In my extensive experience in the foundry industry, I have witnessed a significant shift towards resin sand casting over the past few decades. This process has been widely adopted by numerous casting enterprises globally, providing immense momentum for the advancement of resin sand casting technology. Compared to conventional green sand casting, resin sand casting offers superior dimensional accuracy, clear casting contours, and dense microstructure in produced castings. Additionally, it reduces molding costs due to the excellent flowability and high strength after molding of resin sand, which allows for a reduction in production equipment and improves the process yield rate, thereby enhancing competitive edge in the industry. The focus of this article is on applying resin sand casting to produce large lathe beds, detailing the optimization of process parameters based on structural analysis.
The lathe bed casting, made from HT250 material with a weight exceeding 4 tons, falls under the category of large castings. Its wall thickness ranges between 25 mm and 35 mm, and it features external concave structures that hinder pattern withdrawal during mold making, necessitating the use of loose pieces or cores. The internal cavity is complex, requiring careful core segmentation and assembly. The most critical area is the guide rail section, which serves as the working part of the lathe and demands high precision, free from defects like gas holes, slag inclusions, or sand holes, while also being prone to deformation, thus requiring special attention in process design. From my perspective, resin sand casting is particularly suitable for such components due to its ability to achieve tight tolerances and minimal defects.
When selecting molding materials for resin sand casting, I prioritize cost-effectiveness and casting quality. The base sand primarily consists of rounded sedimentary silica sand with specific chemical requirements: SiO2 content ≥90%, grain size group of 40/70, clay content ≤0.2%, moisture content between 0.1% and 0.2%, and angularity factor ≤1.3. Prior to use, the sand undergoes scrubbing for clay removal and drying to meet these specifications. The binder is a medium-nitrogen furan resin, paired with p-toluenesulfonic acid as the hardener. The binder addition rate ranges from 0.8% to 1.5% of the resin sand weight. Determining the hardener amount involves balancing hardened strength, hardening speed, and practical production needs. I have found that when the hardener addition is 0.35% to 0.40% of the resin sand, the strength peaks, but for optimal production efficiency, the usable time of the resin sand should not be too long. Typically, the hardener is added at about 40% of the resin quantity to achieve a usable time of 5–8 minutes and a stripping time of 25–40 minutes, with a ratio of usable time to stripping time around 0.35–0.6. To enhance strength, a coupling agent like KH-550 aminosilane is incorporated at 0.2% to 0.5% of the resin weight, added during sand mixing to prevent hydrolysis. The actual sand mix proportion I use is summarized in the table below:
| Component | Percentage or Ratio |
|---|---|
| New Sand | 10% |
| Old Sand | 90% |
| Furan Resin | 0.8%–1.5% of sand weight |
| Hardener (p-toluenesulfonic acid) | 40% of resin weight |
| Coupling Agent (KH-550) | 0.5% of resin weight |
Mixing is performed using continuous or bowl-type mixers that emphasize agitation to ensure homogeneity. This formulation has consistently yielded reliable results in my resin sand casting applications.
In designing the casting process for the lathe bed, I first determine the pouring position and parting lines. The guide rail surface, being the most critical machining area, is placed at the bottom during pouring to minimize defects and ensure dense structure. Two parting lines are established: one at the fillet of the casting to facilitate rounding and place most of the casting in the middle flask, and another at the guide rail section to guarantee quality, with the lower mold housing the guide rail core. Core segmentation is essential for forming internal cavities and hard-to-draft external features. For instance, cores are designated for the guide rail structure, internal cavity, and side concave areas, ensuring each core has sufficient cross-section for strength and ventilation. The core arrangement is illustrated in process diagrams, but here I emphasize that resin sand casting allows for precise core making due to its high strength and stability.
The gating system is a critical aspect of resin sand casting. Given the bed’s height and complex structure, I opt for a step gating system, which introduces molten metal in layers to reduce impact, ensure smooth filling, and position hot metal at the top for better feeding and venting. To mitigate complexities in mold making, refractory ceramic materials are used for the gating channels, improving production efficiency. The design calculations for the step gating system are based on established principles to ensure non-full conditions in the distribution sprue and adequate pressure heads. Key calculations include:
1. Sprue Height: The sprue height should equal the upper mold height to provide sufficient pressure for feeding and sharp edges. This is verified using the minimum residual pressure head formula:
$$ H_m \geq L \tan \alpha $$
where \( H_m \) is the minimum residual pressure head, \( L \) is the distance from the sprue center to the highest and farthest point of the casting, and \( \alpha \) is the pressure angle. For the bed, with \( L = 2736 \, \text{mm} \) and \( \alpha = 7^\circ \), \( H_m \geq 336 \, \text{mm} \), confirming adequacy.
2. Pouring Time and Rise Velocity: The pouring time \( t \) is calculated to prevent cold shuts and excessive turbulence:
$$ t = S_1 \sqrt[3]{G} $$
where \( G \) is the total metal weight in the mold (including gating and risers), and \( S_1 \) is a coefficient based on wall thickness. For the bed, \( G = 4416 \, \text{kg} \) (considering an 85% yield), average wall thickness \( \delta = 30 \, \text{mm} \), and \( S_1 = 1.8 \), giving:
$$ t = 1.8 \sqrt[3]{4416} \approx 87 \, \text{s} $$
The rise velocity \( \nu_L \) is then:
$$ \nu_L = \frac{C}{t} $$
where \( C = 996 \, \text{mm} \) is the casting height in pouring position, resulting in \( \nu_L \approx 11.5 \, \text{mm/s} \), within recommended limits for resin sand casting.
3. Minimum Choke Area: This determines the smallest cross-section in the gating system to control flow:
$$ F_{\text{阻}} = \frac{G}{0.31 \mu_1 t \sqrt{H_1}} $$
where \( F_{\text{阻}} \) is the choke area, \( \mu_1 = 0.76 \) is the flow loss coefficient from pouring cup to choke, and \( H_1 = 25 \, \text{cm} \) is the head distance. Substituting values:
$$ F_{\text{阻}} = \frac{4416}{0.31 \times 0.76 \times 87 \times \sqrt{25}} \approx 43.09 \, \text{cm}^2 $$
4. Distribution Sprue Area: Based on step gating design, the ratio \( F_{\text{阻}} : F_{\text{分直}} = 1 : 1.5 \), so:
$$ F_{\text{分直}} = 1.5 \times 43.09 \approx 64.6 \, \text{cm}^2 $$
5. Total Ingate Area per Layer: For the bottom layer, considering flow dynamics in resin sand casting:
$$ F_{\text{内(底)}} = \frac{\mu_1}{\mu_2} \times \frac{\sqrt{H_1}}{K \sqrt{H_0}} \times F_{\text{阻}} $$
where \( \mu_2 = 0.4 \) for dry sand molds with high resistance, \( K = 0.4 \), and \( H_0 = 400 \, \text{mm} \) is the distance between ingate layers. Thus:
$$ F_{\text{内(底)}} = \frac{0.76}{0.4} \times \frac{\sqrt{25}}{0.4 \times \sqrt{400}} \times 43.09 \approx 102.34 \, \text{cm}^2 $$
To promote layered filling in resin sand casting, upper ingate areas are set at 1–2 times the bottom area; here, I use 1 times, so each of the two upper layers also has \( 102.34 \, \text{cm}^2 \).
6. Runner Area: With \( F_{\text{阻}} : F_{\text{横}} = 1 : 1.3 \):
$$ F_{\text{横}} = 1.3 \times 43.09 \approx 56.02 \, \text{cm}^2 $$
These calculations are summarized in the table below for clarity in resin sand casting applications:
| Parameter | Symbol | Value | Unit |
|---|---|---|---|
| Total Metal Weight | \( G \) | 4416 | kg |
| Pouring Time | \( t \) | 87 | s |
| Rise Velocity | \( \nu_L \) | 11.5 | mm/s |
| Choke Area | \( F_{\text{阻}} \) | 43.09 | cm² |
| Distribution Sprue Area | \( F_{\text{分直}} \) | 64.6 | cm² |
| Bottom Ingate Area | \( F_{\text{内(底)}} \) | 102.34 | cm² |
| Upper Ingate Area (each layer) | \( F_{\text{内(上)}} \) | 102.34 | cm² |
| Runner Area | \( F_{\text{横}} \) | 56.02 | cm² |
Other process parameters in resin sand casting include using Grade I wooden patterns, a shrinkage allowance of 0.8%, machining allowances of 13 mm for sides and 15 mm for guide rails (based on CT13 tolerance and MA(H) class), draft angles of 0°20′ for sidewalls, parting line allowances of 2 mm each for upper and lower molds, and a reverse deformation allowance of 2.5 mm/m due to the step gating system in resin sand casting.
Quality inspection results from implementing this resin sand casting process have been highly satisfactory. The castings meet stringent requirements, as shown in the table below, which compares specifications against actual outcomes. This demonstrates the efficacy of resin sand casting in producing high-integrity components.
| Inspection Item | Technical Requirement | Test Result | Verdict |
|---|---|---|---|
| Surface Roughness | 25–12.5 μm | 18.3 μm | Qualified |
| Dimensional Accuracy | GB6414-86 CT11 | GB6414-86 CT10 | Qualified |
| Brinell Hardness | 170–255 HB | 186 HB | Qualified |
In conclusion, the demand for high-quality castings continues to grow, and resin sand casting has proven to be a robust solution. This process significantly improves casting precision from CT13 in green sand to CT9 in resin sand casting, with surface roughness reaching Ra 12.5 μm. Moreover, resin sand casting simplifies molding operations, reducing skill requirements for operators. However, the process parameters differ from those of green sand casting, necessitating continuous optimization and technical exchange within the industry. From my experience, factors such as sand composition, hardener ratios, and gating design require meticulous attention to fully leverage the benefits of resin sand casting. Future advancements may focus on enhancing resin formulations and automation to further boost efficiency and consistency. Ultimately, resin sand casting offers a reliable pathway to achieving superior castings, particularly for complex and large-scale components like machine tool beds, and its adoption is likely to expand as technology evolves.
Reflecting on my practice, I have observed that the success of resin sand casting hinges on a holistic approach—from material selection to process control. The use of formulas and tables, as presented here, aids in standardizing procedures and ensuring repeatability. For instance, the step gating calculations prevent common issues like mistruns or turbulence, while proper sand mixing guarantees adequate strength and workability. As the foundry sector progresses, sharing such insights will be crucial for elevating casting quality globally. In summary, resin sand casting is not just a method but a transformative technology that aligns with modern manufacturing demands, and I am confident that its application will continue to drive innovation in the casting industry.