Uniform and stable coating composition is the basis to ensure the coating process performance, which belongs to static control for the user, and the stability of incoming materials is guaranteed by the coating supplier. However, due to the change of water evaporation, refractory powder segregation, viscosity and density, it is very important to control the stability and consistency of the coating in the process of use. It belongs to dynamic control. The user should timely adjust and control the process parameters such as density and suspension, otherwise it is easy to cause batch quality accidents, It brings great economic loss to enterprises.
Density is the most reliable parameter to control the performance of coating, and it is an important means to control the coating thickness. Strengthening the density control can obtain the ideal coating thickness, and the density is proportional to the coating thickness. It is the key measure to control the coating stably in a certain numerical range. For the convenience of control, hydrometer or Baume meter are often used on site, but neither of them can fully reflect the true value of density, so the balance method is used for regular calibration.
In the process of soaking and coating 4GB cylinder sand core with ZHY casting, the low density could not meet the process requirements, resulting in the decrease of coating viscosity, suspension and gas generation, and the coating thickness was also reduced to less than 0.15 mm, which could not meet the process requirements, and the leakage defects of water jacket and oil passage increased significantly, as shown in Figure 1. Sand core coating can reduce the thermal effect of high-temperature molten iron on sand core, delay the phase transformation of silica sand and resin decomposition, prolong the time of keeping high-temperature strength of sand core, slow down the gas generation of resin and sand core breaking, gain time for the cooling and solidification of molten iron to block the formation of penetrating defects of sand particles or pores, and effectively inhibit the leakage defects of water jacket, otherwise, it is difficult to play the above role. Therefore, stable control of coating density and reasonable coating thickness are the key to improve casting quality. Because the density of the coating is inversely proportional to the fluidity, the higher the density is, the worse the fluidity is. Therefore, on the premise of meeting the casting quality requirements, the coating density should be reduced as far as possible, which can not only solve the coating accumulation defects, but also reduce the tendency of porosity defects.
The control of coating density is also affected by the environmental temperature on site. If the temperature is too low, the Brownian motion of active molecules is weak, the apparent viscosity of the coating is high, and the coating thickness will increase. During the dip coating process, it is easy to produce accumulation and bubble defects, and the permeability is poor. The adhesion between the coating and the sand core is low. During the pouring process, the coating is easy to peel off, resulting in casting peeling defects (as shown in Figure 2), In this case, the coating density should be controlled at the lower limit of technology or the lowest value of coating density should be re explored. Because the coating thickness is affected by the temperature, although the density is reduced, the coating thickness will not be reduced, which can still meet the process requirements. ZHY casting’s many production experiences show that when the field ambient temperature is lower than 8 ℃, the coating will accumulate and bubble (as shown in Figure 3), and the coating thickness will increase by 0.03 ~ 0.05 mm. Figure 4 shows the bubble burst trace of the crankcase coating after drying. When the field ambient temperature is controlled above 10 ℃, the above phenomenon will disappear. Therefore, the foundry in the North should control the temperature of the field environment or adjust the density according to the change of the coating temperature in winter.