Molding & Casting Processes

Molding processes

1. According to the method used:

• Floor molding
• Bench molding
• Pit molding
• Machine molding

2. According to the mold materials:

• Sand molding

a. Green sand molding
b. Skin dried sand molding
c. Dry sand molding
d. Core sand molding
e. Loam molding
f. Cement bonded sand molding
g. Carbon dioxide molding
h. Shell molding
• Plaster molding
• Metallic molding

Casting processes:

1. Sand mold casting

2. Plaster mold casting

3. Metallic mold casting

• Gravity or permanent mold casting
• Slush casting
• Presses casting
• Die casting

4. Centrifugal casting

5. Precision casting

6. Carbon dioxide mold casting

7. Continuous casting

SAND MOLDING

Green sand molding: 

Molding which is carried out in green sand i.e. when pouring is done in the sand mold with the sand in its green or natural state is known as Green sand molding. For this first a green sand mixture is prepared, consisting of silica sand, 10-15% clay and 4-6% moisture content, mixed and riddled and given the required condition by proper tempering. 
If molding board is to be used, it is first placed on the floor followed by the pattern. The drag part of the flask is then kept over it and rammed. Excess sand is parted off by the strike off bar and the box turned over. 

The other half of the pattern is placed in position followed by the cope. Runner and riser are placed in position and the box filled with sand and rammed properly. Runner and riser taken out and the pouring basin is made. Vent wire is then applied to provide the vents. 
The cope is then rolled over and the pattern pieces are taken out of both the boxes. Repairs, if any, are made and the gates cut. The boxes are reassembled after dusting the mold surfaces by powdered blacking and placing the dry sand core. The mold is then ready for pouring. Molten metal is poured into the mold through down spruce hole or runner. There is no need for baking the mold before pouring. 
Most of the small and medium sized castings, particularly non-ferrous ones, are made in green sand work. It is very commonly employed in match plate work in bench molding.

Dry sand molding: 

This process  of molding is almost similar to green sand except that the composition of sand constituents differ in this case. In preparing a dry sand mixture special binding materials such as resin, clay, or molasses are added to give strong bond to the sand. 

Grains of the sand used in making a dry sand mixture are comparatively coarser. Here again the process starts with the sand preparation. The sand is properly mixed, riddled, tempered and given the desired bond. After the sand is well prepared, molding is carried out in the same way as for green sand. 
Parting sand is applied around the surface of the pattern and between the matting surface of drag and cope. They are separated and pattern withdrawal. Repairs, if any, are made and the mold faces and cavity coated with liquid blacking. After this the molds, contained in the flasks are sent to drying oven or stove for baking.
Here the moisture content of the sand is driven out by baking the mold. Time taken in baking depends upon the size of the mold and the type of the oven used. Since the boxes are to remain in the oven so long as the baking process continues, only metal boxes are used in dry sand molding. After the mold is dried, it is taken out and cleaned by blowing the sand and ash etc. out of the cavity with the help of bellow. 
The two halves of the mold are then reassembled in position for pouring. If the casting is too big, the pressure of the gases produced inside the mold may be too high and cope is subjected to this pressure. To prevent the cope from being blasted by this pressure extra weight are placed over it. Almost all ferrous castings particularly larger ones are made in dry stand. 
Dry sand molds are costlier than green sand because of baking operation, but at the same time has an advantage that the casting obtained from them are much superior in quality having cleaner and smoother surfaces with very less defect inside i.e. they render sound castings. A typical sand composition for dry sand is old floor sand 64%, coal dust 13%, new sand 20%, and clay 3%, although it varies with different foundries.

Loam molding: 

Here the process of molding is absolutely different from those described above. First a rough structure of the article is made by hand, using brick and loam sand.
It is then given the desired shape by means of sticklers and sweeps, thus avoiding use of pattern. After preparing the mold, it is baked to give strength to resist the flow of molten metal. This method of molding is used only for very large castings to be made in very small numbers. 
This enables the saving in time, labor and material which would have been spent in making the pattern, but this system in not very much popular for  the reason that it takes a lot of time in preparing the mold and requires special skills in the molders. 
The sand to be used known as loam sand or loam mortar which is a mixture of molding sand and clay made in the form of a paste by suitable addition of clay water. 
The cope part and the core of the mold are constructed separately on two different iron bases, using different sizes of sticklers and sweeps etc. and are assembled together after baking.