A certain zinc alloy casting company has an annual demand of 1.1 million pieces. Its forming molds require continuous die casting production day and night, in addition to routine maintenance. This poses challenges to stability of mold forming and yield of castings. Previously, company developed a mold with one mold per cavity, W1.2344 core steel, and a design life of 500,000 cycles. However, after approximately 300,000 cycles, severe burrs and bubbles appeared on the surface of castings, and cores experienced localized fractures, making it difficult to meet production requirements. By replacing core material with ASSAB 8407 Supreme electroslag remelted steel, which offers superior overall performance, continuing one-mold-one-cavity forming scheme to improve casting yield and mold life, mold still experienced core fracture failure and bubble/burr defects after approximately 170,000 molding cycles. Compared to its design life and lifespan of a previously developed mold set, fracture indicates early mold failure, causing significant losses to company's production efficiency and economic benefits. To thoroughly address poor forming stability and short lifespan of mold, to provide a reference for improving forming methods and failure mechanisms of die-casting molds for similar thin-walled castings, failed mold was analyzed and optimized, aiming to provide a reference for production of related products.
Figure 1 shows a scale model of a heavy-duty truck, made of cast zinc alloy ZZnA14Y, with dimensions of 91.32 mm * 32.01 mm * 37.83 mm and a wall thickness of 1 mm. Wall is thin and structure is complex. Casting has two ϕ2 mm mounting holes at the top (referred to as top mounting holes); and a rectangular through-slot at the front end (see shaded area of section A-A in Figure 1, referred to as front through-slot). Casting needs two different shape designs: one with top mounting holes and front through-slot, and the other without these features. In both designs, holes and slot features are switched using interchangeable inserts and interchangeable lateral core-pulling sliders on mold. Casting surface requires a liquid electrostatic spraying process (paint baking and curing temperature of 130~140 ℃) followed by a pad printing decoration process. Casting surface must be free of cold lines, cold shuts, shrinkage cavities, and other die-casting filling defects, with a bubble rate ≤3%.

 

Figure 1. Casting Structure Diagram
Fracture failure occurred at raised, island-like waistline on mold core, as shown in Figures 2 and 3. From fracture morphology, main crack originated in step transition area near middle of raised core (A2 in Figure 2b). Crack extended and spread from front face A2 to both sides (A3 in Figure 3a). Step had a 0.5 mm radius transition fillet, and fracture surface showed no obvious signs of aging. To further analyze fracture characteristics and metallographic changes, a sample was taken from fracture origin area A4 (Figure 3a) by wire EDM. After ultrasonic cleaning and drying, sample was observed using a metallographic microscope.
Hardness of failed mold core was tested using a ZWICK Rockwell hardness tester. Hardness value (HRC) was 52.9, which meets design requirements. Chemical composition analysis was performed on samples of failed mold core using an OXFORD Foundry-Master Pro spectrometer. Results are shown in Table 1. Chemical composition was compared with that of ASSAB 8407 Supreme hot work die steel manufactured by ASSAB, and results met factory requirements.

 

Figure 2: Schematic diagram of core fracture location and morphology (A1. Crack location; A2. Main fracture)

 

Figure 3: Schematic diagram of sampling location and fracture morphology (A3. Side fracture; A4. Sampling location; A5. Metallographic observation location; A6. Crack origin; A7. Residual EDM marks)

Table 1: Chemical composition of failed mold core (%)
Observation using a Leica DMI3000M metallographic microscope revealed that main crack was located at rounded corner of approximately right-angled step transition region, with a rounded corner radius of R0.5 mm. Multiple microcracks, residual EDM marks, a significant discharge alteration layer, and associated microcracks were observed near main crack (see Figures 4 and 5). Workpiece exhibited normal microstructure after heat treatment, with no microscopic shrinkage or porosity defects detected. Inclusions and segregation levels of material were within acceptable limits.
Based on observation and analysis, causes of mold fracture include: ① Stress concentration at corners due to an insufficient transition radius (R0.5 mm); ② High core hardness leading to insufficient toughness, making it prone to instantaneous impact fracture under high-temperature molten metal filling at high speeds; ③ Discharge white layer and altered layer generated by electrical discharge machining at corners resulted in poor toughness in this area, and resulting microcracks increased risk of fatigue cracking.

 

Figure 4: Polished morphology of material matrix and near crack initiation site (A8 solidified Zn alloy, A9 main crack, A10 microcrack)

 

Figure 5: Metallographic structure near crack initiation site (A11 main crack, A12 microcrack, A13 discharge-modified white layer)
Figure 6 shows failed mold, designed as a single-cavity mold. Four corners of main view are marked with guide pillar positions of Lijing 880 kN hot chamber die-casting machine. Analysis of structural diagram 7 reveals that, influenced by mold structure, gate location, and die-casting machine specifications, casting is positioned on one end of mold blank, offset from mold center along its length, leaving a smaller area for forming casting on the other end. Simultaneously, sprue bushing is also offset from mold center by 35 mm along mold width. This significant eccentric placement results in a mold blank utilization rate and die-casting machine efficiency of only about 50%. This leads to an imbalance in the overall filling resistance and ejection system stress within mold, increases risk of deformation and fatigue in weak areas of mold, accelerates wear on mold's guiding and positioning system, and ultimately shortens mold's lifespan.

 

Figure 6. Structure of Failed Mold (One Mold, One Cavity)
1. Casting 2. Core 3. Slider 1 4, 7, 12. Slider Preload Springs 5, 8, 10. Slider Guide Rails 6. Slider 2 9. Locking Module 11. Slider 3 13. Parting Surface Pressure Block 14. Water Nozzle 15. Cavity 16. Sprue Bushing

 

Figure 7. Surface Defect Analysis of Filling Nodes (Time: 0.02153 s)
Failed mold, like old mold, exhibited air bubbles and burrs during die-casting process. Considering potential instability of melt filling process and unreasonable parting of formed parts, parting defects were analyzed and improved. Filling process and defects such as air entrapment and slag inclusions in casting were analyzed using FLOW-3D software. Simulation results are shown in Figures 7-9.
As shown in Figure 7, when melt filling is close to saturation, blank areas exhibit incomplete filling and filling vortices due to high filling resistance (see circles in Figure 7). 3D analysis and sectioning of actual casting revealed that wall thickness in these areas is only 0.3 mm, constituting an uneven wall thickness defect. Even if fully filled, internal stress generated during solidification can easily lead to hot cracks on casting surface.
Figure 8 shows that gas entrapment and oxide accumulation exist at front end and top left and right sides of casting, posing a high risk of bubbles, cold lines, and cold shuts.
Figure 9 shows that due to changes in local hole and groove characteristics of casting, melt filling path is significantly altered. In areas with top assembly holes and front through-slot holes, inward curling occurs at filling end (see Figure 9a), easily leading to gas and oxide accumulation defects. Based on clamping force verification and CAE simulation analysis results, it is believed that casting can achieve a two-cavity mold, thereby improving machine utilization and die-casting efficiency.

 

Figure 8. Analysis of gas entrapment and oxides during filling process (A and B represent gas entrapment and oxide accumulation areas, time: 0.02053 s).

 

Figure 9. Analysis of the end-filling path (time: 0.02153 s).
To address wall thickness design defect, as seen in section A-A of Figure 1, this gradually recessed area serves a decorative function on casting. Therefore, to eliminate filling defect caused by unreasonable wall thickness design, wall thickness was increased to 0.7 mm while ensuring appearance effect. Simultaneously, gate size of sprue bushing was increased from ϕ8 mm to ϕ10 mm, ingate thickness was increased from 0.5 mm to 0.6 mm, and draft angle on both sides of ingate was increased from 25° to 40° to improve melt flowability. Overflow and slag venting bags were added to both sides of top of casting, and slag bags were optimized into a stepped overflow system, i.e., slag bags were designed as stepped shapes with thicknesses of 5, 1, and 0.3 mm, respectively. Figure 10 shows distribution of shrinkage porosity and inclusions within 5% range. Simulation analysis of improved two-cavity mold design revealed that only trace amounts of shrinkage porosity and oxides were concentrated in slag pocket, with a small portion located at casting wall thickness corners and reinforcing rib intersections. These amounts had no substantial impact on casting's appearance quality or bubble rate. Figure 11 shows simulated distribution of 0.01 mg gas content. It can be seen that only trace amounts of gas less than 0.01 mg were found in slag pocket, which are almost negligible. The overall forming scheme is superior to failed one-cavity mold design.

 

Figure 10: Shrinkage porosity and inclusion distribution within 5% range

 

Figure 11: 0.01 mg gas mass distribution
Core fracture failure is main failure mode of this mold. Furthermore, rationality of parting line design on mold directly affects casting surface quality, mold strength, mold maintenance, and mold life. To eliminate risk of fracture failure caused by parting line design from design stage, typical parting line defects were identified after examining casting formed by failed mold and 3D design drawings of mold, and corresponding optimization methods were provided. Schematic diagrams of improved parting line are shown in Figures 12-14. Corners on both sides of top of casting, formed by junction of sliders 1 and 3 with cavity, have a 32.18° sharp angle on two sliders. This sharp angle is prone to local deformation or cracking of sliders due to pressure contact during mold closing, resulting in burrs or excess material in casting. Improved parting line increases angle of sharp angle to 80° to reduce local contact stress, prevent local deformation and cracking of sliders.

 

Figure 12: Parting Line Improvement Schematic 1
1. Casting 2. Slider 1 3. Core 4. Slider 3

 

Figure 13: Parting Line Improvement Schematic 2
1. Core 2. Slider 1 3. Slider 2 4. Casting 5. Slider 3

 

Figure 14: Parting Line Improvement Schematic 3
1. Core 2. Casting
Eccentric design of single-cavity mold in a failed mold causes an imbalance in the overall filling resistance and ejection system force within mold, accelerating deformation and fatigue risks in weak areas of mold, ultimately reducing mold life. Based on combined verification of forming parameters and CAE simulation analysis results for both one-cavity and two-cavity molds, it was found that two-cavity mold scheme exhibits superior forming effects in terms of zinc alloy melt filling, gas content distribution, and oxide inclusions compared to one-cavity mold. Furthermore, two-cavity mold significantly improves space utilization and production efficiency of mold blank and die-casting machine. Therefore, two-cavity mold scheme was adopted to improve casting arrangement and mold structure, as shown in Figure 15.
Temperature control system of failed one-cavity mold consisted of single-group cooling water circuits connected in series for cavity, core, and slide block, using room temperature water as cooling medium. This series connection resulted in limited local temperature regulation and poor cooling effect.
Following two-cavity mold scheme, the longer melt and solidification cooling times correspondingly extend mold forming cycle. To improve surface finish of castings and minimize molding cycle, two-cavity mold design employs three independently controlled cooling systems for fixed mold, moving mold, and side core-pulling system (see Figure 16). In fixed mold section, except for sprue bushing which uses room temperature water cooling (see Figure 16a, IN1/OUT1), the other two temperature control loops (IN2/OUT2, IN3/OUT3) use high-temperature resistant, flame-retardant, and heat-conducting oil. An independent mold temperature controller maintains fixed mold temperature between 170 and 180 ℃. Moving mold and side core-pulling system, which experience significant thermal expansion at high temperatures, are prone to jamming of moving parts; therefore, they continue to be cooled by natural circulation of room temperature water from molding machine.

 

Figure 15 Improved mold structure diagram (one mold, two cavities)
1. Slider 12, 9, 15, 27. Slider preload spring 3, 10, 17, 21, 57. Slider bottom wear-resistant plate 4, 7, 14, 18, 22, 26. Slider guide rail 5, 11. Parting surface pressure block 6. Water nozzle 8. Slider 2 12. Mold support pad 13. Slider 3 16. Slider 4 19. Zero-degree positioning edge lock 20. Slider 5 23. First reset guide block 24. Lifting block 25. Slider 6 28, 47. Core 29. Cavity 30. First reset guide rod 31. Sprue bushing 32. Fixed mold plate 33, 37, 52, 59. Slider back wear-resistant plate 34, 36, 51, 60. Angled guide pillars 35, 50, 61. Angled guide pillar pressure blocks 38, 48, 53, 58. Slider stroke limit screws 39. Moving mold plate 40. Moving mold support plate 41. Ejector pin fixing plate 42. Ejector pin plate base plate 43. Moving mold fixing plate 44. Ejector pin plate guide pillar 45. Ejector pin 46. Support pillar 49. Casting 54. Ejector pin plate return spring 55. Main channel ejector pin 56. Ejection limit pillar 62. Cavity insert pin 63. Flange 64. Diverter cone 65. First return rocker arm 66. First return bearing block 67. Ejector pin plate return rod 68. Positioning pin 69. Return rod pad

 

Figure 16 Schematic diagram of improved mold temperature control (One mold produces two cavities)
Conclusions
(1) Formed parts of failed mold have high hardness and brittleness. Corner transition fillets cannot effectively reduce stress concentration caused by machining and high-temperature melt impact. Microcracks attached to residual deteriorated white layer from EDM form fracture sources, causing fracture under internal and external stress. Large-scale eccentric placement of casting in mold's length and width directions reduces utilization rate of mold blank and machine, accelerates wear of mold guiding and ejection system, and indirectly induces early mold failure. Sudden changes in local wall thickness of casting increase melt filling resistance and risk of air entrapment.
(2) Changes in local hole and groove characteristics of casting significantly alter melt filling path and filling result. For two or more designs with same main shape but different and significantly different local hole and groove characteristics, avoid switching between schemes by designing interchangeable inserts or interchangeable core-pulling sliders on same mold. This is to balance melt filling effect between different schemes and ensure stable casting appearance quality.
(3) Unreasonable parting design leads to local deformation and cracking of formed parts, causing burrs and excess material in castings, requiring additional rework processes, which affects production costs and efficiency; optimizing mold processing and die casting process can significantly extend mold life, improve stability of casting quality.