In the mass production of the WP12 cylinder block, a high‑strength gray iron casting with complex internal geometry and a net weight of 310 kg, we encountered severe sand casting defects, specifically peeling and sand expansion (scabbing). At peak periods these sand casting defects accounted for over 60 % of total scrap, with reject rates as high as 10.9 %. The defects occurred predominantly on large flat surfaces and at the bottom of the casting, manifesting as layered peeling and local swelling of the mold surface. Through systematic analysis and multi‑step process optimization, we successfully reduced the scrap rate to 2.1 % and stabilized subsequent production. This paper presents our first‑person experience in diagnosing and eliminating these persistent sand casting defects.
1. Defect Characteristics and Root‑Cause Analysis
The typical peeling scab defect appeared as thin layers of metal separated from the casting surface, often accompanied by a rough, sandy interface. The defect was most frequent at the large planar areas of the cylinder block and at the bottom cavity. A representative view of such sand casting defects is shown in the image below. During pouring, the intense thermal shock from the molten iron (1385 °C–1395 °C, pouring time ≈30 s) caused the sand surface to expand rapidly. When the thermal stress exceeded the hot‑wet tensile strength of the condensation zone, the sand layer delaminated, buckled, and eventually broke away.
To quantify the problem, we collected monthly scrap data during September and October 2009. Table 1 shows the severity of these sand casting defects.
| Month | Production (pieces) | Defective pieces (peeling scab) | Defect rate (%) |
|---|---|---|---|
| September | 3492 | 456 | 13.06 |
| October | 5174 | 492 | 9.50 |
Through close observation of the entire manufacturing line and statistical analysis of process parameters, we identified seven primary contributors to these sand casting defects:
- High return sand temperature (average >55 °C), causing large moisture fluctuations.
- Inconsistent return sand properties from two different molding lines.
- Aged dust‑collection equipment leading to erratic removal of fines and dead clay.
- Variable quality of raw materials, especially bentonite.
- Excessive temperature of the main core assembly when placed into the bottom mold (up to 50 °C in summer).
- Imperfect gating system design.
- Inherent structural complexity of the WP12 cylinder block (large flat areas, numerous ribs).
Each of these factors directly affected the mold’s resistance to thermal expansion. The classic mechanism of scabbing can be expressed by the balance between the thermal stress and the hot‑wet tensile strength:
$$
\sigma_{th} = E_s \cdot \alpha_s \cdot \Delta T
$$
where \(\sigma_{th}\) is the thermal stress, \(E_s\) the elastic modulus of the sand, \(\alpha_s\) the linear expansion coefficient, and \(\Delta T\) the temperature gradient. When \(\sigma_{th} > \sigma_{hwt}\) (hot‑wet tensile strength), the mold surface fails, producing sand casting defects.
2. Corrective Measures for Sand Casting Defects
We implemented a series of countermeasures targeting each root cause. The following subsections detail the key actions.
2.1 Control of Molding Sand Properties
Optimizing the sand system was the first priority. After extensive data collection, we established specific formulation targets for the return sand and the single‑sand system, as shown in Tables 2, 3, and 4. These tables were designed to maximize hot‑wet tensile strength while maintaining adequate permeability and compactability.
| Parameter | Target range |
|---|---|
| Bentonite content (%) | 6.8 – 7.8 |
| Coal dust content (%) | 4.8 – 5.6 |
| Clay content (%) | 11 – 13 |
| Grain fineness (AFS, core fraction) | 70 |
| Dead clay (%) | < 2.5 |
| Pre‑mix water spray | Required |
| Property | Target value |
|---|---|
| Wet compression strength (×10⁵ Pa) | 1.60 – 1.80 |
| Bentonite addition (kg) | 19 – 30 |
| Moisture (%) | 3.0 – 3.4 |
| Compactability (%) | 39 – 43 |
| Hot‑wet tensile strength (×10² Pa) | ≥ 40 |
| Permeability | 100 – 130 |
| Property | Target value |
|---|---|
| Moisture (%) | 4.2 – 5.0 |
| Wet compression strength (×10⁵ Pa) | ≥ 1.45 |
| Permeability | ≥ 80 |
| Hot‑wet tensile strength (×10² Pa) | ≥ 45 |
We also improved the consistency of bentonite by selecting a single, reliable supplier. In the mold, we applied facing sand manually on the large planar areas most prone to sand casting defects before the automated shooting cycle. This significantly increased local hot‑wet tensile strength.
The relationship between moisture content and hot‑wet tensile strength is well known. For our sand system, we derived an empirical formula:
$$
\sigma_{hwt} = k_1 \cdot (M – M_0)^2 + k_2 \cdot C_b + k_3
$$
where \(M\) is moisture content, \(M_0\) the optimum moisture (~3.2 %), \(C_b\) the active clay content, and \(k_1, k_2, k_3\) constants determined by regression. Maintaining \(M\) within 0.2 % of \(M_0\) kept \(\sigma_{hwt}\) above 40 × 10² Pa, effectively suppressing sand casting defects.
2.2 Core Quality Control
The WP12 cylinder block uses 24 separate sand cores. The main core assembly (six cylinder bores plus front and rear cores) was dipped and dried in a tunnel dryer. In summer, the core temperature at the setting station often reached 50 °C, which baked the underlying sand mold and created a weak condensation zone. We installed forced‑air cooling (fans and extraction) on the core conveyor to ensure core temperature ≤40 °C before placement.
Furthermore, we standardized drying parameters for each core type (Table 5). Residual moisture in the cores had to be strictly controlled to prevent additional steam generation during pouring.
| Core type | Oven temp. (°C) | Dwell time (min) | Coating density (g/mL) | Residual moisture (%) |
|---|---|---|---|---|
| Water jacket core | 180–210 | 10 | 1.35–1.4 | ≤0.3 |
| Tappet core assembly | 180–210 | 15 | 1.35–1.4 | ≤0.4 |
| Main bore core assembly | 180–210 | 25 | 1.15–1.2 | ≤0.5 |
The moisture migration distance in the mold can be approximated by:
$$
d_{cond} = \sqrt{2 \cdot D \cdot t}
$$
where \(D\) is the diffusion coefficient of water vapor in sand (~0.2 mm²/s) and \(t\) is the time of thermal exposure. Lowering the core temperature from 50 °C to 40 °C reduced \(t\) by ~20 %, thereby decreasing the condensation zone depth and mitigating sand casting defects.
2.3 Gating System Redesign
Original design used a single mid‑plane gating with ingates at the bearing saddle cores. Although this minimized direct sand erosion, the filling was turbulent and gas evacuation poor. We converted to a hybrid bottom‑gate system: the main spruce fed a bottom runner that introduced iron uniformly under the large flat areas. This promoted smooth, quiescent filling and quickly covered the mold bottom, reducing localized overheating. The ingate positions were also shifted away from the defect‑prone zones, as illustrated conceptually in the modifications below (no figure numbers used).
The improvement in filling velocity can be expressed by the continuity condition:
$$
v_{rise} = \frac{Q_{pour}}{A_{cavity}}
$$
where \(Q_{pour}\) is the pour rate (~10 kg/s for a 30 s pour) and \(A_{cavity}\) the horizontal cross‑section of the cavity. By lowering the ingate height, we increased the effective \(A_{cavity}\) early in pour, reducing \(v_{rise}\) from ~0.12 m/s to ~0.06 m/s, thereby minimizing sand expansion forces.
We also enlarged the venting risers to allow better escape of hot gases, which helped shift the moisture condensation zone deeper into the mold wall. The riser area was increased by 40 % in the critical area where sand casting defects had been most frequent.
2.4 Structural Modifications and Use of Anti‑Scabbing Nails
The WP12 cylinder block has several large planar surfaces reinforced by thin ribs. These flat areas are especially vulnerable because they lack structural support in the sand. We added small reinforcing ribs on the pattern (approximately 5 mm high, 3 mm wide) at the most sensitive locations. These ribs act as stress‑relieving features and also partition the large sand surface into smaller cells, reducing the effective expansion area. After this change, the scab defects at the ribbed zones disappeared, although the planar areas between ribs still required attention.
To further strengthen the mold locally, we introduced anti‑scabbing nails – small, sharpened steel pins inserted into the pattern at the most defect‑prone points. During molding, the nails create fine holes in the sand. When molten iron fills the cavity, the nails provide mechanical anchorage that resists the upward expansion force. The heat transfer around the nail also creates a local metallurgical bond. The holding force of a single nail can be modeled as:
$$
F_{nail} = \tau_{sand} \cdot \pi \cdot d \cdot L
$$
where \(\tau_{sand}\) is the shear strength of the sand (~0.2 MPa), \(d\) the nail diameter (3 mm), and \(L\) the embedment depth (10 mm). Each nail thus contributed approximately 19 N of restraining force. A grid of 8–10 nails per planar area effectively stabilized the mold surface and eliminated sand casting defects in those regions.
3. Results and Effectiveness
All countermeasures were implemented in November and December 2009. The resulting scrap rates due to peeling sand casting defects dropped dramatically, as shown in Table 6.
| Period | Production (pieces) | Defective pieces | Defect rate (%) |
|---|---|---|---|
| September 2009 (baseline) | 3492 | 456 | 13.06 |
| October 2009 (baseline) | 5174 | 492 | 9.50 |
| November 2009 (actions began) | 4850 | 150 | 3.09 |
| December 2009 (full implementation) | 5100 | 107 | 2.10 |
The defect rate reduction was statistically significant (p < 0.01). Subsequent months remained stable at approximately 2 %, demonstrating that the combination of sand control, core cooling, gating redesign, and structural enhancements effectively eliminated the dominant sand casting defects.
4. Conclusions
This case study illustrates that sand casting defects such as peeling scabs are rarely caused by a single factor; rather, they result from the interaction of thermal, mechanical, and material variables. Our systematic approach – from raw sand formulation to core temperature management, gating optimization, and pattern modification – successfully addressed each contributor. The key lessons learned are:
- Strict control of sand moisture, compactability, and hot‑wet tensile strength is essential; the empirical formulas derived helped maintain process capability.
- Core temperature should be kept below 40 °C to avoid preheating the mold and creating undesirable moisture gradients.
- A bottom‑gating system with optimized ingate locations reduces turbulence and localized overheating.
- Simple pattern modifications (ribs, anti‑scabbing nails) provide cost‑effective local reinforcement against thermal expansion.
By applying these measures, we reduced the scrap rate from 10.9 % to 2.1 %, lowered manufacturing costs, and improved product quality. The methodology can be adapted to other castings prone to similar sand casting defects.