casting ch 6

7/29/2019 Casting Ch 6

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CORES Cores are sand shapes which form the casting

contours that cannot be molded with a pattern. Exterior contours ----- pattern

Internal shapes/cavities ---- cores

Cores provide the casting process;

* its ability to make the most intricate of shapes,

* eliminate much machining,

* produce shapes which would be impossible to

machine.


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Core may be made of;

- metal

- plaster- ceramic materials

- core sand

Cores must be collapsible

Metal cores ---- shape limitations

Sand cores ------ most frequently used


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Other purposes:

i) Complete molds may be assembled of core-sand

forms. Intricate shapes

ii) May be used to form a part of a green-sand

mold.

Pattern contours with backdraft or projectionswhich cannot be molded can be formed by placing

a core in the mold after the pattern is drawn.

iii) To strengthen or improve a mold surface.iv) May be used as a part of the gating system

v) Ram-up cores are used for several purposes.

* Located on the pattern and rammed up along with the molding sand,the core then forming a part of the mold face.


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Cores


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CORE MAKING

Core sand mixtures ------ sand grains and organic binders

The binders provide green strength, baked strength andcollapsibility.

Because the strength comes from core oil, an organic

binder, the strength is lost and the core becomes collapsible

when hot metal is poured around the core. Core-making ------ manually or with machines

Cores with no flat surfaces must be supported on a core

drier, until they are baked. Core venting

Reinforcing wires


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CORE BAKING:

Temperature up to about 500 F.

Moisture is driven off first.

Then, the core oil or other binder changeschemically and molecularly from a liquid to a solidby oxygen absorption as the temperature rises 400

to 500 F. Backing cycle --- 2 to 6 hrs.

Unbaked cores ---- gives of much gas.

Overbaked cores may collapse too soon. When baked, a core-oil bonded core assumes a

nut-brown color, darkness indicating overbaking

and lightness underbaking.


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Core-baking Equipment:

A. Core ovens

1. Batch-type

2. Continuous

a. Horizontalb. Vertical

B. Dielectric bakers

C. Radiant bakers


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Core Ovens:

Batch Type:

Drawer type Gas or oil fired or coal

Fresh air -- oxygen is needed for many core oils to

harden.

Continuous Type:

Conveyor belt

Cores are loaded on racks


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Dielectric Baking:

Principle: heating of non-conducting materials on a

molecular scale can be caused in a rapidlyfluctuating electrostatic field.

A pair of flat plate electrodes to which is applied a

rapidly oscillating alternating-current voltage.

Plate Voltage = 1000 to 5000 volts

Frequencies up to 20 million cycles

Metal driers or core plates are not used to supportcores.

Driers can be made of plastics

A rapid process


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FINISHING OF CORES:

Finishing work may be classified as follows;

1. Cleaning

2. Sizing

3. Core assembly

4. Inspection


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Sizing Operation:

To make cores dimensionally accurate.

Gauges may used to check critical dimensions.

Sagging or slumping during baking.

For exact size ----- Cores may be made slightly

oversize and ground to correct height.


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Core Assembly:

Some cores may be of one piece

Other cores may be assembled of two or more pieces

Cores assemblies may be held together by pasting, boltingor leading.

Avoid sealing off vent holes and groves when cores arepasted

Pasted or mudded cores are best if dried to avoid thedanger of blow defects in the castings.

Cores may be bolted together --- strong assembly.

Joining ------ by leading (molten lead)Inspection:

The cores should be smooth, free of loose sand orprojections, mudded if necessary, and dry, to be ready for

core-setting operation.


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casting ch 6

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