GB2193666A

GB2193666A - Casting metal using polystyrene pattern in sand mould

Info

Publication number: GB2193666A
Application number: GB08615793A
Authority: GB
Inventors: Dan Taylor
Original assignee: AUTO ALLOYS
Current assignee: AUTO ALLOYS
Priority date: 1986-06-27
Filing date: 1986-06-27
Publication date: 1988-02-17
Anticipated expiration: 2006-06-27
Also published as: GB2193666B; GB8615793D0

Abstract

The pattern incorporates a material which is oxidising to carbon under the casting conditions so that carbon generated by destruction of the polystyrene is converted to carbon dioxide and/carbon monoxide and does not contaminate the metal. Alternatively or additionally, the pattern incorporates a material which provides for modification of the metalerical properties of the molten metal, e.g. by introducing an alloying element therein, during casting.

Description

SPECIFICATION Casting of metals The present invention relates to the casting of metals.

It is known to produce cast metal parts by a so-called 'lost foam' process. In this process, a model of the part to be cast (and also including a 'pouring' projection) is produced in expanded polystyrene and provided with a refractory coating. The model is buried in loose sand with its pouring projection protruding from the sand. Liquid metal is then poured onto the protruding pouring projection. The heat of this liquid metal destroys the polystyrene, transforming it into gasses and, if the temperature of the liquid metal is sufficiently high, carbon deposit. The resultant gas pressure (which may or may not be partly relieved by applying a vacuum to the sand to draw the gasses through the refractory coating) holds the sand in position and the liquid metal fills the space originally occupied by the model to produce the metal casting.

A first problem with which this invention is concerned with the carbon residues which may be obtained from the destruction of the polystyrene. These residues, which are generally only produced when the liquid metal has a temperature greater than 900"C, can be a serious contaminate in some metals and alloys.

It should however be appreciated that the carbon may be produced at temperatures less than 900"C. This is because the heat of the liquid metal destroys polystyrene in advance of the liquid metal front and it is in the area of polystyrene destruction (which may be at less than 900"C) that carbon is generated.

It is an object of the first aspect of the invention to obviate or mitigate this problem.

According to a first aspect of the present invention there is provided a method of producing a cast metal part comprising casting liquid metal into an expanded polystyrene model of the part embedded in a refractory sand wherein the model incorporates a material which is oxidising to carbon under the casting conditions.

The material which is oxidising to carbon (hereinafter referred to as the oxidising agent) converts the carbon to carbon monoxide and/or carbon dioxide. The removal of these cases may be helped by the application of a vacuum to the sand, as described above, and thus contamination of the cast metal part by carbon is avoided.

The oxidising agent may be incorporated in the model in a variety of ways. For example, the oxidising agent may be incorporated in the polystyrene during manufacture thereof, or during the pre-foaming of the polystyrene, or by introducing it into a pressure or vacuum mould filling system which is part of the machine for making the polystyrene model. Alternatively, the oxidising agent may be incorporated in a refractory coating of the polystyrene model.

Oxidising agents for use in the invention may, for example be iron oxide (Fe203). More preferably the oxidising agent is manganese dioxide (mono2) which for preferance is used in combination with potassium dichromate.

For best results the particular oxidising agent used for any casting operation will be one which only releases oxygen when the carbon is at a temperature where it is most readily converted to carbon dioxide and/or monoxide. Thus controliing the release of oxygen over a specific temperature range is the most effective way to oxidise the carbon. Factors which need to be considered in deciding on the temperature at which oxygen should be released (and hence deciding on the oxidising agent to be used) are the particular metal being cast, the temperature of casting, and the shape of the part being cast.A particularly suitable way of obtaining controlled oxygen release is to use, as oxidising agent, a mixture of manganese dioxide and potassium dichromate which will promote various rates of oxygen release over the temperature range 200 800"C (depending on the relative proportions of the two components in the mixture).

The amount of oxidising agent incorporated in the model will depend of the particular conditions of the casting operation but will gener aliy be 5-20% (more preferably 8-15%, e.g.

10%) by weight based on the total weight of the model (including oxidising agent).

The first aspect of the invention is applicable to the casting of a wide range of metals but is particularly suitable for casting spheroi dal graphite (S.G.) irons at temperatures of 1350-1480"C. In this case the oxidising agent is preferably manganese dioxide, most preferably used in an amount of about 10% by weight of the model.

The first aspect of the invention has the additional advantage in that the oxidising agent may provide additives in the cast metal which may be beneficial metallurgically. Thus, for example, in the case of casting S.G. irons using manganese dioxide as oxidising agent, manganese is assimilated into the liquid metal (as an alloying element) and provides beneficial properties for the cast part.

This leads to the second aspect of the invention which also relates to the casting of metals using destructible polystyrene models and concerns the improvements of the properties of the cast metal.

According to the second aspect of the invention there is provided a method of producing a cast metal part embedded in a refractory sand wherein the model incorporates a material which provides for modification of the metallurgical properties of the metal.

The material incorporated in the model may, for example, provide alloying elements for the metal. Suitable materials are, for example, any of the oxidising agents discussed with reference to the first aspect of the invention and which lead to the incorporation of alloying elements in the cast metal. In the case where the liquid metal results in generation of carbon from the polystyrene, the oxidising agent will provide the dual function of oxidising the carbon as weli as leading to the introduction of an alloying element into the metal. Such oxidising agents may also be used in the case of liquid metals which are not at a sufficiently high temperature to produce carbon from the polystyrene, in which case they serve merely for the introduction of alloying elements into the metal.

Alternative materials which may be incorporated in the model are for example powdered elemental magnesium, silicon, chromium, titanium or niobium.

A specific example of the second aspect of the invention is in the production of long thin castings (e.g. cam shafts or crankshafts) for which the polystyrene model incorporates a steel wire or steel tube to give the model rigidity. Such metal inserts normally result in undesirable chilling of the liquid metal leading to brittleness. To remedy this problem, the polystyrene model may incorporate a material leading to the introduction of tellurium into the metal. Telurium has the effect of preventing chilling of the metal thus preventing brittleness as well as promoting a better bond between the metal and the insert.

The method of the second aspect of the invention may also be used for: (i) the introduction of magnesium and silicon to promote the formation of S.G. Iron; (ii) the introduction of hexachloroethane to remove hydrogen from aluminium alloys; and (iii) the stablisation of stainless steels by additions of titanium or niobium.

Claims

1. A method of producing a cast metal part comprising casting liquid metal into an ex panded polystyrene model of the part em bedded in a refractory sand wherein the model incorporates a material which is oxidis ing to carbon under the casting conditions.

2. A method as claimed in claim 1 wherein the material which is oxidising to carbon is incorporated in a refractory coating of the model.

3. A method as claimed in claim 1 or 2 wherein the material which is oxidising to car bon is selected from iron oxide and mangan ese dioxide.

4. A method as claimed in claim 1 or 2 wherein the material which is oxidising to car bon comprises manganese dioxide and potassium dichromate.

5. A method as claimed in any one of claims 1 to 4 wherein the model comprises 5 20% by weight of the material which is oxidising to carbon.

6. A method as claimed in any one of claims 1 to 5 wherein the liquid metal is one which produces spheroidal graphite cast iron.

7. A method of producing a cast metal part comprising casting liquid metal into an expanded polystyrene model of the part embedded in a refractory sand wherein the model incorporates a material which provides for modification of the metallurgical properties of the metal.

8. A method as claimed in claim 7 wherein the material provides an alloying element for the metal.

9. A method as claimed in claim 7 wherein the material is selected from iron oxide, manganese dioxide, iron, manganese, titanium, tellurium, and niobium, magnesium, silicon, and chromium.

10. A method as claimed in claim 7 wherein the metal is aluminium and the material is hexachloroethane.

11. A method as claimed in claim 7 wherein the model incorporates a metallic insert.