Pretreatment of Cast Iron Enamel Substrate
The surface pretreatment of the cast iron enamel substrate is different from that of the steel plate. If the pickling method is used, the acid reacts with the iron to produce hydrogen, which is absorbed in the cast iron, which is likely to cause bubbles and other defects in the fired product. Therefore, all cast irons are processed by sandblasting.
Sandblasting of cast iron can be divided into two types:
1. Use the pressurized airflow of an air compressor to spray wear-resistant particles on the surface of the casting.
2. Use mechanical methods such as centrifugal force to make wear-resistant particles impact the casting.
The cast iron surface must be sandblasted before coating to remove surface inclusions, mold sand, iron scale, oxides, etc. Small-size castings can also be cleaned by chemical degreasing. The shape of sand particles used for sandblasting should be irregular and sharp. The hardness of cast steel and quenched cast iron sand particles are above HV650 and have good edge stability, so they are mostly used for sandblasting on the surface of cast iron. Brittle sand particles such as quartz sand, zircon sand, corundum, etc. can also be used.
Generally avoid the use of plastic metal particles or shots, because they are easy to pellet and strengthen the cast iron surface, thereby sealing the inner crystal grains, enveloping impurities, and causing blistering on the enamel surface during the later enameling. The particle size distribution of the sandblasting used should be strictly controlled to ensure uniform cleaning of the entire surface of the workpiece, especially the cleaning of depressions.
The particle size distribution and level of blasting depends on the type of blasting equipment and blasting equipment. The sandblasting process should eliminate all burrs on the workpiece, and the arc of the corner of the workpiece should not be less than 6.5 mm. The cleaned work pieces are best to be coated and used immediately. If they are reused after long-term storage, they need to be pre-burned before use.