In the manufacturing sector, the foundry industry holds a pivotal role, especially within machinery and equipment manufacturing. Promoting green production in casting is essential for achieving sustainable development. As such, our team has focused on optimizing production processes for steel castings used in railway locomotives, which are critical components ensuring safety and performance. Historically, these steel castings were produced using conventional sodium silicate sand as facing sand and green sand as backing sand, a method prone to defects like gas holes and cold cracks due to high gas evolution from green sand upon contact with molten metal. This led us to explore and implement ester-hardened sodium silicate sand for molding and modified sodium silicate sand for core-making, aiming to enhance quality, reduce labor intensity, and improve the working environment. This article details our journey in optimizing these processes, leveraging advanced equipment and rigorous testing to meet mechanized production demands for high-quality steel castings.
The original process for manufacturing steel castings involved a dual-sand system: ordinary sodium silicate sand as the facing sand and green sand as the backing sand. The molding was performed using a Z2520 molding machine, where the sodium silicate sand was applied to a thickness of 30–50 mm over the pattern surface. The backing sand, composed of green sand, was compacted with air rammers at a pressure of at least 0.5 MPa. After molding, the backing sand surface was leveled, and vent holes were created to facilitate gas escape. For core-making, ordinary sodium silicate sand was employed, with manual techniques used to place core reinforcements, sand blocks, straw ropes, and lifting lugs. The sand mixtures were prepared using specific ratios: for facing sand, refined quartz sand was mixed in a S112 sand mixer with sodium silicate added at up to 6% by weight, followed by 8–10 minutes of mixing; for backing sand, reclaimed sand was mixed in a S115 sand mixer with bentonite and water, with mixing times of 1–3 minutes. However, this process resulted in poor collapsibility of cores, difficult cleaning of steel castings, and increased defect rates, prompting our shift toward more advanced methods.
Ester-hardened sodium silicate sand, developed in the mid-to-late 1990s, represents a significant technological advancement over traditional sodium silicate sand processes. It involves the use of organic esters as hardeners, which react with sodium silicate to form a strong, dimensionally stable mold. The hardening mechanism can be divided into three stages, which we describe using chemical equations to illustrate the process. First, the organic ester hydrolyzes in an alkaline aqueous solution, producing organic acid or alcohol; this stage determines the usable time of the sand mixture. The reaction can be represented as:
$$ \text{RCOOR}’ + H_2O \rightarrow \text{RCOOH} + \text{R}’\text{OH} $$
where R and R’ represent organic groups. Second, the organic acid reacts with sodium silicate, increasing its modulus through a dehydration reaction, leading to solidification. This can be modeled as:
$$ \text{Na}_2\text{O} \cdot n\text{SiO}_2 + \text{RCOOH} \rightarrow \text{higher modulus silicate} + \text{byproducts} $$
Third, further dehydration strengthens the bond, promoting full curing. The advantages of this system are manifold: improved sand flowability for mechanization; dimensional stability after hardening, enhancing precision of steel castings; lower moisture content, reducing gas hole defects; better collapsibility for easier shakeout and recycling of used sand; and compatibility with automation, lowering labor intensity and improving safety. These benefits align with our goals for producing high-integrity steel castings for locomotives.
To implement the ester-hardened sodium silicate sand process, we invested in a dedicated molding line equipped with advanced machinery. The key equipment and their technical parameters are summarized in the table below, which highlights their roles in optimizing production for steel castings. This setup allows for continuous mixing and high-efficiency molding, crucial for batch production of locomotive components.
| Equipment | Specifications | Purpose |
|---|---|---|
| Sand Boxes | Various sizes: 1600 mm × 1000 mm × 450 mm, 1200 mm × 1200 mm × 350 mm, 1000 mm × 1000 mm × 300 mm, 1600 mm × 1000 mm × 320 mm | Accommodate different steel casting patterns |
| Continuous Mixer | Double-arm continuous mixer, capacity 25 t/h, with sand and binder dosing systems | Mix ester-hardened sand on-demand for molding |
| Vibration Table | Load capacity 6 t, table size 2000 mm × 1300 mm, low-amplitude high-frequency vibration | Compact sand uniformly in molds |
| Turnover and Stripping Machine | Hydraulic-driven with clamping mechanisms | Strip molds from patterns after hardening |
| Flow Coating Machine | Clamp range 1500–2500 mm, with coating circulation system | Apply coatings to molds off-line |
| Hot Air Drying Oven | Temperature 120–200°C, uniformity ±5°C, natural gas heating | Dry mold surfaces to reduce residual moisture |
The formulation of sand mixtures is critical for achieving desired properties in steel castings. For ester-hardened sodium silicate sand used in molding, we determined optimal ratios through experimentation. The base sand is bagged refined quartz sand, with specifications tailored for molding and core-making. The ester-hardened sodium silicate is added at 2.8–4.0% of sand weight, significantly lower than the 6% used in the original process, while the organic ester hardener is added at 15–18% of the sodium silicate weight. This reduction in binder content improves collapsibility and reduces costs. For modified sodium silicate sand used in core-making, the modified sodium silicate is added at 4.0–6.0% of sand weight, depending on the core complexity. The mixing times are optimized: for molding sand, the continuous mixer ensures homogeneous blending; for core sand, a batch mixer is used with mixing times of 30–60 seconds. The performance requirements for these sands are quantified as follows: modified sodium silicate sand should have a permeability of at least 180 cm³·cm/(g·min), dry tensile strength over 1.3 MPa, and moisture content of 2.0–4.0%; ester-hardened sodium silicate sand should have permeability above 100 cm³·cm/(g·min) and dry tensile strength exceeding 0.1 MPa. These parameters ensure robust molds and cores for producing defect-free steel castings.
Our trial production involved specific steel castings: the 102-type coupler and new eight-axle locomotive axle box bodies. We selected these components due to their complex geometries and high demand in railway applications. The trial aimed to validate the new processes: two boxes per casting type were produced using ester-hardened sodium silicate sand for molding, and cores were made with modified sodium silicate sand. For the 102-type coupler, each box contained two pieces, while for axle boxes, each box had one piece. The core-making for the 102-type coupler utilized an 80 kg shooting machine (SXJ-80-1S) for automated binder addition, whereas other cores were made manually. This approach allowed us to assess adaptability across different production scales. The key steps included sand preparation using the continuous mixer for molding sand and batch mixers for core sand, followed by molding, core setting, coating, drying, and pouring. We monitored parameters such as hardening time, which averaged 30 minutes for cores, ensuring adequate strength before handling. The use of automated systems reduced manual intervention, aligning with our goal of mechanization for steel castings production.
The quality of raw materials directly impacts the performance of sand mixtures and, consequently, the integrity of steel castings. We sourced high-purity silica sand, ester-hardened sodium silicate, organic esters, and modified sodium silicate from reputable suppliers. The specifications and actual test results are summarized in tables to provide a clear comparison. For silica sand, we used two grades: ZGS 98-30/50 (35C) for molding and ZGS 98-40/70 (50D) for core-making. Their properties, such as SiO₂ content, clay content, and grain fineness, are critical for achieving optimal strength and permeability. The table below details these requirements and our verification data.
| Property | Molding Sand (ZGS 98-30/50) | Core Sand (ZGS 98-40/70) | Testing Frequency |
|---|---|---|---|
| SiO₂ Content (%) | ≥ 98 | ≥ 98 | Per batch |
| Clay Content (%) | ≤ 0.3 | ≤ 0.3 | Per batch |
| Loss on Ignition (%) | < 0.5 | < 0.5 | Annually |
| Shape Factor | ≤ 1.45 | ≤ 1.45 | Annually |
| Fine Powder Content (%) | ≤ 1.0 | ≤ 1.0 | Annually |
| Moisture Content (%) | < 0.2 | < 0.2 | Annually |
| Refractoriness (°C) | > 1700 | > 1700 | Annually |
| Average Fineness | 31–39 | 45–55 | Per batch |
| Three-Screen Content (%) | ≥ 75 | ≥ 75 | Per batch |
| Four-Screen Content (%) | ≥ 85 (30/40/50/70) | ≥ 85 (30/40/50/70) | Per batch |
For binders, we evaluated ester-hardened sodium silicate and organic esters based on density and modulus. The modified sodium silicate for core-making was also tested. The results, compared to standard requirements, are shown in the following table. These materials ensure consistent hardening and strength development, vital for producing durable molds and cores for steel castings.
| Material | Property | Standard Requirement | Actual Test Result | Compliance |
|---|---|---|---|---|
| Ester-hardened Sodium Silicate | Density (g/cm³) | 1.45–1.54 | 1.49 | Yes |
| Modulus | 2.0–2.5 | 2.24 | Yes | |
| Organic Ester | Density (g/cm³) | 1.05–1.25 | 1.1–1.2 | Yes |
| Solid Content & Appearance | ≥ 98%, colorless or light yellow | ≥ 98%, acid value ≤ 1%, colorless | Yes | |
| Modified Sodium Silicate | Density (g/cm³) | 1.47–1.52 | 1.49 | Yes |
| Modulus | 2.0–2.35 | 2.14 | Yes |
The production trial yielded positive outcomes for the steel castings. Using the ester-hardened sodium silicate sand molding line, we achieved consistent sand mixtures with binder addition rates of about 3% for sodium silicate and 15–18% for organic ester. The modified sodium silicate sand for cores had a binder addition of approximately 4%, with hardening times around 30 minutes. After molding, the mold cavities were clean, and core surfaces were free of loose sand. Dimensional inspections via layout checking confirmed that all steel castings met drawing specifications, with no deviations beyond tolerances. Pouring was conducted at 1575°C, with pouring times of 20–30 seconds for the 102-type coupler boxes. After cooling, shakeout was notably easier due to improved collapsibility; the steel castings showed minimal sand adherence, and internal cavities like the coupler ears and axle box spring seat holes were smooth, meeting assembly requirements. Non-destructive testing on the coupler traction surfaces revealed no cracks, and density tests on a sample coupler indicated acceptable defect levels. These results demonstrate that the optimized processes enhance the quality and manufacturability of steel castings for locomotives.
Beyond the trial, we analyzed the economic and environmental impacts of the new processes. The reduction in sodium silicate usage from 6% to 2.8–4.0% translates to material cost savings, while the improved collapsibility reduces energy consumption during shakeout and cleaning. The ester-hardened system also supports sand reclamation; we estimate that up to 80% of used sand can be recycled after thermal or mechanical reclamation, lowering raw material costs and waste disposal. The automation potential of the molding line reduces labor requirements by approximately 30%, based on our preliminary assessments. Furthermore, the lower moisture content in molds decreases the risk of gas-related defects, potentially improving yield rates for steel castings by 5–10%. We quantified these benefits using a simple cost model: if Coriginal represents the total cost per ton of steel castings with the old process, and Cnew with the new process, the savings ΔC can be expressed as:
$$ \Delta C = C_{\text{original}} – C_{\text{new}} = (M_{\text{sand}} + M_{\text{binder}} + L + E)_{\text{original}} – (M_{\text{sand}} + M_{\text{binder}} + L + E)_{\text{new}} $$
where M denotes material costs, L labor costs, and E energy costs. Our estimates suggest ΔC is positive, justifying the investment in new equipment. Additionally, the working environment improved due to reduced dust and fumes from lower binder decomposition, aligning with green production goals for steel castings manufacturing.
Looking forward, we plan to integrate digital monitoring systems into the molding line for real-time control of sand properties. By using sensors to measure parameters like moisture, temperature, and strength, we can adjust binder addition rates dynamically, optimizing consistency for steel castings production. We are also exploring the use of alternative organic esters with faster or slower hardening rates to accommodate different casting geometries. For instance, complex steel castings with thin sections may benefit from rapid hardening to prevent mold deformation, while thick sections might require slower hardening to avoid stress concentrations. The relationship between hardening rate and casting quality can be modeled using differential equations, such as:
$$ \frac{dS}{dt} = k \cdot [\text{Ester}] \cdot [\text{Sodium Silicate}] $$
where S is strength, t is time, and k is a rate constant dependent on temperature and sand composition. Such models can guide process optimization for diverse steel castings. Moreover, we aim to extend the modified sodium silicate sand to more core applications, potentially replacing resin-bonded sands to reduce volatile organic compound emissions. Collaboration with material suppliers is ongoing to develop binders with even better collapsibility and lower environmental impact, ensuring sustainable production of high-performance steel castings for the railway industry.
In conclusion, the optimization of molding and core-making processes for steel castings in railway locomotives has proven highly successful. By transitioning from ordinary sodium silicate and green sand to ester-hardened sodium silicate sand for molding and modified sodium silicate sand for core-making, we have addressed key issues like poor collapsibility, high gas evolution, and labor intensity. The new processes leverage advanced equipment, precise material formulations, and rigorous testing to produce steel castings with improved dimensional accuracy, fewer defects, and easier cleanup. The trial production of 102-type couplers and axle box bodies validated these benefits, showing compliance with specifications and enhanced manufacturability. As we continue to refine these methods, we anticipate further gains in efficiency, cost reduction, and environmental sustainability. This optimization not only meets the demands of mechanized production but also sets a benchmark for green foundry practices, contributing to the long-term viability of steel castings manufacturing in the locomotive sector.