Coating processes – Direct application of electrical – magnetic – wave – or... – Electrostatic charge – field – or force utilized
Patent
1996-06-24
1998-04-21
Griffin, Steven P.
Coating processes
Direct application of electrical, magnetic, wave, or...
Electrostatic charge, field, or force utilized
118308, 118310, 118313, 118315, 118324, 118668, 118669, 118679, 118683, 118696, 118621, 427475, 427479, 427483, 427424, 239 67, 239 69, B05D 102, B05D 106, B05B 500, B05C 1900
Patent
active
057415587
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to an improved method and apparatus for powder coating three dimensional articles having surface irregularities which vary in angle, curvature, and/or surface area.
BACKGROUND OF THE INVENTION
One common method and apparatus for coating three dimensional articles involves spray coating powder particles onto the external surface of the articles as they are conveyed by a conveyor past a spray gun. Typically, a sensor detects when an article carried by the conveyor moves into the spray pattern of the gun and signals the apparatus to spray a preset uniform quantity of powder particles to coat the article. Thus, to coat a plurality of articles, the apparatus intermittently sprays a preset, uniform quantity of coating material onto each article as the articles pass the gun.
This manner of coating three dimensional articles is generally acceptable if all of the articles have a flat coating surface, or a surface which is parallel to the conveyor and perpendicular to the orientation of the gun.
However, with three dimensional articles which have surface irregularities such as cut-out regions, angled or curved surfaces, protrusions, indentations, or bent edges, etc., these surface irregularities make it difficult to uniformly coat the entire external surface of the article. One reason for this difficulty relates to the angular orientation of the surface irregularities with respect to the direction of the gun. The more the surface varies from an orientation perpendicular to the spray direction, the more difficult it becomes to adequately coat the surface. For curved or angled surfaces difficulty in coating occurs in part because an angled or curved surface has a greater density of surface area than a flat surface. This means that as the conveyor moves the three dimensional articles past the spray gun, the surface area per unit time which passes the gun is greater for angled or curved surfaces than for flat surfaces. Additionally, some surface irregularities are actually cut-out regions, which require no coating at all. Continued operation of a spray gun as a cut-out region passes by represents a waste of coating material.
Thus, as the topography of the three dimensional article varies, it becomes more difficult to uniformly coat the entire surface area, particularly for conveyors commonly used in the industry which convey such articles past the spray gun at a relatively constant speed.
One way to assure that the entire surface is coated is to operate the spray gun at a sufficiently high pressure to discharge a quantity of coating material which is greater than that which is actually necessary to coat the surface, with the pressure being determined by the portion of the surface which is most difficult to coat. This assures some coating on the most steeply angled or curved surfaces. However, a coating applied in this manner is generally not uniform due to the surface irregularities. This manner of coating also results in a tremendous amount of wasted energy and coating material.
It is an objective of this invention to improve uniformity in coating three dimensional articles with surface irregularities.
It is another objective of this invention to adequately coat the irregular surface areas of a three dimensional article while minimizing the amount of wasted coating material.
SUMMARY OF THE INVENTION
The above-stated objectives are achieved by a method and apparatus which control the discharge rate of coating material according to the irregularity and/or area of the surface of a three dimensional article as it is conveyed past a spray gun. For powder spray coating applications, the discharge rate is controlled by regulating the air pressure input to the powder pump. This controls the volume of powder mixed into the conveying air stream moving through the pump and the rate at which the mixed powder-air stream is discharged from a spray gun toward the three dimensional article. For flat surfaces, a standard reference discharge rate is used. For steeply curved surface po
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Otani Tatsuo
Sekiguchi Makoto
Tsukamoto Hidetaka
Griffin Steven P.
Nordson Corporation
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