Die casting time
Die-casting time refers to the entire process from the injection of molten metal into the die chamber to the removal of the die-casting from the mold. It is an essential parameter in the die-casting process, directly impacting die-casting quality, production efficiency, and mold life. Die-casting time can be further divided into filling time, holding time, and mold opening time. Timing control for each stage has specific significance and requirements. Filling time is the time required for molten metal to enter the mold cavity and completely fill it. It is typically measured in seconds. For thin-walled, complex parts, this time may be as short as 0.05-0.2 seconds, while thick-walled, large parts may require 0.5-2 seconds. Holding time refers to the time that the molten metal is maintained under pressure after filling the mold cavity. This is to prevent backflow of the molten metal before solidification and to provide compensation for shrinkage during solidification. It is generally 1-5 seconds. Opening time is the time from the end of holding time to the mold opening and removal of the die-casting. It is determined based on the solidification conditions of the die-casting and is typically 5-30 seconds.
Properly selecting the filling time is crucial to the quality of die-cast parts. Excessively short filling times can cause violent eddies and impacts in the high-speed flow of the molten metal, which can easily entrap gases and form defects such as porosity and oxide inclusions. This can also increase wear on the mold cavity. Excessively long filling times can reduce the fluidity of the molten metal due to rapid cooling during the filling process, leading to defects such as underfill and cold shuts. This is particularly true for high-melting-point alloys (such as aluminum and magnesium alloys), where excessive filling times significantly increase the probability of defects. For example, when the filling time for thin-walled aluminum alloy parts exceeds 0.2 seconds, cold shut streaks are very likely to appear on the surface of the die-cast part, while filling times shorter than 0.05 seconds significantly increase internal porosity. Therefore, the filling time must be precisely calculated based on the wall thickness, complexity, and alloy type of the die-cast part. Generally, the principle of “short time for thin walls, long time for thick walls” should be followed, and continuous optimization should be carried out through mold trials.
Controlling the hold time is closely related to the density of die-cast parts. During the hold phase, the molten metal is under high pressure. As the metal solidifies in the mold cavity, its volume shrinks. Insufficient hold time can lead to inadequate shrinkage compensation, resulting in defects such as shrinkage cavities and porosity, and reduced mechanical properties of the die-cast part. Excessive hold time prolongs production cycles and reduces efficiency. It can also complicate demolding due to excessive solidification of the molten metal and even cause cracks in the die-cast part as it cools and shrinks within the mold. For example, for a 5mm thick aluminum alloy die-cast part, the hold time should be controlled within 2-3 seconds. If shortened to 1 second, the internal porosity of the die-cast part will increase by over 30%. Extending it to 5 seconds increases adhesion between the die-cast part and the mold, making it more susceptible to strain during demolding. The hold time should be determined based on the wall thickness of the die-cast part, the solidification rate of the alloy, and the cooling capacity of the mold. Generally, the hold time should be increased by 0.5-1 second for every 1mm increase in wall thickness.
The mold opening time must be set based on the die-casting being fully solidified and possessing sufficient strength. Opening the mold too early, while the die-casting is not yet fully solidified, can lead to insufficient strength and deformation or cracking during demolding. This is especially true for complex shapes and slender features, which can cause localized fractures. Opening the mold too late can extend production cycle time and reduce equipment utilization. Furthermore, the die-casting cools excessively within the mold, shrinking more, which can cling tightly to the core, making demolding more difficult and even requiring greater ejection force, resulting in surface damage. For example, the mold opening time for zinc alloy die-castings is typically 5-10 seconds. If the mold opening time exceeds 15 seconds, the clamping force between the die-casting and the core increases by over 20%, resulting in noticeable ejector marks during demolding. A mold opening time shorter than 3 seconds can cause deformation due to insufficient solidification. The mold opening time can be determined by observing the demolding state and surface quality of the die-casting and adjusting it gradually until the optimal result is achieved.
Optimizing the total die-casting time is of great significance to production efficiency and cost control. Under the premise of ensuring the quality of die-casting parts, shortening the total die-casting time can increase the output per unit time and reduce the energy consumption and labor cost per unit product. For example, when the total die-casting time is shortened from 60 seconds to 40 seconds, the daily output of the production line can be increased by 50%, significantly improving economic benefits. However, the time reduction must not sacrifice quality as a prerequisite. It is usually achieved by optimizing the mold cooling system, increasing the injection speed, improving the demolding mechanism, etc. At the same time, the stability of the die-casting time is also very important. The die-casting time fluctuation of the same batch of die-casting parts should be controlled within ±2 seconds, otherwise it will lead to uneven quality of the die-casting parts. Modern die-casting production lines are usually equipped with automated control systems. By precisely controlling the time parameters of each stage, the stable and efficient operation of the die-casting process is achieved, ensuring the consistency of the quality of the die-casting parts.
