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Casting

Sand casting (sand-mould casting)

Sand casting is considerably simple and economical process. In many branches of machine building (automobile production, machine tool building, car building, etc.) this method is most commonly used for mass production of castings.

Its features the following engineering capabilities:

Investment casting

- it is a process, wherein single precise non-detachable ceramic investment moulds are used for manufacture of castings. The moulds are fabricated according to temporary patterns using fluid sand.

Investment casting provides manufacturing castings of complicated shapes weighing from several grams to dozens of kilograms, with wall thickness of 0.5 mm and above, surface class of accuracy equalled to 2–5 (GOST 26645-85), and high accuracy of dimensions as compared to other casting methods.

Investment casting is used for manufacture of turbine blades, cutting tools (milling cutters, drills), brackets, clevises, small parts for motor-cars and tractors.

Overall dimensions: maximum diameter, height, length, width is 300 mm; wall thickness is above 3 mm.

Weight: from 2 g to 20 kg (ornamental castings are not restricted in weight)

Grades of metals used:

Use of investment casting is advisable for manufacture of the following parts:

Chill casting

Chill casting –is a metal casting, which is performed as an open filling of chill moulds. Chill mould is a die with natural or forced cooling, which is filled with melt under gravity. After hardening and chilling, a chill mould is opened and article is extracted. Then chill mould can be used for casting a part of the same shape.

This method is widely used for series and large-scale production.

Accuracy of castings usually corresponds to class 5–9 for castings made of nonferrous metals and class 7–11 for castings made of ferrous metals (GOST 26645-85). Accuracy of castings produces in a chill mould by weight is approximately one class higher as compared to sand moulds.

Chill casting is restricted by possibility to produce large-sized chill moulds and usually weight of castings does not exceed 250 kg.

A wide range of articles for all branches of industry (engine components, blanks of gear wheel rims, case-shaped part, etc.).

Grades of metals used:

High-pressure casting

The principle of high-pressure casting is based on the forced filling of metal mould working cavity with melt and shaping of casting under a force of press piston moving in the compression box filled with melt.

High accuracy, class 1–4 as per GOST 26645-85 (quality class 10), fine finish (virtually no machining is required). Availability of manufacturing castings of large area with little wall thickness (less than 1 mm).

Alloys used for casting:

High-pressure casting is the most advanced method of manufacturing castings from nonferrous alloys (zinc, aluminium, magnesium, brass). Lately it is widely used in precise instrument making, automobile, tractor, electrical engineering and other industries. Design features of castings produced by high-pressure casting are quite various: from simple base plates, furnace bars, billets and sleeves to complicated engine crankcases, cylinder head components, corrugated motor frames and plough stands. High-pressure casting is used for manufacture of parts having special properties: enhanced airtightness, wear-resisting properties (for example, parts made of cast iron with surface and local chilling effect), scale resistance, etc. It ought to be noted that high-pressure casting is used for manufacturing parts of different designations including essential components.

High-pressure casting is reasonable only in series and mass production due to difficulties in fabricating moulds and its high cost.

Controlled pressure casting

Controlled pressure casting includes casting methods based on filling a mould cavity with melt and hardening under action of manometric pressure of air or gas.

In practice, the following controlled pressure casting processes have found wide application: low-pressure casting, low-pressure casting with backpressure, vacuum suction casting, and vacuum suction casting with piezocrystallization (vacuum compression casting).

Main advantages are the possibility to produce blanks with minimum allowance for machining or without machining and minimum roughness of unmachined surfaces, as well as high rate of productivity and low labour intensity at fabrication operations.

This casting method is used for manufacture of pistons, cylinder head components from aluminium alloys, etc., sleeves, bearing elements.

Shell casting

Shell casting appeared as an attempt to automatize manufacture of breakable moulds. The heated mould made of metal is filled with a mixture of sand with particles of unpolymerized thermosetting material. Having held this mixture on the surface of heated blank for some time, a layer of mixture with plastic particles melted and polymerized forming solid crust (shell) on the mould surface is obtained. When the construction is overturned, excessive mixture is fallen and the shell is removed from the mould by means of special extractors. Further, shells obtained in such a way are connected to each other by glueing with silicate cement, installed in mould boxes and filled with sand to ensure hardiness at metal pouring. Ceramic rods for shaping inner cavities of castings are produced the same way.

Shell casting as compared to sand-mould casting has substantial advantage – simplicity of automation in shaping. It should be noted, however, that investment casting is an inappropriate method for producing large-size castings and articles of specially irregular shape.

Shell casting is used for manufacture of: steam and hot-water radiators, parts for motor-cars and a number of machines.

Centrifugal casting

The principle of centrifugal casting is based on filling the mould with melt and shaping of castings by rotating the mould along horizontal, vertical or inclined axis, or rotating it along complicated trajectory.

The centrifugal casting technique provides a number of advantages, which are unachievable with the use of other casting methods, for example:

Centrifugal casting is used for manufacture of castings having revolutional shape:

Centrifugal casting is widely used in manufacturing sleeves from copper alloys, predominantly tin bronzes.

As compared to the fixed mould castings, centrifugal casting has a number of advantages: filling rate of moulds, density and mechanical properties of castings are increased. However, it requires installation of special equipment. Disadvantages of this casting method: inaccuracy of measurement of available casting surfaces, increased tendency to liquation of alloying components, exclusive standards applied to durability of casting moulds.

Cavityless casting

The cavityless casting technique is one of the most promising and developing casting techniques at the present time. This technique can be compared to a method of investment casting, but the difference is that the pattern is removed (gasifies) not before filling but in the course of filling the mould with metal, which removes (replaces) “evaporating pattern” out of the mould and takes free space of the mould cavity.

Basic advantages of castings produced by this method are as follows:

Cavityless casting is used for manufacture of castings of various seriation from piece-production to industrial series.

Materials used are almost all grades of cast iron from SCh15 to VCh-50, wear-resistant IChХ, plain carbon 20-45 to high-alloyed, hot-working and refractory steels, practically all casting grades of bronzes.

Basic weighing of castings is from 1 to 300 kg. Piece-production is up to 1 t.

Continuous casting

The principle of this casting method is based on uniform and continuous pouring of liquid metal into chilled mould from one end and in the form of consolidated ingot (bar, pipe, blank of square, rectangular or other section). Then it is drawn by special mechanism from the other end. This method is can be used for producing castings from all known ferrous and nonferrous alloys.

Continuous casting is suitable for manufacture of ingots, pipes, sections of unlimited length and required cross-section.

Continuous casting method is also used for manufacture of ingots from nonferrous and ferrous alloys. Almost all aluminium alloys for further rolling into sheets, sections and other articles are poured into ingots by this method.

Casting by cold-hardening mixtures

COLD-BOX-AMIN process. Cold-hardening mixtures are special mixtures, which do not require heating in drying furnaces after being manufactured. Due to binding and hardening agents they are self-hardened in the air within 10-15 min. This technique is very similar to the traditional one (sand-mould casting), the difference is that artificial resins are used as binding agent for sand mixtures. Blowing of core boxes with various tertiary amines is applied for resins hardening. It is possible to produce castings of accuracy class 7 as per GOST 26645-85.

Cold-hardening mixtures are extremely rare used as general moulding materials due to high cost of binding agents and difficult regeneration of mixtures. Application of cold-hardening mixtures for manufacture of shapes is economically feasible in the case when the shape weight to melt weight ratio does not exceed 3:1. That is why these mixtures are used primarily for manufacture of rods that allow shaping cavities in the casting.

The technique of casting by cold-hardening mixtures ensures high quality of casting surface, absence of gas defects and foreign inclusions in the casting.

Casting method

Casting materials

Shape and dimensions

Accuracy (quality class), roughness (Rz, µm)

Sand-mould casting

Cast iron, steel, nonferrous metals

Large-size, complicated shape

Rougher than quality class 14

Rz =300

Investment casting

Cast iron, steel, nonferrous metals

Weight is less than 1 t. Dimensions and shape are restricted

Rougher than quality class 14

Rz =300

Chill casting

Aluminium and copper alloys

Weight is up to 250 kg, shape is restricted by conditions of a casting removal from a chill mould

quality class 12-14

Rz >40

High-pressure casting

Aluminium, zinc alloys, rarer copper alloys

Weight is up to 200 kg, shape is restricted by conditions of press mould disclosure

quality class 7-12

Rz =0.63…40

Investment casting

Steel, special alloys, copper alloys

Shape is restricted, weight is up to 20 kg, ornamental castings are not restricted in weight

quality class 10-14

Rz =2.5…40

Centrifugal casting

Cast iron, steel, nonferrous metals

Revolutional shape, tubes with diameter of up to 1200 mm and length of 7000 mm

quality class 9-11

Cavityless casting

Cast iron, steel, bronze

Weight is from 1 to 300 kg. Availability to produce parts of complicated shape

quality class 12-14

Rz =40 (for steel Rz =80)

Casting by cold-hardening mixtures

Cast iron, steel, nonferrous metals

Weight is from 5 kg to 5 t. Maximum overall dimensions are 2500x2200x1200

quality class 10-12

Rz>40