43.2 PAINTING
43.2.1 Powder Coating
Basically a power is distributed over the surface of a part. Subsequent heat then melts the plastic to leave behind a high quality surface.
Advantages,
- energy and labor cost reductions
In contrast, paint uses solvents that dissolve whereas powder coatings are applied and then heat cured.
The basic steps are,
1. apply a finely ground powder coating to a part.
2. Heat the part to melt and fuse the powder.
The parts can be coated with a fluidized bed. The hot part is dipped in a fluidized vat of powder where it coats and hardens. A part curing process is also done.
Parts can also be coated with electrostatically charged powder that is oven cured.
Thermoplastic powders are commonly used.
- They melt and reset at elevated temperatures, but do not change chemically
- The molecular weight is high.
- The materials are hard to grind into fine powders.
- The thermoplastic coatings tend to be thicker (0.008" to 0.04")and applied by the fluidized bed method.
- thermoplastic polyester
Thermosets are another common material,
- in uncured state the components are low molecular weight solid resins. When heated the resins chemically bond to longer molecular chains.
- typical coating thicknesses are 0.001" to 0.003"
Pigments can be added to modify basic colors.
Additives can be used for other properties,
Mechanical Surface Preparation,
- the surface is mechanically worked to remove unwanted coatings, and roughen the surface to help the coating stick.
- airblasting with sand or slag abrasives in open or closed environment.
- centrifugal wheel blasting.
Chemical surface preparation,
- often used on galvanized steel, steel and aluminum
- typical cleaners include,
In mold powder coating,
- powders are sprayed into mold cavities before the part is molded.
- before/after/during molding the cavity is heated to 280-350°F and the coating chemically bonds to the part making better adhesion.
- advantages of this method are,
- chip and impact resistance
- conductive primers can be applied with this method to permit electrostatic coatings.
- other painting facilities can be eliminated
- shelf lives for materials is over a year
- good coverage, including complex geometries, uniform thickness.
2. spray guns (electrostatic) are moved into the mold and spray powder in to coat the empty mold
3. the powder cures on the surface that has been preheated to 280-350°F
- A typical cycle time, including the coating, is less than half a minute
- This technique is most often used with thermoset compression molding.
- typical applications include,
- machine and electrical housings
- multiple molds can be coated at once
- robotic coaters are available
- plastics can be hard to direct into the mold
- parting lines of the mold build up extra material
- the powder between the mold halves must be removed after the mold halves are brought together.
Booths can be used to recover powder that is sprayed but does not adhere to the part.
Systems can be used to remove most of the unbonded particles from the air. The extra components include,
Belt Booths -The booths have many of the components of a normal booth, except that the unbonded powder is drawn onto a conveyor belt - the circulating air is drawn though the belt, but the powder is filtered out.
Self contained booth - an all in one unit that allows fast changeovers for new colors.
Two types of ovens used,
Advantages,
- no incinerators or air scrubbers (no exhaust)
- better properties than paint
Disadvantages,
- color changeovers limited
- some materials damaged by UV
- deep recesses not well covered
