concepts used in foundry

Concepts used in Foundry

Dept of Mechanical Engg. 1

CONCEPTS USED IN FOUNDRY

Meaning of Cores:

Core is a sand shape or form which makes the contour of a casting for which no provision

has been made in the pattern for moulding.

Core may be made up of sand, plaster, metal or ceramics.

Core is an obstruction which when positioned in the mould, does not permit the molten

poured metal to fill the space occupied by the core hence produce hollow casting.

Cores are used as inserts in moulds to form design features which are difficult to be

produced by simple moulding.

Fig: The pattern, mould and core used for producing a short pipe

When the molten metal is poured, it flows around the core and fill the rest of the mould

cavity.

Cores are subjected to extremely severe conditions, and they must, therefore, possess

very high resistance to erosion, exceptionally high strength, good permeability, good

refractoriness, and adequate collapsibility.

Functions of core:

1. Core provides a means of forming the main internal cavity for hollow casting.

2. Core provides external undercut feature.

3. Cores can be inserted to obtain deep recesses in the casting.

4. Cores can be used to increase the strength of the mould.

5. It can be used as a part of gating assembly.

6. It can form a part of green sand mould and can also be used to improve the mould

surface.



Essential characteristics of core:

A dry sand core must possess following properties :

It should have sufficient strength to support itself without breaking.

It should have high permeability and high refractoriness.

It should have smooth surface to ensure a smooth casting.

It should have high collapsibility, to assist the free contraction of the solidifying metal.

It should have those ingredients which does not generate mould gases.

STEPS IN CORE MAKING PROCESS:

Consists of the following operation:

1. Core sand preparation

2. Core molding

3. Baking and

4. Core finishing

The first step in core making is to prepare a homogeneous mixture so that the core will be

of uniform strength throughout, generally using roller mills or core mixers suitably.

Cores are then molded manually or with machines using core boxes.

Then the prepared cores are placed on core carriers and baked in core ovens to remove

the moisture and to develop the strength of the binder at temperatures from 1500C to

4000C, depending on the type of binder used, the size of cores, and the duration of

baking time.

The baked cores are smoothened by filing the rough places and removing unwanted fins.

Cores made in two or more pieces must be glued together with dextrin or other water

soluble binders.

Finally a fine refractory coating (applied by brushing, dipping, or spraying) or core wash

(finely ground graphite, silica, mica, zircon, flour and a rubber-base chemical spray are

used as core washes) to the surface is applied to prevent the metal penetration into the

core to have smoother surface to the casting.



Fig: Core making

Fig: Complete assembly of mould and core

TYPES OF CORES:

Some common types of dry-sand cores are:

i. Horizontal Core:

ii. Vertical Core

iii. Balance core

iv. Drop core

v. Kiss core

i. Horizontal Core:

The horizontal core is the most common type of core and

is positioned horizontally at the parting surface of the

mould. The ends of the core rest in the seats provided by

the core prints on the pattern. This type of core can

withstand the turbulence effect of the molten metal

poured. A horizontal core for gear blank mould is shown

in Fig.



ii. Vertical Core:

The vertical core is placed vertically with some of their

portion lies in the sand. Usually, top and bottom of the core

is kept tapered but taper on the top id greater them at

bottom. A vertical core is shown in Fig.

iii. Balance Core:

The balance core extends only one side of the mould. Only

one core print is available on the pattern for balance core.

This is best suitable for the casting has only one side

opening. This is used for producing blind holes or recesses

in the casting. A balance core is shown in Fig.

iv. Drop Core:

The drop core is used when the core has to be placed either

above or below the parting line. A drop core is shown in Fig.

This core is also known as wing core, tail core, chair core, etc.

v. Kiss Core:

The kiss core is used when a number of holes of less

dimensional accuracy is required. In this case, no core prints are

provided and consequentially, no seat is available for the core.

The core is held in position approximately between the cope and

drag and hence referred as kiss core.

CORE BINDERS:

Core binders serve to hold the sand grains together and impart sufficient strength to the cores and

to provide the desired degree of collapsibility. They are classified as:

i. Organic binders

ii. Metallo organic binders

iii. Inorganic binders



i. Organic binders:

These are combustible, and are destroyed by heat. ƒ

Hence they contribute a good degree of collapsibility to the core sand. ƒ

Core oils (manufactured by blending linseed oil, soya oil, fish oil and petroleum oil),

cereals, resins, plastics, dextrin, molasses, lignin, etc. are some of the types used.

ii. Metallo organic binders:

Compounds such as sodium perborate and magnesium dioxide are used as binders.

Sometimes the baking can be eliminated completely.

iii. Inorganic binders:

Bentonite, fire clay, sodium silicate are some of the compounds used.

These are not preferred due to their poor collapsibility.

CORE PRINTS

Core prints are basically extra projections provided on the pattern.

They form core seats in the mould when pattern is embedded in the sand for mould

making.

Core seats are provided to support all the types of cores.

Though the core prints are the part of pattern, they do not appear on the cast part.



CONCEPT OF GATING AND RISERING SYSTEMS

GATING SYSTEM:

The concept of gating is very important, as it helps one to learn the controlled flow of molten

metal from the crucible (ladle) into the mold cavity.

The term gating refers to all the channels or cavities through which the molten metal flows to

reach and fill the mould cavity.

Figure below shows a simple gating system which consists of the following components.

Pouring cup

Sprue

Runner

Gates or Ingates

1. Pouring basin

It is the conical hollow element or tapered hollow vertical portion of the gating system which

helps to feed the molten metal initially through the path of gating system to mould cavity. It may

be made out of core sand or it may be cut in cope portion of the sand mould. It makes easier for

the ladle operator to direct the flow of molten metal from crucible to pouring basin and sprue.

It helps in maintaining the required rate of liquid metal flow. It reduces turbulence and vertexing

at the sprue entrance. It also helps in separating dross, slag and foreign element etc. from molten

metal before it enters the sprue.

2. Sprue

It is a vertical passage made generally in the cope using tapered sprue pin. It is connected at

bottom of pouring basin. It is tapered with its bigger end at to receive the molten metal the



smaller end is connected to the runner. It helps to feed molten metal without turbulence to the

runner which in turn reaches the mould cavity through gate.

It sometimes possesses skim bob at its lower end. The main purpose of skim bob is to collect

impurities from molten metal and it does not allow them to reach the mould cavity through

runner and gate.

3. Gate

It is a small passage or channel being cut by gate cutter which connect runner with the mould

cavity and through which molten metal flows to fill the mould cavity. It feeds the liquid metal to

the casting at the rate consistent with the rate of solidification.

4. Runner

The runner is a horizontal passage way through which the molten metal flows into the gates. The

cross section of the runner may be square or trapezoid, and its length is very large compared to

its width.

5. Runner Extension

It is a small portion of the runner that extends beyond the last gate. It is used to trap the slag in

the initial molten metal.

RISERING SYSTEM:

We know that as the molten metal solidifies, it shrinks in volume. At this stage, it does not have

a source of more molten metal to feed as it shrinks, voids appear leading to defects in castings.

This problem is overcome with the use of riser. (Refer figure shown above).

A riser or feeder head is a vertical passage made in the cope, to store the liquid metal and supply

(feed) the same to the casting as it solidifies.

Referring to fig shown above, molten metal flows into the mould cavity through the gating

system, fills the cavity and then rises up through the riser till its top. At this moment, the pouring

of the molten metal is stopped.

However during solidification, the metal in the cavity shrinks in volume and hence there will be

no additional metal to be supplied into the mould cavity to compensate for the shrinkage.



PRINCIPLES OF GATING & RISERING SYSTEM:

A gate is a channel which connects runner with the mould cavity, through which molten

metal flows to fill the mould cavity.

The location and size of the gates are so arranged that, they can feed liquid metal to the

casting at a rate consistent with the rate of solidification.

More than one gate is employed to feed a fast freezing casting.

The gate should not have sharp edges as they may break during pouring and thus carried

with the molten metal into the cavity.

The gates should be located where they can be easily removed without damaging the

casting.

lngate is the end of the gate where it joins the mould cavity and through which the molten

metal will be introduced into the mould cavity.

The leading edge of the molten metal flowing in a stream follows the path of least

resistance and continues to build up kinetic energy. If a runner extension is used, the

Kinetic energy may be absorbed hence causing a smoother flow of metal in the runners

and into the mould cavity.

Gate ratio is the ratio of sprue base area to the addition of total runner area and the total

ingate area.

TYPES OF GATES:

1. Top Gate

2. Bottom gate

3. Parting gate

1. Top gate:

Liquid metal will enter into the cavity directly from the

bottom of sprue at atmospheric pressure. The velocity of

liquid metal which is entered into the mould cavity will be

very high. There is a possibility of turbulence, splashing of

the liquid metal and mould erosion. Time taken to fill the

cavity will be less.



2. Bottom gate:

This Gate is provided at bottom of the mould cavity. The

velocity of liquid metal in the cavity will be very less and will

become zero. There is no possibility of turbulence, splashing

liquid metal and mould erosion. Time taken to fill the mould

cavity will be maximum.

3. Parting line gate:

Gate will be provided along the parting line such that

cavity above the parting line can be filled by assuming the

bottom gate and the cavity below the parting line can be filled

by assuming top gate. To get advantages of both top and

bottom gate it is most commonly used type of gating.

RISERING SYSTEM

A riser is a passage of sand made in the cope to permit the molten metal to rise above the

highest point in the casting after the mould cavity is filled up.

This metal in the riser compensates the shrinkages as the casting solidifies. The functions

of risers are as follows :

− To feed metal to the solidifying casting, so that shrinkage cavities are got rid of.

− It permits the escape of air and mould gases as the mould cavity is being filled

with the molten metal.

− It promotes directional solidification.

− Also, it shows that the mould cavity has been completely filled or not.

A casting solidifying under the liquid metal pressure of the riser is comparatively sound.

TYPES OF RISERS

Open riser

Blind riser



Open riser:

Top gate is also called as drop gate because the molten metal just drops on the sand in the

bottom of the mould: Refer Fig below.

A top gate simplifies the moulding with low consumption of additional metal.

There is lot of turbulence in this system.

The top surface of the riser will be open to the atmosphere. The open riser is usually

placed on the top of the casting.

Blind riser:

A bottom gate is provided in the drag half of the mould. Refer Fig below.

In this, liquid metal fills rapidly the bottom portion of the mould cavity and rises steadily

and gently up the mould walls.

Blind Riser is completely enclosed in the mould and not exposed to the atmosphere. The

metal cools slower and stays longer promoting directional solidification.

The liquid metal is fed to solidifying casting under the force of gravity alone.

Bottom gates provide less turbulence and erosion in the mould cavity.

It is not used in large and deep casting because the metal cools gradually as it rises up.



PREPARATION OF SAND MOULDS:

When large number of castings is to be produced, hand moulding consumes more time, labor

intensive and also accuracy and uniformity in moulding varies. To overcome this difficulty,

machines are used for moulding.

Based on the methods of ramming, moulding machines are classified as follows;

Jolt machine

Squeeze machine

Sand slinger

JOLT MACHINE:

A jolt machine consists of a flat table mounted on a piston-cylinder arrangement. The

table can be raised or lowered by means of compressed air. Refer fig shown below.

In operation, the mould box with the pattern and sand in it is placed on the table. The

table is raised to a short distance and then dropped down under the influence of gravity

against a solid bed plate. The action of raising and dropping (lowering) is called jolting.

Jolting causes the sand particles to get packed tightly above and around the pattern, the

number of jolts may vary depending on the size and hardness of the mould required.

Usually less than 20 jolts are sufficient for a good moulding.

The disadvantage is that, the density and hardness of the rammed sand at the top of the

mould box is less when compared to its bottom portions.



SQUEEZE MACHINE:

In squeeze machine, the mould box with pattern and sand in it is placed on a fixed table

as shown in fig below. A flat plate or a rubber diaphragm is brought in contact with the

upper surfaces of the loose sand, and pressure is applied by a pneumatically operated

piston.

The squeezing action of the plate causes thee sand particles to get packed tightly above

and around the pattern.

Squeezing is continued until the mould attains the desired density. In some machines, the

squeeze plate may be stationary with the mould box moving upward.

The disadvantage is that the density and hardness of the rammed sand at the bottom of the

mould box is less when compared to its top portions.

SAND SLINGER:

A sand slinger is an automatic machine equipped with a unit that throws sand rapidly and

with great force into the mould box.

Sand slinger consists of a rigid base, sand bin, bucket elevator, belt conveyor, ramming

head and a swinging arm.

In operation, the pre-mixed sand mixture from the sand bin is picked by the bucket

elevator and is dropped onto the belt conveyor. The conveyor carries the sand to the

ramming head, inside which there is a rotating impeller having cup-shaped blades

rotating at high speeds (around 1800 rpm).

The force of the rotor blades imparts velocity to the sand partickes and a s aresult the

sand is thrown with very high velocity into the mould box thereby filling and ramming

the sand at the same time.



SAND MOULDING PROCESS:

Sands which are used to make mould is called moulding sand. Almost every

manufacturing industries uses moulding sand to make moulds. Casting of materials without use

of moulding sand is impossible. Here we are going to discuss about different types of moulding

sand.

These sand moulding processes are classified based on mould materials are as follows:

1. Green sand moulding

2. Dry sand moulding

3. Carbon dioxide moulding

4. Shell moulding

5. Investment Moulding

6. Sweep moulding

1. GREEN SAND MOULDING

It is the most widely used moulding process.

The green sand is used for moulding process which consists of silica sand, clay, water

and other additives.

Green sand mixture contains 10 to 15% clay binder, 3 to 6% water and remaining silica

sand.



Green sand mixture is prepared and mould is made by packing the sand around the

pattern.

Cope and drag are assembled and the molten metal is poured when the mould cavity is

neither dried nor baked.

This method is mostly preferred for making small and medium casting and suitable for

non-ferrous casting.

The parts like railing and gates, moulding boxes, grills, weights, etc. can be made by this

method.

2. DRY SAND MOULDING

This method is almost similar to green sand moulding except that the composition of sand

constituents is different in this case.

While preparing dry sand mixture, special binding materials like resin, clay or molasses

are added to give strong bond to the sand.

Hence, the dry sand mould possesses high strength.

Dry sand moulds are more permeable than green sand moulds.

Casting produced by this method possesses clean and smooth surfaces.

Dry sand moulding provides better overall dimensional accuracy to the moulds.

But the main disadvantage of this method is, it requires more labour and consumes more

time in completing the mould and mould baking is also an extra work.

Due to high cost and time consuming process, it is not used in mass production.

It is used for producing parts like larger rolls, gear housings, machinery components, etc.

3. CO2 MOULDING PROCESS:

Carbon dioxide moulding also known as sodium silicate process is one of the widely used

process for preparing moulds and cores. In this process, sodium silicate is used as the binder. But

sodium silicate activates or tends to bind the sand particles only in the presence of carbon di

oxide gas. For this reason, the process is commonly known as CO2 process.



Steps involved in making CO2 mould:

a) Suitable proportions of silica sand and sodium silicate binder (3-5%) are mixed together

to prepare the sand mixture. Additives like aluminium oxide, molasses, etc, are added to

impart favorable properties, and to improve collapsibility of the sand.

b) The pattern is placed on a flat surface with the drag box enclosing it. Parting sand is

sprinkled on the pattern surface to avoid sand mixture sticking to the pattern.

c) The drag box is filled with the sand mixture and rammed manually till its top surface.

Rest of the operations like placing sprue and riser pin, and ramming the cope box are

similar of green sand moulding process. Vent holes are made at various locations with

the help of the wire of suitable diameter. At this stage, the carbon dioxide gas is passed

through the vent holes for a few seconds as shown in fig.

d) Sodium silicate reacts with carbon dioxide gas to form silica gel that binds the same

particles together. The chemical reaction is given by:

Na2SiO3 + CO2 Na2CO3 + SiO2

e) The sprue, riser and the pattern are withdrawn from the mould, and gates are cut in the

usual manner. The mould cavity is finished and made ready for pouring.

Advantages :

1. The development of strength takes place immediately after carbon dioxide gassing is

completed.

2. Very little gas evolution during pouring of molten metal.

Disadvantages:

1. Poor collapsibility of moulds.

2. The sand mixture has the tendency to stick to the pattern and has relatively poor

flowability.



4. SHELL MOULDING PROCESS

Shell moulding process was developed by Herr Croning in Germany during World war-II and

hence is sometimes referred to as the Croning shell process.

Shell moulding involves various steps. These are given below

Step 1: Making of Metal Pattern

The first and most important step involves in every casting process is pattern making. Pattern is

replica of the final product. It can made by wood, plastic, metal etc. Shell moulding uses a metal

pattern along with all pattern allowance. This pattern is made by either aluminium of cast iron.

One of the major advantage of using metal pattern is that it gives high accurate casting and can

be used for multiple casting.

Step 2 : Heating of Pattern

The metal pattern created by other casting process is now heated at a temperature range between

180 – 250 degree centigrade. This heating is essential which allows to solidify resin mixed silica

when poured on it. After heating, a small layer of lubricant (Mostly Silicon) sprayed over

metallic pattern which allows easy removal of metallic pattern from shell.

Step 3 : Shell Mould Creation

This step is major step in this casting process. This step can be further divided into following

steps. Pattern is clamped over a dump box. Now this assembly is turned face down. A mixture of

sand and resin is filled into dump box. Mostly fine grade green sand is used for mixture.

http://www.mech4study.com/2017/04/what-is-pattern-what-are-different-types-of-pattern.html

http://www.mech4study.com/2017/04/different-types-of-pattern-allowance-in-casting.html



Now this whole assemble is inverted which allow to sand resin mixture fall over heated pattern.

A layer of mixture, which is in direct contact with pattern is become hard and form a shell. The

thickness of shell is mostly depends on the temperature of the pattern and time duration of

contact.

Now the dump box is again inverted which allow to remove extra sand resin mixture.

After it, metallic pattern is removed from dump box and shell is separated from it. The other half

of the mold is also created using same technic.

Step 4: Mold Assembly

This step assembles all shells created by the shell moulding. The all required shell assembled

into a flask and supported by a baking material. All desirable part like cores, runner, riser etc. are

attached to it.



Step 5 : Casting formation

Now the cavity is filled with a molten metal and allows to solidify. After solidification the metal

cast is removed by breaking the shell. The casting formed by this process is highly accurate and

well finished. Generally it does not required further machining.

Advantages:

1. Thin and complex section can be easily cast.

2. High dimension accuracy and good surface finish.

3. Easily automated.

4. It can be operate by an semi skilled operator.

5. No further machining required.

Disadvantages:

1. Special metal pattern required which makes it expensive for large casting.

2. It is not suitable for small batch production.

3. Shell moulds are less permeable compare to green sand mould.

4. Size and weight limitation.

Application: Most of industrial products like gearbox housing, connecting rod, small size

boats, truck hoods, cylindrical head, Camshaft, valve body etc. are made by shell moulding.

http://www.mech4study.com/2014/03/what-is-gear-box-what-are-main-components-of-gear-box.html



5. INVESTMENT MOULDING:

Investment casting is an industrial process based on lost-wax casting, one of the oldest known

metal-forming techniques. The term "lost-wax casting" can also refer to modern investment

casting processes.

STEPS INVOLVED:

1. Die and pattern making

A wax pattern is prepared by injecting liquid wax into a pre fabricated die having

approximately the same geometry of the cavity of the desired cast part. Refer fig (a). several such

patterns are produced in the similar manner and then attached to a wax gate and sprue by means

of heated tools or melted wax to form a tree as shown in fig (b).

Fig (a) & fig (b)

2. Pre coating wax patterns

The tree is coated by dipping into refractory slurry which is a mixture of finely ground silica

flour suspended in ethyl silicate solution which acts as a binder. The coated tree is sprinkled with

silica sand and allowed to dry. Refer fig (c).

Fig (c) & fig (d)



3. Investment

The pre-coated tree is coated again (referred as investment) by dipping in a more viscous slurry

made of refractory flour and liquid binders and dusted with refractory sand. The process of

dipping and dusting is repeated until a solid shell of desired thickness (about 6-10mm) is

achieved.

NOTE: the first coating is composed of very fine particles that produce a good surface finish,

whereas the second coating which is referred as Investment is coarser so as to build up the shell

of desired thickness.

4. De-waxing

The tree is placed in an inverted position and heated in an oven to about 300ºF. The wax melts

and drops down leaving a mould cavity that will be filled later by the molten metal. Refer fig (e).

5. Reheating the mould

The mould is heated to about 1000-2000F (550-1100C) to remove any residues of wax and at

same time to harden the binder.



6. Melting and pouring

The mould is placed in a flask supported with a backing material, and the liquid metal of the

desired composition is poured under gravity or by using air pressure depending on the

requirement. Refer fig (f).

Fig (f) & Fig (g)

After the metal cools and solidifies, the investment is broken by using chisels or hammer and

then the casting is cut from the gating systems, cleaned and finished. Refer fig (g).

Fig (g)

6. SWEEP MOULD:

The moulding is done by using a sweep pattern, is called sweep moulding. A sweep that can be

rotated around an axis is used for producing a surface of revolution. The casting produced

involves less time and reduced expenses in making a full pattern.

concepts used in foundry

Documents