GB2273679A

GB2273679A - Joining sheet plastics

Info

Publication number: GB2273679A
Application number: GB9326381A
Authority: GB
Inventors: Michael Charles Short
Original assignee: Tipper Hire J & W Ltd
Current assignee: Tipper Hire J & W Ltd
Priority date: 1992-12-23
Filing date: 1993-12-23
Publication date: 1994-06-29
Anticipated expiration: 2013-12-23
Also published as: GB9326381D0; GB9226795D0; WO1994014599A1; GB2273679B; AU5710094A

Abstract

Method of welding together plastic materials, such as two or more panels (1, 2) of plastics foam material, or plastics sheets e.g. for use in making load-bearing inflatable structures, lining of tanks etc, comprises locating a resistive heating element (5), which may be in the form of a flexible tape, in the region (3, 4) of a welded joint to be formed therebetween, resistively heating the element (5) by direct application of current or inductively to melt the plastic materials in the region thereof, and subsequent cooling. A profiled jointing strip (11, 21) may be used in the welding methods. To improve joint integrity, a power supply arrangement may be used whereby the heating current is controlled in a cycle according to the nature of the materials used in the joint. <IMAGE>

Description

WELDING TOGETHER PLASTICS MATERIALS DESCRIPTION This invention relates to the jointing of plastics materials by welding, as well as to a jointing strip for forming such a welded joint and to a method of manufacturing the strip. The invention is especially, but not exclusively, related to the jointing together of flexible thermoplastics sheets and/or plastics foam material sheets, as well as other generally planar articles incorporating an outer plastics layer.

Many comparatively large flexible plastics sheet products are manufactured from smaller sheets or panels of flexible plastics material which have to be jointed together by various known methods, such as, adhesives or welding by RF heating or ultrasonic techniques, stitching or any combination thereof.

However, it has been found that the known methods of manufacturing comparatively large flexible plastics sheet products, sometimes as large as several thousands of square meters in area, are slow, expensive and, in many cases, unreliable.

The failure of welds, stitching and other types of jointing between sheets or panels of plastics materials can, in some cases, be life-threatening and, in certain circumstances, it is extremely difficult both to check such welds or other types of connection for integrity and strength at the production stage and to repair them if subsequently found to be faulty.

Faults are most often found in service and, in many cases, cannot be repaired unless the faulty product is taken out of commission and returned to the source of manufacture when, in certain circumstances, even repairs at that stage are impossible or not commercially viable.

It is an object of the present invention to improve the joining together of sheets or panels of plastics materials, resulting in joints which will be at least substantially as strong as the parent material. Such joints are preferably fluid tight.

Accordingly, a first aspect of the invention provides a method of welding together plastics materials, such as, two or more panels of flexible plastics sheet and/or plastics foam material, comprising locating a resistive heating element in the region of a welded joint to be formed therebetween, resistively heating the element such that the temperature of the plastics materials in the region thereof is raised to at least the respective melting points of the plastics materials, and subsequently cooling the material or allowing the materials to cool to below their melting points, whereby the plastics material in the region of the heating element is welded together to form a joint.

A second aspect of the invention resides in a resistive heating element for use in welding together plastics materials, such as, sheets or panels thereof, the element comprising an electrically conductive component associated with an elongate member of a plastics material which is compatible with plastics materials to be welded together.

The word ~associated" in this context is used to embrace an arrangement of an electrically conductive component, such as, a preformed electrically conductive member, for example, a metallic tape or braid, extending along the length of, and preferably embedded in, the elongate plastics member or an electrically conductive filler, for example, an electrically conductive particulate material, distributed along the length of the elongate plastics member to render the member electrically conductive.

Also, the word 'associated" may be used to embrace, alternatively, an electrically conductive plastics material from which the elongate plastics member is made and which, by its very composition and structure, is inherently electrically conductive.

Compatibility between the plastics material of the elongate member and the plastics materials to be welded together is preferred in respect of, inter alia, their respective melting points which should be generally the same or similar and their molecular structures, so that the welded joints formed between the plastics materials are strong and of sound integrity. The plastics materials to be welded together, may be the same, in which case, they will have the same melting point, or different,in which case, they will almost certainly have different melting points.

Preferably, the resistive heating element is arranged to be located between or adjacent respective overlapping or abutting surfaces of the plastics material to be welded together and, in a preferred embodiment of the inventive method, the overlapping or abutting surfaces of the plastics materials to be welded together are placed under pressure at least during the welding operation.

Preferably, also, the electrically conductive component of the resistive heating element may be of copper or any other suitable conductive metallic material which may be in the form of a flexible tape or woven braid, and which in a preferred embodiment in accordance with the second aspect of the invention and for use in the method in accordance with the first aspect of the invention, is embedded in a plastics material which is the same or is compatible with the plastics materials to be joined together by welding.

Also, two electrically conductive components may be used, preferably extending in parallel relationship, with one acting as a live conductor and the other as a return conductor for welding current passed therethrough. The two elements are electrically insulated from one another, preferably by a plastics material which is, again, the same as or compatible with the plastics materials to be joined together by welding, such that the insulating plastics material is also incorporated into the welded joint.

If metallic braid is used as the electrically conductive component in the resistive heating element and is woven comparatively openly or, say, perforated or expanded flexible metallic tape is used, then the plastics material in which the metallic braid or perforated or expanded tape is embedded or the plastics material to be welded together, is able to flow through the braid or tape during the resistive hot melt welding operation, to reinforce the strength of the so-formed joint. A similar arrangement may be used by suitably shaping and/or perforating any suitable conductive polymer used as the resistive heating element.

Any suitable method may be used to hold confronting or abutting surfaces of the plastics materials together prior to and during at least the initial stages of the actual welding operation, to maintain the correct orientation and alignment between the materials, optionally applying a pressure thereto, until after the weld has been made and, preferably, subsequently cooled.

In one such method, which is particularly useful in the manufacture and/or repair of inflatable devices manufactured from welded-together panels of flexible plastics sheet and/or plastics foam material, the edges of two pieces of flexible, thermoplastics-coated sheeting to be welded together, are brought into overlapping and confronting relationship with each other, with a resistive heating element located therebetween. This structure may be held temporarily together by any suitable means, for example, adhesive or a clamping arrangement.

Plastics material of the resistive heating element can be extruded or otherwise formed in a variety of sections such as, a generally S section.

Other possible sections may include generally rectangular, C, E, H, L, T U, X sections or any combination thereof, to facilitate the welding together of the plastics materials for differing purposes. With such sections for the plastics material of the resistive heating element, edges of the plastics materials to be joined together by the hot melt welding method of the first aspect of the invention can be inserted in respective recesses defined by adjacent arms and/or other portions of such sections, to provide additional welded surfaces.For instance, with an S section, resistive heating components, such as, metallic braid, tape or the like, may extend along the top, intermediate and base portions of the S, to provide three lines of weld, with respective edges of the plastics material to be welded together, inserted in the recesses defined between the top and bottom and intermediate portions of the S.

Accordingly, a third aspect of the invention provides a jointing strip for welding together sheets of plastics material, which strip comprises an elongate outer member of a first plastics material defining therealong a slot arranged to receive therein an edge portion of a plastics sheet to be joined to an edge portion of another plastics sheet, an elongate inner member of a second plastics material which has a melting point less than that of the first plastics material but comparable with that of the material of the plastics sheets to be joined together and which extends along an inner side of the length of the slot in contact with the elongate outer member, and an electrically conductive resistive heating component which is at least partially embedded in each of the elongate inner and outer members along the length of the slot.

Preferably, the resistive heating component which may be of any suitable shape and dimensions and may be made of any suitable electrically conductive material, such as, copper, is embedded partially in each of the inner and outer members, such that it is totally embedded therein with no gap therebetween. A preferred form of resistive heating component is an expanded metal (copper) tape with opposed side portions thereof being at least partially embedded in respective inner and outer elongate members of the jointing strip. Other forms of resistive heating component may be used, for instance, a corrugated metal tape with respective peaks and troughs thereof being at least partially embedded in the elongate inner and outer members of the jointing strip along the length of the slot.

In one embodiment of inventive jointing strip, which is preferably generally flat, the elongate outer member is substantially H-shaped in cross-section, to define a pair of slots in back-to-back relationship with each other. In another embodiment of the inventive jointing strip, the elongate outer member has a substantially S-shaped cross-section, to define a pair of slots in side-by-side relationship with one another.

However, and as mentioned above, the elongate outer member of the inventive jointing strip may have other cross-sections, for example, C, E, L, T, U, X sections or any combination thereof.

Preferably, also, the opposed inner sides of the or each slot have respective elongate inner plastics members extending therealong, such that, when the edge portion of a plastics sheet is received in the slot for subsequent jointing to the edge portion of another plastics sheet, the subsequent welding operation, to be described in more detail hereinbelow, is effected on both sides of the plastics sheet.

Further, at least one side of the or each slot is curved or otherwise extends inwardly of the slot, such that a gripping action provided by the preferred resilient nature of the outer member, takes place upon the edge portion of a plastics sheet received in the slot. This resilient gripping action assists in maintaining the sheet edge portion in correct position in the slot prior to and during the subsequent plastics welding operation for joining two sheets together.

The heating element of the second aspect of the invention and/or the jointing strip of the third aspect of the invention may be heated inductively, as an alternative to resistive heating thereof.

Similarly, the respective inventive methods may employ an inductive heating technique rather than a resistive heating technique.

A fourth aspect of the invention resides in a method of joining together sheets of plastics material, which method comprises providing a jointing strip as defined above in accordance with the third aspect of the invention, inserting edge portions of the plastics sheets in respective ones of the slots of the strip, passing an electric current along the or each resistive heating component to raise the temperature thereof to one between the melting points of the inner and outer elongate members of the strip, to cause the plastics material of the inner member and sheet(s) to melt, and reducing the electric heating current sufficiently to allow the melted plastics materials to cool below their respective melting points and thus weld the plastics sheets to the strip, thereby joining the sheets together.

The welding together of different plastics materials of differing thicknesses, lengths and other physical and chemical parameters by electrically resistive methods requires optimisation of voltage and current, as well as time, type of conductor and shape thereof to be taken into consideration, in order to enhance the integrity of the final weld. It is possible by mathematical modelling to predict accurately such operating parameters for each type of plastics material to be welded. Such information can be reduced to a range of settings in chart form for operator information or programmed with a computerised control system as may variations caused by ambient conditions.

Accordingly, a fifth aspect of the invention provides electrical power supply apparatus for use in or with the other aspects of the invention defined above in welding/joining together plastics material(s), such as, sheets or panels thereof, which apparatus comprises: (a) means arranged to be programmed with data relating to a plurality of different plastics materials to be joined together; (b) means arranged to bring together respective edges of plastics material(s) to be joined together in overlapping or generally abutting relationship with each other; (c) means arranged to locate a resistive heating element or component, as the case may be, in operating relationship with the overlapping or generally abutting edges of the plastics material(s) to be joined together, either during or after the bringing together of said edges; and (d) means for supplying a heating current to the resistive heating element located in operating relationship with the overlapping or generally abutting edges of the plastics material(s) to be joined together, to join the plastics material(s) edges together by hot melt welding, wherein said heating current supply means is arranged to supply current to the resistive heating element or component in dependence upon data programmed into said programmable means and relating to the plastics material(s) to be joined together by welding.

Preferably, said heating current supply means is arranged to supply current to the resistive heating element or component as: (i) a primary increasing heating current which raises the temperature of the plastics material(s) to be joined together to one greater than their melting point(s), as well as the melting point(s) of any other plastics material associated with the resistive heating element or component and the hot melt welding operation, with such increase in heating current preferably being over a predetermined time period; and (ii) a secondary, substantially constant heating current which maintains the temperature of the plastics material(s) to be joined together, as well as that of any plastics material( 6 ) associated with the resistive heating element or component and the hot melt welding operation, substantially constant above their melting point(s) to effect hot melt welding thereof.

Optionally, a tertiary decreasing heating current may be used to control cooling of the welded-together plastics material(s) at least partially to ambient temperature and, preferably, for a given time period.

The data-programmable means may be used to store data for controlling the heating current in respect of not only physical and chemical properties of the plastics material(s) to be joined together by welding but also other operating parameters, such as, the thickness(es) of the material(s) and ambient temperatures. In such a case, the apparatus may also include corresponding detecting means for, say, determining the thickness(es) of the plastics material(s) to be joined together, their length and the ambient temperature. Thus, the heating current can be adjusted accordingly. Similarly, the predetermined and given time periods during which the heating current is increased and decreased may be socontrolled in dependence upon such detected operating parameters, in order to ensure integrity of the soformed welded joint between the plastics material(s) involved.

As indicated above, the welding current through the resistive heating element or component may be increased or "ramped up" such that the desired welding temperature is attained. The eventual welding current may then be maintained for a given time period to allow the melted plastics materials to merge and/or mix together to form a molten bond therebetween and, subsequently, cooling may be carried out in a controlled manner by "ramping down" the current, such that the molecular structures of the plastics materials involved, including that of any outer elongate plastics member, are substantially maintained, thereby enhancing the integrity of the finally-welded joint. Alternatively or additionally, natural cooling may be at least partially employed, depending upon the properties of the plastics materials involved.

Advantageously, the respective plastics materials of the inner elongate plastics member of any jointing strip and of the sheets to be joined together by the jointing strip, are such that, when in the molten state, they are miscible with each other. This provides a plastics weld between the two plastics materials involved, thereby enhancing the strength and integrity of such.

The melting points of the plastics materials of any such inner elongate plastics members may be different, to compliment those of the plastics materials of the sheets to be joined together by any jointing strip. Thus, plastics sheet of different melting points can be readily joined together using this inventive technique.

A sixth aspect of the invention provides a method of manufacturing the jointing strip defined above in accordance with the third aspect of the invention, which method generally comprises bringing the resistive heating element into contact with the elongate outer plastics member along the length of the slot defined therein and inserting the elongate inner plastics member into the slot to bring it into contact with at least the resistive heating element, wherein the temperatures of the outer and inner elongate members when being brought into contact with the resistive heating component are greater than their respective melting points, such that the component is at least partially embedded in each of the outer and inner plastics members along the length of the associated slot.

The outer plastics member is preferably extruded through a die, such as, a crosshead die, with the resistive heating component being fed or otherwise inserted into the slot whilst the temperature of the first plastics material of the elongate outer member is greater than its melting point. Downstream thereof, the inner plastics member may be inserted into the slot at a temperature greater than its melting point. Alternatively, both the resistive heating component and inner plastics member may be inserted into the slot in juxtaposition with respect to each other simultaneously.

Cooling of the so-formed jointing strip may be controlled, to maintain the molecular integrity of the plastics materials involved, as well as the strength of the strip itself.

In order that the various aspects of the invention can be more fully understood, embodiments in accordance therewith will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 is a sectional view, in elevation, of a welded joint to be formed between two plastics sheets; Figure 2 is a sectional, diagrammatic view of a first embodiment of jointing strip, although not to scale; Figure 3 is a sectional, diagrammatic view of a second embodiment of jointing strip, again not to scale; Figure 4 is a sectional, diagrammatic view of the first embodiment of jointing strip of Figure 2, when used to form a joint between two plastics sheets; Figure 5 of a sectional view, on an enlarged scale, of part of the joint shown in Figure 4, as identified by V in that Figure;; Figure 6 is a sectional, diagrammatic view of the second embodiment of jointing strip of Figure 3, when used to form a joint between two plastics sheets; and Figure 7 is a partial view of the joint shown in Figure 6, taken along the line Vil-Vil in that Figure.

Referring firstly to Figure 1 of the drawings, two pieces 1, 2 of flexible thermoplastics sheet to be joined together, are arranged with their adjacent edges 3, 4 arranged in confronting, overlapping relationship. Between these overlapping edges 3, 4 is located a resistive heating element indicated generally at 5, which comprises an elongate copper braid 6 embedded in a sheath of thermoplastics material which is the same or is compatible with the parent material of the plastics sheets 1, 2 to be joined together by welding.

With this arrangement, current is passed through the resistive copper braid 6 until the temperature of the surrounding thermoplastics material, namely, that of the sheath 7 and of the sheet edges 3, 4 reaches a temperature which is just greater than that of its melting point, whereby the hot-melt plastics material joins together by welding. Subsequently, the soformed resistive hot-melt welded joint is allowed to cool, thereby providing a homogeneous mass of plastics material constituting a welded joint between the sheets 1, 2 with the copper braid 6 embedded therein.

The comparatively open weave of the copper braid 6 permits a certain degree of flow of hot-melt plastics material therethrough during the welding operation, thereby avoiding, or at least substantially reducing, the possibility of voids being formed in the final welded joint.

Prior to the welding operation,the plastics sheets 1, 2 to be joined together at their respective edges 3, 4 are maintained in place with respect to each other and to the resistive heating element 5 by any suitable means, such as, by adhesive, clamps or the like, and these are preferably kept in place until the welded joint has cooled.

It has been found in practice that the voltage applied to the braid 6 should be increased gradually to the maximum required, to ensure the integrity of the final weld. Otherwise, if the maximum welding voltage is applied to the braid 6 instantaneously, damage to the plastics material to be welded together can occur, particularly at the front or beginning of the length of the elongate weld along the path of the resistive heating element 5.

The resistive heating element 5 may be manufactured by any suitable method, such as, by extrusion coating the braid, or indeed any other elongate conductor which may be perforated, such coating possibly containing layers of different plastics material with differing melting points.

Also, any suitable clamping means may be used to maintain the plastics sheets 1, 2 to be joined together by welding, in correct positional relationship with respect to each other and to the resistive heating element 5 prior to and during the actual welding operation which may include any necessary cooling period such cooling being effected artificially or naturally.

Referring now to Figure 2 of the drawings, a jointing strip indicated generally at 11, comprises an elongate outer member 12 of a first plastics material which has a generally H-shaped cross-section defining a pair of back-to-back generally U-shaped slots 14 separated by a central spine 13. Pairs of side arms 15 of the outer member 12 which define the sides of each U-shaped slot 14, each have inwardly turned ends 16 which, together with the arms 15 and central spine 13 define four generally rectangular recesses 17 extending along the inner sides of the slots 14.

In each recess 17 is located a resistive heating component in the form of an expanded copper tape 18 which, at 19, is partially embedded in the plastics material of the corresponding arm 15 of the slot 14 defined by the outer member 12, as shown more clearly in Figure 5.

Also located in each recess 17 is an inner elongate member in the form of a strip 9 of a second plastics material also extending along the length of each slot 14. The melting point of the plastics material of the outer member 12 is greater than that of the plastics material of the inner strip 9, for reasons which will be discussed in more detail hereinbelow.

Again, the expanded copper tape 18 is partially embedded in the plastics material of the inner strip 9, as indicated at 8 in Figure 5, and a line 7 of juncture between the plastics materials of each arm 15 of the outer member 12 and the inner strip 9 is indicated at 6 in Figures 2 and 5. Here, there may be some bonding between the two plastics materials along that line, although certainly contact therebetween is substantially total along the length of each slot 14 in any event.

Each arm 15 is slightly arcuate such that it curves inwardly with respect to the other associated arm, this arrangement providing a gripping action with sheets of material to be joined together by the jointing strip, as will be discussed hereinbelow.

The second embodiment of jointing strip is indicated generally at 21 in Figure 3 and comprises an elongate outer member of a first plastics material, which is indicated generally at 22 and which defines a pair of slots 24, this time in side-by-side relationship with each other. The general configuration of the outer member 22 is that of one with a generally laterally inverted S-shaped crosssection.

The slots 24 are separated by a central spine 23 which constitutes also a common side arm of each Ushaped slot 24. The other outer side arm 25 of each slot 24 has an inwardly turned end 26 which, together with the associated arm 25 and central spine 23, defines a pair of opposed recesses 27 in each slot 24 extending along the inner sides thereof.

In each recess 27, which is generally rectangular in cross-section, is located a resistive heating component in the form of an expanded copper tape 28 which is partially embedded in the plastics material of the corresponding arm 25 of the slot 24 defined by the outer member 22, as in accordance with the arrangement discussed above in relation to the embodiment of Figure 2.

Also located in each recess 27 is an inner elongate member in the form of a strip 39 of a second plastics material also extending along the length of each slot 24. Again, and as in the case of the first embodiment described above with reference to Figure 2, the expanded copper tape 28 is partially embedded in the plastics material of the inner strip 39, with a line 37 of juncture between the two plastics materials of each arm 25 and the inner strip 39 being provided.

Again, there may be some bonding between these two plastics materials along that line 37. Each arm 25 is slightly arcuate such that it curves inwardly, due to the inherent resilience of the plastics material of the outer member 22, once again providing a gripping action upon plastics sheets of material to be joined together by the jointing strip 21, again to be described hereinbelow in more detail.

Manufacture of the embodiments of jointing strip 11, 21 is effected by extruding through a crosshead die the outer members 12, 22 and, whilst the plastics material of those two members is still sufficiently molten, the respective resistive heating components in the form of the expanded copper tapes 18, 28 are applied to the recesses 17, 27 in the respective slots 14, 24, such that the tapes 18, 28 become partially embedded in the plastics material of the respective outer members 12, 22.Subsequently or simultaneously, depending upon the nature and melting points of the plastics materials employed, the inner strips 9, 39 of second. plastics materials are applied to the respective recesses 17, 27 of the slots 14, 24, these strips also having been extruded, such that, whilst they are still sufficiently molten, the remaining portions of the expanded copper tapes 18, 28 which are not embedded in the first plastics material of the outer members 12, 22, become embedded in the second plastics material of those strips. At least some bonding between the two plastics materials along the juncture lines 7, 37 may occur, although contact therebetween is substantially total along the whole length of the respective slots 14, 24.Cooling of the so-formed jointing strips 11, 21 is preferably controlled, such that the plastics materials involved maintain their original molecular structure and physical properties.

In use of the embodiment of jointing strip 11, as shown in Figures 4 and 5, two sheets of plastics material 101, 102 have their edge portions 103, 104 inserted lengthwise in respective slots 14 of the strip 11, the sides of the slots 14 now being defined by the inner surfaces of respective pairs of inner strips 9 of the lower melting point plastics material.

This lower melting point of the second plastics material of the strips 9 is comparable with those of the plastics material from which the sheets 101, 102 to be jointed together, are made.

Such melting points may be different, in which case, the melting points of the respective pairs of inner strips 9 may be adjusted accordingly.

In order to insert the edge portions 103, 104 of the plastics sheets 101, 102 into the associated slots 14, the in-turned ends 16 of the respective arms 15 of the outer member 12 are urged apart against the inherent resilience of the plastics material from which that member 12 is made. Upon such insertion, the respective pairs of ends 16 effect a gripping action upon the edge portions 103, 104 of the plastics sheets 101, 102, to retain them in the correct position with respect to the slots prior to and during the welding operation.

Once the edge portions 103, 104 of the plastics sheets 101, 102 have been received and positioned within the slots 14, an electric current is passed through the expanded copper tapes 18, such that they, as resistive heating components, heat up to a temperature which is sufficient to melt the inner plastics strips 9 and plastics material in the region of the edge portions 103, 104 of the plastics sheets 101, 102 but not the higher melting point plastics material of the outer member 12 in the region thereof.

In this manner, the plastics strips 9 become welded to the edge portions 103, 104 of the plastics sheets 101, 102, with the sheets 101, 102 being jointed together in a fluid tight manner.

Cooling of the so-formed joint between the plastics sheets 101, 102 is carried out in a controlled manner by "ramping down" the current being passed through the expanded copper tapes 18, such that the molecular structures of the plastics materials in the joint are substantially maintained, thereby enhancing the strength and integrity thereof.

In use of the embodiment of jointing strip 21, as shown in Figure 6, two sheets of plastics material 201, 202 have their edge portions 203, 204 inserted lengthwise in the respective slots 24 of the strip 21, the sides of the slots 24 now being defined by the inner surfaces of respective pairs of inner strips 39 of the lower melting point plastics material.

This lower melting point of the second plastics material of the strips 39 is comparable with that or those of the plastics material from which the sheets 201, 202 to be jointed together, are made. Such melting points may be different, in which case, the melting points of the respective pairs of inner strips may be adjusted accordingly.

In order to insert the edge portions 203, 204 of the plastics sheets 201, 202 into the associated slots 24, the in-turned ends 26 of the respective arms 25 of the outer member 12 are urged outwardly against the inherent resilience of the plastics material from which they are made. Upon such insertion, the respective ends 26 effect a gripping action upon the edge portions 203, 204 of the plastics sheets 101, 102, to retain them in the correct position with respect to the slots 24 prior to and during the welding operation.

Once the edge portions 203, 204 of the plastics sheets 201, 202 have been received and positioned within the slots 24, an electric current is passed through the expanded copper tapes 28 and "ramped-up", such that they, as resistive heating components, heat up to a temperature which is maintained for a predetermined period of time by maintaining the current at a corresponding level which is sufficient to melt the inner plastics strips 39 and plastics material in the region of the edge portions 203, 204 of the plastics sheets 201, 202 but not the higher melting point plastics material of the outer member 22 in the region thereof. In this manner, the plastics strips 39 become welded to the edge portions 203, 204 of the plastics sheets 201, 202, with the sheets being jointed together in a fluid tight manner.

Again, cooling of the so-formed joint can be controlled by tramping down" the current passing through the expanded copper tapes 28, thereby maintaining the molecular structure of the plastics materials of the joint, as well as the strength and integrity thereof.

In the embodiments of jointing strip discussed above in relation to the accompanying drawings, the expanded copper tapes 18, 28 may be provided with electric terminals at the appropriate ends thereof.

Also, the pairs of tapes 18, 28 associated with respective ones of the slots 14, 24 may constitute live and return conductors for a heating circuit which may be controlled to increase and decrease the current passing through, and hence the temperature of the welding operation, depending upon the particular plastics materials being used, which may include thermoplastics and thermosetting plastics materials, as well as other synthetic rubber materials and, in some instances, natural rubber materials.

In Figure 7, which is a section along the line Vil-Vil in Figure 6, there is shown the expanded copper tape 28 in plan, with the plastics material of the outer arm 25 of the outer member 22 shown having that tape partially embedded therein. As discussed above, the remainder of the tape 28 which is not embedded in the plastics material of the outer member 22, is embedded in the plastics material of lower melting point of the strip 39.

It has been found that the various aspects of the invention can be used to join together very long lengths of flexible thermoplastics sheet or foam in a single operation, such lengths being, in practice, up to 100 metres or more.

Further, it has been found that the weld obtained is at least as strong as the parent material with uniformity of strength throughout. For all product shapes, manufacturing time is substantially reduced, due mainly in part to greatly reduced welding time and the virtual elimination of unnecessary handling by operatives.

Also, it is to be appreciated that products manufactured by using the inventive jointing method and associated jointing strips and techniques, incorporate welds whose strength and integrity are such as to render them extremely durable and, in certain cases, of high load bearing capabilities.

In particular, inflatable, high load bearing structures can be used in certain fields to replace conventional load bearing structures, for instance, in the manufacture of vehicle doors which are normally strengthened by internal metal beams or struts to bear the force of side impacts. Such inflatable, load bearing structures could be designed to deflate on front or rear vehicle impacts, so that the associated door does not become jammed against the vehicle frame, as in the case of sold metal door-reinforcing beams or struts.

Similarly, such inflatable load bearing structures may be used in the construction and shipbuilding industries, to replace conventional metal load bearing or reinforcing components, where deemed applicable.

Additionally, the inventive method enables large areas of thermoplastics sheeting to be welded on-site and this is particularly applicable to civil engineering applications, such as, the lining of sewage tanks, reservoirs, storage tanks and the provision of gas or liquid-proof membranes in a variety of structures.

As indicated above, the inventive jointing strip can be used to join together long seams between the edges of large sheets of plastics material, in which case, apparatus in accordance with the fifth aspect of the invention may be used to provide a mobile welding station which moves along the adjacent edges of the plastics sheets to be joined together, to insert them in the slots 14, 24 of the jointing strips 11, 21 in a continuous manner while simultaneously or subsequently carrying out the welding operation.

It is to be appreciated that the various aspects of the invention, as defined and described above, can be applied to all thermosetting and thermoplastics polymeric materials, together with some types of natural and synthetic rubbers and other suitable materials, and, also, that the resistive heating elements and jointing strips in their various forms may also be heated by an inductive technique as an alternative to the resistive heating thereof.

Claims

1. A method of welding together plastics materials, such as, two or more panels of flexible plastics sheet and/or plastics foam material, comprising locating a resistive heating element in the region of a welded joint to be formed therebetween, resistively heating the element such that the temperature of the plastics materials in the region thereof is raised to at least the respective melting points of the plastics materials, and subsequently cooling the materials or allowing the materials to cool to below their melting points, whereby the plastics material in the region of the heating element is welded together to form a joint.

2. A method according to claim 1, wherein the resistive heating element is of an electrically conductive metallic material.

3. A method according to claim 1 or 2, wherein the resistive heating element is in the form of a flexible tape or woven braid.

4. A method according to claim 1, 2 or 3, wherein the resistive heating element is a flexible perforated or expanded tape.

5. A method according to claim 1, wherein the resistive heating element comprising an electrically conductive component associated with an elongate member of a plastics material which is compatible with the plastics materials to be welded together.

6. A method according to claim 5, wherein the electrically conductive component extends along the length of the elongate plastics member.

7. A method according to claim 5 or 6, wherein the electrically conductive component is preformed.

8. A method according to claim 5, 6 or 7, wherein the electrically conductive component is a metallic tape or braid.

9. A method according to any of claims 5 to 8, wherein the electrically conductive component is embedded in the elongate plastics member.

10. A method according to claim 5 or 6, wherein the electrically conductive component is a conductive filler distributed along the length of the elongate plastics member.

11. A method according to claim 10, wherein the conductive filler is an electrically conductive particulate material.

12. A method according to claim 1 or 5, wherein the resistive heating element comprises an electrically conductive plastics material.

13. A method according to any preceding claim, wherein the resistive heating element is located between or adjacent respective overlapping or abutting surfaces of the plastics materials to be welded together.

14. A method according to any preceding claim, wherein surfaces of the plastics materials to be welded together are placed under pressure at least during the welding operation.

15. A method according to any preceding claim, wherein two resistive heating elements or electrically conductive components, as the case may be, are used, with one acting as a live conductor and the other as a return conductor for welding current passed therethrough.

16. A method according to claim 5 or any of claims 6 to 15 when dependent upon claim 5, wherein the plastics material of the resistive heat element is in the form of a elongate member of generally rectangular, C, E, H, L, S, T, U or X section or any combination thereof, with edges of the plastics materials to be joined together being inserted in respective recesses defined by adjacent arms and/or other portions of such sections.

17. A method according to any preceding claim, wherein the voltage applied to the resistive heating element or electrically conductive component, and hence the welding current passed therethrough, is increased gradually to the maximum required, to ensure the integrity of the finally welded joint.

18. A method according to any preceding claim, wherein the voltage applied to the resistive heating element or electrically conductive component after the welded joint has been formed, is reduced gradually to control cooling of the joint to ensure the integrity thereof.

19. A resistive heating element for use in welding together plastics materials, such as, sheets or panels thereof, the element comprising an electrically conductive component associated with an elongate member of a plastics material which is compatible with plastics material to be welded together.

20. An element according to claim 19, wherein the electrically conductive component extends along the length of the elongate plastics member.

21. An element according to claim 19 or 20, wherein the electrically conductive component is preformed.

22. An element according to claim 19, 20 or 21, wherein the electrically conductive component is a metallic tape or braid.

23. An element according to any of claims 19 to 22, wherein the electrically conductive component is embedded in the elongate plastics member.

24. An element according to claim 19 or 20, wherein the electrically conductive component is a conductive filler distributed along the length of the elongate plastics member.

25. An element according to claim 24, wherein the conductive filler is an electrically conductive particulate material.

26. An element according to claim 19, wherein the resistive heating element comprises an electrically conductive plastics material.

27. An element according to any of claims 19 to 26, wherein the plastics material of the element is in the form of a elongate member of generally rectangular, C, E, H, L, S, T, U or X section or any combination thereof.

28. An element according to any of claims 19 to 27 including two electrically conductive components, with one arranged to act as a live conductor and the other arranged to act as a return conductor for welding current passed therethrough.

29. A jointing strip for welding together sheets of plastics material, which strip comprises an elongate outer member of a first plastics material defining therealong a slot arranged to receive therein an edge portion of a plastics sheet to be joined to an edge portion of another plastics sheet, an elongate inner member of a second plastics material which has a melting point less than that of the first plastics material but comparable with that of the material of the plastics sheets to be joined together and which extends along an inner side of the length of the slot in contact with the elongate outer member, and an electrically conductive, resistive heating component which is at least partially embedded in each of the elongate inner and outer members along the length of the slot.

30. A jointing strip according to claim 29, wherein the resistive heating component is embedded partially in each of the inner and outer members, such that it is totally embedded therein.

31. A jointing strip according to claim 29 or 30, wherein the resistive heating component is an expanded metal tape with opposed side portions thereof being at least partially embedded in respective elongate inner and outer members of the strip.

32. A jointing strip according to claim 29 or 30, wherein the resistive heating component is a corrugated metal tape with respective peaks and troughs thereof being at least partially embedded in respective elongate inner and outer members of the strip.

33. A jointing strip according to any of claims 29 to 32, wherein the elongate outer member is substantially H-shaped in cross-section, to define a pair of slots in back-to-back relationship with each other.

34. A jointing strip according to any of claims 29 to 32, wherein the elongate outer member has a substantially S-shaped cross-section, to define a pair of slots in side-by-side relationship with one another.

35. A jointing strip according to any of claims 29 to 32, wherein the elongate outer member has a generally C, E, L, T, U or X cross-section or any combination thereof.

36. A jointing strip according to any of claims 29 to 35, wherein opposed inner sides of the or each slot have respective elongate inner plastics members extending therealong.

37. A jointing strip according to any of claims 29 to 36, wherein at least one side of the or each slot is curved or otherwise extends inwardly of the slot.

38. A method of joining together sheets of plastics material, which method comprises providing a jointing strip in accordance with any of claims 29 to 37, inserting edge portions of the plastics sheets in respective ones of the slots of the strip, passing an electric current along the or each resistive heating component to raise the temperature thereof to one between the melting points of the inner and outer elongate members of the strip, to cause the plastics material of the inner member and sheet(s) to melt, and reducing the electric heating current sufficiently to allow the melted plastics materials to cool below their respective melting points and thus weld the plastics sheets to the strip, thereby joining the sheets together.

39. A method of welding together plastics materials using a resistive heating element according to any of claims 19 to 28 or a jointing strip according to any of claims 29 to 37.

40. A method of manufacturing a jointing strip according to any of claims 29 to 37, comprising bringing the resistive heating element into contact with the elongate outer plastics member along the length of the slot defined therein and inserting the elongate inner plastics member into the slot to bring it into contact with at least the resistive heating element, wherein the temperatures of the outer and inner elongate members when being brought into contact with the resistive heating component are greater than their respective melting points, such that the component is at least partially embedded in each of the outer and inner plastics members along the length of the associated slot.

41. A method according to claim 40, wherein, the outer plastics member is extruded through a die, such as, a crosshead die, with the resistive heating component being fed or otherwise inserted into the slot whilst the temperature of the first plastics material of the elongate outer member is greater than its melting point.

42. A method according to claim 41, wherein, downstream of the die, the inner plastics member may be inserted into the slot at a temperature greater than its melting point.

43. A method according to claim 40 or 41, wherein both the resistive heating component and inner plastics member is inserted into the slot in juxtaposition with respect to each other simultaneously.

44. A method according to any of claims 40 to 43, wherein cooling of the so-formed jointing strip is controlled.

45. An electrical power supply apparatus for use in a method in accordance with any of claims 1 to 18, 38 and 39 or with a resistive heating element according to any of claims 19 to 28 or with a jointing strip according to any of claims 29 to 37, which apparatus comprises: (a) means arranged to be programmed with data relating to a plurality of different plastics materials to be joined together; (b) means arranged to bring together respective edges of plastics material(s) to be joined together in overlapping or generally abutting relationship with each other; (c) means arranged to locate a resistive heating element, component or jointing strip, as the case may be, in operating relationship with the overlapping or generally abutting edges of the plastics material(s) to be joined together, either during or after the bringing together of said edges; and (d) means for supplying a heating current to the resistive heating element or component, as the case may be, located in operating relationship with the overlapping or generally abutting edges of the plastics material(s) to be joined together, to join the plastics material(s) edges together by hot melt welding, wherein said heating current supply means is arranged to supply current to the resistive heating element or component in dependence upon data programmed into said programmable means and relating to the plastics material(s) to be joined together by welding.

46. Apparatus according to claim 45, wherein said heating current supply means is arranged to supply current to the resistive heating element or component as: (i) a primary increasing heating current which raises the temperature of the plastics material(s) to be joined together to one greater than their melting point(s), as well as the melting point(s) of any other plastics material associated with the resistive heating element or component and the hot melt welding operation, with such increase in heating current preferably being over a predetermined time period; and (ii) a secondary, substantially constant heating current which maintains the temperature of the plastics material(s) to be joined together, as well as that of any plastics material(s) associated with the resistive heating element or component and the hot melt welding operation, substantially constant above their melting point(s) to effect hot melt welding thereof.

47. Apparatus according to claim 46, including a tertiary decreasing heating current which may be used to control cooling of the welded-together plastics material(s) at least partially to ambient temperature and, preferably, for a given time period.

48. A method in accordance with any of claims 1 to 18, 38 and 39 including using an electrical power supply apparatus according to any of claims 45 to 47.

49. A method according to any of claims 1 to 18, 38, 39 and 48, wherein the heating element is heated using an inductive heating technique.

50. A heating element according to any of claims 19 to 28 or a jointing strip according to any of claims 29 to 37, wherein the electrically conductive component is capable of being heated using an inductive heating technique.

51. A method of welding together plastics materials, substantially as hereinbefore described.

52. A resistive heating element or jointing strip substantially as hereinbefore described with reference to the drawings.

53. A method of manufacturing a jointing strip substantially as hereinbefore described.

54. An electrical power supply apparatus for use in joining together plastics materials, substantially as hereinbefore described with reference to the accompanying drawings.