An efficient way to coat flat materials

Spray coating is widely used in industrial coating.
It atomizes coating material and sprays it onto the workpiece, making it relatively adaptable to complex shapes.

However, spray coating is not necessarily the best method for every product.

For flat products such as plates, sheets, panels, and boards in particular, more efficient methods are available.

One of these is the roller coater.

A roller coater is a coating system that uses rotating rollers to transfer coating material onto the workpiece surface.
It applies a controlled quantity to flat material, much as a roller applies ink to paper.

Rather than projecting coating material into the air as in spray coating, it transfers the material through a roller. This makes it easier to reduce material loss and maintain consistent film thickness.

However, a roller coater is not suitable for every application.
It suits flat materials, but may be unsuitable for pronounced surface contours or three-dimensional components.
Unsuitable roller settings, coating viscosity, feed speed, or drying conditions can also cause streaks, roll marks, uneven film thickness, or uneven gloss.

A roller coater is an effective method for coating flat materials efficiently, but the process requires proper control of its conditions.

1. A roller coater transfers coating material through rollers

A roller coater supplies coating material to a roller and transfers it to the workpiece through that roller.

Spray coating atomizes the material into a mist and projects it onto the workpiece.
A roller coater, by contrast, holds material on the roller surface and applies it through contact with the workpiece.

This difference has major implications for the process.

Spray coating produces overspray.
Some of the coating material disperses into the air without depositing on the workpiece.

A roller coater transfers material directly from the roller to the workpiece, making it easier to reduce material loss when conditions are suitable.

It also suits continuous production because coating takes place while the workpiece is conveyed at a constant speed.

2. Products suited to roller coating

Roller coaters are generally suited to products with relatively flat shapes.

Examples include the following.

• Metal sheets
• Aluminum sheets
• Steel sheets
• Building panels
• Wood-based boards
• Plywood
• MDF
• Plastic sheets
• Paper materials
• Decorative boards
• Flat components
• Panel-shaped products

These products have flat surfaces that allow consistent roller contact, making continuous coating easier.

Conversely, products with pronounced contours, many curved surfaces, three-dimensional shapes, or steps may be unsuitable.

When considering a roller coater, first examine the workpiece geometry.

Before asking “Is the coating material suitable?”,
check “Does the geometry allow consistent roller contact?”
This is important.

3. Advantages of roller coaters

Roller coaters offer several major advantages.

Typical advantages include the following.

• Efficient coating of flat materials
• Suitability for continuous production
• Easier control of consistent film thickness
• Easier reduction of coating material loss
• Easier reduction of differences caused by operator skill
• Less spray mist
• Easier improvement of the working environment
• Ease of automation
• Suitability for high-volume production

In spray coating, film thickness changes with the operator’s gun distance, travel speed, and overlap.
A roller coater makes it easier to reduce reliance on the operator if roller conditions and feed speed can be standardized.

Less spray mist can also reduce booth contamination and material loss.

These advantages depend on suitable conditions.
A roller coater can still produce defects if process conditions are poorly controlled.

4. Roller coaters make numerical film thickness control easier

A major characteristic of roller coaters is that the applied quantity is relatively easy to control.

The main conditions affecting film thickness are as follows.

• Coating viscosity
• Coating solids content
• Roll gap
• Roller pressure
• Roller rotation speed
• Workpiece feed speed
• Coating material supply rate
• Roller surface condition
• Absorption by the workpiece surface
• Number of coats

Controlling these factors can, in some cases, make it easier to achieve higher repeatability than with spray coating.

Consistent film thickness requires control of viscosity and solids content as well as roller conditions.

A change in viscosity changes the amount of coating material carried by the roller.
A change in solids content changes the film thickness remaining after drying.

Roller coating requires application conditions and material conditions to be controlled together.

5. Coating viscosity is important

Coating viscosity is extremely important in roller coating.

If viscosity is too low, the material flows more readily and may cause insufficient film thickness, sagging, edge pooling, or uneven application.
If viscosity is too high, supply to the roller becomes unstable, making streaks, roll marks, and a rough finish more likely.

Check the following items.

• Application viscosity
• Dilution ratio
• Viscosity changes with temperature
• Coating solids content
• Leveling
• Foaming of the coating material
• Drying rate
• Wetting of the roller

In hot environments such as Vietnam, coating viscosity may change readily.
Evaporation of solvent or water during operation can also change viscosity over time.

In roller coating, check viscosity changes during operation as well as viscosity at the start of work.

6. Roll gap and pressure determine film thickness

The gap and pressure between rollers have a major effect on the applied quantity.

An excessively wide roll gap tends to apply too much coating material.
An excessively narrow gap may result in insufficient material or streaks.

Excessive roller pressure may squeeze material out, causing uneven film thickness or edge pooling.
Insufficient pressure may prevent adequate transfer, causing skips or uncoated areas.

Check the following items.

• Roll gap
• Roller pressure
• Roller hardness
• Roller surface material
• Roller wear
• Roller eccentricity
• Variation in workpiece thickness
• Relationship with feed speed

Roller conditions are important machine settings that determine film thickness.
They must be checked according to product thickness, coating viscosity, and required film thickness, rather than treated as a one-time setup.

7. Changes in feed speed also change film thickness

A roller coater applies coating while conveying the workpiece at a constant speed.
This feed speed also affects film thickness.

If feed speed is too high, insufficient material may transfer, causing inadequate film thickness or skips.
If it is too low, excessive material may be applied, causing an overly thick film, unevenness, or inadequate drying.

Consider feed speed together with the following conditions.

• Coating viscosity
• Roller rotation speed
• Roll gap
• Required film thickness
• Drying oven capacity
• Workpiece absorption
• Production cycle time

Increasing feed speed to improve productivity can change film thickness or appearance.
Conversely, reducing speed to achieve film thickness affects drying conditions and production capacity.

Set roller coating conditions with a balance between quality and productivity.

8. Roll marks and streaks are common defects

Roll marks and streaks are among the common defects in roller coating.

Roll marks are traces left by the roller on the film surface.
Streaks are linear variations along the application direction.

The main causes are as follows.

• Excessively high coating viscosity
• Insufficient leveling
• Worn roller surface
• Foreign matter on the roller
• Incorrect roll gap
• Excessively high feed speed
• Unstable coating material supply
• Excessively fast drying
• Contamination on the workpiece surface

Roll marks and streaks are not caused by the coating material alone.
They result from a combination of roller condition, machine settings, viscosity, drying rate, and workpiece surface condition.

When defects occur, check the rollers and process conditions before changing the coating material.

9. Bubbles also require attention

Air bubbles can enter the coating material in a roller coater.
Bubbles remaining in the film can lead to pinholes, particle defects, or other appearance defects.

Common causes of bubbles include the following.

• Excessively vigorous stirring
• Air entrainment from roller rotation
• High coating viscosity
• Insufficient defoaming performance
• Bubbles returning through material circulation
• Bubbles forming on the roller surface
• Air remaining on the workpiece surface

Bubble prevention requires checks of stirring, material supply, and roller conditions as well as defoamers.

Foaming can be particularly problematic with waterborne coatings.
Waterborne roller coating requires a balance between defoaming performance, drying characteristics, and leveling.

10. Workpiece surface condition affects the finish

The workpiece surface condition has a major effect on the roller-coated finish.

Spray coating can sometimes reach around minor surface contours.
In roller coating, however, the roller contacts the surface, so contours or foreign matter can directly cause defects.

Check the following items.

• Surface smoothness
• Oil contamination
• Dust
• Moisture
• Scratches
• Surface contours
• Substrate absorption
• Surface preparation condition
• Warping
• Thickness variation

Workpiece warping or thickness variation changes roller contact and can cause uneven film thickness.

In roller coating, workpiece dimensional accuracy and surface condition are important alongside the coating material and machinery.

11. Drying conditions also determine appearance

A film applied by a roller coater is completed through drying or baking.

Excessively fast drying may set the film before it levels, leaving roll marks or streaks.
Excessively slow drying may cause dirt contamination, sagging, pooling, or blocking.

Check the following items.

• Flash-off time
• Drying oven temperature
• Actual product temperature
• Air velocity
• Humidity
• Drying time
• Conveyor speed
• Film leveling time
• Coating drying rate

A roller-applied film may need time to level naturally immediately after application.
Rushing drying makes roll marks or surface texture more likely to remain.

Application and drying conditions must be designed together.

12. Roller maintenance is quality control

The rollers themselves determine quality in roller coating.

Poor roller condition causes defects even when the coating material is normal.

Check the following items.

• Scratches on the roller surface
• Wear
• Contamination
• Adhering foreign matter
• Changes in hardness
• Eccentricity
• Swelling
• Resistance to solvents and water
• Cleanliness
• Replacement interval

Small scratches or foreign matter on the roller surface can appear as continuous streaks in the film.
Roller wear changes the applied quantity.
Roller swelling changes the gap and contact conditions.

Rollers are consumable components and quality-critical equipment.
They need regular inspections and replacement criteria.

13. Common problems in roller coating

Roller coating is susceptible to the following problems.

Investigate these issues through the rollers, machine settings, workpiece, and drying conditions as well as the coating material.

14. A practical shop-floor checking sequence

If defects occur in roller coating, the following checking sequence helps organize the possible causes.

1. Identify the form of the defect

Is it a streak?
A roll mark?
Uneven film thickness?
Bubbles?
Uneven gloss?
First, distinguish the form of the defect.

2. Check the direction in which it occurs

Does it run along the conveying direction?
Does it appear across the width?
Does it occur at a specific position?
Does it occur continuously?

Continuous streaks in the same position may indicate roller scratches or foreign matter.

3. Check coating viscosity

Check whether viscosity is within the standard range
and whether it changes during operation.

4. Examine roller condition

Check for scratches, wear, contamination, foreign matter, swelling, and cleanliness.

5. Check roll gap and pressure

Check that the settings suit product thickness and required film thickness.

6. Check feed speed

Check whether changes in speed are related to the occurrence of defects.

7. Examine the workpiece surface

Check oil contamination, dust, warping, thickness variation, and absorption.

8. Check drying conditions

If roll marks remain, check whether drying is too fast or leveling time is insufficient.

Examining the form and direction of a roller coating defect helps narrow down its cause.

15. Directions for improvement

The following approaches are important for stabilizing roller coating.

Standardize coating viscosity

Control application viscosity, dilution ratio, and temperature changes.

Control the roll gap

Set it according to product thickness, required film thickness, and material conditions.

Standardize feed speed

Set the speed according to film thickness and drying capacity.

Inspect rollers

Regularly check for scratches, wear, contamination, swelling, and eccentricity.

Prepare the workpiece surface

Check oil contamination, dust, moisture, warping, and thickness variation.

Review drying conditions

If roll marks or streaks remain, check leveling time and drying rate.

Address bubbles

Check stirring, material supply, roller conditions, and defoaming performance.

Create process conditions sheets

Organize viscosity, gap, pressure, speed, and drying conditions for each product.

16. When to consider a roller coater

Even a factory currently using spray coating should consider a roller coater if the following conditions apply.

• Flat materials are coated in large quantities
• Coating material loss is high
• Film thickness varies greatly between operators
• Spray mist causes substantial booth contamination
• Uniform appearance is desired
• A continuous coating process is desired
• Operator workload needs to be reduced
• The coated surface is almost flat
• A drying oven can be combined with continuous conveying

However, testing on actual products before installation is important.

Roller coating excels with flat materials, but defects will occur if the substrate, surface condition, coating material, film thickness, and drying conditions are unsuitable.

Before installing the equipment, conduct application trials and verify film thickness, appearance, and drying.

17. How CoatureVietnam can help

CoatureVietnam evaluates roller coaters as a way to improve flat-material coating processes, rather than simply as equipment.

We can provide the following support.

Applicability assessment

We assess suitability from product geometry, production volume, required film thickness, and appearance requirements.

Coating material suitability checks

We check viscosity, solids content, leveling, bubbles, and drying characteristics.

Support with process settings

We organize roll gap, pressure, feed speed, material supply rate, and drying conditions.

Isolation of defect causes

We investigate streaks, roll marks, uneven film thickness, bubbles, and uneven gloss from the 4M perspectives.

Equipment installation support

We consider the roller coater together with conveying, the drying oven, supply equipment, and ease of cleaning.

Work standardization

We document product-specific process conditions sheets, daily inspections, roller cleaning, and replacement criteria in SOPs.

Internal training

We explain roller coating principles and control points clearly to shop-floor teams.

18. Conclusion: Roller coating efficiently coats flat materials

Roller coating uses rollers to transfer coating material to flat substrates.
Instead of projecting material as a spray does, it applies the material directly through rollers, making it efficient for flat materials.

The advantages are as follows.

• Continuous coating of flat materials
• Easier control of consistent film thickness
• Easier reduction of material loss
• Easier reduction of operator dependence
• Reduced spray mist
• Ease of automation

There are also points that require attention.

• Limited suitability for contoured or three-dimensional shapes
• Viscosity control is important
• Roll gap and feed speed determine film thickness
• Roll marks or streaks may occur
• Roller scratches and wear directly affect quality
• Drying conditions affect appearance

A roller coater is not a universal replacement for spray coating, but one coating method suited to flat materials.

Coating the Future with Science and Care

Science means controlling product geometry, coating viscosity, roller conditions, feed speed, and drying conditions numerically.
Care means establishing a process that produces consistent, uniform films without relying on operators’ intuition or burdensome manual work.

CoatureVietnam evaluates roller coating as a process encompassing product geometry, materials, conveying, drying, and quality control, and supports improvements suited to the shop floor.

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