Abstract:
A cornerbead for installation at a drywall outside corner includes a substrate forming a nose and first and second elongated flanges. Each flange has an inner edge and an opposing outer edge and joins an opposing side of the nose at the flange&#39;s respective inner edge. The outer edge of each flange is uneven.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims benefit of U.S. Provisional Application No. 61/232,062 filed Aug. 7, 2009, the entire disclosure of which is hereby incorporated herein by reference for all purposes. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    In building construction, interior walls are generally formed by attaching preformed sheets of gypsum wallboard to a supporting frame. These sheets of wallboard are susceptible to damage, especially at outside corners. In order to protect an outside corner of a drywall structure, a cornerbead is often installed on the corner prior to painting. A conventional cornerbead is typically a thin, elongate strip of metal or plastic with a right angle bend along the strip&#39;s longitudinal axis. The right angle bend forms a nose and two flanges are joined to respective opposing sides of the nose at the flanges&#39; respective inner edges. Conventionally, each flange is 1¼ inch wide. Holes may be provided in the flanges for nailing or screwing the cornerbead to the underlying drywall structure. 
         [0003]    During the construction process an installer must place the cornerbead on the desired outside corner, hold the cornerbead in place with one hand and attach the cornerbead to the drywall using nails or screws with the other hand. Joint compound is then applied over the flange and the adjacent wallboard to provide a smooth surface transition for painting. This process is very time consuming and must be repeated many times in a typical new construction project. 
         [0004]    A conventional form of cornerbead that avoids some of the difficulties described above is made of synthetic plastic material and has a coating of water-activated adhesive on the interior surfaces of the flanges. During installation, the installer wets the interior surfaces of the flanges and then places the cornerbead against the outside corner. The adhesive allows repositioning of the cornerbead during a short interval before the adhesive dries and bonds the cornerbead to the drywall. When the adhesive has dried, the installer applies joint compound to provide a smooth surface for painting. A practical disadvantage of this type of cornerbead becomes apparent in the event of an unexpected rain shower while the installer is transporting a load of cornerbead to a job site on the open bed of a pick-up truck: the adhesive is activated and handling of the cornerbead then becomes difficult. 
         [0005]    The outer edges of the flanges of conventional cornerbead are straight. In the event that nose of the cornerbead is struck after the joint compound has hardened, the impact may cause the flange to move relative to the underlying drywall structure, which may stress the joint compound to such an extent as to create a crack that can propagate a foot or more along the straight outer edge of the flange. 
         [0006]    It has previously been proposed that a cornerbead made of synthetic plastic material should be provided with a strip of fiberglass mesh tape on each flange. The mesh tape adheres to the flanges and, when the cornerbead is placed against the drywall corner, adheres to the drywall in a manner that allows repositioning for an extended period of time. The mesh tape, being perforated, allows joint compound to penetrate through the openings so that the mesh tape is sandwiched between an inner film of joint compound adhering to the drywall and an outer film overlying the mesh tape. The mesh tape reinforces the joint compound and substantially eliminates the problem of cracking along the outer edge of the flange. However, this type of cornerbead is subject to disadvantage because it is expensive to manufacture. 
       SUMMARY OF THE INVENTION 
       [0007]    According to a first aspect of the subject matter disclosed in this application there is provided A cornerbead for installation at a drywall outside corner, the cornerbead comprising a substrate forming a nose and first and second elongated flanges, each flange having an exterior surface and an interior surface and having an inner edge and an opposing outer edge, each flange joining an opposing side of the nose at the flange&#39;s respective inner edge, and wherein the outer edge of each flange is uneven. 
         [0008]    According to a second aspect of the subject matter disclosed in this application there is provided a cornerbead for installation at a drywall outside corner, the cornerbead comprising a substrate forming a nose and first and second elongated flanges, each flange having an exterior surface and an interior surface and having an inner edge and an opposing outer edge, each flange joining an opposing side of the nose at the flange&#39;s respective inner edge, and the first flange being formed with holes penetrating from the exterior surface to the interior surface, and wherein the outer edge of each flange is sufficiently uneven that in the event that after installation of the cornerbead the nose of the cornerbead should be struck sufficiently hard to form a crack in hardened drywall compound at the outer edge of the flange, propagation of the crack along the outer edge of the flange is substantially avoided. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: 
           [0010]      FIG. 1  is a schematic view of a first length section of drywall cornerbead embodying subject matter disclosed in this application, 
           [0011]      FIG. 2  illustrates part of an extrusion and embossing apparatus for forming the cornerbead shown in  FIG. 2 , 
           [0012]      FIG. 3  is a schematic view of a second length section of cornerbead embodying subject matter disclosed in this application, and 
           [0013]      FIG. 4  is a schematic view of a third length section of cornerbead embodying subject matter disclosed in this application. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]      FIG. 1  shows a cornerbead structure comprising a nose  2  and two flanges  4  that extend from the nose. Each flange has an inner edge  8  at which it joins the nose and an outer or free edge  10  remote from the nose. It is preferred that the flanges be wider than the flanges of conventional cornerbead. Preferably, the flanges are 1¾ inch to 1⅞ inch wide. As shown, each flange is formed with multiple rows of perforations  14 . Generally, the perforations in a given row are all the same size and shape, but the perforations in one row may be of different shape and/or size from the perforations of another row. The perforations shown in  FIG. 1  are square but they may alternatively be circular or some other shape. The perforations need not be all the same shape, i.e. mixed circular and square perforations may be employed. The perforations may be as small as 1/16 inch across, i.e. 1/16 inch on a side in the case of square perforations or 1/16 inch in diameter in the case of circular perforations, but it is preferred that the perforations be about ⅛ inch across. 
         [0015]    The cornerbead shown in  FIG. 1  may be manufactured by extruding a heated softened polymer material from a die (not shown) having an orifice that is shaped to produce the cornerbead with the desired cross-sectional form. As the cornerbead is extruded from the die, each flange passes through a pair of stamping rollers  18 A,  18 B ( FIG. 2 ) that are shaped to form the desired pattern of perforations  14  in the flanges. The cornerbead is then cut into suitable lengths, e.g. of 8 feet, for sale. 
         [0016]    In use of a length of the cornerbead shown in  FIG. 2 , an installer applies an adhesion layer of drywall joint compound to the outside corner of a drywall structure, places a length of cornerbead against the corner so that it is held in position by the adhesion layer, and adjusts the position of the cornerbead before the joint compound hardens. When satisfied with the position of the cornerbead, the installer applies a finish layer of joint compound over the exterior surfaces of the flanges to provide a smooth transition from the nose to the drywall structure. The adhesion layer and the finish layer are joined together by joint compound penetrating the apertures  14  in the flanges  4 . The adhesion layer, the finish layer, and the joint compound penetrating the perforations form a monolithic structure that bonds the cornerbead securely to the drywall structure and is highly resistant to cracking in the event that the nose of the cornerbead receives an impact. 
         [0017]    The perforations in the flanges serve two main purposes. First, the perforations allow flow of joint compound through the flange, so the finish layer and the adhesion layer are able to flow together and merge to form the monolithic structure referred to above, with the flange sandwiched between the inner adhesion layer of joint compound and the outer finish layer that overlies the flange. Second, the perforations provide a crumple zone in the flange. Thus, in the event that the nose of the cornerbead is struck, the perforations allow the flange to yield slightly so that the full force of the impact is not transmitted to the outer edge of the flange. Therefore, the likelihood that the joint compound will crack along the outer edge of the flange is reduced. 
         [0018]    In order to ensure that the installer does not displace excessive joint compound by pressing the cornerbead too firmly against the cornerbead structure, so that insufficient joint compound remains between the flanges and the drywall structure to form an adequate adhesion layer, it is preferred that the flanges be provided at their inner surface with projections that space the interior surface of the flanges from the drywall structure. Preferably, the projections are narrow ribs  22  ( FIG. 1 ) extending lengthwise of the flanges, so that channels are formed between the ribs. The ribs may be formed when the cornerbead profile is extruded, by suitably designing the extrusion die orifice. Alternatively, the ribs may be formed after extrusion by passing the extruded profile between embossing rollers before the extrusion cools and hardens. 
         [0019]    The several strips of joint compound, each in a channel between two adjacent ribs, adhere firmly to the drywall structure. 
         [0020]    The ribs are aligned with the rows of perforations, so that each perforation communicates with two adjacent channels between the flange and the drywall structure. This is advantageous because it improves flow of joint compound into the perforations from the adhesion layer during installation. In addition, when the joint compound has hardened, the body of joint compound in a given perforation is connected with two strips of the adhesion layer, which provides a firmer connection between the joint compound in the perforation and the drywall structure. 
         [0021]    The ratio of the area of perforations to the area of the flange should be large enough that the joint compound located in the perforations will firmly connect the adhesion layer to the finish layer. Conversely, if the ratio is too large the lands of flange material between adjacent perforations may be too narrow, such that the cornerbead is flimsy and easily damaged. 
         [0022]    Notches that are about ⅛ inch across are provided along the outer edge of the flange. As shown in  FIG. 1 , the notches are square, but they may be of another shape. The notches may be formed by the same stamping rollers  18 A,  18 B as the perforations. The notches result in the outer edge of the flange being uneven to a sufficient degree to avoid propagation of a crack along the outer edge of the flange. Thus, in the event that the nose should be struck hard enough to crack the joint compound at the outer edge of the flange, the crack will not readily propagate along the outer edge of the flange. 
         [0023]    In accordance with a development of the subject matter described with reference to  FIGS. 1 and 2 , a pressure sensitive adhesive may be applied to the inner surface of each flange. The pressure sensitive adhesive may be in the form of several narrow strips extending lengthwise of the flange over the interior surface of the flange. The adhesive may be applied to the cornerbead after formation of the perforations and notches. Thus, after passing through the stamping rollers  18 A,  18 B, the flanges pass an adhesive application station at which the pressure sensitive adhesive is applied to the flanges. A protective backing sheet is applied over the adhesive. 
         [0024]    In the event that the cornerbead has ribs at the interior surface of the flange, as described with reference to  FIG. 1 , the adhesive may be provided on the ribs, but care must be taken to ensure that the adhesive does not block the perforations. 
         [0025]    The use of pressure sensitive adhesive allows the installer to install the cornerbead without first applying an adhesion layer of joint compound to the cornerbead structure. The installer removes the protective backing sheets from the interior surfaces of the flanges and places the length of cornerbead against an outside corner of a drywall structure. The pressure sensitive adhesive holds the length of cornerbead in position. The pressure sensitive adhesive permits ready repositioning of the cornerbead provided that the cornerbead is not pressed too firmly against the drywall structure. The installer then applies drywall joint compound to the cornerbead in order to provide a smooth transition surface from the exposed drywall to the nose  2 . The perforations ensure that the joint compound is able to penetrate the flange readily and fill any hollows beneath the flange, forming an adhesion layer and preventing voids which might lead to cracking. In the event that the cornerbead has ribs at the interior surfaces of the flanges, the ribs provide channels allowing joint compound that penetrates the perforations to flow away from the perforations, thus increasing the area of the drywall structure that is wetted by the joint compound and is bonded to the joint compound. 
         [0026]    Referring to  FIG. 3 , in a further development of the subject matter described with reference to  FIGS. 1 and 2 , the flange is composed of an inner strip  28  formed with one or two longitudinal rows of perforations  30  and an outer strip  32  formed with a row of larger trapezoidal holes  36 , with each two larger holes separated by a transverse row of smaller perforations  40 . The larger holes  36  allow ready flow of joint compound through the flange to form the adhesion layer, and the combination of the larger holes  36  and the transverse row of smaller perforations  40  results in the outer strip  32  being somewhat flexible so that it will conform readily to minor unevenness in the surface configuration of the drywall structure. It will be noted that each transverse row of smaller perforations ends at a notch  42  in the outer edge of the flange, so that the outer edge of the flange is uneven. 
         [0027]      FIG. 4  illustrates a further development of the subject matter described with reference to  FIGS. 1 and 2 . In the case of the cornerbead shown in  FIG. 4 , each flange is formed with several longitudinal rows of circular perforations. The center-to-center spacing of the perforations is the same for each row, but the perforations in the outer rows are larger than those in the inner rows. Thus, the proportion of the area of the flange that is given over to the perforations increases with distance from the nose of the cornerbead. It has been found that this configuration is advantageous because the flexibility of the flange increases towards the outer edge of the flange and the firmness with which the outer finish layer is secured to the adhesion layer increases toward the outer edge of the flange. 
         [0028]    Generally, the synthetic polymer material that is suitable for extrusion and stamping to form a cornerbead is not readily wetted by joint compound, and so the finish layer of joint compound might not adhere strongly to the flanges of the cornerbead. Stronger adhesion of the joint compound to the cornerbead may be achieved if a strip of paper is provided on the exterior surface of the flanges. The paper strips may be attached to the exterior surfaces of the respective flanges at an attachment station upstream of the stamping rollers, so that the stamping rollers will cut perforations in the paper strips also. 
         [0029]    It will be appreciated that the invention is not restricted to the particular embodiment that has been described, and that variations may be made therein without departing from the scope of the invention as defined in the appended claims, as interpreted in accordance with principles of prevailing law, including the doctrine of equivalents or any other principle that enlarges the enforceable scope of a claim beyond its literal scope. Unless the context indicates otherwise, a reference in a claim to the number of instances of an element, be it a reference to one instance or more than one instance, requires at least the stated number of instances of the element but is not intended to exclude from the scope of the claim a structure or method having more instances of that element than stated. The word “comprise” or a derivative thereof, when used in a claim, is used in a nonexclusive sense that is not intended to exclude the presence of other elements or steps in a claimed structure or method.