CAUSES AND CURES FOR COMMON CASTING DEFECTS
Defects in castings do not just happen. They are caused by faulty procedure (1) in one or more of the operations involved in the casting process, (2) in the equipment used, or (3) by the design of the part. A casting defect is often caused by a combination of factors which makes rapid interpretation and correction of the defect difficult.
Casting defects arise from many causes and have many names. One of the most prominent causes of defects does not appear on any formal list of defects, it is CARELESSNESS. Its remedy is obvious.
NAMES OF DEFECTS
The table in the summary of this chapter lists the most common types of casting defects, their causes, and their cures. Causes and cures are discussed in more detail later in this chapter. The names of common defects are explained as follows.
A blow or blowhole is a smooth cavity caused by gas in the molten metal. A pinhole is a type of blow that is unusual because of its small size. It is common to find a single large gas hole (or blow) in a casting, but pinholes usually occur in groups.
A shrink or shrinkage cavity is a rough cavity caused by contraction of the molten metal. It is quite often impossible to tell whether a particular hole in a casting is a shrink or a blow. Gas will aggravate a shrink defect, and shrinkage will aggravate a gas defect. The distinction can usually be made that gas pressure gives a cavity with smooth sides (blow) and contraction or lack of feeding gives a cavity with rough sides (shrink). When either a blow or shrink occurs, it is a good idea to correct for both if the cause cannot be determined for sure.
A rat tail, buckle, and scab all originate in the same way and differ mainly in degree. They are caused by uncontrolled expansion of the sand. If the condition is not too bad, a rat tail is formed. The surface of the sand buckles up in an irregular line that makes the casting look as though a rat has dragged his tail over it. If sand expansion is even greater, the defect is called a buckle. If it is still worse so that molten metal can get behind the buckled sand, it is a scab.
A misrun or cold shut occurs when the mold does not completely fill with metal, or when pouring is interrupted so that the metal does not fuse together properly.
Metal penetration causes rough castings. The metal seeps in between the sand grains and gives a rough surface on the casting. Such castings are difficult to clean because sand grains are held by little fingers of metal.
A sticker occurs when sand sticks to the pattern, as the pattern is drawn from the mold.
A runout, bleeder, or breakthrough is a casting in which the mold has failed so that the metal runs out before the casting is solid.
A cut or wash is erosion of the sand by the stream of molten metal. It often shows up as a pattern around the gates and usually causes dirt in some part of the casting.
A swell is an enlarged part of a casting resulting frequently from soft ramming. It is often found in connection with metal penetration.
A crush or drop occurs when part of the sand mold is crushed or drops into the mold cavity. It usually causes dirt in some other part of the casting.
A shift is a mismatching of cope and drag or of mold and cores.
Hot cracks or hot tears are usually irregular and oxidized so that the fracture appears dark. A dark fracture usually shows that the crack or tear occurred while the casting was still hot and contracting. A bright fracture indicates that the break occurred when the casting was cold.
A fin is a thin projection of metal usually found at core prints or parting lines. Fins are common on castings and not too harmful if small. If large, they can cause a runout, or small shrinkage cavities at the junction of the fin with the casting.
Inclusions or dirt are just what the name implies. They are often accompanied by other defects which provide loose sand in the mold.
The most common defects caused by casting design are hot tears and hot cracks. A hot tear is usually recognized by its jagged discolored fracture. It occurs when the stresses
in the casting are greater than the strength of metal shortly after the casting has solidified. A hot crack occurs by the same method except that it takes place after the casting has cooled considerably. A hot crack is also recognized by a discolored fracture, but is smooth, as corn-pared with the jagged fracture of the hot tear.
Hot tears and hot cracks both are caused by improper design that does not provide adequate fillets at the junction of sections or that joins sections of different thicknesses without providing a gradual change in section size by tapering. Inadequate fillets (sharp corners) produce planes of weakness at the junctions of the sections and cause failure at these points. Failure from improper joining of heavy and thin sections is caused by the early solidification of the thin section before the heavy section has solidified. The contraction of the thin section produces a stress which is greater than the strength of the partially solidified heavy section. Something has to give; it is usually the heavy section.
The cure for hot cracks and hot tears caused by poor casting design is to provide adequate fillets at all junctions and to use tapered sections where sections of different thicknesses must be joined. Refer to Chapter 2, “Designing a Casting.”
Shrinkage cavities, misruns, cold shuts, pinholes, blows, drops, scabs, and metal penetration can also be caused by poor casting design as well as by other factors. Shrinkage cavities may be caused by using fillets large enough to produce a section that cannot be properly fed, or by heavy sections that are so located in the casting that they cannot be properly fed. The latter condition should be corrected by redesigning the casting, the use of chills on heavy sections, or by making the part as two castings which can be welded together.
Misruns and cold shuts are caused by a low pouring temperature for the sections involved, inadequate gating, or inadequate venting of the mold. Redesigning for the use of tapered sections can be used to eliminate these defects. Pinholes can be caused by nonuniform section size. A high pouring temperature necessary to overcome cold shuts and misruns in thin sections may result in pinholes in the heavier sections. This situation requires redesign for uniform section thickness, re-gating to permit lower pouring temperatures, or the use of chills on heavy sections.
Glows due to design can often be traced to insufficient means for the escape of core gas. This may be due to a core print which is too s mall or inadequately vented. Corrective measures call for an increase in the size of the core print and adequate venting, and the use of core coatings.
Sharp corners in the cope or on protruding sections may become weakened during the drawing of the pattern and cause drops. This is corrected by the use of fillets, increased draft on the pattern, and rounded corners. Deep pockets or overhanging sections in the cope cause drops because of the weight of the sand. If these cannot be overcome by changing the position of the pattern in the flask, reinforcements must be used to give the sand adequate support. Sharp corners also cause scabs because they aggravate the conditions in large flat surfaces, which cause scabs. The use of fillets and round corners will minimize the effect of sharp corners on scab formation. To minimize casting defects caused by improper design, maintain (1) the casting as simple as possible, (2) tapered sections to promote directional solidification, (3) corners rounded or filleted, and (4) plenty of draft on the pattern.
The most common defect which can be traced to pattern equipment is the shift. A shift is easily recognized by the mismatching of the cope and drag sections of the casting at the parting. This type of shift is caused by worn pattern equipment. Loose or worn dowel pins in a pattern will permit movement of the pattern parts during molding and cause a shift in the casting. A shift caused by a defective pattern can only be corrected by repairing the pattern. Good pattern maintenance will go a long way toward minimizing the occurrence of shifts due to worn patterns. The recognition of this defect is especially important in repair ship work because the great majority of castings are made with loose patterns.
Another defect frequently caused by a poor pattern is the sticker. A sticker is due to a poor pattern surface, which causes the sand to stick to the pattern. Poor pattern surface can be remedied by smoothing the rough spots and refinishing the pattern. A sticker which is not noticed in the molding operation will have the appearance of a drop in the completed casting.
Other defects that may be caused by pattern equipment include misruns, cold shuts, drops, and metal penetration. Worn pattern equipment, which causes sections to be thinner than designed, may produce misruns and cold shuts in the casting. A drop will be caused by a pattern having insufficient draft. Improper draft will cause the sand to crack when the pattern is drawn, and will cause a drop because of the weakened condition of the sand. Increase in the draft of the pattern is the cure for this defect.
Metal penetration may occur because an irregular parting line has prevented proper ramming of the sand. Metal penetration of this type can be corrected by remaking the pattern with a flatter parting line.
FLASK EQUIPMENT AND RIGGING
Crushes and shifts are the defects most commonly caused by defective flask equipment and rigging. A displacement of sand in the mold after it has been made causes a crush. Improperly aligned flask equipment, warped or uneven flask joints, bad-fitting jackets, and bad bottom boards all result in an unequal pressure on the mold, with the resulting displacement of sand which produces the crush. Properly maintained equipment is the only solution to crushes of this type.
Shifts are also caused by defective flask equipment. Worn pins or defective bushings in a flask allow movement of the cope to occur when closing the mold. Proper maintenance of equipment again is the solution for this defect.
Stickers are often caused by faulty flask equipment. The defective flask prevents a clean pattern draw and, as a result, some sand sticks to the pattern. The sticker shown in figure 198 was caused by loose pins and bushings.
Swells, fins, runouts, bleeders, metal penetration, hot tears, and hot cracks can often be traced to faulty equipment and rigging. Swells and fins are likely to occur when the mold weights are not heavy enough for the casting being poured. Because of the light weight of sand, the molten metal is able to displace the sand and produce a swell in the casting. If this displacement occurs at the parting line or a core print, the molten metal is able to penetrate the joint and a fin is the result. Swells and fins can be remedied by using enough mold weights or clamps to resist the ferrostatic force of the molten metal. Remember that iron, steel, brass, and bronze are heavier than sand, so the cope will tend to float off when these metals are used. For double security, use both mold weights and clamps.
Runouts and bleeder s occur when the molten metal penetrates the parting line and reaches the outside of the mold. A breakout may occur anywhere on the mold and may be caused by insufficient sand between the pattern and the flask, or by soft ramming. Runouts and bleeders at the parting line are often caused by uneven matching of the cope and drag. This mismatch may be caused by bad pins and bushings, dirt in the flask joint, bad bottom boards, or uneven clamping. The cures are self-evident.
Metal penetration (rough surface of the casting) is caused by ramming the mold too soft, as when there is too little space between the pattern and the flask. A larger flask will permit harder ramming between the pattern and the flask and reduce penetration of the metal between the grains of sand.
Hot tears and hot cracks can often be traced to a lack of collapsibility in sand which has been excessively reinforced. Excessive reinforcement prevents the sand from collapsing and obstructs free contraction of the casting. Remember that metals contract when they solidify and that the mold must be weak enough to allow the casting to contract. If the mold is too strong, the casting may crack. Reinforcement which is placed too near a sprue or riser has an even greater effect than that mentioned above. The reinforcement in this case restrains the sprue and riser from any free movement with the casting and is almost sure to cause hot tears or hot cracks. If hot tears or hot cracks occur near the point where risers or sprues are attached to the casting, the reinforcement of the mold should be checked as a possible cause.
GATING AND RISERING
Shrinkage cavities, inclusions, cuts, and washes are the defects most frequently caused by gating and risering. If a riser is too small for the section to be fed, there will not be enough metal to feed the section and a shrink will occur in the casting. The gross shrink shown in figure 199 was caused by inadequate feeding. Surface shrinks caused by improper feeding are shown in figures 200 and 201. Improper location of gates and risers for directional solidification can also lead to shrinks. Figure 202 shows an internal shrink which was exposed when the riser was removed. This defect resulted from an improper gating system which resulted in cold metal in the riser. When the casting was gated so as to put hot metal in the riser (as shown in figure 203), the shrink defect was eliminated.
Connections which freeze off too early between a riser and casting produce a shrink by the same method as a small riser because there is no molten metal available to feed the casting. In such a case, the connections should be made larger. The location of the riser with respect to the section it is feeding can also cause a shrink as shown in figure 204. In this casting, the location of the ingate prevented proper feeding of the casting even though the riser contained molten metal.
Inclusions are caused when the gating system permits dirt, slag, or dross to be carried into the casting. The method of eliminating inclusions is to provide a choking action at the base of the sprue by using a tapered sprue of correct cross-sectional area. If it is impossible to provide proper choking action in the gating system, a skim core should be used at the base of the sprue to trap dirt and slag. Dross inclusions in a fractured aluminum casting are shown in figure 205.
Cuts and washes are defects which are also caused by the gating system. lithe ingates of a mold are located so that the metal entering the mold impinges or strikes directly on cores or a mold surface, the sand will be washed away by the eroding action of the stream of molten metal. The defect will then appear on the casting as a rough section, usually larger than the designed section thickness. Sand inclusions are usually associated with cuts and washes as a result of the sand eroded and carried to other parts of the casting by the stream of metal.
Improper risering and gating can also cause blows, scabs, metal penetration, hot tears, hot cracks, swells, fins, shifts, runouts, bleeders, misruns, and cold shuts. Blows or gas holes are caused by accumulated or generated gas or air which is trapped by the metal. They are usually smooth-walled rounded cavities of spherical, elongated, or flattened shape. If the sprue is not high enough to provide the necessary ferrostatic heat to force the gas or air out of the mold, the gas or air will be trapped and a blow will result. An increase in the height of the sprue or better venting of the mold are cures for a blow of this type. A similar blow may also occur if the riser connection to the casting freezes off too soon and the metal head in the riser is prevented from functioning properly. To cure this situation, the connections should be made larger, placed closer to the casting, or the connection area should be checked for possible chilling from gaggers or improperly placed chills.
Scabs can be caused by the gating system if the gating arrangement causes an uneven heating of the mold by the molten metal. The cure for a scab from this cause is to regate the casting to obtain a uniform distribution of metal entering the mold.
Metal penetration (rough surface on the casting) occurs if the sand is exposed to the radiant heat of the molten metal for too long a time so that the binder is burned out. An increase in the number of ingates to fill the mold more rapidly will correct this situation. Any gating arrangements which cause the sand to be dried out by excessive radiation will result in metal penetration. A sprue which is too high will cause a high ferrostatic pressure to act on the mold surfaces and cause metal penetration. Metal penetration of this type can be cured by decreasing the height of the cope.
The location of gates and risers can cause hot tears and hot cracks. If the gates and risers restrict the contraction of the casting, hot tears and cracks will occur. If the defects are near the ingates and risers, this cause should be investigated as a possible trouble spot.
Swells, fins, runouts, and bleeders may also be caused by improper gating. Risers which are too high cause an excessive ferrostatic force to act on the mold, with the result that these defects occur. A reduction in cope height will correct defects of this type. Run-outs and bleeders may also occur if any part of the gating or risering system is too close to the outside of the mold. In such a situation, there is insufficient sand between the gates, runners, risers, and flask to permit proper ramming. This results in weak sand which cannot withstand the force of the molten metal. The selection of flasks of proper size for the casting being made is the method of overcoming these defects.
Misruns and cold shuts are caused by any part of the gating and risering system which prevents the mold from filling rapidly. Gates or runners which are too small restrict the flow of molten metal and permit it to cool before filling the mold. Improperly located in-gates will have the same result. If the pressure head of a casting is too low, the mold will not fill completely and a cold shut will result. Increasing the size of the gating system and relocation of ingates are methods used to correct defects due to the gating system. Increasing the height of the sprue will produce a greater pressure head and help to fill the mold rapidly.
By itself, the molding sand can cause all of the casting defects that a molder will encounter. This is one of the reasons that it is difficult to determine the cause of some defects. Blows can be caused by sand that is too fine, too wet, or by sand that has low permeability so that gas cannot escape. If the sand contains clay balls because of improper mixing, blows will be apt to occur because the clay balls are high in moisture. A blow caused in an aluminum casting by high moisture content of the sand is shown in figure 206. To remedy this situation, the sand should be mulled to break up the clay balls. If the sand contains too many fines, it will have a low permeability and the moisture or gas will have a difficult time flowing through the sand away from the casting. Fines should be reduced by adding new sands.
Too high a moisture content in the sand makes it difficult to carry the excessive volumes of water vapor away from the casting. Use of the correct moisture contents and the control of moisture content by routine tests with sand testing equipment is the best way to correct this cause. When the permeability of sand is low, it is difficult for even small amounts of moisture to escape through the sand. The addition of new sands and a reduction in clay content serve to “open up” a sand and increase its permeability.
Drops are often caused by low green strength. Such sand does not have the necessary strength to maintain its shape, so pieces fall off. Corrective measures call for an increase in binder, increase in mixing time, or an increase in both binder and mixing time. Don’t overlook the possibility of reinforcing a weak section of a mold with wires, nails, or gaggers.
A scab will be caused on a casting when the sand mold cannot expand uniformly when it is heated by molten metal. The individual sand grains have to expand. If the mold does not “give,” the surface of the mold has to buckle and cause a scab. An expansion scab is shown in figure 207. The main cause of a sand being unable to expand properly is the presence of too many fines in the sand. These fines cause the sand to pack much harder so that its expansion is restricted. Addition of new sands to properly balance the sand grain distribution and reduce the percentage of fines is used to obtain better sand properties. Another remedy is to add something to the sand to act as a cushion. Cereal flour, wood flour, and sea coal are all used for this purpose.
A molding sand may y have acceptable thermal-expansion properties, but a low green strength may also cause an expansion scab. The cure in this situation is to increase the clay content. A high dry strength and a high hot strength can also cause expansion scabs. The sand will be too rigid because of the high strengths, and proper expansion of the sand will be restricted. A reduction in the clay or binders which cause the high strengths will correct scabs due to these causes. If a scab is present on a casting surface where sand shakeout and cleaning was difficult, high hot strength of the sand was probably the cause. The binder should be reduced, cushioning materials added, or fines reduced by adding coarser sand.
The principal cause for cuts and washes is low hot strength. When the sand is heated by the molten metal, it does not have the necessary strength to resist the eroding action of the flowing metal and is washed away. If an increase in the amount of binder does not cure cuts and washes, a different type of binder may be required. An addition of silica flour may also be used to correct low hot strength. A low green strength and a low dry strength may also lead to cuts and washes. These properties are corrected by increasing the binder. A defect closely related to cuts and washes is the erosion scab. It is also caused by a molding sand having a low hot strength. A combination of other factors such as high moisture and hard ramming can also cause an erosion scab. Hard ramming makes the escape of moisture difficult when hot metal is poured into the mold. As a result, the expanding vapor loosens the sand grains and they are washed away by the molten metal. Sand inclusions in some part of a casting are always found when expansion scabs occur.
A metal-penetration defect occurs when the molten metal penetrates into the sand and produces an enlarged, rough surface on the casting. If the metal penetration is not too deep, it may have the appearance of a swell. Coarse sand, high permeability, and low mold hardness are the principal sand properties which cause metal penetration. A sand that is too coarse will have larger openings between sand grains (this accounts for the high permeability). Because of the openings, the molten metal does not have any difficulty in penetrating into the sand. A low mold hardness is caused by soft ramming of the mold. This condition offers a soft surface to the molten metal which, again, can easily penetrate into the sand. To correct penetration due to coarse sand and high permeability, fine sand must be added to the base sand to get a finer sand distribution and reduce the permeability. Harder and improved ramming technique is the cure for metal penetration caused by low mold hardness. If permeability of the sand cannot be reduced, a mold wash may be used to eliminate penetration. An example of metal penetration is shown in figure 209. The left side of the casting has a good surface – the result of using a mold wash to prevent penetration.
It is caused when the sand cracks and the crack is filled by the molten metal. A sand that collapses rapidly under the heat of molten metal will produce veining. This can be corrected by the addition of more binder or silica flour to the sand.
Hot tears and hot cracks are usually caused by poor sand properties. A high percentage of fines and a high hot strength are the principal causes. A high percentage of fine sand grains produces a more closely packed sand, with the result that it cannot contract properly when the casting itself contracts during and after solidification. The reduction of fines can be accomplished by additions of coarse sand. A high hot strength will also prevent the sand from contracting or collapsing properly. A reduction in the content of fines is also a corrective measure for hot cracks and tears due to a high hot strength. A reduction in the binder content may be required to correct a high hot strength.
Pinholes are caused by a high moisture content in the sand. Pinholes are recognized by their small size and location on the casting surface as shown in figure 211. The cure for pinholes is to use the correct amount of moisture. This can be determined by proper sand testing and control. There are other minor causes of pinholes, but high moisture content in the sand is by far the greatest source of trouble.
An expansion of the sand so that a part of the mold surface is displaced in an irregular line produces a rattail defect. These defects are shown in figures 212 and 213. Rattails do not always occur as severely as shown in these two examples. They may be as fine as hairlines on the surface of a casting. A sand of improper grain size distribution, high hot strength, and that has been rammed hard are the major contributing causes to rattails. To cure this situation, greater care must be taken to ram the mold to make a uniform mold surface of correct hardness. The high hot strength can be corrected by reducing the binder. Better expansion properties can be obtained by proper grain size distribution in the sand, or by adding cushioning materials.
Buckles are similar to expansion scabs and rattails. When an expansion scab is removed from the surface of a casting, an indentation in the casting surface will be revealed. A rattail is sometimes called a minor buckle. The cure for a buckle is the same as for an expansion scab.
Stickers due to sand are caused by too high a moisture content or by a low green strength. A high moisture content will cause the sand to stick to the pattern. A reduction in the moisture content is necessary to overcome stickers from this cause. If the green strength is low, the sand will not have the necessary strength to permit drawing from a pocket or along a vertical surface. Additions of binder or improvement of the mixing procedure by using a muller to produce a more uniform distribution of binder are corrective measures which can be taken to eliminate stickers of this type. Proper use of parting compounds will minimize sticking.
The molding sand conditions which contribute to casting defects also apply to cores. Among these conditions are low permeability (which causes blows), low binder content (which
leads to cuts and washes), and hot tears (which are caused by cores having low collapsibility). Figure 215 shows a casting that cracked because the core was too hard.
The baking of cores can also cause casting defects. An underbaked core will still contain a large amount of core oil, which may cause a blow when the casting is poured. Such a blow can be cured by baking the core properly and by using the correct amount of binder.
Overbaking of cores causes defects because it results in burned-out binders. An over-baked core will have a weak and soft surface. Cuts, washes, and metal penetration result from overbaking. Correct baking time for the type of binder used and for the size of the core is the method for correcting these defects.
Another contributing factor to the occurrence of hot tears may be over-reinforcement. This is especially true of larger cores where reinforcement is necessary. The use of reinforcing wires and rods only when they are required and only in amounts necessary is the way to overcome hot tears from over-reinforcement.
Core shifts cause runouts, bleeders, misruns, coldshuts, and castings that are dimensionally inaccurate. If cores are improperly fitted in the core print, molten metal can run in between the core and the mold and cause a bleeder or runout. Molten metal may also fill the vent and cause a blow. Incorrectly pasted cores, cores with vents too close to the surface, and cores with inert backing material too close to the surface provide an easy path for the molten metal to run out of the mold. Correct fitting and pasting of cores, relocation of vents toward the center of the core, and central location of inert backing material are the steps required to correct runouts and bleeders from these causes. A core shift may reduce the section thickness of a mold with the result that the section will not be completely filled. A misrun caused by a core shift is shown in figure 216. To correct such a defect, it is necessary to provide better support for the core, either by a better-designed core print or by the use of chaplets.
Molding practice, along with the other operations involved in foundry work, contributes to casting defects if not properly done. Blows are caused by a combination of hot sand and cold cores and flasks. This combination causes moisture to condense and to give a localized concentration of moisture which causes a blow. To prevent this type of blow, sand should be cool before making a mold. Do not use hot sand.
Hard ramming of the sand can cause blows and expansion scabs as shown in figure 217. The blows occur because moisture is prevented from escaping by the closely packed sand. Expansion scabs occur because the hard rammed sand expands and buckles. The casting shown in figure 217 was of such a design that hard ramming could not be avoided. In this case, wood flour additions were made to provide a cushion for the hard rammed sand so that it would expand without buckling.
Improved technique corrected this defect.
Metal penetration can take place because of soft or uneven ramming, which produces a soft mold surface. Harder and more uniform ramming is the cure for this type of defect. Soft ramming may also result in swells or fins. The sand is soft at the mold surface and cannot retain its shape against the pressure of the molten metal, with the result that the mold cavity is forced out of shape and the defect occurs.
Poor molding practice is probably the major cause for crushes. Careless closing of a mold will result in displaced sand or cores, which in turn result in the crush. If the bottom board is not properly bedded, a high spot of sand on the board will cause pressure against the mold and a displacement of sand in the mold cavity. This again will cause a crush. Incorrectly placed chaplets, or chaplets of incorrect size, will also result in pressure being exerted either on the mold or on the core and cause a crush. Any defect due to poor molding practice can be corrected by only one method; improve the molding technique. Care and attention to the various operations involved can go a long way toward minimizing defects caused by molding practice.
The defects caused most often by pouring practice are blows, misruns, coldshuts, and slag or dross inclusions. Blows are caused by using green ladles or ladles with wet patches. Severe blows caused by use of a green ladle are shown in figure 219. A defect from this cause is remedied by using ladles which are thoroughly dried after lining and after any patching is done. Misruns and cold shuts are caused by pouring when the metal is too cold or by interrupting the pouring of the mold. With immersion and optical pyrometers in proper operating condition, misruns and coldshuts due to cold metal are minimized. If either of these defects occur when temperature readings indicate hot metal, a defective instrument is indicated. Sometimes
more than one ladle of metal will be required to pour a mold. In such a case, pouring with the second ladle should start- before the first ladle has been emptied. Otherwise the short interval allowed for the start of pouring from a second ladle is sufficient to chill the metal in the mold and to cause a cold shut, or slag inclusions.
Slow pouring may produce uneven heating of a mold surface by the radiant heat from the molten metal and cause a scab. Faster pouring will fill the mold more rapidly and minimize the radiant heating effects in the mold cavity.
Pouring should always be as fast as the sprue will permit. if a slower or faster pouring rate is indicated, a different sprue size should be used.
Pouring from high above the mold results in an increased metal velocity in the mold until the sprue is full and can lead to washing defects. Also, pouring from a ladle which is held high above the mold permits easy pickup of gases by the molten metal, as well as agitation in the stream of metal.
The use of rusty or damp chills and chaplets almost always causes blows. The rust on chills and chaplets reacts readily with the molten metal and a large amount of gas is produced in the casting at this point. The localized high gas content cannot escape and a blow is produced. A similar situation is brought about by the use of damp chills or chaplets. The moisture on the chills or chaplets forms steam which results in a blow. Figure 220 shows a blow which was caused in an aluminum casting by using a bad chill.
Careless handling of a mold can result in wasted effort on the part of the molder. Rough treatment may result in drops. The careless placing of mold weights can also result in drops from excess pressure on the cope. A drop due to rough handling of the mold is shown in figure 221. The left side of the figure shows the cope side of the casting. The rough lump of metal which filled the cavity by the displaced sand can easily be seen. The right side of the figure shows the drag side of the casting with the hole at the center core caused by the sand which dropped from the cope. The sand that dropped may float in heavy metal castings and cause a second defect in the cope.
Cracks and tears can be caused by shaking out the casting too early. This causes chilling of the casting and high stresses are produced. The casting usually has a low strength when hot. Dumping of hot castings into wet sand can also cause cracks and tears. Careless grinding of the casting may cause localized overheating, high stresses, and cracks. “Cleaning Castings,” for correct grinding techniques.
The use of moist, dirty, or rusty melting tools may cause the introduction of moisture into the melt. This source of moisture can result in pinholes in the completed casting. Every effort should be made to maintain good melting practice to prevent the rejection of casting because of carelessness in the melting operation.
When determining the cause of casting defects, it must always be kept in mind that defects are more often due to a combination of causes rather than to one isolated cause. The use of properly kept records of previous castings, good sand control, and development of a good molding procedure can go far in making the job of eliminating casting defects an easier one. A chart has been included which indicates the causes of the various defects and possible cures and should be used as a convenient reference.