Abstract: This paper focuses on the technical research and practical application of lost foam casting for semi-closed castings, addressing common defects such as sand crushing and expansion, and proposing corresponding improvement measures. The practice shows that by incline the upper mouth plane of semi-closed castings at a 45° angle for molding and fabricating an external negative pressure tube to ensure balanced pressure inside and outside the mold cavity during pouring, the casting qualified rate can be significantly improved.
1. Introduction
Semi-closed castings, which are common in casting products and serve as essential components in machinery, primarily function to support and contain. Characterized by thin walls forming various shaped cavities, these castings are complex in structure with uneven wall thickness and internal cavities. Conventionally, sand mold casting was employed, but due to complicated production processes, the need for sand cores, and high costs, the industry has gradually shifted to lost foam casting.
Table 1: Roasting Parameters Requirements
| Process Step | Coating Times | Drying Temperature (°C) | Remarks |
|---|---|---|---|
| First Coat | 1 | 35~45 | – |
| Drying | – | – | 20 hours |
| Second Coat | 2 | 40~50 | – |
| Drying | – | – | 24 hours |
| Third Coat | 3 | 45~50 | Uniform flow of coating at corners and angles, smooth surface |
| Drying | – | – | 26 hours |
2. Production Implementation Process
2.1 Process Characteristics
Semi-closed castings, with their internal cavities and uneven wall thickness, face challenges in achieving the required vacuum level within the mold cavity during lost foam casting. Inadequate negative pressure can lead to defects such as sand crushing and expansion. The key technical points include model foaming density, coating properties, molding, and process methods.
2.2 Lost Foam Casting Process Design
- Process Principle: The process adopts a double-loop separation line design layout, mainly comprising mold making, model bonding and coating, sand processing, casting cleaning, electric furnace melting, and machining.
- Mold Material: Foam boards with a specific gravity of 10 kg/m³ are used for the slag box. The slag box body and pouring risers are manually molded and cut to ensure no gaps or holes. Defects such as size deviations, burn marks, poor continuity, and damage are repaired using a patching knife, and depressions are filled with glue.
- Coating: A water-based coating is selected, mixed according to process requirements, stirred for a certain period, and tested for consistency. After meeting the requirements, the body undergoes three coats, and the pouring system undergoes four coats, with each baking step involving position changes to prevent deformation.
Table 2: Chemical Component Indicators
| Element | Composition (%) |
|---|---|
| C | ≤0.25 |
| Cu | ≤1 |
| Mo | ≤0.6 |
| V | ≤0.035 |
| Mn | ≤0.4 |
| Si | ≤0.35 |
| Ni | ≤0.4 |
| Cr | ≤0.15 |
| P | ≤0.05 |
| S | ≤0.035 |
- Raw Materials: Carbon steel scrap, carbon steel plates, and other materials are used, with strict control over composition. Clean furnace charges are added first, preheated using low power, followed by larger materials melted using high power. Flux materials are added during the process to cover the molten steel and prevent oxidation.
- Pouring: Pouring is done according to the principle of one box per piece. The placement of the yellow mold in the sand box is determined based on the dimensions of the sand box and the yellow mold. The sand is first laid at the bottom to fix the model, then clamped and vibrated on the vibrating table. The total vibration time is not less than 600s at a frequency of 40-50 Hz.
- Pouring Temperature: The tapping temperature of the molten steel is controlled at around 1650°C, and after a 2-3 minute standstill, slag is removed again before pouring at a temperature of 1550-1600°C, with the pouring time controlled within 5 minutes. Each box is pressure-maintained for 10 minutes after pouring.
3. Casting Defects
During production trials, casting defects such as deformation, sand crushing, and expansion were observed, severely impacting product qualification rates.
4. Cause Analysis
4.1 Deformation Defect Analysis
Deformation defects can occur due to discrepancies between the assembled module and the actual drawing, improper coating preparation, improper sand filling and vibration, uneven temperature distribution during pouring, and unreasonable vacuum distribution within the mold. The primary cause identified was insufficient vacuum in the casting cavity, resulting in deformation due to the pressure from the molten metal and pattern gasification.
4.2 Sand Crushing and Expansion Defect Analysis
Sand crushing and expansion defects in semi-closed castings are mainly caused by insufficient sand consumption at the top of the mold, inadequate vacuum, uneven metal filling speed, mismatch between mold pressure and sand pressure, vacuum loss during pouring, excessively high sand temperature, and excessive filling speed and impact force damaging the risers.
5. Improvement Measures
The defects in semi-closed castings are due to insufficient vacuum inside the casting cavity or excessively high vacuum on the outside, causing a significant pressure difference.
5.1 Optimizing Negative Pressure Parameters
The role of negative pressure in lost foam casting includes compacting dry sand, preventing mold collapse and wall movement, accelerating exhaust speed, reducing interface pressure, improving metal filling ability, enhancing casting contour clarity, and improving the working environment under sealed conditions.
5.2 Solutions
Due to the hollow structure of the castings, achieving the required negative pressure solely through the bottom and sides of the sand box is difficult, leading to insufficient compaction of the sand in the cavity and inability to resist the erosion and buoyancy of molten metal. To address this, the casting can be buried at an angle or additional negative pressure piping can be added within the cavity.
Table 3: Actual Measurement of Processing Size (mm)
| Item | Measured Size | Deviation |
|---|---|---|
| Inner Cavity Width | 900 | -30 |
| Total Length | 1400 | -14 |
| Casting Total Length | 1320 | -12 |
| Inner Cavity Length | 1308 | -12 |
| Width | 980 | -36 |
| Casting Total Width | 954 | -36 |
Burying the casting at a 45° angle facilitates uniform sand flow and compaction. The internal negative pressure piping, with an inner diameter of 150 mm and holes of 2 mm diameter, 15 mm horizontal spacing, and 10 mm vertical spacing, is connected to the main negative pressure line to ensure balanced pressure inside and outside the mold cavity during pouring.
6. Implementation Effects and Analysis
After implementing the measures, relevant process standards and specifications were established. Twelve batches of semi-closed castings were trial-produced, with four pieces per batch. A fixed bracket was made to ensure a 45° burial angle, and the external negative pressure tube was tested before use to ensure no clogging.
The results showed that burying the casting at a 45° angle facilitated uniform sand flow and compaction, effectively solving the bulging defect. The qualified rate for 48 castings was 96%. The use of external negative pressure piping balanced the internal and external pressure, effectively controlling sand crushing and expansion defects, with a qualified rate of 94% for 48 castings. The overall qualified rate was 90%.
7. Conclusion
By summarizing and analyzing the defects in the lost foam casting process of semi-closed castings, taking measures and following up on improvements, the following conclusions were drawn:
Burying semi-closed castings at a 45° angle to the upper plane facilitates sand filling and compaction.
Adopting a vibration compaction technique during the molding process can effectively eliminate air voids and improve the compactness of the molding sand.
The use of appropriate coatings can significantly reduce the adhesion between the foam pattern and the molding sand, thereby facilitating the removal of the foam pattern and reducing defects.
Strict control of the pouring temperature and speed is crucial for preventing the collapse of the foam pattern and ensuring the quality of the castings.
Furthermore, regular maintenance and inspection of equipment, as well as training of operators, are indispensable for maintaining the stability and reliability of the lost foam casting process.
In summary, through targeted measures and continuous improvement, the defects in the lost foam casting process of semi-closed castings can be effectively reduced, thereby improving the quality and yield of the castings.