Wednesday, 23 October 2013

Sand Casting



•Most widely used casting process, accounting for a significant majority of total tonnage cast
•Nearly all alloys can be sand casted, including metals with high melting temperatures,
  such as steel, nickel,and titanium
•Parts ranging in size from small to very large
•Production quantities from one to millions






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Categories of Metal Casting Processes



1. Expendable mold processes - mold is sacrificed to remove part
--Advantage: more complex shapes possible
--Disadvantage: production rates often limited by time to make mold rather than casting itself


2. Permanent mold processes - mold is made of metal and can be used to make many castings
--Advantage: higher production rates
--Disadvantage: geometries limited by need to open mold+




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Casting Terms and Definitions Related to Foundry Production

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.

Metal Casting Terms

Alloy — A substance containing two or more chemical elements, at least one of which is metal. The substance usually has metallic properties and possesses qualities different from those of the individual components.

AOD — A refining treatment used to control the carbon content of steel.

Binders — Binders are added to mold materials in order to create a mold of sufficient hardness. Binders can be either organic or inorganic materials.

CAD — Computer Aided Design.

CAM — Computer Aided Manufacturing.

Captive Foundry — A foundry operation that is wholly incorporated into a larger manufacturing operation. A captive foundry only produces castings for the operation that it is a part of.

Casting — A process used to form solid metal shapes out of molten metal. The molten metal is poured into a cavity or a mold.

Charge — Metal and alloy materials that make up the composition of a melt.

Cope — The top half of a horizontally parted mold.

Core — The part of a mold used in the casting process that forms the internal shapes of a casting.

Crucible Furnace — A furnace that melts metals in a refractory crucible. The furnace is typically fueled with coke, oil, gas or electricity.

Cupola Furnace — The traditional furnace used for melting metal. The furnace is typically fueled with coke.

Direct-Arc Furnace — An electric arc furnace in which the metal being melted is one of the poles.

Draft — Taper on the vertical sides of a pattern or corebox that permits the core or pattern to be removed without distorting or tearing of the sand.

Drag — The bottom half of a horizontally parted mold.

Ferrous Metal — An alloy that has iron as the predominant metal.

Flash — A thin section of metal formed at the mold, core, die joint or parting in a casting. Flash usually forms when the cope and drag do not match completely or when the core and the coreprint do not match.

Gas Porosity — A condition in a casting that occurs when gas is trapped in molten metal or as a result of mold gasses that evolved when the casting was poured.

Gating Systems — The channel(s) that allow the molten metal to enter the mold cavity.

Green Sand — Moist sand that is bonded by a mixture that contains silica, bentonite clay, carbonaceous material, and water.

Gross Weight of Casting — The weight of the casting which includes the actual product plus the metal in the gating system as poured.

Hot Isostatic Pressing (HIP) — The process of using high hydrostatic pressure and high temperature to compress fine particles into coherent parts.

Impurity — An element that is allowed into a metal or alloy. Impurities slightly change the properties of the material in some circumstances and can completely damage it in others.

Inclusion — Nonmetallic materials that become part of a metal matrix usually through reoxidation, refractories, slag, and deoxidization products.

Indirect-Arc Furnace — An alternating current electric-arc furnace. The metal being melted is not one of the poles.

Induction Furnace — An alternating current melting furnace that uses the heat of electrical induction.

Investment Casting — A casting process that uses a wax or thermoplastic pattern and is surrounded in a refractory slurry to form a mold. Once the refractory slurry is dry, the wax or plastic is melted out and the mold is formed. Molten metal is then poured in to fill the area that was previously filled with wax.

Jobbing Foundry — A foundry that creates a wide variety of castings, in small quantities for a range of customers.

Ladle — The name for a variety of receptacles used to move and pour molten metal during the casting process.

Lining — The refractory layer of firebrick, clay, sand or other materials that coat the inside of a furnace or ladle.

Lost Foam Process — A casting process that uses foam to form the pattern. The foam is eventually melted out of the mold when the molten metal is poured in.

Metal Yield — The difference between the weight of a finished casting and the total weight of the metal poured.

Mold — The cavity that the molten metal is poured in to form the final shape. A mold usually consists of a top and bottom piece made of sand or ceramic material.

Net Weight of Casting — The final weight of a casting that is determined once all of the excess metal from the gating system has been removed.

Non-ferrous Metal — An alloy that doesn’t have iron as the predominant metal.

Oxidation Losses — The loss of metal or alloy through the process of oxidation.

Oxidizing Atmosphere — Furnace atmosphere which gives off oxygen under certain conditions or where there is an excess of oxygen in the product of combustion, or the products of combustion are oxidizing to the metal being heated.

Pattern — The wood, metal, foam or plastic replica of the final product to be made. Patterns usually include gating systems.

Pattern Draft — The taper allowed on the vertical faces of a pattern to enable removal of the mold or die.

Pigging — Pouring molten metal back into lined containers so that it can be returned to the furnace.

Rapid Prototyping — The computerized equipment that builds a three-dimensional model of a casting from a CAD drawing.

Reducing atmosphere — Furnace atmosphere which absorbs oxygen under suitable conditions or in which there is insufficient air to completely burn the fuel, or the product of combustion is reducing to the metal being heated.

Sand Casting — Producing metal castings out of sand molds.

Sand Reclamation — Processing used sand grains into usable forms so that they can be used in the casting process as a replacement for new sand.

Scrap — All non-product metal produced during the casting process.

Shell Molding — Bringing a resin-bonded sand mixture into contact with a pre-heated metal pattern to form a mold.

Shell Process — The process in which clay-free silica sand coated with a thermostatic resin or mixed with resin is placed on a heated metal pattern for a short period of time to form a partially hardened shell. The bulk of the sand mixture inside the resulting shell is removed for further use. The pattern and shell are heated further to harden or polymerize the resin-sand mix, and the shell is removed from the pattern. Frequently, shell cores are made using this process.

Shotblasting — A process for cleaning castings that involves using a metal abrasive that is propelled by centrifugal or air force.

Shrinkage — The reduction in the volume of metal that occurs as it solidifies.

Shrink Hole — A cavity that forms in a metal part when there was not enough source metal fed into the mold during the casting process.

Slag — A film that forms on top of molten metal as a result of impurities. Slag is composed of non-metal elements.

Slag Inclusions — Imperfections of the surface of metal caused by slag (impurities in the molten mix).

Slurry — The watery mixture such as the gypsum mixture for plaster molding, the molding medium used for investment casting, core dips, and mold washes.

Sprue — The opening in the mold where the metal is first poured.

V-process — A molding process, developed in Japan, in which the mold is formed by stretching a sheet of mylar plastic over a heated metal pattern so that it conforms to the shape of the pattern. A box of loose sand is placed over the pattern, and a vacuum is applied to the sand, which then conforms to the shape of the mylar film. Thus supported, the sand-backed film is removed from the pattern and is used as one part of mold. When the metal is poured, the vacuum is released, and the loose sand falls away from the casting.

Vacuum Casting — The process of casting that uses a vacuum to draw molten metal into a mold that is placed into it.





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METAL CASTING PROCESSES

Tuesday, 22 October 2013

Spark Ignition Engines

Internal combusiton engines are divided into spark ignition engines and com-
pression ignition engines. Almost all automobiles today use spark ignition
engines while trailers and some big trucks use compression ignition engines.
The main di erence between the two is the way in which the air to fuel mix-
ture is ignited, and the design of the chamber which leads to certain power
and e ciency characteristics.
Spark ignition engines use an air to fuel mixture that is compressed at high
pressures. At this high pressure the mixture has to be near stoichiometric
to be chemically inert and able to ignite. Stoichiometric means that there
is a one to one ratio between the air and fuel mixture. So the mixture in
order to ignite needs not to be either with too much fuel or too much air
but rather have an overall even amount. There are several components to
the spark ignition engine. Chamber design, mixture and the injection system
are some of the most important aspects of the spark ignition engine. The
importance of the chamber design will be discussed. The four basic designs
for combustion chambers are as follow:
--the distance travelled by the
ame front should be minimised
--the exhaust valve and spark plug should be close together
--there should be su cient turbulence 17
--the end gas should be in a cool part of the combustion chamber.




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INTERNAL COMBUSTION (IC) ENGINES

An IC engine is one in which the heat transfer to the working
fluid occurs within the engine itself, usually by the combustion
of fuel with the oxygen of air.
In external combustion engines heat is transferred to the
working fluid from the combustion gases via a heat
exchanger. e.g. steam engines; Stirling engines.
IC engines include spark ignition (SI) engines using
petrol as a fuel, and compression ignition (CI) engines
(usually referred to as Diesel engines) using fuel oil,
DERV, etc as a fuel.
In these engines there is a sequence of processes:
· compression
· combustion
· expansion
· exhaust / induction

There are two basic mechanical designs to achieve these four processes in either:-
The basic difference between the petrol engine and the diesel
engine is in the method of ignition and the combustion process,

four strokes of the piston - hence the 4-stroke engine, or
two strokes of the piston - hence 2-stroke engines.





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Physics basic inventions and inventors

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