Method, mold insert and injection mold for producing a plastics molding

Abstract

Method for producing a plastic molded part (1), wherein, in the method, a mold insert (3) with a diffractive surface relief (32) is provided, the mold insert (3) is inserted into one mold half of an injection mold (5) which, together with at least one further mold half, forms a cavity for producing the plastic molded part (1), wherein the mold insert (3) is inserted into the injection mold (5) such that the diffractive surface relief (32) forms a partial area of the surface of the cavity formed by the mold half (5), and the plastic molded part (1) is molded by injection molding by means of the injection mold (5). The invention furthermore relates to a mold insert as well as an injection mold for such a method, as well as a plastic molded part produced in this way.

Claims

1. A method for producing a plastic molded part, wherein, in the method, a mold insert with a diffractive surface relief is provided, the mold insert is inserted into one mold half of an injection mold which, together with at least one further mold half, forms a cavity for producing the plastic molded part, wherein the mold insert is inserted into the injection mold such that the diffractive surface relief forms a partial area of the surface of the cavity formed by the mold half, and the plastic molded part is molded by injection molding by means of the injection mold, wherein, during the provision of the mold insert, first a provisional diffractive surface relief is formed in a surface of a substrate comprising a flat metal part, and then the substrate is reshaped to form the mold insert, wherein, during the reshaping, the provisional surface relief is deformed into the diffractive surface relief of the mold insert, wherein the substrate is reshaped by deep drawing.

2. A method according to claim 1, wherein the diffractive surface relief is formed by an additive or subtractive superimposition of a diffractive microstructure and a curved macrostructure.

3. A method according to claim 2, wherein the macrostructure describes a free-form surface which is curved, at least in areas, with one or more curvatures, wherein the one or more curvatures in each case have a radius of curvature which is at least 100 times and at most 0.1 times the lateral extent of the diffractive surface relief, and/or the one or more curvatures in each case have a radius of curvature in the range of from 10,000 mm to 10 mm.

4. A method according to claim 1, wherein for the deep drawing, a tool is used the hardness of which is less than a hardness of the material of the substrate.

5. A method according to claim 1, wherein for forming the provisional surface relief, first a master element, comprising a photoresist, into which the provisional surface relief is molded is provided and then the substrate is molded by the master element.

6. A method according to claim 1, wherein the provisional surface relief is molded, by lithography and/or by means of a laser or electron beam and/or by illumination by means of a mask, into a master element and/or into the substrate.

7. A method according to claim 1, wherein the provisional surface relief is molded into a master element and/or the substrate as a computer-generated hologram and/or kinoform and/or Fourier hologram.

8. A method according to claim 5, wherein after the molding of the provisional surface relief, the substrate is produced by galvanic deposition of a metal comprising nickel onto the master element.

9. A method according to claim 8, wherein from the produced substrate, further substrates are produced by galvanic deposition of a metal comprising nickel.

10. A method according to claim 1, wherein the substrate is produced in a thickness of from 0.05 mm to 1 mm.

11. A method according to claim 1, wherein for molding the provisional surface relief, a correction function (K) is ascertained, and applied to a function (F1) describing the diffractive effects to be achieved, in order to determine a function (F2) describing the provisional surface relief.

12. A method according to claim 11, wherein the correction function (K) describes or at least partially balances out the modification of the diffractive effects of the provisional surface relief due to the later deformation of the provisional surface relief by the reshaping.

13. A method according to claim 1, wherein the mold insert is secured in the injection mold by means of a clamping element.

14. A method according to claim 1, wherein the mold insert is held on an undercut of the injection mold.

15. A method according to claim 1, wherein the mold insert is adhered to the injection mold.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is explained by way of example below with reference to an embodiment example and the drawing. There are shown in:

(2) FIG. 1 An embodiment example of a mold insert, which is secured in an injection mold by means of a clamping;

(3) FIG. 2 An embodiment example of a mold insert, which is secured in an injection mold by means of an adhesive securing;

(4) FIG. 3 An embodiment example of a mold insert during the reshaping by deep drawing;

(5) FIG. 4 A schematic representation of a plastic molded part in operation;

(6) FIG. 5 A schematic representation of a die with a mold insert;

(7) FIG. 6 A schematic representation of the curvature geometry of an embodiment example of a mold insert.

DETAILED DESCRIPTION OF THE INVENTION

(8) In order to mold a diffractive surface relief 2 into a plastic molded part 1, a mold insert 3 is inserted into a receiver 4 of an injection mold 5. The mold insert 3 has, on its surface 31 facing a cavity 51 of the injection mold in the inserted state, the surface relief 32 to be molded. After sealing of the injection mold 5, the cavity 51 is filled with the molding compound, with the result that the surface relief 32 is molded in the plastic molded part 1.

(9) The surface relief 32 forms only a partial area of the surface 52 of the cavity 51. In other words a part of the surface 11 of the plastic molded part 1 remains smooth.

(10) The diffractive surface relief 2 can be a computer-generated hologram, a Fourier hologram or a kinoform. In transmitted light it is thus possible to produce a floating image of a motif such as a symbol, logo, sign, picture or the like. The relief preferably has a spatial frequency between 100 lines/mm and 3000 lines/mm and/or a relief depth of from 100 nm to 10 m.

(11) FIGS. 1 and 2 show different ways of securing the mold insert 3 in the injection mold 5. In the variant according to FIG. 1 the mold insert 3 is held by non-positive locking by a die 6 on an undercut 53 of a die sleeve 60. In the embodiment shown, between mold insert 3, a step or gap forms between the adjacent surface of the die sleeve 60 and the surface relief 32 on the mold insert 3. In the variant according to FIG. 2, the mold insert 3 is secured in the die sleeve 60 by means of an undercut, by positive locking and advantageously fitting flush with the adjacent surface of the die sleeve 60, and additionally adhered to the die 6. This securing by means of an undercut has the advantage that, unlike in the variant according to FIG. 1, no gap which results in a step on the plastic molded part 1 remains. The additional adhesion leads to an even better fixing of the mold insert 3 on the die 6. The die sleeve 60 receiving the die 6 is inserted in the receiver 4 of the injection mold 5 and secured there, preferably screwed.

(12) FIG. 5 shows the die 6 with the mold insert 3 secured thereon. The die 6 is formed such that it can be inserted, so that it fits into a correspondingly molded die sleeve 60. For this, on its underside the die 6 has a stop 62, in order that the die 6 sinks into the die sleeve 60 only up to a desired depth and the mold insert 3 on the opposite side thereby interacts with the adjacent surface of the die sleeve 60 in the desired manner. The die sleeve 60 is sealed, in particular screwed, with a cover 61 on the underside, in order to also secure the die 6 in the die sleeve 60. The die sleeve 60 is then inserted into the receiver 4 in the injection mold 5 and secured there in turn by means of screwing. The die sleeve 60 also has a stop 63, so as to sink into the receiver 4 of the injection mold 5 only up to a desired depth and, in the cavity 51, to interact with the adjacent surface of the cavity 51 as desired.

(13) The tolerance or clearance between receiver 4 and die sleeve 60 and between die sleeve 60 and die 6 is preferably approximately 0.01 mm to 0.05 mm, in particular 0.02 mm to 0.03 mm.

(14) For the production of the mold insert 3 a flat master is first produced, which comprises a layer of a photoresist. A provisional surface relief 7 is imprinted into this layer. This can be effected for example by means of a laser or electron beam or by illumination with a mask. This makes it possible to achieve a detail resolution of approximately one micrometer. Single or multiple exposures are possible, which can in particular result in two-, four- or eight-step surface profiles. The photoresist is then developed, with the result that the provisional surface relief is formed.

(15) The photoresist is then coated with a conductive varnish. In a galvanizing bath an electrical voltage is applied to the conductive varnish layer and a metal, preferably nickel, is deposited on the master. The layer thickness is 0.05 mm to 1 mm. The thus-produced metal body now also has the provisional surface relief. Copies of this metal body can in turn be prepared galvanically.

(16) In order to be able to provide plastic shaped bodies 1 molded as desired, with a diffractive surface structure 2, this metal body now has to be adapted to the shape of the plastic shaped body to be produced. As FIG. 3 shows, this is effected by deep drawing. Here the metal body is deformed between a die 8 and a counterholder 9, until it obtains the desired shape. The thus-obtained mold insert 3 can optionally be yet further cut and provided with securing elements. For die 8 and counterholder 9 a material is used which is softer than the metal body. If this consists of nickel, deep drawing tools made of steel, for example, are used. The provisional surface relief 7 is not damaged thereby.

(17) In the case of deep drawing, curved surfaces with radii of curvature which are at least 100 times and at most 0.1 times, preferably at least 10 times and at most 0.25 times, particularly preferably at least 5 times and at most 0.33 times the lateral extent of the diffractive surface relief can be produced. The radii of curvature can lie in the range of from 10,000 mm to 10 mm, preferably from 1000 mm to 25 mm, particularly preferably from 500 mm to 33 mm, in particular in the case of a lateral extent of 100 mm.

(18) For surface reliefs 2 with a diameter of 50 mm, for example, deep drawing of 4.8 mm can be achieved in the case of a thickness of the mold insert 3 of from 0.5 mm to 1 mm, or deep drawing of 2.4 mm can be achieved in the case of a thickness of the mold insert 3 of 0.5 mm. In the case of a diameter of the surface relief 2 of 100 mm, deep drawing of 10 mm can for example be achieved in the case of a thickness of the mold insert 3 of 0.5 mm.

(19) The relationship between the height h of the peak of the resulting surface above a notional base plane of the mold insert 3 and the radius of curvature r is represented in FIG. 6 for a simple spherical reshaping geometry. In the case of a lateral extent s of the curved area there results a radius of curvature of

(20) $r=\frac{4h^2+s^2}{8h}.$

(21) Thus, in the case of a diameter of the surface relief of 100 mm and a drawing depth h of 11 mm a radius of curvature of approx. 119 mm results, in the case of a drawing depth of 33 mm a radius of curvature of 54 mm results. However, in the case of more complex free-form surfaces this simple relationship does not necessarily apply.

(22) The provisional surface relief 7 is deformed by the deep drawing and thus modifies its optical properties. This must be taken into account in the design of the provisional surface relief 7. For this reason, a provisional surface relief 7 has to be produced, which results in the desired diffractive surface relief 2 after the deformation.

(23) In order to achieve this, a function F1 is first ascertained, which expresses the desired diffractive effect. Such functions can for example be calculated and represented as follows:

(24) The calculation method for the CGH (CGH=Computer-Generated Hologram) is based on the point light source principle, wherein the element to be projected (e.g. star) is broken down into self-luminous point light sources, and a hologram is then calculated for each of these point light sources. The entire hologram is then calculated from the superimposition of all the individual holograms. The thus-resulting total phase function is converted to a diffractive phase function for the design wavelength (e.g. red LED, wavelength 640 nm). Depending on the resolution of the method of lithography used for producing the diffractive surface relief based on the resulting diffractive phase function, for example laser-beam lithography with a resulting minimum feature size of approx. 1 m, the diffractive surface relief is then approximately imprinted into a photoresist system. The minimum feature size here determines the smallest possible structure sizes and thus the largest possible diffractive diffraction angles or largest possible additional focus functions.

(25) Further, a correction function K is ascertained, which characterizes the modification of the provisional surface relief during the deformation or the optical effect brought about by the deformation.

(26) In the case of a simple spherical curvature geometry of the mold insert 3, for example a correction function K, in the form of a correspondingly conversely molded spherical curvature geometry, results.

(27) The correction function K is applied in the calculation of the diffractive total phase function by means of CGH such that a correction phase function formed conversely to the curvature geometry of the mold insert 3 is taken into account.

(28) The correction is particularly simple if the symmetry of the curvature of the mold insert 3 is matched to the symmetry of the motif to be represented by the diffractive surface relief 2. For example, a rotationally symmetrical motif and a correspondingly similar rotationally symmetrical curvature of the mold insert 3 can be superimposed such that the two centers of symmetry lie one above the other. A specific example is a curvature in the form of a spherical sector and a motif in the form of a star. The center of the spherical sector and the center of the star preferably lie one above the other. Distortions of the star due to the additional curvature thereby remain optically largely unproblematic and influence or impair the optical effect only very slightly. In other specific cases it is possible, by analyzing the symmetry of the curvature and of the motif, to find similar solutions in which the curvature only slightly interferes with the desired optical effect.

(29) Critical variables in the correction of the distortion are in particular the radius of curvature R of the diffractive surface relief 2, the lateral extent Ld of the diffractive surface relief 2, the lateral extent Li of the intensity pattern projected in transmission and the divergence angle of the light source. For practically relevant cases it can be assumed that Ld approximately corresponds to Li. Within the range Ld/R<=2 a correction is possible without difficulty.

(30) By applying the correction function K to the function F1 it is possible to calculate a further function F2 which represents the provisional surface relief 7. If the thus-calculated provisional surface relief 7 is introduced into the metal body, a diffractive surface relief 2 which shows the desired diffractive effect is obtained in the finished mold insert 3 and thus also in the plastic molded part 1.

(31) In the calculation of the surface reliefs 2 and 7 yet further parameters which are important for the desired intended use of the plastic molded part 1 can furthermore be taken into account. If the plastic molded part is to be attached, for example, to an illumination device, with the result that an optically floating symbol is produced in transmitted light, observation distance, observation angle, distance from the light source, divergence angle of the light source and any remaining focusing optics can also be taken into account here. A focusing function of its own can also be computationally incorporated into the diffractive surface relief 7. Particularly good results are achieved by a combination of an incorporated focusing function with a partial focusing by external optics. The divergence angle (half angle) of the light source should lie within the range of from 5 to 60.

(32) After the mold insert 3 has been produced in the described manner and inserted into the injection mold, the plastic molded part 1 can be produced by injection molding. If the diffractive effect is to be visible in transmitted light, transparent plastics, such as for example PMMA (transmittance 92%), PET-G (transmittance 91%), ABS (transmittance 85%) or PC (transmittance 88%), must be used. However, it is also possible to introduce surface structures which can be observed in reflected light and which can be used with opaque or semitransparent plastics or in the insert-molding method. It is to be borne in mind here that the structure depths of the diffractive surface relief must be many times greater for elements in transmission than for elements that work in reflection. For example structure depths of the diffractive surface relief for elements in transmission are approximately 0.5 m to 3 m and the structure depths of the diffractive surface relief for elements in reflection are approximately 0.1 m to 0.5 m. In addition, such opaque plastics can advantageously contain a reflection-enhancing layer in order to increase the visibility and brilliance of the optical effect of the diffractive surface relief.

(33) Pressures of 800 bar to 2000 bar and temperatures between 220 C. and 320 C. usually occur during injection molding. The described mold insert 3 withstands such conditions without difficulty. At the most it needs to be changed after approximately 10,000 to 50,000 molding operations, in particular within the framework of a usual maintenance of the injection molding tools which is also otherwise necessary.

(34) The use of the mold insert 3 does not have a disadvantageous influence on the cycle time of the molding operation. This depends predominantly on the geometry and size, as well as on the wall thickness of the plastic molded part 1, because the cooling time of the injection molding material is determined by these sizes, in particular above all by the wall thickness. The cycle time of the molding operation is usually between 5 seconds and 180 seconds, in particular between 10 seconds and 180 seconds.

(35) FIG. 4 shows a plastic molded part 1 according to the invention in operating position, i.e. with a light source 7, here an LED (LED=Light Emitting Diode) with an optical focusing element, here a double-convex converging lens 8. The diffractive surface relief 2 is thereby transilluminated by means of light which, by means of the focusing element, has the desired divergence. Light source 7 and diffractive surface relief 3 are arranged at a distance L. The distance L is preferably approximately 0.5 cm to approximately 10 cm. Due to the light transmitted through the diffractive surface relief 2 an optical effect 73 is produced which is recognizable at the observation distance A with the naked human eye, i.e. without further aids. The observation distance A is preferably approximately 20 cm to 500 cm. However, the effect 73 is also visible outside this range.

(36) In order to produce the specifically shown effect 73, a five-pointed star, a quasi-continuous, diffractive phase function which results in diffractive relief structures with the smallest local grating periods of from approx. 5 m to 10 m and structure depths of from approx. 1 m to 1.5 m is calculated as a computer-generated hologram. As already described, the diffractive surface relief 2 includes the remaining correction, of the beam path of the light, necessary for the desired divergence of the light or the desired optical effect, wherein this correction is adapted to the course of the macrostructure (in this example the component curvature is a spherical deformation).

(37) Thus the lighting device 7 shown in FIG. 4 results. A red LED with a divergence angle (half angle) of 45 degrees is used as light source 71 for the illumination or backlighting of the transparent plastic material of the plastic shaped body 1. For the production of approximately collimated light or for reducing the divergence angle of the LED light, with the result that the light is adapted to the course of the macrostructure, the additional focusing optics 72 are formed as a convex lens with a focal length of 40 mm and a diameter of 20 mm. The distance between the LED 71 and the focusing optics 72 is 10 mm, the distance between the focusing optics 72 and the diffractive surface relief 2 is approx. 40 mm.

(38) In the area of the diffractive surface relief 2 the surface of the plastic molded part 1 forms a spherical shell with a 120 mm radius of curvature. The diffractive surface relief 3 is circular with a diameter of approximately 35 mm. An optical effect 73 results which is in the form of a star with a size of approx. 30 mm30 mm and which, viewed by the observer, appears to lie approx. 5 cm to 10 cm behind the diffractive surface structure 2. The ideal observer distance is approx. 4 m to 5 m in front of the diffractive surface structure 2; the observer can be positioned slightly offset relative to the optical axis, e.g. slightly raised or also lower.

(39) Various plastic molded parts 1 can be produced in the described manner. As the diffractive surface relief 2 can be arranged on any free-form surface, the design freedom is not impaired during design of the plastic molded part. In particular, diffractive effects can thus be incorporated into already existing designs without further modifications becoming necessary.

(40) As the diffractive surface relief 2 occupies only a partial area of the surface 11 of the plastic molded part 1, still further optical or other functions can also be incorporated into the plastic molded part.

(41) Possible applications are for example lighting devices which project optically floating motifs (appearing to lie in front of or behind the plane of the component), back-lit switches or control elements the function of which is displayed by an optically floating symbol, and the like.