Technology

Gravitational casting application

A. Gravitational casting application principle:
Centrifugal casting is to pour metal solution into a rotating mold. Due to centrifugal force, metal solution adheres tightly to the interior of casting mold. As the mold is rotating, the metal solution gradually solidifies and forms tubular castings. The casting molds for centrifugal casting are mostly made from steel, iron or graphite, mainly for tubular or cylindrical castings. Especially, long cast tube, made by centrifugal casting, does not need large sand core to produce tubular castings of uniform thickness.
As two rollers rotate on gravitational casting machine, centrifugal casting mold also rotates.
Metal solution is poured and carried to the rotary track by the rotary mold and distributed onto the mold surface. Upon depositing on the mold, the metal solution immediately solidifies and further casting will increase thickness and achieve the target thickness.

B. Centrifugal casting classification:
Centrifugal casting is usually divided to three types: 1. True centrifugal casting 2. Semi-centrifugal casting 3. Centrifugal pressure casting.
1. True centrifugal casting does not use sand core, does not need to set up riser and runner. So it is suitable for tubular and cylindrical castings such as cast iron pipe, steel pipe, cylinder and sleeve.
2. Semi-centrifugal casting uses the symmetric axis of workpiece as rotary axis, charges metal solution from central low runner, pushes the metal solution outward by centrifugal force to fill the mold. Semi-centrifugal casting is used for symmetric castings, such as wheel.
3. Centrifugal casting has radial horizontal runners on central low runner. After the metal solution is charged, due to centrifugal force, it fills up the cavity through the horizontal runners. Centrifugal pressure casting is similar to semi-centrifugal casting and only different in the choice of rotary axis. Since its rotary axis is not symmetric axis of workpiece, centrifugal pressure casting is more suitable for workpiece of complicated shape.

C. Workpiece features for centrifugal casting
Due to excellent casting condition, centrifugal casting can obtain highly dense workpiece with fine texture and grains and uniform composition. The benefits mainly come from the quenching effect for metal mold and lower casting temperature than general casting methods, contributing to satisfactory metal flow. The metal mold by true centrifugal casting shows miniaturization effect, while the workpiece by centrifugal pressure casting is not very different from that by general casting methods. However, centrifugal pressure casting can reduce defects like interior slag and gas void. Centrifugal casting workpiece is prone to thermal crack especially under high rotation speed. Thermal crack is the crack from incomplete closure at the interface between solid and liquid metal due to solidification and contraction. When workpiece contracts and the stress generated on casting mold due to thermal expansion is larger than the strength of workpiece at the temperature near solid phase line, axial thermal crack will occur.
Overly high rotation speed is the main reason for thermal crack. Quenching on workpiece exterior layer also effectively minimizes thermal crack. This is achieved by lower casting temperature and slower casting speed. Slower casting speed leads to higher temperature gradient and reduces adverse impact of liquid metal pressure to solidified layer. When the temperature gradient is smaller, the chance for any section of the workpiece to become brittle and weak is higher as well as thermal crack.

Related equipment information >>“Vertical centrifugal casting machine”  >> “Horizontal centrifugal casting machine”