Basic technical requirements for the most popular

2022-07-24
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Basic technical requirements for lost foam casting production as a near static forming method of castings, lost foam casting technology has developed rapidly in recent years. In foreign countries, due to the establishment and operation of mechanized and automatic lost foam casting production lines and the remarkable economic and social benefits, the lost foam casting technology has shown strong vitality. Although the application of lost foam casting technology in China made slow progress some time ago, it has developed rapidly in recent years. Especially because of the less investment in EPC equipment and the short process route, many original small and medium-sized foundry enterprises are increasingly using this technology. However, some enterprises fail to pay attention to some operation problems, which causes some problems in the production process and has a great impact on the quality of castings

1. Model making

in the lost foam casting process, model making is a very important link. The selection of EPS raw materials, the processing technology of the model, the dimensional accuracy, the model density, and the amount of pyrolysis products during pouring are the prerequisites for obtaining high-quality castings. The existing small and medium-sized enterprise model can be made in the following ways:

(1) it is made by cutting and bonding with packaged EPS plates

(2) self made molds and entrusted to other factories for processing

(3) self made simple pre launch molding equipment

when the above method is used to make the model, it is common to ignore the change of pattern density, especially when the model is commissioned to be processed by an external factory, the moisture is not easy to control, the molten iron is often sprayed back from the gate during pouring, or the casting has cold shut and insufficient pouring. Therefore, in the production process, we should strengthen the inspection of the model density and increase the drying time of the model; After the EPS beads are selected through the process sensitive and reliable experiment, the raw material manufacturer cannot be changed at will; In advance, use weighing tools to control bead density, and change the method of controlling bead density based on manual experience; After adopting the above methods, the manufacturing technology level and overall strength were further improved, and the problem was solved

2. Vibration compaction is one of the four key technologies of EPC. The function of vibration is to make dry sand flow dynamically in the sand box, improve the filling property and density of dry sand, and prevent casting defects. When dry sand is vibrated for filling, the ideal condition is that the dry sand flows orderly during the vibration process, and evenly fills all parts of the model on the premise of ensuring that the model is not deformed, so that the molding sand in the sand box can obtain a higher and more uniform filling density. Most of the vibration tables of lost foam casting in small and medium-sized enterprises are self-made equipment. During vibration, the most common phenomenon is due to improper vibration operation, resulting in pattern deformation and coating layer cracking, resulting in corresponding casting defects. Some vibration tables are easy to deform due to excessive exciting force and unbalanced polarization blocks of the same group of motors. For this reason, the exciting force, amplitude and vibration time shall be mainly adjusted; For castings with large size and simple structure, the three-dimensional vibration of six motors can be changed into the vertical or horizontal vibration of two motors; In particular, the parameters of the shaking table are tested and adjusted by the testing instrument to make it meet the design requirements

3. Problems in the use of coating

in the lost foam casting process, the use of coating can improve the rigidity and strength of the pattern, enable EPS to realize the strategic deployment of the coordinated development of traditional metal materials and new energy materials, isolate the pattern from the mold, and prevent sand sticking and mold collapse; During the casting process, the high-temperature decomposition products of the pattern are allowed to be discharged smoothly through the coating in time. The coating is generally composed of refractory, binder and suspending agent. The proportion of each component has a great impact on the performance of the coating. However, some enterprises are not very clear about the role of coating composition. They change the coating formula and preparation process at will, or continue to prepare and use when there is a lack of a component, resulting in a significant decline in coating performance; Some enterprises have problems in the pattern dip coating and drying process. Sometimes, in order to shorten the time, the next dip coating is carried out before the first coating is dry, resulting in insufficient drying inside the model, in which there is water; However, in summer, only air drying method is used, so there is instability in the process, resulting in backflow or porosity during pouring; The coating thickness does not change according to different castings, pouring temperature and hot metal indenter

only by paying attention to and solving the above problems, and working hard on the operation details, will there be no casting defects caused by filling the flexible packaging film with paint, which is usually called bubble film

4. There are problems in the pouring process

during the pouring of EPC, in order to exhaust gas and pattern gasification residue, the sprue should have enough height to make the molten metal have enough pressure head to promote the stable and rapid mold filling of molten metal flow and ensure the integrity and clarity of the casting surface. In practice, some enterprises use the original sprue cup for sand casting. Due to its small size, it is easy to cause the workpiece to be scrapped due to unstable liquid flow. In order to ensure that there is enough flow to keep the pouring process flowing and to quickly establish the starting head, a larger sprue cup can be used; The sprue shall be made hollow to reduce gas generation and reverse injection and increase the pressure head at the beginning of pouring

EPC adopts negative pressure dry sand vibration molding, and the mold strength is much greater than that of green sand. The stability of the mold can be improved by using negative pressure pumping method, and the pyrolysis and gasification products produced during pattern gasification can be pumped away in time. However, in the production process, some factories only pay attention to the surface negative pressure before pouring, but the change of negative pressure is often ignored during pouring, resulting in casting defects. This problem can be solved by adjusting the negative pressure in the pouring process according to the size of the casting and the amount of pyrolysis products

5. Conclusion

the lost foam casting process has developed rapidly in recent years due to its characteristics of less equipment investment and low cost. Many original small and medium-sized foundry enterprises are increasingly using this technology. However, some enterprises fail to pay attention to the model, coating, vibration method and some operation details in the pouring process, which makes the casting quality unstable in the production process

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