1. Introduction
3D printing, a recently emerged technology, has witnessed rapid development and expanding application ranges. The casting industry has a close connection with 3D printing, with the latter being widely used in sand casting and investment casting for new product development, small-batch production, and personalized customization. It features digitization, flexibility, precision, and environmental friendliness. This paper delves into the application of 3D printing technology in sand casting, particularly focusing on its impact on the production of shell components.
2. Overview of 3D Printing Technology
3D printing technology has various applications in sand casting processes, with two primary techniques being three-dimensional binder jetting and selective laser sintering. The general casting process flow involves three-dimensional modeling, model importation, 3D printing of sand molds and cores, mold assembly and pouring, and finally, casting production. This paper mainly discusses the three-dimensional binder jetting technology.
2.1 Three-Dimensional Binder Jetting Technology
2.1.1 Process Principle
The computer slices the prepared three-dimensional CAD model of the sand mold (core) into horizontal layers of a certain thickness, creating a collection of two-dimensional slice information recognizable by the printer. The printer’s computer then directs the nozzles to jet the binder onto the surface of the laid sand mixed with a curing agent, and the layers are bonded together through the binder, repeating this process until the sand mold (core) is complete.
2.1.2 Forming Equipment
A 3D printer primarily consists of a jetting system, sand supply system, motion control system, and computer hardware and software.
2.1.3 Forming Materials
The materials used include base sand, curing agent, binder, and auxiliary materials.
2.1.4 Process Precision
The dimensional accuracy of printed sand molds (cores) can generally be controlled within DCTG10 to DCTG12, with a surface roughness of Ra ≤ 25 µm. The tensile strength at room temperature is 1.0 to 2.0 MPa, and the flexural strength is 2.8 to 4.5 MPa. The gas evolution (at 1000°C) is 6 to 20 mL/g, meeting the requirements of most casting sand molds (cores).
3. Application Case of Shell Components
Shells are crucial components in faucets, supporting the installation of various faucet accessories. Due to their irregular geometries and internal connecting ribs and shaped holes, these structures are difficult to machine post-casting. Therefore, a combined casting and machining approach is employed in actual manufacturing.
3.1 Traditional Casting Process and Its Limitations
The traditional casting process for shells involves mixing sand, molding and coring, steelmaking and pouring, mold breaking and cleaning, and inspection. Given the small-batch and single-piece production of shells, manual molding is used with furan resin self-hardening sand. The gating system adopts a bottom-pouring design, aiming for smooth metal filling, reduced impact on the sand mold and core, minimized metal oxidation, and effective gas exhaust within the mold cavity. The upper part accommodates the riser, compensating for potential shrinkage during casting formation to prevent shrinkage pores, and serving additional functions like gas exhaust, slag collection, and guiding filling. The pouring temperature is set at 1590°C.
During processing, cracks were found at the bottom of the shells, with irregular, discontinuous, and scattered shapes. Some cracks were not visible to the naked eye and required magnetic particle inspection to confirm defect locations. Analysis revealed that the defects were closely related to the design of the gating system. The primary causes of casting defects include:
- Insufficient in-gates in the gating system, hindering the dispersion and uniform filling of metal liquid.
- High temperatures in the lower part of the casting, impeding feeding and consuming more metal liquid.
- Small riser size, insufficient for feeding the casting.
Due to limitations in traditional molding processes and the complexity of the shell mold, modifications to the mold or gating system would necessitate lengthy process trials, making it difficult to ensure the molding requirements. As a result, the defect issue in shells persisted without an effective solution.
3.2 Implementation of 3D Printing for Shells
The shell has a maximum outline dimension of 628 mm × 483 mm × 432 mm, a basic wall thickness of 26.5 mm, and technical requirements of DCTG10 dimensional accuracy and Ra ≤ 25 µm surface roughness for the casting. The sand mold is an assembled type, comprising upper and lower sand molds, a body sand mold, and middle cores.
The 3D printing forming process flow is as follows: casting parting analysis, casting process design, casting pouring simulation, casting process optimization modeling, 3D printing, sand mold cleaning, sand mold coating application, core assembly, box filling and compaction, pouring, mold breaking and cleaning, and inspection. The shell casting weighs 265 kg, requiring a poured steel liquid weight of 350 kg and a pouring time of 20 to 25 seconds. An open bottom-pouring gating system is adopted, with a sectional area ratio of ΣA直 : ΣA横 : ΣA内 = 1 : 1.3 : 1.8. The gating system is formed by leaving voids during sand mold printing.
The surface floating sand thickness of 3D printed sand molds is approximately 0.4 mm, and the coating layer thickness is about 0.3 mm. Relevant sand cores are surface-infiltrated and dried in a drying oven along with other sand cores for 70 minutes, followed by manual core assembly and fastening with screws. During 3D printing, the shell’s gating system is optimized by increasing the riser size for enhanced feeding and adding more in-gates with a dispersed layout to facilitate solidification and feeding of the casting. Additionally, subsidies are placed in thick regions of the casting. Simulations of metal liquid flow, filling, and solidification during pouring (as shown in Figures 6 and 7) reveal a smooth filling process and orderly solidification. After several process scheme explorations and improvements, the 3D printed shell castings fully meet the technical requirements (as shown in Figures 8 to 11), significantly enhancing overall performance and appearance quality.
Table 1. Comparison of Traditional and 3D Printing Casting Methods for Shells
| Aspect | Traditional Casting Method | 3D Printing Casting Method |
|---|---|---|
| Molding Precision | Lower, affected by manual operations | Higher, controlled by computer algorithms and precision printing equipment |
| Casting Defects | More common, especially at the bottom of castings | Fewer defects, optimized gating system design through simulation |
| Production Efficiency | Lower, longer cycle time and higher labor intensity | Higher, shortened production cycle and reduced labor intensity |
| Material Utilization | Lower, more waste during molding and pouring | Higher, precise control of material usage during printing and pouring |
| Design Flexibility | Lower, limited by mold manufacturing capabilities | Higher, capable of producing complex shapes and structures |
| Cost | Lower initial investment, but higher long-term costs | Higher initial investment in equipment, but potential for cost savings in long run |
4. Discussion
4.1 Advantages of 3D Printing Technology in Sand Casting
- Improved Casting Quality: By simulating the pouring process, potential defect locations can be identified and addressed, reducing casting defects and improving product quality.
- Enhanced Design Flexibility: 3D printing technology enables the production of complex shapes and structures that are difficult or impossible to achieve through traditional casting methods.
- Shortened Production Cycle: Automated 3D printing processes reduce labor intensity and shorten production cycles, improving production efficiency.
- Material Savings: Precise control of material usage during printing and pouring leads to higher material utilization rates and reduced waste.
4.2 Challenges and Limitations
- High Initial Investment: 3D printing equipment and materials are relatively expensive, posing a significant initial investment.
- Technical Complexity: 3D printing technology requires professional technical personnel for operation and maintenance, increasing technical complexity.
- Cost-Effectiveness: For large-scale production, the cost of 3D printing may not be as competitive as traditional casting methods due to factors such as equipment depreciation and material costs.
5. Conclusion
The introduction of 3D printing technology into sand casting has addressed the limitations of traditional casting methods, particularly in the production of complex shell components. By simulating the pouring process and optimizing the gating system design, 3D printing technology has significantly improved the yield and quality of castings. Although 3D printing involves higher initial investment and technical complexity, its advantages in terms of casting quality, design flexibility, production efficiency, and material utilization make it a promising technology for the future development of the casting industry.