COLOR CHANGE FOR POWDER COATING
MATERIAL APPLICATION SYSTEM
 This application claims the benefit of U.S. Provisional patent application Ser. No. 60/577,223 filed on Jun. 3, 2004 for AUTOMATED POWDER COLOR CHANGE SYSTEM, the entire disclosure of which is fully incorporated herein by reference.
TECHNICAL FIELD OF THE INVENTION
 The invention relates generally to material application systems, such as for example powder coating material application systems, and more particularly to apparatus and methods for improved material change operations such as for example quick color change.
BACKGROUND OF THE INVENTION
 A typical powder coating material application system includes one or more sources or supplies of powder coating material, a pump arrangement and a spray applicator such as a spray gun. Usually the powder coating materials are sprayed within a spray booth that contains powder overspray and also has an overspray recovery system to collect powder overspray and either reclaim it for further use or disposal. Spray guns are typically either manual guns that are hand held during operation, or automatic guns that are mounted on a support and are triggered and controlled by an electronic control system. The spray guns may be electrostatic such as corona or tribo-charging, or non-electrostatic. A supply hose is commonly used to connect a powder source such as a hopper to a pump inlet, and a feed hose is commonly used to connect a pump outlet to a spray gun inlet or multiple gun inlets. These hoses are typically flexible plastic hoses.
 Many powder coating material application systems are designed to apply a wide variety of powder coating materials to an even wider variety of objects. Different powder coating materials usually involve different colors, but may further include different types of material such as polymeric, such as for example epoxies, polyesters and hybrids of epoxies and polyesters, or metallic, for example polyester with aluminum flake. In order to change over from spraying one type or color powder coating material to another, the application system must be thoroughly cleaned of the previous material before the next material is sprayed, in order to prevent contaminating the new spraying operation. This involves not only cleaning exterior surfaces such as the spray booth and spray guns, but also the entire powder flow path from the supply to the pump and through the outlet of all of the spray guns that were used in the previous spraying operation. These color change or material change operations are time and labor intensive and therefore are a significant cost factor.
SUMMARY OF THE INVENTION
 The invention contemplates in one aspect a powder coating material application system having a material changer function that is fast and efficient, such as for example for a color change operation. The changer function allows for material flow in one direction and a purge flow in an opposite direction. In one embodiment, a material
changer is provided that has a common feed passage connected to a plurality of material supplies, with each supply having an associated inlet passage that opens to the common passage at a port that is sealed by a valve member. The valve member seals the port with a near bore line seal. In a particular embodiment the port is formed in the wall that defines the common feed passage. In a further embodiment the valve member is inflatable by air pressure and a portion of the valve member slightly protrudes into the common feed passage, in effect creating a "zero cavity" or near bore line seal. The common feed passage can be reverse purged with all of the inlet valves closed to an outlet that may be connected to a waste receptacle or other powder collector such as the spray booth. An optional forward purge function may also be used.
 In accordance with another aspect of the invention, a powder coating system includes a material changer function as described above, a pump and an applicator. In a supply mode of operation for the changer function, the pump produces a negative pressure to suck powder from the changer and positive pressure to push powder to the applicator. In a purge mode of operation of the changer function, the pump produces a positive pressure back to the changer, and optionally positive pressure to the applicator. In one embodiment, the pump produces a soft purge function and a hard purge function, and the changer may be purged to a waste or dump outlet and also through the last inlet used during a coating operation.
 The invention further contemplates a powder coating material supply having a changer function and a pump function wherein the pump function sucks material from a selected supply during a supply mode of operation and provides compressed air by reverse flow to the changer function during a purge mode of operation.
 The invention in another aspect contemplates a material changer, such as can be used for example for color change, for a powder coating system. The changer in one embodiment includes control valves that form near bore line seals with a common feed passage, and a reverse purge flow feature. The reverse purge feature may be realized in the form of a reverse flow purge through the common feed passage to an outlet, and optionally through the previous used inlet. An optional purge feature in the forward direction may also be provided. In another embodiment the changer, or at least the material flow path within the changer, is made from low impact fusion material, for example, PTFE (TEFLONTM) or high density polyethylene. In another embodiment each control valve includes a valve member such as a bladder that is made of elastic material, such as for example natural rubber, and expands under air pressure to seal a port that joins an inlet to a common feed passage. The invention further contemplates use of a changer function in accordance with the invention in combination with material application system that includes a pump, applicator such as a spray gun for example and may further include a spray booth. In one embodiment the pump may be a dense phase pump.
 The invention in another aspect contemplates the various methods embodied in the use of such functions as the material changer and powder coating system as described above, as well as in another embodiment a method for reverse purging a material changer. In another method