Abstract:
A multi-layer bell is formed from the outer shell of a multi-layer pipe in a secondary process, thereby allowing the extrusion process to be conducted at normal speeds. The bell may include a strain limiting membrane fused or mechanically secured between the outer shell extrusion layer and the inner liner extrusion layer, increased hoop or circumferential stiffness. This invention allows the extrusion process to be in its simplest form, with no adjustments to the corrugator or extruder speeds.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/186,871, filed Jun. 14, 2009, the disclosure of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates in general to plastic pipe, and more particularly to bell designs for plastic pipe and methods of making bell designs for plastic pipe. 
       BACKGROUND OF THE INVENTION 
       [0003]    Bell and spigot joints are commonly used to join pipes, including extruded plastic pipes. Bell and spigot joints typically have three components; a bell on an end of a pipe, a spigot on an end of another pipe, and a gasket. These systems typically form a water tight joint. 
         [0004]    Typical extruded multi-wall pipe includes a corrugated layer made using an extrusion process including corrugators. Bell and spigot joints are formed during the extrusion process using pipe corrugators incorporating pipe molds and a bell blocks. For example, see U.S. Pat. No. 5,405,569. The preferred process is to apply a heated gas or fluid between the outer shell and inner liner extrusion layers to form the bell and spigot. 
         [0005]    There are two well known methods for forming a bell on the end of an extruded multi-wall corrugated pipe during the extrusion process. The first is a single extrusion layer bell, which is formed from the outer shell extrusion layer. Single layer bell extrusion processes often include complicated corrugators and extruder controls to help thin or thicken the bell, slowing down the pipe extrusion process. 
         [0006]    The second method for forming a bell on the end of an extruded multi-wall corrugated pipe during the extrusion process results in a bell comprised of two plastic layers formed from the outer shell and an inner liner extrusion layer being fused together. In this process, the bell is formed by evacuating the air from between the two layers during the extrusion process. This process is complicated and is also known to slow down the extrusion speed of the corrugators. 
         [0007]    Bell design involves several issues which have caused problems in the past. Control of the bell finish diameter is significant in the performance of a bell and spigot joint. For example, the bell must have adequate strength, through reinforcement or otherwise, to maintain a cylindrical shape during transportation and usage. The bell must be able to hold its shape during spigot and gasket insertion and subsequent pressurization of the pipe assembly. 
         [0008]    One method used in the past to add strength to a pipe bell was to use reinforcing stiffeners, such as annular ribs molded into the bell. These stiffeners add strength and help maintain roundness, but typically create undulations in the inner surface of the bell. Undulations or irregularities have been known to cause problems of gasket rolling when a bell and spigot joint are assembled, as the gasket may be caught on the reinforcing ribs. 
         [0009]    It is well known that plastic materials can have numerous variables affecting the shrinkage rates during processing. In both of the known methods of forming an inline bell discussed above, the sealing surface of the inner bell is subject to the shrinkage variability. This can cause significant dimensional control issues. For example, rapid cooling of the bell may create internal thermal stresses which may result in deformation. Differential deformation between the bell and spigot of the pipe joint may also result in leakage of a pipe joint. 
         [0010]    Controlling the circumferential strain in the bell is important to prevent deformation of the bell during the pipe joining process. Controlling bell strain is also important for bells subjected to internal pressure. Bell expansion caused by sustained internal hydraulic pressure, for example, may result in loss of gasket seating pressure and of a water tight seal. 
         [0011]    In the past, hose clamps and other external devices have been used to reinforce bell and spigot joints as a field fix for problem or leaking joints. It is desirable to eliminate the need for such external sealing aids. 
       SUMMARY OF THE INVENTION 
       [0012]    A multi-layer bell is formed from the outer shell of a multi-layer pipe in a secondary process, thereby allowing the extrusion process to be conducted at normal speeds. The bell is designed with increased hoop or circumferential stiffness to alleviate deformation during the installation process. This invention may be used for dual wall, triple wall, or other multiple layer pipes. The bell design may include a strain limiting membrane mechanically secured between the outer shell extrusion layer and the inner liner extrusion layer, thereby enabling the use of a wider range of high strength membrane materials that are not necessarily compatible with the base resin of the pipe. This invention allows the extrusion process to be in its simplest form, with no adjustments to the corrugator or extruder speeds in an effort to control bell wall thickness. Production speeds may be increased by allowing a thinner outer shell extrusion layer at the pipe bell. The present invention may be used in conjunction with existing pipe extruding technology, minimizing the capital investment and reducing complexity of the pipe corrugating process as compared to current multi-layer bell forming technologies performed as part of the pipe extrusion corrugating process. 
         [0013]    Various aspects of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a cross-sectional view of a typical prior art watertight bell and spigot pipe joint. 
           [0015]      FIGS. 2A-D  are cross-sectional views of a pipe bell of the present invention during various stages of the forming process. 
           [0016]      FIG. 3  is a cross-sectional view of mold blocks used to form the pipe bell of  FIG. 2 . 
           [0017]      FIG. 4  is a cross-sectional view of a first alternative embodiment of the present invention. 
           [0018]      FIGS. 5A and 5B  are cross-sectional views of a second alternative embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0019]      FIG. 1  shows a typical multi-layer extruded plastic pipe bell and spigot joint  10 . The watertight joint is formed from two pipe sections  12 ,  14  having a bell  16  and spigot  18 , respectively. Bell pipe  12  includes an outer shell  20  and an inner liner  22 . A bell  16  is formed from the outer shell extrusion layer. The bell  16  includes annular stiffening ribs  17  near the pipe end to maintain roundness. The bell  16  also includes annular stiffening ribs  19  on its outer surface which are relatively small to avoid deforming the inner surface of the bell. Spigot pipe  14  includes an outer shell  20  and an inner liner  26 . A hollow polyisoprene or thermoplastic elastomer gasket  28  provides a watertight seal between the bell and spigot. When assembled, the inner layers  22  and  26  preferably abut to provide a smooth inner surface, but this is not essential for most applications. 
         [0020]    Referring to  FIG. 2A , a two-layer corrugated pipe  30  having an outer layer  32  fused to an inner liner  34  is extruded in a typical manner well known in the art. Preferably the pipe is made of high density polyethylene, but other materials may be used as well, such as polyvinyl chloride or polypropylene. A bell and spigot section is formed in the extruded pipe using a traveling mold block, again as is well known in the art. However, the mold block of the present invention ( FIG. 3 ) has cavities for forming the bell section with reinforcing or stiffening rings  36  adjacent the end of the bell section, and an annular reinforcing bell membrane recess  38  inward of the reinforcing rings  36 . 
         [0021]    A typical pipe has a forty-eight inch inside diameter, an outer shell wall thickness of about 0.100 inches, and an inner liner wall thickness of about 0.030 inches. Such a pipe may be extruded at a rate of about one foot/minute. The bell section length of a forty eight inch diameter pipe is about ten inches. With the present invention, there is no need to slow the extrusion process to thicken the outer shell bell section. 
         [0022]    The extrusion process is conducted with the material at a temperature of 270 to 425 degrees Fahrenheit. The material must be cooled to the glass transition temperature of the base resin material of the pipe so that the outer shell will release from the mold and hold its shape. For example, a temperature of about 225 degrees Fahrenheit may allow the outer shell bell section to release from its mold. The exact temperature may vary depending on the base resin material of the pipe. Once the pipe is cooled and removed from the mold, a secondary bell reinforcing process takes place. 
         [0023]      FIG. 2B  shows a high tensile strain limiting annular band or membrane  40  positioned in the bell membrane recess  38 . The membrane  40  may be inserted into the recess  38  without difficulty when the outer layer  32  is still pliable from the molding process. The membrane  40  is preferably formed from a fiber reinforced polymer. Preferred fibers include but are not limited to nano carbon fibers, glass fibers, propylene fibers, and polyester fibers. Preferred polymers include but are not limited to high density polyethylene, polypropylene and polyvinylchloride (PVC). The preferred fiber reinforcement is long strand glass fiber. The membrane preferably is 10% glass fiber content by weight, but can be 5% to 25% of the membrane by weight for certain applications, with the remainder being the polymer resin. The reinforcing membrane has a relatively high tensile strength, with a preferred modulus of elasticity of 1.5 to 15 times the modulus of elasticity of the base polymer used to make the pipe. The glass fiber membrane has little to no creep, which is important in maintaining the circumference and diameter of the bell and in keeping associated gasket compression for long term water tightness. 
         [0024]    The preferred embodiment of the reinforcing membrane is an extruded polypropylene. It can be extruded in eight inch wide strips having thicknesses varying from 0.05 to 0.25 inches and cut into a preferred width for various applications. The membrane strips are also cut to proper length, with the ends fused or mechanically joined together to form an annular membrane. Of course, the membrane may be formed of many other materials which are not necessarily fusible with the pipe resin. For example, a steel membrane could be used in certain applications. 
         [0025]    The width and thickness of the membrane may vary depending on the strength needed for any particular application, but it is preferred that the membrane width is about 40% of the bell length, or 4 inches in the present example. The membrane  40  provides a precise diameter, not subject to the shrinkage variability of the pipe bell during the extrusion process and minimizes bell strain during spigot and gasket insertion. The reinforcement membrane  40  will have significantly closer tolerances than that which can be achieved by manufacturing a single layer bell. When the membrane  40  is compressed between the outer shell and inner liner, closer tolerances can be achieved than what is capable with currently known processes. 
         [0026]      FIG. 2C  shows the inner liner  34  reformed to the outer shell  32  in a secondary process. After the strain limiting membrane  40  is inserted, the inner liner extrusion layer  34  is heated and formed to the contour of the outer shell extrusion layer  32 . The inside diameter of the reinforcing membrane  40  is generally identical to the inside diameter of the outer layer adjacent to the recess  38  to provide a consistent inside diameter of the ring/outer layer assembly, and a smooth inside diameter of the inner liner after it is formed to the outer layer, even under the reinforcing ribs  36 . 
         [0027]    The inner liner  34  is heated until its surface reaches a temperature above the glass transition temperature and below the melt temperature of the inner liner&#39;s thermoplastic resin material. The heating process will allow the reforming of the inner liner extrusion layer as shown in  FIG. 2D . Reforming the inner liner  34  is accomplished by applying radial force to the inner liner during or after the secondary heating process, forming the inner liner  34  to the outer layer  32 . Alternatively, the pipe ends can be temporarily capped as is well known in the art, and pressure or vacuum can be applied to radially force the inner liner outwardly to engage and form with the outer shell. In any event, reforming the inner liner  34  in close contact with the outer layer  32  traps the strain limiting membrane  40  between the two layers in the bell recess. 
         [0028]    If the outer shell  32  is also heated until its inner surface reaches a temperature above the glass transition temperature and below the melt temperature of the outer shell&#39;s thermoplastic resin material, the reforming of the inner liner  34  to the outer layer  32  may result in a binding or fusion of the two layers. This is preferred for certain applications, but is not necessary. Alternatively, the inner layer  34  and outer shell  34  may be attached together by a bonding agent or adhesive, but this too is not necessary in all applications. 
         [0029]    It is clear from  FIG. 2D  that the inner liner conforms to the shape of the inside surface of the outer layer/reinforcing ring assembly, except for the region under the reinforcing ribs  36 . During the step of forming the inner liner to the outer layer, the force applied to the inner layer  34  to expand it against the outer shell  32  is not great enough under the stiffening ribs  36  to conform the inner liner to the shape of the reinforcing ribs. 
         [0030]    It is not essential that the inner liner  34  retains a perfect cylindrical shape underneath the reinforcing ribs  36 . Even a small smoothing out the reinforcing ribs will alleviate previously known gasket rolling problems when a bell and spigot joint are assembled. The inner liner bridging the gaps formed by the stiffener ribs will enable the gasket to pass under the bell stiffener profiles, allowing bells to be designed with additional or more pronounced reinforcing stiffeners than previously used without affecting the inner gasket sliding and sealing surface. 
         [0031]      FIG. 3  shows the traveling mold  41  comprised of mold blocks  41   a,    41   b,  and  41   c.  Mold blocks  41  and  41   c  include convolutions  42  for forming corrugations on the outer pipe layer. Mold block  41   b  includes a bell shaping section  44  having annular or spiral recesses  46  for forming annular stiffening ribs in the outer pipe layer, and an annular recess  48  for forming a reinforcing membrane recess. The continuously extruded pipe will be cut in the region generally near the abutment of mold blocks  41   b  and  41   c.    
         [0032]      FIG. 4  shows an alternative embodiment of the present invention. In this embodiment, the process is the same, except that the portion of the inner liner  34 ′ adjacent the bell is trimmed or removed and replaced by a separate plastic cylinder  50  made of the same or similar material as the inner liner  34 ′ which is bondable with the outer shell  32 ′. The process of heating, expanding and attaching the plastic cylinder  50  to the outer shell  32 ′ may be accomplished in the same manner as previously described when the inner liner is used. The cylinder  50  will maintain a cylindrical shape after being joined to the outer shell  32 ′ even below the reinforcing ribs  38 ′ as previously described. Optionally, a reinforcing recess such as  38  may be formed in the cylinder  50  or the outer shell  32 ′ and a reinforcing ring  40  may be applied as previously described. 
         [0033]      FIG. 5A  shows a triple wall composite bell  60  having an outer layer  62 , an inner liner  64 , and an intermediate corrugated layer  66 . In this alternative embodiment, after the initial extrusion process and after cooling of the pipe and removal from the mold, the intermediate layer  66  is trimmed or cut near an end of the pipe section  68  as shown in  FIG. 4B . The outer shell  62  is then heated and formed in the shape of a bell, optionally with reinforcing stiffeners or ribs and a reinforcing ring recess similar to those shown in  FIG. 2A . The bell may then be finally formed by expanding inner liner  64  to conform to the outer shell in the same manner as previously described, with or without a reinforcing ring. 
         [0034]    This invention is useful for pipe diameters of 4 to 120 inches, although pipes having diameters of 60 to 120 inches are typically made by extruding flat multi-layer strips which are helically or spirally wound and bonded to form what is commonly referred to as profile wall pipe. The bells for profile wall pipe is generally roll formed, and such bells are commonly called roll formed bells. 
         [0035]    The outer shell of pipe may range in thickness from 0.070 to 0.250 inches, depending on pipe diameter, with the inner liner generally about 30% of the thickness of the outer shell. The reinforcing membrane of can vary in thickness from 10% of the outer shell thickness to 100% of the outer shell thickness and width from 10% of the bell length to 100% of the bell length depending on the pipe diameter and strength requirements. 
         [0036]    The bell design of this invention may be used with manufacturing methods other than those of the preferred embodiments. For example, the design may be used with injection molded bells, and with non-corrugated pipe. 
         [0037]    The principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.