EME3016 WORKSHOP TECHNOLOGY

TRIMESTER2 (2013/2014)

CHAPTER 2: Casting

[TEXTBOOK CHAPTER 10 & 11]

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Learning Outcome of Subject

• LO3 - Explain the principle of hot &

cold working processes and apply to

the forming processes and casting.

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OFFICIAL SYLLABUS: Casting

• Introduction to casting processes. Sand

casting: castability, quality of sand for

casting. Casting process. Other

processes: permanent mould die casting,

precision investment casting, shell and

plaster moulds.

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OVERVIEW

A. FUNDAMENTALS OF METAL CASTING

B. METAL-CASTING PROCESSES AND

EQUIPMENT

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INTRODUCTION

• Casting was first used about 6000 years ago, to make ornaments

and copper arrowheads.

• Casting basically involves pouring molten metal into a mold cavity

where (upon solidification) it takes the shape of the cavity. The part

is then removed from the mold.

• A wide variety of products can be cast, including one piece complex

shapes with internal cavities such as engine blocks.

• Casting processes are most often selected over other manufacturing

methods, for the following reasons:

– Casting can produce complex shapes and with internal cavities

or hollow sections.

– Very large parts can be produced in one piece.

– Casting can utilize materials that are difficult or uneconomical to

process by other means.

– The casting process is competitive with other manufacturing

processes.

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Examples of cast parts.

(a) A die-cast aluminum transmission housing.

(b) A tree of rings produced through investment casting.

Source: (a) Courtesy of North American Die Casting Association, (b) Courtesy of

Romanoff, Inc.

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Cast parts in a typical automobile.

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Outline of metal-casting processes

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A. FUNDAMENTALS OF METAL CASTING.

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• Important considerations in casting operations are as follows:

– Flow of the molten metal into the mold cavity.

– Solidification and cooling of the metal in the mold.

– Influence of the type of mold material.

FLUID FLOW

• Fluid flow is important to achieve good casting.

• The solidification process is affected by the fluid flow.

• The design of a proper gating system will affect:

– Contaminants to be trapped (oxides) and removed from the

molten metal when they adhere to the walls of the gating system

before reaching the mold cavities.

– Avoidance or mimimize premature cooling, turbulence or gas

entrapments in the system.

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Schematic illustration of a typical riser-gated casting.

Risers serve as reservoirs, supplying molten metal to the casting as it shrinks during

solidification.

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FLUIDITY OF MOLTEN METAL

• The capability of the molten metal to fill mold cavities is called

fluidity. The following characteristics of molten metal influence

fluidity:

– Viscosity – fluidity decreases when viscosity increases.

– Surface tension – fluidity will be reduced when the liquid metal

has a high surface tension.

– Inclusions – insoluble particles will lower the fluidity of the liquid

since the viscosity would have increased.

– Solidification pattern of the alloy – fluidity is inversely

proportional to the freezing range. The shorter the range (pure

metal), the higher the fluidity.

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HEAT TRANSFER

• The heat transfer during the complete cycle (from pouring, to

solidification, and to cooling to room temperature) is an important

consideration in metal casting.

• Heat flow at different locations depends on many factors such as

cast material, mold material and process parameters.

• For instance, in casting thin sections, the metal flows rates must be

high enough to avoid premature chilling and solidification.

• On the other hand, the flow rate must not be so high as to cause

excessive turbulence

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Aluminum piston for an internal combustion engine:

(a) as cast and

(b) after machining.

The part on the left is as cast, including risers, sprue, and well, as well as a machining

allowance; the part on the right is the piston after machining.

Source: After S. Paolucci.

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B. METAL-CASTING PROCESSES AND EQUIPMENT

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• Various casting processes have been developed over time, each

with its own characteristics and applications, to meet specific

engineering and service requirements.

• Two trends have had a major impact on the casting industry.

– The first is the mechanization and automation of the casting

process, which has led to significant changes in the use of

equipment and labor. Advanced machinery and automated

process-control systems have replaced traditional methods of

casting.

– The second major trend has been the increasing demand for

high-quality castings with close dimensional tolerances.

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• The 3 major classifications are:

– Expendable molds

• Typically are made of sand, plaster or ceramics and generally

mixed with various binders (bonding agents) for improved

properties.

• Typical sand mold contains sand, clay and water. These

materials are refractories (capable of withstanding the high

temperatures of molten metal). After the casting is solidified,

the mold is broken up to remove the casting.

– Permanent molds

• Made up of metals that maintain their high strength at high

temperatures.

• Used repeatedly.

• Designed that castings can be removed easily and the mold

used again.

• Metal molds are better heat conductors than expendable

nonmetallic molds. The solidifying process is at a higher rate

of cooling which then affects the microstructure and grain

structure of the casting.

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– Composite molds

• Made of two or more different materials (such as sand,

graphite and metal) which combines the advantage of each

material. Have permanent and expendable portion.

• Used in processes to improve mold strength, control cooling

rates and optimize overall economics of the casting process.

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(i) EXPENDABLE-MOLD CASTING PROCESSES

(a) Permanent -pattern

SAND CASTING

• Traditional method of casting.

• Still the most prevalent form of casting.

• Typical applications include machine bases, propellers, plumbing

fixtures and industry equipment.

• The basic processes of sand casting are:

– Placing a pattern (having the shaped of the desired casting) in

sand to make an imprint.

– Incorporating a gating system.

– Removing the pattern and filling the mold cavity with molten

metal.

– Allowing the metal to cool until it solidifies

– Breaking away the sand mold.

– Removing the casting.

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• SANDS

– Most sand casting operations use silica sand (SiO2) as mold

material.

– Sand in inexpensive and suitable for mold materials because it

has high-temperature characteristics and high melting point.

– For proper functioning, sand must be clean and preferably new.

– There are 2 general types of sand:

• naturally bonded (bank sand)

• synthetic (lake sand) – preferred for most foundries since its

composition can be controlled more accurately.

– Fine and round sand grains can be packed closely and form

smooth mold surface. Although this enhances mold strength, the

fine-grained sand lower the mold permeability (penetration

through pores). Good permeability of molds allows gases and

steam to escape easily.

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• TYPES OF SAND MOLDS

– Sand molds are categorized by the types of sand and by the

methods used to produce them:

• Green-sand mold process

– Most common mold material which is a mixture of sand,

clay and water.

– The term ‘green’ refers to the fact that the sand in the mold

is moist or damp while the material is being poured into it.

– Least expensive method of making mold.

– Sand is recycled easily for subsequent use.

• Cold-box mold process

– Various organic and inorganic binders are blended into the

sand to bond the grains chemically for greater strength.

– More accurate dimensionally but more expensive than

green-sand molds.

• No-bake mold process

– Synthetic liquid resin is mixed with the sand, and the

mixture hardens at room temperature.

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– MAJOR FEATURES OF THE SAND MOLD are:

• Flasks – supports the mold itself. Two-piece molds consist of a

cope on the top and a drag on the bottom. The parting line is the

seam between these 2 pieces. When more than two pieces are

used in a sand mold, the additional parts are called cheeks.

• Pouring basin (cup) – into which molten metal is poured.

• Sprue – through which molten metal flows downwards.

• Runner system – which has channels that carry the molten metal

from the sprue to the mold cavity.

• Gates – inlets into the mold cavity.

• Risers – which supply additional molten metal to the casting as it

shrinks during solidification. There are two types of risers: blind

riser and open riser.

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• Cores – which are inserts made from sand. There are placed in

the mold to form hollow regions or otherwise define the interior

surface of the casting. Cores are also used on the outside of the

casting to form features such as lettering on the surface of a

casting or deep external pockets.

• Vents – which are placed in molds to carry off gases produced

when then molten metal comes into contact with the sand in the

mold and the core. Vents also exhaust air from the mold cavity as

the molten metal flows into the sand.

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Schematic illustration of a sand mold, showing various features.

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• PATTERNS

– Patterns are used to mold the sand mixtures into the shape of

the casting and may be wood, plastic or metal.

– Coated with parting agents to facilitate the removal of the casting

from the molds.

– Patterns can be designed for specific applications:

• One-piece patterns – generally used for simpler shapes and

low-quantity production.

• Split patterns – 2-piece patterns that individually form each

portion of the cavity for the complex-shaped casting.

• Match-plate patterns – 2-piece patterns that are mounted on

either side of a single plate. Gating system is mounted on the

drag side. It is used with molding machines and large

production runs to produce smaller castings.

– The development of rapid prototyping applications and

equipment have enabled faster (thus cheaper) patterns can be

fabricated for castings.

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A typical metal match-plate pattern used in sand casting.

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• CORES

– Cores are utilized for castings with internal cavities or passages.

– Placed in the mold cavity to form the interior surfaces of the

casting and removed from the finished part during shakeout.

– Made out of sand aggregates that have strength, permeability,

ability to withstand heat and collapsibility.

– Core prints and Chaplets can be used to support and anchor

the core in place in the mold cavity.

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Examples of sand cores, showing core prints and chaplets to support the cores.

Schematic illustration of the sequence of operations for sand casting.

(a) A mechanical drawing of the part is used to generate a design for the

pattern. Considerations such as part shrinkage and draft must be built into the

drawing.

(b–c) Patterns have been mounted on plates equipped with pins for alignment.

Note the presence of core prints designed to hold the core in place.

(d–e) Core boxes produce core halves, which are pasted together. The cores

will be used to produce the hollow area of the part shown in (a).

(f) The cope half of the mold is assembled by securing the cope pattern plate to

the flask with aligning pins and attaching inserts to form the sprue and risers.

(g) The flask is rammed with sand, and the plate and inserts are removed.

(h) The drag half is produced in a similar manner with the pattern inserted. A

bottom board is placed below the drag and aligned with pins.

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(i) The pattern, flask, and bottom board are inverted, and the pattern is

withdrawn, leaving the appropriate imprint.

(j) The core is set in place within the drag cavity.

(k) The mold is closed by placing the cope on top of the drag and securing the

assembly with pins. The flasks are then subjected to pressure to counteract

buoyant forces in the liquid, which might lift the cope.

(l) After the metal solidifies, the casting is removed from the mold.

(m) The sprue and risers are cut off and recycled, and the casting is cleaned,

inspected, and heat treated (when necessary).

Source: Courtesy of Steel Founders’ Society of America

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

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SHELL MOLD

• Produce castings with close dimensional tolerances and good

surface finish at low costs; such as high precision molding cores,

gear housings, cylinder heads and connecting rods.

• A mounted pattern (ferrous metal or aluminum) is heated (range

175

C to 370

C) and coated with a parting agent (silicone).

• It is then clamped to a box and chamber which contains fine sand,

mixed with 2.5 to 4% of a thermosetting resin binder that coats the

sand particles.

• The box is then rotated upside down allowing the sand mixture to

coat the pattern. The assembly is then placed in an oven to

complete the curing of the resin. Two half-shells are then bonded or

clamped to make a mold.

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The shell-molding process, also called the dump-box technique.

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PLASTER MOLD

• This is a precision casting process due to its high dimensional

accuracy and good surface finish. Typical parts made are valves,

fittings and tooling. Castings usually weigh less than 10kg.

• The mold is made of plaster of paris with the addition of talc and

silica flour to improve strength and to control the plaster setting

duration. Water is mixed with the mold materials mentioned and the

slurry produced is poured over the pattern.

• After the plaster sets, the mold is dried at higher required

temperatures to remove moisture. The mold halves are then

assembled to form the mold cavity.

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(b) Expendable-pattern

EVAPORATIVE-PATTERN CASTING (LOST-FOAM PROCESS)

• It is unique in that a mold and pattern must be produced for every

casting. Earlier casting methods utilized reusable patterns.

• Typical applications are cylinder heads, engine blocks and brake

components.

• This lost-foam casting process uses a polystyrene pattern which

evaporates upon contact with molten metal to form a cavity for the

casting. This process has become important for casting ferrous and

nonferrous metals in the automotive industry.

• The polystyrene pattern is made in die cavities and after post-

preparation, is coated with slurry and placed in the flask. Sand is

then filled into the flask and compacted.

• When molten metal is poured into the mold, it would evaporate the

pattern and fill the mold cavity.

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• Advantages over other casting methods:

– Relatively simple process – no parting lines, risers or cores are

required.

– Inexpensive flasks

– Polystyrene is inexpensive and easily processed to produce

complex pattern shapes.

– Casting requires minimum finishing and cleaning operation.

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Schematic illustration of the expendable-pattern casting process, also known as lost-

foam or evaporative-pattern casting.

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INVESTMENT CASTING

• The pattern is made of wax, plastic (polystyrene) or rapid

prototyping techniques. It is then coated with refractory material

such as fine silica and binders.

• The term ‘investment’ derives from the fact that the pattern is

invested (surrounded) with the refractory material.

• Unlike plastic patterns, wax can be recovered and reused.

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Schematic illustration of the investment-casting (lost-wax) process.

Castings produced by this method can be made with very fine detail and from a variety

of metals.

Source: Courtesy of Steel Founders’ Society of America.

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(ii) PERMANENT -MOLD CASTING PROCESSES

PERMANENT-MOLD CASTING

• In hard-mold casting, two halves of a mold are made from materials

with high resistance to erosion and thermal fatigue, such as cast

iron, steel, bronze, graphite or refractory metal alloys.

• The mold cavity and gating system is machined into the mold and is

an integral part of it.

• To produce castings with internal cavities, cores made of metal are

placed in the mold prior to casting.

• The molds are clamped together and preheated before molten metal

is poured through the gating system. After solidification, molds are

opened and casting is removed.

• To increase the life of permanent molds, the mold cavity surfaces

are usually coated with a refractory slurry or sprayed with graphite

every few castings. These also serve as parting agents and thermal

barriers.

• Mechanical ejectors (pins) can be used for removal of complex

castings. vii. This process is used mostly for aluminum, magnesium

and gray iron which have generally lower melting points. 40

VACUUM CASTING

• This is an alternative to investment, shell-mold and green-sand mold

casting and is suitable for thin-walled (0.75mm) complex shapes

with uniform properties.

• The vacuum system will draw the molten metal into the mold cavities

through a gate in the bottom of the mold.

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Schematic illustration of the vacuum-casting process.

Note that the mold has a bottom gate.

(a) Before and

(b) after immersion of the mold into the molten metal.

Source: After R. Blackburn.

DIE CASTING

• In this process, molten metal is forced into the die cavity .

• Although equipment costs are high but automation has made this

process economical.

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Various types of cavities in a die-casting die.

Source: Courtesy of American Die Casting Institute.

SQUEEZE CASTING

• This is a combination of casting and forging processes.

• The process involves the solidification of molten metal under high

pressure.

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Sequence of operations in the squeeze-casting process. This process combines the

advantages of casting and forging.

Summary of Casting Processes

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General Characteristics of Casting Processes

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