Coating apparatus – Projection or spray type – Applying solid particulate material
Reexamination Certificate
2001-06-25
2004-05-04
Crispino, Richard (Department: 1734)
Coating apparatus
Projection or spray type
Applying solid particulate material
C118S308000, C118S319000
Reexamination Certificate
active
06730167
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to powder coating spray systems which use powder containment spray booths. More particularly, the invention relates to a powder spray booth that facilitates cleaning and quick color change by the operation of a rotating floor and a powder overspray extraction duct, which results in very little powder remaining in the spray booth and minimizes the amount of powder in process during a spraying operation.
BACKGROUND OF THE INVENTION
Powder coatings are commonly applied to objects by powder spray guns that may be manually operated or automatic. In an automatic system, one or more spray guns are controlled to spray powder onto the objects as the objects are conveyed past the guns. In a manual gun operation, typically the object is suspended or otherwise positioned near a spray gun and the operator controls when the gun starts and stops spraying. A powder spray gun may be selected from a wide variety of gun designs. Since a spraying operation is intended to coat an object evenly, a common technique for spraying powder is to apply an electrostatic charge to the powder particles which causes the powder to better adhere to the object and also results in a more uniform application. Electrostatic spray guns include corona guns and tribocharging guns. In a corona type spray gun, a high voltage electrode is positioned in or near the powder flow path, either within the gun itself or just outside the gun near or at the gun nozzle. In a tribocharging type gun, the powder flow path through the gun body is made of suitable materials that impart an electrostatic charge to the powder as it is forced through the gun body.
The object being sprayed is electrically grounded such that the charged powder is attracted to and adheres to the object. This electrostatic attraction increases the transfer efficiency by increasing the amount of powder that adheres to the object. Transfer efficiency refers to the relationship between the amount of powder that adheres to the object being sprayed versus the amount of powder sprayed from the gun.
In most electrostatic spray systems, the powder is ejected from the gun nozzle as a cloud. This permits the powder spray to envelope the object to coat all the surfaces of the object, even when the object is irregular in geometric shape. Multiple guns may be positioned on different sides of the object and/or directed at different angles to increase the uniformity of the powder applied thereto. However, due to the inherent nature of the powder spray pattern, there is a substantial amount of powder that does not adhere to the object and ends up either falling to the floor or collecting on other objects and structures in the immediate area. This non-adherent powder residue is generally referred to as powder overspray.
Because powder overspray is generated during each spraying operation, spraying operations typically are performed within a spray booth. The spray booth is used for powder containment and may only be partially enclosed. Most spray booths have an air flow system that contains the powder overspray within the structure of the booth by producing a negative pressure zone that draws air from the powder booth along with powder overspray that is entrained in the air flow. The powder laden air is then transferred to a cartridge filter system or cyclone separator system outside the spray booth to recover the powder. However, in known spray booth systems, the powder overspray still tends to collect on the booth walls, ceiling and the booth floor. In electrostatic systems especially, the powder overspray will also tend to be attracted to and collect on any structure that is electrically grounded. The powder particles tend to be very small and well dispersed and therefore can collect in the smallest of recesses, seams and crevices and irregular spray booth wall structures.
Powder overspray presents a two-fold challenge. First, if possible it is usually desirable to try to reclaim or recover powder overspray so that the powder can be reused during subsequent spraying operations. Known powder recovery systems typically work on the basis of a large air volume that entrains the powder overspray. These air flow volumes are routinely generated by conventional high volume exhaust fans. The powder laden air is then filtered, such as for example using cartridge type air filters or cyclone separators. The separated powder is then sieved to remove impurities and returned to a hopper or powder feed center where it is supplied once again to the spray guns. In known systems the actual reintroduction of recovered powder to the powder spray application system is usually accomplished by a positive air pressure conveyance system back to a powder feed center through a series of hoses, valves and pumps.
Besides the challenge of recovering powder overspray for subsequent use or disposal, powder overspray that collects within the spray booth must be removed from the booth when changing over the powder coating color. In order to switch from one color to another the guns, booth and powder recovery system must be as completely purged of the previous colored powder as possible to prevent contamination of the subsequent colored powder. The operation of changing from one color to another is generally known as a “color change” operation and it is an ongoing challenge in the art to make spraying systems that are “quick color change” meaning that the goal is to keep reducing the down time when the spraying system is off line in order to clean the spraying apparatus and system. Thus, the amount of in-process powder, as well as the amount of powder overspray that remains in the spray booth, have a significant impact on the amount of time and effort it takes to perform a color change operation.
A powder coating booth and application system must be completely cleaned and purged of one color of powder coating material prior to a successive coating operation using a different powder color. Cleaning a powder coating spray booth can be a labor-intensive effort. Powder coating materials, in varying degrees, tend to coat all the internal surfaces of the spray booth during a powder coating spray operation, which directly impacts color change time. In a production powder coating environment, minimizing the system down time to change from one color of powder coating material to another is a critical element in controlling operational costs. Seams between booth panels and recessed ledges, such as where access doors or automatic or manual spray application devices may be located, are typically hard to clean areas and tend to hold concentrations of oversprayed powder coating material that could present a contamination risk after a color change. In addition to seams and ledges and other recesses within the booth, charged powder can adhere to booth interior surfaces.
In typical powder coating booth construction, an outer steel framework is provided for supporting individual panel members which form the roof, side and end walls of the booth. These panel members are known to be made of a fabricated or thermoformed plastic, such as polypropylene, polyvinyl chloride (PVC), polyvinyl carbonate or polycarbonate. The floor may also be of thermoformed plastic or stainless steel construction. In other known embodiments, powder coating spray booths can have metallic walls, ceilings and vestibule ends, as well a metallic floor and exterior support framework.
U.S. Pat. No. 5,833,751 to Tucker is an example of a powder coating spray booth intended to reduce powder particle adhesion to the interior surfaces of the booth during an electrostatic powder spray operation. Tucker discloses a booth chamber comprising a pair of thermoformed plastic shells with smooth curvilinear interior surfaces that are intended to inhibit oversprayed powder particle adhesion. Two identical ends connect with the shells and an external support frame is disclosed, but not shown. Possible booth materials disclosed include polycarbonate.
Known booth materials are available in limited sizes requiring some met
Jones Edward L.
Mancuso David
Peddie Andrew M.
Shutic Jeffrey R.
Calfee Halter & Griswold LLP
Crispino Richard
Nordson Corporation
Tadesse Yewebdar T
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