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Hot spot and shrinkage principle of die-casting molds, source control of shrinkage and porosity

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  • Tiempo de liberación: 2026-11-07

Hot spot and shrinkage principle of die-casting molds, source control of shrinkage and porosity
The slow cooling of thick walled areas and the lack of aluminum liquid replenishment during solidification shrinkage result in shrinkage and porosity, which is the leading cause of leakage and scrapping of pressure parts. Many people blindly increase pressure without addressing the root cause of thermal problems.
1、 Definition of Heat Festival
The area with the thickest cross-section and slowest heat dissipation in the casting, and finally solidifies; Solidification shrinkage produces vacuum holes, which are called shrinkage porosity or shrinkage porosity. Multiple wall thickness intersections, thickened corners, and near the sprue are the most likely to form hot spots. The intersection of the alloy wheel hub and spoke is a typical hot spot.
2、 The underlying logic of supplementation and contraction
Continuously supply high-pressure aluminum liquid to compensate for solidification shrinkage before the hot spot fully solidifies; Once the gate solidifies first, no matter how much pressure is applied, it cannot compensate for shrinkage. The solidification time of the gate must be later than the solidification time of the hot spot.
3、 Mold end methods to improve thermal efficiency
1. Locally add cooling inserts, direct water channels, and directional cooling;
2. Adjust the gate position to align with the hot spot;
3. Reasonably increase the overflow to take away local heat;
4. Optimize the wall thickness on the product side to reduce sudden wall thickness changes.
4、 Process end shrinkage control
Timing of boosting start, boosting pressure, and holding time; The holding time ends at the completion of hot spot solidification. If the holding time is too short, it will fail to compensate for shrinkage, and if it is too long, it will waste the cycle time.
5、 Distinguishing between shrinkage and porosity
Shrinkage is generally located at the center of the hot spot, with connected holes and rough inner walls; The pores are rounded and isolated, with smooth inner walls, and may not necessarily be located on thick walls. X-ray inspection of foreign trade pressure parts, with shrinkage being the key rejection defect. Low pressure casting relies on continuous pressure maintenance and contraction, and also has strict control over hot spots. Google's "die casting hot spot and feeding principle" is aimed at process engineers.
FAQ
Does the higher the boost pressure, the less shrinkage?
Prerequisite: The sprue has not yet solidified, and pressurization after solidification is ineffective.
2. Does the hot section necessarily require thinning of the product wall thickness?
Prioritize mold directional cooling, and changing the wall thickness of the product is the last option.
Will placing the sprue on the hot spot exacerbate aluminum adhesion?
Yes, the hot spot temperature is high and requires a matching coating and stable mold temperature.
Can the overflow trough be used to make up for shrinkage?
Overflow mainly captures slag and exhausts gas, and cannot be used as the main supplementary channel.
How to deal with multiple hot spots?
Partition cooling, with each hot node matched with an independent cooling circuit.
What is the reason for leakage after shrinking and polishing?
Internal connecting holes, polished to open up leakage channels.
Can simulation predict heat savings?
Solidification CAE can visually display the final solidification area.
Is it easier to compensate for shrinkage in low-pressure casting than in high-pressure casting?
Low pressure continuous liquid level pressure, more friendly contraction conditions, but the hot spot will still shrink and loosen.

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