As a near-static forming method of castings, lost foam casting technology has been gradually recognized by foundry companies in recent years. However, various problems may arise in the actual casting process. Sanzhuji made the following conclusions based on his own experience in producing casting machinery:
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 control of dimensional accuracy, the density of the model, the amount of pyrolysis products during pouring and other factors are the prerequisites for obtaining high-quality castings. There are several ways to make existing SME models:
1.1 It is made by cutting and bonding the packaging EPS sheet.
1.2 Self-made molds, outsourced processing.
1.3 Self-made simple pre-hair molding equipment.
Using the above method to make models, there is a common phenomenon of not paying attention to the change of pattern density, especially when the model is entrusted to an external factory for processing, the water content is not easy to control, and the molten iron is often sprayed back from the gate during pouring, or the casting has cold shut, insufficient pouring, etc. . For this reason, in the production process, the inspection of the model density should be strengthened, and the drying time of the model should be increased; after the EPS beads are selected through the process experiment, the raw material manufacturer cannot be changed at will; Bead density, changing the method of controlling bead density by manual experience.
2. Problems with vibration
Vibration compaction is one of the four key technologies of lost foam casting. The function of vibration is to generate dynamic flow of dry sand in the sand box, improve the filling and density of dry sand, and prevent casting defects. When dry sand is vibrated and filled, the ideal situation is that the dry sand flows in an orderly manner during the vibration process of the three-dimensional vibrating platform. The molding sand obtains a higher and more uniform filling density.
The lost foam casting vibrating tables of small and medium-sized enterprises are mostly self-made equipment. When vibrating, the most common phenomenon is that due to improper vibration operation, it will cause deformation of the shape, cracking of the coating layer, etc., resulting in corresponding casting defects. Some vibrating tables are easily deformed due to excessive excitation force and unbalanced polarizing blocks of the same group of motors. For this reason, the exciting force, amplitude and vibration time should be adjusted; for castings with large size and simple structure, the three-dimensional vibration of six motors can be changed to vertical or horizontal vibration of double motors; The parameters of the platform are tested and adjusted to meet the design requirements.
3. There are problems with the use of paint
In the lost foam casting process, the use of coatings can improve the rigidity and strength of the pattern, isolate the EPS pattern from the mold, prevent sand sticking and mold collapse; allow the pyrolysis products of the pattern to be discharged through the coating in time and smoothly during the casting process. Coatings are generally composed of refractory materials, binders, suspending agents, etc. The proportion of each component has a great influence on the performance of the coating.
However, some companies are not very clear about the effect of coating composition, and they change the coating formula and preparation process at will, or continue to prepare and use it due to the lack of certain components, resulting in a significant decline in coating performance; some companies have problems in the dip-coating and drying process. Sometimes in order to shorten the time, the next dip coating is carried out before the first paint is dry, resulting in insufficient drying inside the model, and there is moisture in it; in summer, only the drying method is used, and there is instability in the process, resulting in Back spray or porosity during pouring; coating thickness has not been noticed to vary according to castings, pouring temperature and molten iron pressure head.
4. There is a problem in the pouring process
During pouring of lost foam casting, in order to discharge gas and pattern gasification residue, the sprue must have a sufficient height so that the molten metal has sufficient pressure head to push the molten metal flow to fill the mold stably and quickly, ensuring that the surface of the casting is complete and clear. In practice, some enterprises use the original sprue cups for sand casting. Due to the 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 pressure head, a larger sprue cup can be used; the sprue is made hollow to reduce gas generation and back injection, and increase the pressure at the beginning of pouring. head.
Lost foam casting adopts negative pressure dry sand vibration molding, and the mold strength is much greater than that of green sand when molding with this method. The negative pressure pumping method can improve the stability of the mold, and the pyrolysis and gasification products produced during the gasification of the mold can be sucked out in time. However, in the production process, some factories only pay attention to the negative pressure on the surface before pouring, but often ignore the change of negative pressure during the pouring process, resulting in casting defects. This problem can be well solved by adopting the method of adjusting the negative pressure during the pouring process according to the size of the casting and the amount of pyrolysis products.
5. Defects and prevention in mold heat treatment
5.1 Soft spots on the surface of the mold: Soft spots on the surface of the mold after heat treatment will affect the wear resistance of the mold and reduce the service life of the mold.
5.11 Causes
Before heat treatment, the surface of the mold has oxide skin, rust spots and partial decarburization. After quenching and heating, the cooling and quenching medium is not properly selected, and there are too many impurities or aging in the quenching medium.
5.12 Precautions
The scale and rust spots should be removed before heat treatment of the mold, and the surface of the mold should be properly protected during quenching and heating. Vacuum electric furnace, salt bath furnace and protective atmosphere furnace should be used for heating as much as possible. When cooling after quenching and heating, a suitable cooling medium should be selected, and the cooling medium used for a long time should be filtered frequently or replaced regularly.
5.2 Poor structure of the mold before heat treatment: the final spheroidized structure of the mold is coarse and uneven, the spheroidized structure is not perfect, and the structure has network, band and chain carbides, which will make the mold prone to cracks after quenching and cause the mold to be scrapped.
5.21 Causes
Severe carbide segregation exists in the original structure of the die steel material. Poor forging process, such as high forging heating temperature, small deformation, high forging stop temperature, slow cooling rate after forging, etc., make the forging structure coarse and have network, band and chain carbides, and make spheroidizing annealing difficult to eliminate. Poor spheroidizing annealing process, such as too high or too low annealing temperature, short isothermal annealing time, etc., can cause uneven spheroidizing annealing structure or poor spheroidizing.
5.22 Precautions
Generally, good quality mold steel materials should be selected according to the working conditions of the mold, production batches and the strengthening and toughening performance of the material itself. Improve the forging process or adopt normalizing preparatory heat treatment to eliminate the inhomogeneity of network and chain carbides and carbides in raw materials.
Solid solution refining heat treatment can be carried out for high-carbon die steel with severe carbide segregation that cannot be forged. To formulate the correct spheroidizing annealing process specifications for the forged mold blank, quenching and tempering heat treatment and rapid uniform and fine spheroidizing annealing can be used. Reasonably install the furnace to ensure the uniformity of the mold blank temperature in the furnace.
5.3 Quenching cracks in the mold: Cracks in the mold after quenching are the biggest defect in the heat treatment process of the mold, which will make the processed mold scrapped and cause great losses in production and economy.
5.31 Causes
There is severe network carbide segregation in the mold material. There is machining or cold plastic deformation stress in the mold. Improper heat treatment operation (heating or cooling too fast, improper selection of quenching cooling medium, cooling temperature is too low, cooling time is too long, etc.).
The complex shape of the mold, uneven thickness, sharp corners and threaded holes, etc., cause excessive thermal stress and tissue stress. Quenching heating temperature is too high to produce overheating or overburning. Tempering is not timely after quenching or the tempering holding time is insufficient. When repairing and quenching heating, heating and quenching are performed again without intermediate annealing. Heat treatment, improper grinding process. When EDM after heat treatment, high tensile stress and microcracks exist in the hardened layer.
5.32 Precautions
Strictly control the internal quality of mold raw materials, improve forging and spheroidizing annealing processes, eliminate mesh, band, and chain carbides, and improve the uniformity of spheroidizing structures. After machining or cold plastic deformation, the mold should be subjected to stress relief annealing (>600°C) and then heating and quenching. For molds with complex shapes, asbestos should be used to block the threaded holes, and dangerous sections and thin walls should be wrapped, and graded quenching or austempering should be used.
Annealing or high temperature tempering is required when repairing or refurbishing molds. Preheating should be taken when quenching and heating, precooling measures should be taken when cooling, and a suitable quenching medium should be selected. The quenching heating temperature and time should be strictly controlled to prevent the mold from overheating and overburning.
After the mold is quenched, it should be tempered in time, and the holding time should be sufficient, and the high-alloy complex mold should be tempered 2-3 times. Choose the correct grinding process and suitable grinding wheel. Improve the mold EDM process, and carry out stress relief tempering.
5.4 Coarse structure of the mold after quenching: It seriously affects the mechanical properties of the mold, and the mold will break when used, which seriously affects the service life of the mold.
5.41 Causes
The mold steel is confused, and the actual steel quenching temperature is much lower than the quenching temperature required for the mold material (for example, GCr15 steel is regarded as 3Cr2W8V steel). The correct spheroidizing process is not carried out before the steel is quenched, and the spheroidizing structure is poor. The quenching heating temperature is too high or the holding time is too long. Improper placement in the furnace can cause overheating near electrodes or heating elements. For molds with large cross-section changes, improper selection of quenching and heating process parameters will cause overheating at thin cross-sections and sharp corners.
5.42 Precautions
The steel should be strictly inspected before storage to prevent the steel from being mixed up. Correct forging and spheroidizing annealing should be carried out before die quenching to ensure a good spheroidizing structure. Correctly formulate mold quenching heating process specifications, and strictly control quenching heating temperature and holding time. Regularly test and calibrate the temperature measuring instrument to ensure the normal operation of the instrument. When heating in the furnace, keep an appropriate distance from the electrodes or heating elements.
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