Open shoes are currently very widely used which are substantially constituted by an upper made of plastics, leather or fabric, by an insole generally made of EVA (ethyl vinyl acetate) and by a sole with a tread, also made of a plastic material that is slightly more rigid than the insole, generally made of injected expanded EVA.
As mentioned, the insoles are generally made of EVA, which ensures high standards of softness and strength and further allows to avoid absorbing water in the case of beach-type slippers or sandals.
After its production, the insole must be coupled to the previously formed sole.
Currently, the only method used to couple the insole to the sole is the interposition of adhesive between them.
The process for adhesive bonding of the insole to the sole is extremely complex and difficult.
In the case of a sole made of EVA, for example, the two components are washed with solvents, a primer is applied, irradiation with UV rays is performed in order to activate the primer, and adhesives are applied to the two parts; this is followed by reviving in an oven and finally by coupling with subsequent pressing.
The upper can be manufactured separately and fixed during the adhesive bonding of the insole and the sole, optionally inserting its lateral edges between said insole and the sole or by gluing said lateral edges to the sides of the sole; generally, with this procedure the outer side of the insole is exposed and must therefore ensure an aesthetic finish.
Another system for fixing the upper to the sole is of the mechanical type and uses mushroom-shaped tabs, which protrude from the edges of said upper and are inserted in appropriately provided undercut slots formed in the sole; in this case, the insole is glued inside a tray, which is shaped complementarily to said insole, and accordingly the outer side of the insole does not remain exposed.
It can be noticed that these manufacturing systems for open shoes have aspects that can be improved.
First of all, it is evident that the method for adhesive bonding of the insole to the sole is extremely laborious, long and expensive.
This is even more true for materials such as EVA, which require particular care during adhesive bonding steps.
Secondly, the coupling between the insole and the sole that is provided by means of adhesive is easily affected by external agents, which therefore can reduce it.
Moreover, the materials of said shoes are difficult to recycle, since it is particularly difficult to separate the insole from the sole, which are typically made of different materials that are often not compatible during recycling.
International Application WO2004/026062 discloses an improvement of the above described additional fixing system, in which the mushroom-shaped tabs that protrude from the edges of the upper pass through corresponding passages provided in the insole and engage the corresponding undercut slots in the sole.
This improvement allows to avoid gluing the insole to the sole and also locks said insole mechanically.