1. The manufacturing process and inspection process of valve blank are complex. The casting blank of the valve is a thin-walled shell with complex structure. Castings require smooth surface and clear casting characters, especially compact structure, and do not allow defects such as porosity, shrinkage, cracks and sand inclusion. In order to meet the following requirements, a series of technological measures should be taken during casting, such as choosing high refractory moulding materials, controlling moisture content of moulding sand, layering to ensure hardness of moulding sand, adopting reasonable riser system and strictly controlling pouring speed and temperature, etc. Due to the high technical requirements, the casting process of valve gross defect is much more complicated than that of general castings.

In addition, besides checking the size, position accuracy and appearance of the valve, some of them also need to be checked for metallographic structure, mechanical properties, corrosion resistance and non-destructive testing, so the inspection process of the valve is more complicated.

2 It is difficult to add machinery. Due to the variety of valve materials, except for various cast iron and carbon steel, most of the high strength, corrosion resistance and high hardness materials have poor cutting performance, so it is difficult to make the parts meet the required precision and surface roughness. The requirement of geometric shape accuracy and surface roughness of valve sealing surface is very high, which makes it more difficult to add mechanical pressure to the valve.

At the same time, the poor cutting performance of the valve material brings many new problems to the method of parts addition, tool angle and geometry, tool material, cutting parameters and equipment.

It is difficult to install valve parts under machine tools. The structure and shape of the key parts of the valve are complex, such as thin-walled, slender parts and poor rigidity. It is difficult to locate and clamp under the machine tool, so complex special fixture is often needed.

Some valve parts have lower accuracy and higher surface roughness, and sometimes even use non-bottom surface positioning. The precision and surface roughness of the seals and other parts are very high, so it is difficult to guarantee the quality of the seals. Therefore, in order to meet the needs of the next art, it is often necessary to improve the accuracy of the positioning base surface and reduce the surface roughness. Or the positioning base surface is added under the non-lower surface, which increases the complexity of valve manufacturing.