Sand casting of metals - Gating system for sand casting mould
Post on 10-Apr-2017
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Sand Casting of Metals
Prof. Ms. Amruta A. Rane
DJSCE, University of Mumbai,
Sand Casting of Metals
Topics covered: Sand Casting of Metals
Designing and manufacturing of gating system
The gating system refers to all those elements which are concerned with the flow
of molten metal from ladle to the Mould Basin cavity.
The various elements that comes under gating system are:
o Pouring basin/cup
o Sprue base well
o Runner extension
Requirements / function of the gating systemA gating system should,
Fill the mold cavity completely before freezing.
Introduce the liquid metal into the mold cavity with low viscosity and littleturbulence, so that mold erosion, metal oxidation and gas pick up is prevented.
Help to promote temperature gradient favorable for proper directionalsolidification.
Incorporate traps for separation of nonmetallicminclusions which are eitherintroduced with the molten metal or are present in the gating system.
The molten metal is poured into the a pouringbasin which acts as a reservoir from which itmoves smoothly into the sprue.
The pouring basin may be cut into the copeportion directly or a separate dry sandpouring basin may be prepared and used.
The molten metal in the pouring basin shouldbe full during the pouring operation to avoidthe atmospheric air and slag from enteringinto mold cavity.
The molten metal is not directly poured intothe mold cavity because it may cause themold erosion.
The pouring basin also stops the slag fromentering the mold cavity by means ofskimmer or skim core. It holds back the slagor dirt which floats on the top and allows onlyclean metal underneath it into the sprue. 6
A strainer core: It is a strainer or screen with many small holes.
It is utilized to maintain the constant conditions of flow.
The strainer restricts the flow of molten metal into the sprue, thus helps in quickfilling of the pouring basin and restricts the flow of slag into the mold.
It allows only clean metal to enter into the sprue.
Sprue Sprue is the channel through which the molten metal is brought to the parting
plane where it enters the runners and gates to ultimately reach the mold cavity.
If the sprue is straight and cylindrical, then metal flow would not be full atbottom, but some low pressure area would be created around the metal in thesprue. Atmospheric air would be breathed into this low pressure area whichwould be then carried to the mold cavity.
To eliminate this problem tapered sprue is used.
Sprue Base Well
This is reservoir of the metal at the bottom of the sprue to reduce themomentum of the molten metal.
The molten metal gains velocity while moving down the sprue, some of which islost in the sprue base well by which the mold erosion is reduced.
Runner It is located in parting plane and connects the sprue to the in-gates.
The runners are normally made trapezoidal in cross-section.
The slag trapping takes place in the runner, when runner flows full. If the amountof molten metal coming from sprue base is more than the amount flowingthrough the in-gates.
A partially filled runner causes slag to enter the mold cavity.
Runner While designing the runner system, care should be taken to reduce the sharp
corners or sudden change of sections.
From heat-loss factor circular cross-section runners are preferable.
Also runner is generally cut in cope and in-gate in drag to trap the slag.
It is also good practice to have half of the runner in the cope and the rest in thedrag which effectively reduces the slag inclusion.
The runner is extended little further after it encounters the in-gate.
This extension is provided to trap the slag in the molten metal.
Gates or In-gates
These are openings through which moltenmetal enters the mold cavity.
In this type of gate metal enters the cavityfrom top.
Cavity is filled very quickly. Therefore, topgates are not advisable for those materialswhich are likely to form dross (turbulence,waste, slag, etc.).
This type of gate is used when the moltenmetal enters the mold cavity from bottomof the cavity.
It takes more time to fill the mold.
Gates or In-gatesBottom Gates
Gates or In-gates
The metal enters the mold at theparting plane when a part of thecasting is in the cope and a part ofthe casting is in drag.
They are used for heavy and largecastings.
The molten metal enters moldcavity through a number of in-gates, which are arranged invertical steps.
The size of in-gates is normallyincreased from top to bottom.
This ensures the gradual filling ofthe mold without mold erosionand produces sound casting.
Gates or In-gates
The gates are generally made wider comparing to depth, up to a ration of 1:4.
The gates should be placed near the core-print or chill
Riser Most of the foundry metals and alloys shrink during solidification, as a result of
volumetric shrinkage, the voids are likely to form in the casting.
Additional molten metal is fed into these places which is termed as hot spotssince it remains hot till the end.
Hence, a reservoir of molten metal is maintained from which the metal can flowreadily into the mold cavity when the need arises, this reservoir is called riser.
Different materials have different shrinkages hence the risering requirementsvary for the materials.
e.g. Grey cast iron sometimes may have negative shrinkage. This happensbecause with higher carbon and silicon contents, graphitization occurs whichincreases the volume and counteracts the metal shrinkage. Thus risering may bvery critical in such situations.
For metals like aluminium and steel, the volumetric shrinkage being very high,elaborate risering is required.
The solidified metal in the riser is cutoff from the main casting and melted for reuse.
The higher the riser volume, the lower is the casting yield.
The requirement of the riser depends on the type of metal poured and thecomplexity of the casting.
Riser During solidification metal experience shrinkage which results in void formation
creation of hot spots.
This can be avoided by feeding hot spot during solidification.
Riser are used to feed casting during solidification.
Riser must solidify after casting.
Riser should be located so that directional solidification occurs from the extremities of mold cavity back toward the riser.
Thickest part of castinglast to freeze, riser should feed directly to these regions.
RiserThe shrinkage occurs in three stages,
1. When temperature of liquid metal drops from pouring to freezing temperature.
2. When the metal changes from liquid to solid state, and
3. When the temperature of solid phase drops from freezing to room temperature.
The shrinkage for stage 3 is compensated by providing shrinkage allowance on pattern,
while the shrinkage during stages 1 and 2 are compensated by providing risers.
RiserTypes of Risers1. Top Risers: They are open to atmosphere. They Tare most conventional and
convenient to make.
2. Blind Riser: are completely concealed inside the mold cavity. It loses heat slowlysince it is surrounded by the molding sand and thus would be more effective.
3. Internal Risers: They are enclosed on all sides by the casting. They are normallyused for the castings which are cylindrical in shape or have hollow portions.
Chills The chills are used to provide progressive solidification and to avoid the
Chills are large heat sinks.
When the geometry of the molding cavity prevents solidification from occurringnaturally, a chill can be strategically placed to help promote it.
Whenever the thickness of the walls of the casting is unequal, the chills areplaced close to the wall with larger thickness, so that the heat is quickly absorbedby the chill from larger mass making the cooling rate equal to that of thinsections.
It does not permit the formation of shrinkage cavity.
Materials of Chills Chills can be made of many materials, including iron, copper, bronze, aluminium,
graphite and silicon carbide.
Sand materials with higher densities, thermal conductivity or thermal capacitycan also be used as a chill.
For example, chromite sand or zircon sand can be used when moldingwith silica sand.
Types of ChillsInternal chills:
They are pieces of metal that are placed inside the molding cavity.
When the casting cavity is filled, part of the chill will melt and ultimately becomepart of the casting, thus the chill must be made of the same material as that ofthe casting.
They are masses of material that have a high heat capacity and thermalconductivity.
They are placed on the edge of the molding cavity and effectively become part ofthe wall of the molding cavity.