THERMOPLASTICS WELDING ARRANGEMENT

Abstract: An arrangement for welding thermoplastics sheets together which uses medium wavelength infrared transmission located within a focusing reflecting chamber in which an outlet gap constrains the direction of radiation to a relatively small range of radiant direction within approximately 45 to 90 degrees incident angle 10 relative to a sheet of plastics. The arrangement discloses a convex back reflector shape within a reflector chamber and converging step sides to a narrowest gap to provide an outlet for the radiant heating.

Description: THERMOPLASTICS WELDING ARRANGEMENT This invention relates to joining of plastic. There are many occasions in the handling of plastics and manufacture of items using plastics that plastics must be joined together. One of the more preferred methods is to use a welding technique where the faces of the respective plastics are raised to a temperature of above a melt temperature of the plastic and then the plastics are pushed together under substantive pressure. This presumes, of course, that we are talking of thermoplastics materials and it presumes that the plastics materials are compatible materials for effecting such a heated weld join. There are a number of problems that occur, however, in respect of obtaining effective and sufficient heating of any plastics material of this type in that as soon as it is raised to such a melting point the plastic loses its integrity. Perhaps one of the most successful methods of joining plastics is where they are caused to be heated by mutual relative motion one with the other which by friction raises the respective temperatures where there are immediately joining faces to a melt temperature where upon welding can take place. Such a joining technique, however, is only appropriate for substantially rigid articles and where it is appropriate to move or spin the respective parts one with respect to the other and not be concerned that a weld will occur somewhat randomly through any degree of such spin. A further method of joining sheet plastics is a hot stamp die. The problem with such an arrangement is that for any join or complex joining shape a die of that appropriate shape must be manufactured and appropriately heated which is especially expensive. if the joining is to be in relation to a constantly travelling strip of plastic sheet material, then what has been hitherto required is an endless band of metal such as stainless steel which is then heated and, of course, brought into close contact with the external plastics surfaces. Such stainless steel bands are found to be very vulnerable to breaking after a very short period of service and are otherwise extremely expensive to provide and maintain. The object of this invention is, therefore, to propose means and a method of effecting a joining of plastics of the type discussed which will avoid substantially the problems as discussed. This is achieved in accord with this invention by effecting a heating of thermoplastics material by directing a radiant heating through reflector means such that radiation emanating from such a radiant heater is directed substantially perpendicularly to the plastics surface and effecting such heating with such an intensity of radiation that will quickly raise the temperature of the plastics being subjected to the radiation to a temperature of above the melt point of the plastics. In preference, when such heating is being effected, the heating is maintained in an aligned orientation longitudinally with respect to a direction of travel of the tape so that a portion of the tape to be heated will be maintained substantially beneath the radiant heater throughout the length of the heating means. In preference, such plastics materials as are being drawn past such heating means are held so that the parts adjacent the heated portion are retained in constant alignment and are kept in such constant alignment subsequent to such heating effect so that where there are two or more thicknesses of plastics material, these will be held together and to an extent that they will join by being sufficiently heated. In preference, the sheets of plastics material are held in adjoining relationship face upon face and there are radiant heaters with an outlet slot aligned along both an upper side and a lower side of such passing two sheets or more of such plastics material. Each of the radiant heaters is arranged so that by reflection or otherwise they provide a high intensity of radiant heat which also because of the reflection is substantively aligned to be directed only along and through a narrow gap. The shape of reflector can thereby including a plurality of reflective faces each of which reflect the radiant output of a strip heater such that the radiation is substantially directed further away from the direction of the strip heater and such that an outlet from a body housing such heater has a gap aligned in the elongate direction of the strip heater but such that a high proportion of the out part of the strip heater is thus directed to issue from the strip in a direction which is substantively away from the strip heater source. By careful design of such a reflector, a substantial percentage of heating radiation can be so directed and if the slot is of the same width as the source or less than this, then very high concentrations of radiant heat can be effected through such a narrow gap. Such an output gap can be located in very close vicinity to the plastics material to be welded and in the case where the plastics material is a sheet passing along the direction which is parallel to the strip source then this can be held within a millimetre or so away from the plastics outer surface. The surprising discovery in relation to the technique and to this apparatus is that by using this, plastics materials can be heated throughout their thickness very quickly to a temperature above melt temperature, that this temperature appears to be reached throughout the thickness of the plastics material only where substantially aligned with the output from the focused radiant heater source and that where two plastics sheets are adjoining there is a resulting intermix which is effective to cause a very strong and effective heat weld. It is not understood fully at this stage why this is so effective. In one application, it is applied to the heating for purposes of joining polythene plastic sheet for irrigation purposes. The sheets pass beneath upper and lower focused radiant heaters in the manner described and are simply kept, while being heated, in an adjoining surface relationship whereas it is discovered that there is an equivalently narrow band that is melted to melt temperature of the plastics material without substantively losing the integrity of the adjacent plastics parts, that there is uniform melting through mutual thicknesses of the plastics sheets and that subsequent cooling finds the sheets joined by intermoulding of the respective plastics of the sheets providing a substantive bond. There is difficulty in providing an appropriate arrangement to effect such focused radiant heating and in accord with this invention there is proposed a thermoplastics welding arrangement including a filament source of infrared radiation, a reflector body providing an elongate reflector chamber having a substantially constant cross-sectional shape along the elongate length of the reflector chamber, the shape including a wide chamber part, and a first side extending from a first side of the wide chamber part to a first side defining an outlet gap, and a second side extending from a second side of the wide chamber part to a second side defining the outlet gap, the respective first and second sides converging closer together from the position at the wide chamber part as they progress toward the outlet gap to define thereby a narrow outlet gap, the filament source being located within the wide chamber part and extending substantially the length of the reflector chamber, and means to locate the thermoplastics sheet to be welded together, across a mouth of the outlet gap. In preference, the wide chamber part includes a part of convex shape positioned opposite the outlet gap. in preference, the infrared radiation is within a range of medium wave length and, preferably, within the range of 2.2 to 2.8 microns wave length. Further, it would appear that significant advantage is gained by providing that the incident angle 10 of any radiant energy failing on the thermoplastics sheet to be welded would lie within the range of 45 degrees to 90 degrees. For a better understanding of this invention, it will now be described with reference to a preferred embodiment which shall be described with the assistance of drawings in which:- FIG. 1 is a cross-sectional view through an arrangement organised in accordance with the preferred embodiment and comprising two radiant heaters enclosed within respective reflector arrangements and respectively directing focused radiant heating onto a composite plastics strip, FIG. 2 illustrates the manner in which the focused radiant heating intersects and is caught between the respective surfaces of the plastic sheeting, and, FIG. 3 is a side elevation of the arrangement as in FIG. 1. Referring in particular to the drawings, it is first noted that the application is in respect of a thermoplastics material specifically polythene and the radiant heating source temperature is chosen so as to effect a substantial penetration into the plastics material and, to this end, source temperature is chosen as between 900-1000 degrees centigrade, the heating element is chosen for this purpose to be an aluminium alloy, and the element is housed within a quartz glass housing 1. To this end, the source is providing a medium wave length infrared radiant source within the wave length of 2.2 to 2.8 microns. The quartz glass 1 is chosen so as to be substantially capable of transmitting without substantial loss infrared transmission in this wave length. The reflector bodies each cast from metal and having a gold plated inner reflective chamber 2 includes a rearwardmost convex part 3, two concave parts 4 and 5, defining a wide chamber part and then in stepping fashion, a plurality of convergingly located reflector surfaces 6 provided by a first side extending from a first side of the wide chamber part 2 to an outlet gap 8 each substantially parallel one to the other and each aligned so that they are substantially transverse or perpendicular to the target tape 7 which comprises the polythene thermoplastics material in sheet form held together as three composite layers. The shape in cross section is constant along the elongate length of the reflector body. At an outermost end there is the elongated outlet gap 8 through which the concentrated radiant heating is adapted to pass and the gap is positioned and shaped such that only radiation with the relatively perpendicular alignment (approximately 45 degrees to 90 degrees incident angle 10 to the sheet surface) will pass through the gap and be directed at the tape. By using such an arrangement, that is of substantially focused radiant heating and causing this to be concentrated within gap location which is of approximately the width of the source filament 9, has provided the substantially beneficial results discovered. It is a feature of the arrangement that the tape is being caused to constantly move through the radiated area as is shown particularly in FIG. 3, and the length of the radiated area is chosen in respect of the tape speed so as to effect at the end of the passage of the tape a sufficient selected melting such that the respective tapes will have been caused to flow together and weld. Two features of consequence arise from the arrangement described namely that almost all of the direction of reflection is such that the infrared radiation rays will almost never reflect back into the radiation source. This is achieved by providing firstly the divergent reflection at the back convex part 3 of the source, and then having the shape of the reflector is substantially peariike with the outlet gap 8 located at an apex of the pearlike shape with convex reflective surface directly opposite the gap 8 and behind the radiant source, namely filament 9. By arranging that most of the radiation is not reflected back into the radiant source, means in the simplest sense, that more radiant energy is, therefore, available for transmission through the gap. By avoiding reflection onto the heating element, this essentially means that the radiant energy is being mostly transmitted out from the reflector area. The next issue is that the direction of radiant energy is such that it will be predominantly transverse to the sheet of thermoplastics material which is to be subjected to the heating effect. Broadly, this means that the angle of incidence would be substantially within the range of 45 degrees to 90 degrees and would have predominance within the range of 60 degrees to 90 degrees relative to the planar alignment of a target surface. By providing, therefore, a reflector housing which defines a longitudinal space having a longitudinal heating element, a back reflector surface of convex shape extending along the length of the reflector surface, then extending from both sides of the convex shape, parts which are of concave shape then terminating to each of two sides which are then converging to a narrowest outlet gap which is spaced a substantial distance away from the supplying radiant filament.

Claims: 1. A thermoplastics welding arrangement including a filament source of infrared radiation, a reflector body providing an elongate reflector chamber having a substantially constant cross-sectional shape along the elongate length of the reflector chamber, the shape including a wide chamber part, and a first side extending from a first side of the wide chamber part to a first side defining an outlet gap, and a second side extending from a second side of the wide chamber part to a second side defining the outlet gap, the respective first and second sides converging closer together from the position at the wide chamber part as they progress toward the outlet gap to define thereby a narrow outlet gap, the filament source being located within the wide chamber part and extending substantially the length of the reflector chamber, and means to locate the thermoplastics sheet to be welded together, across a mouth of the outlet gap. 2. A thermoplastics welding arrangement as in claim 1 further characterised in that the wide chamber part includes a part of convex shape positioned opposite the outlet gap. 3. A thermoplastics welding arrangement as in either of preceding claims 1 or 2 wherein the respective first side and second side extending from the wide chamber part to the outlet gap each includes steps located and shaped whereby to at least in the main reflect radiation from the filament source further toward the narrow outlet gap. 4. A thermoplastics welding arrangement as in any one of the preceding claims further characterised in that the filament source of infrared radiation is adapted to provide radiation within the range of 2.2 to 2.8 microns wave length. 5. A thermoplastics welding arrangement as in any one of the preceding claims further characterised in that the location of the filament source, and the shape of the reflector chamber together with the outlet gap are such that there will be caused focused radiant heating effect against any thermoplastics sheet located across the mouth of the outlet gap and in which the incident angle 10 of such radiant energy will be substantially within the range of 45 degrees to 90 degrees relative to a plane defined by thermoplastics sheet substantially aligned across the outlet gap. 6. A thermoplastics welding arrangement in which there are two reflector bodies and a filament source as characterised in any one of the preceding claims one of which is aligned to cause focused radiant heating on one side of a thermoplastics sheet located across the outlet gap of the first said body, and the second reflector body is oppositely located so as to effect focused radiant heating on the opposite side of the same part of thermoplastics sheet located across the respective outlet gap of the second said reflector body. 7. A thermoplastics welding arrangement as in any one of the preceding claims in which the fiiament source of infrared radiation is adapted to be operated within the range of 900-1000 degrees centigrade. 8. A thermoplastics welding arrangement as in any one of the preceding claims in which the filament source of infrared radiation is contained within a surrounding protective cylinder of quartz type glass. 9. A thermoplastics welding arrangement as in any one of the preceding claims in which the reflected chamber has a reflector coating comprised of gold. 10. A thermoplastics welding arrangement as in any one of the preceding claims in which the outlet gap is defined by parallel sides. 11. A thermoplastics welding arrangement as in any one of the preceding claims in which thermoplastics sheet is adapted to be moved past the outlet gap, the direction of movement being aligned with the direction of elongation of the reflector chamber. 12. A method of welding thermoplastics sheet material which comprises the steps of directing the output of focused radiant heating onto a surface of such plastics sheeting in the vicinity of the location to be welded, in which the angle of incidence of such focused radiant heating is substantially within the range of 45 degrees to 90 degrees to the planar surface defined by the plastic sheeting. 13. A method of welding thermoplastics sheet material which comprises locating an arrangement substantially as described in any one of the preceding apparatus claims in the vicinity of a thermoplastics sheet, and subjecting the surface of the sheet to such focused radiant heating for a sufficient time to effect a substantial melting at an interface between respective sheets to be welded. 14. A thermoplastics welding arrangement substantially as described in the specification with reference to and as illustrated by the accompanying drawings. 15. A method of welding thermoplastics sheeting substantially as described in the specification with reference to and as illustrated by the accompanying drawings.

Inventors: SNELLING PETER JOHN, SNELLING, PETER, JOHN
Applicants: VINIDEX TUBEMAKERS PTY LTD, VINIDEX TUBEMAKERS PTY. LIMITED
Application year: 1988
Main classification: B29C65
IPC codes: B29C35, B29C65