What Makes a Coil Coating Line Continuous?

Did you know that the modern continuous coil coating line can run the metal strip at speeds of 700 feet per minute? The whole process that involves cleaning, pretreating, priming, top-coating, and curing two layers of paint (double coat, double bake) is completed in a single pass that lasts less than 60 seconds.

Achieving such a feat requires a combination of precision engineering and modern technology. High productivity and automation are essential in modern manufacturing, making continuous coil coating lines increasingly popular due to their efficiency. These lines coat metal coils with paints or plastics at a steady, non-stop speed, maintaining uniform temperature and tension throughout.

This article will focus on how the continuous operation of the coil coating line is achieved. We will explore the key steps the metal strip undergoes and the vital components that make this continuity possible.

1. Achieving Continuity: The Need for Steady-State Processing

The process of coating a metal surface has a number of steps that we will discuss here in detail. They also require steady-state process conditions that have to be provided so that the coating will be completed without any defects, such as inconsistent coatings, peeling, or surface blemishes. Let's start with the process requirement:

Step-by-Step Processing Stages

Step 1: Uncoiling & Joining

The process starts with uncoiling the metal coil. Coil Coating The start of the coil is seamlessly attached to the previous coil, so that it won't stop its process. This will be done by welding the new coil to the end of the previous coil, or they may be attached by a mechanical press-type joint. 

Step 2: Cleaning

After uncoiling has been done next most important thing is to clean the surface from dirt and any contaminant to have a better surface finish. Chemical conversion has also been performed in cleaning to enhance paint adhesion and corrosion resistance.

Step 3: Pretreatment

Pretreatment is very important to have a clean and chemically active surface that will make a stronger bond between the metal and the coating. This will be done by degreasing and removing oil from the surface. After that, chemicals are coated, such as phosphate or chromate, to improve the corrosion resistance of the metal surface. After application of the desired chemicals, the surface is washed again to remove excess chemicals.

Step 4: Prime Coating

Primer coating is also done to control corrosion and bond the paint with the metal. This step is performed by roll coaters to achieve an even coating across the surface.

Step 5: Top Coating

After the primer is cooled, the top coating is done with the help of roll coaters. This coating will provide the final color and texture desired also provide durability and the required properties. This coating will prevent the metal from wearing under harsh weather conditions. Some companies provide details of the method used for top coating for customer satisfaction.

Step 6: Curing

After the priming and top coat have been completed successfully, the metal will pass through an oven called a curing oven, where the metal strip is heated to a certain temperature to bake the top coat and have the desired properties of the metal, and initiate polymerization in the paint resin.

Maintaining Stability: Critical Steady-State Conditions

Constant Line Speed

Constant line speed provides a smooth flow of the metal strip from start to end, making the coating process smooth. This constant provides Uniform Coating, Consistent Curing, Minimized Defects, stable oven temperature: oven temperature should remain stable to provide better curing of coating and provide the desired hardness of the coating, stable oven temperature provides Consistent Coating Adhesion, Optimal Coating Properties, Prevention of Coating Defects.

Stable Oven Temperature

For paint to effectively polymerize and form a layer that is homogenous and defect-free, the temperature of the oven should remain stable. The oven starts the polymerization in paints and ensure high quality result.

Controlled Tension

A pulling force is applied to the metal strip throughout the coating process to have a better surface finish and increase production efficiency. This pulling force provides some added benefits that including prevention of metal wrinkling, uniform coating application, maintaining proper alignment, and reducing stretching and distortion.

Continuous Paint Feed

Continuous supply of paint during the coating process is very important as it ensures the uninterrupted supply of coating material to the metal strip, making the downtime less, and providing Consistency in Color and Finish, and using paint more efficiently.

2. The Key to Non-Stop Production: Accumulators

Accumulators are a very important part of the coil coating process. Without them, we have to stop the process when the coil ends, and we need to join the new coil with the previous coil, which would lead to poor coating quality. To accomplish the continuous coating, modern technology uses a series of accumulators at entry and exit, both have different advantages, so they are used at both ends. Let's discuss some details and their advantages

The Entry Accumulator: Bridging Coils

Entry accumulators are a series of rollers movable with a hydraulic mechanism and store an extra length of coil to make the coating continuous even if the coils end at the upstream. Let's discuss how it works. During normal operation, accumulators have a task to keep the strip in multiple loops arranged in a zigzag pattern. This extra strip will help keep the coating continuous. When the sensor detects that the coil has ended, the uncoiler stops automatically, and the signal is sent to the accumulators will move and provide the coating machine with the metal strip at this time.

Workers attach the new coil before the strip in the accumulators ends. Modern accumulators can provide up to 200m strip and provide approximately 80 seconds (about 1.3 minutes), as coating these days is performed at a speed of 150 m/minute. After the new coil has been attached, the uncoiler will operate at about 200% speed to fill the accumulator again

The Exit Accumulator: Cushioning the Recoiler

Same as the entry accumulator, the construction of the exit accumulator is almost the same, but the function they perform is a bit different, as they provide a cushion to the recoiler at the end of the coating process.

Exit accumulators are normally empty, but when the recoiler stops to remove the finished strip, the accumulators start accepting the strip of metal, so that the coating process won't stop after the new recoiler is installed and accepting the metal strip. They start to empty as we have discussed at the start. 

3. The Payoff: Why Continuous Coating Matters

In a continuous coating process, everything remains constant from the speed of the metal strip passing through all the process, the temperature of the oven, pressure, and tension on the strip, and all these things are essential since they provide an environment to have a better product quality in terms of better quality and durability.

A continuous coil coating line has a lot of advantages, but most people only think about continuous production. No doubt production capacity increases if the process is continuous, but let's discuss some aspects too:

• Stable Coating Quality: This is accomplished with constant temperature maintained at the oven with constant stress on the strip. This combination will make the coating surface better in terms of uniform thickness with better quality.

• Improved Material Efficiency: since the production is continuous, it will make the material of the strip better after coating than a non-continuous process, it will reduce the waste produced due to various kinds of defects, making coating more economical.

• Improved Adhesion and Corrosion Resistance: continuity of the coating process also makes the metal strip more resistant to corrosion. Corrosion is a function of pretreatment quality, primer adhesion, topcoat curing, and film thickness uniformity.

• Quality: The quality of the product made with continuous coil coating is very consistent in terms of the thickness of the coating and the finishing of the strip.

4. Widespread Use: Applications of Pre-Coated Metal

A coated metal strip has applications in different industries, some of which are:

• Construction: Used for the roof, wall cladding, gutters, downpipes, panels, and garage doors.

• Appliances: Most of the casings for refrigerators, washing machines, and microwaves are made with coated metal strips.

• Automotive: In the Automotive industry, Certain non-visible components, fuel tanks, and interior parts are made from coated metals.

• Furniture and Fixtures: Pre-coated metal is more reliable in comparison to painting. The process can be challenging, and some corners may be left uncoated while performing manually.

Conclusion

With the help of automated equipment and precise control of the equipment and the process, metal coating is made continuous, providing us with a better quality of metal strip. The coil coating line is continuous with the addition of accumulators at the entry and exit of the process. This will make speed, temperature, stress, curing time, paint finish, and everything involved in the process stable, making the finished product better in every aspect. Continuity of coil designing of the process  reshaped  the coating industry.