Processes for coating manufactured goods which utilize powdered coating materials and electrostatic attraction have found increasing popularity and an ever increasing number of applications in recent years. In accordance with such processes, a resin coating in powder form ("powder") is sprayed onto the articles to be coated using a spray gun in much the same manner as liquid paint is sprayed. The articles to be coated are electrically connected to one electrode of a DC power supply while the powder is provided with an opposite charge by connecting the other electrode of the DC power supply to the spray gun proximate the orifice from which the powder is sprayed. The resulting electrostatic attraction causes the powder to adhere to the articles sprayed. The articles are then heated to melt the powder, resulting in a permanent bonding between the powder coating and the article. Because there are no liquid solvents present as in conventional paint, the curing time is minimal. There are several advantages these processes over those which utilize liquid paint. The resulting coating has a more even thickness because the powder simply falls off when the coating reaches a desired thickness at which the electrostatic attraction falls to zero. This self-limiting characteristic of the process facilitates the coating of hard to reach areas without overcoating adjacent areas of the articles being sprayed. Thickness of the coating can be varied by varying the DC voltage of the power source and, therefore, the resulting electrostatic attraction. Unlike liquid paint, the oversprayed powder can be collected and reused so long as it is not contaminated.
Articles coated according to the above-described process must be sprayed in a spray booth specially designed to contain the oversprayed powder. The articles are usually transported through the booth using a conveyor system. Prior art booths have been designed to be used with either overhead conveyors or floor mounted spindle conveyors. Powder spray booths are generally provided with an air flow system, an air filtration system and a powder collection system. These systems generally cooperate to contain the powder within the booth and/or collection system and prevent contamination of the air outside the booth. Clean air is normally drawn into the booth through openings provided for conveying the articles to be coated into the booth as well as openings in the booth through which the spray guns are inserted. Some oversprayed powder : will simply fall by gravity to a surface below the spraying area where it is directed to the collection system. The remaining powder which is entrained in the air is drawn through a filtration system where the powder is collected on filter media and the air is exhausted to a clean air plenum. Air flow is normally maintained by a blower which maintains a negative pressure in the clean air plenum. In order to maintain desired air flow through the booth, it is necessary to periodically "purge" the filters by directing a reverse pulse of compressed air through the filters to eject the collected powder. The ejected powder is of sufficient weight and concentration that it can be directed to a powder collection system.
Although equipment constructed in accordance with the foregoing description functions well for so long as it is being used to apply the same color coating, a problem arises when it becomes necessary to change the color of the coating being applied. A cross color contamination of the powder will result in an unacceptable coating. In order to avoid such contamination, it is necessary to conduct a thorough cleaning of the interior surfaces of the booth, the air filtration system, and the powder collection system. It is also necessary to replace the filter media whenever a color change is required because the periodic purges of the filters are not one hundred percent effective. Completing these steps which are necessary to effect a color change is a messy and time consuming operation. In the past, these changeovers resulted in down times of as much as several hours with prior art booths.
A number of prior art powder spray systems have been designed with the objective of reducing the down time associated with color changes. However, the results have been less than satisfactory. These systems incorporate filter and/or powder recovery components or modules which are separable from the spray booth portion of the system. For example, roll-away filter/recovery modules are disclosed in U.S. Pat. Nos.: 4,590,884; 4,378,728; 4,227,260; and 4,401,445. A removable self-contained recovery/filter system is disclosed in U.S. Pat. No. 4,409,009. In each of the above-mentioned patents, powder spray systems are disclosed in which down time for color changes is reduced by providing for replacement of the filter/recovery modules with a pre-cleaned module. This method of dealing with the problem is less than satisfactory. It increases the complexity and therefore the cost of the systems. Further, it requires that each system be provided with at least one spare filter/recovery module. These modules are expensive and still must be cleaned and the filter elements in them must be replaced before they can be reused for a different color powder.
In a powder spraying system, it is extremely important that the interior surfaces of the booth or spraying area be completely cleaned with all powder removed before changing the color of the powder being sprayed. The applicant is unaware of any prior art system which adequately addresses the problem of providing quick and complete access to the interior surfaces of the spray booth for cleaning.
In most larger prior art booths, it is necessary for someone to physically get inside the booth to accomplish and assure a complete cleaning. This is a less than satisfactory situation. Although down time can be reduced in prior art booths incorporating separable filter/recovery modules when a clean spare module is maintained, it is still necessary to clean the modules when they are changed out. The modules in these prior art booths also present problems of access. Accordingly, the time required to change out the filters in the modules and thoroughly clean the interior of the modules results in a substantial increase in the labor cost of the powder spraying operation.
Therefore, it is an object of the present invention to provide a powder spray booth which reduces down time associated with color changeovers.
It is a further object of the present invention to provide a powder spray booth which includes an integrally constructed filter/recovery system, whereby the interior of the spray booth and the filter/recovery system can be cleaned at the same time during color changeovers without increasing down time.
It is another object of the present invention to provide a powder spray booth having the advantage of quick and easy removal and replacement of filter media, thereby reducing down time.
It is yet another object of the present invention to provide a powder spray booth having wall panels which are light in weight and are quickly and easily removable and replaceable, thereby providing improved access to the interior of such booth and reducing downtime required for cleaning.
It is still another object of the present invention to provide a powder spray booth having an improved powder recovery system located immediately below the spray area of the booth, whereby the percentage of oversprayed powder which is recoverable is substantially increased.
It is also an object of the present invention to provide a powder spray booth having a removable hopper or drawer for the collection of oversprayed powder.
It is another object of the present invention to provide a powder spray booth which is designed to minimize the risk of explosion.
It is yet another object of the present invention to provide a powder spray booth which is relatively simple and inexpensive to manufacture and maintain.