The die casting mold consists of two parts, the cover part and the movable part, and the part where they meet is called the parting line. In hot chamber die casting, the cover part has the gate, while in cold chamber die casting, it is the 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 movable part usually includes push rods and runners, which are the channels between the gate and the mold cavity through which the molten metal enters the mold cavity. The cover part is usually connected to the fixed platen or the front platen, while the movable part is connected to the movable platen. The mold cavity is divided into two mold cavity inserts, which are independent parts and can be removed or installed from the mold relatively easily by bolts.
The mold is specially designed so that the casting will remain in the movable part when the mold is opened. This will push the casting out with the push rods in the movable part. The push rods are usually driven by the platen, which will drive all the push rods at the same time with exactly the same amount of force, so as to ensure that the casting is not damaged. When the casting is ejected, the platen retracts to retract all the push rods and prepare for the next die casting. Since the casting is still hot when it is ejected from the mold, only if there are enough push rods can the average pressure on each push rod be small enough to avoid damaging the casting. However, the push rods still leave marks, so they must be carefully designed so that the position of the push rods does not affect the operation of the casting too much.
Other parts in the mold include core slides. Cores are parts used to make holes or openings in castings. They can also be used to add details to the casting. There are three main types of cores: fixed, movable and loose. Fixed cores are parallel to the direction of the casting out of the mold. They are either fixed or permanently connected to the mold. Movable cores can be arranged in any direction except the ejection direction. Before the mold is opened after the casting solidifies, the movable core must be removed from the mold cavity using a separation device. Sliders and movable cores are very similar. The biggest difference is that slides can be used to create undercut surfaces. Using cores and slides in die casting will greatly increase costs. Loose cores, also called ejector blocks, can be used to create complex surfaces such as threaded holes. Before each cycle, the slide is manually installed and then ejected with the casting. The loose core is then removed. Loose cores are the most expensive cores because they are labor-intensive to make and they increase cycle time.
The ejector is usually thin and long (about 0.13 mm) so that the molten metal cools quickly and reduces waste. Risers are not required in the die casting process because the molten metal is under high pressure, which ensures a continuous flow from the gate into the mold.
Due to the temperature, the most important material properties for the mold are resistance to thermal vibration and flexibility. Other characteristics include hardenability, machinability, resistance to hot cracking, weldability, availability (especially for large molds), and cost. The life of the mold is directly dependent on the temperature of the molten metal and the time of each cycle. The molds used for die casting are usually made of hard tool steel. Because cast iron cannot withstand the huge internal pressure, the molds are expensive, which also leads to high mold opening costs. Metals die cast at higher temperatures require harder alloy steels.
The main defects that can occur during die casting include wear and erosion. Other defects include thermal cracking and thermal fatigue. Thermal cracking occurs when defects appear on the mold surface due to large temperature changes. After too many uses, defects on the mold surface will cause thermal fatigue.
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