Die casting process and related problems
Tue Jun 27 14:32:16 CST 2023
In die casting production, die casting machine, die casting alloy and die casting mold are the three major elements. The die-casting process is a process of organically combining and applying the three major elements, so that various process parameters can meet the needs of die-casting production.
1. Selection of pressure and speed
The selection of injection specific pressure should be determined according to the structural characteristics of different alloys and castings.
For the selection of filling speed, generally for castings with thick walls or high internal quality requirements, a lower filling speed and high boost pressure should be selected; for castings with thin walls or high surface quality requirements and complex castings, you should choose High ratio and high filling speed.
2. Pouring temperature
The pouring temperature refers to the average temperature of the liquid metal when it enters the mold cavity from the pressure chamber. Since it is inconvenient to measure the temperature of the liquid metal in the pressure chamber, it is generally expressed by the temperature in the holding furnace.
If the pouring temperature is too high, the shrinkage will be large, which will easily cause cracks, large grains, and stickiness of the casting; if the pouring temperature is too low, defects such as cold shut, surface pattern and insufficient pouring will easily occur. Therefore, the pouring temperature should be considered together with the pressure, die casting mold temperature and filling speed.
3. The temperature of the die-casting mold
Die-casting molds should be preheated to a certain temperature before use. Generally, gas, blowtorch, electric appliances or induction heating are used.
In continuous production, the die-casting mold temperature tends to rise, especially for die-casting high-melting point alloys, which rise rapidly. If the temperature is too high, in addition to causing the liquid metal to stick to the mold, the cooling of the casting is slow and the grains are coarse. Therefore, when the temperature of the die-casting mold is too high, cooling measures should be taken. Usually compressed air, water or chemical media are used for cooling.
4. Filling, holding pressure and mold opening time
(1) Filling time
The time required from the time the liquid metal begins to enter the cavity until the cavity is filled is called the filling time.
The length of filling time depends on the size and complexity of the casting. For large and simple castings, the filling time is relatively longer; for complex and thin-walled castings, the filling time is shorter. The filling time is closely related to the cross-sectional area of the ingate or the width and thickness of the ingate, and must be determined correctly.
(2) Holding pressure and mold opening time
From the liquid metal filling the cavity to the complete solidification of the ingate, the duration of continuing to be under the action of the injection punch is called the holding time. The holding time depends on the material and wall thickness of the casting.
After holding the pressure, the mold should be opened to take out the casting. The time from the end of the injection to the opening of the die casting is called the mold opening time. The mold opening time should be controlled accurately. If the mold opening time is too short, due to the low strength of the alloy, it may cause deformation when the casting is ejected; but if the mold opening time is too long, the temperature of the casting will be too low, the shrinkage will be large, and the resistance to core pulling and ejection of the casting will also be large. Generally, the mold opening time is calculated based on the casting wall thickness of 1 mm and takes 3 seconds, and then adjusted appropriately.
5. Coatings for die casting
During the die-casting process, coatings are used in order to avoid welding between the casting and the die-casting mold, reduce the frictional resistance of the casting ejection and prevent the die-casting mold from being overheated. Requirements for paint:
(1) Good lubricity at high temperature;
(2) The volatilization point is low, and the diluent can volatilize quickly at 100-150°C;
(3) It has no corrosive effect on die-casting molds and die-casting parts;
(4) The performance is stable, and the diluent should not volatilize too fast and thicken in the air;
(5) Harmful gas will not be precipitated at high temperature;
(6) No fouling will occur on the surface of the die-casting mold cavity.
6. Casting cleaning
The cleaning of castings is a very heavy work, and its workload is often 10-15 times that of die casting. Therefore, with the improvement of the productivity of the die casting machine and the increase of the output, it is very important to realize the mechanization and automation of the casting cleaning work.
(1) Cut gate and flash
The equipment used for gate and flash removal is mainly punch press, hydraulic press and friction press. Under mass production, a special mold can be designed according to the structure and shape of the casting, and the cleaning task can be completed on the punching machine at one time.
(2) Surface cleaning and polishing
Surface cleaning mostly uses ordinary polygonal rollers and vibration embedded cleaning devices. For simple small pieces with small batches, multi-angle cleaning rollers can be used, and decorations with high surface requirements can be polished with cloth or leather polishing wheels. For mass-produced castings, a shell-type vibration cleaning machine can be used.
Cleaned castings can also be surface treated and impregnated according to the requirements of use to increase gloss, prevent corrosion and improve air tightness.