Abstract:
A method of and apparatus for spraying a molten thermoplastic polymer composition onto a substrate. The thermal spray apparatus of the present invention includes a source of pressurized molten polymer material, a source of pressurized hot gas, and a spray head which is in fluid communication with the source of pressurized molten polymer material and a source of pressurized hot gas. The pressurized hot gas forms a flowstream as it exits the spray head and acts to atomize and transport the molten polymer material, in a molten state, to the substrate so that the substrate is coated. The molten polymer is atomized into relatively uniform particulates of molten plastic which aids in applying a uniform coating to the subject substrate. It is emphasized that this abstract is provided to comply with the rules requiring an abstract which will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. 37 C.F.R. § 1.72(b).

Description:
This application is a continuation-in-part application of Application Ser. No. 09/253,565, filed Feb. 19, 1999, which is abandoned, and claims priority to the provisional application No. Ser. 60/194,837, filed Apr. 5, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a spray apparatus and methods of applying coatings of polymers to application surfaces. More particularly, this invention relates to a method and apparatus for transforming a solid polymer into its molten state and transporting the molten polymer to a spray head for subsequent delivery, in combination with a heated pressurized gas stream, in the form of molten droplets. When the molten polymer droplets strike the application surface, they adhere and combine to form a solid coat of polymer upon cooling. 
     2. Description of Related Art 
     It has long been appreciated that thermoplastic polymer coatings offer advantages over solvent-based coatings for providing protection afforded the substrate and to the process (in elimination of solvent vapors to the environment). The coated substrate enjoys enhancement in adhesion, chemical resistance, flex strength/modules, impact resistance, and repairability, as well as providing a broader range of material properties in the polymer coating. The substrate to be coated can be any material relatively resistant to heat, including wood, metals, glass, fibrous glass reinforced synthetic resin, or even cardboard without damaging the material surface. Employing the invention apparatus for applying a polymer coating instead of methods employed in applying a solvent-based coating material offers the environmental advantages of (1) safe and easy transportation, storage, and handling of non-hazardous raw materials; (2) no volatile organic compound (VOC) emissions during application; (3) no hazardous waste generated; and (4) no toxic organic chemical solvents or thinners, as well as the advantages of (5) no messy overspray (with attendant product loss) and (6) no shelf or pot life restrictions. 
     The earliest thermoplastic polymer coatings were electrostatic powder coatings, which involved electrostatic attraction/attachment of the thermoplastic polymer in powder form onto the metallic surface and heating to temperatures causing the polymer to melt and flow to form a continuous film. While effective, this process suffers practical limitations. The coating cannot be applied in the field. The size of the item to be coated is limited to the size of the curing/melting oven. Further, the thickness of the coating is limited by the electrical insulation (reducing or eliminating the electrostatic attraction force) as the powder thickness increases. 
     Alternatively, it is known to coat substrate surfaces using flame (or thermal) coating technology. Known thermal spray processes are characterized by chemical combustion heating including powder flame spraying, wire/rod flame spraying, and detonation/explosive flame spraying, and by electrical heating processes including plasma flame spraying. Plasma flame spraying involves the use of an ionized gas consisting of free electrons, positive ions, atoms, and molecules as a means of heating a material, such as metal powder, to a molten state at a high temperature and depositing the metal as a coating on a substrate, such as a chrome plate on an automobile part. 
     There are a number of known devices for spraying powders of high temperature thermoplastics or other high temperature polymer coatings to a variety of surfaces such as U.S. Pat. No. 3,676,638, which discloses a nozzle whereby powder is fed into the plasma stream downstream from the arc. U.S. Pat. No. 2,774,625 teaches an apparatus which uses detonation waves in spraying powders. U.S. Pat. No. 3,111,267 teaches a thermal spray gun apparatus for applying heat fusible coatings on solid objects wherein powder material is fed directly through a heating zone in the spray in which it reaches a molten or, at least, a hot plastic condition and is then propelled at a relatively high velocity onto the object to be coated. U.S. Pat. No. 3,627,204 discloses a spray nozzle arrangement for plasma gun wherein powder material is fed into a spray nozzle downstream of an arc chamber. U.S. Pat. Nos. 4,004,735 and 4,231,518 teach apparatuses for a detonating application of coating with powdered material. U.S. Pat. No. 4,290,555 teaches a method for introducing powder into a gas stream to be provided to a burner. U.S. Pat. No. 4,370,538 teaches an apparatus for spraying heated powder and the like wherein the apparatus includes a combustion chamber which is cooled by air flowing through an annular passage. U.S. Pat. No. 4,688,722 discloses a nozzle assembly for a plasma spray gun. Also, U.S. Pat. No. 4,911,363 teaches a flame spray apparatus including a combustion head provided with radially spaced longitudinal channels extending inwardly from the periphery thereof along which water passes to cool the combustion head. Finally, U.S. Pat. No. 5,520,334 discloses an air and fuel mixing chamber for a tuneable, high-velocity, thermal spray gun. 
     While overcoming some of the limitations of electrostatic polymer coating processes, flame coating is inefficient in that it creates new concerns and presents practical limitations of its own. These concerns and limitations relate to the common requirements of all conventional thermal spray systems: first, an open flame (or the equivalent thereof) to melt the thermoplastic polymer; and, second, the necessity that the polymer fed to the spray system be in powder form. In addition to its high-cost, plastic powder is difficult to handle and is conducive to material loss. 
     It is manifest that any open flame is dangerous and presents serious hazards, both to the applicator and to anyone in his general vicinity. The industrial use of flame spray coating processes essentially amounts to placing flame throwers into the hands of workers in a manufacturing facility. Another impediment to the efficiency of such processes is that plastic is a good insulator. Melting the plastic presents a heat transfer problem. Transferring heat energy into plastic by way of conduction is inefficient. Even a very hot flame is a slow, inefficient solution to the basic heat transfer problem. As a result, most flame systems can spray only about ten (10) pounds of plastic per hour or less. To compound the inefficiency of this slow delivery, most flame spray systems result in only a part of the delivered material being applied to the target substrate material. The application process is dangerous, expensive, and slow. 
     The velocity of a low velocity flame spray chemical process produces a coating of low bond strength and uneven particle melt; wherein some of the thermoplastic particles are amorphous, and overheated particles are crystalline. The plastic particle&#39;s exposure to heat energy is limited to its residence time in the flame. Each particle must reach its melt/sticky temperature during this residence time. Too short a residence time results in particles, that do not achieve this temperature, and thus do not stick to the target surface. The particles that do not stick to the surface fall off and become waste/scrap material. Too long a residence time results in particles that melt and then bum, or crystallize. 
     The problem of slow delivery has been addressed by one practitioner. Weidman, in U.S. Pat. Nos. 5,041,713 and 5,285,967, discloses high velocity thermal spray guns for spraying a melted powder of thermoplastic compounds onto a substrate to form a coating thereon. The latter patent, in particular, discloses a gun including a high velocity, oxygen fueled (HVOF) flame generator for providing an HVOF gas stream to a fluid cooled nozzle. The heat transfer problem is addressed by diverting a portion of the gas stream for preheating the powder, with the preheated powder being injected into the main gas stream at a downstream location within the nozzle. This method/apparatus approach to overcoming the heat transfer problem to produce a higher velocity spray still leaves concerns associated with the high temperature arc/flame exposure danger and the reliance on a thermoplastic polymer powder as the raw material. 
     The powder form of the thermoplastic polymer has continued as the material of choice for several reasons. Inasmuch as the powder is the only acceptable form of the material for the earlier electrostatic process for coating substrates, it was logical that the later developed high velocity delivery equipment be designed for the same form of material. Also, manufacturers and marketers of plastic flame coating equipment normally also manufacture and market thermoplastic polymer powder “specifically designed” for their equipment. For example, one company&#39;s flame coat powder “No. III,” manufactured by Dupont and sold as Nucrelo™, sells for $10.50 per pound. The same Nucrelo™ material can be purchased in pellet form for $2.00 per pound. Therefore, the ability to use a larger particle size thermoplastic polymer material can provide a significant economic advantage. 
     The most common application of flame sprayed thermoplastic coatings is for the protection of metals against corrosion. A properly applied polymer coating is perhaps the most effective corrosion barrier available. For this performance, industries involved with corrosive materials, applications, and/or environments are willing to accept the various disadvantages discussed above. Nevertheless, there is a need for an improved method and/or apparatus for applying thermoplastic polymer compositions on substrate surfaces. 
     In particular, there is a need for the ability to apply a high volume of thermoplastic polymer coating in a short time. There is a need for a clean and efficient system that applies accurately with little or no waste from over spraying. There is a need for a system that is safe in an industrial environment, both from the perspective of safety for the user and for the facility. There is a need for the ability to apply a wide range of materials in various forms, such as pellets, regrind, recycled, or blended plastic materials, as well as powdered. A system which meets all these objectives is necessarily safer, environmentally friendlier, and more economical than currently available thermal spray systems. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an apparatus and method for spraying a molten thermoplastic polymer composition onto a substrate, preferably in the absence of a flame or a high-temperature arc. The thermal spray apparatus of the present invention includes a source of pressurized molten polymer material, a source of pressurized hot gas, and a spray head which is in fluid communication with the source of pressurized molten polymer material and the source of pressurized hot gas. The pressurized hot gas forms a flowstream as it exits the spray head and acts to atomize and transport the molten polymer material, in a molten state, to the substrate so that the substrate is coated. The molten polymer is atomized into relatively uniform particulates of molten plastic which aids in applying a uniform coating to the subject substrate. 
     The spray head has an input coating passage, a separate input air passage, and a nozzle assembly. The input coating passage is in fluid communication with the source of pressurized molten polymer coating material and the input air passage is in fluid communication with the source of pressurized hot gas. The nozzle assembly has a spray surface, a hot air receiving chamber, a plurality of air delivery conduits, and a coating material conduit. The hot air receiving chamber is in fluid communication with the input air passage. The air delivery conduits extend from the air receiving chamber to the spray surface of the nozzle assembly and define a plurality of air orifices. The air delivery conduits are in fluid communication with the hot air receiving chamber. Thus, when operational, hot pressurized gas exits the air orifices to form the flowstream which is at both a high temperature and high velocity. 
     The coating material conduit extends from the input coating passage to the spray surface of the nozzle assembly and defines a material orifice. The plurality of air orifices surround at least a portion of the material orifice so that, when operational, molten polymer exits the material orifice and subsequently interacts with the hot pressurized gas exiting the air orifices. The molten polymer is subsequently atomized by and transported to the substrate by the flowstream. 
    
    
     DETAILED DESCRIPTION OF THE FIGURES 
     FIG. 1 is a schematic view of one embodiment of a thermal spray apparatus showing a source of pressurized molten polymer and a source of pressurized hot gas in fluid communication with a spray head. 
     FIG. 2 is a partial cross-sectional view of the thermal spray apparatus showing the spray head of FIG. 1 with a first embodiment of a nozzle assembly. 
     FIG. 3A is a front view of the spray head and nozzle assembly of FIG. 2 showing a plurality of air delivery conduits arranged in an arcuate pattern surrounding a portion of a material orifice. 
     FIG. 3B is an enlarged detail section taken at inset circle  3 A in FIG.  2 . 
     FIG. 4 is an exploded perspective view of a second embodiment of a nozzle assembly showing a plug member insertable therein a hollow shell. 
     FIG. 5 is a partial cross-sectional view of the thermal spray apparatus showing the second embodiment of the nozzle assembly of FIG. 4 secured to a mounting surface of the spray head. 
     FIG. 6 is a front view of an embodiment of the spray surface of the plug member of the second embodiment of the nozzle assembly showing the skew angle B. 
     FIG. 7 is a front view of an embodiment of the spray surface of the plug member of the second embodiment of the nozzle assembly. 
     FIG. 8 is a top view of an embodiment of the spray surface of the plug member of the second embodiment of the nozzle assembly showing an air mixing conduit in combination with a plurality of air delivery conduits and the coating material conduit. 
     FIG. 9 is a partial cross-sectional view of the thermal spray apparatus taken across line  9 — 9  of FIG. 8 showing the second embodiment of the nozzle assembly secured to a mounting surface of the spray head. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is more particularly described in the following examples that are intended as illustrative only since numerous modifications and variations therein will be apparent to those skilled in the art. As used in the specification and in the claims, “a,” “an,” or “the” can mean one or more, depending upon the context in which it is used. The preferred embodiment is now described with reference to the figures, in which like numbers indicate like parts throughout the figures. 
     Referring generally to FIGS. 1-9, the thermal spray apparatus  10  of the present invention includes a source of pressurized molten polymer material  20 , a source of pressurized hot gas  30 , and a spray head  40  which is in fluid communication with the source of pressurized molten polymer material  20  and the source of pressurized hot gas  30 . The pressurized hot gas  30  forms a high-energy flowstream F as it exits the spray head  40  and acts to atomize and transport the molten polymer material, in a molten state, to the substrate so that the substrate is coated with the polymer material. The molten polymer is atomized into relatively uniform fine particles of molten plastic, which aids in applying a uniform coating to the subject substrate. 
     The spray head  40  has an input end  42 , a spray end  44 , an input coating passage  46 , a separate input air passage  48 , and a nozzle assembly  50 . The input coating passage  46  extends therein to the input end  42  of the spray head  44  and is in fluid communication with the source of pressurized molten polymer material  30 . The input air passage  48  also extends therein to the input end  42  of the spray head  44 . The input air passage  48  is separate from the input coating passage  46  and is in fluid communication with the source of pressurized hot gas  30 . 
     The nozzle assembly  50  forms the spray end  44  of the spray head  40  and has a spray surface  52 , a hot air receiving chamber  54 , a plurality of air delivery conduits  56 , and a coating material conduit  60 . The hot air receiving chamber  54  is in fluid communication with the input air passage  48 . The air delivery conduits  56  extend from the hot air receiving chamber  54  to the spray surface  52  of the nozzle assembly  50  and define a plurality of air orifices  58 . The air delivery conduits  56  are in fluid communication with the hot air receiving chamber  54 . Thus, when operational, hot pressurized gas exits the air orifices  58  to form the high-energy flowstream F which is at both a high temperature and high velocity. The hot pressurized gas discharged through the air orifices  58  exit each air orifice  58  on an axis generally aligned with the major longitudinal axis L a  of the respective air delivery conduits  56 . 
     The coating material conduit  60  extends from the input coating passage  46  to the spray surface  52  of the nozzle assembly  50  and defines a material orifice  62 . The molten polymer preferably exits the material orifice  62  in a stream that is generally co-axial to the major longitudinal axis L c  of the coating material conduit  60 . The molten polymer is subsequently atomized by and transported to the substrate by the high energy flowstream F. The air orifices  58  are preferably in close proximity to the material orifice  62  for increased efficiency in atomizing the molten polymer and transporting it to the substrate after exiting the spray end of the spray head. The spacing between the air orifices  58  and the material orifice  62  may vary within a relatively wide range, depending on several factors including dimensions of the air delivery conduits  56  and the coating material conduit  60  and operating conditions. Preferably the spacing should be less than 3 inches, with less than 1.5 inches being more preferred. 
     Referring to FIG. 3A, a number of different geometries of the air orifices  58 , relative to the material orifice  62 , are contemplated. Preferably, the plurality of air orifices  58  surround at least a portion of the material orifice  62 . In one embodiment, the plurality of air orifices  58  may be arranged in a line pattern oriented to one side of the material orifice  62 . Alternatively, two opposing line patterns oriented to the sides of the material orifice  62  may be utilized. This opposing line patterns may be parallel to each other, or may have a “V” shape in front end view. In another example, the plurality of air orifices  58  may have an arcuate pattern or shape oriented to one side of the material orifice  62 . Additionally, square, rectangle, circular, triangle, and other such geometric patterns of air orifices  58  surrounding the material orifice  62  may be utilized. 
     Referring to FIGS. 2-3B, in one embodiment of the nozzle assembly  50 , the plurality of air orifices  58  are arranged in a pattern, such as an arcuate pattern or a line pattern, oriented to one side of the material orifice  62 . In this embodiment, it is preferred that the nozzle assembly  50  have a substantially “L” shape in cross-section, in which the “L” shape is formed by an integrally connected upright portion  70  and a base portion  72 . The material orifice  62  is intermediate the preferred pattern of the plurality of air orifices  58  and the longitudinally-extending base portion  72  of the nozzle assembly  50 . A portion of the upright portion  70  includes the spray surface  52  of the nozzle assembly  50 . In this “L” shape, the base portion  72  extends longitudinally outwardly away from the spray surface  52 . It is preferred that the base portion  72  be parallel to the longitudinal axis L 1  of the coating material conduit  60  so that when the molten polymer material initially exits the material orifice  62  along the longitudinal axis L c  of the coating material conduit  60 , the stream of molten polymer material is preferably initially discharged generally parallel to the base portion  72 . 
     In this embodiment, it is preferred that each of the air delivery conduits  56  formed in the upright portion  70  of the nozzle assembly  50  be inclined downwardly toward the material orifice  62  of the coating material to form an acute angle A relative to the longitudinal axis L c  of the coating material conduit  60 . The acute angle A is defined by: 1) the longitudinal axis L a  of the air delivery conduit  56 ; and 2) a plane passing through the longitudinal axis L c  of the coating material conduit  60  and the air orifice  58  of the air delivery conduit  56 . Thus, the pressurized hot gas exiting the air orifices  58  converges with and entrains the molten polymer exiting the material orifice  62  at an intermediate point a predetermined distance from the spray surface  52 . The molten polymer material which thereby becomes entrained in the high-energy flowstream F comes into contact with a portion of the base portion  72  of the nozzle assembly  50  where it is atomized and subsequently defected toward and transported onto the substrate. 
     In this embodiment of the spray head  40 , the nozzle assembly  50  may be detachably secured to the body of the spray head  40  by mechanical fasteners  80  or the like. In one example, the body of the spray head  40  has a mounting surface  41  defining an air opening  43  and a material opening  45 . As one skilled with the art will appreciate, the air opening  43  is the distal end of the input air passage  48  and the material opening  45  is the distal end of the input coating passage  46 . The mounting surface  41  further defines at least one mounting bore  47  that extends at least partially therein. The nozzle assembly  50  has at least one aperture  74  that extends through the nozzle assembly  50  module generally traverse to the spray surface  52  of the nozzle assembly  50 . Each mounting bore  47  is co-axial with one aperture  74  when the nozzle assembly  50  is detachably secured to the mounting surface  41  of the spray head  40 . When the nozzle assembly  50  is secured to the mounting surface  41  of the spray head  40 , the air opening  43  of the mounting surface  41  abuts the hot air receiving chamber  54  of the nozzle assembly  50  so that the input air passage  48  is in fluid communication with the hot air receiving chamber  54 . Also, the material opening  45  abuts the coating material conduit  60  so that the input coating passage  46  is in fluid communication with the coating material conduit  60 . 
     As one skilled in the art will appreciate, the nozzle assembly  50  may be connected to the spray head  40  by any suitable means, such as, for example, a mechanical fastener  80 , such as, a screw or bolt. In this example, the mechanical fastener  80  is inserted into the aperture  74  of the nozzle assembly  50  and is detachably engaged within the mounting bore  47  of the mounting surface  41 . To accommodate the use of a treaded mechanical fastener  80 , the mounting bore  47  and/or the aperture  74  may have a complementary threaded surface. 
     Referring now to FIGS. 4-9, in a second embodiment of the nozzle assembly  50 , the coating material conduit  60  has a longitudinal axis L c  and the plurality of air delivery conduits  56  has a longitudinal axis L a . Each air delivery conduit  56  is inclined and skewed inwardly toward the material orifice  62  of the coating material conduit  60  at a compound angle. As one skilled in the art will appreciate, the molten polymer exits the material orifice  62  as a molten steam traveling along an axis which in generally co-axial to the longitudinal axis L c  of the coating material conduit  60 . In this embodiment, each of the air delivery conduits  56  have a major direction component that is in the direction radially inwardly with respect to the longitudinal axis L c  of the coating material conduit  60 . Thus, the radially inwardly component is skewed at a skew angle B with respect to the radial direction of the longitudinal axis L c  of the coating material conduit  60 . The skew angle B is illustrated in FIG. 6, as being the acute angle defined by: 1) the plane passing through the longitudinal axis L a  of the same air delivery conduit  56 ; and 2) a plane passing through the longitudinal axis L c  of the coating material conduit  60  and the center of the respective air orifice  58 . The skew angle B is preferably between about 20° and 80°, more preferably between about 40° and 75°, and most preferably between about 50° and 70°. 
     Additionally, as shown in FIG. 9, it is preferred that each of the air delivery conduits  56  are inclined at an acute angle C which is defined by: 1) the longitudinal axis of the air delivery conduit  56 ; and 2) a plane passing through the axis L c  of the coating material conduit  60  and the center of the respective air orifice  58 . In other words, the longitudinal axis L a  of each of the air delivery conduits  56  defines the angle C with a line L c . (co-axial to the longitudinal axis L c  of the coating material conduit  60 ) passing through the center of the air orifice  58 . The acute angle C is referably between about 10° and 70°, more preferably between about 30° and 60°, and most preferably between about 40° and 50°. 
     Discharged hot pressurized gas exits each of the air orifices  58  generally along the axis of the air delivery conduits  56  and, because of the compound angle of the air delivery conduits  56 , formed by the combination of the acute angle C and the skew angle B, the discharged gas avoids the axis of the exiting molten polymer stream. Instead, the exiting hot-pressurized gas forms a high-energy flowstream F that, in this embodiment, is characterized by a swirling motion. This swirling motion creates a tornado effect. This air circulation of the tornado effect creates a low-pressure area near the material orifice  62  which acts to draw the molten polymer steam to the high-energy flowstream F and to atomize the molten polymer. The atomized molten polymer is subsequently entrained in the high-energy flowstream F which transports the atomized molten polymer, in a molten state, to the subject substrate to provide a continuous film coating thereon. The heat of the polymer and air keeps the plastic in its molten state until it strikes the target. 
     As shown in FIGS. 8 and 9, the spray head  40  may also have an air mixing conduit  90 . The air mixing conduit  90  has a longitudinal axis L m  and extends from the hot air receiving chamber  54  to the spray surface  52  to define an air mix orifice  92 . The air mixing conduit  90  is inwardly inclined toward the material orifice  62  at an acute angle D with respect to the longitudinal axis L c  of the coating material conduit  60 . The acute angle D defined by the acute angle formed by the intersection of the longitudinal axis L c  of the coating material conduit  60  and the longitudinal axis L m  of the air mixing conduit  90 . In this embodiment, the hot gas that discharges from the air mixing orifice  92  converges with the gas discharges from the plurality of air orifices  58  and the axis of the exiting molten polymer a predetermined distance from the spray surface  52  and aids in uniformly dispersing the molten polymer droplets within the high-energy flowstream F. 
     In this embodiment, the nozzle assembly  50  is preferably formed from a generally cylindrical plug member  100  and a hollow shell  120 . The plug member  100  is sized to be complementarily received and seated within the hollow shell  120 . Referring to FIGS. 4,  5  and  9 , the plug member  100  has a first end  102  and a second end  104 , the first end  102  forming the spray surface  52  and defining the air orifices  58  and the material orifice  62 , and the second end  104  defining the proximal end of the coating material conduit  60 . The plug member  100  further defines a first circumferentially-extending groove  106  near the second end  104  of the plug member  100  that forms a first waist  110  having a diameter less than the diameter of the second end  104  of the plug member  100  and substantially similar to the diameter of the first end  102  of the plug member  100 . Still further, the plug member  100  defines a second circumferentially-extending groove  108  intermediate the first waist  106  and the first end  102  to form a second waist  112 . The second waist  112  has a diameter less than the diameter of the first waist  106  and the first end  102  of the plug member  100 . The plug member  100  also has a channel  114  extending partially therein the circumferential edge of the second end  104  and the first waist  110 . 
     The hollow shell  120  has a first side  122  and an opposite second side  124 . The hollow shell  120  defines a stepped-bore  126  extending traversly through the hollow shell  120  from the first side  122  to the second side  124 . The stepped-bore  126  has a first portion  128  proximate the first side  122  and a second portion  130  extending from the first portion  122  to the second side  124 . The first portion  128  of the stepped-bore  126  has a diameter substantially equal to the diameter of the second end  104  of the plug member  100  so that the second end  104  of the plug member  100  may be complementarily secured within the first portion  128  of the stepped-bore  126 . The second portion  130  of the stepped-bore  126  has a diameter substantially equal to the diameter of the first end  102  and the first waist  110  of the plug member  100  so that the first waist  110  and the first end  102  of the plug member  100  may be complementarily secured within the second portion  130  of the stepped-bore  126 . 
     When the plug member  100  is complementarily seated within the shell  120 , the plug member  100  is secured relative to the shell  120  so that the first side  122  of the shell  120  is preferably substantially planar to the second end  104  of the plug member  100  and the second side  124  of the shell  120  is preferably substantially planar to the first end  102  of the plug member  100 . Additionally, and as one skilled in the art will appreciate, when the plug member  100  is complementarily seated and secured within the shell  120 , the second waist  112  of the plug member  100  and a portion of the interior surface of the second portion  130  of the stepped-bore  126  forms the hot air receiving chamber  54  of the spray head  40  and the channel  114  of the plug member  100  and the surrounding portions of the first and second portions  128 ,  130  of the stepped-bore  126  form an air duct  132  that extends from the second end  104  of the plug member  100 , where it abuts the air opening  43  therein the mounting surface  41 , to the formed hot air receiving chamber  54  to fluidly communicate hot pressurized gas from the input air passage  48  to the air delivery conduits  56  and, if used, the air mixing conduit  90 . 
     Similar to the first embodiment, as one skilled in the art will appreciate, the nozzle assembly  50  may be detachably secured to the mounting surface  41  of the spray head  40  by any suitable means, such as, for example, a mechanical fastener, such as, a screw or bolt. In one example, to detachably secure the nozzle assembly  50 , the hollow shell  120  has at least one aperture  134  that extends traversly through the shell  120  from the first side  122  to the second side  124 . Each mounting bore  47  within the mounting surface  41  is co-axial with one aperture  134  when the nozzle assembly  50  is detachably secured to the mounting surface  41  of the spray head  40 . When the nozzle assembly  50  is secured to the mounting surface  41  of the spray head  40  (i.e., when the second side of the shell  120  and the substantially co-planar first end of the plug member  100  of the nozzle assembly  50  is secured to the mounting surface  4 1), the air opening  43  of the mounting surface  41  abuts the formed air duct  132  of the nozzle assembly  50  so that the input air passage  48  is fluidly connected to the hot air receiving chamber  54 . Also, the material opening  45  abuts the proximal end of the coating material conduit  60  so that the input coating passage  46  is fluidly connected to the coating material conduit  60 . In this example, the mechanical fastener is inserted into the aperture  134  of the shell  120  of the nozzle assembly  50  and is detachably engaged within the mounting bore  47  of the mounting surface  41 . To accommodate the use of a treaded mechanical fastener, the mounting bore  47  and/or the aperture  134  may have a complementary threaded surface. 
     The source of pressurized molten polymer coating material preferably includes a colliquation means for converting a solid polymer to a molten polymer state and a heated supply conduit  26 . One example of a suitable colliquation means is an extruder  22 . The extruder  22  may be any commercially available extruding device, such as, for example, those wherein the material is forced through the extruder barrel with a screw, a ram, or plunger. An example of a suitable extruder  22  is a Davis Standard Extruder, Model No. 9159. The force employed to move the material through the extruder barrel and the heat energy generated from the friction resulting from the rapid movement along interface of the material and the internal wall of the extruder body causes the colliquation of the thermoplastic material, converting it from its initial solid state to a molten liquid state. 
     The colliquation means may include apparatus for melting polymer material such as, for example, thermoplastic material. The polymer material may be in the form of various shaped and sized pellets. It may be regrind, recycled, or powdered material. The thermoplastic material may be a composition of a single polymer component or a blend of multiple components (such as those disclosed in the prior art patents earlier discussed). In order to minimize the cost incurred in the use of the thermal spray apparatus, it is preferred that the polymer material utilized is in pelletized form. 
     The heated supply conduit  26  is fluidly connected to the colliquation means and the proximal end of the input coating passage  46  of the spray head  40 . The heated supply conduit  26  can maintain the temperature of polymer material within the conduit  26  at a predetermined range so that the polymer remains in the desired the molten polymer state. Heated supply conduits of this type are known in the art. For example, the heated supply conduit  26  may comprised of an electrically heated, thermostatically controlled, supply conduit supplied by Diebolt (CH 6-15, J-220-J). 
     As needed, depending on the melt point of the material to be sprayed, the degree of liquefaction required by the substrate to be coated, and/or by the desired thickness of coating to be applied, heat may also be applied externally through the extruder barrel wall (such as with a thermal jacket  28 ). In the instance of a screw extruder  22 , the output of the delivered molten polymer material can be controlled by adjustment of the rpm of the screw. Also, there may preferably be included an adjustable back pressure valve on the extruder screw or ram. The liquefied thermoplastic material is then transferred, through the heated supply conduit  26 , to the spray head  40  for application for coating a substrate material. 
     The source of pressurized molten polymer coating further includes a means for feeding the polymer material, such as the preferred solid pelletized polymer material, into the colliquation means. For example the means for feeding may comprise a hopper  24  which directs polymer material into the colliquation means in a controlled manner. 
     The source of pressurized hot gas preferably includes a source of pressurized gas  36 , a gas heater  32 , and an insulated gas line  34  coupled to the gas heater  32  and the proximal end of the input air passage  48  so that gas may be delivered to the spray head  40  under pressure and at an elevated temperature. The gas heater  32  is known to one skilled in the art and is in fluid communication with the source of pressurized gas  36 . The gas heater  32  increases the temperature of the pressurized gas to a predetermined temperature. The predetermined temperature of the pressurized gas is preferably in excess of the predetermined temperature of the molten polymer. The insulated gas line  34  allows the hot gas to be delivered to the proximal end of the input air passage  48  with limited temperature loss. Air is preferred, but other gases are contemplated such as nitrogen, argon, and the like. 
     The pumps and/or motors used in conjunction with the aforementioned equipment may be hydraulic, electric or gas powered. The horsepower of the selected motor powering the extruder  22  component will, in part, determine the capacity of the device. Thus, the greater the horsepower, the greater the potential volume of plastic sprayed per hour. 
     Although the present invention has been described with reference to specific detail of certain embodiments thereof, it is not intended that such details should be regarded as limitations upon the scope of the invention except as and to the extent that they are included in the accompanying claims.