Abstract: The shell molding process and technological flow of coated sand, as well as the raw materials, mold (core) sand formulations, and process properties used. It analyzes the factors influencing the quality of coated sand applied in steel casting production and summarizes the economic benefits created for enterprises after introducing the coated sand process.
Keywords: steel casting; coated sand
Introduction
Our company is a large state-owned enterprise with an annual production capacity of 10,000 tons of steel castings. Our products primarily focus on rail transit equipment, characterized by high quality requirements, complex product structures, and product diversity. Recently, our company underwent relocation, transitioning from a CO2-hardened sodium silicate sand process in the old plant to an ester-hardened sodium silicate sand process. For sand cores of coupler knuckle tongues and small coupler parts, we adopted the coated sand process for production. After the process update, less sodium silicate was used to obtain higher mold (core) sand strength, effectively improving the recycling and regeneration of sodium silicate sand and the cleaning and grinding of castings. Coated sand exhibits excellent strength performance, surface quality, and collapsibility, making it easy to clean castings and preventing sand cores from absorbing moisture, which is beneficial for storage, transportation, and use. Additionally, it plays a positive role in reducing raw material costs and improving production efficiency in the casting workshop.
1. Production Equipment and Technological Process
1.1 Mold Making and Core Forming Equipment
Mold making and core forming are handled by two Z8957CE vertical hot-box core shooters. Before production, operators check if the molds are securely fastened and if there is any exposed or leaky wiring on the heating thermocouple. Before core shooting, parameters are set, with a core shooting temperature of 260270°C and a holding time of 33.5 minutes. After removing the sand cores, the sand nozzles are cleaned, and any flash is ground smooth. One coat of zircon powder-based alcohol coating is applied. Subsequent steps include core setting, mold closing, and pouring. The coating is applied on-site, cores are assembled for fixed positioning management, pouring is done in a pouring pit, and a special simplified tooling is used for core assembly.
1.2 Raw Material Requirements for Production
1.2.1 Performance Parameters of Raw Materials
Silicon sand adopts locally selected sand, with performance indicators shown in Table 1. The parameters of the coated sand binder and hardener are shown in Tables 2 and 3.
Table 1: Performance Indicators of Regionally Selected Silicon Sand
| Category | SiO2 (%) | Angularity Factor | Mud Content (%) | Fine Powder Content (>200 mesh) (%) | Moisture Content (%) | Particle Size 70/100 (%) |
|---|---|---|---|---|---|---|
| Indicator | 98~99 | 1.1 | <0.3 | 0.73~0.8 | 0.01 | 80.44 |
Table 2: Binder Performance Indicators
| Category | Softening Point (°C) | Polymerization Speed (s) | Fluidity (mm) | Free Phenol (%) |
|---|---|---|---|---|
| Indicator | 95 | 60 | 61-65 | 0.003 |
1.2.2 Coated Sand Process Formula
The amount of binder and other parameters are determined based on the raw sand conditions and binder properties.
Table 3: Coated Sand Process Parameters
| Category | Raw Sand | Resin Addition (%) | Surfactant Addition (%) | Calcium Stearate Addition (%) | Hardener Addition (%) | Coating Cycle (min) |
|---|---|---|---|---|---|---|
| Indicator | 100 | 2.7-2.9 | _ | 0.1-0.5 | 0.1-0.6 | 1-5 |
(Note: Surfactant addition is expressed as a percentage of resin mass)
1.2.3 Performance Indicators of Coated Sand Mold (Core) Sand
The performance indicators of the coated sand mold (core) sand are shown in Table 4.
Table 4: Performance Indicators of Coated Sand Mold (Core) Sand
| Performance Indicator | Cold Bending Strength (MPa) | Hot Bending Strength (MPa) | Ignition Loss (%) | Melting Point (°C) | Particle Size (mesh) |
|---|---|---|---|---|---|
| Value | 7.5~7.7 | 4.0~4.2 | 3.2~3.6 | 103~106 | 70/100 |
2. Factors Influencing the Quality of Steel Casting Products
The quality of coated sand is directly tied to the final quality of steel castings, serving as a pivotal factor in guaranteeing casting integrity. This relationship is further underscored by the rational proportioning of high-quality raw materials like silicon sand and binders, along with other process parameters. Variations in any of these elements can significantly impact the performance of the mold (core) sand.
2.1 Influence of Silicon Sand
The composition of silicon sand has a notable effect on the strength of the mold (core) sand. In silicon sand, components other than silicon dioxide (SiO2) are generally considered impurities in industrial applications. Aluminum oxide (Al2O3) and iron compounds are the most common impurities, with iron compounds being particularly detrimental. These impurities can melt at high temperatures and react with various inorganic binders and oxides like iron oxide on the metal liquid surface, forming complex compounds with relatively low melting points. When the iron oxide content exceeds a certain limit, it not only leads to severe erosion and melting of quartz grains but also increases the channels for oxide slag and metal liquid infiltration, ultimately resulting in sand burning on the casting surface. Therefore, during the casting process, it is desirable for silicon sand to have a high SiO2 content and low content of other components. This ensures a high refractoriness of the silicon sand. In fact, SiO2 content is often the primary criterion for accepting casting silicon sand. Consequently, it is recommended to strictly control the performance indicators of silicon sand to ensure optimal results, as outlined in Table 1 (referring to the original document for specific performance indicators).
2.2 Influence of Binders
The coated sand used in the shell method employs thermoplastic phenol-formaldehyde resin, whose performance characteristics categorize it into various types, including high-strength, fast-hardening, low-expansion, easy-collapse, low-odor, heat-resistant, and laser sintering-selective types. In trials for automotive component housings, the high-strength type was selected. The strength of coated sand primarily depends on the bonding strength between the phenol-formaldehyde resin and the sand, as well as the cohesive strength of the resin itself. The bonding strength correlates with the relative molecular weight of the resin; smaller relative molecular weight phenol-formaldehyde resins exhibit good fluidity, facilitating adequate wetting and bonding between the resin and the sand, thereby enhancing strength. To improve the toughness of phenol-formaldehyde resins, appropriate modifiers can be introduced during the resin synthesis process.
2.3 Influence of Tooling
The coated sand process places high demands on tooling, requiring molds to be made of metal. During the mold design process, considerations must be given to feeding, venting, shell thickness, and sand reduction, as well as the match between the mold backplate and the casting, and the size of the heating tubes. The pouring method adopted is series casting, specifically either stacked series casting or horizontal series casting. Given the company’s experience with horizontal series casting and corresponding tooling, as well as the potential for the lowest 1-2 shells in stacked series casting to collapse due to gravity, leading to scrap castings, and the current low production volume, horizontal series casting is employed.
2.4 Influence of Equipment
The equipment used is a coated sand core shooting machine. This machine requires parameter settings tailored to different shell cores. These parameters include sand injection pressure, controlled via a pressure reducing valve on the intake pipe. Insufficient sand injection pressure slows the sand flow, resulting in poor filling, while excessive pressure can cause sand blasting and excessive airflow in dead corners, hindering filling. The sand injection time is typically set in the program and is usually set to 3 seconds. The shell formation time depends on the size and thickness of the sand core and the heating duration. For instance, for small components like coupler hooks, a shell thickness setting of 10mm within a 20mm range is suitable. The hardening time is determined by the shell core thickness; for coupler hook small parts, the actual hardening time is usually between 180 and 210mm. The temperatures of the left and right core boxes also need adjustment, typically set within the range of 240-260°C based on the characteristics of the coated sand and the shell thickness of the sand core. The sand filling pressure is adjusted via a pressure reducing valve on the pipeline, generally maintained at 0.5-0.6MPa. As for the sand filling time, after each sand core is shot, the sand shooting cylinder needs to be replenished with sand in a timely manner for the next production cycle, with this time typically set to 3ms.
These factors, along with appropriate process control measures such as shell strength management, coating application, and pouring temperature regulation, are crucial in ensuring the quality of steel castings produced using coated sand.