DE102019207421B4 - Process for smoothing a surface of a component by processing with energetic radiation - Google Patents

Abstract

Method for smoothing a surface of a component, in particular an additively manufactured plastic component (1), by processing with energetic radiation, in which the surface is exposed several times along a processing path (2) in an edge layer by several passes with at least one energetic beam (5) or is remelted, with a melt bath temperature of a melt bath (3) currently being produced by the energetic beam (5) during the run being recorded during each pass and at least one processing parameter influencing the melt bath temperature being controlled during the pass as a function of the recorded melt bath temperature in order to predeterminable profile of the molten bath temperature along the processing path (2), andwhereby a roughness of the respectively melted or remelted surface is measured in a spatially resolved manner during each pass and compared with a target roughness, where the processing is terminated when the target roughness is reached within predeterminable tolerances,characterised that from the comparison with the target roughness, a course of the melt bath temperature along the processing path (2) is determined and specified for the subsequent pass, through which the roughness of the surface comes as close as possible to the target roughness after the subsequent pass, whereby to determine the course of the melt bath temperature , which is specified for the subsequent pass, a previously determined relationship between the melt bath temperature and the reduction in roughness achieved for the component material is used.

Description

Technical field of application

The present invention relates to a method for smoothing a surface of a component, in particular a plastic component, by processing with energetic radiation, in particular laser radiation, in which the surface is melted or remelted several times along a processing path by several passes with at least one energetic beam in an edge layer.

In the past, components made of plastic were mainly produced by means of injection moulding. This process is increasingly being supplemented by additive manufacturing processes, also known as 3D printing. Processes that are frequently used for plastics are the SLS process (SLS: Selective Laser Sintering) and the FDM process (FDM: Fused Deposition Modeling). However, the surface of additively manufactured plastic components has a surface roughness that is too high for many applications. Therefore, post-processing of the surface of the additively manufactured plastic components is usually necessary to reduce the surface roughness.

State of the art

Various methods are known for smoothing the surfaces of additively manufactured plastic components, for example chemical etching, manual grinding and polishing or vibratory grinding, which, however, each have different disadvantages in terms of time and cost as well as the influence on the surface. Another known method for smoothing plastic surfaces, which avoids the disadvantages of mechanical or chemical treatment, is smoothing using energetic radiation, in particular laser radiation.

That's it DE 102 28 743 A1 a method for smoothing and polishing surfaces by processing with energetic radiation is known, in which the surface to be smoothed is remelted using energetic radiation in order to achieve the smoothing. The processing is carried out by scanning the surface using an energetic beam, in particular a laser beam, with the surface being remelted several times in succession for optimal smoothing results. The method described in this publication is primarily suitable for smoothing metal surfaces. However, it is also proposed to use the method with other materials, for example with workpieces made of plastic. However, this process has so far only been suitable for reducing the roughness of flat surfaces of components of uniform thickness. In the case of free-form surfaces and/or varying component thicknesses, material burns or impermissible edge rounding can occur. A variation of the material thickness leads to a changed heat distribution and thus also changes the result of the smoothing. Thick component areas are not melted sufficiently and thin component areas heat up so much that material can be vaporized.

The DE 10 2018 105 877 B3 describes a laser processing system with an optical coherence tomograph, which can be used to measure various process parameters during laser processing. The U.S. 2005/0040147 A1 deals with applications in which a surface is remelted using laser radiation, in particular to obtain a monocrystalline surface. In the US 4,814,578A describes the planarization of metallic layers by remelting with laser radiation, but without corresponding process monitoring.

The DE 10 2017 002 986 A1 discloses a method for producing a transmittive optics, in which a smoothing according to the preamble of patent claim 1 takes place.

The object of the present invention is to specify a method for smoothing a surface of a component, in particular a plastic component, by processing with energetic radiation, which also provides good smoothing results in the case of free-form surfaces and varying component thicknesses.

Presentation of the invention

The object is achieved with the method according to claim 1. Advantageous configurations of the method are the subject matter of the dependent patent claims or can be found in the following description and the exemplary embodiment.

In the proposed method, the surface is repeatedly melted or remelted by several passes with at least one energetic beam, preferably a laser beam, in a surface layer along a processing path, which is guided over the surface along one or more paths with each pass. With each pass, the molten bath temperature of the molten bath currently generated by the energetic beam during the pass is recorded, as well as at least one processing parameter influencing the molten bath temperature regulated as a function of the recorded melt pool temperature during the passage in order to obtain a definable profile of the melt pool temperature along the processing path. The course of the melting bath temperature is not to be understood as a time course but rather as a local course T(x,y). In this case, the radiation power is preferably regulated as the processing parameter. However, there is also the possibility of regulating other processing parameters such as the movement or scanning speed of the energetic beam relative to the surface. Furthermore, in the proposed method, the roughness of the respectively melted or remelted surface is measured in a spatially resolved manner during each pass and compared with a target roughness that corresponds to the desired smoothing result. The method is terminated as soon as the measured surface roughness corresponds to the target roughness within predefinable tolerances. Depending on the initial roughness of the component, component material and possible processing parameters, this can already be the case after the first or second pass or it can require a larger number of passes. The method is characterized by the fact that, by comparing the measured roughness with the target roughness, a profile of the molten bath temperature along the processing path is determined, through which the roughness of the surface comes as close as possible to the target roughness after the next pass with the energetic beam. This determined course of the melt pool temperature is then specified for the next pass. To determine the course of the melt pool temperature, which is specified for the subsequent pass, a previously determined relationship between the melt pool temperature and the reduction in roughness achieved for the component material is used.

In order to improve the polishing or smoothing result, a process control is proposed in the present invention, which controls both the process and the result during each pass. The control takes place via two controlled variables (temperature and roughness) and thus represents a multi-variable system. The outer control loop consists of the roughness measurement of the surface obtained during the respective pass after solidification of the melt. With the help of a suitable measuring device, the roughness per pass is recorded and evaluated in a spatially resolved manner. The process ends when the target roughness is reached within the specified tolerances. In the inner control loop, the melt bath temperature is used as the control variable to maintain a specified temperature profile along the processing path. For this purpose, a constant temperature profile is preferably specified for the first pass. For all subsequent passes, the temperature profile is then preferably determined by evaluating the surface roughness measured during the previous pass and then specified for the respective pass. Thus, from the second pass, the jump in roughness to be generated by this pass (difference between roughness before and roughness after the pass) is adjusted in a targeted manner. Taking into account the available processing parameters, this jump in roughness is preferably selected in such a way that the target roughness is as close as possible with this jump in roughness without damage to the material or the surface due to excessively high temperatures. The melt bath temperature can be measured using an optical temperature measuring method, for example by means of pyrometry or thermography.

This regulation using two control variables also enables reliable smoothing of free-form surfaces and plastic components with varying component thicknesses. During the first pass, the melt bath temperature along the processing path can be specified so that it remains within a temperature range in which, on the one hand - especially in the case of plastic components - no combustion or evaporation of material takes place, but on the other hand a melting depth sufficient for smoothing is reached , for example by specifying a correspondingly constant temperature value in this area. In this way, the processing parameters for each processing location or each processing area are automatically adjusted by the control, even with varying component thickness, so that there is sufficient melting and no excessive heating occurs. In this way, components with free-form surfaces and different or varying thicknesses can advantageously be processed. A preferably coaxial integration of the measurement components for the temperature measurement in the beam path of the energetic beam ensures that the temperature measurement always takes place at the processing point. In the preferred embodiment, with the second pass, the temperature can then already be predefined in a location-dependent manner via the outer control circuit in such a way that the target roughness is reached as quickly as possible. Of course, this temperature specification is again within the material-specific limits, within which damage to the material is avoided.

In order to determine the temperatures to be specified for the next pass or the corresponding temperature profile, in the proposed method a connection between the melt bath temperature and the resulting improvement in surface roughness (roughness jump), i.e. the roughness difference of the surface, is determined in advance for the respective component material determined before processing or crossing and after processing or crossing. This relationship can, for example, be stored in a table (LUT: Look-Up-Table), which the method then accesses during processing to determine the molten pool temperatures required for a specific roughness jump.

Since the surface roughness can vary over the component, a spatially resolved roughness measurement is carried out and a kind of roughness map is thus obtained over the surface to be smoothed. The temperature profile along the processing path is then determined as a function of the location-dependent difference between the measured roughness and the target roughness.

In the present method, the measurement of the roughness of the respectively melted or remelted surface layer is already carried out during the passage, ie in each case directly behind the molten pool currently produced by the energetic beam after it has solidified. This is preferably made possible by the fact that the measuring device for measuring the roughness is rigidly coupled to the processing head, via which the energetic beam is directed onto the surface and guided over the surface. The measuring device for the roughness can be integrated into this processing head, attached to this processing head or rigidly coupled to this processing head, which is moved relative to the surface along the processing path during processing. For this purpose, an optical measuring device for measuring the roughness is preferably used as the measuring device. Triangulation or white-light interferometry techniques, for example, are suitable for this purpose. A measuring device for optical coherence tomography (OCT) is particularly preferably used for measuring the roughness. This measurement during the pass avoids a separate work step and thus a two-stage process in which the surface to be smoothed is first completely melted or remelted during a pass and the surface is only then measured with regard to surface roughness.

In order to improve the smoothing result, the processing parameters are preferably selected in such a way that the current molten pool has an extent of at least 25 mm ² in each case. This can be achieved by using an energetic beam with a correspondingly large beam diameter or by suitably fast scanning of a correspondingly large surface area with an energetic beam of a smaller beam diameter. In the latter case, the processing head is moved along the processing path over the smoothing surface, while the energetic beam is additionally guided over a small area at a time by a scanning device in the processing head. In the case of plastic components, the molten pool is understood to be a continuous area in which the plastic is heated to a value above the glass transition temperature T _G , at which it has a sufficiently low viscosity for the surface roughness to flow out. A particularly good smoothing result is achieved by ensuring a sufficiently large coherent melt pool.

The proposed method is preferably carried out with laser radiation. However, other types of energetic radiation, for example microwave radiation, electron beams or infrared radiation, can also be used instead of laser radiation. When using a laser beam, a continuous, a pulsed or a time-modulated beam source can be used. The laser radiation must have a suitable wavelength in order to be absorbed near the surface of the material to be processed. This is preferably achieved using a CO ₂ laser beam source.

A device suitable for carrying out the method has a processing head with an optical device, with which at least one laser beam can be directed onto the surface of the component, and a temperature measuring device. The temperature measuring device is integrated into the processing head in such a way that it can measure the temperature of a melt pool generated on the surface of the component with the laser beam. The processing head can be guided with a movement device along a processing path over an area of the surface of the component that is to be smoothed. A measuring device for measuring the roughness of the surface of the component is attached to the processing head, integrated into the processing head or rigidly coupled to the processing head, with which the roughness of the surface in the processing direction behind the melt pool can be measured. In order to carry out the proposed method, the device is connected to a correspondingly designed open-loop and closed-loop control device.

The method can be used in particular to smooth one or more surface areas of plastic components that have a surface finish that is obtained immediately after additive manufacturing of the plastic component, in particular by means of SLS or FDM, without further surface processing. Plastic components with a surface condition according to a different manufacturing process for shaping the component are of course also possible. In many areas, such as medical technology, the surface roughness is of great importance. The additive manufacturing processes specified above do not achieve the required surface properties, so that the proposed process offers great advantages here. The method is suitable, for example, for smoothing plastic components made from a thermoplastic material, for example a plastic component made from PA12, PA6, PLA, PP, PC or PEEK.

character list

• 1 eine schematische Darstellung der Glättung einer Oberfläche eines Kunststoffbauteils mit dem vorgeschlagenen Verfahren.

The proposed method is explained in more detail below using an exemplary embodiment in conjunction with the drawings. This shows:

• 1 a schematic representation of the smoothing of a surface of a plastic component with the proposed method.

Ways to carry out the invention

The proposed method is explained again below using an exemplary embodiment in which a laser beam is used to smooth a surface area of a plastic component, with which the surface of the 1 Schematically illustrated component 1 is melted or remelted along a processing path 2 . Through the interaction of the processing laser beam 5 with the surface, a melt pool 3 is generated in the area of the point of impact of the laser beam 5 by melting the plastic material, which then solidifies again after the laser beam 5 is guided on the surface of the component 1. The processing parameters are preferably selected in such a way that a continuous molten pool of at least 25 mm ² area is produced. For this purpose, either the processing laser beam 5 can have a correspondingly large beam diameter or, as shown schematically in the figure, quickly scan a correspondingly large surface area on one or more paths while it is guided further along the processing path 2 by moving the processing head 6. For this purpose, an additional scanning device, for example a galvanometer scanner, is arranged inside the processing head 6 in order to quickly scan the corresponding surface area for the melt pool. 1 1 shows an enlarged area in which the molten pool 3 and the corresponding path 4 of the processing laser beam 5 for generating the molten pool 3 are indicated schematically. The figure also shows the processing head 6, which is guided along the processing path 2 over the surface of the component 1 in order to remelt the surface of the component along this processing path. In this case, the processing head 6 contains the laser beam source and the necessary optics in order to focus the processing laser beam 5 in a suitable manner onto the surface of the component.

In the present example, during the respective crossing, i.e. during the movement of the processing head 6 along the processing path 2, a temperature measurement is carried out with infrared sensors integrated in the processing head 6 coaxially to the processing laser beam 5. The current temperature of the molten bath 3 at the processing location is thus constantly measured. At the same time, OCT is used to measure the roughness of the surface immediately after the melt pool has solidified, i.e. in the wake of the respective processing location, as shown in 1 with the optical radiation 7 for the OCT measurement is indicated schematically. The OCT measuring device is integrated into the processing head 6 in this example. The surface roughness measured in a spatially resolved manner is compared with a specified target roughness and from the comparison, location-dependent melt pool temperatures are determined as the temperature profile for the next pass. During the next pass, the temperature of the molten pool is then regulated via the processing parameters, in particular the laser power, so that the corresponding temperature is maintained at the respective processing location. This multi-variable control on the one hand and the roughness measurement during the pass on the other hand avoids a multi-stage process and improves the smoothing result of the surface through better temperature management. This means that free-form surfaces can also be processed reliably.

reference list

1

plastic component

2

machining path

3

melt pool

4

Trajectory of the machining laser beam

5

processing laser beam

6

processing head

7

optical radiation for OCT

Claims (6)

Method for smoothing a surface of a component, in particular an additively manufactured plastic component (1), by processing with energetic radiation, in which the surface is treated several times along a processing line by several passes with at least one energetic beam (5) in an edge layer line (2) is melted or remelted, with each pass recording a melt bath temperature of a melt bath (3) currently generated by the energetic beam (5) during the pass and at least one processing parameter influencing the melt bath temperature as a function of the recorded melt bath temperature during the passage is controlled in order to obtain a predeterminable course of the molten bath temperature along the processing path (2), and wherein a roughness of the respectively melted or remelted surface is measured in a spatially resolved manner during each passage and compared with a target roughness, with the processing when the target roughness is reached within specified tolerances, characterized in that a profile of the melt bath temperature along the processing path (2) is determined from the comparison with the target roughness and is specified for the subsequent pass, as a result of which the roughness of the surface comes as close as possible to the target roughness after the subsequent pass, A relationship between the melt bath temperature and the resulting reduction in roughness, determined in advance for the component material, is used to determine the course of the melt bath temperature, which is specified for the subsequent pass. procedure after claim 1 , characterized in that the energetic beam (5) is directed onto the surface via a processing head (6) guided along the processing path (2) over the surface and the roughness is integrated into the processing head (6) with a device attached to the processing head (6). or is measured with the processing head (6) rigidly coupled measuring device in the processing direction behind the molten pool (3). procedure after claim 2 , characterized in that an optical measuring device is used as the measuring device for measuring the roughness. procedure after claim 3 , characterized in that the roughness is measured by OCT. Procedure according to one of Claims 1 until 4 , characterized in that the processing parameters are selected in such a way that the melt pool (3) currently produced with the energetic beam (5) has an extent of ≥ 25 mm ² . Procedure according to one of Claims 1 until 5 , characterized in that a continuous, pulsed or time-modulated laser beam is used as the energetic beam (5).

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