The Physics of Compaction: A Comprehensive Analysis of Online Squeezing Compactability as the Master Variable in Green Sand Process Control

The Rheological and Granular Mechanics of Green Sand

The Clay-Water System: From Platelet Hydration to Cohesive Bonding

The mechanical behavior of green sand is fundamentally governed by the microscopic interactions within the clay-water binder system. The cohesive strength of the sand mixture originates from the unique structure of bentonite clay, which consists of layered aluminosilicate platelets. When water is introduced during the mulling process, its polar molecules are adsorbed into the inter-packet spaces of these platelets, causing them to swell significantly.¹ This hydration process creates viscous, cohesive "clay-water bridges" that coat the silica sand grains and bind them together upon compaction.

The process of mulling is the critical energy input required to activate this bonding mechanism. It applies both compressive and shearing forces to the sand mass, which serves two purposes: it breaks down agglomerates of clay, exposing new platelet surfaces for hydration, and it ensures that the resulting tenacious clay-water "putty" is smeared uniformly onto the sand grains.³ The efficiency of this process determines the quality of the bond. Within this system, water exists in two primary states: "temper water," which is adsorbed by and structurally integrated into the clay platelets, and "free water," which acts more as a lubricant between coated grains.⁴ Only temper water contributes directly to the cohesive strength of the mold; excess free water serves primarily to generate large volumes of steam upon contact with molten metal, a primary cause of gas-related casting defects.⁶

To quantify the state of the binder, foundry technologists use derived properties such as "Available Bond" and "Working Bond." Available Bond, calculated from Green Compression Strength (GCS) and moisture content, indicates the total potential bonding capacity of the clay in the system. In contrast, Working Bond, derived from GCS and compactability, reflects the amount of clay that is effectively producing bond strength under the current molding conditions. A large discrepancy between these two values can indicate inefficient mulling or issues with the clay-to-water ratio.⁸

Influence of Bentonite Type: A Comparative Analysis of Sodium vs. Calcium Bentonites

The choice of bentonite type has a profound impact on the final properties of the green sand mold, as sodium and calcium bentonites exhibit distinctly different behaviors. Sodium (Western) bentonite is characterized by its exceptional thermal stability, which translates to superior hot compressive strength and wet tensile strength. This makes it the preferred binder for high-temperature ferrous alloys like steel and ductile iron, where it is critical to prevent thermal-related defects such as sand erosion, inclusions, and expansion scabs.³ Conversely, calcium (Southern) bentonite is known for its ability to develop green properties rapidly, achieving higher green compression strength at lower moisture levels. It also offers better flowability and lower deformation, making it advantageous for producing molds with intricate details, deep pockets, or for lower-temperature non-ferrous applications.³ The quantitative differences are summarized in Table 1.

Table 1: Comparative Properties of Sodium vs. Calcium Bentonite Systems

Property	100% Sodium Bentonite	100% Calcium Bentonite	50/50 Blend
Green Compression Strength	11.8 psi	14.3 psi	12.6 psi
Green Deformation	1.3%	0.95%	1.1%
Wet Tensile Strength	0.466 N/cm2	0.071 N/cm2	0.346 N/cm2
Hot Compressive Strength (1800∘F)	575 psi	110 psi	320 psi
Thermal Stability	Very Good	Poor	Good
Flowability	Less Good	Good	Moderate

Data synthesized from sources.³ All values are for systems at equivalent compactability.

This data illustrates the fundamental trade-off: calcium bentonite provides superior "as-molded" properties (GCS, flowability), while sodium bentonite provides the necessary thermal durability to withstand the rigors of iron and steel pouring.

The Role of Granulometry: How AFS Grain Fineness Number (GFN) Dictates Baseline Properties

While the binder system creates cohesion, the base sand itself dictates the foundational characteristics of the mold. The American Foundry Society Grain Fineness Number (AFS-GFN) is a weighted average that quantifies the particle size distribution of the sand.¹² More critically, the GFN serves as a direct proxy for the total surface area of the sand in the system.³ This relationship is paramount; a higher GFN indicates finer sand, which corresponds to a significantly larger total surface area per unit weight. This increased surface area demands a greater volume of the clay-water binder to achieve the same coating thickness on each grain, and therefore requires higher moisture and clay additions to reach a target compactability.³

The selection of an appropriate GFN involves a critical engineering trade-off:

• High GFN (Fine Sand): The smaller interstitial spaces between grains produce a smoother mold surface, resulting in a superior casting surface finish. However, these same small, tortuous pathways lead to inherently low permeability, which elevates the risk of gas-related defects as evolving steam and binder gases cannot easily escape.¹⁴

• Low GFN (Coarse Sand): The large, well-connected voids between grains provide excellent permeability, allowing gases to vent freely. This advantage comes at the cost of a rougher casting surface finish and an increased susceptibility to metal penetration, where metallostatic pressure forces liquid metal into the voids between sand grains.¹¹

Therefore, the stability of the GFN is a prerequisite for the stability of the entire green sand system. Uncontrolled changes, such as a shift in new sand supply or an accumulation of thermally degraded fines, will alter the system's surface area. This change will disrupt the established relationship between water additions and compactability, making process control exceptionally difficult. The GFN must be considered the master compositional variable that sets the stage for all subsequent physical property control.

Furthermore, the properties of a sand mix are not static after discharge from the muller. Even in a sealed container with no moisture loss, compactability can decrease over time.¹ This occurs because the mulling process may not achieve complete hydration of all clay platelets. Subsequently, during transport and storage, less-tightly-bound "free" water continues to slowly absorb into the interior of the clay platelets. This migration transforms water from a lubricant to a bonding agent, making the sand mixture stiffer and less plastic. This time-dependent behavior underscores the necessity of testing the sand at the point of use—the molding machine—to capture its true state at the moment of compaction.

Paradigms of Measurement: From Standardized Specimens to In-Process State Analysis

The AFS 3-Ram Method: Principles, Procedures, and Interpretive Limitations

The traditional standard for green sand testing is the AFS 3-ram method, a procedure dating back to the 1920s designed to replace subjective "hand-feel" evaluations with a quantifiable metric.¹⁷ The procedure involves riddling a loose mass of sand into a standard 2-inch diameter specimen tube, striking off the excess to create a fixed initial volume, and applying a fixed energy input by dropping a 14-pound weight from a height of 2 inches three successive times.⁷ The resulting percentage decrease in height is defined as the compactability.

Historically, the primary utility of this method within a laboratory workflow has been to create a standardized 2-inch by 2-inch cylindrical specimen for subsequent tests, such as green compression strength and permeability. To achieve this fixed geometry, the operator must vary the initial weight of the sand in the tube until the 3-ram procedure yields a specimen of the precise target height.⁷ While invaluable for benchmarking material properties, this approach has a fundamental limitation: it answers the question, "How much of this sand is required to create a specimen of standard density?" It does not, however, answer the more practical manufacturing question, "What density will my molding machine achieve with this sand?"

The Online Squeezing Method: Replicating the Moulding Machine for True Process Insight

The online squeezing method represents a paradigm shift in both measurement technique and control philosophy. Instead of impact, this method uses a calibrated pneumatic cylinder to apply a controlled squeeze pressure to a fixed mass of sand within a test chamber.¹⁷ This action directly simulates the compaction process of modern high-density and jolt-squeeze molding machines.¹⁹

The crucial difference is that the final height of the specimen is allowed to vary, and this variation becomes the primary output of the test. The compactability is calculated using the formula:

C(%)=(Hinitial​Hinitial​−Hfinal​​)×100

This method directly measures how a fixed quantity of sand responds to a fixed energy input, which is precisely what occurs within the flask on a production molding line.7 By deploying these systems online, directly at the point of use, foundries can obtain real-time data that reflects the sand's actual condition, accounting for any changes like moisture loss or continued clay hydration that may have occurred during transport from the muller.13 Table 2 provides typical target property ranges for a high-density iron molding system controlled with this methodology.

Table 2: Green Sand Property Target Ranges for High-Density Iron Moulding

Parameter	Typical Target Value	Allowable Range
Compactability	40%	± 2%
Moisture	3.2%	± 0.1%
Green Compression Strength	31-36 psi (214−248 kN/m2)	-
Permeability	> 100	-
Active Clay (MB)	> 8%	-
AFS Clay	11-14%	-
Loss on Ignition (LOI)	3.5-7.5%	-

Data synthesized from sources.⁴

This philosophical divide between the two methods is critical. The traditional lab method is a fixed geometry system where density variation is eliminated to produce a standard test piece. The online squeezing method is a fixed energy system that embraces the resulting density variation as the most important signal. It is not merely a faster, automated version of the lab test; it is a more accurate physical simulation of the manufacturing process itself, providing a direct window into the mold's future state.

Data Correlation: Bridging Measurements from Laboratory and Online Systems

Despite their philosophical differences, studies conducted by the AFS have demonstrated a strong statistical correlation between results from the 3-ram and pneumatic squeeze methods.¹⁷ Typically, for the same sand sample, the pneumatic squeezer will yield a slightly higher compactability reading. This strong correlation allows foundries to transition from legacy lab-based control to modern online systems by establishing new, equivalent target ranges, ensuring that historical process data remains relevant.

Furthermore, utilizing both systems can be a powerful diagnostic tool. By comparing the automatic tester reading at the muller with a manual test performed on sand taken from the molding machine hopper, a foundry can precisely quantify the degree of property change—such as drying or continued hydration—that occurs during transport.¹³ This data can inform adjustments to muller targets or highlight inefficiencies in the sand transport system.

Bulk Density: The Unifying Master Variable of the Compacted Mould

The Direct Link: How Squeezing Compactability Quantifies Achievable Bulk Density

The primary value of the online squeezing compactability measurement lies in its direct and unambiguous relationship to the final bulk density (ρb​) of the compacted sand. For a fixed initial mass of sand (m) placed in a cylindrical test chamber of a fixed radius (r), the bulk density is solely a function of the final compacted height (Hf​):

ρb​=Vfinal​m​=πr2Hf​m​

Since compactability is also a direct function of Hf​, it serves as a precise, real-time indicator of the bulk density that the sand will achieve under a standard molding force.17 A higher compactability value corresponds directly to a smaller

Hf​ and therefore a higher bulk density. This density is the master physical variable that dictates all other critical mechanical and thermal properties of the finished mold.

The Density Effect on Electrical Moisture Measurement: Unmasking a Common Source of Control Error

Automated online moisture sensors typically operate by measuring the electrical conductivity or capacitance of the compacted sand specimen. The electrical current primarily flows through the continuous films of temper water coating the sand grains and clay platelets.⁵ The efficiency of this conduction is highly dependent on the proximity of the conductive particles.

As compactability increases, the resulting bulk density rises, forcing the sand grains closer together. This densification creates more numerous, shorter, and more intimate conductive pathways. The consequence is that for two sand samples with identical gravimetric water content, the sample compacted to a higher density will exhibit a higher apparent moisture reading. Published foundry research confirms that an increase in bulk density of just 0.1 g/cm3 can cause the electrical moisture reading to increase by 0.1% to 0.2%.

This phenomenon can create a pernicious feedback loop in automated control systems. If a transient event causes compactability to rise, the denser specimen will report a higher apparent moisture. A control system programmed to maintain a fixed moisture setpoint will interpret this as the sand being "too wet" and will reduce the water addition in the subsequent batch. This new batch will then be genuinely drier, leading to low compactability and a risk of friability and erosion defects, causing the system to oscillate out of control. This demonstrates that controlling a sand system based on an electrical moisture reading alone is fundamentally flawed; the primary control target must be compactability.

The Physics of Permeability: An Inverse, Non-Linear Function of Inter-granular Void Volume

Permeability is the property that allows gases generated during pouring—steam from moisture and pyrolysis gases from binders—to escape through the mold. It is a measure of the interconnectedness and volume of the void spaces between sand grains, and its value is governed by Darcy's Law.⁷

The effect of bulk density on permeability is direct and severe. As higher compactability leads to higher bulk density, the sand grains are packed more tightly, which drastically reduces the total volume of the voids and constricts the channels that connect them. This relationship is both inverse and highly non-linear. Data from AFS and foundry literature consistently show that while a change in compactability from 35% to 40% may only modestly decrease permeability, a further increase from 45% to 50% can cause a precipitous drop, potentially pushing the sand over a "permeability cliff".¹⁸ For a typical iron system sand, this change can reduce permeability from a safe value of 140 to a critically low 100, creating a high probability of trapped gas defects.

The Mechanics of Green Strength: Exponential Gains Through Inter-particle Contact

The Green Compression Strength (GCS) of a sand mold is the maximum compressive stress it can withstand before fracture. This strength is derived from the cumulative cohesive forces of the millions of clay-water bridges acting at the contact points between adjacent sand grains.⁷

The influence of bulk density on GCS is profound and exponential. As density increases, the number of grain-to-grain contact points per unit volume increases dramatically. This creates a much more robust, interlocked structure capable of resisting higher loads. Consequently, GCS is strongly and directly correlated with bulk density.²⁷ A sand mixture that appears weak may simply be in a low-density state. For example, the same sand composition could exhibit a GCS of

120 kN/m2 (approx. 17 psi) at 35% compactability, but when compacted to 45% compactability, its strength could rise to 170 kN/m2 (approx. 25 psi) without any change in its constituent recipe. This highlights that strength is not just a function of composition, but of the compacted state, which is best measured by compactability.

A Framework for Defect Prediction and Mitigation

By understanding that compactability is a direct measure of the mold's final density, and that density governs all other properties, a powerful framework for predicting and preventing casting defects can be established. This framework moves beyond simply reacting to defects and enables proactive control of the mold's physical state. The most practical application of this knowledge is a diagnostic framework, as detailed in Table 3.

Table 3: Comprehensive Casting Defect Analysis Framework

Defect Name	Visual Appearance	Primary Causal Sand Condition	Key Compactability Indicator	Secondary Sand Parameter Indicators	Formation Mechanism	Recommended Corrective Action
Blows / Pinholes	Smooth-walled, spherical or oval cavities, often subsurface, revealed after machining. 29	Low Mold Permeability	High (>48-50%)	High Apparent Moisture, Low Permeability (<100)	Evolved steam and binder gases are trapped by the dense mold structure. Gas pressure exceeds metallostatic pressure, forming bubbles in the liquid metal. 6	Reduce compactability target to increase permeability. Verify muller water additions. Check for excessive fines or combustibles (LOI). 18
Sand Erosion / Inclusions	Irregularly shaped sand grains embedded in the casting surface or interior. 15	Low Mold Strength / High Friability	Low (<35-38%)	Low GCS, High Friability (>11%)	Weak, low-density mold surface cannot resist the erosive force of flowing metal. Sand particles are scoured away and trapped in the casting. 6	Increase compactability target to improve mold density and strength. Verify clay content (Methylene Blue test) and mulling efficiency. 15
Metal Penetration	Rough casting surface with sand grains embedded in a metallic matrix. 32	High Mold Permeability / Low Density	Low (<35-38%)	Low GCS, Low Mold Hardness	Large voids between loosely packed sand grains allow liquid metal to be forced into the mold wall by metallostatic pressure. 18	Increase compactability target to create a denser mold face. Consider using a finer base sand (higher GFN) or mold coating. 32
Swell / Mould Wall Movement	Casting is oversized, dimensionally inaccurate, and may show signs of "false shrinkage." 18	Low Mold Strength	Low (<35-38%)	Low GCS, Low Mold Hardness	The weak, low-density mold wall cannot resist the metallostatic pressure of the liquid metal head, causing the mold cavity to expand. 6	Increase compactability target significantly to increase mold density and rigidity. Ensure adequate flask support and clamping. 18
Expansion Scab	A thin, metallic layer separated from the casting body by a layer of sand. 18	Low Hot Strength / Poor Thermal Stability	Any (often in range but with poor binder)	Low Wet Tensile Strength	At the mold-metal interface, a layer of sand expands rapidly. If the condensation zone behind it has low strength, this layer can buckle and break away, allowing metal to flow behind it. 3	Ensure adequate Sodium Bentonite content for high WTS. Verify seacoal/carbonaceous additive levels. Avoid excessive compactability. 18

The High-Compactability / High-Density Regime: Predicting and Preventing Gas-Related Defects

When online testers indicate a consistently high compactability (e.g., 42-45%), the resulting mold will be hard, dense, and strong, but critically, it will have low permeability. As molten metal fills the cavity, the intense heat instantly vaporizes the moisture in the sand, creating a massive volume expansion as water turns to steam. Simultaneously, any carbonaceous additives and organic binders pyrolyze, generating additional gases.²⁵ In a low-permeability mold, these gases cannot escape through the sand. The internal gas pressure builds until it exceeds the local metallostatic pressure of the liquid metal, forcing bubbles into the casting. These trapped bubbles manifest as defects such as

blows, pinholes, and gas porosity.⁶ A case study from a ductile iron foundry demonstrated this link clearly: a process drift that caused a drop in permeability led to a 40% increase in gas-related scrap, which was resolved by restoring permeability through sand system corrections.²⁵

The Low-Compactability / Low-Density Regime: Predicting and Preventing Mould Stability Failures

Conversely, a low compactability reading (e.g., <35-38%) signals a soft, porous, and structurally weak mold. While such a mold may have excellent permeability, its mechanical integrity is insufficient to withstand the forces of pouring. The low density means there are fewer grain-to-grain contact points, resulting in a weak cohesive network.¹⁵ This state leads to several distinct failure modes:

• Erosion and Sand Inclusions: The kinetic energy of the flowing metal stream physically scours away loose sand from the mold walls and runners. This dislodged sand is then carried into the casting cavity, becoming trapped as inclusions.⁶

• Metal Penetration: The large, interconnected voids of the low-density structure provide an easy path for liquid metal to be forced into the mold wall under metallostatic pressure, resulting in a rough, sand-encrusted surface that is costly to clean.³²

• Swells and Mould Wall Movement: The low green strength is incapable of resisting the static pressure exerted by the liquid metal. The mold cavity walls deform and expand, producing castings that are dimensionally inaccurate and overweight.⁶

In this low-density regime, the high permeability reading is a misleading and irrelevant parameter. The mold fails mechanically from erosive forces long before gas pressure becomes the primary concern. This illustrates a critical hierarchy of properties: sufficient green strength is a prerequisite for a sound casting. Only after mechanical stability is achieved does permeability become the next limiting factor.

Interpreting the Process Window: A Quantitative Analysis of Four Sand Batches

The following analysis of four distinct sand batches illustrates how to use compactability as the primary interpretive key to predict casting outcomes. The target process window is represented by Batch A.

Batch	Compactability (%)	Bulk Density (g/cm³)	Moisture (Online Reading)	Permeability	Strength (GCS, kN/m²)	Analysis and Predicted Casting Outcome
A	40	1.58	3.3%	145	150	Target Process Window. Properties are well-balanced. The GCS is sufficient to ensure dimensional stability and resist erosion, while the permeability is adequate to evacuate evolving gases. EXPECT GOOD CASTINGS.
B	50	1.68	3.5%	105	190	DANGER: Over-Compacted. The high strength is deceptive. The critically low permeability (105) creates a high probability of trapped steam and core gases, leading to pinholes and blows, especially in complex or poorly vented sections.
C	34	1.49	3.1%	180	115	DANGER: Under-Compacted. The mould is structurally unsound. The low GCS (115) will not resist metallostatic pressure, leading to metal penetration, sand erosion, inclusions, and casting swell. The high permeability is irrelevant due to mechanical failure.
D	45	1.61	3.4%	130	165	Process Drift - Caution. This batch is denser and stronger than the target. While likely acceptable for simple castings, it is trending towards the gas-defect risk zone of Batch B. This could be an early warning of changes in fines or active clay content.

Batch D highlights a more subtle use of compactability data. If operators notice a consistent trend where more water is required over time to maintain the target compactability of 42%, it serves as a powerful leading indicator. This implies that the sand's total surface area is increasing, likely due to an accumulation of fines or dead clay. This allows engineers to investigate and correct the root cause (e.g., adjust dust collection, increase new sand additions) before the compositional drift becomes severe enough to cause defects.

Advanced Topics in Green Sand Control and Characterization

The Influence of Carbonaceous Additives on Thermal Stability and Gas Evolution

Beyond the primary sand-clay-water system, additives play a crucial role. Seacoal (pulverized bituminous coal) and other carbonaceous materials are added to iron foundry sands to improve casting surface finish. When exposed to the heat of the molten metal, these additives volatilize, creating a gaseous cushion of reducing gases at the mold-metal interface. This phenomenon, known as the formation of "lustrous carbon," prevents the liquid metal from wetting the silica grains, thereby preventing sand-metal reactions and producing a smooth, clean casting surface.³⁵

However, this benefit comes with a significant consequence: these additives are a major source of gas. Research has shown that increasing seacoal content from 1% to 2% can increase the total evolved gas volume by more than 15%.³³ This additional gas volume must be safely vented through the mold's permeable structure. This creates a systemic balance: a foundry cannot increase seacoal to improve finish without considering the impact on gas load. To compensate, the compactability target may need to be lowered to intentionally increase permeability, providing a larger escape route for the additional gas. This demonstrates that no variable in a green sand system can be adjusted in isolation.

Dynamic Mechanical Properties: Friability, Wet Tensile Strength, and Thermal Erosion

While GCS is a primary measure of strength, other dynamic properties provide a more nuanced understanding of mold performance during the harsh conditions of casting.

• Friability: This test measures the surface brittleness of the compacted sand, indicating its resistance to abrasion on mold edges and corners. Friability is inversely and sensitively related to compactability; a small decrease in compactability (drying) can cause a sharp increase in friability, elevating the risk of sand inclusion defects.¹⁰

• Wet Tensile Strength (WTS): During pouring, moisture from the green sand is driven away from the hot interface, forming a zone of condensation just behind the dry sand layer. WTS measures the tensile strength of this critical zone. High WTS, a key characteristic of sodium bentonite, is essential for preventing expansion defects like scabs and buckles.³

• Thermal Erosion: This advanced test measures the bulk surface abrasion resistance of a sand specimen at elevated temperatures, providing a more realistic simulation of the erosive action of flowing metal than room-temperature tests like friability.¹⁰

The Future of Sand Control: Insights from Computational Modeling (DEM/FEM)

The future of green sand process optimization lies in computational modeling, which allows for virtual testing and refinement of molding processes. Two primary methods are employed:

• Discrete Element Method (DEM): This approach models the green sand as a collection of millions of individual, interacting particles. DEM is exceptionally well-suited for simulating the granular flow of sand during the filling of a mold cavity, predicting initial density variations, and understanding how sand behaves during a "sand shot" in high-speed molding processes.²⁰

• Finite Element Method (FEM): This method treats the bulk sand as a continuous medium with defined mechanical properties. FEM is used to simulate the compaction or squeezing phase of molding, predicting the final distribution of density, stress, and mold hardness throughout the mold based on the pattern geometry and applied pressure.⁴⁰

Together, these simulation tools enable foundry engineers to optimize pattern layouts, gating designs, and molding machine parameters to achieve uniform compaction and minimize the risk of density-related defects before a single physical mold is ever produced.

Conclusion

The transition from traditional, laboratory-based sand testing to online, in-process measurement by the squeezing method represents a fundamental evolution in green sand control. This shift is rooted in a change of philosophy: from creating a standardized specimen of fixed geometry to accurately simulating the fixed-energy compaction of the molding machine itself. The variability in final specimen density, once treated as experimental noise, is now correctly identified as the critical signal.

This analysis has established that compactability, as measured by the online squeezing method, is the most direct and practical indicator of the final bulk density of the sand in the mold. This bulk density is the master physical variable that unifies and dictates all other critical mold properties. The apparent moisture reading, the permeability to gas, and the green compression strength are not independent variables to be chased individually; they are secondary effects of the density achieved during compaction.

By embracing compactability as the anchor for interpretation, foundries can avoid the pitfalls of naive control strategies, such as chasing an apparent moisture setpoint, and begin to control the actual physical state of the mold. This holistic approach provides a powerful framework for proactively predicting casting outcomes. High compactability warns of an impending risk of gas defects due to low permeability, while low compactability signals the danger of mold stability failures like erosion, penetration, and swells. By maintaining compactability within a well-defined process window, foundries can stabilize their sand systems, align process control with metallurgical reality, and achieve a significant reduction in costly casting defects.