CASTING

SAND CASTING:

• Sand is used for moulds, the sand is especially prepared to contain oils that act as binders to help hold its shape while the hot metal is being cast into it.

- Advantages of sand casting:

1. Complex 3-D shapes can be produced.
2. Core can be used to produce hollow sections.
3. It's appropriate for small runs.
4. Automated processes are suitable for longer production runs.

- Disadvantages of sand casting:

1. Due to the poor surface finish, some machining will be necessary.
2. It's not as accurate as die or investment casting.
3. It has a low rate of output and is therefore suitable only for small production runs.

DIE CASTING:

• Die casting is the term used for the processes of casting metals with a low melting point into alloy steel dies (or moulds). It is known as a permanent mould process, and the molten metal either enter the mould under the action of gravity or it is forced into the mould under pressure.
• The processes involved in die casting vary due to the amount of pressure/force applied to the molten metal as it enter the mould. In general, the higher the force applied, the quicker the process and the finer the detail being produced.

--> GRAVITY DIE CASTING:

• The molten metal is poured into the dies through runners.
• The process uses the force of gravity to ensure the molten metal reaches all parts of the metal mould.

- The dies are made from alloy steel and are split to allow for removal of the completed product.
- Gas rings around the outside of the die keep the mould heated, ensuring even cooling of the cast metal.
- Fluxes are also used to prevent oxidation of the meta as it is being cast.

INVESTMENT CASTING:

• Old process

--> Process = (STEP 1) A wax pattern is produced to a high degree of accuracy. (STEP 2) This is then coated in a high temperature ceramic material, by dipping the wax pattern into the ceramic slip. When a sufficient thickness of ceramic material is achieved, it is left to dry. (STEP 3) Once dry it can be fired in a kiln. This will course cause the wax pattern to melt (hence the alternative term for this process 'lost wax casting') leaving the cavity to be cast into. (STEP 4) When the ceramic mould has cooled, the molten metal is poured in. This is generally done using gravity to help fill the mould. (STEP 5) When the cast has cooled, the ceramic mould is broken open thereby destroying it and leaving only the cast products.

Typical products of investment casting are:

• turbine blades for jet engines
• tools and dies for a variety of applications
• motorcycle steering head components
• valves and controls for the food industry

- Advantages of investment casting:

1. Good finishes can be obtained along with a fair degree of accuracy.
2. Complicated shapes that cannot be produced by other casting process can be made.
3. Complicate shapes can be produced in materials that cannot be machined.
4. There is no split line showing on the product.

-Disadvantages of investment casting:

1. The cost is very high.
2. The size of components is limited by weight.