Products
Pinholes
Causes:
A. Bubbles which appear during the application process and break during the drying process, leaving a hole.
B. Organic particles which burn out during firing leaving a hole in their place, such as partially dissolved vehicle components.
C. Too much heat, too fast. Causes fusion before the organic vehicle has completely burned away. Causes a rupturing of the surface skin from the combustion gases.
D. Particles of dust and other foreign matter on the substrate or in the coating. Air born dirt of all types. Minute cracks in the coating.
E. Soluble chemical compounds in the make-up of inorganic pigments which decompose in heat.
F. Small fish eyes.
G. Metal particles.
H. Agglomerated particles of pigment.
I. Particles of refractory.
J. Glass chips.
K. Poor dispersion of pigments in mix.
L. Printing screen plugged, preventing passage of coating materials.
M. Poor mixing of coating materials.
N. Static electricity.
Solutions:
A. Reduce viscosity of coating by thinning or adding flow control agents which reduce surface tension.
B. B, F, I, L are processing variables which can be controlled by the supplier to a limited degree. H and J also lie with the supplier. Due to high temperature processing, refractories are a must for containment. Also, the high temperatures required for the chemical reaction of pigments sometimes produces small metallic particles in the batch which cannot always be separated out.
C. Use preheat if at all possible. Use higher melting ceramic. Slower speed at lower temperature.
D. Particles of dust and other foreign matter on the substrate or in the coating. Air born dirt of all types. Minute cracks in the coating.
E. See B above.
F. See blister.
G. See B and D above.
H. See B above.
I. See B and D above.
J. Same as D above
K. See B above.
L. Clean or replace screen.
M. Improve mixing procedures.
N. Ground coating equipment and substrate.
