In zinc alloy die-casting process, filling mode of molten metal is one of key factors affecting quality of casting. Many die-casting defects, such as underfill, cold shut, pores, shrinkage, etc., are related to filling mode. At the same time, filling mode is also one of key factors affecting life of mold. Therefore, using simulation software to accurately predict casting filling process can effectively test rationality of gating system design and optimize settings of inner gate position, runner size and die-casting process parameters.
This article takes a zinc alloy die-casting part as an example, using cloud mold flow service provided by domestic die-casting simulation software Intelligent Casting Chaoyun 2.0 version to study impact of different pouring systems on filling state of casting. By optimizing design form of pouring system, problem of insufficient pouring of castings is solved.

Structural characteristics of this zinc alloy casting are typical special-shaped structures. Convex part in the middle is slightly complicated, structure on both sides is simple, and there is no obvious plane parting surface. Because casting is small in size, a mold structure with two cavities is considered. Left design is designed as casting A, and right side is designed as casting B. In this way, a set of parts can be produced at the same time, which is also more advantageous from perspective of organizational production.

 

Figure 1. Casting structure diagram
Combined with structural characteristics of casting, partition filling method can be used to transport molten metal to each inner gate as evenly as possible, ensuring that molten metal entering from inner gate corresponding to each partition fills mold cavity quickly and smoothly in same direction. An overflow tank is provided in final filling area to ensure effectiveness of exhaust. Based on above analysis, gating system of casting was designed. The total area of inner gate is about 82.6mm, and area of inner gate in each zone is shown in Figure 2.

 

Figure 2. Original casting pouring system

Designed castings were imported into Intelligent Casting Chaoyun V2 version for meshing. Minimum unit size was 0.25mm, resulting in a total of 8,429,056 cells. After simulation calculation is completed, following filling process can be obtained. It can be seen from mold filling process that when mold is filled to 80%-85%, filling speeds of two bars above and below left casting are different. Finally, two streams of molten metal converge at the corner of casting. Since converging point was far away from overflow tank, converging air volume could not be discharged from mold cavity. Overall evaluation of this filling scheme shows that after molten metal enters mold cavity, effect of filling mold cavity forward simultaneously is not achieved, which is likely to cause defects such as air entrainment and cold isolation inside casting.
Based on experience, we know that above phenomenon of inconsistent flow rates may lead to air entrainment and inclusions. The most likely reason is that area of ingates at A and D is much smaller than those at B and C. Result is that resistance of molten metal at A and D is much greater than that at B and C.
Through simulation, it can be found that original design of gating system did not meet original intention of the entire molten metal to move forward smoothly and synchronously during injection process, and position of overflow groove did not play its due role. Eventually, air entrainment occurred inside casting. .

Based on above analysis results, casting gating system will be optimized. Expand inner gate area at A and D from 3.3mm to 8.6mm to reduce resistance of molten metal in these two places. Optimized pouring system is shown in Figure 3.

 

Figure 3. Optimized casting pouring system
Optimized pouring system was re-imported into Intelligent Casting Chaoyun V2 version for simulation calculations. Results are as follows:
After recalculation, it was found that original inconsistent flow rates had been significantly improved, and converging point at the end of molten metal filling had also been moved to position of overflow groove, which can effectively discharge gas out of mold cavity.

After some simulation analysis and demonstration, mold design was optimized and completed (see Figure 4), and die-casting production verification was carried out. Die-cast casting (see Figure 5) has a full shape and clear outline.

 

 

Figure 4. Completed mold

 

Figure 5. Product photos of zinc alloy castings

Die-casting simulation analysis predicts locations of casting defects related to mold filling process, which are effectively improved and controlled before casting production. It guides mold designers to optimize die-casting mold pouring system step by step to avoid design errors and shorten time. Trial production and finalization cycle of zinc alloy casting is shortened, which not only saves time and cost, but also improves product qualification rate.