EPC has good casting quality and low cost. Material is not limited, size is appropriate; High dimensional accuracy, smooth surface; Less internal defects, dense tissue; It can be achieved on a large scale, mass production; It can greatly improve the working environment, reduce labor intensity, reduce energy consumption and many other advantages, and has been widely used in many foundries.
1. Existing problems and specific defects of EPC process
Because the EPC process is different from other casting processes, there will be some special defects, such as increasing C, increasing H, etc., at the same time, other process methods also exist however, in the process of EPC, its formation mechanism and appearance characterization have different defects.
1.1 Increased segregation of C, H and C components
EPS shape under the action of high temperature cracking reaction, in the process of complete cracking, C-H bond is opened, the formation of active or free state of C and H, this part of the material in the whole the filling process mainly exists in the “air gap” between the shape and the metal liquid, one is through the coating and molding sand in the form of gas or liquid droplets discharged from the mold; The other is residue
Non-metallic inclusion defects are formed inside or on the surface of the casting; Third, it diffuses into the metal liquid under the action of air pressure and is absorbed by the metal liquid, thus polluting the metal liquid and making it C, H increased.
1.2 White defects
The white defect is formed when the dry sand enters the casting under negative pressure in the process of EFC.
1.3 air hole
There is no obvious difference in appearance and characterization between the pores produced by EPC and those produced by other casting processes, but the formation mechanism is different.
(1) appearance material and adhesive type pores. (2) air entrainment type pores. (3) water and foaming dosage stoma.
1.4 Inclusions generated by pyrolysis
As part of liquid and solid products appear in the cracking process of the shape material, it is possible to form lost mold casting in the inside of the casting if they cannot be eliminated and distributed centrally in time
Some slag inclusion defects.
1.5 Other defects
It is easy to analyze and judge these defects due to deformation, collapse of the box, insufficient pouring, size and poor surface, sticky sand, etc., generally in the case of reasonable process and effective prevention measures, much caused by accident or operating error.
2. Influencing factors and prevention measures of specific defects
With the development of vanishing mode technology, more types of defects may appear. In the future research work, we should not only know how to identify and analyze the common defects it is more important to put forward effective methods to eliminate these defects.
2.1 Appearance factor
The most fundamental difference between EPC casting process and other casting processes is that the casting process of EPC process in the “cavity” position of other processes is full of the appearance to be gasified off
Taking out the shape is the biggest charm of the EPC process, but it is also the root of many problems, because the shape gasification process for the whole pouring process is unable to affect predict.
2.1.1 Appearance quality evaluation and control
Among many factors affecting casting quality, shape quality accounts for 50%, 30% depends on coating and related technology, and 20% depends on molding and casting process. (1) The less dense the shape is,
The larger the ratio to the weight of the casting, the smaller the possibility of defects such as carburization, so it is most important to control the appropriate shape density as low as possible; (2) take the reasonable
Process measures, process layout, so that carburizing as far as possible evenly distributed in the casting, carburizing caused by casting composition, performance change will be small, and can be adjusted by the melting process, chemical composition can be controlled; (3) By artificially controlling the air gap pressure between molten iron and the shape, making it as low as possible, and improving the pouring speed, the carbon molecules produced by cracking can be molten iron the absorption rate dropped significantly; (4) PMMA and copolymer resin in the use of EPS density carburization ratio is smaller, but elected with a higher density than EPS PMMA appearance, carburization a marked increase in the proportion of (5) greatly increase the temperature, so that the appearance of full cracking, because the temperature is high, carbon absorption fast, and the amount of increase, at the same time the gas increase, the pressure increases.
2.1.2 Conclusion from the analysis of appearance
It is the most effective and basic way to solve the problem of carburizing that the low density of shape and the reasonable technological measures are adopted to effectively control the distribution of the optimal flow field and temperature field in the pouring process.
2.2 Coating, molding sand and other factors
2.2.1 The reason why the research of coating has become the first hot spot
Some progress has been made in the study of shape materials for a period of time. Coatings cost in the total cost of a high proportion, choose a good and low-cost coating for casting enterprises the industry has a big impact. Coating quality is closely related to casting quality.
2.2.2 Some experience of improving coating performance
The process details or the amount of key additive is an important factor that hinders the development of EPC. (1) The aggregate with porous granular structure, diatomite and aluminum vanadium were selected soil, etc.; (2) The thixotropic properties of coatings can be significantly improved by using fiber as the particle shape; (3) the type and particle size distribution of composite aggregate; (4) as low as possible with inorganic binder amount, appropriate increase in organic binder content; (5) choose high refractoriness coating and reduce coating thickness; (6) reduce the density of paint; Application of new nanometer technology in EPC coatings.
2.3 The importance of casting scheme is understood and understood by every casting worker. A good casting scheme is firstly to obtain high-quality castings, and secondly
Higher process yield. EPC and traditional casting have certain differences in the process design principles: (1) to ensure enough pressure head, so that the metal head more than liquid metal and EPS between the pressure of the air gap can ensure that the filling model does not choke; (2) Try to use a closed gating system to reduce the formation of turbulent flow in the process of filling, and to ensure adequate supply of metal liquid flow, prevent box collapse; (3) simplify the composition of the gating system as far as possible, can achieve the best straight-inside structure.
2.4 Negative pressure factors
Negative pressure usually has the following functions: (1) enhance the strength and stiffness of the casting mold, prevent sand washing and collapse; (2) accelerate gas discharge, and improve the workshop environment; (3) supply metal liquid filling capacity. Negative pressure also brings many problems: (1) cause liquid iron turbulence, so that the filling process part of the gasification products involved in liquid iron to form non-metallic inclusions or porosity defects; (2) the metal liquid is easy to move near the mold wall and the surface of the mold wall cools too fast. It is very easy to form the shell, which makes it more difficult to fill the mold at some positions, and forms the white opening on the surface of ductile iron and gray cast iron. even part of the casting into the “shell”; (3) Negative pressure is easy to make molten iron move to the wall, the high temperature strength of the coating has higher requirements, and make the casting appear mechanical sticky sand.
2.5 Temperature factors
EPC usually needs to adopt a casting temperature 30~50℃ higher than sand casting, but the temperature control should be appropriate, the adverse factors of high temperature are mainly as follows:
(1) high temperature often makes the metal metallurgy quality decline, resulting in coarse grain, cast iron white tendency to increase and other problems; (2) the coating and sand refractoriness requirements, easy to form clay sand and other defects; (3) The cost increases, the smelting cost increases, the alloy element burning serious.
Post time: Jul-16-2021