Causes of Cold Shuts in the Die-cast and Its Solutions
Abstract: what is a cold shut? What is the forming principle of a cold shut? What are the influencing factors of cold shuts? What are the reasons for the formation of cold shuts? What are the prevention measures for cold shuts in die castings? What are the solutions to cold shuts?
1. What is a cold shut?
A cold shut of aluminum alloy die castings is also called a cold water mark. At the junction of the metal flow in the die casting, if the molten metal is not perfectly fused, surfaces of the die casting will have obvious irregular and sinking linear patterns, resulting in penetrating or unpenetrating gaps in the die casting, which are small and narrow. Some of the junction edges are smooth. They may develop under the action of external force. The shape of the cold shut is like the shape of the initial flow of the liquid, which is smooth and has rounded edges. Therefore, it often appears on radiographs as a smooth strip like black line with a relatively uniform width and no changes. The width of the line appears to be relatively big, and the blackness seems to change in the width direction.
2. The forming principles of cold shuts
The parts where the cold shut occur in aluminum alloy die castings are usually the wide surface and thin wall of the die casting being away from the pouring gate. The reason is that when the metal flow is divided into several streams of flows, the front of each flow has been in a solidified state, which is called the solidification front, but it is still in the filling process under the push of the subsequent metal flow. When the metal flow that meets it also has a solidification front, the two solidified layers which meet can no longer be fused, resulting in gaps at the joints. The two metal flows meet, but they are not completely fused and there is no intermingling between them. The bonding force of the two metal flows is very weak. The die casting with serious cold shuts will seriously affect use of the die-cast. Measures must be taken to eliminate the flow marks and cold shuts in the aluminum alloy die casting process, so as not to affect quality and normal use of die castings.
3. The reasons for cold shuts of die castings
1. Mold temperature is too low during die casting.
2. Alloys are selected improperly, resulting in poor fluidity and low temperatures of molten metal.
3. The chilling of mold surfaces or chilled iron is much.
4. The filling speed is too low or the gas is not exhausted well.
5. Casting pressure is too low (injection specific pressure).
6. Spraying of the release agent is excessive or the release agent is not dried.
7. The process is too long or the pouring system is unreasonable.
8. The fast injection point is not set unreasonably.
9. The gas is not exhausted well.
4. Solutions to the cold shut
The measures to prevent cold shut and water mark defects in die castings are to increase the die casting temperature.
1. Properly increase the pouring temperature and mold temperature.
2. Adjust the alloy composition to improve fluidity.
3. Improve the gating system, especially the number and distribution of the inner gate; increase the speed of the inner gate, and improve the filling conditions.
4. Improve the overflow conditions; increase the overflow and improve the exhaust conditions.
5. Increase injection speeds; improve filling conditions; increase specific pressure and improve mobility of molten aluminum.
5. Preventive measures for cold shuts of die castings
1. The product becomes black with flow marks. Properly increase the pouring temperature and mold temperature; observe the mold temperature and reduce paint spraying.
2. Check the flow direction of molten aluminum in the hot mold. The cold shut produced by the collision of the molten metal generally has a vortex with flow marks. Improve the gating system and filling direction of the inner gate. In addition, a slag collecting bag can be set on the edge of the aluminum alloy die casting to improve the filling conditions.
3. Change the gate position and cross-sectional area to improve the overflow conditions and increase the overflow.
4. Change the flow of molten metal and increase the injection speed.
5. Increase the specific pressure (try not to use it), and if possible, it is best to use a machine with large tonnages.
The cold shut of aluminum alloy die castings must be prevented as early as possible to ensure the quality of aluminum alloy die castings.
1. What is a cold shut?
A cold shut of aluminum alloy die castings is also called a cold water mark. At the junction of the metal flow in the die casting, if the molten metal is not perfectly fused, surfaces of the die casting will have obvious irregular and sinking linear patterns, resulting in penetrating or unpenetrating gaps in the die casting, which are small and narrow. Some of the junction edges are smooth. They may develop under the action of external force. The shape of the cold shut is like the shape of the initial flow of the liquid, which is smooth and has rounded edges. Therefore, it often appears on radiographs as a smooth strip like black line with a relatively uniform width and no changes. The width of the line appears to be relatively big, and the blackness seems to change in the width direction.
2. The forming principles of cold shuts
The parts where the cold shut occur in aluminum alloy die castings are usually the wide surface and thin wall of the die casting being away from the pouring gate. The reason is that when the metal flow is divided into several streams of flows, the front of each flow has been in a solidified state, which is called the solidification front, but it is still in the filling process under the push of the subsequent metal flow. When the metal flow that meets it also has a solidification front, the two solidified layers which meet can no longer be fused, resulting in gaps at the joints. The two metal flows meet, but they are not completely fused and there is no intermingling between them. The bonding force of the two metal flows is very weak. The die casting with serious cold shuts will seriously affect use of the die-cast. Measures must be taken to eliminate the flow marks and cold shuts in the aluminum alloy die casting process, so as not to affect quality and normal use of die castings.
3. The reasons for cold shuts of die castings
1. Mold temperature is too low during die casting.
2. Alloys are selected improperly, resulting in poor fluidity and low temperatures of molten metal.
3. The chilling of mold surfaces or chilled iron is much.
4. The filling speed is too low or the gas is not exhausted well.
5. Casting pressure is too low (injection specific pressure).
6. Spraying of the release agent is excessive or the release agent is not dried.
7. The process is too long or the pouring system is unreasonable.
8. The fast injection point is not set unreasonably.
9. The gas is not exhausted well.
4. Solutions to the cold shut
The measures to prevent cold shut and water mark defects in die castings are to increase the die casting temperature.
1. Properly increase the pouring temperature and mold temperature.
2. Adjust the alloy composition to improve fluidity.
3. Improve the gating system, especially the number and distribution of the inner gate; increase the speed of the inner gate, and improve the filling conditions.
4. Improve the overflow conditions; increase the overflow and improve the exhaust conditions.
5. Increase injection speeds; improve filling conditions; increase specific pressure and improve mobility of molten aluminum.
5. Preventive measures for cold shuts of die castings
1. The product becomes black with flow marks. Properly increase the pouring temperature and mold temperature; observe the mold temperature and reduce paint spraying.
2. Check the flow direction of molten aluminum in the hot mold. The cold shut produced by the collision of the molten metal generally has a vortex with flow marks. Improve the gating system and filling direction of the inner gate. In addition, a slag collecting bag can be set on the edge of the aluminum alloy die casting to improve the filling conditions.
3. Change the gate position and cross-sectional area to improve the overflow conditions and increase the overflow.
4. Change the flow of molten metal and increase the injection speed.
5. Increase the specific pressure (try not to use it), and if possible, it is best to use a machine with large tonnages.
The cold shut of aluminum alloy die castings must be prevented as early as possible to ensure the quality of aluminum alloy die castings.
