NZ244411A - Thermoplastic welding using thermoplastic welding rod incorporating electric heating element - Google Patents

Description

Priority DUv(s}: CoiW**?* 9;/. -..-'rs {.1....!.:.}.]^. cs.« ;■■; e>l'.tLb5./3Hv..6£>Kifc/.& ....U.kL.a/sl "7rrrzri2iMMIlll'III' fo: ......vagi Patents Form No. 5 Number PATENTS ACT 1953 Dated COMPLETE SPECIFICATION ELECTRIC FUSION WELDING OF THERMOPLASTIC MATERIALS We, E 0 BUTTS CONSULSTANTS LTD, a Canadian company, of 78-B Jamie Avenue, Nepean, Ontario K2E 6T6, Canada do hereby declare the invention for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: 1a FIELD OF THE INVENTION The present invention relates to the welding of members of fhermoplastic materials including polyethylene, polyvinyl chloride, nylons, polypropylene, acetals, and the like.

BACKGROUND OF THE INVENTION Thermoplastic materials including polyethylene, polyvinyl chloride, nylons, polypropylene, acetals, and the like have become increasingly important as materials of construction because of their moldability, ease of handling, light weight, and ability to withstand corrosion.

In many applications however, mechanical joints such as threaded joints are unacceptable because they are not fluid-tight. This is particularly of concern in the handling of hazardous materials and waste.

Numerous techniques have therefore been developed for the joining of thermoplastic materials including welding and the use of adhesives. The use of adhesives or solvents for making joints in plastic pipe, etc. is inconvenient in field( i applications where conditions are not ideal and where the t.ime ] "v - 8 OCT I992' _ . c and other factors required to develop the maximum bond strength nay not be available. Moreover, it may be impossible to find a suitable adhesive for the thermoplastic material.

Polyethylene has in the past been welded by techniques such as extrusion welding using a tool which applies heat to overlapped sheets of the thermoplastic material, and extrudes a bead of thermoplastic material at the edge of the lap. The bead of thermoplastic material is intended to fuse with the preheated sheets of the thermoplastic to form a liquid fusion of the two sheets and the extrudate to weld the overlapped sheets. This technique has not proven entirely satisfactory in practice because of a lack of uniformity in the welding process especially where hand held extrusion welding tools are used on large sheets of ma t er i a 1.

Another technique known in the prior art involves the use of a so-called "speed-tip" for welding sheets of thermoplastic material in which a welding rod of the thermoplastic material is fed through the nozzle of a hot air c;un. The stream of hot air together with the melted thermoplastic are applied to the area to be welded. Once again problems of uniformity and completeness of weld are evident. Frequently such welds include pinholes or unwelded portions and can also include portions in which excess heat has been applied and the thermoplastic material damaged. Obviously, damaged and incomplete welds are most undesirable.

A further technique utilizes a hot wedge which is passed bewteen the sheets of thermoplstic material to heat the opposite faces which are then pressed together to form a bond.

A number of electrica11y heated techniques for forming either lap or butt welds between two members of a thermoplastic^, material have been attempted in the past. • , 1 L -8 OCT 1992 3 One technique involves the vise of a bare wire or metal strip which is placed between the two members of thermoplastic material. This method is disclosed in the teachings of the fo11owing United States patents: 2,243,506 (Mitchell), 2,647,072 'Smith), 2,742,390 (Beck), 2,974,566 (Hurley), 3,049,465 (Wilkins), 3,061,503 (Gould), 3,348,640 (Thompson), 4,416,713 'Brooks), 4,176,274 (Lippera), and 4,375,591 (Sturm).

The present inventors have had experience in the welding of thermoplastic members without the supply of new material and have found that the wire can actually burn holes in the thermoplastic material of the members being welded. Accordingly it is very important to supply new material when welding two thermoplastic members.

Some other forms of welding rods have been disclosed in which wire or metal strips are coated in a relatively thick layer of thermoplastic material. This type of welding rod is disclosed in the following patents: US 2,983,306 (Resnick), US 3,506,519 and CA 837,562 (B1umenkranz), US 3,378,672 and CA8 11,837 (B1umenkranz), and FR 1.072.800.

The present inventors have found that this type of welding rod results in the element being too remote from the surfaces to be welded. The plastic immediately surrounding the wire or metal strips burns before the material of the. thermoplastic members melts. The weld, if formed, is of poor qua 1i t y.

French patent number 1.072.800 also describes the use of an adhesive to hold the welding rod in place on one sheet before applying the second sheet and welding the sheets together. The present inventors have tried this method of holding the welding rod in place. They have found that the use of adhesives or double-sided tape adversely affects the integrity of the weld1 ■'«' r 'v \ /' ' / •."-3 OCT 1992 % -V >. • rv r 4 because the adhesive and double-sided tape act as a contaminants. These contaminants prohibit the intermolecu 1ar bonding that is desired between the thermoplastic members being joined and the welding rod.

Many of the prior art techniques have only limited applications and are limited to the welding of certain shapes.

SUMMARY OF THE INVENTION The present invention aims to overcome the disadvantages of the prior art by providing an improved welding rod, a method and apparatus for forming the welding rod, and methods of use of the welding rod. The welding rod is formed of a resistance element and a solid homogeneous core 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 number of wires either wrapped in a helical pattern or braided around the solid core.

The resistance element is then embedded in the solid core to draw the wires just below the surface of the solid core. Alternatively, the resistance element and the solid core are coated in a thin layer of thermoplastic material . The welding rod of the present invention provides a better distribution of heat and reduces the risk of overheating and burning of the thermoplastic, both of the solid core and the members being joined, which can adversely affect the quality of the weld. A double welding rod is also disclosed which can be formed into a variety of shapes to adapt to many different applications.

In use the welding rod is positioned between the thermoplastic members to be welded and an electrical current is applied to the resistance element. Simultaneously, the members - > u n ? G \ •V v ^ • - 8 OCT 1992 ' 1 ji r /■■' A /f •I t ■ t I to be welded are pressed together. The electrical current is applied for a time and at an intensity to cause the solid core and portions of the thermoplastic material of the adjacent members to soften and fuse thereby welding the two members together. The resistance element forms a mechanical reinforcement in the weld thereby rendering a superior weld between the thermoplastic members.

There is also disclosed a method for welding of rtd.M'-ent members of thermoplastic material wherein the welding rod ;s pre-attached to one of the adjacent members of thermop1 astic material.

The welding rod and process for electric fusion welding of the present invention is very versati1e, providing a means for the welding together of different shapes and sizes of thermoplastic members.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings which illustrate embodiments of the present invention, Figure 1 is a perspective view of a welding rod in accordance with the present invention, Figure 2 is a cross-sectional view of the welding rod of Figure 1 taken along the line 2-2, Figure 3 is a schematic diagram of an apparatus for forming the welding rod, Figure 4 is a perspective view of a welding rod at stage A of production shown in Figure 3, Figure 5 is a cross-sectional view of the welding rod of Figure 4, ■ <\ Figure 6 is a perspective view of a terminal pin, c^ -3 OCT I992-n' V ' Figure 7 • ]Just rates the genera) procedure used to form a iar> loint of t hermopl as t i c material using the welding rod of F:gure 1, Figure 8 is a cross-sectional view of the completed wtid of Figure 7, Figure 9 i 1 lustrates a butt weld between curved members such as for a longitudinal butt weld on a pipe, Figure 10 illustrates the weld of Figure 9 on comp1et i on, Figure 11 is a perspective view illustrating the use of the invention in performing a transverse butt weld on piping, Figure 12 is a perspective view of an alternative embodiment of the welding rod of Figure 11, Figure 13 is a top plan view of a welding rod pre-attached to a thermoplastic member, Figure 14 is a perspective view of an alternative embodiment of a welding rod pre-attached to a thermoplastic member, Figure 15 is a perspective view illustrating the use of a fusion ring for a transverse but.t weld with a socket fitting of pipe sections, Figure 16 is a cross-sectional view of Figure 15, Figure 17 is a perspective view of an alternative embodiment of the fusion ring, Figure 18 is a front elevational view of the fusion ring of Figure 17, Figure 19 is a cross-sectional view of Figure 15 with a rabbet fitting of the pipe sections, Figure 20 is a perspective view of a flattened double welding rod, and Figure 21 illustrates the use of the flattened double welding rod of Figure 20 to repair a seam between two thermoplastic sheets. [7''^ * * /* r* •. n * o r - 8 OCT 1992 v v*- C I M 7 DESCRIPTION OF THE PREFERRED EMBODIMENTS Figures 1 and 2 illustrate the welding rod 10 of the present invention. The welding rod 10 is formed of a solid core 11 of thermoplastic material and a resistance element 12. The resistance element. 12 is connected to terminal pins 15 at each end of the welding rod 10 for subsequent connection to a power supply (not shown).

The choice of materials and the dimensions of the sol id core 11 and the resistance element 12 are dependent on the requirements of a particular application. For example the solid core 11 may be formed of thermoplastic materials including polyethylene, polyvinyl chloride, nylons, polypropylene, acetals, and the like. In turn, the resistance element 12 may be formed of conducting metals including sta-inless steel, nichrome, iron, tin-copper alloy, and the like.

For the purposes of this description, the solid core 11 is formed of high density polyethylene (HDPE) and the resistance element 12 is formed of stainless steel. The dimensions of the solid core 11 and the resistance element 12 presented in this discussion are chosen to provide 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.

In the prior art "speed-tip" technique of joining thermoplastic members discussed above, a welding rod of thermoplastic material is fed through a nozzle. One type of this prior art welding rod is an extruded HDPE rod having a diameter of 5 millimeters. This welding rod is an ideal solid core 11 for the present invention. The solid core 11 should be homogeneous and free of any significant voids as discussed hereinafter.- i "3 OCT 1992 8 To form the welding rod 10 with the 5 millimeter diameter HDPE solid core 11, the resistance element 12 is comprised of sixteen strands of stainless steel wire 17 having a diameter of 0.006 inch. As shown more clearly in Figure 4, the wires 17 are wound in eight pairs in a helical pattern around the solid core 11. The size and number of wires 17 of the resistance element 12 are factors which dictate the length of this helical pattern along the length of the solid core 11. In the present example, one revolution of a pair of wires 17 is approximately 1.25 inches long. In all cases the configuration of the resistance element 12 should provide a distribution of energy from the power source to cause uniform heating of the solid core 1 1 Often the limiting factor in the use of the welding rod 10 is the voltage permissible at the welding site. For safety reasons the voltage is often limited to a maximum of 120 volts. Accordingly the current, is dependent on the resistance of the resistance element 12. One relatively large diameter wire .17 cannot be used as a resistance element 12 since it would tend to overheat and cause the solid core 11 to burn. By using a number of smaller diameter wires 17 distributed evenly around the solid core 11, there is better heat distribution in the solid core 11 without overheating and greater lengths of welds are achievable than would be possible with a single conductor.

Once the wires 17 are wound around the solid core 11, the resistance element 12 is embedded in the solid core 11 by holding the wires 17 in tension while softening the surface of the solid core 11 so that the wires 17 are drawn below the surface of the solid core 11. By embedding the resistance element 12, it is not exposed to the atmosphere on the surface of the solid core 11. If the resistance element 12 is exposed to the atmosphere when connected to a power source, a hot. spot, can be created and the thermoplastic material of the solid core , - 8 OCT 1992 11 and the thermoplastic members being welded will be burned. There are also advantages in the handling of the welding rod 10 having an embedded resistance element 12. It will be understood by those skilled in the art that the wires 17 might catch and get tangled if they are exposed on the surface of the solid core 11. Furthermore, the exposed wires 17 are more likely to get covered by dirt, grease, and other contaminants which could adversely affect the quality of the weld. A welding rod 10 which has an embedded resistance element 12 is more readily cleaned of these contaminants.

While it is desirable to embed the resistance element 12 in the solid core 11, it is also desirable to control the depth of the embedding. If the resistance element 12 is embedded too deeply, the element 12 is too remote from the surfaces being welded. The thermoplastic material immediately surrounding the resistance element 12 will burn before the thermopiastic materia 1 of the members being joined can soften, resulting in a poor weld. Moreover, the sixteen strands cf wire 17 would then begin to act as one larger diameter wire which is also unacceptable as it could cause overheating and therefore burning of the thermoplastic material of the solid core 11. Preferably the resistance element 12 is embedded just below the surface of the sol id core 11.

Figure 3 illustrates schematically an apparatus 20 which can be used to form the welding rod 10. The solid core 11 is uncoiled from a core spool 21 and drawn through a wire winding machine 22. The wire winding machine 22 winds sixteen wires 17 in eight pairs in a helical pattern about the solid core 11. One revolution of a helix of a pair of wires 17 is approximately 1.25 inches long. Figures 4 and 5 show the welding rod 10 in more detail at stage A of the process of forming the welding rod 10 with the wires 17 wound in a helical pattern about the solid-core^ 11■ x " ' " ' ' '- 8 OCT 1992 The solid core 11 and the resisfanrfl element [ ? are held in tension and drawn through a heating chamber 25. The tension of the wires 17 and the softening of the solid core 11 ?ause the resistance element 12 to be drawn below the surface of th^ solid core 11. Subsequently the welding rod 10 is drawn through a cooling chamber 2 6 and wound on a welding rod spoo' 27. In this continuous process of forming the welding rod 10, it is advisable to engage the movement, of the core spool 21 with the welding rod spool 2 7 by a common drive mechanism 28 so that the appropriate tension is maintained throughout the process.

An example of the operating parameters of the apparatus in is: the solid core 11 is unwound from the core spool 21 at a rate of 6 to 7 feet/minute? the heating chamber 25 is controlled at a temperature of 400 "C and has a length of 1.5 feet: and the '•^oling chamber 26 is a water bath controlled at a temperature <>f 38 'C and has a length of 1.0 foot.

As shown in Figure 1, the welding rod 10 has a spiral pa'tern on its surface defining the path of the resistance element 12 as it is embedded in the softened solid core 11. If desired the apparatus 20 can be adapted to include a die (not shown) after the heating chamber 2 5 to smooth the surface of the welding rod 10 or to mold the welding rod 10 into another shape.

The resistance element 12 could also be formed of wires 17 braided around the solid core 11.

The apparatus 20 could also be adapted to replace the heating chamber 25 with a cross-head extruder (not shown) to apply a thin coating of thermoplastic material over the resistance element 12. This adaptation is particularly useful when the resistance element 12 is braided around the solid core 11 and/or when the solid core 11 is formed of a relatively sq.f-tr: +*** " material such as polyvinyl chloride. 0 - 8 OCT 1992 ^ - / r.O >• 11 Some methods of producing the solid core M use a fast cooling cycle of the thermoplastic extrudate. However, if the core 1] is cooled too quickly in the manufacture thereof, the exterior of the core 11 is hardened while the interior is still molten so that voids are formed in the interior of the core 11. Accordingly, as the core 11 is heated in the heating chamber 25 of the apparatus 20 during formation of the welding rod 10, the core i 1 collapses at the voids so that the quality of the welding rod in is adversely affected. Preferably, the solid core 11 should be homogeneous and free of any significant voids.

In preparation of the welding rod 10 for use in the welding together of thermoplastic members, the required length of welding rod 10 js cut from the welding rod spool 27 and the terminal pins 15 are attached to the resistance element. 12 at efi'.-'n end of the welding rod 10.

As shown in Figure 6, the substantially trumpet-shaped terminal pin 15 could be formed of a thin-walled brass tube Preferably the terminal pin 15 has a mouth 16 of a diameter slightly less than that of the welding rod 10. The terminal pin 15 is heated and the end of the welding rod 10 is forced into the mouth 16 of the terminal pin 15. As the welding rod 10 is fiji ther forced into the neck 14 of the terminal pin 15, the thermoplastic material of the solid core 11 is stripped from the resistance element 12. The terminal pin 15 is then crimped onto the resistance element 12 at the neck 14.

Alternatively, a portion of the resistance element 12 at each end of the welding rod 10 is exposed by heating and stripping approximately 1/4 inch of the solid core 1J from each end of the welding rod 10 using, for example, a hot air gun and a pair of wire strippers. The exposed wires 17 of the resistance clement 12 are then twisted together and a terminal pin 15 is connected to the resistance element 12. After the t ermi na 1. ; pT/K>^x o " «< '-3 OCT 1992 1 i 1 : 1 12 is attached, the thermoplastic at the base of the terminal pin 15 is reheated and the softened thermoplastic is pushed over the base of the terminal pin 15.

These methods of attaching the terminal pin 15 ensure that the resistance element 12 is not exposed to the atmosphere when connected to a power source thereby preventing overheating of the welding rod 10 at that point during welding due to the lack of thermoplastic material to absorb the heat. Tf these steps are not taken, the thermoplastic material of the members being welded will burn rather than soften on contact with the resistance element 12.

The welding rod 10 is very versatile and can be bent and formed into essentially any shape to effect welding of thermoplastic members. For example, the welding rod 10 can be ns-d to fuse tubular or planar thermoplastic members with butt ci lap joints.

Figure 7 illustrat.es the general procedure for forming a welded lap joint between thermoplastic members. A pair of sheets of thermoplastic material 32 and 33 are positioned on either side of a welding rod 10. A variable voltage transformer 34 is connected to a source of power at 35 and f\irther connected via a suitable switch 36 to the resistance element 12 throu.gh tho t ermina1 pins 15.

An electrical current is passed through the resistance element 12 and simultaneously pressure is applied above and below on the sheets 32 and 33. As the electrical current passes through the resistance element 12, the solid core 11 and the thermoplastic material of the thermoplastic sheets 32 and 33 in the vicinity of the welding rod 10 begin to soften. The electrical current is continued for a time and intensity to cause the thermoplastic material of the welding rod 10 and , the^x o -8 OCT 1992 .: •> \ ^ ^ O ✓ 3 thermoplastic sheets 3 2 and 3 3 t"o so ft-en and m»= 1 t . After the electrical current is discontinued, the application of pressure above and below the thermoplastic sheets 32 and 33 is continued so that the solid core 11 and the two thermoplastic sheets 32 and 3 3 fuse and solidify thereby forming a unified weld. The resistance element 12 remains in the weld and reinforces or "stitches" the weld mechanica 11 y. Another advantage to having the resistance element 12 in place in the weld is to allow reheating of the weld in the event of a weld failure due to insufficient heat or pressure application during the initial weld 1 e • Figure 8 is a cross-sectional view of the weld formed by the process i 1 lust rated in Figure 7. As can be seen from this • i s-sec t i ona 1 view, the materia] of the solid core 11 is now i ruiist innuishahie from the welded materia] of the sheets 32 and and the weld is reinforced by the resistance element 12 "inh^dded there) n Figure 9 illustrates a butt weld between curved members •c' dud 39 of. for example, a longitudinally slit thermoplastic pipe. The welding rod 10 is positioned between the curved members 38 and 39- Pressure is applied to urge the edges of the curved members 38 and 39 into contact and an electrical current is passed through the resistance element 12. The weld is completed when the welding rod 10 and curved members 38 and 39 are fused and the resistance element 12 is embedded in the weld. The structure of the weld is such that the solid core 11 becomes part of the body of material of the two curved members 38 and 39 that are joined and the weld is simultaneously reinforced by the presence of the resistance element 12. This latter completed configuration of the butt weld is illustrated in Figure 10.

Figure 11 is a perspect i ve vi ew illustrating the manner^^f"; in which a pair of tubular thermoplastic members, 41 and 42, such \ / "■ ) if "8 OCT 1992;' 14 as sections of piping., can be welded end to end with a butt joint. The welding rod 10 is formed into a ring shape and is positioned between the tubular members 41 and 42. An electrical current is passed through the resistance element 12 and the tubular members 41 and 42 are pressed together to complete the we Id.

Alternatively, referring to Figure 12, a welding ring 43 is used to weld the tubular thermoplastic members, 41 and 42. The welding ring 43 is formed of two welding rods 10, each having a length slightly longer than half of the circumference of the tubular thermoplastic members 41 arid 42. The resistance element 12 is exposed at each end of the welding rods 10 by heating and stripping the solid core 11 using, for example, a hot air gun and a pair of wire strippers, so that the length of each welding rod 10 is then approximately equal to half of the circumference of the tubular members 41 and 42.

The exposed wires 17 of the resistance element 12 at a first end of the first welding rod 10 are then twisted together with the exposed wires 17 of the resistance element 12 at a first end of the second welding rod 10. Further, the exposed wires 17 of the second ends of the first and second welding rods 10 are also twisted together to form a double welding rod (not shown). The terminal pins 15 are subsequently attached to the resistance element 12. The welding rods 10 are then pulled apart, without separating them from the terminal pins 15, to form the welding ring 43. This type of welding ring 43 is particularly useful in the welding together of large diameter thermoplastic pipe sections and in situations where the available voltage is 1imi ted.

The power supply used to effect electrofusion welding of thermoplastic members with the welding rod 10 of the present 7;" r . / invention preferably offers control of the current in the range ' " -8 OCT 1992 'I i 'i of 0 to 2 0 amperes . often the limiting factor in the use of the welding rod 10 is the voltage permissible ^t I he welding site. For safety reasons the voltage is often limited to a maximum of 12 0 volts. The voltage then becomes the limiting factor to the length of the weld that can be achieved.

For example, a 6 inch weld will take the same length of time to complete as a 30 foot weld at the same current using a 4 volt and a 240 volt source, respectively. Furthermore, the length of time required to complete a weld can be varied by changing the current; at lower amperages the required welding time is increased. Generally, the integrity of the weld benefits by using lower amperages for longer periods of time because the heat distribution is improved and overheating of the thermoplastic of the welding rod 10 and the thermoplastic members is less likely.

Preferably, the welding process is conducted at a current between fi amperes and IS amperes for the single welding rod 10, and between 12 amperes and 20 amperes for the double welding rod (not shown) or the welding ring 43. The length of time required to effect the weld may be determined by using a temperature probe 37 placed, for example, on the upper surface of the thermoplastic sheet 32 in Figure 7, directly over the welding rod 10. When the temperature probe 37 indicates that the temperature has reached 120'C, the power source 35 is disconnected and the weld is allowed to fuse and solidify while the weld is still under pressure. The thickness of the thermoplastic sheet 32 and the resulting temperature gradients must be compensated for in the determination of temperature as the probe 37 will take longer to detect the temperature when thicker pieces of thermoplastic are welded. It will be understood by those skilled in the art that, the conditions at the welding site will affect the time required to complete the welding operation. A single welding rod 10 might require a s oct \cm 1 B welding time of eight minutes at 6 amperes or two minutes at 12 amperes.

An alternative approach to the method of welding described herein is to substitute the 0.006 inch stainless steel wire i 7 of the resistance element 12 with 0.008 inch magnetic wire and to use a radio frequency (RF) generator (not shown) to induce a current in the magnetic wire. This method enables the .•>. r.p 1 et i on of very long welds by simply sliding the RF generator eel 1 {-.long the length of the weld rather than having to supply c. series of terminal pins 1 5> for connection to an electrical power source at various points along the weld.

It is important to apply pressure to the weld during t:: welding operation. If the welding rod 10 is simply placed b-tween the thermoplastic sheets 32 and 33, it will "snake" or move out of position on application of electrical current. A weight (not shown) placed on the weld holds the thermoplastic sheets 32 and 33 and the welding rod 10 in relative position to each other. Furthermore, it is important to maintain the pressure until the weld is fused and solidified. Depending on the situation, the weld should generally be left to cool under pressure for approximately four minutes. If the pressure is discontinued before the thermoplastic material has solidified, the sheets 32 and 33 and the welding rod 10 will, in fact, tend to separate from each other while in the softened state.

Preferably the weight has a relatively soft base which will conform to the surface of the member being welded. A rigid surface on the base of the weight can effect unequal pressure on both sides of the weld. This problem is compounded if the underlying surface is uneven. The weight should be designed to distribute the load equally along the length of the weld particularly in the case of a relatively long weld. The design of the weight can incorporate an insulating effect, allowing' ,. \ j • - 3 OCT 1992 ■:>. 0 / 1 7 welds to be completed at ambient temperatures below 40C.

For example, the base of the weight may be comprised of a length of plywood (3/4 inch thick and 4 inches wide) having a layer of 1/2 inch thick neoprene rubber attached to its underside. The base is placed over the weld and weights of, for example, 10 pounds per 1inear foot are then placed on the base.

This arrangement, provides even distribution of pressure over the length of the weld.

It is often desirable to pre-attach the welding rod 10 to one of the thermoplastic members to be welded. The present inventors however, have found that the use of adhesives or double-sided tape to hold the welding rod 10 in position adversely affects the integrity of the weld because the adhesive -•.nd double-sided tape act as contaminants which prohibit the desired intermo1 ecu 1 ar bonding between the thermoplastic members b^mg joined and the welding rod 10.

Accordingly, the welding rod 10 should be held in Ii ion by fusing the welding rod 10 in part, or in whole, to • uie the two thermoplastic members to be joined. Referring now to Figure 13, the length of welding rod 10 can be pre-attached, m any desired shape, to the thermoplastic member 44 either with or without the terminal pins 15 connected to the resistance element 12. This can be done, for example, by using a hot air gun and the prior art "speed-tip" technique to soften the thermoplastic material of the solid core 11 and to tack the welding rod 10 in place.

Referring now to Figure 14 which illustrates the pre-«ttachinq of Figure 13 with an alternative embodiment of the ■welding rod 10 formed into a patch welding rod 45. This particular patch welding rod 45 is formed in the same manner as...^ the welding ring 43 discussed previously and is particularly cs - 8 OCT !992 ::sefui for patching holes in sheets of r hermop J as r 1 r. material.

The patch welding rod 45 is formed of two welding rods 10 substantially equal in length. The resistance element 12 is exposed at each end of the welding rods 10 by heating and dripping the solid core 11 using, for example, a hot air gun and n pair of w: re strippers. The exposed wires 17 of the resistance •.lernent 12 at a first end of the first weldinq rod 10 are then twisted together with the exposed wires 17 of the resistance -.I'.-iTient 1? at a first end of the second welding rod 10. The •--x posed wires 17 of the second ends of the first and second welding rods 10 are then twisted together to form a double welding rod (not shown). The terminal pins 15 are attached to t resistance element 12 and the patch welding rod 4 5 is formed in'o the desired shape by pulling apart the pair of welding rods ' without separating them from the terminal pins 15. The patch welding rod 45 is then pre-attached to the thermoplastic sheet ■t •• by simultaneously applying a current and pressure.

In both of the applications shown in Figures 13 and 14, :t is preferable to pre-attach the rods 10 and 45, respectively, so that the terminal pins 15 do not extend beyond the edge of tho thermoplastic member 44 thereby preventing overheating of the exposed metal of the terminal pins 15. This is also a safety feature as contact with the terminal pins 15 can result in an electrical shock.

In applications as shown in Figure 14 where two welding rods 10 are connected at a terminal pin 15, there may be a 1 oca 1 ized overheat ing of the patch welding rod 45 at the terminal pins 15. In this case it is advisable to provide, for example, pieces of foil tape 46 on the opposite side of the thermoplastic sheet 44. The foil tape 46 acts to draw off the heat from the terminal pin 15 and conducts it over the area of the foil tape 46 to prevent burning of the thermoplastic material immedi£( ^" o - 8 OCT i992 1 I 19 ;-i cen f the terminal pin ] 5 An alternative fn pre-a 11 acl» i ng the welding r •: »*1 in i :> to provide a channel (not shown) in one of the t hermop I a-j t ic members to be joined to hold the welding rod JO in place during the welding operation.

Referring now to Figures 15 and lb. an application of the welding rod 10 and the process of forming a weld between two thermoplastic members is a fusion ring 50 for welding together two sections of pipe 51 and 52 with a butt joint.

The fusion ring 50 is formed of a welding rod 10 and a collar 54 of thermoplastic pipe having an inside diameter substantially equal to the outside diameter of the sections of pipe 5J and 52 being joined. The distance between the mouths 16 of the terminal pins 15 of the welding rod 10 is subs tantia J 1v equal to the inside circumference of the collar 54. The welding rod 10 is shaped along the inside surface of the collar 54 and the terminal pins 15 are pushed to the outside through ho:es in the col 1ar 54.

The welding ring 53 is then pre-attac'ned to the inside wall of the collar 54. The shaping and pre-at tach i ng of the welding rod 10 can be done, for example, with a pair of non-conductive, heat resistant rings (not shown) which are pressed together as a current is passed through the resistance element 12 of the welding rod 10.

The fusion ring 50 is positioned between the two sections of pipe 5] and 52 as in a socket fitting and the two sections of pipe 51 and 52 are then welded together while pressure is applied to push the pipes 51 and 52 together. The fusion ring 50 supplies a third thermoplastic member, in the forTtf""v~'\ , ., ?• * •- ,v r - of a collar 54, to the weld. This is particularly advantageous c / 8 OCT 1992; in situations where containment of the fluids transported in the pipe r> i and 5 2 is a priority.

The welding ring 43 of Figure 12 can be substituted for the welding rod 10 in the fusion ring 50 for larger diameter and/or thicker walled pipes. Alternatively, the welding rod 10 could be formed with a loop as shown in Figures 17 and 18. The fusion ring 50 shown in Figures 17 and 18 is formed by pre-at Caching the welding rod 10 to a disc 56 of relatively thin thermoplastic material and subsequently attaching the disc 56 to the collar 54. The heat generated by this arrangement of the welding rod 10 sufficiently melts the thermoplastic material of the disc- 5*"' as well as softening and melting the thermoplastic material of the pipes 5J and 52 adjacent the fusion ring 50.

Referring nov to Figure 19, the fusion ring 50 may also formed with a collar 54 of thermoplastic pipe having an outside diameter substantially equal to the outside diameter of the sections of pipe 51 and 52 being joined. The pipe sections 51 and 52 are rabbeted to a depth substantially equal to the walJ thickness of the collar 54. This enables the joint between the sections of pipe 51 and 52 to be flush along the inside and outside surfaces of the pipe sections 51 and 52.

The welding rod 10 and process for electric fusion welding of the present invention is very versatile, providing a means for the welding together of different shapes and sizes of thermoplastic members, a number of which have been described herein. Because of this versatility, these welding operations can be carried out in otherwise less than ideal conditions underwater, in ponds, sewers, and the like. Under these conditions, allowances must be made for the dissipation of heat in the aqueous environment. \ "8 OCT 1992 £ V ^ xlvfi V * 2 1 For example, in the case of the application shown in sire 14. the patch welding rod 45 is pre-attached to a thermoplastic sheer 44. To reduce the dissipation of heat through the aqueous environment, the pre-attached patch welding rod 45 is flattened by applying pressure and current to the patch •.-.•elding rod 45. Flattening the patch welding rod 4 5 increases trie contact ares between the patch welding rod 45 and the thermopiastic member under repair (not shown). Accordingly the 'neat generated :n the resistance eJement 12 during the weld cycle evaporates any moisture remaining between the patch welding rod 4 5 and the thermoplastic member under repair.

The patch welding rod 45 is flattened after pre-a'*aching to the thermoplastic sheet 44. for example, by placing the thermoplastic sheet 44, patch welding rod 4 5 side down, on h sheet of heat resistant glass. A weight, sufficient to cause flattening of the patch welding rod 45 in the softened state before burning of the thermoplastic sheet 44, is placed on top of the thermoplastic sheet 45. For example, the weight could be .• i [ ■ r' i ■ i m a t e ] y r ^ pounds for an 8 inch by 8 inch sheet of thermoplastic material. A current of J7 amperes is applied to the resistance element 12 for 2.5 minutes. The power is disconnected and the fused patch welding rod 45 and thermoplastic sheet 44 is left to cool with the weight applied for 2 minutes. The resulting patch has a flattened welding rod 45 with a smooth glass-1 ike f i n i sh.

Referring now to Figures 20 and 21, there are also applications where a long thin patch is required, for example, to repair a faulty searn in a pond liner. In this case it is advisable to use a double welding rod 58.

The double welding rod 58 is formed as described in the discussion of the welding ring 43 and the patch welding rod 45.

The resistance element 12 is exposed at each end of a pai-i"':^iiv=^s . H O ■ / /" -8 OCT 1992 £ V „s c 22 e"V: r ir^ds in --'f subs'ani iaj|y e-pial length by heat iruj and - r ; ;• p1 m ■ b~ s'MH ^c>re l1 using, for example, a hot air gun and a pair of wire strippers. The exposed wires 17 of the resistance element 12 at a first end of the first welding rod 10 are then s ~d together with the exposed wires 17 of the resistance ~ 1-menr. 12 a: fhe first end of the second welding rod 10. The exposed wire* 17 of the second ends of the first and second welding rods 10 are then twisted together to form a double eld-i;rod 58 The two welding rods 10 are not separated but pre-attached to the thermoplastic sheet 59 as previously .i: s cussed The double welding rod 58 is flattened by the simultaneous application of pressure and current on a non-stick surface, such as heat resistant glass.

When the flattened patch welding rod 4 5 and double w»-id i !in rod So are used to effect welds in an aqueous environment i ■ i :■ import ant to take into consideration the temperature of the environment the buoyancy of the weight used to apply pressure, arid the dissipation of heat through the-aqueous environment. The f l-i t t ened pa t ch we!dingrod 4 5 and doub 1 e wel d i ng rod 58 increase the contact area between the rods 45 and 58 and the thermoplastic number under repair (not shown). Accordingly, the he^t generated m the resistance element 12 during the weld cycle evaporates any moisture remaining between the patch welding rod 45 and the thermoplastic member under repair.

Figure 21 illustrates the use of the double welding rod 58 to repair a seam between two thermoplastic sheets 60 and 61. The heat generated by the resistance element 12 is sufficient to fuse the thermoplastic sheet 59. the double welding rod 58, and the two thermoplastic sheets 60 and 63 into a unitary structure. This type of patch repair is very strong and very effective. -8 OCT 1992 «

Claims (40)

WHAT WE CLAIM IS:

1. A welding rod for welding adjacent members of thermoplastic material comprising; a solid homogenous core of said thermoplastic material, and a resistance element comprised of a plurality of wires, said resistance element being embedded in said solid core proximate the surface of said core.

2. A welding rod as claimed in Claim 1 wherein said resistance element is wound in a helical pattern around said solid core.

3. A welding rod as claimed in Claim 1 wherein said resistance element is braided around said solid core.

4. A welding rod as claimed in any one of claims 1 to 3 wherein said resistance clcmcnr is embedded by coating said resistance element and said solid core in a thin layer of said thermoplastic material.

5. A double welding rod for welding adjacent members of thermoplastic,material comprising: 4 * a first welding rod, and 2 0 j AH 1395 a second welding rod, said first and said second welding rods comprising: a solid homogeneous core of said thermoplastic material, and 24 4 4 i 24 a resistance element comprised of a plurality of wires, said resistance element being embedded in said solid core proximate the surface of said solid core, said resistance element at a first end of said first welding rod connected to said resistance element at a first end of said second welding rod, and said resistance element at a second end of said first welding rod connected to said resistance element at a second end of said second welding rod, and said resistance element at a second end of said first welding rod connected to said resistance element at a second end of said second welding rod. h.

A double welding rod as claimed in Claim 5 wherein said resistance element is wound in a helical pattern or braided around said solid core.

7. A double welding rod as claimed in Claim 5 or Claim 6 wherein said resistance element is embedded by coating said resistance element and said core with a thin layer of said thermoplastic material. <

S. A double welding rod as claimed in any one of claims 5 to 7 wherein said first welding rod is distanced from said second welding rod between said first and said second ends.

9. A method of forming a welding rod for welding adjacent members of thermoplastic material comprising the steps of: providing a solid homogeneous core of thermoplastic material, placing a resistance element comprised of a plurality of wires around said solid core, holding said plurality of wires in tension, and heating said solid core and said plurality of wires, thereby embedding said resistance element in said solid core proximate the surface of said solid core.

10. A method of forming a welding rod for welding adjacent members of thermoplastic material comprising the steps of: providing a solid homogeneous core of thermoplastic material, placing a resistance element comprised of a plurality of wires around said solid core, and embedding said resistance element in said solid core by coating said resistance element and said solid core with a thin layer of said thermoplastic material.

11. A method of forming a welding rod as claimed in Claim 9 or Claim 10, wherein said step of placing a resistance element around said solid core comprises winding said resistance element in a helical pattern about said solid core.

12. A method of forming a welding rod as claimed in Claim 9 or Claim 10 wherein said step of placing a resistance element around said solid core comprises braiding said resistance element around said solid core. jrfV - e •qoi; - i J jJ O '/

13. An apparatus tor terming a welding rod tor welding adjacent members of thermoplastic material comprising: a drive means connected to a supply of a solid homogeneous core of said thermoplastic material, a means for placing a plurality of wires around said solid core, a means for holding said plurality of wires in tension, and a heating chamber, whereby said solid core is fed through said means for placing a plurality of wires around said solid core and said plurality of wires are embedded while in tension into said solid core, when heated, proximate the surface of said solid core.

14. An apparatus for forming a welding rod for welding adjacent members of thermoplastic material comprising: a drive means connected to a supply of a solid homogeneous core of said thermoplastic material, a means for placing a plurality of wires around said solid core, and a means for embedding said plurality of wires in said solid core by coating said plurality of wires and said solid core with a thin layer of said thermoplastic material.

15. An apparatus as claimed in Claim 13 or Claim 14 wherein said plurality^sWes ^ 6* are placed around said solid core by winding in a helical pattern or by braiding. " * r, L J / •, ; ; , J O

16. A method of welding adjacent members of a thermoplastic material, comprising the steps of: providing a welding rod having a solid homogeneous core of said thermoplastic material and a resistance element embedded proximate the surface of said solid core, positioning said welding rod between said adjacent members of said thermoplastic material, applying a current to said resistance element, applying a pressure to said members such that the facing surfaces of said members are in contact with said welding rod, said current being applied to said resistance element lor a time and intensity sufficient to cause said solid core of said welding rod and portions of said members proximate said welding rod to soften and melt, discontinuing said current, and maintaining said pressure until portions of said resistance element penetrate into each of said adjacent members to bind them together as a mechanical coupler and until said members are fused and solidified.

17. A method of welding adjacent members of a thermoplastic material as claimed in Claim 16, wherein said current is an electrical current.

1S. A method of welding adjacent members of a thermoplastic material as claimed in Claim 17, further including the step of providing a heat dissipation means at the connection between the current source and said resistance element.

19. A method of welding adjacent members of a thermoplastic material as claimed in Claim 16, wherein said current is generated by a radio frequency generator.

20. A method of welding adjacent members of a thermoplastic material, corrffitising ■ ^ c the steps of: providing a welding rod having a solid homogeneous core of said thermoplastic material and a resistance element embedded proximate the surface of said solid core, positioning said welding rod on a first of said adjacent members of said thermoplastic material, applying a current to said resistance element, applying pressure to said first member such that said first member is in contact with said welding rod, said current being applied to said resistance element for a time and intensity sufficient to cause said solid core of said welding rod and portions of said first member proximate said welding rod to soften and melt, discontinuing said current, maintaining said pressure until said solid core of said welding rod and said portions of said first member are fused and solidified, whereby said welding rod is pre-attached to said first member, and welding said first member to an adjacent member of thermoplastic material via said pre-attached welding rod.

21. A method of welding adjacent members of a thermoplastic material as claimed in Claim 20, wherein said current is an electrical current.

22. A method of welding adjacent members of a thermoplastic material as claimed in Claim 21, further including the step of providing a heat dissipation means at the connection between the current source and said resistance element.

23. A method of welding adjacent members of a thermoplastic material as claimed in t N r Claim 20, wherein said current is generated by a radio frequency generator.

24. A method of welding adjacent members of a thermoplastic material as claimed in Claim 20, wherein said first member is welded to said adjacent member by: positioning said welding rod pre-attached to said first member, on said adjacent member of said thermoplastic material, applying a current to said resistance element, applying pressure to said first and said adjacent members such that the facing surfaces of said first and adjacent members are in contact with said welding rod, said current being applied to said resistance element for a time and intensity sufficient to causc said solid core of said welding rod and portions of said first and said adjacent members proximate said welding rod to soften and melt, discontinuing said current, and maintaining said pressure until portions of said resistance element penetrate into each of said first and said adjacent members to bind them together as a mechanical coupler and until said first and said adjacent members are fused and solidified.

25. A method of welding adjacent members of a thermoplastic material as claimed in Claim 24 wherein said current is an electrical current.

26. A method of welding adjacent members of thermoplastic material as claimed in Claim 25, further including the step of providing a heat dissipation means at the connection between the current source and said resistance element.

27. A method of welding adjacent members of a thermoplastic material as claimed in \ t i'i ; Claim 24, wherein said current is generated by a radio frequency generator. 24 4 4 30

28. A method ot welding adjacent members ot thermoplastic material as claimed in Claim 20, further comprising the steps of: positioning said welding rod pre-attached to said first member between a second and a third of said adjacent members of said thermoplastic material, applying a current to said resistance element, applying a pressure to said second and third members such that the facing surfaces of said second and third members are in contact with said welding rod, said current being applied to said resistance element for a time and intensity sufficient to cause said solid core of said welding rod and portions of said first, second and third members proximate said welding rod to soften and melt, discontinuing said current, and maintaining said pressure until portions of said resistance element penetrate into each of said second and third members to bind them together as a mechanical coupler and until said second and third members are fused and solidified.

29. A method of welding adjacent members of a thermoplastic material as claimed in Claim 2«S, wherein said current is an electrical current.

30. A method of welding adjacent members of a thermoplastic material as claimed in Claim 29, further including the step of providing a heat dissipation means at the connection between the current source and said resistance element.

31. A method of welding adjacent members of a thermoplastic material as claimed in Claim 28, wherein said current is generated by a radio frequency generator.

32. A method ot welding adjacent members consisting of a thermoplastic material, comprising the steps of: providing a welding rod having a solid homogeneous core of said thermoplastic material and a resistance element embedded proximate the surface of said solid core, applying heat to said welding rod to soften said solid core, attaching said welding rod to the first of said adjacent members of said thermoplastic material, whereby said welding rod is pre-attached to said first member, and welding said first member to an adjacent member via said pre-attached welding rod.

33. A method of welding adjacent members of a thermoplastic material as claimed in Claim 32, further comprising the steps of: positioning said welding rod pre-attached to said first member on a second adjacent member of said thermoplastic material, applying a current to said resistance element, applying pressure to said first and second members such that the facing surfaces of said first and second members are in contact with said welding rod, said current being applied to said resistance clement for a time and intensity sufficient to cause said solid core of said welding rod and portions of said first and second members proximate said welding rod to soften and melt, discontinuing said current, and maintaining said pressure until portions of said resistance element penetrate into each of said first and second members to bind them together as a mechanical coupler and until said first and second members are fused and solidified. o. v* +■ I \ or l C- • , j 5 | sj J J r- ^ /. /i /. 1 £. S- "»■ 4 I 32

34. A method of welding adjacent members of a thermoplastic material as claimed in Claim 33, wherein said current is an electrical current.

35. A method of welding adjacent members of a thermoplastic material as claimed in Claim 34, further including the step of providing a heat dissipation means at the connection between the current source and said resistance element. 3f>.

A method of welding adjacent members of a thermoplastic material as claimed in Claim 33, wherein said current is generated by a radio frequency generated by a radio frequency generator.

37. A method of welding adjacent members of a thermoplastic material as claimed in Claim 32, further comprising the steps of: positioning said welding rod pre-attached to said first member between a second and a third of said adjacent members of said thermoplastic material, applying a current to said resistance element, applying pressure to said second and third members such that the facing surfaces ot said second and third members are in contact with said welding rod, said current being applied to said resistance element for a time and intensity sufficient to cause said solid core of said welding rod and portions of said first, second and third members proximate said welding rod to soften and melt, discontinuing said current, and maintaining said pressure until portions of said resistance element penetrate into each of said second and third members to bind them together as a mcchanical o coupler and until said second and third members are fused and solidified. 33

38. A method ot welding adjacent members ot a thermoplastic material as claimed in Claim 37, wherein said current is an electrical current.

39. A method of welding adjacent members of a thermoplastic material as claimed in Claim 38, further including the step of providing a heat dissipation means at the connection between the current source and said resistance element.

40. A method of welding adjacent members of a thermoplastic material as claimed in Claim 37, wherein said current is generated by a radio frequency generator. WEST-WALKER, NfcCABS K A Per: ATTORfWs0 Fd'R THE APPLICANT \ 6 f Q\ ^ -• r ■ • * r r*. f <c. ^ «.* * * >\ i^JO ( r; ■'/