In powder coating, powder may deposit neatly on external surfaces while failing to enter recesses or internal areas readily.

Examples include the following defects.

The coating inside a box-shaped part is thin. Powder does not reach deep inside a hole. Only recesses have insufficient film thickness. External surfaces and edges are thickly coated, but deep areas remain uncoated. Increasing voltage does not improve thickness in recesses.

Operators may interpret these situations in the following ways.

“Is the operator’s spraying technique poor?” “Is the powder paint at fault?” “Should we discharge more powder?” “Will higher voltage make the powder reach deeper?”

However, the causes of poor powder penetration into recesses are not that simple.

Powder coating uses electrostatic attraction to deposit powder on the workpiece. How this force acts changes with workpiece shape.

The electric field may have difficulty reaching recesses, box-shaped sections, internal surfaces, and deep areas. This phenomenon is called the Faraday cage effect.

1. What is the Faraday cage effect?

The Faraday cage effect is the difficulty of the electric field reaching deep into recesses or enclosed areas.

Powder coating charges powder particles and attracts them to a grounded workpiece. The electric field tends to concentrate on external surfaces and edges.

It is weaker inside recesses and internal areas.

This produces the following phenomena.

• Good powder deposition on external surfaces
• A tendency for thick deposits at edges
• Poor powder penetration deep into recesses
• Insufficient internal film thickness
• Uncoated deep areas
• Large film thickness differences between external and internal surfaces

Poor penetration is therefore not simply a matter of operator skill. It is a process characteristic arising from electrostatic application.

2. Why does powder have difficulty entering recesses?

In powder coating, charged powder particles are attracted toward the grounded workpiece.

However, the electric field is not uniform around complex shapes.

The field tends to concentrate at corners and projections. Powder therefore tends to deposit first on edges and external surfaces.

The field is weaker deep inside recesses, making it harder for powder to penetrate.

Simply forcing powder inward with air is not necessarily a solution either. Airflow transports powder, but electrostatic attraction ultimately deposits it on the workpiece. If the field is concentrated externally, penetration into recesses is difficult.

Poor recess coverage therefore requires consideration of both airflow and electrostatic forces.

3. Higher voltage does not necessarily improve coverage

Operators may be tempted to increase voltage when powder fails to enter recesses.

However, increasing voltage does not guarantee improvement.

Excessive voltage instead makes powder concentrate even more on external surfaces and edges. The outside can become thickly coated while recess penetration remains poor.

Excessive thickness can also cause back-ionization and surface roughness.

Excessive voltage is likely to cause the following problems.

• Thick films at edges
• Larger thickness differences between external and internal surfaces
• Worsening recess coverage
• Increased powder waste
• Surface roughness
• Back-ionization
• Appearance defects after baking

Increasing voltage is therefore not necessarily the correct response to poor recess coverage.

What is needed is to create conditions that allow powder to enter recesses more readily, rather than simply increasing voltage.

4. Increasing powder output may only thicken the outside

Increasing powder output is another common response to poor recess penetration.

However, this also requires care.

Unless conditions favor entry into the recess, increased powder output may simply deposit more powder on external surfaces and edges.

This can cause the following problems.

• Only external surfaces become thickly coated
• Recesses remain thinly coated
• Powder consumption increases
• Recovered powder increases
• Surface roughness develops
• Appearance defects develop during baking
• Costs increase

Before increasing output, determine why powder does not enter the recess.

Is powder not being transported deep enough? Does the electric field fail to penetrate the recess? Is the gun angle unsuitable? Is the hanging arrangement poor? Is grounding insufficient?

Increasing only the powder quantity without investigating these points can worsen defects in another form.

5. Product shape strongly affects recess coverage

The Faraday cage effect is strongly affected by workpiece shape.

The following shapes require particular attention.

• Box-shaped sections
• Deep recesses
• Hole interiors
• U-shaped sections
• Pipe interiors
• Bent sections
• Pocket-shaped structures
• The interiors of welded frames
• Narrow gaps
• Complex bent products

With these shapes, powder may deposit readily on the outside but fail to enter internal areas.

In deep recesses, in particular, powder may attach near the opening without reaching the back. Thick deposition at the opening’s edges can further hinder penetration.

During product and fixture design, it is important to consider whether the shape allows powder to enter readily.

Shapes that are difficult to coat may be hard to address later through operator skill alone.

6. Gun angle and coating sequence are important

Gun direction and coating sequence are important for improving recess coverage.

Applying a thick external coat first can hinder later entry into recesses. Targeting recesses and internal areas first can therefore be more effective.

Check the following points.

• Are recesses coated first?
• Is the gun angled toward the back of the recess?
• Are external surfaces being coated too thickly first?
• Can product orientation be changed during coating?
• Is there a procedure for corrective spraying?
• Does the coating sequence differ between operators?

Coating sequence affects powder film thickness distribution.

For products with recesses, standardize which areas are targeted and in what order, as well as coating flat surfaces uniformly.

7. Check gun distance and airflow as well

Airflow transports powder, so gun distance and airflow also affect recess coverage.

If the gun is too close, powder tends to concentrate on the outside. The electrical effect is also stronger and can hinder entry into recesses.

If the gun is too far away, powder may not reach the workpiece sufficiently, causing insufficient thickness or reduced transfer efficiency.

Similar considerations apply to airflow.

Excessive airflow can cause powder blowback or external dispersion. Insufficient airflow cannot carry powder deep into recesses.

Check the following items.

• Gun distance
• Powder conveying air
• Pattern air
• Output
• Gun angle
• Powder penetration into recesses
• Powder blowback
• Booth airflow

Poor recess coverage requires checks on powder transport by air as well as voltage.

8. Poor grounding also causes poor recess coverage

Workpiece grounding is important in powder coating. Insufficient grounding prevents stable deposition.

Poor grounding causes the following problems.

• Poor powder deposition
• Film thickness variation
• Localized failure to deposit powder
• Increased powder dispersion
• Unstable electrostatic effects
• Operators feel a need to increase voltage or output

Increasing voltage or output while grounding is poor does not address the root cause.

Check the following items.

• Is coating accumulating on fixtures?
• Are hanger contact points cleaned?
• Is reliable contact maintained between the workpiece and fixture?
• Is grounding resistance checked?
• Are fixtures rusty?
• Does the hanging arrangement make contact unstable?

When recess coverage is poor, always check grounding as well as gun settings.

9. The difference between corona and tribo guns also matters

Corona guns and tribo guns are typical powder coating gun types.

Corona guns charge powder using high voltage. They are common and easy to use, but recesses can be particularly susceptible to the Faraday cage effect.

Tribo guns, by contrast, charge powder through friction. They are comparatively less affected by a strong external electric field, so powder can enter recesses more easily in some cases.

However, switching to a tribo gun does not guarantee improvement. Powder compatibility, flowability, moisture absorption, and cleaning inside the gun are important.

When poor recess coverage is frequent, check the following points.

• What gun type is currently used?
• Is the voltage setting too high?
• Is there scope to use a tribo gun?
• Is the powder suitable for triboelectric charging?
• Has film thickness distribution been checked in trial coating?

Gun type should be selected according to product shape and quality requirements.

10. Measure film thickness in recesses, on internal surfaces, and at edges

Measuring only external flat surfaces can miss poor recess coverage.

Check the following locations.

• External flat surfaces
• Edges
• Recess openings
• Deep inside recesses
• Internal surfaces
• Areas around holes
• Welds
• Areas around fixture contacts

The Faraday cage effect produces a thickness distribution with thicker external films and thinner internal films. Average thickness alone is therefore insufficient for assessment.

Assess film thickness through the following questions.

• How large is the difference between external and internal surfaces?
• Are only the edges excessively thick?
• Is the required thickness present deep inside recesses?
• Is the thin area a corrosion risk?
• Is the difference acceptable against customer requirements?

Powder coating quality assurance requires measuring areas prone to problems rather than simply areas that are easy to measure.

11. Problems likely to arise from poor recess coverage

Poor recess coverage can lead to the following problems.

Poor recess coverage is more than an appearance problem. It affects corrosion protection, durability, inspection workload, and recoating costs.

12. A practical checking sequence

When powder fails to enter recesses, the following sequence makes causes easier to identify.

1. Identify the affected locations

Which recesses are affected? Only internal surfaces? Deep inside holes? Corners in a box-shaped section? Only particular products?

First, identify the affected locations.

2. Measure film thickness

Measure external surfaces, edges, recess openings, and deep inside recesses. Thickness differences are important.

3. Check voltage

Check whether voltage is excessive. High voltage can worsen poor recess coverage.

4. Check powder output

Check whether excessive output is producing thick films only on external surfaces.

5. Examine gun angle and sequence

Are recesses targeted first? Is the gun angled toward their back? Check whether the working sequence is standardized.

6. Examine airflow

Check whether the air conditions suitably transport powder into deep areas.

7. Check grounding

Check that grounding through fixtures, hangers, and the workpiece is satisfactory.

8. Consider the gun type

If improvement is difficult with a corona gun, consider a tribo gun or changes in operating conditions.

Poor recess coverage is investigated through a combination of conditions rather than a single setting.

13. Directions for improvement

The following approaches can improve recess coverage in powder coating.

Optimize voltage

Reducing excessive voltage can improve penetration into recesses.

Review powder output

Prevent external thick films caused by excessive output.

Coat recesses first

Target recesses before applying thick films externally.

Change the gun angle

Spray from a direction that carries powder into the recess.

Adjust airflow

Review the air conditions used to transport powder into recesses.

Change product hanging orientation

Consider whether the product can be oriented to make powder entry easier.

Improve grounding

Clean fixtures, verify contacts, and control grounding resistance.

Consider a tribo gun

Consider it as a measure for poor recess coverage when compatible with product shape and the powder.

Consider coatability during design

Deep recesses, pocket-shaped structures, and narrow gaps readily cause coating defects, so account for them during product design.

14. How CoatureVietnam can help

CoatureVietnam does not view poor recess coverage as a powder paint problem alone. We assess product shape, gun type, voltage, grounding, work conditions, and baking together.

We can provide the following support.

Distinguishing the causes of poor recess coverage

We check the Faraday cage effect, voltage, powder output, grounding, and gun angle.

Planning film thickness measurement locations

We set measurement locations in recesses, on internal surfaces, at edges, and around holes as well as externally.

Standardizing application conditions

We document voltage, output, gun distance, gun angle, and coating sequence in condition sheets.

Evaluating gun types

We assess corona and tribo gun suitability according to product shape and powder compatibility.

Reviewing fixtures and hanging arrangements

We assess fixtures for grounding, spraying direction, and ease of powder penetration into recesses.

Operator training

We explain the Faraday cage effect in terms that production teams can easily understand.

15. Summary: Poor recess coverage is not solely a matter of operator skill

Operator skill alone does not explain why powder fails to enter recesses.

The electric field tends to concentrate on external surfaces and edges and can have difficulty reaching recesses and deep areas. This is the Faraday cage effect.

Simply increasing voltage or output may only thicken external surfaces without improving recess coverage. It can instead cause thick films, back-ionization, surface roughness, and increased costs.

Improving recess coverage requires the following checks.

• Product shape
• Voltage
• Powder output
• Gun distance
• Gun angle
• Airflow
• Grounding
• Coating sequence
• Gun type
• Film thickness measurement locations

Coating the Future with Science and Care.

Science means distinguishing causes through electrostatics, shape, thickness distribution, grounding, and work conditions rather than judging by intuition. Care means establishing a process that coats recesses consistently without forcing operators to make repeated corrective passes.

CoatureVietnam assesses poor recess coverage through product shape, gun type, equipment, fixtures, and work conditions as well as the powder paint, and supports improvements suited to daily operations.

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