With the advancement of national emission standards and the growing demand for lightweight components, ductile iron castings have gained widespread application in engine blocks and cylinder heads due to their exceptional mechanical properties. A critical indicator for transitioning from prototype to mass production of ductile iron castings is the consistency of casting dimensions. Compared to gray iron, ductile iron inherently exhibits poorer machinability, leading customers to impose stricter requirements on dimensional accuracy and stability, thereby increasing the difficulty of casting process control. This article analyzes and proposes solutions for factors influencing the dimensional precision of ductile iron engine blocks and cylinder heads, based on the characteristics of high-volume production using green sand molding lines.
The production of engine blocks and cylinder heads involves cold-box resin sand cores and high-pressure green sand molding lines, with an annual demand of approximately 100,000 tons, of which ductile iron accounts for about 70%. The material grades include RuT450 and RuT500, with a minimum wall thickness of 4.5 mm. The dimensional tolerances for rough castings adhere to DIN 1686-1 GTB15, as specified in Table 1. Initially, dimensional fluctuations in ductile iron castings were significant, resulting in customer complaints and substantial tooling compensation claims. To enhance dimensional accuracy, improvements were made in process design, core dimension control, and melting processes, ultimately meeting customer requirements.
| Dimension | <18 | 18–30 | 30–50 | 50–80 | 80–120 | 120–180 | 180–250 | 250–315 | 315–400 | 400–500 | 500–630 | 630–800 | 800–1000 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Tolerance | ±0.85 | ±0.95 | ±1.0 | ±1.1 | ±1.2 | ±1.3 | ±1.4 | ±1.5 | ±1.6 | ±1.7 | ±1.8 | ±1.9 | ±2.0 |
Casting shrinkage, also known as linear casting shrinkage, refers to the linear contraction of a casting from the start of solidification to room temperature, expressed as a percentage of the difference between pattern length and casting length relative to the pattern length. The linear shrinkage rate accounts for various influencing factors, including the metal’s shrinkage characteristics, starting temperature of solidification, casting structure, mold type, and gating system design. Initially, due to the lack of specific data for ductile iron castings, we referenced similar gray iron components and measured representative dimensions after pouring ductile iron melt to identify patterns. Tables 2 and 3 present subsets of this data.
| Position | Design Value (mm) | Measured Value (mm) | Shrinkage Rate (%) | Average Shrinkage Rate (%) |
|---|---|---|---|---|
| Width Direction | 576 | 569.2 | 1.12 | 1.13 |
| Width Direction | 535 | 529.2 | 1.08 | |
| Width Direction | 489 | 483.1 | 1.21 | |
| Width Direction | 206 | 203.8 | 1.07 | 1.02 |
| Width Direction | 200 | 197.9 | 1.05 | |
| Width Direction | 161.6 | 160.1 | 0.93 | – |
| Height Direction | 409 | 405.1 | 0.95 | 0.98 |
| Height Direction | 348.5 | 344.8 | 1.06 | |
| Height Direction | 410 | 406.2 | 0.93 |
| Position | Design Value (mm) | Measured Value (mm) | Shrinkage Rate (%) | Average Shrinkage Rate (%) |
|---|---|---|---|---|
| Length Direction | 1040 | 1026.8 | 1.27 | 1.23 |
| Length Direction | 800 | 790.2 | 1.22 | |
| Length Direction | 356 | 351.7 | 1.21 | |
| Width Direction | 324 | 320.2 | 1.17 | 1.18 |
| Width Direction | 256 | 252.9 | 1.21 | |
| Width Direction | 189 | 186.8 | 1.16 | |
| Height Direction | 156 | 154.2 | 1.15 | 1.16 |
| Height Direction | 120 | 118.7 | 1.08 | |
| Height Direction | 88 | 86.9 | 1.25 |
The shrinkage percentage is calculated using the formula: $$ \text{Shrinkage Percentage} = \frac{\text{Pattern Length} – \text{Casting Length}}{\text{Pattern Length}} \times 100\% $$ Based on the average shrinkage data, we designed the process and conducted formal sample production. Full-dimensional measurements revealed that shrinkage rates at某些位置 did not meet expectations, with significant variations across different locations. After adjustments and process corrections, we established a design specification for casting shrinkage in ductile iron engine blocks and cylinder heads, as shown in Table 4.
| Product | Length Direction Shrinkage (%) | Other Directions Shrinkage (%) | Process Compensation |
|---|---|---|---|
| Engine Block | 1.1 | 1.05 | Separate design for water jacket, cylinder liner, and crank partition |
| Cylinder Head | 1.2 | 1.15 | Separate design for valve seat and injector holes |
For complex engine blocks and cylinder heads, sand cores are crucial in forming casting dimensions, and their precision directly affects the final dimensional accuracy. A typical engine block consists of 12 sand cores, while a cylinder head has 14, all produced using cold-box resin sand with over 80% automation in core shooting and assembly. Although cold-box core making improves dimensional accuracy, high automation demands greater stability in core dimensions. Core assembly dimensions are influenced by individual core dimensions and assembly processes, leading to situations where individual cores meet specifications but assembled cores do not. Therefore, these factors must be considered comprehensively during core process design.
In core process design, coating thickness must be accounted for, as it affects core dimensions. The thickness varies with coating type and Baume degree, typically ranging from 0.2 mm to 0.5 mm based on product structure and coating characteristics. Initial core strength is essential for automated handling and assembly to resist clamping deformation. Strength depends on sand composition, ratio, binder type, and quantity. For instance, cylinder liner and frame cores may use standard resins, while water jacket and air passage cores require high-strength resins. Initial strength control standards are outlined in Table 5.
| Core Type | Main Cores (e.g., Thick Sections) | Thin-Wall Cores (e.g., Water Jacket) | Intake/Exhaust Passage Cores |
|---|---|---|---|
| Initial Strength (MPa) | ≥0.7 | ≥1.2 | ≥1.0 |
Core assembly requires specific配合间隙 and positioning references to ensure dimensional accuracy. In process design, positioning gaps are set at 0.15 mm, other配合间隙 at 0.3 mm, and maximum contour gaps at 0.5–1 mm. The parting negative, similar to the mold parting negative, involves removing a certain value from the core box surface to prevent oversized cores. This is influenced by core size, sand type, drying method, and core box structure. During production, factors like core box deformation, clamping force during shooting, and seal usage can cause cores to be larger in the parting direction. To address this, a parting negative is designed, typically (0.6 ± 0.1) mm, adjusted based on actual conditions. Core assembly dimensions are also affected by drying processes, fastening methods, and torque, requiring control during production.
Core box quality significantly impacts core dimensional accuracy. Material selection is critical for wear and deformation resistance. We chose 4Cr5MoSiV1 for the main body with a surface hardness of 38–40 HRC, and QT500 cast iron for shooting plates and blow plates. Machining精度 relies on precision equipment, with EDM used for sharp corners and lapping for配合面. Forming surfaces are prohibited from polishing to minimize deviations. Carburizing or nitriding may be applied, with carburizing layer thickness of 0.5–1.2 mm or nitriding layer of 0.1–0.3 mm. Key technical requirements for core box machining are listed in Table 6.
| Core Box Part | Main Body | Surface Roughness | Fitting Accuracy |
|---|---|---|---|
| Technical Requirement | ±0.1 mm | Ra 1.6 | ≤0.05 mm |
Maintenance of core boxes is vital, as wear on surfaces and定位销套 can cause dimensional variations. We implemented a specialized maintenance protocol, requiring online cleaning every 200–300 cycles and offline cleaning every 500–600 cycles.定位销套 must be replaced if wear exceeds 0.2 mm. Similar regulations apply to molding patterns, flasks, and fixtures.
The mold is another key factor influencing casting dimensions, affected by pattern accuracy, mold strength, and sand properties. In production, high-pressure molding ensures mold hardness ≥16 on planes and ≥11 on vertical surfaces (measured with PFP hardness tester), providing the necessary rigidity to resist casting contraction and expansion. Additionally, high-quality composite sands with specific properties are used: permeability 130–170, green compression strength 0.12–0.15 MPa, and compactability 30–34%. Controlling these parameters effectively ensures dimensional accuracy in ductile iron castings.
The process window for producing ductile iron is narrow, and fluctuations in vermiculizer composition, particularly magnesium, significantly affect molten metal contraction and dimensional stability. We monitored the impact of different composition combinations on dimensional stability, as illustrated in Figure 1, and found that elements like magnesium have a substantial influence. Based on data analysis, we established a melting process to stabilize casting dimensions, strictly controlling magnesium and other elements. The relationship between element content and dimensional change can be modeled as: $$ \Delta D = k \cdot \Delta C $$ where $\Delta D$ is the dimensional change, $k$ is a material constant, and $\Delta C$ is the composition variation.
Casting distortion is a common issue in engine cylinder heads, caused by铸造应力 from hindered contraction during cooling after solidification. Under stress, elastically stretched sections undergo compression deformation, while compressed sections undergo tensile deformation, resulting in warping. For example, in uniform cross-section castings, uneven cooling causes slower cooling parts to concave and faster cooling parts to convex. Plate castings with slower cooling centers develop tensile stress in the center and compressive stress at the edges, leading to distortion, as shown in Figure 2.
To reduce distortion, measures include preventing or eliminating casting stress by slowing cooling to minimize temperature differences. Increasing the cooling time in the mold after pouring helps reduce distortion. We tested the effect of in-mold cooling time on cylinder head distortion, with results shown in Figure 3. Statistical data and trend analysis indicate that as cooling time延长, distortion decreases, stabilizing after 6 hours. Extending in-mold cooling time effectively reduces distortion, as confirmed by tracking machining results. For specific dimensions, additional process anti-distortion measures may be necessary.
In conclusion, the dimensions of engine block and cylinder head ductile iron castings are highly sensitive to production variations, but effective control can be achieved through the following工艺设计 and measures: (1) Select different shrinkage rates for different dimensions. (2) Design appropriate initial core strength and parting negatives based on process requirements and site conditions. (3) Specify material control requirements for core boxes and molds. (4) Account for the impact of molten metal contraction on dimensional changes. (5) Extend in-mold cooling time to reduce distortion. These approaches ensure that ductile iron castings meet DIN 1686 GTB15 tolerance requirements consistently, supporting high-volume production of precision components.
The consistent dimensional accuracy of ductile iron castings is paramount for mass production, as it directly impacts machining efficiency and product quality. By integrating advanced process controls and material science, we have optimized the manufacturing of ductile iron castings for engine applications. Further research could focus on predictive modeling of shrinkage and distortion using finite element analysis, enhancing the robustness of ductile iron casting processes. The formula for overall dimensional deviation can be expressed as: $$ \delta_{\text{total}} = \sqrt{\delta_{\text{shrinkage}}^2 + \delta_{\text{core}}^2 + \delta_{\text{mold}}^2 + \delta_{\text{melting}}^2} $$ where $\delta$ represents the standard deviation of each factor. This holistic approach ensures that ductile iron castings achieve the desired precision for demanding automotive applications.