modern casting / September 2001 29

asting defect recogni-tion is one of the mostdifficult tasks facing a

metalcaster. With the multitudeof processes (coremaking, mold-ing, melting, etc.) used to manu-facture a casting, determiningwhich is responsible for a defectrequires analysis, testing and, mostimportantly, experience.

Of all the departments within afoundry, more defects can be at-tributed to molding and the sandsystem than any other. This is duein part to the high number of com-ponents that make up a greensand mold. From the sand, clayand water to the carbon, cereal and other additives, eachcomponent has properties that serve to reduce or controlspecific defects in castings. However, when the amount ofany one component is out of balance with regard to thecasting being poured, the potential for defects arises.

This article will examine the causes of common green sandcasting defects related to expansion, metal penetration, gasand weak sand while offering possible remedies. Althoughevery casting operation is different, common themes tiemany of these defects together, allowing a foundry to followa simple step-by-step remedy progression to determine whicharea(s) of mold development is responsible for the defect.

Expansion DefectsExpansion defects are a family of defects that include

rattails, buckles and scabs. Thesedefects originate, in part, from theexpansion of the sand gains whenheated by the metal entering themold. Silica sand expands the great-est amount when in contact withthe molten metal, as compared toolivine, chromite and zircon sands,which expand less.

Beyond sand expansion, thesedefects also are moisture related.As molten metal runs over the sur-face of a green sand mold, moisturein the sand is converted to steamthat permeates between the sandgrains. When the steam reaches apoint in the mold where the sand

temperature is less than 212F(100C), it re-condenses, creatinga wet layer. This wet layer isweaker than the normal greensand or the hot, dry sand layerdirectly beneath the metal. Asthe hot sand expands, the wetlayer shears to allow the expan-sion. The small ridge of sand thatextends into the mold cavity as aresult of the expansion can cre-ate a line on the surface of thecasting called a rattail (Fig. 1).This defect usually is formed onthe drag portion of the casting.

In further filling of the moldcavity, the molten metal radiates

heat toward the cope casting surface. The moisture on thissurface vaporizes and permeates into the sand where itcondenses to form the wet layer. In the same manner as in thedrag portion of the mold, as the molten metal nears the copesurface of the mold, the intensity of the radiant heat increasesand the sand in the dry sand layer expands. The wet layersplits or shears to accommodate this expansion.

As the metal completes the filling of the mold cavity, thesand buckles, creating a deep groove on the casting surfacecalled a buckle (Fig. 2). Sometimes the buckle will open up,allowing the metal to run through the crack in the sand andfill the void behind the buckle to create a scab (Fig. 3).

Although the rattail is synonymous with the drag and thebuckle and scab with the cope, the three expansion defectsmay be found on either casting surface. When foundries are

faced with these defects, the fol-lowing remedy progression shouldbe applied to the sand system: make an addition of cellulose or

cereal to the sand to provide a placefor expansion to occur;

lower the moisture content of themolding sand, which increasesthe overall mold strength;

lower the pouring temperature ofthe metal (eliminate excess super-heat), which reduces the amountof sand expansion;

lower the temperature of the mold-ing sand from the return sand sys-tem to increase the strength prop-erties of the sand;

Its Time to Play,

Foundrymen often are stumped by the origin of castingdefects. To aid in the analysis, this article explores commongreen sand defects, their causes and possible remedies.

Ian Kay and Mark Nagel, Cast Metals Institute (CMI), Des Plaines, IllinoisAlfred T. Spada, Executive Editor

Fig. 1. RATTAILCaused by expansion, it is a small ridgeof sand that extends into the mold cavity and makes animpression on the casting surface.

Fig. 2. BUCKLESDue to a weak wet layer in the mold,the sand can buckle and form a deep groove on thecasting surface.

Name ThatGreen Sand Casting Defect!

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modern casting / September 2001 30

increase the clay content ofthe sand, especially sodium(western) bentonite, for betterhot strength properties;

improve the sand distributionto at least three screens to stag-ger the expansion and createa linear expansion curve;

decrease the amount of finesin the sand. Fines tend to soakup water, increasing overallmold moisture without increas-ing mold strength;

avoid over-ramming or over-squeezing the mold. Thispushes the moisture closer tothe mold surface, increasingthe probability for defects;

improve the sand mulling prac-tice to create a more homog-enous sand mixture with better de-veloped bond;

fill the mold faster by increasingthe flow rate of the gating systemto leave less time for the heat toact on the sand without pressurefrom the metal.

Adhering Sand DefectsAdhering sand defects are com-

mon to all alloys poured in greensand and are characterized by arough casting surface or by sand stick-ing to the casting surface. These de-fects may be found at a specific spoton the casting such as a hot spot orover the entire casting surface. Twoof the most common ways these de-fects are produced are mechanicalpenetration and chemical reaction.

Mechanical PenetrationThis isthe penetration of metal into the moldmaterial due to the metallostatic pres-sure of the molten metal. It usually isseen when the sprue height is toolarge. The greater the height of themetal in the mold from the top of thepouring cup to the bottom of the cast-ing, the greater the pressure exertedon the liquid. High metal pressuresforce molten metal between the sandgrains where the metal solidifies, hold-ing sand on the casting surface.

Mechanical penetration also canoccur where the metal impinges onthe mold wall. This dynamic pres-sure of the metal can force it into the sand grain openings.This often occurs near the gate entrances to the casting. Highmetal velocities found at the gates can produce the pressurenecessary to create penetration defects.

Two factors that affect mechanical penetration are the sandsfineness and the metal pouring temperature. In general, thecoarser the sand, the larger the voids between the sand grains.It takes less pressure to force metal into larger voids.

In regard to pouring temperature, when the molten metal

contacts the mold surface, itquickly loses heat and a thin,solid skin forms against themold. This skin prevents mol-ten metal from penetrating intothe voids between the sandgrains. When metal is poured athigher temperatures, the extraheat in the metal diffuses intothe sand, delaying the skin for-mation. Without the rapid for-mation of the skin, molten metalhas more time to penetrate intothe sand, creating the defect.

Chemical ReactionIn thismechanism, a reaction occursbetween the liquid metal andthe molding material. These re-actions may produce productsthat act as glue, adhering the

molding sand to the casting.This reaction usually is limited to

ferrous alloys (especially steel).When these alloys are exposed to asource of oxygen (O) such as air orwater, the O may react with the iron(Fe) in the ferrous alloy to form ironoxide (FeO), also called wustite.Once formed, FeO can react withsilica to form iron silicate or fayalite.This is a liquid at metal pouring tem-perature that easily wets the surfaceof the silica and runs between thesand grain. Fayalite then solidifiesaround the grains, gluing the sandonto the surface of the casting.

The defect has two differentformsburn-on and burn-in. Thedifference is how tight the sand isadhering to the casting surface,which is a result of how fast the sili-cate cools as it is being formed. Burn-on (Fig. 4) sand isnt held as tightand usually can be removed duringshotblasting. Burn-in usually requiresa grinding operation to remove it, ifit can be removed at all.

Remedies for sand-adhering de-fects include: reduce the moisture content of

the sand because the moisture fillsthe space between the sand grains.When the moisture evaporates af-ter being hit by molten metal, itthen leaves open space for themetal to penetrate;

improve mold compaction to increase density and leave lessroom for the metal to penetrate;

reduce metal velocity because higher velocities create morepressure, allowing the metal to penetrate more easily;

improve casting design and avoid metal re-entrant angles.Sharp internal corners create hot spots, which are areaswhere penetration is more likely occur;

reduce the metallostatic head pressure because the higherthe pressure, the easier metal can penetrate the mold wall;

Fig. 3. SCABSometimes the buckle in the sand willopen up, allowing the metal to run through the crack inthe sand and fill the void behind the buckle to create ascab defect.

Fig. 4. BURN-ONThis defect usually can beremoved during shotblasting.

Fig. 5. BLOWSA gas defect, it is denoted bylarge voids in the casting due to entrapped,soluble or reactive gas.

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modern casting / September 2001 31

use a mold coating as a pre-ventative barrier against themetal at the mold interface;

add a finer sand to the mix ifthe GFN is too coarse for bet-ter mold compaction;

check metal chemistry andtemperature (especially iniron) to ensure proper fluidity;

improve the mold filling withbetter gating to reduce veloci-ties and avoid hot spots;

increase carbon additives(seacoal) in the mold to cre-ate the reducing atmosphere inthe mold that produces better sur-face finish.

Gas DefectsGas Defects are divided into two

major categoriesblows (Fig. 5),which are large voids in the casting,and pinholes (Fig. 6), which arenumerous small holes. For the mostpart, these gases can occur in cast-ings due to two mechanismsen-trapped gas and soluble gas.

Entrapped GasEntrapped gas isderived from the thermal decompo-sition of mold and core materials orair and mold gas washed into thecasting from the gating system. Entrapped gases are free gasesthat float to the top of the molten metal as the casting solidifies.

Entrapped gas from core or mold binders occurs whenthese organic materials degrade as they are exposed to theheat of the molten metal. The greater the amount of resinused to manufacture the cores and molds, the greater theamount of gas to be generated. Entrapped gas from the gasdesign occurs at the sprue or as metal flows through thedownsprue, runners and ingates. During pouring, care mustbe taken to ensure that the sprue remains full and gas isntpulled down with the metal. Also, if the poured metal under-goes excessive turbulence while flowing through the gatingsystem, air and gas can become entrained and flushed intothe casting cavity.

If an entrapped gas bubble floats to the top of the moldcavity, the gas should permeate into the molding sand beforethe metal can solidify around it. If the permeability is notsufficient, the gas may not have enough time to leave themetal before solidification takes place.

The force that pushes gas into the sand is the metal headpressure. If enough head pressure is not above the top of thecasting, then the gas may not be expelled quick enough.

The pouring temperature also is critical in ensuring trappedgases are out of the casting. If a mold is poured too cold, themetal quickly forms a solid skin. If the gas reaches the top of thecasting cavity after a skin has formed, the gas cannot permeatethe solid metal skin and enter the sand.

Another consideration with entrapped gas is mold/coreventing. Gas is lazy. It simply follows the easiest route awayfrom where it is formed (path of least resistance). If gas isformed in the molding sand and the easiest way out is throughthe metal, than that is where it will go. If gas can go through themetal easier than through a core, then it will follow that route.

The venting of molds and cores provides an open path for

the gases released from the de-composing mold and core ma-terials to follow, rather thanthrough the metal. Venting alsohelps in situations where lowpermeability is a problem.

Another factor related to vent-ing is core print size. Smallerprints make the flow of gasthrough the prints difficult. Of-ten, venting of the cores throughthe print area can reduce oreliminate gas defects in castings.Another aid can be core washes

or coatings, which will help reduceentrapped gas defects by sealingthe surface of the core, forcing thegas out through the core prints.

Although venting molds and corescan help reduce gas defects, a ventonly is effective while it is open. Ventson top of the casting cavity will allowgases to escape prior to the completefilling of the cavity or possibly whilethe metal is in the molten state. Oncethe vent fills with metal, it quicklyfreezes off. At that point, no furtherremoval of gas from the mold cavitycan occur. This idea also applies tocore print vents if the metal leaksinto the print area. Sloppy fitting cores

often show gas related defects.Soluble GasThis refers to gases that dissolve in molten

metal. Aluminum alloys will dissolve hydrogen. Iron alloys willdissolve hydrogen and nitrogen. Copper base alloys will dis-solve hydrogen and oxygen. Steel alloys will dissolve hydrogen,nitrogen and oxygen. The problem is that molten metal canhold a greater amount of gas in solution than solid metal can.

This means that large amounts of gas that may dissolve inthe liquid metal during melting, pouring and mold filling willbe expelled from the metal as it solidifies. During solidifica-tion, the dissolved gases will precipitate into tiny bubbles ofgas, forming pinholes in the casting.

Pinholes also may result from soluble gases near thecasting surface. High sand moisture and combustible levelsin the mold may lead to the formation of these defects. Aswith entrapped gas, increasing sand permeability and moldand core venting can reduce these problems.

Remedies for gas related defects include: reduce the combustible level of the sand because com-

bustibles create gas during pouring; reduce the moisture content of the sand because moisture

means more steam (gas); increase the sand permeability to allow the gases to es-

cape through the mold; with entrapped gases, increase the metal pouring tempera-

ture to increase metal fluid life, which provides entrappedgases more time to escape the mold and/or metal;

with soluble gases, reduce the pouring temperature toreduce the chance for gases to be dissolved in the metal;

vent molds and cores to provide the gases a highway toescape through;

fill the mold quickly but quietly to avoid turbulence andentrained oxides. Also, the quicker a mold fills, the less timeallowed for gases to form;

Fig. 6. PINHOLESA gas defect, it is denoted by numeroussmall holes in the casting due to entrapped, soluble orreactive gas.

Fig. 7. STICKERSThis defect results from the mold-ing sand sticking to the pattern as it is drawn fromthe mold.

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modern casting / September 2001 32

reduce the binder level ofcores to reduce the gas-pro-ducing materials in the mold;

use a mold or core coating toprevent gases from escapinginto the molten metal.

Weak Sand DefectsTwo sand properties relate to

weak sand defectslow greenstrength and low hot strength.

Low Green CompressiveStrengthTear-ups, stickers,drops and crush are defects re-sulting from low green sandstrength. This strength is affectedprimarily by the type of clay used,the amount of clay used and themoisture content of the sand.

If sand does not have sufficientgreen strength, the mold may tearup when the pattern is stripped.Loose sand also may result, leadingto inclusions in the casting.

Stickers (Fig. 7) are defects re-sulting from the molding sand stick-ing to the pattern as it is drawn fromthe mold. The molten metal thenforms the sticker defect when it fillsthe mold cavity. While stickers alsomay be blamed on tooling that lacksthe proper draft angle or has im-proper use of mold release agents, many problems occurwhen sand with a low green tensile strength is used.

Crush is a defect that occurs when two mold surfaces fittogether poorly. If the sand is not strong enough, one matingsurface may give-in or crush. Loose sand from crushes oftenleads to sand inclusions in other parts of the casting.

In general, increasing the percentage of clay in thesand increases its green compressive strength. This is truefor clay levels up to 12%. Calcium (southern) bentonitedevelops higher green strength than equal amounts ofsodium bentonite, so adding clay for green strength prob-lems is a logical choice.

To improve sand green strength, foundries can: increase calcium bentonite levels in the mold to increase

mold green strength; introduce better mulling practices to allow for better and

more consistent distribution of clays, sand and other addi-tives throughout the mold;

reduce the moisture level in the sand. Excess moistureweakens the mold.Low Hot StrengthThis is the strength of the sand during

and just after pouring, and is a common cause of defects foriron and steel foundries where higher metal temperaturesare required. The defects that occur due to this problem areswell, erosion and run outs.

Swell refers to swollen, oversized castings that result whenthe mold wall is not capable of holding the castings shapewhile the molten metal is in the mold. Swell occurs when themold wall is pushed back due to the head pressure on themetal. Although this may occur with an inadequately rammedmold with low sand compactability, swell typically occurswhen the sand hot compressive strength is too low.

Mold erosion (also called cuts or washes) can be caused

Fig. 8. EROSIONThis defect is excess metal on the castingsurface at places where high metal velocity exists, such asin front of a gate.

Fig. 9. RUN OUTThis defect occurs when metalleaks out of the mold through the parting line.

by sand with a low hot com-pressive strength. Erosion (Fig.8) is excess metal on the cast-ing surface at places where highmetal velocity exists, such as atthe front of a gate. Without suf-ficient hot strength, high metalvelocity washes the mold ma-terial away, leaving an erosionscab. The loosened sand maybe found as inclusions in otherparts of the casting. Besides in-creasing the hot strength, re-ducing ingate metal velocitiesalso solves this problem.

Run out (Fig. 9) describes thedefect where metal leaks out ofthe mold at the parting line.

While a majority of run outs occurdue to a lack of mold weight beingplaced in the mold prior to pouring,a run out also can occur with low hotstrength. Metal pressure on the moldmay push back the weak sand at theparting line. This exposed area thenlifts the cope, allowing the metal toflow freely out of the mold cavity.

Increasing the sand clay contentwill increase hot strength properties.The correct mix of clay is critical assodium bentonite has higher hotstrength than calcium bentonite.

Another option (which does not apply to green strength)is to increase the sand moisture, which increases hot strength.However, due to the host of other problems high moisturecontent causes, this is not recommended as a solution toinadequate sand hot strength.

To improve sand hot strength, foundries can: increase sodium bentonite levels in the mold to provide

better hot strength properties; introduce better mulling practices for more consistent

distribution of clays, sand and other additives throughoutthe mold;

reduce the combustibles in the sand mix because theyincrease the mold moisture content and gas-producingability, but usually do not strengthen the moldIt is of the utmost importance that the metalcaster under-

stand the complex relationships that exist between the com-ponents of a green sand system. While looking at the rem-edies suggested in this article, some of the solutions seem tocontradict one another. To ensure success, sand propertiesshould be compared to casting scrap (and to good castings),and limits should be set up to ensure that the sand systemruns at a level that does not produce either defect. Control-ling a sand system can be like walking a tightropetoo farone way or the other means trouble, but if you stay on thestraight path (within pre-established ranges), things comeout OK at the other end of the line. This article was adapted from the CMI class Practical Green SandControl. For more information, contact CMI at 800/537-4237.

For a free copy of this article circle No. 344 on the Reader Action Card.For More Information

International Atlas of Casting Defects, AFS, Des Plaines, IL (1993).Analysis of Casting Defects, AFS, Des Plaines, IL (1997).Casting Defects Handbook, AFS, Des Plaines, IL (2000).

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