In the field of machine tool casting, components are typically large in size and demand high standards of appearance and performance. Traditionally, most machine tool castings in our country have been produced using wood pattern processes. This conventional sand casting method is mature and yields stable quality, offering advantages such as ease of machining, low cost for batch production, and light weight for easy handling. However, the wood pattern process has inherent drawbacks, including high costs for single-piece production, low dimensional accuracy, and susceptibility to deformation under temperature and humidity variations.
With the continuous deepening of research and exploration into the application of 3D printing in foundry technology, a range of production methods suitable for our national conditions have emerged. Many large and medium-sized enterprises have seized the opportunity to develop along a path that combines basic theory with their specific circumstances, achieving significant advancements in 3D printing for casting. As 3D printing sand casting technology, auxiliary materials, and supporting equipment continue to improve, the application has evolved from small castings to gradually include medium and large castings, such as machine tool components. The advantages of applying the 3D printing sand casting process to machine tool castings are substantial.
The primary benefits include: shortening the casting production cycle by eliminating the time and cost of creating sand molds from patterns, thereby reducing new product development lead times and costs, and securing a competitive edge in new markets. It enhances casting quality and production efficiency by allowing the consolidation of small, separate sand cores into a single printed unit, which avoids cumulative errors from manual core making, molding, and assembly. This improves the dimensional accuracy of the sand mold and prevents casting defects due to dimensional mismatches, while also simplifying some assembly steps. The process offers high design flexibility, cost savings, and lower manufacturing complexity. Using digital files for 3D printing allows for quick and flexible design modifications, and it enables the printing of complex cavity structures that are difficult or impossible to achieve with manual molding, thus reducing production difficulty. Furthermore, it aligns with principles of human-centric, green, and intelligent manufacturing. 3D printing significantly improves the workshop environment, reduces operator labor intensity, replaces manual core making and molding to save labor costs, and promotes cleaner and smarter production in the foundry industry. Based on these advantages, the proportion of machine tool castings produced using the 3D printing sand casting process is steadily increasing.
One specific casting, the worktable, is a critical component. Its table surface is exposed on the machine tool, requiring high surface finish and dimensional precision. It must bear heavy loads and pressure, and its surface is strictly prohibited from having any defects or requiring weld repairs. Similarly, the T-slots must be free from shrinkage cavities and porosity defects.
| Feature | Advantage of 3D Printing Sand Mold in Sand Casting Process |
|---|---|
| Development Speed | No pattern making required, reduces product development cycle and cost for new machine tool worktable castings. |
| Dimensional Accuracy | Integral printing minimizes cumulative errors from traditional core assembly in sand casting, improving precision. |
| Design Flexibility | Enables complex internal geometries for T-slots, difficult for traditional methods, directly from digital files. |
| Operational Efficiency | Simplifies the mold-making process in sand casting, reducing manual steps and operational complexity. |
| Environment & Safety | Reduces manual labor and resin sand waste, promoting a greener sand casting environment. |
Casting Design and Production Process
1. Casting Basic Information
The casting in question is a worktable for a vertical machining center. It serves as a planar surface for machining operations, featuring holes and T-slots for clamping workpieces and removing chips. After machining, the overall dimensions are 2,000 mm × 900 mm × 190 mm. The material is HT300 gray iron, with a casting weight of 1.4 tons. This represents a relatively large worktable for a vertical machining center. The casting has a single-layer structure, making it relatively simple in form. Given the casting’s dimensions and our available 3D printing equipment, we opted for an integral printing of the entire sand mold.
2. Casting Process Design
2.1 Gating and Feeding System Design
The gating system was designed with a single long side for pouring molten metal and an opposing long side for pressure-fed blind risers. To prevent material shortage in the casting during riser removal, a small extension wall with a thickness of 8 mm and length of 30 mm was designed. The riser necks into this wall, which is subsequently ground off after solidification.
The total pouring time was designed to be within 25 seconds. The cross-sectional area ratio of the gating system components was set as: $\Sigma S_{\text{sprue}} : \Sigma S_{\text{runner}} : \Sigma S_{\text{ingate}} = 1 : 1.6 : 0.8$. This configuration is a semi-closed, semi-open gating system, designed for rapid pouring to ensure quick filling of the large worktable surface.
The theoretical pouring time can be estimated using the following formula derived from fluid dynamics and casting practice:
$$ t = \frac{V_{\text{casting}}}{\mu \sqrt{2g H_p}} \cdot \frac{1}{\Sigma S_{\text{ingate}}} $$
where:
- $t$ = pouring time (s)
- $V_{\text{casting}}$ = volume of the casting (including risers) (m³)
- $\mu$ = flow coefficient (dimensionless, typically 0.3-0.5 for sand casting)
- $g$ = acceleration due to gravity (9.81 m/s²)
- $H_p$ = effective pouring head height (m)
- $\Sigma S_{\text{ingate}}$ = total cross-sectional area of ingates (m²)
For this worktable, the critical issue is the solidification time in the T-slot area. The wall thickness in the T-slot region is approximately 60 mm. The local solidification time, $\theta$, is proportional to the square of the casting modulus ($M$), which is the ratio of volume ($V$) to cooling surface area ($A$).
$$ \theta \propto M^2 = \left( \frac{V}{A} \right)^2 $$
A thicker section, like the T-slot base, has a larger modulus, leading to a longer solidification time and a higher risk of shrinkage porosity. This requires special measures to create a favorable temperature gradient or to provide an effective feeding path. The feeding distance using a single riser can be estimated by empirical rules. For gray iron castings, a common rule of thumb for a plate-like casting is:
$$ L_{\text{feeding}} \approx 4.5 \cdot t_{\text{wall}} $$
where $t_{\text{wall}}$ is the wall thickness. For a 60 mm thick wall, the feeding distance is approximately 270 mm. This must be considered when positioning risers for the T-slot region.
2.2 Tooling Design for Integral 3D Printing
To address the high risk of shrinkage porosity in the thick T-slot region, we needed to incorporate chills. Traditional placement of individual chills in the drag mold is inefficient and increases the height of the drag box, wasting resin sand. Our innovative solution integrated the chill directly into the tooling design.
We designed a specialized steel tooling plate, functioning as a large chill. Its external dimensions are 6,000 mm × 2,500 mm × 300 mm. The top wall of the tooling has a thickness of 100 mm. On this top surface, a 10 mm deep recess was machined within an area of 200 mm from the periphery. This recess is filled with sand during molding to provide a backing layer and is a key feature for controlling heat transfer. In the width direction, 9 T-slots were machined at a spacing of 260 mm. In the length direction, 17 dovetail grooves were machined at a spacing of 200 mm. These grooves are essential for mechanically locking the printed sand core to the tooling, preventing it from floating during pouring.
| Tooling Parameter | Value |
|---|---|
| Overall Dimensions | 6,000 mm × 2,500 mm × 300 mm |
| Top Wall Thickness | 100 mm |
| Recess Depth | 10 mm (area within 200 mm from perimeter) |
| T-slots (Width direction) | 9 grooves, spacing 260 mm |
| Dovetail Grooves (Length) | 17 grooves, spacing 200 mm |
The tooling serves multiple purposes in the 3D printing sand casting workflow. Firstly, it acts as a massive chill. The 100 mm thick steel plate has a high heat capacity and thermal conductivity, which rapidly extracts heat from the T-slot region, accelerating solidification and reducing the risk of shrinkage. Secondly, the 10 mm deep recess, filled with sand, creates a layer of “chilled sand” that provides proper rigidity for the mold. This technique allows for precise control over the cooling rate. The thermal field can be optimized by adjusting the chill’s thermal properties. The Fourier number ($F_o$) is a dimensionless parameter that characterizes the transient heat conduction:
$$ F_o = \frac{\alpha t}{L^2} $$
where $\alpha$ is thermal diffusivity of the sand, $t$ is time, and $L$ is a characteristic length (e.g., sand layer thickness). A thicker sand layer (higher $L$) in the recess increases the cooling effect’s resistance, while a thinner layer enhances it. The matching of the chill’s thermal effusivity ($e$) to the casting is critical:
$$ e = \sqrt{k \rho C_p} $$
where $k$ is thermal conductivity, $\rho$ is density, and $C_p$ is specific heat capacity. The high thermal effusivity of the steel chill ($e_{steel} \propto \sqrt{50 \cdot 7800 \cdot 450}$) compared to the sand mold ($e_{sand} \propto \sqrt{0.5 \cdot 1500 \cdot 1000}$) ensures a much faster cooling rate, which is essential for preventing shrinkage in the thick T-slots.
2.3 Sand Printing Parameters
Our available 3D printing equipment has a build volume of 2,500 mm × 1,500 mm × 1,000 mm. Based on the casting’s dimensions, we decided to print the entire mold as a single unit. This integral printing approach eliminates assembly errors and improves overall efficiency.
| Parameter | Specification |
|---|---|
| Sand Grain Size | 70-140 mesh (AFS) |
| Moisture Content | ≤ 0.1% |
| Clay Content | ≤ 0.2% |
| Loss on Ignition | <0.2% |
| Bulk Density | ≥ 1.35 g/cm³ |
| SiO₂ Content | ≥ 95% |
The resin content for the printed sand was 2.1%. The resulting 24-hour tensile strength of the printed sand was measured to be between 2.5 and 3.0 MPa. The mold wall thickness was designed with a side wall of 120 mm and a top wall of 80 mm to ensure structural integrity during handling and pouring. The printed sand mold was then coated using a water-based zirconia wash in a flow-coating process, applied in two coats.
2.4 Assembly Process
After printing and coating, the sand mold was dried to achieve a moisture content of ≤ 0.3%. The assembly procedure was as follows:
- Place the steel tooling plate on the foundry floor.
- Apply a layer of sand over the central area of the tooling, leveling it to the top of the 10 mm deep recess. Allow this sand to cure.
- Seal the perimeter of the tooling with a clay strip to prevent metal run-out.
- Carefully lower the printed sand mold onto the tooling. The T-slots and dovetail grooves in the tooling interlock with the printed sand.
- Clamp the printed sand mold tightly to the tooling using clamps.
- Fill the area around the printed mold with backup resin sand to secure its position.
The entire assembly process, after coating, can be completed within 20 minutes, demonstrating the efficiency gained by integrating the chill into the tooling.
3. Melting and Pouring
To minimize costs while maintaining quality, we employed a short-flow process combining a medium-frequency induction furnace with molten iron from a cupola. The charge consisted of 15% cupola iron, 70% steel scrap, and the remainder being returns of the same material. The cupola iron was treated in a ladle before use. The target chemistry for the HT300 material is given in the following table.
| Element | C | Si | Mn | S | P | Cr | Sn |
|---|---|---|---|---|---|---|---|
| Target (wt.%) | 3.0 – 3.1 | 2.3 – 2.4 | 0.7 – 0.8 | 0.06 – 0.08 | ≤ 0.06 | 0.2 – 0.3 | 0.06 – 0.08 |
The carbon equivalent ($CE$) is a critical parameter for predicting solidification behavior and shrinkage tendency in gray iron. It is calculated as:
$$ CE = C\% + \frac{1}{3}(Si\% + P\%) $$
Using our target values, we find:
$$ CE \approx 3.05 + \frac{1}{3}(2.35 + 0.05) \approx 3.05 + 0.8 = 3.85\% $$
A higher $CE$ promotes graphite precipitation during solidification, which provides self-feeding through graphitic expansion and counteracts shrinkage. This is a primary preventive measure against shrinkage porosity. The degree of saturation ($S_c$) is another useful index:
$$ S_c = \frac{C\%}{4.26 – \frac{Si\%}{3.0}} $$
For our composition, this yields approximately:
$$ S_c \propto 0.95 \text{ to } 0.98 $$
indicating a hypoeutectic iron close to the eutectic point. The closer the composition is to the eutectic, the more significant the graphitic expansion during solidification, which is beneficial for producing sound castings.
The melting temperature was held between 1,510 °C and 1,540 °C followed by a 5-minute holding period. Inoculation was performed in three stages:
- Pre-inoculation: 0.1% silicon carbide was added 2 minutes before tapping.
- Ladle inoculation: 0.3% barium-calcium-silicon long-acting inoculant was added during tapping.
- Stream inoculation: 0.1% of a sulfur-oxygen inoculant with a grain size of 0.2-0.7 mm was added during pouring.
The pouring temperature was precisely controlled within the range of 1,360 °C to 1,380 °C.
4. Defect Prevention and Analysis
During the production of these worktable castings using the 3D printing sand mold process, we encountered and addressed three common defects: shrinkage porosity at the T-slot base, subsurface pinholes on the worktable surface, and gas porosity at the slide mounting surface.
4.1 Shrinkage Porosity at T-slot Base
Shrinkage porosity appears after machining the T-slots. The root cause is the thick section (60 mm) acting as a thermal node that solidifies last without adequate feeding. The prevention strategy has two key aspects. First, the innovative tooling chill design effectively modifies the temperature gradient. The steel chill, with its high thermal diffusivity ($\alpha_{steel} >> \alpha_{sand}$), extracts heat rapidly from the thick T-slot area, reducing its solidification time. Second, we increased the carbon equivalent ($CE$). A higher $CE$ near the eutectic composition maximizes graphitic expansion, which fills the interdendritic cavities formed during solidification.
4.2 Gas Porosity
Gas porosity was observed as subsurface pinholes in the thicker casting sections like the worktable surface and slide mounting surfaces, only visible after machining. The primary causes were identified as nitrogen segregation in the last-to-solidify areas, leading to nitrogen pinholes, and localized moisture in the sand mold reacting with the molten metal to form hydrogen and nitrogen gases. The molten metal can absorb nitrogen ($N_2$) from the atmosphere or raw materials. The solubility of nitrogen in liquid iron follows Sievert’s law:
$$ [wt.\% N] = K_N \sqrt{p_{N_2}} $$
where $K_N$ is the equilibrium constant and $p_{N_2}$ is the partial pressure of nitrogen in the atmosphere. During solidification, the solubility of nitrogen drops drastically, causing it to come out of solution. The tendency to form nitrogen bubbles is related to the partition coefficient:
$$ k_N = \frac{C_{s, N}}{C_{l, N}} $$
where $C_{s, N}$ is the nitrogen content in solid iron and $C_{l, N}$ is the content in liquid iron. Since $k_N < 1$, nitrogen segregates to the remaining liquid, increasing its concentration and the risk of bubble nucleation. Our preventive measures focused on two areas. Firstly, we enforced strict drying of the printed and coated sand mold, ensuring a moisture content of $\leq 0.3\%$. Secondly, we rigorously controlled the nitrogen content in the charge materials. We carefully selected steel scrap and returns, ensuring they were shot-blasted to remove sand and rust which can contribute to nitrogen pickup.
| Defect Type | Root Cause | Preventive Measure in Sand Casting with 3D Printing |
|---|---|---|
| T-slot Shrinkage | Thick section thermal node; lack of liquid feeding during solidification. | Use of integral tooling chill to accelerate cooling; increase carbon equivalent ($CE$) to 3.85%. |
| Surface Pinholes | Nitrogen segregation in thick sections; local mold moisture causing H/N gas reactions. | Thorough drying of printed sand (moisture ≤ 0.3%); strict control of nitrogen in raw materials. |
5. Results and Quality Performance
The production of the worktable castings for vertical machining centers using the integral 3D printing sand mold process, in conjunction with the specialized chilling tooling, has been highly successful. The initial trial castings were machined and inspected. The worktable surface, T-slots, and slide mounting surfaces were completely free of casting defects. This consistent quality was then replicated across a full production run of 56 worktable castings, all of which passed the rigorous quality inspection.
This application demonstrates the significant advantages of integrating 3D printing technology with smart tooling design in the sand casting foundry. The process yielded the following key outcomes:
- Improved Quality and Dimensional Precision: Integral printing eliminated assembly errors, leading to more accurate and consistent castings.
- Increased Efficiency: The combined tooling, acting as both a chill and a locator, drastically reduced the assembly time to under 20 minutes.
- Cost Reduction: The elimination of pattern costs and the reduction in manual operations lowered the overall cost for this batch of castings.
- Enhanced Working Conditions: The process aligns with green and intelligent manufacturing, reducing dust, noise, and physical labor in the workshop.
The 3D printing sand casting process has proven to be a superior method for producing large, high-quality machine tool worktable castings, offering distinct advantages in lead time, quality, and cost, especially for the growing demand for specialized and customized machine tool components. The tooling design, acting as an integral part of the sand casting process, is crucial for achieving defect-free castings with minimal process steps.
