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GIVE AN EXPLATION OF FOLLOWING TERMS:

by either air or hydraulic pressures, which forces the metal into the dies to In die casting, molten metal is forced by pressure into a metal mold

Known as a die. Because the metal solidifies under a pressure from 80 to 4000 psi, the casting confirm to the die cavity in shape and surface finish. The usual pressure is from 1500 to 2000 psi.

Die casting is most widely used of the permanent mold process. Two methods are employed (1) hot chamber (2) cold chamber.

In the hot chamber process a melting pot is included with the machine and the injection cylinder is immersed in the molten metal all the time.

The injection cylinder is actuated complete the casting. Machine using cold chamber uses a separate melting furnace and metal is introduced into the injection cylinder by hand or mechanical means. Hydraulic pressure then forces the metal into the die.
 

In die casting it is desirable to provide vents and small overflow wells on one side of the die to facilitate the escape of air and to catch surplus metal that has passed through the die cavity. There is amount of flash metal found at the sectional mating surface that must be trimmed in finishing process.
  In die casting when the molten metal solidifies then after opening the dies casted part is then ejected. For this purpose the dies are mounted with an ejector rod that comes in action and ejects the casting from the movable half of the die.
 
  Stalk is a heated refractory through which the molten metal is forced in the furnace. This type of refractory is used in Low Pressure Permanent Mold casting. It is a casting process which resembles both the gravity and slush processes but differs somewhat in procedure. A definite amount of metal is poured into an open ended mold and a close fitting core is forced into the cavity causing the metal to be forced into the mold cavities with some pressure. The core is removed as soon as the metal set, leaving a hollow, thin walled casting. This process was developed by corthias and is limited in use mainly to ornament casting of open design. In semicentrifugal casting the mold is completely full of metal as it is spun about its vertical axis. The center of casting is usually solid but because the pressure is less their, the structure is not so dense and inclusions and entrapped air are always present. This method is always used for parts in which the center of casting will be removed by machining. In these types of casting the molds which are used are called stack molds. Investment casting employs techniques that enable very smooth, highly accurate casting to be made from both ferrous and nonferrous alloys.The process is useful in casting unmachinable alloys and radioactive metals. There are a number of are a number of processes employed, but all incorporate a sand, ceramic, plaster or plastic shell made from an accurate pattern into which metal is poured. Although most castings are small, the investment process has been used to produce castings weighing over 100 lb. In lost wax process the wax pattern used is melted from the mold leaving a cavity having all the details of the original pattern. This mold is made from wax and is called lost wax. Dump box is used in Shell molding process. It’s also called blowing machine. The sand, free from clay is mixed with either urea or phenol formaldehyde resin ; the mix is then put into dump box. The pattern is preheated before putting it into dump box, the box is then inverted so that the sand may drop on the pattern. The pattern is then removed by ejecting the casting by an ejector. This process consist of continuously pouring molten metal into a mold that has a facility for rapid chilling the metal to the point of solidification, and then withdrawing it from the mold. The following processes are typical:

reciprocating mold process

EFFECT OF MELTING TEMPERATURE ON DIE LIFE:

Dies have negative effect of temperature, these were once limited to low melting alloys, but with gradual improvement of heat resisting metals for dies, this process can now be used for numerous alloys. But still die life decreases by operating it on high temperatures.

PRESSURE APPLIED TO COLD CHAMBER DIE CAST PART:

Die casting brass, aluminum, and magnesium requires higher pressures and melting temperatures. In cooled chamber die cast the pressure is applied under a hydraulic press. Machines operating by this method are built to be very strong and rigid to withstand the heavy pressure exerted on the metal as it is forced into the dies. Of the two machines in general use, one has plunger in vertical position, the other in a horizontal position.
 

CHARACTERISTICS OF ZINC:

Zinc have low casting temperature, alloy of zinc gives considerable strength and are of low cost. Zinc reduces the cost of bronze if it is added in greater percentage with brass. Bronze having 20% brass and 36% zinc is more cheaper because of zinc, having better machinability and slightly greater strength.
 
 

OVER 75% USE OF ZINC IN DIE CASTING:

Over 75% of die castings produced are the zinc base type. These alloys cast easily with a good finish at a fairly low temperature, and are of low cost. Nominal composition of two standard Zinc die casting alloys are indicated in the table. These alloys are much similar in composition except for the copper content, and in most cases they can be used interchangeably.

Typical zinc die casting alloy:

Alloy As cast tensile As cast elongation

Alloy number SAE strength (%)

AG40A 3 903 41000 10

AG41A 5 925 47600 7

---------- 7 903 41000 14
 
 

METALS AND ALLOYS CAST IN HOT CHAMBER MACHINE:

The list of metals and alloys used in hot chamber machine is given below:

USE OF INERT GAS IN LOW PRESSURE PERMANENT MOLD CASTING: Inert gas is used to force the metal in the furnace up through a heated refractory into the cavity. Inert gas is so used because it do not react with any alloy or metal and also because it is very lighter to escape and is easily escape with vacuum pump.
 
 

METALS AND ALLOYS CAST IN COLD CHAMBER MACHINE:

The list of metals and alloys used in cold chamber machine is given below:

DIFFERENCE BETWEEN MULTIPLE CAVITY DIE AND COMBINATION DIE:

Multiple cavity die have two or more same cavities, these are used when a number of small objects are to be casted. Whereas combination die have two or more different cavities these are frequently made insert boxes that can be removed so that other die block can be substituted.
 

EJECTOR PINS OR RODS IN DIE CASTING:

Ejector pins or rods are required in die casting as to eject the casted parts from the dies. They mar the surface as the ejector hit the casted part to eject it from the die which increases the cost of machining.

PRODUCTS MADE BY SLUSH CASTING PROCESS:

Slush casting is a process by which ornamental objects, statuettes, toys and other novelties are made. In this process the molten metal is poured into the die which is rotated over immediately so that the metal remaining remaining as liquid may run out. A thin walled casting results, the thickness of which depends on the chilling effect from the mold and and the time of operation. The casting is removed by opening the two halves of the die.

BRASS DIE CASTING IS DIFFICULT:

Brass have high melting temperature and is not melted in self contained pot, because the life of pot would be very short. The usual procedure is to heat the metal in an auxiliary furnace and ladle it to the plunger cavity next to the die. It is then forced into the dies under the hydraulic pressure.

VACUUM PUMP:

The air from the die is removed by using vacuum pumps. These are mounted to the dies and come in action before the metal is poured into the die.

FUNTION OF STALK IN LOW PRESSURE MOLD:

Stalk is a heated refractory through which the molten metal is pushed into the die. This is used so as to keep the metal in pure molten state until it reaches the die.

LARGE CASTINGS IN TRUE CENTRIFUGAL CASTING METHOD:

Vertical cast parts are usually spun at 90 to 100 g’s. Vertical cast parts are much smaller in size and weight because of the instability of a spinning vertical cylinder, the higher g force necessary to overcome the parabolic shape and the increased pressure on the mold.

Therefore we cannot cast large castings in true centrifugal casting method.
 
 

COMPARISON BETWEEN ADVANTAGES OF:

GRAVITY TYPE PERMANENT MOLD:


DIE CASTING:

SLUSH CASTINGS:

Alloys of pure metals may be used but they should be of low melting temperatures, like lead, zinc etc. Parts casted in this fashion are either painted or finished to represent bronze.

FERROUS METALS FOR PLASTER MOLD:

The gypsum based plaster used as a casting investment dry quickly with good porosity but are not permanent, being destroyed when the casting is removed.

Plaster molds are suitable only for nonferrous alloys. The wide variety of small casting made by this process includes miscellaneous airplane parts and numerous intricate parts. In general, the process competes more successfully with die casting using high temperature alloys such as brass rather than metals such as zinc and aluminum. At high temperatures metal molds have relatively short life; with plaster molds.

ATTACHMENT OF WAX PATTERN:

In multiple cavities investment casting the wax patterns are joined to the gates by wax welding. This is done by heating a wire, then touching it with the wax pattern, after which the wax pattern is stuck with the gates

VACCUM POURING:

Generally the molten metal is poured into the mold by air pressure or by gravity force. Another process by which we can pour the molten metal is the vacuum pouring. In this process a vacuum pump is mounted to the mold cavity, which produces a vacuum in it by which the molten metal is sucked into the mold.

SHRINKAGE IN CONTINOUS CASTING:

In continuous casting the casted parts are cooled rapidly which causes the shrinkage. This is reduced by giving a shrinkage allowance or by giving a riser. In continuous casting the upper end in molten metal acts as a riser and compensates for any shrinkage that might take place during solidification.

PROCESS ADAPTABLE TO MASS PRODUCTION:

In shell molding process the mold is dried in oven whereas in co2 processes the molds are dried by co2 gas. Hence for mass production co2 processes are more adaptable as to reduce the cost of huge ovens required for drying the molds in shell molding processes.

SPECIAL EXPENCES IN SHELL MOLDING PROCESS:

In shell mold the following extra things are required which increases the cost of process:

Hence small production runs cannot afford these expenses.

PROCESS USED FOR FOLLOWING CASTINGS:


SAND CASTING (THEORITICAL CONCEPT):

The theoretical concept of sand casting that we took with us was that , metal will be burned at a high temperature and poured into a flask having the mold cavity through an opening called sprue .There are two parts of flask ,cope and drag. The flask will contain green sand in which a mold cavity would have already been formed by putting in the pattern. There would be some permeability for letting gases escape. The soldification of molten metal would take place and later on the flask shall be opened, and the formed product would be taken out.

TYPES OF SAND CASTINGS :

There are two different methods by which sand castings can be produced. Classified according to the type of pattern used, they are:

In the method employing a removable pattern, sand is packed round the pattern. Later the pattern is removed and the cavity produced is filled with the molten metal. Disposable patterns are made from polystyrene and, instead of being removed from the sand, are vaporized when the molten metal is poured into the mold.

To understand the foundry process, it is necessary to know how a mold is made and what factors are important to produced a good casting. The principal factors are molding procedure, patterns, sand, cores, mechanical equipment, the metal and pouring and cleaning the casting.
 

MOLDING PROCEDURE:

Molding are classified according to the materials used.

This most common method, consisting of forming the mold from damp molding sand , is used in both of the processes previously described. The term green sand does not refer to the color of the sand, which is dark brown in color, but rather to the fact that the sand is uncured. Two general methods are used in preparing the skin-dried molds.

In one, the sand around the pattern to a depth of about half in 12.7mm is mixed with a binder, so that when it is dried it will leave a hard surface on the mold. The remainder of the mold is ordinary green sand.

These molds are made entirely from fairly coarse molding sand mixed with a binding material similar to those already mentioned. Since they must be oven-baked before being used, the flasks are of metal. A dry-sand mold holds its shape when poured and is free from gas problems caused by moisture. In this process clean sand is mixed with sodium silicate and the mixture is rammed about a pattern. When co2 gas is pressure fed to the mold, the sand mixture hardens. Very smooth and intricate castings are obtained by this method, although the process was originally developed for making cores. Plastics, cement, plaster, paper, wood and rubber are all mold materials used to fit particular applications.

MOLDING PROCESSES:

These, in conventional foundry may be classified as:

Bench molding is for small work done on a bench at a height convenient to the molder.
 
  When castings increases in size with the resultant difficulty in handling, the work is done on the foundry floor. This type of molding is used for practically all medium sized and large castings. Extremely large castings are frequently molded in a pit instead of a flask. The pit acts as the drag part of the flask, and a separate cope is used above it. The sides of the pit are brick lined, and on the bottom there is a thick layer of cinders with connecting vent pipes to the floor level. Machine have been developed to do a number of the operations that the molder ordinary does by hand. Ramming the sand, rolling the mold over, forming the gate, and drawing the pattern can be done by these machines much better and more efficiently than by hand.

TYPES OF SAND:

Silica sand(SiO2), found in many natural deposits, is well suited for molding purposes because it can withstand a high temperature without decomposition. This sand is low in cost, has a high expansion rate when subjected to heat and has some tendency to fuse with the metal. If it contains a high percentage of fine dust, it may constitute a health hazard.

Pure silica sand is not suitable for molding because it lacks binding qualities. The binding qualities may be obtained by adding 8% to 15% clay. The three types of clay commonly used are kaolinite, illite, and bentonite. The latter, used most often, is weathered volcanic ash.

Some natural molding sands are bonded with clay when quarried, and only water must be added to have an adequate molding sand for nonferrous castings. The large amount of organic material found in natural sands prevents them from being sufficiently refractory for high-temperature applications such as in the molding of higher melting point metals and alloys.

MECHANICAL MOLDING EQUIPMENT:

Machines can eliminate much of the labor in molding and at the same time produce better molds. Molding machines, varying considerably in design and method of operation, are named according to the way the ramming operation is performed.

The plain jolt molding machine is equipped with adjustable flask lifting pins to permit the use of flasks of various size within the capacity of the machine. Molds weighing up to 13000 lb can be made on the larger machines. In the operation of this machine the table is raised a short distance by means of air pressure and then dropped. This action causes the sand to be packed evenly about the pattern. The density of the sand is greatest around the pattern. The uniform ramming about the pattern gives added strength to the mold and reduces the possibility of swells, scabs or runouts. Castings produced under such conditions vary little in size or weight. The lifting pins on the machine engage the flask and raise it from the match plate after the mold is complete. Jolt machines quite obviously take care of only one part of a flask at a time and are especially adapted to production runs. Squeezer machines press the sand in the flask between the machine table and an overhead platen. Greatest mold density is obtained at the side of the mold from which the pressure is applied. Because it is impossible to obtain uniform mold density by this method, squeezing machine are limited to molds only a few inches in thickness. Many machines, use both jolt and squeeze. To produced a mold in this machine the flask is assembled with the match plate between the cope and drag, and the assembly is placed upside down on the machine table. Sand is shoveled into the drag and leveled off and a bottom board is placed on top. The jolting action then rams the sand in the drag. The assembly is turned over and the cope filled with sand and leveled off. A pressure board is placed on the top of the flask and the top platen of machine is brought into position. By application of pressure the flask is squeezed between the platen and table, packing the sand in the cope to the proper density. After the pressure is released the platen is swung out of the way. The cope is then lifted from the match plate while the plate is vibrated, after which the plate is removed from the drage. This machine eliminates six separate hand operations: ramming, smoothing the parting surfaces, applying parting sand, swabbing around the pattern, rapping the pattern, and cutting the gate.

GATES AND RISERS:

The passageway for bringing the molten metal to the mold cavity, which is known as the gating system, is usually made up of a pouring basin, a downgate or vertical passage known as a sprue, and a gate through which the metal flows from the sprue base to the mold cavity. In large castings a runner may be used that takes the metal from the sprue base and distributes it to several gate passageways around the cavity. The design of the gating system is important and involves a number of factors.

Risers are often provided in molds to feed molten metal into the main casting cavity to compensate for the shrinkage. They should be large in section so as to remain molten as long as possible, and should be located near heavy sections subject to large shrinkage. If they are placed at the top of the section, gravity will assist in feeding the metal into the casting properly.

Blind risers are domelike risers found in the cope half of the flask that are not the complete height of the cope. They are normally placed directly over the gate where the metals feeds into the mold cavity and thus supply the hottest metal when pouring is completed.

PATTERN ALLOWANCES:

For the good finishing and to obtain the object of same size which is required the allowances should be made in the pattern. Some of which are shrinkage, draft finish, distortion and shake.

When any pure metal and most alloy metals cool, they shrink. To compensate for shrinkage a shrink rule must be used in laying out the measurementsfor the pattern. A shrink rule for cast iron is 1/8 in per foot. When a removable pattern is drawn from a mold, the tendency to tear away the edges of the mold in contact with the pattern is greatly decreased if the surface of the pattern, parallel to the direction it is being withdrawn, are slightly tapered. This tappering of the sides of the pattern is known as draft, is done to provide a slight clearance for the for the pattern as it is lifted up. Draft is provided to exterior dimensions of a pattern and is usually 1/8 to ¼ in per foot. On a drawing of the details of a part to be cast, each surface that will ultimately be machined is indicated by a finish mark. This mark shows the pattern maker where additional metal must be provided so that there will be some metal to machine, hence a finish allowance. The amount that is to be added to the pattern depends on the size and shape of the casting but in general the allowances for small and average-sized castings is 1/8 in per foot. Distortion allowances applies only to those castings of irregular shape that are distorted in the cooling process because of shrinkage of metal. A horse shoe-shaped piece would be an example. When a removable pattern is rapped in the mold before it is withdrawn, the cavity in the mold increases slightly. In an average-sized casting this increase in size can be ignored. In large castings or in ones that must fit together without machining, a shake allowance should be considered by making the pattern slightly smaller.

SAND TESTING:

Various tests are performed in order to determine the quality of sand and its usefulness for casting processes. Some of which are defined below:

 Porosity of the sand enables the escape of gas and steam formed in the mold. Sand must be cohesive to the extent that it has sufficient bond; both water and clay content affect the cohesive properties. Sand must resist high temperatures without fusing. Sand must have a grain size commensurate with the surface to be produced, and grains must be irregular to the extent that they will have sufficient bonding strength.

TEST FOR MOISTURE CONTENTS:

The moisture content of foundry sands varies according to the type of mold being made and the metal being poured. For a given condition there is a close range within which the moisture percentage should be held to produce satisfactory results.

The moisture teller contains electric heating units and a blower for forcing warm air through a filter pan containing the sand sample. By weighing the sand after it is dried and noting the difference in the initial and final readings, the percentages of moisture can be determined. The moisture content should vary from 2% to 8%, depending on the type of molding being done.

CLAY CONTENT TEST:

The equipment necessary for determining the percentages of clay in molding sands consists of a drying oven, a balance and weights, and a sand washer.

A quantity of sand is dried and a water based caustic soda solution is added. Following a timed mixing the caustic soda solution, which has absorbed the clay, is siphoned off. This process is repeated three times. The sample is dried, weighted, and compared to the original sample weight to determine the loss in clay.
 

PERMEABILITY TEST:

One of the essential qualities of molding sand is sufficient porosity to permit the escape of gases generated by the hot metal. This depends on several factors including the shape of sand grains, fineness, degree of packing, moisture contents, and amount of binder present. Permeability is measured by the quantity of air that passes through a given sample of sand in a prescribed time and under standard conditions. Coarse-grained sands are naturally more permeable,