Manufacturing Technology 1

Manufacturing TechnologyManufacturing Technology

LO #3 Selection of Manufacturing Process Part 2 – Casting & MoldingReference pages in the e-text : 230 ~ 286 & 327 ~ 343

Other Casting Methods

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1. Shell Molding2. Investment Casting3. Lost foam Casting4. Pressure Die casting

Casting Ingots

Casting – Casting – Expandable pattern & Permanent pattern Expandable pattern & Permanent pattern

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Expandable pattern : Pattern is consumed during the process and a new one must be created. Ex. : Investment casting, Expanded Ploystyrene Casting (=lost- foam process, lost-pattern process, evaporative-foam process, and full-mold process)

Expandable Mold Casting – Expandable Mold Casting – See page 257 ~ 260See page 257 ~ 260

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Shell moldingThe dump box is then return to its upright position and surplus mixture falls back into the dump box, leaving the

pattern covered with a layer of sand bonded with resin.This layer or shell is then hardened or cured by the further

application of heat, before being removed from the pattern.The shell is removed with the aid of ejector pins.The most widely used materials for the pattern are cast iron

and steel.Patterns are costlier than the wooden pattern used in green

sand casting and so, to be economic, the process requires larger numbers of casting –seldom less than 200.

When the shells are cured, they are placed together and secured, ready for metal pouring.

Shell molding:

Shell molding comparison Shell molding can generally be expected to produce small castings more

accurately and with smoother surfaces than other forms of sand casting. It is comparable with gravity die-casting in Aluminum but not good as pressure die-casting or investment casting.

Although sand used is more expensive than green sand , the volume used is less due to the thin shell and this can show savings in the amount purchased.

It is estimated that the a simple shell-molded casting will cost around 10-15%more than the simple sand casting. However, by taking advantage

of the greater accuracy and reducing or eliminating machining operation, the use of a shell-molded casting can often result in a cheaper finished component.

Commonly used materials include bronze. aluminum, magnesium, copper and ferrous alloys

Investment Casting Investment casting and all other listed methods(Slide2)are employed

where greater accuracy is required than can usually be achieved by sand casting, especially with small intricate castings and those made from materials otherwise difficult to work with.

The molds in each process are expandable, as in sand casting, and, in comparison with sand casting, these processes are often referred to as “ Precision Casting”

Equipment and materials are more expensive than those required in sand casting and therefore require the production of large quantities to be economical.

The higher cost however, often be offset by saving as a result of the ability to produce a high degree of dimensional accuracy and surface finish

Which can reduce or eliminate subsequent machining operations. Any metal capable of being cast can be cast by these methods, although

advantage is most readily gained when using those difficult to work by other methods.

Investment Casting – (Lost –Wax process)Investment Casting – (Lost –Wax process)

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1. Wax patterns are produced by die casting (See section 11.3.3 in the e-text page 266 ~ 269).

2. Several patterns are attached together to form tree pattern.3. The pattern tree is coated with thin layer of refractory

materials.4. Sufficient rigid layer of refractory coat added to the pattern

tree.5. The mold then held in an inverted position and heated to let

the wax drip out.6. The mold preheated to high temperature to remove any

contaminations or wax.7. The mold if filled with the molten metal while hot.8. The mold left to cool and cleaned after solidify.

Investment Casting – Investment Casting – Process / StepsProcess / Steps

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Investment Casting – Investment Casting – ProductsProducts

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Complex machinery parts, blades, and other components for turbine engines, jewelry, and dental fixtures.

See a video clip

Investment Casting – Investment Casting – Advantages & DisadvantagesAdvantages & Disadvantages

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1. Parts of great complexity and intricacy can be cast.2. Close dimensional control-tolerances of +/-0.075 mm are

possible.3. Good surface finish is possible.4. The wax can usually be recovered for reuse.5. Additional machining is not normally required.

1. Because many steps are involved in this casting operation, it is a relatively expensive process.

Disadvantage

Advantages

Metals for investment castingThe ferrous range covers low alloy, carbon

and tool steels as well as corrosion and heat resistant steel.

The non ferrous range covers aluminum and zinc alloys and the copper-based alloys, brasses, bronzes and gunmetal.

Application of Investment casting:The process is used in the automotive

industry in the production of cylinder heads, engine blocks, crankshafts, and exhaust manifolds.

In the electric industry for electric motor frames.

Other application includes main water valves.

Evaporative-Foam Casting (Lost foam casting)

Expanded polystyrene casting process steps:1. Foam Pattern of polystyrene is coated with

refractory liquid compound and left to dry in many layers.

2. The coated foam pattern is placed in mold box, and sand is compacted around the pattern.

3. Molten metal is poured into the portion of the same pattern, causing the foam to vaporize.

4. The molten metal will fill the foam shape and left to cool and solidify.

5. The solid casting is cleaned from the coat.

Evaporative-Foam Casting (Lost foam casting)

Evaporative-Foam Casting (Lost foam casting)

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Characteristics of lost foam casting:Design freedom to produce complex shapes.No parting lineNo cores are neededLow draft angles can be usedClose dimensional tolerances can be achievedUses dry un-bonded sand which is reusableHigh production rates are possibleMachining can be eliminatedMinimum finishing operations are needed, e.g. Shot

pinning, grinding.Expensive tooling restrict the process to long production

runs.

Die CastingDie Casting

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Die casting is a permanent-Mold Casting process in which the molten metal is injected into the mold cavity under high pressure (7 to 350 MPa).

The pressure will be maintained during solidification, after that the mold is opened and the part is removed.

The mold in this casting process is called dies. Die casting operations are carried out in special die casting

machines. There are two types of those machines:

Hot-chamber machines Cold-chamber machines

Die Casting – Die Casting – Advantages & Disadvantages (In General)Advantages & Disadvantages (In General)

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Advantages of die casting : High production rates possible

Economical for large production quantities

Close tolerances possible

Good surface finish

Complex shape casting is possible

Disadvantages of die casting :

High die costs.

Offers limited part size / Shape restriction

Limited to non-ferrous metals(low melting point)

Requires long lead time

Die Casting – Die Casting – Hot Chamber Hot Chamber Die Casting MachinesDie Casting Machines

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In this method the material is melted in a container attached to the machine.

A piston is used to inject the liquid metal under high pressure (7 – 35 MPa) into the die.

This type of process is limited to low melting-point materials, since the injection system is submerged in the molten metal.

Also it is limited to material that show low chemical reactivity with the injection system parts.

Metals types used with Hot-chamber machines are: Zinc, Tin, Lead, and sometimes magnesium.

can produce 500 parts per hour.

Die Casting – Die Casting – Hot Chamber Hot Chamber Die Casting MachinesDie Casting Machines

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Die Casting – Die Casting – Cold Chamber Cold Chamber Die Casting MachinesDie Casting Machines

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The molten metal is poured into an unheated chamber from an external melting container.

A piston is used to inject the metal under high pressure into the die cavity.

Injection pressure required is higher in the cold machines, and varies between 14 – 140 MPa.

The cold-chamber machine has lower production rate, cycle rates are slower than hot-machines.

Cold-Chamber casting usually used for Aluminum, Brass, and magnesium casting.

The molds or dies used in die casting are made from a higher melting-point metals such as tool steel, mold steel, and others.

Die Casting – Die Casting – Cold Chamber Cold Chamber Die Casting MachinesDie Casting Machines

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Die casting MetalsZinc alloysAluminum AlloysMagnesium alloysCopper AlloysLead & Tin alloysAll alloys are having low melting temperature

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Factors influencing choice of die-casting method.

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