Mold and method for manufacturing two-layered preforms by injection overmolding, and two-layered preform

Abstract

The mold includes a first molding cavity connected to a first hot channel for supplying a primary molding material, a second overmolding cavity connected to a second hot channel for supplying an overmolding material; and a core configured to be first introduced in the first cavity for a molding operation of a first layer, and to then be introduced, with the first layer thereon, in the second cavity for an overmolding operation of a second layer. At least one supporting member is arranged to support the core with respect to the first cavity during the entire molding operation of the first layer. An opening left in the first layer by the supporting member is filled and / or covered by the second layer.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS[0001]This application is a Continuation-in-Part (CIP) Application of PCT International Application No. PCT / ES2008 / 000226, filed Apr. 11, 2008, which claims benefit of priority from PCT International Application No. PCT / ES2007 / 000202, filed Apr. 11, 2007. The contents of these applications are incorporated herein by reference.TECHNICAL FIELD[0002]The present invention relates to a mold and a method for manufacturing two-layered preforms by plastic injection overmolding, including means for stably supporting a mold core inside a corresponding mold cavity while an injection step for injecting a plastic material into said cavity is performed. The present invention also relates to a two-layered preform provided with an inner layer of a material capable of providing a barrier effect against light by absorption and an outer layer of a material capable of providing a barrier effect against light by reflection, such two-layered preform being applicable...

AI Technical Summary

Benefits of technology

[0015]With this construction, the supporting member, which is at least one in number, holds and stabilizes the core in a centered position inside the primary molding cavity and prevents the elastic deformation of the core during the injection operation of the first layer, whereby the thickness of the first layer is substantially uniform around the circumference thereof according to accepted dimensional manufacturing tolerances. The fact that the first layer has a uniform thickness contributes to the second layer, subsequently obtained by injection overmolding in the overmolding cavity, also being substantially uniform. On the other hand, the problem of elastic deformation of the core during the overmolding operation of the second layer is generally less pronounced, and in many cases the second layer has a greater thickness than the first layer and / or has a less relevant function, for example, as a barrier material, whereby a small irregularity in the thickness of the second layer is less relevant than in the first layer.

[0023]The use of a percentage of titanium oxide (TiO2) in both the inner and outer layers has the dual interest of favoring the adhesion of the two layers to one another and of mixing a proportion of white in the black colorant of the inner layer to cause a gray effect facilitating the optical coating by the outer layer and thus give a deep white appearance to the bottle obtained by blow molding the two-layered preform without this involving a significant decrease in the capacity of the inner layer to offer an efficient barrier against light by absorption. Nevertheless, it must be indicated that the present invention is not limited to the mentioned base materials and colorants, being it possible to use any other plastic materials which are suitable for manufacturing preforms and / or any other colorants.

Problems solved by technology

A common problem in the molds of any type of apparatus for manufacturing two-layered preforms by injection overmolding is that the cores, due to their slenderness and to their long cantilevered portion, can be slightly deformed elastically inside the primary molding cavities during the injection of the primary molding material of the first layer due to the differences in the temperature, and therefore in the fluidity, of the plastic material injected in an initial phase and in a final phase of the injection process, and to the random distribution of this material with different degrees of fluidity inside the empty space defined between the primary molding cavity and the core.

Given that the thicknesses of the layers of the two-layered preform are subject to strict dimensional tolerances, a slight deformation of the core during the injection operation of the first layer results in variations in the thickness of both layers in opposite sides of the two-layered preform which may be unacceptable, taking into account that the thickness of the layers is reduced to significantly small values during the subsequent blow molding process to form the bottle or another container from the two-layered preform.

However, this arrangement has several drawbacks.

On one hand, the incorporation of the injection valve arranged in the closed bottom of the cavity involves a relatively complex construction and operation, and on the other hand, the fact that the core is no longer supported during the final phase of the molding operation does not ensure a perfect alignment of the core in the molding cavity during the entire injection process of the preform.

These supports leave holes in the preform.

A drawback of this arrangement is that the incorporation of the cylinder and piston assembly at the end of the blow molding cavity is complex and expensive.

Furthermore, if, as usual, the preforms are to be allowed to cool in order to remove them from the core and store them for a subsequent distribution and use in a plurality of blow molding apparatuses, this would limit the use of such preforms with holes to only those blow molding apparatuses which are equipped with the mentioned cylinder and piston assembly to extrude the protrusion and close the holes before the blow molding operation.

However, there is not described nor suggested that the two-layered plastic product obtained by such process is useful to be subsequently blow molded, and in fact the alternate arrangement of monolayer regions of different materials in the hollow plastic product is a serious drawback since the joining areas therebetween are substantially parallel to the pressure direction during a blow molding operation and this involves a risk of said joining areas to be broken.

A drawback related with the sequential co-injection process in comparison with the overmolding process I that the former is not suitable for injecting only two layers forcing to inject an odd number thereof, in general three or five.

It neither permits to accurately control the extent of the intermediate layer or layers nor the uniformity in the width of the layers.

A lack of precision in the preform can result in irregularities in a container obtained by blow molding the preform, and such irregularities will be the more accentuated the more thinner are the layers of the preform.

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