Dry Sand Mould and Its making

The molding process involved in making dry sand molds are similar to those employed in green-sand molding except that a different sand mixture is used and all parts of the mold are dried in an oven before being reassembled for casting. 

The green sand mold depends upon the moisture and the natural clay binder in the sand to retain its shape. But the sand used for dry sand molds depends upon added binding material such as flour, resin, molasses, or clay. The materials are thoroughly mixed and tempered with a thin clay water. The amount of binder is determined by the size of  the casting being made. 

Metal flasks must be used for dry sand molds to withstand the heat in the oven. Before drying, the inside surfaces of the dry sand mold are coated with wet blacking - a mixture of carbon black and water with a small addition of a gum. This gives a smoother surface to the casting. These molds can be held for any length of  time before pouring, provided they are kept dry.

Advantages of dry sand molds are :

1. They are stronger than green sand molds, and thus are less susceptible to damage in handling.

2. Over-all dimensional accuracy of the mold is better than for green sand molds.

3. Surface finish of castings is better, mainly because dry sand molds are coated with wash. 

Disadvantages Of dry sand molds are :

1. Castings are more susceptible to hot tears.

2. Distortion is greater than for green sand molds, because of the baking.

3. More flask equipment is needed to produce the same number of finished pieces, because processing cycles are longer than for green sand molds.

4. Production is slower than for green sand molds.

Dry-sand molds are often used for large work such as engine cylinders, engine blocks, rolls for rolling mills, etc, to avoid spoilage. Castings of intricate design and those requiring special smoothness, soundness, and accuracy are also used in dry sand molds.