Abstract: In our production of a series of cylinder block castings, casting holes defects were identified on the K-face after machining, leading to high scrap rates and impacting production delivery. To address this, an internal task force was established. This paper analyzes and validates the issue from multiple aspects of high-pressure die casting, including human, machine, material, and method factors. Through systematic investigation, the primary causes of the casting holes formation were identified and improvements were implemented, effectively reducing the incidence of K-face casting holes and achieving the desired improvement target.
The high-pressure die casting process is widely utilized for manufacturing complex automotive components such as engine blocks due to its efficiency and ability to produce near-net-shape parts. However, defects like casting holes, which encompass sand inclusions, gas porosity, and shrinkage cavities, can severely compromise the structural integrity and machinability of castings. In our facility, the emergence of casting holes on the K-face of a specific cylinder block series became a critical quality concern, especially as模具 were required to operate beyond their intended lifespan to meet supply demands. This situation necessitated a thorough investigation to understand the root causes and implement corrective actions to mitigate these casting holes.
The background of this project stems from an engine model transition within our company. To conserve costs and utilize resources effectively, certain cylinder block模具 were extended in service life. Concurrently, a high rejection rate due to casting holes on the K-face was observed post-machining, averaging 6.58%. This defect not only affected the first-pass yield in machining but also threatened the timely delivery of engines. The K-face is a critical mating surface for engine assembly, and the presence of casting holes can lead to leakage or reduced sealing performance, making the parts non-conforming.
A detailed survey of the production process from January to April 2022 confirmed that casting holes were the predominant defect. Statistical process control data indicated a consistent defect rate, with peaks aligning with specific production batches. The financial impact was significant due to material waste and reprocessing costs. Furthermore, customer feedback from the machining department highlighted operational delays caused by the need for additional inspection and sorting of defective castings.
The project goal was explicitly set to reduce the incidence of casting holes on the K-face to ≤3% for模具 operating beyond their standard life, thereby ensuring product quality and meeting production schedules.
Root cause analysis was conducted using a systematic approach, focusing on the four primary categories of manufacturing variables: Man, Machine, Material, and Method. A cause-and-effect diagram (Ishikawa diagram) was constructed to visualize potential sources of the casting holes. Initial screening ruled out several factors: aluminum alloy feedstock met specifications with consistent chemical composition and cleanliness, operators were experienced and followed standardized procedures, and the die casting machines operated without significant faults or downtime. This directed attention to process parameters and模具 conditions.
The key factors suspected of influencing the formation of casting holes included: vacuum start position, injection velocity, cooling time, casting pressure,模具 variation (due to extended life), and模具 temperature. Each factor’s impact on fluid flow, solidification, and defect formation was theorized. For instance, improper vacuum timing could entrain cold material or air, while模具 temperature gradients affect the solidification sequence, potentially trapping gases or creating shrinkage casting holes.
To quantify these effects, a series of controlled production trials were designed and executed. The response variable was the defect rate of casting holes on the K-face, measured post-machining. The following sections detail the analysis and validation for each factor, supported by data tables and theoretical models.
Analysis of模具 Variation (X1)
模具 with different service lives exhibit wear, minor geometrical changes, and potential surface degradation, which can alter the molten metal flow and heat transfer. The hypothesis was that模具 differences lead to turbulent flow, air entrapment, and inclusion entrainment, promoting casting holes. Trials were conducted using模具 with distinct lifetimes. The results are summarized in Table 1.
| Mold ID | Number of Castings with Casting Holes | Number of Acceptable Castings | Total Machined Castings | Defect Rate (%) |
|---|---|---|---|---|
| 5# | 38 | 538 | 576 | 6.60 |
| 8# | 16 | 236 | 252 | 6.35 |
| 9# | 569 | 7419 | 7988 | 7.12 |
The data shows no consistent or significant improvement across different模具, suggesting that模具 variation within the tested range is not the dominant factor for these casting holes. However, wear patterns specific to the K-face region might require更 detailed inspection.
Analysis of Injection Velocity (X2)
Injection velocity directly influences the filling pattern. High velocities can cause turbulence and jetting, leading to gas entrapment, while low velocities may result in premature solidification and cold shuts. The relationship between injection velocity and defect formation can be modeled by fluid dynamics. The momentum equation for the molten metal flow in the shot sleeve and die cavity can be expressed as:
$$ \rho \left( \frac{\partial \mathbf{u}}{\partial t} + \mathbf{u} \cdot \nabla \mathbf{u} \right) = -\nabla p + \mu \nabla^2 \mathbf{u} + \mathbf{f} $$
where \( \rho \) is density, \( \mathbf{u} \) is velocity vector, \( p \) is pressure, \( \mu \) is dynamic viscosity, and \( \mathbf{f} \) represents body forces. Turbulence can be assessed using the Reynolds number \( Re = \frac{\rho u L}{\mu} \), where \( L \) is a characteristic length. Trials varied velocity within the process window (5±1 m/s). Results are in Table 2.
| Injection Velocity (m/s) | Number of Castings with Casting Holes | Number of Acceptable Castings | Total Machined Castings | Defect Rate (%) |
|---|---|---|---|---|
| 4.0 | 68 | 857 | 925 | 7.35 |
| 5.0 | 73 | 1012 | 1085 | 6.73 |
| 6.0 | 62 | 798 | 860 | 7.21 |
The defect rates remain relatively stable across velocities, indicating that injection velocity alone, within this range, is not a key driver for the observed casting holes. This suggests that other factors interact with flow dynamics to cause defects.
Analysis of Vacuum Start Position (X3)
The vacuum system evacuates air from the die cavity before and during injection to reduce gas porosity. Starting vacuum too early can draw cold material or lubricant vapors into the cavity, potentially creating inclusions or casting holes. The vacuum start position, defined as the plunger position when vacuum is initiated, was varied across its allowable range (350 mm to 880 mm). The effectiveness of vacuum in reducing cavity pressure can be described by:
$$ P_{cavity}(t) = P_0 e^{-t/\tau} $$
where \( P_0 \) is initial pressure, \( t \) is time, and \( \tau \) is a time constant dependent on system design. The start position influences the time available for evacuation before metal arrival. Trial results are shown in Table 3.
| Vacuum Start Position (mm) | Number of Castings with Casting Holes | Number of Acceptable Castings | Total Machined Castings | Defect Rate (%) |
|---|---|---|---|---|
| 380 | 40 | 599 | 639 | 6.26 |
| 430 | 63 | 924 | 987 | 6.38 |
| 480 | 50 | 797 | 847 | 5.90 |
| 530 | 45 | 877 | 922 | 4.88 |
| 580 | 37 | 812 | 849 | 4.36 |
| 630 | 27 | 691 | 718 | 3.76 |
| 680 | 19 | 723 | 742 | 2.56 |
| 730 | 25 | 843 | 868 | 2.88 |
| 780 | 22 | 695 | 717 | 3.07 |
| 830 | 28 | 861 | 889 | 3.15 |
| 880 | 28 | 834 | 862 | 3.25 |
The data reveals a clear trend: as the vacuum start position increases (later initiation), the defect rate due to casting holes decreases significantly, reaching a minimum around 680 mm. This indicates that starting vacuum too early is detrimental, likely because it entrains cold material or contaminants from the shot sleeve into the cavity, which later manifest as casting holes. Optimizing this parameter is crucial for minimizing casting holes.
Analysis of模具 Temperature (X4)
模具 temperature governs the heat transfer during filling and solidification. Low temperatures can cause rapid chilling, leading to misruns and cold shuts, while high temperatures may prolong solidification, increasing shrinkage porosity and gas absorption. The temperature distribution affects thermal gradients, which influence defect formation. The heat conduction equation is:
$$ \frac{\partial T}{\partial t} = \alpha \nabla^2 T $$
where \( T \) is temperature, \( t \) is time, and \( \alpha \) is thermal diffusivity. A uniform temperature field is desired to promote directional solidification away from critical areas like the K-face. Trials evaluated模具 temperature impact on casting holes, as shown in Table 4.
| Mold Temperature (°C) | Number of Castings with Casting Holes | Number of Acceptable Castings | Total Machined Castings | Defect Rate (%) |
|---|---|---|---|---|
| 100 | 77 | 889 | 966 | 7.97 |
| 120 | 59 | 896 | 955 | 6.18 |
| 140 | 32 | 984 | 1016 | 3.15 |
| 160 | 19 | 829 | 848 | 2.24 |
| 180 | 23 | 916 | 939 | 2.45 |
| 200 | 25 | 789 | 814 | 3.07 |
| 220 | 48 | 904 | 952 | 5.04 |
| 240 | 69 | 947 | 1016 | 6.79 |
| 260 | 84 | 969 | 1053 | 7.98 |
The results demonstrate a parabolic relationship: defect rates are high at low and high temperatures, with an optimal range around 160°C to 180°C where casting holes are minimized. Low temperatures likely cause cold material defects, while high temperatures promote gas porosity and shrinkage casting holes. Thus, precise模具 temperature control is vital to prevent casting holes.
Analysis of Cooling Time (X5)
Cooling time, the duration the casting remains in the die before ejection, affects solidification completeness and internal stresses. Insufficient cooling can lead to hot tearing or deformation, while excessive cooling may not reduce defects further. The solidification time can be estimated using Chvorinov’s rule:
$$ t_s = B \left( \frac{V}{A} \right)^n $$
where \( t_s \) is solidification time, \( V \) is volume, \( A \) is surface area, \( B \) is a constant dependent on material and模具 conditions, and \( n \) is an exponent (typically ~2). Trials varied cooling time, with results in Table 5.
| Cooling Time (s) | Number of Castings with Casting Holes | Number of Acceptable Castings | Total Machined Castings | Defect Rate (%) |
|---|---|---|---|---|
| 40 | 194 | 2562 | 2756 | 7.04 |
| 45 | 111 | 1568 | 1679 | </td6.61 |
| 50 | 252 | 3517 | 3769 | 6.68 |
| 55 | 107 | 1557 | 1664 | 6.43 |
| 60 | 153 | 2141 | 2294 | 6.67 |
The defect rates show no consistent trend with cooling time, indicating that within the tested range, cooling time is not a significant factor for the formation of these casting holes. This suggests that the primary solidification phenomena leading to casting holes occur earlier in the process.
Based on the above analysis, two factors were identified as having a statistically significant impact on the incidence of casting holes: vacuum start position and模具 temperature. The interaction between these factors was also considered. For instance, an optimal模具 temperature ensures proper fluidity, while correct vacuum timing minimizes gas entrapment, collectively reducing the risk of casting holes.
The improvement measures were thus formulated: set the vacuum start position to 580 mm (within the optimal range identified) and maintain the模具 temperature at 160°C. These settings were selected based on the trial data showing low defect rates while providing a robust operating window. Implementation involved updating the die casting machine parameters and enhancing模具 temperature monitoring using additional thermocouples and closed-loop control systems.
After implementing the changes, a sustained production run was monitored. The results demonstrated a significant reduction in casting holes on the K-face. The defect rate dropped from an average of 6.58% to below 2.5%, consistently meeting the target of ≤3%. A summary of the before-and-after comparison is presented in Table 6.
| Period | Average Defect Rate (Casting Holes) | Number of Castings Sampled | Improvement |
|---|---|---|---|
| Before Improvement | 6.58% | ~8500 | Baseline |
| After Improvement | 2.32% | ~5000 | 64.7% Reduction |
The reduction in casting holes directly improved the first-pass yield in machining, reduced scrap and rework costs, and ensured on-time delivery of engines. Furthermore, the extended模具 life did not compromise quality, validating the approach of parameter optimization to mitigate age-related effects.
In conclusion, through this project, we successfully identified and addressed the key factors causing casting holes on the K-face of die-cast cylinder blocks. The systematic analysis, involving design of experiments and data-driven decision-making, highlighted the critical roles of vacuum start position and模具 temperature in defect formation. The implemented measures effectively minimized the occurrence of casting holes, achieving the project goal. This experience deepens our understanding of how process parameters and模具 conditions influence casting quality, particularly in managing defects like casting holes. It underscores the importance of continuous monitoring and optimization in high-pressure die casting, especially when operating under constraints such as extended模具 life. Future work may explore advanced simulation tools to predict casting holes formation and further refine process windows for other critical casting regions.