Casting is a manufacturing method that involves pouring liquid material into a mold and allowing it to solidify. This process is said to have had its origin in 1838 and was used for the first time in the printing industry to manufacture some of its moving parts. The mold, in this case, gets filled with molten metal by no force other than gravity alone, hence the name Gravity Die Casting. The mold is an exact replica of the object to be manufactured. Casting or cast is another name for the hardened component that comes from the mold. The casting materials consist of metals or different types of time-setting substances that cure after combining with two or more ingredients. Examples include epoxy, concrete, plaster, and clay.
Different types of casting processes are as follows :
- Sand Casting
- Gravity Casting
- Pressure Die Casting
- Investment Casting
- Plaster Casting
- Centrifugal Casting
- Lost-Foam Casting
- Vacuum Casting
- Squeezing Casting
- Continuous Casting
- Shell Molding
Out of this, we will focus mainly on GRAVITY DIE CASTING. It is among the oldest known processes for fabricating parts with metals and alloys. It is also known as “PERMANENT MOULD CASTING”.
Parts Of Gravity Casting:
The basic molding process uses metallic molds made in two sections suitable for easy opening and closing. Steel is usually used to construct the molds. The casting system that includes cavity, gating, and rising is accurately machined to have a good surface finish. Metals like Aluminium, Magnesium, Copper base alloys, and cast iron are cast using steel molds.
The various components used in a gravity die-casting process are :
- Mold – The mold includes the whole system where the molten metal is poured. It can be a temporary or permanent setting based on the material used. Permanent gravity casting uses molds made of iron or steel tooling. Temporary casts are made from sand. And they are normally broken to obtain a new shape and cannot be re-used.
- Pouring basin – It is the path through which the molten metal flows. Its main purpose is to direct the flow of molten metal into the runner and then further into the gate and casting.
- Clamp – Due to the movement of molten material, there can be thermal expansion and contraction in the mold system. Clamps keep the system stationary throughout the process. This process is called “Clamping”.
- Mold cavity – It is the cavity in which the molten material is filled. It produces the casting and has the shape of the product. The dimensions of the mold cavity could be adjusted accordingly to the size of the product.
- Runners – It is the connecting link that transfers molten metal to various mold sections. It takes the molten metal from the pouring basin into the casting. Gates are also used in gravity casting to lead the molten metal into the die cavity.
- Risers Or Feeders – Solidification of the molten metal causes shrinkage. This creates cracks and irregularities in the mold surface. This is prevented by the use of an additional component called Risers. They carry additional molten metal and supply them to the casting in a continuous flow. This prevents shrinkage when solidification takes place during the casting process. It is also known as feeders.
- Cores – In some cases, the product can contain cavities inside. This is achieved using cores. Cores are made from metals or sand, based on application and ease of removal. Usually, metallic cores are used in permanent molds. When the removal of metallic cores is difficult, sand cores are used. Then, the process may be called semi-permanent mold casting.
Working Of Gravity Casting:
The mold is preheated (150 – 200 degrees Celsius), and lubricant coatings are sprayed before pouring liquid metal. This is to prevent the molten metal from sticking into the mold cavity in the initial stage. The surfaces of the mold cavity are typically coated with a refractory slurry (such as sodium silicate and clay) or sprayed with graphite after each casting. It prolongs the life of permanent molds. The mold halves are then closed under external pressure and clamped together until the solidification is completed.
The gating system is then filled with molten metal through the pouring basin. No outside pressure is used to press the liquid metal into the mold cavity. The movement occurs by the effect of gravity alone. The molten metal enters the casting via runners and begins to solidify. A certain amount of molten metal also enters the riser. During solidification, they flow down into the cavity. This ensures a smooth surface finish without defects. After solidification, the molds are opened and the casting is separated. If the product needs an internal cavity, cores should be used. And they are to be removed from the cast during the separation process. For the removal of intricate castings, mechanical ejectors (such as pins placed in various areas of the mold) may be necessary.
Once the casting is removed, it is then machined to obtain a good surface finish. The machining process includes grinding and cutting to bring precision and accuracy to the metal parts.
Pros And Cons Of Gravity Casting:
Gravity Die Casting, like any other, has its Pros and Cons. Though it is a cost-effective way to make metal parts in large quantities, it might be difficult to generate high-quality parts with good dimensional precision. The technique also draws several disadvantages such as limited design flexibility, and restricted part sizes. In general, organizations that need to manufacture metal parts in bulk and are prepared to make the initial investment in tools and equipment may choose gravity die casting.
Advantages Of Gravity Casting:
- Permanent-mold casting produces castings with a good surface finish, close dimensional tolerances, with uniform and good mechanical properties.
- Due to automation, labor expenses are kept low even while die prices can be expensive for equipment.
- In the casting, a fine and dense-grained structure is attained, thus improving the strength of the final product. Rapid Cooling in gravity casting also gives a fine-grain structure.
- This process produces castings without blow holes.
- Casting flaws seen in sand castings are removed with permanent gravity die casting.
Disadvantages Of Gravity Casting:
- Due to the difficulties in removing the cast from the mold, the procedure is not ideal for complicated shapes.
- This method is not cost-effective when the production rate is low.
- The price of a metallic mold is more expensive than a sand mold, so the initial cost is large for permanent mold casting.
- The process is impractical for large castings.
- The surface of the casting becomes hard due to the chilling effect and requires more surface machining.
- Refractoriness of the high melting point alloys also needs to be considered. This increases the cost of the overall process.
Applications Of Gravity Casting:
Gravity dies casting is employed in a variety of industries. They are effectively employed in several industries such as automotive, lighting, and other manufacturing segments to shape a wide range of parts and appliances.
In the automotive industry, a wide range of Parts and components are being made using gravity casting. That includes engine pistons, cylinder blocks, cylinder heads, brake calipers, and much more. Gravity Die Casting is very effective as it provides sufficient strength and the components are not geometrically complex. They are also useful to produce automobile housing that is made from steel and aluminum.
Gravity Die Casting is extensively used in Aerospace Industry to produce lightweight body components. Then they are machined further for fine precision.
In a kitchen setting, utensils, pipe fittings, and several tools are manufactured using gravity die casting. Different components used in the kitchen like spoons, pots, etc. are also manufactured by gravity casting.
Gravity Die Casting is preferred for industries where the production rate is high but the designs are not complicated. The process could also be automated, reducing labor expenses. The final product has better thermal and mechanical properties, improving its life and durability.