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Chapter 11

Metal-Casting Processes

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Manufacturing Processes Alternatives

Material Removal ProcessesForming ProcessesAssembly

And..Casting Processes

Similar to?

Examples?

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Metal Casting ProcessesThe process involves:

Pour molten metal into a mold patterned after the part to be manufacturedAllow it to solidifyRemove the part from the mold

Advantages of Metal Casting Processes:Produce part in single step (“near-net” shape)Produce large partsCan produce a wide variety of shapes and sizesComplex shapes are possibleInternal cavities are possible

Any Disadvantages?

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Examples of Cast ProductsNote how difficult it might get to produce these parts using other manufacturing processes

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

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Categories of Metal-Casting ProcessesExpendable Molds:

Typically made out of sand, plaster, ceramics and similar materialSuch material are refractory i.e., capable of withstanding high temperatures. 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 removed easily

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Categories of Metal-Casting ProcessesComposite Molds:

Made of two or more materials (sand, graphite, and metal) in order to combine the advantages of eachHave both permanent and expendable portions

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First: Expendable Mold CastingThere’s a variety of casting processes that use expendable molds.We will focus on discussing 2 of them in detail:

Sand CastingInvestment Casting

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Sand CastingProcess outline:

Place a pattern (having the shape of the desired casting) in sand to make an imprintIncorporate a gating systemRemove the pattern and filling the mold cavity with molten metal Allow the metal to cool until it solidifiesBreak away the sand metal Remove the casting

ExampleCan 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 materialHigh melting point

Factors affecting sand selectionFine round grains can be closely packedFine grained sand has lower permeabilityMold 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; and

the pattern is withdrawn

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Pattern Plate

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Pattern Considerations/AllowancesShrinkage 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 passagesRemoved from the finished part during shakeoutMust be strong, collapsible, permeable, heat resistantMade in a similar manner to mold making

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Investment Casting ProcessAnother common casting process with expendable molds.Also known as lost-wax process

The mold material and labor is costlyAdvantages

Suitable for high-melting-point alloys Good surface finishFew or no finishing operations thus reducing cost

Typical parts Mechanical components such as gears, cams, valves.Lately, titanium aircraft-engineParts 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 CastingDepending on the casting process used, and specially for expendable mold casting processes discussed in this lecture:

Production rate can be slowFinished Casting usually needs finishing operations

Casting Defects

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Casting Defects and How to Avoid them

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Next TimePermanent Molds Casting ProcessesOther Technicalities in various casting processes