PRODUCTION OF PLASTIC COMPONENTS WITH REDUCED MICRODEFECTS

Description

[0001] The invention relates to a device for controlling or regulating the production of plastic parts, in particular optical elements, in a plastic injection molding process or plastic injection compression process, a method for producing plastic parts, in particular optical elements, by plastic injection molding or plastic injection compression, a computer program and a data carrier signal.

[0002] The production of plastic parts, and especially optical elements, using a plastic injection molding or plastic injection compression molding process has been known for a long time.

[0003] Since the advent of smartphones and tablets, the performance of their camera modules has played a crucial role in sales. Images are expected to be increasingly sharp, lighting conditions better balanced, and color quality even more brilliant. Given that individual camera modules consist of numerous lenses, the importance of lens quality becomes even more apparent.

[0004] The importance of the injection molding process is already evident in the currently dominant choice of lens materials. High-performance transparent plastics offer essential advantages over other potential materials, such as glass. Key benefits include lower energy consumption and thus lower manufacturing costs, low weight, and the ability to reproduce complex structures or finely structured areas.

[0005] Lenses, however, offer two types of potential for defects. On the one hand, defects such as streaks can occur on the lens surface, while on the other hand, the lens volume itself also provides space for voids or smaller defects. While voids can be eliminated through appropriate process control in currently feasible standard processes, streaks and smaller defects often pose problems for the user.

[0006] In its experiments, the applicant identified another problem that is currently not widely recognized in the industry: Injection-molded or injection-pressed plastic parts are riddled with numerous micro-defects. Detecting these so-called micro-defects is challenging because they cannot be visualized using conventional light microscopy.

[0007] For example, experiments conducted by the applicant have shown that smartphone lenses which appear flawless under a light microscope can have over 1000 micro-defects per lens.

[0008] It is known from the prior art to compress the melt after metering and before injection to stabilize the shot weight (see, for example, US 2013 0095199 A1 and JP 3529771 B2).

[0009] In the paper "Michler, GH and Schmeling, H.-HK-B. von: "The physics and micromechanics of nano-voids and nano-particles in polymer combinations", Polymer 54.13 (2013), pp. 3131-3144", it was shown that the main cause of micro-defects is the distribution of undissolved larger air inclusions on the one hand and the agglomeration of even smaller air inclusions on the other.

[0010] It is an object of the invention to provide a device and a method that enables the production of plastic parts, in particular optical elements, with higher quality. A further object is to provide a computer program for carrying out the method and a data carrier signal.

[0011] This problem is solved by a device having the features of claim 1, an injection molding machine or injection press having the features of claim 7, a method having the features of claim 10, a computer program having the features of claim 15 and a data carrier signal having the features of claim 16.

[0012] A device for producing a predetermined number of plastic parts, in particular optical elements, per injection cycle of an injection molding machine or injection press, has at least the following features: at least one signal output via which commands can be transmitted by at least one processing unit to at least one actuator of an injection molding machine or injection molding press, causing the at least one actuator to perform compression of a melt provided in an injection cylinder of the injection molding machine or injection molding press; at least one processing unit configured to transmit commands via the at least one signal output to at least one actuator of an injection molding machine or injection molding press, causing the at least one actuator to specify, with respect to compression of a melt provided in an injection cylinder of the injection molding machine or injection molding press, the magnitude of each compression and / or the number of compressions performed and / or the duration of each compression. to carry out in such a way as to result in a reduction of micro-defects.

[0013] A method for producing a predetermined number of plastic parts, in particular optical elements, by plastic injection molding or plastic compression molding, comprises at least the following steps: Providing a melt in the injection cylinder of an injection molding machine or injection press in a quantity determined by the number of plastic parts to be produced; applying compression to the metered melt in the injection cylinder to a degree and / or a certain number and / or duration such that a reduction in micro-defects is achieved in the produced plastic parts; injecting the metered melt into at least one mold to produce the specified number of plastic parts

[0014] A microvoid is a defect with a size on the order of micrometers (µm range). Typical microvoids, for example, have a size of approximately 10–20 µm.

[0015] The reduction of micro-defects can refer to: a reduction in the volume fraction of micro-defects in the plastic part and / or a reduction in the number of micro-defects in the plastic part and / or a reduction in the size of the micro-defects in the plastic part

[0016] The verification of whether the manufactured plastic parts show a reduction in micro-defects, in particular micro-defects below a specified limit value, can be carried out in a manner known per se by means of statistical investigations of test parts.

[0017] Of course, the numerical definition of a limit value can also encompass all better (in the sense of stricter) limits, i.e., for example, a limit value in the form of a reduction of micro defects to 50% also includes all use cases where a reduction of more than 50%, for example to 30%, results.

[0018] The specified limit value can, in principle, be chosen arbitrarily.

[0019] However, the limit value can also be chosen depending on the type and / or nature and / or size and / or the intended use of the plastic parts to be manufactured.

[0020] Alternatively, the limit value can be chosen in relation to a previously used manufacturing process for a specific plastic part, e.g., the statement: "half as many micro-defects as before".

[0021] Regarding the unit of the limit, for example, it is possible to specify the following: Volume fraction of micro-defects in the total volume of the plastic part (expressed in vol%) and / or number of micro-defects per unit volume of the plastic part (expressed in 1 / mm³< ) mean size of micro-defects in the plastic part (expressed in mm³< or mm) Table 1 with examples: Criterion: Plastic part 1: Plastic part 2: Art lens gear Nature PMMA PA Size Diameter 3 mm Diameter 35 mm Purpose of use Smartphone lens in combination with a predefined evaluation electronics Use in the gearbox of a model helicopter limit 0.1 vol% 0.5 vol%

[0022] Depending on the type of plastic part, the determination of micro-defects may not be relevant for the entire part. For example, with regard to optical elements, it may only be relevant in optically active areas, or with regard to mechanically stressed plastic parts, only in areas of highest mechanical stress. In some plastic parts, however, the determination of micro-defects may be relevant for the entire part.

[0023] Of course, it is not always necessary to measure the entire plastic part; examining a section and then statistically extrapolating the results may suffice.

[0024] Until now, the dosing process using a plasticizing cylinder always consisted of three steps: an (optional) compression release before dosing, the actual plasticizing process (applying back pressure during dosing), and an (also optional) compression release after dosing. These steps often do not offer sufficient flexibility to eliminate gases present in the melt.

[0025] According to Henry's Law, there is a direct relationship between the pressure under which a melt is subjected and the amount of gas that can be dissolved in it. The invention provides for the re-dissolving of any gas present by pressurizing the melt before injection into the injection mold or compression mold, thereby preventing micro-defects.

[0026] Advantageous embodiments of the invention are defined in the dependent claims. Unless explicitly stated otherwise, the following information refers to embodiments of both the device and the method.

[0027] Plastic parts whose production according to the invention is particularly advantageous include, besides optical elements, especially mechanically stressed components. Particularly under cyclic loading, cracks often originate from micro-defects in the material. Reducing these micro-defects leads to improved fatigue strength.

[0028] Examples of optical elements include lenses (especially those suitable for smartphones and tablets, and thick-walled lenses in the automotive sector) and light guides.

[0029] Preferred transparent plastics are polycarbonate (PC), cycloolefin copolymers (COC), liquid silicone rubber (LSR), polystyrene (PS) and polymethyl methacrylate (PMMA).

[0030] Preferred non-transparent plastics are polyamide (PA), polyoxymethylene (POM) and polypropylene (PP).

[0031] To optimally utilize the invention, it is advantageous to select the number, size, and duration of the compression applications depending on the plastic part. However, it has been shown that a selection within the range of the following specifications is sufficient to achieve a reduction of micro-defects of at least 50%: a number of compression applications, of two or three compression applications of the same melt in one and the same injection cycle, a height of 100%, preferably 150%, particularly preferably 200%, measured against the height of the back pressure during metering (the height can be selected differently for different compression applications), a total duration of all compression applications together in a range of approximately 2.5 to 3.5 seconds, preferably 2.9 to 3.1 seconds, preferably a total duration of 3 seconds

[0032] There is, of course, some possibility of compensation insofar as, for example, a shorter duration of compression can be compensated for by increasing the number and / or intensity of the compressions. This applies to each of the three parameters.

[0033] Preferably, the specified limit value is a certain percentage, preferably 50% or 30%, of the volume fraction of the originally present micro-defects (i.e., that the volume fraction of the micro-defects is below 50% of the initial value, where this initial value corresponds to the value that results in the plastic component without the at least one compression application), (example for limit value = 50%: originally 0.6 vol% -> limit value = 0.3 vol%; example for limit value = 30%: originally 0.6 vol%, limit value = 0.18 vol%) and the at least one control device is configured to transmit commands via the at least one signal output to the at least one actuator, causing it to perform the at least one compression application such that micro-defects are below this value, preferably below 0.3 vol%, particularly preferably below 0.18 vol%.

[0034] Preferably, the device has at least one signal input (especially if the device is intended not only to control but also to regulate), via which signals from at least one sensor of an injection molding machine or injection press can be transmitted to the at least one computing unit.

[0035] Preferably, the device has at least one operating unit configured to display information provided by the at least one computing unit to an operator by means of an operating program and to transmit operator inputs to the at least one computing unit, wherein it is preferably provided that the operating program is configured to accept operator inputs relating to a size of each compression application and / or the number of compression applications performed and / or the duration of each compression application to accept and transmit to at least one computing unit which, depending on these inputs, transmits commands to at least one actuator of an injection molding machine or injection press.

[0036] This embodiment provides the operator with a multitude of new modules for expanding the plasticizing process. The computer program offers additional modules with which the machine sequence can be customized. The core element here is a freely configurable plasticizing sequence that can be programmed into the sequence as an add-on. This sequence can include process steps such as metering, screw reversal to close a positively closing Smartshut gate, or compression relief. At least two options for applying back pressure are also possible, enabling the compression of the melt after metering as provided for in the invention. This pressure application is, in turn, freely adjustable with regard to magnitude, duration, and timing.

[0037] Preferably, the injection cylinder of the injection molding machine or injection press is a plasticizing cylinder in which a plasticizing screw is mounted as an actuator in a known manner, allowing both translational displacement and rotation, so that both plasticizing and metering can take place in the injection cylinder. Alternatively, it would also be conceivable to carry out the plasticizing and / or metering in a separate device, for example in an extruder, and to introduce the prepared melt, optionally after intermediate storage in a buffer, into the injection cylinder, which in this case is preferably designed as a piston-cylinder unit with an injection piston that is translationally movable and rotatably stationary as the actuator.

[0038] In addition to generating back pressure by shifting the plasticizing screw ("back pressure via screw position"), pressure can also be generated by rotating the plasticizing screw ("back pressure via screw rotation"). While the back pressure variant via screw position involves an axial stroke, the screw preferably remains axially stationary in the variant via screw rotation.

[0039] Preferred detection method for micro-defects in at least partially transparent plastics:

[0040] For plastic parts that are at least partially transparent, especially optical lenses, it is advisable to perform quality control using light microscopy. Many defects that can occur during the injection molding process are easily detectable this way, such as sink marks, surface defects, and black spots. The focus here is on the actual lens body (diameter approx. 3 mm) or, depending on the application, other transparent and optically active areas, as defects in the technical peripheral areas (assembly function) are less relevant.

[0041] Due to market developments towards increasingly higher-resolution sensors with greater light sensitivity and camera modules used for very large digital zoom ranges, the size of acceptable defects is decreasing to areas that are no longer visible to the human eye, even under a microscope. To make these defects visible, special measurement methods are necessary, with optical coherence tomography (OCT) being a preferred option due to its performance in terms of resolution and measurement time. An alternative measurement method is, for example, X-ray CT.

[0042] Optical coherence tomography (OCT) is used to create high-resolution 2D or 3D images in organic and inorganic materials. In principle, the method can be considered the optical equivalent of ultrasound measurements. By utilizing white-light interferometry, it uses interference from a reference beam and backscattered light from the sample to create a depth profile. A light source shines through a beam splitter, forming a reference beam and a beam directed to the sample. Interference is detected after spectral decomposition using a grating at the detector. Depending on the light source used, depth resolutions of 1 µm and lateral resolutions of 0.5 µm are possible.

[0043] To count microdefects, a volume scan of the optical element is divided into sections, and the number of microdefects is determined for each section. However, OCT analyses present the challenge of speckles, which arise from unavoidable internal scattering of the radiation within the sample. These appear as white spots in the acquired images, which are difficult to distinguish from the targeted microdefects. However, an evaluation routine makes it possible to reliably differentiate between speckles and actual microdefects, thus enabling the measurement of the actual microdefects. This evaluation routine takes advantage of the fact that speckles and microdefects differ in their transition to the surrounding area in the imaging: while speckles are sharply defined, microdefects appear with a gradual gradient.Differentiation is easily possible, for example by using edge detection.

[0044] Preferred detection method for micro-defects in non-transparent plastics:

[0045] The same measures and algorithms can be used as for transparent plastics, but based on an X-ray CT procedure (X-ray computed tomography procedure).

[0046] Exemplary embodiments of the invention are discussed with reference to the figures. Figure 1 shows an example of an injection molding machine according to an embodiment of the invention. Figure 2A and 2B Each schematically shows a progression of the pressure application to a melt in exemplary embodiments of the invention. Figure 3 schematically shows a process of pressurization of a melt in a further embodiment of the invention. Figures 4A and 4Bshow a reduction of micro-defects in one embodiment of the invention. Figure 5 shows one way of differentiating between speckles and micro-defects. Figure 6 schematically shows selection options through a computer program according to an embodiment of the invention.

[0047] In Figure 1Figure 1 depicts a known injection molding machine, which differs from the prior art only in that it includes a device 1, which can, for example, be in the form of a machine control unit for the injection molding machine. The device 1 comprises a processing unit 2, a signal output 3, and a signal input 7. Signal output 3 and signal input 7 are connected, as shown only schematically, to the actuator 4 (specifically, of course, to motors that drive the actuator 4) and serve to determine the position and / or speed of the actuator 4. Naturally, further signal outputs 3 to additional actuators (not shown) and further signal inputs 7 to sensors (e.g., for pressure measurement) (not shown) can be provided.

[0048] In the illustrated embodiment, a rotatable and displaceable plasticizing screw arranged in an injection cylinder is provided as actuator 4.

[0049] Figure 2AFigure 1 schematically shows the pressure application process of a melt (referred to as the first back pressure control) in an embodiment of the invention, wherein the compression is applied via the position of an actuator 4. The screw speed and the screw position of the actuator 4 are also shown. After a metering process, during which a back pressure of 100% is present, an optional delay phase occurs. Subsequently, a first and only compression application (referred to as the second back pressure control) is carried out to reduce micro-defects.

[0050] In Figure 2B For example, instead of a single compression application to reduce micro defects, two compression applications (referred to as second and third dynamic pressure control) are provided.

[0051] Figure 3Figure 1 schematically shows a process of pressurizing a melt in an embodiment of the invention, wherein the compression is effected by the rotation of an actuator 4 in the form of a plasticizing screw. Figure 3 corresponds to Figure 2A However, here the first and only compression is generated by rotating the plasticizing screw (without axial movement of the screw). Of course, it could also be done as in Figure 2B The procedure will be shown.

[0052] In Figure 4A A section of a lens is shown, revealing a multitude of micro-defects 5 (speckles 6 have already been removed). By considering different section planes 8, the volume fraction of the micro-defects 5 can be automatically determined. In comparison (namely to the Figure 4A ) is in the Figure 4B, which shows the result of a method according to the invention, a significantly reduced number of micro-defects 5 are recognizable.

[0053] Figure 5 shows that a speckle 6 has a sharp boundary to its surroundings, while a micro defect 5 has a gradient (the pixel grid is only shown in the vicinity of the speckle 6 and the micro defect 5).

[0054] Figure 6 shows an extension of the design of a method for manufacturing plastic parts made possible by an embodiment of the invention, in which an individual design of the plasticizing process is now also possible. Reference symbol list:

[0055] 1 Device for controlling or regulating the production of a predetermined number of plastic parts 2 Computing unit 3 Signal output 4 Actuator 5 Micro defect 6 Speckle 7 Signal input 8 Section plane

Claims

1. Device (1) for open-loop or closed-loop controlling the production of a predefined number of plastic parts, in particular of optical elements, in each injection cycle of an injection moulding machine or transfer mould, having: - at least one processing unit (2) - at least one signal output (3), via which commands can be transmitted, by the at least one processing unit (2), to at least one actuator (4) of an injection moulding machine or transfer mould, which commands prompt the at least one actuator (4) to carry out at least one application of compression to a melt provided in an injection cylinder of the injection moulding machine or transfer mould, wherein the at least one processing unit (2) is configured to transmit commands to at least one actuator (4) of an injection moulding machine or transfer mould, via the at least one signal output (3), which commands prompt the at least one actuator (4) to carry out the at least one application of compression before an injection in relation to - a level of each application of compression and / or - a number of applications of compression and / or - a duration of each application of compression, such that there is a reduction in microdefects (5).

2. Device according to the preceding claim, wherein the at least one application of compression is chosen such that microdefects are present in the plastic parts produced below a predefined threshold value.

3. Device according to one of the preceding claims, wherein the at least one processing unit is configured to transmit commands to at least one actuator of an injection moulding machine or transfer mould, via the at least one signal output, wherein the commands prompt the at least one actuator to perform more than one application of compression to the same melt in one injection cycle.

4. Device according to at least one of the preceding claims, wherein the at least one processing unit is configured to transmit commands to at least one actuator of an injection moulding machine or transfer mould, via the at least one signal output, which commands prompt the at least one actuator to carry out: - a number of one application of compression, of two or three applications of compression to the same melt in one and the same injection cycle and / or - at least one application of compression with a level of 100%, preferably 150%, particularly preferably 200%, measured against the level of the backpressure during the metering and / or - at least one application of compression with a total duration in a range of from approx. 2.5 to 3.5 seconds, preferably 2.9 to 3.1 seconds, preferably with a total duration of 3 seconds.

5. Device according to at least one of the preceding claims, wherein the at least one regulating device is configured to transmit commands to the at least one actuator, via the at least one signal output, which commands prompt the at least one actuator to carry out the at least one application of compression such that the volume share of the microdefects is below a particular percentage, preferably 50% or 30%, of the initial value, wherein this initial value corresponds to the value which is present in the plastic component part without the at least one application of compression, wherein a volume share of the microdefects is preferably below 0.3 vol.-%, preferably below 0.18 vol.-%.

6. Device according to at least one of the preceding claims having at least one operating unit, which is configured to display information provided by the at least one processing unit to an operator by means of an operating program and to transmit inputs by the operator to the at least one processing unit, wherein it is preferably provided that the operating program is configured to accept inputs by an operator in relation to - a level of each application of compression and / or - a number of applications of compression and / or - a duration of each application of compression and to transmit them to the at least one processing unit, which transmits commands to at least one actuator of an injection moulding machine or transfer mould in dependence on these inputs.

7. Device according to at least one of the preceding claims having at least one signal input, via which signals of at least one sensor of an injection moulding machine or transfer mould can be transmitted to the at least one processing unit.

8. Injection moulding machine or transfer mould which is in operative connection with a device according to at least one of the preceding claims or has such a device.

9. Injection moulding machine or transfer mould according to the preceding claim, wherein the injection cylinder is formed by a plasticizing cylinder, in which an actuator in the form of at least one displaceably and rotatably mounted plasticizing screw is arranged, wherein the at least one regulating device is configured to transmit commands, via the at least one signal output, to the at least one plasticizing screw wherein the commands prompt the at least one plasticizing screw to carry out the at least one application of compression before an injection via - a displacement of the at least one plasticizing screw and / or - a rotation of the at least one plasticizing screw.

10. Process for producing a predefined number of plastic parts, in particular of optical elements, by plastic injection moulding or plastic transfer moulding by means of an injection moulding machine or transfer mould, comprising at least the following steps: - providing a melt in an injection cylinder of the injection moulding machine or transfer mould in an amount which is geared to the number of plastic parts to be produced - carrying out at least one application of compression to the metered melt in the injection cylinder - injecting the metered melt into at least one mould to produce the predefined number of plastic parts wherein the step of the at least one application of compression to the melt in the injection cylinder before the injection is chosen in relation to - a level of each application of compression and / or - a number of applications of compression carried out and / or - a duration of each application of compression such that there is a reduction in microdefects (5).

11. Process according to the preceding claim, wherein the step of the at least one application of compression to the melt in the injection cylinder is carried out such that microdefects are present in the plastic parts produced below a predefined threshold value.

12. Process according to one of the two preceding claims, wherein the following are used: - a number of one application of compression, of two or three applications of compression to the same melt in one and the same injection cycle and / or - a level of 100%, preferably 150%, particularly preferably 200%, measured against the level of the backpressure during the metering and / or - a total duration in a range of from approx. 2.5 to 3.5 seconds, preferably 2.9 to 3.1 seconds, preferably a total duration of 3 seconds.

13. Process according to one of the two preceding claims, wherein the predefined threshold value relates to a volume share of the microdefects and the volume share of the microdefects is below a particular percentage, preferably 50% or 30%, of the initial value, wherein this initial value corresponds to a volume share of microdefects which is present in the plastic component part without the at least one application of compression, and is preferably equal to 0.3 vol.-%, preferably 0.18 vol.-%.

14. Process according to at least one of the preceding claims, wherein the at least one application of compression is carried out via - a displacement and / or - a rotation of at least one plasticizing screw of the injection moulding machine or transfer mould.

15. Computer program comprising commands, which, when the program is executed, prompt a computer to perform the process according to at least one of claims 10 to 14 by means of the device according to at least one of claims 1 to 7.

16. Data carrier signal which carries the computer program according to the preceding claim.