The manufacturing landscape is undergoing a profound transformation, driven by the convergence of advanced digital technologies with traditional industrial processes. Among these, Additive Manufacturing (AM), commonly known as 3D printing, has emerged as a pivotal force. In the foundry sector, sand binder jetting 3D printing, specifically, has catalyzed a paradigm shift, enabling a new rapid casting production model. This model is particularly transformative for low-volume, high-complexity components where traditional pattern-making is prohibitively expensive and time-consuming. This article delves into a detailed application case, exploring from a first-person engineering perspective, the integrated use of “3D Printing + Conventional Mold” methodology for the rapid development and successful first-time production of a critical, large-scale component: an ultra-supercritical steam turbine’s medium-pressure outer cylinder casting, manufactured from heavy-section nodular cast iron.
The conventional production cycle for such massive castings, often exceeding 50 tons in weight, is typically measured in many months, with a significant portion dedicated to the design, fabrication, and verification of complex wooden or metal pattern equipment. For new product introductions or design iterations, this represents a major bottleneck. The project in focus involved a new variant of a medium-pressure outer cylinder where the core geometry remained consistent with a previous model, but with localized modifications. The challenge was to produce a single-piece order cost-effectively and rapidly without investing in an entirely new, full-scale pattern set that would be uneconomical for a one-off piece. This scenario presented the perfect opportunity to leverage a hybrid manufacturing strategy.
Component Characteristics and Demanding Specifications
The component is a quintessential large, pressure-retaining housing for power generation equipment. Its primary characteristics and the stringent quality requirements dictated our technical approach.
Geometric and Material Profile
The casting is a massive, drum-shaped shell structure with significant variations in wall thickness. The base material is a high-silicon molybdenum alloyed nodular cast iron (QTRSi3Mo), selected for its excellent high-temperature strength, oxidation resistance, and thermal fatigue properties—critical for ultra-supercritical service conditions. The key features are summarized below:
| Feature | Specification |
|---|---|
| Component | Medium-Pressure Outer Cylinder (Lower Half) |
| Material | High-Si-Mo Nodular Cast Iron (QTRSi3Mo) |
| Overall Dimensions (L x W x H) | 5,485 mm x 4,320 mm x 2,150 mm |
| Poured Weight | ~62,000 kg |
| Flange Wall Thickness | 380 mm |
| Cylinder Wall Thickness | 65 – 100 mm |
| Nozzle/Port Thickness | ~190 mm |
The structural modification from the previous design involved changing the number and arrangement of extraction ports on the shell body. While the main body was identical, the new design featured four ports instead of three: two new ports added diagonally opposite each other, the central port was removed and replaced with a small boss, and the original side ports were retained.
Technical and Quality Requirements
The quality benchmarks were exceptionally high, reflecting the component’s critical function:
- Dimensional Tolerances: General dimensions to CT13 per ISO 8062 (equivalent to GB/T 6414), and wall thickness to CT14.
- Non-Destructive Testing (NDT): 100% Magnetic Particle Inspection (MT) and 100% Ultrasonic Testing (UT) were mandatory. UT acceptance levels, per EN 12680-3, were stringent: Level 2 for high-stress areas (mounting flanges, extraction nozzles) and Level 4 for all other regions of the nodular cast iron casting.
- Mechanical Properties: The high-silicon molybdenum nodular cast iron had to meet specified tensile strength, elongation, and impact toughness values at room and elevated temperatures, with strict control over nodule count and morphology.
The Hybrid “3D Printing + Conventional Mold” Methodology
Faced with the need to adapt the existing pattern for the modified design without damaging it or incurring high costs for new core boxes, we devised an innovative hybrid strategy. The philosophy was to use the substantial existing pattern for the majority of the mold geometry and employ 3D printed sand cores and expendable polystyrene (EPS) patterns to create only the new or altered features.
Deconstruction of the Challenge
The modifications presented two distinct problems:
- Addition of Two New Diagonal Extraction Ports: These were entirely new external and internal features.
- Modification of the Central Region: Removing an existing port and adding a small boss.
A traditional approach would require machining new loose pieces for the pattern and entirely new sections for the large core box—a costly and destructive process. Our hybrid solution broke down as follows:
| Design Change | Solution for Pattern (Cope & Drag) | Solution for Core Box (Large Cavity Core) |
|---|---|---|
| New Diagonal Ports | EPS patterns attached to the existing wooden pattern via precision locators. | 3D printed sand cores, designed to slot into the existing core print locations in the large core. The core box itself was modified with EPS blocks to create voids for these new 3D printed core prints. |
| Central Port Removal & Boss Addition | A 3D printed “fill” core with a core print designed into the cope pattern. | The corresponding cavity in the large core box was filled with an EPS block. The 3D printed core would sit in the cope and fill the space, simultaneously forming the new boss feature. |
Detailed Process Design and Engineering
The success of this method hinged on meticulous design for assembly and process control.
1. For the New Extraction Ports:
The internal geometry of the new ports—complex passages integrated into the main cavity—was ideal for 3D printing. We designed two sand cores for each port, split horizontally due to the printer’s build volume constraints. The mating surfaces featured interlocking “step” joints for precise alignment. Crucially, the lower core print was designed with a tapered “slide fit” to ensure it would locate positively and securely in the void created in the large core. The cores included integrated handling lugs, which were removed after assembly. The external shape of the ports was created by accurately machined EPS patterns fixed to the main wooden pattern.
2. For the Central Modification:
A single 3D printed sand core was designed to perform two functions: plug the hole left by the removed port and form the new boss protruding into the cavity. This core was designed with a core print that seated only in the cope (top) mold half. During molding, this core was placed into the cope box. When the large core was lowered into the drag, the 3D printed core’s lower face would mate against the flat surface of the large core (where the EPS block had filled the old port cavity), effectively sealing it and creating the boss.
The gating and feeding system remained unchanged from the proven original process: a bottom-gated, ceramic-lined system with multiple ingates along the lower flanges to ensure smooth, controlled filling of the massive nodular cast iron volume. Risering design was critical for soundness in the heavy sections. The feeding requirements can be approximated using the modulus method. The modulus (Volume/Surface Area ratio) of the heaviest section (the flange, ~380mm thick) dictates the riser size needed to ensure directional solidification towards the riser.
For a cylindrical riser, the modulus relationship is:
$$ M_{riser} = k \cdot M_{casting} $$
where \( M_{riser} = \frac{D}{6} \) for a side riser (ignoring the contact surface), \( M_{casting} \) is the modulus of the hot spot, and \( k \) is a safety factor (typically 1.1 to 1.2). For our thick flange, this calculation governed the riser diameter and height. The solidification time, \( t \), according to Chvorinov’s rule, is:
$$ t = B \cdot \left( \frac{V}{A} \right)^n = B \cdot (M)^n $$
where \( B \) and \( n \) are constants dependent on the mold material and metal being poured. This underscores why controlling the modulus is paramount for defect-free nodular cast iron castings, as shrinkage porosity is a key risk.
Production Execution and Critical Control Points
Translating the hybrid design into a physical casting required stringent controls at multiple stages.
Pre-Production Validation
- Pattern and Core Box Modification Verification: After attaching the EPS patterns and installing EPS blocks in the core box, the entire assembly was scanned using photogrammetry. The resultant 3D point cloud was compared directly against the CAD model to validate dimensional accuracy and locator fit before any sand was rammed.
- 3D Printed Core Validation: Each 3D printed sand core was also scanned post-printing and post-curing. This verified not only their intrinsic accuracy but also, critically, the fit-up of the interlocking joints and the accuracy of the core prints designed to interface with the traditional mold components.
Molding and Core Assembly
The process followed this sequence:
1. The modified drag pattern (with EPS port patterns) was used to produce the drag mold half.
2. The modified large core box (with EPS filler blocks) was used to produce the primary cavity core.
3. The 3D printed sub-cores for the new diagonal ports were assembled using adhesives and positioned into their designated locations on the large core. The assembly was then lifted as one unit.
4. The modified cope pattern (with the core print for the central “fill” core) was used to produce the cope mold half. During this process, the central 3D printed fill core was placed into its core print in the cope box.
5. The massive core assembly was carefully lowered into the drag mold.
6. Finally, the cope was closed onto the drag. During closing, the central 3D printed core in the cope descended to meet and seal against the flat face of the large core.
A paramount concern was venting the 3D printed cores. Unlike traditionally vented cores, the permeability of 3D printed sand can differ. We designed explicit vent channels into the 3D printed cores during the CAD stage, connecting to the core prints. Corresponding vents were cut from the mold at these print locations to ensure any gases generated could escape freely into the mold atmosphere and through the mold walls.
Melting, Pouring, and Solidification Control
The melting of the high-silicon molybdenum nodular cast iron was performed in a high-capacity induction furnace. The process required precise control of chemistry, particularly low sulfur content prior to nodularizing treatment. Treatment was done via the sandwich method using a FeSiMg alloy in a dedicated treatment ladle, followed by post-inoculation. The pouring temperature was carefully maintained within a narrow range (typically 1,350°C – 1,370°C) to ensure fluidity for the thin sections while avoiding excessive thermal shock and sand burn-on.
The thermal gradient during solidification was managed by the pre-designed risering and the use of exothermic riser sleeves and topping compounds to maximize feeding efficiency. The cooling curve for this heavy-section nodular cast iron is critical, as the long solidification time influences the final matrix structure (ferrite/pearlite ratio) and nodule characteristics. Monitoring was essential to meet mechanical property targets.
Results, Verification, and Comparative Analysis
The casting was cleaned, heat-treated (ferritizing anneal to achieve the required ductility and toughness), and subjected to full inspection.
- Dimensional Accuracy: Final layout inspection confirmed all critical dimensions were within the specified CT13/CT14 tolerances. The interfaces between the traditional mold and the 3D printed features showed no mismatch or significant deviation.
- NDT Performance: The casting passed 100% MT inspection. More significantly, it passed the rigorous 100% UT inspection, meeting the required Level 2 and Level 4 criteria throughout. This demonstrated the structural integrity and soundness achieved by the hybrid method, with no shrinkage or cold lap defects at the hybrid interfaces.
- Mechanical Properties: Coupons from attached test blocks met all specified tensile strength, yield strength, elongation, and impact energy requirements for the high-silicon molybdenum nodular cast iron grade.
The benefits of the hybrid approach versus traditional methods for this low-volume scenario are quantitatively stark:
| Metric | Traditional New Pattern Approach | “3D Printing + Conventional Mold” Hybrid Approach |
|---|---|---|
| Tooling Lead Time | 12-16 weeks | 3-4 weeks (for 3D model prep, printing, EPS pattern fabrication) |
| Tooling Cost | Very High (new core box sections, complex loose pieces) | Reduced by ~70-80% (cost of printing sand cores & EPS patterns only) |
| Pattern Risk | Permanent alteration/damage to existing pattern possible if adapted. | Zero damage to existing pattern; EPS attachments are non-invasive. |
| Design Flexibility | Low; changes require physical re-machining. | Very High; design changes only affect digital files for printing. |
Conclusion and Forward Perspective
This project serves as a compelling testament to the practical viability and immense potential of integrating sand 3D printing with established foundry practices. The “3D Printing + Conventional Mold” strategy successfully enabled the low-cost, rapid, and first-time-right manufacture of a massive, high-integrity nodular cast iron casting. It effectively addressed the core dilemma of prototyping and low-volume production of large components: eliminating the need for high capital investment in full tooling for geometrically evolving designs.
The technical keys to success were a synergistic design philosophy, rigorous pre-production validation using digital metrology, and meticulous control over core venting and interface integrity. This hybrid model is not merely a stopgap but a forward-looking production methodology. It significantly compresses the development cycle for new power generation equipment, industrial machinery, and other sectors reliant on large, complex castings. As 3D printing binder systems and sand materials continue to advance, offering improved surface finish and higher strength, the scope and reliability of such hybrid applications will only expand. This convergence is more than an efficiency gain; it is a fundamental enabler for agile manufacturing, sustainable production through reduced waste, and accelerated innovation in heavy industry, firmly establishing its role in the future of nodular cast iron casting production.