Powder
coating Painting
Powder coating is an advanced method of applying a decorative and protective
finish to a wide range of materials and products that are used by both
industries and consumers. The powder used for the process is a mixture of
finely ground particles of pigment and resin, which is sprayed onto a surface
to be coated. The charged powder particles adhere to the
electrically grounded surfaces until heated and fused into a smooth coating
in a curing oven. The result is a uniform, durable, high-quality, and
attractive finish.
Purpose:
The
basic purpose is to carry out pre-treatment before powder coating. Normally 7-10
tanks are used for hot process for cleaning before applying the powder coat. General
practice is to follow the best of standards in the implementation of “Seven
Tank Process”.
Typical
size: seven tanks are made of Poly Propylene and the size of each tank is (2600
L X 1400 B X 2500 D) mm
TANK-1
DEGREASING
TANK-2
WATER RINSE-I
TANK-3
DERUSTING
TANK-4
WATER RINSE-II
TANK-5
SURFACE ACTIVATION
TANK-6
PHOSPHATING
TANK-7
WATER RINSE-III
TANK-8
PASSIVATION
1.
DEGREASING
Room
Temperature Process, Duration-30-40 Minutes, For-Cleaning the Sheet Metal
Surface of grease, oils, soils, lubricants, oxide films, Heat Treatment/welding
Scales etc. by ALKALINE DEGREASER.
2.
WATER RINSE-I
Rinsing
Process is Carried out.
3.
DERUSTING
Room
Temperature Process, Duration-20 Minutes, For-Cleaning The Sheet Metal Surface
of Rust by Mixed HYDROCHLORIC ACID.
4.
WATER RINSE-II
Rinsing
Process is carried out.
5.
SURFACE ACTIVATION
Room
Temperature Process, Duration-20 Minutes, For-Activating the Sheet Metal
Surface for Zn & Mn phosphate Coating by ACTIVATION CHEMICALS.
6.
PHOSPHATING
Room
Temperature Process, Duration-5-20 Minutes, For-Smooth & Uniform Coating of
Sheet Metal Surface with Zn Phosphate by ZINC PHOSPHATE SOLUTION
7.
WATER RINSE-III
Rinsing
Process is carried out.
8.
PASSIVATION
Room
Temperature Process, Duration-15-20 Minutes, For-Sealing Pores of Sheet Metal
Surface for obtaining Maximum Corrosion Resistance over Phosphate Coating by
DECXYLITE SOLUTION
OVEN
DRYING
150
Deg c Process, Duration-10-20 Minutes, For- Drying Sheet Metal Parts by hot
air.
POWDER
COATING
Done
within 24 Hrs of Oven Drying, Sheet Metal parts are Coated with Powder of shade
desired by client through Spraying in Spray Booths, and then Epoxy-Powder
Coated/Sprayed Sheet Metal parts are baked in Electrically Fired Oven at
140-150 Deg c for 10-15 Minutes
Surface preparation.
Surface
preparation includes
Cleaning –
mechanical or chemical
Mechanical
cleaning includes methods like scratch brushing and sand blasting. This by abrasive
action not only removes the surface impurities but also eliminates scratches
and surface irregularities. Cleaning is very good; however, coating must be
done immediately because the cleaned surface is in a highly reactive state and
corrosion occurs very soon. Chemical cleaning includes removal of dirt, oil and
grease, and the oxidation products present on the surface by means of
chemicals. The chemicals may be applied by wiping, spraying or dipping. The
nature of chemicals used depends upon the base metal.
Application of conversion coating:
These are
applied for three purposes:
a) To provide
temporary in process corrosion protection before application of powder coating.
b) To promote good adhesion of the powder
coating to the substrate.
c) To impart
under paint corrosion & thereby improve the life of the powder coating.
The type of
the conversion coating used also depends upon the base.
PRETREATMENT FOR MILD STEEL
TENDENECY OF MATERIAL: - picking up rust when exposed to atmosphere
bare.
NEED OF CLEANING - Presence of oil, grease, rust and
black scale on the surface. Mild Steel material is known for its tendency of
going under rapid oxidation when exposed to atmosphere bare. The result of
oxidation is the formation of oxide layer on the surface which we commonly call
as ‘RUST’.
To avoid this oxidation or rusting, the mild steel
material is not allowed to get directly exposed to atmosphere. For this, we
generally find rust preventive oil on the surface of mild steel material. The
oil does not allow the material to come in direct contact with air and thus delays
oxidation. However, this doesn’t entirely eliminates the problem of rusting and
it only delays the process of rust formation.
Some mild steel materials are hardened for strengthening
and similarly some are welded during fabrication. Both these processes leave
black scale on the surface.
It concludes that mild steel parts generally have rust
and/or grease-oil and/or black scale on the surface. All these cause severe
problems of bonding if not removed before application of paint.
NEED OF CONVERSION COATING – The bare metal being
extremely susceptible to atmospheric corrosion should be applied with
conversion coating Before painting, once the rust, oil and scale are cleaned,
the bare metal is again exposed for oxidation. To avoid this, the material has
to be coated with some type of conversion coating which prevents further
oxidation before applying paint and also gives adhesion for paint.
DEGREASING –
Removal of oil and grease from the
surface-
This can be done by Solvent Based Degreasing:- Petroleum based solvents
have excellent degreasing abilities but are not commonly used because they are
highly inflammable.
Alkaline degreasing powders:-
This is generally a blend of alkalies and surfactants. This is advised to
be used under hot conditions. However, alkaline powders are also used under
cold conditions as knock out degreasing stage.
Acidic Degreasing:-
These are solvent based liquids which are acidic in nature. They remove
rust as well as grease under cold conditions. The bath of these chemicals is
however, much expensive as their concentration is much higher than alkaline
degreasing.
DERUSTING – Removal of rust and light scale from the surface.
These are
necessarily acidic chemicals unlike degreasing, which may be alkaline in
nature. The chemical is generally a blend of mineral acids like phosphoric acid,
sulphuric acid and hydrochloric acid with added inhibitors. Higher acidity will
result in faster removal of rust. However, this will affect the life of the
coating applied later. Phosphoric acid based de-rusting chemicals are most
suggested for pre-treatment of mild steel before applying paint.
CONVERSION COATING FOR MILD STEEL
The most
common types of conversion coating include:
- Zinc
phosphate coating
- Iron
phosphate coating
Zinc phosphate coating is crystalline heavy coating
gray in colour. While iron phosphate coating is amorphous conversion coating
that ranges in colour from iridescent blue to gray.
Iron
phosphate coating gives minimum sludge formation and hence smoother finish than
zinc phosphate. Apart from this the bonding characteristics are also good. Iron
phosphate processes are much easier to operate than zinc phosphate processes
and require fewer process stages (zinc phosphate coating requires a pre-dip in
activation chemical prior to phosphating to have micro crystalline grain
structure), but iron phosphates do not provide the degree of corrosion protection imparted by zinc
phosphates.
Iron phosphate systems are therefore used for a range of products
requiring a durable finish that are not exposed to severely corrosive
environments. Zinc phosphating processes have been developed to provide
exceptional painted part durability in corrosive requirements. Typical
industries using zinc phosphate processes include automotive, appliance and
truck and bus.
POST-TREATMENT
After a
metal surface receives a conversion coating, the surface is water rinsed to
remove un-reacted conversion coating chemicals and a post-treatment is applied.
The post-treatment can provide a two- to ten-fold increase in corrosion resistance
and humidity resistance when compared to conversion coatings without final
rinses. Post-treatments are generally based on chromic acid.
Problems Because Of Improper Cleaning:-
1.
Appearance of oily patches.
2.
Appearance of pin holes.
3.
Appearance of rusty surface beneath powder coating film.
PROBLEMS
BECAUSE OF IMPROPER CONVERSION COATING:-
1. Reduced
resistance to weather.
2. Reduced
mechanical resistance.
3. Poor
finish of the powder coating film.
PROBLEMS
BECAUSE OF IMPROPER DRYING:-
1.
Appearance of pin holes.
2. Poor
bonding of the powder coating film.
These
problems can be avoided by the choice of right process – right chemicals and right maintenance of the chemicals.
