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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.
Table of Contents
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1. Structure and Composition of PPGI1. Structure and Composition of PPGI
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2. Overview of the Production Process2. Overview of the Production Process
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3. Detailed Steps of PPGI Manufacturing3. Detailed Steps of PPGI Manufacturing
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4. Key Quality Control Points and Defect Prevention4. Key Quality Control Points and Defect Prevention
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5. Surface Finishes, Coating Types, and Differentiated Technologies5. Surface Finishes, Coating Types, and Differentiated Technologies
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6. PPGI vs. PPGL and Other Prepainted Metals6. PPGI vs. PPGL and Other Prepainted Metals
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7. Applications and Market Trends7. Applications and Market Trends
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8. Conclusion and Future Outlook8. Conclusion and Future Outlook
1. Structure and Composition of PPGI
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
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
| Process | Possible Defect | Preventive Measures |
|---|---|---|
| Pretreatment | Oil stains, peeling | Improve degreasing, optimize chemical baths |
| Conversion Coating | Poor adhesion | Apply uniform film, monitor bath concentration |
| Coating | Uneven thickness | Adjust roller pressure, use online gauges |
| Baking | Under-curing, blistering | Control oven temperature/time profiles |
| Cooling | Warping, cracking | Ensure controlled cooling and tension balance |
| Packaging | Scratches, corrosion | Use 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
| Aspect | PPGI (Prepainted Galvanized) | PPGL (Prepainted Galvalume) |
|---|---|---|
| Corrosion Resistance | Good for general use | Excellent, especially in marine/industrial environments |
| Heat Reflection | Standard | Higher, due to Al-Zn alloy |
| Cost | More economical | Higher cost |
| Durability | Sufficient for most applications | Longer 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.
7. Applications and Market Trends
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
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Growing demand for energy-efficient, durable, and eco-friendly coated steel
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Shift towards chromium-free, low-VOC coatings to meet global environmental standards
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Increasing adoption of PVDF and HDP coatings for high-end projects
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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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