MOLD FOR INJECTION MOLDING PROCESSES AND MOLDING PROCESS USING THE MOLD

Abstract

A mold for injection molding processes and a molding process that uses the mold. The mold has, at at least one part of the surface of its cavity raised portions which are longitudinally extended, are arranged side by side and are oriented substantially in the direction of the flow of the melted plastic material that is injected. During the process, the surface of the cavity of the mold is kept at a temperature that is lower than the glass transition temperature, in the case of molding of plastic material constituted by amorphous polymers, and lower than the crystallization temperature, in the case of molding of plastic material constituted by semicrystalline polymers.

Claims

1.-9. (canceled)

10. A mold for injection molding processes, having mold cavity and comprising, at least one part of a surface of said cavity, raised portions which are longitudinally extended, are arranged side by side and are oriented substantially in a direction of a flow of a melted plastic material that is injected.

11. The mold according to claim 10, wherein said raised portions consist of undulations formed by an alternation of ridges and hollows.

12. The mold according to claim 10, wherein said raised portions are repeated in a direction at right angles to the flow of the melted plastic material.

13. The mold according to claim 10, wherein said raised portions are provided by laser ablation of the surface so as to obtain a periodic nanostructure.

14. The mold according to claim 11, wherein each one of said undulations has a width and wavelength of less than 1 μm.

15. The mold according to claim 10, wherein it is made of steel.

16. The mold according to claim 10, wherein it is made of coated steel, the coating being obtained by means of a process selected among chromium plating, nickel plating, physical vapor deposition (PVD) and chemical vapor deposition (CVD).

17. The mold according to claim 10, wherein it is made of steel treated by means of a process chosen from hardening, nitriding, cementing and oxidation.

18. An injection molding process, using a mold according to claim 10, wherein the surface of the cavity is kept at a temperature that is lower than a glass transition temperature, in the case of molding of plastic material constituted by amorphous polymers, and lower than a crystallization temperature, in the case of molding of plastic material constituted by semicrystalline polymers.

Description

[0030] Further characteristics and advantages of the invention will become better apparent from the description of a preferred but not exclusive embodiment of the mold according to the invention, illustrated by way of nonlimiting example, in the accompanying drawings, wherein:

[0031] FIG. 1 is a sectional view of a mold;

[0032] FIG. 2 is an enlarged-scale view of a surface nanostructure of a mold according to the invention;

[0033] FIG. 3 is another enlarged-scale view of the nanostructure shown in FIG. 2;

[0034] FIG. 4 is a view of a first example of a molded object;

[0035] FIG. 5 is a view of the structure of the surface of the mold according to the invention, used in the molding of the object according to FIG. 4;

[0036] FIG. 6 is a view of a second example of a molded object;

[0037] FIG. 7 is a view of the structure of the surface of the mold according to the invention, used in the molding of the object according to FIG. 6;

[0038] FIG. 8 is a view of a third example of a molded object;

[0039] FIG. 9 is a view of the structure of the surface of the mold according to the invention, used in the molding of the object according to FIG. 8.

[0040] With reference to the figures, the mold according to the invention is designated generally by the reference numeral 10.

[0041] A mold 10 is shown in cross-section in FIG. 1, in which a cavity 11, to be filled with the thermoplastic material and which, with its impression, gives shape to the object to be produced, and a sprue 12, at the injection point, are indicated.

[0042] As can be seen from the enlarged-scale views of FIG. 2 and FIG. 3, photographed on the surface of the cavity of the mold 10, said mold has, at at least one part of the surface of its cavity 11, raised portions 13 which are longitudinally extended, are arranged side by side and are oriented substantially in the direction of the flow of the melted (thermoplastic) plastic material that is injected.

[0043] It is evident that the raised portions 13 consist of undulations formed by an alternation of ridges 14 and hollows 15, which are indicated in FIG. 3.

[0044] Preferably, each one of the undulations has a width and a wavelength of less than 1 μm.

[0045] The geometric structure is in fact repeated almost identically at regular intervals of less than 1 μm.

[0046] The raised portions 13, therefore the undulations, are provided by laser ablation of the surface of the mold 10 so as to obtain a periodic nanostructure, where the term “periodic” is understood to reference the almost identical repetition of the single undulation. This treatment is known in the field as LIPSS (Laser-Induced Periodic Surface Structures) and can be obtained by means of known methods for laser ablation of the surface, with femtosecond sources. In particular, NLL (Non-linear Laser Lithography) is the technology to be preferred, since it allows higher productivity.

[0047] The mold 10 according to the invention is preferably made of steel.

[0048] As an alternative, it can be made of aluminum or coated steel, the coating being obtained with a process selected preferably from chromium plating, nickel plating, physical vapor deposition (PVD) and chemical vapor deposition (CVD). Moreover and as an alternative, the mold 10 can be made of steel treated by means of a process chosen from hardening, nitriding, cementing and oxidation.

[0049] These coatings and treatments, as is known, increase the hardness of the surface of the cavity and consequently its wear resistance. Laser treatments, adapted to obtain a periodic nanostructure, only modify the topography of the surface without altering its mechanical properties and are therefore compatible both with coatings and with treatments.

[0050] The raised portions 13, in addition to being arranged side by side, are repeated in the direction at right angles to the flow of the plastic material.

[0051] Substantially, the orientation of the raised portions 13 (and therefore of the undulations), which as mentioned have a longitudinal extension, is parallel to the direction of the flow of the polymer, i.e., to the direction of advancement in the mold from the injection point.

[0052] This concept is explained in the illustrations in FIGS. 4 to 9, with three examples of molded objects and corresponding nanostructures of the mold.

[0053] In particular, FIG. 4 shows a disk-like product 16, with the sprue at the center, which corresponds to the region where injection occurs.

[0054] The arrows indicated by 17 indicate the radial direction of the filling flow.

[0055] As a function of the radial flow direction, the orientation of the nanostructure to be obtained on the surface of the cavity 11 of the mold 10 is the one shown schematically in FIG. 5, which also shows an enlarged-scale microscope view of a part of said structure. This orientation is also correspondingly radial, with a number of raised portions 13 that is conveniently larger for greater distances from the center.

[0056] The same FIG. 5 also indicates the flow front of the polymer, with the reference numeral 18, where the flow substantially passes from the injection direction to the radial direction for filling the cavity, and the numeral 17 designates the arrows that represent the filling flow beyond the front 18.

[0057] FIG. 6 provides as an example a product 19 that is rectangular in plan view, with the direction of the filling flow indicated by the arrows 17 which move radially away from a point of one side of the product.

[0058] The direction of the flow corresponds to the orientation of the nanostructure as shown schematically in the FIG. 7.

[0059] The orientation is mixed, i.e., the raised portions are arranged radially close to the injection region and are then mutually parallel.

[0060] FIG. 8 is a view of an example of a product 20 in which the filling flow, designated again by 17, is unidirectional since it is distributed in a parallel manner from multiple points.

[0061] Correspondingly, the orientation of the raised portions 13 of the nanostructure is unidirectional, as shown in the subsequent FIG. 9.

[0062] The temperature of the internal surface of the mold 10 during injection is lower than the glass transition temperature, in the case of molding of plastic material constituted by amorphous polymers, and lower than the crystallization temperature, in the case of molding of plastic material constituted by semicrystalline polymers.

[0063] The present invention also relates to an injection molding process, in which a mold 10 is used, wherein the surface of the cavity is kept at a temperature that is lower than the glass transition temperature, in the case of molding of plastic material constituted by amorphous polymers, and lower than the crystallization temperature, in the case of molding of plastic material constituted by semicrystalline polymers.

[0064] Periodic surface nanostructures of the described type induce the flow of the polymers in the melted state at the mold wall and therefore allow significant reductions of the injection pressure. This is due substantially to the orientation of the nanostructure, therefore of the raised portions 13 and accordingly of the undulations, with respect to the direction of advancement of the flow of plastic material, i.e., with respect to the direction of advancement in the mold with respect to the injection point. The adsorption properties of the surface of the mold are reduced, facilitating the flow of the melted plastic material.

[0065] Laser ablation is used to modify the surface nanostructure of the mold, providing periodic undulations arranged parallel to the direction of advancement of the flow of plastic material, which depends on the impression of the mold and on the position of the injector with respect to said mold.

[0066] Tests performed in laboratory have verified that the wall flow speed of the melted plastic material depends not only on the orientation of the undulations but also increases as the temperature of the surface of the mold decreases. It is preferable to keep the nanostructured surface of the mold at the lowest possible temperature, to the extent allowed by the processability of the material.

[0067] In practice it has been found that the invention achieves the intended aim and objects, providing a mold and proposing a molding process by virtue of which it is possible to reduce the injection pressure in molding processes, facilitating the flow of the melted plastic material in said mold, at the same time keeping the temperature of the mold lower than the injection temperature of the melted plastic material, to the benefit of higher productivity.

[0068] It should also be noted that the reduction of the filling pressure facilitates the production of parts made of plastic material that are thinner than currently possible, with a consequent reduction of the consumption of materials and power and therefore also of production costs and environmental impact.

[0069] The invention thus conceived is susceptible of numerous modifications and variations, all of which are within the scope of the appended claims; all the details may furthermore be replaced with other technically equivalent elements.

[0070] In practice, the materials used, so long as they are compatible with the specific use, as well as the contingent shapes and dimensions, may be any according to the requirements and the state of the art.

[0071] The disclosures in Italian Patent Application No. 102018000001348 from which this application claims priority are incorporated herein by reference.