Preventing cracking in plastic coating through the relationship between internal stress and solvents

Introduction

Fine cracks sometimes appear after plastics are coated.

The product looked normal immediately after coating.
Fine, whitish cracks appeared after drying.
Cracks appeared only at edges or near gates.
Only certain molded parts cracked despite using the same coating material.
The film appears cracked, but the cause is unknown.

These problems are not uncommon in plastic coating.

The coating material may be suspected first on the shop floor.

“Could the coating be too hard?”
“Could the drying conditions be too harsh?”
“Could the thinner be unsuitable?”
“Should we switch to a more flexible coating?”

Coating materials, thinners and drying conditions are, of course, important.
However, cracking in plastic coating does not necessarily occur only in the film.

It can result from an interaction between residual internal stress in the molded plastic and solvents in the coating material or thinner.

This article organizes the basics of examining plastic coating cracks through substrate, molding, internal stress, solvents and drying conditions, rather than the coating material alone.

1. Cracks after plastic coating are not necessarily cracks in the coating film

When cracks are seen after coating, they are often assumed to be in the coating film.

However, the visible cracks in plastic coating may be in the plastic substrate rather than the film.

This makes cracking after plastic coating difficult to diagnose.

Is the coating film cracked?
Is the plastic substrate cracked?
Are both the film and substrate affected?

These possibilities need to be considered separately.

If the plastic substrate is cracked, changing the coating material alone may not solve the problem.
Molding conditions, internal substrate stress, solvent aggressiveness, drying temperature and product shape also need to be checked.

2. What is solvent cracking?

Solvent cracking is an important phenomenon in plastic coating.

“Solvent” refers to the solvent component.
“Cracking” means the formation of cracks.

Solvent cracking is therefore cracking in a plastic substrate caused by solvents.

Coating materials, thinners and wiping solvents may contain components that soften, swell or attack plastics.
When these solvents act on the plastic surface, fine cracks may form.

Areas with residual molding stress are particularly susceptible to solvent effects.

Solvent cracking is therefore more than simply a problem of an aggressive solvent.
It arises through the combination of internal substrate stress and solvent compatibility.

3. What is internal stress?

Internal stress is something like a force remaining within a molded part.

Plastic parts are heated, flow within a mold and are cooled into shape.
Molding conditions or part geometry can leave strain inside the material during this process.

This residual strain is internal stress.

Internal stress may be difficult to recognize from appearance.
A product may look normal while its interior remains susceptible to cracking.

When solvents in the coating or thinner act on it, cracks tend to begin where stress is concentrated.

Cracking in plastic coating therefore needs to be considered back through the molding process, not only the coating process.

4. Areas prone to cracking

Cracks after plastic coating are not necessarily distributed evenly over the whole product.
They may concentrate in particular locations.

The following locations require particular attention.

• Near gates
• Edges
• Corners
• Areas with different wall thicknesses
• Around ribs
• Around bosses
• Areas subjected to bending stress
• Areas where plastic flows meet during molding
• Internal corners of the product
• Areas held by jigs

A gate is the inlet through which plastic enters the mold.
This area is readily affected by flow and cooling during molding and may retain internal stress.

Edges and corners also need care.
Coating buildup, solvent exposure and stress concentration can occur together there.

When cracks occur, recording their locations is an important first step.
The locations indicate where to look among molding stress, solvents, drying conditions and product shape.

5. Solvent resistance differs between substrates

Different plastics have substantially different resistance to solvents.

The same coating material or thinner produces different results on different substrates.

ABS, PC and PS, for example, require care regarding solvents.
Depending on solvent type, concentration and contact time, whitening, swelling, poor surface texture or cracking may occur.

Some substrates are relatively resistant to solvents.
However, solvent resistance cannot be judged completely from the material name alone.

It also varies with molding conditions, additives, batch, internal stress and part geometry.

Plastic coating should therefore always be checked in a small batch beforehand.

Check the following points.

• Plastic type
• Grade
• Presence of additives
• Molding conditions
• Solvent compatibility
• Thinner type
• Wiping solvent
• Cracking after coating
• Appearance changes after drying

Coating without knowing the substrate type increases the risk of cracking.

6. Examine both coating solvents and wiping solvents

Examining only the solvents in the coating material is insufficient when considering crack causes.

Wiping solvents used before coating are also important.

IPA, thinner or cleaning agents are sometimes used to wipe off mold release agents or oil before coating.
If the wiping solvent attacks the substrate strongly, the plastic surface may weaken before coating.

Subsequent coating adds coating solvents and drying heat, which can make cracks apparent.

Cracks may therefore occur through the following combined factors.

• Internal molding stress
• Effects of wiping solvents
• Effects of solvents in the coating material
• Thinner selection
• Drying temperature
• Drying time
• Product shape

In plastic coating, cleaning agents, thinner, coating material and drying conditions need to be considered as one system.

7. Drying conditions can promote cracking

Drying conditions also affect cracking in plastics.

Unlike metal, some plastics are sensitive to heat.
Excessive drying temperature can cause deformation, warpage, dimensional changes or changes in internal stress.

Furthermore, rapid surface drying before solvents have fully escaped can place stress on the film or substrate surface.

The following conditions require particular attention.

• Drying temperature is too high
• Heating is rapid
• Drying time is too short
• The coating is applied too thickly
• Solvents are too aggressive
• The product has large wall thickness differences
• Jigs place the product under stress
• Deformation occurs during drying

When determining drying conditions, check whether the plastic substrate can withstand the temperature and time, as well as whether the film dries.

8. Annealing may be effective

Annealing may be effective when molded parts have high internal stress.

Annealing treats molded plastic parts at a defined temperature for a defined time to relieve internal stress.

Lower internal stress may make cracking less likely even when solvents act on the material.

However, annealing is not a universal solution.

Check the following points.

• Is the treatment temperature suitable for the material?
• Will heat cause deformation?
• Will dimensions change?
• Is treatment time appropriate?
• Can it be incorporated into mass production?
• Do the benefits justify the cost?
• Does the cracking rate actually decrease?

Annealing must be checked through trials before use.
Some plastic parts may undergo dimensional or appearance changes during annealing.

9. Do not try to solve cracking through coating materials alone

A material change may be considered when cracks occur in plastic coating.

This can be effective in some cases.

For example, switch to a coating with milder solvents.
Switch to a low-temperature drying type.
Switch to a flexible film.
Use a dedicated primer.

Such measures may be necessary.

However, changing the coating material alone may not solve the problem.

If the cause is molding stress, molding conditions need to be reviewed.
If the wiping solvent is too aggressive, surface preparation needs to be reviewed.
If drying temperature is too high, drying conditions need to be reviewed.
If stress is concentrated in particular areas, product shape or jigs may need to be reviewed.

Crack prevention requires separate checks of coating material, substrate, molding, surface preparation and drying.

10. Checking procedure when cracks occur

Checking in the following order makes the causes of cracking after plastic coating easier to organize.

10-1. Check the crack locations

• Over the entire surface?
• Only in particular areas?
• Near gates?
• At edges?
• At areas with different wall thicknesses?
• At jig contact points?
• At internal corners?

If cracks concentrate in particular areas, suspect molding stress or shape-related factors.

10-2. Check when they occurred

• Immediately after coating?
• After drying?
• 1 day later?
• After assembly?
• After use?
• Is there whitening or cracking immediately after wiping?

The timing changes where to look among surface preparation, coating, drying and the service environment.

10-3. Check the substrate type

• ABS?
• PC?
• PS?
• PP?
• PA?
• FRP?
• Another plastic?
• What grade?
• Are there additives?

Without the substrate type, accurate root-cause analysis becomes difficult.

10-4. Check the solvents used

• Wiping solvent
• Solvents in the coating material
• Thinner
• Cleaning agent
• Drying time before coating

Check whether the solvents are compatible with the substrate.

10-5. Check molding conditions

• Gate location
• Molding temperature
• Cooling conditions
• Wall thickness differences
• Molding batch
• Change of molding manufacturer
• Mold release agent type

Compare molding conditions with crack locations.

11. Points to note in Vietnamese factories

The following points require care when coating plastics in Vietnamese factories.

11-1. Molding information is not shared with the coating process

When molding and coating are performed by different companies or departments, the coating team may not know the material type or molding conditions.
This makes it easy to attribute cracks to the coating material alone.

11-2. Wiping work is not standardized

Differences between operators in wiping solvent amount, force, number of passes or drying time cause different effects on the plastic surface.

11-3. Excessive use of aggressive solvents

Using aggressive solvents to remove mold release agents or oil may damage the substrate.
Cleaning power needs to be balanced with avoiding substrate damage.

11-4. Drying conditions for metal are used unchanged

Using metal coating drying conditions unchanged for plastic parts may mean excessive temperatures.
The heat distortion temperature of each plastic needs to be checked.

11-5. Decisions are based only on defect photographs

Attributing cracks to the coating material from photographs alone overlooks molding stress or solvent effects.
The material type, molding information, surface preparation and drying conditions need to be checked alongside photographs.

12. Basic measures for preventing cracking

The following measures are effective for preventing cracks in plastic coating.

12-1. Check the substrate type

Without knowing the plastic, the suitability of solvents and drying conditions cannot be judged.
First identify the substrate.

12-2. Use milder solvents or change them

If solvents are too aggressive for the substrate, thinner and cleaning agents need to be reviewed.
However, the balance with coatability and drying characteristics also needs checking.

12-3. Standardize wiping conditions

Standardize solvent amount, number of wiping passes, cloths and drying time.
Reducing variation between operators is important.

12-4. Review drying conditions

Check drying temperature, drying time, heat-up rate and actual product temperature.
The film needs to be dried within a range that does not deform the plastic.

12-5. Check molding stress

If cracks concentrate near gates or edges, suspect molding stress.
Consider molding conditions or annealing where necessary.

12-6. Evaluate small batches beforehand

Small-batch trials before mass production are important for plastic coating.
Check substrate, solvents, drying conditions, adhesion and cracking before proceeding to mass production.

13. Questions for customer visits

For the factory manager

• Are there complaints about cracking or whitening in plastic coating?
• Are defects concentrated in particular products or batches?
• Are molding and coating performed in the same factory or by different companies?
• Does cracking cause rework or disposal?

For production engineering

• What is the plastic type?
• Are gate locations and wall thickness differences checked?
• Are cracks concentrated near gates or edges?
• Is internal molding stress considered?
• Has annealing been considered?

For quality assurance

• Is it checked whether cracks are in the film or substrate?
• Is magnified observation used?
• Are molding batches linked to crack occurrence?
• Is evaluation performed when wiping solvents or thinners change?
• Are drying conditions and defect rates recorded?

For purchasing

• Are changes in plastic material or molding manufacturer shared with the coating process?
• Is coatability checked when the plastic grade changes?
• Are substrate changes made based on cost alone?
• Is quality information on molded products obtained?

For coating shop-floor managers

• Does the wiping solvent cause substrate whitening?
• Is coating applied immediately after wiping?
• Does the post-coating drying temperature suit the substrate?
• Are crack locations recorded?
• Are the substrate and molding conditions checked as well as the coating material?

14. How CoatureVietnam can help

CoatureVietnam can support investigation of plastic coating cracks through substrate, molding, surface preparation and drying conditions, as well as material selection.

Specifically, the following support is available.

• Organizing plastic substrate information
• Classifying crack locations
• Organizing defect photographs
• Checking wiping solvents
• Checking thinner effects
• Checking drying conditions
• Considering the possibility of molding stress
• Considering annealing
• Designing small-batch trial conditions
• Preparing improvement proposal materials for customers

For plastic cracking, causes need to be separated into substrate-side and process-side factors, rather than only changing the coating material.

Conclusion

Cracking in plastic coating is not necessarily a film problem alone.

Visible cracks may be in the plastic substrate rather than the film.
Solvent cracking becomes especially likely when internal molding stress combines with solvents in coating materials or thinners.

The following checks are important for preventing cracks in plastic coating.

• Substrate type
• Molding conditions
• Gate location
• Edges
• Internal stress
• Wiping solvent
• Solvents in the coating material
• Thinner
• Drying temperature
• Need for annealing

When cracks are seen, check the condition inside the substrate rather than immediately concluding that the coating film has cracked.

CoatureVietnam’s principle, “Coating the Future with Science and Care,” means separating these less visible causes and organizing them so that the shop floor can prevent recurrence.

Science means considering plastic cracking separately through coating material, solvents, substrate, molding stress and drying conditions.
Care means creating checking procedures and standard conditions so the shop floor does not face repeated cracking.

Cracking in plastic coating cannot be solved by examining only the film.
Understand the relationship between substrate and solvents.
This is the basis for preventing cracking in plastic coating.