For many years, our foundry operation relied on traditional green sand molding using coal dust (black sand) for the production of various sand casting parts. While effective for preventing burn-on and ensuring surface finish, the use of coal dust presented significant challenges: a dirty working environment, airborne particulate matter, emission of volatile gases during pouring, and the accumulation of ash which degraded the sand system’s properties over time. Our most produced sand casting part is a refrigerator compressor cylinder block, a critical component with challenging geometry. Persistent burn-on defects within its internal cavities, despite high coal dust additions, prompted a fundamental review of our process. This document details our first-person journey in developing, testing, and implementing a stable, coal-free green sand system for the serial production of these high-volume sand casting parts.
The primary function of coal dust in green sand is multifaceted. Upon contact with molten iron, it generates a reducing atmosphere, produces a plastic coke layer that buffers thermal expansion, and most importantly, forms a layer of lustrous carbon (glossy carbon) that prevents metal penetration. Historically, some foundries achieved clean surfaces on thin-walled sand casting parts using naturally fine, clay-bonded sands without any coal dust. This historical precedent, combined with our own preliminary trials where increasing the sand’s Average Fineness Number (AFS) significantly reduced burn-on, led us to hypothesize that a sufficiently fine, well-controlled sand matrix could replicate the anti-penetration function of lustrous carbon. The key was to create a dense, sintered surface layer on the mold itself.
Our initial feasibility test focused on comparing a coal-free sand mix with our standard black sand. The goal was to produce the problematic cylinder block sand casting parts and evaluate surface quality. The base formulation for the coal-free sand was designed for high fineness and controlled bonding.
| Parameter | Coal-Free Sand (Trial 1) | Coal-Free Sand (Trial 2) | Standard Black Sand |
|---|---|---|---|
| Compactability (%) | 44 | 49 | 38 |
| Moisture (%) | 2.5 | 2.4 | 3.0 |
| Permeability | 70 | 78 | 83 |
| Green Compression Strength (kPa) | 134 | 137 | 158 |
| Active Bentonite (%) | 8.4 | 8.57 | – |
| Loss on Ignition (%) | 1.4 | – | 3.7 |
| Clay Content (%) | 8.18 | – | 11.38 |
| Average Fineness (AFS) | 89.18 | – | 65.83 |
$$ \text{Sand Mix Composition (Coal-Free)} = S_{100/200} + B_{9-12\%} + H_2O_{2.4-3.3\%} $$
Where $S_{100/200}$ is silica sand of 100/200 mesh, $B$ is bentonite, and $H_2O$ is added moisture.
The molding and pouring parameters were kept identical for both sands to isolate the variable. The coal-free sand exhibited slightly lower green strength and required careful moisture control to avoid stickiness during pattern stripping. Crucially, upon visual inspection after casting and shot blasting, the internal cavities of the sand casting parts produced with the high-AFS, coal-free sand were completely free of burn-on, matching the quality of parts from black sand. Internal quality checks via non-destructive testing also showed no shrinkage or porosity, and mechanical properties were within specification. This first trial confirmed that eliminating coal dust was technically feasible for producing sound sand casting parts, provided the sand’s granulometry was tightly controlled.
Encouraged by these results, we proceeded to eliminate coal dust from one of our high-pressure molding lines for mass production. Initially, sand casting parts showed excellent surface finish without burn-on. However, after a short period of sand system recycling, a new defect emerged: subsurface porosity, primarily in the gating system and certain sections of the sand casting parts. This indicated that while the new sand worked initially, its properties degraded upon recycling. The root cause was identified as the “Oolitization Rate” or “Dead Clay Buildup.” Oolitization refers to the formation of a hard, sintered layer of dead bentonite firmly cemented onto sand grains during the thermal cycling of pouring and cooling. This layer is inert, provides no bonding strength, and negatively impacts sand properties.
We quantified this phenomenon. The oolitization rate ($R_o$) is determined by a sequential chemical digestion process that removes all reactive clay, leaving only the inert, sintered shells on the grains. The mass of this residue relative to the original sample defines the rate.
$$ R_o = \frac{M_{\text{residue after KOH digestion}}}{M_{\text{original baked sand sample}}} \times 100\% $$
A higher $R_o$ indicates a higher degree of sand system degradation. We correlated this metric with a key molding property: shear strength. Shear strength is more sensitive to surface condition of sand grains than compression strength.
| Production Line | Sand Shear Strength (kPa) | Oolitization Rate, $R_o$ (%) |
|---|---|---|
| Line A | 27 | 20.11 |
| Line A | 45 | 17.11 |
| Line B | 29 | 22.47 |
| Line B | 46 | 17.33 |
The inverse relationship is clear: $$ \text{Shear Strength} \propto \frac{1}{R_o} $$
Higher oolitization directly led to lower shear strength, weaker mold surfaces, and ultimately, gas-related defects in the sand casting parts. To manage this, we initiated a controlled experiment by systematically increasing the percentage of new sand additions to the returning system sand. The results definitively linked oolitization to defect rate.
| New Sand Addition Rate (%) | System Sand Oolitization Rate, $R_o$ (%) | Cylinder Block Sand Casting Parts with Porosity (%) |
|---|---|---|
| 0.30 | 19.86 | 6 |
| 0.50 | 17.34 | 3 |
| 0.70 | 15.66 | 1 |
| 1.00 | 14.82 | 0 |
The data established a critical control limit: to prevent gas porosity in these sand casting parts, the system sand’s oolitization rate must be maintained below 15%. This finding became the cornerstone for sustainable coal-free sand recycling.
The final phase involved optimizing the virgin coal-free sand mix for better performance in a closed-loop system. We hypothesized that a higher initial clay content could provide a larger buffer against the inevitable buildup of dead clay. A new mix with increased bentonite was formulated and tested.
| Parameter | Coal-Free Sand (High Clay) | Standard Black Sand |
|---|---|---|
| Compactability (%) | 34 / 50 | 30 |
| Moisture (%) | 2.4 / 3.3 | 3.3 |
| Permeability | 85 | 97 |
| Green Compression Strength (kPa) | 177 / 174 | 171 |
| Active Bentonite (%) | 11.2 | 8.0 |
| Loss on Ignition (%) | 1.6 | 3.2 |
| Clay Content (%) | 11.56 | 12.08 |
| Average Fineness (AFS) | 83.41 | 65.12 |
This high-clay, high-fineness, coal-free sand produced sand casting parts with exceptional surface finish, again with no burn-on in critical areas. More importantly, when this sand was introduced into the recycling loop with the disciplined addition of 1.0% new sand to control $R_o < 15\%$, the system stabilized. We achieved consistent, mass production of high-quality cylinder block sand casting parts without the use of coal dust. The process was cleaner, with no volatile smoke during pouring, and the sand itself was more responsive to conditioning.
In conclusion, the successful implementation of a coal-free green sand process for demanding sand casting parts hinges on two interdependent pillars: Granulometry Control and Oolitization Management. First, a high Average Fineness Number (AFS > 80) is essential to create a naturally dense mold surface that resists metal penetration, functionally replacing the lustrous carbon layer from coal dust. Second, the inevitable thermal degradation of bentonite, measured as the Oolitization Rate ($R_o$), must be rigorously controlled through sufficient new sand dilution. Our operational limit is $R_o < 15\%$. This ensures adequate shear strength and prevents gas porosity. This holistic approach transforms green sand from a traditional, coal-dependent mix into a precisely engineered, environmentally friendlier system capable of producing excellent sand casting parts reliably and at high volume.