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You are an expert at summarizing long articles. Proceed to summarize the following text: 
FIELD OF THE INVENTION 
     This invention relates to the field of door manufacture and more particularly to the manufacturing of an insulated door having a glazing or window section installed therein, where the frame of said window section is installed during manufacture and is flush with both faces of the door. 
     BACKGROUND OF THE INVENTION 
     The manufacture of decorative insulated doors for residential use is a process that marries a coloured and decorative skin or sheathing to both sides of a perimeter frame. The frame usually constructed from wood, has specially formed recesses into which the longitudinal edges of the decorative sheathing can be inserted after they have been in turn correspondingly bent to fit said recesses. Frictional engagement between the frame and sheathing components secure this assembly and lend rigidity to the door structure. This door and frame unit is typically insulated with an expanding insulating foam material which is applied through very small holes drilled through the wooden frame to provide access to the internal void. 
     As the insulating foam expands to fill the internal door cavity, it is likely to flow outwardly through any minute perimeter imperfections between sheathing and frame resulting in time consuming cleanup by production personnel. 
     As a result of the foregoing, frames for glazing or window units have not generally been installed at the time of manufacture. It is common that a portion of an insulated door is cut out after manufacture to accommodate pre-made glazing units available in a variety of shapes and sizes. Said units comprise a frame, having an outer portion which is slipped into the cut out portion from the outside and which bears against the outer face of the door. An inner portion is installed in opposed facing relation so as to bear against the inner face of the door. The inner and outer two portions are connected by screws passing from the inside portion to the outer. The two parts when assembled retain a separate thermally insulated glass window unit. 
     This process is typically not cost effective since the part cut out from the door, once removed, have no further use and are scrapped. The insulating value of the door is obviously compromised since any retro-fitted frame will be subject to thermal transmission. Further, the window frame must be located on the outside faces of the door surrounding the window opening which can be susceptible to moisture infiltration. The door face loses the smooth linear expression by being interrupted by a bulky frame. U.S. Pat. No. 4,897,975 discloses a door light which includes a frame incorporated structurally into the door and a glass removable from the frame. Glazing stops are provided to secure within the frame to allow for the insertion and removal of glass panels. While this patent does provide an integral door light frame, it may not adequately provide a weather-proof scal about the glass panel, nor does it provide an adequately insulated door where the insulation is continuous with the glass support portion of the door. There was therefore a need to develop a simple and efficient method for making an insulated door which is rigid and strong and can accommodate a suitable glass window that has a weather-proof seal. As will be evident, the present invention obviates the problems associated with the prior art methods. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention there is provided a method and apparatus for manufacturing a door, wherein the door has a rigid wooden frame having interior and exterior faces, metal sheathing secured to the rigid wooden frame by means of preformed grooves or recesses therein which securely accept suitably formed edge portions of the sheathing, a window frame, which may be of a suitable extruded plastic, the window frame for placing within a window opening defined by the wooden frame and sheathing, that is, formed by interior and exterior sheathing assembled onto the wooden frame, the window frame for accepting for mounting therein a thermally insulated window. 
     The window frame has integral, outwardly facing recesses to accept suitably formed edges of the interior and exterior sheathing, such as edges bent at right angles to the sheathing. 
     A resilient insulation seal is integrally formed at, or mounted to, the extremity of each of the outwardly facing recesses such as along the distal edge of the recess. The seal engages the inner faces of the door sheathing thereby preventing expanding foam insulation from escaping around the perimeter of the window frame. The insulation seals may be components which are separate from the window frame and are installed around the inner and outer perimeter of the window frame during assembly. An integrally formed shoulder is formed around an exterior perimeter, parallel to the length of each window frame member, to retain a double glazed window therein. The shoulder has a longitudinal internal void or thermal break. The interior perimeter of the window frame has formed therearound a slot or channel. A shoulder, notch or step is formed along at least one sidewall of the channel for mating engagement with a corresponding shoulder, notch or step in a mating resilient clip. The clip may be a channel member having opposed side walls, where a first side wall is mated into the slot or channel in the window frame to interlock the corresponding notches on the frame and clip. The second side wall is thereby pressed against the face of the double glazed window pane retained within the window frame. 
     Advantageously, the second side wall may be formed of a length to allow double glazed window units of different thickness to be accommodated within window frames having the same dimensions. In order that the clip functions as a spring clip to urge the second side wall against the window pane, the first side wall and the web extending between the first and second side walls may be formed at a slightly acute included angle therebetween. 
     According to an object of the present invention there is provided a flush glazed door comprising; 
     a rigid wooden frame having an opening for mounting an insulated window therein; 
     a metal sheathing secured on each side of said frame; 
     a window frame unit mounted within said wooden frame within said opening; 
     an insulated window positioned within said window frame unit; and 
     means for retaining said window within said frame in an air-tight and weatherproof manner. 
     The rigid wooden frame has a hollow internal cavity for the introduction of insulating foam therein such that when the window frame unit is mounted within said opening it is partially embedded within the introduced insulating foam as it cures. 
     According to another aspect of the present invention is a method for assembling a flush glazed door, said method comprising the steps of: 
     constructing a rigid wooden perimeter frame having an internal cavity therein; 
     fitting an exterior and interior metal sheathing to said frame; 
     securing a window frame within said perimeter frame; 
     filling said interior cavity with expanding foam insulation such that said window frame is partially embedded within said perimeter frame; and 
     mounting a window within said window frame. 
     The present invention provides a flush glazed door that is durable and thermally insulating and which allows one to manufacture the door without the glass insert therein such that no damage will occur to the glass as it can be easily installed at a job site. The door can accommodate varying sizes and thickness of glass and by virtue of the design of the window frame, a window can be installed therein and tightly secured in an airtight and weatherproof manner. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A detailed description of the preferred embodiments are provided herein below with reference to the following drawings in which: 
     FIG. 1 is a perspective view of one embodiment of the flush glazed door of the present invention; 
     FIG. 2 is an enlarged view of one corner identified as “ 2 ” of the door assembly; 
     FIG. 3 is an enlarged sectional view along line  3 — 3  in FIG. 1; 
     FIG. 4 is an enlarged view of the exterior shoulder potion of a window frame member of the present invention. 
     FIG. 5 is an enlarged view cross-sectional through a window frame member; 
     FIG. 6 is a cross-sectional view of a retainer clip; and 
     FIG. 7 is a view of the retainer clip installed against a window pane. 
    
    
     In the drawings, preferred embodiments of the invention are illustrated by way of example. It is to be expressly understood that the description and drawings are only for the purpose of illustration and as an aid to understanding and are not intended as a definition of the limits of the invention. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As seen in FIGS. 1-3, door  10  comprises perimeter frame members  12 , exterior and interior faces  14  and  16  and a window unit  18 . Frame  12  has preformed vertical grooves  20  formed in both the exterior and interior faces to accept and securely retain suitably formed edge portions  22  of the sheathing. An extruded plastic window frame  24  is positioned within door frame  12  and sheathing panels are trimmed to a size that will permit their preformed edges  22  to engage both the recesses in the perimeter frame  12  and corresponding recesses  26  formed around the exterior and interior perimeter of the window frame  24 . It is preferred that each sheathing panel be made of a single piece of suitable steel that is punched and flanged to add rigidity and strength to the door. While the sheathing is fixed via grooves it is preferred that the sheath be bent at 90° angles to form lips  27  over the top and bottom edges of the door at either sides as desired. 
     It is understood by one skilled in the art, that the extruded plastic frame can be made of any suitable material. Such materials may include but are not restricted to plastic resin such as polyvinyl chloride of a suitable weatherability characteristic and of any suitable colour. 
     As can be seen in FIGS. 3-7, the extruded plastic window frame  24  has an integrally formed shoulder  28  formed around its exterior perimeter which restrains window  18  against outward movement. Shoulder  28  has a continuous cavity along its length to provide a thermal break. The interior face  30  of shoulder  28  is formed at right angles to the window surround  32 , as can be seen by angle A in FIG.  5 . Shoulder  28  terminates at its upper end in an inwardly projecting lip  34 . Lip  34  contacts the glass pane of window unit  18  to provide a watertight seal when window unit  18  is inserted into the frame. Flanges  36  are integrally formed with window frame  24  to define recesses  26  for securely retaining therein suitably formed edge portions  22  of the door sheathing. 
     Flexible insulation seals  38  are formed along the free edge of flanges  36 . Seals  38  press tightly against the inner faces of the door sheathing when installed in recesses  26  to prevent escape of expanding foam insulation  39  injected into the door frame cavity around the perimeter of the window frame. Seals  38  may be of a suitably flexible resilient material which may be attached to, or formed as part of, flanges  36  so as to be cantilevered outwardly of the edges of flanges  36 . After the door unit is fully assembled the internal cavity is completely filled with expanding foam insulation  39 . 
     As seen in FIG. 5, the inner edge of window frame  24  has channel  40 , parallel to surround  32 , and opening toward the inside face  16  of the door. One or both side walls of channel  40  may be notched to form shoulder  42 , although in FIG. 5 only the radially outer side wall (radially outer relative to the center of the window and lying in the plane of the window) is notched. Resilient retaining clip or strip  44 , illustrated in FIG. 6, resiliently snaps into place to mate step  50  with shoulder  42  as side wall  46  is slid into channel  40 . The clip can be fabricated in any suitable resilient and durable material such as plastic. The clip is cut to a length to suit the window frame dimensions. Side wall  46  may also have a raised projection  48  on its inside surface, opposite to step  50 . Web  52  extends from the side wall  46  and is formed so as to be canted slightly toward the window when installed as indicated by angle B, that is, side wall  46  and web  52  form a slightly acute angle between them. Web  52  extends from side wall  46  to side wall  54 , which also projects toward the window when clip  44  is installed. Resilient retaining clip  44  may be manufactured so as to provide clips having side walls  54  of different lengths so as to accommodate double glazed window units of different thickness, that is, having different dimensions. As will be noted from FIG. 7, the retaining clip  44  cannot be completely seated within channel  40  so as to bring step  50  and shoulder  42  into locking engagement until side wall  54  engages the inner pane of glass  53  causing web  52  to be resiliently forced into a generally perpendicular orientation relative to side wall  46 . Clip  44  thus acts as a spring clip in frictional engagement with glass  53 , that is, with the inside face of the window unit. The glass for use in the present invention is preferably a thermally insulated glass. 
     The double glazed window is generally manufactured with dimensions which are slightly undersize to that of the window frame. It is also understood that the window can be manufactured of varying widths and still be mounted within the window frame. Setting blocks  56  enable the window to be easily centred between the upper and lower portions of the frame. 
     Construction of an insulated door using the previously described process requires only that a rigid wooden perimeter frame be constructed, a window frame be positioned within said frame, exterior and interior sheathing strips be fitted thereto and the internal cavity be filled with expanding foam insulation. This method and the construction of the insulated door does not allow the expanding insulating foam to flow outwardly from the cavity. Thus the door of the present invention does not require any additional cleanup of any perimeters. The double glazed window need not be installed within the frame until the door is ready to be hung, thereby reducing damage or breakage during shipping. The method of manufacturing the door of the present invention does not compromise the insulating value of the finished door in any manner. 
     Installation of the window within the frame simply requires that double-sided glazing tape  60  as seen in FIG. 4, similar to CASCADE 2021™ (glazing tape) for example, be applied to the inside perimeter of shoulder  28 . and the protective covering strip removed. Setting blocks  56  are then placed in the bottom portion of the window frame and the bottom of the double glazed window is positioned on the blocks and the unit centred within the opening. The window is then pressed firmly against the glazing tape. Lengths of retaining clip  44  are then cut to suit the frame dimensions, side wall  46  is inserted into channel  40  formed in the frame and step  50  snapped into position behind shoulder  42 . 
     It is understood by those skilled in the art that the door of the present invention can be fabricated in various dimensions to accommodate various sized window units therein. The door has excellent insulating valve and is not susceptible to moisture infiltration about the window. 
     As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof.

Summary:
The invention is directed to an insulated door and a method for manufacturing such have a glazing or window section installed therein. The frame of the window section is installed during manufacture and is flush with both faces of the door. The flush glazed door comprises a rigid wooden frame having an opening for mounting an insulated window therein, a metal sheathing secured on each side of the frame, a window frame unit mounted within the wooden frame within the opening and an insulated window positioned within the window frame unit. The window is retained within the frame in air-tight and weatherproof manner.