CO2 Moulding• This method is widely used in making cores

and occasionally for moulds • Sodium silicate is used as a binder• A quick process of core or mould preparation• Mould preparation Mixture of sodium silicate and sand is treated

with CO2 for 2 – 3 minutes till we get a dry compressive strength of over 1.4 MPa

How does dry compressive strength increase on treatment with CO2?

• CO2 is expected to form a weak acid, which hydrolyses the sodium silicate resulting in amorphous silica, which forms the bond

• The introduction of CO2 gas starts the reaction by forming hydrated sodium carbonate (Na2CO3 +H2O)

• This gelling reaction increases the viscosity of the binder till it becomes solid

• The compressive strength of the bond increases with standing time due to dehydration

Advantages of CO2 Moulding

• Because of high strength of the bond, the core need not be provided with any other reinforcements

• The equipment required is fairly simple and inexpensive

- CO2 easily available- No expensive core baking equipment needed

• Better dimensional accuracies are achieved since it does not involve distortions due to baking

Limitations of CO2 Moulding

• Sand mixture after preparation has very little shelf life and therefore should be immediately used

• Shake-out properties of CO2 moulding are poor compared to normal moulding

• Moulds and cores deteriorate from water pick-up if kept stored for long periods before use

• Difficult to achieve uniform gassing which results in nonuniform strength

CASTING PROCESSES

Two Categories of Casting Process

1. Expendable mould processes – uses an expendable mould which must be destroyed to remove casting– Mold materials: sand, plaster, and similar materials,

plus binders2. Permanent mould processes – uses a permanent

mould which can be used many times to produce many castings– Made of metal (or, less commonly, a ceramic

refractory material

EXPENDABLE MOULD CASTING

Shell Moulding

• Shell moulding is a casting process in which the mould is a thin shell (typically 9 mm) made of sand held together by a thermosetting resin binder

• Developed in Germany during the early 1940s• Examples of parts made using shell molding include

gears, valve bodies, bushings, and camshafts.

Steps in Shell Moulding

1. A match-plate or cope-and-drag metal pattern is heated and placed over a box containing sand mixed with thermosetting resin

Steps in Shell Moulding

2. Box is inverted so that sand and resin fall onto the hot pattern, causing a layer of the mixture to partially cure on the surface to form a hard shell

Steps in Shell Moulding

3. Box is repositioned so that loose, uncured particles drop away

Steps in Shell Moulding

4. Sand shell is heated in oven for several minutes to complete curing.

Steps in Shell Moulding

5. Shell mould is stripped from the pattern

Steps in Shell Moulding6. The two halves of the shell mold are

assembled, supported by sand or metal shot in a box, and pouring is accomplished

In Summary…

Advantages of Shell Moulding

• The surface of the shell mould cavity is smoother than a conventional green-sand mould, and this smoothness permits easier flow of molten metal during pouring and better surface finish on the final casting

• Good dimensional accuracy is also achieved, with tolerances of 0.25 mm possible on small-to-medium-sized parts

• The good finish and accuracy often precludes the need for further machining

• Collapsibility of the mould is generally sufficient to avoid tearing and cracking of the casting

Disadvantages of Shell Moulding

• More expensive metal pattern is required than the corresponding pattern for green-sand molding

• Size of the casting obtained by shell moulding is limited (upto 200 kg)

• Highly complicated shapes difficult to obtain• More sophisticated equipment is needed for

handling the shell mouldings such as those required for heated metal pattern

Plaster Mould Casting

• Plaster mould casting is similar to sand casting except that the mould is made of plaster of instead of sand.

• Additives such as talc and silica flour are mixed with the plaster to control contraction and setting time, reduce cracking, and increase strength.

• To make the mould, the plaster mixture combined with water is poured over a plastic or metal pattern in a flask and allowed to set

Plaster Mould Casting

• Wood patterns are generally unsatisfactory due to the extended contact with water in the plaster

• The fluid consistency permits the plaster mixture to readily flow around the pattern, capturing its details and surface finish. Thus, the cast product in plaster molding is noted for these attributes

Limitations of Plaster Mould Casting

• Curing of the mould is required, at least in high production

• The mould must set for about 20 minutes before the pattern is stripped

• The mould is then baked for several hours to remove moisture

Dilemma faced by foundrymenMould strength is lost when the plaster becomes too

dehydrated, and yet moisture content can cause casting defects in the product

Limitations of Plaster Mould Casting

• Plaster mould is not permeable, thus limiting escape of gases from the mould cavity

• Plaster molds cannot withstand the same high temperatures as sand molds. They are therefore limited to the casting of lower-melting-point alloys, such as aluminum, magnesium, and some copper-base alloys.

Applications

• Applications include metal moulds for plastic and rubber molding, pump and turbine impellers, and other parts of relatively intricate geometry

• Casting sizes range from about 20 g to more than 100 kg • Parts weighing less than about 10 kg are most common

Investment Casting

• In investment casting, a pattern made of wax is coated with a refractory material to make the mold, after which the wax is melted away prior to pouring the molten metal

• It is a precision casting process, because it is capable of making castings of high accuracy and intricate detail

• The process dates back to ancient Egypt and is also known as the lost-wax process, because the wax pattern is lost from the mold prior to casting

Steps in Investment Casting

1. Wax patterns are produced

Steps in Investment Casting

2. Several patterns are attached to a sprue to form a pattern tree

Steps in Investment Casting

3. The pattern tree is coated with a thin layer of refractory material

Steps in Investment Casting

4. The full mold is formed by covering the coated tree with sufficient refractory material to make it rigid

Steps in Investment Casting5. The mold is held in an inverted position and

heated to melt the wax and permit it to drip out of the cavity

Steps in Investment Casting6. The mold is preheated to a high

temperature, which ensures that all contaminants are eliminated from the mold; it also permits the liquid metal to flow more easily into the detailed cavity; the molten metal is poured; it solidifies and………

Steps in Investment Casting

7. The mold is broken away from the finished casting. Parts are separated from the sprue.

In Summary…

Advantages of Investment Casting• Parts of great complexity and intricacy can be

cast• Close dimensional control—tolerances of 0.075

mm are possible• Good surface finish is possible• The wax can usually be recovered for reuse• Additional machining is not normally required—

this is a net shape process• All types of metals, including steels, stainless

steels, and other high temperature alloys, can be investment cast

Disadvantages of Investment Casting

• Because many steps are involved in this casting operation, it is a relatively expensive process

• Investment castings are normally small in size, although parts with complex geometries weighing up to 75 lb have been successfully cast

Applications

• Examples of parts include complex machinery parts, blades, and other components for turbine engines, jewelry, and dental fixtures

A one-piece compressor stator with 108 separate airfoils made by investment casting