One Question per Day (20100108): Mesh-like Flaws on Die-cast Parts

Reticular trichomes, also known as “turtle cracks,” are mainly caused by 1: cracks on the surface of the die cavity in die-casting molds； 2. Uneven preheating of the die casting mold. Solutions and preventive measures include: 1. The die-casting mold should be subjected to annealing treatment periodically, or after a certain number of castings, in order to eliminate stresses within the mold cavity. 2. If cracks have appeared on the cavity surface, the shaped surface should be polished to remove the cracked layer. 3. The preheating of the mold must be even.

1. Cracking on the surface of the die cavity. 2. Inappropriate material for the die or incorrect heat treatment process. 3. Excessive temperature fluctuations between hot and cold conditions in the die. 4. Too high pouring temperature. 5. Insufficient preheating of the die. 6. Rough surface of the die cavity. 7. Thin walls of the die or presence of sharp corners

1. Cracks on the surface of the die cavity of the die-casting mold； 2. Uneven preheating of the die casting mold.

1. First, the molten metal that enters the cavity forms a very thin and incomplete metal layer; the traces left behind are those resulting from the subsequent molten metal filling in that area. 2. The mold temperature is too low. 3. Excessive splashing occurs due to an overly small cross-sectional area of the internal runner and an improper placement of it. 4. Insufficient pattern due to insufficient pressure on the molten metal: Excessive amount of coating used. 1. Cracking on the surface of the die cavity in die casting 2. Improper material for the die casting mold or incorrect heat treatment process 3. Excessive temperature differences between hot and cold areas in the die casting mold 4. Too high pouring temperature 5. Insufficient preheating of the die casting mold 6. Rough surface of the die cavity 7. Thin walls of the die casting mold or sharp corners 1. The alloy is overheated too much or the holding time is too long 2. Severe undercooling, resulting in very fine crystals 3. Excessive impurities such as zinc and iron in the aluminum alloy 4. Copper content in the aluminum alloy exceeds the specified range. Cracks in castings caused by stress or external forces 1. Cracks in zinc alloy castings: (1) The levels of harmful impurities such as lead, tin, iron, and cadmium in the zinc alloy exceed the specified limits; (2) The casting is removed from the die casting mold too late; (3) Uneven force when extracting or pushing out the core; (4) Sharp changes at the junctions where the thickness of the casting varies; (5) Too high melting temperature. 2. Cracks in aluminum alloy castings: (1) Excessively high iron content or too low silicon content in the alloy; (2) High levels of harmful impurities in the alloy, which reduces its ductility; (3) In aluminum-silicon alloys and aluminum-silicon-copper alloys, excessive zinc or copper content; in aluminum-magnesium alloys, excessive magnesium content; (4) Too low temperature of the mold, especially the core; (5) Areas with sharp changes in wall thickness of the casting; (6) Excessive time the casting remains in the mold; (7) Uneven force during ejection. 3. Cracks in magnesium alloy castings: (1) High aluminum-silicon content in the alloy; (2) Low mold temperature; (3) Sharp changes in wall thickness of the casting; (4) Uneven force during ejection and core extraction. 4. Cracks in copper alloy castings: (1) Excessive zinc content in brass (causing cold cracks) or insufficient zinc content (causing hot cracks); (2) High silicon content in silicon brass; (3) Delayed mold opening, especially for castings with multiple cores. Shrinkage holes are voids that occur in die castings due to insufficient compensation during cooling. 1. Too high pouring temperature 2. Low injection pressure 3. Areas in the casting structure where metal accumulates or where there are sharp changes in cross-section 4. Small size of the feed channel. Preventive measures: 1. Increase the mold temperature. 2. Adjust the cross-sectional area or position of the feed channel. 3. Adjust the speed and pressure of the feed channel. 4. Select appropriate coatings and adjust their usage amounts. 1. Choose the right material for the die casting mold and use the correct heat treatment process. 2. The pouring temperature should not be too high, especially for alloys with high melting points. 3. The mold should be thoroughly preheated. 4. The die casting mold should be tempered regularly or after a certain number of castings, and the surface of the molded parts should be polished. 1. The alloy should not be overheated. 2. Increase the mold temperature and reduce the pouring temperature. 3. Strictly control the alloy composition within acceptable limits. 1. Pay attention to the impurity content in the alloy mixture; it should not exceed the specified limits. 2. Adjust the mold opening time properly. 3. Ensure even force distribution on the ejector pins. 4. Reduce unevenness in wall thickness. 1. Properly control the alloy composition. In some cases, pure aluminum ingots can be added to the alloy to reduce magnesium content; or aluminum-silicon intermediates can be added to increase silicon content. 2. Increase the mold temperature. 3. Change the structure of the casting. 4. Adjust the core extraction mechanism or ensure even force distribution on the ejector pins. 1. Add pure magnesium to the alloy to reduce aluminum-silicon content. 2. Keep the mold temperature within the required range. 3. Improve the structure of the casting to eliminate areas with large thickness variations. 4. Adjust the core and ejector pins to ensure even force distribution. 1. Ensure the correct chemical composition of the alloy; use the lower limits for alloy elements. When preparing silicon brass, both silicon and zinc contents should not exceed their upper limits simultaneously. 2. Increase the mold temperature. 3. Properly control and adjust the mold opening time. 1. Change the structure of the casting to eliminate areas where metal accumulates or where there are large cross-sectional changes. 2. Reduce the pouring temperature if possible. 3. Increase the injection pressure. 4. Improve the gating system to ensure better pressure transmission

Reticular burrs are marks and protruding metal spikes on the surface of castings caused by thermal cracking in mold parts; since thermal cracking in molds often takes a reticular (radial) pattern, when the degree of cracking is mild, the resulting marks on the castings are reticular in shape； When the degree of thermal cracking is severe, cracks often form, resulting in a network of burrs on the casting. The higher the melting point of the alloy, the more severe the phenomenon caused by thermal cracking becomes. For example, in molds made of copper alloys, thermal cracking is relatively severe. And the situation is even more severe in ferrous metal die casting. The mesh-like marks on die-casting are generally not subject to any restrictions. And mild cases of reticular burrs are usually acceptable ； When it reaches a severe level, it depends on the operating conditions. The reasons for thermal cracking of molds include: 1. The frictional resistance is highest near the internal gate, which suffers the most from the erosion by molten metal, making it most prone to thermal cracking. 2. Parts molded by molds have large flat surfaces that are weak areas (with low solid thickness). 3. Improper adjustment of the cooling system. 4. The aqueous coating was not preheated, or the spraying was improper, resulting in excessive rapid cooling of the mold. 5. Coatings have chemical corrosive effects (such as sodium fluoride). 6. Weak areas resulting from inlays on the molded parts (including holes in the core that are too small near the edges) can also cause early thermal cracking, but this cracking appears in a striated pattern. Similarly, two types are also reproduced: traces and burrs. 7. When the push rods and cores (small circular holes in die-cast parts) are located in areas subject to severe erosion by the metal flow, such as gates and runners, early thermal cracking occurs at the upper edge of the corresponding hole openings, with the cracks spreading in a radial pattern. It also causes marks and burrs on the surface of the die-cast parts. 8. Potential cracks caused by inherent defects in the mold material, improper forging processes, or incorrect heat treatment methods.

Cracks are present on the surface of the die cavity of the die-casting mold； The die preheating is uneven.

Visual inspection: The surface of the die-cast parts shows web-like protrusions or depressions, which become larger and more extensive with each casting. Causes: 1. Cracks on the surface of the die cavity; 2. Inappropriate material for the die or incorrect heat treatment processes; 3. Excessive temperature fluctuations between hot and cold conditions in the die; 4. Too high pouring temperature; 5. Insufficient preheating of the die; 6. Rough surface of the die cavity; 7. Thin walls of the die or sharp corners. Preventive measures: 1. Select appropriate materials for the die and use proper heat treatment processes; 2. Keep the pouring temperature low, especially for alloys with high melting points; 3. Ensure thorough preheating of the die; 4. Periodically or after a certain number of castings, anneal the die and polish the surface of its molded parts