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Production Process of Internal and External Plastic Coated Composite Steel Pipe
Created on 05.17
Internal and external plastic coated composite steel pipe is a widely used anti-corrosion pipeline product in municipal water supply, fire protection, sewage treatment, and industrial fluid transportation. It combines the high mechanical strength of steel pipe with the excellent corrosion resistance, hygiene performance, and smooth inner wall of plastic materials (such as epoxy resin or polyethylene), making it an ideal choice for various corrosive and sanitary medium conveying projects. The production process directly determines the quality, service life, and performance of the finished pipe.
1. Steel Pipe Surface Pretreatment
The first and most critical step is to thoroughly clean the inner and outer surfaces of the steel pipe. The pipe is first degreased to remove oil, grease, and other contaminants using alkaline cleaning agents or organic solvents. Then, it undergoes pickling or shot blasting to eliminate rust, scale, welding slag, and debris. This process achieves a clean, rough surface (typically Sa2.5 standard) with controlled roughness to ensure strong mechanical adhesion between the plastic coating and the steel substrate.
2. Preheating Treatment
After pretreatment, the steel pipe is preheated to a specific temperature in an oven or heating line. The preheating temperature is precisely controlled according to the coating material:
- For epoxy resin (EP) coating: usually 180–220°C
- For polyethylene (PE) coating: usually 160–200°C
Preheating is essential to ensure the plastic powder melts evenly, flows, and forms a continuous, dense coating during the subsequent spraying process.
3.Internal and External Coating Application
The coating process is the core step, typically using the electrostatic spraying method:
- Outer Coating: Plastic powder is sprayed onto the preheated outer surface of the steel pipe using electrostatic guns. The powder is electrostatically charged and adheres evenly to the pipe surface, then melts under high temperature to form a uniform outer coating.
- Inner Coating: A special inner spraying nozzle moves linearly inside the rotating pipe, spraying plastic powder onto the preheated inner wall. The powder melts and flows evenly under gravity and centrifugal force, forming a smooth, seamless inner lining.
Some processes also use the fluidized bed dipping method for the outer coating, where the preheated pipe is dipped into a fluidized powder bed to achieve a thick, even coating.
4. Curing and Cross-linking
After coating, the pipe enters a curing oven to complete the cross-linking reaction of the plastic material:
- Epoxy resin coatings are cured at 200–220°C for 15–30 minutes to form a rigid, three-dimensional cross-linked structure with excellent chemical resistance.
- Polyethylene coatings are cured at 160–190°C for 10–20 minutes to ensure full melting and fusion of the coating.
Proper curing ensures the coating is fully bonded to the pipe surface and free of pinholes, bubbles, or peeling.
5. Cooling and Finishing
The cured pipe is gradually cooled to room temperature in a controlled environment to avoid internal stress and coating cracking. After cooling, the pipe is trimmed, and the coating is inspected for uniformity. The ends of the pipe are usually capped or protected to prevent damage during transportation.
6.Strict Quality Inspection
Every finished pipe undergoes comprehensive quality checks before delivery:
- Visual Inspection: Check for coating uniformity, smoothness, and defects such as pinholes, bubbles, or peeling.
- Thickness Test: Verify that both inner and outer coatings meet the design requirements (typically 0.3–2.0mm depending on the application).
- Adhesion Test: Confirm the bonding strength between the coating and the steel pipe to ensure no delamination.
- Spark Test: Use a high-voltage spark detector to detect pinholes or weak spots in the coating.
- Hydraulic Pressure Test: Test the pipe under pressure to check for leaks and structural integrity.
Only pipes that pass all inspections are marked with specifications, batch numbers, and certifications, then packaged for delivery. With mature production technology and strict quality control, internal and external plastic coated composite steel pipe provides a reliable, long-lasting solution for water supply, fire protection, and industrial pipeline projects worldwide.
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