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UNIT 5: Casting Processes Manufacturing Engineering Unit 5 Copyright © 2012 MDIS. All rights reserved. 1
Pure metals It gives off the latent heat of fusion during the process solidification front moves through the molten metal from the mold wall inwards grains of the micro structure grow in direction opposite to the heat transfer grains with the preferred direction of growth are called the columnar grains grains have absolutely different orientations even in the same region of the mold, is called homogeneous nucleation 5-2Unit 5 Copyright © 2012 MDIS. All rights reserved.
Alloys solidification begins at temperatures beow the liquid line and completes at the solid line columnar dendrites form that have three dimensional arms and branches that interlock Effects of various parameters on Casting slower cooling rates of 100K/s results in coarse dendritic structure Successful casting requires that the fluid flow components of the pattern must be designed properly for the flow quantity, materials viscocity, and rate of solidification Mold Design includes the critical dimensions of the cavity, the tapers, and design of the sprue, runners, gates, rises 5-3Unit 5 Copyright © 2012 MDIS. All rights reserved.
Material Shrinkage As the material solidifies, it begin to shrink in volume. A casting that is good needs to have its dimensions calibrated to this shrinkage Casting Defects Porosity Caused by shrinkage, by gases or both casting has small regions without any material,. Micro porosity is when the materials shrinks in very tiny regions at the dendritic branches. It affects the castings surface Projections there are metallic fins, flash or projections 5-4Unit 5 Copyright © 2012 MDIS. All rights reserved.
Cavities caused by round or rounded rough internal cavities examples include blow holes, pinhoiles, and shrinkage cavities Discontinuities cracks, cold or hot tearing caused by metal constrained from shrinking freely 5-5Unit 5 Copyright © 2012 MDIS. All rights reserved.
Metal Casting Process Major categories of the casting process are expendable mold, permanent mold, and composite molds. Expendable molds sand ; shell mould, plaster mold, ceramic mold, evaporative pattern and investment casting Sand casting casting of molten metal into a sand pit of speical grades of sand offers least accuracy, and is good for rough precision casting 5-6Unit 5 Copyright © 2012 MDIS. All rights reserved.
Parts of mold cope and drag- hold the cavity on the top and bottom sides of the mould pouring cup or basin -molten metal enters the mold Sprue - to allow the melt to reach the cavity level from the top surface of cope runner- which leads the melt to the gates of individual cavities gate -to constrict flow and prevent particles from entering the cavity risers which allow excess melt to retain melt to compensate for shrinkage as melt cools core- to provide for hollows in the casting Pattern - used to create the hollow in the cope and drag of the sand casting 5-7Unit 5 Copyright © 2012 MDIS. All rights reserved.
Shell casting can produce higher dimensional tolerances than sand casting used for small parts with lower weight pattern of ferrous or aluminium is heated to about 370C and coated with parting agents. Then it is clamped to the casting box. The sand and binder is introduced the box is rotated about two axis thus coating the pattern. This is then placed in an oven for a complete curing of the resin binder. The shell hardens around the pattern, and can now be ejected off the pattern. A sand replica of the patterns is obtained 5-8Unit 5 Copyright © 2012 MDIS. All rights reserved.
Plaster Cast molding plaster of paris with addition of talc and silica are stirred in a slurry state and poured over a pattern. plaster is dried at 120C. The mold halves are assembled to form the mold cavity and kept preheated to 120C. Patterns used are made of aluminium, plastics etc. Wood is not a suitable pattern material here due to its moisture absorbency. Plaster mold casting can only be used on Aluminium, magnesium, zinc and copper based alloys due to the temperature limit of 1200C for plaster. They also cool slowly and result in uniform grain structure and reduced warpage 5-9Unit 5 Copyright © 2012 MDIS. All rights reserved.
Lost foam casting (Also called evaporative pattern casting) pattern is made from an embedded pattern material that will eventually evaporate leaving no residue. Raw expandable styrene (EPS) beads are placed in a preheated die to take the shape of the die. As the beads fuse together, the die is cooled and the pattern removed, Complex patterns may be made in this way. This pattern is coated with a refractory slurry, and then placed in a flask with dry fine sand. The sand is compacted and then molten metal poured in to the cavity. As molten high temperature materials enters the cavity, the EPS vaporizes and leaves the cavity in its place into which the metal flows, thus creating a replica of the pattern. The process is simple, and there are no parting lines giving design flexibility. The pattern although lost is itself inexpensive. The casting requires minimal finishing and cleaning. And the process can be automated. 5-10Unit 5 Copyright © 2012 MDIS. All rights reserved.
The process is simple, and there are no parting lines giving design flexibility. The pattern although lost is itself inexpensive. The casting requires minimal finishing and cleaning Investment casting pattern made of a lasting material is created by a lost wax method. This pattern is then used in the process of casting and is thus called the investment casting method. Investment here refers to the surrounding of the pattern being surrounded with the refractory materials. 5-11Unit 5 Copyright © 2012 MDIS. All rights reserved.
The mold made around the wax is then held at elevated temperatures for few hours and inverted. This causes the wax to melt out. The hollow cast then is used to pour metal for the final casting process. Vacuum Casting This is a counter-gravity process. Molten metal is sucked by the vacuum pressures in the mold cavity to fill up against gravity. Resulting parts can be intricate and complex, with little chance of cavities. 5-12Unit 5 Copyright © 2012 MDIS. All rights reserved.
Mixture of fine sand ahd urethane is molded over metal dies and cured with amine vapor. It is then immersed into molten metal in an induction furnace. Gas turbine parts and thin walled components of 0.5mm are made using this process. Slush Casting As molten metal comes in contact with lower temperature form a thin skin like structure. molten metal is poured into a metallic mold. desired skin thickness is achieved, the mold is inverted to remove the excess metal, still in molten state. Ornaments, toys and low melting point metals can be used in this process. 5-13Unit 5 Copyright © 2012 MDIS. All rights reserved.
Control of Media Text - Tutorial Pressure Casting As in the vacuum casting, using pressure to pack molten metal into a cavity results in the same ability of intricate and complex mold Molten metal is force upwards by gas pressure into a graphite or metal mold. This method is used in precision and high-quality parts. Die Casting A die similar to the injection molding die in plastics is created and molten metal is forced into the die by hydralulic pressure on an injecting plunger. 5-14Unit 5 Copyright © 2012 MDIS. All rights reserved.
The arrangement can be horrizontal or vertical. High melting point alloys are used in this process. In the hot chamber process, the plunger pushes molten metal retained in a hot chamber such as a furnace through a goose neck path into the cavity. In the cold chamber process the metal is poured into the injecting chamber which is not heated hence this name. Centrifugal Casting the parts to be cast are normally revolved sections like pipes, bearing housings etc. Molten metal is poured into a rotating cavity with a horizontal axis of rotation. Vertical axis is not normal but can be used for short castings. Unit 5 Copyright © 2012 MDIS. All rights reserved. 5-15
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