In recent years, the development of the machine tool manufacturing industry has led to increasing market demand for large-scale machine tools. As a researcher in this field, I have focused on studying the lost foam casting process, specifically for the bed of a large CNC machine, designated as CTW6180. This bed features a complex structure and requires high surface hardness on the guideways. Based on design calculations, the net weight of the bed ranges from approximately 39.0 to 40.5 metric tons. Given its large dimensions and heavy weight, it is essential to develop a rational and economical lost foam casting solution. Lost foam casting, also known as EPC (Expanded Polystyrene Casting), offers significant advantages for such applications by enabling the production of intricate shapes with reduced machining requirements. In this article, I will detail the entire process, from pattern making to final quality inspection, emphasizing the use of lost foam casting and EPC techniques to achieve optimal results.
The pattern material selected for this study is expanded polystyrene (EPS), which is commonly used in lost foam casting due to its lightweight and easy formability. The EPS foam boards had a density of 18.7 kg/m³ and a compressive strength of 0.13 MPa. These boards were manually cut and bonded together to create a foam pattern weighing approximately 120 kg. The use of EPS in lost foam casting ensures that the pattern vaporizes upon contact with molten metal, minimizing residue and facilitating a smooth casting process. The structural design of the CTW6180 bed includes long and wide dimensions, with accuracy requirements meeting CT7-8 grade standards. The guideways, consisting of a vee rail and a flat rail, have substantial thicknesses with thermal sections of approximately 180 mm and 200 mm, respectively. The material specified is HT300 cast iron, with guideway surface hardness required to be (185 ± 10) HBS, and defects such as shrinkage cavities and sand inclusions must be avoided in these areas. This highlights the importance of precise control in lost foam casting and EPC methods to meet these stringent requirements.
Moving to the casting process, I designed the allowances for various parts of the machine tool bed to ensure proper machining later. The bed was oriented with the guideways facing downward. The machining allowances were set as follows: 20 mm for the upper part, 15 mm for the sides, 20 mm for the guideways, and 20 mm for the lower section. A casting shrinkage rate of 1% was applied, which is typical for cast iron in lost foam casting. This can be expressed using the formula for linear shrinkage: $$ L_f = L_p \times (1 – s) $$ where \( L_f \) is the final dimension after shrinkage, \( L_p \) is the pattern dimension, and \( s \) is the shrinkage rate (0.01 in this case). For the CTW6180 bed, the initial pattern dimensions were adjusted accordingly to compensate for this shrinkage, ensuring the final cast part meets design specifications in lost foam casting applications.
For molding, I opted for pit molding due to the limited lifting capacity in the workshop. Pit molding is a practical approach in lost foam casting for large castings, as it provides stability and reduces handling risks. The process involved several steps: First, a pit was excavated, and equipment for locking the mold was placed at the bottom. Then, a 150 mm layer of crushed coke was laid down, with nylon vent ropes of 12 mm diameter installed every 1–2 meters to serve as exhaust channels leading to the pit edge. Pre-made segmented sand cores were positioned accurately, along with chill plates, before proceeding with molding. The molding material used was resin sand with a grain size of 20 mesh. The sand thicknesses were carefully controlled: 250 mm for the upper part, 300 mm for the sides, and 300 mm for the bottom. This configuration ensures adequate support and venting in the lost foam casting process, preventing issues like mold collapse or gas entrapment.
The gating and riser system is crucial in lost foam casting for such a massive and thick-walled casting. I designed a bottom-gating system with four sprue channels to ensure smooth metal flow and uniform heat distribution. The specific dimensions of the gating system are summarized in the table below. The sprues were made of refractory ceramic tubes to withstand high temperatures, and they were inclined at an angle of 12° to 14° within the sand mold to prevent metal “backfiring” during pouring. The cross-sectional area ratio of the gating system was set as \( F_{\text{sprue}} : F_{\text{ingate}} : F_{\text{runner}} = 1 : 2.5 : 2 \), which promotes laminar flow and reduces turbulence in lost foam casting. The pouring basins were positioned at the four corners of the bed casting, with capacities of 20 t, 15 t, 10 t, and 10 t, respectively, to facilitate coordinated pouring.
| Group | Sprue (Number and Diameter, mm) | Ingate (Dimensions, mm) | Runner (Dimensions, mm) |
|---|---|---|---|
| 1 | 2, Ø100 | 20 × 70 | 120 × 120 |
| 2 | 2, Ø70 | 20 × 70 | 80 × 80 |
| 3 | 2, Ø70 | 20 × 70 | 80 × 80 |
| 4 | 1, Ø80 | 20 × 70 | 70 × 70 |
For riser design, I used blind risers with dimensions of 80 mm × 40 mm × 160 mm, uniformly distributed in 30 locations to feed the casting and compensate for solidification shrinkage. Additionally, 12 vent holes and barrier-type vents were evenly placed to allow gases to escape during the lost foam casting process. To enhance the surface hardness of the guideways and address the significant thickness variation between thin sections and thermal hotspots, I incorporated external chill plates with a thickness of 40 mm. The use of chills in lost foam casting promotes directional solidification and refines the microstructure, which is critical for achieving the desired mechanical properties. The heat transfer effect of chills can be modeled using Fourier’s law: $$ q = -k \frac{dT}{dx} $$ where \( q \) is the heat flux, \( k \) is the thermal conductivity, and \( \frac{dT}{dx} \) is the temperature gradient. By optimizing this, the lost foam casting process ensures minimal defects in critical areas.
The molten metal requirements were calculated based on the EPS pattern weight of 120 kg. Given that the density of molten iron is approximately 430 times that of EPS, the total iron needed was: $$ m_{\text{iron}} = m_{\text{EPS}} \times 430 = 120 \, \text{kg} \times 430 = 51,600 \, \text{kg} = 51 \, \text{t} $$ This substantial amount underscores the importance of efficient melting and pouring in lost foam casting. For melting, I employed three cold-blast cupola furnaces with melting rates of 8 t/h, 10 t/h, and 5 t/h, respectively. The tapping temperature was maintained at \( 1,440 \pm 10 \, ^\circ\text{C} \), and the total melting time was about 3 hours. The charge composition and proportions were carefully controlled, as detailed in the table below. The charge mix consisted of 40% pig iron, 30% steel scrap, and 30% returns, with 65% Si-Fe alloy used as an inoculant at the furnace front to improve graphite formation in the cast iron. This approach is vital in lost foam casting to ensure consistent metal quality and performance.
| Material | C | Mn | Si | S | P |
|---|---|---|---|---|---|
| Pig Iron (Tangshan) | 4.3 | 0.5 | 1.8 | 0.03 | 0.07 |
| Pig Iron (Zibo) | 4.1 | 0.2 | 1.2 | 0.02 | 0.05 |
| Steel Scrap | 0.14 | 0.5 | 0.2 | 0.03 | 0.01 |
The pouring process was meticulously planned for the lost foam casting operation. Four ladles were used, with capacities as mentioned earlier. After 90 minutes of simultaneous melting in the three cupolas, the first ladle (20 t) was filled and held for temperature stabilization. The subsequent ladles were filled in sequence, with temperatures measured at \( 1,340–1,350 \, ^\circ\text{C} \) for the first ladle and \( 1,370–1,380 \, ^\circ\text{C} \) for the others. Pouring commenced with the first ladle emptied in 4 minutes, followed by the second and third ladles over 6 minutes, and the fourth ladle last. The entire pouring process took 15 minutes and proceeded smoothly without any incidents of metal backfiring, demonstrating the effectiveness of the lost foam casting setup. The pouring rate can be analyzed using the equation: $$ Q = A \cdot v $$ where \( Q \) is the flow rate, \( A \) is the cross-sectional area of the sprue, and \( v \) is the velocity of the metal, which was controlled to ensure complete filling and minimal turbulence in the EPC process.
After casting, I conducted thorough quality inspections on the bed. The dimensional accuracy and machining allowances met the drawing specifications, and no defects such as shrinkage cavities or sand inclusions were found on the guideways. The casting showed no significant distortion, indicating proper control during solidification in lost foam casting. Chemical composition analysis of the cast part revealed the following results, which align with HT300 requirements. The microstructure primarily consisted of pearlite, contributing to the desired mechanical properties. Tensile tests on samples yielded a strength of 330 MPa, exceeding the typical values for HT300. Hardness measurements at four points on the guideways averaged \( 190 \pm 10 \) HBS, satisfying the specified range and ensuring good machinability. Although some wrinkles and carbon/slag accumulation occurred on the sides and base of the bed, these did not affect the functionality, highlighting the robustness of the lost foam casting and EPC methods.
| C | Mn | Si | S | P | Cu | Cr |
|---|---|---|---|---|---|---|
| 3.20 | 0.97 | 1.55 | 0.07 | 0.05 | 0.18 | 0.22 |
In conclusion, the lost foam casting process, particularly EPC, proved highly effective for producing large CNC machine tool beds. The bottom-gating system ensured stable metal flow and even heat distribution, reducing the risk of deformation. Pit molding with adequate venting and sand thicknesses prevented mold failure issues like “bursting” or run-outs. The integration of chills and optimized risering contributed to the high surface hardness and soundness of the guideways. Overall, this study demonstrates that lost foam casting is a viable and economical method for heavy-section castings, with potential for further refinement in future applications. The success of this EPC approach underscores its value in advancing manufacturing capabilities for large-scale components.