Abstract:
A process for producing a pipe fitting of thermally weldable material for attachment to or repair of conduits of thermally weldable material. The first injection mold has channels formed as windings and having channel walls. A heating wire is formed to a wavy heating strand which is laid into the first injection mold between the channel walls. A first welding material is introduced into the channels so that a first broad side of the wavy shape of the heating strand is pressed against a first mold surface of the injection mold and an intermediate product with embedded wavy heating strand is formed. The outside surface is formed by the first mold surface against which the heating strand is pressed and provides the contact surface with the conduit in the end product. The first broad side of the wavy shape of the heating strand remains visible at the outer surface of the intermediate product. The intermediate product is placed into a second injection mold for producing a pipe fitting so that the outer surface rests on a second mold surface of the second injection mold. A second welding material is introduced and the contact surface of the pipe fitting is formed. The first broad side of the heating strand remains visible at the contact surface after solidification of the second welding material.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention pertains to a process for the production of a pipe fitting, at least certain areas of which consist of thermally weldable material (weld material), which fitting is designed for attachment to, or for the repair of, conduits which also consist of thermally weldable material, wherein several windings of at least one electrical heating wire are first laid in the desired welding area of the future contact surface between the pipe fitting and the conduit, wherein contacts are attached, if desired, to the two ends of the heating wire laid in windings, to which contacts an electrical power source is connected during the later welding of the pipe fitting, and then the position of the windings of the heating wire and possibly the position of the contacts are fixed by welding material which is applied in the injection-molding process. Pipe fittings are used for various purposes to be mounted on a previously installed conduit for media such as gas or water, possibly while the conduit is under the pressure of the medium. Appropriate pipe fittings are also used for repair purposes or to connect sections of pipe. At least certain areas of the pipe fitting and of the conduit consist of thermoplastic resin. In these cases, the pipe fitting has a welding area at its contact surface with the conduit, where several windings of at least one electric heating wire are laid. Contacts which serve to connect the wire to a source of electric power when the pipe fitting is welded on at a later time are usually attached to the ends of the heating wire. 
     2. Description of the Related Art 
     In the known process, windings of a heating wire of the type in question can be laid directly in the injection mold used to produce the pipe fitting; this wire is thus integrated into the welding material of the pipe fitting during the injection-molding process (CH 528,697 C). The heating wire is surrounded by a plastic jacket, which hinders the flow of heat to the contact surface between the pipe fitting and the conduit. The windings of the heating wire can be laid in the mold as a monofilar or bifilar spiral or as a meander. 
     It is also known that a plate-shaped intermediate product can be produced out of the heating wire. This is done by winding the heating wire into a bifilar spiral and by connecting these spiral windings to each other by plastic webs (DE 3,810,845 A1). Finally, it is also known that the heating wire can first be brought into the form of a helix, and that the helix can then be embedded in a star-shaped intermediate product of thermoplastic material (DE 7,121,715 U). When the pipe fitting is injection-molded, this star-shaped intermediate product is then integrated into the area of the contact surface with the conduit. The current flowing through the helix leads to maximum heating in the interior of the helix, however, and this beat is therefore unavailable for the welding process with the conduit. The heat which reaches the contact surface is inadequate. In these known pipe fittings, the strength of the weld to the conduit is unreliable. To achieve satisfactory welding results, it is necessary to use a relatively large amount of electrical energy, which is uneconomical. 
     SUMMARY OF THE INVENTION 
     The invention is based on the task of providing an economical process for producing pipe fittings of the aforementioned kind, the end products of which can be welded quickly and reliably to conduits. This is achieved according to the invention in that the heating wire, or at least certain sections thereof, is formed into a flat, wavy shape, as a result of which a wavy heating strand is produced as a first step; in that only then are the windings of the wavy heating strand laid in the future welding area of the pipe fitting; wherein, after the injection-molding of the pipe fitting and the solidification of the welding material, one of the broad sides of the wavy shape extends along the contact surface. 
     The process according to the invention proceeds in two stages. First, the heating wire is brought into the form of a flat, wavy line, as a result of which a special intermediate product is obtained, which is to be referred to below in brief as the “heating strand”. The special feature of this heating strand is therefore to be found in its wavy shape, which can also be described as a sine wave. Then, in the second stage of the process, windings of this wavy heating strand are laid in the injection mold in such a way that one of the broad sides of its wavy shape lies directly at the contact surface with the pipe fitting. The wavy shape and its position at the surface ensure the rapid, intense heating of the contact surface even when only a moderate amount of current flows through the strand. The softening and coalescence of the material of the pipe fitting and the conduit lead to a reliable weld after solidification. 
     Additional measures and advantages of the invention can be derived from the subclaims, from the following description, and from the drawings. The invention is intended to cover all of the novel features and combinations of features which can be derived from them, even if these are not explicitly stated in the claims. The invention is illustrated in the form of an exemplary embodiment in the drawings: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a top view of an intermediate product for a pipe fitting produced in accordance with the invention; 
     FIG. 2 shows a schematic cross section of the intermediate product of FIG. 1; 
     FIG. 3 shows, on an enlarged scale, a bottom view of part of the intermediate product shown in FIG. 1; 
     FIG. 4 shows, at the same degree of enlargement, a top view of the part of the intermediate product shown in FIG. 3; 
     FIG. 5 shows a cross section through the part of the intermediate product shown in FIG. 4 along line V—V of FIG. 4; 
     FIG. 6 shows a schematic diagram of several components of a device which is used to give the heating wire a wavy shape, as a result of which a wavy heating strand is obtained; 
     FIG. 7 shows a perspective view, on an enlarged scale, of part of the wavy heating strand of FIG. 6, which is integrated into the intermediate product of FIGS. 1-5; 
     FIGS. 8 and 9 show schematic diagrams of parts of an injection mold for the production of the intermediate product shown in FIGS. 1-5, in two different working stages; 
     FIG. 10 shows a schematic diagram of a cross section through an additional injection molding device, which holds the intermediate product of FIGS. 1-5 and is used to produce a pipe fitting; 
     FIG. 11 shows, on an enlarged scale, a part of the device shown in FIG. 10; 
     FIG. 12 shows a cross section through a part of the pipe fitting which can be produced in the device of FIG. 10, the position of this end product while in service on a conduit also being indicated; 
     FIG. 13 shows a side view of a different exemplary embodiment of a intermediate product similar to that of FIG. 1; 
     FIG. 14 shows a cross-sectional view of the intermediate product of FIG. 13 along III—III; and 
     FIG. 15 shows a side view of the end product made from the intermediate product of FIGS. 13 and 14, the figure also showing how the end product is used to join two sections of conduit. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The goal of the invention to produce a pipe fitting  50  such as that shown by way of example in FIG. 12, at least certain areas of which consist of a weldable material  51 , namely, a thermoplastic material, such fittings being designed to be welded to a desired location on a conduit  56 , which also consist of a thermally weldable material. This material is to be referred to below in brief as “welding material”. To achieve its purpose, pipe fitting  50  has a saddle  53 , which, in the mounted state, comes into contact by its contact surface, designated  54 , with conduit  56 . To produce the weld, a heating wire  10 , to be described in greater detail further below, is integrated into a certain welding area  55 . This heating wire  10  is first given a special shape, to be presented in greater detail below, and is a component of a special intermediate product  20 , which is shown in FIGS. 1-5. During the injection-molding of pipe fitting  50 , this intermediate product  20  is integrated into the desired welding area  55 , as can be seen in FIG. 12. A pipe fitting of this type can have one or more pipe sockets  52 . After the pipe fitting has been welded in place, socket  52  offers access to conduit  56  so that a hole can be drilled through it at the spot indicated by reference number  57 . 
     As shown in FIG. 6, the starting point is a bare heating wire  10 , which does not have any insulating jacket but which does have sections  12 , which have been brought into the form of a flat, wavy line  11 . Heating wire  10  has at most a conventional, minimal enamel finish. There can be straight sections  13  of wire between wavy sections  12 . This wavy intermediate product  15  is shown in perspective in FIG.  7  and is referred to in brief in the following as the “heating strand”. The wavy shape  11  of heating strand  15  is produced by conducting the originally straight wire, as can be seen in FIG. 6, between two intermeshing gear wheels  14 . Gear wheels  14  have a toothed pro file in the form of the same wavy shape  11 . In the present case, the wavy shape is in the form of a sine wave. Straight sections  13  of wire between wavy sections  12  are produced by moving the two gear wheels  14  away from each other while section  13  is passing between them in the direction of the arrow in FIG.  6 . Thus the teeth of gear wheels  14  are no longer able to deform heating wire  10 . 
     An injection mold with two mold halves  31 ,  32  is used to produce intermediate product  20 . FIGS. 8 and 9 show enlarged cross sections of this mold. Whereas mold half  31  is designed as an essentially flat plate, the other half  32  of the mold has a channel  33 , which is designed here as a flat, monofilar spiral. Channel  33  can be divided into two partial cross sections  30 ,  34 , which serve different functions. One part  30  of the cross section has the form of a trapezoid, the larger trapezoidal side being open toward the other half  31  of the mold. This partial cross section  30  serves to accept wavy heating strand  15 . The channel walls of this partial cross section  30  are undercut and grip opposing peaks  16  of heating strand  15 , as can be seen in FIG.  7 . Inserted heating strand  15  has a certain transverse elasticity and is held in place in partial cross section  30  even while the two halves  31 ,  32  of the mold are open. In correspondence with the pattern of the windings of channel  33 , heating strand  15  itself assumes the form of spiral windings  70  according to FIG.  3 . 
     A flow cross section  34  in the form of a semicircular arc, which lies above the previously mentioned, trapezoidal partial cross section  30 , also belongs to channel  33 . After the injection mold has been closed, this partial cross section serves according to FIG. 9 to guide the flow of welding material  21 , illustrated by the arrows. When intermediate product  20  is being made, welding material  21  flows toward the upward-facing, broad side  18  of heating strand  15 , as a result of which heating strand  15  is pressed against mold surface  37  of adjacent half  31  of the mold. After welding material  21  has solidified in injection mold  31 ,  32 , therefore, intermediate product  20  shown in FIGS. 1-5 is obtained. FIG. 3 shows a bottom view, where wavy shape  11  remains visible on outside surface  27 . At the same time, the wavy shape ensures that heating strand  15  is firmly anchored in the welding material of intermediate product  20 , which also has the following additional special design feature. 
     After the plastic material has solidified, a band  23  with the corresponding pattern of windings is formed in the individual spiral turns of channel  33 . Width  19  of the wavy shape visible in FIG. 7 is essentially the same as width  39  of the channel used to produce the band, which can be seen in FIG.  9 . Therefore, wavy shape  11  passing around individual turns  70  of band  23 , as shown in FIG. 3, extends all the way across width  29  of the band, proceeding from one edge of the band to the other. This is also easy to see in the cross section shown in FIG.  5 . 
     As illustrated in FIG. 9, the individual windings of channel  33  are a certain radial distance  35  away from each other, for which reason open gaps  25  are formed in intermediate product  20  between individual windings  23  of the band. These gaps  25  are bridged in intermediate product  20  by connecting ribs  24 , as shown in FIG.  1 . These connecting ribs  24  are formed by distribution lines, not shown in detail, provided in injection mold  31 ,  32 ; these lines proceed from a central gate  44  shown in FIG.  1  and extend essentially in the radial direction across intermediate product  20 , which here is in the form of a circular disk. These distribution lines conduct welding material  21  from gate  44  to the individual windings  33  of the channel and ensure that the masses flow primarily into the described flow cross sections  34 , which have a rounded profile, which is offset with respect to the cornered profile of receptacle cross sections  30 . In the finished band  23 , curved, longitudinal beads  43 , which are visible in FIG. 4, are thus formed in the area of these flow cross sections  34 . As can be seen from completed intermediate product  20  in FIG. 5, the cross section of the distribution line which produces connecting rib  24  is taller than channel  33  serving to produce band  23 , for which reason connecting ribs  24  project beyond inside surface  28  of intermediate product  20  visible in FIG.  1  and extend without a break. Connecting ribs  24 , which are of uniform width, have an arc-like curvature, which is favorable for the flow of material. 
     Intermediate product  20 , finally, also has radial, transverse webs  36 , which project beyond the circular outline of intermediate product  20 . These are formed by appropriately designed transverse chambers (not shown) in injection mold  31 ,  32 , which terminate in expanded chamber areas (not shown). Previously mentioned straight wire sections  13  are laid in these transverse chambers up as far as the chamber expansions when heating strand  15  is laid in the mold, as can be derived from FIGS. 3-5. At the same time, contacts  40  are also laid in the chamber expansions, so that, after the injection and solidification of welding material  21 , cups  46 , visible in FIGS. 1 and 2, are formed at end of transverse webs  36  to hold contacts  40 . As can be derived from FIG. 5, straight wire sections  13  pass at a sufficiently high level  38  above wavy heating strand  15 ; this applies especially to straight section  13  of long transverse web  36 , which proceeds from the wavy section of the heating wire of innermost winding  23 ′ of the band shown in FIG.  1 . Straight wire section  13  can, as shown in the figure, extend either in the interior of transverse web  36  or on the top surface of this web  36 . 
     In the completed intermediate product, according to FIGS. 1-5, the position of flat wavy shape  11  of heating strand  15  and the position of contacts  40  are established by welding material  21 . This intermediate product thus serves as a carrier for the windings of special heating strand  15  and can therefore be described below as the “heating strand carrier”. In the flow direction indicated by the arrows in FIG. 1, a flat, in this case ring-shaped, boundary zone  18  is provided inside of inner most winding  23 ′ of the band, based on gate  44  as the center; this zone is formed by a appropriate starting chamber (not shown) in injection mold  31 ,  32 . This starting chamber is supplied with welding material via the previously mentioned distribution lines, which serve to form connecting ribs  24  in heating strand carrier  20 . This cross section of this chamber is flatter than overall cross section  34 ,  30  of channels  33  serving to produce band  23 . This is significant for the further processing of heating strand carrier  20  according to the invention in pipe fitting  50  to be obtained, as can be seen on the basis of FIGS. 10 and 11. Central part  49  in FIGS. 1 and 2 is cut out so that flat boundary zone  18  with its circular opening  22  begins there. In the present case, heating strand carrier  20  is a flat, level product. Because of open gaps  25  remaining between windings  23  of the band and connecting ribs  24 , heating strand carnier  20  has the character of a mat; it is flexible, and its original flat contour shown in FIGS. 1 and 2 can be easily deformed into a cylinder. This is significant for the following stages of the processing of heating strand carrier  20 . 
     To produce pipe fitting  50 , a second injection mold  41 ,  42  shown in FIGS. 10 and 11, is used. It consists of a mold half  41  on the nozzle side, with a nozzle  45  for the introduction of the previously mentioned welding material  51 , and a mold half  42  on the ejector side, serving to eject finished pipe fitting  50 . The previously produced heating strand carrier  20  of FIGS. 1-5 is first inserted into ejector-side mold half  42  after the injection mold has been opened. Suitable receptacles for projecting contacts  40 , previously mentioned, are also provided there. When injection mold  41 ,  42  is brought into the closed position shown in FIGS. 10 and 11, central circular opening  22  of heating strand carrier  20  is automatically set down onto a nozzle bush  48  of opposite mold half  41 , This nozzle bush  48  surrounds central nozzle  45 , which supplies the flow of material, and rises above prominent saddle-shaped mold surface  47  of mold half  41 . When, as FIG. 11 shows, welding material  51  flows in the direction of the arrows into the cavity between the two mold halves  41 ,  42 , it first arrives at the side of flat boundary zone  71  of heating strand carrier  20  facing away from mold surface  47  and presses outside surface  27  of the boundary zone against saddle-shaped mold surface  47  of mold half  41 . The originally flat mat of heating strand carrier  20  is therefore deformed on this saddle surface  47  into a partial cylinder. It is therefore impossible for incoming welding material  51  to pass into circular opening  22  between nozzle bush  48  and flat boundary zone  71  and arrive at outside surface  27  of heating strand carrier  20 . 
     The two mold halves  41 ,  42  have suitable coolant channels  58  to ensure the rapid solidification of injected welding material  51 . After the mold has been opened, it is then possible in the present case to remove previously described end product  50 , shown in FIG. 12, by means of a suitable ejector (not shown) from mold half  42 . Previously mentioned mold surface  47  of nozzle-side mold half  41  produces previously mentioned contact surface  54  on end product  50 . In this area, integrated heating strand carrier  20  with its described outside surface  27  remains free of welding material  51 , because this material is able to flow only into remaining gaps  25 , which it then closes. The entire outward-facing broad side  17  of heating strand  15 , in the form of a wavy line, therefore remains with visible. It would also be possible to use a different injection mold in which heating strand carrier  20  would be formed into a complete cylinder. 
     It is therefore guaranteed that outward-facing, broad side  17  of wavy shape  11  remains visible on outside surface  27  of intermediate product  20  and thus also at contact surface  54  of end product  50 . In practice, wavy shapes  11  actually project slightly beyond outside surface  27 , which can be explained by the shrinkage of welding material  51 . Exposed wavy shape  11  is very effective at increasing the efficiency of the later welding operation by which pipe fitting  50  is attached to conduit  56 . Electrical insulation problems cannot arise, because the lateral surface of conduit  56  to be welded also consists of electrically nonconductive, thermoplastic material. 
     FIGS. 13 and 14 show a different exemplary embodiment of a heating strand carrier  20 ′ according to the invention. Elements analogous to those of mat-like heating strand carrier  20  of the preceding exemplary embodiment are designated here by the same reference numbers, for which reason the previous description also applies here to that same extent. It is sufficient to describe only the differences. 
     One difference consists in that band windings  23  provided here are in the form of a helix from the very beginning and therefore present a completely cylindrical form right in intermediate product  20 ′ itself, as is especially easy to see in FIG.  14 . What cannot be seen in FIGS. 13 and 14, is that, of course, the wavy heating strand extends along these band windings  23  in the previously described manner. Individual windings  23  of the band are joined here, too, by connecting ribs  24 , which, however, can also be parallel to the axis of the cylinder. This design is presented in the form of two axial cross sections  59  of intermediate product  20 ′ , which has the form of a cylindrical basket. 
     Cylindrical basket  20 ′ thus obtained serves to produce end product  60 , shown in a side view in FIG.  15 . End product  60  represents in this case a connecting sleeve  60 , which is used to connect the ends of two pipes  61 ,  62  together. During the injection-molding of this connecting sleeve  60 , cylindrical basket  20 ′ comes to rest on inside sleeve surface  63  illustrated in FIG. 15, where, in the manner already described in the preceding case, one of the broads side of the wavy shape rests is exposed on the inside  64  of the sleeve. Heating strand  15  used in the present case has three of the straight wire sections  13  explained in conjunction with FIG. 6, two of which, as in the preceding case, serve to connect contacts  40 , provided here at the ends of cylindrical basket  20 ′, but which are not visible in FIG.  13 . The third wire section  13  is located between the above-mentioned axial sections  59  and is shown in FIG.  13 . Here there is a central ring  65 , which is produced out of welding material  21  of cylindrical basket  20 ′ and which, in the case of end product  60  of FIG. 15, ends up in the center  66  of the sleeve between the two end surfaces of pipes  61 ,  62  to be connected. 
     When FIG. 15 is in service, an inside stop  67 , which fixes the position of pipes  61 ,  62  in the interior of the sleeve, is located between the two end surfaces of pipes  61 ,  62 . This stop  67  could be produced by welding material  21  of cylindrical basket  20 ′. But it is also possible for stop  67  to be formed later on out of the welding material of connecting sleeve  60 , by providing an appropriate opening in central ring  65  and by designed the injection mold for connecting sleeve  60  in a corresponding manner. This opening in central ring  65  is filled up by the welding material of connecting sleeve  60 , with the result that inside stop  67  is formed in the interior of the sleeve. Gaps  25  between bands  23  extending around the helix, however, are filled up flush by the welding material of connecting sleeve  60 , so that here, too, the wavy lines in tapes  23  remain exposed in interior  64  of the sleeve. Contacts  40  of cylindrical basket  20 ′ in FIG. 13 end up in plastic shoulders  69  of finished connecting sleeve  60  of FIG.  15 . 
     A wavy heating strand  15  could also be used to produce a mat-like intermediate product, which later forms a contact surface of a so-called multi-shell “muff”, the shells of which form a complete cylinder which enclose the conduit. In the production of an end product of this type, at least some areas the intermediate product mats used here are formed into more than half-cylinders in the injection mold. The special pattern of the windings of the band in a mat such as this is illustrated in German patent Application No. 196-23,353.4. 
     List of Reference Numbers 
       10  heating wire 
       11  wavy shape 
       12  wavy section of  15   
       13  straight section of  15   
       14  gear wheels for  15   
       15  heating strand 
       16  peak area of  11   
       17  outward-facing broad side of  11   
       18  inward-facing broad side of  11   
       19  width of wavy shape  11   
       20  intermediate product, mat-like heating strand carrier 
       20 ′ intermediate product, cylindrical basket 
       21  welding material of  20   
       22  circular opening in  20  at  18   
       23  band, windings of the band 
       23 ′ innermost windings of the band on  20   
       24  connecting rib between  23 ,  23 ′ 
       25  open gap between  23   
       26  longitudinal corner of band  23   
       27  outside surface of  20  and  25   
       28  inside surface of  20   
       29  width of band 
       30  partial cross section of  33 , receptacle cross section for  15   
       31  injection mold for  20 , first half 
       32  injection mold for  20 , second half 
       33  channel in  32 , channel winding 
       34  partial cross section of  33 , flow cross section 
       35  distance between  33   
       36  transverse web of  20   
       37  mold surface in  31   
       38  differences in height between  13  and  15   
       39  channel width of  33   
       40  contact 
       41  injection mold for  50 , mold half on the nozzle side 
       42  injection mold for  50 , mold half on the ejector side 
       43  longitudinal bead on  23   
       44  gate for  24  on  20   
       45  nozzle on  41   
       46  cup for  40  (FIGS. 1,  2 ) 
       47  saddle-shaped mold surface of  41   
       48  nozzle bush on  45   
       49  central part of  20   
       50  pipe fitting, final product 
       51  welding material for  50   
       52  pipe socket on  50   
       53  saddle of  50   
       54  contact surface on  53   
       55  welding area of  50  to  53   
       56  conduit for  50   
       57  drilling point for  50  to  56   
       58  coolant channel in  41 ,  42   
       59  axial section of  20 ′, with openings 
       60  connecting sleeve, pipe fitting, end product 
       61  first pipe 
       62  second pipe 
       63  inside surface of sleeve 
       64  interior of sleeve of  20 ′ 
       65  ring of  20 ′ for  13   
       66  middle of sleeve  60   
       67  inside stop in  64   
       68  end ring of  20 ′ 
       69  shoulder for  40  (FIG. 15) 
       70  winding of  15  (FIG. 3) 
       71  flat boundary zone of  20