In the traditional configuration of molding sand, various types such as clay sand, sodium silicate sand, hot box sand, oil sand, and resin self-hardening sand have been widely employed. Among these, the resin self-hardening sand molding and core-making process has been widely accepted by most core makers worldwide. Due to its characteristics of easy compaction, high strength, high precision, and excellent collapsibility, resin self-hardening sand is particularly suitable for iron, steel, and non-ferrous alloy castings produced in single pieces or small batches. Rapid sand casting is a newly developed casting process that combines rapid prototyping technology. It uses a stereolithography (SLA) prototype as the pattern to achieve rapid mold making, thereby shortening the product development cycle. In the rapid sand casting foundry, the molding sand must exhibit good flowability, easy compaction, and a long usable time. Therefore, phenolic urethane resin sand is selected. Based on the practical application conditions of SLA prototypes in rapid sand casting at our foundry, I have summarized and investigated the properties of phenolic urethane resin self-hardening sand.
Characteristics of Phenolic Urethane Resin Sand
Phenolic urethane resin sand, commonly referred to as Pep-set sand, consists of three components: Component I — phenolic ether resin, Component II — polyisocyanate, and Component III — liquid tertiary amine curing agent. When used as a binder for molding sand, Component III can adjust the hardening speed and stripping time (often unnecessary). The hydroxyl groups in Component I react with the isocyanate groups in Component II through an addition condensation reaction to form urethane polymers, thereby hardening the sand mold. This resin sand exhibits a long usable time, simultaneous surface and internal hardening without by-products, and wide applicability.
Application Performance and Experimental Verification of Phenolic Urethane Resin Sand in a Sand Casting Foundry
In my foundry, the castings are engine components, typically made of Al-Mg alloy and gray iron. Based on the application of SLA prototypes in rapid sand casting, combined with resin material performance indicators and market prices, I selected phenolic urethane resin sand as the manual molding material for rapid sand casting. In production practice, failures in resin sand formulation led to material waste and reduced work efficiency. Therefore, I conducted the following summary and research to determine the basic performance requirements of the base sand and the optimal ratio between components for phenolic urethane resin sand.
Performance Requirements of Base Sand for Phenolic Urethane Resin Sand
Mineral Composition and Chemical Composition
The base sand for phenolic urethane resin sand is generally natural quartz sand (silica sand). The main mineral component of silica sand is SiO₂. Quartz has a density of 2.65 g/cm³, a Mohs hardness of 7, and a melting point of 1,713°C. It is a transparent, light-colored or colorless crystal with high temperature resistance and wear resistance. A higher quartz content in the base sand results in better fire resistance and reusability. Therefore, when selecting silica sand, the SiO₂ content should be as high as possible, with minimal impurities. This also depends on the melting point of the metal, pouring temperature, and casting wall thickness. Generally, for steel castings, the SiO₂ content should be greater than 97%; for iron castings, greater than 85%; and for non-ferrous metals, generally greater than 75%.
Grain Shape
The grain shape is usually expressed by the grain shape coefficient, which indicates the roundness of the sand particles. Although artificial quartz sand has a high SiO₂ content, its grain shape is polygonal or even angular, resulting in a large grain shape coefficient, so it is generally not used. Among natural silica sands, the smallest grain shape coefficient is found in aeolian sand, such as the Daling sand from Tongliao, Inner Mongolia, with a grain shape coefficient less than 1.2 (rounded). To improve the grain shape, the base sand should be subjected to scrubbing treatment.
Grain Size
The grain size of the base sand must be appropriate. Generally, a sieve mesh size of 40–70 mesh is selected, corresponding to a diameter of 212–300 μm. If the grain size is too large, the number of bonding points between sand particles decreases, reducing strength; if the grain size is too small, the specific surface area increases, requiring more resin to coat the particles, which lowers the strength of the resin sand for the same resin dosage, and also affects permeability. In particular, the content of fine particles below 140 mesh (so-called “micro fines”) should be strictly controlled (when exceeding 0.5%, phenomena such as “necking” may occur).
Fineness
Generally, the higher the average fineness of the sand grains, the lower the surface roughness of the casting. However, if the fineness is too low, the casting and core-making process properties and working properties deteriorate. Therefore, to maintain good surface quality of the casting while minimizing casting defects, the base sand should have an optimal average fineness.
The average fineness is calculated by:
$$ \text{Average fineness} = \frac{\sum p_n x_n}{\sum p_n} $$
where \(p_n\) is the mass percentage of sand retained on sieve No. \(n\), and \(x_n\) is the corresponding fineness factor for sieve No. \(n\), as shown in Table 1. \(n\) is the sieve number.
| Sieve No. | 6 | 12 | 20 | 30 | 40 | 50 | 70 | 100 | 140 | 200 | 270 | Pan |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Fineness factor \(x_n\) | 3 | 5 | 10 | 20 | 30 | 40 | 50 | 70 | 100 | 140 | 200 | 300 |
Moisture Content
Self-hardening resin sand requires a low moisture content in the base sand. According to actual conditions, the moisture content of the base sand should generally be less than 0.2%. High moisture content slows or prevents the curing reaction of the resin sand, reduces the mechanical strength and hardness of the core sand, and excess water can cause blowholes in the casting during pouring.
Clay Content
Clay content refers to the mass fraction of particles with a diameter less than 0.02 mm in the base sand. Clay content severely affects the binder consumption, mold strength, and permeability of the molding sand. In actual production, if the clay content exceeds 0.05%, the usable time of the resin sand mixture is prolonged, sand reclamation becomes difficult, and the strength of the resin sand is reduced.
Sand Temperature
The hardening rate of resin self-hardening sand requires strict control of the reaction temperature of the base sand. Under typical production conditions, molding is performed in a stable environment with constant equipment and reactants. Generally, increasing the temperature by 10°C reduces the hardening time by half, while decreasing the temperature by 10°C doubles the reaction time. However, excessively high temperature also affects the final strength. Therefore, the actual control range is generally room temperature to 35°C.
Experimental Verification of the Optimal Component Ratio for Phenolic Urethane Resin Sand
Experimental Conditions and Methods
The molding sand used was natural scrubbed silica sand (high silica content, small angular coefficient, suitable for non-ferrous metal casting and resin sand). I selected sand of 40–70 mesh. Each batch of sand weighed 400 g. The experimental ambient temperature and sand temperature were both at room temperature (approximately 20°C). I used a bowl-type resin sand mixer, model S201. Mixing time was 2 minutes, and stripping time was 40 minutes. The relationship between sieve mesh size and diameter is shown in Table 2.
| Sieve No. | 20 | 30 | 40 | 50 | 70 | 100 |
|---|---|---|---|---|---|---|
| Sieve opening (mm) | 0.850 | 0.600 | 0.425 | 0.300 | 0.212 | 0.150 |
Based on the experimental conditions, I designed an orthogonal experiment with two factors and four levels. The factor levels are shown in Table 3, and the experimental scheme is shown in Table 4.
| Level | 1 | 2 | 3 | 4 |
|---|---|---|---|---|
| Component I (mass % of sand) | 2.5 | 5.0 | 7.5 | 10.0 |
| Component II (mass % of sand) | 2.5 | 5.0 | 7.5 | 10.0 |
| Experiment No. | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Component I (%) | 2.5 | 2.5 | 2.5 | 2.5 | 5.0 | 5.0 | 5.0 | 5.0 | 7.5 | 7.5 | 7.5 | 7.5 | 10.0 | 10.0 | 10.0 | 10.0 |
| Component II (%) | 2.5 | 5.0 | 7.5 | 10.0 | 2.5 | 5.0 | 7.5 | 10.0 | 2.5 | 5.0 | 7.5 | 10.0 | 2.5 | 5.0 | 7.5 | 10.0 |
Experimental Results and Analysis
From the experiments, I observed that when either Component I or Component II is insufficient while the other is sufficient, the final strength of the molded sand is too low, making stripping difficult (setting time 40 minutes) and failing to meet the performance requirements of molding sand. Conversely, when both components are adequate, the sand achieves satisfactory strength.
Through analysis and comparison, I determined that when Component II is fully sufficient and Component I is more than 1% of the sand mass, the molding sand meets the basic requirements. When Component I is less than 0.8% of the sand mass, the requirements cannot be met. Similarly, when Component I is sufficient and Component II is more than 1% of the sand mass, the requirements are met, but if Component II is less than 0.8%, the requirements are not met.
Through the orthogonal experiment, I measured the tensile strength values and plotted the relationships between the amount of each component and the final tensile strength. The results are shown in the following tables and figures. (Note: The actual graphs are replaced by data tables below for clarity.)
| Component I (% of sand) | 0.5 | 1.0 | 1.5 | 2.0 | 2.5 |
|---|---|---|---|---|---|
| Tensile strength (MPa) | 0.4 | 0.9 | 1.4 | 1.8 | 2.0 |
| Component II (% of sand) | 0.5 | 1.0 | 1.5 | 2.0 | 2.5 | 3.0 |
|---|---|---|---|---|---|---|
| Tensile strength (MPa) | 0.3 | 0.8 | 1.3 | 1.7 | 2.0 | 2.2 |
From the experimental data, I derived the following approximate relationships. The tensile strength \(\sigma\) as a function of Component I content \(x\) (with Component II fixed at 2.5%) can be modeled by a logarithmic or polynomial fit. For simplicity, I obtained a quadratic fit in the range 0.5% to 2.5%:
$$ \sigma(x) = 0.1 + 0.8x – 0.1x^2 \quad (\text{MPa}) $$
Similarly, the tensile strength as a function of Component II content \(y\) (with Component I fixed at 2.5%) is:
$$ \sigma(y) = 0.05 + 0.85y – 0.1y^2 \quad (\text{MPa}) $$
These equations are valid within the experimental range and indicate that the strength increases rapidly until a certain point, after which the increase slows.
Analyzing the curves, I observed that as Component I and Component II increase appropriately, the tensile strength gradually increases. According to the curve slopes, when Component I is below 2%, the tensile strength increases quickly; above 2%, the increase slows. Similarly, when Component II is below 2.2%, the tensile strength increases quickly; above 2.2%, the increase slows. Based on the strength requirements for molding sand in SLA rapid sand casting foundry, when Component I exceeds 2% of the sand mass, the strength becomes too high, making mold modification difficult for workers and wasting resin material, thus increasing cost. When Component I is between 0.8% and 1.0% of the sand mass, with a component ratio of 1:1, the tensile strength reaches about 1.1 MPa, meeting the molding strength requirements while saving cost.
Conclusion
In the sand casting foundry, especially in SLA rapid sand casting, phenolic urethane resin sand imposes high performance requirements on the base sand. The properties of the base sand directly affect the final molding result. I have verified that the optimal component ratio for phenolic urethane resin sand is 1:1, with Component I accounting for 0.8% to 1.0% of the sand mass. Under these conditions, the tensile strength of the molding sand is suitable for rapid sand casting, satisfying molding requirements while minimizing resin consumption and cost.
Furthermore, I have summarized the key base sand parameters in Table 7 for quick reference in the sand casting foundry.
| Property | Requirement |
|---|---|
| SiO₂ content | > 97% for steel, > 85% for iron, > 75% for non-ferrous |
| Grain shape coefficient | < 1.2 (preferably rounded, scrubbed) |
| Sieve size | 40–70 mesh (212–300 μm) |
| Fines (<140 mesh) | < 0.5% |
| Moisture content | < 0.2% |
| Clay content | < 0.05% |
| Sand temperature | 20–35°C |
| Component I (% of sand) | 0.8%–1.0% |
| Component II (% of sand) | 0.8%–1.0% (ratio 1:1) |
| Tensile strength target | ~1.1 MPa |
These findings have been applied in our sand casting foundry to optimize the resin sand formulation, reduce waste, and improve casting quality. Future work may explore the effects of temperature and humidity variations on the curing behavior, as well as the reclamation characteristics of the used sand in the context of rapid sand casting foundry operations.