Basic Process Flow of Die Casting Products

Die castingtechnology is the combination of three major elements: die-casting machine, die-casting die and alloy. The process of filling the cavity with metal in die casting is to unify the process factors such as pressure, speed, temperature and time. The die structure design, heat treatment process, die manufacturing and die assembly will affect the life of die casting die of aluminium alloy.

Die casting process
1. Die Casting Machine Debugging
2. Die Casting Die Installation
3. Design and Manufacture of Die Casting Die
4. Mould preheating and coating
5. Paint preparation
6. Mould cleaning
7. Closing (Mould Closing)
8. Alloy melting heat preservation
9. Insert preparation
10. Casting Injection
11. pressure maintaining
12. Mould opening and core pulling
13. Surface Quality Inspection
14. Cleaning (renovation)
15. Aging treatment
16. Casting Infiltration and Shot Peening

Die Casting Process Principle
The principle of die casting technology is to use high pressure to press liquid metal into a precise metal mould cavity at high speed, and the liquid metal is cooled and solidified under pressure to form a casting. Cold chamber and hot chamber die casting are two basic ways of die casting process, and their principle is shown in Figure 1-1. In cold chamber die casting, liquid metal is poured into the chamber by manual or automatic pouring device, then the injection punch advances, and the metal hydraulic pressure is put into the chamber. In the hot chamber die casting process, the chamber is perpendicular to the crucible, and the liquid metal flows into the chamber automatically through the inlet of the chamber. The injection punch moves downward, pushing the liquid metal through the gooseneck tube into the cavity. After the molten metal solidifies, the die casting die opens and the castings are taken out to complete a die casting cycle.
Characteristics of Die Casting Technology
Advantages
(1) Dimensional precision of die castings is high. The surface roughness is Ra0.8-3.2um and the interchangeability is good.
(2) High utilization rate of materials. Because of the high precision of die castings, they can be assembled only after a small amount of mechanical processing. Some die castings can be directly assembled and used. Production efficiency is high. Because of high-speed filling, short filling time, rapid solidification of metal industry and fast cycle of die casting operation. It is easy to use inserts.
Disadvantage
(1) Due to the high speed filling, rapid cooling, gas cavity could not be drained, as a result, there are often holes and oxidized inclusions in die casting, which reduce the quality. Heat treatment is not suitable.
(2)Die-casting machine and die-casting die are expensive and unsuitable for small batch production.
(3) The size of die castings is limited. The types of die casting alloys are limited. Mainly used for die casting zinc alloy, Aluminum alloy, magnesium alloy and copper alloy.

Application Scope of Die Casting Technology
Die casting has high production efficiency, complex shape, precise size, clear outline, high surface quality, strength and hardness, so it is widely used and developed rapidly. At present, the output of aluminium alloy die castings is more, followed by zinc alloy die castings.

Die Casting Alloys

Die-casting alloy is one of the key factors of die-casting production. In order to produce good die-casting parts, besides reasonable parts structure, well-designed die-casting die and die-casting machine with superior technological performance, there is also a need for alloy with good performance. The section thickness of die casting depends on the stress it bears and the strength of the alloy material itself. High strength is one of the advantages of die casting alloy. When choosing die-casting alloys, many factors such as service performance, process performance, application occasion, production conditions and economy should be fully considered.

Various Die Cast Aluminum Alloys_
Because Al-Si alloy has the characteristics of small crystallization temperature interval, large latent heat of solidification, large specific heat capacity and small linear shrinkage coefficient, its casting performance is generally better than other aluminum alloys, its filling capacity is better, and its tendency of hot cracking and shrinkage is also smaller. Al-Si alloy is the most widely used die-casting aluminum alloy at present.

Al-Mg 
The properties of Al-Mg alloys are as follows: good mechanical properties at room temperature; strong corrosion resistance; poor casting performance, great fluctuation of mechanical properties and wall thickness effect; for long-term use, the plasticity of the alloys decreases due to aging effect, even cracking of die castings.

Al-Zn
The mechanical properties of Al-Zn alloy die castings can be improved by natural aging. When the mass fraction of zinc is more than 10%, the strength of Al-Zn alloy die castings can be improved significantly.

Structure Design of Die Castings
Functional Structural Design
Functional structure design of die castings is the core of structural design of die castings. It determines the size, wall thickness and shape needed to realize the function of die castings, and checks the deformation, fatigue and wear of die castings during the use of static or dynamic loads to meet the safety of their use.
To design the functional structure of die castings, not only the qualities and technical abilities of mechanical structure design and processing, but also the comprehensive basic knowledge of die casting alloy, die casting forming process and die casting die design are needed so that the designed functional structure can meet the technical requirements of die castings and the functions within the service life. Performance is reliable, safe and economical.

Die Casting Technology 
Pressure is the basic feature of the die-casting process. The filling flow and compaction of the molten metal are all done under pressure. Pressure is divided into dynamic and pressurized injection forces. The function of dynamic injection force is to overcome all kinds of resistance and ensure the liquid metal to reach a certain speed when filling. The effect of supercharged injection force is to compact the die castings after filling, improve the density of the die castings, and make the outline of the die castings clear. The injection force exerts pressure on the liquid metal through the injection punch.

Expansion force
In the process of die casting, under the action of specific pressure, when liquid metal fills the cavity, it gives certain pressure to the cavity wall and parting surface, which is called expansion force. In the final stage of the die casting process, when the specific pressure is transferred to the die casting die through liquid metal, the bulging force is the largest, which is an important parameter for checking the strength and stiffness of the die casting machine and the supporting plate.  

In the process of speed die casting, speed is directly affected by pressure, and together with pressure, they play an important part in internal quality and surface profile clarity. There are two forms of velocity: injection velocity and gate velocity.

Injection velocity, also known as punch velocity, is the speed that pressure punch in the chamber to push metal liquid. That is, the velocity of the punch. The injection speed is variable in the process of injection. It can be divided into two stages: low-speed and high-speed. Stepless speed regulation can be carried out through the speed regulating valve of the die casting machine. The velocity of inner gate is the linear velocity at which the liquid metal enters the cavity through the inner gate. Higher inner gate speed, even with lower specific pressure, can fill the cavity quickly before solidification, obtain clear outline and smooth surface of the die casting, and increase the dynamic pressure of the metal liquid. When the inner gate speed is too high, a series of problems will be brought about, such as easy to envelop gas to form bubbles; metal liquid enters the cavity in the form of fog, adheres to the wall of the cavity and can not fuse with the subsequent metal liquid to form surface defects and oxide inclusions, and accelerates the wear of the die casting die.