MANUFACTURING PROCESSES

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MOULD, CORE AND GATING SYSTEM

The Mold in Casting

• The Mold contains a cavity whose geometry determines the shape of the cast part.

• Mold is a hollow container(cavity) used to give shape to molten or hot liquid material (such as wax or metal) when it cools and hardens.– Actual size and shape of cavity must be slightly

oversized to allow for shrinkage of metal during solidification and cooling

– Molds are made of a variety of materials, including sand, plaster, ceramic, and metal

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Open Molds and Closed Molds

Two forms of mold: (a) open mold, simply a container in the shape of the desired part; and (b) closed mold, in which the mold geometry is more complex and requires a gating system (passageway) leading into the cavity. 3

Sand Casting Mold (Closed)

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Forming the Mold Cavity

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Forming the Mold Cavity• Mold cavity is formed by packing sand around a

pattern, which has the shape of the part• When the pattern is removed, the remaining cavity

of the packed sand has desired shape of cast part• The pattern is usually oversized to allow for

shrinkage of metal during solidification and cooling • Sand for the mold is moist and contains a binder to

maintain its shape

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Desirable Mold Properties

• The sand used to generally make molds is required to meet four primary requirements –

• Refractoriness – ability to withstand high temperature,

• Cohesiveness – ability to retain a given shape when packed into the mold,

• Permeability – allow hot air and gases to pass through voids in sand,

• Collapsibility – ability to accommodate metal shrinkage after solidification and free the casting by disintegration.

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Use of a Core in the Mold Cavity

• The mold cavity provides the external surfaces of the cast part

• In addition, a casting may have internal surfaces, determined by a core, placed inside the mold cavity to define the interior geometry of part

• In sand casting, cores are generally made of sand

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Core

• Core is a Full scale model of interior surfaces of ‑the part .

• It is inserted into the mold cavity prior to pouring• The molten metal flows and solidifies between

the mold cavity and the core to form the casting's internal surfaces

• May require supports to hold it in position in the mold cavity during pouring, called chaplets .

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Core

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(a) Core held in place in the mold cavity by chaplets, (b) possible chaplet design, (c) casting with internal cavity.

Chaplets

• Chaplets are the supports provided to hold the core in its position in the mold cavity during pouring.

• Because the chaplets are positioned within the mold cavity, they become an integral part of the finished casting.

• Chaplets should therefore be of the same, or at least comparable, composition as the material being poured.

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Chaplets

• They should be large enough that they do not completely melt and permit the core to move,

• Since chaplets are one more source of possible defects and may become a location of weakness in the finished casting, efforts are generally made to minimize their use.

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Core Print

• Where coring is required, provision should be made to support the core inside the mold cavity.

• Core prints are used to serve this purpose. • The core print is an added projection on the

pattern and it forms a seat in the mold on which the sand core rests during pouring of the mold.

• The core print must be of adequate size and shape so that it can support the weight of the core during the casting operation.

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Core Print• Depending upon the requirement a core can be placed

horizontal, vertical and can be hanged inside the mold cavity.

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Core PrintCoreCore Print

Gating System

• The gating system in a casting mold is the channel, or network of channels, through which molten metal flows into cavity from outside of mold.

• The pouring cup(or pouring basin) is the portion of the gating system that receives the molten metal from the pouring vessel and delivers it to the rest of the mold.

• Pouring cup is often used to minimize splash and turbulence as the metal flows into downsprue.

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Gating System

• From the pouring cup, the metal travels down a downsprue also called simply the sprue (the vertical portion of the gating system),

• Then along horizontal channels, called runners , and finally through controlled entrances, or gates, into the mold cavity.

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Gating System

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Gating System

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Sprue well

Runner well Gate

Sprue

Pouring Basin

Runner

Gating System

• The gates are usually attached to the1. thickest or heaviest sections of a casting to control

SHRINKAGE 2. to the bottom of the casting to minimize

TURBULENCE AND SPLASHING. • For large castings, multiple gates and runners

may be used to introduce metal to more than one point of the mold cavity.

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Gating System

• Turbulent flow is generated while pouring the molten metal into the mold which causes the following problems:– absorption of gases,– oxidation of the metal, and – erosion of the mold.

• Therefore gating systems should be designed to minimize turbulent flow.

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Gating System

• Short sprues are desirable, since they minimize the distance that the metal must fall when entering the mold.

• Rectangular pouring cups prevent the formation of a vortex or spiraling funnel, which tends to suck gas and oxides into the sprue.

• Tapered sprues also pre-vent vortex formation.

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Gating System

• A large sprue well can be used to dissipate the kinetic energy of the falling stream and prevent splashing and turbulence as the metal makes the turn into the runner.

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Gating System

• The choke , or smallest cross-sectional area in the gating system, serves to control the rate of metal flow. If the choke is located:– near the base of the sprue, flow through the runners

and gates is slowed and flow is rather smooth.– at the gates, the metal might enter the mold cavity

with a fountain effect, an extremely turbulent mode of flow, but the small connecting area would enable easier separation of the casting and gating system.

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Gating System

• Gating systems can also be designed to trap dross (slag) and sand particles and keep them from entering the mold cavity.

• Screens or ceramic filters of various shapes, sizes, and materials can also be inserted into the gating system to trap foreign material.

• Wire mesh can often be used with the nonferrous metals, but ceramic materials are generally required for irons and steel.

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Sprue well Runner well

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Sprue well Runner well

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Sprue well Runner well

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Sprue well Runner well

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Sprue well Runner well

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Sprue well Runner well

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Sprue well Runner well

Riser

• A riser is an additional void in the mold that also fills with molten metal.

• Riser is a reservoir of additional molten metal that can flow into the mold to compensate for shrinkage of the part during solidification .

• The riser must be designed to freeze after the main casting in order to satisfy its function.

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Riser

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Riser

• Live risers (also known as hot risers) receive the last hot metal that enters the mold and generally do so at a time when the metal in the mold cavity has already begun to cool and solidify.

• Thus, they can be smaller than dead (or cold) risers, which fill with metal that has already flowed through the mold cavity.

• As shown in Figure, top risers are almost always dead risers.

• Risers that are part of the gating system generally live risers.

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Riser

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Dead

Live

Dead

Riser

• Open risers have the danger of solidifying first, therefore they must be sized properly (larger) for proper function.

• An open riser helps exhaust gases from the mold during pouring, and can thereby eliminate some associated defects.

• A blind riser that is not open to the atmosphere may cause pockets of air to be trapped, or increased dissolution of air into the metal, leading to defects in the cast part.

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Problems with Too Large Riser

• The material in the riser is eventually scrapped and has to be recycled; the riser has to be cut off, and a larger riser will cost more to machine.

• An excessively large riser slows solidification.• The riser may interfere with solidification

elsewhere in the casting.• The extra metal may cause buoyancy forces

sufficient to separate the mold halves, unless they are properly weighted or clamped

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Problems with Too Small Riser

• The drawbacks to having too small riser are mainly associated with defects in the casting, either due to insufficient feeding of liquid to compensate for solidification shrinkage.

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Riser Location

• A riser should be located in such a way that directional solidification is obtained.

• Since the heaviest section of the casting solidifies last, the riser should be located to feed this section.

• The heaviest section will now act as a riser for other sections which are not so heavy or thick.

• For small castings, a single riser can feed the entire casting, but more than one riser is required for large castings.

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Vent holes

• As the metal flows into the mould, the air that previously occupied the cavity, as well as hot gases formed by reactions of the molten metal, must be evacuated so that the metal will completely fill the empty space.

• In sand casting, for example, the natural porosity of the sand mould permits the air and gases to escape through the walls of the cavity.

• In permanent-metal mould, small vent holes are drilled into the mould or machined into the parting line to permit removal of air and gases.

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