In the field of construction machinery fluid power transmission, the integral hydraulic multi-way valve serves as the core control element due to its compact structure, simplified piping connections, and low pressure losses. However, the large-scale integral hydraulic multi-way valve features extremely complex internal flow channels with numerous overhanging geometries and interconnected oil passages. During the casting process, these characteristics impose stringent requirements on the strength of sand cores. Traditional casting methods typically achieve a success rate of only about 60% for such valves, and the high cost of metal molds along with long processing cycles significantly delay the time-to-market for new products. Three-dimensional (3D) printing casting technology, as an advanced intelligent manufacturing approach, enables the rapid fabrication of sand molds and cores with complex geometries without the need for metal tooling. This technology offers the advantages of low cost and high efficiency, making it highly beneficial for research, development, and design of new products.
In this work, we systematically investigate the rapid manufacturing of large-scale integral hydraulic multi-way valves using 3D printing casting. We first compare two typical 3D printing technologies for sand mold fabrication: selective laser sintering (SLS) and binder jetting (3DP). Through comprehensive performance testing, we identify the most suitable 3D printing casting process for the target valve geometry. Using finite element simulation, we analyze the mold filling behavior of single-layer and composite gating systems, and develop a composite pouring system tailored for 3D printing casting. We also design conformal exhaust systems for both internal sand cores and external molds, reinforce the cores with optimized core rods, and apply high-temperature-resistant coatings. The proposed rapid casting solution, combining “composite gating system + conformal exhaust + strength enhancement + temperature-resistant coating,” is validated through actual casting experiments and mass production trials.
1. Introduction
The integral hydraulic multi-way valve is characterized by a compact design with multiple internal oil passages arranged in a three-dimensional intersecting network. A typical valve body has external dimensions of 420 mm × 320 mm × 320 mm and a weight of approximately 195 kg. The main valve bore is 420 mm long with a diameter of only 20 mm, yielding an aspect ratio of 21:1. Some internal oil passages have diameters as small as 12 mm with lengths up to 220 mm, resulting in aspect ratios exceeding 18.3:1. During conventional sand casting, the cores for these slender passages are prone to bending or fracture, leading to casting defects. For example, the main valve core may bend during pouring, causing angular deviation of the bore and eventual scrapping. In addition, many overhanging oil channels lack adequate support, making them susceptible to breakage during core handling and casting.
The traditional manufacturing route for such valves relies on metal dies and core shooting processes, which involve high upfront costs and long lead times. More importantly, the casting success rate hovers around 60% due to the difficulty in controlling core strength, gas evacuation, and metal flow. As a result, rapid prototyping and low-volume production of new valve designs remain challenging.
Additive manufacturing, particularly 3D printing casting technology, has emerged as a promising solution. Both SLS and 3DP processes can directly produce sand molds and cores without the need for physical patterns. However, the as-printed sand has inherently lower density and strength compared to conventionally shot cores. The tensile strength of 3D-printed sand typically ranges from 2 to 3 MPa, which is only 50%–77% of that achieved by traditional core shooting. This limitation has hindered the widespread industrial application of 3D printing casting for complex hydraulic components.
In this study, we address these challenges by systematically optimizing the 3D printing casting process for a large integral hydraulic multi-way valve. We compare the properties of SLS and 3DP sand molds, select the more suitable process, design a composite gating system, implement conformal exhaust paths, reinforce cores with metallic inserts, and apply a high-temperature coating. Through simulation and experimental validation, we demonstrate that the proposed technique achieves a casting success rate exceeding 95% and meets stringent quality requirements for internal defects and dimensional accuracy.
2. Materials and Methods
2.1 Selection of 3D Printing Casting Process
We prepared standard 8-shaped tensile test specimens using both SLS and 3DP processes. For SLS, we used a self-developed SLS1200 machine with phenolic resin-coated silica sand (grit size 106–150 μm). For 3DP, we used a commercial binder jetting system with ceramic sand (150–212 μm) and furan resin binder. At least 30 specimens were produced for each process, and the tensile strength was measured using a hydraulic strength tester (model XQY-Ⅱ). The average results, along with other key properties, are summarized in Table 1.
Table 1. Comparison of properties for 3D printing casting sand molds
| Property | SLS (phenolic) | 3DP (furan) |
|---|---|---|
| Tensile strength (MPa) | 3.85 | 2.73 |
| Mold accuracy | ±0.25 mm/100 mm | ±0.35 mm/100 mm |
| Gas generation rate (mL·g⁻¹·s⁻¹) | 1.0 | 1.2 |
| Total gas emission (mL/g) | 12.1 | 9.6 |
| Printing efficiency | Low | High |
Although the SLS samples exhibited higher tensile strength (approximately 1.4 times that of 3DP), the strength uniformity across the entire core volume was inferior due to the post-curing process required for SLS. For large integral valves with varying channel diameters, achieving uniform strength after secondary curing is extremely difficult. In contrast, the 3DP process provides better overall uniformity, higher printing speed, and lower total gas emission (79.3% of SLS). Therefore, we selected the 3DP process for manufacturing the sand molds and cores for the integral multi-way valve.
2.2 Composite Gating System Design
We designed a double-layer composite gating system, as illustrated conceptually in Figure 1 (not shown in text). The ingates were positioned to avoid direct impingement on the slender oil channel cores. The three critical slender bores were placed opposite to the ingates to reduce erosion. The cross-sectional area ratios of the gating system and riser locations were determined based on conventional casting principles to ensure a balanced temperature field and uniform linear contraction.
To evaluate the filling behavior, we performed finite element simulations using commercial casting software (ProCAST). The pouring temperature was set to 1380 °C, and the pouring time was 40 s. The valve material was ductile iron QT550. We compared the single-layer gating system (traditional bottom gating) with the proposed double-layer composite gating system. The simulation results are discussed in Section 3.1.
2.3 Exhaust System and Core Reinforcement
When molten metal contacts the sand core, the resin binder decomposes and generates gas. Inadequate venting can lead to gas porosity defects. Since the 3D-printed sand has lower density than shot sand, its intrinsic permeability is higher. We exploited this characteristic by designing conformal exhaust channels along all main valve bores and blind holes connected to the external surface. Figure 2 (not shown) illustrates the exhaust paths inside the core. Additionally, we machined exhaust grooves on the external mold surfaces to guide gases out. The risers were semi-open insulating types with small top holes to allow gas escape while maintaining feeding capability.
To enhance the bending strength of the slender cores, we inserted reinforcing core rods made of steel wire into the exhaust channels. The core rods had a hollow lattice structure to permit gas flow. The diameter of the exhaust channel was designed to be 5.6 mm, and the core rod diameter was 5.4 mm, providing optimal interference for secure placement. The rods were pre-positioned in the 3D-printed cores after printing, as the additive process cannot embed them during layer-by-layer deposition.
2.4 High-Temperature-Resistant Coating
To further improve the thermal stability of the 3D-printed cores and reduce the risk of veining defects, we applied a zirconium-based refractory coating by dipping. The coated cores were then dried before assembly. This coating provides a dense surface layer that resists erosion and reduces gas evolution from the core.
3. Results and Discussion
3.1 Flow Analysis of Gating Systems
We extracted the metal flow velocity at two representative nodes: one in the lower half of the mold (Node L: X=15, Y=28, Z=0) and one near the second-layer ingate in the upper half (Node U: X=28, Y=2, Z=8). The results are summarized in Table 2 and Table 3.
Table 2. Metal flow velocity at lower section node (Node L)
| Filling percentage (%) | Single-layer gating (m/s) | Composite gating (m/s) |
|---|---|---|
| 0–25 | 0.50–0.80 | 0.50–0.80 |
| 25–100 | 0.05–0.10 | 0.05–0.10 |
Table 3. Metal flow velocity at upper section node (Node U)
| Filling percentage (%) | Single-layer gating (m/s) | Composite gating (m/s) |
|---|---|---|
| 0–70 | 0.05–0.10 | 0.05–0.10 |
| 70–90 | 0.05–0.10 | 0.86 (peak for ~6 s) |
| 90–100 | 0.05–0.10 | 0.11 |
In the lower mold region, both systems showed similar stable velocities after initial filling. The composite gating system effectively reduced the flow rate at the bottom ingates because the total flow was divided between two layers. This reduction minimized the continuous thermal and mechanical impact on the bottom cores, which are particularly vulnerable when using 3D printing casting (core strength only 50–77% of conventional). The brief velocity surge at Node U (0.86 m/s) during the 70–90% filling stage did not cause defects because the upper cores were already partially submerged and supported by the surrounding sand.
3.2 Casting Success Rate and Quality Inspection
We conducted a batch casting trial of 10 units using the optimized 3D printing casting process. All 10 castings were successfully produced with no visible defects. After the process was solidified on the production line, the annual statistical success rate for this specific valve model reached 95.67%, a substantial improvement over the traditional process.
The castings were sectioned and inspected using a coordinate measuring machine (Hexagon MicroPlus10.12.08) and industrial computed tomography (Y.CT Modular, 600 kV, 580 kV tube voltage, 1.75 mA tube current, 1.27× magnification). The results are summarized in Table 4.
Table 4. Quality inspection results of integral hydraulic valve castings via 3D printing casting
| Parameter | Measured value | Requirement |
|---|---|---|
| Main bore straightness | ≤0.28 mm/100 mm | CT5 grade |
| Internal pores/shrinkage (≥0.3 mm) | None detected | Zero tolerance |
| Average hardness deviation | ≤5% | Industrial standard |
Hardness was measured on three internal cross-sections using a portable ultrasonic hardness tester (KT-C). The average hardness deviation was computed as:
$$ \Delta H = \frac{|H_i – \bar{H}|}{\bar{H}} \times 100\% \leq 5\% $$
where H is the average hardness of all measurement points. The uniform hardness distribution confirms the effectiveness of the gating and feeding design.
3.3 Discussion of Core Strength Enhancement
The relationship between core bending strength and rod reinforcement can be approximated by composite beam theory. The core rod contributes a bending moment resistance proportional to its modulus and cross-sectional area. For the slender main bore core (diameter 20 mm, length 420 mm), the maximum bending stress under metal static pressure was reduced by approximately 40% after inserting the 5.4 mm diameter steel rod, as estimated by:
$$ \sigma_{\text{max}} = \frac{M y}{I_{\text{eff}}} $$
where M is the bending moment, y is the distance from neutral axis, and I is the effective moment of inertia of the composite core-rod system.
4. Conclusion
In this work, we successfully developed a rapid manufacturing technology for large integral hydraulic multi-way valves using 3D printing casting. The key findings and contributions are summarized below:
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Through comparative analysis, the 3DP (binder jetting) process was selected as the preferred 3D printing casting method for sand mold fabrication due to its superior strength uniformity, lower gas emission (9.6 mL/g vs. 12.1 mL/g), and higher printing efficiency, despite having 1.4 times lower tensile strength than SLS.
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A double-layer composite gating system was designed and validated via finite element simulation. Compared to single-layer gating, the composite system reduced the flow rate at bottom ingates during the critical early filling stage, protecting the weak bottom cores from prolonged thermal and mechanical impact. The flow velocity at the lower mold region stabilized at 0.05–0.10 m/s after 25% fill.
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An integrated core performance enhancement strategy was proposed, consisting of conformal exhaust channels, steel core rods (optimized diameter 5.4 mm for 5.6 mm exhaust hole), and zirconium-based high-temperature coating. This combination increased the effective bending strength of slender cores and eliminated defects such as veining and gas porosity.
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Batch casting trials achieved a success rate of 95.67%. The produced valve bodies exhibited main bore straightness ≤0.28 mm/100 mm, no internal defects ≥0.3 mm, and average hardness deviation ≤5%. These results confirm that the proposed 3D printing casting process meets industrial requirements for complex hydraulic components.
The developed technology demonstrates the feasibility of replacing traditional die-based manufacturing with 3D printing casting for large integral hydraulic valves, enabling rapid prototyping and low-volume production at reduced cost and lead time. This work provides a practical reference for applying sand mold additive manufacturing in construction machinery, aerospace, and other industries requiring complex internal cavities.
