Tool for injection molding a Fresnel stepped lens

Abstract

Tool (100) for injection molding a Fresnel stepped lens, wherein a mold surface (10) of the tool (100) has annular and / or annular segment-shaped steps (11) in a concentric arrangement, wherein the tool (100) comprises at least one die (2) with a casting surface (12) and an ejector element (3) received in a bore of the die (2) with a casting surface (13), wherein the mold surface (10) is formed by the casting surfaces (12, 13) of the die (2) and the ejector element (3), characterized in that the casting surface (13) of the ejector element (3) has a circular outline and forms a centrally arranged segment of the mold surface (10), wherein a circumferential gap (S) between the ejector element (3) and the die (2) separates two steps (11) of the mold surface (10) from each other.

Description

[0001] The present invention relates to a tool for injection molding a Fresnel stepped lens. STATE OF THE ART

[0002] Fresnel lenses represent a volume-reduced type of optical lens and feature ring-shaped or at least ring-segment-shaped steps, the entirety of which forms a section-wise approximation of a conventional lens of the same focal length. Fresnel lenses are used as converging lenses, for example, in automotive lighting systems.

[0003] Fresnel lenses are typically manufactured by injection molding from an optical plastic. One challenge lies in ejecting the lenses from the injection mold without compromising their optical functionality. The ejector elements commonly used can cause lens distortion or leave indentations on the optical lens surfaces.

[0004] For the state of the art, reference is also made to publications US 2013 / 0 120 707 A1 and US 5 840 352 A. REVELATION OF THE INVENTION

[0005] The object of the present invention is to propose a tool for injection molding a Fresnel stepped lens, which in particular enables gentle ejection of the cast lenses.

[0006] This problem is solved starting from a tool according to claim 1. Advantageous embodiments of the invention are specified in the dependent claims.

[0007] The technical teaching of the invention discloses that the mold surface of the tool has annular and / or annular segment-shaped steps in a concentric arrangement, wherein the tool comprises at least one die with a casting surface and an ejector element received in a bore of the die with a casting surface, wherein the mold surface of the tool is formed by the casting surfaces of the die and the ejector element, wherein the casting surface of the ejector element has a circular outline and forms a centrally arranged segment of the mold surface, wherein a circumferential gap between the ejector element and the die separates two steps of the mold surface from each other.

[0008] The invention is based on the idea of ​​integrating the ejector element into the die in such a way that its cavity-side surface forms a segment of the mold surface, i.e., during injection molding, it represents an impression surface for the injected plastic. The segment is selected such that the necessary gap between the ejector element and the die does not cause any casting artifact that significantly compromises the optical functionality of the manufactured lens. Since the casting surface of the ejector element forms a segment of the mold surface, and is therefore in contact with the injected plastic melt and the resulting lens, the ejection process does not create a new contact surface that could lead to an undesirable indentation in the lens. Due to the centric arrangement of the ejector element, the lens is subjected to uniform stress during ejection, thus minimizing the risk of induced distortion.The gap around the ejector element runs along the boundary line between two adjacent steps of the mold surface, corresponding to a section of the associated lens that does not perform a dedicated optical function. Therefore, the formation of a burr on the manufactured lens in the area of ​​the gap is not critical with regard to its optical functionality.

[0009] In particular, the steps in the mold surface of the tool each feature a step surface for molding a lens surface and a step flank with a draft angle, with the gap between the ejector element and the die replacing one step flank. The step surfaces correspond to the annular surface segments of the manufactured stepped lens, the entirety of which approximates the optical effect of a conventional lens of the same focal length. The step flanks formed between the step surfaces occupy only a very small proportion of the mold surface, and in the manufactured stepped lens, the corresponding surface sections contribute practically nothing to beam shaping. Advantageously, the gap between the ejector element and the die therefore runs in the transition region between two adjacent step surfaces.On both sides of the gap, the adjacent stepped surfaces are designed in such a way that the desired stepped lens segments can be molded true to function. During injection molding, molten plastic is forced into the gap to a certain extent, so that a ring-shaped burr is formed on the manufactured lens. However, this burr does not contribute to the optical functionality of the lens in any practical way and is therefore not considered problematic.

[0010] In particular, the ejector element is displaceable along an ejector axis relative to the die, the ejector axis intersecting the mold surface orthogonally at its centroid. The ejector axis thus corresponds, in particular, to the optical axis of a lens molded in the tool. In a casting position, the ejector element is fully enclosed in the die bore, and the cavity-side surface forms the casting surface as a segment of the mold surface. Starting from the casting position, the ejector element is displaceable along the ejector axis in the direction of the mold cavity to eject a manufactured lens from the tool. Preferably, the ejector element has a cylindrical shape, the axis of which forms the ejector axis.

[0011] A clearance fit is preferably present between the ejector element and the die, allowing the ejector element to move with minimal friction and without jerking. For example, the gap between the ejector element and the die has a width in the range of 1 micrometer to 20 micrometers.

[0012] In an exemplary embodiment of the tool according to the invention, the mold surface for molding a Fresnel stepped lens has a diameter in the range of 20 millimeters to 100 millimeters, and / or the segment of the mold surface formed by the casting surface of the ejector element has a diameter in the range of 5 millimeters to 10 millimeters. PREFERRED EXAMPLE OF THE INVENTION

[0013] Further measures improving the invention are illustrated below together with a description of a preferred embodiment of the invention with reference to the figures. The figures show, in schematic form: Fig. 1: a top view of one half of a tool according to the invention, and Fig. 2: a cross-sectional view of Fig. 1.

[0014] Fig. Figure 1 shows a top view of one half of a tool 100 according to the invention, which together with an associated nozzle-side tool half (not shown) is designed for injection molding a Fresnel stepped lens. Fig. Figure 2 shows a corresponding cross-sectional view along section line II.

[0015] The mold surface 10, designed for molding a stepped lens, has concentric annular steps 11, wherein the mold surface 10 is formed by the casting surfaces 12, 13 of the die 2 and the ejector element 3 received in a bore of the die 2. According to the invention, the casting surface 13 of the ejector element 3 has a circular outline and forms a centrally arranged segment of the mold surface 10, wherein the circumferential gap S between the ejector element 3 and the die 2 separates two steps 11 of the mold surface 10 from each other.

[0016] The steps 11 of the mold surface each feature a step surface 11a for molding a lens surface and a step flank 11b with a draft angle. The gap S between the ejector element 3 and the die 2 replaces a step flank 11b. The gap S is preferably dimensioned such that a clearance fit exists between the ejector element 3 and the die 2. However, the width of the gap S should be as small as possible to minimize the ingress of molten plastic during injection molding. The step surfaces 11a, which slope upwards from the inside to the outside, correspond to the lens surface of a stepped lens molded on them, and the formation of a burr in the area of ​​the gap S is considered non-critical for the optical functionality of the manufactured lens.

[0017] The ejector element 3 has a cylindrical shape, the axis of which forms the ejector axis Z. The ejector axis Z intersects the mold surface 10 orthogonally at its centroid, i.e., at the concentric center of the steps 11. The ejector axis Z thus also corresponds to the optical axis of a stepped lens molded in the tool 100. Starting from the casting position shown in the figures, the ejector element 3 can be displaced along the ejector axis Z relative to the die 2. Due to the position of the ejector axis Z, the manufactured lens is subjected to a uniform circumferential load during ejection, thus minimizing the risk of distortion.

[0018] The invention is not limited in its implementation to the embodiment described above. Rather, a number of variants are conceivable, which utilize the solution presented even in fundamentally different designs. All features and / or advantages arising from the claims, the description, or the drawings, including design details and spatial arrangements, can be essential to the invention, both individually and in various combinations. Reference symbol list 100 tools 10 Mold surface Level 11 11a Step area 11b Step flank 12 Casting surface of the die 13 Casting surface of the ejector element 2 die 3 Ejector element S gap Z ejector axis II Intersection line

Claims

[1] Tool (100) for injection molding a Fresnel stepped lens, wherein a mold surface (10) of the tool (100) has annular and / or annular segment-shaped steps (11) in a concentric arrangement, wherein the tool (100) comprises at least one die (2) with a casting surface (12) and an ejector element (3) received in a bore of the die (2) with a casting surface (13), wherein the mold surface (10) is formed by the casting surfaces (12, 13) of the die (2) and the ejector element (3), characterized by , that the casting surface (13) of the ejector element (3) has a circular outline and forms a centrally arranged segment of the mold surface (10), wherein a circumferential gap (S) between the ejector element (3) and the die (2) separates two steps (11) of the mold surface (10) from each other. [2] Tool (100) according to claim 1, characterized by, that the steps (11) each have a step surface (11a) for molding a photometric lens surface and a step flank (11b) with a draft angle, wherein the gap (S) between the ejector element (3) and the die (2) replaces a step flank (11b). [3] Tool (100) according to claim 1 or 2, characterized by , that the ejector element (3) is displaceable along an ejector axis (Z) relative to the die (2), wherein the ejector axis (Z) intersects the mold surface (10) orthogonally at its centroid. [4] Tool (100) according to claim 3, characterized by , that the ejector element (3) has a cylindrical shape, the cylinder axis of which forms the ejector axis (Z). [5] Tool (100) according to any of the preceding claims, characterized by , that there is a clearance fit between the ejector element (3) and the die (2). [6] Tool (100) according to any of the preceding claims, characterized by, that the gap (S) between the ejector element (3) and the die (2) has a width in the range of 1 micrometer to 20 micrometers. [7] Tool (100) according to any of the preceding claims, characterized by , that the mold surface (10) has a diameter in the range of 20 millimeters to 100 millimeters, and / or the segment of the mold surface (10) formed by the casting surface (13) of the ejector element (3) has a diameter in the range of 5 millimeters to 10 millimeters.

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