Coating has become a rather wide area. It includes
coating the metal components with other materials by different means to
enhance their physical properties like corrosion resistance, appearance
and durability. When a material has to be coated the grinding buffing
and polishing are not so much required as in un-coated products. Coating
is used for
- Brightening
- Burr removal
- Increased corrosion resistance
- Removal of directional lines
- Reduced surface friction
- Stress relieved surface
- Removal of hydrogen
The process originally started with the expensive
metals being coated over the cheaper ones to enhance the appeal and in
affordable ranges. Gradually the other aspects were discovered. They are
electrochemical processes mainly known as electroplating and powder
coating.
Electroplating - electroplating defined as the electrolytic
deposition of a metal from a solution onto the surface of a substrate
immersed in that solution. It requires a plating bath that has metallic
ions dissolved in an acid on an alkaline base. The metal part to be
plated is submerged in the bath in the middle and an electric current is
passed. This metal part acts as a cathode and the anodes are at the
edges. The electrons flow from the current source to the cathode and
then to the anodes through the bath causing the positive ions to be
plated over the cathode. The process sequence is:
- Cleaning/ degreasing
- Electroplating
- Post-plate treatments,
- Rinsing after each operation
Powder Coating - It is also a kind of electroplating but
gives a different kind and texture of finish. It is also known as plasma
spray coating.Powder coating is a method by which electrically charged
powder coating material is spray-applied to a grounded work piece
The powder coating process involves:
- Cleaning/ degreasing
- Removing the oxide layers by etching
- Chromating and lacquer coating
- Rinsing after each operation
- Drying prior to powder coating
- Powder spray coating
Powder coating is done in the spray booths. It has powder guns on both
the sides. An electrode is applied at the front of the gun. As the
powder leaves the gun, the electrode emits a field charge that is
imparted to the coating material as it is propelled toward the part. The
charged powder particles are drawn and attach themselves to the grounded
work piece.
Coating Pans - they are used for tablet coating. It is a tilted
stainless steel pan with the material to be coated. The formed tablets
are hopped into the pan and the pan is then rotated about its axis. The
tablets are coated and discharged. An infrared heater may be added for
faster drying.
The tablets are then sent to the polishing pan. This pan is similar to
the coating pan but with removable felt lining that polishes the tablets
when rotated in it.
