Quality control of coating for valve castings

1. Coating performance requirements

The selection of different formulations, components and the proportion of various components have a great impact on the performance of the coating. On the one hand, the coating should have certain strength and fire resistance. When pressurized, the coating layer effectively separates high-temperature stainless steel water from dry sand, so as to prevent metal liquid from penetrating into dry sand and forming bonded sand. On the other hand, the coating should have good air permeability, so that the residual gas of the foam pattern gasification can be discharged smoothly and the casting defects such as porosity and porosity can not be penetrated into the high temperature stainless steel water. Of course, the higher the permeability of the coating, the better. If it is too high, it will cause a convex starting point on the surface of the valve casting after pressurization. In serious cases, it will cause the stainless steel water to overflow and penetrate into the dry sand, so that the valve casting can not be cleaned, resulting in the scrapping of the valve casting.

The air permeability of the coating layer is related to the coating composition and the thickness of the coating. The key to controlling the thickness of the coating is to control the viscosity and hanging times of the coating. Therefore, the type and amount of inorganic and organic binders should be selected to prepare coatings with high strength and coating performance. First, stir at high speed for at least 30 minutes, and then stir at low speed. In order to obtain a uniform coating and ensure that the coating does not drip and flow, it should be stirred and used continuously within 24 hours.

2. Coating drying process

The EPC drying room should have good ventilation, heating system and moisturizing function, which can meet the thorough drying of appearance and coating layer. The temperature of the drying room should be controlled at 45~ 50 degrees C, and the drying time is related to the main wall thickness of the foam pattern. The thin wall foam pattern can be dried for more than 24 h, and the thick wall foam pattern can reach 24~ 72 h to completely dry. If the appearance is not dry, it is easy to cause the increase of gas generation during pouring, resulting in reverse spray, resulting in casting defects such as air hole and cold shut of valve castings and scrapping.

In addition to temperature control, pay attention to humidity control during pattern drying. Set ventilation equipment in the drying room to control the humidity no more than 30%. During the drying process, the valve casting pattern needs to be placed and supported reasonably to prevent the pattern from deformation. The dried pattern should be placed in a place with low humidity before modeling to prevent moisture absorption.

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