INTRODUCTION ON METAL CASTING

Introduction on Metal Casting

In metalworking, casting involves pouring liquid metal into a mold, which contains a hollow

cavity of the desired shape, and then allowing it to cool and solidify. The solidified part is also

known as a casting, which is ejected or broken out of the mold to complete the process. Casting

is most often used for making complex shapes that would be difficult or uneconomical to make

by other methods.

Molding or moulding (see spelling differences) is the process of manufacturing by shaping liquid

or pliable raw material using a rigid frame called a mold or matrix. This itself may have been

made using a pattern or model of the final object.

A mold or mould is a hollowed-out block that is filled with a liquid or pliable material like

plastic, glass, metal, or ceramic raw materials. The liquid hardens or sets inside the mold,

adopting its shape. A mold is the counterpart to a cast. The very common bi-valve molding

process uses two molds, one for each half of the object. Piece-molding uses a number of different

molds , each creating a section of a complicated object. This is generally only used for larger and

more valuable objects.

The manufacturer who makes the molds is called the moldmaker. A release agent is typically

used to make removal of the hardened/set substance from the mold easier. Typical uses for

molded plastics include molded furniture, molded household goods, molded cases, and structural

materials.

Casting processes have been known for thousands of years, and widely used for sculpture,

especially in bronze, jewellery in precious metals, and weapons and tools. Traditional techniques

include lost-wax casting, plaster mold casting and sand casting.

Metal casting is one of the most common casting processes. Metal patterns are more expensive

but are more dimensionally stable and durable. Metallic patterns are used where repetitive

production of castings is required in large quantities.

Casting is a 6000 year old process.The oldest surviving casting is a copper frog from 3200 BC.

The modern casting process is subdivided into two main categories: expendable and nonexpendable

casting. It is further broken down by the mold material, such as sand or metal, and

pouring method, such as gravity, vacuum, or low pressure.

Casting is a manufacturing process by which a liquid material is usually poured into a mold,

which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified

part is also known as a casting, which is ejected or broken out of the mold to complete the

process. Casting materials are usually metals or various cold setting materials that cure after

mixing two or more components together; examples are epoxy, concrete, plaster and clay.

Casting is most often used for making complex shapes that would be otherwise difficult or

uneconomical to make by other methods.

Epoxy is the cured end product of epoxy resins, as well as a colloquial name for the epoxide

functional group. Epoxy resins, also known as polyepoxides are a class of reactive prepolymers

and polymers which contain epoxide groups. Epoxy resins may be reacted (cross-linked) either

with themselves through catalytic homopolymerisation, or with a wide range of co-reactants

including polyfunctional amines, acids (and acid anhydrides), phenols, alcohols, and thiols.

These co-reactants are often referred to as hardeners or curatives, and the cross-linking reaction

is commonly referred to as curing. Reaction of polyepoxides with themselves or with

polyfunctional hardeners forms a thermosetting polymer, often with strong mechanical properties

as well as high temperature and chemical resistance. Epoxy has a wide range of applications,

including metal coatings, use in electronics / electrical components, high tension electrical

insulators, fiber-reinforced plastic materials, and structural adhesives. Epoxy resin is employed

to bind gutta percha in some root canal procedures.

Epoxy resins are low molecular weight pre-polymers or higher molecular weight polymers which

normally contain at least two epoxide groups. The epoxide group is also sometimes referred to as

a glycidyl or oxirane group.

A wide range of epoxy resins are produced industrially. The raw materials for epoxy resin

production are today largely petroleum derived, although some plant derived sources are now

becoming commercially available (e.g. plant derived glycerol used to make epichlorohydrin).

Epoxy resins are polymeric or semi-polymeric materials, and as such rarely exist as pure

substances, since variable chain length results from the polymerisation reaction used to produce

them. High purity grades can be produced for certain applications, e.g. using a distillation

purification process. One downside of high purity liquid grades is their tendency to form

crystalline solids due to their highly regular structure, which require melting to enable

processing.

An important criterion for epoxy resins is the epoxide content. This is commonly expressed as

the epoxide number, which is the number of epoxide equivalents in 1 kg of resin (Eq./kg), or as

the equivalent weight, which is the weight in grams of resin containing 1 mole equivalent of

epoxide (g/mol). One measure may be simply converted to another:

Equivalent weight (g/mol) = 1000 / epoxide number (Eq./kg)

Plaster, Concrete, Or Plastic Resin

Plaster and other chemical setting materials such as concrete and plastic resin may be cast using

single-use waste molds as noted above, multiple-use 'piece' molds, or molds made of small rigid

pieces or of flexible material such as latex rubber (which is in turn supported by an exterior

mold). When casting plaster or concrete, the finished product is, unlike marble, unattractive,

lacking in transparency, and so it is usually painted, often in ways that give the appearance of

metal or stone. Alternatively, the first layers cast may contain colored sand so as to give an

appearance of stone. By casting concrete, rather than plaster, it is possible to create sculptures,

fountains, or seating for outdoor use. A simulation of high-quality marble may be made using

certain chemically-set plastic resins (for example epoxy or polyester) with powdered stone added

for coloration, often with multiple colors worked in. The latter is a common means of making

attractive washstands, washstand tops and shower stalls, with the skilled working of multiple

colors resulting in simulated staining patterns as is often found in natural marble or travertine.

Resin Casting

Resin casting is a method of plastic casting where a mold is filled with a liquid synthetic resin,

which then hardens. It is primarily used for small-scale production like industrial prototypes and

dentistry. It can be done by amateur hobbyists with little initial investment, and is used in the

production of collectible toys, models and figures, as well as small-scale jewelry production.

The synthetic resin for such processes is a monomer for making a plastic thermosetting polymer.

During the setting process, the liquid monomer polymerizes into the polymer, thereby hardening

into a solid.