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How is PPGI coil manufactured?

PPGI stands for Prepainted Galvanized Iron/Steel, also known as color-coated galvanized steel coils. Unlike conventional painting methods applied after forming, PPGI is coated and finished before shaping while the steel is still in coil form. This process ensures:

  • Uniform coating thickness

  • Stable and consistent quality

  • High production efficiency

  • Cost-effective large-scale manufacturing

According to the coil coating industry standard, PPGI is produced by a continuous coil coating line (CCL) where galvanized steel is cleaned, pretreated, coated, baked, and recoiled in a fully automated sequence.

PPGI Coil Production Process3

Table of Contents

1. Structure and Composition of PPGI

PPGI Coil Production Process2

A typical PPGI coil or sheet consists of multiple functional layers designed for strength, corrosion resistance, and aesthetic appeal:

Layer Function Typical Materials / Properties
Base Metal Provides strength and rigidity Cold-rolled steel, hot-dip galvanized steel
Zinc Coating Primary corrosion protection Zinc layer (Z30, Z80, Z120, Z275, etc.)
Chemical Conversion Film Enhances paint adhesion and corrosion resistance Chromium-free passivation, titanium-zirconium conversion coatings
Primer Improves adhesion, barrier protection Epoxy primer, polyester-modified primer
Topcoat Decorative and protective finish PE, SMP, HDP, PVDF, PU coatings
Backcoat Balances stress, protects reverse side Simple, thinner protective coatings

Different coating systems are customized depending on performance needs. For instance:

  • Polyester (PE): cost-effective, widely used for general roofing and cladding

  • SMP / HDP: enhanced UV resistance for longer outdoor service life

  • PVDF: premium performance with excellent weatherability and anti-fading properties

Hengze Steel offers full customization of coating systems, colors (Brick Pattern, Woodgrain, Flower Painted, Matt Textured Prepainted
, etc.), and thickness options to meet global market demands.

2. Overview of the Production Process

The production of PPGI typically follows these steps:

Raw material → Galvanizing → Surface cleaning & pretreatment → Primer coating → Topcoat application → Baking & curing → Cooling → Quality testing → Slitting / cutting → Packaging

This sequence is completed on a continuous coil coating line, ensuring high-speed and uniform output.

3. Detailed Steps of PPGI Manufacturing

PPGI Coil Production Process

3.1 Raw Material Selection and Preparation

  • Cold-rolled or galvanized steel is used as the base material.

  • Thickness, flatness, and internal stress must meet strict standards.

  • Uncoiling, leveling, tension control, and oil removal are performed before processing.

3.2 Galvanizing (Base Metal Treatment)

  • Steel strips are coated with zinc using hot-dip galvanizing (HDG) or continuous galvanizing.

  • The zinc coating provides sacrificial protection against corrosion.

  • Air knives ensure uniform zinc thickness across the strip.

3.3 Surface Cleaning and Chemical Pretreatment

  • Degreasing: removes oil and lubricants

  • Rinsing & pickling: eliminates residues and rust

  • Conversion coating: applies a thin film to enhance adhesion and corrosion resistance

  • Pretreatment is critical to ensure paint adhesion and long-term durability.

3.4 Coating System Design (Primer + Topcoat / Backcoat)

  • Primer layer enhances adhesion and corrosion resistance.

  • Topcoat provides color, durability, and weather resistance.

  • Backcoat protects the inner side of the coil and balances mechanical stress.

  • Coating selection depends on the end-use environment (roofing, appliances, automotive, etc.).

At Hengze Steel, we provide multi-layer coating systems designed to withstand aggressive environments such as coastal regions and industrial zones.

3.5 Coil Coating / Roller Coating Technology

  • Roller coating ensures uniform application of primer and topcoat.

  • Production speeds often exceed 100–200 m/min depending on the line.

  • Film thickness is controlled with online measuring systems for accuracy.

3.6 Baking, Curing, and Film Formation

  • Coated steel passes through high-temperature ovens at 200–270°C.

  • Heat induces crosslinking and solidification of the paint film.

  • Proper curing ensures strong adhesion, hardness, and resistance to UV and chemicals.

3.7 Cooling, Testing, and Post-Treatment

  • Coils are gradually cooled to stabilize stresses.

  • Quality checks include:

    • Film thickness and adhesion test

    • Salt spray resistance test

    • Surface inspection for scratches, color variation, or pinholes

3.8 Slitting, Cutting, and Packaging

  • Finished coils are slit or cut to customer specifications.

  • Protective films, anti-rust oil, and moisture-proof wrapping are applied.

  • Products are carefully packed for safe transportation worldwide.

4. Key Quality Control Points and Defect Prevention

PPGI Coil Production Process4
ProcessPossible DefectPreventive Measures
PretreatmentOil stains, peelingImprove degreasing, optimize chemical baths
Conversion CoatingPoor adhesionApply uniform film, monitor bath concentration
CoatingUneven thicknessAdjust roller pressure, use online gauges
BakingUnder-curing, blisteringControl oven temperature/time profiles
CoolingWarping, crackingEnsure controlled cooling and tension balance
PackagingScratches, corrosionUse protective wraps and moisture-proof packing

5. Surface Finishes, Coating Types, and Differentiated Technologies

PPGI can achieve a wide variety of surface effects, such as:

  • Glossy, semi-gloss, or matte finishes

  • Wood grain, marble, or stone textures

  • Anti-fingerprint, anti-bacterial, and scratch-resistant coatings

  • Functional coatings for UV resistance, fire resistance, or self-cleaning surfaces

These advanced finishes allow Hengze Steel PPGI coils to serve both industrial and decorative applications with superior aesthetics and performance.

6. PPGI vs. PPGL and Other Prepainted Metals

AspectPPGI (Prepainted Galvanized)PPGL (Prepainted Galvalume)
Corrosion ResistanceGood for general useExcellent, especially in marine/industrial environments
Heat ReflectionStandardHigher, due to Al-Zn alloy
CostMore economicalHigher cost
DurabilitySufficient for most applicationsLonger lifespan, premium option

At Hengze Steel, we supply both PPGI and PPGL coils, helping customers choose the right solution for their specific environment and budget.

Applications

  • Construction: roofing sheets, wall cladding, sandwich panels

  • Home Appliances: refrigerators, washing machines, microwave ovens

  • Automotive: body panels, components, signage

  • Industrial & Agricultural: machinery casings, silos, storage tanks

Market Trends

  • Growing demand for energy-efficient, durable, and eco-friendly coated steel

  • Shift towards chromium-free, low-VOC coatings to meet global environmental standards

  • Increasing adoption of PVDF and HDP coatings for high-end projects

  • Rising customization in color, texture, and functionality

Hengze Steel stays ahead by offering eco-friendly, high-performance PPGI coils tailored for global markets.

Conclusion and Future Outlook

The production of Prepainted Galvanized Steel Coils (PPGI) involves a highly controlled process of galvanizing, pretreatment, coating, curing, and finishing. Each step plays a critical role in delivering a durable, high-quality, and aesthetically appealing product.

With advanced coil coating lines and strict quality systems, Hengze Steel ensures superior PPGI solutions for roofing, cladding, appliances, and industrial applications. Looking ahead, the trend towards eco-friendly coatings and differentiated surface finishes will continue to shape the PPGI market.

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