Controlling excess coating material

In coating, attention naturally tends to focus on how to apply the material.

Which coating material should be used?
Which gun should be used?
What film thickness should be applied?
Under what conditions should it be dried?

These are, of course, important questions.

However, the coating process has another important aspect.

That is how to remove excess coating material.

More coating material is not necessarily better.
Excessive application leads to sagging, pooling, uneven film thickness, inadequate drying, popping, parts sticking together, and higher costs.

What is needed is an appropriate film thickness that meets the required performance.
This requires a means of controlling excess material as well as applying it.

Centrifugal coating and wipe coating are methods related to this approach.

Centrifugal coating uses rotation to spin off excess material.
Wipe coating physically wipes off excess material, leaving a controlled amount behind.

Neither is simply a process for applying material.
Both adjust the applied material to the required quantity.

1. Applying too much coating material also causes defects

Discussions of coating defects sometimes focus on insufficient film thickness or missed areas.
However, excessive film thickness is also a defect.

Too much coating material causes the following problems.

• Sagging
• Pooling
• Excessive film thickness
• Inadequate drying
• Popping
• Pinholes
• Blistering
• Parts sticking together
• Film cracking
• Appearance defects
• Higher coating material costs
• Longer drying time

Excess material is particularly likely to remain after dip coating or batch coating of small parts.

On pipes, rods, wire, and cylindrical components, material can also collect locally or produce uneven film thickness.

Coating processes therefore need to consider how to remove excess material as well as how to apply it.

2. What is centrifugal coating?

Centrifugal coating uses rotational force to remove excess coating material.

Basically, the parts are coated and then rotated to spin the excess material outward.
This reduces pooling and adjusts film thickness.

The principle is similar to spinning a wet object to remove water.
However, coating requires more than simply spinning material off: the required film must remain while only the excess is controlled.

Centrifugal coating may be used for the following purposes.

• Removing excess material
• Making film thickness more uniform
• Preventing pooling
• Reducing sagging
• Processing small parts in batches
• Stabilizing material consumption
• Preventing inadequate drying

Rotation conditions have a major effect on centrifugal coating quality.

3. Products suited to centrifugal coating

Centrifugal coating does not suit every product.
It generally suits small parts or products from which excess material can readily be removed by rotation.

It may be considered for products such as the following.

• Small metal components
• Screws
• Bolts
• Nuts
• Washers
• Small stamped parts
• Small cast parts
• Parts processed together in batches
• Small parts prone to pooling
• Small components requiring corrosion-protection coatings

If parts readily contact one another, pay attention to scratches, impact marks, film chipping, and sticking.

Centrifugal coating may process small parts efficiently, but part geometry and appearance requirements must be checked carefully.

4. Rotation conditions determine film thickness in centrifugal coating

Rotational speed, rotation time, part loading, and coating viscosity are important in centrifugal coating.

If rotation is too weak, excess material will not be removed sufficiently.
This causes pooling, excessive film thickness, sagging, and inadequate drying.

Conversely, excessive rotation may remove too much material and leave insufficient film thickness.
It may also cause parts to collide forcefully, creating scratches or film chipping.

Check the following items.

• Rotational speed
• Rotation time
• Part loading quantity
• Part geometry
• Contact between parts
• Coating viscosity
• Coating solids content
• Quantity of coating material
• Draining time
• Timing of the start of drying
• Film thickness after rotation
• Appearance after rotation

Standardize rotation conditions by checking film thickness and appearance, rather than setting them by intuition.

5. Common problems in centrifugal coating

Centrifugal coating may present the following problems.

In centrifugal coating, the way parts move and contact each other affects quality as well as material conditions.

6. Confirm appearance requirements before adopting centrifugal coating

Centrifugal coating may process small parts efficiently.
However, parts may contact one another, making the method unsuitable for products with demanding appearance requirements.

Particular care is needed with the following products.

• Products with strict visible-surface requirements
• Products on which scratches are unacceptable
• Products for which gloss or surface texture is important
• Products on which film chipping is unacceptable
• Products subject to strict customer appearance inspection
• Components with visible surfaces after assembly

Before adopting centrifugal coating, confirm acceptable appearance limits.

The method may suit components coated for corrosion protection with relatively modest appearance requirements.
For decorative parts where appearance is important, another method may be more suitable.

When selecting a method, consider functional requirements and appearance requirements separately.

7. What is wipe coating?

Wipe coating removes excess coating material by wiping, leaving only the required amount on the surface.

Wiping means passing a coated workpiece through a controlled gap or wiping element to remove excess material.

For example, wire, rods, and tubular components may be coated and then passed through a wiping section to adjust film thickness.

Wipe coating serves the following purposes.

• Removing excess material
• Making film thickness more uniform
• Preventing pooling
• Stabilizing material consumption
• Improving the surface
• Supporting continuous production

In wipe coating, wiping conditions determine film thickness.

8. Products suited to wipe coating

The suitability of wipe coating depends on product geometry.

It generally suits relatively continuous products with a nearly constant cross-section.

Examples include the following.

• Wire
• Rods
• Pipes
• Tubes
• Cylindrical components
• Long, narrow components
• Continuous materials with a constant cross-section
• Components shaped to pass through a wiping section

For these products, applying material and then controlling thickness through a wiping section may be more practical than spray coating.

However, the method may be unsuitable for pronounced contours, complex shapes, or products that scratch easily.

9. Wiping-section conditions determine quality

The wiping-section conditions have a major effect on wipe coating quality.

Check the following items.

• Wiping-section gap
• Wiping-section material
• Wiping pressure
• Workpiece feed speed
• Coating viscosity
• Coating solids content
• Variation in workpiece outside diameter
• Surface roughness
• Wear of the wiping section
• Adhering foreign matter
• Cleanliness

An excessively wide gap leaves too much material, causing excessive film thickness or sagging.
An excessively narrow gap may remove too much material and leave insufficient film thickness.
Wear or foreign matter on the wiping section also causes streaks or scratches.

The wiping section is a quality-critical component in wipe coating.
It needs inspection and replacement criteria as a consumable component.

10. Common problems in wipe coating

Wipe coating may present the following problems.

Investigate these defects through the wiping-section machine settings, workpiece dimensions, feed speed, and drying conditions, as well as the coating material.

11. What centrifugal and wipe coating have in common

Centrifugal coating and wipe coating use different methods.

Centrifugal coating uses rotational force to spin off excess material.
Wipe coating physically removes excess material through a gap or wiping element.

However, they share an essential principle.

Both are processes that control excess material after application.

The following items are important to both.

• Coating viscosity
• Coating solids content
• Deposited quantity
• Conditions for removing excess material
• Film thickness measurement
• Drying conditions
• Product geometry
• Work standards
• Control of wear in equipment components

Stable coating quality requires control of both material application conditions and excess-material removal conditions.

12. Coating viscosity and solids content are important

Coating viscosity and solids content are important in both centrifugal and wipe coating.

Excessively low viscosity can cause too much material to be removed or leave insufficient film thickness.
Excessively high viscosity makes excess material more likely to remain, causing thick films or pooling.

High solids content leaves more film after drying.
Low solids content produces a thinner dry film even with the same deposited quantity.

Check the following items.

• Application viscosity
• Dilution ratio
• Solids content
• Coating material temperature
• Viscosity changes during operation
• Drying rate
• Leveling
• Sagging tendency

Material conditions can change during operation, particularly in mass production.
Viscosity may rise, solvent or water may evaporate, material temperature may change, or foreign matter may enter.

Film thickness and appearance will not be consistent unless these changes are controlled.

13. Drying conditions are also related to film thickness control

Removing excess material does not prevent defects if drying conditions are unsuitable.

If drying is too slow, material flows and causes sagging or pooling.
If it is too fast, only the surface may dry, leaving solvent or water inside.

For parts prone to excessive film thickness in particular, pay attention to the following defects.

• Popping
• Pinholes
• Blistering
• Inadequate drying
• Parts sticking together
• Surface wrinkling
• Insufficient internal curing

In centrifugal and wipe coating, the excess-material removal process and drying process must be designed together.

Check drying conditions especially in thick-film areas, areas prone to pooling, and locations where parts readily contact each other.

14. A practical shop-floor checking sequence

If defects occur in centrifugal or wipe coating, the following sequence helps organize the possible causes.

1. Identify the type of defect

Is film thickness insufficient?
Is it excessive?
Is there pooling?
Are there streaks?
Scratches?
Are parts sticking together?
First, distinguish the phenomenon.

2. Measure film thickness

Check thickness in normal areas, defective areas, edges, and locations prone to pooling.

3. Check coating viscosity

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

4. For centrifugal coating, examine rotation conditions

Check rotational speed, rotation time, part loading, and contact between parts.

5. For wipe coating, examine the wiping section

Check the gap, wear, foreign matter, material, and pressure.

6. Examine product geometry

Check whether the shape is prone to pooling
and whether air venting or drainage is needed.

7. Check drying conditions

Check whether thick-film areas dry adequately
and whether sticking or popping occurs.

8. Check work standards

Check whether loading quantity, rotation time, or feed speed differs between operators.

Methods that control excess material require materials, machinery, geometry, and drying to be considered together.

15. Directions for improvement

The following approaches are important for stabilizing centrifugal and wipe coating.

Standardize coating viscosity

Control application viscosity, dilution ratio, and temperature conditions.

Define film thickness measurement locations

Include locations prone to pooling or insufficient film thickness.

Standardize rotation conditions for centrifugal coating

Define rotational speed, rotation time, and part loading for each product.

Control the gap in wipe coating

Control the wiping-section gap, wear, and cleanliness.

Reduce contact between parts

Check loading quantity and part geometry to prevent scratches, film chipping, and sticking.

Review shapes prone to pooling

Consider fixture orientation, air venting, drainage, and drying orientation.

Review drying conditions

Verify conditions that adequately dry thick-film areas and pooled areas.

Create work standards

Prepare process conditions sheets and inspection sheets rather than relying on intuition.

16. Points to check when selecting a method

When considering centrifugal or wipe coating, check the following items.

1. Product geometry

Check whether the shape is suitable for rotation or wiping.

2. Appearance requirements

Check whether scratches or contact marks are acceptable.

3. Required film thickness

Confirm the target film thickness and the acceptable range of variation.

4. Production volume

Check whether quantities are sufficient to benefit from batch or continuous processing.

5. Coating material properties

Check whether viscosity, solids content, drying characteristics, and sagging tendency suit the method.

6. Drying equipment

Check whether thick-film and pooled areas can be dried adequately.

7. Quality inspection

Define how film thickness, appearance, adhesion, and corrosion protection will be checked.

8. Repair feasibility

Define repair and recoating methods in the event of defects.

Method selection must take account of quality assurance as well as efficiency.

17. How CoatureVietnam can help

CoatureVietnam evaluates centrifugal and wipe coating as means of improving the process by controlling excess material, rather than merely as specialized methods.

We can provide the following support.

Applicability assessment

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

Material condition checks

We check viscosity, solids content, drying characteristics, sagging tendency, and leveling.

Film thickness control planning

We organize measurement locations, acceptance limits, and recording methods.

Equipment condition checks

We check rotational speed, rotation time, wiping-section gap, feed speed, and equipment wear.

Isolation of defect causes

We investigate insufficient or excessive film thickness, pooling, streaks, scratches, and sticking from the 4M perspectives.

Work standardization

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

Internal training

We share with shop-floor teams the concept of controlling excess coating material as well as applying it.

18. Conclusion: Controlling excess material also determines coating quality

How coating material is applied is important in the coating process.
However, that alone is insufficient.

How should excess material be removed?
How can the required film thickness be retained?
How can pooling be prevented?
How can excessive film thickness and inadequate drying be prevented?

These are also important factors determining coating quality.

Centrifugal coating uses rotational force to spin off excess material.
Wipe coating removes excess material through a gap or wiping element.

Neither method is intended to apply more coating material.
Both adjust the material to an appropriate quantity.

The basic items to check are as follows.

• Coating viscosity
• Solids content
• Rotational speed
• Rotation time
• Wiping-section gap
• Feed speed
• Part geometry
• Pooling
• Film thickness
• Drying conditions
• Acceptable appearance limits

Coating the Future with Science and Care

Science means controlling excess material through rotation conditions, wiping conditions, viscosity, and film thickness rather than intuition.
Care means creating a process that consistently produces the required film thickness without forcing operators to make adjustments based on experience alone.

CoatureVietnam views coating as a process that establishes the required film thickness, as well as a task of applying material, and supports improvements suited to product geometry and production conditions.

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