Consistent quality in plastic-part coating through conductive treatment and jig grounding

Introduction

Electrostatic coating uses electrical force to attract coating material to the workpiece.

Improve transfer efficiency.
Stabilize film thickness.
Reduce coating material loss.
Improve wraparound coverage.
Stabilize appearance quality.

Electrostatic coating is used in many industrial coating operations for these purposes.

The principles are relatively straightforward for metal parts.
Metal conducts electricity and is easily grounded, making the electrostatic effect easier to obtain.

However, plastic parts require care.

Plastics generally conduct electricity poorly.
Using electrostatic coating in the same way as for metal parts may therefore result in insufficient attraction, inconsistent film thickness or no improvement in transfer efficiency.

Electrostatic coating of plastics requires consideration of conductive treatment on the workpiece and jig grounding, rather than the coating machine alone.

This article explains the basics of conductive treatment, conductive solutions, conductive primers, surface modification and jig grounding for electrostatic coating on plastics.

1. The workpiece condition changes the electrostatic coating effect

Electrostatic coating charges coating particles and attracts them to the workpiece.

The workpiece needs to be properly grounded.
Grounding creates a path for electrical charge to escape.
Metal parts can readily be grounded through jigs or hangers.

However, because plastics conduct electricity poorly, charge tends to accumulate on their surfaces and may make the electrostatic effect inconsistent.

The following problems can result.

• Poor wraparound coverage
• Variable film thickness
• Different deposition amounts in different areas
• Coating material loss does not decrease
• The effect is weak despite using an electrostatic coating machine
• Dirt and dust are readily attracted
• Static charge remains on the surface
• Appearance after coating is inconsistent

Electrostatic coating is therefore not determined by machine performance alone.
The electrical condition of the plastic workpiece strongly affects quality.

2. Untreated plastics make the electrostatic effect difficult to obtain

Plastics are generally highly insulating materials.
High insulation means that electricity does not pass through readily.

For metal parts, charge escapes through the jig and ground during coating.
A stable electrostatic field is therefore formed, making coating particles readily attracted to the workpiece.

For plastic parts, charge cannot escape readily and may accumulate on the surface.
This makes the electrostatic coating effect inconsistent.

For example, material may deposit initially, but its deposition changes partway through.
Only certain areas become thick.
Coating material has difficulty entering deep areas.
Film thickness differs between areas near and far from the jig.

These phenomena can occur.

In electrostatic coating of plastics, first check whether the part is in an electrical condition suitable for electrostatic coating.

3. What is conductive treatment?

To apply electrostatic coating to plastics, the surface may be made conductive.

This can be understood as conductive treatment.

Conductive treatment brings a poorly conducting plastic surface closer to a condition that allows charge to flow during coating.

Typical methods include the following.

• Use a conductive solution
• Use a conductive primer
• Apply a conductive undercoat
• Use a substrate containing conductive materials
• Ground through jigs or contact points
• Combine with surface modification

The purpose is more than simply making electricity flow.
It is to improve transfer efficiency, film thickness consistency, wraparound coverage and appearance quality during electrostatic coating.

However, conductive treatment may add process steps, costs, drying requirements and management items.
It needs to be considered with a balance between quality improvements and process burden.

4. Using conductive solutions

Using a conductive solution is one way to make a plastic surface conductive.

A thin application to the plastic surface lowers surface resistance and is intended to improve the electrostatic coating effect.

Conductive solutions offer a relatively simple way to provide conductivity.
However, certain conditions need to be managed.

Check the following items.

• Conductive solution type
• Application amount
• Uneven application
• Drying condition
• Time before coating
• Humidity effects
• Compatibility with the substrate
• Compatibility with the coating material
• Effects on adhesion
• Effects on appearance

The effect of a conductive solution is not necessarily stable immediately after application.
It may be affected by humidity or elapsed time.

For example, coating immediately after application may produce a different electrostatic effect from coating after a delay.
Surface resistance may also differ between days with high and low humidity.

When using conductive solutions, the time before coating and environmental conditions therefore need to be managed alongside application conditions.

5. Using conductive primers

Another common method uses conductive primers.

A conductive primer is an undercoat applied to a plastic surface to provide the conductivity needed for electrostatic coating.

Conductive primers have the following roles.

• Provide conductivity to the plastic surface
• Improve the electrostatic coating effect
• Stabilize film thickness
• Improve wraparound coverage
• Assist topcoat adhesion
• Stabilize appearance quality

Conductive primers may be effective for plastic parts requiring good appearance and consistent film thickness, such as automotive parts.

However, using a conductive primer adds process steps.

An undercoating step is added.
A drying step is needed.
Film thickness control is needed.
Compatibility with the topcoat must be checked.
Costs increase.

Conductive primers should therefore be judged not only as an added cost, but also through transfer efficiency, reduced rework, consistent appearance and fewer quality defects.

6. Points to note with conductive primers

Inadequate management may prevent conductive primers from delivering the expected effect.

The following points require attention.

6-1. Insufficient film thickness

Insufficient conductive primer thickness may fail to provide enough conductivity.
Thickness variation between areas also produces variation in the electrostatic effect.

6-2. Insufficient drying

Applying a topcoat before the conductive primer has dried sufficiently may cause poor adhesion, appearance defects or drying defects.

6-3. Topcoat compatibility

Poor compatibility between the conductive primer and topcoat may cause intercoat delamination or appearance defects.

6-4. Adhesion to the substrate

If the conductive primer itself does not adhere to the plastic substrate, the whole system is at risk of peeling even with an attractive topcoat.

6-5. Inadequate conductivity checks

It is risky to assume that applying a conductive primer makes everything acceptable.
Surface resistance and the electrostatic effect need to be checked where necessary.

Conductive primers are a useful option, but perform only after proper application, drying and evaluation.

7. Inadequate jig grounding makes the electrostatic effect inconsistent

Jig design and grounding are also important in electrostatic coating of plastics.

The workpiece needs to be properly grounded for electrostatic coating.
Because the plastic itself conducts poorly, grounding through the jig and contact points needs to be as stable as possible.

Inadequate jig grounding causes the following problems.

• A weaker electrostatic effect
• Film thickness variation between areas
• Good deposition only near the jig
• Coating material has difficulty entering deep areas
• Increased coating loss
• Inconsistent appearance

Check the following points.

• Is the jig conductive?
• Is contact between the jig and part stable?
• Is coating or contamination accumulating at contact points?
• Is the grounding wire appropriate?
• Is jig cleaning frequency appropriate?
• Is grounding included in routine daily checks?
• Is there jig wear or deterioration?

Check jig grounding before treating poor electrostatic performance as a machine problem alone.

8. Consider surface modification and electrostatic coating together

Surface modification is sometimes used for plastic coating.

It changes the plastic surface to a condition more compatible with coating material.

Typical methods include the following.

• Flame treatment
• Corona treatment
• Plasma treatment
• Sanding treatment
• Chemical treatment
• Primer treatment

Surface modification is primarily intended to improve adhesion.
Conductive treatment, on the other hand, is intended to improve electrostatic coating performance.

These two measures serve different purposes, but need to be considered together in an actual process.

For example, substrates with poor coating adhesion, such as PP, may need surface modification or a dedicated primer.
If electrostatic coating is also used, conductive treatment and jig grounding are needed as well.

Thus, the following three factors need to be considered together for electrostatic coating of plastics.

1. Adhesion
2. Conductivity
3. Grounding capability

Any one of these alone is insufficient for consistent quality.

9. Humidity and time management are also important

Humidity and elapsed time may affect electrostatic coating of plastics.

Surface resistance may be affected by humidity, particularly when a conductive solution is used.
The time between solution application and coating may also change its effect.

Coating immediately after preparation.
Coating several hours after preparation.
A day with high humidity.
A dry, air-conditioned environment.

These differences may change the electrostatic effect and deposition condition.

Check the following items.

• Time between surface preparation and coating
• Waiting time after conductive solution application
• Factory humidity
• Dryness of the part surface
• Changes in surface resistance
• Film thickness variation
• Times of day when defects occur
• Differences between rainy and dry seasons

Humidity in Vietnamese factories may change substantially between the rainy and dry seasons.
It therefore cannot be ignored in electrostatic coating condition control.

10. Common defects in electrostatic coating

Electrostatic coating of plastics is prone to defects such as the following.

• Film thickness variation
• Insufficient deposition
• Insufficient wraparound coverage
• Dirt deposition
• Uneven gloss
• Poor surface texture
• Sagging in thick areas
• Excessive film thickness at edges
• Insufficient film thickness in deep areas
• Poor adhesion
• Defects in the conductive primer layer
• Defects around jig contact points

Adjusting only machine voltage or gun settings may not solve these defects.

Workpiece conductivity.
Jig grounding.
Surface preparation.
Conductive solution.
Conductive primer.
Humidity.
Time before coating.
Part shape.

These factors need to be included in the checks.

11. Do not manage plastics as though they were metal parts

Shop floors familiar with electrostatic coating of metal parts may apply the same thinking to plastics.

However, the points to examine differ between metal and plastic.

For metal parts, the following items are mainly checked.

• Grounding condition
• Film thickness
• Coating conditions
• Gun settings
• Drying conditions
• Surface preparation condition

For plastic parts, the following additional checks are needed.

• Substrate conductivity
• Surface resistance
• Conductive solution effectiveness
• Use of a conductive primer
• Jig contact design
• Use of surface modification
• Dirt attraction due to static electricity
• Effects of humidity and time

Conditions that work consistently on metal parts do not necessarily produce the same results when applied unchanged to plastics.

12. Basic items to check for electrostatic coating of plastics

When using electrostatic coating on plastics, check the following items.

About the substrate

• Plastic type
• Whether conductivity is present
• Surface resistance
• Need for surface modification
• Variation between substrate batches

About surface preparation

• Mold release agent removal
• Cleaning method
• Static neutralization
• Surface modification
• Whether a conductive solution is used
• Conductive solution application conditions
• Time before coating

About conductive primers

• Whether one is used
• Film thickness
• Drying conditions
• Compatibility with the topcoat
• Adhesion
• Conductivity
• Quality improvement effects

About jigs and grounding

• Jig conductivity
• Contact point stability
• Grounding checks
• Jig cleaning
• Jig wear
• Coating defects around contact points

About coating conditions

• Voltage
• Gun distance
• Coating delivery rate
• Air pressure
• Film thickness
• Wraparound coverage
• Transfer efficiency
• Defect locations

About the environment

• Humidity
• Temperature
• Dust
• Waiting time after surface preparation
• Differences between rainy and dry seasons

13. Points to note in Vietnamese factories

Certain points require particular attention when electrostatically coating plastics in Vietnamese factories.

13-1. Humidity changes

Humidity changes between rainy and dry seasons can affect static electricity, conductive solutions, dirt deposition and coating deposition.
Seasonal changes need to be recorded and compared with defect occurrence.

13-2. Inadequate jig cleaning

Coating buildup on jigs may cause poor grounding.
Jigs become dirtier with continued use.
Regular cleaning and inspection are necessary.

13-3. Waiting time after surface preparation

Long waiting times before coating allow surface changes or dirt deposition.
When conductive solutions are used, their effects may also change over time.

13-4. Condition variation between operators

Differences between operators in solution application amount, wiping, static neutralization or jig loading cause variation in film thickness and appearance.

13-5. Adjusting only the electrostatic coating machine

When defects occur, adjusting only voltage or delivery rate may not resolve the root cause.
Workpiece conductivity and grounding must be checked.

14. Questions for customer visits

For the factory manager

• Does electrostatic coating of plastic parts show film thickness variation or insufficient deposition?
• Are improved transfer efficiency or reduced material loss priorities?
• Are the costs and benefits of adding a conductive primer step compared?
• Do defects increase in the rainy season or periods of high humidity?

For production engineering

• What is the plastic type?
• Is conductive treatment performed before electrostatic coating?
• Is a conductive solution used?
• Is a conductive primer used?
• Is grounding through the jig reliable?
• Is surface resistance checked?

For quality assurance

• Where does film thickness vary?
• Are defect rates compared with and without conductive treatment?
• Are there defects around jig contact points?
• Does quality change with the time elapsed after conductive solution application?
• Is the relationship between humidity and defects recorded?

For purchasing

• Are decisions based only on conductive primer or solution costs?
• Are material-loss reductions through improved transfer efficiency considered?
• Is the impact on electrostatic coatability checked when the plastic substrate changes?
• Are jig and surface preparation equipment costs also considered?

For coating shop-floor managers

• Is jig grounding checked daily?
• Is coating building up on the jigs?
• Does conductive solution application amount vary?
• Are conductive primer thickness and drying conditions followed?
• Are areas with weak electrostatic effects recorded?

15. How CoatureVietnam can help

CoatureVietnam can support improvements in electrostatic coating of plastics through workpieces, jigs, surface preparation and conductive treatment, not only coating machines.

Specifically, the following support is available.

• Organizing plastic substrate information
• Initial checks of electrostatic coatability
• Considering conductive primers
• Clarifying conductive solution use conditions
• Checking jig grounding
• Organizing the causes of film thickness variation
• Considering transfer efficiency improvements
• Preparing surface preparation SOPs
• Preparing jig cleaning and inspection criteria
• Preparing improvement proposal materials for customers

Installing a coating machine alone is insufficient for electrostatic coating of plastics.
Quality becomes consistent only when conductive treatment, grounding, surface preparation and shop-floor management are included.

Conclusion

Electrostatic coating of plastics should not be approached in the same way as metal parts.

Plastics conduct electricity poorly.
Workpiece conductive treatment and jig grounding are therefore important for a consistent electrostatic coating effect.

Check the following points.

• Plastic substrate conductivity
• Surface resistance
• Conductive solution use conditions
• Conductive primer thickness and drying
• Jig grounding
• Jig cleaning
• Humidity and time management
• Surface condition after preparation
• Film thickness variation
• Transfer efficiency

Adjusting only machine conditions cannot make quality consistent if the workpiece side is not properly prepared.

CoatureVietnam’s principle, “Coating the Future with Science and Care,” means examining the entire process in this way.

Science means considering electrostatic coating through plastic surface conductivity, jig grounding, surface preparation, humidity and film thickness, rather than the machine alone.
Care means turning these into reproducible shop-floor conditions that reduce material loss, rework and quality variation.

In electrostatic coating of plastics, examine the workpiece before the machine.
Establishing conductive treatment and jig grounding is the first step toward consistent quality.