In recent decades, the automotive industry in my country has experienced rapid growth and become a vital pillar of the national economy. However, challenges such as irrational industrial structure, low technical level, and weak independent R&D capability persist. A key bottleneck limiting independent innovation is insufficient rapid manufacturing ability. Critical components like engine cylinder heads, cylinder blocks, and intake/exhaust manifolds are complex thin-walled box or shell parts with intricate internal and external shapes, multiple curved surfaces, many isolated hot spots inside, and high casting precision requirements, making casting extremely difficult. Traditional development methods using wooden or metal patterns for trial production and modification lead to long development cycles, high costs, and high risk of mold scrap due to unsuccessful engine development. The combination of stereolithography (SLA) rapid prototyping technology with conventional sand casting—forming the SLA-based rapid sand casting technique—typically uses SLA prototypes to produce patterns and core boxes. After assembly of SLA prototype molds and sand mold preparation using traditional sand casting processes, castings are produced. This technology offers low cost and short manufacturing cycles, making it suitable for new product development in the automotive industry.
Here I focus on the rapid casting process and mold design for a specific engine cylinder head. Finally, I use ProCAST software for casting process analysis to achieve new product development.
1 Rapid Sand Casting Process Design
Using the SLA-based rapid sand casting technology, the rapid casting flow for the cylinder head is as follows: establish a 3D model of the casting → complete the CAD model of the mold and casting process design → use casting simulation software to verify the designed casting process → manufacture the SLA pattern using a stereolithography apparatus → use the SLA prototype equipment to produce the SLA pattern and core box → assemble the SLA prototype mold → make the sand mold and perform casting. The CAD model of the mold can be established using a casting process CAD system, which includes the design of casting process parameters, gating system, and feeding system. The molding material can be phenolic urethane resin self-hardening sand based on silica sand, which offers easy compaction, high strength, high precision, and good collapsibility.
1.1 3D Model of the Cylinder Head and Casting Characteristics
The engine cylinder head is made of ZL105 aluminum alloy. During casting, precise dimensional and positional accuracy between cavities (such as intake and exhaust ports) must be guaranteed, requiring precise core design and core positioning. The complex internal structure demands careful consideration of core placement and stability.
1.2 Casting Process Design
Casting process design mainly includes core design and casting parting, gating system design, riser design, exhaust system design, and casting process parameters (machining allowance, draft angle, linear shrinkage rate, minimum cast hole, etc.). The principles for casting parting include: (1) ensure smooth core production and removal; (2) minimize the number of parting surfaces to maintain structural and dimensional accuracy; (3) use flat parting surfaces as much as possible to reduce deformation errors from curved parting lines. Based on the cylinder head structure, the cores can be divided into: lower left oil gallery core, lower right oil gallery core, water jacket core, upper oil gallery core, intake port core, and exhaust port core.
Each core has a unique shape and requires high strength. All cores use the shell core process. The core surface is coated with a water-alcohol-based coating to prevent sand inclusion defects during mold filling. Slender sections of cores are reinforced with core rods to ensure strength.
| Core Type | Function | Key Feature |
|---|---|---|
| Lower left oil gallery core | Forms left oil passage | Slender, requires reinforcement |
| Lower right oil gallery core | Forms right oil passage | Slender, requires reinforcement |
| Water jacket core | Forms water cooling cavity | Complex geometry |
| Upper oil gallery core | Forms upper oil passage | Slender, requires reinforcement |
| Intake port core | Forms intake port | Curved shape |
| Exhaust port core | Forms exhaust port | Curved shape |
For casting molding, three gating schemes were considered: single-side top gating, symmetrical top gating, and single-side bottom gating. The advantages and disadvantages of each scheme are summarized in the following table.
| Scheme | Gating Type | Advantages | Disadvantages |
|---|---|---|---|
| 1 | Single-side top gating | Ensures smooth mold filling; top ingates facilitate directional solidification and feeding; simple mold construction. | High impact force, unstable filling, splashing, oxidation and air entrapment in the cavity; prone to sand inclusion, blowholes, and inclusions. |
| 2 | Symmetrical top gating | More uniform inlets; relatively stable liquid level rise. | High impact force; metal front converges in the middle, causing inclusions, turbulence, and gas entrapment. |
| 3 | Single-side bottom gating | Avoids direct impact on the sand mold; relatively gentle liquid level rise. | May suffer from insufficient static pressure, leading to misruns. |
After evaluation, I adopted the single-side bottom gating system. The metal enters from the bottom, avoiding direct impact on the sand mold, and the liquid level rises smoothly. Since the material is ZL105 aluminum alloy, an open gating system can be used, characterized by the sprue cross-section area being smaller than the runner area and the runner area smaller than the ingate area, i.e.,
$$A_{\text{sprue}} < A_{\text{runner}} < A_{\text{ingate}}$$
The choke section is at the sprue. After pouring begins, the mold filling is gentle, reducing erosion on the cavity walls. Using the cross-section ratio design method, I calculated the runner and sprue cross-sectional areas. Based on experience, I set the predetermined ratio
$$A_{\text{sprue}} : A_{\text{runner}} : A_{\text{ingate}} = 1:2:4$$
I used flat ingates, distributed multiple ingates to regulate temperature differences and solidification sequence, preventing slag absorption. The runner cross-section was trapezoidal to minimize heat loss.
For the feeding system, I employed the modulus method. According to Chvorinov’s formula, the solidification time \( T_c \) of the casting and \( T_r \) of the riser are:
$$T = k \left( \frac{V}{A} \right)^2$$
where \( V \) is volume, \( A \) is surface area, and \( k \) is a constant. The feeding condition requires \( T_r \ge T_c \). Based on the casting’s feeding geometry, I selected standard waist-shaped top open risers located in the cope. Two types of risers were designed according to feeding location, shape, and volume.
| Riser Type | Shape | Location | Purpose |
|---|---|---|---|
| Riser A | Waist-shaped top open | Over thick sections | Feed primary hot spots |
| Riser B | Waist-shaped top open | Over secondary hot spots | Supplement feeding |
The casting process parameters such as machining allowance, draft angle, linear shrinkage rate, and minimum cast hole were determined according to relevant standards in the casting process handbook. However, the draft angle and shrinkage rate were adjusted for SLA characteristics: SLA resin patterns are prone to deformation, brittleness, and breakage, so a larger draft angle was used. The photopolymer resin shrinks during curing, so the linear shrinkage rate for the casting was also increased accordingly.
The assembly of the rapid sand casting mold for the engine cylinder head is shown in an exploded view (not illustrated). After completing the mold design, I prepared the molding material (phenolic urethane resin self-hardening sand: silica sand, benzyl ether phenolic resin, and polyisocyanate; the mass fraction of benzyl ether phenolic resin is 0.8%–1% of the sand, and the mass ratio of resin to hardener is 1:1). Using the SLA prototype as a pattern, manual molding was performed, and the casting was poured.
2 ProCAST Simulation Analysis and Production Verification
I used ProCAST software to simulate the mold. The simulation involved mesh generation, parameter settings, and analysis. In the Viewcast module, I examined the filling and solidification processes and analyzed casting defects to assess the rationality of the casting process design. The locations and porosity levels of shrinkage cavities and shrinkage porosity under the filling conditions are displayed in the simulation results. The dark regions indicate areas with shrinkage defects, and the color scale corresponds to the volume fraction of shrinkage. The simulation showed that only uniformly dispersed shrinkage cavities and shrinkage porosity existed inside the casting, with overall porosity below 10%, indicating good casting quality.
| Defect Type | Location | Porosity Level (%) |
|---|---|---|
| Shrinkage cavity | Distributed in thick sections | <5 |
| Shrinkage porosity | Scattered throughout | <10 |
Based on the casting process design and numerical simulation conclusions, I completed the procedures of mold making, sand finishing, cleaning, coating, and pouring to obtain the engine cylinder head casting. The actual casting shows clear surface contours without visible defects. Cross-sections at locations prone to casting defects were examined under a metallographic microscope, revealing a dense microstructure with fine grains and no significant casting defects. Production practice demonstrates that SLA-based rapid sand casting offers short production cycles and high casting precision, making it suitable for rapid new product development.
The occurrence of sand casting defects such as shrinkage and porosity was minimized through careful design and simulation. I emphasize that understanding the formation mechanisms of sand casting defects is critical to avoiding them in complex castings. The simulation predicted that sand casting defects would be uniformly distributed and insignificant. The final product confirmed that sand casting defects were well controlled, with porosity levels under 10%. The combination of SLA rapid prototyping and simulation drastically reduces the risk of severe sand casting defects in the early development stage.
Below is a reference image illustrating typical sand casting defects for context.
3 Conclusion
(1) The casting process and mold design for the engine cylinder head are reasonable. The casting contains only uniformly dispersed shrinkage cavities and shrinkage porosity, with overall porosity below 10%, resulting in good casting quality.
(2) SLA-based rapid sand casting has the advantages of short production cycles and high casting precision, making it suitable for rapid manufacturing of new products. By integrating numerical simulation, sand casting defects can be predicted and mitigated effectively, further enhancing the reliability of the process.