Coating apparatus – Projection or spray type – With hood or offtake for waste material
Reexamination Certificate
2000-06-19
2002-10-08
Crispino, Richard (Department: 1734)
Coating apparatus
Projection or spray type
With hood or offtake for waste material
C118S309000, C454S050000
Reexamination Certificate
active
06461431
ABSTRACT:
This invention relates to powder spray coating and, in particular, to the recovery of powder which has not adhered to the article to be coated from a powder spray booth.
The powder which does not adhere to an object to be coated in a powder spray booth is known as the “overspray”. Overspray powder may miss the object to be coated, rebound from a surface of the object, or be deflected by the electrostatic fields in the spray booth away from the object.
It is known to recover overspray powder by exhausting the air from the booth and hence the air-borne overspray. Filter elements have been used to recover overspray from powder spray booths and are capable of removing substantially all the overspray from the spray booth exhaust air so that the powder may be reused. Such filter elements require frequent cleaning in order to prevent them from becoming blocked or clogged with powder and this is usually carried out by reverse pulses of pressurised air which dislodge powder adhering to the filter elements.
It is also known to use cyclone separators instead of, or in conjunction with, filter elements for collection of overspray. Cyclone separators are effective in removing the majority of the overspray from the spray booth exhaust air and are easier to clean than filters because very little powder is deposited inside the cyclone housing.
In known arrangements, the cyclone separators used in powder recovery have either been very large single or dual side by side cyclones or multi cyclones having tangential or axial feed inlets. In cyclones of the vertical feed type, the air entrained overspray powder is fed vertically downwards from a common inlet manifold into the separators and a circumferential velocity is imparted thereto by a number of radial veins. In such vertical cyclones, the overspray powder is separated from the air by the combined effect of centrifugal and gravitational forces and falls to the bottom of the separators there to be collected and removed. The cleaned air is then directed vertically upwards through ducts, one per cyclone separator, which pass through the centre of the veins and into an exhaust manifold. The cleaned air then passes through a further powder recovery unit containing a number of filter elements to remove any fine powder particles entrained in the air.
Such vertical feed cyclone separators, when arranged in a module of a cluster of small cyclones, render the module difficult to clean thoroughly as power tends to impinge and adhere to the inlet areas of the cyclone and the veins directly below the inlet areas and to the vertical exhaust ducts passing through the centre of the veins. These surfaces are difficult to clean properly, particularly where there is a large number of cyclone separators because these areas are generally not easily accessible. The cleaning of such a powder recovery module, when changing the colour of powder to be recovered thereby, for example, is a lengthy process.
An arrangement intended to reduce the amount of overspray retained in the cyclone separators of a recovery module and facilitate cleaning of the retained overspray is disclosed in European Patent Application Number 94929600.8. A powder recovery module is provided comprising at least two tangential feed cyclone separators, each with a horizontal tapered feed channel having a dedicated inlet, the inlet lying in a common plane disposed near the ceiling of the powder spray booth when the module is mounted to one of the side walls of the powder spray booth. An overspray intake is located near the floor of the powder spray booth and a channel connects the intake to the inlet. The arrangement has the advantage that a greater proportion of overspray is separated from the spray booth exhaust air through use of the tangential feed cyclone separators in combination with the tapering feed channels. The geometrical arrangement of the inlets to the cyclone separator feed channels and the configuration of the tangential feed cyclone separators facilitates the cleaning of the surfaces to which overspray may adhere by means of an air hose, for example.
The above described arrangement has been found to work well in practice. However, changing the powder colour may still be a time-consuming task because of the time required to clean the booth. The booth needs sweeping to remove powder which was deposited on the walls and ceiling and on the floor and cannot be extracted by air exhaustion. In addition the cyclone separators and the channels leading thereto need to be cleaned which can be a time-consuming and difficult operation because of the limited access to the interiors thereof.
Proper cleaning of the walls and floor of a powder spray booth between colour changes is very important to ensure acceptable production quality. Such cleaning may be carried out manually by an operator using a scraping and/or compressed air jet apparatus. Booths have been proposed which are intended to automate the deposited overspray powder cleaning process. US Pat. No. 4153008 discloses a booth with a floor in the form of a conveyor belt which continuously transports powder to the exterior of the booth where the powder is removed from the belt by a suction nozzle and then passed to a separator. US Pat. No. 4430956 discloses a booth with a rotating arm on the floor which sweeps up deposited powder and passes it to a hopper positioned below the floor. European Patent Application No. 0701868 also discloses an arrangement with a floor scraper but in this case scrapers are also provided for the walls and ceiling, the collected powder again being transferred to a collection hopper located below the booth.
Even with the relatively sophisticated and therefore costly arrangements proposed for cleaning of deposited powder, the cleaning times for each change of powder colour are still considerable with consequently long periods during which the booth is out of operation. Furthermore, effective cleaning is still not guaranteed because of the difficulties involved in total removal of the powder which may be very fine.
One solution involves the use of a number of booths each dedicated to spraying of a particular powder colour. In an alternative solution, two or more booths are provided which can be moved alternately in and out of the production line to allow cleaning of one whilst the other is operational and vice versa.
British Patent Application No. 2141642 proposes that the deposited powder cleaning problems discussed above should be overcome by use of a booth in which the roof and/or walls are provided with replaceable coverings. The booth includes a collapsible suction tube for removal of air-borne overspray and also has a floor in the form of a rotatable disc by means of which deposited overspray passes outside the booth to a suction collector which is also connected to the collapsible suction tube. The proposed arrangement like the others discussed above suffers from the disadvantage that it is complicated and therefore costly and prone to failure.
German Utility Model G8907538.2 describes a powder spray booth with a primary extraction system comprising a filter around the spraying area and a secondary extraction system comprising extraction channels near the booth floor through which deposited overspray is sucked. The two extraction systems have separate suction devices.
It is an object of the present invention to provide a colour spray apparatus in which powder colour can be changed quickly and easily and which, moreover, is of relatively simple construction.
It is another object of the present invention to provide a colour spray apparatus in which both air-borne and deposited overspray can be recovered by a system involving a small number of relatively simple parts which are themselves readily cleaned.
It is a further object of the present invention to provide a powder recovery system for recovering both air-borne and deposited overspray employing suction means in which the amount of suction applied to the air-borne overspray and the deposited overspray can be varied.
It is a still further object of the present
Ainsworth James
Eastwood Christopher
Perrin Robert
Calfee, Halter & Criswold, LLP
Crispino Richard
Nordson Corporation
T. Yewebdar T.
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