Lost foam shell casting

Lost foam shell casting combines the advantages of foam lost foam casting and wax loss precision casting, and is highly consistent with new casting concepts such as “clean casting”, “green casting” and “environmental protection casting”. It is called the new century green casting project. On the basis of foam molding, the lost foam casting process adds the pre heated foam liquefaction discharge process, which realizes the empty shell casting without foam, completely avoids the problems of slag inclusion, wrinkling, carburization, etc., and can be used to manufacture low-carbon alloy steel castings with high requirements such as low-carbon steel, stainless steel, etc. Lost wax precision casting is an advanced process for near net forming, but it is powerless for slightly larger parts. This is because, on the one hand, the molding process and cost of large-sized castings are too high, and on the other hand, the thickness of the mold shell when precision casting large-sized parts is as high as ten centimeters, which far exceeds its bearing range, thereby limiting the application range of precision casting [8]. Unlike traditional precision casting, lost foam shell casting has been used for large-sized and complex shaped low-carbon steel and stainless steel parts by using special coatings to make the shell. Lost foam shell precision casting is a green and energy-saving new casting process with broad application prospects.

After the coating is brushed and fired, it forms a complex shaped hollow shell. During the pouring process, on the one hand, it must have sufficient strength to resist the erosion of high-temperature metal liquid, and on the other hand, it must have good breathability to ensure full gas discharge. These are closely related to the viscosity and rheological properties of the coating. Production practice and research results show that under the same smelting quality and pouring process, hollow shell coating is a key factor affecting product quality. Zhang Liang et al. studied the rheological properties of magnesium alloy casting coatings and analyzed the effect of shear dilution on the performance of coatings. Fan Lipeng et al. studied coatings for titanium alloy casting and analyzed the effect of coating viscosity on improving casting quality. Guo Guangsi and others studied the effect of sodium based bentonite content on the viscosity and brushing performance of coatings. Yang Zhe et al. optimized the coating composition using orthogonal design method, significantly improving the quality of castings. In order to ensure that the coating forms sufficient thickness during the brushing process, the coating must have sufficient viscosity, but excessive viscosity can reduce the smoothness of the coating. To ensure smooth application, it is best to reduce the viscosity of the coating to increase its fluidity; After completing the coating, it is hoped that the viscosity of the coating can quickly rebound to prevent the coating from flowing and ensure the uniformity of the mold shell thickness. The requirements for the use of the above-mentioned coatings are directly related to the apparent viscosity and rheological properties of the coatings. Most coatings need to be stirred and mixed evenly before use, and there are more or less requirements for placement during use, during which the performance of the coating will change. To ensure effective use, it is necessary to understand the effects of mixing time and placement time on the viscosity and rheological properties of the coating.

The viscosity of the lost foam shell coating is composed of two parts: structural viscosity and plastic viscosity. The plastic viscosity is mainly determined by the intermolecular attraction of the coating, collision between powder particles, internal friction, etc. Shear force has a relatively small impact on the plastic viscosity. The structural viscosity is influenced by certain special structures formed inside the coating, such as the chain like structure formed by polymer and the hydration film network structure formed by bentonite. The viscosity changes during coating mixing and placement in this study are mainly caused by changes in structural viscosity. The components of the powder and powder liquid have not only been fully mixed, but the network structure has also been uniformly broken, and the structural viscosity has reached its lowest point. The viscosity value of the coating is also approaching a stable value. When placed for a short period of time, the viscosity of the coating increases with the increase of placement time. This is because the network structure that was cut off during the stirring process gradually recovers, and some free water is bound again and cannot move freely. After being left for a long time, the viscosity of the coating began to decrease. In this study, the viscosity of the coating began to decrease after being left for more than 40 hours. This is due to the weakened effect of the effective ingredients in the coating and the sedimentation of the coating particles.


The coating for lost foam shell precision casting was taken as the research object, and the effects of stirring time and placement time on the apparent viscosity of the coating were analyzed. The specific conclusions are as follows:
(1) In order to obtain a uniform coating, the stirring time should not be less than 50 minutes.
(2) To ensure the performance of the coating, the mixing and stirring time of the coating should not exceed 40 hours.

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