A Welding Method for Plastic Elements, in particular for Door and Window Frames, and Frames obtained using the Method.

The invention relates to a welding method for elements made of plastic material, in particular for realising door and window frames.

The invention also relates to frames obtained via the above method.

For some time now, as is known, apart from tradition wooden or aluminium frames, the market has offered frames realised by elements made of plastic, especially PVC, which exhibit a high degree of reliability, require less maintenance and guarantee, above all, a greater heat insulation with a consequent energy saving.

Manufacturing of this new type of mounted frame elements begins from standard bars which are cut into profiled elements and then welded. The welding operations are at present performed using a process, known as hotplate, in which the two ends of the elements are placed in contact, or in close proximity, with a hot plate for a sufficient time to cause melting of the material over a certain tract of the two ends thereof; these are then pressed together with pressures in opposite directions with times and modes that cause joining by welding. During this operation, inevitably, following the pressure exerted, an irregularly-shaped bead of melted material forms along the welding line; the removal of the bead has to be done with a subsequent process of mechanical working that is very complex and expensive.

Considering the consequent overall length of the process and the energy consumption connected to the need to keep the hot plate constantly hot, given the associated heat inertia, it is clear that any opportunity of developing and setting up new welding processes that are more advantageous from the industrial and ecological points of view is welcome. The main aim of the present invention is to provide a welding method for elements made of plastic material which comprises the use of less massive machines and has production costs which are more contained with respect to what is usual today.

A further aim of the invention is to provide a manufacturing method for mounting elements by welding of relative elements of plastic material, in particular PVC, with which structures can be obtained that are ready to be used after the welding operation, without there being any need for further industrial working, and with a reduction of working times required by the welding.

The above aims are attained with a welding method for plastic elements, in particular PVC, which comprises:

interposing an element which is sensitive to electromagnetic fields (susceptor) between the two ends of the elements to be joined by welding; reciprocal nearing of the two ends of the elements, before melting and welding;

heating the susceptor element by application of an electromagnetic field in the joining zone of the elements for a sufficient time to cause heating up to melting point and welding of the two ends of the elements in contact with the susceptor.

The welding of the elements is done by passing the molten material through the interposed susceptor element or by amalgamation of the material of the susceptor element with the molten material of the elements. In this second case the susceptor element essentially comprises plastic material and ferro-magnetic material. The application of the electromagnetic field is advantageously obtained by use of at least a circular coiled inductor arranged around the element welding zone; the concentration of the electromagnetic field in the welding zone can be obtained by use of variously-shaped ferro-magnetic nuclei on which coiling has been mounted.

The limitation to passage of the electromagnetic field in particular regions can advantageously be obtained by interposing, between the field applicator and the susceptor, screens (of an appropriate thickness and shape) made of a conductor material. The conductor material can be, preferably, copper or aluminium.

During the heating stage, the contacting of the two element ends is done by application of suitable pressure in opposite directions on the free ends of the elements. The application of pressure continues beyond the application time of the electromagnetic field, necessary for melting the plastic material of which the two contact surfaces of the elements by means of the susceptor, such as to prevent formation of internal tensions during cooling; the pressure exerted can vary in intensity during this stage of cooling.

Frames for doors or windows made of plastic material, made as described above, have good aesthetic qualities and contained realisation costs with respect to those actually in production, as they do not require the use of high energy-consuming apparatus nor machine tools for performing the mechanical operations specifically connected or associated to the welding operation, such as, for example cleaning operations.

These and other characteristics and advantages of the present invention will more fully emerge from the detailed description that follows of a preferred but not exclusive embodiment of the invention, illustrated purely by way of non-limiting example, in the accompanying figures of the drawings, in which: figure 1 is a perspective view of an end section of a profiled element for doors and windows to be welded according to the method of the invention and a possible embodiment of a susceptor element;

figure 2 schematically illustrates an apparatus usable for actuating the method of the present invention;

figures 3 and 4 are views relating to a machine usable for actuating the method of the invention;

figure 5 is a possible embodiment of the susceptor used when the welding is performed by passage of molten material through the susceptor;

figure 6 illustrates a possible embodiment of the susceptor used when the welding is performed by amalgamation of molten material of the elements with the material of the susceptor;

figure 7 relates to a possible production stage comprised in or associated to the method of the invention;

figure 8 is a further embodiment of a susceptor element of the present invention.

With reference to figure 1, 10 denotes an element for doors and windows made of a plastic material, in particular PVC. The shape of the transversal section of the element is well illustrated in the front part of the figure; in proximity of the transversal section a layered element 11 is illustrated, in this embodiment a grid, a perimeter of which is a copy of the perimeter of the transversal section of the element 10 to be welded, and which remains on the inside of the perimeter. In particular the external perimeter of the susceptor follows the perimeter of the section of the element at a distance of preferably a few tenths of a millimetre. According to the method of the invention, the layered element 11 is interposed, as schematically illustrated in figure 2, between the two ends of the plastic elements 10, 10' that are to be joined by welding. The element 1 1 is a susceptor, i.e. an object which when interested by electromagnetic waves heats up and is brought to a high temperature, consequently causing a heating by conduction of the adjacent ends of the elements 10, 10', up to their melting point. The structure of the susceptor element 11 is such as to enable passage of the molten material from one to another of the elements, thus realising a joining by welding thereof.

The useful action of the susceptor element 11 occurs at the moment when it is interested by an electromagnetic field. In the example of figure 2 this happens following supply to the inductors 12 which generate a magnetic flow of a variable intensity over time which crosses the ferromagnetic circuit 13, affects the joining zone of the two elements 10, 10', where the element 11 is located. Note that in order to obtain the concentration of the electromagnetic field at determined points of the air gap small ferromagnetic plates 14 are used, for example made of ferrite, specially conformed and positioned in the air gap zone itself, and to prevent propagation of the field in certain directions, suitably thick and shaped screens 15 made of a conductor material are positioned, as illustrated in figure 2, for example.

In the example of figures 3 and 4, the inductor 16 is a circular coil inductor of such a size as to surround the joining zone of the two plastic elements 17, 17'. In this case, the elements form between them a 90° angle and are located on special support guides 18, 18', which proceeds towards the joining zone where the susceptor element 20 is located. The susceptor element 20, as is well illustrated in the section view of figure 4, has, in this example of application of the invention, an empty central part 21; it is sufficient for the heat produced thereby to be transmitted to the perimeter structural part of the two elements 17, 17', up to causing fusion of the plastic material present therein and the relative subsequent passage through the links of the element 20 in order to realise the welding.

The susceptor element can certainly take on different shapes from the above-described ones; by way of example, the shapes might be dependent, or not, on the material the susceptor element is made of.

If for example sheet metal is used, the susceptor element 22, illustrated in figure 5, can exhibit comb-shaped edges to enable transfer of the molten material between the two ends of the elements. It can also be easily jointed and anchored to at least one of the two elements, using for example metal tabs 23 which are bent to functionally realise the joint with the internal edge of the plastic element.

The susceptor element can also be realised in a carbon fibre textile shaped the same as the section of the elements to be welded, or it can be constituted by ferromagnetic powders mixed with other type of powders, such as for example PVC powders, and pressed together such as to form a shaped element. If ferromagnetic powders are used, the frequency at which the electromagnetic field strikes the susceptor has to be varied could be greater than 10 MegaHertz.

If a metal grid made of a ferromagnetic material is used, it can be coated with a layer of protective material to prevent oxidation problems during storing and after realising the welding operation.

With the aim of reducing the realisation times of the welding, according to the method of the invention, two or more layered susceptor elements, perfectly superposed, might be used. A core 26 made of a plastic material, for example PVC, might be interposed between two layered elements 25 and 25', as illustrated in figure 6. The layered elements 25, 25' can be fixed to the plastic core 26 by mechanical anchoring or gluing. The layered elements 25, 25' can be made using selective removal techniques from surfaces created by deposition, using spray nozzles 28, on a plastic support 27, such as is illustrated for example in figure 7; the support 27 can coincide (or not) with the core made of a plastic material 26 as described above. The deposition is realised preferably with the technique known as thermal spray.

The shape and arrangement of the layered heating element, in particular if made of metal, can obviously be specially varied to produce a uniform heating of the plastic material of the elements to be welded.

Specifically, this result is obtained as illustrated in figure 8, which shows a case in which the heating element is subdivided horizontally into several parts 29', 29", 29"'.

The inductor can also be varied with respect to the illustrated embodiments; for example a circular two-piece inductor can be used, formed by two semicircles hinged to one another, and easily actionable to open and close in order to be easily located about the join zone of the two elements and also easily removed once the welding operation has been completed.

In reference to figure 2, the number of inductors can also be greater than 2 and can be arranged differently about the transverse section of the element, for example having a horizontal orientation with respect to the vertical orientation of the two inductors represented in the figure.

Other inductors can obviously be used, also suitable for heating the susceptor element located between the plastic elements; the heating can be obtained by parasite electric currents generated in the element by action of the electromagnetic field produced by the inductor, and which can thus be due both to the Joule effect produced by the parasite currents, and to dielectric or magnetic leakage. In reference to the case illustrated in figure 2, the angle formed by the two elements can be 180 degrees, i.e. the two elements can be aligned.

The advantages inherent to industrial costs for manufacturing frames obtained by the method of the present invention are in all cases clear, and it is also clear that these advantages remain even if further modifications or variants are brought to the embodiments described herein.

The dimensions of the frames can certainly vary very flexibly as the use of uprights and crossbars having variable transversal sections does not lead to changes in the welding process, nor to the equipment or machinery used. Other modifications of a practical-applicational nature can be brought to the invention and to the frames for doors and windows obtained, without forsaking the ambit of protection of the inventive idea as claimed herein below.