DE102008034153C5 - Method of manufacturing optical lenses - Google Patents

Abstract

A method for producing an optical lens by injecting plasticized plastic into a mold cavity, the production of the optical lens comprising at least three consecutive injection processes, the following steps being carried out: - injecting a first partial quantity of plastic, whereby a basic body is produced, - injecting a second partial quantity of plastic , as a result of which the base body is at least partially coated,- injecting a third partial quantity of plastic, as a result of which the base body coated with the second partial quantity of plastic is at least partially coated, the greatest occurring wall thickness (t2) of the second partial quantity of plastic applied to the basic body (2) being smaller than the greatest occurring wall thickness (t1) of the base body (2) and the greatest occurring wall thickness (t3) of the third plastic subset applied to the base body (2) is smaller than the greatest occurring wall thickness (t1) of the base body (2), characterized in that- the third partial quantity of plastic is injected into a mold cavity spaced apart from the mold cavity for the second partial quantity of plastic by the first partial quantity of plastic and- during the production of the base body (2) a convex-curved surface is produced and during the coating of the base body (2) and/or the with the second plastic subset at least partially coated base body (2) varying wall thicknesses are generated, so that starting from an edge area of the optical lens, the wall thickness increases up to a lens center.

Description

The present invention relates to a method of manufacturing an optical lens according to the preamble of claim 1.

It is known in the prior art to replace precision lenses made of glass with lens bodies that are produced in the course of an injection molding process. For reasons of cost, the optical lenses produced in this way are used, for example, as lenses or objectives of camera cell phones or digital cameras. From the DD 298 620 A5 An injection molding process for plastic molded parts has become known, in which a molded part is produced in a first process step, which differs from the final part in that certain functional surfaces are set back at a parallel distance. In a second injection molding cycle, this molded part is overmoulded with molding compound, resulting in the final shape of the molded part.

The EP 0 696 955 B1 describes a method for producing photochromic spectacle lenses by injection molding, with two or more injection processes being provided to form the lens body.

The EP 0 839 636 B1 also describes a method for producing optical lenses by injection molding, the lens body being produced in the course of three separate injection processes, so that a homogeneous lens is formed.

The EP 0 884 155 B1 describes a method for producing a multicolored lens system that is produced by repeatedly injecting plastic into the mold cavity.

Finally, in the JP 2001-191365 A describes the production of optical lenses by injection molding, with a first partial quantity of plastic being injected into a first mold cavity, a further partial quantity of plastic being injected into a second mold cavity and a further partial quantity of plastic being injected into a third mold cavity. Although this method already has significant advantages, it has been found that the production time of the entire lens is relatively long.

The DE 10 2007 011 338 A1 discloses a method and an apparatus for producing thick-walled plastic optical components by a multi-stage injection molding process.

WO 2008/150 988 A1 discloses a method and a device for producing thick-walled plastic components using an injection molding process.

The production of thick-walled optical lenses made of transparent plastics (PMMA, PC, COC, PS) in the injection molding or injection compression process is associated with a number of challenges today. In the field of light optics, contour deviations of 5 to 30 µm are tolerated today, and even less in the field of imaging optics. Differences in shrinkage between thick-walled and thin-walled areas have a negative effect on contour accuracy. A plano-convex lens thus tends to have a sink mark in the center in the area of its greatest wall thickness. According to the prior art, this can only be counteracted with very long holding pressure times in injection molding or long embossing times in injection compression molding. However, the duration of holding pressure is limited by the freezing of the sprue, in the case of embossing by the formation of a solidified surface layer. In injection molding, a very thick sprue is usually appropriate, which results in higher material consumption.

The cooling time in the injection molding process increases with the square of the wall thickness. The typical wall thickness for injection molding can be specified as around 2.5 mm. Optical lenses sometimes have wall thicknesses of 5 to 20 mm at the thickest points. Cycle times of several minutes are common.

The most important factors influencing the contour accuracy of optical components have been found to be the holding time (or embossing time) and the remaining cooling time. High quality often means low profitability.

The object of the present invention is to specify an improved injection molding process for the production of--in particular thick-walled--optical lenses, with the overall cycle time for the production of the optical element being able to be shortened.

According to the invention, this is achieved by the features of claim 1 .

In this case, it is provided that not only a single surface coating is applied to a lens base body produced in the course of an injection process to form a lens surface, but that the coating takes place in the course of several separate injection processes in several partial layers, so that the shrinkage is also relatively low.

The at least three consecutive injection processes are not necessarily to be understood as being immediately consecutive in terms of time. Rather, additional structure-changing process steps can be provided between the individual injection processes, such. B. an embossing process or the like. Also can wh Diffractive structures can be formed in or on the lens surface using the method according to the invention, for example in the manner of a Fresnel lens.

According to the present invention, it is proposed to produce a basic lens body in a first method step by injecting plastic into the mold cavity, with this remaining in the mold for the longest and also being cooled for the longest. This circumstance can be used to optimize the cycle time. With the method described, both the contour accuracy of the surface is improved and the cycle time is minimized. Furthermore, the risk of free jet formation is significantly reduced, since the side gating takes place in comparatively thin cavities. According to an advantageous development of the invention, it can also be expedient for the production of the optical lens to also include more than three successive injection processes.

Favorably, it can be provided that a method known from multi-component technology is used (rotary table, transfer, index plate, slide technology). Since an optical surface can be formed in the first layer, it can be expedient to leave it in the same cavity during the further process steps in order to avoid damage. For this reason, a turntable method can advantageously be used.

In a preferred embodiment of the method according to the invention, it is envisaged that at least between two of the at least three injection processes the injection process is delayed until the respectively injected plastic has at least partially cured.

In order to achieve favorable optical properties, it is provided according to the invention that the largest occurring wall thickness of the second amount of plastic applied to the lens body is smaller than the largest occurring wall thickness of the lens body. In this context, it is in accordance with the invention if the greatest wall thickness of the third plastic subset is smaller than the greatest wall thickness of the lens body. In other words, an optical lens is produced that has a plurality of layers applied to the lens base body and in a plurality of separate injection processes.

Although the same plastic can always be used in the method according to the invention in order to produce a thick-walled lens body, different types of plastic can also be injected instead. This means that compared to the first injected plastic, a different type of plastic can also be injected into the mold cavity at a later point in time. However, the plastic used should be selected in such a way that it is sufficiently transparent, depending on the intended use.

According to an exemplary embodiment of the invention, it can be provided that the mold cavity is arranged between a fixed and a movable mold part. When carrying out the method, it can be advantageous if the mold cavity is essentially completely filled with each injection process. In this context, it can also be advantageous if the mold cavity is enlarged before or during each injection process.

• 1 eine durch das Verfahren hergestellte plankonvexe Linse,
• 2 eine durch das Verfahren hergestellte Linse mit ausgefüllten Außenflächen,
• 3 eine durch das erfindungsgemäße Verfahren hergestellte Linse mit Außenflächen, die in getrennten Schritten gespritzt werden.

Further details and advantages of the present invention result from the following description of the figures. It shows:

• 1 a plano-convex lens made by the method,
• 2 a lens made by the method with filled outer surfaces,
• 3 discloses a lens made by the method of the present invention having exterior surfaces molded in separate steps.

1 shows in a schematic way a plano-convex lens 1 not according to the invention, which was produced from at least three layers in the rotary table method. In a first method step, a lens base body 2 with the convex surface is produced by injecting plastic into the mold cavity of the injection molding machine. The lens base body 2 has a wall thickness t1, which is the largest occurring wall thickness of the individual layers. Due to the difference in wall thickness between the center and the edge, there will be more shrinkage in the center, so care must be taken to ensure that sink marks appear on the side that will later be overmolded. This can e.g. This can be achieved, for example, by choosing a lower tool temperature on the convex side in order to obtain a stable, frozen surface layer more quickly than on the opposite side. The shrinkage will therefore have an effect more on the side with the weaker surface layer.

The lens base body 2 is rotated into the second station with a turntable and overmolded there with a layer 3a. The wall thickness t2 at the center (along the optical axis A) of this second layer 3a is made smaller than the wall thickness t1 of the lens body 2, the total cooling time of the second layer 3a being shorter and because the lens body 2 acts as an insulator. The sink marks on the overmolded boundary layer are compensated by the newly injected material of layer 3a. It is now rotated to the third station, where sink marks from the second layer 3a are compensated for and the planar lens surface (layer 3b) is formed. The wall thickness t3 of the layer 3b is in turn less than the wall thickness t2 of the second layer 3a.

2 1 shows a further embodiment of a lens 1 produced by a method not according to the invention, the two outer layers 3a and 3b of the lens 1 being injected first. In a further process step, plastic is injected between the outer surfaces 3a, 3b to form the lens body 2 and assembled to form a molded part. The thickness of the outer layers 3a, 3b is selected in such a way that they are sufficiently stable so that they are no longer deformed in the subsequent connection process.

3 shows an exemplary embodiment of a biconvex lens 1 according to the invention, the lens base body 2 being produced in a first method step by injecting plastic into the mold cavity. In subsequent method steps, the two optical layers 3a, 3b are injected in separate steps. In this context, it would also be possible in principle for a relatively imprecise lens base body 2 to be produced in a first method step, which is then converted into a further cavity. Following this, the lens base body 2 is overmoulded on the surfaces.

All variants described could be carried out in one or more stations with injection compression molding instead of injection molding. The injection of the individual layers can preferably take place with separate injection units, the use of a single unit for all stations is also conceivable with special sprue constructions. Of course, the optical surfaces 3a, 3b of the lens 1 can also be reworked, in particular ground or polished. In one embodiment variant, an embossing step can be provided between individual injection processes.

Claims (10)

Method for producing an optical lens by injecting plasticized plastic into a mold cavity, the production of the optical lens comprising at least three successive injection processes, the following steps being carried out: - injecting a first partial quantity of plastic, whereby a base body is produced, - injecting a second partial quantity of plastic , as a result of which the base body is at least partially coated, - injecting a third partial quantity of plastic, as a result of which the base body coated with the second partial quantity of plastic is at least partially coated, the largest occurring wall thickness (t2) of the second partial quantity of plastic applied to the basic body (2) being smaller than the greatest occurring wall thickness (t1) of the base body (2) and the greatest occurring wall thickness (t3) of the third plastic portion applied to the base body (2) is smaller than the greatest occurring wall thickness (t1) of the base body (2), characterized in that - the third partial quantity of plastic is injected into a mold cavity spaced apart from the mold cavity for the second partial quantity of plastic by the first partial quantity of plastic and - a convex-curved surface is produced during the production of the base body (2) and during the coating of the base body (2) and/or the with the second plastic subset at least partially coated base body (2) varying wall thicknesses are generated, so that starting from an edge area of the optical lens, the wall thickness increases up to a lens center. procedure after claim 1 , characterized in that the largest occurring wall thickness (t3) of the third plastic part quantity is smaller than the largest occurring wall thickness (t2) of the second plastic part quantity applied to the base body (2). procedure after claim 1 or 2 , characterized in that the injection process is delayed at least between two of the at least three injection processes until the respectively injected plastic is at least partially cured. Procedure according to one of Claims 1 until 3 , characterized in that the first, second and third plastic subset is formed from the same plastic. Procedure according to one of Claims 1 until 3 , characterized in that the first, second and third plastic subset is formed from a different plastic. Procedure according to one of Claims 1 until 5 , characterized in that a substantially transparent plastic is used for the first, second and third subset of plastic. Procedure according to one of Claims 1 until 6 , characterized in that the mold cavity is essentially completely filled during each injection process. Procedure according to one of Claims 1 until 7 , characterized in that the mold cavity is enlarged before or during each injection process. Procedure according to one of Claims 1 until 8th , characterized in that the shape and/or size of the mold cavity is changed with each injection process by a rotary table process. Procedure according to one of Claims 1 until 9 , characterized in that at least one embossing process is provided before a further injection process.

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