The die-casting mold is composed of two parts, namely the covering part and the moving part. The combined part is called the parting line. In hot chamber die casting, the covering part has a gate, while in cold chamber die casting, it is an injection port. Molten metal can enter the mold from here, and the shape of this part matches the injection nozzle in hot chamber die-casting or the injection chamber in cold chamber die-casting. The moving part usually includes the push rod and the runner. The so-called runner is the channel between the gate and the mold cavity, through which molten metal enters the mold cavity. The covering part is usually connected to the fixed pressing plate or the front pressing plate, while the movable part is connected to the movable pressing plate. The mold cavity is divided into two mold cavity inserts, which are independent components and can be easily removed or installed from the mold by bolts.
The mold is specially designed. When the mold is opened, the casting will remain in the moving part. In this way, the movable part of the push rod will push the casting out. The push rod is usually driven by the press plate. It will accurately drive all the push rods with the same force at the same time, so as to ensure that the casting will not be damaged. When the casting is pushed out, the pressing plate shrinks and retracts all the push rods to prepare for the next die casting. Since the casting is still at high temperature when demoulding, only if the number of push rods is enough can the average pressure on each push rod be small enough to prevent damage to the casting. However, the push rod will still leave traces, so it must be carefully designed so that the position of the push rod will not have too much impact on the operation of the casting.
Other parts in the mold include core slide plate, etc. The core is used to make holes or openings in the casting, and they can also be used to increase the details of the casting. There are three types of cores: fixed, movable and loose. The direction of the fixed core is parallel to the direction of the casting coming out of the mold. They are either fixed or permanently connected to the mold. The movable core can be arranged in any direction except the ejection direction. After casting solidification, before opening the mold, the movable core must be taken out of the mold cavity by using the separation device. The slider is very close to the movable core, and the biggest difference is that the slider can be used to manufacture the concave surface. The use of core and slider in die casting will greatly increase the cost. Loose cores, also known as extraction blocks, can be used to make complex surfaces, such as threaded holes. Before the start of each cycle, it is necessary to install the slider manually, and then push it out with the casting. Then take out the loose core. Loose core is the most expensive core, because it requires a lot of labor to manufacture, and it will increase the cycle time.
The outlet is usually thin and long (about 0.13 mm), so the molten metal can be cooled quickly to reduce waste. Risers are not required in the die casting process, because the molten metal pressure is very high, which can ensure a continuous flow from the gate into the mold.
Due to the temperature, the most important material characteristics for the mold are thermal shock resistance and softness. Other characteristics include hardenability, machinability, thermal crack resistance, weldability, usability (especially for large molds) and cost. The die life directly depends on the temperature of molten metal and the time of each cycle. Dies used for die casting are usually made of hard tool steel. Because cast iron can not bear huge internal pressure, the mold is expensive, which also leads to high cost of mold opening. The metal cast at higher temperature needs to use harder alloy steel.
The main defects in die casting process include wear and erosion. Other defects include hot cracking and thermal fatigue. When the mold surface has defects due to too large temperature changes, hot cracks will occur. However, after too many times of use, the defects on the surface of the mold will cause thermal fatigue.