In our manufacturing facility, we have successfully implemented the full molding process, also known as lost foam casting, to produce high-quality machine tool castings. This method has proven particularly effective for large and complex components, such as machine tool beds, which require precise dimensions and minimal defects. By utilizing expanded polystyrene (EPS) foam patterns, we have achieved significant improvements in efficiency and cost-effectiveness for producing machine tool castings. The process involves creating a foam model of the casting, coating it with a refractory material, and embedding it in unbonded sand before pouring molten metal. This approach eliminates the need for traditional cores and complex molding techniques, making it ideal for single-piece or small-batch production of machine tool castings. In this article, I will share our experiences and detailed methodologies, focusing on key aspects like gating system design, riser calculations, and material selection, all aimed at optimizing the production of durable and accurate machine tool castings.
The full molding process begins with the creation of a foam pattern that replicates the final machine tool casting. For a machine tool bed measuring 6100 mm in length, 900 mm in width, and 700 mm in height, with a rough weight of 8500 kg, we constructed the pattern by bonding foam boards together. Given the complexity of the bed, which includes 30 support cavities and thick guide rails, deformation was a major concern. To mitigate this, we designed an auxiliary mold for the drive shaft and lead screw mounting surfaces, which added a parting plane and reduced the need for internal supports. This step ensured dimensional stability and prevented warping during casting. The foam material selected had a density range of 0.04–0.05 g/cm³, as it offers a balance between strength and gasification properties. Lower densities risk model damage during handling, while higher densities increase smoke and residue during pouring, potentially leading to defects in the machine tool casting. The physical properties of the foam boards we used are summarized in Table 1.
| Property | Value Range |
|---|---|
| Compressive Strength (MPa) | 0.30–0.35 |
| Impact Strength (MPa) | 0.07–0.08 |
| Flexural Strength (MPa) | 0.09–0.11 |
| Tensile Strength (MPa) | 0.22–0.34 |
| Shear Strength (MPa) | 1.1–1.5 |
| Heat Resistance (°C) | 70–80 |
| Gas Permeability (g/m²·h) | 0.38 |
| Water Absorption (g/cm²) | 0.01–0.012 |
For bonding the foam boards, we chose urea-formaldehyde resin with a hydrochloric acid catalyst (water-to-acid ratio of 10:1) due to its low gas generation, minimal residue, and fast curing time of under 30 seconds. This adhesive ensured strong joints without compromising the integrity of the machine tool casting. Additionally, we used tape to seal any gaps or imperfections in the foam model, preventing coating penetration and sand inclusion during molding. The model was fabricated using simple tools like band saws, planers, milling machines, and hot-wire cutters to shape the foam into the desired geometry for the machine tool casting. After assembly, the model was inspected and coated to prepare for the next stages.
The gating system design is critical in full molding for machine tool castings, as it influences metal flow, gas evolution, and defect formation. During pouring, the molten metal vaporizes the foam, creating gases that can impede flow if not properly managed. We based our calculations on conventional gating principles but increased the dimensions by 50–100% to account for resistance from decomposition products and heat loss. The cross-sectional area of each ingate was set at 4 cm² (20 mm × 20 mm), with a total of 20 ingates, resulting in a combined area of 80 cm². The ratio of ingate-to-runner-to-sprue was maintained at 1:2:22 to ensure balanced flow. We employed a combination of shower-type and layered gating systems. The shower-type gates promote uniform gasification of the foam and stable metal rise, while layered gates along the guide rails help flush away carbon residues and ensure overheating in critical areas, reducing slag and gas pores in the machine tool casting. The general formula for calculating the gating dimensions can be expressed as:
$$ A_g = k \cdot \frac{W}{\rho \cdot v \cdot t} $$
where \( A_g \) is the total gating area, \( W \) is the weight of the machine tool casting, \( \rho \) is the metal density, \( v \) is the flow velocity, \( t \) is the pouring time, and \( k \) is an amplification factor (typically 1.5 to 2.0 for full molding). For our machine tool bed, this approach minimized turbulence and enhanced the quality of the machine tool castings.
Riser design for machine tool castings focuses on compensating for shrinkage during solidification. We used the modulus method to calculate riser sizes based on the thermal characteristics of the casting. For the machine tool bed, the riser diameter was determined to be 100 mm, with 16 risers placed strategically. These were designed as blind risers to contain smoke and allow controlled gas escape, preventing atmospheric combustion and ensuring a smooth metal rise. The modulus \( M \) of a section is given by:
$$ M = \frac{V}{A} $$
where \( V \) is the volume and \( A \) is the surface area. For a cylindrical riser, the diameter \( D \) can be derived from:
$$ D = \frac{6 \cdot M}{\pi} $$
This calculation ensured adequate feeding for the thick sections of the machine tool casting, reducing shrinkage defects and improving integrity.
Pouring temperature is another vital parameter in producing high-quality machine tool castings. It must be high enough to vaporize the foam completely but not so high as to cause burn-on or sand adhesion. We set the pouring temperature at 1400°C, with a minimum threshold of 1350°C to avoid slag inclusions and a maximum limit to prevent surface defects. Temperature control was achieved using a 5-ton cupola furnace, and for the 8500 kg bed, we employed two 7-ton ladles. The first ladle delivered 4 tons of metal at an initial temperature of 1480°C, cooling to 1440°C in the ladle, while the second provided 7 tons at 1490°C, cooling to 1440°C. A subsequent top-up from the first ladle added 3 tons at 1500°C, cooled to 1420°C. Pouring times were 180 seconds and 164 seconds for the respective ladles, maintaining an average temperature of 1400°C to ensure proper foam gasification and metal fluidity for the machine tool casting.
The molding materials and coatings play a crucial role in the success of machine tool castings. We used resin sand with a grain size of 50–100 mesh, mixed with 1.2% resin by weight and 60% p-toluenesulfonic acid solution as a curing agent. This yielded a sand strength exceeding 100 N/cm², sufficient to withstand metal pressure without distortion. The foam model was coated with a refractory coating to prevent sand penetration and improve surface finish. Our alcohol-based coating composition included black graphite powder (60–70%), flake graphite (25–30%), bentonite (3–5%), rosin (8–10%), and industrial alcohol as a solvent. This mixture provided high refractoriness, good adhesion, and adequate permeability. Coating thickness was optimized to 1–1.5 mm through permeability tests; samples with this thickness showed optimal gas escape without compromising the integrity of the machine tool casting. Permeability was measured using standard φ50 mm × 50 mm sand specimens, and the results confirmed that double-sided coatings with 1–1.5 mm thickness offered the best performance for machine tool castings.
In the molding process for machine tool castings, we began by placing the auxiliary mold on a platform, setting the flask with a sand margin of 150–200 mm, and incorporating 45° × 40 mm angle irons to prevent run-outs. After compacting the sand, especially at corners, we removed the auxiliary mold and positioned the foam model. Vent channels were created using φ12 mm iron rods spaced 200 mm apart to facilitate gas escape and reduce residue entrapment. The middle flask was then added, followed by the top flask, which included core reinforcements to prevent sand drop. Gating elements and risers were arranged, and vent rods were inserted before sand filling. After compaction, the rods were withdrawn, and the flasks were clamped securely. This method ensured a robust mold capable of producing precise machine tool castings.
Post-casting, the machine tool bed was cleaned and inspected, meeting international standards such as ISO 8062-1984 for dimensional accuracy (grades 8–9), with surface flatness within 1 mm per 600 mm and roughness Ra 25–50. Chemical composition and mechanical properties aligned with design specifications, demonstrating the effectiveness of the full molding process for machine tool castings. The economic benefits are substantial, as shown in Table 2, which compares traditional wood pattern methods with full molding for producing machine tool castings. The data highlights reductions in material usage, labor hours, and overall costs, making it a viable option for single-piece and small-batch production of various machine tool castings, including circular guide tables, headstocks, and grinding machine beds.
| Factor | Traditional Wood Pattern | Full Molding (Foam Pattern) |
|---|---|---|
| Material Volume (m³) | 10.5 | 5 |
| Material Cost (Currency Units) | 12,780 | 2,250 |
| Pattern Making Time (hours) | 1,100 | 370 |
| Pattern Making Cost (Currency Units) | 11,000 | 3,700 |
| Molding Time (hours) | 280 | 57 |
| Molding Cost (Currency Units) | 1,400 | 285 |
| Total Cost (Currency Units) | 25,180 | 3,985 |
In conclusion, the full molding process is highly feasible for manufacturing machine tool castings, offering economic advantages and quality improvements. By controlling pouring temperature, ensuring proper coating thickness, and designing adequate supports, we can avoid common defects like sand adhesion and slag inclusions in machine tool castings. This method supports the production of complex geometries with high dimensional stability, making it a valuable technique for the foundry industry. As we continue to refine this process, we anticipate broader applications in producing various types of machine tool castings, enhancing overall manufacturing efficiency and product performance.