It is reasonable to assert that the use of continuously mixed self-hardening sands for mold and core making represents the optimal production method for medium and large castings such as those for machine tool casting, presses, compressors, turbines, and rolling mill housings. From a designer’s viewpoint, the primary task is to correctly select the appropriate molding method, as this decision critically influences the overall mechanization strategy for the molding department.
In practical applications, several types of self-hardening sands suitable for this production scale are employed. The common characteristic of these materials is the elimination of the traditional drying process for molds and cores, though a brief surface drying might still be necessary when using water-based coatings. These materials, however, possess a series of distinct differences. The most critical factors from a design and operational standpoint include: the cost of molding materials, the number and nature of their components, the initial setting time (minimum time from filling to pattern withdrawal or core box stripping), the total hardening time (time from filling to mold closing or pouring), knock-out properties after casting, and the possibility of reclamation.
The following four families of self-hardening sands are most relevant for machine tool casting and similar heavy-section work:
- Furan resin-bonded sand.
- Dry, self-hardening sodium silicate sand (often called “air-set”).
- Fluid self-hardening sodium silicate sand.
- Fluid self-hardening cement sand.
For core making, especially for complex cores in machine tool casting, another fluid sand is used, known as the “C” process, which uses a resin binder. Due to its higher cost, its application is typically confined to core production.
The table below provides a comparative analysis of the key properties of these materials relevant to the design of a mechanized system.
| Property / Material | Furan Resin Sand | Dry Sodium Silicate Sand | Fluid Sodium Silicate Sand | Fluid Cement Sand | “C” Process Fluid Sand |
|---|---|---|---|---|---|
| Typical Compressive Strength (MPa) | 1.2 – 2.0 (after 1h) | 0.6 – 1.0 (after 1h) | 0.4 – 0.7 (after 1h) | 0.3 – 0.6 (after 24h) | 1.0 – 1.8 (after 1h) |
| Initial Setting Time, ti (min)* | 5 – 30 | 10 – 45 | 15 – 60 | 60 – 180 | 5 – 20 |
| Total Hardening Time, th (min)* | 30 – 120 | 60 – 180 | 90 – 360 | 1440+ | 30 – 90 |
| Knock-out Property | Good | Difficult | Difficult | Fairly Difficult | Excellent |
| Reclaimability | Very Difficult | Difficult | Difficult | Possible (with processing) | Very Difficult |
| Relative Material Cost Index | 100 (Base) | 20 – 30 | 25 – 35 | 15 – 25 | 120 – 150 |
* Highly dependent on ambient temperature, catalyst/hardener type and amount, and geometry of the mold/core.
Furan resin sand offers distinct advantages, including high early strength and excellent knock-out characteristics, which are crucial for complex machine tool casting geometries. However, it has significant drawbacks: high and often volatile cost, limited availability, and the presence of nitrogen in cheaper variants, which can lead to casting defects. Nevertheless, its use can be economical in specific high-value applications. The high strength of furan sand even allows for flaskless molding, significantly reducing handling requirements and sand consumption per casting. The strength development can be modeled as a function of time and temperature:
$$ \sigma_c(t, T) = \sigma_{\infty} \cdot (1 – e^{-k(T) \cdot t}) $$
where $\sigma_c$ is the compressive strength, $\sigma_{\infty}$ is the ultimate strength, $k(T)$ is a temperature-dependent rate constant, and $t$ is time.
Water glass-based sands (both dry and fluid) and cement sand are more economical and widely available binders. Their slower hardening profiles, especially for cement, dictate the layout and timing of the production flow. The fluid sands offer the advantage of excellent flowability, filling complex patterns without the need for heavy ramming, which is ideal for large, intricate machine tool casting molds. The flowability can be characterized by a spread ratio test, but the required workability for filling a mold segment of volume $V_m$ relates to the sand’s yield stress $\tau_y$:
$$ V_m \propto \frac{1}{\tau_y} $$
Lower yield stress indicates better flow.
The design of mechanized systems must adapt to the chosen sand system. Continuous mixers are central to productivity. For furan or similar two-component systems, single-chamber mixers are standard. For sands with multiple dry additions (like cement, additives) followed by liquid components, two-stage continuous mixers are required. A typical configuration involves a primary chamber for dry blending and a secondary chamber for liquid incorporation and final mixing. The required mixer throughput $Q$ (kg/h) is determined by the molding cycle and box size:
$$ Q = \frac{n \cdot m_s}{t_c} $$
where $n$ is the number of molds per cycle, $m_s$ is the sand mass per mold, and $t_c$ is the cycle time.
Layout Strategies for Mechanized Lines
The selection of sand defines the process timeline, which in turn dictates the plant layout. Key considerations are space utilization, handling of patterns and flasks, and logistics for pouring and cooling.
1. Horizontal Layout with Flask Molding (Furan Resin): A classic layout for heavy machine tool casting (e.g., beds up to 30 tons) uses a movable continuous mixer serving fixed molding stations equipped with jib cranes. Mold filling, hardening, and pouring occur at the station. Overhead cranes transport flasks for closing and moving to the pouring area. This layout offers great flexibility for very large, single-piece production.
2. Linear Flow with Conveyor Hardening (Dry Sodium Silicate): A modern line for machine tool casting utilizes a central continuous mixer positioned over a roller conveyor. After filling, flasks are transferred to a belt conveyor for the hardening period, isolating them from vibration. A transfer car then moves the flask to a stripping station. The pattern returns via a separate conveyor, while the mold is lifted by a jib crane to a gravity roller conveyor for coating, closing, and pouring. This decouples the mixing/stripping cycle from the slower hardening and downstream operations.
3. Vertical Layout for Space Optimization: Urban constraints and poor soil conditions often motivate multi-story layouts. The molding floor is placed on an upper level to avoid pits. Two continuous mixers (one for backing sand, one for facing sand) fill flasks on a roller line equipped with a vibrating table for compaction at the facing station. A roll-over device strips the pattern. The empty mold proceeds for further processing, while the pattern is lowered via a lift to a ground-floor storage and preparation circuit, served by another lift for the next cycle. This minimizes the footprint and simplifies foundation work. The vertical transport energy for a pattern of mass $m_p$ is:
$$ E = m_p \cdot g \cdot h $$
where $g$ is gravity and $h$ is the lift height. This cost is often negligible compared to the savings in floor space.
4. Integrated Lines for Fluid Sands: A typical line for fluid self-hardening sand (water glass or cement) features a mixer feeding a filling station with a vibratory table. A rotating index table or a long linear conveyor provides the necessary hardening time ($t_h$). The required length $L$ of the hardening conveyor is:
$$ L = v_c \cdot t_h $$
where $v_c$ is the conveyor speed. After hardening, molds undergo stripping, coating, surface drying, and are then transferred to a pouring and cooling zone.
5. Mechanized Core Making with the “C” Process: For large cores essential in machine tool casting, a dedicated “C” process line is highly effective. A mixer with a distribution arm serves multiple core boxes on roller conveyors or turntables. After filling and a short hardening period on a powered roller conveyor, cores are stripped, often using a dedicated unit. They are then transferred via monorail or conveyor through a coating station and a drying oven. The excellent collapsibility of these cores justifies their cost for complex internal geometries.
Technical and Economic Analysis
The mechanization strategy must be evaluated against technical performance and overall cost. The total cost per ton of casting $C_{total}$ can be broken down as:
$$ C_{total} = C_{material} + C_{labor} + C_{energy} + C_{capital} + C_{disposal} $$
Material Cost (Cmaterial): This is dominated by the binder system. While furan resin is expensive, its high strength can reduce overall sand consumption through flaskless molding or thinner walled flasks. The reclamation potential also plays a major role. Sodium silicate and cement sands have lower binder costs but often higher new sand make-up rates if reclamation is inefficient.
Labor Productivity: Mechanization drastically reduces direct labor. The productivity gain $\Delta P$ can be estimated by comparing manual and mechanized cycle times:
$$ \Delta P \approx \frac{t_{manual} – t_{mech}}{t_{manual}} \times 100\% $$
For a typical machine tool casting molding cycle, $\Delta P$ can range from 200% to 400%.
Energy Consumption: The elimination of large drying ovens (stoving) represents massive energy savings. The energy saved $E_{save}$ per ton of sand previously dried is:
$$ E_{save} = m_s \cdot c_s \cdot \Delta T + m_w \cdot h_{fg} $$
where $m_s$ is sand mass, $c_s$ is specific heat of sand, $\Delta T$ is the temperature rise, $m_w$ is water mass evaporated, and $h_{fg}$ is the latent heat of vaporization.
Capital Investment & Floor Space: Mechanized lines require higher initial investment but use floor space more efficiently. The vertical layout is particularly effective in reducing the required area $A$:
$$ A_{vertical} \approx 0.6 \cdot A_{horizontal} $$
for equivalent capacity.
Environmental & Disposal Costs (Cdisposal): This is increasingly critical. The reusability of sand is a key differentiator. Poorly reclaiming sands lead to high disposal costs and environmental liability. The specific disposal cost per ton of waste sand $c_d$ directly impacts the total:
$$ C_{disposal} = m_{waste} \cdot c_d $$
Processes with better reclamation characteristics (some fluid sands after thermal reclamation) have a significant long-term advantage.
The following table summarizes the economic and operational drivers for selection in the context of machine tool casting.
| Decision Factor | Furan Resin | Dry Sodium Silicate | Fluid Sodium Silicate / Cement | “C” Process |
|---|---|---|---|---|
| Primary Driver | High Precision, Complex Geometry, Fast Cycle | Good Strength, Moderate Cost, Widely Available | Excellent Flowability, Low Equipment Wear, Low Binder Cost | Superior Core Collapsibility, Complex Cores |
| Optimal Batch Size | Small to Medium | Medium | Medium to Large | Small (Cores only) |
| Labor Skill Required | Medium | Medium | Low | High (Process Control) |
| Payback Justification | High-Value Castings, Reduced Machining Allowance | General Purpose, Reliable Quality | Large Castings, Reduced Ramming Labor | Prevents Hot Tearing, Enables Complex Internal Features |
Material Properties and Process Control
The consistent quality of machine tool casting relies on tight control of sand properties. Key parameters include:
- Strength Development: As shown in the first formula, understanding the kinetics is vital for scheduling. The rate constant $k(T)$ often follows an Arrhenius relationship:
$$ k(T) = A e^{-E_a/(R T)} $$
where $A$ is a pre-exponential factor, $E_a$ is activation energy, $R$ is the gas constant, and $T$ is absolute temperature. - Gas Evolution & Permeability: Core sands must allow gases to escape. Permeability $P$ is critical and is a function of grain size distribution and binder amount:
$$ P \propto \frac{d^2}{\eta \cdot L} \cdot \frac{\phi^3}{(1-\phi)^2} $$
where $d$ is mean grain size, $\eta$ is gas viscosity, $L$ is path length, and $\phi$ is porosity. - Thermal Deformation & Expansion: Sand must resist deformation at high temperatures to maintain dimensional accuracy in machine tool casting. The thermal expansion coefficient $\alpha_s$ and high-temperature strength are key.
Reclamation of self-hardening sands, while challenging, is essential for economic and environmental sustainability. The performance of reclaimed sand $R$ can be expressed as a function of new sand properties $N$ and the number of cycles $n$:
$$ R(n) = N \cdot f(n) $$
where $f(n)$ is a degradation function, typically exponential $f(n)=e^{-\lambda n}$, with $\lambda$ being a loss coefficient specific to the binder system. Furan sands have a high $\lambda$, meaning properties degrade rapidly with recycling without intensive thermal reclamation.
Conclusion and Future Outlook
The transition to self-hardening sands and associated mechanization is the definitive path for modern, small-batch production of medium and large castings, with machine tool casting being a prime beneficiary. This approach delivers:
1. High Productivity: Through continuous mixing and streamlined material flow.
2. Improved Working Conditions: Significant reduction in physical labor, noise (compared to pneumatic rammers and shakeout), and dust.
3. Flexibility: Quick pattern changeovers accommodate the small-batch nature of the business.
4. Space Efficiency: Modern layouts, especially vertical ones, optimize valuable factory floor area.
5. Energy Savings: Elimination of mold drying ovens.
The main unresolved challenge remains the environmental and economic cost of spent sand disposal. Therefore, the development of efficient, cost-effective sand reclamation technologies for all self-hardening binder systems is the most critical area for future advancement. The choice between furan resin, sodium silicate, or cement-based processes ultimately depends on a detailed analysis of casting geometry, required quality, production volume, and local economic factors (material cost, labor cost, environmental regulations). However, the underlying principle is clear: a well-designed mechanized system based on self-hardening sands is fundamental to the competitive and sustainable production of high-quality machine tool casting and other heavy industrial components.