Forming Process Discussion for Plastic-Lined Steel Pipes and Fittings

Overview of the Literature

This paper, published in China Plastics (2001, Vol. 15, No. 8, pp. 60-61) by Jiang Jihong from Jiangxi Agricultural Machinery Factory and Zhang Renkun from Jiangxi Mafang Steel Factory, provides a concise technical discussion of the manufacturing processes for plastic-lined steel pipes and fittings. The work addresses the characteristics and applications of galvanized steel pipes and malleable iron fittings with internal plastic lining.

Core Technical Content

Plastic-lined steel pipes represent a hybrid construction approach that combines the mechanical strength and dimensional stability of steel with the corrosion resistance and smooth interior surface of plastic linings. This technology was particularly important in the early 2000s for water distribution systems, chemical processing applications, and building services where both structural integrity and corrosion protection were required.

Product Configuration

Component	Base Material	Lining Material	Typical Application
Straight pipe	Hot-dip galvanized carbon steel	PE, PTFE, or EPDM	Water supply, chemical transfer
Elbows	Malleable iron (玛钢)	PE or PTFE	Pipe routing changes
Tees	Malleable iron (玛钢)	PE or PTFE	Branch connections
Caps/plugs	Malleable iron (玛钢)	PE or PTFE	System termination
Flanges	Carbon steel	PTFE or EPDM	Flanged connections

Forming Process Overview

The plastic lining process involves several sequential operations:

Surface preparation: The steel or iron substrate undergoes cleaning, degreasing, and surface roughening (sandblasting or chemical etching) to ensure adequate adhesion.
Lining application: The plastic material is applied through one of several methods depending on the geometry and material selection.
Curing/bonding: Thermal or chemical bonding ensures permanent adhesion between the plastic lining and the metal substrate.
Quality inspection: Visual inspection, adhesion testing, and dimensional verification confirm product conformity.

Lining Methods by Geometry

Method	Applicable Geometry	Plastic Material	Process Description
Extrusion lining	Straight pipes	PE, PVC	Continuous extrusion onto rotating pipe
Dip coating	Small fittings	PE, PTFE	Fitting immersed in molten plastic
Spray coating	Large fittings	PTFE, EPDM	Powder or liquid spray application
Extrusion blow molding	Complex fittings	PE	Melt extruded into fitting mold
Roll bonding	Long pipes	EPDM, FEP	Sheet bonded to pipe interior

Process Parameters and Quality Considerations

Surface Preparation Parameters

Parameter	Specification	Quality Impact
Surface roughness (Ra)	6.3–12.5 μm	Optimal adhesion range
Cleaning method	Solvent degreasing + acid pickling	Removes oxides and contaminants
Sandblast grit size	30–60 mesh	Creates mechanical interlocking
Surface cleanliness	No visible oil, rust, or scale	Critical for adhesion

Lining Quality Parameters

Parameter	Acceptance Criteria	Test Method
Adhesion strength	>1.0 MPa (peel test)	ASTM D3330
Lining thickness	±10% of nominal	Micrometer measurement
Holiday detection	No pinholes at 5 kV	Spark test
Water permeability	<0.1 mL/m²·day	ASTM D1434
Impact resistance	No cracking at 5 J	Indentation test

Application Characteristics

The paper highlights several key application characteristics of plastic-lined steel products:

Corrosion resistance: The plastic lining provides complete barrier protection against the conveyed medium, extending service life significantly compared to bare galvanized steel.
Flow efficiency: The smooth plastic interior reduces friction losses by 20-30% compared to galvanized steel interiors, enabling smaller pipe diameters for the same flow rate.
Temperature limitations: The maximum operating temperature is governed by the plastic lining material (typically 60-80°C for PE, 150°C for PTFE).
Pressure rating: The steel substrate maintains the original pressure rating of the pipe, unlike solid plastic pipes which have lower pressure capacity at elevated temperatures.
Joint integrity: The primary concern is maintaining lining continuity at connection points, particularly at threaded or flanged joints.

Engineering Challenges and Countermeasures

Challenge	Description	Countermeasure
Lining delamination	Loss of adhesion at high temperatures or under cyclic pressure	Optimize surface preparation; select appropriate liner material
Joint lining discontinuity	Lining interrupted at connection points	Use lined coupling nuts or internal gasket rings
Impact damage	Mechanical damage to lining during handling	Protective coatings on exterior; careful installation
Thermal expansion mismatch	Differential expansion between steel and plastic	Allow for expansion at fittings; select compatible materials
Aging degradation	Long-term chemical and thermal degradation of lining	Select lining material with adequate environmental resistance

Standards and Quality Assurance

Standard	Scope	Relevance
GB/T 28897	Plastic-lined steel pipes	Chinese national standard
ISO 14692	Plastic-lined steel tubes	International standard
EN 10345	Thermoplastic-lined steel pipes	European standard
ASTM F1220	Plastic-lined steel pipe	American standard
SY/T 0575	Plastic-lined steel pipe for oil/gas	Chinese industry standard

Integration with Engineering Practice

The plastic lining technology addresses a fundamental limitation of bare metal piping in corrosive service environments. In practice, engineers must consider:

System design: Account for the thermal expansion coefficient of the composite pipe, which differs from bare steel.
Installation practices: Avoid excessive mechanical stress that could damage the lining during bending, threading, or welding of adjacent components.
Welding considerations: When welding to plastic-lined pipes, use indirect heating methods or remove the lining at weld zones and re-apply after welding.
Inspection protocols: Include lining integrity checks during commissioning and periodic maintenance.
Material compatibility: Verify that the lining material is chemically compatible with the conveyed fluid throughout the expected service life.

Key Technical Reflections

This paper, while brief, captures an important manufacturing technology that bridges the gap between metal and plastic piping systems. The approach of combining steel strength with plastic corrosion resistance represents a pragmatic engineering solution for applications where neither material alone satisfies all requirements. The malleable iron (玛钢) fittings mentioned in the paper are particularly relevant to Chinese manufacturing practice, where this material was widely used for water distribution fittings before being largely replaced by ductile iron and stainless steel in more recent decades.

The technology discussed here has evolved significantly since 2001. Modern plastic lining processes incorporate advanced adhesion promoters, improved surface preparation techniques, and automated quality inspection systems that were not available at the time of publication. However, the fundamental principles of surface preparation, adhesion optimization, and lining integrity maintenance remain unchanged and continue to guide current practice.

Study Insights and Implications

The plastic-lined steel pipe technology exemplifies the engineering philosophy of combining the best properties of different materials through intelligent construction design. For engineers specifying piping systems, the key decision factors include the severity of the corrosive environment, the operating temperature and pressure, the mechanical loading conditions, and the required service life. When these factors cannot be satisfactorily addressed by a single material, composite constructions such as plastic-lined steel provide an economically viable and technically effective solution. The forming process considerations discussed in this paper remain relevant for manufacturers developing and maintaining plastic-lined pipe and fitting production capabilities, and the quality assurance principles apply equally to modern implementations of this technology.