Type II urea resin sand, as a type of resin sand, is widely used in precision casting due to its excellent collapsibility and dimensional stability. However, its recycling requires a targeted design that combines common resin sand treatment technologies with the specific characteristics of urea resin. The core objective of its recycling is to remove the failed binder film on the sand grain surface through physical or chemical means, restoring the original sand's flowability and surface properties. Simultaneously, it's crucial to control the loss on ignition, particle size distribution, and micron content of the recycled sand to meet the high performance requirements of precision casting molding sand.
The recycling of Type II urea resin sand must begin with the used sand recovery process. Used sand generated during casting production typically contains incompletely degraded resin films, carbonized residues, and metallic impurities. It must first undergo pretreatment steps such as vibration crushing, magnetic separation, and sieving to remove large impurities and iron filings. Then, it needs further separation of micronized powder and light contaminants using air separation or inertial separation devices. The key at this stage is controlling the sand grain breakage rate to avoid over-crushing, which leads to angular sand grains and affects the flowability and compactness of the recycled sand.
Dry regeneration is a common method for regenerating Type II urea resin sand. This method removes the residual resin film from the surface of sand particles through mechanical friction, and is suitable for used sand with a brittle binder film. Specific equipment includes vibratory crushing regeneration machines and centrifugal impact regeneration machines. Their working principle is to peel off the failed film layer by utilizing the mutual impact between sand particles or the friction with a target plate. Dry regeneration is simple and has low energy consumption, but the removal rate is greatly affected by the hardness and thickness of the resin film. For thermosetting resins like urea resin, the regeneration effect needs to be improved by optimizing equipment parameters (such as vibration frequency and impact speed).
Thermal regeneration completely decomposes the resin film through high-temperature roasting, and is suitable for precision casting applications where high-quality recycled sand is required. When the used sand is heated to above 800℃, the organic components in the urea resin decompose into carbon dioxide, water, and a small amount of nitrogen oxides. Residual inorganic matter can be removed by subsequent screening. While thermal regeneration can achieve a high removal rate, it has higher energy consumption and requires a complete exhaust gas treatment system to avoid environmental pollution. In actual production, thermal and dry methods are often combined to form a composite process of "thermal + mechanical regeneration." This involves softening the resin film at high temperatures and then mechanically peeling it off, balancing efficiency and quality.
Wet regeneration is less common in Type II urea resin sand regeneration, primarily because it is suitable for used sand where the binder film is water-soluble, while urea resin requires chemical decomposition for removal. If a wet process is used, it requires chemical treatment with acidic or alkaline solutions, but this process may introduce new impurities and has high wastewater treatment costs. Therefore, it is mostly used in special scenarios where extremely high surface cleanliness of the sand particles is required.
Quality control of regenerated sand is a crucial aspect of Type II urea resin sand regeneration. The loss on ignition (reflecting residual organic matter content), particle size distribution, and micron content of the regenerated sand need to be tested regularly to ensure compliance with precision casting standards. For example, excessive loss on ignition may lead to porosity defects in the casting, while excessive micron content will reduce the permeability of the sand mold. The performance of recycled sand can be effectively controlled by adjusting recycling process parameters (such as calcination temperature and mechanical friction intensity) or by incorporating virgin sand.
The application of recycled sand requires optimized proportioning based on casting process requirements. In precision casting, recycled sand is often mixed with virgin sand in a certain proportion to balance cost and performance. For example, the surface sand (the sand layer in direct contact with the casting) may use a higher proportion of virgin sand to ensure surface quality, while the back sand (the sand layer supporting the surface sand) can increase the proportion of recycled sand to reduce costs. Furthermore, the reuse of recycled sand requires appropriate sand mixing processes to ensure that the resin binder uniformly coats the sand particles, avoiding uneven hardening due to differences in surface activity of the recycled sand.
The recycling of Type II Urea resin sand requires combining dry and hot processes, involving pretreatment, demolding, quality inspection, and proportion optimization to achieve efficient reuse of old sand. The application of recycled sand in precision casting not only reduces production costs but also reduces waste emissions, aligning with the trend of green manufacturing. In the future, with the continuous optimization of recycling technology, the recycling rate and performance of Type II Urea resin sand will be further improved, providing a more sustainable molding sand solution for the precision casting industry.
