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 |  موضوع: casting notes الخميس 07 أكتوبر 2010, 3:38 pm | |
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 Introduction
How are designs turned into products? What resources, materials and methods used and
what set of activities that goes under the heading of ‘manufacturing’ ? This unit will
introduce manufacturing as a system and will describe some of the many different ways of
making products. We will illustrate how the required properties of the materials in a
product influence the choice of manufacturing process used.
Learning Outcomes
After studying this unit you will be able to:
• Explain the difference between industrial and engineering design with reference
to familiar products; and for specific products explain whether it is the product’s
form or its function that enhances its value in the marketplace.
• Understand the concept of a product design specification (PDS), and be able to
indicate some to the factors which should be included in producing one.
• Describe the role of marketing in developing the PDS for a product.
• Classify products simply in terms of their basic shape.
• Describe the difference between the not and cold working of metals and give the
advantages of each.
• Indicate which types of manufacturing process are suited to producing different
shapes of product.
• Indicate which processes are likely to be used for producing a particular product
using a specific material or class of material.
• Describe the advantages and disadvantages of the different classes of
manufacturing processes.
• Outline the concept of surface engineering for improving the properties of a
component.
 
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| | موضوع: رد: casting notes الخميس 07 أكتوبر 2010, 3:39 pm | |
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2. Casting
2.1 Introduction
Casting is one of the easiest classes of process to understand. Casting is simply a process
where a mould is filled with a fluid, which then solidifies in the shape of the mould cavity.
Provided the liquid is capable of undergoing a liquid-to-solid transition, by freezing or
chemical reaction for instance, then casting can be used. Making ice cubes and jellies are
useful analogies here. The production of the mould is one of the most important stages in
making a casting. The casting, when solidified, must be of the right shape for the final
product. In making the mould, often a ‘pattern’ made in the shape of the final component
is used. This might be a wooden mock-up, for example.
Complex 3D shapes can be made using casting processes. Casting can be used to make a
vast array of products, from gas-turbine blades to cheap plastic toys. Cast parts can range
in size from fractions of centimetres and grams (such as the individual teeth on a zipper),
to over 10 metres in length and many tonnes (such as the propellers of ocean liners).
Using one of the available casting processes almost anything can be manufactured. It is a
2
matter of optimising materials to be cast, the mould material and the pouring method (see
Properties for processing – casting).
Generally, during casting, the fluid flows into the mould under gravity, but sometimes the
fluid may need some extra force to push it into the cavity.
Casting is not restricted to metals (or jellies). Glass and plastics can also be cast using a
variety of processes, each being dependent on the raw starting material, and the manner
by which it can be made to flow when it is in its liquid state. Casting processes can be
classified into three types depending on the nature of the mould used.
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Admin
مدير المنتدى
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| | موضوع: رد: casting notes الخميس 07 أكتوبر 2010, 3:40 pm | |
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2.2 Types of casting
2.2.1 Permanent pattern
This type of casting uses a model, or pattern, of the final product to make an impression
which forms the mould cavity. Each mould is destroyed after use but the same pattern is
used over and over again. Sand casting is a typical example of a permanent pattern
process, where a pattern is placed into a special casting sand to form the right shape of
cavity. Permanent pattern processes are usually cheaper than other methods, especially
for small quantity production or ‘one-offs’, and are suitable for a wide range of sizes of
product.
2.2.2 Permanent mould
In this method the same mould is used for large numbers of castings. Each casting is
released by opening the mould rather than by destroying it. Permanent moulds need to be
made of a material which can withstand the temperature fluctuations and wear associated
with repeated casting. A good example of a product made with methods such of this is the
ubiquitous ‘die-cast’ child’s toy (‘die’ is another word for ‘mould’).
2.2.3 Expendable mould and pattern
With this type of casting, a pattern is made from a low melting point material and the
mould is built around it. The pattern is then melted or burnt out as the metal is poured in.
The mould has to be destroyed to retrieve the casting.
This method is used to make moulds for casting high melting-point alloys like those used
for jet engine turbine blades (Figure 19). A model (the pattern) of the blade is made in
wax. The pattern is then coated in a thick slurry containing ceramic particles. The slurry
dries, and is then fired in an oven: this hardens the ceramic (like firing a pot) and melts
out the wax, leaving a hollow ceramic mould. The metal is then poured in to the mould,
which is broken away after the metal has solidified and cooled.
2.3 Casting processes
Casting is used to produce ingots which are then used as the raw materials for forming
processes such as rolling or extrusion. As an intermediate processing step, casting needs
to be less carefully regulated (than other processes to make engineering products), as the
properties of the final product are controlled by the forming processes which follow
casting. Therefore we will concentrate on casting processes which make components.
Casting processes vary depending on the type of solid to be produced and the type of fluid
used to fill the mould. The type of mould required depends on the material to be cast, and
in particular, on the temperature at which it is sufficiently viscous to flow into the mould.
Metals are cast when molten, so we should consider their melting point, whereas for
polymers (whose fluidity can change markedly with temperature) we need to know the
temperature where viscosity is low enough for reasonable flow.
2.4 Casting metals
Sand casting is illustrated in Figure 20. A solid replica of the required object is made: the
‘pattern’. Sand is then rammed around the pattern in a ‘moulding box’. When the pattern
is removed it leaves a shaped cavity behind. The runners (where the fluid is poured in) and
risers (where excess fluid can escape) also act as reservoirs of liquid to top up the casting
as the metal contracts on cooling.
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Admin
مدير المنتدى
عدد المساهمات : 18724
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| | موضوع: رد: casting notes الخميس 07 أكتوبر 2010, 3:41 pm | |
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The process can be used, perhaps surprisingly, to make hollow castings. To do this, ‘cores’
are inserted into moulds to produce shapes that would be difficult or impossible to make
by just using a pattern.
The mould is destroyed when the solid casting is removed. The surface of the castings
produced by this method tend to be rather rough, even though quite fine-grained sand is
used for the moulds. So some machining (cutting) of the surface is generally required
before a finished product is made from a sand-cast route. Certainly the runners and risers
need to be cut away.
Sand casting is particularly useful for casting complex 3D shapes such as automobile
cylinder heads or large castings
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محمد محمد أحمد
مهندس فعال جدا جدا
عدد المساهمات : 654
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تاريخ التسجيل : 14/11/2012
العمر : 32
الدولة : EGYPT
العمل : Student
الجامعة : Menoufia
| | موضوع: رد: casting notes الجمعة 26 يوليو 2013, 4:46 pm | |
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