Mass-production coating beyond spraying

Many people think of spray coating when they hear the word coating.

Spray paint with a coating gun.
Operators apply paint while observing the product.
Use automatic guns or robots.
Coat inside a booth and dry in an oven.

This is a common method in industrial coating.

However, spraying is not the only application system.

Considering product shape, production volume, thickness and appearance requirements, operator workload, and paint loss, methods other than spraying may be effective.

Typical examples include the following systems.

• Dip coating
• Rotational coating
• Curtain flow coating

Instead of atomizing and spraying paint, these systems coat by dipping products into paint, rotating them, or passing them through a sheet of flowing paint.

They do not suit every product.
However, when conditions align, they may coat more efficiently and consistently than spraying, with less operator workload.

Rather than continuing to spray simply because it has always been done, what matters is selecting an application system suited to product shape and production conditions.

1. Spray coating is not always the best option

Spray coating is a highly versatile application system.
It readily accommodates complex shapes and suits high-mix, low-volume production.

However, spray coating also presents the following issues.

• Paint mist disperses
• Overspray occurs
• Paint losses are likely
• Differences in operator skill readily affect results
• Film thickness variation is likely
• Booth cleaning is required
• Working postures place a heavy burden on operators
• Odor and solvent exposure must be controlled

Spraying is not necessarily the most rational option, particularly for coating simple-shaped or small parts in large quantities.

Depending on the product, considering methods such as dipping, rotation, and curtain flow may simplify the process.

When selecting an application system, start with the following question.

Does this product really need to be spray-coated?

2. What is dip coating?

Dip coating forms a coating film by immersing a substrate in a paint tank and withdrawing it.

It is also referred to as dip coating in English.

The basic sequence is as follows.

1. Prepare the substrate surface
2. Immerse it in the paint tank
3. Hold for a defined period
4. Withdraw
5. Drain off excess paint
6. Dry or bake
7. Inspect

Dip coating immerses the entire substrate in paint.
Paint may therefore reach inner and back surfaces that are difficult to spray.

Because parts can readily be processed together, mass-production capability may improve for small or simple-shaped parts.

3. Advantages of dip coating

Dip coating has the following advantages.

• Paint readily covers the entire surface
• Paint readily reaches inner and back surfaces
• Small parts can easily be processed together
• Less paint mist than spraying
• Less dependence on operator gun handling
• The coating process may be simplified
• It may suit mass-production processing
• Coating equipment may be relatively simple

Dip coating is particularly effective when multiple small parts need to be processed with consistent quality.

For simple-shaped products that drain readily, film thickness and appearance may also be easier to stabilize.

However, dip coating has its own points requiring attention.

4. Watch for pooling and sagging in dip coating

Drainage is important in dip coating.

After the substrate is withdrawn from the paint tank, how excess paint drains changes film thickness and appearance.

Poor drainage causes the following defects.

• Sagging
• Pooling
• Thick film
• Uneven film thickness
• Drying defects
• Popping
• Pinholes
• Sticking
• Appearance defects

Paint particularly tends to collect in recesses, holes, closed-pocket structures, horizontal surfaces, and corners.

In dip coating, draining excess paint after withdrawal is as important as immersion itself.

Items to check are as follows.

• Withdrawal speed
• Drainage time
• Substrate orientation
• Hanging arrangement
• Paint viscosity
• Solids content
• Drying rate
• Presence of recesses and holes
• Locations prone to pooling
• Timing of entry into the drying oven

Fixture design to prevent pooling is also important in dip coating.

5. Watch for air pockets as well

Dip coating immerses the substrate in a paint tank.
However, depending on product shape, air may remain inside.

This trapped air is called an air pocket.

Paint cannot enter an air pocket.
This leads to missed coating, insufficient thickness, and inadequate rust prevention.

Shapes prone to air pockets are as follows.

• Closed-pocket parts
• Box shapes
• Tubular parts
• Structures with few openings
• Deep recesses
• Interiors from which air escapes poorly
• Shapes with many nearly horizontal surfaces

Possible measures include the following.

• Change the hanging angle
• Change the immersion direction
• Provide air vent holes
• Provide drainage holes
• Adjust immersion time
• Add oscillation
• Improve the withdrawal direction

Dip coating requires consideration of both the path for paint to enter and the path for air to escape.

6. What is rotational coating?

Rotational coating applies paint while rotating the substrate or a container.

It is sometimes used for processing small parts together or spinning off excess paint.

The idea is to spread paint uniformly by moving the product rather than coating it while stationary.

Examples of its purposes include the following.

• Coat small parts together
• Distribute paint over the entire product
• Remove excess paint
• Bring film thickness closer to uniformity
• Reduce manual work by operators
• Improve mass-production capability

Rotational coating has various configurations depending on shape and application.
The important point is that rotation controls paint deposition and drainage.

7. Advantages of rotational coating

The advantages of rotational coating are as follows.

• Small parts can easily be processed together
• Manual operator work is easier to reduce
• Excess paint is easier to remove
• Film thickness variation may be easier to limit
• Production efficiency is easier to improve
• It may accommodate shapes that are difficult to spray
• The process may be easier to automate

Rotational coating is particularly worth considering where small parts are spray-coated individually.

However, parts may contact one another during rotational coating.
Scratches, sticking, film chipping, and contact marks therefore require attention.

8. Rotation conditions determine quality in rotational coating

Rotational speed and duration have a major effect on quality.

Insufficient rotation leaves paint poorly distributed, allowing pooling and uneven thickness to remain.
Excessive rotation may throw off too much paint, leave the film too thin, or damage parts.

Items to check are as follows.

• Rotational speed
• Rotation duration
• Rotation direction
• Quantity of parts loaded
• Contact between parts
• Paint viscosity
• Paint quantity
• Drainage time
• Timing of drying startup
• Part shape

Paint and machine conditions must be controlled together in rotational coating.

It is not simply a process in which rotation automatically produces uniformity.
Both excessive and insufficient rotation cause defects.

9. What is curtain flow coating?

Curtain flow coating creates a curtain of flowing paint and coats substrates as they pass through this sheet of paint.

Imagine a uniform sheet of paint flowing downward, with flat material passing beneath it.

This system is effective for continuously coating flat sheet materials and sheet-shaped products.

The basic sequence is as follows.

1. Supply paint at a constant flow rate
2. Let paint fall as a curtain
3. Pass the substrate through at a constant speed
4. Deposit paint uniformly on the surface
5. Recover excess paint
6. Dry or bake

Curtain flow coating produces less mist dispersion than spraying and may continuously coat flat materials at high speed.

10. Advantages of curtain flow coating

Curtain flow coating has the following advantages.

• Flat materials can be coated continuously
• Application quantity is easy to control
• Little spray mist
• Paint loss is easier to limit
• It may suit high-speed coating
• Uniform surface coverage is easier to achieve
• Dependence on operators is easier to reduce
• It is easy to integrate into an automated line

It is particularly effective for sheets and panels requiring uniform application across their width.

However, stable paint flow is important in curtain flow coating.
Disturbances in the paint curtain directly cause uneven thickness or streaks.

11. Paint viscosity and flow rate are important in curtain flow coating

Curtain flow coating requires a stable sheet of flowing paint.

Paint viscosity and flow rate are therefore extremely important.

Excessively low viscosity may destabilize the paint curtain or cause excessive flow.
Excessively high viscosity may make flow unstable and cause streaks or uneven thickness.

Items to check are as follows.

• Paint viscosity
• Solids content
• Flow rate
• Paint temperature
• Supply pressure
• Curtain width
• Substrate feed speed
• Bubbles in the paint
• Foreign matter
• Condition of recovered paint

In curtain flow coating, paint flow itself determines quality.
The paint supply system, filtration, bubbles, and viscosity variation must therefore be controlled.

12. Common defects in curtain flow coating

The following defects may occur in curtain flow coating.

These defects relate to flow rate, viscosity, feed speed, filtration, and drying conditions, as well as paint.

Maintaining a stable paint curtain is central to quality control in curtain flow coating.

13. Comparing the 3 systems

Dipping, rotation, and curtain flow compare as follows.

The best system changes with product shape and production conditions.

The system name is not what matters.
What matters is which system can produce the product’s required quality most consistently and without excessive strain.

14. A practical shop-floor checking sequence

When considering an application system other than spraying, checking in the following order makes the issues easier to organize.

1. Check product shape

Is it flat?
A small part?
A box shape?
Does it have an interior?
Is it prone to pooling?

First establish the shape.

2. Check production volume

Is it high-mix, low-volume production?
High-volume continuous production?
Check the batch size.

3. Check required film thickness

Is the film thin?
Thick?
Check the allowable thickness variation.

4. Check appearance requirements

Is a high-quality appearance required?
Check whether streaks, sagging, pooling, and roll marks are acceptable.

5. Check paint properties

Check viscosity, solids content, drying characteristics, bubbles, and leveling.

6. Check drying conditions

Check whether the drying oven and flash-off conditions can accommodate thick films.

7. Check fixtures and transport

Check hanging arrangements, rotation conditions, feed speed, and product orientation.

8. Define quality risks

Check pooling, air pockets, contact damage, streaks, uneven thickness, and other risks.

System selection therefore requires considering products, equipment, work, and drying together with paint.

15. When to consider systems other than spraying

Alternative application systems are worth considering in the following cases.

• Large quantities of small parts are being coated
• Flat materials are being coated continuously
• Paint losses from spraying are high
• Operator workload is high
• Film thickness variation is large
• Coating booth cleaning workload is high
• The coating cycle needs to be shortened
• Many mass-produced products share the same shape and specification
• Automation and reduced staffing requirements are desired
• The working environment needs improvement

However, changing systems requires equipment investment, trial coating, and quality approval.

The first step is therefore to clarify current process issues and define which issues the system change is intended to resolve.

16. How CoatureVietnam can help

CoatureVietnam supports consideration of application systems suited to product shape and production volume, beyond spray coating alone.

We can provide the following support.

Checking application system suitability

We assess dipping, rotation, and curtain flow against product shape, production volume, film thickness, and appearance requirements.

Comparison with current spray coating

We compare paint losses, labor, film thickness variation, equipment load, and the working environment.

Defining defect risks

We define system-specific risks such as pooling, air pockets, contact damage, streaks, and uneven thickness.

Designing trial coating

We plan trial conditions on actual products, film thickness measurements, appearance checks, and drying checks.

Supporting equipment introduction

We consider paint tanks, rotation equipment, curtain flow equipment, transport, drying ovens, and fixtures together.

Operating standardization

We create SOPs for withdrawal speed, rotational speed, feed speed, paint viscosity, and drying conditions.

In-house training

We explain the features of application systems other than spraying clearly to sales, technical, and shop-floor personnel.

17. Summary: consider application systems through product shape and production volume

Spraying is not the only application system.

Dip coating immerses products in a paint tank to coat the entire surface.
Rotational coating controls paint deposition and drainage by rotating products or a container.
Curtain flow coating continuously coats flat materials by passing them through a sheet of flowing paint.

When conditions align, these systems may be more efficient than spraying.

Each also has its own points requiring attention.

• Dip coating: pooling, sagging, and air pockets
• Rotational coating: contact damage, rotation conditions, and uneven thickness
• Curtain flow coating: viscosity, flow rate, and curtain stability

Application system selection must consider product shape, production volume, quality requirements, operator workload, and equipment conditions, rather than relying on familiar methods.

Coating the Future with Science and Care

Science means selecting an application system through shape, quantity, thickness, drying, and quality risks rather than intuition.
Care means establishing a stable mass-production process without relying on operators’ manual work or strained postures.

CoatureVietnam considers application systems suited to customers’ products and processes together with them, including dipping, rotation, and curtain flow, without limiting the review to spraying.

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• What Is a Roller Coater? An Efficient Method for Coating Flat Sheet Materials
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• Fundamentals of Air Spray Coating: Atomization, Distance, and Air Pressure Change Film Thickness
• Application System Selection Checklist: Consider Product Shape, Production Volume, and Quality Requirements