Die casting holding pressure time and mold retention time
The hold time in die casting refers to the duration of pressure maintained after the molten metal fills the mold cavity and builds up a boost pressure. Its primary function is to prevent shrinkage cavities and porosity during solidification, ensuring the density of the casting. During this hold phase, high pressure is continuously applied to the partially solidified molten metal, allowing it to continuously fill the voids created by solidification shrinkage until the casting shell develops sufficient strength to withstand the internal pressure. If the hold time is too short, the molten metal loses pressure support before sufficient shrinkage compensation is complete, potentially leading to shrinkage cavities in thicker areas of the casting. If the hold time is too long, production cycle time will be prolonged, energy consumption will increase, and over-solidification of the casting may make demolding difficult. For example, for aluminum alloy castings with a wall thickness of 3mm, the hold time is typically between 2 and 5 seconds. However, for thicker-walled castings with a wall thickness of 10mm or more, the hold time may need to be extended to 8 to 15 seconds to ensure adequate shrinkage compensation.
The hold time setting is closely related to the solidification characteristics of the casting, primarily determined by the maximum wall thickness of the casting and the solidification rate of the alloy. The wider the alloy’s crystallization temperature range, the slower the solidification process, and the longer the required hold time, as more time is needed to complete shrinkage feeding. Conversely, alloys with a narrow crystallization temperature range solidify quickly, and the hold time can be appropriately shortened. Furthermore, mold temperature also affects the hold time: at higher mold temperatures, the solidification rate of the molten metal slows down, requiring a corresponding increase in the hold time; at lower mold temperatures, the solidification rate accelerates, requiring a corresponding decrease in the hold time. For example, in the production of aluminum alloy wheels, due to the thicker wall of the rim and the mold temperature typically controlled at around 200°C, the hold time is generally set to 10 to 12 seconds to ensure the density of the rim interior.
The mold retention time refers to the time from the end of holding pressure to the time the casting is ejected from the mold. Its main function is to allow the casting to fully cool in the mold to have sufficient strength and rigidity to avoid deformation or damage during demolding. If the mold retention time is too short, the casting temperature will be too high and the strength will be insufficient, which will easily lead to defects such as warping and cracking during ejection; if the mold retention time is too long, it will reduce production efficiency, and may make demolding difficult due to excessive heat exchange between the casting and the mold, increasing mold wear. For example, the mold retention time of small zinc alloy die castings is usually 1 to 3 seconds, while the mold retention time of large aluminum alloy engine cylinders may require 5 to 10 seconds to ensure that the casting cools to sufficient strength.
There is a relationship of mutual influence and mutual coordination between the holding time and the mold retention time. The length of the holding time will affect the initial temperature of the casting in the mold retention stage: if the holding time is long, the casting has completed more solidification in the holding stage, and the temperature is relatively low when entering the mold retention stage, and the required mold retention time can be appropriately shortened; on the contrary, if the holding time is short, the initial temperature of the casting in the mold retention stage is high, and a longer mold retention time is required to cool. At the same time, the mold retention time will also affect the stability of the holding effect: if the mold retention time is insufficient, the deformation of the casting during demolding may destroy the density formed in the holding stage, resulting in hidden defects; if the mold retention time is too long, although it is beneficial to the strength of the casting, it will offset the efficiency improvement brought about by the optimization of the holding time. Therefore, when setting these two parameters, it is necessary to comprehensively consider the casting quality and production efficiency to seek the best balance.
In actual production, optimizing hold and hold times requires dynamic adjustment based on specific production conditions. By installing temperature sensors in key locations on the mold, temperature changes in the casting at different stages can be monitored in real time, allowing adjustments to be made to the hold and hold times accordingly. For example, if the temperature in thick-walled areas of the casting remains above the solidification temperature at the end of the hold, the hold time should be appropriately extended. If excessive temperature during ejection causes deformation, the hold time should be increased. Furthermore, automated production lines often utilize closed-loop control systems that automatically adjust hold and hold times based on quality feedback from the previous die (such as dimensional accuracy and defect detection results), enabling adaptive parameter optimization. Using this dynamic adjustment approach, an automotive parts manufacturer optimized the hold time of a certain aluminum alloy bracket from 6 seconds to 4 seconds and the hold time from 5 seconds to 3 seconds. While maintaining a high casting yield, the production cycle was reduced by 15%, significantly improving production efficiency. This optimization approach, based on real-time monitoring and feedback, is crucial for achieving efficient and stable operation in modern die-casting production.
