Abstract:
An observation window is provided for use with a spray booth during a spray coating process where the observation window is located in a position to permit an operator to observe the spray coating process. The observation window is controlled to provide light transmission in the window suitable for the specific spray process being performed. The control can be automatic or operator controlled. The window is adapted to have a different light transmission during different spray processes such as plasma spray and HVOF spray.

Description:
BACKGROUND 
       [0001]    Thermal spraying, and particularly plasma spraying and high velocity oxygen fuel (HVOF) spraying, are well established processes used in many metallurgical manufacturing processes. Thermal spraying is done normally in a thermal spray booth or enclosure. 
         [0002]    The primary role of a thermal spray enclosure is to contain and/or control various hazards associated with thermal spray processing of materials. Historically, thermal spray booths were used to shield the shop floor from the intense sound pressures, dust and fumes, and ultraviolet light generated during spray operations. In the past years, the introduction of robotics has led to an expanded role for booths as barriers to protect humans from being struck by fast-moving robotic arms. The most modern spray enclosures are now designed to minimize operator exposure to a variety of thermal spray hazards. Spray boxes and fully automated spray booths that do not require an operator to be present in the spray environment are becoming more common, and in many situations have become an accepted requirement. 
         [0003]    The use of an enclosed space to reduce hazards has considerably increased the safety of thermal spray operators and shop personnel that work in the vicinity of thermal spray devices. However, an enclosure introduces new hazards that must be addressed. A wide variety of gases are used in thermal spray processing. Unless proper care is taken, it is possible to create dangerous situations within an enclosure. An inert gas leak can displace the air, leading to an asphyxiation hazard. A fuel leak or build-up of an explosive metal powder can set up the possibility of a violent explosion. Thermal spray enclosures must be designed to provide simple egress of operators during an emergency situation. Operators inside a spray booth may be unaware of emergency situations on the shop floor, such as fire alarms, unless special provisions are made. 
         [0004]    The thermal spray enclosure is the most important safety device used in thermal spray processing. Because the spray operations are conducted within its confines, all of the energy sources (gas, electricity, and water), the feedstock materials, and all of the process effluents (heat, dust, fumes, sound and ultraviolet light) are present. Proper mitigation of all of these hazards requires careful thought in the design and operation of a spray booth. 
         [0005]    One concern that is important is to protect operators from point source radiation generated by thermal spray processes, particularly plasma and high velocity oxy-fuel (HVOF) spray processes. Operators need to view the process so they do not crash the robotic arm holding the spray device. Window tinting for plasma spray is too dark when the chamber is used for HVOF spraying. Conversely, the HVOF tint can cause eye fatigue and irritation when the chamber is used for plasma spraying. 
         [0006]    The use of the same thermal spray chamber for plasma spray and HVOF spray processes allows for improved efficiency of a spray facility. Accordingly, allowing the operator to view the spray process without eye fatigue or eye damage is important. 
       SUMMARY 
       [0007]    This invention is a window used for viewing the operation of a thermal spray process in a thermal spray chamber. The window has an adjustable tint shade depending on the thermal spray process being used in the chamber. The window uses electro tinting technology to switch between clear, when no thermal spraying is being done, to a first tint suitable for a first thermal spray process such as plasma spraying, and to a second tint suitable for a second thermal spray process such as HVOF spraying. The spray booth or chamber gas/powder control unit will also include the tinting control circuit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a diagram showing the steps of process of this invention. 
           [0009]      FIG. 2  is a schematic view of a thermal spray booth used for multiple spray processes such as plasma spraying and HVOF spraying. 
           [0010]      FIG. 3  is a view of a window adjusted in tint for HVOF spraying. 
           [0011]      FIG. 4  is a view of a window adjusted in tint for plasma spraying. 
           [0012]      FIG. 5  is a table (Table I) that shows transmittance requirements for shade values of the window of this invention. 
           [0013]      FIG. 6  is a section view of a window with the light shade not energized. 
           [0014]      FIG. 7  is a section view of a window with the light shade energized to produce a dark shade window. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    Spray coating of parts is performed in many industries. One example is spray coating of parts for gas turbine engines. Plasma spraying produces one type of coating while HVOF spraying produces a different type of coating. It is desirable to use the same spray booth or chamber for both processes, as well as for other spray processes, because mounting the specific spray gun on the gun mount of the robotic arm is the only change in equipment needed. 
         [0016]    The steps in using the present invention,  10  generally, involve placing a part on a mount in a spray booth or chamber in step  11 . When the specific spray process is known, the coating gun for that process is mounted on a gun mount interface in step  13 . The operator who will be observing and/or controlling the spray process then adjusts the shade of the observation window in step  15 . This can be done manually or automatically. 
         [0017]    In some instances, simply mounting a specific coating gun will send a signal to adjust the shade of the observation window. The robotic control system that runs a predetermined spray control program would call out the shade in a program routine and trigger a relay or controller to switch the shade on or off. A key type system can also do that instead, if manual control is wanted. When the gun is bolted on to the robot in step  13 , a sensor or switch is either tripped or not tripped by the way the gun is mounted onto the robot. 
         [0018]    Step  17  is an optional step to fine tune the precise shade darkness of the observation window using a potentiometer as described below. The part is then spray coated using the coating gun in step  19 . When the spraying is complete, the gun is turned off and the part removed, in step  21 , the shade of the operating window can manually or automatically return to clear since there is no need to protect the operator from the lights. 
         [0019]      FIG. 2  shows a typical spray booth configuration  22 , with a mount  23  for holding a part  25  proximate the spray equipment  27 . Booth controls for controlling the spray process are located in control box  29  and contain the program used by the robotic arm  31  and coating gun  33 . Coating gun  33  may be a plasma spray gun, a HVOF spray gun, or other similar spray guns. 
         [0020]    The operator is positioned at operator control console  35  shown in  FIG. 2  and  FIG. 3  where the operator can view the operation through window  37 .  FIG. 3  shows the relationship between window  37 , which is controlled to have appropriate shading, and the operator control console  35 , outside spray booth  22 .  FIG. 4  illustrates the inside of spray booth  22  as seen from the outside by an operator. Mount  23  holds part  25  so that spray equipment  27  can perform a spray process using robotic arm  31  and coating gun  33 . 
         [0021]    Window  37  can be pre-set at different shades of darkness for different spraying processes. The robotic control system that runs the spraying process according to a recipe that identifies the shade needed for that process and trigger a relay to switch the shade on or off. Alternatively a key type system for the operator can be used. In any case, window  37  is set to operate for the specific spray system such as plasma spray or HVOF spray. The spray system is mounted to a robot via a gun mount bolt and at that point a sensor/switch is either tripped or not tripped by the way the gun is mounted to the robot. 
         [0022]      FIG. 5  is Table  1  showing the transmittance for various shades that the window is set at, from clear to shade number  14 . The specific values of luminous transmittance for shades ranging from 9 to 12 and 6 to 8 can be identified. 
         [0023]    When the booth is used for plasma spray coating, the ultraviolet light produced has a wavelength ranging from 280 to 220 nanometers at greater than 30 W/m 2 . Window  37  would be darkened to a shade ranging from 9 to 12. HVOF ultraviolet light at grater than 30 W/m 2  has a peak wavelength of 280 to 360 nanometers. Window  37  would be darkened to a shade ranging from 6 to 8. 
         [0024]    Liquid crystal windows use liquid crystals, which are a state of matter having properties between those of a conventional liquid and those of a solid a crystal. Liquid crystals find wide use in liquid crystal displays that rely on the optical properties of certain liquid crystalline substances in the presence or absence of an electrical field. In a typical device, a liquid crystal layer sits between two polarizers that are crossed (oriented at 90° to one another). The liquid crystal alignment is chosen so that its relaxed phase is a twisted one. This twisted phase reorients light that has passed through the first polarizer, allowing its transmission through the second polarizer. The device thus appears transparent. When an electric field is applied to the LC layer, the long molecular axes tend to align parallel to the electric field thus gradually untwisting in the center of the liquid crystal layer. In this state, the LC molecules do not reorient light, so the light polarized at the first polarizer is absorbed at the second polarizer, and the device loses transparency with increasing voltage. 
         [0025]    Alternatively, the liquid crystals can be oriented as shown in  FIGS. 6 and 7 , as follows. The light transmission of window  37  is controlled by changing the voltage potential of a window constructed as shown in  FIG. 6 . Light, shown by arrows  41  passes through two panes of glass  43  and  45 , with liquid crystal  47  between the panes. In  FIG. 6 , the light shade is not energized and light (and therefore vision) passes freely through glass  43 , liquid crystal  47  and pane  45 . The liquid crystals have elongated molecules that lay parallel to each other, as shown in  FIG. 6 . 
         [0026]      FIG. 7  illustrates a dark shade energized window. When a current is applied to polarizers  49  and  51 , liquid crystal  47  rotates to a closed position when energized. Thus the amount of light that passes through window panes  43  and  45  is reduced, shown by arrows  53 . Looking through the window when liquid crystals  47  are at 90 degrees to the pane surfaces, the darkest shade is seen. If viewed through an angle of less than 90 degrees, the shading appears lighter, having values such as those in  FIG. 5  Table 1. This essentially is a venetian blind effect, known in the art as angle dependency. 
         [0027]    A voltage potential between polarizers  49  and  51  makes window  37  darker or lighter, depending on the liquid crystal density or alignment needed to block light. The more voltage used and the polarity of the voltage would cause liquid crystals  47  to align between panes  43  and  45  to function the way a venetian blind would function, as described above. The more current or voltage, the darker the glass would get because more liquid crystals would be activated. This can be fine tuned with a potentiometer in step  17  of  FIG. 1 , and shown schematically in  FIG. 3  at  35   a.  To reverse the process, the voltage or current flow is reversed to reorient liquid crystals  47  to reorient to their original state. 
         [0028]    A spray booth for plasma spray coating and HVOF spray coating with separate shades for window  37  that is used by the operator to monitor the movement of the spray gun effectively protected the operator&#39;s eyes from any damage. 
         [0029]    While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.