Casting - Casting is a manufacturing process for making complex
shapes that is difficult or costly to make through other methods. Liquid metal
is poured into a mold of the desired shape, and then allowed to solidify. The
solid part is also called a casting and it is ejected from the mold once
solidified.
Foundry - A facility engaged in the manufacture of numerous types
of castings using various casting processes.
Metals
Alloy - A metallic solid created by combining two or more metals
or adding a non-metal to a metal for the purpose of enhancing its properties
for the end use.
Ferrous
Alloys - Alloys with iron as its main
constituent.
Non-ferrous
Alloys - Non-iron-based metals.
Cast
iron - A hard, brittle alloy of iron and
carbon that can be readily cast in a mold and contains a higher proportion of
carbon than steel does. It is firm and unchangeable.
Ductile
iron - A cast iron that has been treated
while molten with an element such as magnesium or cerium to induce the
formation of free graphite as nodules or spherulites. This imparts a measurable
degree of ductility (easily manipulated) to the cast metal. The ductile iron
family offers the design engineer a unique combination of strength, wear
resistance, fatigue resistance, and toughness, as well as excellent ductility
characteristics.
Aluminum
Castings - Aluminum is a silvery-white,
ductile metallic element. As the most abundant metal in nature, it is
extensively used for castings and foundry patterns, and is also utilized as a
deoxidizer in iron and steel making.
Steel
Casting - A specialized form of casting
that involves various types of carbon steel and alloy steels.
Steel
Alloys - Steel that has been alloyed with
other metals from 1.0% to 50% by weight to improve the mechanical properties of
the steel. Alloy steels are broken down into two groups: low-alloy steels and
high-alloy steels
Carbon
Steel Casting - Cast, low carbon steels contain
mainly carbon as the principal alloying element. Other elements are present in
small quantities, including those added for de-oxidation. They are produced to
a great variety of properties because composition and heat treatment can be
selected to achieve specific combinations of properties, including hardness,
strength, ductility, fatigue resistance, and toughness.
Stainless
Steel Casting - A casting made from stainless
steel. Stainless steel is a steel alloy with a minimum of 10.5 or 11% chromium
content by mass. Stainless steel does not stain, corrode, or rust as easily as
ordinary steel (it stains less, but it is not stain-proof). There are different
grades and surface finishes of stainless steel to suit the environment to which
the material will be subjected in its lifetime.
CASTING
PROCESSES
Investment
Casting - Often thought of as the ultimate
technique for low weight, precision steel casting production; investment
casting is used to make parts that cannot be produced by normal manufacturing
techniques. It is done by casting metal into a mold produced by surrounding, or
investing, an expendable pattern with a refractory slurry coating that sets at
room temperature, after which the wax or plastic pattern is removed through the
use of heat prior to filling the mold with liquid metal. It is also called
precision casting or lost wax process.
Centrifugal
Casting - A casting process performed by
rotating the mold so as to produce a centrifugal force in the molten metal. The
end product is often more reliable -- free from gas and shrinkage porosity.
Die
Casting - A rapid, water-cooled permanent
molding casting process that is quite limited to non-ferrous metals. The metal
is melted, and then injected into a die. It is cooled rapidly to form the part.
The die-cast part can then be taken through a number of finishing steps for a
wide variety of end products.
No-Bake
Molding / Sand Casting - A
versatile process for metal casting designers in which sand is mixed with a
chemical binder/catalyst system and then molded around a pattern. It is an
ideal option for production runs from 1 - 5,000 castings / year.
PATTERN
AND PRODUCTION TERMS
Casting
Drawing - Contains all of the information
for the foundry operations involved including machining allowances, tolerances
and other specifications.
Machining - Process such as abrading, cutting, drilling, forming,
grinding, and/or shaping of a piece of metal or other material performed by
machine tools such as lathes, power saws, and presses. In casting, the supplier
foundry is responsible for giving the designer a cast product that is capable
of being transformed by machining to meet the specific requirements intended
for the function of the part.
Degassing - A step in the casting process, usually necessary to
reduce the amount of hydrogen dissolved in the liquid metal. If the hydrogen
concentration in the melt is too high, the resulting casting will be porous as
the hydrogen comes out of solution as the aluminum cools and solidifies.
Porosity often seriously deteriorates the mechanical properties of the metal.
Machining
Allowance - The amount by which dimensions of
a casting are made oversize to provide excess stock for machining.
Pattern - A replica of the object to be cast, used to prepare the
cavity into which molten material will be poured during the casting process.
Mounted
Pattern - A pattern that is fastened
permanently to a board or match plate. Mounted patterns cost more than loose
patterns - but when many castings are to be made from a pattern, the time saved
in operation warrants the cost of mounting it. Mounted patterns have the shape
of the casting often with forms for sprues, risers, etc. attached.
Loose
Pattern - The cheapest pattern to make, has
the shape of the casting without forms for sprues, risers, etc. attached. May
be made in one or more pieces.
Core
Box - A type of pattern into which sand
is rammed or packed to form a core.
Core - A separate piece (often made from molding sand) placed
inside the mold to create openings and cavities, which cannot be made by the
pattern alone. Every attempt should be made to eliminate or reduce the number
of cores needed for a particular design, which proportionately reduces the cost
of the casting.
Tolerance - An allowance, given as a permissible range, in the
nominal dimensions of a finished product. The acceptable dimensional tolerances
must be indicated when a drawing is provided. Tolerances are normally decided
by agreement between the supplier foundry and customer. Close cooperation
between the customer's design engineers and the supplier foundry is essential
to optimize the casting design.
Soundness - In metal components, refers to the level of freedom from
impurities and/or discontinuities such as sand inclusions, slag inclusions,
macro porosity (pores greater than 50 nm in diameter), and shrinkage.
Shrinkage - Volumetric contraction that occurs as a steel casting
begins to solidify. Testing should be done to ensure the end product will meet
specifications in castings when the design is likely to result in shrinkage.
Minimum
Section Thickness - The minimum thickness to which a
section can be designed. In custom cast work, the specifications in the bid
should include this information. A minimum thickness of 0.25 in (6 mm) is
suggested for design use when conventional steel casting techniques are
employed. Wall thicknesses of 0.060 in (1.5 mm) are common for investment
castings and sections tapering down to 0.030 in (0.76 mm) can readily be
achieved.
Draft - The amount of taper (or the angle) which must be allowed
on all vertical faces of a pattern to permit its removal from the sand mold
without tearing the mold walls. Draft should be added to the design dimensions
while maintaining minimum metal thickness.
Parting
Line - The line where parting surface
meets with the casting surface of the mold. Patterns with straight parting
lines (that is, with parting lines in one plane) can be produced more easily
and at lower cost than those with irregular parting lines. Casting shapes,
which are symmetrical about one centerline or plane readily suggest the parting
line. Such casting design simplifies molding and coring, and should be used
wherever possible. They should always be made as "split patterns"
(separate cope and drag) which require a minimum of handwork in the mold,
improve casting finish, and reduce costs.
