Types and Classification of Casting Processes | Manufacturing Science

This article throws light upon the nine types of casting processes. The types are: 1. Green Sand Mould Casting 2. Dry Sand Mould Casting 3. Shell Mould Casting 4. Investment Casting 5. Gravity Die Casting 6. Die Casting 7. Centrifugal Casting 8.Slush Casting and 9. CO₂ Process.

Types and Classification of Casting Process # 1. Green Sand Mould Casting:

The material for a green sand mould is a mixture of sand, clay, water, and some organic additives, e.g., wood flour, dextrin, and sea coal. The percentage of these ingredients on weight basis is approximately 70-85% sand, 10-20% clay, 3-6% water, and 1-6% additives. This ratio may vary slightly depending on whether the casting is ferrous or non-ferrous.

Sand is an inexpensive refractory material, but natural sand may not have all the desirable qualities of a moulding material. For example- it normally has higher clay content than desired. The sand used as a moulding material should have a specified clay, water, and additive content in addition, it must have a specific grain size distribution. The importance of the grain size distribution would be clear from the discussion that follows.

Both the shape and the size of sand grains vary over a wide range. The grains may be smooth and round in shape or may have sharp angular corners. The bulk density of a sand-mix is very low if the grains are of almost equal size with smooth round shape. Such grains result in an increased void and a higher permeability.

Higher permeability permits an easy outflow of the gases (produced dining the casting operation) which may otherwise be entrapped within the casting. The situation gets reversed if the grains are of various sizes and have sharp corners. To study the grain size distribution, the screening test is performed.

This is done by taking a fixed sample weight of sand and screening it through standard sieves. The screening is accomplished by shaking the sieves. The amount of sand that collects in the different sieves is then plotted. Finally, from this plot, the distribution of grain size and the average grain size are computed.

Clay, together with water, acts as a bonding agent and imparts tensile and shear strength to the moulding sand. The organic additives burn out at high temperatures and make room for the moulding sand to expand, and thus save the mould from crumbling.

The success of a casting process depends greatly on the properties of the moulding sand. These include- (i) strength, (ii) permeability, (iii) deformation, (iv) flowability, and (v) refractoriness. (Standard specimens and tests are rec­ommended for an evaluation of these properties.) Strength refers to the com­pressive strength and deformation indicates the change in length of a standard specimen at the point of failure.

Permeability is expressed as the gas flow rate through the specimen under a specified pressure difference across it. Flowability refers to the ability of the sand to flow around and over the pattern when the mould is rammed. Refractoriness measures the ability of the sand to remain solid as a function of temperature.

For a given sand-clay ratio, the nature of variation of these properties with water content is as shown in Fig. 2.2. It is obvious, both from strength and permeability considerations, that there is an optimum water content.

At a low water content, dry clay powder, being finer than sand grains, fills up the void between the sand particles, and thus reduces the permeability. With higher water content, moist clay forms a coating over the sand particles keeping them further away, thus enhancing the permeability. Beyond the opti­mum water content, water itself fills up the void and reduces the permeability.

Types and Classification of Casting Process # 2. Dry Sand Mould Casting:

The dry sand mould casting uses expendable moulds, i.e., each mould is used only once. A dry sand mould is basically a green sand mould baked in an oven at 100-250°C for several hours. The sand-mix contains 1-2% of pitch. The oxidation and polymerization of pitch increases the hot strength of the mould. As the water is driven out from the sand-mix by heating, the defects caused by the generation of steam, e.g., blows and porosity, are less frequent in dry sand mould casting.

Types and Classification of Casting Process # 3. Shell Mould Casting:

The shell mould casting is a semi precise method for producing small castings repetitively in large numbers. The mould material contains phenolic resin mixed with fine, dry silica. These are mixed either dry or in the presence of alcohol; no water is used. Normally, a machined pattern of gray iron, aluminium, or brass is used in this process. First, the pattern is heated to 230-260°C, and then the sand-resin mixture is either dumped or blown over its surface.

This way, the heated pattern melts and hardens the resin which, in turn, bonds the sand grains closely together. After a dwell time of 20-30 sec, the pattern and sand are inverted. When this happens, a layer of sand adheres to the pattern in the form of a shell of about 6 mm thickness. The rest of the sand is cleaned off. The thickness of the shell can be accurately controlled by controlling the dwell time. The thickness of the shell is so decided that the shell has the required strength and rigidity to hold the weight of the liquid metal to be poured into the mould.

Then the mould is heated in an oven (at 300°C) for 15-60 sec. This curing makes the shell rigid when it can be stripped off by means of ejector pins mounted on the pattern. The shell thus formed constitutes one-half of the mould. Two such halves, placed one over the other, make the complete mould. While pouring the liquid metal, the two halves are clamped down together by clamps or springs.

It should be noted that in this process, the smoothness of the mould wall is independent of the moulder’s skill. This contributes to a better dimensional accuracy and consistency when compared with green sand moulding. Smooth mould walls also offer less resistance to the flow of liquid metal in the mould. This is why smaller gates can be used. Moreover, thin sections, sharp corners, small projections, which are not possible in green sand moulds, can be accommodated. Further, subsequent machining operations are also reduced.

Often, only grinding can produce the finished product. The increased cost of the metal pattern (as compared with the wooden pattern used in green sand moulding), however, can be justified only if the casting is produced in large enough numbers.

Types and Classification of Casting Process # 4. Investment Casting:

The process of investment casting is suitable for casting a wide range of shapes and contours in small-size parts, especially those that are made of hard-to- machine materials. The process produces excellent surface finish for the casting. Here, the mould is made in a single piece, and consequently there is no parting line to leave out fins. This also adds to the dimensional accuracy of the casting. As will be apparent from the description of the process, no complication arises when withdrawing a pattern from the mould.

Though the process is elaborate and expensive, it has been found very suitable for casting turbine and jet engine parts made of high temperature and high strength alloys. We now describe the steps to be followed in this process.

A rather accurately dimensioned metal pattern is used. The dimensions of the pattern are calculated to compensate for the several size adjustments which take place in the process—in the die, in the wax, in the investment material, and, finally, in the casting material. The determination of the pattern dimensions is a tedious task and requires considerable experimentation. This makes the pattern in an investment casting very costly.

This pattern is used to make a die out of a soft material, e.g., aluminium. Thereafter, wax or plastic is injected into the die to form an expendable pattern. The expendable pattern is rinsed in alcohol to remove grease and dirt. After drying, the pattern is dipped in a slurry composed of silica flour, water, and some bonding agent. Then, the pattern is taken out of the slurry and rotated to produce a uniform coating, to fill inside corners and to drain out the excess slurry.

Sometimes, a number of expendable patterns are assembled as a ‘tree’ for economy. Finally, fine-grain silica sand is sprinkled onto the wet slurry surface. The coating thus produced on the expendable pattern after drying is called a pre coat.

The pattern with the pre coat is then placed on a steel base and is covered by an open-ended steel can. Both the pattern and the can are secured to the base by molten wax. Then, the can is filled with a slimy of heavy, self-hardening refractory concentrate. The concentrate sets in after a lapse of 24 hours when the can is placed in an oven. Thus, most of the wax or plastic melts and flows out of the mould, leaving a cavity with the shape of the intended casting. The residual wax is removed by firing the mould in a furnace for about 24 hours.

The liquid metal is poured into this mould while it is still hot. This saves the liquid metal from acquiring the moisture and avoids high thermal gradient between the liquid metal and the mould. In critical cases, the pouring is conducted in a vacuum chamber or in a protective inert atmosphere (such as argon). Frequently, the mould is clamped to a special type of furnace which is then inverted for pouring directly from the furnace into the mould.

After cooling, the can is removed and the hard refractory investment is knocked off by a hammer or other vibratory means. Finally, the adhered investment material is removed from the casting surface by sand-blasting or a tumbling operation.

Types and Classification of Casting Process # 5. Gravity Die Casting:

In gravity die casting, a permanent mould is used. The liquid metal is poured into a non-expendable mould under the force of gravity. The process is normally used for cast iron and, occasionally, for a nonferrous casting. The mould is made of heat-resistant cast iron, and is provided with fins on its outer surface for efficient air-cooling. The inner surface of the mould cavity is sprayed with an oil-carbon-silica mixture before each pouring.

Types and Classification of Casting Process # 6. Die Casting:

In the die casting process, unlike in gravity casting, the liquid metal is forced into the mould cavity under high pressure. The process is used for casting a low melting temperature material, e.g., aluminium and zinc alloys. The mould, normally called a die, is made in two halves, of which one is fixed and the other moving. Medium carbon, low alloy tool steel is the most common die material. The die is cooled by water for an efficient cooling of the casting. This also increases the die life.

The process is referred to as a hot chamber or a cold chamber process, depending on whether or not the melting furnace is an integral part of the mould. Since the liquid metal is forced into the die with high external pressure, much thinner sections can be cast by this process. The process, when applied to a plastic casting, is called injection moulding.

Types and Classification of Casting Process# 7. Centrifugal Casting:

The centrifugal casting process is normally carried out in a permanent mould which is rotated dining the solidification of a casting. For produc­ing a hollow part, the axis of rotation is placed at the centre of the desired casting. The speed of rotation is maintained high so as to produce a centripetal acceleration of the order of 60g to 75g. The centrifuge action segregates the less dense nonmetallic inclusions near the centre of rotation. It should be noted that the casting of hollow parts needs no core in this process.

Solid parts can also be cast by this process by placing the entire mould cavity on one side of the axis of rotation. The castings, produced by this method, are obviously very dense. By having several mould cavities, more than one casting can be made simultaneously.

Types and Classification of Casting Process# 8. Slush Casting:

A slush casting is produced by pouring the liquid material into an open-top permanent mould and inverting the mould after a thin layer of the liquid has solidified on its surfaces. Thus, the liquid at the centre of the mould is drained out. The process results in shell-like castings which are widely used for ornamental objects, e.g., lamp shades and toys. Normally, low temperature materials such as tin, lead, and zinc are used in this process.

Types and Classification of Casting Process # 9. CO₂ Process:

The CO₂ process is essentially a sand moulding process where the sand-mix does not contain any oil, resin, or clay as the bonding agent. This eliminates the use of driers and the heating cycle. Instead, the sand-mix contains 2-6% of sodium silicate solution. This sand-mix has a very high flow ability to fill up corners and intricate contours. The sand is hardened by passing CO₂ for about one minute. The CO₂ gas forms a weak acid that hydrolyzes the sodium silicate (Na₂O, SiO₂) solution to form amorphous silica which acts as the bond. Sodium silicate itself also provides some bonding action.