METHOD FOR PROVIDING A PRINTING TEMPLATE

Description

[0001] The invention relates to a method for providing a printing stencil according to claim 1.

[0002] Stencil printing, a sub-type of screen printing, is a new process for producing magnetic sheets for electric motors. Starting with metal powders, a printing paste is first produced, which is then processed into a green body in the form of a thick film using a stencil printing technique. The resulting green body is then converted into a metallic structured sheet by thermal treatment, on the one hand for debinding and on the other hand for sintering.

[0003] Due to their inherent principle, magnetic sheets manufactured using this process are characterized by increased thickness tolerance compared to sheets produced using a rolling process and subsequently punched. This leads, in particular, to thickness variations within a sheet, which manifest themselves, for example, in a wedge shape or a bowl-shaped, concave depression across the sheet surface. This, in turn, leads to a less than optimal fill level of magnetically active material in a laminated core, which constitutes a rotor or stator of an electric motor.

[0004] To even out the thickness of the printed layer and thus the resulting magnetic sheet, various parameters can be optimized. Firstly, the composition of the printing medium, i.e., the printing paste, can be changed; secondly, the process conditions, such as the squeegee speed or the squeegee contact pressure, can be adjusted. However, all of these modifications still allow for further optimization.

[0005] The prior art is US 2011094398 A1, which describes a method for producing a printing stencil.

[0006] The invention is based on the object of generating a method for providing a printing stencil which is suitable for reducing the thickness differences in the printed thick layer.

[0007] The solution to the problem consists in a method for providing a printing stencil with the features of patent claim 1.

[0008] The printing stencil according to claim 1 for a stencil printing device has open areas for the passage of a printing medium, in particular a printing paste, as well as cover areas to prevent the passage of the printing medium. The printing stencil comprises at least two connecting webs between at least two cover areas. The invention is characterized in that two adjacent connecting webs have an average width that differs by at least 5%.

[0009] According to the invention, it has been found that the given process parameters, in particular the squeegee speed and the squeegee contact pressure, but also the paste availability, are subject to continuous change across the surface to be printed, despite process optimization. The average width of the connecting webs, which correlates linearly with the web area, is to be continuously adjusted by a small amount between each two adjacent connecting webs, so that the layer thickness remains constant or becomes constant for the given process parameters. It has been found that the change in the average width between two connecting webs should be at least 5% in order to keep the thickness of the layer to be printed as constant as possible across its surface area.

[0010] The mean width of the connecting bars is defined as the average of a large number of width measurements—ideally an infinite number—perpendicular to a longitudinal axis of the connecting bar. Assuming the connecting bars are of equal length, the mean width of the connecting bars correlates with the bar area. The bar area is defined as the area visible when viewed perpendicularly to the template.

[0011] It has proven particularly advantageous that a squeegee path is defined with respect to the printing stencil in its intended installation position, and that a squeegee start area and a squeegee turnaround area opposite this are defined with respect to the squeegee path. The squeegee start area and the squeegee turnaround area each essentially account for approximately 25% of the squeegee path. Furthermore, there is a central area arranged between these two areas, with the connecting webs in the central area being at least 5% narrower in average width than the connecting webs in the turnaround area or in the start area.

[0012] It has been found that, particularly in the described central region, the layer thicknesses of the printed layer are thinner compared to the layer thicknesses in the start and turnaround regions. By selecting narrower connecting webs in the central region, the area of ​​the open regions, i.e. the regions where a printing medium is pressed onto the substrate to be printed, is increased. This also increases the absolute amount of printing medium in this region, which in turn leads to a thicker layer thickness. Thus, the layer thickness, which in conventional state-of-the-art printing stencils is thinner in the central region than in the turnaround and start areas, is increased by the described measure. It is thus adapted to the greater thickness in the turnaround and start areas. In return, the absolute thickness deviation across the entire printed layer is reduced.

[0013] The absolute thickness deviation is defined as the quotient of the difference between the highest and lowest measured thickness values ​​relative to the highest measured value. For example, if the highest measured layer thickness is 100 µm and the lowest measured layer thickness is 80 µm, the absolute thickness deviation from the maximum (highest) measured thickness is 20%.

[0014] To achieve the described advantageous printing stencil, it is expedient for the connecting webs to be at least partially wedge-shaped along their length. This also allows thickness differences in the layer to be compensated from a center outward. It may also be expedient for the connecting webs to be wedge-shaped in opposite directions along their length.

[0015] In a further preferred embodiment of the invention, a printed image of the printing stencil is essentially rotationally symmetrical, with connecting webs projecting radially outwards. In this case, a first axis is initially defined along the doctor blade path, and a second axis is defined that is perpendicular to the first axis. The first axis is set at an angle of 0°, and a second axis accordingly has an angle of 90°. In this rotationally symmetrical embodiment, the printing stencil has the special feature that the average width of the connecting webs increases from 0° to 90°, in particular continuously decreases. This is advantageous because, in the rotationally symmetrical embodiment of the printed image, the central region is located in particular at the described 90° angle to the doctor blade path axis.

[0016] The invention describes a method for providing a printing stencil with various surface units, namely open areas for the passage of a printing medium and cover areas for preventing the passage of the printing medium, wherein the cover areas are connected by at least two connecting webs that have a surface in a vertical projection. The method comprises the following steps: a) inserting a base printing stencil into a stencil printing device, b) moving a squeegee along a squeegee path from a squeegee start area to a squeegee turning area opposite thereto, c) with simultaneous printing of a printing medium through the open areas onto a substrate in the form of a printed layer, d) measuring the resulting thickness 38 of the printed layer at several measuring points, e) determining a target area size of the connecting webs which causes a smaller absolute thickness deviation of the layer, f) changing the area of ​​the connecting webs on the basis of the determined target area size, g) repeating steps a) to d).

[0017] The method according to the invention essentially has the same advantages as those already described with regard to the printing stencil. It is, in particular, an iterative method, wherein the average web width is changed, in particular, to change the area. This iterative method can be stopped after the first pass if performed optimally. Should the thickness deviation continue to be insufficient, steps a) to d) and steps e) to f) can be repeated several times, if necessary, preferably until the absolute thickness deviation of the layer is less than 15%, particularly preferably less than 10%.

[0018] The target area size is the area of ​​each connecting bridge required to ensure that the thickness of the layer at a position adjacent to the bridge exhibits a smaller absolute thickness deviation than the previously measured thickness deviation. The target area size can be determined using a mathematical algorithm or by retrieving stored empirical data.

[0019] Particularly preferably, the surface of the connecting webs is changed by means of a mechanical processing method, for example a milling method, a laser cutting method or a water jet cutting method.

[0020] In a further embodiment of the invention, the area of ​​the connecting webs is reduced if the thickness of the layer between two connecting webs is less than the highest measured thickness of the layer. As already mentioned, with a smaller area of ​​the connecting webs, the adjacent open area is served with a higher amount of printing medium or printing paste, which is why the layer thickness of the printed layer increases there. Furthermore, from a process engineering perspective, it is easier to reduce the area of ​​the connecting webs than to enlarge them. This would require the production of a new stencil. If the connecting webs can be advantageously reduced, this can be done easily on an existing stencil using the described method.

[0021] Further embodiments and further features of the invention are explained in more detail with reference to the following figures. These are purely schematic, highly simplified representations of the invention that do not represent a limitation of the scope of protection. Features with the same designation are provided with the same reference numerals even in different embodiments.

[0022] Showing: Figure 1a typical print image of a magnetic sheet or a printed layer, which is however shown in a highly simplified manner without the usual contours, Figure 2a stencil printing device in cross section, Figure 3a graphic representation of thickness measurements related to the print image in Figure 1, Figure 4 shows a printing stencil according to the prior art, Figure 5 shows a typical magnetic sheet with a printed image that can be used in a rotor of an electric machine, Figure 6 shows a printing stencil that is modified compared to the prior art, Figure 7 shows a further modified printing stencil and Figure 8 shows an enlarged section of the dashed area VIII from Figure 7 .

[0023] In Figure 1 A very schematic illustration of a printed image 52 of a printing stencil 2 is shown, which may represent a printed layer 36 or a magnetic sheet 48 sintered therefrom. This is a rotationally symmetrical object that has a ring-shaped configuration. The description below can also be applied to other structures, for example, square ones. In this ring-shaped configuration according to Figure 1This is a highly simplified form that serves to explain the further manufacturing processes. As an example of a real magnetic sheet 48 with a corresponding print image 52, the illustration in Figure 5 Such a magnetic sheet 48 is stacked together in a stack (not shown here) and thus forms the magnetic core of a rotor of an electrical machine. Figure 5 The recesses visible in the magnetic sheet serve as channels in the magnetic sheet stack for laying electrical conductors in the form of windings.

[0024] To produce a magnetic sheet 48 or its precursor, a printed layer 36, a stencil printing device 4 with a printing stencil 2 is used. This is shown in Figure 2shown schematically. The printing stencil 2 is mounted in a frame 50, and a printing medium 8, in the form of a printing paste 8, is printed onto a substrate 34 by means of a squeegee 32 along a squeegee path 16. This creates a printed layer 36.

[0025] The doctor blade path 16 is divided into three parts: the doctor blade start area 18, a middle area 22 and a doctor blade turning area 20. The doctor blade start area 18 and the doctor blade turning area 20 each make up approximately 25% of the entire doctor blade path 16. The middle area 22 therefore corresponds to approximately 50% of the doctor blade path 16. These sections 18, 20 and 22 are auxiliary definitions to describe the nature of the printing stencil according to the Figures 6 and 7 to describe in more detail.

[0026] In Figure 3a spider web-shaped representation of a respective thickness 38 of the printed layer 36 at a plurality of measuring points 40 is given. The thickness 38 is rotationally symmetrical with respect to a print image 52 of the printed layer 36 according to Figure 1 entered in the diagram. The angle in the rotationally symmetrical course of the respective measuring point 40 corresponds to its position in the printed image 52 according to Figure 1 analog. The position of the measuring point 40 in the radial direction, in turn, represents the measured layer thickness. The measurement is carried out at a defined point with a constant radius around a center point of the rotationally symmetrical printed image 52. The individual chains of points connected by lines each represent different measurements on different printed layers 36. If each printed layer 36 were to have a uniform distribution of the layer thickness 38, the image would be Figure 3also essentially circular. However, the image is Figure 3 for all measured printed layers 36, the thickness is compressed in an axial direction, resulting in an approximately elliptical shape. Each measurement series has at least one highest measured layer thickness 46. All other measurement points 40 of the respective measurement chain are correspondingly lower.

[0027] The image of the individual layer thicknesses according to Figure 3 results from the use of a stencil 30 according to the prior art. This stencil is a basic printing stencil, from which printing stencils 2, which are in the Figures 6 and 7 The basic printing template 30 according to Figure 4is characterized in particular in that it has open areas 6 which encompass the printing paste 8 in the subsequent printed image 52. The connecting webs 12 arranged therebetween serve to cover covering areas 10 which are not intended to be covered by the printing paste in the subsequent printed image 52. Since the covering areas 10 are spaced apart areas, they are connected to one another in the base printing stencil via these connecting webs 12. The connecting webs 12 are preferably designed to be thin in relation to the open areas 6; they are just wide enough for the covering areas 10 to be mechanically securely connected to one another. In the areas of the connecting webs 12, no printing paste 8 is therefore printed onto the underlying substrate 34.However, after the stencil is removed, the rheological properties of the printing paste 8 cause this printing paste to flow into the areas previously covered by the connecting webs 12. This creates the printed image 52 shown in FIG. Figure 1 , in which the connecting webs 12 of the printing template 30 are no longer visible.

[0028] Due to production-technical conditions, such as the speed of the doctor blade 32 along the doctor blade path 16 and its contact pressure on the stencil 30 and the printing paste 8, the inhomogeneous distribution pattern of the thickness 38 of the layer 36 at different measuring points 40 results according to Figure 3 .

[0029] To avoid this inhomogeneous distribution pattern according to Figure 3Starting from the base printing stencil 30, a printing stencil 2 is created whose connecting webs 12 have different average widths 44. The average widths 44 of two adjacent connecting webs 12 of the printing stencil 2 at any location on the printing stencil 2 differ by at least 5%.

[0030] The average width 44 of the connecting bars 12 in Figure 8 illustrated. Figure 8 shows section VIII in Figure 7 as an enlargement and is also an exemplary representation of possible designs of a connecting web 12. The web 12 in Figure 8 or in Figure 7 VIII has a double conical shape along the course 24 of the connecting web 12. The web 12 has the smallest width according to Figure 8in the middle of its course 24 and widens towards the outer ends, i.e. in the direction of the cover areas 10. The web 12 therefore does not have a constant width. However, if one calculates its average width 44, which is indicated by the dashed lines in Figure 8 is shown, this average width 44 differs by at least 5% from the average width 44 of the adjacent connecting web. Furthermore, Figure 8 The profile 24 of the web 12 is specified, which corresponds to its length. Thus, the product of the average width 44 and the profile 24 results in the area 42 of the web 12.

[0031] Regarding the printing template 2 according to Figure 6In addition to the doctor blade path 16, a first axis 26 is defined in its direction, which is intersected at right angles by a second axis 28, preferably in the center of the annular printed image 52. The two axes 26 and 28 describe a 90° angle, with the average width 44 of the connecting webs 12 continuously decreasing from 0°, i.e. parallel to the first axis 26, up to 90°. This is expedient because it has been found that, as in Figure 3 As can be seen, the thickness 38 of the printed layer 36 becomes continuously smaller in the vicinity of the 90° line. By reducing an area 42 of the connecting web 12, which results in a linear correlation to the average width 44 at a constant length, more printing paste is pressed into the open areas 6, so that Figure 6 The measure described increases the thickness 38 in the area mentioned. It should be noted that the surface 42 of the connecting webs 12, as shown in Figure 6is shown is considerably exaggerated for the sake of clarity.

[0032] In Figure 7 This change in the area 42 or the mean width 44 of the connecting webs 12 also applies to the same coordinate system. Only the already Figure 8 The double wedge shape described or the simple wedge shape of the connecting webs 12 is in Figure 7 more pronounced than in Figure 6 . Due to the wedge shape of the connecting webs 12, a thickness deviation along the radius can also be adjusted according to the desired specification.

[0033] To produce a printing stencil 2 according to the Figures 6 and 7 The base printing template 30 is preferably used. This is printed according to Figure 2 inserted into a stencil printing device 4 as described and the printing process is carried out. This results in, as shown in Figure 1 described, a corresponding print image 52 and the Figure 3 shown thickness distribution. For this purpose, a large number of measuring points 40 are recorded, whereby the highest layer thickness 46 is determined. Analogously, a lowest measured layer thickness 47 is determined and the difference is related to the highest measured layer thickness. In Figure 3 The highest layer thickness 46 is approximately 110 µm, while the lowest measured layer thickness 47 is only approximately 70 µm. The difference is therefore 40 µm. This difference is disadvantageously high for a subsequent magnetic sheet 48. It amounts to approximately 36% of the highest measured layer thickness 46. This is referred to as the absolute thickness deviation.

[0034] In order to minimize the absolute thickness deviation, the base printing stencil 30 is cut according to the Figures 6 and 7modified. Preferably, the web area 42 in the central region 22 is continuously reduced by a mechanical process, whereby the reduction occurs to a target area size. The target area size, in turn, is determined either by means of a corresponding mathematical algorithm or by using an empirical database. Once the target area size of the connecting webs 12 is reached, the modified base stencil 30, now referred to as printing stencil 2, is reinserted into the stencil printing device 4, and the printing process is repeated. This is followed by another thickness measurement analogous to that in Figure 3shown, and it is checked whether the modification to the printing stencil 2 has had the desired effect. If the absolute thickness deviation of the layer continues to be too high, in particular more than 15%, the described optimization steps for the printing stencil 2 are repeated until this preferred maximum thickness deviation value is reached in a subsequent printing process. The absolute thickness deviation should particularly preferably be less than 10%.

[0035] When calculating the target area size of the connecting webs 12, it is also expedient to take into account the parameters of the squeegee speed along the squeegee path 16 and the contact pressure of the squeegee 32 on the substrate 34. List of reference symbols

[0036] 2Printing stencil 4Stencil printing device 6Open areas 8Printing medium 10Covering areas 12Connecting webs 14Installation position 16Squeegee path 18Squeegee start area 20Squeegee turning area 22Central area 24Web path 26First axis 28Second axis 30Base printing stencil 32Squeegee 34Substrate 36Printed layer 38Thickness 40Measuring points 42Area webs 44Mean web width 46Highest measured layer thickness 47Lowest measured layer thickness 48Magnetic sheet 50Frame stencil 52Printed image

Claims

1. Method for providing a printing screen (2) with a variety of area units, namely open areas (6) to allow a printing medium (8) to pass through and cover areas (10) to prevent the printing medium (8) from passing through, wherein the cover areas (10) are connected by way of at least two connecting webs (12) which have an area (42) in a perpendicular projection, comprising the following steps: a) inserting a basic printing screen (30) into a screenprinting apparatus (4), b) moving a doctor blade (32) along a doctor blade path (16) from a doctor blade starting area (18) to a doctor blade turning area (20) lying opposite thereto, c) simultaneously printing a printing medium through the open areas (6) onto a substrate (34) in the form of a printed layer (36), characterised by d) measuring the resulting thickness (38) of the printed layer (36) at a number of measuring points (40), e) ascertaining a target area size of the connecting webs (12) which results in a smaller absolute thickness deviation of the layer (36) f) changing the area (42) of the connecting webs (12) on the basis of the target area size ascertained, g) repeating steps a) to d).

2. Method according to claim 1, characterised in that an average web width (44) is changed.

3. Method according to claim 1 or 2, characterised in that steps a) to d) and e) to f) in claim 1 are repeated until an absolute thickness deviation of the layer is less than 15 %.

4. Method according to claim 3, characterised in that steps a) to d) and e) to f) in claim 1 are repeated until the absolute thickness deviation of the layer is less than 10 %.

5. Method according to one of claims 1 to 4, characterised in that the change to the area (42) of the connecting webs (12) is carried out by means of a mechanical processing method.

6. Method according to one of claims 1 to 5, characterised in that the area (42) of the connecting webs (12) is reduced when the thickness (38) of the layer (36) between two connecting webs (12) is less than the greatest measured thickness of the layer.

7. Method according to one of claims 1 to 6, characterised in that the target area size is determined by means of a mathematical algorithm or by calling up stored empirical data.