Injection moulding process for soles in expansible and crosslinking "EVA" based compounds

Abstract: This invention concerns an injection moulding process for soles in expansible and cross-linking "EVA" based compounds characterised in that during the cooling stage of the moulded sole (1) the spontaneous shrinkage of the same is arrested by means of a template (3) consisting of a rigid shaped plate placed on the sole (1) so that its edge connects with a perimeter edge (2) realised specifically on the upper surface (1a) of the sole (1), whose dimensions reduce no further either during the remaining cooling phase or after cooling.

Description: This patent application concerns an injection moulding process for soles in expansible and cross-linking "EVA" based compounds. The patent protection is also extended to the sole obtained with the process according to the invention. The abbreviation "EVA" refers to "vinyl-acetate ethylene copolymer" which, according to the type of additive used, may be thermoplastic or expansible and cross-linking. In the case of the latter type, an expanding powder additive is used which decomposes at a specific temperature to produce gases which give rise to a typical phenomenon which occurs in injection moulding using this type of "EVA", consisting of the instantaneous expansion of the part as soon as the mould is opened. In particular the dimensions of the part increase rapidly while the proportions and shape remain unaltered thus maintaining a ratio of perfect similarity with the impression of the matrix. Expansible and cross-linking "EVA", thanks above all to its low cost, is widely used in the footwear sector for the production of inexpensive injection moulded soles. It is not however currently possible with this material to mould soles with a raised trim along the sides since the expansion of the part when the mould is opened requires the perimeter of the sole to be milled in order to reduce the sole to the necessary dimensions. In other words, since it is not possible to control this expansion precisely, it is currently necessary to dimension - according to the expansion ratio of the material and that of its shrinkage during cooling - the impression of the mould so that the cooled moulded sole is slightly oversized with respect to the nominal dimensions of the sole sizes in question, which are obtained by milling away a surface layer of material along the entire perimeter of the sole. It is thus obvious why this material and moulding system can not be used for soles which have a raised trim, such as a welt, a part or all of which would be removed when the sides of the sole are milled. FR 2 628 300 patent refers to a moulding process for soles produced with thermoplastic synthetic material, such as PVC; said soles are provided with an insert made of soft and light material produced with EVA, which is housed in a suitable housing provided, if necessary, on the upper surface of the sole. The above mentioned process is characterised in that said EVA insert is located inside said housing immediately after the mould is opened, when the sole is still hot, so that said insert firmly remains embedded in its housing thanks to the simple shrinkage of the PVC during the cooling phase of the sole. Therefore, the above mentioned French patent describes a process in which the insert has a function which is completely different from that of the template applied in the process according to the invention. As a matter of fact, in the French patent the insert is destined to remain embedded inside the sole and the cooling of the PVC is exploited in order to embed said insert into its housing, without the necessity of applying other fixing means, such as adhesives. In the moulding process according to the invention, instead, the template only serves for preventing the spontaneous shrinkage of the sole, until the sole itself has become completely cold and has reached a stable position; after that, the template itself must be detached from the sole in order to be used again during the subsequent moulding phase. In the French patent the sole is moulded in PVC, while the insert is made of soft and light EVA, since said insert must constitute the supporting surface of the foot. In the moulding process according to the invention, instead, the sole is made of soft and light EVA, while the template must consist of a thin flexible plate made of rigid material. The rigidity of the material is imposed by the condition that the template must not lose its shape when it is dosed inside the perimeter edge of the sole in process of cooling, so that the template can efficaciously prevent the shrinkage of the sole. At the same time, said template must be flexible since it must be moulded in order to become similar to the profile of the upper surface of the sole and then it must be placed over the sole, inside the above mentioned perimeter edge. This means that, once a rigid material has been chosen, one has to use a plate made of said rigid material whose thickness is thin enough to make the plate flexible. For example, the template in question should consist of a thin plate of PVC. In conclusion, said French patent does not include any teaching concerning the way of controlling the shrinkage of a sole moulded in expansible and cross-linking EVA, during the cooling phase, since the sole described in said patent is moulded in PVC which is a material that, during the shrinkage, behaves in a way which completely differs from that of the expansible and cross-linking EVA. The purpose of this invention is to design an injection moulding process for soles made of expansible and cross-linking "EVA" that makes it possible to produce a perfectly sized sole and to eliminate the costs and limitations of the milling which is currently necessary to size soles made of expansible and cross-linking "EVA" compounds. The process according to the invention derives from an idea for a solution which is, in general terms, exactly opposite to that on which current production techniques are based. According to this innovative idea, based on the expansion ratio of the material and that of its shrinkage during cooling, the impression of the mould is designed so that the moulded sole is slightly undersized after cooling and shrinkage with respect to the nominal dimensions of the size in question, which are however strictly respected by arresting the spontaneous shrinkage of the sole during cooling. For this purpose, the process according to the invention involves the use of a template consisting of a thin and flexible plate of rigid material placed during cooling so as to adhere on the moulded sole which in turn has a raised perimeter edge along its upper surface, that surrounds and holds the edge of the template before cooling is completed, thereby preventing additional shrinkage of the sole whose dimensions reduce no further either during or after cooling. For major clarity the description of the process according to the invention continues with reference to the enclosed drawings which are intended for purposes of illustration and not in a limiting sense and which schematically show the moulded sole before and after shrinkage following cooling, whereby: fig. 1 is a cross-section along a longitudinal plane of the sole injected with expansible and cross-linking "EVA" compounds immediately off the mould in maximum expansion dimensions; fig. 2 is a top view of the sole in fig. 1 on which the template designed to arrest the spontaneous shrinkage of the sole during the post-moulding cooling stage, is placed; fig. 3 is a cross-section of the sole in fig. 2 along a longitudinal plane III-III; fig. 4 is a cross-section along a longitudinal plane of the sole according to the invention at the stage in which the perimeter edge of the sole and template join, during the spontaneous shrinkage phase; fig. 5 is a cross-section along a longitudinal plane of the sole produced with the moulding system according to the invention. Before describing the operating stages of the moulding process according to the invention, it is important to remember that in order to realise the process according to the invention, the moulding system must be designed to ensure that the sole off the mould after complete and spontaneous shrinkage is slightly undersized with respect to the nominal dimensions of the size in question. Moreover the impression of the mould must be designed so that the sole off the mould, regardless of its shape or pattern of the treading surface and sides, has a raised perimeter edge on its upper surface. This being so, the actual moulding process is attained according to current technology by injecting granules of expansible and cross-linking "EVA" compound into a mould, maintained at a specific temperature to ensure that during the time the material remains in the mould, cross-linking and decomposition of the expanding filler, occur. This conventional moulding phase produces a sole of the type illustrated in figure 1 which, as mentioned above, shows the sole in the maximum dimensions of expansion, that the sole attains immediately the mould is opened. As can be seen in figure 1, sole (1) is characterised by a raised perimeter edge (2) along its upper surface (1a). According to the process in question, a template (3) is placed on the sole (1) before cooling is completed, said template consisting of a thin flexible plate made of rigid material such as rigid PVC, having a profile similar to that of the sole (1) but sized to fit into the perimeter edge (2), which, before completion of the cooling process, encircles and holds the edge (3a) of said template (3), so as to prevent additional shrinkage of the sole whose dimensions reduce no further either during or after cooling. After cooling, template (3) is prised off the edge (2) by which it is encircled. The sole (4) thus obtained, is shown in fig. 5, with the same shape but reduced dimensions with respect to the sole (1) shown in figure 1. The dimensions of the template (3) must therefore be sized to ensure that the shrinkage of the sole (1), during cooling, is stopped when the dimensions of the sole correspond exactly to the nominal dimensions of the sole size in question. Finally, in order to ensure adhesion of the template (3) on the sole (1), the process according to the invention provides that a pressure be created on the template, during cooling, by suitable means, the most convenient and simplest of which may consist of small weights placed on the template (3).

Claims: An injection moulding process for soles produced with expansible and cross-linking "EVA" based compounds; said process applies a mould whose impression is shaped so that the moulded sole has a raised perimeter edge (2) on its upper surface; said process having the steps: expansible and cross-linking "EVA" based compound granules are injected into a mould whose impression is sized to ensure that the sole (1) after spontaneous and complete shrinkage, is slightly undersized with respect to the nominal dimensions of the sole size in question. a template (3), consisting of a thin flexible plate made of a material having rigidity comparable to rigid PVC, said template having a profile similar to that of the sole (1) but sized to fit into the perimeter edge (2), is placed on the upper surface of the sole (1) before cooling is completed; said template (3) remains in said edge (2) until cooling is completed; the template (3) is extracted from the sole after cooling. A process according to the previous claim characterised in that the template (3) is subject to a pressure in order to ensure adhesion on the upper surface of the sole (1) in the perimeter edge (2). An injection moulded sole made of expansible and cross-linking "EVA" compounds produced with the process according to the previous claims.

Inventors: BISCONTI BRUNO, BISCONTI, BRUNO
Applicants: FINPROJECT SPA, FINPROJECT - S.P.A.
Application year: 1997
Main classification: B29D31
IPC codes: B29C44, B29C65, A43B13, B29D35