Misalignment between the die-casting mold's sprue bushing and diverter nozzle can significantly affect the flow stability of the molten metal. This impact begins when the molten metal initially enters the diverter nozzle and gradually spreads throughout the entire process of filling the mold cavity, ultimately leading to a series of casting defects. It's important to understand that the coaxial design of the sprue bushing and diverter nozzle essentially provides a transition channel with a uniform cross-section and stable direction for the molten metal. After flowing from the pressure chamber, the molten metal first passes through the sprue bushing, then smoothly enters the diverter nozzle, and is finally distributed to the various mold cavities by the nozzle's diverter holes. Misalignment between the two causes a "step" or "deflection" to appear on the inner wall of this transition channel, disrupting the molten metal's original linear flow path.
When molten metal enters a misaligned channel at high pressure and velocity, a noticeable difference in flow velocity occurs at the misalignment point. For example, if there is radial misalignment between the two, the gap on one side of the channel will become smaller and larger on the other. The molten metal will flow faster in the area with the smaller gap due to compression, while it will flow slower in the area with the larger gap due to ample space. This uneven flow rate will immediately disrupt the flow balance within the molten metal and induce localized eddies. The generation of eddies not only disrupts the overall flow direction of the molten metal but may also trap air within the molten metal, forming bubbles. If these bubbles enter the mold cavity with the molten metal, they will eventually form pores within the casting, affecting the mechanical properties of the casting.
Secondly, misalignment can increase the impact of the molten metal on the diverter nozzle. In normal coaxial conditions, the molten metal flows along the axis, evenly impacting the nozzle inner wall, resulting in balanced forces and uniform wear. However, when misalignment occurs, the molten metal will flow toward one side of the nozzle inner wall, causing concentrated impact. This localized, concentrated impact causes a rapid increase in the temperature of the nozzle's inner wall. The high-speed impact of the molten metal converts kinetic energy into heat. Simultaneously, the molten metal in the impact area, obstructed by the impact, retains its position for a longer period, further accumulating heat and potentially causing localized overheating and softening of the nozzle's inner wall. Over time, this can exacerbate nozzle wear, leading to irregular pitting or deformation. This deformation, in turn, further increases concentricity deviation, creating a vicious cycle of "deviation-wear-further deviation," further deteriorating the flow stability of the molten metal.
Furthermore, this loss of flow stability can affect the uniformity of the molten metal's distribution within the diverter nozzle. The core function of the diverter nozzle is to evenly distribute the molten metal to multiple cavities, a process that relies on maintaining stable pressure and flow rate within the nozzle. If misalignment causes turbulence in the molten metal at the nozzle inlet, the pressure and flow rate of the molten metal will vary when entering the nozzle diverter orifices. Diverter orifices closer to the impact point may fill too quickly due to excessive pressure, while diverter orifices farther from the impact point may fill slowly due to insufficient pressure. This ultimately leads to inconsistent filling of castings in different cavities, such as missing material in some parts, flash in others due to overfilling, or even cavity cracking due to localized excessive filling pressure.
Furthermore, unstable molten metal flow increases temperature loss during flow. When turbulent flow of molten metal due to eddy currents or impact increases the contact area with the gate bushing and the inner wall of the diverter nozzle, heat dissipation accelerates, potentially causing some parts of the molten metal to cool too quickly before reaching the cavity, reducing fluidity. If the temperature falls below the solidification point of the molten metal, a "cold shut" defect can occur—in which the molten metal fails to fully fuse within the cavity, resulting in noticeable delamination or gaps. This defect not only affects the appearance quality of the casting but also severely weakens its structural strength, rendering it unable to meet service requirements.
Misalignment between the gate bushing and the diverter nozzle can directly impair the flow stability of the molten metal by disrupting its flow path, causing uneven flow velocity and eddy currents, exacerbating localized impact wear, disrupting diversion uniformity, and accelerating temperature loss. This loss of flow stability is a key factor in casting defects such as porosity, cold shuts, material shortages, and flash. Therefore, strict control of the coaxiality between the two must be achieved during die-casting mold design and assembly to ensure stable molten metal flow and casting quality.
