gating design criteria for sound casting - … int. j. mech. eng. & rob. res. 2014 mazhar iqbal,...

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Int. J. Mech. Eng. & Rob. Res. 2014 Mazhar Iqbal, 2014

GATING DESIGN CRITERIA FOR SOUND CASTING

Mazhar Iqbal1*

*Corresponding Author: Mazhar Iqbal � [email protected]

Casting as a manufacturing process to make complex shapes of molten metal in mass productionmay experience many different defects such as porosity and incomplete filling. How to improvethe casting quality becomes important. Gating/riser system design is critical to improving castingquality. The objective of the research presented in this thesis is to optimize gating systems withthe goal of improving casting quality such as reducing incomplete filling area, decreasing largeporosity and increasing yield

Keywords: Gating system, sprue, runner, ingates, shrinkage,

INTRODUCTION

As defined earlier, gating systems refer to allthose elements, which are connected with theflow of molten metal from the ladle to the mouldcavity. The various elements that areconnected with a gating system are (Rao P N,xxxx).

Pouring basin, Sprue, Sprue base well,Runner, Runner Extension,In-gate.

Any gating system designed should aim atproviding a defect free casting. This can beachieved by making provision for certainrequirements while designing the gatingsystem. These are as follows.

The mould should be completely filled in thesmallest time possible without having to raise

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Int. J. Mech. Eng. & Rob. Res. 2014

1 NIFFT, National Institute of foundry And Forge Technology, Ranchi, Ranchi university.

the metal temperature or use higher metalheads. The metal should flow smoothly into themould without any turbulence. A turbulencemetal flow tends to form dross in the mould.Unwanted material such as slag, dross andother mould material should not be allowed toenter the mould cavity. The metal entry into themould cavity should be properly controlled insuch a way that a aspiration of the atmosphericair is prevented. A proper thermal gradientshould be maintained so that the casting iscooled without any shrinkage cavities ordistortions. Metal flow should be maintainedin such a way that no gating or mould erosiontakes place. The gating system should ensurethat enough molten metal reaches the mouldcavity. The gating system design should be

Research Paper

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economical and easy to implement andremove after casting solidification. Ultimately,the casting yield should be maximized. Agating system is the conduit network throughwhich liquid metal enters a mold and flows tofill the mold cavity, where the metal can thensolidify to form the desired casting shape. Thebasic components of a simple gating systemfor a horizontally parted mold are shown in Fig.1. A pouring cup or a pouring basin providesan opening for the introduction of metal from apouring device. A sprue carries the liquid metaldown to join one or more runners, whichdistribute the metal throughout the mold until itcan enter the casting cavity through ingates.

DESIGN VARIABLES

Methods used to promote any of the desirabledesign considerations discussed below oftenconflict with another desired effect. Forexample, attempts to fill a mold rapidly canresult in metal velocities that promote molderosion. As a result, any gating system willgenerally be a compromise among conflictingdesign considerations, with the relativeimportance of the consideration beingdetermined by the specific casting and itsmolding and pouring conditions (Wallace J Fand Evans E B, 1959; Sylvia J G, 1972).

Rapid mold fillingcan be important forseveral reasons.Especially with thin-sectioncastings, heat loss from the liquid metal duringmold filling may result in prematurefreezing,producing surface defects (forexample, cold laps) or incompletely filledsections (misruns). Superheating of themoltenmetal will increase fluidity and retard freezing,but excessive superheat can increaseproblems of gas pickup by the molten metaland exaggerate the thermal degradation of themold medium. In addition, the mold filling timeshould be kept shorter than the mold producingtime of the molding equipment to maximizeproductivity.

Minimizing Turbulence

Turbulent filling and flow in the gating systemand mold cavity can increase mechanical andthermal attack on the mold. More important,turbulence may produce casting defects bypromoting the entrainment ofgases into theflowing metal. These gases may by themselvesbecome defects (for example bubbles), or theymay producedross or inclusions by reactingwith the liquid metal. Turbulent flow increasesthe surface area of the liquid metal exposedto air within the gating system. Thesusceptibility of different casting alloys tooxidation varies considerably. For those alloysthat are highly sensitive to oxidation, such asaluminum alloys; magnesium alloys; andsilicon, aluminum, and manganese bronzes,turbulence can generate extensive oxide filmsthat will be churned into the flowing metal, oftencausing unacceptable defects.

Avoiding Mold and Core Erosion

High flow velocity or improperly directed flowagainst a mold or core surface may produce

Figure 1: Elements of Gating System(Sylvia J G, 1972)

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defective castings by eroding the mold surface(thus enlarging the mold cavity) and byentraining the dislodged particles of the moldto produce inclusions in the casting.

Removing Slag, Dross, andInclusions

This factor includes materials that may beintroduced from outside the mold (for example,furnace slags and ladle refractories) and thosethat may be generated inside the system.Methods can be incorporated into the gatingsystem to trap such particles (for example,filters) or to allow them time to float out of themetal stream before entering the mold cavity.

Promoting Favorable ThermalGradients

Because the last metal to enter the mold cavitywill generally be the hottest, it is usuallydesirable to introduce metal in those parts ofthe casting that would already be expected tobe the last to solidify. One obvious method ofaccomplishing this is to direct the metal flowfrom the gating system into a riser, from whichit then enters the mold cavity. Because the riseris generally designed to be the last part of theriser/casting system to solidify, such a gatingarrangement will help promote directionalsolidification from the casting to the riser. If thegating system cannot be designed to promotesome desirable thermal gradient, it should atleast be designed so that it will not produceunfavorable gradients. This will often involveintroducing metal into the mold cavity throughmultiple ingates so that no one locationbecomes a hot spot.

Maximizing Yield

A variety of unrecoverable costs must go into

the metal that will fill the gating system andrisers. These components must then beremoved from the casting and generallyreturned for remelt, where their value isdowngraded to that of scrap. Production costscan be significantly reduced by minimizing theamount of metal contained in the gatingsystem. The production capacity of a foundrycan also be enhanced by increasing thepercentage of salable castings that can beproduced from a given volume of melted metal.

Economical Gating Removal

Costs associated with the cleaning andfinishing of castings can be reduced if thenumber and size of ingate connections withthe casting can be minimized. Again, it maybe advantageous to introduce metal into themold cavity through a riser, because the riserneck can also serve as an ingate.

Avoiding Casting Distortion

It is especially important with rangy, thin-wallcastings, in which uneven distribution of heatas the mold cavity is filled may produceundesirable solidification patterns that causethe casting to warp. In addition, the contractionof the gating system as it solidifies can pull onsections of the solidifying casting, resulting inhot tearing or distortion.

Compatibility with ExistingMolding/Pouring Methods

Modern high-production molding machinesand automated pouring systems often severelylimit the flexibility allowed in locating andshaping the pouring cup and sprue forintroducing metal into the mold. They alsogenerally place definite limits on the rate atwhich metal can be poured.

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Controlled Flow Conditions

A steady flow rate of metal in the gating systemshould be established as soon as possibleduring mold filling, and the conditions of flowshould be predictably consistent from onemold to the next.

PRINCIPLES OF FLUID FLOW

Proper design of an optimized gating systemwill be made easier by the application ofseveral fundamental principles of fluid flow.Chief among these principles are Bernoulli’stheorem, the law of continuity, and the effect ofmomentum.

Bernoulli’s Theorem

This basic law of hydraulics relates thepressure, velocity, and elevation along a lineof flow in a way that can be applied to gatingsystems. The theorem states that, at any pointin a full system, the sum of the potential energy,kinetic energy,pressure energy, and frictionalenergy of a flowing liquid is equal to a constant.The theorem can be expressed as:

wZ +wPv+ wV2/2g+ wF =K ...(1)

where w is the total weight of the flowing liquid(in pounds), Z is the height of the liquid (ininches), P is the static pressure in liquid (inpounds per square inch), v is the specificvolume of liquid (in cubic inches per pound), gis the acceleration due to gravity (386.4 in. /s2), V is the velocity (in inches per second), Fis the friction loss per unit weight, and K is aconstant.

If Equation (1) is divided by w, all the termsreduce to dimensions of length and willrepresent:

• Potential head Z

• Pressure head, Pv

• Velocity head V2/2g

• Frictional loss of head F

• Equation 1 allows prediction of the effect ofthe several variables at different points inthe gating system, although severalconditions inherent in foundry gatingsystems complicate and modify its strictapplication. For example:

• Equation 1 is for full systems, and at leastat the start of pouring, a gating system isempty. This

• Indicates that a gating system should bedesigned to establish as quickly as possiblethe flow conditions of a full system

• Equation 1 assumes an impermeable wallaround the flowing metal. In sand foundrypractice, the permeability of the moldmedium can introduce problems, forexample, air aspiration in the flowing liquid

• Additional energy losses due to turbulenceor to friction (for example, because ofchanges in the direction of flow) must beaccounted for

• Heat loss from the liquid metal is notconsidered, which will set a limit on the timeover which flow can be maintained. Also,solidifying metal on the walls of the gatingsystem components will alter their designwhile flow continues

• Bernoulli’s theorem (Eq 1) is illustratedschematically in Figure 2, and severalpractical interpretations can be derived.The potential energy is obviously at amaximum at the highest point in the system,that is, the top of the pouring basin. As metal

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flows from the basin down the sprue,potential energy changes to kinetic energyas the stream increases in velocity becauseof gravity. As the sprue fills, a pressure headis developed.

Once flow in a filled system is established,the potential and frictional heads becomevirtually constant, so conditions within thegating system are determined by the interplayof the remaining factors. The velocity is highwhere the pressure is low, and vice versa.

The Law of Continuity

This law states that, for a system withimpermeable walls and filled with anincompressible fluid, the rate of flow will be thesame at all points in the system. This can beexpressed as:

Q = A1μ1 = A2μ2 ...(2)

where Q is the rate of flow (in cubic inches persecond), Ais the cross-sectional area of thestream (in square inches), is the velocity of thestream (in inches per second), and thesubscripts 1 and 2 designate two differentlocations in the system. Again, the permeabilityof sand molds can complicate the strictapplication of this law, introducing potentialproblems into the casting process.

One practical implication of the law ofcontinuity is illustrated in Figure 3, whichillustrates the flow of metal from a pouringbasin. As indicated in Eq 1, potential energyis high but velocity is low as the stream leavesthe basin. Velocity increases as the streamfalls, so the cross-sectional area of the stream

Figure 2: Illustration of Bernoulli'sTheorem (Sylvia J G, 1972)

Figure 3: Schematic Showing The Advantages Of A Tapered Sprue Over A Straight-sidedSprue. (a) Natural Flow Of A Free-falling Liquid. (b) Air Aspiration Induced By LiquidFlow In A Straight-sided Sprue. (c) Liquid Flow In A Tapered Sprue (Sylvia J G, 1972)

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must decrease proportionately to maintain thebalance of the flow rate. The result is thetapered shape typical of a free-falling streamshown in Figure 3(a).

If the same flow is directed down a straight-sided sprue (Figure 3b), the falling stream willcreate a low-pressure area as it pulls awayfrom the sprue walls and will probably aspirateair. In addition, the flow will tend to be unevenand turbulent, especially when the streamreaches the base of the sprue. The taperedsprue shown in Figure 3(c) is designed toconform to the natural form of the flowingstream and therefore reduces turbulence andthe possibility of air aspiration. It also tends tofill quickly, establishing the pressure headcharacteristic of the full-flow conditionsrequired by Eq 1. Many types of high-

production molding units do not readilyaccommodate tapered sprues, so the gatingsystem designerwill try to approximate theeffect of a tapered sprue by placing arestriction, or choke, at or near the base of thesprue to force the falling stream to back upinto the sprue (Figure 4).

Momentum Effects

Newton’s first law states that a body in motionwill continue to move in a given direction untilsome force is exerted on it to change itsdirection.

Reynolds’s Numbers and Types ofFlow

The flow of liquids can be characterized by aspecial measurement called the Reynolds’snumber, which can be calculated as follows:

NR= vdr/m ...(3)

where NR is the Reynolds’s number, v is thevelocity of the liquid, d is the diameter of theliquid channel, ρ is the density of the liquid,and μ is the viscosity of the liquid.

Reynold’s number, NR, and its relationshipto flow characterization. (a) NR < 2000,laminar flow. (b) 2000 £NR < 20,000, turbulentflow. (c) NR³ 20,000, severe turbulent flow(Sylvia J G, 1972).

If a fluid flow system has a Reynold’s numberbetween 2000 and 20,000, some mixing andturbulence will occur but a relativelyundisturbed boundary layer will be maintainedon the surface of the stream. This type ofturbulent flow, common in most foundry gatingsystems, can be considered relativelyharmless so long as the surface is not ruptured,thus avoiding air entrainment in the flowingstream. With a Reynold’s number of about

Figure 4: Choke MechanismsIncorporated Into Straight-sided

Sprues To Approximate Liquid Flow inTapered Sprues (a) Choke Core (b)Runner Choke (Sylvia J G, 1972)

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20,000, flow will be severely turbulent. This willlead to rupturing of the stream surface with thestrong likelihood of air entrainment and drossformation as the flowing metal reacts withgases.

Abrupt Changes in Flow ChannelCross Section

As shown in Figure 6, low-pressure zones—with a resulting tendency toward airentrainment—can be created as the metalstream pulls away from the mold wall. With asudden enlargement of the channel (Figure 6a),momentum effects will carry the stream forwardand create low-pressure zones at theenlargement. With a sudden reduction in thechannel (Figure 6b), the law of continuity showsthat the stream velocity must increase rapidly.This spurting flow will create a low-pressurezone directly after the constriction. Theproblemsillustrated in Figure 5 can be

Figure 5: Schematic ShowingThe Formation Of Low-pressure AreasDue To Abrupt Changes In The Cross

Section Of A Flow Channels(a) Sudden Enlargement of the Channel

(b) Sudden Reduction of the Channel

Figure 6: Schematic Illustrating FluidFlow Around Right-angle And Curved

Bends In A Gating System (a) TurbulenceResulting From A Sharp Corner (b) MetalDamage Resulting From A Sharp Corner(c) Streamlined Corner That Minimizes

Turbulence And Metal Damage

minimized by making gradual changes in theflow channel cross section; abruptchangesshould be avoided.

Abrupt Changes in Flow Direction

As shown in Figure 6, sudden changes in thedirection of flow can produce low pressurezones, as described above. Problems of airentrainment can be minimized by making thechange in direction more gradual.

Abrupt changes in flow direction, in additionto increasing the chances of metal damage,will increase the frictional losses during flow.As shown in Figure 7, a system with high

Figure 7: Effect of Pressure Head andChange in Gate Design on the Velocity of

Metal Flow: (a) 90° Bend; (b) r/d = 1;(c) r/d = 6; (d) Multiple 90° Bends.The Variables r and d are the Radius

of Curvature and the Diameterof the Runner, Respectively

Source: Sylvia J G, 1972

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frictional losses will require a greater pressurehead to maintain a given flow velocity.

Using a Runner Extension

Use of a runner extension beyond the lastingate is illustrated in Figure 1. The first metalentering the gating system will generally be themost heavily damaged by contact with the moldmedium and with air as it flows. To avoidhaving this metal enter the casting cavity,momentum effects can be used to carry it pastthe ingates and into the runner extension. Theingates will then fill with the cleaner, lessdamaged metal that follows the initial moltenmetal stream.

Equalizing flow through ingates by decreasingthe runner cross section after the ingate isillustrated in Figure 8; this is done for systemswith multiple ingates. As noted earlier, in the filledsystem shown, potential and frictional energiesbecome constants, so they can be dropped from

consideration in Eq 1 to show the interaction ofpressure and velocity effects.

At the first ingate, velocity is high asmomentum effects carry the flowing streampast the gate. At the second ingate, velocitydecreases in the runner as it reaches the end,causing higher pressure and resulting in higherflow through the gate. By stepping down therunner after the first ingate, metal velocitiesand pressures at the two ingates can beequalized. Thiseffect can be achieved bygradually tapering the runner to a smaller crosssection along its length, but patternmakinglimitations usually make it simpler toincorporate actual steps in the runner.

DESIGN CONSIDERATIONS

In applying fluid flow principles to the designof a specific gating system, several designdecisions must be made before the actualsizes of the various components can becalculated.

Runner and Ingate

Figures 1 and 2 show gating systems withingates coming off the top of the runner andthen into the casting. This arrangement of copeingates and drag runners is common and hasthe advantages that the runner will be full beforemetal enters the ingates. This establishes thefull-flow conditions discussed earlier in thisarticle. A full runner will reduce turbulence andwill help to allow any low-density inclusions inthe flowing stream to float out and attachthemselves to the mold wall.A system of coperunners and drag ingates (or ingates comingoff the base of a cope runner) is also commonand has strong proponents (Sylvia J G, 1972).The basis of this design is that momentumeffects will carry the first metal past the ingates,

Figure 8: Applying Bernoulli’s Theorem toFlow From a Runner at two Ingates For aFilled System and Comparingvelocity and

Pressure at the Ingates for two RunnerConfigurations (a) Same Runner Cross

Section at Bothingates (b) SteppedRunner Providing Two Different Runner

Cross Sections at Each Ingate


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and if the runner can be quickly filled (at leastabove the level of the ingates), clean metal willflow from the bottom of the runner, whileinclusions carried along in the metal streamwill float above the ingates. A common elementof this system is that the total cross-sectionalarea of the ingates should be smaller than thecrosssectional area of the runner. Such apressurized system is intended to force themetal to back up at the ingates and rapidly fillthe runner, although complete filling of a coperunner will often depend on at least partial fillingof the casting. During this period of incompletefilling, turbulence and the potential for airentrainment and dross generation areincreased.

Pressurized Versus UnpressurizedSystems.

The difference between these two systems isin the choice of the location of the flow-controlling constriction, or choke that willdetermine the ultimate flow rate for the gatingsystem. This decision involves thedetermination of a desired gating ratio, thatis, the relative cross-sectional areas of thesprue, runner, and gates. This ratio, numericallyexpressed in the order sprue: runner: gate,defines whether a gating system is increasingin area (unpressurized) or constricting(pressurized). Common unpressurized gatingratios are 1:2:2, 1:2:4, and 1:4:4. A typicalpressurized gating ratio is 4:8:3. Both typesof systems are widely used.

The unpressurized system has theadvantage of reducing metal velocity in thegating system as it approaches and enters thecasting. Lower velocities help encouragelaminar (or less turbulent) flow, sounpressurized systems are recommended for

alloys that are highly sensitive to oxide anddross formation. Pressurized systemsgenerally have the advantage of reduced sizeand weight for a given casting, thus increasingmold yield. The single greatest disadvantageof pressurized systems is that, by design,stream velocities are highest at the gatesjustas the metal enters the casting. This increasesthe potential for mold or core erosion andplaces a premium on proper location of ingatesto minimize such damage.

Some gating ratios1 used in practice are

Aluminium 1:2:11:3:31:4:4

Aluminium bronze 1:2.88:4.8

Brass 1:1:11:1:31.6:1.3:1

Copper 2:8:13:9:1

Ductile Iron 1.15:1.1:11.25:1.13:1

Grey Cast Iron 2:1.5:12:3:1

Magnesium 1:2:21:4:4

Malleable Iron 1:2:9.51.5:1:2.52:1:4.9

Steels 1:1:71:2:11:2:1.5

Gating System and Types

A mould cavity must be filled with clean metalin a controlled manner to ensure smooth,uniform and complete filling, for the casting tobe free of discontinuities, solid inclusionsandvoids. This can be achieved by a well-designed gating system. The first stepinvolvesselecting the type of gating system andthe layout of gating channels: the orientationandposition of sprue, runner and ingate(s). Themost critical design decision is the idealfillingtime, based on which the gating channels aredesigned.The main objective of a gatingsystem is to lead clean molten metalpoured

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from ladle tothe casting cavity, ensuringsmooth, uniformand completefilling. Cleanmetal impliespreventing the entry of slag andinclusions into the mould cavity, and minimizingsurfaceturbulence. Smooth filling impliesminimizing bulk turbulence. Uniform fillingimpliesthat all portions of the casting fill in acontrolled manner, usually at the sametime.Complete filling implies leading moltenmetal to thin and end sections with minimumresistance [4]. The major elements of a gatingsystem include pouring basin, sprue, well,runner and ingate, in the sequence of flow ofmolten metal from the ladle to the mould cavity.The pouring basin or bush or cup is a circularor rectangular pocket that accepts the moltenmetal from the ladle. The sprue or down sprue,usually circular in crosssection, leads moltenmetal from the pouring basin to the sprue well.The sprue well or base changes the directionof molten metal by right-angle and sends it tothe runner. The runner takes the metal from thesprue to close to the casting. Finally, the ingateleads the metal to the mould cavity. Anothermajor element is filter or slag trap, usuallyplaced in the runner or between the runner andingate, meant for filtering out slag and otherinclusions.

The sprue is always vertical. The well, runnerand ingate are usually located in the partingplane. Depending on the orientation of theparting plane, the gating systems can beclassified as horizontal and vertical gatingsystems. Thus in horizontal gating systems, thesprue is perpendicular to the parting plane,whereas in vertical gating systems, the sprueis parallel to the parting plane.Gating systemscan be classified depending on the orientationof the parting plane (which contains the sprue,

runner and ingates), as horizontal or vertical.Depending on the position of the ingate(s),gating systems can be classified as top,parting and bottom.

Horizontal gating systems are suitable forflat castings filled under gravity. They are widelyused in sand casting of ferrous metals, as wellas gravity diecasting of non-ferrous metals.

Vertical gating systems are suitable for tallcastings. They are employed in high-pressuresand mould, shell mould and diecastingprocesses, where the parting plane is vertical.

Top gating systems, in which hot moltenmetal enters at the top of the casting, promotedirectional solidification from bottom to top ofthe casting. These are however, suitable onlyfor flat castings to limit the damage to metalas well as the mould by free fall of the moltenmetal during initial filling.

Bottom gating systems have the oppositecharacteristics: the metal enters at the bottomof the casting and gradually fills up the mouldwith minimal disturbances. It is recommendedfor tall castings, where free fall of molten metal(from top or parting gates) has to be avoided.

Middle or side or parting gating systemscombine the characteristics of top and bottomgating systems. If the gating channels are atthe parting plane, they are also easier toproduce and modify if necessary, during trialruns. The most widely used system is thehorizontal gating with ingates at the partingplane. In vertical gating systems, ingates maybe positioned at top, bottom andside.

Gating Channel Layout

The most important decision here is thenumber and location of ingate(s). Let us

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consider horizontal gating systems with sideingates. Their location is governed by thefollowing considerations

Side feeders

If side feeders are employed, then theirefficiency can be improved by filling with thefirst stream of hot molten metal throughingates. It also reduces the fettling effort andthe resulting marks on the casting, since theingates do not have to be removed separately.

Thick Sections

The next best position after a side feeder is athick section, which will allow molten metal toflow to other sections with minimal cooling. Itwill also reduce occasional breakage duringfettling of ingates.

Clear Path

In sand casting, the molten metal should beallowed to flow with minimal obstructions and

change of direction (particularly at sharpcorners) to avoid turbulencerelated problems.Ingates should never be placed directlyopposite a core.

Low Free fall

The ingate should be located where the freefall of molten metal inside the mould cavity islow. This minimizes oxidation during fall anderosion at the point of impact of molten metal.

The number of ingates must be sufficientenough, so that the distance of flow from anyingate to the farthest point filled by that ingateis less than the fluidity distance.

The sprue conducts the molten metal fromthe pouring basin at its top to the plane in whichthe runners and ingates are located. Its locationis governed by the following considerations:

Flow Distance

The sprue location must minimize the total flowdistance within the gating channels, to reduceheat loss as well as maximize yield.

Heat Concentration

Since the hottest metal flows through the sprue,it must be away from hot spots (essentiallythick sections) in the casting.

Mould Layout

The sprue must be located to minimize the sizeof the bounding box enclosing the entirecasting (including the gating channels), so thata smaller mould is required. This also appliesto multi cavity layout, where the sprue andrunner(s) are shared by multiple cavities.Therunner layout is simply given by the shortestpath to connect the ingates with sprue.

Figure 9: Heuristics for Ingate Location


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MOULD FILLING

PHENOMENON

Fluidity

It is not a physical property. It is a technologicalcharacteristic. It indicates theability of liquidmetal to flow through a given mould passage– even as it is solidifying – and fill the cavity toreproduce the design details. It is quantifiedin terms of the solidified length of a standardspiral casting.mThe fluidity as defined by thefoundry community is different that defined byphysicists(as the reciprocal of viscosity). Thecasting fluidity is driven by metallostaticpressure and hindered by: viscosity andsurface tension of molten metal, heat diffusivityof mould, back pressure of air in mould cavityand friction between the metal-mould pair.

Metallostatic Head

The metallostatic pressure is given by ρ g hwhere ρ is the metaldensity and h is the heightof liquid metal column above the filling point. Ahigher metallostatic pressure gives highervelocity of molten metal, and there by higherfluidity.

Viscosity

Viscosity depends on the metal family,composition and the instantaneoustemperature. For most metals, the viscosity atthe pouring temperature is close to that ofwater(1 centistokes); aluminum: 1.2 and iron: 0.9centistokes. In comparison, the viscosity oftypical mineral oils is about 600.

Surface Tension

For a flat plate of thickness t, the relationbetween head, thickness and surface tensionis given by: ρ g h= γ / t, where ρ is the surfacetension. At the pouring temperature, the surface

tension of aluminum and iron is 0.5 and 0.9 N/m respectively; similar to mercury at roomtemperature (0.46 N/m), but higher than water(0.07 N/m).

Heat Diffusivity

Moulds with high heat diffusivity transfer heatfaster from the moltenmetal, causing it to freezeearlier and stop flowing. It is given by √ (Kmρm Cm), where Km is thermal conductivity, ρmis density and Cm is specific heat of the mouldmaterial.

Back Pressure

As molten metal advances in the mould, theback pressure of air that isbeing compressedin the cavity ahead effectively reduces themetallostatic pressure, andthus hinders filling.The back pressure depends on the cavityvolume, mould permeability and the velocityof the advancing front. Venting helps.

Friction

The rough surface of sand mould hinders metalflow. Thus mould coating(usually water based,containing silica flour and graphite) reducesthe friction betweenthe metal and mould,contributing to higher fluidity. In general, fluidityof pure metals is higher than alloys. Withinalloys, eutectics havehigher fluidity than non-eutectics. The fluidity of grey iron rangesbetween 0.5-1.0 m,and can be estimated bythe empirical equation:

Fluidity = 14.9 CE + 0.05 T –p - 155 inch.

where CEis the carbon equivalent given byCE= %C + 0.25 %Si + 0.5 %Pand Tp is thepouring temperature in Fahrenheit.

Turbulence

It implies irregular, fluctuating flow with

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disturbances. It is observed when:(1) inertiaforces (which make the fluid continue in thesame direction), are much higherthan the dragforces (which tend to stop the fluid motion), and(2) there are obstructions in the path of flow,such as a sharp corner or a change of sectionthickness. The drag forces include thosecaused by viscosity and surface tension. Theviscous forcesmainly operate in the bulk of theliquid metal, whereas surface tension forcesoperate near the mould wall. Thus we have twotypes of turbulence: bulk and surface.

Bulk Turbulence

It is quantified by Reynolds number Re, whichis the ratio of inertia toviscous pressure in afluid. It is given by ρ V d / μ where ρ is thedensity, μ is the viscosityand V is the velocityof the liquid; dis a characteristic dimension ofthe flow path. If Re ismore than 2000, then theflow is usually turbulent.

Surface Turbulence

It is quantified by the Weber number We, whichis the ratio of inertiato surface tension pressurein a fluid. It is given by ρ V2 r / γ where ris theradius ofcurvature of the free liquid surface.For Weis less than 1, surface turbulence isabsent.When it is 100 or more, surfaceturbulence is prominent, leading to violentmixing ofsurface layers with the bulk of themolten metal. The path of molten metal duringcasting process comprises mainly four parts:

1. Pouring of molten metal from ladle to thecup in the mould

2. Flow within the gating channels, frompouring basin to ingate

3. Jet of molten metal emerging from ingateand entering the mould cavity

4. Filling of mould cavity by liquid movementsin the bulk as well as near the surface.Ingeneral, the entire path of molten metal,within the gating system as well as themould cavity, is turbulent in most castings.This can be readily ascertained bycalculating thevalue of Reynolds number fora typical casting. A major purpose of thegating system (instead of pouring metaldirectly into the mould cavity) is to reducethe turbulence, though it cannot becompletely eliminated.

There are mainly three major classes ofcasting defects related to mould filling:incomplete filling, solid inclusions and gaseousentrapments

INCOMPLETE FILLING

This is primarily caused by poor fluidity ofmolten metal, andmanifests in the form of acold shut or mishrun. A cold shut occurs whentwo streams ofmolten metal coming fromopposite directions meet, but do not fusecompletely. A misrun occurs when the moltenmetal does not completely fill a section of themould cavity (usually an end section far fromthe entry point). The presence of surface oxidesand impurities on the advancing front of liquidmetal aggravates such defects.

Solid Inclusions

This is primarily caused by the turbulence inmolten metal, and manifests in the form ofsand inclusion or slag inclusion. Sandinclusions are mainly caused by bulkturbulence in gating channels or mould cavity,which dislodges sand particles from the mouldwall. Slag inclusions can be caused by surfaceturbulence any where along the path of molten

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metal, leading to mixing of surface oxide layerswith the rest of molten metal.

Gaseous Entrapments

This class of casting defects includes air andgas entrapment, usually in form of blow holeand gas porosity, respectively. They occurwhen the air orgas inside the mould cavitycannot escape through the mould. The majorsource of gases includes dissolved gases inthe molten metal, vaporization of mould sandmoisture andcombustion of binders in core ormould sand. The occurrence of these defectsincreases when the amount of air entrappedor gas generated is high, filling andsolidification of molten metal are fast, theventing of the mould is poor.

Optimal Filling Time

A casting that fills too slow can havediscontinuities such as cold shuts and misruns.Too fast filling can lead to solid and gaseousinclusions. The higher limit of filling time(slowest filling) is governed by the need toavoid premature freezing in thin sectionsbefore complete filling. The lower limit of thefilling time (fastest filling) is governed by theonset of surface turbulence. The correct fillingtime lies some where in between, and is afunction of cast metal, weight, minimum sectionthickness and pouring temperature.

Several empirical equations fordetermining the correct filling time for majormetals have been developed by castingresearchers, based on experimentalinvestigations. The filling timetfis expressed asa function of casting weight W in kg, sectionthickness t in mm and fluidity length Lf in mm.

A generalized equation for filling time canbe written as:

τ f = K0 (KfLf / 1000) ( Ks + Ktt / 20)

(Kw W )P

There are five coefficients: K0 is an overallcoefficient, and Kf, Ks, Kt, Kw are thecoefficients for fluidity, size, thickness andweight, respectively. For grey iron the followingvalues may be used: K0 = 1.0, Kf= 1.0, Ks =1.1(for castings of size 100-1000 mm),Kt=1.4(for wall thickness up to 10 mm), Kw =1and P = 0.4. Based on individual experience,an expert casting engineer can set the valuesof the coefficients for each metal-processcombination. These form a valuable part of theknowledge base of a foundry specializing inspecific castings.

Metal Velocity

The optimal filling time is determined such thatgating channels can be designed to avoidsurface turbulence and minimize bulkturbulence within the gating channels as wellas the mould cavity. This mainly depends onthe velocity of the molten metal, which varieswidely within the gating channels as well asinside the mould cavity. For a given location inthe casting, the velocity also changes with time,from the start to end of filling. The mostimportant event is that of molten metalemerging from the ingate, just after the fillingof gating channels and before the filling ofmould cavity. The metal is both hot and fast atthis location and instant, and can lead toconsiderable damage if not controlledproperly. The velocity of molten metal at theingate depends on mainly two parameters: (1)the metallostatic head and (2) the ratio ofcross-sections of sprue exit, runner(s) and

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ingates(s), referred to as the gating ratio. Ingeneral, the velocity of molten metal must bekept lower than 1 m/s for ferrous metals and0.5 m/s for aluminum alloys.

GATING ELEMENT DESIGN

The gating system can be designed to fill agiven casting in a predetermined time, bykeeping a constant level of liquid metal in thepouring basin during pouring, to achieve acontrolled rate of flow through the choke. Thechoke is the smallest cross-section in thegating system that controls the flow rate ofmolten metal. The element (sprue exit, runners

or ingates) with the smallest value in the gatingratio is considered the choke. The choke areaAcis given by:

Ac = W / (ρ c τ f V c)

where, W is the total casting weight (includingfeeders and gating channels), ρ c is the metaldensity, tf is the total filling time and Vc is thechoke velocity.

The choke velocity is given by:

Vc = Vp+ cf√ (2 g H)

where H is the metallostatic pressure head,given by the vertical distance between the

Figure 10: Flow Chart of Gating Element Design

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liquid level in pouring cup and the centerline ofthe choke. The value of pouring velocity Vp isnon-zero, if poured from a height or if bottompouring ladles are used. The friction factor cfwithin the gating system depends on itsgeometry and surface finish, and rangesbetween 0.6-0.9.

Note that the weight of the gating system isunknown at the time of calculating the mouldfilling time and choke area. This can beovercome by determining the total castingweight after a gating design and repeating thecalculations.During actual filling, themetallostatic pressure head graduallydecreases after the molten metal starts risingabove the level of choke. Thus the averagevalue of actual choke velocity is less than theone used above, leading to slower filling. Thiscan be compensated by estimating the actualfilling time (as described in a later section),and then correcting the choke area.

The cross-sectional area of sprue exit,runners and ingates, is initially determinedbased on the choke area, gating ratio and thenumber of individual elements. Then thesectional area of individual elements, as wellas their shape and dimensions are determinedas follows.

Sprue

It usually has a circular cross-section, whichminimizes turbulence and heat loss.The cross-sectional area at the sprue exit or bottom iscalculated from the choke area and gatingratio. The area of the sprue top should becalculated using mass and energy balanceequations, to prevent flow separation in thesprue. Essentially,

A1 √ H1 = A2 √ H2 where, H1 and H2 arethe metallostatic pressure head at the top andbottom of the sprue, respectively; A1 and A2being the respective cross-sectional areas.The ideal sprue must be larger at the top andsmaller at the bottom. Since this leads to anundercut, such a sprue can not be created bythe pattern during moulding operations, andmust be formed by a core. If this is noteconomical, then the choke can be created inthe beginning of runner.

Sprue well

It arrests the free fall of molten metal throughthe sprue and turns it by a right angle towardsthe runner. It must be designed to minimizeturbulence and air aspiration. Therecommended shape of a sprue well iscylindrical, with diameter twice that of sprueexit and depth twice that of runner. A filletbetween the well and runner will facilitatesmooth transfer of molten metal.

Runner

The main function of the runner is to slow downthe molten metal, which speeds up during itsfree fall through the sprue, and take it to all theingates. This implies that the total cross-sectional area of runner(s) must be greaterthan the sprue exit. In general, a ratio of 1:2 isrecommended. A much higher ratio (such as1:4) may lead to flow separation in the runner.The second implication is that the runner mustfill completely before letting the molten metalenter the ingates. Finally, in casting where morethan one ingate is present, the runner cross-section area must be reduced after eachingate connection (by an amount equal to thearea of that ingate), to ensure uniform flow.

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Ingate

The ingate leads the molten metal from thegating system to the mould cavity. A number ofconflicting requirements apply to the designof ingates, as listed below. Ingate section mustbe designed to reduce the metal velocity belowthe critical limit.This implies that in general, theingate area must be more than the sprue exit(choke). Ingate must be easy to fettle. Thisimplies a smaller cross-section, preferably aflat section (against a square one), ispreferred. Ingate must not lead to a local hotspot. This implies that the ingate modulus (ratioof volume to cooling surface area) must besmaller than that of the connected section.Flow of molten metal through an ingate (andtherefore its cross-sectional area) mustbeproportional to the volume of the connectedcasting region.The number, shape (aspectratio) and dimensions of ingates must becarefully designed to optimize the aboverequirements.

Optimization and Validation

Several iterations of gating system design andmould filling analysis may be carried out untilfilling related problems are eliminated. Ingeneral, several different gating designs(essentially, the number, location anddimensions of gating channels) may lead todefect free castings. We will therefore, developa set of criteria to assess a given gatingdesign,which can be used in an optimizationexercise. Finally we describe differentexperimental techniques to observe mouldfilling for validating the gating design.A givendesign of gating system can be assessedusing the following criteria. All criteria havebeen normalized and are sought to bemaximized.

Mould Filling Time

The actual filling time as determined bycomputer simulation or actual experiment mustbe close to the optimal filling time for whichthe gating system was designed. This criterionis expressed as follows:

CG1 = 1 (/τ f-actual - τ f-optimal /) /

τ f-optimal

Note that if a casting is found to have filling-related defects at the optimal filling time, butis defect-free at some other filling time, thenthe empirical equation for optimal filling timemay be corrected for the particularcombination of geometry, metal and process.

Ingate Velocity

The velocity of molten metal emerging from theingate must be as low as possible to minimizeturbulence.

CG2 = 1 (Vingate/ Vcritical)

where, Vcritical is the recommended limit ofvelocity depending on the metal: about 1 m/sfor iron, and 0.5 m/s for aluminum.

Impingement

The velocity and direction of the first stream ofmolten metal emergingfrom an ingate andstriking a mould face affect mould erosion atthat location. A faststream striking in a directionperpendicular to the face of impingement shouldbeavoided. This is expressed as follows:

CG3 = Vimp-limit / ( Vimp( nimp. nf))

where, Vimp-limitis the limiting value ofimpingement velocity for the onset of moulderosion, Vimp is the velocity of impingement;nimp and nf are the unit vectors along thedirection of impingement and normal to thecasting face of impingement, respectively.

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Gating Yield

The volume of the gating system must beminimized to increase the yield.

The criterion is given by:

CG4 = Ncvc/ ( Ncvc+ vg )

Where, Nc is number of casting cavities permould, vc is the volume of each cavity, and vgis the volume of the common gating systemfor all the cavities in the mould.

Fettling

The size of an ingate must be small comparedto the connected portion of the casting to avoidcasting breakage or cracks during fettling.When several ingates are present, one that ismost likely to cause damage determines thecriteria assessment value.

CG5 = mini (1(tgi/ tci)

where, tgi is the thickness of ingate i and tci isthe thickness of the connected portion ofcasting. The gating design can be validatedby various techniques. Visualization of mouldfilling – g,)g – even if indirect (since the mouldsare opaque)– g,) g – provides a useful pointerto filling related defects and their causes. Othertechniques are briefly described here.

Shop Floor Trials

Sample castings are produced using the

materials and processes that will be finally used

for production castings. Then their surface,

sub-surface and internal quality may be

observed by visual, destructive and non-

destructive testing. Destructive testing

includes machining and cutting the sections

through critical regions.

High-speed Radiography

This involves recording the mould-fillingphenomenon using a high-speed x-raycamera. This is most useful for observing allmajor phenomenon in mould filling, includinginitial filling of the gating system, the sequenceof filling through different ingates, branchingand rejoining of streams, etc. It is however,limited to low density metals and smallcastings (in terms of thickness along thedirection of rays).

Partial Filling

Several moulds are prepared; and say only10% of metal is poured in the first mould, 20%in the second mould, and so on. The sequenceof partially filled and solidified castingsfacilitates visualizing the mould filling. This issuitable for thin castings in which the mouldfilling time is comparable to the castingsolidification time.

Open Mould

This is suitable for castings that are primarilyin the drag. A portion of cope directly abovethe casting cavity is cut away, leaving the gatingsystem. A standard video camera is used torecord the molten metal stream emerging fromthe ingate and the gradual filling of the mould.The video can later be played back in slowmotion. The absence of back pressure of airin the mould may lead to some errors.

Contact Wire Sensing

Contact wires can be placed in different partsof the mould. The completion of circuit whenthe metal reaches a particular wire is recordedby a multi channel recorder. Based on thesequence of observations, the time taken for

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the metal to reach different parts of the mouldcan be assessed. This is however, useful onlyto record the initial flow of metal to differentparts of the mould.

Water in Transparent Mould

Since the viscosity of water is close to that ofmost molten metals, the flow of water in atransparent mould (constructed by Perspex orother transparent polymers) provides a veryuseful indicator. A color marker (if turbulenceis low), oil droplets or particles are introducedfor better visualization and determination ofvelocity in different sections. This is however,not suitable for studying flow in thin castings inwhich the flow of molten metal is affected bythe onset of solidification.

CONCLUSION

1. Pouring basin, sprue , sprue base well,runner and runner extension serves thepurpose of allowing clean molten metal toenter the mould cavity.

2. Parting gate is the most widely used gatewhile the top and bottom gates aresometimes used for specific applicationthat favors them.

3. Fluid mechanics law, together with empiricalrelations, is applied to the design theoptimum gating system.

4. It is important to make sure that slag enteringthe gating system be removed completelybefore the metal enters the mould cavity.

5. Sometimes chills may be need to be addedto reduce porosity at isolated sections thatare not fed by risers.

REFERENCES

1. Atlas of Casting Defects, Institute ofBritish Foundry men.

2. Guleyupoglu S (1995), “A generalapproach to the parting direction problemfor sand castings”, Ph.D. Thesis, TheUniversity of Alabama, Tuscaloosa, AL.

3. Guleyupoglu S and Hill J L (1995),“Parting direction and parting planeselection criteria forsand castings”, AFSTransactions, Vol. 103, pp. 259-264.

4. Guleyupoglu S, Yu K O and Hill J L (1996),“Analysis and optimal design of risersbased on section modulus method”, InProc. 9th World Conference onInvestment Castings, San Francisco,CA, October.

5. Karsay S I (1981), Ductile Iron III: Gatingand Risering, QIT—Fer et Titane, Inc.

6. Khanna O P (2011), Foundry Technology,15th Reprint. Principles of Gating, pp.203-237.

7. Rao P N (2007), “ManufacturingTechnology”, Vol. 1, Gating System forCasting, pp. 126-133.

8. Richins D S and Wetmore W O (1951),AFS Symposium on Principles ofGating, p. 1, AFS, Des Plaines, IL.

9. Ruddle R W (1956), The Running andGating of Sand Castings. The Institute ofMetals, London, England.

10. Ruddle R W (1978), “Risering—past,present and future”, British Foundryman,Vol. 71(a), p. 197.

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11. Ruddle R W (1982), “A computer programfor steel risering”, AFS Transactions, Vol.90, pp. 227-237.

12. Swift R E, Jackson J H and Eastwood LW (1949), “A study of principles of gating”,AFS Transactions, Vol. 57, pp. 76-88.

13. Sylvia J G (1972), Cast MetalsTechnology, Addison-Wesley.

14. Wallace J F and Evans E B (1959),“Principles of Gating”, Foundry, Vol. 87,Oct ober.

15. WWW.emt-india.net

gating design criteria for sound casting - … int. j. mech. eng. & rob. res. 2014 mazhar iqbal,...

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