Centrifugal Casting (Spinning)

This method is used to manufacture hollow castings, which have a shape of rotating bodies. Two modifications of the method are distinguished: with vertical and horizontal axes of rotation (Fig. 3.13).

Fig. 3.13. Centrifugal casting with horizontal (a) and vertical(b) axes of rotation:

1 – mould; 2 – liquid metal; 3 – spout; 4 – ladle; 5 – casting

 

Ingot moulds are made of cast iron. They are heated (150...450°C) and painted by refractory mixture (paint) before pouring of metal.

Method with vertical axis of rotation is used for production of castings small in height, because inner surface of the casting acquires paraboloid shape and lower diameter of hollow is less than upper one.

The second method is used for castings of significant length production, e.g. hydrostatic pressure and overflow pipes. They have length from 3 to 12 meters. Advantages of the methods:

- high density of metal due to the action of centrifugal forces;

- reduced metal consumption, because a gating system is absent;

- inner hollow in castings is received without using of cores;

- a mould is easily filled by molten metal under the action of centrifugal forces;

- a patternless method.

The disadvantages are as follows:

- low dimensional accuracy of inner surface;

- inner surface is contaminated by segregation products and non-metallic inclusions.

 

Pressure-Die Casting

Method has the highest output from 200 to 400 castings per hour. Sand mixtures are not applicable for the method. Crystallization of castings is carried out in a metal mould, named press-mould. Essence (main point) of the method is that liquid or both liquid and solid metal is pressed into press-mould under high pressure (30...300 MPa) and with high speed (0.5...140 m/sec). Mass of castings may be from a few grams to several tens kilograms. Alloys of zinc (very often), aluminium (often), magnesium (often), copper (sometimes), iron (very seldom) base are used.

The casting's nomenclature ranges from part of a zipper to automobile block of cylinders.

Automatic or semi-automatic plunger machines with cold and hot pressing chambers are employed (Fig. 3.14).

Fig. 3.14. Plunger machines with cold (a, b, c) and hot (d) pressing chambers: a…c:

1- plunger; 2 – pressing chamber; 3 – pusher; 4 – metallic core; 5 – removable half-mould;

6 – irremovable half-mould; d: 1 – heating crucible; 2 – metal-conductor; 3 – press-mould

 

In the first type machine the metal together with air by a plunger 1 is pressed into mould 2 and solidifies under pressure. Compressed gases dissolve in metal and the casting acquires normal density. But further heat-treatment is not available, because gas holes are formed at high temperature.

In the second type machine the metal is pressed into a mould 3 without any gases, but plunger and chamber work at high temperature, in liquid metal and by this reason they are rapidly damaged.

The advantages of the method:

- very high productivity;

- sand mixture isn't used at all;

- dimensional accuracy is very high (for example, screw thread may be received);

- thickness of casting may be 1 mm and less;

- patternless method.

The disadvantages of pressure die-casting:

- high cost of press-moulds;

- short service life of press-moulds;

- powerful machines are required to produce medium mass castings.

Investment Casting

 

This is one of the oldest casting methods. Expendable (investment) patterns are mostly prepared of waxlike materials. The essence of the method:

- a pattern is made of easily meltable material;

- the pattern is covered by refractory layer;

- the pattern is removed (melted);

- a shell mould is preheated to remove the pattern rest;

- a metal is poured in hot mould.

The method enables to manufacture castings very complicated in shape. That is why it is used, first of all, for casting of monuments. In the prehistoric, ancient and the Middle Ages and later the wax was used as a pattern material. Now oil product, such as paraffin, stearin; pine-tree product, such as rosin, and so on are used.

Investment casting appeared in foundry practice 50 years ago when new binder ethylsilicate was found out.

Manufacturing process consists of (Fig. 3.15):

- pattern-making in metallic moulds; the pattern mix, which has melting point about 50°C, is pressed into a mould;

- removing of patterns from the mould after solidification;

- assembly of paraffin-stearin patterns in blocks by soldering method (using a heated knife);

- dipping a pattern complete set in refractory suspension, which consists of silica flour and ethysilicate;

- dipping the pattern complete set in fluidized sand;

- chemical drying of suspension and sand layer;

- manufacturing of new layer (from 5 to 15);

- dipping of the pattern complete set with refractory shell into boiling water to remove the pattern mix;

Fig. 3.15. Investment casting process: a – part; b – pattern-making; c – pattern; d-pattern complete;

e – dipping in suspension; f – dipping in fluidized sand; h – chemical drying; i – removing the pattern mix;

j – setting of the block into steel box; k – heating of box; l – pouring of a metal;

m – castings with gating system

- settling of shell moulds into steel box and filling box by sand;

- heating of boxes with the moulds to 800...850°C to remove pattern remains from the moulds;

- pouring of a metal into heated moulds;

- destroying of the moulds and cutting off the gating, inspection, heat treatment and finishing operations.

Investment casting provides high dimensional accuracy (similar to pressure-die casting), but it is the most complicated method of foundry. Hence, using of it is profitable in the mass production, especially in production of castings of alloys with high melting point (steels, Ni-alloys, Mo-alloys).

Mass of castings varies from several grams to a few kilograms.

Advantages:

- high dimensional accuracy and surface quality;

- possibility to produce castings, which have a very complicated shape;

- possibility to mechanize and automatize practically all operations.

Disadvantages:

- very complex process;

- small mass of castings.