The economic disadvantage of any of the expendable-mold processes is that a new mold is required for every casting. In permanent-mold casting, the mold is reused many times.
In this section, we treat permanent-mold casting as the basic process in the group of casting processes that all use reusable metal molds. Other members of the group include die casting and centrifugal casting.
6.3.1 THE BASIC PERMANENT-MOLD PROCESS
Permanent-mold casting uses a metal mold constructed of two sections that are designed for easy, precise opening and closing. These molds are commonly made of steel or cast iron. The cavity, with gating system included, is machined into the two halves to provide accurate dimensions and good surface finish. Metals commonly cast in permanent-molds include aluminum, magnesium, copper-base alloys, and cast iron. However, cast iron requires a high pouring temperature, 1250C to 1500C (2300F to 2700F), which takes a heavy toll on mold life. The very high pouring temperatures of steel make permanent molds unsuitable for this metal, unless the mold is made of refractory material.
Cores can be used in permanent-molds to form interior surfaces in the cast product.
The cores can be made of metal, but either their shape must allow for removal from the casting or they must be mechanically collapsible to permit removal. If withdrawal of a metal core would be difficult or impossible, sand cores can be used, in which case the casting process is often referred to assemipermanent-mold casting.
Steps in the basic permanent-mold casting process are described in Figure 6.8. In preparation for casting, the mold is first preheated and one or more coatings are sprayed
on the cavity. Preheating facilitates metal flow through the gating system and into the cavity. The coatings aid heat dissipation and lubricate the mold surfaces for easier separation of the cast product. After pouring, as soon as the metal solidifies, the mold is opened and the casting is removed. Unlike expendable-molds, permanent-molds do not collapse, so the mold must be opened before appreciable cooling contraction occurs in order to prevent cracks from developing in the casting.
Advantages of permanent-mold casting include good surface finish and close dimensional control, as previously indicated. In addition, more rapid solidification caused by the metal mold results in a finer grain structure, so stronger castings are produced. The process is generally limited to metals of lower melting points. Other limitations include simple part geometries compared to sand casting (because of the need to open the mold), and the expense of the mold. Because mold cost is substantial, the process is best suited to high-volume production and can be automated accordingly.
Typical parts include automotive pistons, pump bodies, and certain castings for aircraft and missiles.
FIGURE 6.8 Steps in permanent-mold casting: (1) Mold is preheated and coated; (2) cores (if used) are inserted, and mold is closed; (3) molten metal is poured into the mold; and (4) mold is opened. Finished part is shown in (5).
(Credit:Fundamentals of Modern Manufacturing,4thEdition by Mikell P. Groover, 2010. Reprinted with permission of John Wiley & Sons, Inc.)
6.3.2 VARIATIONS OF PERMANENT-MOLD CASTING
Several casting processes are quite similar to the basic permanent-mold method. These include slush casting, low-pressure casting, and vacuum permanent-mold casting.
Slush Casting Slush casting is a permanent-mold process in which a hollow casting is formed by inverting the mold after partial freezing at the surface to drain out the liquid metal in the center. Solidification begins at the mold walls because they are relatively cool, and it progresses over time toward the middle of the casting. Thickness of the shell is controlled by the length of time allowed before draining. Slush casting is used to make statues, lamp pedestals, and toys out of low-melting-point metals such as zinc and tin. In these items, the exterior appearance is important, but the strength and interior geometry of the casting are minor considerations.
Low-Pressure Casting In the basic permanent-mold casting process and in slush casting, the flow of metal into the mold cavity is caused by gravity. In low-pressure casting, the liquid metal is forced into the cavity under low pressure—approximately 0.1 MPa (15 lb/in2)—from beneath so that the flow is upward, as illustrated in Figure 6.9.
The advantage of this approach over traditional pouring is that clean molten metal from the center of the ladle is introduced into the mold, rather than metal that has been exposed to air. Gas porosity and oxidation defects are thereby minimized, and mechani- cal properties are improved.
Vacuum Permanent-Mold Casting This process is a variation of low-pressure casting in which a vacuum is used to draw the molten metal into the mold cavity. The general configuration of the vacuum permanent-mold casting process is similar to the low- pressure casting operation. The difference is that reduced air pressure from the vacuum in the mold is used to draw the liquid metal into the cavity, rather than forcing it by positive air pressure from below. There are several benefits of the vacuum technique relative to low-pressure casting: air porosity and related defects are reduced, and greater strength is given to the cast product.
FIGURE 6.9 Low- pressure casting. The diagram shows how air pressure is used to force the molten metal in the ladle upward into the mold cavity. Pressure is maintained until the casting has solidified.
(Credit:Fundamentals of Modern Manufacturing, 4thEdition by Mikell P.
Groover, 2010. Reprinted with permission of John Wiley & Sons, Inc.)
e
6.3.3 DIE CASTING
Die casting is a permanent-mold casting process in which the molten metal is injected into the mold cavity under high pressure. Typical pressures are 7 to 350 MPa (1000 to 50,000 lb/in2). The pressure is maintained during solidification, after which the mold is opened and the part is removed. Molds in this casting operation are called dies; hence, the name die casting. The use of high pressure to force the metal into the die cavity is the most notable feature that distinguishes this process from others in the permanent- mold category.
Die-casting operations are carried out in special die-casting machines that are designed to hold and accurately close the two halves of the mold and keep them closed while the liquid metal is forced into the cavity. The general configuration is shown in Figure 6.10. There are two main types of die-casting machines: (1) hot-chamber and (2) cold-chamber, differentiated by how the molten metal is injected into the cavity.
In hot-chamber machines, the metal is melted in a container attached to the machine, and a piston is used to inject the liquid metal under high pressure into the die. Typical injection pressures are 7 to 35 MPa (1000 to 5000 lb/in2). The casting cycle is summarized in Figure 6.11. Production rates up to 500 parts per hour are not uncommon.
Hot-chamber die casting imposes a special hardship on the injection system because much of it is submerged in the molten metal. The process is therefore limited in its applications to low-melting-point metals that do not chemically attack the plunger and other mechanical components. The metals include zinc, tin, lead, and sometimes magnesium.
Incold-chamber die-casting machines, molten metal is poured into an unheated chamber from an external melting container, and a piston is used to inject the metal under high pressure into the die cavity. Injection pressures used in these machines are typically 14 to 140 MPa (2000–20,000 lb/in2). The production cycle is explained in Figure 6.12. Compared to hot-chamber machines, cycle rates are not usually as fast because of the need to ladle the liquid metal into the chamber from an external source.
Nevertheless, this casting process is a high production operation. Cold-chamber machines are typically used for casting aluminum, brass, and magnesium alloys. Low- melting-point alloys (zinc, tin, lead) can also be cast on cold-chamber machines, but the advantages of the hot-chamber process usually favor its use on these metals.
Molds used in die-casting operations are usually made of tool steel, mold steel, or maraging steel. Tungsten and molybdenum with good refractory qualities are also being used, especially in attempts to die cast steel and cast iron. Dies can be single-cavity or
FIGURE 6.10 General configuration of a (cold-chamber) die-casting machine. (Credit:
Fundamentals of Modern Manufacturing,4thEdition by Mikell P. Groover, 2010. Reprinted with permission of John Wiley & Sons, Inc.)
multiple-cavity (single-cavity dies are shown in Figures 6.11 and 6.12). Ejector pins are required to remove the part from the die when it opens, as in our diagrams. These pins push the part away from the mold surface so that it can be removed. Lubricants must also be sprayed into the cavities to prevent sticking.
Because the die materials have no natural porosity and the molten metal rapidly flows into the die during injection, venting holes and passageways must be built into the dies at the parting line to evacuate the air and gases in the cavity. The vents are quite small; yet they fill with metal during injection. This metal must later be trimmed from the part. Also, formation of flashis common in die casting, in which the liquid metal under high pressure squeezes into the small space between the die halves at the parting line or into the clearances around the cores and ejector pins. This flash must be trimmed from the casting, along with the sprue and gating system.
Advantages of die casting include (1) high production rates possible; (2) eco- nomical for large production quantities; (3) close tolerances possible, on the order of 0.076 mm (0.003 in) for small parts; (4) good surface finish; (5) thin sections are possible, down to about 0.5 mm (0.020 in); and (6) rapid cooling provides small grain size FIGURE 6.11 Cycle in hot-chamber casting: (1) With die closed and plunger withdrawn, molten metal flows into the chamber; (2) plunger forces metal in chamber to flow into die, maintaining pressure during cooling and solidification; and (3) plunger is withdrawn, die is opened, and solidified part is ejected. Finished part is shown in (4). (Credit:Fundamentals of Modern Manufacturing,4thEdition by Mikell P. Groover, 2010. Reprinted with permission of John Wiley & Sons, Inc.)
and good strength to the casting. The limitation of this process, in addition to the metals cast, is the shape restriction. The part geometry must allow for removal from the die cavity.
6.3.4 SQUEEZE CASTING AND SEMISOLID METAL CASTING
These are two processes that are often associated with die casting.Squeeze castingis a combination of casting and forging (Section 13.2) in which a molten metal is poured into a preheated lower die, and the upper die is closed to create the mold cavity after solidification begins. This differs from the usual permanent-mold casting process in which the die halves are closed prior to pouring or injection. Owing to the hybrid nature of the process, it is also known asliquid-metal forging. The pressure applied by the upper die in squeeze casting causes the metal to completely fill the cavity, resulting in good surface finish and low shrinkage. The required pressures are significantly less than in forging of a solid metal billet and much finer surface detail can be imparted by the die than in forging. Squeeze casting can be used for both ferrous and nonferrous alloys, but aluminum and magnesium alloys are the most common due to their lower melting temperatures. Automotive parts are a common application.
Semisolid metal castingis a family of net-shape and near net-shape processes performed on metal alloys at temperatures between the liquidus and solidus (Section 5.3.1). Thus, the alloy is a mixture of solid and molten metals during casting, like a slurry;
it is in the mushy state. In order to flow properly, the mixture must consist of solid metal globules in a liquid rather than the more typical dendritic solid shapes that form during freezing of a molten metal. This is achieved by forcefully stirring the slurry to prevent dendrite formation and instead encourage the spherical shapes, which in turn reduces the viscosity of the work metal. Advantages of semisolid metal casting include the FIGURE 6.12 Cycle in cold-chamber casting: (1) With die closed and ram withdrawn, molten metal is poured into the chamber; (2) ram forces metal to flow into die, maintaining pressure during cooling and solidification; and (3) ram is withdrawn, die is opened, and part is ejected. (Gating system is simplified.) (Credit:Fundamentals of Modern Manufacturing, 4thEdition by Mikell P. Groover, 2010. Reprinted with permission of John Wiley & Sons, Inc.)
following [15]: (1) complex part geometries, (2) thin part walls, (3) close tolerances, (4) zero or low porosity, resulting in high strength of the casting.
There are several forms of semisolid metal casting. When applied to aluminum, the termsthixocastingandrheocastingare used, and the production equipment is similar to a die-casting machine. When applied to magnesium, the term isthixomolding, and the equipment is similar to an injection-molding machine (Section 8.6.1).
6.3.5 CENTRIFUGAL CASTING
Centrifugal casting refers to several casting methods in which the mold is rotated at high speed so that centrifugal force distributes the molten metal to the outer regions of the die cavity. Here we describe the process used to cast tubular parts, called true centrifugal casting.
In true centrifugal casting, molten metal is poured into a rotating mold to produce a tubular part. Examples of parts made by this process include pipes, tubes, bushings, and rings. One possible setup is illustrated in Figure 6.13. Molten metal is poured into a horizontal rotating mold at one end. In some operations, mold rotation commences after pouring has occurred rather than beforehand. The high-speed rotation results in cen- trifugal forces that cause the metal to take the shape of the mold cavity. Thus, the outside shape of the casting can be round, octagonal, hexagonal, and so on. However, the inside shape of the casting is (theoretically) perfectly round, due to the radially symmetric forces at work.
Orientation of the axis of mold rotation can be either horizontal or vertical, the former being more common. Let us consider how fast the mold must rotate inhorizontal centrifugal castingfor the process to work successfully. Centrifugal force is defined by this physics equation:
Fẳmv2
R (6.1)
whereFẳforce, N (lb);mẳmass, kg (lbm);vẳvelocity, m/s (ft/sec); andRẳinside radius of the mold, m (ft). The force of gravity is its weightWẳmg, whereWis given in kg (lb), andgẳacceleration of gravity, 9.8 m/s2(32.2 ft/sec2). The so-called G-factorGF is the ratio of centrifugal force divided by weight:
GFẳ F W ẳ mv2
Rmgẳ v2
Rg (6.2)
FIGURE 6.13 Setup for true centrifugal casting. (Credit:Fundamentals of Modern Manufacturing,4thEdition by Mikell P. Groover, 2010. Reprinted with permission of John Wiley & Sons, Inc.)
Velocityvcan be expressed as 2pRN/60ẳpRN/30, whereNẳrotational speed, rev/min.
Substituting this expression into Eq. (6.2), we obtain GFẳR pN30 2
g (6.3)
Rearranging this to solve for rotational speedN, and using diameterDrather than radius in the resulting equation, we have
Nẳ30 p
ffiffiffiffiffiffiffiffiffiffiffiffiffi 2gGF
D r
(6.4) whereDẳinside diameter of the mold, m (ft). If the G-factor is too low in centrifugal casting, the liquid metal will not remain forced against the mold wall during the upper half of the circular path but will ‘‘rain’’ inside the cavity. Slipping occurs between the molten metal and the mold wall, which means that the rotational speed of the metal is less than that of the mold. On an empirical basis, values ofGFẳ60 to 80 are found to be appropriate for horizontal centrifugal casting [2], although this depends to some extent on the metal being cast.
Invertical centrifugal casting, the effect of gravity acting on the liquid metal causes the casting wall to be thicker at the base than at the top. The inside profile of the casting wall takes on a parabolic shape. Consequently, part lengths made by vertical centrifugal casting are usually no more than about twice their diameters. This is quite satisfactory for bushings and other parts that have large diameters relatively to their lengths, especially if machining is used to accurately size the inside diameter.
Castings made by true centrifugal casting are characterized by high density, espe- cially in the outer regions of the part where centrifugal force is greatest. Solidification shrinkage at the exterior of the cast tube is not a factor, because the centrifugal force continually reallocates molten metal toward the mold wall during freezing. Any impurities in the casting tend to be on the inner wall and can be removed by machining if necessary.