Foundry process - metal casting

FOUNDRY PROCESS - METAL CASTING

Foundry Process consists of pattern making, making molds, melting the metal and pouring the metal into the molds to make castings. Foundry or Casting is the process of forming metallic products by melting the metals, pouring it into a cavity of the required shape, known as the mold and allowing it to solidify. In this way the molten metal takes the shape of the mold.

The product of the foundry is the casting which may vary from a fraction of a kilogram to several tonnes. Practically all ferrous and non-ferrous metals and alloys can be: cast. Foundry is one of the oldest industries in the metal forming field and dates back to approximately 2000 B.C. A shop where molding, casting and their related processes are conducted is called a Foundry Shop.

Basic steps in the Foundry Process are:

- Pattern Making

 - Molding

- Metal Melting and Pouring, i.e., Casting

 

1. PATTERN MAKING

Before any product is produced through casting process, a model, which is an exact facsimile of the product to be produced, known as pattern is prepared. The quality of casting produced depends on the design, construction, accuracy and the material of the pattern.

Pattern Materials

The pattern is made from suitable materials, such as wood, aluminium, plaster or plastics, waxes, etc. The material of pattern depends upon the number of castings to be produced, size of casting and the casting metal used.

Wood (Teak or Mahogony) is the material very widely used for pattern making. It can be given desired shape and finish easily. Aluminium pattern is used when large numbers of castings are to be made. The aluminium pattern itself is cast from a wooden pattern, called Master Pattern. Aluminium is the best all-round metal for pattern making due to light weight, easy to work and high resistance to corrosion.

A poor casting may be produced from a good pattern, but a good casting will not be made from a poor pattern.

Pattern Allowances

The size of a pattern is never kept the same as that of the desired casting. This is because of the fact that during cooling, the casting is subjected to various metallurgical effects. Hence, shrinkage, distortion and machining allowances are provided in the pattern to compensate for contraction during solidification, machining, etc.

 

2. MOLDING

Mold is the cavity of the required shape made in molding sand into which molten metal is poured. Molding process consists of making a mold. Pattern, molding box, molding sand and molding tools are necessary for molding process.

Molding Sand

Foundry sand contains silica, clay and moisture in suitable proportions. Silica sand in the form of granular quartz is the main constituent of molding sand. Clay acts as a bonding medium around the silica grains.

Molding Procedure

The procedure for green sand molding (floor or bench molding) is explained in the following steps:

1. See Fig. (i). A two piece (split) pattern is used. One half of the pattern is placed on the molding board. The molding box (drag) is placed with the dowel pins down..

2. Molding sand (i.e., sand with proper amount of moisture and clay content) is filled in the molding box to cover the pattern. It is pressed firmly around the pattern with fingers.

3. The drag is completely filled with sand upto the top and rammed by the peen end of the hand rammer. During ramming, if necessary, additional sand can be put into the drag. Later, the sand around the pattern as well as the edges of the molding box are rammed with the help of the butt-end of the rammer, so that packing is perfect.. Remember loose packing and too hard packing, both are undesirable. The proper amount of ramming comes through experience only. If the mold is not sufficiently rammed, it is difficult to hold the packed sand. On the other hand, if it is rammed too hard, the escape of gases and steam becomes difficult, while pouring the molten metal.

4. Then the excess sand is leveled by a strike-off bar.

5. Small vent holes are made through the sand using vent wire, so that when the molten metal is poured, the gases and steam so generated can escape out.

6. A little sand is sprinkled over the mold and smoothened by a trowel.

7. See Fig. (ii). The drag is tilted upside down.

8. Dry parting sand is sprinkled such that the pattern and surface are completely covered.This is done to prevent the sand of the drag from sticking with the sand of the cope.

9. The top half of the pattern is now placed correctly in position over the first half.

10. See Fig. (iii). The cope is placed correctly in position on the drag and aligned using dowel pins.

11. The sprue pin is placed vertically by the side of the pattern for making sprue hole for pouring the molten metal.

12. The riser is placed vertically over the highest point of the pattern so that the gases and - steam in the molten metal will escape.

13. The operations of filling the sand, ramming and venting of the cope are done in the same manner as in the drag.

14. See Fig. (iv). The riser and the sprue pin are removed. A funnel shaped opening called Pouring Basin is cut at the top to facilitate easy pouring of the metal.

15. The cope is lifted, turned over and placed on the floor.

16. The pattern pieces are carefully removed after moistening the edges with a swab and loosening the pattern by a draw spike.

17. Then, a small passage known as gate is cut connecting the sprue basin (bottom of the sprue hole) and the mold cavity. A bellow is used to clean the sand or dirt, fallen into the mold cavity.

18. The mold surfaces are coated with graphite, using a swab, to give smooth surface to the casting.

19. Bake the mold, in the case of Dry Sand Mold.

20. The cope is placed over the drag carefully again to get the two halves of the mold together. Now the mold is ready for pouring, as illustrated in Fig. (iv).

21. The molten metal is poured through the pouring basin. It flows through the sprue hole, sprue basin, runner and gate, until the metal comes out on the top of the riser hole. The contents in the molding box are allowed to cool.

 

3. FLOW OF MOLTEN METAL IN THE MOLD

See Fig. 3. The molten metal is poured at the funnel-shaped pouring basin. It flows into the sprue hole to the runner through sprue basin. The runner is a horizontal passage which carries the molten metal to one or more gates. Molten metal enters the mold through the gate(s). After the mold is filled up, the molten metal rises into the riser and comes to the top of the riser hole.

 

4. ADVANTAGES AND DISADVANTAGES OF CASTING PROCESS

Advantages

1. Versatility in Production:

a) Casting is one of the most versatile form of manufacturing processes, for producing components, because there is no limit to the size and shape of the component that can be produced by casting.

b) A wide range of metals and alloys with different compositions and properties can be cast easily.

2. Low Cost: It is found to be the cheapest method of metal forming.

3. Complexity: Most complex shaped products, which cannot be easily produced by other means, can be produced by casting.

4. Rate of Production: It gives a faster rate of production. Thus the total time per unit production is minimized.

5. Production of Prototype: By casting process, it is possible to produce prototype products as desired.

6. Surface Finish: The surface finish can be controlled depending on the metal to be cast. Hence, in most of the cases, the process of casting reduces or eliminates subsequent machining operations.

Disadvantages

1. Casting Defects: Castings are susceptible to various defects such as blow holes, shrinkage, scab, swell, hard spots, runout, honey combing, cracks, shifts, etc.

2. Economy: It is not economical when the parts to be made are very small in quantity.