
Priority date: 1984-06-20
Filing date: 1984-06-20
Publication date: 1985-12-24
Also published as: GB2160454B; GB8415740D0

Abstract

Shrinkage in a casting is reduced or modified by introducing the metal or alloy under pressure into a frozen sand mould. Solidification begins quickly which results in more even distribution of porosity in the casting, and because the need is avoided for metal or alloy to be fed to the casting as it solidifies defects and distortion are reduced. The method is particularly advantageous applied to metals or alloys cast in the temperature range 400 DEG -800 DEG ; it may be applied to zinc-aluminium alloys, and to the manufacture of castings having fibre reinforcement.

Description

SPECIFICATION Improvements in the manufacture of castings This invention relates to improvements in the manufacture of castings.

A problem accociated with casting is shrinkage which can result in defects in the castings produced and distortion, as well as dimensional inaccu racies.

The present invention seeks to reduce or modify shrinkage in castings.

According to the present invention there is provided a method of manufacturing a casting from a metal or alloy, comprising the steps of making and freezing a sand mould and introducing the molten metal or alloy into the frozen mould under pressure.

As the metal or alloy is introduced under pressure the mould is quickly filled. The metal or alloy is rapidly cooled by the frozen mould and so solidification begins quickly. This reduces shrinkage in the casting, or at least reduces the occurence of un-even shrinkage in the casting because a more even distribution of porosity throughout the casting can be achieved. Furthermore, introducing the metal or alloy under pressure reduces or avoids the need to feed metal or alloy to the casting as it solidifies, which minimises the occurrence of defects in the casting and the possibility of distortion.

A good quality casting may be obtained, therefore, with dimensions which are contained within close tolerances.

With the additional advantage of a good surface finish on the casting, which results from the use of the frozen mould, it will be appreciated that the need for fettling or other surface finishing treatment of the casting after removal from the mould is reduced.

The pressure at which the metal or alloy is introduced into the mould is dependent upon the metal or alloy to be cast. In general the pressure may be relatively low, for example of the order of 20 p.s.i.

(1.4 kg/cm2) or less, but for some metals or alloys the pressure may have to be higher. It is probable that the lighter the metal or alloy is that is to be cast the higher will be the pressure required. With a relatively low pressure of introduction the inherent strength of the frozen mould may be adequate to withstand such pressures. If reinforcement should be required the mould may be supported externally by a flask or clamp.

The more quickly the metal or alloy solidifies in the frozen mould the more effective may be the control of the shrinkage in the resultant casting.

The temperature of the frozen mould and the temperature at which the metal or alloy is cast are determining factors in this. Accordingly the effectiveness of the present method in controlling the shrinkage of casting may increase the lower the temperature of the mould and the lower the temperature of casting the metal or alloy.

Preferably the mould is frozen to a termperature considerably below the freezing point. It may be frozen by means of liquid nitrogen or other suitable liquid gas.

The mould may be made of a sand which produces a self-supporting mould before it is frozen.

Alternatively the mould may be made to become self-supporting as it is frozen. It may, for example, be made by the known 'Effset Process' developed by W.H. Booth % Co. Limited in which the sand is bonded into the self-supporting state by freezing by the use of liquid nitrogen.

The method is most effective when applied to metals or alloys which are cast at a temperature of 1300"C or less. It is particularly advantageous when applied to the casting of metals and alloys which are cast in the temperature range 400-800"C, or, such as lead, at an even lower temperature. It will be appreciated, therefore that aluminium, zinc and copper and their alloys, and gun and Babbit-metals, for example, may benefit from being cast by the present method.

Zinc-aluminium alloys have low melting points (550-650 C). There are alloys within this group, such as the ZA alloys introduced in recent years, which are readily cast, and the castings produced from them have strengths which are comparable with castings made of mild steel. They have been used increasingly instead of cast iron and brass for the manufacture of air cylinders and small components for general engineering because they are lighter and cheaper. However, a problem has been their tendency to shrink excessively and un-evenly and distort when cast by conventional sand mould ing methods. In order to contain the shrinkage within reasonable limits it has been the general practice hitherto to cast these alloys in permanent moulds. Such moulds are expensive and for their use to be economically acceptable manufacture of castings in high volume is required.The application of the method according to the present invention to these alloys reduces the problem of shrinkage and distortion. In addition, because the frozen mould can be made relatively cheaply, the use of the method makes the manufacture of small numbers of castings, and even one-off castings, of the alloys a viable proposition.

Because the metal or alloy is introduced into the mould under pressure the method may be applied to the manufacture of a casting which has fibre-reinforcement. The pressure of the metal or alloy assists its passage between the fibres so that a satisfactory dispersion of the fibres in the casting can be obtained. A reinforcing fibre which may be used is, for example, the alumina fibre made by Imperial Chemical Industries PLC sold under the Trade Mark "Saffil". This may be used with a range of metals and alloys. In light metal alloys of aluminium and zinc, for example, the inclusion of this fibre singnificantly improves the strength of the casting at operating temperatures above room temperature and so the temperature range in which the casting may be satisfactorily employed is extended. The inclusion of the fibre can also assist in reducing creep. A zinc-aluminium alloy with fibre reinforcement may be introduced at a pressure of about 20 p.s.i. (1.4 kgicms) into the mould when frozen by liquid nitrogen.

It will be appreciated from the foregoing that not only does the present invention enable shrinkage in castings to be controlled so that the accurec r the castings is improved, but also it can extend :ne usefulness of some metals and alloys and enable castings to be manufactured at reasonable cost.

After a casting has been made the sand of the mould can be knocked out and made ready for r & use with minimal preparation.

Claims

1. A method of manufacturing a casting from a metal or alloy comprising the steps of making and freezing a sand mould and introducing the molten metal or alloy into the frozen mould under pressure.

2. A method according to claim 1 in which the metal or alloy is molten at a temperature of 1300 C or less.

3. A method according to claim 2 in which the metal or alloy is molten in the temperature range 400-800 C.

4. A method according to claim 1 in which the molten metal or alloy is introduced into the mould under a pressure of up to 20 p.s.i. (1.4 kgtcm2).

5. A method according to any preceding claim in which the metal or alloy has fibre reinforcement.

6. A method according to claim 4 in which the mould is frozen by means of liquid nitrogen and molten zinc- aluminium alloy is introduced into the mould under a pressure of 20 p.s.i. (1.4 kg/cm2).

New claim filed on 18 June 1985

7. A method of manufacturing a casting from a metal or alloy substantially as described herein.