0102030405
Die Casting Defect Series - Cold Shut
2025-10-18
Cold shuts/cold joints are also common defects in die castings, particularly magnesium alloys, which are particularly susceptible due to their material properties. A cold shut is an irregular, linear gap formed when metal flows at lower temperatures butt against each other due to poor liquid metal flow, without fusing. There are two types of cold shuts: penetrating and non-penetrating and they tend to develop under the influence of external forces.
Cold shut defects may include undercasting, cold connection, butt joint, flow marks, patterns, pitting, wrinkles, cold beans, etc. Although their appearance characteristics are different from those of cold shut, the causes and solutions are mostly the same, so they can be analyzed and discussed together for rectification.
Defect characteristics:
There are obvious, irregular, smooth and sunken linear lines or gaps (including penetrating and non-penetrating) on the surface of the die-casting. The shape is small and narrow, and some of the intersection edges are smooth. There is also the possibility of disconnection. It may have a tendency to develop under the action of external force.
Cold shuts typically occur in areas far from the ingate. When the die-casting metal flow splits into several streams, the temperature at the flow front of each stream drops rapidly, initially becoming semi-solid or condensed. However, driven by the following metal stream, the flow continues to fill. When the metal stream it encounters is also solidified, the solidified layers cannot fuse, and a gap forms at the junction, known as a cold shut. If the cold shut is caused by eddy currents, the turbulent cold material is engulfed into an irregularly shaped mass.
Severe cold shut affects the performance and surface quality of castings. The qualification of castings should be determined based on the use conditions of castings and the degree of cold shut.
Cause:
(1) Poor alloy flow leads to:
①. The pouring temperature or mold temperature is too low and the alloy liquid cools too quickly.
②. The injection speed is too low, the gate filling speed is low, and the filling time is too long, causing the alloy liquid to cool down.
③. The injection chamber is large, the injection pressure ratio is low, the filling pressure is insufficient, and the molten metal flows slowly.
④. The filling degree of the injection chamber is low, the alloy liquid cools down too much in the injection chamber, and the pouring material is insufficient.
⑤. Improper design such as the casting wall being too thin or having a large difference in thickness, the alloy liquid cools down quickly during the cavity filling process, or the alloy liquid flows irregularly.
⑥ The casting structure is not easy to be filled with alloy liquid, such as corners, deep cavities, blind holes and cylinders and other similar shapes, which affect the filling of alloy liquid.
(2) Poor design of the pouring system causes:
① Improper selection of gate location, diversion method, and number of ingates prevents quick and direct filling. The metal flow is divided into several streams for filling in the cavity. The temperature is too low at the end of filling, and the two metal streams butt against each other but are not completely fused.
② The cross-sectional area of the ingate is too small, the filling flow is insufficient, and the filling time is too long. The ingate velocity is too high, causing the alloy liquid to be directly sprayed onto the distal mold wall (without flowing through the bottom of the cavity), and then flow back to fill other parts of the cavity, resulting in undercasting or cold shut.
③. The cross-sectional area of the ingate is too large, the filling speed of the ingate is too low, and the alloy liquid solidifies prematurely.
④. The runner and ingate are not in the correct position, or the filling process is too long, and the cold shut is far away from the ingate.
⑤. When the alloy liquid flows in the pouring system or cavity, the turbulence is severe and there is too much entrained gas.
(3) Poor exhaust conditions cause:
① The cross-sectional area of the exhaust groove is small or the exhaust groove is blocked by mold release agent dirt, resulting in poor exhaust. Too much gas is involved in the alloy liquid, hindering filling.
② The exhaust groove is not positioned correctly, and the gas blocked in the cavity can’t be discharged, resulting in a large air cushion back pressure on the filled alloy liquid. Gas obstruction, cold shut, and undercast parts have smooth surfaces but irregular shapes.
③. It is difficult to set up a drainage system in the area where gas accumulates and exhaust is difficult.
④. There is too much release agent and punch oil, which has not been dried and burned out.
Improvement measures:
(1) The main measures to improve the fluidity of the alloy are as follows:
① Use the correct smelting process to refine the alloy liquid, purify the gas and non-metallic inclusions contained in the alloy liquid as much as possible, and improve the quality of the alloy liquid; avoid overheating of the alloy liquid, otherwise the alloy liquid will be severely oxidized, with excessive slag inclusions and poor fluidity.
②. Appropriately increase the casting temperature or increase the boost pressure to prolong the time the alloy liquid remains in the high-temperature liquid state and improve the fluidity of the alloy liquid.
③. Increase the injection speed, gate speed and filling flow rate to shorten the filling time.
④. Increase the gate area, increase the high-speed injection speed without increasing the gate speed, and shorten the filling speed appropriately.
Fill in the time;
⑤. Appropriately adjust the injection speed and the gate filling speed to change the flow state of the molten metal filling the cavity.
⑥. Appropriately increase the slow injection speed to reduce the time the alloy liquid stays in the injection chamber.
⑦. Check the high-speed injection stroke and adjust the high-speed starting position according to the surface defect position of the casting.
Rapid Injection stroke to change the flow pattern of molten metal filling the cavity.
⑧. Use quantitative pouring to ensure the thickness of the cake.
⑨. Appropriately increase the mold temperature to prevent the alloy liquid from condensing prematurely and losing fluidity.
(2) The reference scheme for improving the pouring system is as follows:
①. Improve the pouring system and the location of the inner gate as well as the diversion method and flow direction to reduce the collision of the alloy liquid.
For good-shaped castings or large castings, multiple ingates are used to ensure that all parts of the casting can flow in sequence and be filled at the same time.
The filling mode and effect of the deep center cavity first and then the outside. Auxiliary gates can also be opened for direct filling in thin-walled areas.
②In order to eliminate cold shut and increase the injection speed; if there is a sticky gate phenomenon, the cross-sectional area of the gate can be increased.
When the parts allow, the stock runner is used for auxiliary filling.
③ When die casting short planes or castings with right angles, the injection speed should be appropriately reduced and the widest possible inner gate should be used.
(3) Reference directions for improving exhaust conditions:
① It is necessary to increase the overflow groove volume and exhaust groove cross-sectional area to improve the exhaust capacity of the cavity; or change the exhaust groove and overflow groove
The position of the vent should coincide with the point where the metal streams converge. A vent plug can be opened in the deep cavity.
②. Add overflow grooves and exhaust grooves. Deep concave cavities can be vented by using push rod gaps or core gaps.
③. Appropriately select a release agent that produces less oil smoke and water vapor, adjust the ratio or reduce the amount of release agent.