Green Sand Mold , Its Process And Types

Green-sand molds are prepared with natural molding sands or with mixtures of silica sand, bonding clay, and water. These materials are thoroughly mixed in proportions which will give the desired properties for the class for work being done.

To make the green-sand mold the sand must be properly tempered before it can be used. If the sand is too dry, additional water is added if too wet, dry sand is added until it has the proper temper. To check the sand for proper temper, a handful is grasped in the first. The pressure is released, and the sand is broken in two sections. The sections of sand should retain their shape and the edges of the break should be sharp and firm. 

The surface of the mold  which comes in contact with the molten metal forms the most important part in green-sand molds. In order to give the casting a clean and bright surface and to prevent the sand from the burning on the face of the mold , a layer of the facing sand is given surrounding the pattern. Facing-sand mixtures for iron castings generally contain some finely ground bit luminous coal known as sea-coal, and new sand in addition to used molding sand. One part by volume of sea-coal, and new sand in addition to used molding sand. One part by volume of sea-coal to ten parts of molding sand is a common ratio in mixtures for molds for iron castings. The sea-coal aids in preventing the sand from fusing to the surfaces of the castings, whereas the new sand increases the bond in the facing mixture, and thereby prevents cutting of sand surfaces by the liquid metal. 

It is common practice to coat the surfaces of  sand mold with refractory material to produce a smooth skin on the castings. The material ordinarily used for this purpose are graphite, coke, charcoal, gas carbon, plumbago, black lead, silica, mica, and talc. These materials may be placed in two groups ; the carbonaceous materials known as blackings, and the other materials are designated as mineral coatings. They may be applied wet or dry. For use in the wet state some adhesive is employed-clay, gum and other substances being mixed with water are used. Blackings or mineral coatings used dry are dusted over the mold face.

Advantages of green sand mold are listed below :

1. Green sand molding is the least expensive method of producing a mold. 

2. There is less distortion than in dry sand molds, because no baking is required.

3. Flasks are ready for reuse in minimum time.

4. Dimensional accuracy is good across the parting line.

5. There is less danger of hot tearing of casting than in other types of mold.

Disadvantages Of Green sand mold are listed hereunder : 

1. Sand control is more critical than in dry sand molds.

2. Erosion of the mold is more common in the production of large castings.

3. Surface finish deteriorates as the weight of the casting increases.

4. Dimensional Accuracy decreases as the weight of the casting increases.

The principal methods of green-sand molding are as follows :

1. Open-sand method.

2. Bedded-In Method.

3. Turn-over Method.