Castings are made by pouring molten metal into refractory molds and allowing the metal to solidify. The solidified metal will retain the shape of the mold cavity and can be removed from the mold when the metal is solid. A mold is made by shaping a suitable sand mixture around a pattern of the desired form. A metal or wood box (flask) is used to retain the sand. The pattern is then removed from the sand, leaving a cavity in the sand into which the molten metal can be poured.
The molder’s skill is the basic skill of the foundry. He must know how to prepare molds with the following characteristics:
- Strong enough to hold the weight of the metal.
- Resistant to the cutting action of the rapidly moving metal during pouring.
- Generate a minimum amount of gas when filled with molten metal.
- Constructed so that any gases formed can pass through the body of the mold itself rather than penetrate the metal.
- Refractory enough to withstand the high temperature of the metal, so it will strip away cleanly from the casting after cooling.
- Collapsible enough to permit the casting to contract after solidification.
The refractory material normally used by foundries is silica sand bonded with clay. The material usually provided for the variety of castings made aboard repair ships is a washed and graded silica sand mixed with clay and cereal bond as described in Chapter 4, “Sands for Molds and Cores.”
MOLDING TOOLS AND ACCESSORIES
Flasks are wood or metal frames in which the mold is made. They must be rigid so that distortion does not take place during ramming of the mold or during handling. They must also resist the pressure of the molten metal during casting. The pins and fittings should be continually checked for wear and misalignment to avoid mismatched or shifted molds.
The use of steel flasks is preferred, but cases will arise requiring a size of flask not available. Under such circumstances, a flask may be constructed of wood. It should be husky enough to stand wear and tear. If it is planned to use the flask for several molds, allowance should be made for some burning of the wood, which will often occur when the metal is poured.
A flask is made of two principal parts, the cope (top section) and the drag (bottom section). When more than two sections of a flask are necessary, either because of the size or design of the casting, intermediate flask sections, known as cheeks, are used.
RIDDLES are used for sifting the sand over the surfaces of the pattern when starting a mold. The size of the riddle is given by the number of meshes to the inch. A No. 8 riddle has eight meshes per inch, a No. 4 riddle has four meshes per inch, etc. The particular riddle used depends on the kind and character of casting to be made; castings with fine surface detail require finer sand and a finer riddle.
RAMMERS are used for tamping the sand around the pattern in the flask. For the heavier class of molding, they are made of iron. Sometimes they are made with a wooden handle with a cast iron butt on one end and a cast iron peen on the other. The small rammers used in bench work are usually made of maple, although sometimes they are made of cast iron or aluminum.
STRIKES are used to scrape the extra sand from the top of the cope or drag after ramming. They are usually a thin strip of metal or wood. They should have one straight edge and should be light but sturdy.
CLAMPS are used for holding together the cope and drag of the completed mold or for clamping together the mold-board and the bottom-board on either side of the drag when the latter is rolled over. They are of many styles and sizes. Some are adjustable and are tightened on the flask by means of a lever. Other types use wedges to secure them on the flask. The WEDGES are usually of soft wood, but for the heavier work are either of hard wood or iron.
BELLOWS are used to blow excess parting materials from the pattern and also to blow loose sand and dirt from the mold cavity. Compressed air hoses have almost replaced bellows for this purpose.
TROWELS are of many different styles and sizes to suit the individual taste of the molder and the particular requirements of the job. The trowel is used for making joints and for finishing, smoothing, and slicking the flat surfaces of the mold.
VENTS – Thin, rigid steel strips are used for making vents. Hacksaw blades are suitable for this purpose. Rods are also used for vents but they often cause a shrinkage depression at their base on the casting.
BOSHES or SWABS are made of hemp, tasselled to a point at one end and bound with twine at the other to hold it together. They are used for placing a small amount of water on the sand around the edge of the pattern before the pattern is rapped for drawing from the mold. Bushes will hold considerable water and the amount which they deliver to the sand can be regulated by the pressure the molder applies when squeezing them. Boshes are also used to apply wet blacking to dry-sand molds when they are to be blacked before the mold is dried.
SOFT BRUSHES are used to brush the pattern and the joint of the mold. The hard brush is used to spread beeswax or tallow on metal patterns and to brush and clean out between the teeth of gears and similar patterns.
CAMEL’S HAIR BRUSHES are used to brush dry blacking on the face of the mold.
RAPPING and CLAMPING BARS are usually bars of steel about 3/4 inch in diameter and 2 feet long. They are pointed at one end to enter rapping plates in a pattern and are flattened and turned up at the other end for convenience in tightening clamps on a flask.
DRAW SCREWS are eye-bolts threaded on one end. They are used for drawing large wooden patterns from the sand by screwing into holes drilled for that purpose in the rapping plate. They are also used for drawing metal patterns where pointed spikes could not be used.
DRAW SPIKES are steel rods which are sharpened at one end for driving into a wooden pattern to rap and draw it and are principally used in bench work for drawing small patterns.
LIFTERS are used for removing loose sand from deep cavities in molds. They are of different lengths and sizes, one end being turned at right angles to the stem; this portion is called
the heel. The straight, flattened portion is known as the blade and is used to slick the sides of the mold where they cannot be reached by the trowel or slicker. The heel is also used to slick the bottom of deep recesses after the sand has been removed.
SLICKERS are formed with blades of varying widths, sometimes with one end of the tool turned to form a heel somewhat similar to the lifter. It is used for lifting loose sand from shallow parts of the mold, for patching, and to form corners to the proper shape. This tool is widely used by molders.
SPOON SLICKERS have spoon-shaped ends and are used to slick rounded surfaces in a mold. They are usually made with one end larger than the other.
The DOUBLE ENDER has a slicker at one end and a spoon at the other. They are usually made to the molder’s order and are used on small molds.
CORNER TOOLS are used to slick the corners of molds where a slicker or the heel of a lifter is not satisfactory. Corner tools are made with different angles for special work.
Various specialized tools such as flange tools, pipe tools, and hub tools are also used.
WOODEN GATE-PINS or SPRUES are round tapered pins used to form the sprue or down-gate through which metal is poured into the mold. The size depends on the size of the mold.
GATE CUTTERS are pieces of sheet brass bent to a semicircle on one edge. They are used to cut the ingate in the drag leading from the base of the sprue to the mold cavity.
SPRUE CUTTERS are cylindrical metal tubes used to cut the sprue in the cope when the sprue-stick is not used. Tapered sprue cutters are available for making the more desirable tapered sprue. They must be pressed down from the cope side before stripping the mold from the pattern.
CALIPERS are used more often by the core maker than the molder. The molder uses them to verify the sizes of cores in order to insure proper fit in the core print and also to obtain the length of smaller cores. The calipers in this case are set at the proper dimension and the core filed to fit. This is important in dry-sand work to prevent crushing of the mold if the core is too large when the mold is closed.
CUTTING NIPPERS are used to cut small wires to the desired length for use in cores or molds.
Facing nails are used: (1) to reinforce mold surfaces and to prevent washing of the mold face, (2) to mechanically lock the sand on the face with that deeper in the body of the mold, and (3) to act as a means for slightly accelerating solidification at internal corner s of the casting. These nails are similar to “roofing nails,” having a flat, thin head of large diameter and shanks of various lengths. Caution must be exercised to see that no galvanized, rusty, oily, or dirty nails are used. The use of anything but clean dry nails will result in defective castings.
Gaggers are used to give support to hanging masses of sand which would break under their own weight unless they were supported. Gaggers should be cleaned and are coated with clay before use to provide a better bond with the sand. Care must be taken in the placing of gaggers in the mold so that they are not too close to a mold surface, where they would cause a chilling of the metal where it is not wanted. Many times, a casting defect can be traced to a gagger located too near to a mold surface.
Chaplets are metal supports used to hold a core in place when core prints are inadequate. They are too often used to compensate for poor design, improper pattern construction, or bad core practice. In all castings (especially in pressure castings), chaplets are a continual source of trouble and should be avoided whenever possible. Figures 73, 74, and 75 show typical chaplets. It is absolutely necessary that they be clean. Rust, oil, grease, moisture, or even finger marks, cause poor fusion or porosity. Sandblasting immediately before use is a good practice if no other protection is used. Copper and nickel plating is a good method of protecting chaplets from rusting but does not eliminate the need for absolute cleanliness. Their size must bear a direct relationship to the type and section of metal in which they are to be used. Soft-steel chaplets are used in iron and steel, and copper chaplets in brass and bronze castings. Chaplets should be the same composition as the casting, if possible. The strength of the chaplet must be enough to carry the weight of the core until sufficient metal has solidified to provide the required strength, but it should be no heavier than necessary. The use of an oversize chaplet results in poor fusion and often causes cracks in the casting. A chaplet which can be made in the machine shop for emergency use is shown in figure 76. Chaplets should not have any sharp, internal corners because metal will not fill a sharp internal groove.
It is well to consider the forces which a chaplet must resist. In all metals except aluminum and the light alloys, a core tends to float
when molten metal is poured into the mold. It is buoyed up by a force equal to the weight of the displaced molten metal. A core with dimensions of 12 inches by 12 inches by 12 inches, or one cubic foot, will weigh approximately 100 pounds. Immersed in molten gray iron, which weighs 450 pounds per cubic foot, the core will tend to remain in place until it has displaced 100 pounds of iron, and then it will tend to float. In order to keep it submerged (displacing 450 pounds of cast iron) it will be necessary to exert 350 pounds of force on it (450 – 100 = 350). It takes no more force to keep it submerged at greater depths than just below the surface. A greater head does not increase the lifting effect, although it does increase the pressure on the core.
The ratio of 100 to 350, or 1 to 3.5, holds good for cores of any size, so we can make the rule that the force resulting from the tendency of a sand core to lift in cast iron is roughly 3.5 times its weight; for steel, 3.9 times; for copper, 4.5 times; etc.
Where chaplets are used on large cores with extensive surface areas exposed to the metal, the usual practice is to use ordinary chaplets in the drag (since they are only required to hold the core in place until the metal is poured around them) and to use stem chaplets in the cope. Stem chaplets, instead of bearing on the mold face, pass through the mold body and are brought to bear against a support placed across the top of the flask. They are thus able to withstand very high forces, such as imposed when large cores tend to float on the metal. Figure 77 illustrates this method. It also shows a useful method for increasing the load-carrying ability of the green sand mold. A dry-sand core is used as a chaplet support in the mold. A dried oil sand core will safely support a load of 70 to 90 p.s.i. while the strength of green sand is 5 or 6 p.s.i.
Metal wedges or shims must be used to hold the stem chaplet down because the force of the molten metal acting on the core and transmitted through the stem of the chaplet will force it into a wooden wedge and thus allow the core to rise.
A detailed description of the use of chills will be found in Chapter 7, “Gates, Risers, and Chills.”
Chills used in making molds are internal chills and external chills. Internal chills are set so they project into the mold cavity. They are expected to fuse with the solidifying metal and become a part of the casting. Extreme care should be taken to make sure the chills are clean. Any grease, finger marks, film, or dirt will prevent good fusion between the chill and the casting. External chills are rammed up with the mold to anchor them firmly in the sand. They also should be rust free and clean when used without special treatment. Many times, when external chills fuse to a casting, the condition can be overcome by coating the chill surface with a thin coat of shellac, or other adhesive material, applying a very thin layer of fine, dry sand, and then drying the chill to drive off any moisture. Many commercial chill coating materials are available also. Torch drying of coated chills in the mold should be avoided because moisture from the flame will condense on the chill. Moisture will condense on cold chills in green sand molds if the molds are closed and allowed to stand for an appreciable time before pouring.
CLAMPS AND WEIGHTS
Clamps and weights are used to hold the cope and drag sections of a mold together and to prevent lifting of the cope by the force of the molten metal. It is safe practice to use a weight on small molds, but when the molds are of considerable size, both weights and clamps should be used. The use of insufficient weights is a common cause of defective castings.
TYPES OF MOLDS
The types of molds which are made aboard repair ships are (1) green-sand molds, (2) dry-sand molds, and (3) skin-dried molds. These three types of molds differ mainly in their sand mixture content.
Molds made from tempered sand (see chapter on foundry terminology) and not given any further treatment are called green-sand molds. Green-sand molds are used for normal foundry work aboard ship. They have the necessary green strength and other properties which make them suitable for a great variety of castings. Green sand gives less resistance to contraction of a casting than does dry sand, and thereby tends to prevent hot cracks in the casting. Green-sand molds are the easiest to make.
Dry-sand molds, as the name implies, are molds made with tempered sand and then thoroughly dried by baking. Dry-sand molds are used when a mold of high strength is needed, or when low moisture content is important. Dry-sand molds are not recommended for complicated castings unless special care is taken to obtain sand mixtures which will give good collapsibility, so as to prevent hot cracks or tears.
Skin-dried molds are green-sand molds which have been dried only on the mold surface by the use of a torch or some other source of heat. Skin-dried molds are used where a mold surface low in moisture content is necessary. The mold surface is usually sprayed with additional special binding materials and then dried by the use of a torch. This type of mold combines the firm sand face obtained from a dry-sand mold with the collapsibility of a green-sand mold in the backing sand. In general the sand used for skin-dried molds has a moisture content higher than for a green-sand mold and dry sand molds require a still higher original moisture content.
MOLDING LOOSE-PIECE PATTERNS
Loose-piece patterns are in one piece or are split to make molding easier. Molding with a split pattern will be described here. Molding with a single-piece pattern (and the use of broken parts) usually involves the cutting of a parting line and will be described under the section, “False-Cope Molding,” later in this chapter.
In making a mold, a flask should be selected so that sufficient room is allowed between the pattern and flask for risers and the gating system. There must also be enough space over and under the pattern to prevent any break-outs of the metal during pouring or straining of the mold. Many castings are lost, or require extra cleaning, and many injuries to personnel are caused by the use of undersized flasks. It is better to err on the side of safety and choose too large a flask, rather than to use a flask that is too small. In addition to the safety factor, an undersized flask makes positioning of the gages and risers difficult. Gates and risers placed too close to a steel flask will be chilled by the flask and will not perform their function properly. Safe practice in the selection of a flask is shown in figure 78.
For a split pattern, such as that for a pump housing, a smooth ram-up board and a bottom board are needed. The ram-up board should be
of sufficient size to project an inch or two beyond the flask. A one-piece board, such as 3/4-inch plywood, is preferred. The use of such a ram-up board keeps mold finishing and slicking to a minimum.
Before use, the pattern should be checked for cleanliness and the free working of any loose pieces which must seat securely. Any chills which will be required should be clean and on hand ready for use. The chills should be checked to make sure that they fit the pattern correctly and have the proper means for anchoring them.
When using a split pattern, the drag part of the flask is turned upside down and placed on the ram-up board. If the flask is not too large, the ram-up board and drag can be placed on the cope of the flask. The drag pattern is placed with the parting surface down on the ram-up board along with any pieces used for the gating and risering system. Figure 78 shows a pump-housing pattern set in the drag with the parts of the gating system. The facing sand is then riddled to a depth of about one inch on the pattern and the ram-up board. Riddling of the sand is absolutely necessary for good reproduction of the pattern. The riddled sand is then tucked into all pockets and sharp corners and hand packed around the pattern as shown in figure 79.
Backing sand is then put into the flask to cover the facing sand to a depth of three or four inches. The backing sand should be carefully rammed into any deep pockets as shown in figure 80. The remainder of the mold is then rammed with a pneumatic or hand rammer, care being taken to avoid hitting or coming too close to the pattern. The mold must be rammed uniformly hard in order to obtain a smooth, easily cleaned casting surface and to avoid metal penetration into the sand, swelling, break-outs, or other casting defects. When this ramming is completed, five or six more inches of sand are added at a time and rammed until the flask is filled to a point about one inch above the top of the flask.
Next, the excess sand is “struck off,” by means of a straight edge or strike as shown in figure 81. Instead of striking the sand in one motion, it is often easier to loosen the sand by a series of short strokes and then remove it with one motion. When the struck-off surface is smooth, a scattering of a small amount of loose sand on the struck surface helps to give better contact with the bottom board. The bottom board is placed into position with a slight circular motion. Good, full, and solid contact between bottom board and drag sand is important if the mold and pattern are to have adequate support when they are rolled over. The drag section is then clamped between the bottom board and the ram-up board and turned over. The ram-up board (which is now on top) is removed and the mold face cleaned and slicked. Figure 82 shows the drag of the mold ready for the cope.
A parting material is sprinkled over the mold joint and pattern. The parting material prevents the sand in the cope from sticking to the sand in the drag when the cope is rammed up. The parting material for large castings is usually fine silica sand. For medium and small castings, finely ground powders (such as talc or silica flour) are used.
The cope of the flask is set on the drag and seated firmly with the aid of the flask pins. The cope pattern, riser forms, and any other parts of the gating system are set in their proper positions. Figure 83 shows the mold with the cope pattern, sprue, whirl-gate, and cross-gate pieces set.
The facing sand is riddled into the cope and hand packed around corners and in deep pockets. At this point, any gaggers which are necessary are placed. Care should be taken not to set the gaggers too close to the pattern risers or parts of the gating system. Gaggers set too close to the mold surface will cause undesired chilling of the metal. Any mold showing exposed gagger s in the cope after the cope pattern has been drawn, should be shaken out and made over. The number of gaggers and supporting bars will depend upon the size of the casting. Large flasks will require cope bars to support the sand. Gaggers may be fastened to the cope bars. The flask is then filled with sand and rammed as in making the drag. The sand should be packed by hand around any riser forms or raised portion of the pattern and care should be taken to avoid striking any forms or patterns during ramming. The partially filled cope is shown in figure 84. Notice that the cope has been peened around the inside edge of the flask. This procedure should be followed for both cope and drag, as it serves to pack the sand tightly against the flask and to prevent the sand from dropping out during handling. Also notice that the sprue and swirl gate form are slightly below the top of the cope of the flask. The cope is filled the same as the drag with successive fillings and uniform ramming. The completed mold is then struck off. With the riser, gating, and sprue forms a little below the top of the flask, the excess sand can be struck off without disturbing them.
After the mold has been struck off, it is vented in the cope as shown in figure 85. The cope is then removed from the drag, set on its side, and rolled over to facilitate drawing the cope pattern. By tapping the runner, riser, and sprue forms lightly on the parting side of the cope, they will come free easily and can be removed.
Any cope pattern pieces are also withdrawn at this time. The pieces used for the gating system are drawn from the drag. Cutting of in-gates is done before drawing the pattern if possible. Loose sand should be cleaned from the drag. The drag pattern is drawn by the use of eyebolts or draw pins as dictated by the pattern. A light rapping of the pattern and eye bolt before and at the beginning of the draw will make this operation easier. (NOTE: avoid excessive rapping.) The beginning of the pattern draw for the pump housing is shown in figure 86. The drawn pattern is shown in figure 87. Notice that this pattern was drawn with both hands. Such procedures give the molder better control over the pattern. The cope and drag are both inspected and cleaned, if necessary. Slicking of the mold should be kept to a minimum, but the pouring gate in the cope should be compacted and smoothed so as to eliminate loose sand and prevent washing out by the molten metal.
If any facing nails are required to resist washing of the mold face, they should be placed at this time. Any sharp corners or fins of sand in the mold cavity or in the gating system should be carefully removed. Any such projections will be washed out by the stream of molten metal and result in defective castings.
Once the cope and drag have been properly finished, the cores should be set in place. It is good practice to rest the arms against the body while setting the core to make the operation easier and smoother and to avoid damage to the mold. Both hands should be used for setting all but the smallest cores. The handling of the core for the pump housing is shown in figure 88. The cope and drag with the core set and ready for closing are shown in figure 89.
The mold is closed carefully by using pins to guide the cope. The cope should be lowered slowly and kept level. Any binding on the pins because of cocking of the cope should be avoided. A jerking motion caused by binding pins often causes sand to drop from the cope. This is one reason why flask equipment should be kept in top condition. After the mold is closed, it is clamped, the weights are placed, and the pouring cup or basin set over the sprue. The mold, ready for pouring, is shown in figure 90.
The proper pouring techniques are discussed , “Pouring Castings.”. A block of iron was used to hold the pouring basin down. Notice that the lip of the ladle is close to the basin and that the basin is kept full of metal.
MOLDING MOUNTED PATTERNS
A mounted pattern is one which is attached to a ram-up board. It is called a match-plate pattern if the cope pattern is mounted on one side of the board and the drag pattern on the other. For a match-plate, the cope and drag patterns must be aligned perfectly.
The molding of a mounted pattern is much easier than the molding of loose patterns. Mounted patterns are usually metal and are used for quantity production, but their use is often justified when quite a few castings of one design are required. Mounted patterns may also be of wood, but these require proper care and storage facilities to prevent warping.
The advantages of a mounted pattern are that the parting-line surface can be rammed much harder than with loose-pattern molding and a vibrator can be attached to the pattern plate to make the drawing of the pattern much easier. Another important advantage is that the gating and risering system can be made a fixed part of the pattern. As a result, smooth hard surfaces will be obtained and sand-erosion problems reduced.
The sequence of operations for molding with mounted patterns is the same as for molding loose patterns. The pattern is set between the cope and drag parts of the flask and held in place by the flask pin. The drag is rammed up first, the flask rolled over, and the cope rammed up. When the mold is completed, the cope is drawn off the pattern and then the pattern drawn from the drag. Core-setting operations and closing the mold are the same as for loose pattern molding.
FALSE-COPE MOLDING AND THE USE OF BROKEN PARTS AS PATTERNS
Some patterns do not have a straight or flat parting line that permits them to be placed solidly against a ram-up board. Broken castings or parts which are to be used as a pattern usually fall into this class. Very often castings of this type would be impossible to mold by the common cope-and-drag method. The difficulties come from the facts that: (1) the parting line is not straight, and (2) the pattern or broken part requires special support while being molded. The false-cope method provides this extra support and makes it possible to have a very irregular parting line. Essentially the method consists of molding the part or pattern roughly into a false cope that is used to support the part while the final drag section is molded. The false cope is then removed and a final cope section molded Lo take its place in the final mold assembly.
The cutting of an irregular parting line is probably the most important step in false-cope molding and will be described here. A small boat propeller is used as the pattern.
The propeller is set in the drag, on the ram-up board, as shown in figure 93. The facing sand is riddled onto the pattern, and the drag filled with sand and rammed in the conventional manner. The bottom board is set and the flask rolled over. The cope of the flask and ram-up board are removed. The parting line is then cut with the use of spoons and slicks. The sand must be removed to provide a gradual slope from the casting parting line to the flask parting line. A 45° slope usually is the maximum that can be tolerated and prevent sand from dropping. The completely cut parting line is shown in figure 94. The cope section of the flask is set in position, parting compound sprinkled on the drag, and the cope rammed. Extreme care must be taken in ramming to prevent damage to the drag. After the cope is completed, it is carefully removed from the drag. The drawn cope is shown with the drag in figure 95. The pattern is then drawn, and the sprue, gates, and risers cut. The mold ready for closing is shown in figure 96. The as-cast propeller is shown in figure 97.
In false-cope molding, the false cope provides a bearing surface for the pattern when ramming up the drag. It has the advantage that the finished mold is not disturbed in cutting the parting line.
The preparation of a false cope consists of molding the pattern in the cope and cutting the parting line. The sand is rammed as hard as possible to provide a good bearing surface when ramming the drag. An alternative way of preparing a false cope is to ram the cope without the pattern. The pattern is then bedded into the parting line side of the cope. The parting line can be cut into the cope or built up with additional sand, or it may be a combination of both.
The drag section of the flask is then placed into position, parting material sprinkled over the mold joint, and the drag made as described for loose-pattern molding. The flask is then rolled over and the cope drawn. Extreme care must be taken in drawing the cope. The original cope is discarded, the cope section returned to the drag, parting compound sprinkled over the mold joint and a new cope made. Extreme care must be taken in ramming the cope to prevent any damage to the drag. The cope is then carefully drawn, the pattern drawn, sprues, gates, and risers cut, and the mold closed. This type of molding provides a firm, sharp, parting line without any loose sand particles that might wash into the mold cavity.
If several castings are required from a pattern with an irregular parting line, a more permanent type of false cope, or “follow board” can be used. A shallow box, the size of the flask and deep enough for the cope section of the pattern, is made from wood. The box should be made so that it is held in place by the flask pins. The pattern should be given a light grease coating to prevent any sticking. It is then positioned in the box, cope side up, in the manner previously described. Plaster is poured around the pattern and permitted to set firmly but not hard. The false cope and follow board are then turned over together. The pattern is worked back and forth slightly so that it can be drawn easily. While the plaster is still workable, the parting line is cut and the plaster permitted to harden. After the plaster has dried completely, it may be coated with shellac to prevent any moisture pickup. Nails may be used through the sides of the frame to help support the plaster. A follow board may be made in a similar manner by building up the required backing with fireclay mixed to the consistency of heavy putty, and working it around the pattern. A fireclay match has the disadvantage that it must be kept slightly moist to keep the fireclay from cracking.
The follow board is used in place of the false cope in providing the necessary support when ramming the drag. The pattern is set in the follow board, and the drag rammed up as for molding a loose pattern. After the bottom board is set, the drag is rolled right side up and the match plate drawn, exposing the pattern in the drag with the parting line made. The cope is then placed and the molding completed as for loose -pattern molding.
SETTING CORES, CHILLS, AND CHAPLETS
In the setting of cores, it is important to check the size of the core print against the core itself. A core print is a depression or cavity in the cope or drag, or both. The print is used to support a core and, when the core is set, is completely filled by the supporting extensions on the core. A typical example of a core print in use is shown at the extreme left of the mold in figure 89. An oversize print or an undersize core will cause fins on the completed castings, which may lead to cracks or chilled sections in the core area. An oversize core or an undersize print may cause the mold to be crushed and result in loose sand in the mold and a dirty casting.
Setting simple cores in the drag should be no problem to a molder. Care should be taken in handling and setting the core. After a core has been properly set, it should be seated by pressing it lightly into the prints. Another item which should be checked is the venting of cores through the mold. Many times, the cores themselves are properly vented but the molder forgets to provide a vent through the mold for the core gases to escape.
In come instances, the cores may have to be tied to the cope. In such a case, they are attached to the cope by wires extending through the cope. The wires are wound around long rods resting on the top of the cope to provide additional support. The rods should rest on the flask to prevent crushing or cracking of the cope.
Such operations should be done with the cope resting on its side or face up. The tieing should be done with as little disturbance as possible to the rammed surface. The core should be drawn up tight to prevent any movement of the core while the mold is being closed. Before closing the mold, the cope should be checked to make sure it is free of any loose sand.
Chills are rammed in place with the mold and are described under “Molding Tools” in this chapter. Again it is emphasized that chills must be clean and dry. Even chills which have just been removed from a newly shaken-out mold should be checked before immediate reuse.
The use of chaplets was described earlier in this chapter under “Molding Tools.” It must be remembered that chaplets should be used only when absolutely necessary. Preferably, another method for support (for example, core prints) should be used, if at all possible. The
use of chaplets in pressure castings should be completely avoided.
The most important factor in the proper and easy closing of molds is to have flask equipment in good condition. Clean pins and bushings and straight sides on the flasks are the factors that make the closing of molds an easy operation. The opening of a mold after it has been closed is sometimes recommended. This procedure may prove useful. By using an excess of parting compound, the molder can then determine, with a fair degree of certainty, any mismatch or crushing of the mold. Nevertheless, the fewer times a mold is handled, the fewer chances there are to jar it and cause sand to drop.
The molding operation aboard ship depends primarily on the molder and his ability to do his job. Skill in this type of molding can be attained only through experience, but a high level of skill can be reached in a shorter length of time by following correct molding techniques. For a beginning molder, it may appear much easier to patch molds that have been made haphazardly, than to take the time to make them properly. A molding technique based on careful attention to the various details involved in making a mold is by far the best approach to attaining molding skill. As with many other trades, speed in molding comes about by itself, if proper attention is given to the basic techniques.