In the context of rising national emission standards and the growing demand for lightweight components, ductile iron castings have become increasingly prevalent in engine blocks and cylinder heads due to their superior mechanical properties. A critical factor in transitioning from prototype to mass production of ductile iron castings is the consistency of casting dimensions. Compared to gray iron, ductile iron castings inherently exhibit poorer machinability, leading to heightened customer expectations for dimensional accuracy and stability. This places greater demands on casting process control. Based on our experience with high-volume production using high-pressure molding lines for ductile iron castings, this article analyzes the factors affecting dimensional accuracy in engine block and cylinder head castings and proposes effective control methods.
We specialize in the production of heavy-duty truck engine blocks and cylinder heads utilizing cold-box resin sand cores and green sand high-pressure molding line processes. The annual production volume reaches approximately 100,000 tons, with ductile iron castings comprising about 70% of the output. The material specifications correspond to grades like RuT450 and RuT500, featuring a minimum wall thickness of 4.5 mm. The dimensional tolerances for rough castings adhere to the DIN 1686-1 GTB15 standard, as detailed in Table 1.
| Dimension (mm) | Tolerance (mm) |
|---|---|
| < 18 | ±0.85 |
| 18–30 | ±0.95 |
| 30–50 | ±1.0 |
| 50–80 | ±1.1 |
| 80–120 | ±1.2 |
| 120–180 | ±1.3 |
| 180–250 | ±1.4 |
| 250–315 | ±1.5 |
| 315–400 | ±1.6 |
| 400–500 | ±1.7 |
| 500–630 | ±1.8 |
| 630–800 | ±1.9 |
| 800–1000 | ±2.0 |
Initially, we faced significant dimensional fluctuations in our ductile iron castings, resulting in customer complaints and substantial tooling compensation claims. To address these issues, we implemented comprehensive improvements in process design, core dimension control, and melting practices, ultimately meeting customer requirements for dimensional precision in ductile iron castings.
Casting shrinkage, also referred to as linear shrinkage rate, defines the contraction of a casting from the onset of solidification to room temperature. It is expressed as the percentage difference between the pattern length and the casting length relative to the pattern length. The actual linear shrinkage rate accounts for various influencing factors, including the metal’s inherent shrinkage, the starting temperature of linear contraction, casting geometry, mold type, and gating system design. Initially, lacking specific data for ductile iron castings, we utilized gray iron engine blocks and cylinder heads with similar structures, poured ductile iron molten metal, and measured representative dimensions to identify shrinkage patterns. Partial data from these trials are summarized in Tables 2 and 3.
| Position | Design Value (mm) | Measured Average (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 Average (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 casting shrinkage rate is calculated using the formula:
$$ S = \frac{L_p – L_c}{L_p} \times 100\% $$
where \( S \) is the shrinkage rate, \( L_p \) is the pattern length, and \( L_c \) is the casting length after cooling. Based on the average shrinkage data, we designed the initial process and conducted formal sample production. Full-dimension measurements revealed that shrinkage rates at certain locations did not meet expectations, with significant variations across different positions. After further adjustments and process compensations, we established a standardized design protocol for casting shrinkage in ductile iron engine blocks and cylinder heads, as outlined in Table 4.
| Product | Length Direction Shrinkage (%) | Other Directions Shrinkage (%) | Process Compensation |
|---|---|---|---|
| Block | 1.1 | 1.05 | Separate design for water jacket, cylinder liner, and crank partition |
| Head | 1.2 | 1.15 | Separate design for valve seat ring hole and injector hole |
For complex engine block and cylinder head castings, sand cores are crucial in defining the casting geometry, and their dimensional accuracy directly impacts the final casting dimensions. Our engine block comprises 12 sand cores, and the cylinder head consists of 14 sand cores, all produced via the cold-box resin sand process. Core shooting and assembly are fully automated using robots, achieving an automation rate exceeding 80%. While cold-box core making enhances core dimensional accuracy, the high degree of automation necessitates greater stability in core dimensions. The overall dimensions of core assemblies are influenced by individual core dimensions and the core assembly process, sometimes resulting in compliant individual cores but non-compliant assemblies. Therefore, these factors must be integrally considered during core process design for ductile iron castings.
Cold-box sand cores typically require coating by immersion, and the coating thickness varies significantly with coating type and Baume degree. This thickness can alter core dimensions. During process design, coating thickness must be accounted for to prevent adverse effects on casting dimensions. Coating thickness is measured post-immersion and drying, and generally ranges from 0.2 mm to 0.5 mm, depending on product structure and coating type.
Automated core shooting and assembly demand that sand cores possess sufficient initial strength to resist deformation during handling and clamping. Core strength is determined by sand composition, mix ratio, binder type, and quantity. For instance, cylinder liner cores and frame cores may use standard resins, while water jacket cores and intake/exhaust passage cores require high-strength resins. The initial strength design criteria are specified in Table 5.
| Core Type | Initial Strength (MPa) |
|---|---|
| Main cores, etc. (thick large cores) | ≥0.7 |
| Water jacket cores, etc. (thin-walled cores) | ≥1.2 |
| Intake and exhaust passage cores | ≥1.0 |
Engine blocks and cylinder heads incorporate numerous cores with diverse geometries. To ensure dimensional accuracy of core assemblies, combined cores must have defined fit clearances and positioning references. During process design, the positioning fit clearance is set at 0.15 mm, other fit clearances at 0.3 mm, and the maximum contour fit clearance at 0.5–1 mm.
Core box negative, analogous to mold negative, involves removing a specific value from the core box surface to prevent oversized core dimensions after assembly. This negative is influenced by core size, sand type, core drying method, and core box structure. In practice, factors like core box deformation, clamping force during shooting, and sealing strip usage can cause core dimensions in the clamping direction to exceed design specifications. To mitigate this, a core box negative is incorporated during design and manufacturing, typically set at \( (0.6 \pm 0.1) \) mm, though exact values are adjusted based on production conditions for ductile iron castings.
Overall core assembly dimensions are also affected by core drying parameters, assembly fastening methods, and torque settings, all of which require stringent control.
Core dimensional accuracy is heavily dependent on core box precision, which in turn is influenced by material selection. To balance wear resistance and deformation resistance, we use 4Cr5MoSiV1 for core box bodies, with a surface hardness of 38–40 HRC after machining. Shooting plates and blow plates are preferably made from QT500 cast iron.
Core box machining accuracy is ensured through precision equipment. Sharp corners are processed via EDM, mating surfaces are lapped for fit, and forming surfaces are avoided from polishing and grinding to minimize deviations. When necessary, forming surfaces undergo carburizing (or nitriding), with carburizing layer thickness of 0.5–1.2 mm or nitriding layer thickness of 0.1–0.3 mm. Key technical requirements for core box machining are listed in Table 6.
| Core Box Position | Core Box Body | Surface Roughness | Fit Accuracy |
|---|---|---|---|
| Technical Requirements | ±0.1 mm | Ra 1.6 | ≤0.05 mm |
During operation, core box surfaces and positioning pins/bushings experience wear, and blocked vent channels can lead to incomplete core formation, causing dimensional variations. We have implemented dedicated tooling maintenance protocols: online cleaning every 200–300 cores, offline cleaning every 500–600 cores, and replacement of positioning pins/bushings when wear reaches 0.2 mm. Similar maintenance schedules apply to molding patterns, flasks, and fixtures to ensure consistency in ductile iron castings.
The mold is another critical element in achieving casting dimensional accuracy. Beyond pattern dimensions, mold strength and sand properties significantly affect casting surface finish and dimensions. In our high-pressure molding line, we control mold hardness to ensure plane hardness ≥16 and vertical hardness ≥11 (measured with a PFP mold hardness tester), providing the necessary stiffness to resist casting contraction and expansion. Additionally, we use high-quality composite sands to maintain optimal casting performance. Key molding sand parameters are controlled as follows: permeability: 130–170, green compression strength: 0.12–0.15 MPa, compactability: 30%–34%. Through rigorous control of molding sand and mold hardness, we effectively ensure dimensional precision in ductile iron castings.
The production of ductile iron castings involves a narrow process window, where variations in nodularizer composition, particularly magnesium, significantly impact molten iron contraction and casting dimensions. Over extended production runs, we monitored the effects of different composition blends on dimensional stability, as summarized in Table 7. Our analysis revealed that fluctuations in elements like magnesium markedly influence casting dimensions. Based on this data, we defined a melting process that stabilizes dimensions, implementing strict controls on magnesium and other critical elements.
| Melting Recipe | Dimension 500 mm (mm) | Dimension 105 mm (mm) |
|---|---|---|
| Recipe 1 | 497 | 104.5 |
| Recipe 2 | 498 | 104.8 |
| Recipe 3 | 499 | 105.0 |
| Recipe 4 | 500 | 105.2 |
| Recipe 5 | 501 | 105.5 |
The relationship between magnesium content and dimensional change can be modeled as:
$$ \Delta D = k \cdot \Delta [Mg] $$
where \( \Delta D \) is the dimensional deviation, \( k \) is a proportionality constant, and \( \Delta [Mg] \) is the change in magnesium concentration. This emphasizes the need for precise composition control in ductile iron castings.
Casting deformation is a common issue in engine cylinder heads, arising from casting stresses generated during constrained contraction after solidification. Under stress, elastically stretched regions undergo compression, while elastically compressed regions experience tension, leading to warping: slower-cooling areas concave, and faster-cooling areas convex. Even castings with uniform cross-sections can deform if cooling rates are uneven. For example, in a plate casting, the center cools slower than the edges, inducing tensile stress in the center and compressive stress at the edges, resulting in warping.
To address deformation, we focused on mitigating casting stress by promoting uniform cooling. One approach is extending the cooling time within the mold to reduce temperature gradients. We experimented with varying in-mold cooling times for cylinder heads and observed the effects on deformation, as shown in Table 8.
| Cooling Time (h) | Deformation (mm) |
|---|---|
| 3 | 1.6 |
| 3.5 | 1.4 |
| 4 | 1.2 |
| 4.5 | 1.0 |
| 5 | 0.8 |
| 5.5 | 0.6 |
| 6 | 0.4 |
| 6.5 | 0.2 |
| 7 | 0.1 |
| 8 | 0.05 |
| 10 | 0.02 |
| 12 | 0.01 |
The data indicate that deformation decreases with longer cooling times, stabilizing after approximately 6 hours. Implementing extended in-mold cooling times confirmed reduced deformation in machined castings. For specific dimensions where cooling time alone is insufficient, we apply process anti-deformation measures to compensate.
In summary, the dimensions of engine block and cylinder head castings are highly susceptible to production process variations, but through systematic process design and controls, we can achieve consistent dimensional accuracy in ductile iron castings. Key measures include:
- Selecting appropriate shrinkage rates for different dimensions and directions.
- Designing robust core initial strength and core box negatives based on process requirements and operational conditions.
- Specifying stringent material and machining criteria for core boxes and molds.
- Accounting for molten iron contraction effects on dimensional changes.
- Extending in-mold cooling times to minimize deformation.
These strategies collectively enhance the dimensional stability and quality of ductile iron castings, ensuring they meet rigorous tolerance standards like DIN 1686 GTB15. Continued focus on process optimization is essential for advancing the performance and reliability of ductile iron castings in high-volume applications.