MXPA97002031A - Molding process by injection for soles in the compounds based on "eva" expansible and of reticul - Google Patents

Abstract

The present invention relates to an injection molding process for producing molded soles from expandable and reclinable "EVA" -based compounds, comprising: injecting granules of the "EVA" compound expandable and reclined within a mold to form the molded sole, the impression of which is on one side dimensioned so that the sole, when removed from the mold after spontaneous and complete contact, is of a size slightly smaller with respect to the size dimensions of the desired sole and, on the other hand side, it is shaped in such a way that the molded sole has a perimeter edge raised on its upper surface, placing a template consisting of a thin flexible plate made of solid material that has a profile similar to that of the sole, but sized for fit within the perimeter edge on the upper surface of the sole when it is removed from the mold during cooling; a at the perimeter edge until the cooling of the molded platen is completed, and removing the platen from the molded sole after cooling the sole molds

Description

MOLDING PROCESS FOR INJECTION FOR SOLES IN COMPOSITIONS BASED PE "EVA" EXPANSIBLE AND RETICULATED DESCRIPTION OF THE INVENTION This patent application relates to an injection molding process for soles in the expandable and crosslinking "EVA" based compounds. The patent protection also extends to the sole obtained with the process according to the invention. The abbreviation "EVA" refers to the "vinyl acetate-ethylene copolymer" which, according to the type of additive used, can be thermoplastic or expandable and crosslinking. In the case of the latter type, an expansion powder additive is used, which decomposes at a specific temperature to produce gases, which produce a typical phenomenon, which occurs in injection molding using this type of "EVA". ", which consists of the instant expansion of the part, as soon as the mold is opened. In particular, the dimensions of the part increase rapidly, while the proportions and shape remain unaltered, thus maintaining a relationship of perfect similarity with the impression of the matrix.

The "EVA" expandable and crosslinking, thanks to all the above, at low cost, is widely used in the footwear sector for the production of cheap injection molded soles. However, currently it is not possible with this material to mold soles with a raised cut along the sides, since the expansion of the part when the mold is opened, requires that the perimeter of the sole be laminated to reduce the sole to the necessary dimensions. In other words, since it is not possible to control this expansion accurately, it is now necessary to diress - according to the expansion ratio of the material and that of its contraction during cooling - the printing. of the mold (in such a way that the cooled molded sole is slightly overstressed with respect to the nominal dimensions of the sizes of the sole in question, which are obtained by rolling, separating a surface layer of the material throughout the entire Thus, it is obvious why this material and the molding system can not be used for soles, which have a raised cut, such as a vira, a part or all of which should be removed when sides of the sole are laminated.

The purpose of this invention is to design an injection molding process for soles made of expandable and crosslinking "EVA", which makes it possible to produce a perfectly dimensioned sole and to eliminate the costs and limitations of lamination, which is currently necessary for size soles made of expandable and crosslinking "EVA" compounds. The process, according to the invention, is derived from an idea for a solution, which is in general terms, exactly opposite to that on which the current production techniques are based. According to this innovative idea, based on the ratio of expansion of the material and that of its contraction during cooling, the impression of the mold is designed in such a way that the molded sole is slightly below its size, after cooling and contraction with with respect to the nominal dimensions of the size in question, which, however, are strictly respected by stopping the spontaneous contraction 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 that it adheres on the molded sole, which in turn has a perimeter edge raised along its upper surface, which surrounds and retains the edge of the template before the cooling is finished, thus avoiding additional contraction of the sole, whose dimensions are no longer reduced, either during or after cooling. For clarity, the description of the process according to the invention, continues with reference to the attached drawings, which are intended for purposes of illustration and not in a limiting sense and which schematically shows the molded sole before and after the contraction after cooling, whereby: Figure 1 is a cross section along a longitudinal plane of the sole injected with expandable and crosslinking "EVA" compounds immediately outside the mold in the maximum expansion dimensions; Figure 2 is a top view of the sole in Figure 1, in which the template designed to stop the spontaneous contraction of the sole during the post-casting cooling stage is placed; - Figure 3 is a cross section of the sole in Figure 2 along a longitudinal plane lilili; Figure 4 is a cross section along a longitudinal plane of the sole, according to the invention, in the stage in which the perimeter edge of the sole and the insole are joined during the spontaneous contraction phase; - Figure 5 is a cross section along a longitudinal plane of the sole produced with the molding system according to the invention. Before describing the steps of the operation of the molding process according to the invention, it is important to remember that in order to carry out the process according to the invention, the molding system must be designed to ensure that the sole is out of the mold, after which It ends and the spontaneous contraction is slightly smaller than the nominal dimensions of the size in question. In addition, the impression of the mold must be designed in such a way that the sole outside the mold, without considering its shape or pattern of the surface in contact with the floor and the sides, has a perimeter edge raised on its upper surface. By doing this, the actual molding process is achieved, according to current technology, by the injection of granules of the expandable and crosslinking "EVA" compound into a mold, maintained at a specific temperature to ensure that during the time the material remains in the mold, the cross-linking and decomposition of the expansion filling material occurs.

This conventional molding phase produces a sole of the type illustrated in FIG. 1, which, as mentioned above, shows the sole in the maximum expansion dimensions that the sole immediately achieves that the mold is opened. As can be seen in figure 1, the sole (1) is characterized by a perimeter edge (2) raised along its upper surface (the). According to the process in question, a jig (3) is placed on the sole (1) immediately outside the mold, the jig consists of a flexible, thin plate made of rigid material, such as rigid PVC having a similar profile to that of the sole (1), but sized to fit within the perimeter edge (2), which, before finishing the cooling process, encompasses and retains the edge (3a) of the template (3), to avoid additional contraction of the sole, whose dimensions are no longer reduced either during or after cooling. After cooling, the template (3) is removed from the edge (2) by which it is covered. The sole (4) thus obtained, is shown in figure 5, with the same shape, but reduced dimensions with respect to the sole (1) shown in figure 1.

The dimensions of the insole (3) must therefore be digested to ensure that the contraction 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, to ensure the perfect adhesion of the template (3) on the sole (1), the process according to the invention, provided that a slight pressure is created on the template, during cooling, by an appropriate means , the most convenient and simplest of which may consist of small weights placed on the template (3).

Claims (3)

1. An injection molding process for soles produced with expandable and crosslinking "EVA" based compounds, characterized in that: the granules of the expandable and crosslinking "EVA" based compound are injected into a mold whose printing is for a side dimensioned to ensure that the sole outside the mold, after spontaneous and complete contraction, is slightly smaller in size with respect to the nominal dimensions of the sole size in question and, on the other hand, conform in such a way that the molded sole has a perimeter edge raised on its top surface. - a template, consisting of a flexible, thin plate made of rigid material having a profile similar to that of the sole, but sized to fit within the edge of the perimeter, is placed on the upper surface of the sole outside the mold and during cooling; - the template remains on the edge until cooling is complete; - the template is extracted from the sole after cooling.

2. The process according to the previous claim, characterized in that the template is subjected to a light pressure to ensure perfect adhesion on the upper surface of the sole at the edge of the perimeter.

3. An injection molded sole made of expandable and crosslinking "EVA" compounds produced with the process in accordance with the previous claims.