The production process of galvanized color-coated coil refers to a
procedure that takes galvanized steel sheets as the base material
and forms a uniform, firm organic coating on their surface through
multiple continuous and refined processing steps. Its core goal is
to balance the corrosion resistance of the base material with the
decorativeness and functionality of the coating, while meeting the
efficiency and quality requirements of industrial continuous
production. The complete process can be divided into three core
stages: "Base Material Pretreatment", "Coating Application &
Curing", and "Post-Processing & Finished Product Inspection". The
specific processes and key details are as follows:
This stage serves as the "foundation" for ensuring tight bonding
between the coating and the galvanized base material. It requires
thorough removal of impurities such as oil stains, oxide layers,
and dust from the base material surface, while improving surface
activity through chemical treatment to prevent subsequent coating
peeling or blistering. The main processes include:
The raw material is galvanized steel coils (hot-dip
galvanized/electro-galvanized sheets, usually with a thickness of
0.15-2.0mm). The steel coil is smoothly unrolled by an uncoiler, and then the bent or wavy deformations of the steel sheet are
corrected by a leveler (multi-roll structure). This ensures the steel sheet runs smoothly
in subsequent processes and the coating thickness remains uniform.
During rolling and storage, the surface of galvanized steel sheets
may retain oil stains such as rolling oil and anti-rust oil, which
need to be treated in a degreasing tank: Alkaline degreasing agents (e.g., sodium hydroxide, sodium
carbonate solutions) are injected into the tank. After the steel
sheet is continuously soaked or sprayed, the oil stains are
emulsified and decomposed, and then rinsed off with high-pressure
water to prevent oil stains from affecting coating adhesion.
If slight oxidation spots (white rust) appear on the galvanized
layer surface due to improper storage, the oxide layer needs to be
removed using a pickling tank (dilute hydrochloric acid or dilute sulfuric acid solution), while
slightly activating the galvanized surface. After pickling, the
steel sheet must be immediately rinsed with clean water to prevent
residual acid from corroding the base material.
This is a core step to improve coating adhesion. A very thin
(several nanometers to tens of nanometers), dense conversion film
is formed on the galvanized layer surface through chemical
reactions (common types include "chromate conversion film" or
"chromium-free conversion film", with the latter being more
environmentally friendly).
Functions of the conversion film:
① Isolating air to prevent secondary oxidation of the galvanized
layer;
② The conversion film has a porous structure and strong polarity,
which can form chemical bonds with resin molecules of the
subsequent coating, significantly enhancing the bonding strength
between the coating and the base material and avoiding coating
peeling during long-term use.
Treatment method: The steel sheet continuously passes through a conversion
treatment tank (by soaking or spraying). After the reaction is
completed, it is preliminarily dried with hot air to ensure the
surface is dry and free of moisture.
This stage is the core of "endowing the product with appearance and
functionality". It requires precise control of coating thickness,
curing temperature, and time to ensure the coating has uniform
color and meets performance standards (e.g., weather resistance,
impact resistance). According to the coating structure (usually
"primer + topcoat", with "backcoat" added for some high-end
products), the process involves multiple coating-curing cycles. The
specific steps are as follows:
Function of primer: Acting as an "intermediate bridge", it not only bonds tightly
with the base material’s conversion film but also provides a good
adhesion foundation for the topcoat. Meanwhile, it enhances the
overall corrosion resistance of the coating (some primers contain
anti-rust pigments such as zinc powder and zinc phosphate).
Application method: A roll coater is used (the most mainstream method, divided into "forward roll
coating" and "reverse roll coating"). By adjusting the pressure and
rotation speed between the coating rolls and the base material, the
wet film thickness of the primer (usually 5-10μm) is controlled to
ensure no missing coating or sagging.
Pre-curing: The steel sheet with primer applied enters a hot-air drying oven (first furnace section), where it is dried at 100-150℃ to remove
solvents (e.g., xylene, ethyl acetate) from the primer. This allows
the primer to be pre-cured (not fully cured, with some residual
activity to facilitate bonding with the topcoat).
Function of topcoat: Determines the product’s appearance (color, texture, gloss) and
core functions (weather resistance, stain resistance, wear
resistance). The topcoat formula is adjusted according to
application scenarios (e.g., topcoats for outdoor use need to add
"weather-resistant resins" such as PVDF fluorocarbon resin and
high-weather-resistance polyester resin; ordinary polyester resin
can be used for indoor applications).
Application method: Same as the primer, a roll coater is still used. For texture
effects such as "wood grain" or "stone grain", an "embossing roll"
is used for simultaneous texture pressing, or "digital printing" is
adopted to achieve customized patterns.
Full curing: The steel sheet with topcoat applied enters a curing oven (second furnace section, with a higher temperature, usually
200-250℃). At high temperatures, the resin in the topcoat (e.g.,
polyester, fluorocarbon) undergoes a cross-linking reaction to form
a dense, hard cured coating, and solvents are completely
volatilized. The curing time must be precisely controlled (usually
30-60 seconds, adjusted according to coating thickness):
insufficient time leads to incomplete curing (coating is easy to
scratch), while excessive time causes resin aging and
discoloration.
Backcoat Application (Optional): For some products (e.g., color-coated coils for construction), a
"backcoat" (usually epoxy or polyester resin) needs to be applied
to the back of the steel sheet. Its function is to prevent rust on
the back of the steel sheet (especially when used for roofs or
walls, where the back contacts air or insulation layers). The
application and curing process is the same as that of the topcoat.
This stage mainly involves physical treatment and quality screening
of the cured color-coated coils to ensure the finished products
meet standards.
The temperature of the color-coated steel sheet coming out of the
curing oven is over 200℃, so it must immediately enter a cooling system: First, it undergoes "air cooling" (preliminary cooling with
high-pressure cold air), then "water cooling" (cooling with cold
water rolls or cold water spraying) to lower the temperature to
below 50℃. This prevents steel sheet deformation or coating
adhesion caused by high temperatures.
To prevent the coating from being scratched during transportation
and storage, a protective film is applied to the topcoat surface of
some color-coated coils (transparent PE protective film, with a
thickness of 20-50μm). The film application is completed
simultaneously by a film laminator, and the film’s adhesiveness must be moderate (easy to peel off
during subsequent construction without leaving adhesive residue).
The cooled color-coated steel sheet is guided to a coiler by a tension control system (to ensure uniform coiling tension)
and rolled into finished steel coils that meet customer
requirements (inner diameter is usually 508mm or 610mm, and outer
diameter is adjusted according to order requirements). If customers
need cut-to-length flat sheets, the steel sheet is cut into
fixed-size flat sheets by a slitting machine (for width slitting) and a cutting machine (for length cutting) before coiling.
Each finished coil must pass multi-dimensional inspections before
leaving the factory. The inspection items include:
- Appearance inspection: Visual inspection for coating defects such as missing coating,
pinholes, sagging, color difference (compared with standard color
cards), and scratches;
- Performance inspection: Sampling and testing of coating thickness (using a coating
thickness gauge, total thickness of primer + topcoat is usually
20-50μm), adhesion (cross-cut test: cutting a cross grid with a
cutter, no coating peeling after tape adhesion), impact resistance
(drop ball test: no cracks or peeling of the coating when a steel
ball falls from a specified height), and weather resistance
(accelerated aging test: simulating ultraviolet rays, high
temperature, and high humidity to test coating discoloration and
chalking);
- Dimensional inspection: Testing the thickness, width, length of the steel sheet, and coil
diameter to ensure compliance with order requirements.
The entire production line operates in a closed-loop continuous
mode, with a speed of usually 30-100 meters per minute and a daily
output of up to several thousand tons, meeting the needs of
large-scale industrial production.
Traditional processes partially use chromate conversion films and
solvent-based coatings (containing VOCs). Currently, the mainstream
trend is to adopt "chromium-free conversion films" and "water-based
coatings" (reducing VOC emissions by more than 80%), while
equipping with waste gas treatment systems (e.g., RTO regenerative
thermal oxidizers) to reduce environmental pollution.
Through the above processes, galvanized color-coated coils
ultimately achieve comprehensive performance of "base material
corrosion resistance + coating decorativeness + processing
adaptability", becoming a core material in fields such as
construction, home appliances, and transportation.
