In our foundry, we produce large and medium-sized gray iron machine tool castings using the Full Mold Casting (FMC) process, which is a branch of the expendable pattern casting technology. The typical castings include machine tool beds, columns, slide carriages, etc., with weights ranging from 500 kg to 5,000 kg, main wall thicknesses from 20 mm to 30 mm, and maximum dimensions up to 4 meters. A persistent problem we have encountered is the presence of dark carbonaceous inclusions, especially on the top surfaces and side top areas of the castings after machining. This sand casting defect manifests as black slag inclusions, often accompanied by surface wrinkles and carbon deposition, seriously degrading the quality and density of the castings. In 2014, the overall scrap rate due to this sand casting defect was approximately 8%, accounting for more than half of all casting rejects. Therefore, it became urgent to understand the root causes and develop effective countermeasures.
The pattern material we use is expanded polystyrene (EPS) foam board with a density of 18 g/L. During pouring, the EPS undergoes thermal decomposition. The decomposition process first breaks the main chain into styrene monomer, dimers, trimers, toluene, etc. Then these intermediate products further decompose into benzene, ethylbenzene, and other small molecules. The final products depend heavily on the extent of secondary decomposition. Insufficient secondary decomposition yields a viscous, tar-like liquid residue. This liquid residue wets poorly with the molten iron and forms a carbon film at the metal-coating interface, leading to the characteristic sand casting defect of carbon slag and surface imperfections. The decomposition behavior of EPS is strongly temperature-dependent, as shown in the following relationship.
$$ \text{Liquid residue mass fraction} = f(T) \quad \text{where} \quad T \text{ is temperature in } ^{\circ}\text{C} $$
During filling, the advancing molten iron front dissolves and gasifies the foam pattern, consuming considerable thermal energy. Consequently, the metal temperature drops as it moves away from the ingates. In our FMC process, we typically employ bottom or side-bottom gating systems to provide sufficient ferrostatic pressure against the gasification pressure of the foam. This means that the top regions and areas far from the ingates experience the lowest temperatures, leading to incomplete secondary decomposition and a higher volume of liquid residue. This residue can either become trapped inside the metal, causing internal sand casting defect, or remain on the surface, creating carbonaceous films and slag. Therefore, the severity of this sand casting defect is influenced by several process parameters: pouring temperature, pouring time, coating thickness, and EPS density.
Orthogonal Experiment on Process Parameters
To quantify the influence of these factors, we designed and conducted a four-factor, two-level orthogonal experiment using stepped test blocks. The test block geometry is shown schematically below (we omit the figure reference). The block dimensions are 300 mm × 300 mm with stepped thicknesses of 20 mm, 40 mm, 60 mm, and 80 mm. The gating system was placed at the bottom. We prepared 16 test blocks (numbered #1 to #16) with the parameters summarized in Table 1.
| Group | Block No. | Pouring Temp. (°C) | Choke diameter (mm) | Coating thickness (mm) | EPS density (g/L) |
|---|---|---|---|---|---|
| A | #1 | 1430 | 60 | 1 | 21 |
| #2 | 1430 | 60 | 2 | 21 | |
| #3 | 1430 | 60 | 1 | 18 | |
| #4 | 1430 | 60 | 2 | 18 | |
| B | #5 | 1430 | 80 | 1 | 21 |
| #6 | 1430 | 80 | 2 | 21 | |
| #7 | 1430 | 80 | 1 | 18 | |
| #8 | 1430 | 80 | 2 | 18 | |
| C | #9 | 1370 | 60 | 1 | 21 |
| #10 | 1370 | 60 | 2 | 21 | |
| #11 | 1370 | 60 | 1 | 18 | |
| #12 | 1370 | 60 | 2 | 18 | |
| D | #13 | 1370 | 80 | 1 | 21 |
| #14 | 1370 | 80 | 2 | 21 | |
| #15 | 1370 | 80 | 1 | 18 | |
| #16 | 1370 | 80 | 2 | 18 |
After pouring, cleaning, and machining the top surfaces to depths of 5 mm, 10 mm, and 15 mm, we examined the blocks for sand casting defect occurrence. The locations and frequencies of slag defects are shown in Table 2.
| Machining depth (mm) | 20 mm section | 40 mm section | 60 mm section | 80 mm section |
|---|---|---|---|---|
| 5 | 6 | 11 | 15 | 11 |
| 10 | 3 | 7 | 10 | 5 |
| 15 | 0 | 3 | 7 | 4 |
The key findings from the experiment are:
- At a machining depth of 5 mm, all 16 blocks showed visible sand casting defect on the top surface, predominantly in the 40–80 mm thick sections. The 20 mm section had fewer occurrences. This indicates that without any slag-collecting measures, it is nearly impossible to completely eliminate carbon slag defects on the top surface under any combination of parameters tested.
- After machining to 10 mm depth, we measured the percentage of slag-affected area using a 1 cm² grid. The best six blocks (in order of increasing defect area) were #3, #7, #2, #4, #5, and #6, with defect area percentages of 3.3%, 4.7%, 6.7%, 7.3%, 8.7%, and 10%. The remaining ten blocks had defect areas ranging from 12% to 28.7%. This comparison clearly shows that higher pouring temperature (1430 °C) significantly reduces the severity of the sand casting defect. Additionally, slower pouring (smaller choke diameter) and thinner coating tend to produce less slag.
- At a machining depth of 15 mm, blocks #1–#8 (poured at 1430 °C) showed no visible slag on the top surface. Blocks #9–#10 (poured at 1370 °C) still had less than 10% defect area. Penetrant testing (PT) of blocks #1–#8 revealed that slag existed only in a thin layer (3–10 mm) near the edges, while the central areas were entirely sound. This suggests that when pouring temperature exceeds 1430 °C, the carbon slag sand casting defect at the top of castings with wall thickness up to 80 mm is confined to a depth less than 15 mm, and sidewall slag depth is about 3–10 mm.
- The frequency of slag defects correlates strongly with section thickness: the thicker the section, the more liquid slag residue remains, and the higher the severity of the sand casting defect. This is consistent with the thermal decomposition kinetics: thicker sections cool more slowly during filling, leading to a longer residence time of the liquid decomposition products and greater accumulation.
Based on this experimental evidence, we identified the following critical relationships and equations to describe the defect formation:
$$ S_{slag} \propto \frac{1}{T_{pour}} \cdot \left( \frac{t_{wall}}{t_{ref}} \right)^{0.5} \cdot \left( \frac{v_{fill}}{v_{ref}} \right)^{-0.3} $$
where \(S_{slag}\) is the severity index of the sand casting defect, \(T_{pour}\) is pouring temperature, \(t_{wall}\) is local wall thickness, and \(v_{fill}\) is metal filling velocity. The exponents are empirical estimates.
Process and Design Modifications
Guided by the experimental results, we implemented a series of modifications to our FMC process to mitigate the sand casting defect.
1. Gating System Improvement
We changed from a single-point bottom ingate to a multi-point feeding system. The new design provides multiple ingates at different heights, especially near thick sections, to deliver hot metal to those regions and maintain higher temperatures during filling. The cross-sectional area ratio of sprue : runner : ingates was set to 1 : (1.3–1.5) : (3–5). A choke (reduced cross-section) is placed in the runner near the sprue to ensure rapid filling of the sprue and to trap any early slag. The choke area is 0.8–0.9 times the sprue area. Specific diameters were selected based on casting weight:
- For castings 500–1000 kg: sprue diameter 70 mm
- For castings 1000–2000 kg: sprue diameter 80 mm
- For castings >2000 kg: sprue diameter 100 mm
$$ A_{choke} = (0.8 \sim 0.9) \cdot A_{sprue} $$
$$ A_{sprue} : A_{runner} : A_{ingate} = 1 : (1.3 \sim 1.5) : (3 \sim 5) $$
2. Additional Machining Allowance and Pads
We added 10–15 mm of excess material (pads) on top and top-side surfaces that require machining. This sacrificial layer contains the sand casting defect concentration and is removed during subsequent machining, leaving a sound casting beneath.
3. Slag Collecting Risers
At the top of the casting and at thick sections (especially terminal ends), we designed spherical slag-collecting risers. These risers capture the first wave of cold metal and the unburned carbonaceous residue, preventing them from entering the casting body. The riser diameter, \(D_{riser}\), was chosen based on the local modulus:
$$ D_{riser} = 60 \text{ mm} \sim 100 \text{ mm} $$
depending on the available space at the top of the casting.
4. Pouring Temperature Adjustment
We raised the pouring temperature of gray iron from 1380±10 °C to 1440±10 °C. This higher temperature ensures more complete thermal decomposition of EPS, drastically reducing the amount of liquid residue that causes the sand casting defect. We also paired this with a coating that has high permeability and good anti-wetting properties.
5. Hollowing of Thick Sections in the Foam Pattern
Since the sand casting defect severity increases with wall thickness, for very thick sections (e.g., >60 mm), we hollowed out the foam pattern locally to reduce the amount of EPS material that needs to be decomposed. This directly reduces the source of carbonaceous residue. The hollowed volume is typically replaced by a lightweight core or left as a cavity that fills with metal.
The combined effect of these modifications can be expressed by a simple empirical model for the scrap rate \(R_{sand\; defect}\) due to this sand casting defect:
$$ R_{sand\; defect} = R_0 \cdot \exp\left( -\alpha \cdot \Delta T_{pour} \right) \cdot \left( \frac{N_{ingates}}{N_0} \right)^{-0.5} \cdot \left( \frac{A_{riser}}{A_{ref}} \right)^{-0.3} $$
where \(R_0\) is the baseline scrap rate before improvements, \(\Delta T_{pour}\) is the increase in pouring temperature (in °C), \(N_{ingates}\) is the number of ingates, \(A_{riser}\) is the riser area, and \(\alpha\) is a constant determined empirically.
After implementing these changes, the monthly production of FMC castings increased to 240–250 tons. The overall scrap rate dropped from 15–16% to 10–11%, and the specific scrap rate due to the carbon slag sand casting defect fell below 4%. This represents a significant improvement in casting quality and a reduction in rework and waste.
We continue to monitor the process and refine the parameters. The methodology we developed—systematic orthogonal experimentation, quantitative relationship between wall thickness and defect severity, and targeted design modifications—can be applied to similar sand casting defect issues in other foundries using full mold casting or lost foam casting processes. Our experience demonstrates that controlling the thermal decomposition of EPS through optimized gating, risering, pouring temperature, and pattern geometry is the key to eliminating this persistent sand casting defect and achieving sound gray iron castings.