4. Paint vs. Mold-in-color
4
So, why paint plastics?
Volume may not justify a press run
Color/effect may not be available in resin
Improved environmental resistance
Aftermarket refinishing/repair
5. Materials and Coatings
5
In general, coatings are applied to materials…
To change the material’s properties and/or
performance in some way
Examples:
Color, appearance, aesthetics
Environmental resistance
Abrasion, wear, friction
Electrical conductivity
Thermal conductivity
Static dissipation
6. Materials and Coatings
6
But sometimes…
Material properties change in unexpected
ways
Example: Nickel
plating on steel
(J.O. Almen,
1951)
7. Materials and Coatings
7
Materials and coatings interact with…
• Each other
• Their environment
• Other materials and coatings
Example: Zinc-plated
steel nut and nylon-
6,6 blowoff valve
8. Paint and Plastics
8
Paints interact with plastics in three ways…
• Chemical
• Thermal (i.e. cure cycle)
• Mechanical
9. Chemical Interactions
9
Solvents (including paint solvents) can cause …
• Swelling of the polymer
• Dissolution of the polymer
• Environmental stress cracking (ESC)
• Changes in molecular entanglement
Example: ESC of
ABS part due to
threadlocking
compound
10. Thermal Interactions
10
Curing of paint at elevated temperatures can cause …
• Accelerated solvent diffusion into plastic
• Relief of molded-in stresses
• Paint shrinkage (residual stress)
• Thermal degradation of plastic
12. How Can You Predict The Effects?
You can’t.
That’s why you test.
12
13. Experimental Set-up
13
Four plastic substrates…
Typically painted
Not typically
painted
All have similar mold shrinkage
(.006 - .008)
Substrates
General purpose ABS
High impact ABS
Weather resistant ASA
PA-6/PE ionomer blend
(Surlyn)
29. Conclusions
29
Painting can have significant effects on
mechanical strength of plastics.
These effects are complex and (at
present) difficult to predict.
The best way to determine the effect of
a given paint on a given plastic is to test.
31. For More Information
31
“Understanding the Effects of
Paint on Plastics,” Plastics
Technology, October 2014.
“Effects of Painting on Mechanical Properties
of Injection Molded Plastics,” Society of
Plastics Engineers ANTEC 2014.
