In recent years, full mold casting has gained significant attention in the manufacturing industry, particularly for producing machine tool castings. As someone deeply involved in foundry processes, I have observed that this method offers numerous advantages over traditional sand casting, such as simplified pattern making, shorter production cycles, higher dimensional accuracy of castings, and reduced labor hours. However, the process is not without challenges; issues like deformation, cold shuts, and slag inclusions often arise during full mold casting. Through extensive experimentation on various machine tool castings, we have identified effective solutions to these problems, leading to substantial economic benefits and paving the way for broader adoption of this innovative technique.
Full mold casting, which utilizes expandable polystyrene (EPS) foam patterns, has been widely applied in our production of stamping die blanks. The patterns mirror the final casting shape, eliminating the need for complex core assemblies and reducing material and labor costs associated with core box fabrication. For instance, EPS foam sheets cost approximately $500 per cubic meter, compared to red pine wood at $1500 per cubic meter, resulting in significant material savings. Additionally, the use of plastic patterns in full mold casting accelerates production, enhances casting precision, and minimizes labor input. Despite these benefits, the low strength and rigidity of plastic patterns pose challenges, especially for thin-walled, intricate machine tool castings, where deformation during sand filling and compaction is common. Moreover, the vaporization of the foam pattern during molten metal pouring absorbs heat, causing rapid cooling and potential defects like cold shuts and slag inclusions. Initially, these limitations restricted our use of full mold casting to thick-walled stamping components. To expand its application to machine tool castings, we undertook a systematic approach, starting with simpler geometries and progressing to more complex parts.
Our experimentation involved a range of machine tool castings, including discs, flanges, worktable bodies, base frames, slide bodies, fixture bodies, and large conveyor bases, with a total mass of approximately 50 tons. The selection criteria focused on part complexity, wall thickness, and overall dimensions to evaluate the feasibility of full mold casting. For example, we tested components with varying geometries to assess deformation risks and cooling rates. The table below summarizes the key parameters and outcomes of these trials, highlighting the dimensions, masses, and any defects observed, such as local sand burning or minor cold shuts in rib sections. Most castings, except for a base frame requiring minor welding, met quality standards without significant issues, demonstrating the potential of this process for machine tool castings.
| Cast Part Name | Dimensions (mm) | Mass (kg) | Observed Defects | Remarks |
|---|---|---|---|---|
| Disc | Ø600 × 100 | 150 | None | Fully qualified |
| Flange | Ø800 × 120 | 200 | None | Fully qualified |
| Worktable Body | 1000 × 800 × 200 | 300 | None | Fully qualified |
| Base Frame | 1500 × 1000 × 250 | 500 | Local sand burning | Minor repairs needed |
| Slide Body | 1200 × 600 × 180 | 350 | None | Fully qualified |
| Fixture Body | 900 × 700 × 150 | 250 | None | Fully qualified |
| Conveyor Base | 2000 × 1500 × 300 | 800 | None | Fully qualified |
One of the primary challenges in full mold casting for machine tool castings is preventing pattern deformation during sand filling and compaction. Machine tool castings often feature box-like structures with multiple walls, internal ribs, and cavities, making them susceptible to distortion due to the low rigidity of EPS patterns. To address this, we implemented several measures. For castings with internal cavities, we ensured uniform sand filling around both the inner and outer walls, maintaining thin layers and simultaneous compaction to avoid bulging or inward collapse. The compaction force was controlled to achieve even density without excessive pressure. For non-penetrating upper and lower cavities, we used shaped foam support blocks during pattern making; these were removed after mold turnover, allowing the upper cavity to form via cope drag. In multi-cavity castings, we adopted segmented patterns with interlocking joints to facilitate step-by-step sand filling and compaction, preserving geometric accuracy and dimensions.
Cold shuts and slag inclusions are common defects in full mold casting due to rapid heat loss from foam vaporization. To mitigate these, we optimized pouring parameters and gating systems. Increasing the pouring temperature alone was insufficient; we also enlarged the gating system to reduce filling time and ensure rapid cavity filling. The gating ratio was adjusted to enhance metal distribution, with multiple ingates along the length of the casting to minimize flow distance and maintain higher metal temperature in the runners. Specifically, we prioritized bottom gating systems to promote steady upward metal flow, reducing turbulence and cold shut tendencies. The ingates were positioned at thinner sections to retain heat in vulnerable areas. The gating design was based on empirical parameters, including casting mass, wall thickness, and轮廓 dimensions, as summarized in the table below. For instance, the cross-sectional area of the sprue was determined using the formula: $$ A_s = k \cdot \sqrt{M} $$ where \( A_s \) is the sprue area, \( M \) is the casting mass, and \( k \) is an empirical coefficient ranging from 0.6 to 0.8 depending on part geometry.
| Casting Mass (kg) | Gating Ratio (Sprue:Runner:Ingate) | Sprue Cross-Sectional Area (cm²) | Remarks |
|---|---|---|---|
| 100–300 | 1:1.5:1.2 | 8–12 | For shorter runners, use lower ratio |
| 300–600 | 1:2:1.5 | 12–18 | Adjust based on wall thickness |
| 600–1000 | 1:2.5:2 | 18–25 | For longer runners, use higher ratio |
To illustrate the practical application, consider the conveyor base casting with a mass of 800 kg, made from gray iron (Grade HT250). This machine tool casting features a complex structure with internal ribs and multiple cavities. The pattern was segmented to ease sand compaction, and foam support blocks were used for non-penetrating cavities, removed after molding. The gating system was designed along one side of the length, with a continuous runner distributing metal evenly to minimize temperature gradients. The empirical formula for calculating the pouring time \( t \) in seconds is: $$ t = \frac{M}{\rho \cdot A \cdot v} $$ where \( M \) is the casting mass, \( \rho \) is the metal density, \( A \) is the total ingate area, and \( v \) is the flow velocity. For this casting, we achieved a pouring time of under 30 seconds, preventing cold shuts.
The economic benefits of using full mold casting for machine tool castings are substantial. Comparing wood patterns to plastic patterns, the cost savings are evident. For a typical machine tool casting like the conveyor base, wood pattern construction required 1.2 m³ of timber for the main pattern and core boxes, costing $1800, plus 150 hours of labor at $20 per hour, totaling $4800. In contrast, the EPS pattern used 0.8 m³ of foam at $500, with 80 hours of labor, resulting in a cost of $2100. This represents a saving of $2700 per pattern, highlighting the cost-effectiveness of full mold casting for single or large-scale production of machine tool castings. The reduction in lead time and labor further enhances profitability, making it an attractive option for foundries specializing in repair or custom orders.
In conclusion, our experience with full mold casting for machine tool castings has been highly positive. Over a period involving numerous trials, we produced over 50 tons of castings without a single scrap instance, confirming the viability of this process for selected applications. By addressing deformation through careful sand handling and combating cold shuts via optimized gating, we have expanded the scope of full mold casting beyond thick-walled components. The repeated success with various machine tool castings, such as bases and slides, underscores the potential for broader adoption. As we continue to refine these techniques, full mold casting is poised to deliver even greater efficiencies, particularly for manufacturers dealing with large, one-off machine tool castings. The integration of empirical formulas and systematic parameter selection will further enhance reliability, driving innovation in the casting industry.