GB2153272A - Mould for large castings in metals and alloys - Google Patents

Abstract

A mould for casting a large body (5) in an aluminum alloy consists of a closed outer casing (1) corresponding to the outer contours of the body to be cast, the casting being manufactured of a thin steel sheet by welding or riveting. The casing comprises gates (3) and risers (4) in a conventional manner. The entire inner surface of the steel casing is lined with a heat-resistant insulating material (21), a preferred material consisting of asbestos sheeting or woven asbestos cloth up to a thickness of 2 mm, which prevents rapid cooling and permits ready stripping of the mould. <IMAGE>

Description

SPECIFICATION Mould for large castings in metals and alloys The invention relates to the founding of large castings in metals and alloys, more especially in aluminum alloys. It relates particularly to the casting of large and complex bodies which would, with conventional sand moulding, require intricate patterns and cores as well as large moulding boxes.

At present two kinds of moulds are used for the casting of non-ferrous metals, viz. sand mould and metal moulds. Sand moulding requires a pattern of wood or metal which forms the required shape in sand filling one or more moulding boxes, while inner contours and cavities are formed by cores inserted into the mould, these cores being likewise made of sand. Metal moulds are generally in use for smaller, mass-produced articles and consist of metal-usually cast-iron blocks-milled or other-wise machined in the required negative shape of the casting, taking into account the shrinkage during solidification and cooling, which varies in respect of different metals.

Cores are similarly made of cast iron or sand and are held in the mould in a manner suitable for producing the cavities in the casting.

Both methods are suitable in the case of small or medium-sized cast bodies; the moulds become, however, extremely heavy and expensive for casting large and heavy bodies, which is partichularly felt in all cases where the casting is an one-off job, or where only a small number of castings is required.

It is, therefore, one aspect of the present invention to provide a mould for large castings which should be comparatively light and, accordingly, obtainable at lower cost than the mould of the aforesaid kinds.

These moulds should be in one piece for one-off jobs and should be divided, split-type moulds when used for casting a small series of the same product.

It is an object of the present invention to provide moulds for forming the outer contours and cores for forming the inner cavities by the same fabricating method, whereby the cores could be left in the casting because of their small weight if so desired.

A mould, according to the present invention, comprises a casing made of comparatively thin steel plate, the mould being provided at its upper end with gates and risers while its inside surface is lined with a layer of heat-insulating material such as asbestos sheeting or a mixture of asbestos, graphite and binders sprayed onto the steel surface.

The shape of the mould is obtained by preparing a plurality of differently shaped parts of steel sheet and assembling these by welding, riveting or the like. A core is similarly made from a number of steel parts and lined on its outside with a heat-insulating material. The mould may be composite, comprising several parts held together by flanges and bolts for repeat orders or, in the case of a single casting, may be a completely closed casing which is cut off the casting after pouring and solidification of the metal. The this wall of the mould is made possible, even with very big moulds, by the light weight of the metal, aspecially in case of aluminum alloys, on the one hand, and by the fact that large castings are poured in several stages owing to the limited capacity of the furnace and/or the ladle.Between pouring stages the first poured metal layer solidifies except for its upper surface which stays in liquid or in mushy state, which then combines with the molten metal of the next portion. The already solid portion of the casting does not contribute to hydraulic pressure acting on the mould walls and the mould is, for this reason, exposed only to the hydraulic pressure caused by the height of the molten material still liquid above the dividing line between the previous and the new layer of poured metal. Owing to this circumstance only a small section of the mould is stressed by hydraulic pressure at every individual stage of pouring, and this fact permits the fabrication of a mould from steel sheet of less than 1 mm thickness.

The inner layer of heat-insulating serves, on the one hand, to prevent rapid cooling of the metal through the steel walls with its adverse effect on the final quality of the casing. and on the other, to permit ready separation of the mould from the casting. since asbestos or the known sprayed mixtures do not stick to aluminum alloys.

It is evident that for relatively small castings and large series the conventional method of casting in sand by means of patterns of wood or metal, especially in moulding machines, is preferable. However, the present method is better and less expensive for casting large bodies, such as parts of bridges, support pillars for buildings, underwater foundations for sea walls, moulds for pressing of sheet metal in the aircraft and automotive industry and others. It is especially useful for casting moulds employed in making prototype pressings during the development of new aircraft and motor vehicles.

The steel sheet mould lends itself to the forming of intricate shapes since the problems incurred with wooden patterns do not exist while using the present method. Sand forming requires, first of all, a draft along opposite surfaces in order to permit ready withdrawal of the pattern out of the sand mould, whereas the present method permits casting of bodies with parallel sides. Pattern moulding requires forming of reentering and protruding portions by means of split patterns, their loose parts being pulled out of the mould separately, while with the present method the entire mould can be either destroyed in the case of single castings, or the composite mould is opened along the flanged portions, and the several mould portions are taken off the casting.This way of removing a mould is similar to that used with permanent metal block moulds, except that the latter kind would be very heavy and immensely expensive to fabricate.

A preferred material for making the moulds is steel sheet of a thickness of 0.5 mm or more which can be readily riveted and welded. Preferable lining material consists of 2 mm asbestos sheeting, woven asbestos cloth which may be thinner but is obtainable up to a thickness of 2 mm; both materials can be bent to the shape of the mould, especially when wetted, and adhere well to the metal surface of the mould after having been attached thereto by rivetting or gluing. Another insulating method consists in spraying onto the inside of the mould insulating material of the kind used for lining permanent steel moulds for the purpose of ready separation of the casting from the mould.

The cores are similarly fabricated from preformed parts of steel sheeting which should be lined on the outside in order to be readily separated from the casting. On the other hand, since their weight is light, they may be left inside the casting, if so desired, or if they are intended for hollow spaces not communicating with the outside.

For better understanding the invention will now be more particularly described with reference to the accompanying drawings, wherein: Figure 1 is a vertical section through a mould used for casting a hollow column, Figure 2 is an enlarged drawing of the detail "A" of Fig. 1, and Figure 3 is a section through a mould for a different kind of casting.

With reference to Fig. 1 a steel sheet mould comprises an outer casing 1 and an inner casing 2 forming the core, their respective shapes corresponding to the outside and inside contours of the body to be cast in the mould. The top of the mould contains gates 3 serving to pour the metal into the mould, while several risers 4 are provided in the top and in the side walls serving to permit escape of gases.

Fig. 2 shows a portion of the outer and inner casing walls which are formed of steel sheet 10 and 20 lined on the side intended for contact with the molten material with a heat-insulating layer of asbestos sheeting 11 and 21 respectively. The metal casting is denoted by the numeral 5.

The method of fabricating a mould of this kind consists of first building up the outer casing from cut and preformed steel sheet portions assembled into the final shape by welding or riveting, and lining the inner surfaces with a heat-insulating material. It is presumed that the outer casing is of a size permitting a worker to enter it and fasten the insulating layer by gluing or riveting. However, if the inner space is too narrow for a worker to carry out this job. the lining is applied to separate parts before their final assembly, which can be carried out by welding since asbestos is heat resistant. In the case of a composite mould each separate part is lined internally before assembly by means of flanges and bolts.

The inner casing or core 2 is fabricated in a similar manner, and it is evident that there exists no problem of lining with heat-insulating material, since this is done on the outside surfaces after complete build-up of the casing.

In the mould illustrated in Fig. 1 it will be impossible to insert the core through the bottom opening of the outer casing, since the large-diameter parts of the core cannot pass through the inner small-diameter parts of the outer casing. The insertion of the core in this case is carried out by making the outer casing in at least two parts and connecting these after their being placed around the core at correct spacing between the walls, either by a permanent or by a releasable connection. The distance between the two casings is guarded and secured by stay-bolts or rivets 6 preferably made of the same material as the casting.

The stays are inserted either during, or after, complete assembly of the mould.

The shape of the core illustrated in Fig. 1 does not permit its stripping from the completed casting, owing to its shape and form:it can, therefore, be either left inside the casting, since it will not add much to the total weight, or it can be cut into several parts and pulled out of the casting in several parts.

Fig. 1 also illustrates the casting in several stages. The figure shows that three'portions of metal have been poured reaching the levels I, II and Ill, respectively. For ready understanding of the casting process it will be assumed that the casting is of such size that the metal carried by one ladle is just sufficient to fill the mould successively to the different levels.

After pouring the first ladle the level ''I'' has been reached, and until the next ladle is poured the bulk of the metal below level I has solidified, except for the uppermost layer which is still in molten state. When the metal is being poured up to level "II" the solid portion below level ''I'' does not exert any hydraulic pressure on the surrounding mould walls owing to the contracting of the casting.

For this reason the maximum hydraulic pressure will act on the mould wall at the height of level , this pressure being p = y.H, wherein y is the specific gravity of the molten metal (kg/cm3) and H is the distance between the levels "I" and "II" (cm). The freshly poured metal will readily combine with the molten or mushy surface of the lower layer.

The same state of pressure will exist during the following pouring stages up to the top level, and it becomes clear that the maximum pressure on any part of the mould is exerted at the bottom end of each liquid metal portion.

The mould illustrated in Fig. 4 comprises an outer casing 1' in the shape of a cone frustum, and an inner casing 2' in the shape of a cone frustum of the same length and of smaller diameter than that of the outer casing.

Gates 3' and risers 4' are provided in the top and the side walls of the outer casing similarly as in the mould shown in Fig. 1. The inner and the outer mould casings are connected at their top and bottom ends respectively by annular plates 7' and 8' which close the mould. The entire inner space of the mould is lined with an insulating material. The connections between the two casings are preferably of a releasable kind, so as to permit stripping of the inould without the need to destroy it.

It can be readily seen that the assembly as well as stripping the mould parts off the solidified casting is a simple matter, and that this kind of mould is most suitable for casting long, hollow masts, hollow columns and the like.

Claims (11)

1. A mould for a large casting in metal, especially suitable for casting a large body in an aluminurn alloy, comprising a closed outer casing corresponding to the outer contours of said body and having walls fabricated from comparatively thin steel sheeting, said casing being provided with at least one gate and at least one riser and being lined on its inside with a heat-insulating material resistant to the heat of the molten metal to be poured into said mould.

2. The mould as defined in Claim 1 for casting a large body having at least one inner cavity, said mould comprising a core in the shape an inner casing corresponding to the shape of said cavity and having walls fabricated from a comparatively thin steel sheeting. said core being connected to said outer casing and secured at the required distance from the walls of said outer casing, the outside of said casing walls being lined with a heat-insulating material resistant to the heat of the molten material to be poured in said mould.

3. The mould of Claim 1 or 2 comprising an undivided outer casing, used for single casting of a body.

4. The mould of Claim 1 comprising a composite outer casing consisting of several wall parts releasably connected to each other by connecting means such as flanges, bolts and nuts.

5. The mould of Claim 1 comprising a heat-resistant lining fastened to said casing surface by means of riveting.

6. The mould of Claim 1 or 2 comprising a heat-resistant lining fastened to said casing surface by gluing.

7. The mould of Claim 1 of 2 comprising a heat-resistant layer sprayed onto said casing surface.

8. The mould of Claim 5 or 6 wherein said lining consists of asbestos sheeting.

9. The mould of Claim 5 or 6 wherein said lining consists of woven asbestos cloth.

1 0. The mould of Claim 7 wherein said heat resistant layer consists of a mixture of asbestos, graphite and binder sprayed onto the surface of said casing walls.

11. The mould for a large casting in a aluminum alloy, substantially as hereinbefore described and illustrated in the accompanying drawings.