Background of the invention It is known to make fully opaque mailing envelopes from a plastics material, such as polyethylene. The plastics material of the envelope can itself be opaque, or it can be a clear plastics material that is rendered opaque by printing over it with an opaque ink. Where the plastics material is itself opaque, it may consist of a single opaque layer or multiple co-extruded or laminated opaque layers.

Opaque plastics envelopes can be pre-printed with a delivery address but when they are intended to be mailed to different addresses they have to be addressed individually.

Commonly, the envelopes have at least one region on which it is possible to write with a ball point pen or to affix a self-adhesive label. Such a method of addressing is unnecessarily time consuming, especially when the delivery address is already present on a document contained in the mailing envelope.

When a plastics envelope is rendered opaque by printing over a clear plastics material, then a clear window could be created by failing to print over the window. The appearance of such an envelope is not however entirely satisfactory.

Instead, the invention is concerned with window mailing envelopes that are made of an opaque plastics material, be it of a single layer or of a multi-layered type, and that have a clear window through which an address printed on the documentation within the envelope may be read-.

W096/150338 discloses such an envelope designed in particular for mailing computers storage media. A window cut out of the opaque plastics material of the envelope is covered on the inside by a clear sheet that extends over the entire area of the envelope and is secured to the front of the envelope around the periphery of the window. The clear sheet thus forms a partition dividing the envelope into two compartments of which one is restricted by the windows and serves to store the diskette to be mail while the other is unobstructed and retains the accompanying documentation.

Such an envelope is not ideal for general letters and documents because the large clear separator sheet adds to the cost and the weight of the envelope and if the documentation should accidentally enter the wrong compartment then the window would prevent it from being correctly inserted.

US-A-5,533,810 also describes a document carrier but one intended to act as a reusable pouch rather than as a mailing envelope. The embodiment described in Figure 5 of that patent has a window cut out of an plastics envelope that is covered by a transparent plastics sheet secured to the front of the envelope around the periphery of the window. In this case, the plastics sheet covering the window opening is secured to the outside of the envelope, which makes the envelope easier to manufacture but makes unsuitable for mailing because the transparent sheet covering the window opening can peel off accidentally.

Summary of the invention With a view to mitigating the foregoing disadvantages, the present invention provides a method of manufacturing a mailing envelope formed of an opaque plastics material and having a clear window for viewing an address contained on a document placed in the envelope, which method-comprises the

steps of providing a sheet of opaque plastics material having first, second and third regions to form the front and back panels of the envelope and a closure flap, respectively, forming a window aperture in the first region of the sheet, adhering to the inner surface of the first region a clear plastics sheet to cover the window aperture, the clear plastics sheet being preformed as a self-adhesive transparent label carried on a web and coated with an adhesive only around its periphery, folding the first and second regions of the sheet to overlie one another after the window aperture has been covered with the transparent label and subsequently securing the side edges of the first and second regions to one another to form the side seams of the finished envelope.

The invention introduces the step of covering the window opening with a clear sheet into the manufacturing method before the side seams of the envelope have been formed and when the rear surface of the front panel, that is to say the surface that lies inside the finished envelope, is still accessible. If the envelope is made from a flat sheet, then the sheet may be folded and the side seams welded after the windows have been covered with transparent labels. If the envelopes are made from a flattened tube, then before the side seams have been formed, one of the first and second regions of the envelope can be folded back temporarily to allow access to the part of the rear surface of the first region in which the window opening is formed to allow the self-adhesive transparent label to be placed over the window opening.

The invention thus allows an opaque plastics envelope with a clear window to be manufactured by incorporating some simple additional steps in existing production methods so as not to add significantly to the cost of the mailing envelopes. These additional steps can furthermore be implemented simply regardless of whether the envelopes are

formed one at a time by folding over a plastics sheet, two or more at a time by slitting an extruded tube or by welding two separate plastics sheets that form the front and back of the envelope.

The invention can use known technology for affixing labels to a plastics web as it is being advanced through a machine to close the window opening from the inside of the envelope.

Brief description of the drawings The invention will now be described further, by way of example, with reference to the accompanying drawings, in which : Figure 1 is a front view of a mailing envelope manufactured by the method of the invention, Figure 2 is a section on the line II-II in Figure 1, and Figures 3 to 8 show different stages in one method for the manufacture of the envelope shown in Figures 1 and 2.

Detailed description of the drawings The envelope 10 in Figure 1 resembles a conventional paper envelope with a window 12 and differs from it essentially only in the fact that it is made of a plastics material. The preferred plastics material is polyethylene though it is possible to use other thermoplastic materials, such as polypropylene. The envelope has two seams 14 and 16 along which the front panel and rear panel of the envelope 10 are secured to each other, usually by welding.

As can be seen in Figure 2, the envelope has a front panel 22, a rear panel 20 connected to the front panel along the lower edge of the envelope and a closure flap 18 connected to the front panel 22 along the upper edge of the

envelope. A self adhesive glue 24 is applied to seal between the closure flap 18 and the envelope. The window 12 is formed by a cut out aperture that is covered by a clear plastics sheet 28 glued at 26 to the rear surface of the front panel 22. The clear plastics sheet 28 is a label only slightly larger than the cut out and adhered to the envelope about the periphery of the cut out. The transparent label is precoated with an adhesive only around its periphery and is carried as individual labels on a continuous web.

The window 12 is to allow a delivery address printed on documents 25 inserted into the envelope to be viewed, thereby avoiding the need to address the mailing envelope separately.

Figures 3 to 8 show a method for manufacturing the envelope of Figures 1 and 2.

Figure 3 shows in section a flattened continuous extruded tube 30 made of an opaque plastics material, which can comprise one or more co-extruded layers. As is well known in the manufacture of plastics envelopes from such an extruded tube, it will later be welded and separated along transverse seams and slit to form one or more envelopes at a time from each section of the tube 30.

In the first step of the method, shown in Figure 4, a strip is cut and peeled away from the tube 30 to leave a gap 32. The section of the tube exposed by this gap will later form the closure flaps 18 of the two envelopes. The upper side of the tube 30 as viewed will form the rear panels 20 of the two envelopes while the part of the lower side of the tube 30 covered by the rear panels 20 will form the front panels 22.

In the step shown in Figure 5, at least part of the rear panel 20 is folded back to provide access to the rear surface of the region of the front panel in which a clear window is to the formed. The windows 12 are formed by cutting out apertures in the two front panels (Figure 6) then covering the apertures with transparent self-adhesive labels that are only slightly larger than the windows 12.

The labels are coated with adhesive only around their periphery and are carried on a continuous web. The labels are separated from the web by passing the web over a small diameter roller. The front edge of the label peeled from the web in this manner is applied to the surface of the plastics material of the front panel and pressed onto it by a further roller. Stations for applying labels in this manner are known per se and need not be described in greater detail within the present context.

The rear panels 20 are then folded over to cover the windows 12, as shown in the Figure 8 and the tube is further processed. in the usual manner, that is to say transverse welds are made to form the seams 14 and 16 and the tube is slit longitudinally as well as transversely to form the individual envelopes.

The method steps described above are only concerned with the formation of a window in the plastics envelope as proposed in the present invention. Other steps such as the application of glue to the closure flap, covering the glue with a protective strip and printing onto the plastics material are conventional and need not therefore be described in detail.

It is also known to form plastics envelopes one at a time from a flat sheet that is subsequently folded over and seamed. To form window envelopes by such a method merely requires the window apertures to be cut out and covered with a clear plastics material before the rear panel is folded over and welded to the front panel.