Manufacturing Technology II(ME-202)

Sheet Metal Operations

Dr. Chaitanya Sharma

PhD. IIT Roorkee

Core, Core prints & Chaplets

Lesson ObjectivesIn this chapter we shall discuss the following:Core: Need, characteristics, typesCore printsChaplets

Learning Activities1. Look up

Keywords2. View Slides; 3. Read Notes, 4. Listen to

lecture

Keywords:

Cores

• Full-scale model of interior surfaces of part• It is inserted into the mold cavity prior to pouring• The molten metal flows and solidifies between mold cavity

and core to form the casting's external and internal surfaces• May require supports to hold it in position in the mold cavity

during pouring, called chaplets

Figure 11.4 (a) Core held in place in the mold cavity by chaplets, (b)

possible chaplet design, (c) casting with internal cavity.

Core Parts

A core consists of two portions:

a) The body of the core and

b) one or more extensions called prints

The body of the core is surrounded by molten metal

during casting process.

Body of core has all the features which are required in

final internal surface (e.g. hole) of the castings.

The prints are necessary to support core in the mould.

They also conduct the heat (and gases produced by a

sand core) to the mould.

CORE, CORE PRINT & CORE BOX

CORE: a sand shape that is

inserted into the mold to

produce the internal features

of a casting, such as holes or

passages for water cooling

CORE PRINT: region added to

the pattern, core, or mold which

is used to locate and support

the core within mold

CORE BOX: the mold or die

used to produce casting cores

Essential Characteristics of Core (Sand)

A good core must possess followings:

High permeability to allow an easy escape to gases formed.

High refractoriness to withstand high temperature of

molten metal

Smooth surface.

High collapsibility i.e. it should be able to disintegrate

quickly after the solidification of the metal is complete.

Sufficient strength to support itself.

Functions (Purposes) of Cores

Cores are required for following :

The cores are used to form the internal cavities.

Cores are used to form a part of a green sand mould.

Cores are used to strengthen the moulds.

Cores are used as a part of the gating system.

Desired Characteristics of A Core

Cores are the materials used for making cavities and internal features which cannot be produced by the pattern alone.

Cores are generally made of the sand and are even used inpermanent molds.

In general, cores are surrounded on all sides by melt and thereforesubjected to much more severe thermal and mechanical conditionscore sand should be of higher strength than molding sand.

Following are the desired characteristics for a core

1. Green strength: a core made of green sand should be strongenough to retain the shape till it goes for baking.

2. Dry strength: core should have adequate dry strength so thatwhen the core is placed in the mold, it is able to resist the castmaterial pressure acting on it.

3. Permeability: the gases evolving from the melt and from the moldmay have to go through the core to escape out of the mold. Hencecores are required to have adequate permeability.

4. Refractoriness: in most of the cases, core is surrounded all around bythe melt, it is desirable that the core material should have higherrefractoriness.

5. Collapsibility: as the casting cools, it shrinks, and so the core should havegood collapsibility (ability to decrease in size). Lack of collapsibility mayprovide resistance against shrinkage and can cause the casting defect ofhot tears.

6. Smoothness: surface of the core should be smooth so as to provide agood finish to the casting surfaces in contact with the cores.

7. Friability (ability to crumble): after the casting is completely cooled, thecore should be removed from the casting before it is processed further.Hence the friability is also an important consideration.

8. Low gas emission: because the cores are subjected to very hightemperature, the evolution of gases from the inside are very high atthat temperature. These gases are otherwise likely to produce gasinclusion defects. So the cores should be made such that the evolutionof gases is minimum.

Desired Characteristics For A Core

Core Sands

CORE SAND CONSTITUENTS:

• Core sand should contain the sand grains, binders and other additives to providespecific properties.

Sand:

• Silica sand which is completely devoid of clay is generally used for making coresands.

Coarse silica (because of its higher refractoriness) is used in steel foundries

Finer sands are used for cast irons and non-ferrous alloys.

Binders:

Core sands need to be stronger than the molding sand.

Clay used as binder in molding sands is not enough & so organic binders areused.

Generally used binders are, linseed oil, core oil, resins, dextrin, molasses, etc.

Core oil is a mixture of linseed, soya, fish and petroleum oils and coal tar.

These binders are burnt away by the heat of the melt and thus make the corecollapsible during the cooling of the casting.

Amount of binder required depends to a great extent on the fineness of sandgrains. Amount of clay left in the sand increases the consumption of binder.

Organic binders develop strength by means of polymerization andCross-linking.

To effect this, the cores after preparation need to be Baked.

A proper combination of baking time is to be chosen so as toOptimize the core properties (as shown in fig).

General composition of a core sand mixture could be core oil (1%)and water (2.5 to 6%).

Core Sands

Classification of Cores

• The selection of the correct type of core depends on productionquantity, production rate, required precision, required surfacefinish, and the type of metal being used.

Core can be classified as follows:1. Based on material used for making cores

a) Sand cores b) Metal cores

2. Based on nature of use

a) Dispensable (in sand casting) b) Permanent (in die casting)

3. Based on shapes and positions of the cores in prepared moulds

a) Horizontal core b) Vertical core

c) Balanced core d) Hanging or cover core

e) Drop core or stop off core f) Ram up core

g) Kiss core.

Metal And Sand Cores

• Metal Cores are used inpermanent mould casting.

• Metal cores should beparallel to the mould partingline, or can be removedbefore the casting isremoved from the mould, andshaped so that is readilyfreed from the casting.

• Metal cores are typicallymade from cast iron or steel.

• Sand cores are madefrom materials similar tothose used for chemicallybonded sand moulds.

• These cores are formed incore boxes - similar topattern boxes used tomake moulds.

• Sand core are chemicallybonded sand of complexshapes, and used in allmould types.

Based on the material used for making cores are of two types:

Metal cores and sand cores.

Types of Cores

Cores are generally made of sand & are even used in permanent molds.

BASED ON THE TYPE OF SAND USED:

1. Green sand core: these are obtained by the pattern itself duringmolding.

This is used only for those type of cavities which permit thewithdrawal of the pattern.

Though this is the most economical way of preparing core, thegreen sand being low in strength cannot be used for fairly deepholes.

A large amount of draft is to be provided so that the pattern canbe withdrawn.

2. Dry sand cores: are those which are made by means of specialcore sands in a separate core box, baked and then placed in themold before pouring.

Green Sand Core

Types of Cores

3. Horizontal core: the most common type.

Usually in a cylindrical form laid horizontally in the mold.

Ends of core rest in seats provided by the core prints on pattern.

Horizontal core may be made in one piece using a split core box, or intwo halves using a half core box.

4.Vertical core:

The core is placed along a vertical axis in the mould.

The ends of the core at top and the bottom fit into the seats providedin the cope and drag halves of the mold.

Both horizontal and vertical cores are used more frequently thanother cores in the foundry work. For this reason they are called stockcores and are kept ready in various diameters and lengths.

Horizontal core Vertical core

Types of Cores

5. Balanced core:

Balanced core is suitable when the casting has an openingonly on one side and only one core print is available on thepattern.

Core print in such cases should be sufficiently large tosupport the weight of the Core, which extends into the moldcavity, and it should be able to withstand the force ofbuoyancy of the melt surrounding it.

To support core in mold cavity, chaplets are often inserted.

Balanced core

Types of Cores

6. Cover Core:

Cover core is used when the entire pattern isrammed in the drag and the core is required to besuspended from the top of the mold.

Unlike the balanced core, which extends horizontallyin the mold cavity, the cover core stretchesvertically downwards.

Cover core

7.Hanging Core:

If the core hangs from the cope and does not have

any support at the bottom in the drag, it is referred

to as a hanging core.

In this case, it may be necessary to fasten the core

with a wire or rod, which extends through the cope

to a fastening on the top side of the cope.

Types Of Cores

Hanging core

8. Wing core" or stop-off:

Wing core may be used when a hole or recess is to be obtainedin the casting either above or below the parting line.

Wing core is necessitated when it is not possible to place thepattern in the mold such that the recess can be coreddirectly or with the other types of cores.

Since a part of the core placed in seat becomes a stop-off andforms a surface of casting, it is also referred as stop-off core.

It is also known as tail core, chair core, and saddle coreaccording to its shape and position in the mold

Types of Cores

Wing core

9. Ram-Up Core:

Sometimes, the core is set with the pattern in the mold

before the mold is rammed. Such a core is called ram-up core

it is favored when the core detail is located in an

inaccessible position.

It may be used for both interior and exterior portions of a

casting.

Types of Cores

Ram Up core

10. Kiss Cores:

When the pattern is not provided with core prints and noseat is available for resting the core, the core is held inposition between the cope and drag simply by the pressureof the cope.

Kiss cores are useful when a number of holes are requiredin the casting

Dimensional accuracy with regard to the relative location ofthe holes is not important.

Types of Cores

Kiss Core

Core Making

Cores for sand casting are manufactured by packingspecially prepared sand in Core boxes.

Core-making processes include sand preparation, coreshooting, coating/treatment and placement in mould.

The cavity in a core box is a negative replica of thecorresponding part feature.

The core box is made in two segments (with a parting) toenable removal of the core.

Complex cores are prepared by assembling or gluing two ormore cores of simpler shapes.

The core-related activities consume significant resources.

Thus the number and volume of cores must be minimizedto the extent possible, to reduce tooling cost andmanufacturing time.

Core Boxes

Core boxes are used for making cores. A core box is a woodenor metallic type of pattern and are made either single or intwo parts.

They may be classified according to the method of making thecore or shape of core.

The common types of core boxes are described below:

1.Half Core Box

• Half core box is used when a symmetrical core is prepared intwo identical halves which are later on pasted or cementedtogether to form a complete core.

Half Core Box

Core Boxes

2.Split Core Box

It is made in two parts like a split pattern.

Both the parts are joined together by means of dowel

pins to form the complete hollow cavity for making the

core as shown in fig.

Split Core Box

Core Boxes

3.Dump Core Box

For making the slab or rectangular shape of core,

dump core box is used.

In construction, it is similar to half core box. The box

is made with side opening.

Dump Core Box

Core Boxes

4.Loose Piece Core Box

It is used for the preparation of core with the

provisions of boxes or hubs.

This is used when the two halves of a core of which

the halves are not identical in shape and size is to

be prepared in the same core-box as shown in fig.

Loose Piece Core Box

Core Boxes

5. Strickle Type Core Box

Used for making unsymmetrical or irregular shapes of cores.

A strickle core box is used when the core is required to have anirregular shape which cannot be easily rammed by othermethod.

The desired irregular shape is achieved by striking off thecore from the top of the core box with a piece of wood calledstrickle board.

Strickle board is having same contour as that of the core.

Strickle Core Box

Core Prints

Core prints are provided so that the cores are securelyand correctly positioned in the mold cavity.

Design of core prints takes care of the weight of the corebefore pouring and the upward metallostatic pressure ofthe melt after pouring.

Core prints should also ensure that the core is not shiftedduring the entry of the melt into the mold cavity.

Main force acting on the core when melt is poured into themold cavity is due to buoyancy which is the difference in

the weight of the liquid metal tothat of the core material ofthe same volume as that of theexposed core.

Design Of Core Prints

Core prints should be able to take care of weight of core before pouring &upward metallostatic pressure of molten metal after pouring.

The core print should ensure that core is not shifted during the entry ofmetal into mould cavity

The main force acting on the core when metal is poured into mould cavity isdue to buoyancy.

Buoyant force is the difference in the weight of the liquid metal to that ofthe core material of the same volume as that of the exposed core.

Mathematically

For horizontal core P = V(ρ-d)P = Buoyant force, N

V = Volume of the core in the mould cavity, cm3 (Volume = 0.25 π D2 H)

ρ = Weight density of the liquid metal, N/cm3

d = weight density of core material= 1.65x 10-2 N/cm3

For vertical core, Buoyant force P= [0.25 π (D12 - D2 ) H ρ– Vd]

Where V= total volume of the core in the mould

A core should be able to support a load of 35 N/cm2 of surface area to keep core in position . A core must satisfy following condition A= surface area

If above condition is not satisfied than provide additional support by using chaplets.

The Russian practice of dimensioning the core print is to make thepressure acting on the core bearing area( i.e. the core printsurface area) to be less than 50- 75 % of the moulding sandcompression strength Hence

Core Print Dimensions

• Core print dimensions are tabulated below withreference to fig on next slide

Table 1: Core Print Dimensions

Core Print Sizes

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