You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION 
   In the construction of window and door assemblies, it is common to use insulated glass panel units of the general type disclosed in U.S. Pat. Nos. 5,003,747 and 6,675,537 which issued to or are owned by the assignee of the present invention. Usually, the insulated glass units include two parallel spaced rectangular glass panels having peripheral edge portions which receive a rectangular spacer frame. The edge portions and spacer frame are bonded and sealed together by a bonding compound such as a butyl rubber compound or a similar bonding material surrounding the spacer frame. The insulated glass panel unit is assembled into a surrounding rectangular sash frame and is retained by glazing members or beads. Preferably the sash frame and glazing beads are formed from extrusions of plastics material such as polyvinyl chloride (PVC). Such insulated glass panel units are commonly used in fixed window assemblies such as picture windows, single hung windows, sliding windows, bow and bay windows and sliding and swinging patio door assemblies such as disclosed, for example, in U.S. Pat. No. 6,318,036 which issued to the assignee of the present invention. 
   In insulated glass panel units as described above, it is common for the outer edges of the glass panels to be exposed and unprotected during handling and shipping or be covered by a thin layer of the bonding and sealing compound. When the outer edges of the glass panels are relatively unprotected, insulated glass panel units must be carefully handled and carefully protected during shipping. Also, when the edge surfaces of the glass panels are exposed, the personnel handling the insulated glass panel units need to wear gloves in order to avoid cutting their fingers or receiving glass splinters. It is also desirable for a fixed window assembly, such as a picture window assembly, to provide for conveniently removing the insulated glass panel unit in the event of glass breakage or damage or moisture seeps into the space between the glass panels and results in etching the inner surfaces of the glass panels. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to an improved window assembly having insulated glass panels and which is ideally suited for use in a fixed glass window unit such as a picture window, a single hung window and sliding and swinging patio doors assemblies. The window assembly of the invention provides for conveniently handling and shipping an insulated glass panel unit while protecting the unit and also provides for conveniently removing the insulated glass panel unit from the surrounding sash frame and from the inside of the window assembly in the event the unit requires repair or replacement. In addition, when the insulated glass panel unit is inserted into the surrounding sash frame, the unit locks to the sash frame so that it is precisely located and is prevented from shifting laterally relative to the sash frame without the use of spacers. 
   In accordance with one embodiment of the invention, a window assembly constructed in accordance with the invention includes a rectangular outer sash frame formed of sash frame members of extruded rigid plastics material and rigidly connected at the corners of the frame, for example, by miter cuts and welding. The outer sash frame includes a laterally inwardly projecting flange which engages and seals with the outer glass panel of the insulated glass panel unit. A rectangular inner sub-sash frame is formed with sub-sash frame members of extruded rigid plastics material and are also rigidly connected at the corners of the frame such as by miter cuts and welding. The sub-sash frame also includes a laterally inwardly projecting flange which overlaps a peripheral edge portion of one of the glass panels, and the flange is bonded to the glass panel. The sub-sash frame interlocks with the outer sash frame when assembled for precisely locating the sub-sash frame within the surrounding outer sash frame and to limit relative lateral movement. Extruded plastic glazing members have laterally inwardly projecting flange portions which overlap and seal with a peripheral edge portion of the inner glass panel. The glazing members engage a shoulder on an inner wall of the outer sash frame and have spring flange portions which project outwardly between the inner wall and the sub-sash frame and press against the sub-sash frame. 
   Other features and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an elevational view of a fixed window assembly constructed in accordance with the invention; 
       FIG. 2  is an enlarged vertical section taken generally on the line  2 - 2  of  FIG. 1  and with a center portion broken away; 
       FIG. 3  is an enlarge horizontal section taken generally on the line  3 - 3  of  FIG. 1  and with a center portion broken away; and 
       FIG. 4  is a fragmentary section similar to  FIG. 3  and showing a modification of a window assembly constructed in accordance with the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  illustrates a window assembly  10  which includes a rectangular outer sash frame  12  formed by elongated sash frame sections or members  14  and  16  which are extruded of a rigid plastics material such as rigid polyvinyl chloride (PVC). The sash frame members  14  and  16  are rigidly connected at the corners of the frame  12  by mitered cuts and welded corner joints  18 . As used herein, the term rectangular with respect to the shape of the frame  12  also includes a square frame. Also, instead of the mitered cuts and welded corner joints  18 , the frame sections or members  14  and  16  may be rigidly connected by mechanical fasteners such as screws. 
   Referring to  FIGS. 2 and 3 , each of the frame sections or members  14  and  16  are shown with the same cross-sectional configuration, and the outer tubular portion of each sash frame member is shown as substantially rectangular in cross-section. However, it is to be understood that the outer portion of each sash frame section or member may have any desired cross-sectional configuration. As also shown in  FIGS. 2 and 3 , each of the sash frame members  14  and  16  includes a laterally inwardly projecting flange portion  22  having an inwardly projecting longitudinally extending rib  24  and an inwardly projecting flexible lip seal  26 . As generally known in the industry, the flexible lip seals  26  are co-extruded of a PVC material having a lower durometer so that the lip seals are somewhat flexible when compared with the substantially rigid PVC material forming the sash members  14  and  16 . Each of the sash frame members  14  and  16  also has an inner wall  28  with an offset or step portion  29  forming a longitudinally extending shoulder  31 . 
   An insulated glass panel unit  35  includes a rectangular inner glass panel  36  and a parallel spaced rectangular outer glass panel  38 . In a conventional manner, the glass panels  36  and  38  are spaced parallel by an internal rectangular spacer frame  42  which is commonly formed from a roll formed aluminum sheet metal strip, but may also be formed from another form of spacer material. A bonding compound or material  44 , such as a compound of butyl rubber, surrounds the spacerframe  42  and bonds to the peripheral inner surfaces of the glass panels  36  and  38  to form a sealed air-tight space  46  between the glass panels. The space  46  may be filled with a suitable gas such as argon, and the spacer frame  42  may be used to enclose a desiccant material to absorb any moisture within the space  46 . 
   In accordance with the present invention, a rectangular inner sub-sash frame  50  is enclosed within the outer sash frame  12  and is formed by elongated sub-sash frame members  52  ( FIG. 2) and 54  ( FIG. 3 ) which are rigidly connected at the corners of the sub-sash frame, for example, by mitered cuts and welding or by mechanical fasteners such as screws. Each of the sub-sash frame members  52  and  54  are also extruded of a substantially rigid plastics material, such as rigid PVC, and have an outer base portion  56  with inwardly and outwardly facing slots  58  forming generally an H-shaped cross-sectional configuration. Each of the sub-sash frame members  52  and  54  also includes an integrally extruded flange portion  62  which projects laterally inwardly into overlapping relation with a peripheral edge portion of the outer glass panel  38 . 
   The flange portion  62  has an inwardly projecting inner lip  64  which contacts the outer surface of the outer glass panel  38 , and a bonding compound or material  66 , such as a butyl rubber compound, positively bonds the flange portion  62  to the outer peripheral portion of the glass panel  38 . As shown in  FIGS. 2 and 3 , the base portion  56  of the sub-sash frame members  52  and  54  closely surround the glass panel unit  35  and has substantially the same width in order to cover and protect the outer edges of the glass panels  36  and  38 . 
   In the assembly of the window, the sub-sash frame  50  is placed on a table, and the bonding material  66  is placed on the flange portion  62 . The glass panel unit  35  is then lowered into the sub-sash frame  50  until the frame  50  is bonded and sealed to the outer glass panel  38  of the insulated glass panel unit  35 . The sub-sash frame  50  and attached glass panel unit  35  are then assembled into the outer sash frame  12  by laying the outer sash frame  12  on the table and then lowering the sub-sash frame  50  and glass panel unit  35  downwardly until the peripherally extending ribs  24  of the outer sash frame flanges  22  are received within the mating grooves  58  of the sub-sash frame members  52  and  54 , as shown in  FIG. 3 . During this assembly, the flexible lip seals  26  engage the outer surface of the outer glass panel  38  and form a fluid-tight seal between the outer sash frame  12  and the glass panel unit  35 . 
   The sub-sash frame  50  and the attached insulated glass unit  35  are retained and secured within the outer sash frame  12  by a set of glazing sections or members  72  and  74  which are formed from an extrusion of rigid plastics material such as PVC . The glazing members  72  and  74  are not connected but have mitered or abutting corner joints on the inside of the window assembly  10 . Each of the glazing members  72  and  74  includes a flange portion  76  which projects laterally inwardly in overlapping relation with a peripheral edge portion of the inner glass panel  36  and has an integrally extruded flexible sealing lip  77  which is formed like the sealing lip  26  and engages the inner surface of the inner glass panel  36  to form a fluid-tight seal. Each of the glazing members  72  and  74  also includes an outer slightly curved flange portion  81  which is integrally extruded with the inner flange portion  76  and projects outwardly to engage the outer surface of the corresponding sub-sash frame member  52  or  54 . The flange portion  81  is slightly flexible and spring-like so that a corner portion of the glazing member is forced laterally outwardly into engagement with the shoulder  31  on the inner wall  28  of the corresponding outer sash frame member  14  or  16 . The glazing members  72  and  74  thereby lock the sub-assembly of the sub-sash frame  50  and glass panel unit  35  onto the outer sash frame, as shown in  FIGS. 2 and 3 . 
   Referring to  FIG. 4  which shows another embodiment of the invention, a window assembly  10 ′ includes and outer sash frame  12  and glazing member  72  and  74  as described above in connection with  FIGS. 1-3 . In this embodiment, however, a sub-sash frame  50 ′ includes sub-sash frame members  54 ′ and  52 ′ (not shown) which are formed from an extrusion of rigid plastics or PVC material and have base portion  56 ′ with slots  58 ′. Each of the sub-sash frame members also includes integrally extruded and parallel spaced flanges or wall portions  78  which project laterally inwardly between the glass panels  36  and  38  and are bonded to peripheral edge portions of the glass panels by a suitable bonding material  80  such as a butyl rubber compound. With respect to the construction and attachment of the flanges or wall portions  78  to the inner surfaces of the glass panels  36  and  38 , The disclosures of U.S. Pat. Nos. 6,286,288 and 6,536,182, which are owned by SashLite, LLC, are herein incorporated by reference. 
   From the drawings and the above description, it is apparent that a window assembly constructed in accordance with the present invention, provides desirable features and advantages. For example, after the sash frame  12 , sub-sash frame  50  and glass panel unit  35  are assembled, as shown in  FIGS. 2 and 3 , moisture from the outside is prevented from seeping into the space  46  between the glass panels  36  and  38  by not only the lip seal  26  and bonding compound  66 , but also the bonding compound  44  surrounding the spacer frame  42 . It is also apparent that when the sub-sash frame  50  is attached to the glass panel unit  35 , the frame  50  provides for not only protecting the outer edge portions of the glass panels  36  and  38 , but also for conveniently handling and transporting the glass panel unit  35  after it is assembled and until it is lowered into the outer sash frame  12 . 
   The assembly of the sub-sash frame  50  and attached glass panel unit  35  may also be conveniently removed from inside the building after the outer sash frame  12  has been installed simply by using a sharp blade and prying laterally inwardly on the glazing members  72  and  74  against the bias of the spring flanges  81  until the glazing members are released from the shoulders  31  on the outer sash frame  12 . Such convenient removal of the glass panel unit  35  with the sub-sash frame  50  is occasionally desirable from inside the building, for example, when a glass panel  36  or  38  has been damaged or broken. It is also apparent from  FIGS. 2 and 3  that the interfitting connection of the outer sash frame  12  and the sub-sash frame  50 , by means of the peripherally extending rib  24  within one of the peripherally extending grooves  58 , effectively eliminates or limits any lateral movement of the sub-sash frame  50  and glass panel unit  35  relative to the outer sash frame  12 . The modification shown in  FIG. 4  also provides all of the above advantages in addition to eliminating the spacer frame  42  and surrounding bonding material  44 . 
   While the forms of window assembly and the methods of assembly herein described constitute preferred embodiments of the invention, it is to be understood that the invention is not limited to the precise methods and forms of assembly described, and that changes may be made therein without departing from the scope and spirit of the invention as defined in the appended claims.

Summary:
A rectangular outer sash frame has extruded plastic frame members with inwardly projecting flange portions overlapping a peripheral edge portion of an outer glass panel of an insulated glass unit. A removable inner sub-sash frame interconnects with the outer sash frame and has extruded plastic frame members with inwardly projecting flange portions overlapping and bonded to a peripheral edge portion of at least one of the glass panels. Extruded plastic glazing members have flange portions overlapping a peripheral edge portion of the inner glass panel and include retaining flange portions projecting between the outer sash frame and the inner sub-sash frame. In another embodiment, the sub-sash frame has integrally extruded parallel spaced walls projecting inwardly between and bonded to the peripheral edge portions of both the inner and outer glass panels.