In our work, we focus on developing a novel casting process for the pulping rotor using 3D printing sand mold technology combined with gravity casting. The pulping rotor is a critical component in the papermaking industry, where it operates at high rotational speeds to generate centrifugal forces that break down pulp fibers. The traditional manufacturing approach relies on wooden patterns, which presents significant challenges due to the complex geometry of the rotor, especially the eight streamlined blades and the internal hollow cavity. Wooden pattern manufacturing typically requires at least 20 days and costs tens of thousands of RMB, while 3D printed sand molds can be directly produced from digital models within one day, eliminating the need for expensive pattern tooling. Moreover, 3D printed sand molds do not require draft angles for pattern removal, simplifying molding and core assembly, and improving dimensional accuracy. This makes 3D printing sand casting particularly suitable for producing intricate pulping rotor castings.
The rotor is made of ZG06Cr13Ni4Mo, a medium‑strength martensitic stainless steel with excellent corrosion resistance, wear resistance, and high tensile strength. Its chemical composition and mechanical property requirements are summarized in Table 1 and Table 2, respectively. The rotor has an outer diameter of 945 mm, a height of 158 mm, and a weight of 156 kg. The design includes a central disc and eight outer blades with a streamlined shape, and an internal hollow cavity to reduce weight. The blade dimensions are strictly controlled because they directly affect the dynamic balance and performance of the rotor.
| Element | C | Si | Mn | S | P | Cr | Ni | Mo |
|---|---|---|---|---|---|---|---|---|
| Content | ≤0.06 | ≤0.80 | ≤1.00 | ≤0.035 | ≤0.025 | 11.5–13.5 | 3.5–5.0 | 0.4–1.0 |
| Property | Yield strength Rp0.2 (MPa) | Tensile strength Rm (MPa) | Elongation A5 (%) | Reduction of area Z (%) | Impact energy KV (J) | Hardness HBW |
|---|---|---|---|---|---|---|
| Value | ≥550 | ≥750 | ≥15 | ≥35 | ≥50 | 221–294 |
To ensure the rotor meets the ultrasonic inspection standard ASTM A609 Grade 2, we designed a robust casting process. The casting shrinkage allowance was set to 2.0% based on the martensitic stainless steel. We adopted a bottom gating system with an ingate diameter of 60 mm to allow smooth filling and promote flotation of gas and inclusions into the risers. Machining allowances were 8 mm on the top face, 10 mm on the outer diameter, and 6 mm on the bottom face. The central shaft hole was left uncored, resulting in a rough casting weight of 187 kg. Riser design included a cylindrical riser of φ200 mm × 280 mm with a modulus of 4.5 cm at the rotor center, eight insulating risers of φ70 mm × 100 mm (modulus 1.8 cm) at the blade root, and φ20 mm vent risers at each blade tip. Total poured weight of steel was 312 kg, yielding a process yield of 60%.
We employed a VX2000 3D printer to fabricate the sand molds using GS15 silica sand with 1.2% resin binder and 0.3% curing agent. The printing layer thickness was 0.3 mm, and the achieved sand mold strength was 1.4 MPa with dimensional accuracy ≤0.3 mm. The maximum printable size was 2000 mm × 1000 mm × 1000 mm. The 3D printed upper and lower molds were assembled directly without the need for traditional pattern tooling.
We performed casting process simulation using Procast software to predict solidification behavior and identify potential shrinkage porosity or hot spots. The finite element mesh was generated, and the material was defined as ZG06Cr13Ni4Mo with a heat transfer coefficient of 500 W/(m²·K). The pouring temperature was 1580 °C, initial environment temperature 25 °C, pouring time 6 s, pouring rate 50 kg/s, and cooling in air. After simulation, the temperature distribution during solidification and the porosity distribution were analyzed. The results indicated that the designed riser system was adequate to feed the casting, with no major shrinkage defects exceeding the acceptable level.
The melting process was carried out in a basic electric arc furnace combined with an AOD argon‑oxygen decarburization unit to achieve ultra‑low carbon content (<0.03% C) and strict control of phosphorus and sulfur. After tapping, the molten steel was held in the ladle for more than 5 minutes to allow inclusions to float. The pouring temperature was controlled between 1560 °C and 1580 °C, using a bottom‑pouring ladle with a 50 mm nozzle. Heat treatment consisted of normalizing at 1020±10 °C for 2 hours followed by tempering at 600±10 °C for 4 hours. The heating rate was limited to ≤100 °C/h between 300 °C and 600 °C, and ≤80 °C/h above 600 °C.
After casting, shakeout, cutting of risers, and heat treatment, the rotor was inspected. Dimensional checks using a marking plate showed that all eight blades had consistent heights. Ultrasonic inspection confirmed no internal defects exceeding the Grade 2 criteria. Chemical analysis and mechanical tests met the requirements. The use of 3D printing sand casting significantly reduced lead time: the mold was ready in 1 day versus 20 days for a wooden pattern. For a mold volume of 0.48 m³, the material cost was 0.35 ×10⁴ RMB/m³, giving a total mold cost of 1680 RMB, compared to an estimated 30 000 RMB for a wooden pattern. This makes the 3D printing approach highly economical for small‑batch production of complex parts like pulping rotors.
We derived several important conclusions from this study. First, the Procast simulation validated the casting design and helped optimize riser dimensions, achieving a process yield of 60%. Second, 3D printing sand mold technology offers tremendous advantages in production efficiency and cost savings for complex, small‑batch castings. Third, the uniform compaction of 3D printed sand molds leads to improved dimensional accuracy of the blades, which is critical for rotor performance. The combination of 3D printing sand casting and numerical simulation provides a reliable and efficient route for manufacturing high‑quality pulping rotors.