Various inspections are performed in coating processes, including visual inspection, color checks, gloss checks, film thickness measurement, and adhesion checks.

Inspection is often considered a process for finding defective products.
Preventing their release is, of course, particularly important.

However, inspection in a coating process has a broader role.

It also provides information for correcting the process quickly.

For example, inspection reveals that dirt inclusions are increasing.
Sagging is beginning to occur.
Film thickness is becoming too low.
Color or gloss is starting to deviate from the standard.
A condition suggesting insufficient drying is found.

If this information returns immediately to the coating process, conditions can be corrected promptly.
If it returns slowly, however, the shop floor continues coating under the same conditions.

As a result, it keeps producing the same defect repeatedly.

A layout and operating system that promptly return inspection results to the process are particularly important for stable coating quality.

1. Inspection is more than a process for finding defects

Inspection has two broad roles.

The first is preventing defective products from being released.
This is the final check to avoid sending defective products to customers.

The second is detecting process abnormalities early.
This check prevents continued production of the same defect.

In coating processes, even a small change in conditions can cause defects to occur repeatedly.

Coating material viscosity changes.
Air pressure changes.
Gun distance changes.
Film thickness changes.
Airflow inside the booth changes.
Drying oven temperature changes.
Dirt begins to attach to workpieces before coating.
Cleaning after a color change is insufficient.

These changes initially appear as small defects.
Promptly returning inspection results to the process at this stage prevents the problem from expanding.

Inspection exists not only to stop defects at the exit but also to send information upstream for correction.

2. A distant inspection location delays information

If the inspection area is far from the coating process, information takes time to return.

The inspector finds a defect.
Records it.
Reports it to a manager.
The manager contacts the coating shop.
The shop floor checks the situation.
Conditions are changed.

The coating line may continue operating during this time.

The slower inspection results are communicated, the more defective products are made.

On continuous lines in particular, a single abnormality can create many defects in a short time.
If a defect is found only after products pass through the drying oven, many workpieces may already have been coated under the same conditions.

The inspection area’s location is therefore more than a space issue.
It is a quality control issue involving how quickly defect information can return to the process.

3. Without feedback, the same defect continues to be produced

Some coating defects tend to occur repeatedly.

Examples include the following.

• Contamination and dirt inclusions
• Sagging
• Cratering
• Uneven color
• Gloss defects
• Insufficient film thickness
• Excessive film thickness
• Insufficient drying
• Poor adhesion
• Scratches
• Foreign matter contamination

These defects do not necessarily end with a single product.

Poor gun conditions cause the same defect to continue.
Unsuitable viscosity causes the same appearance defect to continue.
Inadequate oven conditions cause the same curing defect to continue.
Supply air or dust problems cause the same contamination defect to continue.

Even if inspection finds a defect, the shop floor continues using the same conditions unless that information returns to the process.

Recording inspection results alone is therefore insufficient.
A system is needed to return them to the shop floor, check conditions, and, where necessary, stop, correct, and restart the process.

4. Consider inspection locations in terms of process distance and ease of feedback

Simply using an available space for inspection is risky.

The inspection location needs the following conditions.

• Sufficient brightness
• Easy assessment of color and gloss
• Safe placement of coated workpieces
• Inspection without undue operator strain
• Separation of good, defective, and rework products
• Easy return of information to the coating process
• Immediate shop-floor verification where necessary
• A clear return route for rework

A distant inspection location can sometimes work if information is transmitted quickly.
Conversely, a nearby location serves little purpose if no one informs the shop floor.

The important point is the feedback system as well as distance.

Who checks the inspection results?
Which defects require immediate communication?
What threshold triggers a line stop?
Who decides on condition changes?
Is verification performed again after the change?

These matters must be established in advance.

5. Even 100% inspection only increases the burden if it does not lead to process improvement

Coated products may undergo 100% inspection when appearance requirements are high.

This is effective for preventing defective product release.
However, performing 100% inspection does not by itself ensure confidence in quality.

If defects are found but information cannot be returned to the responsible process, the burden on inspectors simply increases.

Find the defect.
Separate the product.
Record it.
Repair it.
Reinspect it.

These tasks increase, but defects do not decrease unless the process changes.

Inspection is not performed solely to sort out defective products.
It is performed to reduce defects.

The more extensive the inspection, the more important a system for returning results to the process becomes.

6. Establish stop decisions when defects are found

In coating processes, it can be difficult to decide whether to stop the line immediately when a defect is found or monitor a certain number of products first.

However, leaving this decision entirely to the shop floor makes responses inconsistent.

One operator reports immediately.
Another waits to observe the situation.
An inspector considers the product defective, while the coating shop considers it acceptable.
The line continues while a manager’s confirmation is awaited.

These conditions make repeated defects difficult to prevent.

Decision criteria must be established in advance.

Examples include the following.

• How many consecutive products with the same defect require a report?
• Which defects require immediate stopping?
• Which defects require condition checks?
• Who decides to stop?
• Who approves restarting?
• How many products are checked after a condition change?
• Where are defective products isolated?

Stopping a coating line is a major production decision.
That is why criteria are necessary.

With criteria, the shop floor can act without uncertainty.

7. Separate locations for good, defective, and rework products

It is important to distinguish workpiece status clearly in the inspection area.

Good products.
Products awaiting a decision.
Defective products.
Repair products.
Products for recoating.
Products awaiting reinspection.

Mixing these increases quality risks.

Defective products mix with good products.
Products awaiting rework move onward as finished products.
Reinspection after repair is missed.
Products awaiting a decision are shipped.
The defect details become unclear.

These are common inspection process problems.

The inspection area needs separate locations by status.
It also needs identification.

Which workpieces are good?
Which are defective?
Which await rework?
Which await reinspection?

The status must be clear to anyone who looks.

Locations separated by status are necessary not only for inspection accuracy but also to reduce shop-floor uncertainty.

8. Design the return route for rework

When defective products occur, repairs or recoating may be performed.

The rework return route is important here.

Where are defective products placed?
Who decides whether repair is possible?
Where is repair performed?
Does recoating return to the regular line?
Is a dedicated repair booth used?
Which oven is used for drying again?
Where is reinspection performed?

If these matters are not established, rework disrupts the flow of regular products.

Rework products are placed in aisles.
They are inserted into regular production.
Coating materials must be prepared urgently.
The oven loading sequence is disrupted.
Inspectors lose track of products requiring reinspection.

Rework follows a different movement pattern from regular products.
It therefore needs a dedicated flow and identification.

Preventing repeated defects requires not only handling the defects produced but also a system that prevents rework from disrupting the shop floor.

9. Make inspection information visible

Making information visible is effective for returning inspection results to the process.

Defect details.
Number of occurrences.
Time of occurrence.
Affected lot.
Process where it occurred.
Coating material used.
Operator.
Coating conditions.
Drying conditions.
Action taken.
Whether it recurred.

Making this information visible helps the shop floor recognize abnormalities.

Merely keeping inspection records may not readily lead to improvement.
What matters is presenting the information so the shop floor can immediately examine it and decide.

Methods include the following.

• Defect occurrence boards
• Shop-floor notices
• Daily defect charts
• Sharing defect photographs
• Checking conditions through QR codes
• Immediate sharing of inspection results
• Aggregation by line, color, and product
• Sharing at morning meetings
• Alerts for abnormalities

It is insufficient for inspection information to remain only within the quality assurance department.
It must return as information the coating shop can use.

10. The inspection environment also affects quality

The inspection environment is also important for correctly returning results to the process.

The inspection area is dark.
The lighting color is unsuitable.
Reflections make the surface difficult to see.
The inspection table is narrow.
Workpieces cannot be placed securely.
Inspectors use strained postures.
The location is dusty.
Heat or noise is excessive.
Good and defective product locations are too close.

Inspection results vary in such environments.

Lighting conditions are particularly important in visual coating inspection.
Contamination, dirt inclusions, sagging, gloss, and color unevenness look different under different lighting.

An inspection area should not simply occupy an available space.
It must be designed as an environment where sound judgments can be made.

If inspection accuracy is unstable, feedback to the process is also unstable.

11. CoatureVietnam’s inspection feedback checkpoints

When examining inspection processes, CoatureVietnam checks how results return to the process as well as inspection itself.

Inspection location

Check distance from the coating process, brightness, storage areas, working posture, and separation of good and defective products.

Information communication

Check who communicates defects, to whom, and when.

Stop criteria

Check which defects require the line to stop and who decides.

Cause investigation

Check whether the defect relates to coating materials, application conditions, drying conditions, surface preparation, dust, or working methods.

Rework routes

Check the flow and locations of repair products, recoating products, and products for reinspection.

Visibility

Check whether occurrence counts, locations, times, and recurrence are visible on the shop floor.

Inspection leads to improvement only when its results return to the process.

Conclusion

Inspection is more than a process for finding defective products.

It detects process abnormalities early, returns information to the shop floor, and prevents continued production of the same defect.

The inspection location is distant.
Communication is slow.
Stop decisions are unclear.
Good, defective, and rework products are mixed together.
The rework return route is undefined.
Inspection information is not visible on the shop floor.

Under these conditions, repeated defects are difficult to prevent even when inspection is performed.

Promptly returning inspection results requires both layout and operation.

Position the inspection area appropriately.
Establish communication rules.
Establish stop decision criteria.
Create locations by status.
Design rework routes.
Make inspection information visible.

These turn inspection from a simple sorting process into a starting point for process improvement.

CoatureVietnam does not consider inspection solely the work of the quality assurance department.
It sees inspection as an information function linked to the coating shop for prompt process correction.

Coating the Future with Science and Care
Promptly returning inspection results to the process is an important improvement that reduces defects, rework, and the burden on people working on the shop floor.