A Report By - WordPress.com Processes 1 A Report By Zaheer ul haque ... In this report we will be focusing on sand molds, ... Sand casting is the most widely used casting process,

Mohammad Ali Jinnah University, Islamabad.

1 Casting Processes

A Report

By

Zaheer ul haque [Me113008]

Nauman Qureshi[Me113032]

Zohaib Ahmad [Me113022]

Talal Ahmad [Me113118]


2 Casting Processes

Table of Contents

CASTING PROCESSES .......................................................................................................................... 4

AN INTRODUCTION ............................................................................................................................ 4

CASTING PROCESS: STEP BY STEP ....................................................................................................... 5

MOLD MAKING .................................................................................................................................. 5

POURING THE METAL ........................................................................................................................... 5

SOLIDIFICATION .................................................................................................................................. 5

METAL CASTINGPROCESSES ............................................................................................................... 6

SAND CASTING.................................................................................................................................... 6

Patterns ..................................................................................................................................... 6

Core ........................................................................................................................................... 7

.................................................................................................................................................. 7

Mold Making .............................................................................................................................. 7

OTHER EXPENDABLE MOLD CASTING PROCESSES ........................................................................................ 8

Shell Molding ............................................................................................................................. 8

Vacuum Molding ........................................................................................................................ 9

Expanded Polystyrene Process .................................................................................................. 10

Investment Casting ................................................................................................................... 11

Plaster-mold casting ................................................................................................................. 12

PERMANENT MOLD CASTING PROCESS ................................................................................................... 12

Slush Casting ............................................................................................................................ 12

Low-Pressure Casting................................................................................................................ 13

Vacuum Permanent-Mold Casting ............................................................................................ 14

DIE CASTING .................................................................................................................................... 14

Hot-Chamber Machines ............................................................................................................ 14

Cold-Chamber Die Casting ........................................................................................................ 15

CENTRIFUGAL CASTING ....................................................................................................................... 15

True Centrifugal Casting ........................................................................................................... 15

Semi Centrifugal Casting ........................................................................................................... 16

Centrifuge Casting .................................................................................................................... 17

CASTING QUALITY ............................................................................................................................ 17

CASTING DEFECTS ............................................................................................................................. 17

INSPECTION METHODS ....................................................................................................................... 19

PRODUCT DESIGN CONSIDERATIONS ............................................................................................... 20


3 Casting Processes

Abstract

In this report a brief overview of casting processes is discussed with step by step processes. The

basic types of casting that is the expendable mold castings and permanent mold castings along

with their subtypes are also discussed. The quality assurance and the defects usually encountered

in this field are also explained. In the end a brief discussion of product design considerations that

are to be kept in mind when casting an object are also explained.


4 Casting Processes

Casting Processes

An Introduction Casting is a process in which molten metal flows by gravity or other force into a mold

where it solidifies in the shape of the mold cavity. The term casting is also applied to the part that

is made by this process. It is one of the oldest shaping processes, dating back 6000 years. The

principle of casting seems simple: melt the metal, pour it into a mold, and let it cool and solidify;

yet there are many factors and variables that must be considered in order to accomplish a

successful casting operation.

A variety of casting methods are available, thus making it one of the most versatile of all

manufacturing processes. Among its capabilities and advantages are the following:

Casting can be used to create complex part geometries, including both external and

internal shapes.

Some casting processes are capable of producing parts to net shape. No further

manufacturing operations are required to achieve the required geometry and dimensions

of the parts. Other casting processes are near net shape, for which some additional shape

processing is required (usually machining) in order to achieve accurate dimensions and

details.

Casting can be used to produce very large parts. Castings weighing more than 100 tons

have been made.

The casting process can be performed on any metal that can be heated to the liquid state.

Some casting methods are quite suited to mass production.

There are also disadvantages associated with casting—different disadvantages for

different casting methods. These include limitations on mechanical properties, porosity, poor

dimensional accuracy and surface finish for some casting processes, safety hazards to humans

when processing hot molten metals, and environmental problems.

Parts made by casting processes range in size from small components weighing only a few

ounces up to very large products weighing tons. The list of parts includes dental crowns,

jewellery, statues, wood-burning stoves, engine blocks and heads for automotive vehicles,

machine frames, railway wheels, frying pans, pipes, and pump housings. All varieties of metals

can be cast, ferrous and nonferrous. Casting can also be used on other materials such as polymers

and ceramics.


5 Casting Processes

Casting Process: Step By Step

Mold Making

Casting logically begins with the mold. The mold contains a cavity whose geometry determines the shape of the cast part. In an open mold, the liquid metal is simply poured until it fills the open cavity. In a closed mold, a passageway, which is carefully designed, called the gating system, is provided to permit the molten metal to flow from outside the mold into the cavity. The closed mold is by far the more important category in production casting operations.

A mold can be reusable i.e. permanent mold or be an expendable mold. The details of both will be discussed under separate headings in the following report.

Pouring the Metal

Before the metal can be poured inside the cavity of the mold, it must be melted. This process requires heat and establishes the fact that the gating system is also to be carefully designed. The energy required to achieve the pouring temperature of the metal is calculated by the following formula.

𝐻 = 𝜌𝑉[𝐶𝑠(𝑇𝑚 − 𝑇𝑜) + 𝐻𝑓 + 𝐶𝑙 (𝑇𝑝 − 𝑇𝑚)]

Where H is total heat required to raise the temperature of the metal to the pouring temperature; r is density; Cs is weight specific heat for the solid metal; Tm is melting temperature of the metal; To is starting temperature; Hf is heat of fusion; Cl is weight specific heat of the liquid metal; Tp is pouring temperature; and V is volume of metal being heated.

Once the metal is sufficiently heated its ready to be poured into the cavity. Introduction of molten metal into the mold, including its flow through the gating system and into the cavity, is a critical step in the casting process. For this step to be successful, the metal must flow into all regions of the mold before solidifying. Factors affecting the pouring operation include pouring temperature, pouring rate, and turbulence.

The pouring rate can be calculated by the simple formula, derived via the Bernoulli equation,

𝑇𝑀𝐹 = 𝑉𝑄⁄

Where TMF is mold filling time; V is volume of mold cavity: Q is volume flow rate

Solidification

Solidification involves the transformation of the molten metal back into the solid state. The solidification process differs depending on whether the metal is a pure element or an alloy. A pure metal solidifies at a constant temperature equal to its freezing point, which is the same as its melting point. The melting points of pure metals are well known and documented while most alloys freeze over a temperature range rather than at a single temperature. The exact range depends on the alloy system and the particular composition.

The time required for the metal to solidify is known as total solidification time and is calculated by the Chvorinov’s rule, which simply is

𝑇𝑇𝑆 = 𝐶𝑚(𝑉𝐴⁄ )

𝑛

Where TTS is total solidification time, V is volume of the casting; A is surface area of the casting: n is an exponent usually taken to have a value 2; and Cm is the mold constant.


6 Casting Processes

METAL CASTINGPROCESSES Metal casting processes divide into two categories, based on mold type: expendable mold

and permanent mold. In expendable mold casting operations, the mold is sacrificed in order to remove the cast part while in permanent mold casting the mold can be used several times.

In this report we will be focusing on sand molds, other expendable molds and the permanent molds.

Sand casting

Sand casting is the most widely used casting process, accounting for a significant majority of the total tonnage cast. Nearly all casting alloys can be sand cast; indeed, it is one of the few processes that can be used for metals with high melting temperatures, such as steels, nickels, and titanium. Its versatility permits the casting of parts ranging in size from small to very large and in production quantities from one to millions.

Sand casting, also known as sand-mold casting, consists of pouring molten metal into a sand mold, allowing the metal to solidify, and then breaking up the mold to remove the casting. The cavity in the sand mold is formed by packing sand around a pattern (an approximate duplicate of the part to be cast), and then removing the pattern by separating the mold into two halves. The mold also contains the gating and riser system. In addition, if the casting is to have internal surfaces (e.g., hollow parts or parts with holes), a core must be included in the mold.

Patterns

Sand casting requires a pattern, a full-sized model of the part, enlarged to account for shrinkage and machining allowances in the final casting. Materials used to make patterns include wood, plastics, and metals.

There are various types of patterns, as illustrated in. The simplest is made of one piece, called a solid pattern, same geometry as the casting, adjusted in size for shrinkage and machining. Although it is the easiest pattern to fabricate, it is not the easiest to use in making the sand mold.

Split patterns consist of two pieces, dividing the part along a plane coinciding with the parting line of the mold. Split patterns are appropriate for complex part geometries and moderate production quantities. The parting line of the mold is predetermined by the two pattern halves, rather than by operator judgment.

For higher production quantities, match-plate patterns or cope-and-drag patterns are used. In match-plate patterns, the two pieces of the split pattern are attached to opposite sides of a wood or metal plate.

Cope and drag patterns are similar to match-plate patterns except that split pattern halves are attached to separate plates, so that the cope and drag sections of the mold can be fabricated independently, instead of using the same tooling for both.

Please refer to figure 1, where a is a solid pattern, b is split pattern, c is match plate pattern and d is cope and drag pattern.

Figure 1


7 Casting Processes

Core

A core is a full-scale model of the interior surfaces of the part. It is inserted into the mold cavity prior to pouring, so that the molten metal will flow and solidify between the mold cavity and the core to form the casting’s external and internal surfaces. The core is usually made of sand, compacted into the desired shape. As with the pattern, the actual size of the core must include allowances for shrinkage and machining. Depending on the geometry of the part, the core may or may not require supports to hold it in position in the mold cavity during pouring. These supports, called chaplets, are made of a metal with a higher melting temperature than the casting metal. For example, steel chaplets would be used for cast iron castings. On pouring and solidification, the chaplets become bonded into the casting.

Mold Making

Foundry sands are silica (SiO2) or silica mixed with other minerals. The sand should possess good refractory properties, capacity to stand up under high temperatures without melting or otherwise degrading. Other important features of the sand include grain size, distribution of grain size in the mixture, and shape of the individual grains. Small grain size provides a better surface finish on the cast part, but large grain size is more permeable (to allow escape of gases during pouring). Molds made from grains of irregular shape tend to be stronger than molds of round grains because of interlocking, yet interlocking tends to restrict permeability.

In making the mold, the grains of sand are held together by a mixture of water and bonding clay. A typical mixture (by volume) is 90% sand, 3% water, and 7% clay. Other bonding agents can be used in place of clay, including organic resins

To form the mold cavity, the traditional method is to compact the molding sand around the pattern for both cope and drag in a container called a flask. An alternative to traditional flasks for each sand mold is flask less molding, which refers to the use of one master flask in a mechanized system of mold production. Each sand mold is produced using the same master flask

Desired Qualities

Several indicators are used to determine the quality of the sand mold: strength; The mold’s ability to maintain its shape and resist erosion caused by the flow of molten metal; it depends on grain shape, adhesive qualities of the binder, and other factors; permeability, capacity

Figure 2


8 Casting Processes

of the mold to allow hot air and gases from the casting operation to pass through the voids in the sand; thermal stability, ability of the sand at the surface of the mold cavity to resist cracking and buckling upon contact with the molten metal; collapsibility, ability of the mold to give way and allow the casting to shrink without cracking the casting; it also refers to the ability to remove the sand from the casting during cleaning; and reusability, can the sand from the broken mold be reused to make other molds? These measures are sometimes incompatible; for example, a mold with greater strength is less collapsible.

Classification

Sand molds are often classified as green-sand, dry-sand, or skin-dried molds.

Greens and molds are made of a mixture of sand, clay, and water, the word green referring to the fact that the mold contains moisture at the time of pouring. Green-sand molds possess sufficient strength for most applications, good collapsibility, good permeability, good reusability, and are the least expensive of the molds. They are the most widely used mold type, but they are not without problems. Moisture in the sand can cause defects in some castings, depending on the metal and geometry of the part.

A dry-sand mold is made using organic binders rather than clay, and the mold is baked in a large oven at temperatures ranging from 200 to 320 degree Celsius Oven baking strengthens the mold and hardens the cavity surface. A dry sand mold provides better dimensional control in the cast product, compared to green-sand molding. However, dry-sand molding is more expensive, and production rate is reduced because of drying time. Applications are generally limited to medium and large castings in low to medium production rates.

In a skin-dried mold, the advantages of a dry-sand mold is partially achieved by drying the surface of a green-sand mold to a depth of 10 to25mm at the mold cavity surface, using torches, heating lamps, or other means. Special bonding materials must be added to the sand mixture to strengthen the cavity surface.

Apart from the above method, several other ways can be used to make the mold thus they have different classifications than the above mentioned classical ones.

Other Expendable Mold Casting Processes

As versatile as sand casting is, there are other casting processes that have been developed to meet special needs. The differences between these methods are in the composition of the mold material, or the manner in which the mold is made, or in the way the pattern is made.

Shell Molding

Shell molding is a casting process in which the mold is a thin shell (typically 9mm) made of sand held together by a thermosetting resin binder.

There are many advantages to the shell-molding process. The surface of the shell mold cavity is smoother than a conventional green-sand mold, and this smoothness permits easier flow of molten metal during pouring and better surface finish on the final casting. Disadvantages of shell molding include a more expensive metal pattern than the corresponding pattern for green-sand molding. This makes shell molding difficult to justify for small quantities of parts. Shell molding can be mechanized for mass production and is very economical for large quantities.

In Shell molding casting, a match-plate or cope-and-drag metal pattern is heated and placed over a box containing sand mixed with thermosetting resin. The box is inverted so that sand and resin fall onto the hot pattern, causing a layer of the mixture to partially cure on the surface to form a hard shell. The box is repositioned so that loose, uncured particles drop away and then the sand shell is heated in oven for several minutes to complete curing. The mold is


9 Casting Processes

stripped from the pattern and the two halves of the shell mold are assembled, supported by sand or metal shot in a box, and pouring is accomplished. See figure 3.

Vacuum Molding

Vacuum molding, also called the V-process. the term vacuum in this process refers to the making of the mold rather than the casting operation itself.

Because no binders are used, the sand is readily recovered in vacuum molding. Also, the sand does not require extensive mechanical reconditioning normally done when binders are used in the molding sand. Since no water is mixed with the sand, moisture related defects are absent from the product. Disadvantages of the V-process are that it is relatively slow and not readily adaptable to mechanization.

Steps in vacuum molding: a thin sheet of preheated plastic is drawn over a match-plate or cope-and-drag pattern by vacuum, the pattern has small vent holes to facilitate vacuum forming. A specially designed flask is placed over the pattern plate and filled with sand, and a sprue and pouring cup are formed in the sand. Another thin plastic sheet is placed over the flask, and a vacuum is drawn that causes the sand grains to be held together, forming a rigid mold. The vacuum on the mold pattern is released to permit the pattern to be stripped from the mold. This mold is assembled with its matching half to form the cope and drag, and with vacuum maintained on both halves, pouring is accomplished. The plastic sheet quickly burns away on contacting the molten metal. After solidification, nearly all of the sand can be recovered for reuse.

Refer to figure 4.

Figure 3


10 Casting Processes

Expanded Polystyrene Process

The expanded polystyrene casting process uses a mold of sand packed around a polystyrene foam pattern that vaporizes when the molten metal is poured into the mold. Also known as lost-foam process, the foam pattern includes the sprue, risers, and gating system, and it may also contain internal cores (if needed), thus eliminating the need for a separate core in the mold. Also, since the foam pattern itself becomes the cavity in the mold, considerations of draft and parting lines can be ignored.

Expanded polystyrene casting process: pattern of polystyrene is coated with refractory compound and foam pattern is placed in mold box, and sand is compacted around the pattern and molten metal is poured into the portion of the pattern that forms the pouring cup and sprue. As the metal enters the mold, the polystyrene foam is vaporized ahead of the advancing liquid, thus allowing the resulting mold cavity to be filled.

Figure 4

Figure 5



A significant advantage for this process is that the pattern need not be removed from the mold. This simplifies and expedites mold making. With the expanded polystyrene process, these steps are built into the pattern itself. A new pattern is needed for every casting, so the economics of the expanded polystyrene casting process depend largely on the cost of producing the patterns. The process has been applied to mass produce castings for automobiles engines. Automated production systems are installed to mold the polystyrene foam patterns for these applications.

Investment Casting

In investment casting, a pattern made of wax is coated with a refractory material to make the mold, after which the wax is melted away prior to pouring the molten metal. The term investment means ‘‘to cover completely,’’ this referring to the coating of the refractory material around the wax pattern. It is a precision casting process, because it is capable of making castings of high accuracy and intricate detail.

Since the wax pattern is melted off after the refractory mold is made, a separate pattern must be made for every casting. Pattern production is usually accomplished by a molding operation, pouring or injecting the hot wax into a master die that has been designed with proper allowances for shrinkage of both wax and subsequent metal casting. In cases where the part geometry is complicated, several separate wax pieces must be joined to make the pattern. In high production operations, several patterns are attached to a sprue, also made of wax, to form a pattern tree; this is the geometry that will be cast out of metal.

Steps in investment casting: wax patterns are produced; several patterns are attached to a sprue to form a pattern tree; the pattern tree is coated with a thin layer of refractory material; the full mold is formed by covering the coated tree with sufficient refractory material to make it rigid and the mold is held in an inverted position and heated to melt the wax and permit it to drip out of the cavity. The mold is preheated to a high temperature, which ensures that all contaminants are eliminated from the mold; it also permits the liquid metal to flow more easily into the detailed cavity; the molten metal is poured. It solidifies and the mold is broken away from the finished casting. Parts are separated from the sprue.

See Figure

Figure 6



Plaster-mold casting

Plaster-mold casting is similar to sand casting except that the mold is made of plaster of Paris (gypsum, CaSO4–2H2O) instead of sand. Additives such as talc and silica flour are mixed with the plaster to control contraction and setting time, reduce cracking, and increase strength. To make the mold, the plaster mixture combined with water is poured over a plastic or metal pattern in a flask and allowed to set. Wood patterns are generally unsatisfactory due to the extended contact with water in the plaster. The fluid consistency permits the plaster mixture to readily flow around the pattern, capturing its details and surface finish. Thus, the cast product in plaster molding is noted for these attributes.

Curing of the plaster mold is one of the disadvantages of this process, at least in high production. The mold must set for about 20 minutes before the pattern is stripped. The mold is then baked for several hours to remove moisture. Even with the baking, not all of the moisture content is removed from the plaster. The dilemma faced by foundrymen is that mold strength is lost when the plaster becomes too dehydrated, and yet moisture content can cause casting defects in the product. A balance must be achieved between these undesirable alternatives. Another disadvantage with the plaster mold is that it is not permeable, thus limiting escape of gases from the mold cavity.

Permanent Mold Casting 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 aluminium, magnesium, copper-base alloys, and cast iron.

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 as semi-permanent-mold 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.

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 centre. 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.



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). Refer to figure 7.

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 from beneath so that the flow is upward. The advantage of this approach over traditional pouring is that clean molten metal from the centre 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 mechanical properties are improved.

Figure 7

Figure 8



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 them old 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.

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. 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 which 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. There are two main types of die casting machines: hot-chamber and cold-chamber, differentiated by how the molten metal is injected into the cavity.

Hot-Chamber Machines

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. 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.

Figure 9



In reference to figure 9. 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).

Cold-Chamber Die Casting

In cold-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. 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 aluminium, 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 favour its use on these metals.

In reference to figure 10. 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.)

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. The group includes (1) true centrifugal casting, (2) semi centrifugal casting, and (3) centrifuge casting.

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. 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

Figure 10



results in centrifugal 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 in horizontal centrifugal casting for the process to work successfully.

Semi Centrifugal Casting

In this method, centrifugal force is used to produce solid castings. The rotation speed in semi centrifugal casting is usually set so that G-factors of around 15 are obtained and the molds are designed with risers at the centre to supply feed metal. Density of metal in the final casting is greater in the outer sections than at the centre of rotation. The process is often used on parts in which the centre of the casting is machined away, thus eliminating the portion of the casting where the quality is lowest. Wheels and pulleys are examples of castings that can be made by this process.

Figure 11



Centrifuge Casting

In centrifuge casting, the mold is designed with part cavities located away from the axis of rotation, so that the molten metal poured into the mold is distributed to these cavities by centrifugal force. The process is used for smaller parts, and radial symmetry of the part is not a requirement as it is for the other two centrifugal casting methods.

Casting Quality There are numerous opportunities for things to go wrong in a casting operation, resulting

in quality defects in the cast product. In this section, we compile a list of the common defects that occur in casting, and we indicate the inspection procedures to detect them.

Casting Defects

Some defects are common to any and all casting processes. These defects are briefly described.

Misruns, which are castings that solidify before completely filling the mold cavity. Typical

causes include fluidity of the molten metal is insufficient, pouring temperature is too low,

pouring is done too slowly, and/or cross-section of the mold cavity is too thin. Figure 13,

a.

Cold Shuts, which occur when two portions of the metal flow together but there is a lack

of fusion between them due to premature freezing. Its causes are similar to those of a

misrun. . Figure 13, b.

Cold shots, which result from splattering during pouring, causing the formation of solid

globules of metal that become entrapped in the casting. Pouring procedures and gating

system designs that avoid splattering can prevent this defect. . Figure 13, c.

Shrinkage cavity is a depression in the surface or an internal void in the casting, caused

by solidification shrinkage that restricts the amount of molten metal available in the last

Figure 12



region to freeze. It often occurs near the top of the casting, in which case it is referred to

as a ‘‘pipe.’’ The problem can often be solved by proper riser design. . Figure 13, d.

Microporosity consists of a network of small voids distributed throughout the casting

caused by localized solidification shrinkage of the final molten metal in the dendritic

structure. The defect is usually associated with alloys, because of the protracted manner

in which freezing occurs in these metals. . Figure 13, e.

Hot tearing, also called hot cracking, occurs when the casting is restrained from

contraction by an unyielding mold during the final stages of solidification or early stages

of cooling after solidification. The defect is manifested as a separation of the metal (hence,

the terms tearing and cracking) at a point of high tensile stress caused by the metal’s

inability to shrink naturally. . Figure 13, f.

Some defects are related to the use of sand molds, and therefore they occur only in sand castings. To a lesser degree, other expendable-mold processes are also susceptible to these problems.

Sand blow is a defect consisting of a balloon-shaped gas cavity caused by release of mold

gases during pouring. It occurs at or below the casting surface near the top of the casting.

Low permeability, poor venting, and high moisture content of the sand mold are the usual

causes. Figure 14, a.

Pinholes, also caused by release of gases during pouring, consist of many small gas

cavities formed at or slightly below the surface of the casting. Figure 14, b.

Sand wash, which is an irregularity in the surface of the casting that results from erosion

of the sand mold during pouring, and the contour of the erosion is formed in the surface

of the final cast part. Figure 14, c.

Figure 13



Scabs are rough areas on the surface of the casting due to encrustations of sand and metal.

It is caused by portions of the mold surface flaking off during solidification and becoming

imbedded in the casting surface. Figure 14, d.

Penetration refers to a surface defect that occurs when the fluidity of the liquid metal is

high, and it penetrates into the sand mold or sand core. Upon freezing, the casting surface

consists of a mixture of sand grains and metal. Harder packing of the sand mold helps to

alleviate this condition. Figure 14, e.

Mold shift refers to a defect caused by a sidewise displacement of the mold cope relative

to the drag, the result of which is a step in the cast product at the parting line. Figure 14,

f.

Core shift is similar to mold shift, but it is the core that is displaced, and the displacement

is usually vertical. Core shift and mold shift are caused by buoyancy of the molten metal.

Figure 14, g.

Mold crack occurs when mold strength is insufficient, and a crack develops, into which

liquid metal can seep to form a ‘‘fin’’ on the final casting. Figure 14, h.

Inspection Methods

Foundry inspection procedures include visual inspection to detect obvious defects such as misruns, cold shuts, and severe surface flaws; dimensional measurements to ensure that tolerances have been met; and metallurgical, chemical, physical, and other tests concerned with the inherent quality of the cast metal.

Tests in category include: pressure testing to locate leaks in the casting; radiographic methods, magnetic particle tests, the use of fluorescent penetrants, and supersonic testing to detect either surface or internal defects in the casting; and mechanical testing to determine properties such as tensile strength and hardness.

If defects are discovered but are not too serious, it is often possible to save the casting by welding, grinding, or other salvage methods to which the customer has agreed.

Figure 14



Product Design Considerations If casting is selected by the product designer as the primary manufacturing process for a

particular component, then certain guidelines should be followed to facilitate production of the part and avoid many of the defects enumerated in Section. Some of the important guidelines and considerations for casting are presented here.

Geometric simplicity. Although casting is a process that can be used to produce complex

part geometries, simplifying the part design will improve its castability. Avoiding

unnecessary complexities simplifies mold making, reduces the need for cores, and

improves the strength of the casting.

Corners. Sharp corners and angles should be avoided, because they are sources of stress

concentrations and may cause hot tearing and cracks in the casting. Generous fillets

should be designed on inside corners, and sharp edges should be blended.

Section thicknesses. Section thicknesses should be uniform in order to avoid shrinkage

cavities. Thicker sections create hot spots in the casting, because greater volume requires

more time for solidification and cooling. These are likely locations of shrinkage cavities.

Draft. Part sections that project into the mold should have a draft or taper. In expendable-

mold casting, the purpose of this draft is to facilitate removal of the pattern from the mold.

In permanent-mold casting, its purpose is to aid in removal of the part from the mold.

Dimensional tolerances. There are significant differences in the dimensional accuracies

that can be achieved in castings, depending on which process is used

Surface finish. Typical surface roughness achieved in sand casting is around 6 mm.

Similarly poor finishes are obtained in shell molding, while plaster-mold and investment

casting produce much better roughness values: 0.75 mm.

Machining allowances. Tolerances achievable in many casting processes are insufficient

to meet functional needs in many applications. Sand casting is the most prominent

example of this deficiency. In these cases, portions of the casting must be machined to the

required dimensions.



References

Fundamentals of Modern Manufacturing, 4th edition, by Mikell P. Groover.

A Report By - WordPress.com Processes 1 A Report By Zaheer ul haque ... In this report we will be focusing on sand molds, ... Sand casting is the most widely used casting process,

Documents