CHAPTER 3

METAL-CASTING PROCESSES

DR . Ahmad Hassan

1-Introduction:

This chapter focuses on the major metal-casting processes and their principles, advantages, and limitations. Two trends currently

are having a large impact on the casting industry. The first is continuing mechanization and automation of the casting process,

which has led to significant changes in the use of equipment and labor. Advanced machinery and automated process-control

systems have replaced traditional methods of casting. The second major trend affecting the casting industry is the increasing demand

for high-quality castings with close tolerances. This demand is spurring the further development of casting processes that

produce high-quality castings, see Table3.1.

Table 3.1 General characteristics of casting processes

ProcessTypical

material castWeight (kg)

min max

Typical surface finish

(m, Ra)Porosity*Shape

complexity*

Dim. Accuracy*

Section Thickness

mmMin max

SandAll0.05 N.L.5-2541-233 N.L.

ShellAll0.05 100+1-342-322 -----

Expandable pattern

All0.05 N.L.5-204122 N.L.

PlasterAl, Zn, Mg,

Cu0.05 50+1-231-221 ------

InvestmentAll0.005 100+1-33111 75

Permanent mold

All0.05 3002-32-33-412 50

DieAl, Cu, Zn,

Mg<0.05 501-21-23-410.5 12

CentrifugalAll--- 5000+2-101-23-432 100

Relative rating: 1 best, 5 worst.

2-Sand Casting: The traditional method of casting metals is in sand molds and has been used for millennia. Although the origin of the sand casting dates to ancient times, it is still the most prevalent form of casting. Simply, sand casting, consists of:1- Placing a pattern (having the shape of the desired casting0 in sand to make an imprint2- Incorporating a gating system3- Filling the resulting cavity with molten metal4- Allowing the metal to cool until it solidifies5- Breaking away the sand mold6- Removing the casting Fig.1 shows an outline of the typical sand casting operation steps.

Pattern makingCore making

Gating systemMolding

MoldSand

Pouring into mold

Metal melting Casting Heattreatment

CleaningAnd

finishingInspection

FurnaceSolidification Shakeout,

Removal ofRisers andGates

AdditionalHeattreatment

Defects,Pressure,dimensions

2-1 Sands: Most sand casting operations use silica sand (SiO2), because it is inexpensive and is suitable as mold material because of its resistance to high temperatures. There are two general types of sand: naturally bonded and synthetic sand. Because its composition can be controlled more accurately most foundries prefer synthetic sand. Several factors are important in the selection of sand for sand molds. Sand having fine, rounded grains can be closely packed and forms a smooth mold surface. Good permeability of molds and cores allows gases and steam evolved during casting to escape easily. The selection of sand involves certain tradeoffs with respect to properties. For example, fine-grained sand enhances mold strength, but the fine grains also lower mold permeability. Sand is typically conditioned before use. Mulling machines are used to uniformly mull (mix thoroughly) sand with additives. For example clay 9bentonite) is used as a cohesive agent to bond sand particles, giving the sand strength.Zircon (ZrSiO4), olivine (Mg2SiO4), and iron silicate (Fe2SiO4) sands are often used in steel foundries for their low thermal expansion. Chromate (FeCr2O4) is used for its high heat transfer property.

2.2 Types of sand molds:

There are three types of sand molds:1- Green sand mold: It is the most common mold material. The term green refers to the fact that the sand in the mold is moist or damp while the metal is being poured into it. Green mold sand is a mixture of sand, clay, and water. Greensand molding is the least expensive method of making molds. In the skin-dried method, the mold surfaces are dried, either by storing the mold in air or drying it with torches. Skin-dried molds are generally used for large castings because of their higher strength. 2- Cold-box mold: In this process, various organic and inorganic binders are blended into the sand to bond the grains chemically for greater strength. These molds are dimensionally more accurate than green sand molds but more expensive.3- No-back mold: In this process, a synthetic liquid resin is mixed with the sand, and the mixture hardens at room temperature. Because molding of the mold in this and the cold-box process takes place without heat, they are called cold-setting processes.

Major components of sand molds as shown in Fig.2 are:1- The mold itself, which is supported by a flask. A two-piece mold consists of a cope on top and a drag on the bottom. When more than two pieces are used, the additional parts are called cheeks.2- A pouring basin or pouring cup, into which the molten metal, is poured.3- A sprue, through which the molten metal flows downward.4- The runner system, which has channels that carry the molten metal from the sprue to the mold cavity.5- Gates are the inlets into the mold cavity.6- Risers, which supply additional metal to the casting as it shrinks during solidification. Fig.2 shows two different types of risers: a blind riser and an open riser.7- Cores, which are inserts made from sand. They are placed in the mold to form hollow regions or otherwise define the interior surface of the casting.8- Vents, which are placed in the molds to carry off gases produced when the molten metal comes into contact with the sand in the molds and cores. They also exhaust air from the mold cavity as the molten metal flows into the mold.

Fig.2 Schematic illustration of a sand mold showing various features.

2.3 Patterns:

Patterns are used to mold the sand mixture into the shape of the casting. They may be made of wood, plastic, or metal. The selection of a pattern material depends on:1- The size and shape of the casting2- The dimensional accuracy3- The quantity of castings required4- The molding process to be used as shown in Table 2.Because patterns are used repeatedly to make molds, the strength and durability of the material selected for pattern must reflect the number of the castings that the mold will produce. They may be made of a combination of materials to reduce wear in critical regions. Patterns are usually coated with a parting agent to facilitate their removal from the molds.

Table 2 Characteristics of pattern materials

E, excellent; G, good; F, fair; P, poor

CharacteristicWoodAluminumSteelPlasticCast iron

Machine-abilityEGFGG

Wear resistancePGEFE

StrengthFGEGG

WeightEGPGP

Repair-abilityEPGGP

Resistance to:CorrosionSwelling

EP

EE

PE

EE

PE

2.3.1 Types of patterns: 1- One-piece patterns, also called loose or solid pattern, are generally used for simple shapes and low-quantity production. They are generally made of wood and are inexpensive.2- Split patterns are two pieces patterns made so that each forms a portion of the cavity for the casting. In this way castings having complicated shapes can be produced.3- Match-plate patterns are a popular type of mounted pattern in which two-piece patterns are constructed by securing each half of one or more split patterns to the opposite sides of a single plate, as shown in Fig.3. This type of patterns is used most often in conjunction with molding machines and large production runs to produce smaller castings.

Fig.3 A typical metal match-plate pattern used in sand casting.

2.3.2 Pattern design: Pattern design is a crucial aspect of the total casting operation.The design should provide for:

1- Metal shrinkage, 2- Ease of removal from the sand mold by means of a taper or draft, Fig.4, and 3- Proper metal flow in the mold cavity.These topics will be discussed in greater details in the next chapter.

Fig.4 Taper on patterns for ease removal from the sand mold.

2.4 Cores:

Cores are used for castings with internal cavities or passages. Cores are placed in the mold cavity before casting to form the interior surfaces of the casting and are removed from the finished part during shakeout and further processing. Like molds, cores must possess strength, permeability, ability to withstand heat, and collapsibility. The cores are anchored by core prints. These are recesses that are added to the pattern to support the core and to provide vents for the escape of the gasses, Fig.5. Cores are generally made in a manner similar to that used in making molds, and the majority are made with shell, no-bake, or cold-box processes. Cores are formed in core–boxes, which are used much like patterns are used to form sand molds. The sand can be packed into the boxes with sweeps or blown into the box by compressed air from core blowers. Core blowers have the advantages of producing uniform cores and operating at a very high production rate.

Fig.5 Different shapes of sand cores supported by core prints.

2.5 Sand-molding machines

The oldest known method of molding, which still used for simple castings, is to compact the sand by hand hammering or ramming it around the pattern. For most operations, however, the sand mixture is compacted around the pattern by molding machines, Fig.6. These machines have the following advantages:Eliminate labor cost,

Offer high quality casting by improving the application and distribution of forces,Manipulate the mold in a carefully controlled fashion,

Increase the rate of production

Fig.6 Various designs of squeeze heads for mold making: (a) conventional flat head; (b) profile head; (c) equalizing squeeze pistons; and (d) flexible diaphragm.

2.5.1 Automatic molding methods 

1 -Jolting the assembly. Jolting the assembly can further assist mechanization of

the molding process. The flask, molding sand, and pattern are placed on a pattern plate mounted on an anvil, and jolted upward by air pressure at rapid intervals, as shown in Fig.7. The inertial forces compact the sand around the pattern. Jolting produces the highest compaction at the horizontal parting line, whereas in squeezing, compaction is highest at the squeeze head, Fig.6. Thus more uniform compaction can be obtained by combining them, as shown in Fig.7b.

Fig.7 (a) Schematic illustration of a jolt-type mold-making machine. (b) Schematic illustration of a mold-making machine combines jolting and squeeze.

2- Vertical flaskless molding. In this method, the halves of the pattern form a vertical chamber wall against which sand is blown and compacted, Fig.8. Then the mold halves are packed horizontally, with the parting line oriented vertically and moved along a pouring conveyor.

Fig.8 Vertical flaskless molding. (a) Sand is squeezed between two halves of the pattern. (b) Assembled molds pass along on assembly line for pouring.

3- Sands lingers molding. In this process the flask is filled uniformly with sand under a stream of high pressure. They are used to fill large flasks and are typically operated by machine. An impeller in the machine throws sand from its blades or cups at such high speeds that the machine not only places the sand but also rams it approximately.

4 -Impact molding. In the impact molding process, the sand is compacted by

controlled explosion or instantaneous release of compressed gasses. This method produces molds with

uniform strength and good permeability .

5- Vacuum molding.

The process is also known as “V” process. In this process, the molding process can be done in several steps:

1 -The pattern is covered tightly by a thin sheet of plastic2 -A flask is placed over the coated pattern and is filled with dry sand

3 -A second sheet of plastic is placed on top of the sand4 -A vacuum action hardens the sand so that the pattern can be

withdrawn5 -Both halves of the mold are made this way and then assembled.

During pouring, the mold remains under a vacuum but the casting cavity does not. When the metal has solidified, the vacuum is turned off and the sand falls away, releasing the casting. Vacuum molding produces castings having

very good detail and accuracy. It is especially will suited for large, relatively flat castings .

2.6 The sand-casting operation: 

Casting procedure: 

1 -After the mold has been shaped2 -The cores have been placed in position

3 -The two halves (cope and drag) are closed4 -The two halves should be clamped

Also the two halves should be weighed down to prevent the separation of the mold sections under the pressure exerted when the molten metal is poured into the mold cavity

The complete sequence of operations in sand casting is shown in Fig.9:1- Mechanical drawing (a) of the part is used to generate a design of the

pattern2- Considerations such as shrinkage and draft must be belt into the drawing

(b-c)3- Patterns have been mounted on plates equipped with pins for Guidant.

Note the presence of core prints designed to hold the core in place 4- Core boxes (d-e) produce core halves, before and after pasted together

5 -The cores will be used to produce the hollow area of the part shown in (a)

6- The cope and the mold is assembled by taking the cope pattern plate, securing it to the flask through aligning pins (f),

7- The drag half is produced in a similar manner, with the pattern inserted (h). A bottom board is under the drag and aligned with pins

8- The pattern, flask, and bottom board are inverted, and the pattern is withdrawn leaving the appropriate imprint (i)

9- The core is set in place (j) within the drag cavity10- The mold is closed (k) by putting the cope on top of the drag and

securing the assembly with pins11 -The metal is left to solidify (l) 

After the metal solidifies the following steps should be preceded:

a)- the casting is removed from the mold,

b)-the sprue, gates and risers are cut off its mold (m) by oxyfuel-gas cutting, sawing, shearing and abrasive wheels and recycled,

c)- the casting is cleaned from sand and oxide layers adhering to the casting by vibration or by sand blasting,

d)- castings may be cleaned by electrochemical means or by pickling with chemicals to remove surface oxides,

e)-depending on the metal used, the casting may subsequently be heat- treated to improve certain properties needed,

f)- finishing operations may involve straightening or forging with dies to obtain final dimensions .

 

2.6 Factors that should be taken into consideration during casting: 

1- The design of the gating system is important for proper delivery of the molten metal into the mold cavity

2-Turbulence must be minimized3- Air and gases must be allowed to escape by such means as vents

4- Proper temperature gradients must be established and maintained to minimize shrinkage and porosity

5- The design of risers is also important in order to supply the necessary molten metal during solidification of the casting.

2.7 Disadvantages of sand casting: 

1 -)The surface finish obtained is largely a function of the materials used in making the mold

2 -)Dimensional accuracy is not so good as that of other casting processes

2.8 Uses and advantages: 

It can be used to produce intricate shapes, such as cast-iron engine blocks and very large propellers for ocean liners.

Sand casting can be economical for relatively small production runs, and equipments cost are generally low .

Fig.9 Schematic illustration of the sequence of operations for sand casting

3-Shell-Mold Casting:Shell-mold casting

was first developed in 1940s and has grown significantly because: 1 -)It can produce many types of castings

2 -)The castings produced having close tolerances and good surface finishes and low cost

3.1 Steps for making the process:1- A mounted pattern made of ferrous metal or aluminum is heated to 175-350 ˚C

2- The pattern is coated with a parting agent such as silicon3 -Then clamped to a box or chamber containing a fine sand containing 2.5 to 4.0 %

thermosetting resin binder as phenol-formaldehyde4- This makes the sand particles to be coated with the binder

5- The box is either rotated upside down, Fig.10, or the sand mixture is blown over the pattern, allowing it to coat the pattern

6- The assembly is then placed in an oven for a short period of time to complete the curing of the resin

7- The shell hardens around the pattern and is removed from the pattern using built-in ejector pins

8- The two halve-shells are made in this manner9 -The two halves are then bonded or clamped together in preparation for pouring.

Fig.10 Common method of making shell molds, called dump-box technique

The thickness of the shell can be accurately determined by controlling the time that the pattern is in contact with the mold. The shells are light and thin, usually 5-10 mm. Shell molds are generally poured with the parting line horizontal and may also be supported with sand.

3.2 Limitations of the process:1- Shell sand have much lower permeability than sand used for green-sand

molding, because a sand of much smaller grain size is used for shell molding 2- The decomposition of the shell-sand binder also produces a high volume

of gas 3- Unless the molds are properly vented, trapped air and gas can cause

serious problems in shell molding of ferrous castings

3.3 Advantages of the process:1- The walls of the molds are relatively smooth2- This offers low resistance to flow of the molten metal3- Producing castings with sharper corners4- Thinner sections with smaller projections than are possible in green-sand molds can be produced5- Shell-mold casting may be more economical than other casting processes6- The high quality of the finished casting can significantly reduce cleaning, machining, and other finishing costs7- Complex shapes can be produced with less labor, and the process can be automated fairly easily.

4-Expandable Pattern Casting (Lost Foam): The expandable pattern casting process uses a polystyrene pattern, which evaporates upon contact with the molten metal to form a cavity for the casting. The process is also known as evaporative-pattern or lost-pattern casting, and under the trade name Full-Mold process. 4.1 Casting procedure:1- Raw expandable polystyrene (EPS) bend, containing 5 to 8 percent petane are placed in a preheated die, usually made of aluminum2- The polystyrene expands and takes the shape of the cavity3- Additional heat is applied to fuse and bond the beads together4- The die is then cooled and opened, and the polystyrene pattern is removed5- The pattern is then coated with a water-base refractory slurry, dried, and placed in a flask6- The flask is then filled with loose fine sand. The sand may be dried or mixed with bonding agents to give it additional strength.7- Then, without removing the polystyrene pattern, the molten metal is poured into the mold. This action immediately vaporizes the pattern and fills the mold cavity.

4.2 Advantages of the evaporative pattern process: 1- It is relatively simple process because there are no parting lines, cores, or riser system; hence it has design flexibility2- Inexpensive flasks are sufficient for the process3- Polystyrene is inexpensive and can be easily processed into patterns having very complex shapes, various sizes, and fine surface detail4- The casting requires minimum finishing and cleaning operations5- The process is economical for long production runs6- The process can be automated

4.3 Typical applications of the process The process can be used to produce cylinder heads, crankshafts, brake components and manifolds for automobiles, and machine bases. The aluminum engine blocks and other components of the automobile are made by this process.

5- Plaster-Mold Casting:  In the plaster mold casting process, the mold is made of plaster of pairs (gypsum, or calcium sulfate), with the addition of talc and silica flour to improve strength and control the time required for the plaster to set.

 5.1 Process procedure:

 1- The above components are mixed with water, and then the

resulting slurry is poured over the pattern2- After the plaster sets, usually within 15 minutes, the pattern is

removed and the mold is dried at 120 –260 ˚C to remove moisture3- The mold halves are then assembled to form the mold cavity

4- Then the assembly is preheated to about 120˚ 5- The molten metal is then poured into the mold

 

5.2 Pattern materials and process uses: Patterns materials used for plaster molding are generally made of aluminum alloys, thermosetting plastics, brass, or zinc alloys. Wood patterns are not suitable for making a large number of molds, because the patterns

are repeatedly subjected to the water-based slurry .Process uses: Since there is a limit to the maximum temperature that the plaster mold can withstand, generally about 1200 ˚C, plaster-mold casting is used only for aluminum, magnesium, zinc, and some copper-base alloys.

5.3 Advantages and disadvantages of the process:Advantages:

1- The castings have fine details with good surface finish2- Because of the low thermal conductivity of the mold, the casting cool

slowly, and more uniform grain structure is obtained.3- High dimensional accuracy and good surface finish obtained

 Disadvantages:

Because plaster molds have very low permeability, gasses evolved during solidification of the metal cannot escape.

5.2 Pattern materials and process uses: Patterns materials used for plaster molding are generally made of aluminum alloys, thermosetting plastics, brass, or zinc alloys. Wood patterns are not suitable for making a large number of molds, because the patterns are repeatedly subjected to

the water-based slurry .Process uses: Since there is a limit to the maximum temperature that the plaster mold can withstand, generally about 1200 ˚C, plaster-mold casting is used only for aluminum, magnesium, zinc, and some copper-base alloys.

5.3 Advantages and disadvantages of the process:Advantages:

1- The castings have fine details with good surface finish2- Because of the low thermal conductivity of the mold, the casting cool slowly, and

more uniform grain structure is obtained.3- High dimensional accuracy and good surface finish obtained

Disadvantages:Because plaster molds have very low permeability, gasses evolved during solidification of the metal cannot escape.

5.4 Casting dimensions and weight:Wall thickness of the parts produced can be 1-2.5 mm. Casting usually weigh less than 10 kg and are typically in the range of 125-250 g, although parts as light as 1.0 g have been made.

6 .Ceramic-Mold Casting:The ceramic-mold casting process is similar to the plaster-mold casting process, with the exception that it uses refractory mold materials suitable for high-temperature applications. The process is also called cope-and-drag investment

casting  .

6.1 Ceramic material composition: The slurry is a mixture of fine-grained zircon (ZrSiO4), aluminum oxide, and fused silica, which are mixed with bonding agents.

 

6.2 Mold preparation:The above mixture is poured over the pattern which may be made of wood or metal, which has been placed in a flask

After setting, the molds are removed and dried Then the mold is burned off to remove the volatile matter, and then bakedThe molds are clamped firmly and used as all-ceramic molds.The sequence of the operation is shown in Fig.11.

The high temperature resistance of the refractory molding materials allows these molds to be used in casting ferrous and other high-temperature alloys, stainless

steels, and tool steels .

Fig.11 Sequence of operations in making a ceramic mold.

6.3 Advantages, disadvantages and uses: Advantages:

The castings have good dimensional accuracy and surface finish over a wide range of sizes and shapes.

 Disadvantages:

But the process is somewhat expensive Uses:

Typical parts made are impellers, cutters for machining, dies for metal forming, and molds for making plastic or rubber components. This process has cast parts weighing as much as 700 kg.

7 .Investment Casting:The investment casting process, also called lost-wax process is one of the first used

casting processes   7.1 Process procedure: 

The pattern is made of wax or a plastic such as polystyrene. The sequences involve in the investment casting are shown in Fig.12, and as follows:

 1- The pattern is made by injecting molten wax into a metal die in the shape of the

pattern.2- The pattern is then dipped into a slurry of refractory material, such as a very fine

silica, including water, ethyl silicate, and acids.3- After this initial coating has dried

4- The pattern is coated repeatedly to increase its thickness5- The one-piece mold is dried in air and heated to a temperature of 90-175 ˚C in

an inverted position to melt out the wax for about 12 hours.6- The mold is then fired to 650-1050 ˚C for about 4 hours, depending on the metal

to be cast7- After the mold has been poured and the metal has solidified, the mold is broken

up and the casting is removed. A number of patterns can be joined to make one mold, called a tree (Fig.12c), thus increasing the production rate .

7.2 Advantages and disadvantages of the process: Advantages:

Although the labor and material involved make the lost-wax process costly;

1- It is suitable for casting high-melting alloys 2 -With good surface finish and close tolerance .

3 -Thus little or no finishing operations are required.4- This process is capable of producing complex shapes

 Disadvantages:

The produced parts weighing from 1 g to 35 kg, for a wide variety of ferrous and nonferrous metals and alloys. Typical parts made are components for office equipment and mechanical components such as gears, cams, valves and ratchets.

7.3 Ceramic-Shell Investment Casting: A variation of the investment-casting process is ceramic-shell casting. It uses the same type of wax or plastic pattern, which is:

1- Dipped first in ethyl silicate gel and then dipped into a fluidized bed of fine-grained fused silica flour.

2- The pattern is then dipped into coarser-grained silica (to build up additional coatings and thickness to withstand the thermal shock of pouring).

3- The rest of the procedure is similar to investment casting. Uses:

This process is economical and is used extensively for precision casting of steels and high-temperature alloys .

 

Fig.12 Schematic illustration of the investment casting process.

8 .Permanent-Mold Casting :In the permanent-mold casting process, also called hard-mold casting, two halves of a mold are made from materials such as cast iron, steel, bronze, or refractory metal alloys. The mold cavity and gating system are machined into the mold and

thus become an integral pert of it . 

8.1 Core materials :To produce castings with internal cavities, cores made of metal or sand are placed in the mold prior to casting. Typical core materials are oil-bonded or resin-bonded, plaster, graphite, gray iron, low carbon steel, and hot-work die steel. Gray iron is the most commonly used, particularly for large molds of aluminum and magnesium castings.

 To increase the life of the permanent molds :

The surfaces of the mold cavity are usually coated with refractory slurry, such as sodium silicate, and clay, or sprayed with graphite every few castings. This coating also serves as parting agents and as thermal barriers, controlling the rate of cooling of the casting. Mechanical ejectors, such as pins located in various parts of the mold, may be needed for removal of complex castings. Ejectors usually leave small

round impressions on the castings .

8.2 Procedure: 

1 -The molds are clamped together by mechanical means and heated to about 150-200 ˚C to facilitate metal flow and reduce thermal damage to the dies.

2- The molten metal is then poured through the gating system3- After solidification, the molds are opened and the casting is

removed.4- Special means employed to cool the mold include water or the use

of fins, similar to those found on motorcycle.

8.3 Advantages and uses: Advantages:

1 -This process produces castings with good surface finish 2- Close tolerances

3- Uniform and good mechanical properties1 -And at high production rates

2- Although the permanent-mold casting operation can be performed manually, the process can be automated for large production runs

3 -Although equipment costs can be high because of die costs, the process can be mechanized, thus keeping labor costs low .

4- Permanent-mold casting is not economical for small production runs. Uses:

This process is used mostly for aluminum, magnesium, and copper alloys and gray cast iron because of their generally lower melting points. Steels can also be cast

using graphite or heat-resistant metal molds .

9 .Slush Casting: We noted in one of the figures of the last previous chapter that the solidified skin first develops in a casting and that this skin becomes thicker with time. Hollow castings with thin walls can be made by permanent-mold casting using this principle. This process is called slush casting.

 9.1 Procedure:

 1- The molten metal is poured into the metal mold

2- After the desired thickness of solidified skin is obtained, the mold is inverted or slung

3- The remaining metal is poured out4- The mold halves are then opened and the casting is removed.

 Uses:

This process is suitable for small production runs and is generally used for making ornamental and decorative objects and toys from low-melting-point metals, such as zinc, tin, and lead alloys.

10 .Pressure Casting: In the two permanent-mold processes that we just described, the molten metal flows into the mold cavity by gravity. In the pressure-casting process, also called pressure pouring or low-pressure casting, Fig.13, the molten metal is forced upward by gas pressure into a graphite or metal mold. The pressure is maintained until the metal has completely solidified in the mold. The molten metal may also be forced upward by a vacuum, which also removes dissolved gases and gives the casting lower porosity. Pressure casting is generally used for high-quality castings. An example for this process is steel railroad-car wheels.

Fig.13 a)- Bottom-pressure casting utilizes graphite molds for production of steel railroad wheels. B)- Gravity-pouring method of casting a railroad wheel.

11 .Die Casting: 

The die casting process, is a further example of permanent-mold casting. The molten metal is forced into the die cavity at pressures ranging from 0.7-700 MPa. Typical parts made by die-casting are carburetors, motors, business-machine and appliance components, hard tools, and toys. The weight of most casting ranges from less

than 90 g to about 25 kg .There are two basic types of die-casting machines: hot-chamber and cold-chamber.

 11.1 Hot-chamber process:

 

The hot-chamber process, Fig.14, involves the use of a piston, which traps a certain volume of molten metal and forces it into the die cavity through a gooseneck and nozzle. The pressures range up to 35 MPa, with an average of about 15 MPa. The metal is held under

pressure until it solidifies in the die .

Fig.14 Sequence of steps in the die casting of a part in the hot-chamber process

To improve the die life and to aid in rapid metal cooling - thus reducing the cycle time - dies are usually cooled by circulating water or oil through various passageways in the die block. Cycle times usually range up to 900 shots per hour for zinc, although very small components can be cast at 18000 shots per hour. This process commonly casts low melting-point alloys such as zinc, tin and lead.

11.2 Cold-chamber process:In the cold-chamber process, Fig.15, molten metal is poured into the injection cylinder (shot chamber) with a ladle. The shot chamber is not heated – hence the term cold chamber. The metal is forced into the die cavity at pressures usually ranging from 20 MPa to70 MPa, although they may be as high as 150 MPa. The machines may be horizontal, Fig.16, or vertical, in which the shot chamber is vertical and the machine is similar to a vertical press.

Fig.15 Sequence of operations in die casting of a parting the cold-chamber process

Fig.16 Schematic illustration of a die-casting machine

The horizontal machines are large compared to the size of the casting because large forces are required to keep the two halves of the dies closed. Otherwise,

the pressure of the molten metal in the die cavities may force the dies apart.

11.2.1 Method applicationHigh-melting-point alloys of aluminum, magnesium, and copper are commonly cast by this method, although other metals (including ferrous metals) can also be cast in this manner. Molten-metal temperatures start at about 600 ˚C for aluminum and magnesium alloys and increase considerably

for copper-base and iron-base alloys .

11.3 Process capabilities and machine selection: Because of the high pressure used, the dies have a tendency to part unless clamped together tightly. Die-casting machines are rated according to the clamping force

that can be exerted to keep the dies closed . 11.3.1 Machine selection: 

The capacities of the commercially available machines range from 25 tons to 3000 tons. Other factors involved in the selection of die-casting machines are die size, piston stroke, shot pressure, and cost.

11.3.2 Types of dies: Die-casting dies, Fig.17, may be made single-cavity dies, multiple-cavity dies (with several identical cavities), combination-cavity dies (with several different cavities), or unit dies (simple small dies that can be combined in two or more units in a

master holding die) .Typically, the ratio of die weight to part weight is 1000 to 1. Thus the die for a casting weighing 2 kg will weigh about 2000 kg. Dies are usually made of hot-work die steel or mold steels. Die-wear increases with the temperature of the molten metal. Heat checking of dies (surface cracking from repeated heating and cooling of the die) can be a problem. When die materials are selected and maintained properly, dies may last more than half a million shots before any significant die wear takes place.

Fig.17 Various types of cavities in die-casting dies.

11.3.4 Die design: Die design includes taper (draft) to allow the removal of the casting, Fig.4. The sprues and runners may be removed either manually or by using trim dies in a press. The entire die casting process can be highly automated. Lubricants (parting agents) are usually applied, as thin coatings on die surfaces. Alloys except magnesium alloys generally require lubricants. These are usually water-base lubricants either graphite or other compounds in suspension. Because of the high cooling capacity of water, these fluids are also effective in keeping die temperatures low.

11.4 Advantages, disadvantages, uses and cost:Advantages:

1- Die casting has the capability for high production rates with good strength2- High quality parts with complex shapes

3- It also produces good dimensional accuracy and surface details4- Thus requiring little or no subsequent machining or finishing operations (net-

shape forming) 5 -Because of the high pressure involved, wall thicknesses as small as 0.5 mm are

produced and are smaller than those obtained by other casting methods6- The castings have a fine-grained, hard skin with higher strength.

Disadvantages:1- Ejector marks remain

2- Also do a small amount of flash (thin material squeezed out between the dies) at the parting line.Uses:Components such as pins, shafts, and threaded-fasteners can be die cast integrally.Cost:

Equipment costs, particularly the cost of dies, are somewhat high, but labor costs are generally low because the process is semi-or fully automated. Die-casting is

economical for large production runs  .The properties and typical applications of common die-casting alloys are given in Table 3.

Table 3 Properties and typical applications of common die-casting alloys

Alloy

Ultimate Tensile

Strength)MPa(

YieldStrength

)MPa(

ElongationIn 50 mm

(%)Applications

Al-3.5% Cu-8.5%Si 3201602.5

Automotive components, electrical motor frames and

housing

Al-12%Si3001502.5

Complex shapes with thin walls, rats requiring strength

at higher temperaturesBrass 858 (60 Cu)

38020015Fixtures, lock hardware,

bushing, ornamental castingsMg+9A+-0.7

Zn2301603

Power tools, automotive parts

Zinc+3% Al280___10Automotive parts, office

equipment, building hardware

Zn+4Al+1Cu

320___7Automotive parts, building

hardware, business equipment

12 .Centrifugal Casting: The centrifugal-casting process utilizes the inertial forces caused by rotation to distribute the molten metal into the mold cavities. There are three types of centrifugal casting: true centrifugal casting, semi-centrifugal casting, and centrifuging.

 

12.1 True centrifugal casting: In the true centrifugal casting, hollow cylindrical parts, such as pipes, gun barrels, and streetlamp posts, are produced by the technique shown in Fig.18, in which molten metal is poured into a rotating mold. The axis of rotation is usually

horizontal but can be vertical for short workpieces .Molds are made of steel, iron, or graphite and may be coated with a refractory lining to increase mold life. The mold surfaces can be shaped so that pipes with various outer shapes, including square or polygonal, can be cast. The inner surface of the casting remains cylindrical because the molten metal is uniformly distributed by centrifugal forces. However, because of density differences, lighter elements such as dross, impurities, and pieces of the refractory lining tend to collect on the inner surface of the casting.

Fig.18 Schematic illustration of the centrifugal casting process.

12.1.1 Advantages and casting dimensions :Advantages:

Castings of good qualityDimension accuracyExternal surface detail are obtained by this process Casing dimensions:

Cylindrical parts ranging from 13 mm to 3 meter in diameter and 16 meter long can be cast centrifugally, with wall thicknesses ranging from 6 mm to 125 mm 

12.2 Semi-centrifugal casting: An example of semi-centrifugal casting is shown in Fig.19a. This method is

used to cast parts with rotational symmetry, such as a wheel with spokes .12.3 Centrifuging: 

In centrifuging, also called centrifuge casting, mold cavities of any shape are placed at a certain distance from the axis of rotation. The molten metal is poured from the center and is forced into the mold by centrifugal forces, as shown in Fig.19b. The properties of the castings vary by distance from the axis of rotation.

Fig.19 (a) Schematic illustration of the semi-centrifugal casting process .(b )Schematic illustration of casting by centrifuging .

13 .Squeeze Casting: The squeeze casting, or liquid-metal forging, process involves solidification of the molten metal under high pressure, Fig.20. The machinery includes, die, punch, and ejector pin. This process combines the advantages of casting and forging.

Fig.20 Sequence of operations in the squeeze casting process.

13.1 The pressure applied:The pressure applied by the punch has the following advantages:

1- Keeps the entrapped gasses in solution2- The contact under high pressure at the die-metal interface

promotes rapid heat transfer3- Due to this rapid cooling, a fine microstructure obtained

4- Good mechanical properties5- The application of pressure also overcomes feeding problems

that can arise when casting metals with along freezing range 

13.2 Process advantages: 

1- Parts can be made to near-net shape2- It can be used to produce complex shapes with fine surface

detail, from both ferrous and nonferrous alloys.

14 .Continuous Casting: This method is used to produce ingots of both ferrous and nonferrous alloys. This process can be used to cast tubes or pipes and plates.

 14.1 Casting procedure:

1- Metal is poured into the mold when the base is firstly upward at the bottom of the mold

2- After the first part of metal is solidified, the base is moved down using the piston

3- The piston takes the solidified metal and in the same time the molten metal is being continuously poured

4- It is important to control the piston speed carefully with the solidification rate 

Fig.21 Continuous casting machine

14.2 Advantages of the process: 1- Minimum losses

2- Produces steel ingots of large sizes and lengths3- Produce ingots of similar and regular specifications

4- Low cost of production

Base

Piston

Mold

Quenching water

Ladle

Molten metal

15 .Inspections and Testing of Castings: 

15.1 Visual inspection: (Nondestructive test)It can be done by using naked eye, or by using lens, or microscope of low magnification.

 15.2 Dimensional inspection: (nondestructive test)

This test can be done by using measuring tools (i.e. venires, micrometer, …….) 

15.3 Mechanical testing: (Destructive test)A specimen is cut from the cast, machined to the standard size required for the special test, and mechanical test (i.e. tension, compression, impact, bending, ….) were performed. The test results should be analyzed to know if the casting has the required properties or not.

 15.4 Metallurgical test: (Destructive test)

A specimen is cut from the cast and prepared for metallugraphic test. The specimen is examined on an optical microscope of high magnification or an electron microscope. Grain size, shape and porosity can be seen in this test.

 

15.5 Sound test: (Nondestructive test) In this test a chain suspends the cast, then it left to swing freely. A hummer then strikes the cast. Notice the obtained sound.

 15.6 Pressure test: (Nondestructive test)

This test is used for valves or tubes that exposed to internal pressure. The cast is put under a pressure equal to twice the working pressure. The pressure can be obtained from a pump. If the casting contains any defects then it will

explode and rejected . 

15.7 Ultrasonic test: (Nondestructive test) The specimen is exposed to the ultrasonic waves and the echo of these waves should be recorded. The wave should have the same size and shape. If there is any change in the wave size at any position this means that there is a defect at this point.

15.8 Radiographic test: (X-ray test) (Nondestructive test) The cast is exposed to X-ray or γ-ray. A film is put under the cast. The defects will appear on the film as dark areas.

15.9 Fluorescent-penetrate test: (Nondestructive test) The test procedure is as follows:1- The surface of the cast should be cleaned2- Apply the penetrate3- Then wash the surface of the cast4- Apply the developer5- Look at the surface of the cast to see any surface defects easily Unacceptable, rejected or defective castings are re-melted for reprocessing. Because of the major economic impact, the types of defects present in the castings and their causes must be investigated. Control of all stages during casting, from mold preparation to the removal of castings from molds or dies, is important in maintaining good quality.

16. Melting Practice and Furnaces: Melting practice is an important aspect of casting operations, because it has a direct bearing on the quality of castings. To protect the surface of the molten metal against atmospheric reaction and contamination –and to refine the melt- the pour must be insulated against heat loss. Insulation is usually provided by covering the surface or mixing the melt with compounds that form a slag. In casting steels, the composition of the slag includes CaO, SiO2, MnO, and FeO.

16.1 Furnace charge: Furnaces are charged with melting stock consisting of metal, alloying elements and various other materials such as flux and slag-forming constituents. Fluxes are inorganic compounds that refine the molten metal by removing dissolved gases and various impurities. Fluxes may be added manually or can be injected automatically into the molten metal. Fluxes have several functions, depending on the metal. For example, for aluminum alloys there are: (a) cover fluxes (to form a barrier to oxidation), (b) cleaning fluxes, (c) drossing fluxes, (d) refining fluxes, (e) and wall cleaning fluxes. Fluxes for aluminum typically consists of chlorides, fluorides, and borates of aluminum, calcium, magnesium, potassium, and sodium. Flux for magnesium consists of a composition of magnesium chloride, potassium chloride, barium chloride, and calcium fluoride.For copper alloys, there are: (a) oxidizing fluxes, (b) refining fluxes, (c) and mold fluxes for semi-continuous casting (to prevent oxidation and improve lubricity).For zinc alloys, such as in die-casting typical fluxes contain chlorides of zinc, potassium, and sodium. Fluxes for cast iron typically include sodium carbonate and calcium fluoride.

16.2 Melting furnaces: The melting furnaces commonly used in foundries are: electric-arc, induction,

crucible, and cupolas .16.2.1 Electric-arc furnaces: In the electric arc furnaces, the source of heat is a continuous electric arc formed between the electrodes and the charged metal, Fig. 22. Temperatures as high as 1925 ˚C are generated in this type of furnaces. There are usually three graphite electrodes, and they can be as large as 750 mm in diameter and 1.5 to 2.5 meter in length. Their height in the furnace can be adjusted depending on the amount of metal present and wear of the electrodes.Working steps:1- Steel scrap and small amount of carbon and limestone are dropped into the electric furnace through the open roof 2- Electric furnaces can also be charged with 100 percent scrap3- The roof is then closed4- The electrodes are lowered5- Power is turned on6- Within a period of about two hours the metal melts7- The current is shut off, and the electrodes are raised8- The furnace is tilted and the molten metal is pored into a ladle

Electric furnace capacities range from 60 to 90 tons of steel per day. The quality of steel produced is better than that of open-hearth or basic-oxygen steels.

For smaller quantities, electric furnaces are of the induction type. The metal is placed in a crucible, a large pot made of refractory material and surrounded with a copper coil through which alternating current is passed, Fig.22c. The induced current in the charge melts the metal. These furnaces are also used for re-melting metal for casting.

Fig. 22 Schematic illustration of types of electric furnaces: (a) direct arc, (b) indirect arc, and (c) induction.

16.2.2 Basic-oxygen furnace: The basic-oxygen furnace (BOF) is the newest and fastest steel-making process. The vigorous agitation of the oxygen refines the molten metal by an oxidation process in which iron oxide is first produced. The oxide reacts with the carbon in the molten metal, producing carbon dioxide. The BOF process is capable of refining 250 tons of steel in 35 to 50 minutes. Most BOF steels, which are of better quality than open-hearth furnace steels and have low impurity levels, are processed into plates, sheets, and various structural shapes, such as I-beams and channels. Working procedure:1- Typically, 200 tons of molten pig iron and 90 tons of scrap are charged (fed) into a vessel, Fig.23a. 2- Pure oxygen is then blown into the furnace for about 20 minutes under a pressure of about 1250 kPa, through a water-cooled lance, which is a long tube as shown in Fig.23b.3- Fluxing agents, such as lime, are added through a chute.4- The lance is retracted and the furnace is taped by tilting it. Note the opening in Fig.23c for the molten metal5- The slag is then removed by tilting the furnace in the opposite direction

Fig. 23 Schematic illustration showing (a) charging, (b) melting, and (c) pouring of molten metal in a basic-oxygen process.

16.2.3 Vacuum furnaces: Steel may be also be melted in induction furnaces from which the air has been removed, similar to the one shown in Fig. 22c. Because the process removes gaseous impurities from the molten metal, vacuum melting produces high-quality steels.

16.2.4 Induction furnaces: 

As shown in Fig.22c, these furnaces are especially useful in smaller foundries and produce composition-controlled smaller melts. There are two basic types .

The coreless induction furnace consists of a crucible completely surrounded with a water-cooled copper coil through which a high frequency current passes. Because there is a strong electromagnetic stirring action during induction heating, this type of furnaces has excellent mixing characteristics for alloying and adding new charge of metal.

The core or channel furnace which uses low frequency (as low as 60 Hz) and has a coil that surrounds only a small portion of the unit. It is commonly used in nonferrous foundries and is particularly suitable for superheating (heat above normal casting temperature to improve fluidity), holding (keeping the molten metal at a constant temperature for a period of time, thus making it suitable for die-casting applications). .

16.2.5 Crucible furnaces: 

These furnaces have been used extensively throughout history, as shown in Fig.24, are heated with various fuels, such as commercial gases, fuel oil, as well as electricity. They may be stationary, tilting, or movable. Many ferrous and nonferrous metals are melted in these furnaces.

Fig.24 Crucible furnace

16.2.6 Cupola furnaces: 

These furnaces are basically refractory-lined vertical steel vessels that are charged with alternating layers of metal, coke, and flux, Fig.25. Although they require major investments and are being replaced by induction furnaces, cupolas operate continuously, have high melting rates, and produce large

amounts of molten metal . 

16.3 Furnaces selection: 

Furnaces selection requires careful consideration of several factors that can significantly influence the quality of castings, as well as the economics of casting operations. So proper selection of a furnace depends on:

1- Economic considerations, such as initial cost and operating and maintenance costs

2- The composition and melting point of the alloy to be cast and ease of controlling metal chemistry

3- Control of the furnace atmosphere to avoid contamination of the metal4- Capacity and the rate of melting required

5- Environmental considerations, such as air pollution and noise6 -Power supply and its availability and cost of fuel

7- Ease of superheating the metal8 -Type of charge material that can be used 

Fig.25 Copula furnace

16.4 Safety in foundries: As all other manufacturing operations, safety in foundries is an important consideration. Safety is particularly important in these operations because

of the following factors: 1- Dust from sand and other compounds used in casting, thus requiring

proper ventilation and safety equipment for the workers2- Fumes from molten metals, as well as splashing of the molten metal during

transfer or pouring3- The presence of fuels for furnaces, the control of their pressure, the proper

operation of valves, etc.4- The presence of water and moisture in crucibles, molds, and other

locations, since it rapidly converts to steam, creating severe danger of explosion

5- Improper handling of fluxes, thus absorbing moisture and creating a danger6- Inspection of equipment, such as pyrometers, for accuracy and proper

calibration7- The need for proper personal safety equipment such as gloves, face

shields, and shoes.