Gravity casting refers to the process of injecting molten metal into the mold under the action of the earth’s gravity, also known as casting.
(1) Gravity casting is also known as metal mold casting. The forming principle is to supplement solidification shrinkage by the gravity action of riser. This gravity casting mold can be reused, also known as permanent mold casting.
(2) For the cold defect mode of metal mold casting, air pressure gas (air-cooled) or air pressure gas water mixed fog (water-cooled) can be used to cool the mold and casting body.
(3) The reserved machining dimension of the casting by metal mold casting is still high, because the casting will deform after heat treatment.
(4) The degassing operation of aluminum water is very important in the process of metal mold casting, otherwise the pinhole is easy to be produced after the casting surface is processed.
(5) The metal mold casting method is more suitable for a small number of various order production processes.
Low pressure casting
Low pressure casting refers to the casting method that makes liquid metal fill the mold and solidify into casting under the action of low pressure gas. The gas pressure is generally 0.6-1.5 PA. The process of low-pressure casting is: add a sealing cover on the crucible furnace of melting metal, install a liquid lifting pipe in the center of the cover, insert the liquid lifting pipe below the metal level, and place the mold on the upper part of the cover. The dry compressed air is sent to the crucible through the air inlet pipe to make the metal liquid enter the mold through the riser pipe from the gate (see Figure), keep the pressure until the metal liquid in the mold solidifies completely, then release the pressure, the metal liquid in the riser pipe will automatically fall back into the crucible, at this time, the mold can be opened and the casting can be pushed out. The gas pressure and flow through the crucible can be controlled, so the filling speed and gas pressure of molten metal can be adjusted according to the casting structure and mold material. The mold for low pressure casting can be sand mold, shell mold, ceramic mold, metal mold, graphite mold, etc. On the basis of low-pressure casting, the method of filling and solidification of liquid metal under differential pressure is called differential pressure casting, which is a special form of low-pressure casting.
Low pressure casting was mainly used in the production of aluminum alloy castings at first, and then expanded its application to produce copper castings, iron castings and steel castings with high melting point. This method has been successfully used in China in the 1970s to cast important parts such as 10000 ton copper alloy propeller and 2000 horsepower diesel engine nodular cast iron crankshaft.
The advantages of low pressure casting are: the filling of liquid metal under pressure is conducive to casting thin-walled castings; the compactness of castings is improved; the bottom filling is stable, which can reduce the oxide inclusions caused by the splashing of liquid metal; the gating riser system is simple, and the metal utilization rate can reach more than 80%; the working conditions are improved, mechanization and automation can be realized, and the production efficiency is high.
A casting method in which liquid metal is poured into a rotating mold, filled with centrifugal force and solidified into a casting. The machine for centrifugal casting is called centrifugal casting machine. According to the different direction of the rotation axis of the mold, the centrifugal casting machine can be divided into three types: horizontal, vertical (see Figure) and inclined. Horizontal centrifugal casting machine is mainly used for casting various tubular castings, such as gray cast iron,water pipe and gas pipe, with the minimum diameter of 75mm and the maximum diameter of 3000mm. In addition, it can pour large-diameter copper roller for paper machine, all kinds of carbon steel, alloy steel pipe and double-layer steel roller with different components inside and outside.
According to the shape, size and production batch of castings, the mold used in centrifugal casting can be non-metallic mold (such as sand mold, shell mold or investment mold shell mold), metal mold or mold with coating layer or resin sand layer in the metal mold. The number of revolutions of the mold is an important parameter of centrifugal casting. It is necessary to have enough centrifugal force to increase the compactness of the casting metal, and the centrifugal force should not be too large, so as not to hinder the shrinkage of the metal. Especially for lead bronze, too much centrifugal force will produce component segregation between the inner and outer walls of the casting. Generally, the speed changes from dozens to 1500 rpm.
Centrifugal casting is characterized by filling and solidification of liquid metal under the action of centrifugal force, good metal feeding effect, dense structure and good mechanical properties of the casting; casting hollow casting does not need to cast riser, the metal utilization rate can be greatly improved. Therefore, centrifugal casting is a kind of material saving, energy saving and high-efficiency process for some special shape castings, but effective safety measures should be taken.