In the context of increasing environmental regulations, the demand for low-energy consumption, low-emission, and low-pollution mechanical products has driven innovations in engine design. As a result, engine blocks are becoming more integrated, incorporating features like water jackets, oil and gas passages, and gear chambers into a single complex structure. This complexity poses significant challenges for casting processes, particularly in core assembly and venting. In our work on a new-generation four-cylinder dry-type engine block, initial trials using traditional casting methods resulted in a 100% defect rate due to blowholes on the upper surface, rendering the castings unsuitable for machining. This article details our first-person perspective on analyzing the issues, optimizing the process, and achieving a qualification rate exceeding 95% for ductile iron castings. We emphasize the importance of reducing core gas generation and enhancing venting in ductile iron castings, supported by tables and formulas to summarize key parameters.
The engine block in question is made of HT300 gray iron, with a weight of 190 kg and dimensions of 516 mm × 551 mm × 432 mm. It features uniform wall thickness of 6 mm and includes integrated structures such as water jackets, oil and gas passages, and a gear chamber. The material requires a tensile strength greater than 250 MPa at specific locations, including the crankshaft bearing areas and bolt bosses. The block’s design involves narrow internal channels and extended sections like the gear chamber, which is prone to mist runs and gas defects. These characteristics make it essential to employ advanced casting techniques to ensure integrity and performance in ductile iron castings.
Our initial process utilized a horizontal molding line with green sand, employing a one-casting-per-mold approach and a flat pouring orientation. The gear chamber was positioned in the drag to prevent mist runs. The core assembly consisted of 11 segments: #1 to #6 were side core plates made with cold-box process, #7 was a header core, #8 was a combined water jacket and passage core using shell process, #9 and #11 were oil and gas passage cores with a frame design to prevent deformation, and #10 was a tappet chamber core produced with furan resin sand for cost-effectiveness. Coating and drying processes were carefully managed to ensure core integrity. The gating system was semi-choked, with a ratio of sprue to runner to ingate areas set at ΣFsprue : ΣFrunner : ΣFingate = 1.1 : 1.4 : 1, designed to avoid critical machining surfaces. Venting included overflow and round risers on the top and ends, with a total vent area 1.7 times the ingate area. Melting was conducted in a 10-ton medium-frequency induction furnace, with a chemical composition controlled within: C 3.3–3.4%, Si 1.8–1.9%, S 0.08–0.09%, Mn 0.8–0.9%, P ≤ 0.05%, Cu 0.5–0.6%, Cr 0.3–0.35%, Mo 0.3–0.4%, Sn 0.045–0.05%, and a pouring temperature of 1,400–1,430 °C.
During the first trial of five castings, poured in 35–40 seconds, all exhibited severe blowhole defects on the upper surface, particularly on bosses and reinforcing ribs. The round risers were incomplete or hollow, indicating gas entrapment. Dissection revealed isolated subsurface blowholes with smooth, oxidized surfaces, characteristic of invasive gas defects. This highlighted the need for improved venting and gas management in ductile iron castings.
Analysis showed that the blowholes were primarily invasive, caused by gases from resin-bonded cores that could not escape quickly enough. Compared to traditional blocks, the new design added oil and gas passage cores, increasing gas generation by 1.08 times, while venting was insufficient. This is common in complex ductile iron castings where core density and venting design are critical. The gas generation volume can be modeled using the formula: $$ G = m \times g $$ where \( G \) is the total gas volume, \( m \) is the core mass, and \( g \) is the gas evolution per unit mass. For the initial cores, \( g \) was as high as 15.6 mL/g, contributing to defect formation.
To address these issues, we implemented several optimizations focused on enhancing venting and reducing gas generation in ductile iron castings. First, we changed the sand for the oil and gas passage cores from 50/100 mesh coated sand to 40/70 mesh zircon sand, which has better sphericity and lower binder requirements. This reduced the gas evolution from 15.6 mL/g to 10.2 mL/g, as the spherical particles allow faster gas escape due to larger inter-particle spaces. The relationship can be expressed as: $$ g_{\text{new}} = g_{\text{old}} \times \frac{\rho_{\text{new}}}{\rho_{\text{old}}} $$ where \( \rho \) represents sand density, illustrating how material changes impact gas behavior in ductile iron castings.
Second, we redesigned the core venting system. For the oil passage cores, we introduced dedicated core prints connected to vent pins, as shown in the image, to create additional pathways for gas escape. We also drilled holes at the junctions between main cores and oil passage cores, ensuring continuous venting channels. During molding, asbestos pads were added at these junctions to prevent metal penetration while maintaining venting. Additionally, we increased the number of vent holes between the water jacket core and side cores, accelerating gas removal. The vent area ratio was recalculated to ensure adequacy, with the new total vent area being 1.48 times larger than before, crucial for ductile iron castings where rapid gas expulsion is needed.
Third, we optimized the gating system to improve filling dynamics and reduce gas entrapment. We increased the sprue and runner areas, changing the ratio to ΣFsprue : ΣFrunner : ΣFingate = 1.5 : 2 : 1, which ensured a filled runner for better slag trapping and higher upper mold temperatures, reducing blowhole tendency. The modified Bernoulli equation for fluid flow in gating systems applies: $$ v = \sqrt{\frac{2gH}{1 + \frac{\Sigma F_{\text{loss}}}{\Sigma F_{\text{ingate}}}}} $$ where \( v \) is flow velocity, \( g \) is gravity, \( H \) is head height, and \( \Sigma F_{\text{loss}} \) accounts for losses, highlighting how area changes affect flow in ductile iron castings. We also enlarged the filter area by using 70 mm × 70 mm × 16 mm ceramic filters placed vertically instead of 60 mm × 60 mm × 16 mm horizontal ones, increasing the flow area by a factor of 1.44 and preventing clogging that could slow filling.
Furthermore, in collaboration with product designers, we added two 20 mm wide reinforcing ribs on the oil passage arc, which allowed for the design of additional vent risers, increasing the cavity vent area. This structural modification was validated through UG 3D simulations to ensure no interference with assembly, demonstrating how integrated design and process improvements benefit ductile iron castings.
The following table summarizes the key changes and their impacts on process parameters for ductile iron castings:
| Parameter | Initial Value | Optimized Value | Impact |
|---|---|---|---|
| Core Gas Evolution (mL/g) | 15.6 | 10.2 | Reduced gas generation |
| Vent Area Ratio (vs. Ingate) | 1.7 | 2.52 (1.48× increase) | Faster gas escape |
| Gating Ratio (Sprue:Runner:Ingate) | 1.1:1.4:1 | 1.5:2:1 | Improved filling and temperature |
| Filter Area (mm²) | 7,200 (2×60×60) | 9,800 (2×70×70) | Reduced clogging, faster pour |
| Pouring Time (s) | 35–40 | 25–30 | Shorter fill time, less gas entrapment |
After implementing these optimizations, we conducted three batches totaling 45 castings. The results showed a significant reduction in blowhole defects, with a qualification rate exceeding 95%. Machining of the upper surface bosses confirmed the absence of blowholes, validating the effectiveness of our approach for ductile iron castings. The success underscores the importance of holistic process design, where material selection, venting, and gating are tailored to the specific challenges of complex geometries in ductile iron castings.
In conclusion, the new-generation engine block casting process demonstrates that reducing core gas generation and ensuring efficient venting are critical for minimizing blowhole defects in ductile iron castings. Key measures include using low-gas evolution sands, enhancing core venting with dedicated channels, optimizing gating systems for rapid filling, and incorporating structural modifications to facilitate venting. These principles are widely applicable to other complex ductile iron castings, contributing to higher yields and better performance in demanding applications. Future work could focus on further refining venting designs and exploring real-time monitoring to adapt processes dynamically for ductile iron castings.