Abstract: This paper analyzes causes of continuous fracture failure of two sets of molds from aspects of zinc alloy thin-walled casting structure, mold failure mode, mold forming scheme, and processing technology. Using physical and chemical testing equipment and casting simulation software, simulation verification was conducted on casting filling, melt filling process, and defects such as gas entrapment and slag inclusions, obtaining effective forming data, solving problems of poor forming stability and short lifespan of casting molds. Compared with failed mold, improved and optimized mold scheme increased utilization rate and die casting efficiency of die casting machine by 100%, extended mold life by more than 2.7 times, eliminated secondary rework process of removing casting burrs and excess material.
A certain zinc alloy casting has an annual demand of 1.1 million pieces, requiring its forming molds to be used for continuous die casting production during both day and night shifts, except for routine maintenance. Therefore, this poses challenges to stability of mold forming and yield of castings. Previously, company developed a die-casting mold with a single mold cavity, using W1.2344 core steel, and a designed life of 500,000 cycles. However, after approximately 300,000 cycles, severe burrs and bubbles appeared on casting surface, and core experienced localized fracture, making it difficult to meet production requirements. Company replaced core material with ASSAB 8407 Supreme electroslag remelted steel, which has superior overall performance, while retaining single-mold molding scheme to improve casting yield and mold life. However, after approximately 170,000 cycles, mold also experienced core fracture and burr/bubble defects. Compared to its designed life and lifespan of previously developed mold, fracture represents early mold failure, causing significant losses to company's production efficiency and economic benefits. To thoroughly address poor molding stability and short lifespan of mold, to provide a reference for improving molding methods and failure prevention 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 outline 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%.

 

Fig.1 Structure diagram of cásfings

Fracture failure occurred at waistline of mold core, which appears as an isolated protrusion, as shown in Figures 2 and 3. From fracture morphology, main crack originated in step transition area near middle of protruding 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 old fracture. 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.

 

Fig.2 Schematic diagram of fracture location and appearance of core

 

Fig. 3 Schematic diagram of sampling position and fracture morphologies
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.
Tab. 1 Chemical composition of failure mold cores %

Observation using a Leica DMI3000M metallographic microscope revealed that main crack was located at rounded corner of near-right-angle step transition region, with a radius of R0.5 mm. Multiple microcracks, residual EDM marks, obvious EDM alteration layer, and associated microcracks were observed near main crack (see Figures 4 and 5). Workpiece's heat-treated microstructure was normal, with no micro-shrinkage defects found. Inclusions and segregation levels of material were within acceptable limits.

 

Fig. 4 Microstructure of matrix and polishing morphology near fracture source

 

Fig. 5 Metallographic structure near fracture source
Based on observation and analysis, reasons for mold fracture include: ① stress concentration at corners and insufficient transition radius (R0.5 mm); ② high core hardness leading to insufficient toughness, making it prone to instantaneous impact fracture under high-temperature melt high-speed filling; ③ discharge white layer and altered layer generated by EDM at corners resulting in poor toughness in this area, and resulting micro-cracks increase risk of fatigue cracking of workpiece.

Figure 6 shows failed mold, designed as a single-cavity mold. Four corners of main view indicate positions of guide pillars of LK 880 kN hot chamber die-casting machine. Analysis of structure in Figure 7 shows that, influenced by mold structure, gate position, and die-casting machine specifications, casting is placed on mold base at one end of length direction away from mold center, leaving less area for forming casting at the other end; simultaneously, sprue bushing is also 35 mm away from mold center in mold width direction. Significantly eccentric placement design results in a mold base utilization rate and die-casting machine efficiency of only about 50%. This leads to an imbalance in the overall filling resistance and ejection system force within mold, increases risk of deformation and fatigue in weak parts of mold, accelerates wear of mold guide and positioning system, and thus shortens mold's service life.

Figure 6. Structure diagram of the 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

Given low mold base utilization rate, die-casting machine efficiency, and productivity caused by mold structure defects, clamping force, a key forming parameter, was verified, and feasibility of a two-cavity mold was compared. Since no significant deformation occurred in mold cavity and casting, strength and rigidity of mold forming parts and template were not checked or analyzed.
Clamping force F_lock is calculated using formula:

 

Where k is safety factor, taken as 1.25.
Mold expansion force F_expansion is:

 

Where A is projected area of casting surface, cm² (to ensure safe and stable mold clamping, surface area of casting and overflow system is used; surface areas of one-cavity, two-cavity castings analyzed using UG software are 91.88 cm² and 201.26 cm², respectively); p is pressure, MPa, with working pressure taken as 34 MPa.
Scheme 1 (one-cavity mold) clamping force: F_expansion = 312.39 kN, therefore F_lock ≥ 390.49 kN. Scheme 2 (One mold, two cavities) clamping force: F_expansion = 684.28 kN, therefore F_locking ≥ 855.35 kN

Failed mold, like old mold, exhibits air bubbles and burrs during die casting. Considering potential instability of melt filling process and unreasonable parting of formed parts, parting defects are analyzed and improved. Filling process and defects such as air entrapment and slag inclusions in casting are analyzed using FLOW-3D software. Simulation results are shown in Figures 7-9.

 

Fig.7 Surface defect analysis of filling nodes
As can be seen from Figure 7, when melt filling is close to saturation, blank areas show insufficient filling and filling vortices due to high filling resistance (see circle in Figure 7). 3D analysis and sectioning of actual casting revealed that wall thickness in this area is only 0.3 mm, which is a wall thickness unevenness defect. Even if filling is complete, internal stress generated during solidification can easily lead to hot cracks on casting surface.

 

Fig. 8 Air entrainment and oxides analysis of filling process
As shown in Figure 8, entrapment of gas and oxides is present on the front end, top left and right sides of casting, posing a high risk of defects such as bubbles, cold lines, and cold shuts.

 

Fig. 9 Analysis of filling path at the end stage
As shown in Figure 9, due to changes in local hole and groove characteristics of casting, melt filling path is significantly altered. Specifically, in the areas with top assembly holes and front through-slot holes, inward curling of filling end occurs (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.
For failed mold, defects such as bubbles and oxide inclusions were found on the front end and top sides of casting in both CAE simulation analysis and actual production process. Corresponding improvement measures were implemented.
To address wall thickness design defects, as can be seen from section A-A in Figure 1, this gradually recessed area serves as a decorative feature on casting. Therefore, to eliminate filling defects caused by unreasonable wall thickness design, wall thickness was increased to 0.7 mm while ensuring appearance effect. At the same time, gate size of sprue bushing was increased from ϕ8 mm to ϕ10 mm, thickness of ingate 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, that is, slag bags were designed as stepped, with thicknesses of 5, 1, and 0.3 mm respectively. Figure 10 shows distribution of shrinkage porosity and inclusions within range of 5%. Simulation analysis of improved two-cavity mold scheme 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. 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 bag, which are almost negligible. The overall forming scheme is superior to single-cavity mold failure scheme.

 

Fig.10 Distribution of shrinkage porosity and inclusion within 5%

Fig.11 Mass distribution of 0.01mg air

Core fracture failure is main failure mode of this mold. Furthermore, rationality of parting design on mold directly affects surface quality of casting, mold strength, mold maintenance, and mold life. To eliminate risk of fracture failure caused by parting design from design stage, typical parting defects were found after examining casting formed by failed mold and 3D design drawings of mold, and corresponding optimization methods were given. Schematic diagrams of parting improvement are shown in Figures 12-14. Area formed by junction of slider 1, slider 3, and cavity at the top two corners of casting has a 32.18° sharp angle on two sliders. This is prone to local deformation or breakage of sliders due to pressure contact during mold closing, resulting in burrs or excess material in casting. Improved parting line enlarges sharp corner area to 80° to reduce local contact stress, prevent local deformation and cracking of slider.

 

Fig. 12 Schematic diagram of modified parting surface 
1. Casting 2. Slider 1 3. Core 4. Slider

 

Fig. 13 Schematic diagram of modified parting surface
1. Core 2. Slider 1 3. Slider 2 4. Casting 5. Slider 3
3D parting line design of slider 1 features a thin steel feature of 0.8141 mm thickness (enlarged circle in Figure 13a), which is weak and has a high risk of deformation and fracture. Analysis of casting appearance quality requirements shows that this area is not on the surface of casting; therefore, local strength of slider can be increased by modifying parting line. Optimized parting line is shown in Figure 13b.
End and middle parts of casting have thin steel feature bosses of 1.94 mm and 1.57 mm width respectively on 3D parting line drawing of mold core (see Figure 14b). These two features frequently deform and crack during actual forming process (see Figure 14c), resulting in burrs and excess material defects in casting, requiring secondary manual processing (Figure 14a). Without affecting appearance of casting and the overall assembly structure, near-high-pair line contact of thin steel bosses at the ends and middle of casting along cavity and core directions during mold opening and closing was changed to low-pair surface contact. Optimized parting surface has a simple planar configuration, avoiding local deformation and chipping defects of core, is also easy to process and grind for assembly.

 

Figure 14 Schematic diagram of modified parting surface

Eccentric design of single-cavity mold in a failed mold will cause an imbalance in the overall filling resistance and ejection system force within mold, accelerating deformation and fatigue risk of weak parts of mold, and ultimately reducing mold life. Based on combined verification of forming parameters and CAE simulation analysis results for one-cavity and two-cavity molds, it was found that two-cavity mold scheme has better forming effects in terms of zinc alloy melt filling, gas content distribution, and oxide inclusions than one-cavity mold. Furthermore, two-cavity mold scheme can significantly improve space utilization and production efficiency of mold base and die-casting machine. Therefore, two-cavity mold scheme was adopted to improve casting arrangement and mold structure, as shown in Figure 15. To ensure melt filling pressure balance as much as possible, avoid direct impact of melt on mold cavity, fully utilize space of mold base and die-casting machine, gating system adopts an S-shaped symmetrical design. Formed part consists of cores 28 and 47, cavity 29, cavity insert 62, slider 1, slider 2, slider 3, slider 4, slider 5, and slider 6, and slider 6, similar to one-mold-one-cavity design. Each cavity's three sliders form undercut features on the left, right, and front sides of casting. Cavity insert 62 is designed as an interchangeable insert. Slider 3, 13, and slider 6, 25 are each formed by two independent interchangeable sliders to switch between having and not having slotted features on the top and front of casting. Each slider is driven out of undercut stroke by its own pre-loaded spring and inclined guide post combination under opening and closing action of die-casting machine, and finally limited by a limit screw until mold closes and resets to enter next die-casting cycle.

 

Fig.15 Structure diagram of modified mold
1. Slider 1; 2. 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 sleeve; 2. Fixed mold plate; 33. 37. 52. 59. Slider back wear-resistant plate; 34. 36. 51. 60. Angled guide post; 35, 50, 61. Inclined guide post pressure block; 38, 48, 53, 58. Slider stroke limit screw; 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 post; 45. Ejector pin; 46. Support post; 49. Casting; 54. Ejector pin plate return spring; 55. Main runner ejector pin; 56. Ejection limit post; 62. Cavity insert pin; 63. Flange; 64. Diverter cone; 65. First reset rocker arm; 66. First reset bearing block; 67. Ejector plate reset rod; 68. Positioning pin; 69. Reset rod pad

Temperature control system of failed mold's one-cavity mold consists of single-group cooling water circuits connected in series for cavity, core, and slide block, with room temperature water as cooling medium. This series connection results in limited local temperature regulation and poor cooling effect.
Adopting a two-cavity mold design will correspondingly extend mold forming cycle due to longer melting and solidification cooling times. To improve surface forming quality of castings and shorten forming cycle as much as possible, two-cavity mold design incorporates three independently controlled cooling systems for fixed mold, moving mold, and side core-pulling system (see Figure 16). For fixed mold, 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.

 

Fig. 16 Temperature controlling diagram of modified mold

For guiding and positioning system, four straight-body precision positioning side locks are designed on four sides of mold base to protect friction pairs where forming parts have small contact angles in mold opening and closing directions. Regarding ejection system, compared to a single-cavity mold, improved ejection and reset system is more balanced and stable, eliminating defects such as guide system wear, ejector pin slippage, and pin burn-out caused by eccentric positioning of casting on mold. Furthermore, due to very large quantity of castings and a wall thickness of only 1mm, if reset spring of ejection system experiences spring fatigue during high-temperature, high-strength forming, ejector pin tip can easily scratch cavity surface or interfere with slider during reset, causing mold damage. To avoid this, a mechanical forced reset mechanism is installed along mold length to ensure that even if spring fatigue causes reset errors, ejection system can still accurately and safely reset.

Fracturing at microcrack locations due to stress concentration at corners of formed parts, high hardness, and reduced toughness caused by discharge white layer generated by EDM demonstrates that defects in workpiece heat treatment and processing are also important factors causing fracture failure. Improved two-cavity mold design optimizes heat treatment process. After drilling and CNC rough milling, formed parts are quenched at 1030℃ and then tempered twice at 580℃ (i.e., tempering treatment), each tempering lasting 2 hours, to refine grains, eliminate internal stress caused by drilling and CNC rough milling, and reduce hardness (HRC) of formed parts to 46-48 (failed mold was quenched at 1020℃ and then tempered once at 250℃ to 52-54). Secondly, CNC semi-finish milling process is split into two steps to reduce risk of excessive machining allowance and increased internal stress caused by a single semi-finish milling operation. Finally, based on failed mold which only required one CNC EDM operation, process is optimized to two steps: semi-finish and finish machining. A CNC mirror EDM machine is used for machining, and after EDM, fine spark marks, tool marks on corners and reinforcing ribs of casting are polished and removed in stages. These measures minimize mold failure caused by improper heat treatment and machining processes. In terms of die casting process, in addition to using high-temperature resistant flame-retardant heat-conducting oil as an independently controlled temperature circuit in fixed mold to improve stability of casting quality and shorten die casting cycle, automatic dropping method of casting after ejection from mold was changed to automatic spraying and picking by a robotic arm. This achieved timed and balanced spraying and forming, further ensuring surface quality of casting and production cycle, avoiding die-casting events caused by occasional sticking or automatic drop failure of casting under automatic dropping method.

(1) Formed parts of failed mold have high hardness and high brittleness. Corner transition fillets cannot effectively reduce stress concentration caused by machining and high-temperature melt impact. Microcracks attached to residual deteriorated white layer of EDM form fracture sources, causing fracture under action of internal and external stress. Large-size eccentric placement of casting in length and width direction of mold reduces utilization rate of mold base and machine, accelerates wear of mold guiding and positioning and ejection system, and indirectly induces early mold failure. Sudden change in local wall thickness of casting increases melt filling resistance and risk of gas inclusion.
(2) Changes in local hole and groove characteristics of casting significantly alter melt filling path and result. For two or more designs with same main shape but different and significantly different local hole and groove characteristics, avoid switching between designs by using interchangeable inserts or core-pulling sliders on same mold. This is to balance melt filling effect between different designs and ensure stable casting appearance quality.
(3) Inappropriate parting design leads to local deformation and cracking of formed parts, causing burrs and excess material in casting, requiring additional rework processes, affecting production costs and efficiency. Optimizing mold processing and die-casting technology can significantly extend mold life, improve casting quality stability.