Repair of parts and hanger stripping in powder coating
The methods of part repair after powder coating can be put into two categories : touch-up and recoat.
Touch-up repair is appropriate when a small area of the coated part is not covered and is unable to meet finishing specifications. When hanger marks are not acceptable, touch-up is required. Touch-up also may be used to repair slight damage from handling, machining, or welding during assembly.
Recoat is required when a part is rejected because of a large surface area defect or when touch-up is not acceptable. At this point, there is a variety of options that should be considered carefully. Usually the rejected part can be re-paired with a second coat. Another option is stripping and repainting the part. Stripping can also clean part hangers to provide a good ground
for electrostatic spray.
Liquid touch-up paint is applied with a small brush, aerosol spray, or airless gun. The paint is air-dried. The drying process can be accelerated with a low-temperature bake. Touch-up paint is used after the powder coating has been fully cured in a bake oven. Hanger marks, light spots in corners and seams, damage from welding or assembly, and other small defects can be touched up. Generally, a color-matched acrylic enamel or lacquer is used. Touch-up paint cannot be used if it will not meet the performance specifications required during the expected life of that part.
Touch-up should not be used to repair a faulty finish unless the resulting product meets inspection standards.
Applying a second coat of powder is the common approach to repair and reclaim rejected parts. However, the defect should be carefully analyzed and the source corrected before recoating. Do not recoat if the reject is caused from a fabrication defect, poor quality substrate, poor cleaning or pretreatment, or when the thickness of two coats together will be out of tolerance. Also, if the part is rejected due to undercure, it merely needs to be rebaked at the required schedule.
A second coat is effective to cover light areas, surface defects from dirt and contamination ,rough spots from heavy film build or gun spitting, and color change from severe overbake. Rough surfaces and protrusions should be sanded smooth before recoating.
Parts inspected on-line can be left on the conveyor to receive a second coat. These parts can pass through the pretreatment stages with raw parts. If the recoated parts show water spots or stains, an adjustment can be made in the final rinse stage.
Chemical suppliers can offer recommendations. When parts for recoat are hung together, cleaning and pretreatment is not necessary. However, if the rejected parts have been stored to accumulate a practical number, they should be checked for dirt and contamination.
Coat Entire Part
When applying the second coat, normal mil thickness should be applied to the entire part. A common mistake is to coat only the defect area. This leaves a rough gritty surface where there is only a very thin overspray layer on the remainder of the part. The same recommended cure schedule is used for the second coat.
Intercoat adhesion can be checked after recoating on selected samples by using the cross hatch test or simply scratching the surface to see if the second coat peels easily from the first. Some powder coatings may need to be lightly sanded to provide a good anchor for the second coat.
When a part is undercured during the first coat, it can be repaired by just returning it to the bake oven for normal cure schedule at the specified time and temperature. Properties will be recovered when the part is properly cured, with some exceptions, such as certain chemically controlled low-gloss coatings. Partial cure will result in a higher gloss, which does not drop to the same level during final cure that would have been obtained with an adequate initial cure.
Stripping is usually the last alternative for part repair since stripping rejected product can add greatly to production cost and disrupt the production line flow. Stripping coated parts becomes necessary, however, when the reject is caused by poor pretreatment or when touch-up or two coats are not acceptable.
On the other hand, stripping plays an important role in effectiveness of the powder coating line by providing clean hangers for a good electrical ground. Hangers should be stripped periodically. Stripping methods are discussed in the following paragraphs. (Note: There is a difference of opinion that chemical stripping is the preferred method.)
Chemical strippers are available to be used hot (raised temperature) or cold (ambient) in a dip tank. There are acid, alkaline, and molten salt types, with selection depending on the type of parts and hangers and the coating to be removed.
The main advantage of chemical strippers is the low initial capital investment for equipment. Disadvantages include safety hazards of handling the chemicals, high costs of chemical replacement and disposal, and chemicals laden with paint. Some parts, such as aluminum alloys, may not be able to withstand corrosion of the chemicals.
Burn off, or pyrolysis, ovens for stripping use high temperatures to incinerate the coating. They can be batch type or on-line ovens that operate at about 800°F (427″C), with the pollution control exhaust operating at temperatures of approximately 1200-1300°F (649-704°C). Burn-off ovens eliminate pollution and disposal problems. They are relatively efficient to operate, but require large capital investment and need some type of post cleaning to remove residual ash. The parts must withstand 800°F (427°C) temperatures. Some coating chemistries are not suitable for this stripping technique. Consult the equipment manufacturer and local regulatory agencies. It should also be noted that repeated stripping of tooling may require some type of alloy to prevent breaking or deforming.
Shot blasting, or abrading, can be used to strip parts or hangers when other methods have been ruled out. This process is very slow due to toughness of the cured powder coating. The disadvantage of this process is that it erodes (thins) the tooling and exposes more surface area,which becomes harder to strip when recoated.
Cryogenic stripping embrittles the film with liquid nitrogen, then uses a nonabrasive shot blast to easily remove the coating. This is a fast, nonpolluting method, but it requires specialized equipment. Parts must endure -100°F (-37°C) be considered for tooling.
Consideration must be given as to whether or not parts can withstand any of the methods described. Chemical and equipment suppliers can temperatures, and some type of alloy may have to assist in that determination. When it comes to tooling, proper design can reduce the amount of cleaning necessary. An inexpensive part hook can become very costly if it must be replaced frequently.
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