The basic process flow of die casting products
The die-casting process is a process in which the three elements of die-casting machine, die-casting mold and alloy are combined organically and used comprehensively. The process of filling the cavity with metal during die casting is a process of unifying process factors such as pressure, speed, temperature, and time. The influence of mold structure design, heat treatment process, mold manufacturing and mold assembly on the life of aluminum alloy die-casting molds.
Principle of Die Casting Process
The principle of die-casting process is to use high pressure to press molten metal into a precision metal mold cavity at high speed, and the molten metal is cooled and solidified under pressure to form a casting. Cold and hot chamber die casting are two basic methods of die casting process. In the cold chamber die-casting, the molten metal is poured into the press chamber by manual or automatic pouring device, and then the injection punch advances to hydraulically press the metal into the cavity. In the hot chamber die casting process, the pressure chamber is perpendicular to the crucible, and the molten metal flows into the pressure chamber automatically through the feed port on the pressure chamber. The injection punch moves downward to push the molten metal into the cavity through the gooseneck. After the molten metal has solidified, the die-casting mold is opened, the casting is taken out, and a die-casting cycle is completed.
1. Features of die casting process
(1) Metal parts with complex shapes, clear outlines, thin-walled deep cavities can be manufactured. . The dimensional accuracy of die castings is high, the surface roughness reaches Ra0.8-3.2um, and the interchangeability is good.
(2) High material utilization rate. Due to the high precision of the die-casting parts, it can be assembled and used only after a small amount of mechanical processing, and some die-casting parts can be directly assembled and used. high productivity. Due to the high-speed filling, the filling time is short, the metal industry solidifies rapidly, and the die-casting operation cycle speed is fast. Easy to use inserts.
(1) Due to high-speed filling and rapid cooling, the gas in the cavity is too late to be discharged, resulting in the presence of pores and oxidized inclusions in the die-casting parts, thereby reducing the quality of the die-casting parts. Can not be heat treated.
(2) Die-casting machines and die-casting molds are expensive and not suitable for small batch production.
(3) The size of die castings is restricted. The types of die casting alloys are restricted. Mainly used for die-casting zinc alloy, aluminum alloy, magnesium alloy and copper alloy.
2. Application range of die casting process
Die-casting has high production efficiency and can die-casting die-casting parts with complex shapes, precise dimensions, clear outlines, high surface quality and strength, and hardness, so it is widely used and develops rapidly. At present, the output of aluminum alloy die castings is relatively large, followed by zinc alloy die castings.
Die casting alloy
Die-casting alloy is one of the elements of die-casting production. To produce excellent die-casting parts, in addition to reasonable part structure, well-designed die-casting mold and die-casting machine with superior process performance, alloys with good performance are also required. The section thickness of die-casting parts depends on the stress it bears and the strength of the alloy material itself. Higher strength is one of the advantages of die-casting alloys. When selecting a die-casting alloy, full consideration should be given to various factors such as its performance, process performance, use occasion, production conditions and economy.
1.1 Al-Si alloy
Because Al-Si alloy has the characteristics of small crystallization temperature interval, large solidification latent heat of the silicon phase in the alloy, large specific heat capacity, and relatively small linear shrinkage coefficient, its casting performance is generally better than other aluminum alloys. The filling ability is also better, and the tendency of thermal cracking and shrinkage porosity is also relatively small. Al-Si alloy is currently the most widely used die-cast aluminum alloy.
1.2 Al-Mg alloy
The performance characteristics of Al-Mg alloy are: good mechanical properties at room temperature; strong corrosion resistance; poor casting performance, large fluctuations in mechanical properties and large wall thickness effects; long-term use, the plasticity of the alloy is reduced due to aging , Even the die-casting parts appear cracking;
1.3 Al-Zn alloy
After natural aging, Al-Zn alloy die castings can obtain higher mechanical properties. When the mass fraction of zinc is greater than 10%, the strength is significantly improved.
Structural design of die casting
1.1 Functional structure design
The functional structure design of die-casting parts is the core of the structural design of die-casting parts. It determines the size, wall thickness and shape required to realize the function of die-casting parts, and checks the deformation and deformation of die-casting parts during static load or dynamic load. The changing state of fatigue, wear, etc., to meet the safety of its use.
Designing the functional structure of die-casting parts requires not only the quality and technical ability of mechanical structure design and machining, but also comprehensive basic knowledge of die-casting alloy, die-casting molding process, and die-casting mold design to make the design The functional structure of the die casting can meet the various technical requirements specified by the die casting and the functions and performance within the service life, and it is reliable, safe and economical.
Die casting process
Pressure is the basic feature of die-casting process. The filling flow and compaction of molten metal are all completed under the action of pressure. Pressure is divided into dynamic injection force and pressurized injection force. The function of dynamic injection force is to overcome various resistances and ensure that the molten metal reaches a certain speed during filling. The function of the pressurized injection force is to compact the die-casting after the filling is completed, improve the density of the die-casting, and make the contour of the die-casting clear. The injection force exerts pressure on the molten metal through the injection punch.
1.2 Expansion force
During the die-casting process, under the action of specific pressure, when molten metal fills the cavity, a certain pressure is applied to the cavity wall and parting surface, which is called the expansion force. In the final stage of the die-casting process, that is, when the pressure-increasing specific pressure is transmitted to the die-casting mold through the molten metal, the swelling force is the largest, which is an important parameter for the primary selection of die-casting machine model and support plate for strength and rigidity checking.
During the die-casting process, the speed is directly affected by the pressure, and together with the pressure, it plays an important role in the internal quality and the clarity of the surface profile. There are two types of speed: injection speed and inner gate speed.
1.4 Injection speed
The injection speed is also called the punch speed, which is the speed at which the injection punch in the pressure chamber pushes the molten metal, that is, the speed of the injection punch. The injection speed changes during the injection process. It can be divided into two stages, low speed and high speed. The speed control valve of the die casting machine can be used for stepless speed regulation.
1.5 Inner gate speed
The gate speed is the linear velocity of the molten metal entering the cavity through the gate. The higher the gate speed, even if the lower specific pressure is used, the molten metal can quickly fill the cavity before solidification, to obtain a die-casting with clear contours and smooth surface, and to increase the dynamic pressure of the molten metal. If the gate speed is too high, it will also bring a series of problems. The main reason is that it is easy to envelop the gas to form bubbles; the molten metal enters the cavity in mist, and adheres to the cavity wall and the subsequent molten metal cannot be fused to form surface defects. And oxidation inclusions, accelerate the wear of the die-casting mold, etc.