Abstract This article focuses on the lost foam casting process for semi-closed castings. It analyzes the production process, defects, and corresponding improvement measures. Through experiments and research, it is found that tilting the semi-closed part at a 45° angle for box molding and using an external negative pressure tube can effectively improve the casting quality and qualified rate. This article also details the key technologies and process parameters of lost foam casting, providing a reference for the production of semi-closed castings.
1. Introduction
Lost foam casting is a modern casting method with many advantages. It simplifies the production process, reduces labor intensity, and is suitable for producing complex-structured parts. Semi-closed castings are common in the casting industry and play important roles in machinery. However, due to their special structure, there are some challenges in the lost foam casting process. This article aims to explore and practice the lost foam casting technology for semi-closed castings to improve the casting quality and production efficiency.
2. Production Process of Semi-Closed Castings in Lost Foam Casting
2.1 Process Characteristics
The internal cavity of semi-closed castings has uneven walls. During lost foam casting, it is difficult to form a vacuum degree inside the cavity. Insufficient negative pressure can easily lead to defects such as sand crushing and expansion tank. Four key technical points are essential: model, coating, molding, and process method. Unreasonable parameters in these aspects can cause various defects, such as model deformation, poor coating performance, and improper molding and process methods, resulting in low casting qualified rates.
2.2 Lost Foam Casting Process Design
2.2.1 Process Principle and Layout
The process principle is lost foam casting, and a double-cycle separation line design layout is adopted on site. It mainly consists of four parts: casting mold making process, casting model bonding and coating process, casting sand treatment, casting cleaning, electric furnace melting process, and machining process.
2.2.2 Model and Coating
The slag box of this casting selects a foam board with a specific gravity of 10 kg/m³. The slag box body and pouring riser are manually modeled, and the cutting model should have no gaps or pits. After cutting, any size deviations, burn marks, or damage are repaired. The model is dried to reduce the moisture content to below 0.8%. A water-based coating is selected for this casting, and it is prepared according to the process ratio. After stirring for a certain time, it is tested for viscosity. The body is coated three times, and the pouring system is coated four times, with position changes during baking to prevent deformation.
2.2.3 Melting and Pouring
Production materials include carbon steel scraps, carbon steel plates, etc. The composition is strictly controlled. The furnace is preheated with clean furnace materials at a low power, and then large materials are gradually added and melted at a high power. During the process, slag-making materials are added to cover the molten steel and prevent oxidation. Before pouring, the slag in the furnace must be removed, and samples are analyzed and adjusted to meet the requirements. The pouring temperature is controlled within a certain range, and the pouring time is also limited. The casting is held under pressure for a certain time after pouring.
2.2.4 Molding and Pouring System
According to the principle of one piece per box, the yellow model is placed in the sand box. First, the bottom sand is laid to fix the model. After the model is fixed, the sand box is clamped on the vibrating table for bottom vibration. Then, sand is added while vibrating. When the sand reaches the middle of the cavity, the addition of sand and vibration stop. After tightening, the addition of sand continues until the sand covers the top by about 80 mm. The total vibration time is not less than 600 s, and the vibration frequency is 40 – 50 Hz.
3. Casting Defects and Analysis
3.1 Defects Observed
During the production test process, casting defects such as deformation (cavity expansion inward), sand crushing, and expansion tank occurred, which seriously affected the product’s qualified rate. The comparison between the designed and actual dimensions of the casting is shown in Table.
| Item | Designed Dimension | Actual Dimension | Deviation Value |
|---|---|---|---|
| Total Length | 1400 | 1386 | -14 |
| Inner Cavity Total Length | 1320 | 1308 | -12 |
| Width | 980 | 954 | -36 |
| Inner Cavity Total Width | 900 | 870 | -30 |
3.2 Cause Analysis of Defects
3.2.1 Deformation Defect
The deformation defect may be caused by various factors. Through the analysis of the whole process, it is found that the main reason is the insufficient vacuum degree in the casting inner cavity. The huge lifting force caused by the metal liquid and the model gasification pressure leads to local deformation and scrapping of the casting.
3.2.2 Sand Crushing and Expansion Tank Defects
The main reasons for sand crushing and expansion tank defects of semi-closed castings are as follows: the sand amount on the top of the mold is too small, the vacuum degree is insufficient; the rising speed of the liquid metal filling is too slow or stops flowing, resulting in a mismatch between the air pressure in the mold and the sand mold pressure; the vacuum degree decreases during the pouring and exhaust process; the sand temperature is too high (>70°C) and burns the model, resulting in sand collapse; the filling speed is fast, the impact force is large, and the riser is damaged. In the actual production process, the sand temperature is measured to be lower than 30°C, and there is no flow interruption or inconsistent speed during pouring. However, the negative pressure is unstable and the vacuum degree is insufficient, resulting in sand crushing and expansion tank of the casting.
4. Improvement Measures
4.1 Optimization of Negative Pressure Parameters
In lost foam casting production, negative pressure plays an important role. It can compact dry sand, prevent mold collapse and wall movement, accelerate the exhaust speed, reduce the interface air pressure, accelerate the metal front speed, improve the filling ability, make the casting contour clearer, and improve the working environment. For semi-closed castings, optimizing negative pressure parameters is crucial for successful casting.
4.2 Specific Improvement Measures
Due to the hollow structure of the casting, the internal cavity only relies on the negative pressure at the bottom and four sides of the sand box, which cannot meet the required negative pressure degree. This may lead to insufficient compaction of the sand in the cavity and inability to resist the scouring and buoyancy of the metal liquid. To solve this problem, two methods can be considered: tilting the box for molding and adding a negative pressure pipeline in the cavity.
After experiments, it is found that when the casting is tilted at a 21° angle, the sand amount on the upper part is insufficient, and it is not suitable for box molding. When tilted at a 30° angle, the bottom position of the casting is not conducive to the natural flow of sand, and the artificial operation space is small, resulting in expansion tank and bulge defects after pouring. When tilted at a 45° angle, the sand flows evenly, the gaps in each part are consistent, and it is also conducive to artificial operation, and the pouring is stable. When tilted at a 60° angle, the gap near the lower side wall of the casting is small, which is not conducive to the flow of sand and the vibration is uneven. Therefore, a 45° angle is selected for tilting the box for molding.
For the internal negative pressure pipeline, the inner diameter is 150 mm. The pipe wall is perforated with small holes with a diameter of 2 mm, and the transverse spacing is 15 mm, and the longitudinal spacing is 10 mm. The pipeline is connected to the negative pressure pipe to ensure that the model inside and outside reaches the corresponding negative pressure degree and holding time during pouring. The layout of the external negative pressure pipe in the hollow interior of the casting is shown in Figure 1.
5. Implementation Results and Analysis
After implementing the improvement measures, relevant process standards and specifications have been formulated. Twelve batches of semi-closed castings have been trial-produced according to the formulated process standards. Each batch has 4 trial-produced parts. A fixed-angle bracket is made to ensure that the box is tilted at a 45° angle, and the external negative pressure pipe is tested before use to ensure that each negative pressure port is not blocked. The results of batch production tests show that tilting at a 45° angle for box molding is conducive to the even flow of sand, artificial leveling, and even vibration, effectively solving the bulge defect. Among the 48 castings poured, 2 have bulge defects, and the qualified rate is 96%. After using the external negative pressure pipe for pouring, the internal and external negative pressures are balanced, and the problems of expansion tank and sand crushing are effectively controlled. Among the 48 castings poured, 3 have expansion tank and sand crushing defects, and the qualified rate is 94%. The overall qualified rate is 90%. The application of the external negative pressure pipe involves fewer processes, simplifies the recovery of relevant models, is reusable, and is easy to handle.
After the external negative pressure pipe is designed and manufactured and put into use, large castings do not collapse after pouring, the deformation amount of shell-type castings meets the requirements, and the casting qualified rate is increased from the original 40% to 90%. The casting quality is effectively improved, surface shrinkage holes and looseness and insufficient pouring defects are eliminated, and the welding and grinding work intensity is reduced.
6. Conclusion
In conclusion, through the research and practice of lost foam casting for semi-closed castings, the following conclusions are drawn:
- Tilting the semi-closed part at a 45° angle for box molding is conducive to the filling and vibration compaction of sand.
- Making an external negative pressure tube and opening both the internal and external negative pressures during pouring can ensure the balance of negative pressures inside and outside the cavity and improve the casting qualified rate. These conclusions provide a reference for the production of semi-closed castings in lost foam casting, which can help to improve the casting quality and production efficiency.