WO1997011577A1

WO1997011577A1 - Electric fusion welding of thermoplastic materials

Info

Publication number: WO1997011577A1
Application number: PCT/US1996/014179
Authority: WO
Inventors: Nicholas E. Butts
Original assignee: Environ Products, Inc.
Priority date: 1995-09-06
Filing date: 1996-09-05
Publication date: 1997-03-27
Also published as: AU7154296A

Abstract

An electric fusion welding tube (10) for welding thermoplastic workpieces comprising a flexible hollow elongate tube (10), said tube (10) comprising an inner layer (11) of flexible thermoplastic material, an electrical resistance heating element (15), said heating element (15) being disposed around said hollow inner layer (11) and along its length, and a coating layer (13) of thermoplastic material covering said resistance heating element (15) along the elongate inner layer (11) such that the heating element (15) is enclosed within the coating layer (13), said elongate inner layer (11), resistance element (15) and coating layer (13) thereby being combined into an integrated construction. The tube (10) is one component of an apparatus for welding thermoplastics, which further comprises a connector plug for the tube and a power source to heat the resistance element (15). The invention includes a welding rope comprising a plurality of strands of thermoplastic, where the strands comprise numerous filaments of thermoplastic yarn and at least one resistance wire.

Description

ELECTRIC FUSION WELDING OF THERMOPLASTIC MATERIALS

Field of the Invention

The present invention relates to the welding of members of thermoplastic materials including polyethylene, polypropylene, nylon, polyvinyl chloride, and the like.

Background of the Invention

There have been many methods developed for bonding together thermo¬ plastic materials. Some plastics, such as polyethylene, can only be properly bonded with a heat weld. There are many mechanical methods for heat welding thermoplastics together. These involve heating the plastic by means of hot air, hot wedges or extrusion type guns which melt rods or pellets of the thermoplastic material in the barrel of a mini extruder, mounted on a glorified drill. The heated plastic is pushed out the end of the gun where it is smeared onto a surface of like thermo- plastic material that has been preheated with a hot air source mounted in front of where the extrudate material exits the gun. All of these methods rely on operator ability to adjust the heat sources according to the current climatic conditions at the weld site.

Every thermoplastic material has its own welding window; that is, an upper and lower temperature where a proper weld bond is formed. Too much heat or not enough heat will result in a poor quality of weld.

Electricity has been employed as the heat source for the welding of thermoplastics. One way to use electrical heating is to wrap a resistance wire around a rod or strip of thermoplastic material. There are many inherent problems in this system in that the exposed wire tends to migrate unevenly around the thermoplastic form it is wrapped, creating hot and cold spots in the weld. The exposed wires also have a tendency to overheat, burning the plastic material immediately adjacent to the wire. It has also been the experience of the present inventor that the heat softened plastic does not necessarily envelope the hot wires, instead, tending to scallop around them leaving the wires exposed which could allow wicking of fluid through the weld.

Another technique employs a bare wire, strip of metal or metal mesh material as a heating element placed between two thermoplastic members being welded together. Other forms of electrofusion of thermoplastics use a strip of metal, a heavy wire or a number of parallel wires imbedded in the center of a matrix of the thermoplastic material. The inventor has found that the thickness of the material over the wire needs to be kept to a minimum because plastic tends to be an insulating material, impeding heat migration rather than conducting it. Combined with the fact that the hot wires tend to knife down through the plastic due to gravity, penetrating into the lower member being welded, this results in uneven heating of the two surfaces being welded together. Fewer, larger diameter wires generally require much larger electrical currents to heat the wire, and are more likely to have hot spots due to nicks in the wire. The present inventor is a co-inventor of another plastic welding system which comprises a homogeneous core of plastic material in which is imbedded a plurality of resistance wires. From his experiences working with this system he is aware of several of its limitations. Due to the limited ability for heat to migrate into an insulating material like plastic, the size of the core must be small. In order to weld a wider area, a number of these welding rods must be laid side by side. In some instances more than 10 rods are needed to weld across a wide pipe wall. It can be very time consuming and laborious to lay down a number of these welding rods. The electrical current requirement is multiplied by the number of rods, which can become a limiting factor to a given power source.

Having to use numerous rods creates the possibility of having air pockets in the welded joint. During the heating cycle of the weld, the rods tend to migrate within the weld. If too much pressure is applied, some of the rods are pushed out of the joint, where, once exposed, they tend to overheat and burn. In some pipe sidewall applications it is necessary to drill out a hole after welding, which means cutting wires that have migrated to protrude into the inside diameter of the lateral pipe. Overheating normally occurs at the electrical connection where three or more wires are bent at right angles to come out of the pipe weld.

The welding rod system usually requires pre-attachment of the rods to one of the two pieces of plastic being welded together. This is usually not practical in field installations. Making a stand-alone ring of two or more rods usually requires embedding the rods into a matrix of like plastic material. Many of the above-described techniques have only limited applications and are limited to the welding of certain shapes.

Summary of the Invention

The present invention is intended to avoid the limitations of earlier ap¬ proaches by providing a welding tube, a method for making a better electrical connection, and methods for using the welding tube. The welding tube is formed of a resistance element and a tube of thermoplastic material which provides a supply of thermoplastic material to the weld between two adjacent thermoplastic members. The resistance element is formed of a plurality of wires either wrapped in a helical pattern or braided around a plastic tube. The resistance element is then embedded within more thermoplastic material, which can be accomplished by placing a co-extrusion of a thin layer of the same thermoplastic material over the wires.

The tubular construction of the welding device permits a flexibility that other welding devices do not exhibit. It may be formed into various shapes to conform to the items that must be welded together. The welding tube may be flattened for use in applications where a flattened welding strip is preferable. The weld tube can be made in various diameters and wall thicknesses resulting in a variation of widths and thicknesses of the flattened tube. A preferred embodiment of the present invention is a thermoplastic welding tube comprising a flexible hollow elongate tube, said tube comprising an inner layer of flexible thermoplastic material, an electrical resistance heating element, said heating element being disposed around said hollow inner layer and along its length, and a coating layer of thermoplastic material covering said resistance heating element along the elongate inner layer such that the heating element is enclosed within the coating layer, said elongate inner layer, resistance element and coating layer thereby being combined in an integrated construction. The electrical resistance element may comprise a plurality of wires, which are disposed along the length of the welding tube and may be arranged in a helical winding, an oblique winding forming a braid, longitudinally, or in another pattern allowing uniform heating of the tube.

The invention includes an apparatus for welding thermoplastic structures comprising a flexible hollow elongate thermoplastic welding tube comprising an inner layer of flexible thermoplastic material, an electrical resistance heating element, said heating element being disposed around said hollow inner layer and along its length, and a coating layer of thermoplastic material covering said resis¬ tance heating element along the elongate inner layer such that the heating element is enclosed within the coating layer, said elongate inner layer, resistance element and coating layer thereby being combined in an integrated construction, said thermoplastic welding tube having first and second ends; said welding apparatus further comprising a tube connector plug connecting the first and second ends of the welding tube. The resistance heating element may comprise a plurality of wires that extend from the ends of the welding tube for connection to an electrical power source. In a preferred embodiment of the invention, tlie tube connector plug comprises a cylinder of thermoplastic material, said cylinder having a diameter sized to fit snugly into the ends of the welding tube. The tube connector plug further comprises a band of thermoplastic material disposed around the circumference of the plug cylinder at its approximate center, said band of thermoplastic material enlarging the diameter of the plug to approximate the diameter of the thermo- plastic welding tube, said band serving to separate the respective ends of the welding tube from each other when said ends are joined by the connector plug. Said welding tube and connector plug apparatus comprises a flexible thermoplas¬ tic welding device for shaping to conform to a profile, said profile being defined by two thermoplastic workpieces to be welded together, each respective workpiece having at least one surface to be welded to a surface of the other respective workpiece, at least one of said surfaces having a defined profile to which the welding device must be shaped to make the weld. The respective ends of the welding tube are stripped of the thermoplastic material to expose a small segment of the resistance wires comprising the heating element, and the apparatus further comprises electrical connectors attached to the respective exposed ends of the resistance element wires, and a power source connected to the respective electrical connectors for introducing a current into the resistance element to heat the element. A variation of the weld tube is a weld rope, where a length of thermo¬ plastic rope is used as the source of plastic for the welding. The new welding process positions resistance wires, used for heating the plastic, throughout the plastic matrix, unlike other electrofusion processes that position the wires around the outside of an extruded profile. The fibers in the rope are very flexible and permit the weld rope to be easily shaped for welding irregularly shaped objects. A variation of this embodiment of the invention would be to braid wires coated individually with a desired plastic material into a rope.

Other electrofusion systems teach the concept of welding only similar plastics. The present invention provides the ability to weld dissimilar plastics together with the use of 'tie' materials. There are certain plastics that are able to adhere to different plastic materials. For instance, one such tie material bonds to both polyethylene and to nylon, although polyethylene and nylon cannot be bonded to each other. Tie materials can be used to form a bond between otherwise incompatible plastics like polyethylene and nylon, polyethylene and PVC, and other combinations. By using tie materials in the formation of an electrofusion system, the ability to weld dissimilar plastics is realized. Brief Description of the Drawings

Figure 1 is a perspective view of a welding tube in accordance with the present invention.

Figure 2 is a cross-section view of the welding tube of Figure 1. Figure 3 is a perspective view of a welding tube after flattening in accor¬ dance with the present invention.

Figure 4 is cross-section view of me flattened tube of Figure 3.

Figure 5 is a perspective view of a welding tube showing its three compo¬ nent layers. Figure 6a is perspective view of a welding tube and two workpieces to be welded together.

Figure 6b is a perspective view of a welding tube between two irregularly shaped workpieces to be welded together.

Figure 7 is a perspective view of a tube connector plug for joining the ends of the welding tube according to the invention, and the electrical connections for heating the tube.

Figure 8 is a schematic diagram of an apparatus for flattening the weld tube.

Figure 9a is a perspective view of a connector block. Figure 9b is a perspective view of the end of a flattened weld tube with the plastic stripped off the end and a wire mesh applied to the resistance wires.

Figure 10 is a cross section of mesh-wrapped resistance wires fitted into a connector block.

Figure 11 is a perspective view of a lap joint of thermoplastic material using the flattened welding tube of Figure 4.

Figure 12 is a cross-section view of the completed weld of Figure 10.

Figure 13 is a cross-section view of a cap strip weld with the flattened welding tube. Figure 14 is a perspective view of a socket tube fabricated from flattened tube weld.

Figure 15 is a cross-section view of a socket tube weld.

Figure 16 is a perspective view of the welding tube formed into a ring before flattening.

Figure 17 is a top plan view of the ring formed after the welding tube has been flattened.

Figure 18 is a perspective view of a weld ring positioned between the ends of two pipes. Figure 19 is a perspective view of a flattened tube weld formed to fit onto a radiused pipe.

Figure 20 is a perspective view of a saddle weld pad shown with a typical saddle tee fitting and a length of pipe.

Figure 21 is a perspective view of a socket weld using a welding tube. Figure 22 is a side view of a braided rope used for welding rope.

Figure 23 is a cross section of the welding rope showing the steel wires.

Figure 24 is a side view of a second embodiment of welding rope, the inner conductance wire and insulating material shown extending from the rope.

Figure 25 is a cross section of a third embodiment of welding rope where monofilament lines are braided together.

Description of the Invention

Figures 1 to 5 illustrate the welding mbe 10 of the present invention. The welding tube 10 is formed of a flexible hollow elongate inner layer 11 of thermo¬ plastic material, a resistance heating element 15 and a coating layer 13 of like thermoplastic material (see Fig. 5). The elements of the welding tube are combined in an integrated construction, forming a flexible hollow elongate welding medium for use in thermoplastic welding.

The choice of particular materials and the dimensions of the tube 10 and the resistant heating element 15 and coating layer 13 are dependent on the requirements of a particular application. For example, the inner layer 11 and coating layer 13 may be formed of thermoplastic materials including polyethyl¬ ene, polypropylene, polyvinyl chloride (PVC), and nylon. In turn, the resistance element 15 may be formed of various conducting metals including stainless steel, nichrome, iron, among others. For the sole purpose of this description of a preferred embodiment of the invention, the inner tube layer 11 and coatmg 13 are formed of high density polyethylene (HDPE) and the resistance heating element 15 is fonned of stainless steel. The dimensions of the inner layer 11 , coating 13 and resistance heating element 15 presented herein are chosen to describe a preferred embodiment of the present invention. It will be understood that these dimensions and the operating parameters discussed hereinafter are dependent on the particular application and the choice of materials.

The welding tube 10 is a combination of about a 0.5 inch diameter flexible hollow HDPE tube, and a resistance heating element 15 comprised of twenty -four strands of stainless steel wire 12 having a diameter of .007 inch. The wires 12 are braided around me inner layer 11 of the tube 10 (see Fig. 5). The size of the tube 10 will dictate the size and number of wires 12 (Figs. 1 and 2) comprising the resistance heating element 15 and also the length of the braid pattern along the length of the tube 10. In all cases the configuration of the resistance heating element 15 should provide a distribution of energy from the power source to cause a uniform heating of the tube 10.

Referring to Fig. 5, once the wires are braided around the inner tube layer 11, the resistance heating element 15 and inner tube 1 1 are coated with a co-extruded 0.05 inch coating layer 13 of HDPE. By coating the resistance heating element 15, it is not exposed to the atmosphere on the surface of the tube 10. If the resistance heating element 15 is exposed to the atmosphere when connected to the power source, a hot spot can be created and the thermoplastic material of the tube 10 and the thermoplastic members being welded (not shown) can be burned. Coating the wires 12 with an outer coating layer 13 serves, several other functions. Coating layer 13 maintains the wires' positioning around the tube 10. Coating layer 13 keeps the tube and resistance element interface clean and prevents entrapment of dirt and grease which would contaminate the weld. A welding tube 10 which has a coating layer 13 is more readily cleaned of contam¬ inants. For most closed welding applications, it is preferable to form the welding tube 10 into a single continuous loop having a length conforming to the profile of the surface(s) to be welded. For instance, to weld two circular workpieces having a circumference of 120 inches on the surfaces to be welded together (see Fig. 6a at A), a welding tube 10 of 120 inches length is needed. The weld mbe 10, which has two ends when cut to length, is closed using a mbe connector plug (described further below).

The welding tube 10 of the present invention is versatile because of its omnidirectional flexibility. Though many simple welded joints may be welded using conventional welding materials, joints of irregular shapes, from circles to polygonal shapes and jagged repair welds in structures, may be welded using a single welding tube shaped to conform to the profile of the workpieces to be welded together. A single welding mbe 10 provides a strong, watertight joint where flat strips or multiple-element welding media risk voids and leaks.

Figure 6b shows the application of a welding mbe shaped to conform to a more difficult profile for welding. The workpieces 76,78 have a concave side, presenting an irregular welding surface A for joining the workpieces. The welding tube 10, its ends joined together by the connector plug 60, can be shaped to conform to the odd profile A of the workpieces 76,78 for welding because a flexible mbe can be flexed in any direction. If the irregular surface A in Figure 6b had further irregularities in three dimensions (e.g., the upper surface being curved toward the viewer, the lower surface being curved away from the viewer), the welding tube could follow these irregularities as well.

Referring to Figure 7, the mbe connector plug 60 comprises a cylindrical portion 61 made of thermoplastic material. The circumference of the cylinder is a dimension suitable for the cylindrical portion 61 to be inserted into the two free ends of the welding mbe 10, as shown, achieving a snug fit into the inside of the mbe 10. This dimension is substantially equal to the circumference of the opening in the hollow inner layer 11 (not indicated) of the welding mbe 10. The connector plug 60 further comprises a band 62 of thermoplastic material near the center of the cylinder 61. The band 62 has a larger circumference than the cylinder 61 , extending out to approximately the circumference of the outer surface of the coating layer 13 (not indicated) of the welding mbe 10.

The connector plug 60 is inserted into the open ends of the welding mbe 10. Before placing the connector plug 60 in place, the thermoplastic on the ends of the welding mbe 10 must be stripped away from the resistance heating element wires to expose a short segment of the wires for connection to a power source. Then the plug 60 may be set in place using a heat gun or other suitable device to spot- we Id the plug 60 into the mbe 10. Once set in place, the mbe 10 and the connector plug 60 comprise a single integrated construction of welding media. When the mbe 10 is ultimately used to weld workpieces together, the plug 60 melts along with the mbe, forming a continuous weld.

In Figure 7 is also illustrated the means by which the welding mbe 10 is energized to accomplish the welding of two or more thermoplastic workpieces (not shown). At each free end of the welding mbe 10, the thermoplastic material of the mbe is stripped away, exposing a short segment of the resistance heating element wires. These wires are gathered and twisted into bundles 64,66 that extend outside the mbe 10 after the connector plug 60 is inserted and fixed in place. A suitable electrical terminal (not shown) may be affixed to each of the wire bundles 64,66, such as terminal posts or sockets. The respective poles of a power supply 70 are connected to the wire bundles 64,66. An electric current is passed through the resistance heating element wires to heat the welding mbe 10, melting it and the thermoplastic material of the workpieces togetiier at the site of the weld.

Returning to Figure 6a, to accomplish the welding of the two workpieces 72,74, the welding mbe 10 is placed in position on the surface of one of the workpieces (72 in the Figure). The remaining workpiece 74 is moved into position against the welding tube 10. As the welding mbe is heated by the means described above, the workpieces 72,74 are compressed together progressively as the welding mbe melts. Care must be taken not to compress the workpieces together with such force that the melting tube material is forced out of the weld. After the weld is made, the resulting joint must be allowed to cool sufficiently to permit the thermoplastics to regain their solidity and strength.

Like the welding mbe, the connector plug may be made of a variety of thermoplastic materials. It is recommended that the plug material match that of the welding mbe with which it will be used. When the connector plug melts during the welding process it becomes an integral part of the weld. The materials used to form the welding mbe may comprise certain "tie" materials that, when melted in a weld, are capable of bonding two polymers together that would otherwise be incompatible for bonding directly. For instance, for welding two workpieces of HDPE togetiier, a welding tube constructed of HDPE is suitable. However, welding a nylon workpiece to a polyethylene workpiece requires selection of a tie material for the welding mbe that will bind with each of the workpieces when melted, thus joining workpieces that otherwise could not have been easily joined together. Of course, where the workpieces are of the same material, such as polyethylene, then the weld mbe should be made of the same material or one compatible to it. As shown in Figures 3 and 4 the welding tube 10 may be flattened for omer welding applications. Care must be taken not to entrap air during the flattening process. To properly flatten the welding mbe 10 a required length of the welding mbe 10 is cut from a spool. Both ends of the tube 10 are pressed together flat and heated. Referring to Fig. 9b, the plastic from inner mbe and coating layer at both ends of the mbe is heated and stripped away, exposing about 0.5 inch of each wire 12. The splayed wires 12 are wrapped in a piece of wire mesh 17 to ensure a positive electrical contact with each wire 12. Referring to Fig. 9a, the mesh- wrapped wires are fitted into a connector block 14. The mbe 10 is then flattened in a press, so that the inside walls of the mbe 10 are touching. The connector block 14 is attached to a power source (nor shown) which is turned on, allowing electrical current to heat the resistance element 15, which in mm heats the thermoplastic of the flattened mbe 10. Any suitable means for keeping the mbe 10 flattened during heating may be employed, such as pumping the air out of the tube, or continuing pressure on the tube. The current is applied until sufficient heat has been generated to fuse the opposing walls of the flattened mbe, so mat the inner walls bond together forming a homogenous flat strip. The power is turned off and the thermoplastic is allowed to cool.

Figure 8 depicts a method of flattening the mbe 10 by the use of radio frequency generation. To do this, the wires of the resistance element are about .008 inch diameter magnetic stainless steel wire. Weld tube 10 is flattened by a pinch roller 51 and then passed under a specially designed radio frequency generator coil 50 which heats the wires of the resistance element. The wires in mm heat the plastic in the mbe and coating. The flattened, heated mbe is then allowed to cool, forming a continuous flat strip 16.

When forming a continuous flat strip weld, it would probably be easier to extrude a flat strip of thermoplastic of a desired width and thickness instead of trying to flatten the mbe 10. However, a pre-formed flat strip cannot be formed into the number of various shapes that the welding mbe can. The mbe is therefore more adaptable to use for welding shaped thermoplastic assemblies. It may be shaped for a unique workpiece profile, then flattened before welding, if necessary.

Figure 9a shows a connector block 14 which was developed to meet the particular requirements for the wide, flattened welding tube. The connector block 14 is made from the same thermoplastic material as the welding mbe, so that the connector block will also melt and become an integral part of the weld. The thickness of the connector block 14 is the same as the flattened welding mbe. The connector block 14 is designed to maintain a separation of the ends of the resistance heating element when the two ends of the flattened weld mbe come together, at the same time ensuring the integrity of the weld is maintained at the electrical connections. In Figure 9b, the stripped wires 12 of the resistance element are folded into a piece of wire mesh 17. In Figure 10, the mesh-wrapped wires 15 of the resistance element are folded again and fitted into one of the two slots 18 of the connector block 14. The resistance heating element wires 12 may be connected to a connector block 14 (Fig. 10) at each end of the welding mbe 10 for subse¬ quent connection to a power supply (not shown). If a welding ring is being formed (as in Figure 17), then the mesh-wrapped wires 12 of the resistance element are fitted into the other slot 18 of the connector block 14.

Referring to Figure 11 , the wire mesh is cut long enough so that about 0.5 inches of the wire mesh extends out beyond the open end of the connector block 14. A metal connector 19 is attached to the wire mesh to enable a power source to be connected to the resistance element within the welding tube 16, and to prevent overheating at the connection during welding. A power source (not shown) is connected to the metal connectors 19 attached to the wire mesh extending out of the connector block 14. When the power is turned on, the electrical current passes through the resistance wires in the welding mbe generat¬ ing sufficient heat to melt the thermoplastic material of the mbe and the thermo¬ plastic of the two parts being welded together, forming a homogeneous bond. The flattened welding mbe 10 is very versatile and can be formed into essentially any shape to effect welding of thermoplastic members. For example, the welding mbe 10 can be used to fuse mbular or planar thermoplastic members with butt, lap or cap strip joints.

Figure 11 illustrates the general procedure for forming a welded lap joint between thermoplastic members. A pair of sheets of thermoplastic material 21 and 22 are positioned on either side of a flattened welding mbe 16. A variable amperage transformer 23 is connected to a source of power at 24 and further connected via a suitable switch 25 to the resistance element (not shown) through the connector block 14.

An electrical current is passed through the resistance element in the flattened mbe 16 and simultaneously pressure is applied above and below on the sheets 21 and 22. As the electrical current passes through the resistance element, the thermoplastic material of the mbe and coating and the thermoplastic sheets 21 and 22 in the vicinity of the flattened welding tube 16 begin to soften. The electrical current is continued for a time and intensity to cause the thermoplastic material of the welding mbe 16 and the thermoplastic sheets 21 and 22 to soften and melt.

After the electrical current is discontinued, the application of pressure above and below the thermoplastic sheets 21 and 22 is continued so that the plastic of the mbe 16 and the two thermoplastic sheets 21 and 22 fuse and solidify thereby forming a homogeneous weld. The resistance element remains in the weld allowing the weld to be reheated in the event of a weld failure due to insufficient heat or pressure application during the initial weld cycle.

Figure 12 is a cross sectional view of the weld formed by the process illustrated in Figure 10. As can be seen from this cross-sectional view, the material of the mbe and the coating are indistinguishable from the welded material of the sheets 21 and 22. The resistance element 15 is permanently sealed within the weld.

Figure 13 is a cross sectional view of a cap strip weld where the flattened welding mbe 16 is laid over two adjacent sheets of thermoplastic material 26 and 27. The flattened tube weld can be made in various widths and thicknesses by adding substantial amounts of thermoplastic material to the mbe 16. To apply pressure to the heated thermoplastic of the flattened tube during tlie welding cycle, a strip of Teflon type material 20 can be used on top of the flattened welding mbe 16. After the joint is cooled, the Teflon type material 20 is removed. Figure 14 shows another variation of the cap strip weld of Figure 13.

The flattened weld mbe is formed into a socket mbe 30 and the two ends are connected into a connector block 14. The ends of two thermoplastic pipes 28 and 29 are pushed into the socket mbe 30. A strip of Teflon-type material (not shown) is wrapped over the socket tube 30 so that pressure can be applied to the heated thermoplastic of the socket tube 30 during the welding cycle. Again the Teflon-type material is removed after the joint has been cooled. Figure 15 shows a cross-section of the welded joint of Figure 14. The thermoplastic of the socket mbe 30 forms a homogeneous bond with the thermo¬ plastic of pipes 28 and 29. The resistance element 15 within the welding mbe remains permanently sealed within the weld. Figure 16 shows a perspective view of an unflattened weld mbe 10 formed into a hoop. The welding mbe 10 may be closed by inserting a connector plug (not shown, see Figure 7) into the open ends of the mbe. In Figure 17 the weld tube 10 of Figure 15 has been flattened to form a weld tube ring 31. The thermoplastic from the mbe has been heated and stripped away from both ends of ring 31. A closed welding ring is formed when a connector block (not shown) is attached to the weld mbe ring 31 forming a closed loop.

In Figure 18 the weld mbe ring 31 is positioned between two mbular members 32 and 33 of a similar thermoplastic material as the weld mbe ring 31. A power supply (not shown) is connected to the metal connector 19 on the connector block 14. An electrical current is passed through the resistance element and the mbular members 32 and 33 are pressed together to complete the weld.

Figure 19 shows a radiused weld tube ring 35 that has been formed to fit the radiused end of a mbular thermoplastic member 36. The radiused weld mbe ring 35 is positioned between the end of a radiused mbular thermoplastic member 36 and the side of a second mbular thermoplastic member 34. An electrical current is passed through the resistance element of the radiused weld mbe ring 35 as the mbular member 36 is pressed against the side of the mbular member 34 to complete a side wall weld or tee fitting. Each of the welds in Figures 11 through 19 can be made using the welding mbe of the invention without flattening the mbe. In each case, the mbe would be closed using a tube connector plug. It is easier to use the unflattened welding mbe because it is so easily formed into whatever shape is needed to conform to the profile of the weld joint surfaces on the workpieces. Flattening the tube first requires an extra operation and, once flattened, the resulting flat strip is not as flexible in all directions as the mbe. The mbe is the most versatile form of the present invention.

Figure 20 shows a variation of Figure 19 where a length of weld mbe 37 is shaped into a closed square and then flattened to a desired radius. This weld mbe saddle pad 37 is fitted with two single connector blocks at right angles to each otiier. The weld mbe saddle pad 37 is designed to weld a saddle tee fitting 38 onto a small diameter pipe 39 as might be found in the gas utility industry. An electrical current is passed through the resistance element of weld mbe saddle pad 37 which is pressed between the saddle tee fitting 38 and the mbular member 39 together to complete the weld.

Figure 21 is an example of using weld mbe 42 that has not been flattened to form a socket weld. This would typically be used as a 'scab' patch on small diameter pipe again that is typically used in the gas utility industry. For this weld application special metal ring connectors 40 are fitted onto the bared wires (not shown) of the resistance element at each end of the mbe 42. In the case of a 'scab' patch the damaged pipe 41 is cut through and the non-flattened weld mbe 42 is pushed completely on one side of the mbular member 40. Then the two ends of the cut-through pipe 41 are re-aligned end to end and the weld tube 42 is pulled back over the joint so that it straddles the pipe 41 equally on both sides of the joint.

A piece of Teflon-type material (not shown) is wrapped over the open mbe weld to allow the softened thermoplastic of mbe 42 to be pressed against the wall of pipe 41 during the welding cycle forming a homogeneous bond of all the thermoplastic members. A sheet of namral rubber (not shown) is stretched over the Teflon-type material to supply a constant elastic pressure on the weld during heating. After the joint is cooled the sheet of rubber material and the Teflon type material are removed.

As with other electrofusion welding systems for thermoplastics, the temperamre of the resistance element is dependent on the electrical current applied to it. The higher the current the hotter the wires of the resistance element get. The temperamre of the wires should never exceed the upper melting temperamre of the thermoplastic being welded or else the thermoplastic adjacent to the wires can be burnt. The temperamre that the wires reach is also dependent on the amount of thermoplastic material around the wires.

If a current was passed through a flattened weld mbe not placed between two thermoplastic members it will reach a much hotter temperamre than if the same flattened weld mbe was placed between two thermoplastic members at the same electrical current. In fact, the mass of the two thermoplastic members has much to do with the welding time and temperamre. The larger the mass, the larger the heat sink effect of the mass, and the more heat that will be required to effect the weld.

When welding thermoplastics there is a bonding temperamre that must be reached to achieve a true chemical bond. Simply melting the plastic to form a weld bead is not sufficient if the bonding temperamre is not reached. Many seemingly good welds have proven to be cold welds where the proper bonding temperamre was not reached during the weld cycle. When welding thermoplas¬ tics it is better to weld at a lower current or temperamre for a longer time, again assuring that an adequate bonding temperamre is reached during the course of the weld. The slower weld results in a wider temperamre gradient within the plastic.

The wires of the resistance element are inherently positioned where they are during the heating cycle. If too much pressure is applied to the joint during welding, the molten thermoplastic of mbe and coating is squeezed out from the weld while the wires continue to heat and migrate into the parent material of the thermoplastic pieces being welded together.

With the ability to vary number and thickness of the wires of the resis- tance element and the ability to vary the size and thickness of mbe and the thickness of coating, it is relatively easy to design a mbe of the appropriate thickness and diameter so that, when flattened in rings, no additional material needs to be added by injection molding to create a stand alone ring.

Another embodiment of the present invention is a welding rope. Figure 22 shows welding rope 46 comprising numerous strands 47. Figure 23 shows a cross section of welding rope 46 showing the individual strands 47. Each strand 47 comprises numerous filaments (not individually shown) of thermoplastic yarn which are twisted together to form a strand 47. In the welding rope 46 of the invention, each strand 47 also contains a resistance wire 48 within the strand 47. The resistance wire 48 heats the thermoplastic rope material which forms a plastic weld when heated between two pieces of plastic. Resistance wire 48 can be stainless steel, nichrome or other suitable wire material. Heating the welding rope requires stripping away a small amount of plastic at each end of the rope to expose the resistance wires. A power source is connected to the exposed wires and a current introduced to heat the wires and make the weld.

Where it would be impractical to connect an electrical power source to two ends of a welding rope, a variant can be used that permits the electrical connection to be made at one end of the rope. Figure 24 shows a side view of welding rope that has a central conductance remrn wire 53 which is covered with a non-conductive, insulating material 49. In this described embodiment, the insulating material 49 is cotton braid. This wire 53 and cotton braid 49 are then used as a core for a welding rope where strands 47 with wires 48 are braided around the core.

To make rope weld, a multistranded rope braid is preferable because the several rope strands 47 are more uniformly positioned in the tighter braid pattern. Stainless steel wires are embedded in the rope 46 as part of each strand 47 that makes up the rope 46. Typically, there are eight filaments of thermoplastic fibers or yarn that are twisted together to form a strand 47. Several of these strands 47 are then braided together to form a rope 46. The size and number of the filaments are variable, as are the size and number of strands to form the rope. Although braid is presently preferred, other rope pattems can be used, such as a simple twisted rope.

This system can also be applied to flat rope. The stainless steel wires of the electrofusion system are spun into the filaments that are braided together to form the rope. Depending on the size of the rope being made, any number of stainless steel wires can be spun into each filament. Rope can be made out of many types of plastic material. The present invention can use any of these types of material, as long as the plastic is able to be heat fusion bonded. Some of the common plastic materials that can be welded with this method include; polyethylene, polypropylene, nylon and polyvinyl chloride (PVC). Most thermoformed plastics can also be welded with this system.

Of particular interest is nylon welding. Nylon is a very difficult material to weld for several reasons. Nylon is very rigid and it tends to turn very viscous over a narrow temperamre range. When formed into a thin walled mbe, like that of the mbe weld, the coextrusion of a second layer tends to melt and deform the inner mbe. A nylon rope weld is much more pliable with a more even source of heating.

Another advantage of the present invention is the ability to include reinforcing fibers braided into the rope. Fibers such as carbon, polyester, fiberglass and Kevlar can be used to strengthen the weld as they become encased in the molten plastic matrix.

The size of the weld is only limited by the size and shape of the rope. If larger welds with more plastic are required then the size and number of filaments in the rope are increased, and the size and number of resistance wires in the strand can also be increased.

The manufacturing process of rope has no heating cycles, which can affect the plastic matrix material. Every time plastic is heated, the heat history of the plastic is increased. The fewer heating cycles plastic is exposed to the better it is for the plastic, which becomes weaker every time it is heated. It is for this reason that there are guidelines on the amount of regrind (old reused plastic) that can be used in the manufacturing of most plastic products. Other electrofusion systems usually require several heating cycles in the formation of the product, as does the mbe weld described in this patent application (though only if it is flattened). The rope weld of the present invention can be pre-formed into rings and other required shapes. When pre-form shapes are made with rope weld, the heating wires are positioned throughout the solid plastic matrix that is formed as the fibers of the rope are melted together.

Electrofusion systems require a power source to be applied across both ends of the resistance wires. There are situations where it is not practical to connect the power source to opposite ends of the wire, such as in a trench liner installation. The rope can be fabricated with an inner conductance wire (copper), around which is braided a namral fiber-like cotton to prevent short circuiting. Around this cotton covered wire, the electrofusion plastic strands with interwoven resistance wires are braided. At one end of the weld rope, which is cut to conform to the length of the weld to be made, the inner conductance wire and the outer resistance wires are twisted together to form an electrical connection. By doing this both leads from the power source can be connected at the same end of the rope weld, opposite the end where the conductance wire and resistance wires are connected to each other. A second variation of the weld rope is made by extruding a thin layer of plastic over one or a pair of resistance wires to form monofilament lines. These monofilament lines are then braided together to form a rope, which can be used as an electrofusion system for welding plastic.

Figure 25 shows a cross section of a welding rope 54 comprised of numerous monofilament lines 52 in which resistance wires 48 are coated with a plastic material, as described above. In this example, two wires 48 are coated in each individual line 52. The lines 52 are twisted together to form a rope that can be formed into various shapes for welding thermoplastic workpieces. The monofilament line type of welding rope is used in the same way as the stranded rope. The thermoplastic material is stripped from a small segment at the ends of the rope, exposing just enough resistance wire to make an electrical connection to heat the rope and make the weld.

The present invention may be embodied in other specific f rms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicating the scope of the invention.

Claims

CLAIMSI claim:

1. A thermoplastic welding mbe for welding plastic workpieces, comprising: a flexible hollow elongate mbe, said mbe comprising an inner layer of flexible thermoplastic material, an electrical resistance heating element, said heating element being disposed around said hollow inner layer and along its length, and a coating layer of thermoplastic material covering said resistance heating element along the elongate inner layer such that the heating element is enclosed within the coating layer, said elongate inner layer, resistance element and coating layer thereby being combined into an integrated construction.

2. The welding mbe of claim 1, wherein said electrical resistance heating element comprises a plurality of wires.

3. The welding mbe of claim 2, wherein the wires of the heating element are disposed longitudinally along the mbe.

4. The welding mbe of claim 2, wherein the wires of the heating element are helically wound around the mbe along its length.

5. The welding tube of claim 2, wherein the wires of the heating element are wound obliquely around the mbe forming a braided wire element.

6. The welding mbe of claim 2, wherein said mbe has at least one end and said wires extend from the end of the mbe for connection to an electrical power source.

7. An apparams for welding thermoplastic strucmres, comprising a flexible hollow elongate thermoplastic welding mbe comprising an inner layer of flexible thermoplastic material, an electrical resistance heating element, said heating element being disposed around said hollow inner layer and along its length, and a coating layer of thermoplastic material covering said resistance heating element along the elongate inner layer such that the heating element is enclosed within the coating layer, said elongate inner layer, resistance element and coating layer thereby being combined into an integrated construction, said thermoplastic welding mbe having first and second ends; said welding apparams further comprising a mbe connector plug connect- ing the first and second ends of the welding mbe.

8. The welding apparams of claim 7 wherein the electrical resistance heating element comprises a plurality of wires.

9. The welding apparams of claim 8 wherein the wires extend from the ends of the thermoplastic welding mbe for connection to an electrical power source.

10. The welding apparams of claim 7 wherein the mbe connector plug comprises a cylinder of thermoplastic material, said cylinder having a diameter sized to fit snugly into the ends of the welding mbe.

11. The welding apparams of claim 10 wherein the tube connector plug further comprises a band of thermoplastic material disposed about the circumference of the plug at its approximate center, said band of thermoplastic material enlarging the diameter of the plug to approximate the diameter of the thermoplastic welding mbe, said band serving to separate the respective ends of the welding mbe from each other when said ends are joined by the connector plug.

12. The welding apparams of claim 7, wherein said welding mbe and connector plug apparams comprises a flexible thermoplastic welding device for shaping to conform to a profile, said profile being defmed by two thermoplastic workpieces to be welded together, each respective workpiece having at least one surface to be welded to a surface of the other respective workpiece, at least one of said surfaces having a defined profile to which the welding device must be shaped to make the weld.

13. The welding apparams of claim 7, wherein the electrical resistance heating element comprises a plurality of wires; the mbe connector plug comprises a cylinder of thermoplastic material, said cylinder having a diameter sized to fit snugly into the ends of the welding mbe, said connector plug further comprising a band of thermoplastic material disposed about the circumference of the plug at its approximate center, said band of thermoplastic material enlarging the diameter of the plug to approximate the outer diameter of the thermoplastic welding mbe, said band serving to space the respective ends of the welding tube from each other when said ends are joined by the connector plug.

14. The welding apparams of claim 13, wherein the respective ends of the welding mbe are stripped of the thermoplastic material to expose a small segment of the resistance wires comprising the heating element, said apparams further comprising electrical connectors attached to the respective exposed ends of the resistance element wires, and a power source connected to the respective electrical connectors for introducing a current into the resistance element to heat the element.

15. A kit for electric thermoplastic welding of plastic workpieces, comprising a flexible hollow elongate thermoplastic welding mbe comprising an inner layer of flexible thermoplastic material, an electrical resistance heating element, said heating element being disposed around said hollow inner layer and along its length, and a coating layer of thermoplastic material covering said resistance heating element along the elongate inner layer such that the heating element is enclosed within the coating layer, said elongate inner layer, resistance element and coating layer thereby being combined into an integrated construction, said thermoplastic welding mbe having first and second ends; said welding kit further comprising a mbe connector plug connecting the first and second ends of the welding mbe, and at least two thermoplastic workpieces for welding together with the thermoplastic welding tube.

16. The thermoplastic welding kit of claim 15, further comprising electrical connectors and an electrical power source having first and second poles for causing an electric current to pass through the electrical heating element of the welding mbe.

17. A connector block for a flat thermoplastic welding strip, said connector block comprising a substantially flat thermoplastic block, said block having first and second ends and first and second sides, each of said first and second ends of the block having a respective cavity for receiving a gathered wire mesh containing exposed ends of resistance wires extending from the thermoplastic welding strip, said respective cavities being open at the end of the connector block from side to side, said cavities at the first and second ends of the block being separated from each other on the inside of the block by a core of material substantially in the center of the block that extends from side to side.

18. The connector block of claim 17 wherein the respective first and second cavities in the block are open on one side of the block for permitting the mesh and resistance wires to extend outside the block for electrical connection to a power source.

19. A thermoplastic welding rope for welding thermoplastic workpieces together, comprising a central conductance wire, a layer of non-thermoplastic non-conductive insulating material covering the central conductance wire, a plurality of strands of thermoplastic welding material, said strands comprising a plurality of filaments of thermoplastic yarn and at least one electri- cal resistance wire, said plurality of strands being wound about the insulated central conductance wire.

20. The thermoplastic welding rope of claim 19, wherem the layer of non-thermoplastic non-conductive insulating material covering the central conduc¬ tance wire comprises cotton.

21. A thermoplastic welding rope for welding thermoplastic workpieces together, comprising a plurality of elongate strands arranged together to form an elongate rope of thermoplastic flexible material, said strands further comprising a plurality of filaments of thermoplastic yarn and at least one resistance wire.

22. The thermoplastic welding rope of claim 21, wherein the arrange¬ ment of the strands forms a braided rope.

23. The thermoplastic welding rope of claim 21, wherein the arrange¬ ment of strands forms a twisted rope.

24. A thermoplastic weldmg rope for welding thermoplastic workpieces together, comprising a plurality of monofilament Imes arranged together to form an elongate rope of flexible thermoplastic material, said monofilament lines comprismg at least one resistance wire that is coated with a thermoplastic material along its length

25. The thermoplastic welding rope of claim 24, wherem the arrange¬ ment of monofilament lines forms a braided welding rope

26. The thermoplastic weldmg rope of claim 24, wherem the arrange¬ ment of monofilament Imes forms a twisted welding rope