This document provides an overview of casting processes and techniques. It begins with learning outcomes and syllabus details for a workshop technology course. The document then covers fundamentals of metal casting including fluid flow, fluidity, heat transfer and solidification. It discusses various casting processes like sand casting, shell mold casting, plaster mold casting, permanent mold casting, investment casting, lost foam casting and vacuum casting. For each process, it provides details on mold materials, production steps, advantages and applications. Diagrams and figures are included to illustrate key aspects of different casting methods.

1. EME3016
WORKSHOP TECHNOLOGY
TRIMESTER2 (2013/2014)
CHAPTER 2: Casting
[TEXTBOOK CHAPTER 10 & 11]
1

2. Learning Outcome of Subject
• LO3 - Explain the principle of hot &
cold working processes and apply to
the forming processes and casting.
2

3. 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.
3

4. OVERVIEW
A. FUNDAMENTALS OF METAL CASTING
B. METAL-CASTING PROCESSES AND
EQUIPMENT
4

5. 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. 5

6. 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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7. Cast parts in a typical automobile.
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8. Outline of metal-casting processes
8

9. A. FUNDAMENTALS OF METAL CASTING.
9

10. • 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.
10

11. Schematic illustration of a typical riser-gated casting.
Risers serve as reservoirs, supplying molten metal to the casting as it shrinks during
solidification.
11

12. 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.
12

13. 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
13

14. 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.
14

15. B. METAL-CASTING PROCESSES AND EQUIPMENT
15

16. • 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. 16

17. • 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.
17

18. – 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.
18

19. (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.
19

20. • 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.
20

21. • 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.
21

22. – 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.
22

23. • 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.
23

24. Schematic illustration of a sand mold, showing various features.
24

25. • 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.
25

26. A typical metal match-plate pattern used in sand casting.
26

27. • 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.
27
Examples of sand cores, showing core prints and chaplets to support the cores.

28. 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.
28

29. (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
29

30. 30

31. Outline of production steps in a typical sand-casting operation.
31

32. 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.
32

33. The shell-molding process, also called the dump-box technique.
33

34. 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.
34

35. (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.
35

36. • 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.
36

37. Schematic illustration of the expendable-pattern casting process, also known as lost-
foam or evaporative-pattern casting.
37

38. 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.
38

39. 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.
39

40. (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

41. 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.
41
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.

42. 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.
42
Various types of cavities in a die-casting die.
Source: Courtesy of American Die Casting Institute.

43. SQUEEZE CASTING
• This is a combination of casting and forging processes.
• The process involves the solidification of molten metal under high
pressure.
43
Sequence of operations in the squeeze-casting process. This process combines the
advantages of casting and forging.

44. Summary of Casting Processes
44

45. General Characteristics of Casting Processes
45