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Metal casting processes Metal casting processes and equipmentand equipment

WECWEC

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Molding is the process of manufacturing by shaping pliable raw material using a rigid frame or model called a pattern.

A mold or mould is a hollowed-out block that is filled with a liquid like plastic, glass, metal, or ceramic raw materials. The liquid hardens or sets inside the mold, adopting its shape.

A mold is the opposite of a cast. 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.

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A foundry is a factory that produces metal castings from either ferrous or non-ferrous alloys.

Metals are turned into parts by melting them into a liquid, pouring the metal in a mold, and removing the mold material or casting after the metal has solidified as it cools.

The most common metal processed are aluminum and cast iron. However, other metals, such as steel, magnesium, copper, tin, and zinc, are also used to produce castings in foundries.

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1. Melting Melting is performed in a furnace. Virgin material, external

scrap, internal scrap, and alloying elements are used to charge the furnace.

Virgin material refers to commercially pure forms of the primary metal used to form a particular alloy.

Alloying elements are either pure forms of an alloying element, like electrolytic nickel, or alloys of limited composition, such as ferroalloys or master alloys.

External scrap is material from other forming processes such as punching, forging, or machining.

Internal scrap consists of the gates, risers, or defective castings.

The process includes melting the charge, refining the melt, adjusting the melt chemistry and tapping into a transport vessel. Refining is done to remove deleterious gases and elements from the molten metal. Material is added during the melting process to bring the final chemistry within a specific range specified by industry and/or internal standards.

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2. Furnace

Furnaces are refractory lined vessels that contain the material to be melted and provide the energy to melt it.

Furnaces in foundries can be any size, ranging from mere ounces to hundreds of tons, and they are designed according to the type of metals that are to be melted.

Also, furnaces must be designed around the fuel being used to produce the desired temperature.

For low temperature melting point alloys, such as zinc or tin, melting furnaces may reach around 327 Celsius. Electricity, propane, or natural gas are usually used for these temperatures.

For high melting point alloys such as steel or nickel based alloys, the furnace must be designed for temperatures over 3600 Celsius. The fuel used to reach these high temperatures can be electricity or coke.

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3. Mold making

Prior to pouring a casting, the foundry produces a mold. The molds are constructed by several different processes dependent upon the type of foundry, metal to be poured, quantity of parts to be produced, size of the casting and complexity of the casting.

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4. Pouring

In a foundry, molten metal is poured into molds. Pouring can be accomplished with gravity, or it may be assisted with a vacuum or pressurized gas.

Many modern foundries use robots or automatic pouring machines for pouring molten metal.

Traditionally, molds were poured by hand using ladles.

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5. Shakeout

The solidified metal component is then removed from its mold. Where the mold is sand based, this can be done by shaking or tumbling.

This frees the casting from the sand, which is still attached to the metal runners and gates - which are the channels through which the molten metal traveled to reach the component itself.

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6. Degating

Degating is the removal of the heads, runners, gates, and risers from the casting. Runners, gates, and risers may be removed using cutting torches, band saws or ceramic cutoff blades.

For some metal types, and with some gating system designs, the sprue, runners and gates can be removed by breaking them away from the casting with a hammer or specially designed knockout machinery.

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7. Surface cleaning

After degating, sand or other molding media may adhere to the casting. To remove this the surface is cleaned using a blasting process. This means a granular media will be propelled against the surface of the casting to mechanically knock away the adhering sand.

The media may be blown with compressed air, or may be hurled using a shot wheel. The media strikes the casting surface at high velocity to remove the molding media (for example, sand, slag) from the casting surface.

Numerous materials may be used as media, including steel, iron, other metal alloys, aluminum oxides, glass beads, walnut shells, baking powder or numerous other materials.

The blasting media is selected to develop the color and reflectance of the cast surface.

Terms used to describe this process include cleaning, blasting, shotblasting and sand blasting of castings.

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8. Finishing

The final step in the process usually involves grinding, sanding, or machining the component in order to achieve the desired dimensional accuracies, physical shape and surface finish.

Removing the remaining gate material, called a gate stub, is usually done using a grinder or sanding. These processes are used because their material removal rates are slow enough to control the amount of material. These steps are done prior to any final machining.

After grinding, any surfaces that require tight dimensional control are machined.

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Definition

A casting may be defined as a "metal object obtained by allowing molten metal to solidify in a mold", the shape of the object conforms to the shape of the mould cavity.”

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The most complicated of shapes, both external and internal, may be cast. As a result, many other operations, such as machining, forging, and welding, can be minimized or eliminated.

Because of their physical properties, some metals can only be cast to shape since they cannot be hot-worked into bars, rods, plates, or other shapes from ingot form as a preliminary to other processing.

Construction may be simplified. Objects may be cast in a single piece which would otherwise require assembly of several pieces if made by other methods.

Metal casting is a process highly adaptable to the requirements of mass production. Large numbers of a given casting may be produced very rapidly. For example, in the automotive industry hundreds of thousands of cast engine blocks and transmission cases are produced each year.

Extremely large, heavy metal objects may be cast when they would be difficult or economically impossible to produce otherwise. Large pump housing, valves, and hydroelectric plant parts weighing up to 200 tons illustrate this advantage of the casting process.

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A sand casting or a sand molded casting is a cast part produced by forming a mold from a sand mixture and pouring molten liquid metal into the cavity in the mold. The mold is then cooled until the metal has solidified. In the last stage the casting is separated from the mold.

There are six steps in this process:

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There are two main types of sand used for molding.

"Green sand" is a mixture of silica sand, clay, moisture and other additives.

The "air set" method uses “Dry Sand” bonded to materials other than clay, using a fast curing adhesive. When these are used, they are collectively called "air set" sand castings to distinguish these from "green sand" castings.

Two types of molding sand are natural bonded (bank sand) and synthetic (lake sand), which is generally preferred due to its more consistent composition.

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Die casting is the process of forcing molten metal under high pressure into mold cavities (which are machined into dies).

Most die castings are made from non-ferrous metals, specifically zinc, copper, aluminium, magnesium, lead, and tin based alloys, although ferrous metal die castings are possible.

The die casting method is especially suited for applications where a large quantity of small to medium sized parts are needed with good detail, a fine surface quality and dimensional consistency.

This level of versatility has placed die castings among the highest volume products made in the metalworking industry.

In recent years, injection-molded plastic parts have replaced some die castings because they are cheaper and lighter. Plastic parts are a practical alternative if hardness is not required and little strength is needed

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When no porosity is required for a casting then the pore-free casting process is used.

It is identical to the standard process except oxygen is injected into the die before each shot.

This causes small dispersed oxides to form when the molten metal fills the dies, which virtually eliminates gas porosity.

An added advantage to this is greater strength. These castings can still be heat treated and

welded. This process can be performed on aluminium, zinc, and lead alloys.

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In centrifugal casting, a permanent mold is rotated about its axis at high speeds (300 to 3000 rpm) as the molten metal is poured.

The molten metal is centrifugally thrown towards the inside mold wall, where it solidifies after cooling. The casting is usually a fine grain casting with a very fine-grained outer diameter, which is resistant to atmospheric corrosion, a typical situation with pipes. The inside diameter has more impurities and inclusions, which can be machined away.

Centrifugal casting as a category includes Centrifugal Casting, Semi-Centrifugal Casting and Centrifuging.

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Only cylindrical shapes can be produced with this process. Size limits are upto 3 m (10 feet) diameter and 15 m (50 feet) length. Wall thickness can be 2.5 mm to 125 mm (0.1 - 5.0 in).

The tolerances that can be held on the OD can be as good as 2.5 mm (0.1 in) and on the ID can be 3.8 mm (0.15 in). The surface finish ranges from 2.5 mm to 12.5 mm (0.1 - 0.5 in) rms.

Typical materials that can be cast with this process are iron, steel, stainless steels, and alloys of aluminum, copper and nickel. Two materials can be cast by introducing a second material during the process. Typical parts made by this process are pipes, boilers, pressure vessels, flywheels, cylinder liners and other parts that are axi-symmetric

Examples: Pipes for water, gas and sewerage bearing bushes, cylinder liners, piston rings, paper making rollers, clutch plates, pullies

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The molds used can be permanent or expendable, can be stacked as necessary. The rotational speeds are lower than those used in centrifugal casting.

This process is used for making wheels, nozzles and similar parts where the axis of the part is removed by successive machining.

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Centrifuging is used for forcing metal from a central axis of the equipment into individual mold cavities that are placed on the circumference.

This provides a means of increasing the filling pressure within each mold and allows for reproduction of intricate details.

This method is often used for the pouring of investment casting pattern.

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It is another variation of permanent mold process where vacuum is used to draw material into the mold cavity

All of the benefits and features of the low pressure process are retained, including the sub-surface extraction on molten metal from the pressure melt, the bottom feed to the mold, the minimal metal disturbance during pouring, the self risering action, and the downward directional solidification.

Thin walled castings can be produced with high metal yield and excellent surface quality. Because of the vacuum, the cleanliness of the metal and dissolved gas content are superior even to that of the low-pressure-process.

Final castings typically range from 0.4 to 10lbs.

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A variety of blow molding processes have been developed, the most common being the used to convert thermoplastic polyethylene, polyvinyl chloride (PVC), polypropylene, and PEEK resins into bottles and either hollow-shape containers.

A round solid-bottom, hollow tube, known as preform or parison, is made from heated plastic by either extrusion or injection molding.

The pre-form is them positioned between halves of the split mold, the mold closes, and the pre-form is expanded against them old by air or gas pressure.

The mold is then cooled, the halves separated, and the product is removed. Any flash is them removed for direct recycling.

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In this process, thermoplastic sheet material is heated to a working temperature and then formed into a finished shape by heat, pressure or vacuum. The starting material can be either discrete sheets or a continuous roll of material.

If continuous material is used, it is usually heated by passing through an oven or other heating device.

The materials emerges between the upper and lower halves of a mold and is formed either up or down by the application of vacuum or pressure.

Cooling occurs upon contact with the mold, and the product hardens in its new shape.

Ejection and trimming follow to complete the process, and the remaining strip material is rewound in preparation of recycling.

Products range from panels for light fixtures to pages for Braille texts for blinds.

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The pattern, almost duplicate of the real part, is to form cavity in the sand.

The pattern is typically made out of wood, sometimes metal or plastic.

The cavity is contained in an aggregate housed in a box called the flask.

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Core is a sand shape inserted into the mold to produce the internal features of the part such as holes or internal passages. Cores are placed in the cavity to form holes of the desired shapes.

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Core print is the region added to the pattern, core, or mold that is used to locate and support the core within the mold.

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A riser is an extra void created in the mold to contain excessive molten material. The purpose of this is feed the molten metal to the mold cavity as the molten metal solidifies and shrinks, and thereby prevents voids in the main casting

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The molten material is poured in the pouring cup, which is part of the gating system that supplies the molten material to the mold cavity. The vertical part of the gating system connected to the pouring cup is the sprue, and the horizontal portion is called the runners and finally to the multiple points where it is introduced to the mold cavity called the gates. Additionally there are extensions to the gating system called vents that provide the path for the built up gases and the displaced air to vent to the atmosphere.

The cavity is usually made oversize to allow for the metal contraction as it cools down to room temperature. This is achieved by making the pattern oversize. To account for shrinking, the pattern must be made oversize by these factors, on the average. These are linear factors and apply in each direction. These shrinkage allowance are only approximate, because the exact allowance is determined the shape and size of the casting. In addition, different parts of the casting might require a different shrinkage allowance.

Sand castings generally have a rough surface sometimes with surface impurities, and surface variations. A machining (finish) allowance is made for this type of defect.

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A full‑sized model of the part, slightly enlarged to account for shrinkage in cooling and solidification, and provide enough metal for the subsequence machining operation(s)

TYPES W.R.T. USAGE

1. Permanent Pattern Sand casting Shell molding

2. Expandable pattern. Investment Casting Expanded polystyrene process (full-mold) Permanent mold casting process

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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

Plaster of paris Wax –precision casting

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(a) solid pattern ( single piece)(b) split pattern ( Two piece)(c) match‑plate pattern(d) cope and drag pattern (e) Sweep pattern(f) Skeleton pattern

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A core is a device used in casting and molding processes to produce internal cavities and reentrant angles. The core is normally a disposable item that is destroyed to get it out of the piece.They are most commonly used in sand casting, but are also used in injection molding.

A core consists of two portions: the body of the core and one or more extensions (called prints)

Cores are used to create internal cavities. Core is a separate entity placed in a mould to produce a

corresponding cavity – hole or undercut – in the casting Cores for sand casting are manufactured by packing

specially prepared sand in core boxes Chaplets- These are small metal supports that bridge the

gap between the mold surface and the core, but because of this become part of the casting.

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A furnace is a device used for heating.The term furnace can also refer to a

direct fired heater, used in boiler applications in chemical industries or for providing heat to chemical reactions for processes like cracking, and is part of the standard English names for many metallurgical furnaces worldwide.

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Several specialised furnaces are used to melt the metal. Furnaces are refractory lined vessels that contain the material to be melted and provide the energy to melt it. Modern furnace types include electric arc furnaces (EAF), induction furnaces, cupolas, reverberatory, and crucible furnaces.

Furnace choice is dependent on the alloy system and quantities produced. For ferrous materials, EAFs, cupolas, and induction furnaces are commonly used. Reverberatory and crucible furnaces are common for producing aluminum castings.

Furnace design is a complex process, and the design can be optimized based on multiple factors.

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Furnaces in foundries can be any size, ranging from mere ounces to hundreds of tons, and they are designed according to the type of metals that are to be melted. Also, furnaces must be designed around the fuel being used to produce the desired temperature. For low temperature melting point alloys, such as zinc or tin, melting furnaces may reach around 327 Celsius. Electricity, propane, or natural gas are usually used for these temperatures. For high melting point alloys such as steel or nickel based alloys, the furnace must be designed for temperatures over 3600 Celsius. The fuel used to reach these high temperatures can be electricity or coke.

The majority of foundries specialize in a particular metal and have furnaces dedicated to these metals. For example, an iron foundry (for cast iron) may use a cupola, induction furnace, or EAF, while a steel foundry will use an EAF or induction furnace. Bronze or brass foundries use crucible furnaces or induction furnaces. Most aluminum foundries use either an electric resistance or gas heated crucible furnaces or reverberatory furnaces.

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A crucible is a heat-resistant container in which materials can be heated to very high temperatures.

Crucibles are available in several sizes and typically come with a correspondingly-sized crucible cover (or lid).

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Crucibles and their covers are made of high temperature-resistant materials, usually porcelain or an inert metal. One of the earliest uses of platinum was to make crucibles. Ceramics such as alumina, zirconia, and especially magnesia will tolerate the highest temperatures. More recently, metals such as nickel and zirconium have been used. The lids are typically loose-fitting to allow gases to escape during heating of a sample inside. Crucibles and their lids can come in high form and low form shapes and in various sizes, but rather small 10–15 ml size porcelain crucibles are commonly used for gravimetric chemical analysis. These small size crucibles and their covers made of porcelain are quite cheap when sold in quantity to laboratories, and the crucibles are sometimes disposed of after use in precise quantitative chemical analysis. There is usually a large mark-up when they are sold individually in hobby shops.

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