Patent Document

BACKGROUND 
     This application relates to a gas dynamic spray unit. More particularly, the application relates to a gas dynamic spray gun and heater core for use therewith. 
     Portable gas dynamic spray guns are being developed to widen their application and reduce the cost of using cold spray technology. Low-pressure cold spray systems are used for spraying powdered material at supersonic velocities. The low-pressure carrier gas is supplied to the spray gun at typically less than 10 bar (150 psi). The carrier gas passes through a heater assembly, which heats the carrier gas to reduce its density. The heated gas then flows through a venturi throat and is accelerated. Powdered material is then introduced into the gas jet and is expelled at a supersonic velocity towards a substrate. The powdered material typically includes a single constituent abrasive, metal, metal alloy or a blend of such materials. The powdered material can be used to prepare (clean or abrade) the surface or deposit a coating onto the substrate. 
     It is desirable to commercialize portable gas dynamic spray units, which has not been done very successfully. Prior art cold spray guns are rather heavy and can pose safety issues to the user due to the high operating temperature of the heater assembly, which may be between 400-650° C. during use. Moreover, packaging the cold spray gun components in a portable size that is also durable can be difficult. For example, the heater assembly in some cold spray guns is susceptible to breakage and electrical shorts due to rough handling. Other heater assemblies, which are rather heavy and not adapted to cold spray technology, generate heat in such a way that would expose the user to very high temperatures. 
     What is needed is a gas dynamic spray unit more suitable for commercialization. 
     SUMMARY 
     A gas dynamic spray unit is provided that includes gun housing halves, which may be a polymer, secured about a heater assembly. The heater assembly includes a one-piece, multi-passage ceramic heater core. The heater assembly is retained within the gun housing using locating features provided on the heater assembly and the gun housing. 
     The heater assembly includes a heater housing at least partially surrounding the heater core. A biasing member biases the ceramic heater core toward a tapered outlet, which is provided by a deflecting cone surrounded by an insulating cone. 
     An outlet fitting is secured to the heater assembly and supports a nozzle having a venturi that accelerates a carrier gas. The carrier gas is supplied to the nozzle by a passageway. A powder feed passage communicates with the nozzle to provide powdered material to the accelerated carrier gas, which is expelled from a tube. The passageway includes an aperture for leaking carrier gas inside the gun housing to pressurize the gun housing and prevent powdered material from infiltrating the gun housing. A shroud is secured to the gun housing about the tube to prevent damage to the tube and protect the user from contacting the hot nozzle and tube. 
     These and other features can be best understood from the following specification and drawings, the following of which is a brief description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic perspective view of an example cold spray unit. 
         FIG. 2  is a partially broken cross-sectional view of an example spray gun, which is illustrated in  FIG. 1 . 
         FIG. 3   a  is a cross-sectional view of a heater assembly taken along line  3   a - 3   a  in  FIG. 3   b.    
         FIG. 3   b  is a side elevational view of the heater assembly shown in  FIG. 3   a.    
         FIG. 3   c  is an end view of the heater assembly shown in  FIGS. 3   a  and  3   b.    
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A cold spray unit  10  is shown in  FIG. 1 . The unit  10  includes a control unit  18  connected to a power supply and a gas source  14  via a gas supply  16 . A spray gun  20  is connected to the control unit  18  by a service cable  22 . The control unit  18  controls and monitors the various inputs and outputs of the unit  10  to obtain desired deposition of powder material onto the substrate. For example, the control unit  18  monitors and regulates the process parameters such as gas pressure, gas flow rate, heater temperature, and powder system sequencing. The control unit  18  allows the operator to monitor and adjust settings and provide data on maintenance status, process efficiency, and communicate this data to a higher order control. 
     A powder feeder  24  having one or more powder containers  26  supplies powder material to the spray gun  20  for deposition onto a substrate. The powder feeder  24  supplies a regulated amount of powder to the spray gun  20 . Example powdered materials include ceramic, metal, metal alloy, or other hard materials. The powdered material is supplied to the spray gun  20  at the times and rates commanded by the control unit  18 . It is desirable for the powder containers  26  to be designed to withstand some pressure, which may be caused by an obstruction downstream during the spraying process. 
     The spray gun  20  is shown in more detail in  FIG. 2 . The spray gun  20  includes a gun housing  28 , which is two plastic halves secured to one another in one example. In one example, the gun housing  28  is constructed from an impact and heat resistant glass-reinforced polymer. Providing two halves simplifies assembly. 
     The service cable  22  is secured to a handle  29  of the spray gun  20  by a strain relief fitting  31 . The service cable  22  includes adequate protection for the internal connections and passageways that it houses. A trigger  33  is provided on a handle  29  and signals the control unit  18  to turn on or off. The control unit  18  directs the flow of carrier gas and, with appropriate feedback signals, allows feeding of powders and performs regulation of the powder-laden gas jet. An indicator on the gun housing  28  (not shown) provides confirmation to the operator of the selected operating mode. 
     A heater assembly  34  is arranged within the gun housing  28  to rapidly heat the carrier gas and reduce its density. The heater assembly  34  includes an inlet fitting  36  that receives a gas inlet  30  secured to a gas line  32 . The gas line  32  provides a carrier gas to the spray gun  20 . Features provided by the gun housing  28  are used to locate the heater assembly without requiring additional fasteners. In one example, the inlet fitting  36  includes an annular groove  38  that receives a protrusion  40  provided by the gun housing  28  to locate the rear portion of the heater assembly  34  within the spray gun  20 . 
     In one example, the inlet fitting  36  includes an aperture  99  that accommodates a heating wire for a heater core  42  ( FIG. 3 ). The aperture  99  is in communication with a passageway the supplies the carrier gas to the heater core  42 . The aperture  99  is designed to create a controlled leak within the gun housing  28  that pressurizes the spray gun  20 , which prevents infiltration of the powdered material into the gun housing  28 . The leaked carrier gas escaped between the gun housing joint halves as well as other areas of the spray gun  20  (such as the front, which is hottest). 
     Referring to  FIG. 3   a , the heater assembly  34  provides the heater core  42  that receives the carrier gas from the gas line  32  and heats it to a desired temperature, typically between 400-650° C. In the example, the heater core  42  is a one-piece ceramic structure that is relatively simple to manufacture. The ceramic heater core  42  better ensures that the gun housing  28  does not become too hot for an operator to handle. The heater core  42  is a multi-passage arrangement. In the example, the heater core  42  includes an outer wall  50  concentrically arranged about first and second spaced apart walls  52 ,  54 . An inner wall  56  is arranged within the second wall  54 . The walls  50 ,  52 ,  54 ,  56  respectively provide an outer passage  58  and first and second passages  60 ,  62 . 
     In one example, support legs  55  extend radially between the inner wall and first wall  56 ,  52 , as shown in  FIG. 3   c . Similar support legs (not shown) extend between the first and second wall  52 ,  54  and second and outer wall  54 ,  50 . In this manner, a one-piece ceramic structure can be provided. In one example, the support legs  55  are continuous the length of the flow passages, which are divided the support legs  55  into circumferentially arranged flow channels  57 , best shown in  FIG. 3C . 
     A heater housing  44 , which is stainless steel in one example, surrounds the heater core  42 . In one example, the heater housing is spin formed to reduce its weight and thermal mass. An end of the heater core  42  is received in a retaining cup  46 , which is biased forward by a biasing member  48  (for example, a spring) arranged between the retaining cup  46  and the inlet fitting  36 . The biasing member  48  accommodates thermal expansion of the heater assembly components without overstressing any of its fragile components, such as the ceramic heater core  42 . Moreover, the biasing spring  48  reduces issues relating to tolerance stack-ups within the heater assembly  34 . An end of the heating core  42  opposite the retaining cup  46  extends axially outward relative to the outer wall  50  and is received in an aperture  69  of an insulating cone  68 . The insulating cone  68  keeps the temperatures at the front of the gun housing  28  to a minimum and reduces any shock transmitted to the ceramic heater core  42 . 
     Heating elements  64  are arranged within the first and second passages  60 ,  62  in the example shown. Additional and/or fewer heating elements can be used depending upon the amount of heat desired and the packaging constraints. In operation, the carrier gas flows into the heater housing  44  through the inlet fitting  36  via the gas inlet  30  ( FIG. 2 ). The carrier gas flows along the inner surface of the heater housing  44  radially outward of the heater core  42 . This first pass of carrier gas also acts to insulate the heated gases at the interior of the heater core  42  and minimize heat transfer to the gun housing  28 . The carrier gas flows through the outer passage  58  and simultaneously through the first and second passages  60 ,  62  where the carrier gas is rapidly heated by heating elements  64 . Additional or fewer passes can be provided to obtain desired heating of the carrier gas within the packaging constraints. 
     The heated carrier gas converges to an outlet  66  where the gas is focused by a deflecting cone  70 . In one example, the deflecting cone  70  is constructed from a stainless steel material. The deflecting cone  70  prevents the erosion of the ceramic insulating cone  68  over time to reduce the service requirements for the heater assembly  34  and extend its life. 
     An outlet fitting  72  is received by an end of the heater housing  44  and secured thereto by a weld bead  74 . The outlet fitting  72  includes an indentation  90  that receives a temperature sensor  96  for temperature feedback to the control unit  18 . The temperature sensor  96  is provided near the outlet  66  for monitoring the temperature of the heater core  42 . The temperature sensor  96  is in communication with the control unit  18  so that the desired carrier gas temperature can be maintained. In one example, the unit  10  can be shut down if no heating of the carrier gas is detected. In another example, the unit  10  can be shut down if undesirably high temperatures are reached. 
     Referring to  FIGS. 2 and 3   b , bosses  92  on the gun housing  28 , which provide the locating features  73  for a supplemental insulating cone  75 . The front portion of the heater assembly  34  is closely fitted with supplemental insulating cone  75  and is retained in this position by the biasing member  48 . This way the heater assembly  34  is maintained in proper orientation within the gun housing  28  without the use of additional fasteners in the example. 
     The outlet fitting  72  receives a nozzle  76  that provides a venturi for accelerating the carrier gas. The outlet fitting  72  includes a hole  94  for receiving a set screw (not shown) that secures the nozzle  76  to the outlet fitting  72 . The nozzle  76  includes a throat  78 . In one example, a converging section is provided upstream from the throat  78 , and a diverging section is provided downstream from the throat. In one example, a powder feed passage  80  is provided in the nozzle  76  downstream from the throat  78  for introducing powder material provided through a powder feed line  82 . A tube  84  is received in an end of the nozzle  76 , which deposits the supersonic powder material on the substrate. 
     A shroud  86  is secured to the gun housing  28  and at least partially surrounds the tube  84 . The shroud  86  prevents the tube  84  from becoming bent or damaged, which would change the powder material deposition characteristics. Moreover, the shroud  86  protects the user from unwanted contact with the tube  84 , which could burn the user. Openings  88  are provided in the shroud  86  to provide cooling to the nozzle  76  and tube  84 . 
     A pressure sensor  98  ( FIG. 3   a ) is in fluid communication with the spray gun  20  to monitor the pressure of the carrier gas. In one example, the pressure sensor  98  is used to ensure that sufficient carrier gas pressure is available to cause adequate flow through the heater core  42  to prevent over heating in the event insufficient gas is present. Pressure sensor  98  is located in the space between the inlet fitting  36  and the retaining cup  46 . In this location the sensitive pressure sensor circuitry is maintained at a sufficiently cool temperature so as to ensure a long service life. 
     Although example embodiments have been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of the claims. For that reason, the following claims should be studied to determine their true scope and content.

Technology Category: b