Analysis of the causes of uneven coating thickness of coated steel pipes

This article briefly analyzes the reasons for uneven coating thickness of coated spiral steel pipes and how to make the coating uniform.

Coated steel pipes process

1. Before coating, remove the grease and dirt on the surface of the steel pipe, and preheat the steel pipe for surface pretreatment. The preheating temperature of the steel pipe is 400C~600C. The quality of surface pretreatment should meet the requirements of Sa21/2 level specified in “Surface Rust Level and Rust Removal Level of Steel Materials Before Coating” , and the anchor pattern depth should reach 50μm~75μm.
2. After surface pretreatment, the dust and abrasive attached to the surface of the steel pipe should be cleaned, and the surface of the steel pipe should be prevented from being damp, rusted or tertiary contaminated before coating. The surface pretreated steel pipe should be coated within 4h; if it exceeds 4h or the surface of the steel pipe rusts again, the surface pretreatment should be repeated.
3. The pretreated steel pipe rotates and passes through the medium frequency induction heating ring to heat the steel pipe to the appropriate coating temperature, and the epoxy powder coating is evenly applied to the surface of the steel pipe.
4. The coating of polyethylene and adhesive can be carried out by longitudinal extrusion process or lateral winding process. For steel pipes with a nominal diameter greater than 500mm, the lateral winding process should be adopted.
5. The longitudinal extrusion process is that the heated steel pipe (2PE) or the heated and powder-coated steel pipe (3PE) passes through the die head of a coating extruder and the die head of a polyethylene extruder, thereby coating the outer surface of the steel pipe with a layer of molten adhesive and a layer of molten polyethylene. In the longitudinal extrusion process, the die head is closer to the steel pipe, the adhesive and polyethylene coated on the steel pipe have a higher temperature, and shrink more after cooling, so that the adhesive, polyethylene and steel pipe are tightly bonded together. When the longitudinal extrusion process is adopted, there should be no voids on both sides of the weld.
6. The lateral winding process is that the steel pipe is heated by the medium-frequency induction heating ring while rotating, and two extruders continuously extrude the molten adhesive film strip and the molten polyethylene film strip. The adhesive and polyethylene film strip are wrapped from the side on the rotating and advancing heated steel pipe (2PE) or the epoxy powder coating (3PE) of the steel pipe. When using the side winding process, ensure that the polyethylene of the overlapping part and the polyethylene on both sides of the weld are completely rolled and compacted to prevent the surface of the polyethylene layer from being crushed. The wall thickness difference of the plastic layer using the side winding process is small.
7. After the polyethylene is coated, water is used to cool the steel pipe to a temperature not higher than 600C. As the polyethylene shrinks after cooling, the adhesive, polyethylene and steel pipe are tightly bonded together.
8. After the anti-corrosion layer is applied, the polyethylene layer at the pipe end should be removed. The reserved length of the pipe end is 100~150mm, and the end face of the polyethylene layer should form a chamfer of less than or equal to 300.