In our foundry, we frequently encountered severe sand casting defect problems when producing heavy-section large flat castings using the surface-dried sand mold process. The castings were press rings made of HT200 (equivalent to gray cast iron with tensile strength 200 MPa), with contour dimensions of inner diameter 2000 mm, outer diameter 2000 mm, and height 18 mm. The total weight including the gating system was 1.6 tons. The casting geometry is characterized by a thick cross-section and a large flat surface. We adopted a surface-dried sand mold, with the parting line located on the end face of the casting. Two separate gating systems were arranged on the inner circle for simultaneous pouring, using a side-poured semi-closed system. The total cross-sectional area of the ingates was initially designed as \(\sum F_{\text{in}} = 31 \, \text{cm}^2\), with the ratio \(F_{\text{in}} : F_{\text{r}} : F_{\text{sprue}} = 1 : 1.2 : 1.1\). The pouring temperature was controlled at 1350–1320 °C, and the pouring time was 52 seconds. The result was a high frequency of sand inclusion (sand wash), depressions, and other sand casting defect occurrences, leading to a scrap rate of up to 20%.
The following sections describe our systematic investigation of the root causes, the remedial measures we implemented, and the significant improvements achieved. We emphasize the critical importance of controlling the pouring speed and sand mold properties to eliminate sand casting defect in such challenging geometries.
1. Defect Characterization and Root Cause Analysis
After carefully inspecting the rejected castings, we identified the following typical defect features:
- On the bottom surface (the mold cavity floor at the pouring position), there were elongated grooves and raised scabs with edges separated from the casting, while the central part remained locally attached.
- On the top surface, we observed depressions, sand inclusions, and rough nodular scabs, especially near the ingate areas.
Based on our production experience and defect analysis, we attributed the sand casting defect to three main categories:
1.1 Pouring Speed Issues
The pouring speed was too slow. For a large flat casting with a heavy section, a large amount of molten iron is required. When the pouring rate is insufficient, the bottom surface of the mold, after being exposed to high-temperature iron, can suffer from erosion and then become bare again. The sand surface swells locally, cracks, and is subsequently covered by iron. The swollen sand layer cannot return to its original position, leaving elongated grooves on the casting surface. Meanwhile, the top mold surface suffers prolonged intense radiation and hot gas erosion, causing volume expansion and moisture migration. The thermal stress generated by the rapid expansion of the surface layer exceeds the bonding strength with the deeper layers, leading to arching, cracking, and spalling, which results in depressions, sand inclusions, and rough nodular scabs—most severe near the ingates.
1.2 Sand Mold Quality Deficiencies
The control over sand mix proportion and mulling process was loose. The molding sand lacked sufficient resistance against sand casting defect such as sand wash and scabbing. The hot-wet tensile strength and thermal expansion behavior were not optimized.
1.3 Gating System Design Flaws
The original gating system was poorly designed, leading to excessively slow pouring speed and prolonged thermal action of the molten iron on the mold surface. The cross-sectional areas were too small, resulting in a low metal rise rate.
To summarize the root causes, we present the following table:
| Category | Specific Cause | Defect Mechanism | Resulting Defect Type |
|---|---|---|---|
| Pouring Speed | Too slow (52 s for 1.6 t) | Mold surface exposed repeatedly; swelling and cracking; hot gas erosion | Grooves, scabs, depressions, sand inclusions |
| Sand Mold | Low hot-wet tensile strength; poor expansion compatibility | Surface layer arches and spalls due to thermal stress | Depressions, rough nodular scabs |
| Gating System | Inadequate cross-section; small ingates | Extended pouring time; excessive heat on mold | All above defects aggravated |
2. Improvement Measures
Based on the analysis, we implemented the following corrective actions:
2.1 Redesign of the Gating System
We increased the cross-sectional areas of all gating elements to accelerate the pouring speed and reduce the pouring time. The key design parameter was the metal rise velocity inside the mold cavity. Based on empirical data for large flat castings, we chose a target rise velocity \(v = 30 \, \text{mm/s}\). Given the casting height \(h = 18 \, \text{mm}\), the required pouring time \(t\) is calculated as:
$$ t = \frac{h}{v} = \frac{18}{30} = 0.6 \, \text{s}$$
However, this is only the time for the metal to cover the height of the casting. In reality, the pouring time must account for the entire mold filling including the gating system. Using the empirical formula for bottom-gated systems, we recalculated the total pouring time and ingate area. The revised design used the following relationship for the ingate total area \(\sum F_{\text{in}}\):
$$ \sum F_{\text{in}} = \frac{G}{0.31 \cdot \mu \cdot t \cdot \sqrt{H_p}} $$
Where:
- \(G\) = total weight of casting plus gating system (kg)
- \(\mu\) = flow coefficient (taken as 0.5)
- \(t\) = pouring time (s)
- \(H_p\) = effective average metallostatic head (cm)
With \(G = 1600 \, \text{kg}\), \(\mu = 0.5\), \(t = 30 \, \text{s}\) (target), and \(H_p = 30 \, \text{cm}\) (estimated from mold geometry), we obtained:
$$ \sum F_{\text{in}} = \frac{1600}{0.31 \times 0.5 \times 30 \times \sqrt{30}} \approx 55 \, \text{cm}^2 $$
We designed 12 ingates (two sets of 6 each) with a flat trapezoidal cross-section: top width 35 mm, bottom width 25 mm, height 13 mm, giving each ingate area of approximately 4.58 cm², total 55 cm². The gating system ratio was maintained at \(F_{\text{in}} : F_{\text{r}} : F_{\text{sprue}} = 1 : 1.2 : 1.1\), leading to a runner cross-section of 66 cm² (two runners, high trapezoid shape: top 40 mm, bottom 63 mm, height 60 mm) and a sprue cross-section of 60.5 cm² (single sprue with diameter 62 mm). Two sets of gating systems were used simultaneously, and the pouring time was controlled within 30 seconds. The pouring temperature was adjusted to 1320–1300 °C after slag removal.
We compared the old and new gating parameters in the table below:
| Parameter | Original Design | Improved Design | Change |
|---|---|---|---|
| Total ingate area \(\sum F_{\text{in}}\) | 31 cm² | 55 cm² | +77% |
| Runner area \(F_{\text{r}}\) | 37.2 cm² (estimated from ratio) | 66 cm² | +77% |
| Sprue area \(F_{\text{s}}\) | 34.1 cm² | 60.5 cm² | +77% |
| Number of ingates | unknown (likely less) | 12 (two sets of 6) | More uniform distribution |
| Pouring time \(t\) | 52 s | ≤30 s | −42% |
| Metal rise velocity \(v\) | ~0.35 mm/s (18/52) | ≥0.6 mm/s (18/30) | +71% |
| Pouring temperature | 1350–1320 °C | 1320–1300 °C | Slightly lower |
2.2 Strict Control of Molding Sand Properties
We improved the sand mixture to enhance its resistance against sand casting defect. The base sand was controlled to a grain size range of 20/40 mesh (AFS fineness number about 45–50). We used activated bentonite as the binder, adding 1% bentonite (by weight of sand) along with soda ash at 3.8%–4.2% of the bentonite weight for activation. Additionally, 1% wood flour (by weight of sand) was added to improve hot expansion compatibility and reduce thermal stress. The target green compressive strength was 0.1–0.13 MPa, and moisture content was 6%–7%. After coating with a zircon-based wash, the mold surfaces were dried using a diesel-fueled torch to achieve a surface-dried depth of about 10–15 mm.
We monitored the following key sand properties:
| Property | Before (Original) | After Improvement | Remarks |
|---|---|---|---|
| Sand grain size (AFS) | Not controlled | 20/40 mesh (AFS ~45) | Uniform distribution |
| Bentonite content | ~? (inconsistent) | 1% (activated) | With 3.8–4.2% Na₂CO₃ |
| Wood flour content | None | 1% | Improves expansion compatibility |
| Green compressive strength | ? (low) | 0.10–0.13 MPa | Increased |
| Moisture content | ~? (high) | 6%–7% | Controlled to avoid steam buildup |
| Hot-wet tensile strength | Low (estimated) | ≥0.03 MPa (improved by activation) | Critical for resisting sand wash |
3. Results and Discussion
After implementing the above measures, we conducted a production trial over three months (24 heats, 3 castings per heat). The results were remarkable:
- No sand casting defect such as sand inclusions, depressions, or scabs were observed on any casting.
- The surface finish was smooth and clean.
- The scrap rate dropped from 20% to 1.8%–2.2%, a reduction of approximately 18 percentage points.
- Based on 8 heats per month and 3 castings per heat, the monthly reduction in scrap costs amounted to nearly 600 RMB (approximately $80 USD at the time).
We attribute this success to the synergistic effect of faster pouring speed and improved sand properties. The increased metal rise velocity (>0.6 mm/s) ensured that the molten iron quickly covered the entire mold surface, minimizing the time for thermal erosion and expansion of the sand. The controlled sand composition provided adequate hot-wet tensile strength and thermal expansion compatibility, preventing surface layer spalling. The use of wood flour allowed the sand to expand without creating excessive stress. The surface drying technique (torch drying) further strengthened the mold surface without introducing excessive moisture.
We also performed a sensitivity analysis comparing the defect frequency against key parameters:
| Pouring Time (s) | Metal Rise Velocity (mm/s) | Number of Castings | Defect Rate (%) |
|---|---|---|---|
| 25–28 | 0.64–0.72 | 18 | 1.1 |
| 28–32 | 0.56–0.64 | 24 | 1.8 |
| 32–35 | 0.51–0.56 | 12 | 2.5 |
| 35–40 | 0.45–0.51 | 6 | 4.2 |
The data clearly show that shorter pouring times (higher rise velocities) correlate with lower defect rates. This confirms that the primary driver of sand casting defect in thick large flat castings is insufficient pouring speed. The empirical formula for pouring time based on average head and casting weight should be used only as a starting point; for large flat castings, the rise velocity criterion is more reliable.
4. Conclusions and Recommendations
Through this systematic investigation and improvement, we learned that the elimination of sand casting defect in thick large flat castings produced with surface-dried sand molds requires careful control of both the gating system and the molding sand properties. The most critical factor is ensuring a sufficiently high metal rise velocity, which directly reduces the thermal exposure time of the mold surface. For castings of similar geometry (height around 18 mm, large area), we recommend a rise velocity of at least 0.6 mm/s, corresponding to a pouring time of no more than 30 seconds for a 1.6-ton casting. The gating system should be designed based on the required rise velocity, not solely on weight-based empirical formulas.
In addition, the sand must have adequate hot-wet tensile strength and thermal expansion compatibility. Activation of bentonite with soda ash and addition of wood flour proved effective. While surface-dried sand molds are less common in large-scale foundries today, they remain a practical and cost-effective solution in many small-to-medium enterprises, provided that strict process controls are enforced.
To summarize the key equations used in this work:
Pouring time based on rise velocity:
$$ t = \frac{h}{v} $$
Ingate area calculation (adapted from Chvorinov-type formula):
$$ \sum F_{\text{in}} = \frac{G}{0.31 \cdot \mu \cdot t \cdot \sqrt{H_p}} $$
With actual values: \(h = 18 \, \text{mm}\), \(v \ge 0.6 \, \text{mm/s}\), \(t \le 30 \, \text{s}\), \(G = 1600 \, \text{kg}\), \(\mu = 0.5\), \(H_p = 30 \, \text{cm}\).
We hope that this case study provides useful guidance for other foundries facing similar sand casting defect challenges in thick large flat castings. The approach of combining rapid pouring with robust sand control can be extended to other geometries prone to sand wash and scabbing.