Abstract:
The shell molding process, technological flow, raw materials, sand formula, and technological properties of coated sand. It analyzes the factors influencing the quality of coated sand used in steel casting production and summarizes the economic benefits created by introducing the coated sand process.
1. Introduction
Our company, a state-owned large enterprise with an annual production capacity of 10,000 tons of steel castings, mainly produces products for rail transit equipment, characterized by high quality requirements, complex product structures, and multiple varieties. After relocating from the old plant, we updated from the CO2-hardened sodium silicate sand process to the ester-hardened sodium silicate sand process. The sand cores for coupler knuckles and small coupler parts are produced using the coated sand process. After the process update, less sodium silicate is used to obtain higher strength cores, effectively improving the recycling and regeneration of sodium silicate sand and the cleaning and grinding of castings. Coated sand features good strength, surface quality, collapsibility, ease of casting cleanup, moisture resistance, and is conducive to storage, transportation, and use. It also plays a positive role in reducing raw material costs and improving production efficiency in the casting workshop.
2. Production Equipment and Technological Process
2.1 Core Making Equipment
Core making is performed using two Z8957CE vertical-parting hot-box core shooters. Before production, operators check the firmness of the molds and ensure there is no exposed or leaking wiring on the heating elements. Before core shooting, parameters are set with a shooting temperature of 260-270°C and a holding time of 3-3.5 minutes. After removing the cores, the sand nozzles are cleared, and any flash is ground smooth. One coat of alcohol-based zirconium powder paint is applied. Subsequent operations include core assembly, mold closing, and pouring. Field paint application, core assembly for fixed-position management, pouring in pouring pits, and the use of specialized tooling for core assembly are adopted. The core making process is illustrated in Figure 1.
Figure 1: Core Making Process Flowchart
<img src=”https://example.com/core-making-process.png” />
3. Raw Material Requirements for Production
3.1 Performance Parameters of Raw Materials
The silicon sand used is locally selected, with performance indicators as shown in Table 1. The parameters of the coated sand binder and curing agent are shown in Tables 2 and 3.
Table 1: Performance Indicators of Regionally Selected Silicon Sand
| Category | SiO2 (%) | Angularity Factor | Mud Content (%) | Micropowder 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: Performance Indicators of Binder
| Category | Softening Point (°C) | Polymerization Speed (s) | Fluidity (mm) | Free Phenol (%) |
|---|---|---|---|---|
| Indicator | 95 | 60 | 61-65 | 0.003 |
3.2 Process Ratio of Coated Sand
The amount of binder and other parameters added 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 (%) | Curing Agent Addition (%) | Coating Cycle (min) |
|---|---|---|---|---|---|---|
| Indicator (as a % of raw sand) | 100 | 2.7-2.9 | – | 0.1-0.5 | 0.1-0.6 | 1-5 |
| (as a % of resin mass) | – | – | (as a % of resin mass) | – | – | – |
3.3 Performance Indicators of Coated Sand Cores
The performance indicators of the coated sand cores are shown in Table 4.
Table 4: Performance Indicators of Coated Sand Cores
| Performance Indicator | Value (MPa/%) |
|---|---|
| Cold Bending Strength | 7.5-7.7 |
| Hot Bending Strength | 4.0-4.2 |
| Ignition Loss | 3.2-3.6 |
| Melting Point (°C) | 103-106 |
| Particle Size (Mesh) | 70/100 |
4. Factors Affecting the Quality of Steel Casting Products
The quality of coated sand is directly related to the final casting quality and is crucial for ensuring casting quality. It is also closely related to the reasonable ratio of high-quality raw sand and binder process parameters. Changes in any of these can affect the performance of the core sand.
4.1 Impact of Silicon Sand
The composition of silicon sand has a significant impact on the strength of the core sand. Components other than silicon dioxide (SiO2) in silicon sand are typically considered impurities in the industry. Aluminum oxide (Al2O3) and iron compounds are the most common impurities, with iron compounds considered one of the most harmful. These impurities melt at high temperatures and react with various inorganic binders and oxides such as iron oxide on the metal liquid surface to form complex compounds. These compounds have relatively low melting points, especially when the iron oxide content exceeds a certain limit. This not only leads to severe erosion and melting of quartz particles but also increases the channels for oxide slag and metal liquid infiltration, ultimately causing sand burning on the casting surface. Therefore, during casting, the desired composition of silicon sand is high SiO2 content and low content of other components, which helps ensure high refractoriness of silicon sand. SiO2 content is usually the main acceptance criterion for casting silicon sand. It is recommended to strictly control the performance indicators of silicon sand to ensure optimal results during casting, as shown in the requirements in Table 1.
4.2 Impact of Binder
Thermoplastic phenolic resin is used in the shell molding process of coated sand, and its performance divides it into multiple types, including high-strength, fast-hardening, low-expansion, easily collapsible, low-odor, heat-resistant, and laser sintering-selective types. After trial production in automotive parts boxes, the high-strength type was selected. The strength of coated sand mainly depends on the bonding strength between phenolic resin and sand and the cohesive strength of the resin itself. The bonding strength is related to the relative molecular weight of the resin. Phenolic resins with smaller relative molecular weights have good fluidity, which facilitates sufficient wetting and bonding between the resin and sand, thereby improving strength. To enhance the toughness of phenolic resins, appropriate modifying substances can be introduced during resin synthesis.
4.3 Impact of Tooling
The coated sand process has high requirements for tooling. The mold must be a metal mold. During mold design, considerations include the feeding and venting of the casting, the shell thickness, sand reduction, matching between the mold backplate and the casting, and the size of the heating tube. Pouring is done in series, either in stacked or horizontal series. The company has experience with horizontal series pouring and corresponding tooling. Additionally, the bottom 1-2 shells in stacked series pouring may collapse due to gravity, causing casting scrap. Currently, production volumes are low, so horizontal series pouring is adopted.
4.4 Impact of Equipment
The equipment used is a coated sand core shooter. The core shooter requires parameter settings for different shell cores. First, the shooting pressure is controlled via a pressure reducing valve on the inlet pipe. Excessively low shooting pressure slows sand flow, leading to poor filling, while excessively high pressure can cause sand blasting and excessive airflow in dead corners, hindering filling. Next, the shooting time is usually set in the program, typically set to 3 mm. The shelling time depends on the size and thickness of the sand core and the heating time. For example, a shell thickness of 10 ms within a 20 mm range is suitable for coupler small parts. The hardening time is determined by the shell core thickness, with an actual hardening time of 180-210 mm for coupler small parts. The temperatures of the left and right core boxes also need adjustment, typically set within the range of 240-260°C based on the properties of the coated sand and the thickness of the sand core shell.