In the mining industry, scraper conveyors are critical equipment for coal extraction, and the central trough is one of their key components, accounting for over 70% of the total mass and being the most used and consumed part. Traditionally, central troughs were manufactured by welding cast channel beams, middle plates, and bottom plates. However, this approach often led to long production cycles, poor wear resistance, and welding failures. In my research, I explored an integrated sand casting process to produce central troughs as single sand casting parts, which effectively avoids the drawbacks of traditional cast-welded structures while saving machining and welding steps, thereby reducing manufacturing costs. This article analyzes the challenges of integral casting for central troughs and presents a reasonable casting process design to overcome these difficulties. Through the use of ester-hardened water glass sand molding, I completed the trial production study for the overall casting of central troughs, focusing on enhancing the quality and efficiency of sand casting parts.
The original structure of the central trough, as shown in historical designs, had maximum dimensions of 750 mm × 1330 mm × 377 mm and was produced via welding of cast components. This sand casting part is large, complex, with uneven wall thickness and multiple hot spots, making it prone to issues like local sand sticking, cracks at transitions, and deformation of the middle and bottom plates. To improve castability while maintaining performance, I made several structural modifications from a casting perspective. The key changes included simplifying certain areas to reduce crack risks and increase feeding channels, reducing mass and minimizing hot spots where possible to enhance solidification control, and thickening transition zones to prevent casting cracks. These adjustments are crucial for optimizing sand casting parts for better integrity and durability. Below is a table summarizing the structural improvements:
| Modification Area | Purpose | Impact on Sand Casting Parts |
|---|---|---|
| Simplification of complex joints | Reduce crack risk and improve feeding | Enhances structural integrity of sand casting parts |
| Mass reduction at hot spots | Minimize shrinkage defects | Improves soundness in thick sections of sand casting parts |
| Thickening of transition zones | Prevent cracking during solidification | Increases durability of sand casting parts |
These modifications ensure that the central trough, as a sand casting part, achieves better performance in terms of strength and wear resistance, which is essential for harsh mining environments.
In traditional cast-welded structures, the channel beam material was typically ZG30MnSi, while the middle and bottom plates used rolled steel like Hardox400. For an integral sand casting part, a single material with comprehensive properties is required to meet both the strength needs of the channel beam and the wear resistance demands of the plates. In my design, I based the material on ZG30MnSi but added microalloying elements such as Re, B, Ti, Zr, and V to enhance strength and wear resistance. The carbon content was controlled at w(C) ≤ 0.35%, with silicon and manganese limited to w(Si+Mn) ≤ 2.5% to ensure weldability and reparability. The carbon equivalent (CE) is a critical parameter to prevent cracking and ensure good mechanical properties, calculated using the formula:
$$ CE = w(C) + \frac{w(Mn)}{6} + \frac{w(Si)}{24} $$
For this sand casting part, I maintained CE ≤ 0.68 to guarantee weldability while achieving tensile strength over 700 MPa and impact toughness suitable for mining applications. This material design ensures that sand casting parts like the central trough exhibit high durability and performance.
For the molding material, I selected ester-hardened water glass sand due to its excellent properties, dimensional accuracy, and surface finish, comparable to alkali phenolic resin sand but with lower tendencies for cracks and gas holes. This green process reduces volatile emissions and allows for dry regeneration and reuse of old sand after washing, making it environmentally friendly. The parameters for this sand mixture are crucial for producing high-quality sand casting parts. Below is a table detailing the key parameters for the ester-hardened water glass sand process:
| Parameter | Range | Importance for Sand Casting Parts |
|---|---|---|
| Quartz sand temperature | 10–35 °C | Ensures proper sand curing and mold strength |
| Modified water glass addition | 2.0–3.0% | Provides binding for sand particles in sand casting parts |
| Organic ester hardener addition | 13–15% of water glass | Controls hardening rate and mold integrity |
| 24-hour tensile strength of mold | 0.2–0.5 MPa (mold), 0.4–0.6 MPa (core) | Guarantees dimensional stability of sand casting parts |
To prevent sand sticking on the cast surface, I used chromite sand with higher refractoriness, placed 30–40 mm thick at critical areas like slot roots and thick sections, before applying the water glass sand. Before mold closing, I brushed the mold surface three times with an alcohol-based zircon flour coating to enhance the surface finish of the sand casting parts. This approach significantly reduces defects in complex sand casting parts.
The design of risers is vital for feeding the solidification shrinkage in sand casting parts. For the central trough, the hot spots are mainly at the dumbbell connection areas of the channel beam ends, totaling four locations. I chose the bottom surface of the trough as the riser face due to its flatness and better feeding ability for the middle and bottom plates. Using open risers and top risers, I applied the modulus method to size them. The modulus (M) is calculated as the volume-to-surface area ratio, and for effective feeding, the riser modulus must exceed that of the casting section. The formula used is:
$$ M_{\text{riser}} = 1.2 \times M_{\text{casting}} $$
Considering the sandbox specifications and the tilt pouring method, I designed four risers with dimensions of ø240 mm × 400 mm. To validate this, I performed solidification simulation using casting process analysis software, which confirmed that the risers adequately feed the sand casting part, though areas like the dumbbell sockets showed tendencies for shrinkage porosity, requiring further attention. This simulation highlights the importance of precise riser design for large sand casting parts to ensure soundness.
The gating system was designed to avoid sand washing, using refractory ceramic tubes for the runners. The sprue was made with ø80 mm tubes, and the runners with ø60 mm tubes, divided into two channels injected from the bottom of the convex end face of the central trough mold. To prevent sand expansion cracks in large planar structures like the middle and bottom plates due to thermal radiation, I employed a tilt pouring method. After mold assembly, the sand mold was placed on an inclined platform with the concave end raised by 8 degrees for pouring. The pouring temperature was controlled at 1560–1590 °C to prevent cold shuts in the plate areas. This gating strategy is essential for achieving defect-free sand casting parts, as it ensures smooth metal flow and reduces thermal stresses.
Heat treatment is critical for enhancing the mechanical properties of sand casting parts. For the central trough, I used a car-type resistance furnace with the following steps: first, normalizing pretreatment at 920 °C for 240 minutes followed by air cooling; then, quenching at 920 °C for 240 minutes followed by cooling in water at 15–30 °C; and finally, tempering at 560–600 °C for 360 minutes followed by air cooling. This regimen ensures a balanced microstructure with high strength and toughness, meeting the demanding requirements for sand casting parts in mining equipment. The heat treatment parameters can be summarized in the table below:
| Heat Treatment Step | Temperature | Time | Cooling Method | Effect on Sand Casting Parts |
|---|---|---|---|---|
| Normalizing | 920 °C | 240 min | Air cooling | Refines grain structure |
| Quenching | 920 °C | 240 min | Water cooling | Increases hardness and strength |
| Tempering | 560–600 °C | 360 min | Air cooling | Improves toughness and reduces brittleness |
This heat treatment cycle is optimized for sand casting parts made from low-alloy steel, ensuring they withstand abrasive and impact loads in service.
Through production trials and inspections via sectioning, dimensional checks, and performance tests, I identified key issues such as local deformation, cracks, and insufficient wear resistance. To address these, I implemented several improvements. First, I added three 16 mm × 50 mm ties at the center between the middle and bottom plates to prevent upward deformation of the middle plate and downward deformation of the bottom plate. Second, I placed 10 mm thick triangular ties on each side at the junction of the bottom plate and channel beam to prevent cracks. Third, during molding, I applied abundant chromite sand at the dumbbell socket areas to promote chilling and avoid shrinkage porosity. Fourth, I added wear-resistant overlay welding on the chain path areas of the middle plate to locally enhance abrasion resistance. These modifications significantly improved the quality of the sand casting parts, as shown in the table below:
| Issue | Improvement Measure | Outcome for Sand Casting Parts |
|---|---|---|
| Deformation of plates | Addition of ties at plate centers | Enhanced dimensional stability of sand casting parts |
| Cracks at junctions | Use of triangular ties at transitions | Increased crack resistance in sand casting parts |
| Shrinkage porosity | Application of chromite sand for chilling | Improved soundness in thick sections of sand casting parts |
| Insufficient wear resistance | Overlay welding on chain paths | Extended service life of sand casting parts |
After these optimizations, the integral sand-cast central trough achieved all design requirements for dimensions, mechanical properties, internal quality, and appearance. This study provides an economical and environmentally friendly new solution for manufacturing central troughs, showcasing the potential of advanced sand casting processes for producing complex sand casting parts. The success of this trial underscores the importance of holistic process design in sand casting, from material selection to post-treatment, ensuring that sand casting parts meet stringent industrial standards.
In conclusion, the integral sand casting of central troughs using ester-hardened water glass sand offers a viable alternative to traditional cast-welded methods. By addressing challenges through structural modifications, precise material engineering, optimized molding, and effective heat treatment, I have demonstrated that sand casting parts can achieve superior performance with reduced costs and environmental impact. Future work could focus on further refining the alloy composition for even better wear resistance or exploring automated molding techniques for mass production of such sand casting parts. This research contributes to the advancement of casting technology in heavy machinery, emphasizing the role of sand casting parts in modern manufacturing.