Selecting the casting process is an important element in the design cycle, even though in some cases, it is a decision that can be left to the foundry. More often than not, the process to be used falls out logically from the product's size, shape and technical requirements. Among the more important factors that influence the choice of casting method are:
Other considerations, such as code requirements, can also play a role in selecting the casting process, but it is primarily the number and size of castings required, along with the alloy chosen, that determine how a casting will be made.
That is not to say that the designer has little choice; in fact, quite the opposite can be true. For example, small parts made in moderate to large quantities frequently lend themselves to several processes, in which case factors such as surface finish, soundness or mechanical properties will bear strongly on the choice of method used. These parameters are set by the designer.
It is convenient to classify the casting processes as being applicable either to general shapes of more or less any form or to specific and usually rather simple shapes. In addition, several new special processes have been commercialized in recent years, one of which is described below.
Sand Casting. Sand casting currently accounts for about 75% of U.S. copper alloy foundry production. The process is relatively inexpensive, acceptably precise and above all, highly versatile. It can be utilized for castings ranging in size from a few ounces to many tons. Further, it can be applied to simple shapes as well as castings of considerable complexity, and it can be used with all of the copper casting alloys.
Sand casting imposes few restrictions on product shape. The only significant exceptions are the draft angles that are always needed on flat surfaces oriented perpendicular to the parting line. Dimensional control and consistency in sand castings ranges from about + 0.030 to + 0.125 in (+ 0.8 to 3.2 mm). Within this range, the more generous tolerances apply across the parting line. Surface finish ranges between approximately 300 and 500 gin (7.7 - 12.9 gm) rms. With proper choice of molding sands and careful foundry practice, surprisingly intricate details can be reproduced. There are a number of variations on the sand casting process.
In green sand casting - still the most widely used process--molds are formed in unbaked (green) sand, which is most often silica, SiO2, bonded with water and a small amount of a clay to develop the required strength. The clay minerals (montmorillonite, kaolinite) absorb water and form a natural bonding system that holds the sand particles together. Various sands and clays may be blended to suit particular casting situations.
The mold is made by ramming prepared sand around a pattern, held in a flask. The patterns are withdrawn, leaving the mold cavity into which metal will be poured. Molds are made in two halves, an upper portion, the cope, and a lower portion, the drag. The boundary between cope and drag is known as the parting line.
Cores, made from sand bonded with resins and baked to give sufficient strength, may be supported within the mold cavity to form the internal structure of hollow castings. Chills of various designs may be embedded in the mold cavity wall to control the solidification process.
Risers are reservoirs of molten metal used to ensure that all regions of the casting are adequately fed until solidification is complete. Risers also act as heat sources and thereby help promote directional solidification. Molten metal is introduced into the mold cavity through a sprue and distributed through a system of gates and runners.
Bench molding operations are performed by hand. Quality and part-to-part consistency depend largely on the skill of the operator. The labor-intensive nature of bench molding usually restricts it to prototypes or short production runs. Patterns are another significant cost factor, especially if their cost cannot be amortized over a large number of castings. Still, bench molding remains the most economical method when only a few castings must be produced.
The machine molding method is 'automated and therefore faster than bench molding, but the casting process is essentially similar. Molding machines sling, ram, jolt or squeeze sand onto patters, which in this case may consist of several parts arranged on a mold board. Dimensional control, surface finish and product consistency are better than those achievable with bench molding. Favorable costs can be realized from as few as several dozen castings. Machine-molded sand casting is therefore the most versatile process in terms of production volume.
Waterless molding aims to eliminate the sometimes detrimental effects of moisture in the molding sand. Clays are treated to react with oils rather than water to-de them bond to the sand particles. The hot strength of the waterless-bonded sand is somewhat lower than that of conventional green sands. This reduces the force needed to displace the sand as the casting shrinks during solidification, which in mm reduces the potential for hot tearing. On the other hand, sands with low hot strength have a greater tendency to be damaged by hot metal flowing into the mold.'
For large castings, molds may be baked or partially dried to increase their strength. The surfaces of skin-dried molds are treated with organic binders, then dried by means of torches or heaters. To make dry sand molds, simple organic bonding agents such as molasses are dissolved in the bonding water when making up the green sand mixture. The entire mold is then baked to develop the desired hot strength. Besides hardening the mold, removing water also reduces the chance for blowholes and other moisture-related casting defects. Baking and skin drying are expensive operations and the dry sand methods are rapidly being replaced by a variety of no-bake processes, described below.
There are three general types of low-temperature-curing, chemical binders: Cement has traditionally been used as a bonding agent in the extremely large molds used to cast marine propellers and similar products. Cement bonded molds are extremely strong and durable, but they must be designed carefully since their inability to yield under solidification shrinkage stresses may cause hot tearing in the casting.
Organic binders utilize resins that cure by reaction with acidic catalysts. Furan-, phenolic-, and urethane-base systems are the most popular of the large variety of currently available bonding agents. Of the inorganic binders, the well-known liquid sodium silicate-CO2 process is most widely used for copper alloy castings.
No Bake (Air Set). In this process, silica sand is mixed with a resin that hardens when exposed to the atmosphere. The process requires no water. It can be used for molds as well as cores. It is applicable to products as small as 20 lb (9 kg), although it is mainly used for large castings weighing up to 20,000 lb (9,100 kg). The no-bake process has become very popular in the past 10 years.
Shell Molding. Resin-bonded sand systems are also used in the shell molding process, in which prepared sand is contacted with a heated metal pattern to form a thin, rigid shell.. As in sand casting, two mating halves of the mold are made to form the mold cavity. Common shellmolding binders include phenolformaldehyde resins, furan or phenolic resins and baking oils similar to those used in cores. Non-baking resins (furans, phenolics, urethanes) are also available; these can claim lower energy costs because they do not require heated partems.
The shell molding process is capable of producing quite precise castings and nearly rivals metal-mold and investment casting in its ability to reproduce fine details and maintain dimensional consistency. Surface finish, at about 125 gin (3.2 gm) rms, is considerably better than that from green sand casting.
Shell molding is best suited to small-to-intermediate size castings. Relatively high pattern costs (pattern halves must be made from metal) favor long production runs. On the other hand, the fine surface finishes and good dimensional reproduceability can, in many instances, reduce the need for costly machining. While still practiced extensively, shell molding has declined somewhat in popularity, mostly because of its high energy costs compared with no-bake sand methods; however, shellmolded cores are still very widely used.
Plaster Molding. Copper alloys can also be cast in plaster molds to produce precision products of near-net shape. Plaster-molded castings are characterized by surface finishes as smooth as 32 gin rms and dimensional tolerances as close as _+ 0.005 in (+ 0.13 mm), and typically require only minimal finish machining. In some cases, rubber patterns can be used. These have the advantage of permitting re-entrant angles and zero-draft faces in the casting's design.
Gypsum plaster (CaSO4) is normally mixed with refractory or fibrous compounds for strength and specific mechanical properties. The plaster must be made slightly porous to allow the escape of gases as the castings solidify. This can be achieved by autoclaving the plaster molds in steam, a technique known as the Antioch process. This produces very fine cast surfaces suitable for such precision products as tire molds, pump impellers, plaques and artwork. It is relatively costly.
Foaming agents produce similar effects at somewhat lower costs. Labor cost remains relatively high, however. Foamed plaster molds produce very fine surface finishes with good dimensional accuracy, but they are better suited to simple shapes.
Most plaster mold castings are now made using the Copaco process, which utilizes conventional wood or metal patterns and gypsum-fibrous mineral molding compounds. The process is readily adapted to automation; with low unit costs, it is the preferred plastermold method for long production runs. On the whole, however, plaster molding accounts for a very small fraction of the castings market.
Refractory Molds. Of the several refractory-mold-based methods, the Shaw process is probably the best known. Here, the wood or metal pattern halves are dipped into an aggregate slurry containing a methyl silicate binder, forming a shell. After stripping the pattern, the shell is fired at a high temperature to produce a strong refractory mold. Metal is introduced into the mold while it is still hot. This aids feeding but it also produces the relatively slow cooling rates and coarse-grained structures that are typical of the process.
Dimensional accuracy as good as + 0.003 in (_+ 0.08 mm) is attainable in castings smaller than about one inch (25 mm), while tolerances as close as + 0.045 in (+1.1 mm) are claimed in castings larger than 15 in (630 mm) in cross section. Additional allowances of about 0.010-0.020 in (0.25-0.5 mm) must be included across the parting line. Surface furlshes are typically better than 80 gin (2 gm) rms in nonferrous castings.
Very fine surface finishes and excellent reproduction of detail are characteristic of the investment casting, or lost wax process. The process was practiced by several ancient cultures and has survived virtually without modification for the production of artwork, statuary and fine jewelry. Today, the process's most important commercial application is in the casting of complex, net shape precision industrial products such as impellers and gas turbine blades.
The process first requires the manufacture of an intricate metal die with a cavity in the shape of the finished product (or parts of it, if the product is to be assembled from several castings). Special wax, plastic or a lowmelting alloy is cast into the die, then removed and carefully finished using heated tools. Clusters of wax patterns are dipped into a refractory/plaster slurry, which is allowed to harden as a shell or as a monolithic mold.
The mold is first heated to melt the wax (or volatilize the plastic), then fired at a high temperature to vitrify the refractory. Metal is introduced into the mold cavity and allowed to cool at a controlled rate.
Investment casting is capable of maintaining very high dimensional accuracy in small castings, although tolerances increase somewhat with casting size. Dimensional consistency ranks about average among the casting methods; however, surface finishes can be as fine as 60 gin (1.5 gm) rms, and the process is unsurpassed in its ability to reproduce intricate detail.
Investment casting is better suited to castings under 100 lbs (45 kg) in weight. Because of its relatively high tooling costs and higher than average total costs, the process is normally reserved for relatively large production runs of precision products, and is not often applied to copper alloys.
Metal-Mold Processes. Reusable or metal-mold processes are used more extensively for copper alloys in Europe and England than in North America; however, they are gaining recognition here as equipment and technology become increasingly available. Permanent mold casting in North America is identified as gravity die casting or simply die casting in Europe and the U.K. The process called die casting in North America is known as pressure die casting abroad.
Permanent mold casting utilizes a metallic mold. The mold is constructed such that it can be opened along a conveniently located parting line. Hot metal is poured through a sprue to a system of gates arranged so as to provide even, low-turbulence flow to all parts of the cavity. Baked sand cores can be provided just as they would be with conventional sand castings. Chills are unnecessary since the metal mold provides very good heat transfer. The nature of the process necessitates adequate draft angles along planar surfaces oriented perpendicular to the parting line. Traces of the parting line may be visible in the finished casting and there may be some adherent flashing, but both are easily removed during finishing.
Permanent mold castings are characterized by good part-to-part dimensional consistency and very good surface finishes (about 70 gin, 1.8 [tm). Any traces of metal flow lines on the casting surface are cosmetic rather than functional defects. Permanent mold castings exhibit good soundness. There may be some micro shrinkage, but mechanical properties are favorably influenced by the castings' characteristically fine grain size. The ability to reproduce intricate detail is only moderate, however, and for products in which very high dimensional accuracy is required, plaster mold or investment processes should be considered instead.
Permanent mold casting is more suitable for simple shapes in mid-size castings than it is for very small or very large products. Die costs are relatively high, but the absence of molding costs makes the overall cost of the process quite favorable for medium to large production volumes.
Die casting involves the injection of liquid metal into a multipart die under high pressure. Pneumatically actuated dies make the process almost completely automated. Die casting is best known for its ability to produce high quality products at very low unit costs. Very high production rates offset the cost of the complex heat-resisting tooling required; and with low labor costs, overall casting costs are quite attractive.
The process can be used with several copper alloys, including yellow brass, C85800, manganese bronzes, C86200 and C86500, silicon brass, C87800, the special die casting alloys C99700 and C99750, plus a few proprietary compositions. These alloys can be die cast because they exhibit narrow freezing ranges and high beta phase contents. Rapid freezing is needed to complement the process's fast cycle times. Rapid freezing also avoids the hot shortness associated with prolonged mushy solidification. Beta phase contributes the hot ductility needed to avoid hot cracking as the casting shrinks in the unyielding metal mold.
Highly intricate copper alloy products can be made by die casting (investment casting is even better in this regard). Dimensional accuracy and part-to-part consistency are unsurpassed in both small (<1 in, 25 mm) and large castings. The attainable surface finish, often as good as 30 gin (0.76 [tm) rms, is better than with any other casting process. Die casting is ideally suited to the mass production of small parts.
Extremely rapid cooling rates (dies are normally water cooled) results in very fine grain sizes and good mechanical properties. Leaded alloys C85800 and C99750 can yield castings that are pressure tight, although lead is incorporated in these alloys more for its favorable effect on machinability than for its ability to seal porosity
Continuous Casting. Picture a mold cavity whose graphite or watercooled metal side walls are fixed, while the bottom wall, also cooled, is free to move in the axial direction as molten metal is poured in from the top. This is the continuous casting process. It is used to produce bearing blanks and other long castings with uniform cross sections. Continuous casting is the principal method used for the large-tonnage production of semifinished products such as cast rods, tube rounds, gear and bearing blanks, slabs and custom shapes.
The extremely high cooling and solidification rates attending continuous casting can, depending on the alloy, produce columnar grains. The continuous supply of molten metal at the solidification interface effectively eliminates microshrinkage and produces high quality, sound products with very good mechanical properties. With its simple die construction, relatively low equipment cost, high production rate and low labor requirements, continuous casting is a very economical production method.
Centrifugal Casting. This casting process has been known for several hundred years, but its evolution into a sophisticated production method for other than simple shapes has taken place only in this century. Today, very high quality castings of considerable complexity are produced using this technique.
To make a centrifugal casting, molten metal is poured into a spinning mold. The mold may be oriented horizontally or vertically, depending on the casring's aspect ratio. Short, squat products are cast vertically while long tubular shapes are cast horizontally. In either case, centrifugal force holds the molten metal against the mold wall until it solidifies. Carefully weighed charges insure that just enough metal freezes in the mold to yield the desired wall thickness. In some cases, dissimilar alloys can be cast sequentially to produce a composite structure.
Molds for copper alloy castings are usually made from carbon steel coated with a suitable refractory mold wash. Molds can be costly if ordered to custom dimensions, but the larger centrifugal foundries maintain sizeable stocks of molds in diameters ranging from a few inches to several feet.
The inherent quality of centrifugal castings is based on the fact that most nonmetallic impurities in castings are less dense than the metal itself. Centrifugal force causes impurities (dross, oxides) to concentrate at the casting's inner surface. This is usually machined away, leaving only clean metal in the finished product. Because freezing is rapid and completely directional, centrifugal castings are inherently sound and pressure tight. Mechanical properties can be somewhat higher than those of statically cast products.
Centrifugal castings are made in sizes ranging from approximately 2 in to 12 ft (50 nlm to 3.7 m) in diameter and from a few inches to many yards in length. Size limitations, if any, are likely as not based on the foundry's melt shop capacity. Simple-shaped centrifugal castings are used for items such as pipe flanges and valve components, while complex shapes can be cast by using cores and shaped molds. Pressure-retaining centrifugal castings have been found to be mechanically equivalent to more costly forgings and extrusions.
In a related process called centrifuging, numerous small molds are arranged radially on a casting machine with their feed sprues oriented toward the machine's axis. Molten metal is fed to the spinning mold, filling the individual cavities. The process is used for small castings such as jewelry and dental bridgework, and is economically viable for both small and large production quantities. Several molding methods can be adapted to the process, and the unit costs of centrifuged castings will depend largely on the type of mold used.
Recent years have seen the introduction of a number of new casting' processes, often aimed at specific applications. While these techniques are still to some extent under development and while they are certainly not available at all job shop foundries, their inherent advantages make them valuable additions to the designer's list of options.
Squeeze Casting. This interesting process aims to improve product quality by solidifying the casting under a metallostatic pressure head sufficient to (a) prevent the formation of shrinkage defects and (b) retain dissolved gases in solution until freezing is complete. This method was originally developed in Russia and has undergone considerable improvement in the U.S. It is carded out in metal molds resembling, the punch and die sets used in sheet metal forming.
After introducing a carefully metered charge of molten metal, the upper die assembly is lowered into place, forming a fight seal. The "punch" portion of the upper die is then forced into the cavity, displacing the molten metal under pressure until it fills the annular space between the die halves.
Proponents of squeeze casting claim that it produces very low gas entrapment and that castings exhibit shrinkage volumes approximately one half those seen in sand castings. Very high production rates, comparable to die casting but with considerably lower die costs, are also claimed. The process produces the high quality surfaces typical of metal mold
casting, with good reproduction of detail. Rapid solidification results in a fine grain size, which in mm improves mechanical properties. It is claimed that squeeze casting can be applied to many of the copper alloys, although die and permanent mold casting alloys should be favored.
A product's shape, size and physical characteristics often limit the choice of casting method to a single casting process, in which case the task simply becomes one of selecting a reliable foundry offering a fair price. If there is a choice of casting methods, it may be worthwhile to consult a trusted foundry, since the foundryman's experience can be a source of cost-saving ideas. In any event, it is advantageous to limit selection of the casting method to a few choices early in the design process so that the design and the casting method meet each other's requirements.
Making the selection is not inherently difficult, although it should be emphasized that the help of a skilled foundryman can be invaluable at this point. The factors listed at the beginning of this chapter determine the best suited and most economical process. Table 20, page 91, adapted from several sources,3, 2, defines the broad limits on process-selection parameters.