gating system :- casting processes

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MANUFACTURING PROCESSES - II CASTING PRACTICES: GATING SYSTEM DESIGN

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Page 1: Gating system :- CASTING PROCESSES

MANUFACTURING PROCESSES - II

CASTING PRACTICES:GATING SYSTEM DESIGN

Page 2: Gating system :- CASTING PROCESSES

THE GATING SYSTEM:-Topics covered:

• Designing and manufacturing of gating system

• Pouring basin

• Sprue

• Runners

• In-gates

• Riser

Page 3: Gating system :- CASTING PROCESSES

MECHANICAL ENGINEERING

NAME ENROLLMENT NO.

PATEL JAIMIN A. 150280119080

PATEL MEET B. 150280119083

4TH SEMESTERB1 BATCH

GUIDED BY:- PROF. V. D. SONARA

Page 4: Gating system :- CASTING PROCESSES

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

o Sprue base well

o Runner

o Runner extension

o In-gate

o Riser

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Page 5: Gating system :- CASTING PROCESSES

Requirements / function of the gating system

A gating system should,

• Fill the mold cavity completely before freezing.

• Introduce the liquid metal into the mold cavity with low viscosity and little turbulence, so that mold erosion, metal oxidation and gas pick up is prevented.

• Help to promote temperature gradient favorable forproper directional solidification.

• Incorporate traps for separation of nonmetallicinclusions which are either introduced with the molten metal or are present in the gating system.

Page 6: Gating system :- CASTING PROCESSES

Pouring Basin

• 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

Page 7: Gating system :- CASTING PROCESSES

Pouring Basin

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 quick filling of the pouring basin and restricts the flow of slag into the mold.

• It allows only clean metal to enter into the sprue.

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Page 8: Gating system :- CASTING PROCESSES

Pouring Basin/Cup

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Page 9: Gating system :- CASTING PROCESSES

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.

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Page 10: Gating system :- CASTING PROCESSES

Sprue Base Well

• This is reservoir of the metal at the bottom of the sprue to reduce the momentum 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.

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Page 11: Gating system :- CASTING PROCESSES

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 amount of molten metal coming from sprue base is more thanthe amount flowing through the in-gates.

• A partially filled runner causes slag to enter the mold cavity.

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Page 12: Gating system :- CASTING PROCESSES

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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 runnerin the cope and the rest in thedrag which effectively reduces the slag inclusion.

Page 13: Gating system :- CASTING PROCESSES

Runner Extension

• The runner is extended little further after it encounters the in-gate.

• This extension is provided to trap the slag in the molten metal.

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Page 14: Gating system :- CASTING PROCESSES

Gates or In-gates

• These are openings through which moltenmetal enters the mold cavity.

Top Gate:

• 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.).

Bottom Gate:

• 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.

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Page 15: Gating system :- CASTING PROCESSES

Gates or In-gatesBottom Gates

Page 16: Gating system :- CASTING PROCESSES

Gates or In-gates

Parting Gate:

• 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.

Step Gate:

• 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.

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Page 17: Gating system :- CASTING PROCESSES

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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 spots

since it remains hot till the end.• Hence, a reservoir of molten metal is maintained from which the metal can flow

readily into the mold cavity when the need arises, this reservoir is called riser.

• Different materials have different shrinkages hence the risering requirements vary 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 cut off from the main casting and melted forreuse.

• The higher the riser volume, the lower is the casting yield.

• The requirement of the riser depends on the type of metal poured and the complexity of the casting.

Page 18: Gating system :- CASTING PROCESSES

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 theextremities of mold cavity back toward the riser.

• Thickest part of casting–last to freeze, riser should feed directly to these regions.

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Page 19: Gating system :- CASTING PROCESSES

RiserTypes of Risers

1. 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 slowly since 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 normally used for the castings which are cylindrical in shape or have hollow portions.

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Page 20: Gating system :- CASTING PROCESSES

Defects in Sand CastingDefects occurring due to improper design of Gating System:

1. Oxidation of metal

2. Inclusion of slag, dross and other foreign matter

3. Cold shuts

4. Mold erosion

5. Rough surfaces

6. Shrinkage

7. Porosity

8. Entrapped gases

9. Misruns

10. Penetration of liquid metal into mold walls

Page 21: Gating system :- CASTING PROCESSES

Gating Ratio

• The gating ration refers to proportion of the cross

sectional areas between the sprue, runner and in-gates

and generally denoted as:

Sprue area : runner area : in-gate area

• Depending on choke area, there can be two types of

gating systems: pressurized and non- pressurized

Page 22: Gating system :- CASTING PROCESSES

A pressurized gating system• The in-gate area is smallest

• Back pressure is there throughout the system

• Metal is more turbulent and dross formation

• Gating system flows full

• Straight sprue can be used

• Higher casting yield

• Used for ferrous castings

Page 23: Gating system :- CASTING PROCESSES

A non-pressurized gating system• A non-pressurized gating system have choke area at the bottom of the sprue

base, total runner area and in-gate areas higher than the sprue area.

• In this system no pressure is existing in the metal flow system and thus it helps

to reduce turbulence.

• It is helpful for casting drossy metals and alloys such as aluminium andmagnesium.

• The gating system should be designed to see all the parts flow full. Otherwise

some elements of the gating system may flow partially allowing for air

aspiration.

• Tapered spruces

• Runners in drag

• Lower casting yield

Page 24: Gating system :- CASTING PROCESSES

Caine’s Method

• Solidification of casting occurs by losing heat from the surface and

amount of heat is given by the volume of the casting. The cooling

characteristics of a casting can be represented by surface-area-

to-volume ratio of the casting.

• Since Riser is also same in solidification behavior as casting, riser

cooling characteristics can also be specified by the ratio of its

surface area to volume.

• If this ratio of casting is higher than it is expected to cool faster.

Page 25: Gating system :- CASTING PROCESSES

Chvorinov’s Rule

• Solidification time of a casting is proportional to square of the

ratio of volume to surface area of the casting.

ts = K ( V / SA )2

• ts = solidification time

• K = mould constant depends on pouring temp., casting and mould thermal characteristics

• V = Volume of the casting

• SA = Surface area

Page 26: Gating system :- CASTING PROCESSES

Freezing Ratio• The freezing ratio (X) of the mould is defined as the ratio of

cooling characteristics of casting to the riser.

X = (SA / V)casting / (SA / V)riser

• In order to feed the casting the riser should solidify last and hence freezing ratio should

be greater than unity.

• Sphere has lowest SA to V ratio and hence that should be used as

riser. In sphere hottest material being at center, it is difficult to use

it for feeding the casting.

• The next best is the cylindrical type of riser.

Page 27: Gating system :- CASTING PROCESSES

Caine’s formula for freezing ratio• Based on Chvorinov’s rule, Caine developed a relationship empirically for the freezing ratio as

follows:

X = a / (Y – b) – c

• Y = Riser volume / Casting volume

• The above equation when plotted will be shown as following graph. The line shows the locus of the

points that separates the sound castings and castings with shrinkages.

Page 28: Gating system :- CASTING PROCESSES

Thank You!