LP systems generally fall into the following three categories:
(a) Unsealed LP system:
In this systems, the metal is transferred from a furnace using a pump. An example of this is the Cosworth process, where an electromagnetic pump is used to lift the metal up and into the mould.
(b) Sealed LP system:
This systems uses a furnace that can be sealed and pressurized. In this furnace, one or more hollow tubes connect to the top of the furnace and extend down into the molten metal. A mould is placed on top of the furnace, the melt in the furnace is pressured and the molten metal is forced up through hollow tubes into the mould.
(c) Vacuum-assisted LP system:
Vacuum-assisted LP systems are similar to the sealed LP systems, except that a vacuum is applied to the casting prior to/at the same time as pressurization of the melt.
Principle of working:
Unlike the traditional die casting process, it has a unique setup and uses several pieces of equipment. Figure 1 illustrates the setup of the LP die casting system. In low pressure casting, the mould is located above the sprue and metal flows 'up' the sprue and into the runner system and the casting cavity.
The process begins with a melting furnace which melts the metal alloys and brings them to the casting temperature. For example, aluminum has a casting temperature of 710-720°C. The molten metal will then go to a holding furnace below the mould, which functions as a container while maintaining the liquid at the casting temperature.
The metal flow for the arrangement shown in Figure 1 is accomplished by pressurizing the furnace, which is located below the mould. Low pressure forces the molten metal through a riser tube into the mould. The liquid metal moves under constant pressure until the molten metal solidifies in the die cavity. The rate of metal flow is controlled by the rate of pressurization of the furnace.
On solidification, the pressure is released, and the remaining molten metal goes back through the riser tube to the holding furnace for recycling. Finally, the casting will be easily removed once the mould is cooled.
Applications:1. LP die casting is mainly used to produce aluminum alloy and magnesium alloy parts, such as automobile hub in the automobile industry, cylinder block, cylinder head, piston, missile shell, impeller, wind guide wheel and other castings with complex shape and high-quality requirements.
2. It can also be applied to small copper alloy castings, such as pipe fittings, faucets in bathrooms, etc.
Advantages:
1. The molten metal quality is often very high, having low levels of inclusions and hydrogen gas.
2. Metal quality is also high because the molten metal used for casting comes from the middle of the furnace; oxides and inclusions tend to settle to the bottom or rise to the surface.
3. High yield is achievable (typically over 90%).
4. Excellent control of process parameters which can be obtained with a high degree of automation
5. Good metallurgical quality due to a slow, non-turbulent filling and controlled solidification dynamics which results in good mechanical and metallurgical properties of the castings, including low porosity.
6. The process is very flexible, and can be used with sand moulds, metal dies, or combinations of both.
7. The castings made are of high quality with very less defects.
8. The post machining costs is less as there are no feeders.
Limitations:
1. LP systems require specialized machines i.e. an integral melting furnace capable of being sealed and pressurized must be present.
2. It has a slower cycle due to the low pressure. Therefore, the number of parts manufactured per unit time is low.
3. Equipment cost is high.
4. Maintenance cost is high.
5. It is unsuitable for making thin-walled parts (less than 3 mm).
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