PERMANENT MOLD OR GRAVITY DIE CASTING
This process is commonly known as permanent mold casting in U.S.A and gravity die casting in England. A permanent mold casting makes use of a mold or metallic die which is permanent. Molten metal is poured into the mold under gravity only and no external pressure is applied to force the liquid metal into the mold cavity. The metallic mold can be reused many times before it is discarded or rebuilt.
The molds are made of dense, fine grained, heat resistant cast iron, steel, bronze, anodized aluminum, graphite or other suitable refractoriness. The mold is made in two halves in order to facilitate the removal of casting from the mold. It may be designed with a vertical parting line or with a horizontal parting line as in conventional sand molds.
The mold walls of a permanent mold have thickness from 15 mm to 50 mm. The thicker mold walls can remove greater amount of heat from the casting. For faster cooling, fins or projections may be provided on the outside of the permanent mold. This provides the desirable chilling effect.
Advantages
(i) Fine and dense grained structure is achieved in the casting. Because of rapid rate of cooling, the castings possess fine grain structure.
(ii) No blow holes exist in castings produced by this method. Good surface finish and surface details are obtained. defects observed in sand castings are eliminated.
(iii) The process is economical for mass production. Fast rate of production can be attained.
(iv) Close dimensional tolerance or job accuracy is possible to achieve on the cast product.
(v) Manpower required is less.
Disadvantages
(i) The cost of metallic mold is higher than the sand mold. The process is impractical for large castings.
(ii) The surface of casting becomes hard due to chilling effect.
(iii) Refractoriness of the high melting point alloys.
Applications
(i) This method is suitable for small and medium sized casting such as carburetor bodies, oil pump bodies, connecting rods, pistons etc.
(ii) It is widely suitable for non-ferrous casting.
PRESSURE DIE CASTING
Molten metal is forced into metallic mold or die under pressure in pressure die casting. The pressure is generally created by compressed air or hydraulically means. The pressure varies from 70 to 5000 kg/cm 2 and is maintained while the casting solidifies. The application of high pressure is associated with the high velocity with which the liquid metal is injected into the die to provide a unique capacity for the production of intricate components at a relatively low cost. This process is called simply die casting in USA. The die casting machine should be properly designed to hold and operate a die under pressure smoothly. There are two general types of molten metal ejection mechanisms adopted in die casting set ups which are:
(a) Gooseneck or air injection management
(b) Submerged plunger management
(ii) Cold chamber type
Die casting is widely used for mass production and is most suitable for non-ferrous metals and alloys of low fusion temperature. The casting process is economic and rapid. The surface achieved in casting is so smooth that it does not require any finishing operation. The material is dense and homogeneous and has no possibility of sand inclusions or other cast impurities. Uniform thickness on castings can also be maintained.
The principal base metals most commonly employed in the casting are zinc, aluminum, and copper, magnesium, lead and tin. Depending upon the melting point temperature of alloys and their suitability for the die casting, they are classified as high melting point (above 540°C) and low melting point (below 500°C) alloys. Under low category involves zinc, tin and lead base alloys. Under high temperature category aluminum and copper base alloys are involved.
Applications
1. Carburetor bodies
2. Hydraulic brake cylinders
3. Refrigeration castings
4. Washing machine
5. Connecting rods and automotive pistons
6. Oil pump bodies
7. Gears and gear covers
8. Aircraft and missile castings, and
9. Typewriter segments
Chapter Contents - Groover
10 FUNDAMENTALS OF METAL CASTING
Chapter Contents
10.1 Overview of Casting Technology
10.1.1 Casting Processes
10.1.2 Sand-Casting Molds
10.2 Heating and Pouring
10.2.1 Heating the Metal
10.2.2 Pouring the Molten Metal
10.2.3 Engineering Analysis of Pouring
10.2.4 Fluidity
10.3 Solidification and Cooling
10.3.1 Solidification of Metals
10.3.2 Solidification Time
10.3.3 Shrinkage
10.3.4 Directional Solidification
10.3.5 Riser Design
The starting work material is either a liquid or is in a highly plastic condition, and parts are created through solidification of the material. Casting and molding processes dominate this category of shaping operations.
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. Casting also means the part that is made by the casting process.
11. METAL CASTING PROCESSES
Chapter Contents
11.1 Sand Casting
11.1.1 Patterns and Cores
11.1.2 Molds and Mold Making
11.1.3 The Casting Operation
11.2 Other Expendable-Mold Casting Processes
11.2.1 Shell Molding
11.2.2 Vacuum Molding
11.2.3 Expanded Polystyrene Process
11.2.4 Investment Casting
11.2.5 Plaster-Mold and Ceramic-Mold Casting
11.3 Permanent-Mold Casting Processes
11.3.1 The Basic Permanent-Mold Process
11.3.2 Variations of Permanent-Mold Casting
11.3.3 Die Casting
11.3.4 Squeeze Casting and Semisolid Metal Casting
11.3.5 Centrifugal Casting
11.4 Foundry Practice
11.4.1 Furnaces
11.4.2 Pouring, Cleaning, and Heat Treatment
11.5 Casting Quality
11.6 Metals for Casting
11.7 Product Design Consideration
Foundry
Principles of Foundry Technology
P. L. Jain
Tata McGraw-Hill Education, 2003 - Founding - 401 pages
Updated on 2 September 2019, 12 August 2018
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