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Die casting shrinkage and porosity defects, optimization method for mold shrinkage system

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  • Release time: 2026-11-07

#Die casting shrinkage and porosity defects, optimization method for mold shrinkage system

Shrinkage and porosity are caused by the solidification shrinkage of aluminum liquid, with a high incidence of thick and large sections. The layout of mold runners and overflow channels directly determines whether defects can be controlled. Xinfeng Machinery has optimized the mold end by combing through the optimization methods.

##1、 The underlying mechanism of shrinkage defects

During the solidification process of aluminum alloy, the volume shrinks by about 3.5% -7%, and the thick walled position finally solidifies. Without continuous shrinkage of aluminum liquid, pores are formed. Industry data shows that 41% of thick walled castings scrapped are due to shrinkage porosity, with hub flanges and valve body thick sections being high-risk areas. Die casting mold relies on overflow channels and slag collection bags as supplementary channels to balance the solidification sequence.

##2、 Design specifications for overflow trough and slag collection bag

The overflow groove needs to be arranged at the position of the thick hot section of the casting, with a cross-sectional area larger than the corresponding hot section, to facilitate shrinkage and accommodate low-temperature impurities. The thickness of the overflow groove is usually set to 1.1-1.3 times the thickness of the hot section wall of the casting. If the size is too small, it cannot achieve the shrinkage effect, and if it is too large, it will increase the consumption of raw materials. The flange position of the alloy wheel mold will be matched with an annular overflow structure to control the loosening of the wheel root.

##3、 Differentiated temperature control strategy for cooling water circuit

Adopting a sequential solidification approach, slow down the cooling of thick sections and accelerate the cooling of thin walls at the far end, guiding the aluminum liquid to replenish and shrink towards thicker areas. The distance between the waterway and the cavity is arranged differently, and the waterway in the thick hot section area is moderately moved outward to avoid local rapid solidification and locking of the contraction channel. Many low-priced molds have evenly distributed waterways, ignoring the need for sequential solidification and continuously exceeding shrinkage standards.

##4、 Differences in shrinkage control during low-pressure casting

Low pressure casting relies on a liquid lift system for continuous static pressure compression, which provides better compression conditions than high-speed die casting and reduces the risk of shrinkage and loosening at the same wall thickness. The low-pressure process of A356 wheel hub, with a reasonable pressure holding curve, is the core to suppress spoke shrinkage and looseness, supplemented by changes in mold structure. It is difficult to achieve long-term pressure holding in die-casting, and it is even more difficult to control the shrinkage and looseness of thick and large parts.

##5、 Tips for avoiding pitfalls in procurement selection

Some purchases have seen thick walled parts, but still prioritize die-casting technology, resulting in a high rate of shrinkage defects in the later stage. Die and mold modifications have upper limits for overflow and cooling structures, and for pressure parts with wall thickness exceeding 12mm, priority should be given to evaluating low-pressure casting solutions. Metal injection mold shrinkage is sintering shrinkage, and the shrinkage control logic is completely independent. The parameter system cannot be interchanged.

## FAQ

How to distinguish between shrinkage and porosity X-ray images?
Clear circular boundary of pores; The shape of the shrinkage is irregular, with continuous pieces distributed at the hot spot position.
Can increasing the injection pressure eliminate shrinkage?
The improvement of high-pressure compaction is limited, and the root cause is still the solidification sequence and shrinkage channels.
What are the consequences of a too small overflow tank?
Pre solidified closed channel, unable to replenish shrinkage, excessive shrinkage at the hot spot position.
How long is the low pressure holding time usually set?
According to the wall thickness of the casting, maintain pressure for about 8-12 seconds per millimeter of wall thickness, and extend the pressure of the hub flange.
5. Can shrinkage porosity be achieved by infiltration treatment of pressure bearing parts?
Fine dispersion, shrinkage and leachability; Even after extensive shrinkage and infiltration, there is still leakage.
Can cold iron be added to the mold to improve shrinkage and looseness?
Cold iron is rarely installed in die-casting molds, and cold iron is relatively more commonly used in low-pressure working conditions.
Which is more prone to shrinkage between ADC12 and A356?
A356 has a wide solidification interval and a longer shrinkage window, making it easier to control shrinkage porosity under reasonable processes.
Can simulation identify the location of hot nodes?
Solidification simulation can mark hot spots and plan overflow and cooling layouts in advance.

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