Cracking Obstacles: Unveiling the Path to Improve Cracks in Die-Casting Parts
I. Common Types of Crack Defects
Shrinkage cracks
This usually occurs during the cooling process of castings due to uneven metal volume shrinkage. It is commonly found in areas of casting with significant thickness variations or where the metal cools rapidly.
Stress crack
These cracks are caused by internal or external mechanical stress during the cooling process. They are commonly found at sharp corners, around holes, or at the transition between thick and thin sections of the casting.
Cracks
When a casting is ejected from the mold, excessive ejection force or unreasonable mold design can cause surface or internal cracks, which are commonly found at the ejector pin position or other stress-prone areas of the casting.
II. Common Causes of Crack Defects
1. Material issues
An improper ratio of Fe, Cu, and Si in the alloy composition leads to unstable material properties and a tendency to crack.
Material defects: Inclusions or porosity can reduce the mechanical properties of castings and increase the risk of cracking.
2. Mold Factors
(1) Improper design: If the mold design is unreasonable, such as poor venting or improper cooling channel setting, cracks may occur.
(2) Ejection system problems: Improper design or operation of the ejection system may lead to cracks. These include uneven ejection force, uneven ejection action, uneven force on the ejector pins, mold displacement or sticking during ejection, all of which may cause cracks on or inside the casting.
3. Mold wear
Long-term use or improper maintenance of the mold can affect the quality of the casting.
(1) Incomplete mold cleaning: Residues that are not cleaned properly may become the source of cracks.
(2) Improper process control of casting temperature: Too high or too low melt temperature will affect the solidification process of castings and lead to cracks.
(3) Improper cooling rate: Too fast cooling rate will cause thermal stress, which in turn will cause cracks. Pouring too fast or too slow will both lead to cracks.
III. Measures to Improve Crack Defects
1. Material selection and control
(1) Alloy composition control
Strict monitoring of the proportions of alloying elements such as Fe, Cu, and Si in aluminum alloys is essential to ensure that the composition of the molten metal remains within a reasonable range, preventing material instability and cracking. Furthermore, effective Si modification treatment is necessary to improve material quality.
(2) Material purity
High-purity raw materials are used in the casting production process, and the materials are treated more thoroughly during smelting to remove gas and slag, thereby improving the strength of the castings and reducing the possibility of cracks.
2. Mold Design and Maintenance
(1) Cooling channel design
Ensure the mold's cooling channels are laid out properly, especially in areas with uneven thickness or those that cool quickly, to prevent localized overheating that could lead to cracks. If the cracks are caused by shrinkage, check if the transition angle at the crack is appropriate. If necessary, adjust the cooling system or increase pressure in that area to improve the cracking problem.
(2) Optimization of the exhaust system
Improve the design of the mold's venting system, and set up reasonable venting grooves or venting holes to prevent gas from accumulating and causing surface defects and cracks in the castings.
(3) Adjustment of the ejection system
Ejection optimization: Ensure the ejection process is straight, stable, and guided to avoid wobbling or uneven thrust that could cause cracks. If sticking or jamming is observed during ejection, slow down the ejection speed, observe, and make necessary adjustments. Use lubricant or treat the mold surface to reduce adhesion.
Guide and slide maintenance: Regularly inspect the mold slide and ejector system to ensure they are free from wear or wobble, and that ejection is smooth and doesn't apply unnecessary stress to the casting. If cracks appear in areas of high contact between the casting and the mold, it may be necessary to adjust the ejector pin position or reduce the ejection force.
(4) Mold maintenance and adjustment
Regularly maintain and service the molds, and promptly repair or replace worn parts. For cracks caused by mold misalignment, it is recommended to check the alignment of the moving and fixed molds to ensure there is no misalignment during mold separation. If necessary, adjust the moving mold support plate or ejector pin pressure to ensure uniform locking force.
3. Process optimization
(1) Control of casting temperature and cooling rate
Based on the characteristics of different aluminum alloys, the pouring temperature of the molten aluminum should be precisely controlled to avoid excessively high or low temperatures affecting the solidification process. For stress cracks, especially in thin-walled castings, cracks can be reduced by appropriately adding reinforcing ribs and adjusting the cooling rate. For thin-walled castings, stress can be alleviated by cooling the runner and ejecting the casting earlier; while for thick-walled castings, stress can be controlled by increasing the wall thickness or enhancing cooling.
(2) Adjustment of pouring speed and pressure
Based on the geometry and dimensions of the casting, the pouring speed and pressure should be adjusted appropriately to ensure stress balance during the casting process. During the holding pressure stage, ensure that the casting receives sufficient feeding during solidification, especially at thick-walled and hot-spot areas, to reduce cracks caused by shrinkage.