Showing posts with label Green Sand. Show all posts
Showing posts with label Green Sand. Show all posts

Friday 1 November 2013

Types of Molding sand in Casting Process

Types of molding sand:

 
types of molding sand in casting process

  Green sand:

                                 Natural sand with moisture

 Dry sand:

                                Not suitable for large castings

Facing sand:

This sand is used directly next to the surface of the pattern and comes into contact with the molten metal when the mould is poured.
As a result, it is subjected to the severest conditions and must possess, therefore, high strength and refractoriness. This sand also provides a smoother casting surface and should be of fine texture. It is made of silica sand and clay, and some additives without the addition of used sand.
Facing sand is always used to make dry sand moulds while system sand is frequently used for green sand molding.

Parting sand:

This sand is used to prevent adhering of two halves of mould surfaces in each molding box when they are separated. Thus, to ensure good parting, the mould surface (at contact of cope and Drag) should be treated with parting sand or some other parting material.
It is also sprinkled or applied on the pattern surface (before the molding sand is put over it) to avoid its sticking and permit its easy withdrawal from the mould. The parting sand is fine dry sand.

Backing or floor or black sand:

This is the sand which is used to back up the facing sand and to fill the whole volume of the flask. Old, repeatedly used molding sand is mainly employed for this purpose.             

  Core sand:

                                The core sand mainly consists of silica sand and an organic binder, with very little, if any, clay content. The presence of clay in core sand reduces its permeability and collapsibility. The core sand may contain small percentages of other constituents also, to enhance its properties.            

 Loam sand:

                                50 % of clay and dried hard and using for large castings
 
 
 

Tuesday 29 October 2013

Green Sand Molding

The most common method used to make metal castings is green sand molding. In this process, granular refractory sand is coated with a mixture of bentonite clay, water and, in some cases, other additives. The additives help to harden and hold the mold shape to withstand the pressures of the molten metal.
The green sand mixture is compacted through mechanical force or by hand around a pattern to create a mold. The mechanical force can be induced by slinging, jolting, squeezing or by impact/impulse.
The following points should be taken into account when considering the green sand molding process:
  • for many metal applications, green sand processes are the most cost-effective of all metal forming operations;
  • these processes readily lend themselves to automated systems for high-volume work as well as short runs and prototype work;
  • in the case of slinging, manual jolt or squeeze molding to form the mold, wood or plastic pattern materials can be used. High-pressure, high-density molding methods almost always require metal pattern equipment;
  • high-pressure, high-density molding normally produces a well-compacted mold, which yields better surface finishes, casting dimensions and tolerances;
  • the properties of green sand are adjustable within a wide range, making it possible to use this process with all types of green sand molding equipment and for a majority of alloys poured.



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Unbonded Sand Processes

Unlike the sand casting processes that use various binders to hold the sand grains together, two unique processes use unbonded sand as the molding media. These include the lost foam process and the less common V-process.
Lost Foam Casting—In this process, the pattern is made of expendable polystyrene (EPS) beads. For high-production runs, the patterns can be made by injecting EPS beads into a die and bonding them together using a heat source—usually steam. For shorter runs, pattern shapes are cut from sheets of EPS using conventional woodworking equipment and then assembled with glue. In either case, internal passageways in the casting, if needed, are not formed by conventional sand cores but are part of the mold itself.
The polystyrene pattern is coated with a refractory coating, which covers both the external and internal surfaces. With the gating and risering system attached to the pattern, the assembly is suspended in a one-piece flask, which then is placed onto a compaction or vibrating table. As the dry, unbonded sand is poured into the flask and pattern, the compaction and vibratory forces cause the sand to flow and densify. The sand flows around the pattern and into the internal passageways of the pattern.
As the molten metal is poured into the mold, it replaces the EPS pattern, which vaporizes. After the casting solidifies, the unbonded sand is dumped out of the flask, leaving the casting with an attached gating system.
With larger castings, the coated pattern is covered with a facing of chemically bonded sand. The facing sand is then backed up with more chemically bonded sand.
The lost foam process offers the following advantages:
  • no size limitations for castings;
  • improved surface finish of castings due to the pattern’s refractory coating;
  • no fins around coreprints or parting lines;
  • in most cases, separate cores are not needed;
  • excellent dimensional tolerances.
V-process—In the V-process, the cope and drag halves of the mold are formed separately by heating a thin plastic film to its deformation point. It then is vacuum-formed over a pattern on a hollow carrier plate.
The process uses dry, free-flowing, unbonded sand to fill the special flask set over the film-coated pattern. Slight vibration compacts the fine grain sand to its maximum bulk density. The flask is then covered with a second sheet of plastic film. The vacuum is drawn on the flask, and the sand between the two plastic sheets becomes rigid.
The cope and drag then are assembled to form a plastic-lined mold cavity. Sand hardness is maintained by holding the vacuum within the mold halves at 300-600 mm/Hg. As molten metal is poured into the mold, the plastic film melts and is replaced immediately by the metal. After the metal solidifies and cools, the vacuum is released and the sand falls away.

 


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Wednesday 23 October 2013

Pattern Types

The Pattern

A full-sized model of the part, slightly enlarged to
account for shrinkage and machining allowances in
the casting

Pattern materials:

Wood - common material because it is easy to work, but it warps
Metal - more expensive to make, but lasts much longer
Plastic - compromise between wood and metal




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Sand Casting Process



Steps in Sand Casting

1.Pour molten metal into sand mold
2. Allow metal to solidify
3. Break up the mold to remove casting
4. Clean and inspect casting
5. Heat treatment of casting is sometimes required to improve metallurgical properties
 

Making the Sand Mold
 
1.The cavity in the sand mold is formed by packing sand around a pattern, then     
  separating the mold into two halves and removing the pattern
2.The mold must also contain gating and riser system
3.If casting is to have internal surfaces, a core must be included in mold
4.A new sand mold must be made for each part






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Casting Patterns QUIZ






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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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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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