What is the biggest disadvantage of the lost foam casting process and how to overcome it?
Correctly understand the characteristics of lost foam process
Compared with other casting technology, the biggest advantage of the lost foam process is that the white mold is no longer needed to take the mold after it is buried in the molding sand, which not only greatly simplifies the casting process, but also greatly improves the dimensional accuracy and surface quality of the casting, and the appearance quality is close to melting. Mold precision castings. However, the lost foam casting process also has its limitations and requirements.
Lost foam process characteristics
(1) No sand core is used, no matter how complicated the casting is, a white mold is enough.
(2) There is no need for parting when modeling, so there is no need for parting surface design, and the process is much more flexible.
(3) The pouring and riser process design is very flexible, the inner gate can be easily set at any position of the casting, and the riser can be easily placed in any direction.
(4) Dry sand is used for modeling, no need to add any binder, casting sand is very convenient, and sand recycling rate is high.
(5) The production efficiency is high, the number of workers is small, mechanization and automation are easy to realize, and the initial investment is small, the area is small, and the labor intensity is low.
(6) Due to negative pressure pouring and solidification, the density of the casting increases.
(7) Since the sand box is pumped with negative pressure during pouring, the white mold gasification gas is easy to be collected, processed and discharged in a centralized manner, and the internal environment of the workshop is good.
Lost foam process requirements
The lost foam casting process has strict requirements on production conditions. The quality of castings is more sensitive to the performance and stability of the raw and auxiliary materials, and the cross-effects of various conditions and the rules are more complicated. So far, it has not been fully understood. Because the process design is more sensitive and the rules are more complicated, the lost foam casting is basically designed for specific castings one by one and one by one. Therefore, it is required to:
(1) The source of raw and auxiliary materials, including foam beads, adhesives, coatings, and dry sand, must be stable, the composition must be stable, and the performance must be stable.
(2) The performance of the vibrating table must meet the requirements of dry sand vibration to fill the complex cavity of the white mold, and the white mold must maintain a stable size and shape.
(3) The cooling method of dry sand and old sand including processing temperature and processing flow rate shall meet the requirements for the white mold not to be deformed by heat.
(4) The bonding of white molds should be accurately and stably aligned (it is recommended to promote machine bonding machines).
(5) The coating thickness should be uniform, so the coating should have good spreadability, leveling, air permeability, normal temperature and high temperature strength, collapsibility, etc. (paint leveling machinery is recommended).
(6) The white mold is required to have high precision, high hardness, good wear resistance and high stability (it is recommended to use an overall white mold and reduce the number of molds and bonding gaps).
In addition, the most important thing is to have a group of responsible engineering and technical personnel who are proficient in the basic principles of lost foam casting and the details of production operations, and know how to design and manage. They must be able to make castings according to different materials, structures, and sizes. Match specific process parameters and requirements.
The scope of application of lost foam process
Manufacturers should clearly realize that the lost foam process is not a universal process. As far as my country’s current technology level is concerned, according to its basic principles, the law of flow of molten iron in the filling process after pouring, the law of white mold gasification and the movement of gas residues and the production The rules of slag inclusion and carbon deposit defects are more suitable for the following types of castings:
(1) Gray iron castings are the most suitable according to the material, followed by ductile iron, again cast steel, and finally cast aluminum.
(2) According to the structure, it is most suitable for castings with uniform wall thickness, complex structure, and wall thickness ranging from 10 to 20 mm, such as box and shell castings.
(3) According to the size, the most suitable medium-sized castings with a weight of more than ten to several hundred kilograms, the most suitable batch is more than 10,000 pieces, the white mold is formed by a mold, and the casting uses a production line.
(4) Steel castings are most suitable for non-processing, less processing, and low requirements for internal defects, such as wear-resistant castings, heat-resistant castings, and corrosion-resistant castings.
(5) Among the large castings made of resin sand, it is most suitable for automobile cover stamping molds and machine tool bed castings. The white molds are processed by plate cutting and automatic processing by CNC machine tools. The casting method is normal pressure casting or negative pressure casting.
The lost foam process must be comprehensively weighed and repeatedly verified. What impact will the emergence of the lost foam casting process bring to small and medium-sized traditional sand casting enterprises? From the perspective of development trend, will it replace traditional foundry enterprises?
Although the lost foam process can be implemented with very little capital, the cost of wooden molds is eliminated, the dependence of sand casting on skilled casting workers is reduced, the labor intensity of workers is reduced, the operating environment of the workshop is improved, and the enterprise is easier to transform and upgrade . However, for large casting companies, the lost foam casting process will not bring great impact. Because the production efficiency and production rhythm of lost foam casting lag far behind the advanced automated vertical molding line or static pressure molding line, and large casting companies often produce certain or certain types of shaped products, and they also have the strength to purchase automated molding lines. From the perspective of development trend, lost foam casting will not replace traditional casting methods.
In fact, it is impossible for every casting process to conquer the world, and they have their suitable product types. Therefore, mastering and using the lost foam casting process has to implement "private customization" based on the characteristics of the enterprise itself.
In addition to the advantages, characteristics and scope of application of the lost foam process described above, the lost foam process also has some shortcomings, shortcomings, and inadequacy. For example, the flow of molten iron is turbulent, which can easily lead to internal slag and gas; a flame retardant is added to the raw materials of the white mold, which will increase the amount of slag; the use of negative pressure solid type, negative pressure makes the flow of molten iron more turbulent , And cause the wall effect to prevent the slag gas from being discharged; during the pouring process, the heat absorption of the white mold causes the temperature difference between the front and back of the molten iron entering the mold to increase, and the low-temperature molten iron at the leading edge causes poor gasification of the white mold, resulting in carbon deposition; A large amount of gas can only be discharged through a narrow paint gap, so it is very dependent on the quality of the paint; using dry sand modeling, the thermal conductivity of dry sand is lower than that of wet sand, which will easily reduce the hardness of the casting; white mold gasification produces toxic gases that pollute the air ; When producing cast aluminum parts, the lower pouring temperature makes the white mold poorly vaporized, and the liquefied white mold pollutes the dry sand, resulting in decreased air permeability and increased porosity in the casting; due to the low strength of the white mold, it is not deformed. Dry sand for castings with complex cavities is not easy to pack tightly during vibration filling, and it is easy to cause local sand sticking in the cavity; the principles and laws have not been clarified; etc.
To this end, companies should comprehensively consider the following aspects in accordance with their own conditions and capabilities, investment return expectations, product market characteristics, etc.:
First, the company's own talent situation. Must have technical personnel who understand lost foam casting technology, or send technical personnel to conduct special learning to master the basic principles, basic techniques and basic operations.
Second, the market situation of the enterprise. It is necessary to analyze whether the customer base needs lost foam castings, what is the demand and the demand period, and whether its own production capacity can meet customer requirements.
Third, the situation of casting products. It is necessary to analyze whether the casting products produced are suitable for the lost foam casting process.
In addition, the enterprise must do a confirmatory process experiment before installing the complete set of lost foam process equipment; if the experiment is successful, a small batch of trial production is required. Until the process is stable and the quality of the castings is qualified, they can consider looking for a major in lost foam casting. Complete sets of equipment or production lines on equipment manufacturing enterprises. It should be noted that generally speaking, during the trial production period, the qualification rate of castings is generally not high, and it is necessary to repeatedly explore the process plan and process parameters suitable for the enterprise conditions. In addition, it is inevitable to encounter difficulties and setbacks in the production process. Don't be discouraged. You must calmly analyze and find the cause of casting waste before taking corresponding measures. After a certain period of practice, continuous exploration and improvement, the lost foam casting process can be gradually applied flexibly.
With the development of my country’s foundry industry, the advantages of lost foam casting technology have become increasingly prominent, and its research and exploration have been continuously deepened. A number of applicable technologies suitable for my country’s national conditions have continued to emerge. A number of large and medium-sized enterprises have seized favorable opportunities to focus on basic theories and their own On the research and development road of combining conditions, continuous exploration and improvement have realized the leapfrog development of lost foam casting technology. However, in general, there is still a period of continuous development of lost foam casting technology. This period of development will increase its speed and achieve a qualitative leap and a quantitative expansion. The so-called quality refers to the gradual increase in high-end casting production enterprises and the gradual breakthroughs in the production of complex and special-shaped castings; and the quantity refers to a considerable number of enterprises that will adopt the lost foam casting process.
