1. Introduction
Semi-closed castings are common in casting products and play important roles such as carrying and containing in machinery. Their structural characteristics are complex shapes, thin and uneven walls, and internal cavities. Traditionally, sand casting was used for semi-closed castings, but due to complex production processes and high costs, the trend has shifted towards lost foam casting. Lost foam casting offers advantages such as suitability for complex parts, small-batch production, simplified processes, and environmental improvement. This article focuses on the research and practice of the lost foam casting process for semi-closed castings, with an emphasis on mold making and casting processes.
2. Production Implementation Process
2.1 Process Characteristics
The internal cavity of semi-closed castings makes it difficult to achieve the required vacuum degree during lost foam casting, leading to potential defects such as sand crushing and expansion tank. Key technical points include model, coating, molding, and process methods. Deficiencies in any of these aspects can result in various defects throughout the production process. To address the issue of insufficient vacuum degree, an internal negative pressure tube is made according to the shape of the casting to improve the uniformity of negative pressure and enhance the casting yield and surface quality.
2.2 Lost Foam Casting Process Design
- Process Principle and Layout: The process principle is lost foam casting, and the site is designed with a double-cycle separation line layout, consisting of casting mold making, casting model bonding and coating, casting sand treatment, casting cleaning, electric furnace melting, and machining processes.
- Model and Coating: The slag box is made of foam boards with a specific gravity. Manual modeling is used for the slag box body and gating system, and the model is repaired and dried to reduce moisture content. Water-based coating is selected, and the casting system and 本体 are coated and baked multiple times according to the process requirements.
- Raw Materials and Melting: High-quality raw materials are used, and the melting process is carefully controlled to prevent oxidation. The chemical composition of the molten steel is adjusted to meet the requirements before casting.
- Casting and Pouring: Casting is carried out according to the principle of one piece per box. The model is placed in the sand box, and the sand is added and vibrated to ensure compactness. The pouring temperature and time are controlled, and the casting is held under pressure for a certain period after pouring.
3. Casting Defects
During the production test, casting defects such as deformation, sand crushing, and expansion tank occurred, seriously affecting the product qualification rate. These defects can occur at any stage of the production process, and a lack of control over any detail can lead to casting scrap.
4. Cause Analysis
4.1 Deformation Defect Cause Analysis
Deformation defects can be caused by factors such as incorrect module combination, improper coating preparation, inappropriate sand filling and vibration, local heating during pouring, and 不合理的 vacuum distribution in the casting mold. Through process tracking, it was found that the main cause of deformation was insufficient vacuum degree in the casting cavity, resulting in a large lifting force caused by the metal liquid and pattern gasification pressure, leading to local deformation and scrap of the casting.
4.2 Sand Crushing and Expansion Tank Defect Cause Analysis
The main reasons for sand crushing and expansion tank defects in semi-closed castings are insufficient sand depth at the top of the casting mold, insufficient vacuum degree, slow or interrupted liquid metal filling speed, a drop in vacuum degree during pouring and exhaust, high sand temperature, and fast filling speed with large impact force. Field measurements showed that the sand temperature was low, and there was no flow interruption during casting, but negative pressure instability and insufficient vacuum degree led to sand crushing and expansion tank.
5. Improvement Measures
5.1 Optimization of Negative Pressure Parameters
Negative pressure plays a crucial role in the lost foam casting process, including compacting the sand, accelerating exhaust, improving filling ability, making the casting contour clearer, and improving the working environment. Optimizing the negative pressure parameters is essential for the success of casting semi-closed castings.
5.2 Solutions
To address the problem of insufficient negative pressure in the internal cavity of the casting, measures such as inclined box burying or adding a negative pressure pipeline in the cavity can be taken. After testing and verification, it was found that burying the casting at a 45° angle was conducive to sand filling and vibration compaction, and adding an external negative pressure tube with specific parameters could ensure the balance of negative pressure inside and outside the cavity during casting.
6. Implementation Effects and Analysis
After implementing the improvement measures, relevant process standards and specifications were formulated. Batch production tests showed that burying the box at a 45° angle improved the uniformity of sand filling and vibration compaction, effectively solving the problem of bulging defects. Using an external negative pressure tube controlled the expansion tank and sand crushing defects, and the overall qualification rate reached 90%. The application of the external negative pressure tube simplified the recycling process of related models and was convenient for reuse. The casting qualification rate increased from 40% to 90%, and the casting quality was significantly improved, reducing the work intensity of welding and grinding.
7. Conclusion
In conclusion, through the research and practice of the lost foam casting process for semi-closed castings, the following conclusions are drawn:
- Burying the semi-closed casting at a 45° angle with the upper plane inclined is beneficial for sand filling and vibration compaction.
- Making an external negative pressure tube and opening both internal and external negative pressures during casting can ensure the balance of negative pressure inside and outside the cavity and improve the casting qualification rate.
These measures effectively solve the problems of casting defects in semi-closed castings, improve the casting quality, and have certain reference significance for the production of similar castings.
Table 1: Roasting Parameters Requirements
| Process | Coating Time/h | Coating Drying Temperature/°C | Second Coating Time/h | Second Coating Drying Temperature/°C | Third Coating Time/h | Third Coating Drying Temperature/°C | Supplementary Coating | Remarks |
|---|---|---|---|---|---|---|---|---|
| Semi-closed Casting | 20 | 35 – 45 | 24 | 40 – 50 | 26 | 45 – 50 | Corner Supplementary Coating | Uniform coating flow at corners and smooth surface |
Table 2: Chemical Component Indicators
| 3005 | 3 | <0.4 | 3035 | 304 | 30.035 | 50.035 | 903 | 025 | Component | |
|---|---|---|---|---|---|---|---|---|---|---|
| A | W | C | C | N | IS | W | C | Element |
Table 3: Actual Measurement of Processing Size
| Project | Drawing Completion Size | Blank Detection | Deviation Value |
|---|---|---|---|
| Total Length | 1400 | 1386 | -14 |
| Casting Total Length | |||
| Inner Cavity Total Length | 1320 | 1308 | -12 |
| Casting Total Width | 980 | 954 | -36 |
| Width | |||
| Inner Cavity Total Width | 900 | 870 | -30 |
