Casting Elwany Full

8/20/2019 Casting Elwany Full

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3/20/2006 56:032 Design for Manufacturing 1

Chapter 11

Metal-Casting Processes


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3/20/2006 56:032 :Design for Manufacturing56:032 :Design for Manufacturing 2

Manufacturing Processes Alternatives

Material Removal Processes

Forming Processes

Assembly

And.. Casting Processes

Similar to?

Examples?


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Metal Casting Processes

The process involves: Pour molten metal into a mold patterned after the part to be

manufactured

Allow it to solidify Remove the part from the mold

Advantages of Metal Casting Processes: Produce part in single step (“near-net” shape)

Produce large parts

Can produce a wide variety of shapes and sizes

Complex shapes are possible

Internal cavities are possible

Any Disadvantages?


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Examples of Cast Products

Note how difficult it might get to produce these parts usingother manufacturing processes

Fig. 11.1 aFig. 11.1 bFig. 11.1 cFig. 11.1 d


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Categories of Metal-Casting Processes

Expendable Molds: Typically made out of sand, plaster, ceramics and similar

material

Such material are refractory i.e., capable of withstanding hightemperatures.

After casting the mold is broken up to remove the casting

Permanent Molds: Made of metals that maintain their strength at high

temperatures.

Are used repeatedly

Designed in such a way that the casting can be removedeasily


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Categories of Metal-Casting Processes

Composite Molds: Made of two or more materials (sand, graphite, and metal) in

order to combine the advantages of each

Have both permanent and expendable portions


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First: Expendable Mold Casting

There’s a variety of casting processes that useexpendable molds.

We will focus on discussing 2 of them in detail:

Sand Casting Investment Casting


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Sand Casting

Process outline: Place a pattern (having the shape of the desired casting) in

sand to make an imprint

Incorporate a gating system Remove the pattern and filling the mold cavity with molten

metal

Allow the metal to cool until it solidifies

Break away the sand metal Remove the casting

Example Can we make hollow parts or parts with internal

cavities? How?


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Sand Casting (contd.)

Sands Silica sands (SiO2 ) is widely used as the mold material

High melting point

Factors affecting sand selection Fine round grains can be closely packed

Fine grained sand has lower permeability

Mold should have good collapsibility


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Sand Mold Terminology

Figure 11.3 Schematic illustration of a sand mold, showing various features.


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Finished Casting


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Sand Mold Preparation

(a) Mechanical drawing of part

(b & c) Pattern Mounting

(d) Core Production


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Sand Mold Preparation (Cont’d.)

(e) Cores are pasted(f ) Assembly of the cope half of the mold

(g) The flask is rammed with sand

(h) The drag half is produced in a similar manner

(i) The pattern , flask, and bottom board are inverted; andthe pattern is withdrawn


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Pattern Plate


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Pattern Considerations/Allowances

Shrinkage Allowance (Why?) Machining Allowance (?)

Draft Allowance (?)

Round corners (?)


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Demonstration of the Draft Allowance


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Sand Cores

Figure 11.6 Examples of sand cores showing core prints and chaplets to support cores.

Cores are used for internal cavities and passages Removed from the finished part during shakeout

Must be strong, collapsible, permeable, heat resistant

Made in a similar manner to mold making


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Investment Casting Process

Another common casting process with expendablemolds.

Also known as lost-wax process

The mold material and labor is costly

Advantages Suitable for high-melting-point alloys

Good surface finish Few or no finishing operations thus reducing cost

Typical parts

Mechanical components such as gears, cams, valves. Lately, titanium aircraft-engine

Parts up to 2500 lbs were successfully produced


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Investment Casting Process (Cont’d)

Figure 11.13 Schematic illustration of investment casting (lost-wax) process. Castings

by this method can be made with very fine detail and from a variety of metals.


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Disadvantages of Metal Casting

Depending on the casting process used, and speciallyfor expendable mold casting processes discussed in thislecture:

Production rate can be slow Finished Casting usually needs finishing operations

Casting Defects


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Casting Defects and How to Avoid them


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Next Time

Permanent Molds Casting Processes Other Technicalities in various casting processes

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