DJM Technical Introduction

    The cause of formation and the solution of the V method of pore defects

    The cause of formation and the solution of the V method of pore defects

    Abstract: V method casting due to conform to the requirements of social development and the national energy saving and environmental protection and rapid development, it is generally assumed that the V-process in pouring process always maintain a vacuum, gas easily from the connection channel and open riser discharge, generally not produced porosity defects. In fact, gas hole defects in all defects is still very prominent a. This paper summarizes the reasons of blowhole, and according to the reasons, puts forward some solutions for the industry colleagues reference.
    Key words: V method casting porosity defects measures
    1 Preface
    V method casting is vacuum sealed molding method also known as negative pressure moulding or vacuum molding method, origin in Japan. A foreign country the first letter of the English words vacuum vacuum with a, referred to as V method. It is using the plastic film vacuum and the dry sand molding, so called the third generation modeling method, namely, physical modeling method is a very promising advanced casting process.
    Casting about, institutions of higher l974 China scientific research units, casting factory on the new technology of test and research and achieved certain results. With this technology continues to develop, foreign production line is introduced, and the domestic foundry workers continue to explore and practice, this technology has been rapid development. According to incomplete statistics, at present our country application V method casting casting factory more than 400, hundreds of production lines, production varieties and different, such as bathtub, a counterweight, railway bolster and side frame, the water pump casing body, gearbox, wear-resisting lining board, count, boiler plate, Jean, automobile rear axle, the anode claw, wrought iron. Materials are cast iron, cast steel and non-ferrous metals.
    The V modelling technology in foundry industry in the future, will be due to its unique advantages, and has more broad application prospects, but we can not assert that, V process is flawless, generally considered the V modelling always maintain the vacuum in the casting process, the gas easily from the connecting channels and open riser discharge, generally does not produce stomatal defects. In fact, the gas is still in the defects of all defects very prominent one. We developed V method is the production of professional paint manufacturers, in the development process of coatings, we have been to a number of V casting manufacturers to learn and test, and with the extension of V method technology and industry colleagues do V equipment are discussed, and also get some harvest. Stomatal defects can be overcome and avoid, there are many pores causes, as long as you find the causes of defects can be solved and avoided.
    2 reasons for the formation of pores
    Porosity is formed as a result of the air entrapped into the liquid metal in the casting surface showed round, oval or teardrop shaped holes, as shown in Figure 1
    Figure 1

    2.1 sources of gas
    The main source of gas, as shown in figure 2:
    2.2 causes of air holes
    After a lot of experience and lessons learned, the reasons for the formation of the pores are as follows:
    1 because the pouring system is not reasonable, the pouring process is involved in the gas, the liquid is not in time to discharge the liquid, that is, the formation of the gas in the hole, as shown in Figure 3
    Fig. 3.
    . paint is not dry, moisture and organic matter content, hair gas quickly; gasification of EVA plastic film is not good, too thick. Core head is fixed at the EVA film did not take cut and puncture guiding gas discharged. Resulting in stomatal response. Such as shown in Figure 4
    Fig. 4 porosity of coatings
    3 water glass sand core, resin sand core, coated sand core is not dry or too thick, large amount of gas generated by the exhaust gas is blocked, the resulting porosity is for the invasion of the pores, as shown in Figure 5.
    Fig. 5 chemical pore
    4 molten metal pouring temperature is low, pouring too fast molten metal liquid to the liquid when the gas is wrapped into the liquid and other reasons for the formation of the pores that are involved in the pores.
    Found the cause of the air holes, then find the solution is not difficult.
    3 solutions
    Through long-term practice, proves that the different gating systems with different particle size of sand and coating, blowhole defect degree also has bigger difference. Open gating system science is to prevent the casting produced the key to blow holes and other defects; blowhole defect generation and gating system design practice has proved that, sand, selection of paint, and EVA films have relatively close relationship.
    First of all, to design a reasonable casting process V, on the one hand to ensure that the metal flow under the turbulent flow, on the other hand, to ensure that a certain casting speed so that the straight runner is always full of molten metal.
    Second, control the smelting process, reduce the gas in the liquid metal.
    Again, as far as possible to reduce the thickness of the EVA film, and strictly control the V core technology, to prevent the invasion of porosity.
    Finally, we are here to summarize the main to prevent the casting porosity, V coating selection.
    On the one hand, the coating should have good adhesion, the coating layer can be firmly stick on the EVA film, on the other hand in ensuring the excellent paint indicators under the premise, to maximize the reduction of coating gas generation. After many tests, we will V special coatings volume control in less than 10mL/g (100 + - 5 DEG C).
    V method casting use alcohol based quick drying coating. As the coating solvent is ethanol. At the same time, we are in the paint with resin to improve the strength and adhesion of the coating. And ethanol, resin and other additives are hydrocarbons, flammable and easy to produce gas. So, (1) in order to reduce the gas, we must control the paint solvents and additives on the quality, of ethanol and resin addition amount and the moisture and impurities must strictly control and the purity of ethanol should be above 0.95, ethanol adding amount should generally be maintained in 75Be. coating Baume
    (2) in the premise of ensuring the quality of castings, as much as possible to reduce the amount of paint coating, the general coating layer thickness should be controlled at 1.5mm, hot junction, in order to prevent the sticky sand, you can brush 2mm thick.
    (3) coating must be dry. If the paint dry quickly is not too good, must use hot air flow drying. Our company produces V special coatings, with good quick drying, can be at room temperature under natural drying. At the same time our paint has good smearing, adhesion, suspension, paint in the spraying process is not flowing, not accumulation, continuous uniform coating thickness, from but coating drying degree of agreement, after pouring can form a dense layer stripping, effectively prevent the gas hole, the so as to make casting surface finish.
    On the basis of numerous experiments and analysis, our company has a number of actual combat summary, the V method of the performance of the paint has a relatively mature standards, see Table 1
    Table 1 performance indexes of casting coatings by V method
    Item name density suspension rate viscosity
    (phi 6 flow cup) abrasion resistant coating
    Exposed thermal cracking performance gas drying appearance
    Index = 1.8g/cm3 = 99% = 15s (2H) <0.4g (64r/min) of less than 10mL/g
    (1000+5 C) no crack coating is uniform
    High speed stirring for 30 minutes, 5 minutes after the static measurement Baume was 7 5Be, the coating grade is also grade I
    4 concluding remarks
    In recent years, V coating casting due to comply with the requirements of the social development and the national energy saving and environmental protection, the momentum of rapid development, almost a summary of all types of casting. Of course because it is the growth phase, in casting process of defects which is normal, however, believe that through all the serious study, will Jiangqi holes and other defects, the greatest degree of reduced.
    Here we have in recent years in V-process casting porosity research, practical experience and industry colleagues and share the hope of V method casting industry development as weak, I believe under the joint efforts of all, V method casting will be faster and more healthy development, at the same time, make a greater contribution to national economic construction of our country.

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