moulding and casting processes in shell moulding, a very fine clay free sand, coated with resin...

2.1 INTRODUCTION

Conventional sand moulding suffers from many limitations such as more material handling, poor surface finish and dust generation, to name a few.

In shell moulding, a very fine clay free sand, coated with resin (thermosetting type) is used. The resin sand is invested on a hot metal pattern to get a thin shell. Two such shells are assembled after locating cores, if required, and clamped. The metal is poured to get a casting.

In a centrifugal casting, no pattern is used, but is limited to production of cylindrical/symmetrical shaped castings.

In die casting, metal moulds are used to produce the castings. The moulds are filled with metal under pressure and the pressure is maintained during solidification as well. Hence, dense and sound casting is the result.

2.2 saND mOUlDINg

Quality of the castings depends on the moulds used in the casting process. Moulding process refers to the method of making the mould and the materials used. Sand moulding is done normally with green sand. Green sand is a mixture of sand grains, clay, water and some additives. Because of the moist condition of the sand this method is called green sand moulding. In general, match plate or cope and drag patterns are used for producing more number of castings. Green sand moulding involves the basic steps of preparation of the pattern, preparation of the mould and placing cores as explained in section 1.4.

advantages

1. Green sand moulding is flexible, rapid and repetitive. 2. Green sand is reused by reconditioning and this method is more

economical. 3. Mechanization is possible due to its ease of adaptability.

2mOUlDINg aND

CasTINg PROCesses

42 Production Technology

2. Copper alloys used for gears, pinions and impellers for small pumps can be cast.

3. Hard alloys of lead storage battery parts can be also cast.

advantages

1. Except steel, all metals can be cast by this process. 2. Close tolerances and good surface finish can be achieved.

Disadvantages

1. Initial cost is more. 2. Operating temperature should be maintained.

2.4 mOUlDINg maChINes

Following are the machines which are used for compaction of moulding sand into molds: 1. Squeezers 2. Jolt machines 3. Jolt-squeeze machines 4. Slingers 5. Blowers 6. Combination of the above-mentioned machines

2.4.1 squeezers

These moulding machines utilize pressure to compact the sand. This is achieved by applying pressure through a squeeze head or plate of different types (Fig. 2.1) by a moulding machine. Hand moulding machine is shown in Fig. 2.2. The squeezing force is given by the following equation:

MF = 2cd

P – W4

p¥

where, MF = moulding force, P = air pressure in squeeze cylinder, N/m dc = piston diameter of squeeze cylinder, m W = weight of the pattern, flask, sand, and other accessories on

work table of the machineHence, moulding force of a squeeze machine depends on the piston

diameter and the air pressure available, usually 63 kN/m2 – 77 kN/m2. The moulding force of the squeeze head is distributed to the entire area of the flask. Though the moulding force is constant for a particular machine, the

Moulding and Casting Processes 43

flask size is not constant; as a result the pressure applied varies depending on the area. The following equation describes this:

where, MP = moulding pressure at flask surface, MF = moulding force applied Af = surface area of the flask under the force MF.

Pattern

Flask

Flat squeeze plate

Contoured squeeze plate

Fluid pressure

Rubber diaphragm

Diaphragm squeeze

Fig. 2.1: Three methods of squeezing sand to compact it


Fig. 2.3: Jolting machine

The power of jolting is given by the following equation:

Power of jolting =

where, M = mass of sand, kg.

V = velocity at instant of jolt = 2gd g = acceleration due to gravity d = height of fall or jolt stroke, m/s A = the jolted area, m2

It is observed from the above equation that power for moulding is independent of flask area, and is determined by jolt stroke which is a machine characteristic. But there is a drawback of such a machine also. The process though achieves better uniformity than squeezing but when sand is jolted the bottommost sand is rammed harder at the parting plane and hardness decreases as we proceed upwards. This is explained as the sand which is added later is rammed and exerts a pressure on the sand below it. As such the sand which is filled last is loosely rammed as compared to the sand below it. Hence, manual ramming is required.

A jolt machine working on compressed air, and the flask supported on the table is called a jolt cylinder. This jolt cylinder is raised and dropped in succession by compressed air on its entry and on its exit respectively, coming from underneath its base. When the cylinder is dropped it presses against a valve which causes it to open and allows compressed air filling the space underneath its base. This air is under high pressure and causes the cylinder to be lifted up till it uncovers the exhaust port. As such the air escapes and the pressure falls which causes the cylinder to fall back and again presses the valve and opens the air entry. This process is continued


till required hardness of the moulding sand is achieved. The load exerted varies from 200 to 1000 kg depending on the size of the jolting machine.

2.4.3 jolt-squeeze machines

To overcome drawbacks in squeezing and jolting and to achieve uniform density and hardness in all the portions of the mould, a combination of squeezing and jolting is often employed. The machine which performs this action is called jolt-squeeze moulding machine. A jolting action is performed first to consolidate sand on the face of the pattern and followed by squeezing action to obtain the desired density in the upper portion of the mould. This squeezing action eliminates the necessity of further ramming of the sand.

The jolt-squeeze machine (Fig. 2.4) is so constructed that the squeezing and jolting action can be obtained one after the other. The table is attached to a cylindrical piston, which is raised and dropped in the jolt cylinder by compressed air. This piston is called the jolt piston. The jolt cylinder is an integral part of the squeeze piston which can move up and down due to air pressure inside a squeeze cylinder. The upper surface of the squeeze piston remains in contact with the bottom of table. During the jolt action, the squeeze piston lies solidly on the base of the cylinder, and the lift of the jolt piston causes the table to lift. During squeezing, the jolt piston and cylinder move along with the squeeze piston to raise the table to the desired height.

Fig. 2.4: Jolt squeeze moulding machine


4. Some lubricants such as calcium stearate and zinc stearate may also be added to the resin sand mixture to improve the flowability of the sand mixture.

Figure 2.5 illustrates the basic steps of the shell moulding process: 1. The resin coated sand is placed in a dump box. 2. Hot cope/drag metal pattern plate maintained at 150°C to 180°C is

clamped on the top of the dump box. 3. The dump box is rotated so that the resin sand is dumped on the

hot pattern plate and kept in contact with pattern for about 15 seconds, to develop 7 mm thickness of shell.

(Note: Incubation period may be increased based on the thickness of the shell needed.)

4. The dump box is brought to the original position so that only partially cured sand adheres to the hot metal pattern and the sand mix not affected by heat is deposited back into the dump box.

5. The pattern along with partially cured shell is heated in an oven to complete the process of curing the shell.

6. The hardened shell is then stripped from the pattern. 7. Two or more shells are then clamped or glued together to produce

a mould. 8. The bonded shells are often placed in a pouring jacket and

surrounded with sand to provide extra support during the pour of molten metal.

applications

1. Cylinders and cylinder heads for IC engines. 2. Transmission parts of automobiles. 3. Gear blanks. 4. Chain seat brackets.

advantages

1. Because the sand is fine and a metal pattern is used, the shell has excellent dimensional accuracy and surface finish.

2. Tolerances of 0.08 – 0.13 mm are possible. 3. Cleaning, machining, and other finishing costs can be reduced

significantly. 4. Productivity can exceed that of conventional sand-casting. 5. Thin sections can be cast.


2.6 CO2 mOUlDINg

CO2 moulding is a special moulding process for making the moulds and cores. The principle of this process is the chemical action between sodium silicate and CO2· Silica gel (SiO2·x·H2O) is formed when CO2 reacts with sodium silicate. Hardening of sand takes place which gives moulding strength to the mould because of the silica gel that has been formed. The bonding strength is sufficient to eliminate the drying or baking of the mould.

The chemical reaction between CO2 and sodium silicate is given by:

Na2O·m SiO2·x H2O + CO2 ———→ Na2 CO3 + m SiO2· x H2O (silica gel)

The sand used for the process must be dry and free of clay. The amount of sodium silicate to be mixed with the sand must be around 3 to 5 per cent. To enhance the collapsibility of the mould, coal powder, wood flour and sea coal may be added as additives. Addition of iron oxide prevents hot deformation of cores and produces smooth interface between the mould and the metal.

In this process CO2 gas is passed through the mould for a predetermined length of time. The amount of CO2 required can be calculated based on the sodium solicate mixed with the sand. The flow rate of CO2 depends on the depth of penetration desired.

applications

1. Parts of valves, pumps and compressors. 2. Parts of machine tools, wheel castings, railway components. 3. Parts of diesel engines and gear castings.

advantages

1. Labour cost and floor space are saved by avoiding drying or baking of the mould.

2. High accuracy and good surface finish of the casting are possible.

3. Withdrawal of the pattern is facilitated since the moulds may be hardened before extracting the pattern.

4. Mould cracking and deformation are prevented. 5. Process can be mechanized for mass production. 6. Less skill is required compared to dry sand moulding. 7. Machining allowances can be reduced.


Disadvantages

1. Moulds and cores are more expensive. 2. Difficult to reclaim the sand. 3. Life of the sand mixture is short. 4. Moulds and cores deteriorate if they are stored under normal

atmospheric conditions.

2.7 CeNTRIFUgal CasTINg

In this casting process, the mould is rotated rapidly about the central axis while molten metal is poured in. The metal fills the cavity by centrifugal force under continuous pressure towards the periphery. Slag, oxides and other impurities, which are light in weight, are pushed towards the centre and form the inner surface of the casting which can be machined out later.

The castings made by this process are dense and completely free from any porosity defect. The use of gates, runners and in some cases cores is eliminated, making the method less expensive. Pipes, pressure vessels, cylinder liners, brake drums, gears, pulleys are conveniently cast by centrifugal casting.Centrifugal casting can be classified into three types: 1. True centrifugal 2. Semicentrifugal 3. Centrifuging

2.7.1 True Centrifugal Casting

In this process, metal mould with green or dry sand lining rotates about either horizontal or vertical axis at speeds of 300–3000rpm. As the molten metal is poured, it is forced against the outer walls of the mould by centrifugal force where it solidifies as shown in Fig. 2.6.When rotation is about horizontal axis the inner surface is always cylindrical. In true centrifugal casting, solidification begins at the outer surface and centrifugal force continues to feed the molten metal, compensating for shrinkage, so no risers are required. Figure 2.7 illustrates the centrifugal casting about vertical axis. True centrifugal casting, when the axis of rotation is horizontal, will have uniform cross-sectional thickness throughout its length. However, when the axis is vertical, the section thickness/ inner diameter will not be uniform.


than the true centrifugal casting. For larger production rates, the moulds can be stacked one over other, all fed from the same central sprue.

Cope

Feeder

Centre core

reservoir

Casting

Holdingfixture

Revolvingtable

Drag

Fig. 2.8: Semicentrifugal casting

2.7.3 Centrifuging

This casting process uses centrifugal action to force the metal from a central pouring reservoir into separate mould cavities that are offset from the axis of the rotation.

Mould

Pouring basin

Ladle

Fig. 2.9: Centrifuging

In centrifuge casting process, several identical smaller moulds are located radially about a vertically arranged central sprue as shown in Fig. 2.9. The centrifugal action forces the metal from the central reservoir into all the moulds through a number of radial gates. The entire mould is


rotated about the central sprue. Relatively low rotational speeds are required to produce sound castings. This type of casting is suitable for small and intricate parts where feeding problems are encountered. Centrifuging is often used to assist in pouring of investment casting trees. Use of individual cores is a must to get the required cavity in the casting.

2.8 DIe CasTINg PROCess

Die casting process consists of producing accurately dimensioned parts by forcing molten metal under pressure into a metallic die. Die casting is closely related to the permanent mould casting as both processes use reusable metallic dies. However, permanent mould casting uses only gravitational force for filling the mould cavity. Die casting is also called pressure die casting as the molten metal is injected under pressure into the metallic die. Within a fraction of a second, the molten metal fills the entire die including all the minute details. In die casting process, the die is made in two halves. First half is called stationary die or cover die and is fixed to the die casting machine. The second half is called ejector die which is moved out for ejecting the casting. This process is particularly suitable for lead, magnesium, tin and zinc alloys.

2.8.1 general Procedure of Die Casting (in metal mould)

1. Lubricant parting agent is sprayed manually.

2. The two die halves closed and clamped.

3. Molten metal is poured/injected under pressure.

4. Pressure is maintained till solidification of metal is completed.

5. Die is opened and the casting is ejected.

Pressure die casting is divided into two groups:

1. Hot chamber process

2. Cold chamber process

The basic difference between both of them is, holding furnace for molten metal is an integral part of die casting machine in hot chamber machine, whereas in cold chamber process, metal is melted in a separate furnace.

2.8.2 hot Chamber Process

Figure 2.10 schematically illustrates the hot chamber process. A “gooseneck”/ injection chamber is partially submerged in a reservoir of molten metal.


Fig. 2.11: Cold chamber die casting

advantages

1. Smooth surfaces and excellent dimensional accuracy are attractive features of the die casting.

2. Die castings require no finish machining except for the removal of small amounts of excess metal fin or flash around the parting line

3. Production rates are high and a set of dies can produce many thousands of castings without significant change in dimensions.

4. Thin sections can be easily produced. 5. It is economical.

Disadvantages

1. High initial investments. 2. Part size is limited. 3. Ferrous metals cannot be cast.

2.9 INVesTmeNT CasTINg

The ‘lost-wax’ or investment casting process as is known today, was developed from the technique that dates back as early as 1766-1122 B.C. during the early Egyptian and the later Shang dynasty periods. This process is developed rapidly, primarily because of the large demand for super charger buckets required for aircraft reciprocating engines.


4. Investment of wax patterns: The term investment is derived from the fact that the wax pattern is invested with the refractory material. In this investment process, the wax pattern (assembly) is dipped into the slurry of the refractory material. Generally used refractiories are very fine silica and small amounts of kaolin and graphite mixed with water.

5. Preparation of mould: After one layer of refractory coat is dried, the process is repeated to get more refractory layers and a thickness of 7 to 10 mm is thus obtained. Refractory mould is obtained after the christmas tree is located in a mould and invested with refractory material.

6. Dewaxing: The mould is heated to a temperature of 90-175oC in a furnace. It is held in an inverted position for about 12 hours to melt down the wax. The melted wax runs through a hole in the bottom plate into a tray and provides a cavity of high dimensional accuracy.

7. Preheating: The mould is then fired to 650–1050oC for about 4 hours to drive off the water of recrystalization and burn any residual wax.

8. Pouring: When the mould is at preheated temperature, the molten metal is gravity-poured into the sprue. Air pressure may then be applied to the sprue to force-fill the mould cavity by centrifuging.

9. Cleaning: After solidification, final castings are obtained by fettling.

Investment castings have good surface finish and exact reproduction of the master pattern. This process is used for making turbine blades, parts of automobiles, typewriters, sewing machines, and various other parts.

advantages

1. Surface smoothness about 40 – 125 microns is possible compared to 80-165 microns for shell mould castings and 200-500 microns for sand castings.

2. Close tolerances are possible. 3. Minimum of machining is required for finished castings. 4. Reproducibility of intricate shapes and sizes are possible that are

difficult to obtain by other methods. 5. Extremely thin sections to the extent 0.75 mm, can be cast easily. 6. Sound and defect-free castings may be obtained. 7. Suitable for mass production of small castings.


Disadvantages

1. Unsuitable for large castings, which are about 5 kg. 2. Expensive in all respects because of larger manual labour involved

in the preparation of the pattern and the mould.

2.10 ReVIeW QUesTIONs

1. Explain the shell moulding process with neat sketches. 2. What are the advantages of shell moulding? 3. What is the differences between chamber die casting and cold

chamber die casting? 4. What are the advantages and limitations of a die casting process? 5. Why the term “ investment” has been assigned to investment

casting? 6. Differentiate between die and investment casting processes. 7. What are the advantages of the investment casting process? 8. What are the different types of die casting? Enumerate the

differences between them (or) give the classification of die casting. Discuss the characteristics of each method.

9. Illustrate and describe the process of semicentrifugal casting. 10. Discuss the shell moulding with the help of a neat sketch. 11. Sketch and explain the constructional and operation of hot

chamber die casting machine. 12. What are the typical situations in which the following casting

processes are used? (i) Precision investment casting (ii) True centrifugal casting (iii) Pressure die casting (iv) Shell moulding 13. What are the advantages of true centrifugal process? Discuss the

influence of average rotational speed upon centrifugal casting? 14. Explain the process of investment casting, emphasis must be

given to identify the operations involved and explain each operation.

15. List the advantages and disadvantages of sand moulding. 16. Describe the advantages and limitations of die casting. 17. Describe the functions of the following: (i) Jolting machine, (ii) Squeezing machine, (iii) Jolt, squeeze machine (iv) Sand slinger


(b) lower melting point metals (c) medium melting point metals (d) non-ferrous metals 12. Hot chamber process can be used for the castings of (a) aluminium (b) brass (c) zinc (d) copper 13. Cold chamber die casting is generally employed for the casting of (a) high melting point metals (b) low melting point metals (c) earth metals (d) ferrous metals 14. In cold chamber die casting, molten metal transferred to the

chamber known as (a) volume/cycle (b) shot (c) sweep (d) gooseneck 15. The other name for investment casting is (a) lost-wax process (b) wax process (c) waxing process (d) C-process 16. In investment casting process, the wax patterns are invested in (a) kerosene (b) gelatin (c) refractory slurry (d) moulding sand

ANSwErS

1. b 2. a 3. a 4. c 5. c 6. a 7. b 8. a 9. c

10. a 11. a 12. c 13. a 14. b 15. a 16. c

moulding and casting processes in shell moulding, a very fine clay free sand, coated with resin...

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