In modern casting production, lost foam casting (LFC) and expanded polystyrene casting (EPC) have emerged as highly efficient methods for manufacturing complex and large-scale components. These techniques are particularly advantageous for single-piece or small-batch production due to their ability to produce castings with smooth surfaces, precise dimensions, high internal density, and no flash or burrs. This paper explores the application of lost foam casting in the production of a large tool set, focusing on process design, simulation, and optimization. We will delve into the CAD modeling, gating system design, riser optimization, and solidification simulation using CAE software, emphasizing the benefits of lost foam casting and EPC in reducing costs and shortening production cycles.
The tool set, designed to support molds, features a unique “U”-shaped structure with side grooves and thick sections. Its overall dimensions are 1920 mm × 1060 mm × 1500 mm, classifying it as a large casting. The material selected is ZG35CrMo cast steel, which requires careful control of solidification to prevent defects like shrinkage porosity and cavities. Given the production scale—single-piece and small-batch—we adopted a lost foam casting approach that combines conventional sand molding with EPC principles. This hybrid method uses STMMA as the pattern material, with a facing layer of alkyd-oil urethane resin sand and dry sand for the bulk filling, optimizing cost and performance. The coating for the cast steel component consists of 70% bauxite and 30% zircon flour, applied to a thickness of approximately 4 mm.
In the initial phase, we performed CAD modeling using UG software to create a precise 3D representation of the tool set. This involved establishing reference planes, defining positional relationships based on drawings, and employing commands like extrusion, Boolean subtraction, and edge blending to generate the solid model. The CAD model served as the foundation for subsequent casting process design and simulation, ensuring accuracy in geometry and facilitating the identification of potential issues. The integration of CAD with lost foam casting allows for rapid prototyping and design validation, which is critical for EPC applications where pattern accuracy directly affects casting quality.
The casting process design for lost foam casting focused on the gating and riser systems to ensure proper metal flow and feeding. We selected an open, middle-pouring gating system with ceramic tubes for connectivity, as this configuration minimizes turbulence and erosion in EPC. The gating system dimensions were calculated using the choke section method, which determines the cross-sectional areas based on the casting’s weight and material properties. For ZG35CrMo, the pouring temperature was set between 1570°C and 1600°C, with a pouring time of 110–130 seconds. The gating components included a sprue with a diameter of 100 mm and height of 1450 mm, a runner of 80 mm diameter and 1560 mm length, and ingates of 50 mm diameter and 150 mm length. To address solidification shrinkage, we initially designed four cylindrical risers (numbered 1# to 4#) with diameters of 300 mm and heights of 500 mm, placed strategically along the “U”-shaped body. Insulating riser sleeves were used to prolong solidification, and chills were incorporated to promote directional solidification. Additionally, a rib was added to the pattern to prevent deformation during cooling, a common consideration in lost foam casting for large structures.
The design of the gating system in lost foam casting can be mathematically expressed using fluid dynamics principles. The flow rate $Q$ through the gating system is given by Bernoulli’s equation, modified for casting applications: $$Q = A \cdot v = A \cdot \sqrt{2gH}$$ where $A$ is the cross-sectional area, $v$ is the flow velocity, $g$ is gravitational acceleration, and $H$ is the metallostatic head. For EPC, the foam decomposition introduces additional complexities, and the effective head can be adjusted as: $$H_{\text{eff}} = H – \Delta P / \rho g$$ where $\Delta P$ is the pressure drop due to foam degradation and $\rho$ is the metal density. The choke area $A_c$ is critical and can be derived from the continuity equation: $$A_c = \frac{W}{\rho \cdot t \cdot v_c}$$ where $W$ is the casting weight, $t$ is the pouring time, and $v_c$ is the critical velocity at the choke. This ensures a controlled fill in lost foam casting processes.
| Riser Number | Diameter (mm) | Height (mm) | Volume (cm³) |
|---|---|---|---|
| 1# | 300 | 500 | 35,343 |
| 2# | 300 | 500 | 35,343 |
| 3# | 300 | 500 | 35,343 |
| 4# | 300 | 500 | 35,343 |
Solidification simulation was conducted using Huacast CAE software to predict defect formation in the lost foam casting process. The simulation input parameters included the material properties of ZG35CrMo, such as thermal conductivity, specific heat, and latent heat of fusion. The governing equation for heat transfer during solidification is the Fourier heat equation: $$\frac{\partial T}{\partial t} = \alpha \nabla^2 T$$ where $T$ is temperature, $t$ is time, and $\alpha$ is thermal diffusivity. For EPC, the foam’s endothermic reaction adds a source term $S$: $$\rho c_p \frac{\partial T}{\partial t} = \nabla \cdot (k \nabla T) + S$$ where $c_p$ is specific heat capacity, $k$ is thermal conductivity, and $S$ represents heat absorption from foam decomposition. The initial simulation revealed significant shrinkage cavities and porosity in the tool set, indicating inadequate riser performance. The defects were concentrated in thick sections, highlighting the need for optimization in the lost foam casting setup.
Based on the simulation results, we embarked on a step-by-step optimization of the risers to improve feeding in the lost foam casting process. First, for riser 1#, we increased the height from 500 mm to 600 mm while keeping the diameter at 300 mm. This change enhanced the metallostatic pressure and extended the solidification time, as described by 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 mold constant, and $n$ is an exponent (typically 2 for sand castings). The modified riser 1# showed improved feeding, eliminating defects in its zone. However, risers 2#, 3#, and 4# still exhibited issues. For riser 2#, we increased the diameter to 400 mm and height to 700 mm, which provided better compensation. The solidification simulation confirmed reduced defects, but risers 3# and 4# required further adjustments. After iterative changes, we finalized risers 3# and 4# with diameters of 500 mm and heights of 800 mm, achieving complete defect elimination. This optimization underscores the importance of riser design in lost foam casting for large castings.
| Riser Number | Diameter (mm) | Height (mm) | Aspect Ratio (H/D) | Solidification Time (s) |
|---|---|---|---|---|
| 1# | 300 | 600 | 2.0 | 850 |
| 2# | 400 | 700 | 1.75 | 920 |
| 3# | 500 | 800 | 1.6 | 1100 |
| 4# | 500 | 800 | 1.6 | 1100 |
The feeding efficiency of risers in lost foam casting can be quantified using the modulus method, where the modulus $M$ is defined as the volume-to-surface area ratio: $$M = \frac{V}{A}$$ For effective feeding, the riser modulus $M_r$ should be greater than the casting modulus $M_c$ by a factor, often 1.2 times: $$M_r \geq 1.2 M_c$$ In EPC, the foam pattern affects heat transfer, so the effective modulus may require correction. The optimized risers satisfied this criterion, with $M_r$ values calculated as follows for riser 3#: $$M_{r3} = \frac{\pi (500/2)^2 \cdot 800}{2\pi (500/2)^2 + \pi \cdot 500 \cdot 800} = \frac{157,079,632.67}{1,256,637.06 + 1,256,637.06} \approx 62.5 \text{ mm}$$ compared to the casting modulus of approximately 52 mm in thick sections. This ensured progressive solidification toward the risers, minimizing shrinkage in the lost foam casting process.
In conclusion, the application of lost foam casting and EPC for the large tool set demonstrated significant advantages in terms of cost reduction, shorter lead times, and improved efficiency. Through comprehensive CAD modeling, rigorous process design, and CAE-based optimization, we achieved a defect-free casting that meets technical requirements. The iterative optimization of riser parameters, guided by solidification simulation, highlights the critical role of computational tools in modern lost foam casting. This approach is highly suitable for single-piece and small-batch production, offering a competitive alternative to traditional sand casting methods. Future work could explore the integration of advanced materials and real-time monitoring in EPC to further enhance the reliability and applicability of lost foam casting in industrial settings.
Overall, lost foam casting and EPC provide a robust framework for manufacturing complex geometries with high integrity. The use of mathematical models and simulations enables precise control over the process variables, ensuring quality outcomes. As the demand for customized large castings grows, the adoption of lost foam casting is expected to expand, driven by its economic and technical benefits. This case study serves as a testament to the effectiveness of lost foam casting in addressing the challenges of modern foundry practices.