In our foundry, we have transitioned from traditional dry mold casting to wet mold casting for producing machine tool castings, which has significantly enhanced our operational efficiency and product quality. Since 1969, we have successfully cast over twenty thousand tons of qualified machine tool castings using this method. This shift has not only boosted productivity but also improved working conditions by reducing high temperatures and dust levels in the workshop, aligning with the principles of sustainable industrial development. The key to wet mold casting lies in ensuring sufficient surface strength and permeability of molds and cores to withstand the impact, heat, and pressure of molten iron, while allowing gases to escape during pouring to prevent defects. Through optimized sand mixtures and process measures, we have effectively applied wet mold casting to complex machine tool castings.
The quality of machine tool castings heavily depends on the properties of mold and core sand, including permeability, wet compressive strength, and moisture content. We conduct daily tests and maintain strict control over these parameters, with re-mixing required if standards are not met. For instance, permeability has been improved from lower values to over 100 for general castings and above 120 for large machine tool castings by coarsening new sand grain size from 50-70 mesh to 40 mesh and increasing the proportion of new sand in the mixture. This enhancement reduces defects like blowholes and sand inclusions. The wet compressive strength is critical for withstanding the weight and complexity of machine tool castings; we achieve this by adding sodium bicarbonate to activate bentonite, which boosts strength. Moisture content is carefully managed to avoid issues such as runouts or sand drops, typically controlled at 4-5% for facing sand and 3-4% for backing sand, with seasonal adjustments.
| Component | Percentage (%) | Property | Target Value |
|---|---|---|---|
| New Sand | 60-70 | Permeability | >100 (General), >120 (Large Castings) |
| Bentonite | 5-8 | Wet Compressive Strength (kPa) | 50-70 |
| Sodium Bicarbonate | 0.5-1 | Moisture Content (%) | 4-5 (Facing), 3-4 (Backing) |
| Other Additives | 2-5 | Sand Hardness (Units) | 80-90 ±5 |
Permeability is a vital factor, as wet sand has lower permeability than dry sand, leading to higher gas evolution. We focus on increasing permeability to prevent defects, using the relationship: $$P = \frac{V \cdot H}{A \cdot t}$$ where \(P\) is permeability, \(V\) is air volume, \(H\) is sample height, \(A\) is cross-sectional area, and \(t\) is time. For wet compressive strength, we apply the formula: $$\sigma_c = \frac{F}{A}$$ where \(\sigma_c\) is compressive strength, \(F\) is the force applied, and \(A\) is the area. By activating calcium bentonite with sodium bicarbonate, we convert it to sodium bentonite, enhancing the thermal strength of the sand and reducing issues like scabbing and sand expansion in machine tool castings.
To achieve uniform compaction, we increased the sand-to-mold distance in flask design to 50-80 mm, ensuring better surface strength and reducing defects such as rat tails and sand drops. Models are made sturdier, often using metal or wood-metal combinations, and height is minimized to facilitate sand ramming. For core boxes, we employ durable structures that allow easy sand compaction and handling, with simplified core bones to avoid hindering compaction. In one example, for large machine tool castings like bed pieces, we eliminated core irons and built cores directly on pouring platforms, improving efficiency and precision.
Coating and surface drying are essential for enhancing surface strength and refractoriness. Our coating mixture consists of 50% small flake graphite and 50% powdered black graphite, with 10% fire clay, mixed into a paste and diluted to a specific consistency. We apply multiple coats for large machine tool castings and use diesel blowtorches for surface drying, achieving a dry layer of about 10-15 mm. This process must be gradual to prevent cracking or shelling. Additionally, natural drying over several days is preferred when space permits, often by suspending cores to save area.
The gating system is designed to minimize冲刷 of molds and cores. We use a semi-closed system with a choke in the runner to control flow velocity, employing a cross-sectional area ratio such as \(A_{\text{runner}} : A_{\text{choke}} : A_{\text{ingate}} = 1.2 : 1 : 1.5\). This reduces metal velocity and prevents defects like冲砂. For complex machine tool castings, dispersed ingates are used to distribute heat evenly. Venting is crucial due to high gas evolution; we incorporate multiple vents, air channels, and materials like coke or straw in cores to facilitate gas escape, ensuring the quality of machine tool castings.
| Process Measure | Specification | Impact on Casting Quality |
|---|---|---|
| Compaction Hardness | 80-90 units ±5 | Reduces sand drops and improves surface finish |
| Sand-to-Mold Distance | 50-80 mm | Ensures uniform compaction and minimizes defects |
| Gating System Ratio | \(A_r : A_c : A_i = 1.2 : 1 : 1.5\) | Controls flow and reduces冲刷 |
| Drying Layer Thickness | 10-15 mm | Enhances surface strength and prevents cracks |
In practical applications, we have implemented wet mold casting for several typical machine tool castings. For a bed piece measuring 2000 mm × 800 mm × 500 mm with a weight of 500 kg, we switched from dry mold pit casting to wet mold with split pattern plate molding. This change increased efficiency by over 50% and improved working conditions by eliminating the need for pit work. Similarly, for a headstock component of 600 mm × 500 mm × 400 mm and 200 kg, we adopted wet mold with split patterns and integrated cores, boosting productivity by 40% and ensuring dimensional accuracy. These examples demonstrate the versatility of wet mold casting in producing high-quality machine tool castings.
The benefits of wet mold casting for machine tool castings are substantial. Productivity has risen by 30-50% due to simplified operations and faster cycles. Labor conditions have improved markedly, with reduced dust and heat exposure. Workshop space and equipment utilization have increased as casting cycles shortened from two days to one. Additionally, fuel consumption has dropped significantly; previously, we used hundreds of tons of coal monthly, but now with diesel for drying, we save approximately $2,000 per month. This method also reduces flask deformation and extends equipment life, contributing to cost-effective production of machine tool castings.
Despite these successes, challenges such as runouts, blowholes, and sand inclusions occasionally arise due to operational inconsistencies. We continue to refine our processes through实践 and learning, aiming to further optimize wet mold casting for machine tool castings. By sharing experiences and adopting best practices, we strive to enhance the reliability and efficiency of our foundry operations, supporting the production of durable and precise machine tool castings for various industrial applications. The ongoing evolution of this工艺 underscores its potential to revolutionize the casting industry for machine tool components.
In summary, wet mold casting has proven to be a transformative approach for machine tool castings, offering a balance of performance, efficiency, and environmental benefits. Through careful control of sand properties and process parameters, we have achieved significant improvements in the quality and output of machine tool castings. As we move forward, we will focus on standardizing operations and exploring further innovations to maintain our leadership in producing high-integrity machine tool castings. The journey from dry to wet mold casting has not only met technical demands but also fostered a safer and more productive foundry environment, paving the way for future advancements in machine tool casting technologies.