In the rapidly evolving machinery industry, the demand for high-performance and superior-quality machine tools has intensified, necessitating advancements in the internal and external quality of machine tool castings. Our exploration into cold-set furan resin sand for producing machine tool castings stems from the critical need to achieve precise dimensions, smooth surfaces, and easy cleaning of cast components. This approach significantly enhances the competitiveness of machine tool products in international markets, where surface finish directly impacts aesthetic appeal and functionality. Over the past few years, we have conducted extensive trials and practical applications, producing over 200 types of machine tool castings with weights ranging from a few kilograms to several tons, cumulatively exceeding 100 tons. These castings have been successfully machined, assembled into machine tools, and exported, demonstrating the viability of this technology.
The selection of raw materials is paramount in cold-set furan resin sand systems for machine tool castings. For base sand, we prioritize silica sand with spherical grain shapes to minimize surface area, ensure complete contact between grains, and enhance flowability. The grain size distribution should concentrate within four adjacent sieve sizes to achieve high strength and smooth casting surfaces. Fines must be kept to a minimum, and alkaline substances should be avoided due to their inhibitory effects on catalysts. Typically, natural or lake sands are preferred over sea sands. In our practice, we utilize sands from Inner Mongolia, such as Dalin and Bahuta sands, which exhibit low clay content (below 0.5%), acid demand value less than 5, and SiO2 content exceeding 90%. The following tables summarize the properties of these sands:
| Sieve Size (Mesh) | Dalin Sand (%) | Bahuta Sand (%) |
|---|---|---|
| 50 | 0.2 | 0.1 |
| 70 | 18.5 | 20.3 |
| 100 | 55.8 | 53.7 |
| 140 | 22.1 | 23.5 |
| 200 | 2.5 | 1.8 |
| Pan | 0.9 | 0.6 |
| Component | Dalin Sand (%) | Bahuta Sand (%) |
|---|---|---|
| SiO2 | 91.5 | 92.0 |
| Al2O3 | 4.2 | 3.8 |
| Fe2O3 | 1.1 | 1.0 |
| CaO + MgO | 0.8 | 0.7 |
| Alkalis | 1.5 | 1.2 |
| Loss on Ignition | 0.9 | 1.3 |
| Property | Dalin Sand | Bahuta Sand |
|---|---|---|
| Grain Shape | Round | Round |
| Clay Content (%) | 0.3 | 0.4 |
| Acid Demand Value | 4.2 | 4.5 |
For the furan resin, we employ types such as ZY-1 and ZY-2, which are furfuryl alcohol-based polymers forming linear molecular chains that cross-link into three-dimensional networks upon catalysis. These resins offer low viscosity, high strength, and minimal nitrogen content to prevent casting defects. The catalyst of choice is p-toluenesulfonic acid, selected for its adjustable hardening speed, high final strength, low hygroscopicity, and compatibility with sand reclamation. The key properties of these materials are outlined below:
| Parameter | ZY-1 Resin | ZY-2 Resin |
|---|---|---|
| Appearance | Brown viscous liquid | Dark brown viscous liquid |
| Density (g/cm³) | 1.18 | 1.20 |
| Viscosity (mPa·s) | 35 | 40 |
| pH | 6.5 | 6.8 |
| Free Formaldehyde (%) | 0.8 | 0.9 |
| Nitrogen Content (%) | 1.5 | 2.0 |
| Shelf Life (months) | 6 | 6 |
| Parameter | Value |
|---|---|
| Total Acidity (%) | 65 |
| p-Isomer Content (%) | 95 |
| Free Sulfonic Acid (%) | 1.0 |
| Moisture Content (%) | 2.5 |
| Melting Point (°C) | 105 |
| Form | Crystalline solid |
The hardening process of cold-set furan resin sand is influenced by several factors, including catalyst addition, resin content, temperature, and humidity. We have derived empirical relationships to optimize these parameters for machine tool castings. The catalyst addition, expressed as a percentage of resin weight, significantly affects the hardening rate and final strength. Let $C$ be the catalyst addition (%), $R$ the resin addition (%), and $S_t$ the tensile strength at time $t$ (MPa). Our experiments show that strength peaks at a specific catalyst level, described by:
$$S_t = k_1 \cdot C \cdot e^{-k_2 \cdot C} + b$$
where $k_1$, $k_2$, and $b$ are constants dependent on resin type and sand properties. For instance, with ZY-2 resin and Dalin sand, we observed maximum 24-hour tensile strength at $C = 40\%$, beyond which strength declines due to excessive gas evolution. Similarly, resin addition directly correlates with strength, as shown by:
$$S_t = \alpha \cdot R^\beta$$
where $\alpha$ and $\beta$ are empirical coefficients. In our system, $R = 1.5\%$ yields $S_{24} \geq 1.2$ MPa, sufficient for most machine tool casting applications. Temperature ($T$ in °C) accelerates hardening, with the rate doubling for every 10°C increase, approximated by:
$$\frac{dS}{dt} = A \cdot e^{-E_a / (R_g \cdot T)}$$
where $A$ is a pre-exponential factor, $E_a$ the activation energy, and $R_g$ the gas constant. We maintain sand temperature between 15°C and 30°C to avoid reduced usable time and final strength. Humidity ($H$ in %) also plays a role; high humidity slows hardening and lowers strength due to hindered water evaporation. The combined effect can be modeled as:
$$S_t = \gamma \cdot \left( \frac{T}{T_0} \right)^\delta \cdot \left( \frac{H_0}{H} \right)^\epsilon$$
where $\gamma$, $\delta$, $\epsilon$ are constants, and $T_0$, $H_0$ are reference values. Based on our findings, we standardized the sand mixtures as follows:
| Component | Face Sand (%) | Backing Sand (%) |
|---|---|---|
| Base Sand | 100 | 100 |
| Furan Resin | 1.5 | 1.2 |
| Catalyst (p-toluenesulfonic acid) | 40% of resin | 40% of resin |
In molding and core-making for machine tool castings, the fluidity of resin sand allows uniform compaction with minimal effort, reducing distortions in patterns and core boxes. We design casting processes with core prints and clearances of 0.5-1.0 mm, similar to green sand practices, leveraging the high dimensional stability post-hardening. Deformation after demolding is typically less than 0.5 mm, ensuring precision. For wooden patterns, we avoid coatings like shellac or nitro-based paints to prevent sticky sand issues, opting instead for polyurethane or epoxy coatings. Cores and molds require controlled drying; we follow a ramp-and-hold schedule: heat to 150°C at 20°C/h, hold for 2 hours, then cool slowly to ambient temperature. This prevents cracks and ensures integrity.
Our production trials involved critical machine tool castings, such as bed frames and headstocks. For example, a bed frame measuring 2500 mm × 500 mm × 300 mm with wall thicknesses of 15-25 mm and weight of 1.2 tons was produced using a two-part mold with resin sand. The casting exhibited excellent surface smoothness, sharp edges, and dimensional accuracy within JIS Grade 4 tolerances. Similarly, a headstock casting of 600 mm × 400 mm × 300 mm, weighing 150 kg and made from FC300铸铁 (equivalent to ASTM A48 Class 30), utilized eight resin sand cores. The chemical composition and mechanical properties met specifications, as shown below:
| Element/Property | Value |
|---|---|
| Carbon (C, %) | 3.2 |
| Silicon (Si, %) | 1.8 |
| Manganese (Mn, %) | 0.7 |
| Phosphorus (P, %) | 0.05 |
| Sulfur (S, %) | 0.02 |
| Tensile Strength (MPa) | 300 |
| Hardness (HB) | 200 |
The quality of machine tool castings produced with resin sand surpasses that of clay sand, flowable sand, and double-fast cement sand, with minimal veining, burn-on, or sand inclusions. However, we encountered minor gas defects in some bed frames due to rapid gas evolution, which we mitigated by optimizing venting designs. Overall, scrap rates for internal and external defects can be reduced to around 2% by combining resin sand with high-temperature melting (e.g., 1500-1550°C pouring temperature) and specialized coatings in critical areas. The following image illustrates a typical machine tool foundry setup using this process:
Regarding environmental and health aspects, we measured hazardous substances during mixing and pouring operations. The results, compared to national standards, highlight the need for protective measures:
| Substance | Measured Concentration (mg/m³) | National Standard (mg/m³) | Remarks |
|---|---|---|---|
| Formaldehyde | 0.8 | 0.5 | Exceeds limit; ventilation required |
| Hydrogen Sulfide | 0.1 | 10 | Within safe range |
| Benzene | Trace | 5 | Negligible |
| Carbon Monoxide | 15 | 30 | Acceptable |
| Ammonia | 0.5 | 20 | Well below limit |
In conclusion, the application of cold-set furan resin sand in machine tool casting production has proven highly effective over two years of practice. It ensures dimensional accuracy, superior surface finish, and easy cleanup, positioning it as a forward-looking technology for the casting industry. However, to fully leverage its benefits, we must intensify research on coatings to prevent metal penetration and further improve surface quality. Additionally, cost reduction through sand reclamation is essential to mitigate the high expenses of resins and catalysts, thereby expanding its applicability. Future work will focus on optimizing process parameters and enhancing environmental controls to sustain the production of high-quality machine tool castings.