1. Introduction
In recent years, the landscape of the casting industry has been revolutionized by the advent of sand mold 3D printing technology. This innovative approach has liberated casting design from the constraints of traditional molding techniques, enabling the production of complex, thin – walled, lightweight, and precise castings. Such castings find extensive applications across diverse sectors, including aerospace, automotive, and marine industries.
The demand for high – quality 3D – printed casting sand molds has grown in tandem with the expansion of 3D printing applications. These molds are required to possess excellent process properties to ensure the high – quality formation of equipment castings. However, the 3D printing process for sand molds presents a unique set of challenges. For instance, in contrast to traditional molding methods that rely on compaction to achieve sand mold strength, 3D printing often necessitates an increase in the content of binders and curing agents to attain the desired strength. This increase, unfortunately, leads to a rise in gas evolution and may compromise the molding accuracy. Additionally, the selection of the printing layer thickness is a crucial factor. A thin layer thickness can enhance the molding accuracy but at the cost of reduced printing efficiency due to increased sand – laying times. Conversely, a thick layer thickness may weaken the bonding force between sand grains, thereby reducing the sand mold strength.
To address these challenges and optimize the sand mold 3D printing forming process, a scientific data – analysis method is essential. This paper aims to comprehensively review the research on manufacturing thin – walled impeller castings through the optimization of the sand mold 3D printing forming process. By doing so, it provides valuable insights and references for the production of similar products in the casting industry.
2. Materials and Equipment Used in the Experiment
2.1 Experimental Materials
The experiment utilized silicon sand as the original sand, 3D – printed furan resin as the binder, and 3D – printed curing agent. The main performance indicators of these materials are presented in Table 1, Table 2, and Table 3.
| Table 1: Technical performance of 3D printed silica sand | ||||
|---|---|---|---|---|
| Average fineness/μm | w(Si)/% | Angular coefficient | Loose bulk density/(g·cm⁻³) | Ignition loss (LOI)/% |
| 90 – 92 | 64 – 72 | <1.25 | 1.35 – 1.45 | ≤0.2 |
| Table 2: Technical performance of 3D printed furan resin | ||||
|---|---|---|---|---|
| Free formaldehyde/% | Viscosity/(mPa·s) | Density/(g·cm⁻³) | Conductivity/(μS·cm⁻¹) | Surface tension/(mN·m⁻¹) |
| 1.12 – 1.18 | 35 – 40 | 6 – 8 | ≤20 | ≤0.1 |
| Table 3: Technical performance of 3D printed curing agent | ||||
|---|---|---|---|---|
| Viscosity/(mPa·s) | Density/(g·cm⁻³) | Total acidity/% | Free sulfuric acid/% | |
| 20 – 40 | 1.30 – 1.35 | 25.5 – 26.5 | <2.5 |
The silicon sand’s characteristics, such as its fineness and angular coefficient, influence the packing density and bonding performance within the sand mold. The furan resin’s properties, including viscosity and free formaldehyde content, play a crucial role in the bonding strength and curing process. The curing agent’s acidity and other properties regulate the rate of the curing reaction.
2.2 Experimental Equipment
The printing equipment employed was the ExOne S – Max Pro. It featured a forming size of 1800 mm×1000 mm×700 mm, a printing resolution of 400 dpi, and an adjustable printing layer thickness ranging from 0.20 – 0.50 mm. This equipment’s capabilities allowed for the production of sand molds with a relatively large size and a certain degree of precision.
For the performance testing of sand molds, a SWY – B digital display hydraulic strength testing machine was used to measure the tensile strength, and a GET – III intelligent gas evolution tester was employed to determine the gas evolution. These testing devices provided accurate and reliable data for evaluating the quality of the printed sand molds.
3. Experimental Design
3.1 Optimization Experimental Design
The Box – Behnken (BBD) response surface method was adopted to design the optimization experiment. The process parameters selected for study were resin inkjet content (%), curing agent content (%), and printing layer thickness (mm). Resin inkjet content refers to the mass fraction of resin in the molding sand within each forming area during 3D printing, while the curing agent content represents the mass fraction of the curing agent in the premixed sand. The tensile strength (MPa) and gas evolution (mL/g) of the sand mold were chosen as the performance responses.
The 取值范围 of the process parameters was determined based on the characteristics of the 3D printing equipment. The factors and level coding of the experimental design are shown in Table 4.
| Table 4: The factors and level coding of experimental design | |||
|---|---|---|---|
| Level | Resin inkjet content (A)/% | Curing agent content (B)/% | Printing layer thickness (C)/mm |
| – 1 | 1.35 | 0.20 | 0.25 |
| 0 | 1.50 | 0.30 | 0.30 |
| 1 | 1.65 | 0.40 | 0.35 |
This three – factor three – level design enabled the investigation of the individual and interactive effects of these parameters on the performance of the sand mold. By adjusting the equipment parameters of the sand mold 3D printer, “8” – shaped test blocks were printed, and their performance was tested in accordance with GB/T 2684 – 2009.
3.2 Casting 3D Printing Casting Process Design
A specific thin – walled impeller casting was selected as the verification object for optimizing the 3D printing sand mold forming process. The overall contour dimensions of the impeller were 318 mm×318 mm×124 mm, with a maximum wall thickness of 44.5 mm and a minimum wall thickness of 1.2 mm. The wall thickness distribution was uneven, and 16 groups of blades surrounded the center to form a complex 异形内腔. The casting, with a mass of 6.3 kg, was made of ZL101A alloy.
Considering the structure and material characteristics of the impeller, sand mold low – pressure casting was chosen as the casting method. The gating system adopted an open – type design, and the cross – sectional area ratio of each unit of the gating system was \(\sum A_{in}:\sum A_{runner}:\sum A_{ingate}=1.0:2.1:2.3\). The design model of the 3D printing casting process is shown in Figure 1. [Insert Figure 1 here: 3D printing casting technique for impeller]
To enhance the exhaust and feeding capacity of the flange surface, the volume of the top of the riser connected to the stepped ingate was increased, and the feeding distance between the risers was set to 167 mm. Chills were placed on the side walls at the top and bottom of the casting to accelerate the solidification rate of thick – walled parts, guiding the sequential solidification of the casting and reducing the risk of concentrated shrinkage. A filter was placed at the bottom of the sprue to stabilize the flow rate of the melt, effectively remove harmful impurities, and improve the casting forming quality.
4. Experimental Results and Analysis
4.1 Response Surface Analysis of Tensile Strength
The results of the BBD experiment were analyzed using ANOVA to fit a second – order model for the tensile strength. The variance analysis results of the tensile strength are presented in Table 5.
| Table 5: Variance analysis for tensile strength | |||||
|---|---|---|---|---|---|
| Source | Sum of Squares | Degrees of Freedom | Mean Square | F Value | P Value |
| Model | 1.47 | 9 | 0.1635 | 56.68 | <0.0001 |
| A | 1.02 | 1 | 1.02 | 351.93 | <0.0001 |
| B | 0.3444 | 1 | 0.3444 | 119.39 | <0.0001 |
| C | 0.0780 | 1 | 0.0780 | 27.07 | <0.0001 |
| AB | 0.0002 | 1 | 0.0002 | 0.69 | 0.4182 |
| AC | 0.0009 | 1 | 0.0009 | 3.13 | 0.0997 |
| BC | 0.0007 | 1 | 0.0007 | 2.41 | 0.1416 |
| A² | 0.0970 | 1 | 0.0970 | 33.61 | <0.0001 |
| B² | 0.0057 | 1 | 0.0057 | 1.97 | 0.1813 |
| C² | 0.0025 | 1 | 0.0025 | 0.86 | 0.3665 |
| Residual | 0.0202 | 7 | 0.0029 | – | – |
| Lack of Fit | 0.0083 | 3 | 0.0028 | 0.9256 | 0.5055 |
| Pure Error | 0.0119 | 4 | 0.0030 | – | – |
| Total Error | 1.49 | 16 | – | – | – |
The model’s \(P<0.0001\) and F value of 56.68 indicate that the model is highly significant. The P value of the lack – of – fit term is 0.5055>0.1, and the F value is 0.9256, suggesting that the lack of fit is not significant, and the experimental results can be predicted by this model. The correlation coefficient \(R^{2}=0.9865\) and the adjusted determination coefficient \(R_{Adj}^{2}=0.9691\) demonstrate a high degree of fit between the measured and predicted values of the tensile strength. The coefficient of variation \(C_{v}=2.58\%\) indicates a low deviation between the measured and predicted values, ensuring the reliability of the experimental results.
The regression equation for the tensile strength was established as follows: \(Y_{1}=2.19 + 0.3563A-0.0037B – 0.2075C+0.0075AB-0.015AC + 0.005BC-0.1518A^{2}-0.0368B^{2}-0.0242C^{2}\)
The contour and response surface plots of the tensile strength, as shown in Figure 2 and Figure 3, visually display the interactive effects of the process parameters on the tensile strength. [Insert Figure 2 here: Response surface plot of tensile strength] [Insert Figure 3 here: Contour plot of tensile strength]
From Figure 2, it can be observed that the resin inkjet content is the most significant factor affecting the tensile strength, followed by the printing layer thickness, while the impact of the curing agent is relatively weak. The contour plot in Figure 3 shows that when the curing agent content is constant, the interactive effect of the resin inkjet content and the printing layer thickness on the tensile strength is the most significant.
The increase in resin content leads to an increase in the number of bonding bridges between sand grains and an expansion of the bonding specific surface area, resulting in a positive linear relationship between the tensile strength of the printed sand mold and the resin inkjet content. The printing layer thickness affects the diffusion behavior of the resin between sand layers. As the layer thickness increases, the tensile strength of the printed sand mold gradually decreases. The curing agent mainly influences the curing reaction process. However, due to the drying treatment and 24 – hour waiting period before testing the tensile strength of the “8” – shaped test block, the impact of the curing agent content on the tensile strength is not significant under the interaction of parameter variables.
4.2 Response Surface Analysis of Gas Evolution
Similar to the tensile strength analysis, the variance analysis of gas evolution was conducted, and the results are presented in Table 6.
| Table 6: Variance analysis for gas evolution | |||||
|---|---|---|---|---|---|
| Source | Sum of Squares | Degrees of Freedom | Mean Square | F Value | P Value |
| Model | 35.81 | 9 | 3.98 | 564.2 | <0.0001 |
| A | 32.16 | 1 | 32.16 | 4559.88 | <0.0001 |
| B | 1.82 | 1 | 1.82 | 258.63 | <0.0001 |
| C | 0.0760 | 1 | 0.0760 | 10.78 | 0.0134 |
| AB | 0.0042 | 1 | 0.0042 | 0.5990 | 0.4643 |
| AC | 0.0020 | 1 | 0.0020 | 0.2871 | 0.6087 |
| BC | 0.0012 | 1 | 0.0012 | 0.1737 | 0.6893 |
| A² | 1.68 | 1 | 1.68 | 238.27 | <0.0001 |
| B² | 0.0030 | 1 | 0.0030 | 0.4272 | 0.5342 |
| C² | 0.0217 | 1 | 0.0217 | 3.07 | 0.1230 |
| Residual | 0.0494 | 7 | 0.0071 | – | – |
| Lack of Fit | 0.0269 | 3 | 0.0090 | 1.59 | 0.3245 |
| Pure Error | 0.0225 | 4 | 0.0056 | – | – |
| Total Error | 35.86 | 16 | – | – | – |
The model’s \(P(<0.0001)<0.005\) and F value of 564.2 indicate a highly significant model. The P value of the lack – of – fit term is 0.3245>0.1, and the F value is 1.59, suggesting that the model is suitable for predicting the experimental results. The correlation coefficient \(R^{2}=0.9986\) and the adjusted determination coefficient \(R_{Adj}^{2}=0.9969\) confirm a high degree of fit between the measured and predicted values of the gas evolution. The coefficient of variation \(C_{v}=0.849\%\) indicates a low deviation between the measured and predicted values, ensuring the reliability of the experimental results.
From Figure 4, it can be seen that the resin inkjet content has a more significant impact on the gas evolution compared to the curing agent. The change in the printing layer thickness has a weak effect on the gas evolution. The contour plot in Figure 5 shows that the combination of the resin inkjet content and the curing agent content has the most significant impact on the gas evolution.
