The present invention relates to an apparatus and a method for making a resilient unit, such as a mattress.

Resilient units, such as mattresses, are usually made using resilient elements, such as springs of coiled wire, located inside individual pockets of material. The pockets are formed around the springs by joining together leaves of pocketing material, which may, for example, comprise non-woven, spun- bonded polyester.

Traditionally, springs would be pocketed individually and then joined together to form an array. As a variant of this method several springs can be inserted between leaves of pocketing material which is then folded over the springs, before the individual pockets are established by joining the leaves of material between, and around, the springs. This is referred to as a "string" of pocketed springs. The linear strings are then joined together, side by side, to form a two-dimensional array.

Another manufacturing method is outlined in our UK Patent No. GB 2426926 B in which rows of springs are sequentially introduced between axially spaced sheets of material that are then joined together at positions between the sheets, and at their edges, to form an array of individually pocketed springs .

Embodiments of the present invention aim to provide an improved apparatus and method for manufacturing pocketed springs . The present invention is defined in the attached independent claims, to which reference should now be made. Further, preferred features may be found in the sub-claims appended thereto .

According to one aspect of the present invention, there is provided a welding apparatus comprising first and second welding tools for welding together sheets of a workpiece, wherein at least one of the welding tools comprises a rotatable member arranged in use to rotate to weld a workpiece positioned between the tools.

The rotatable member may be arranged to rotate with respect to the workpiece in use and/or to rotate with respect to the other tool. The rotatable member may comprise a wheel or a roller .

The welding apparatus may comprise ultrasonic welding apparatus. The at least one tool may comprise an ultrasonic vibrating tool, which may be a sonotrode, or horn. Alternatively the at least one tool may comprise a base or anvil .

In one arrangement the first and second tools may both include a rotatable member.

The or each rotatable member may comprise a profiled surface which may be arranged in use to impart a patterned weld to the workpiece. The profiled surface of the at least one rotatable member may comprise one or more of a knurled, serrated, ridged, stepped or otherwise patterned surface.

According to another aspect of the present invention, there is provided a method of welding together sheets of a workpiece, the method comprising positioning the sheets between first and second welding tools and causing a rotatable member of at least one of the tools to rotate to effect a weld.

The method may comprise causing the rotatable member to rotate with respect to the workpiece and/or with respect to the other tool.

The method may comprise a method of ultrasonic welding of sheets of a workpiece. In a preferred arrangement the method comprises a method of welding together thermoplastic material, more preferably non-woven material.

The method may comprise rolling the rotatable member over the sheet material.

In a preferred arrangement the method comprises rolling a rotatable member over the workpiece, which rotatable member comprises a profiled surface, to impart a patterned weld to the workpiece.

The invention may include any combination of the features or limitations referred to herein, except such a combination of features as are mutually exclusive, or mutually inconsistent . A preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, in which:

Figure 1 shows schematically in perspective view a resilient unit made up of a plurality of coil springs encapsulated between axially spaced sheets of material, according to a prior art example;

Figure 2 shows the unit of Figure 1 in schematic cross section taken along line 2 2 of Figure 1;

Figure 3 shows schematically a welding apparatus according to an embodiment of the present invention; and

Figure 4 is a schematic end view of a part of the apparatus of Figure 3.

Turning to Figures 1 and 2 these show a resilient unit generally at 100, comprising an array of steel wire coil springs 110 inside individual pockets 120 formed by axially spaced sheets 130 of spun-bonded polyester that have been ultrasonically welded at points 140 between the springs and at edges of the unit 100. The resilient unit would typically include more springs, i.e. be longer and wider, than is represented in the drawing.

Figure 3 shows an apparatus for welding a resilient unit such as is shown in Figures 1 and 2, and many other types of pocketed spring unit (not shown) . The welding apparatus is shown generally at 200 and comprises a first tool, in the form of a sonotrode - or "horn" - 210 and a second tool, comprising an anvil 220. Between the welding tools 210 and 220 are located the sheets 130 of material to be welded, as a workpiece. The material may be of any weldable type, but for the purpose of making pocketed spring units it is often of non-woven, spun-bonded polyester.

The sonotrode tool 210 comprises a bracket 210a and a rotatable wheel 210b which is arranged to rotate with respect to the stationary anvil 220. To effect welding an electrical current is applied to the sonotrode which causes it to vibrate at ultrasonic frequencies, typically in the region of 20 KHz. In combination with applied pressure, this causes highly localized friction in the sheets 130 which heats the material locally and welds the sheets together.

As the wheel moves across the sheets 130 in the direction of arrow A, whether between adjacent springs or at the edge of the unit 100, the welding tools join the sheets together.

Figure 4 shows the tool 210 schematically from the front, from which it can be seen that the circumferential surface of the wheel 210b has a pattern P thereon. The pattern can be selected from many alternatives. One reason why a pattern of welds is preferred to a continuous line, is that the latter might introduce a structural weakness into the unit 100. The example shown has the sonotrode with the rotatable part 210b, and the anvil as a stationary component. However, the stationary component could equally be the sonotrode, with the anvil including a rotatable part. Indeed, a more complex arrangement can be envisaged in which both welding tools include rotating parts that bear against one another - with the material sheets therebetween, to effect a weld.

One advantage of the apparatus described is the speed with which the welds can be made. Previous welding apparatus requires the tools to come together and then separate, usually under the control of a pneumatic actuator, before either moving the pair of tools or, more conveniently, indexing the sheets to a new weld position and bringing the tools together again. There are certain operational limitations that must be overcome when seeking to increase the speed of such a process. In contrast, when at least one of the welding tools is rotatable with respect to the other, and to the workpiece, it can be moved quickly and efficiently.

Although the example described above is of a stationary workpiece and a moving weld tool, it will be understood by the skilled person that the workpiece could also be moved while one or both of the tools remain in a fixed position. What is important is that there is relative movement of the material with respect to at least one of the tools. The rotatable member still rotates with respect to the material and/or with respect to the other tool. It may be useful/necessary to move the workpiece while keeping one or more of the welding tools stationary for one direction of weld and to keeping the workpiece stationary while moving one or more of the welding tools for another direction of weld. So, it will be understood that a combination of approaches might be appropriate, for example when welds are to be made in substantially different directions in the workpiece, such as directions that are of the order of 90 degrees apart.

Furthermore, while the example given above is of ultrasonic welding, it will be understood by those skilled in the art that other types of welding can be employed either alone or in combination with ultrasonic welding, whilst still remaining within the scope of the present invention. For example, the sheets of the workpiece could be welded together by heat sealing. This could be achieved by heating one or both of the welding tools, for example electronically. Such heat, optionally in combination with the application of pressure, could effect the weld in a similar way, by causing localised heating in the workpiece, with one or both of the welding tools having a rotatable member as described above.

Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance, it should be understood that the applicant claims protection in respect of any patentable feature or combination of features referred to herein, and/or shown in the drawings, whether or not particular emphasis has been placed thereon.