Abstract:
A wet masking system for applying a maskant to a portion of a workpiece includes one or more pressurized mixing vessels with agitators for continuously blending the ingredients of the maskant, a dispensing system connected to the vessels for supplying metered volumes of maskant to injection valves. An electronic controller operable for controlling the system to coat the portion. A fixture for holding the workpiece with just the portion inside a cavity of the fixture includes one or more inlet ports alignable with injection ports of the injection valves injecting the maskant into the cavity. Heaters are positioned for drying or flash drying the maskant on the portion of the workpiece. Recirculation lines from the injection valves to at least one of the mixing vessels operated by the controller to flow the maskant through the recirculation lines after a period of time when no maskant is flowing through masking system.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This application relates generally to methods and apparatus for masking as may be used prior to vapor phase diffusion coating of airfoils of gas turbine engine components such as turbine blades. 
     2. Description of Related Art 
     Gas turbine engine components such as blades having airfoils often require coatings such as aluminides for environmental protection or as bond coats. Only a portion of the blade, such as the airfoil, that is exposed to the hot gases of the environment requires such protection. A remainder of the blade, such as the root or dovetail of the blade, may not require such protection. Not only does the remainder of the airfoil not require such protection, coating of that portion of the airfoil may be undesirable. 
     Turbine blades have an airfoil section which extend outward into the hot gases resulting from the combustion of fuel in the combustor portion of the engine. Because of the high temperatures and corrosive effects of such gases on the airfoil section, the standard practice has been to coat the airfoil portions of the turbine blades airfoils with protective coatings that provide insulation from the extremely high temperatures and environmental protection from the corrosive effects of the hot gases. The root or dovetail of the blade is assembled into dovetail slots on the disk or rotor portion of the engine. The walls of the root or dovetail contact the walls of the dovetail slots of the disk or rotor and are subject to fretting which may be exacerbated by coatings applied to the airfoil portion of the turbine airfoil. Thus, in order to achieve the desired properties in the various portions of the turbine blade to maximize the life of the turbine blade, it has been necessary to devise methods to properly coat the airfoil portion of the turbine airfoil without affecting the dovetail portion of the turbine airfoil. 
     One of the methods of providing the coating to the desired portion of the turbine blade has been to mask the portion of the turbine blade that does not require coating, that is to say, the dovetail, before inserting the turbine airfoil into a coating apparatus. Masking is generally considered to be useful but it is a time-consuming and labor intensive process. Accordingly, less expensive, less time-consuming and less labor intensive masking methods and apparatus for masking are desired. Less material handling is also desirable for improving quality and mask line repeatability. 
     SUMMARY OF THE INVENTION 
     An automated wet masking system for applying a maskant to a portion of a workpiece, the system includes one or more pressurized mixing vessels wherein each of the mixing vessels includes an agitator for continuously blending the ingredients of the maskant and/or maintaining the maskant in the form of a slurry mixture in suspension. A dispensing system hydraulically is connected to and downstream of the one or more pressurized mixing vessels and is operable to supply volumetrically metered amounts of the maskant to one or more injection valves. An electronic controller may be included for controlling the masking system to automatically coat the portion of a workpiece with the maskant. 
     A fixture may be included for holding the workpiece during the masking process with the portion of the workpiece inside a hollow interior or cavity of the fixture. A top opening in the fixture is operable to place only the portion of the workpiece inside the cavity and one or more inlet ports in one or more sides of the fixture are alignable with one or more injection ports of the one or more injection valves respectively for injecting the maskant into the cavity. 
     One or more heaters may be operably positioned for drying or flash drying the maskant on the portion of the workpiece. 
     One or more controllable recirculation lines hydraulically leading from the one or more injection valves to at least one of the mixing vessels and the controller may be included to flow the maskant through the recirculation lines after a period of time when no maskant is flowing through masking system. 
     The system may further include one or more injection valves may be mounted on one or more powered slides respectively, a rotary table for supporting the workpiece during the masking process at a first angular position of the rotary table, and the one or more heaters operably positioned for drying or flash drying the maskant on the portion of the workpiece at a second angular position of the rotary table. The one or more injection valves may be positioned ninety degrees apart from the one or more heaters respectively. 
     The system may further include one or more controllable recirculation lines hydraulically leading from the one or more injection valves to at least one of the mixing vessels and the controller operable to flow the maskant through the recirculation lines after a period of time when no maskant is flowing through masking system. 
     The maskant may include an alcoholic binder material and may further include Aluminum Oxide with trace amounts of Nickel Oxide and Nickel powder. 
     The system may be used for applying a maskant to a root or dovetail of gas turbine engine blade. The fixture may hold the blade the root or dovetail of gas turbine engine blade inside the hollow interior or cavity of the fixture. The top opening in the fixture is operable for placing only the root or dovetail inside the hollow interior or cavity. 
     A method for applying a maskant to a portion of a workpiece may use the automated wet masking system and includes loading maskant ingredients into a first one of the one or more pressurized mixing vessels, continuously blending the ingredients of the maskant and/or maintaining the maskant in the form of a slurry mixture in suspension using the agitator in each of the mixing vessels, hydraulically flowing the maskant from the one or more pressurized mixing vessels to the dispensing system, metering the maskant from the dispensing system and hydraulically supplying the maskant from dispensing system to one or more injection valves, and coating the portion of a workpiece with the maskant injected by the one or more injection valves. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and other features of the invention are explained in the following description, taken in connection with the accompanying drawings where: 
         FIG. 1  is a perspective view illustration of a turbine blade. 
         FIG. 2  is a perspective view illustration of the turbine blade illustrated in  FIG. 1  mounted in a masking fixture. 
         FIG. 3  is a diagrammatical illustration of an automated wet masking system for masking a root of the turbine blade mounted in the masking fixture illustrated in  FIG. 2 . 
         FIG. 4  an enlarged illustration of maskant injection valves and a rotary table for holding the masking fixture in the automated wet masking system illustrated in  FIG. 3 . 
         FIG. 5  is a perspective view illustration of the injection valves and the rotary table holding the masking fixture in the automated wet masking system illustrated in  FIG. 4 . 
         FIG. 6  is a perspective view illustration of the masking fixture illustrated in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Illustrated in  FIG. 1  is an exemplary turbofan gas turbine engine turbine blade  10  including an airfoil  12  with cooling holes  18  extending radially outwardly from a blade platform  14 . A root or dovetail  16  of the blade  10  extends radially inwardly from the blade platform  14 . The blade is representative of gas turbine engine components or other workpieces for which the wet masking system and apparatus and method disclosed herein may be used in that a portion of the component requires coating while another portion of the component coating should not be coated and, thus, may be masked prior to the coating process. The airfoil  12  requires coating while the root or dovetail  16  should not be coated. 
       FIG. 2  illustrates a fixture  30  in which the blade  10  is mounted during masking. During the masking, a maskant in the form of a slurry is introduced into a hollow interior or cavity  31  of the box like fixture  30  for coating the dovetail  16  and not the airfoil  12 . The fixture  30  includes first and second long sides  32 ,  34  and first and second short sides  36 ,  38 . A top opening  40  (illustrated in greater detail in  FIG. 6 ) of the fixture  30  is designed to allow only the dovetail  16  of the turbine blade  10  to fit into the cavity  31  of the fixture  30  and the airfoil  12  and the blade platform  14  to remain outside of the fixture  30  and sealed off from the cavity  31 . The top opening  40  of the fixture  30  allows access to the cavity  31  for cleaning and fixturing of the blade  10 . A bottom wall  33  of the fixture  30  closes off the cavity  31 . First and second inlet ports  44 ,  46  (illustrated in  FIG. 6 ) in the first and second long sides  32 ,  34  respectively are used to introduce the maskant into the cavity  31  during the masking. 
     An automated wet masking system  60  is illustrated in  FIG. 3  for applying the maskant to the dovetail  16  and forming a solid mask on the dovetail. An electronic controller  20  such as one offered by Allen Bradley electronically controls masking system  60  and an operator&#39;s station  22  allows an operator to start and shutoff the system. Ingredients used to form the maskant or slurry include metallic and metallic oxide powders along with an alcoholic binder material. An exemplary binder is a blend of Isopropyl, Methyl, and Ethyl alcohols. Exemplary powders include Aluminum Oxide with trace amounts of Nickel Oxide and Nickel powder. The maskant ingredients are manually or otherwise loaded into a pressurized pneumatic-powered first mixing vessel  62  from which a pressurized pneumatic-powered second mixing vessel  64  is filled through first and second shutoff valves  72 ,  74 , respectively and a hydraulic line  76  therebetween. Slurry or maskant lines in the system such as the hydraulic line  76  are typically stainless steel braided rubber hoses. 
     The mixing vessels include agitators  66  for continuously blending the maskant ingredients together to form and maintain a homogenous mixture of proper or desired viscosity. The wet maskant in the form of a slurry mixture is kept in suspension by the agitators  66  which, as illustrated herein, includes rotating paddles  68  driven by an electric motor  70  and gearbox. The mixing vessels and the rest of the wet masking system  60  is airtight to prevent volatization and evaporation of the binder. Suitable mixing vessels are stainless steel pressure-vacuum tanks available from Sealant Equipment, 45677 Helm Street, P.O. Box 701460, Plymouth, Mich. 48170 USA. 
     After the first and second mixing vessels  62 ,  64  are filled, the maskant is flowed through third lines  80 , opened and closed by a third shutoff valve  82 , to a volumetric metered dispensing system  88 . When the maskant is required for application, the air pressurized third lines transport the maskant into the dispensing system  88 . The dispensing system  88  disclosed herein includes first and second volumetric metering cells  90 ,  92  each of which includes a cylindrical cavity with pistons driven by a servo-motor and feed screw. The metering cells are capable of supplying and dispensing accurate volumes of the maskant. The metering cells contain pressure transducers, which verify presence of the maskant before dispensing the maskant. 
     The maskant is transferred from the first and second volumetric metering cells  90 ,  92  through high-pressure first and second transfer lines  99 ,  101  to first and second injection valves  94 ,  96  illustrated in greater detail in  FIGS. 4 and 5 . The first and second injection valves  94 ,  96  are activated when the turbine blade  10  is mounted in the fixture  30  and the fixture  30  is mounted on a rotary table  98  of the masking system  60 . The first and second injection valves  94 ,  96  include first and second maskant plenums  95 ,  97  mounted on linear first and second slides  100 ,  102  of pneumatically activated and powered first and second linear actuators  103 ,  105  respectively. The exemplary automated wet masking system  60  illustrated herein includes the first and second injection valves  94 ,  96  being located 180 degrees apart on opposite sides of the rotary table  98 . 
     The first and second maskant plenums  95 ,  97  include first and second injection ports  104 ,  106 . The first and second injection ports  104 ,  106  are aligned with the first and second inlet ports  44 ,  46  respectively of the fixture  30  in which the blade  10  is mounted during masking. After the first and second volumetric metering cells  90 ,  92  are filled with the maskant, the rotary table  98  aligns the first and second injection ports  104 ,  106  with the first and second inlet ports  44 ,  46 . Then, the linear first and second slides  100 ,  102  clamp the first and second maskant plenums  95 ,  97  against the first and second long sides  32 ,  34  respectively of the fixture  30 . This seals the first and second injection ports  104 ,  106  of the first and second injection valves  94 ,  96  against the first and second inlet ports  44 ,  46  respectively. The first and second injection ports  104 ,  106  include seals  107  made of rubber or some other elastomeric material to help seal the first and second injection ports  104 ,  106  against the first and second inlet ports  44 ,  46  respectively. 
     After the fixture  30  with the blade  10  is mounted and secured on the rotary table  98 , the wet masking system  60  uses proximity sensors (not illustrated herein) to detect the blade  10  and begin coating the root or dovetail of the blade with the maskant. The first and second injection valves  94 ,  96  are moved linearly towards and clamped against the first and second long sides  32 ,  34  of the fixture  30  by the linear first and second slides  100 ,  102  respectively. The first and second injection ports  104 ,  106  are aligned and in fluid communication with the first and second inlet ports  44 ,  46  at this point in the process. 
     The electronic controller  20  instructs the dispensing system  88  to pump a metered amount of the maskant at a predetermined or controlled rate from the first and second volumetric metering cells  90 ,  92  into the first and second valves  94 ,  96  respectively. The predetermined rate is controllable by the controller  20 . The electronic controller  20  then opens the first and second valves  94 ,  96  and the maskant is then injected by the first and second valves  94 ,  96  through the first and second inlet ports  44 ,  46  in the first and second long sides  32 ,  34  respectively and into the cavity  31 . 
     After a predetermined metered amount of maskant from the first and second volumetric metering cells  90 ,  92  has been injected into the cavity, the dovetail  16  is coated with the wet maskant. Next, the controller  20  closes the first and second valves  94 ,  96  and linearly retracts the first and second injection valves  94 ,  96  away from the first and second long sides  32 ,  34  of the fixture  30  using the linear first and second slides  100 ,  102  respectively. After the first and second injection valves  94 ,  96  have been unclamped and retracted far enough away from the first and second long sides  32 ,  34  the controller  20  rotates the rotary table  98  from a first angular position ninety degrees to a second angular position to present the first and second long sides  32 ,  34  to first and second heaters  110 ,  112 . The maskant is flash dried on the dovetail  16  by the heaters and then the operator removes the fixture  30  from the rotary table  98 . Thus, completing the automated masking of the dovetail. 
     When the maskant is not being applied, such as between blades, the maskant is recirculated by the masking system  60  to prevent the maskant from drying up in and clogging the system. First and second recirculation lines  114 ,  116  lead from the first and second valves  94 ,  96  respectively to the first mixing vessel  62 . The controller  20  controls how long the maskant isn&#39;t flowing through the system and determines when to open the first and second recirculation lines  114 ,  116  and flow the maskant therethrough, typically when no blade is mounted in the system. A predetermined amount of time with no maskant flowing through the system may be used to open the first and second recirculation lines  114 ,  116  and start flowing the maskant therethrough. An exemplary predetermined amount of time with no flow in the system is about 5 minutes. 
     The present invention has been described in an illustrative manner. It is to be understood that the terminology which has been used is intended to be in the nature of words of description rather than of limitation. While there have been described herein, what are considered to be preferred and exemplary embodiments of the present invention, other modifications of the invention shall be apparent to those skilled in the art from the teachings herein and, it is, therefore, desired to be secured in the appended claims all such modifications as fall within the true spirit and scope of the invention. 
     Accordingly, what is desired to be secured by Letters Patent of the United States is the invention as defined and differentiated in the following claims.