In our pursuit of advanced and economically viable manufacturing techniques, our foundry has successfully adopted the Full Mold Casting (FMC), also commonly known as evaporative pattern casting or lost foam casting, for the production of complex, single-unit, and small-batch machine tool castings. The most significant achievement in this endeavor was the production of a large horizontal machine tool bed with overall dimensions of 6100mm × 900mm × 700mm and a rough weight of 8500 kg. This practice has not only proven the technical feasibility of the process for such critical components but has also yielded substantial economic benefits, solidifying its value proposition for specialized machine tool casting production.
The fundamental principle of this process involves creating a pattern from expandable polystyrene (EPS) foam, coating it with a refractory coating, embedding it in unbonded or bonded sand, and then pouring molten metal directly onto it. The metal’s heat vaporizes the foam pattern, which is replaced precisely by the metal, forming the casting. The success of a machine tool casting using this method hinges on meticulous control over every stage, from pattern making to pouring.
The initial and most critical step is the fabrication of the foam pattern for the machine tool casting. For the large bed mentioned, the pattern was constructed by bonding together machined foam blocks. The geometry of this particular machine tool casting, featuring 30 supporting cavities along its 6100mm length and substantial rail sections, presented a significant challenge: pattern deformation. To counteract this, an innovative approach using an auxiliary molding jig was employed at the parting line corresponding to the drive shaft mounting surfaces. This jig was used to shape the lower mold section initially, which was then removed before placing the final foam pattern. This method simplified internal cavity support and effectively minimized potential casting distortion.
The selection of the EPS raw material is paramount. The density of the foam board directly influences pattern strength, surface finish of the final machine tool casting, and the gaseous decomposition products during pouring. Excessively low density (<0.04 g/cm³) results in a weak pattern prone to damage during handling and molding, and a poor surface texture. Excessively high density (>0.05 g/cm³) increases pattern rigidity but leads to excessive smoke and a higher risk of carbonaceous defects in the cast part due to incomplete vaporization. An optimal density range of 0.04–0.05 g/cm³ is recommended, with the upper limit preferred for large, complex machine tool castings to ensure handling strength. The key physical properties for the selected foam board are summarized below:
| Property | Value Range |
|---|---|
| Density | 0.04 – 0.05 g/cm³ |
| Compressive Strength | 0.30 – 0.35 MPa |
| Flexural Strength | 0.09 – 0.11 MPa |
| Tensile Strength | 0.22 – 0.34 MPa |
| Heat Resistance | 70 – 80 °C |
The bonding agent for assembling foam blocks must have low gas generation, minimal residue, and fast curing. Urea-formaldehyde resin activated with a dilute hydrochloric acid catalyst (water:acid = 10:1) proved effective, achieving hardening within 30 seconds. Surface imperfections on the assembled pattern, such as fine seams or dents from machining or handling, must be sealed with adhesive tape to prevent coating penetration or sand incursion, which would lead to defects in the final machine tool casting.
The design of the gating system is arguably more critical in Full Mold Casting than in conventional sand casting. The molten metal faces resistance from the decomposing foam products and experiences heat loss during the endothermic vaporization process, which can impair fluidity. While conventional gating calculation methods provide a starting point, the resulting cross-sectional areas are typically increased by 50% to 100% to compensate for these unique factors. For the large machine tool bed, individual ingate cross-sections were designed at 20mm × 20mm (4 cm²). Accounting for damping and operational variances, 20 such ingates were used, yielding a total ingate area of 80 cm². The proportional relationship was established as: Ingate : Runner : Sprue = 1 : 2 : 2.2.
$$ A_{total\_ingates} = n \times (w \times h) = 20 \times (2.0 \text{ cm} \times 2.0 \text{ cm}) = 80 \text{ cm}^2 $$
The configuration of the gating system is chosen to facilitate rapid and uniform gas evacuation. A combination of a shower-type (rain淋) and a step-type (分层) gating system was implemented for this machine tool casting. The shower gates, positioned above, allow the metal to contact the foam pattern over a large area, promoting uniform burning and rapid vaporization. This helps maintain equal gas pressure within the mold cavity, leading to a calm, sequential rise of the metal front which helps avoid carburization. The step gates introduced metal from both sides of the machine tool bed’s guide rails. This design ensures a high-temperature metal flush over the critical rail sections, sweeping away any carbonaceous residues formed from the decomposing foam and facilitating gas expulsion, thereby guaranteeing sound rails free from slag and gas holes—a non-negotiable quality requirement for any precision machine tool casting.
| Gating System Parameter | Design Value |
|---|---|
| Individual Ingate Cross-section | 20 mm × 20 mm |
| Number of Ingates | 20 |
| Total Ingate Area (Ai) | 80 cm² |
| Total Runner Area (Ar) | ~160 cm² |
| Total Sprue Base Area (As) | ~176 cm² |
| Area Ratio Ai : Ar : As | 1 : 2 : 2.2 |
Adequate feeding is essential to prevent shrinkage cavities in the thick sections of the machine tool casting. The size of the feeders (risers) was calculated using the modulus method. The modulus (M) of a casting section is its volume (V) divided by its cooling surface area (Ac): M = V / Ac. A feeder must have a larger modulus than the section it feeds and contain sufficient liquid metal volume to compensate for shrinkage. For the identified hot spots on the bed, calculated feeder diameters were approximately 100mm. A total of 16 feeders were employed. Blind (side) feeders were chosen over open top feeders as they help contain the smoke and gases within the mold, preventing their contact with air and subsequent combustion, which contributes to a more stable and controlled metal rise during the pour.
$$ M_{feeder} > M_{casting\_section} $$
$$ M = \frac{V}{A_c} $$
The choice of molding aggregate and the application of the pattern coating are critical for achieving a good surface finish on the machine tool casting. Self-setting resin sand was used, with a grain fineness of 50-100 mesh. The resin addition was 1.2% of the sand weight, catalyzed by a para-toluene sulfonic acid solution at 60% of the resin weight, achieving a mold strength in excess of 1 MPa (100 N/cm²).
The coating applied to the foam pattern serves multiple vital functions: it provides a high-refractory barrier between the sand and the metal, reinforces the foam surface, and allows gases from the decomposing pattern to permeate through it into the sand. Given the lower mold compactness often associated with this process, a thicker coating layer is generally applied to prevent both mechanical and chemical sand burning. A fast-drying alcohol-based coating is ideal. The formulation used successfully is detailed below:
| Component | Percentage (%) |
|---|---|
| Black Graphite Powder | 60 – 70 |
| Flake Graphite Powder | 25 – 30 |
| Bentonite (Clay) | 3 – 5 |
| Rosin | 8 – 10 |
| Industrial Alcohol | Balance (as carrier) |
The optimal coating thickness was determined through permeability tests. Standard resin sand specimens (φ50mm × 50mm) were prepared, and their permeability was measured. The test was repeated on specimens with coating layers of 1–1.5 mm applied on one side and then both sides. The results confirmed that a coating layer of 1–1.5 mm provided the necessary barrier while maintaining adequate gas permeability for the successful production of a sound machine tool casting.
| Test Specimen Condition | Relative Permeability Assessment |
|---|---|
| Uncoated Resin Sand | Highest (Baseline) |
| Coated on One Side (1-1.5mm) | Sufficiently High |
| Coated on Both Sides (1-1.5mm) | Adequate for Process |
The molding process for the large bed involved the strategic use of the auxiliary jig in the drag (lower mold). After placing the jig on the molding platform, the drag flask was positioned with a sand margin of 150-200mm. Angles (45°×40) were placed around the flask perimeter at the parting line to create a seal and prevent run-out. Sand was rammed around the jig, with special attention to corners for adequate compaction. After removing the jig, the finished, coated foam pattern was carefully positioned in the impression. Multiple vent rods (φ12 mm) were inserted at intervals of approximately 200mm in the cope and drag to create channels for gas escape—a crucial step to prevent gas-related defects in the large-volume machine tool casting. The step gates and runners were set up. The cope (upper mold) was then built over the pattern, incorporating necessary core reinforcements (chaplets) to support the sand in complex areas, positioning the shower sprue and the blind feeders, and inserting more vent rods. After ramming, the vent rods were withdrawn, the flasks were clamped, and the pouring basin was prepared.
Pouring parameters are decisive for final quality in Full Mold Casting. The pouring temperature must be high enough to ensure complete and rapid vaporization of the foam pattern, yet not so high as to cause severe mold erosion or penetration. For this gray iron machine tool casting, a pouring temperature of 1400°C was targeted, with a minimum threshold of 1350°C. Temperatures below this risk slag inclusions from incomplete foam decomposition. The 8500 kg bed required two ladles poured simultaneously from both sides. The temperature and timing sequence was meticulously planned and executed as follows:
| Ladle | Tap Weight | Tap Temp. | Ladle Temp. before Pour | Pouring Time | Target Pour Temp. |
|---|---|---|---|---|---|
| I (First Tap) | 4 metric tons | 1480°C | 1440°C (after 40 min hold) | 180 seconds | 1400°C |
| II | 7 metric tons | 1490°C | 1440°C (after 30 min hold) | 164 seconds | |
| I (Second Tap) | 3 metric tons | 1500°C | 1420°C | (Part of first pour) | 1400°C |
Following shakeout and cleaning, the produced machine tool casting was thoroughly inspected. The bed met all specified requirements: dimensional accuracy conformed to ISO 8062-1984 grade CT 8-9; surface flatness was within 1mm over 600mm; surface roughness was Ra 25-50 µm; and the chemical composition and mechanical properties satisfied the drawing specifications. This successful outcome validated the entire process design and execution for large, complex machine tool castings.
The economic advantage of using the Full Mold process for single and small-batch production of machine tool castings is profound and is a major driver for its adoption. Beyond the large bed, we have applied this method to produce dozens of other components like circular guide tables, headstocks, and smaller lathe and grinder beds, totaling over 100 metric tons. The cost savings primarily stem from the elimination of traditional pattern equipment (wood or metal), drastically reduced molding time, and simplified core making. A direct cost comparison for the 8500 kg bed illustrates this starkly:
| Cost Category | Traditional Wood Pattern Method | Full Mold (Foam Pattern) Method |
|---|---|---|
| Pattern Material & Labor | Material: 10.5 m³ wood @ 1200/m³ = 12,600 Labor: 1100 hours @ 10/hr = 11,000 Subtotal: 23,600 |
Material: 5 m³ foam @ 450/m³ = 2,250 Labor: 370 hours @ 10/hr = 3,700 Subtotal: 5,950 |
| Molding Labor | 280 hours @ 5/hr = 1,400 | 57 hours @ 5/hr = 285 |
| Total Direct Cost | 25,000 | 6,235 |
This comparison highlights a reduction in direct pattern and molding costs of approximately 75% for this single piece, a compelling economic argument. The savings are even more significant when considering the storage, maintenance, and modification costs associated with permanent wooden patterns for various machine tool castings.
In conclusion, our extensive practice confirms that Full Mold Casting is a highly viable and advantageous method for producing complex, high-quality machine tool castings, particularly in single-unit and low-volume scenarios. The keys to success are: meticulous control over foam pattern quality and integrity; intelligent gating and feeding system design that accounts for foam decomposition dynamics; the application of a robust, gas-permeable refractory coating; and strict control over pouring temperature and sequence. When these factors are correctly managed, issues like sand burning, slag inclusions, and distortion can be effectively avoided. The substantial economic benefits derived from eliminated pattern costs and reduced labor solidify the Full Mold process as a powerful and strategic tool in the specialized field of machine tool casting manufacturing.