manufacturing engineering guide book

7/30/2019 manufacturing engineering guide book

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CASTING AND MOLDING


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A quick way of producing the

finished parts like.... Engine parts

Pipes

Jewellery

Fire hydrants

Etc...


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CASTING STEPS

Melt metals

Pour / force liquid into hollow cavity

(mold)

Cool / Solidify

Remove Finish


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Why Do We Prefer Casting?

Near net shape

Low scrap

Relatively quick process

Large hollow shapes

No limit to size Reasonable to good surface finish


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Metal Casting Processes

Sand Centrifugal

Shell Die

Ceramic Investment Permanent mold

Plaster mold

Evoparative Pattern


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Metals processed by casting

Sand casting 60%

Investment casting 7%

Die casting 9%

Permanent mold casting 11%

Centrifugal casting 7% Shell mold casting 6%


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Sand Casting


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Manufacturing, Engineering & Technology, Fifth Edition, by Serope Kalpakjian and Steven R. Schmid.ISBN 0-13-148965-8. 2006 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.

Production Steps in Sand-Casting

Figure 11.2 Outline of production steps in a typical sand-casting operation.


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Why Sand Casting?

In metal casting work, as with almost every kind ofmanufacture, high precision means high cost. In acase we are making a small number of objects so wecan't really afford to make a sophisticated metal mold(i.e. we can't do die casting). The usual approach in

this case is to make disposable molds out of sand orplaster.The kind of disposable mold, and the techniques usedto make the mold, are again determined by questions

of cost and quality. For a very intricate, preciseresult, lost wax or lost polystyrene foam casting isused. For less intricate workpieces, or lowerprecision requirements, direct sand casting can be

adequate.


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What is Sand Casting?

All casting, not just sand casting, is about

pouring or squirting a molten substance into a

cavity.

All casting methods require two basic things:

A cavity which is the shape of the desired finalobject, and

A way to put molten substance into the cavity.

In the sand casting process, the cavity is madeof sand which is held together by a binder (a

substance that holds the sand grains together).


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Sand casting is used to produce a wide variety of metal components

with complex geometries. These parts can vary greatly in size and

weight, ranging from a couple ounces to several tons. Some smallersand cast parts include components as gears, pulleys, crankshafts,

connecting rods, and propellers. Larger applications include housings

for large equipment and heavy machine bases. Sand casting is also

common in producing automobile components, such as engine blocks,

engine manifolds, cylinder heads, and transmission cases.


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Die Casting

Die casting is a manufacturing process for

producing accurately dimensioned, sharply

defined, smooth or textured-surface metal parts.


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Die casting hot chamber machine overview

Die casting cold chamber machine overview


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Advantages of Die Casting

Cost of castings is relatively low with highvolumes.

High degree of design complexity andaccuracy.

Excellent smooth surface finish. Suitable for relatively low melting point

metals (1600F/871C) like lead, zinc,

aluminum, magnesium and some copperalloys.

High production rates.


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Disadvantages of Die Casting

Limits on the size of castings - most suitable

for small castings up to about 75 lb.

Equipment and die costs are high.


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Investment Casting

With the advancement in rapidprototyping, investment casting has

become the leading technology for

producing high-quality castings quickly

and inexpensively. Investment casting isalso known as the "lost wax" process.


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This process uses wax patterns assembled

in tree forms on a runner. The completed

assembly is coated with a ceramic slurry,

allowed to dry and then heated to melt out

the wax leaving a ceramic mould into which

the molten alloy is poured.

Parts made with investment castings oftendo not require any further machining,

because of the close tolerances that can be

achieved.


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http://www.custompartnet.com/wu/images/investment-casting/investment-casting.png


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Advantages of Investment Casting

Excellent accuracy and flexibility of design.

Useful for casting alloys that are difficult tomachine.

Exceptionally fine finish.

Suitable for large or small quantities of parts.

Almost unlimited intricacy.

Suitable for most ferrous / non-ferrous metals.

No flash to be removed or parting linetolerances.


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Disadvantages of Investment Casting

Limitations on size of casting. Higher casting costs make it important to

take full advantage of the process to

eliminate all machining operations.


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Shell Mold Casting

Shell molding is a process for producingsimple or complex near net shapecastings, maintaining tight tolerances anda high degree of dimensional stability.Shell molding is a method for making highquality castings. These qualities ofprecision can be obtained in a wider

range of alloys and with greater flexibilityin design than die-casting and at a lowercost than investment casting.


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Process Characteristics

Is superior to other sand casting processes in the

accurate duplication of intricate shapes and

dimensional accuracy

Process can be completely mechanized

Uses a thin-walled non-reusable shell composed

of a sand-resin mixture

Requires a heated metal pattern for producing

the shell molds


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Shell-Molding Process

Figure 11.9 The shell-molding process, also called dump-boxtechnique.


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Centrifugal Casting

In centrifugal casting, a permanent mold isrotated about its axis at high speeds (300to 3000 rpm) as the molten metal ispoured. The molten metal is centrifugallythrown towards the inside mold wall,where it solidifies after cooling. The castingis usually a fine grain casting with a veryfine-grained outer diameter, which isresistant to atmospheric corrosion, a

typical situation with pipes. The insidediameter has more impurities andinclusions, which can be machined away.


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Only cylindrical shapes can be produced withthis process.


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Advantages of Centrifugal Casting

Rapid production rate.

Suitable for Ferrous / Non-ferrous parts.

Good soundness and cleanliness of castings.

Ability to produce extremely large

cylindrical parts.


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Permanent Molding Process

In Permanent Molding Process, instead of using sand asthe mold material, a metal is used as a mold.

Permanent mold casting is typically used for high-volume

production of small, simple metal parts with uniform wallthickness. Non-ferrous metals are typically used in thisprocess, such as aluminum alloys, magnesium alloys, andcopper alloys. However, irons and steels can also be castusing graphite molds. Common permanent mold parts

include gears and gear housings, pipe fittings, and otherautomotive and aircraft components such as pistons,impellers, and wheels.


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Advantages of Permanent Mold
http://www.custompartnet.com/wu/images/permanent-mold-casting/permanent-mold-casting.png


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Advantages of Permanent Mold

Casting Superior mechanical properties.

Produces dense, uniform castings with highdimensional accuracy.

Excellent surface finish and grain structure.

The process lends itself very well to the use ofexpendable cores and makes possible theproduction of parts that are not suitable for thepressure die casting process.

Repeated use of molds.

Rapid production rate with low scrap loss.


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Disadvantages of Permanent Mold

Casting

Higher cost of tooling requires a higher

volume of castings.

The process is generally limited to the

production of somewhat small castings of

simple exterior design, although complex

castings such as aluminum engine blocks and

heads are now commonplace.


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Plaster Mold Casting

In plaster mold casting, a plaster, usually gypsum orcalcium sulfate, is mixed with talc, sand, asbestos, andsodium silicate and water to form a slurry. This slurryis sprayed on the polished surfaces of the patternhalves (usually brass). The slurry sets in less than 15

minutes to form the mold. The mold halves areextracted carefully from the pattern, and then dried inan oven.

The mold halves are carefully assembled, along withthe cores. The molten metal is poured in the molds.After the metals cools down, the plaster is broken andthe cores washed out.


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Parts cast are usually small to medium size,

Low temperature melting materials such as

aluminum, copper, magnesium and zinc can becast using this process.

This process is used to make quick prototype

parts as well as limited production parts.



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Utilizes gravity to feed molten metal into a non-

reusable plaster mold

Is limited to nonferrous metals

Produces smooth surface finishes and fine details

compared to sand casting

Produces minimal scrap material

Can produce thin-walled sections and complex

shapes


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Ceramic Mold CastingSimilar to plaster mold casting, the pattern used in

ceramic mold casting is made of plaster, plastic,wood, metal or rubber. A slurry of ceramic is pouredover the pattern. It hardens rapidly to the consistency

of rubber. This can be peeled of the pattern,reassembled as a mold. The volatiles are removedusing a flame torch or in a low temperature oven. It isthen baked in a furnace at about 1000 C (1832 F)

yielding a ceramic mold, capable of high temperaturepours. Additionally, the pour can take place while themold is until hot.


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This process is expensive, but can eliminate secondarymachining operations. Typical parts made from thisprocess include impellers made from stainless steel,

bronze, complex cutting tools, plastic mold tooling.


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Sequence of Operations in Making a Ceramic Mold

Figure 11.10 Sequence of operations in making a

ceramic mold. Source: Metals Handbook, Vol. 5, 8th ed.


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Evoporative Pattern Casting

Evaporative-pattern casting is a type of casting

process that uses a pattern made from a material

that will evoporate when the molten metal ispoured into the molding cavity.

The two major evaporative-pattern casting

processes are:

Lost Foam Casting

Full Mold Casting


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Lost Foam CastingThe Lost Foam casting process is a cavity-

less casting method, using a polystyrenefoam pattern embedded in traditional greensand. The polystyrene foam pattern left inthe sand is decomposed by the poured

molten metal. The metal replaces the foampattern, exactly duplicating all of thefeatures of the original pattern. Like otherinvestment casting methods, this requiresthat a pattern be produced for everycasting poured because it is evaporated(lost) in the process.


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Expandable-Pattern Casting Process

Figure 11.11 Schematic illustration of the expandable-pattern casting process,

also known as lost-foam or evaporative casting.

Advantages of Lost Foam Casting


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Advantages of Lost Foam Casting

No cores are required.

Reduction in capital investment and operating costs. Closer tolerances and walls as thin as 0.120 in.

No binders or other additives are required for the sand,

which is reusable.

Flasks for containing the mold assembly are inexpensive,

and shakeout of the castings in unbonded sand is

simplified and do not require the heavy shakeout

machinery required for other sand casting methods. Need for skilled labor is greatly reduced.

Casting cleaning is minimized since there are no parting

lines or core fins


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Disadvantages of Lost Foam Casting

The pattern coating process is time-

consuming, and pattern handling requires

great care. Good process control is required as a

scrapped casting means replacement not

only of the mold but the pattern as well.


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Full Mold Casting

Full-mold casting is a technique similar toinvestment casting, but instead of wax as theexpendable material, polystyrene foam isused as the pattern. The foam pattern is

coated with a refractory material. The patternis encased in a one-piece sand mold. As themetal is poured, the foam vaporizes, and themetal takes its place. This can make complexshaped castings without any draft or flash.However, the pattern cost can be high due tothe expendable nature of the pattern.


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General Design Rules for Casting

Design the part so that the shape is cast easily.

Select a casting process and material suitable for the part, size, mechanical

properties, etc.

Locate the parting line of the mold in the part.

Locate and design the gates to allow uniform feeding of the mold cavity with molten

metal.

Select an appropriate runner geometry for the system.

Locate mold features such as sprue, screens and risers, as appropriate.

Make sure proper controls and good practices are in place.


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Examples of Good and Poor Designs

Figure 12.3 Examples of undesirable (poor) and desirable (good) casting designs.

Source: Courtesy of American Die Casting Institute.


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Cost Characteristics of Casting

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