This article details our first-hand experiences, challenges, and solutions encountered during the full-scale transition from traditional clay sand dry sand molds to furan resin self-hardening (no-bake) sand molds for the production of large-section ductile iron (DI) crankshafts. The shift was driven by the well-known advantages of self-hardening sand systems: operational simplicity, energy savings from eliminating mold drying, superior casting surface finish, high dimensional accuracy, and easier shakeout. However, for critical castings like crankshafts, the distinct properties of the furan sand—higher sulfur content, slower cooling rate, lower elevated-temperature strength, and higher gas evolution—presented significant new challenges regarding shrinkage porosity, slag inclusions, and gas defects. Based on extensive foundry trials, laboratory data, and long-term production statistics, this paper provides a comparative analysis of the two mold types and outlines the specific process adaptations required to successfully utilize self-hardening sand for high-integrity sand castings.
The core quality issues manifested initially as internal shrinkage porosity detected by ultrasonic inspection, sporadic gas defects causing violent mold reactions (“boiling”), and slag inclusions. Our systematic investigation and countermeasures focused on each of these areas.
Shrinkage Porosity Control: The original clay sand process used a horizontal pouring/vertical cooling method with a top riser for feeding. For the self-hardening sand, concerns over its lower high-temperature strength led to the adoption of a horizontal pour/horizontal cool, riserless process with distributed gating. This initially resulted in unacceptable shrinkage. The successful strategy combined several elements:
- Modified Gating and Cooling Practice: We implemented a horizontal pour/horizontal cool process to maintain mold geometry and counteract the mold’s tendency to yield. The gating was revised to a combination of concentrated and distributed in-gates. Most metal enters via a side riser at the small end, while smaller flows are introduced at the flange end and intermediate crank throws. This creates a favorable temperature gradient—hotter near the riser, cooler at the far ends—optimizing the natural feeding characteristics of ductile iron.
- Enhanced Mold Rigidity: Strict control of sand mix ratios was enforced. During molding, ramming with hand tools was introduced alongside continuous mixer filling to ensure high and uniform mold compactness, thereby increasing the effective mold rigidity to resist wall movement.
- Strategic Use of Chills: To counteract the slower cooling rate of self-hardening sand and promote a rapid, solid skin, the number and size of chills were increased, particularly on the main journals. Special attention was paid to areas of high heat accumulation, such as the inner surfaces of the first two crank throws adjacent to the riser.
- Metallurgical Control: Carbon equivalent (CE) was tightly controlled within the range of 4.3–4.5%. Effective inoculation was ensured to promote maximum graphitization, leveraging the graphite expansion phase to counteract shrinkage.
The solidification time ($t_f$) for a casting is related to its modulus ($M = V/A$) and the mold material’s properties, often described by Chvorinov’s rule: $$t_f = B \cdot \left( \frac{V}{A} \right)^n = B \cdot M^n$$ where $B$ is a mold constant and $n$ is typically around 2. For self-hardening sand, $B$ is larger than for clay dry sand, leading to longer solidification times for the same modulus. For a crankshaft with a modulus of approximately 2.6 cm, our measurements indicated: $$t_{f,\ self-hardening} \approx 1.3 \times t_{f,\ dry\ sand}$$ This slower cooling, while beneficial for feeding if managed correctly, necessitates the aforementioned use of chills.
Gas Defect Mitigation: The high gas evolution rate and volume from the furan binder were primary culprits. Solutions involved:
- Enhanced Venting: The number and diameter of vent holes were substantially increased. “Half-vents” (vents stopping short of the cavity) were extensively used in the cope to allow gases from the sand to escape outward rather than into the cavity.
- Binder and Sand Control: Resin addition was minimized to the level required for adequate strength. Limits were placed on sand fines content, loss on ignition (LOI), and gas evolution value.
- Coating Practice: Mold coats were formulated with iron oxide powder and applied at a controlled thickness. Both molds and cores, coated with water-based paints, were thoroughly dried.
Slag Inclusion Prevention: To combat slag, a multi-pronged approach was adopted:
- Use of a stopper-equipped ladle to prevent primary slag entry.
- Covering the molten metal surface with cryolite powder to minimize oxidation and secondary slag formation.
- The modified gating system improved the temperature at the far end of the casting, reducing the tendency for slag formation there.
- Increasing pouring speed, using high-quality foundry coke for melting, and desulfurizing base iron to below 0.02% S were also critical. Pouring temperature was maintained between 1300°C and 1320°C.
Comparative Analysis of Mold Properties and Their Impact
The successful implementation required a deep understanding of how self-hardening sand fundamentally differs from clay dry sand molds in the context of large sand castings.
1. Cooling Rate: Laboratory tests measured the solidification time of DI samples with different moduli in both mold types. The results, plotted on a log-log scale, show a linear relationship confirming slower cooling in self-hardening sand.
| Modulus, M (cm) | Solidification Time, Dry Sand (min) | Solidification Time, Self-Hardening Sand (min) | Ratio (Self-Hard./Dry) |
|---|---|---|---|
| 1.0 | ~15 | ~22 | ~1.47 |
| 1.5 | ~32 | ~46 | ~1.44 |
| 2.0 | ~55 | ~78 | ~1.42 |
| 2.5 | ~83 | ~115 | ~1.39 |
Extrapolating this trend to a crankshaft modulus of ~2.6 cm gives a solidification time ratio of approximately 1.35x. This slower cooling significantly influences the volume change behavior during solidification. A slower cooling rate promotes a longer graphitic expansion phase, which can enhance the self-feeding capability of ductile iron, a principle utilized in riserless casting designs for thick-section sand castings.
2. Thermal Strength and Dimensional Stability: Clay sand exhibits increasing hot strength up to ~800°C, leading to poor collapsibility. Self-hardening sand loses strength rapidly above ~300°C due to resin decomposition. This lower high-temperature stiffness is a disadvantage for resisting metallostatic pressure and preventing mold wall movement, which can induce shrinkage porosity. Our countermeasure—intensive ramming to achieve high mold density—places the sand near the mold face in a semi-confined state upon heating, helping it retain sufficient strength longer. Combined with the horizontal cooling practice, this effectively prevented measurable casting wall movement, as confirmed by dimensional checks and density measurements.
3. Gas Evolution: This is the most critical difference. Dry clay sand molds have negligible gas evolution. Furan no-bake sand has high, rapid evolution. The gas evolution rate ($\dot{G}$) can be modeled as a function of temperature and time: $$\dot{G}(T,t) = A \cdot e^{-E/(R T(t))} \cdot f(t)$$ where $A$ is a pre-exponential factor, $E$ is activation energy, $R$ is the gas constant, $T(t)$ is the temperature at the sand/metal interface, and $f(t)$ describes the remaining binder content. The total gas volume ($V_g$) generated per unit area before metal solidification is: $$V_g = \int_{0}^{t_s} \dot{G}(T(t), t) \, dt$$ where $t_s$ is the local solidification time of the skin. For self-hardening sand, both $\dot{G}$ and $V_g$ are orders of magnitude higher than for dry sand, necessitating vastly increased venting capacity in the mold design for successful sand castings.
4. Sulfur Pick-up and Surface Degradation: The acid catalysts (e.g., toluenesulfonic acid) in self-hardening sand introduce sulfur into the mold atmosphere, posing a risk of surface magnesium loss (reversion) in ductile iron sand castings. The diffusion of sulfur into the casting and the counter-diffusion of magnesium can be described by Fick’s laws. The depth of the affected layer ($d$) depends on the diffusion coefficient of S/Mg ($D$), the interfacial sulfur concentration ($C_s$), the initial Mg content ($C_{Mg,0}$), and the time at temperature ($t$). For thick-section castings with long $t$, if $C_{Mg,0}$ is sufficient, magnesium from the interior can diffuse outward to replenish surface losses: $$\frac{\partial C}{\partial t} = D \frac{\partial^2 C}{\partial x^2}$$ In our practice, this issue was mitigated by: a) applying zircon-based mold and core washes as barriers, b) the inherently long solidification time of thick crankshafts allowing for Mg diffusion, and c) using a proprietary high-Mg, low-rare earth, copper-containing inoculant with high anti-fade properties.
Evaluation of Resultant Casting Quality
Direct comparisons between crankshafts produced via the two mold systems confirm the viability of the self-hardening sand process.
Metallurgical Density: Density measurements via the weight-loss method on samples from corresponding locations showed no significant difference, indicating equivalent soundness and absence of gross shrinkage.
Mechanical Properties: A comparison of as-cast properties from keel block tests (from the same heat of iron) revealed no detrimental effect from the mold change. Long-term production data for normalized-and-tempered crankshafts (tensile samples from the body) further supports this, showing that properties from self-hardening sand molds have a narrower scatter band and slightly higher average values.
| Mold Type | Base Iron Condition | Tensile Strength (MPa) | Elongation (%) | Hardness (HB) | ||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Count | Range | Avg. | Count | Range | Avg. | Count | Range | Avg. | ||
| Clay Dry Sand | Met. Coke, No Desulfurization | ~120 | 650-850 | 755 | ~120 | 2-8 | 4.5 | ~120 | 229-277 | 248 |
| Clay Dry Sand | Foundry Coke, Desulfurized | ~150 | 750-900 | 835 | ~150 | 3-10 | 6.0 | ~150 | 241-285 | 262 |
| Furan No-Bake | Foundry Coke, Desulfurized | ~200 | 770-920 | 855 | ~200 | 4-11 | 6.3 | ~200 | 245-289 | 265 |
Graphite Structure: While statistical review of nodularity ratings indicated a slight shift towards more Type II graphite in self-hardening sand castings (attributed to slower cooling), no instances of deteriorated nodularity or unacceptable graphite forms were recorded that would impact mechanical performance.
Surface Quality and Dimensional Accuracy: Visual inspection and dimensional analysis consistently showed that sand castings from self-hardening molds have superior surface finish and higher dimensional precision compared to those from dry sand molds, a direct benefit of the more precise and stable mold medium.
Overall Scrap Rate: After process optimization, the comprehensive scrap rate for crankshafts produced with self-hardening sand was significantly lower than the historical rate for dry sand production, primarily due to the reduction in shrinkage, slag, and gas-related defects.
In conclusion, based on our extensive experimental work and multi-year production experience, furan resin self-hardening sand is a fully viable and advantageous mold material for manufacturing large, high-duty ductile iron crankshafts. The internal quality, as defined by density, microstructure, and mechanical properties, is equivalent to that achievable with clay dry sand molds. Meanwhile, the surface quality, dimensional accuracy, and overall yield are superior. The key to success lies in recognizing and actively compensating for the specific characteristics of the self-hardening sand system—namely its slower cooling rate, lower high-temperature strength, high gas evolution, and potential for sulfur transfer—through integrated modifications in gating and feeding design, chilling practice, mold rigidity control, venting, coating, and metallurgical management. This process knowledge has been successfully transferred to other large crankshaft sizes, proving the robustness of the approach for heavy-section ductile iron sand castings.