Patent Publication Number: US-2021187818-A1

Title: Method and Tool for Embossing

Description:
FIELD OF THE INVENTION 
     The present disclosure relates to a method for embossing a plastic sheet between a first and a second tool half, where the plastic sheet is pressed and heated between the tool halves while a pattern is imprinted on first and second faces of the plastic sheet. 
     TECHNICAL BACKGROUND 
     Such a method and tool are described for instance in WO-2013/002703-A1 where an actively heated tool is used for instance to produce so-called lightguide plates, LGPs, by imprinting a fine pattern on a thin, transparent plastic sheet. 
     One problem with such production methods and tools is how to improve their yields in terms of finished products that fulfil quality requirements. 
     SUMMARY OF THE INVENTION 
     One object of the present disclosure is therefore to provide an embossing method and tool that provides improved yield. This is obtained with an embossing method as defined in claim  1 . More specifically, in a method of the initially mentioned kind, there is imprinted at least a first reference mark on the first face of the plastic sheet and at least a second reference mark on the second face of the plastic sheet. The embossed plastic sheet is evaluated with optical means to determine error data based on the relative position of the first and second reference marks, and the embossing tool adjusted based on the error data for subsequent embossing operations. This allows any occurring misalignment between stampers used on the top and bottom faces of the plastic sheet to be detected immediately. This is particularly important if the stampers used float with respect to their respective tool halves. If so, one of the stampers may begin to drift, and this can be readily detected. 
     It is possible to carry out the adjustment by moving the first tool half by means of a plurality of servos, typically three servos, such that the tool halves are closed in an inclined manner, thereby moving the first stamper relative to the second. This allows to control the relative position of the stampers during pressing such that the misalignment between the stampers can be kept within allowed limits, even without interrupting production. 
     The tool may be configured to make four or more reference marks on the plastic sheet, and first and second cameras evaluate the reference marks at different locations on the plastic sheet. The spaced-apart registering simplifies the detection of a condition where one stamper begins to rotate. 
     Typically, the reference marks are made separate from an area where a fine optical pattern is embossed e.g. for a lightguide plate. 
     There is also considered a method and a corresponding tool for embossing a plastic sheet between a first and a second tool half, wherein the plastic sheet is pressed and heated between the tool halves while a pattern is imprinted on first and second faces of the plastic sheet by means of a first and a second stamper associated with the first and second tool halves respectively. The first tool half is configured to be moved by a plurality of servos, typically three, and the first tool half is moved towards the second tool half in an inclined manner, thereby moving the location of one stamper relative to the other when the tool is closed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically describes an embossing tool. 
         FIG. 2  illustrates schematically an embossing tool modified to compensate for stamper drift. 
         FIG. 3  illustrates a detection method. 
         FIG. 4A-C  illustrate examples of reference markers. 
         FIG. 5  illustrates the offsetting of a stamper by means of an inclined tool half. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure relates to a method and a tool for embossing a plastic sheet. An example of a pressing embossing tool is shown in WO-2013/002703-A1. Such a tool may be used for instance to produce so-called lightguide plates, LGPs, for graphic display devices and TVs. 
       FIG. 1  illustrates schematically an embossing tool  1 . The tool has an upper  3  and a lower  5  tool half, and a plastic sheet blank  7  is pressed with a high force during a part of a process cycle, and during that stage a fine pattern may be embossed in either or both surfaces of the blank by means of upper  11  and lower  13  stampers. As illustrated, that blank may optionally be fed from a blank roll  9 . The pressing may be made more efficient by heating the surfaces coming into contact with the blank  7 , i.e. the stampers  11 ,  13  during pressing, by means of a heating arrangement including a coil  15 . Further, during a subsequent phase, the same surfaces may be cooled by cooling means  17 , such as ducts conveying a cooling medium. The tool is then opened, and the embossed plastic sheet is removed giving room for a new blank  7 . The process cycle may be concluded in e.g. 30 seconds. 
     The stampers  11 ,  13  provide the fine patterns to the upper  19  and lower  21  surfaces of the plastic sheet  7 , and may consist of thin, flat sheet structures in a ferromagnetic material. As described in the aforementioned document, the stampers may float in relation to the underlying tool half (as seen from the blank to be embossed). This allows the stampers to expand and contract freely during the heating and cooling phases of the embossing process. Had the stampers for instance been screwed to the underlying tool half, they could have been deformed to some extent by the screwed connection to the tool half when heated. Instead, the lower stamper  13  may simply rest on the lower tool half  5 , and the upper stamper  11  may be kept at the upper tool half  3  using a vacuum technique. 
     When pressing is applied, the stampers  11 ,  13  may sometimes drift slightly with regard to its tool half, and this drift need not be uniform, that is the stampers may also drift mutually. Even if some misalignment between the patterns embossed in upper  19  and lower  21  surfaces may be allowed, this could imply problems. The drift may be very small, fractions of micrometers, but after a number of cycles the accumulated misalignment may be unacceptable. 
     The present disclosure relates to methods for detecting such a misalignment. The present disclosure also relates to methods for dealing with such a misalignment. 
       FIG. 2  illustrates schematically an embossing tool modified to compensate for stamper drift. As before, a blank  7  is pressed between a first  3  and a second  5  tool half. 
     Here, the first tool half  3  is moveable upwards and downwards by means of three servos/actuators  23 ,  25 ,  27 , while the second tool half  5  is fixed. The use of the three servos will be discussed further later on. 
     When the blank  7  is embossed with e.g. a lightguide plate pattern  29 , reference markers are embossed thereon at the same time. Typically, this is done outside the pattern  29 , although this is not necessary. At least one reference marker  31  (in the illustrated case two), is embossed on the first, top surface of the plastic sheet  7 , and at least one reference  33  mark is embossed on the second, bottom surface of the plastic sheet  7 . 
     Those reference markers are embossed using the upper and lower stampers  11 ,  13 . Thereby any misalignment between the upper and lower stampers can be detected by optically recording a reference marker  31  on the top surface, and a corresponding reference marker  33  on the bottom surface. One is registered directly and the other through the plastic sheet  7  which is at least partially transparent. As illustrated in the enlarged portion of  FIG. 2 , a misalignment may be detected as two patterns slightly displaced, which are recorded by a camera  35 . In the illustrated case two cameras  35 ,  37  are used which are spaced apart, suitably at least 70 mm, e.g. 200 mm. This allows not only offsets in orthogonal directions (x, y) of the plastic sheet plane to be detected, but also the detecting of one of the stampers rotating about an axis orthogonal to that plane and in relation to the other stamper. Detection of a rotating movement can however also be detected using a single camera, as will be discussed. The cameras may be connected to a control unit  39  that adjusts the tool&#39;s operation as will be discussed. 
       FIG. 3  thus describes a basic detection method, where markers are embossed  51  on the top and bottom surfaces. after embossing, the relative positions of those markers are evaluated  53 , and, if needed, the tool is adjusted  55  to deal with any detected misalignment, thereby improving the alignment in subsequent embossing operations on new plastic sheet blanks. The adjusting may be carried out using a multiple-servo technique, as will be described, although other options would be possible. 
       FIGS. 4A-C  illustrate possible reference markers.  FIG. 4A  shows a possible top side marker  31  comprising a cross with a centered circle.  FIG. 4B  shows a possible bottom side marker  33  comprising an identical cross and a centered circle which is larger than the one on the top side.  FIG. 4C  shows a possible image registered by a camera when the top and bottom reference markers are perfectly aligned. It should be noted that this configuration provides an option to detect orthogonal direction errors (x, y) and any rotation which may occur even though this point may be well aligned. A relatively large rotation may however be required to make the bottom side marker&#39;s  33  cross visible, and therefore it may be preferred to use a two-camera solution to test the alignment at two locations on the plastic sheet  7 , preferably spaced apart 70 mm, as mentioned. 
     In this way, any misalignment between the upper and lower stamper can be determined. While it would be possible to temporarily stop production and adjust the position of the stampers, the present disclosure suggests another solution for the adjustment function. 
     With reference again to  FIG. 2 , the upper tool half  3  is raised and lowered by three servos  23 ,  25 ,  27  which may be controlled individually. Therefore, it is possible to lower the upper tool half  3  towards the lower tool half  5  in an inclined manner, such that, when closing the tool, there is formed a wedge-shaped space in between the tool halves  3 ,  5 . 
     This is illustrated in  FIG. 5 , where the upper tool half  3  has been inclined slightly by rotating clockwise about one degree. It should be noted that even this inclination is somewhat exaggerated, an inclination of about 0.1 degrees may be suitable to achieve the desired effect to be described. This inclination makes the space between the tool half  3  and the plastic sheet  7  slightly wedge-shaped. In the case illustrated in  FIG. 5  the rightmost part of the upper stamper  11  will become pressed first. This induces some shear strain in the intervening plastic sheet. When pressing ends, this moves the upper and lower stampers slightly and in mutually opposite directions, as shown. 
     Therefore, by individually controlling the servos  23 ,  25 ,  27  closing the gap between the first and second tool halves, it becomes possible to move the floating stampers  11 ,  13  in relation to each other. The use of three or more servos allows the upper tool half  3  to present any inclination vis-à-vis the lower tool half  5  within a wide range. 
     The control unit can therefore, based on the output from the cameras and e.g. an internal lookup table, produce a suitable servo control sequence, that allows the misalignment between the upper and lower stampers  11 ,  13  to remain within allowed limits for a long set of production cycles. It is also possible to use an algorithm controlling the servos to deal with any misalignment, for instance based on a PID controller. 
     By varying the wedge shape as the tools are closed it is possible to make the upper stamper rotate slightly with respect to the lower stamper, thereby counteracting any rotation offset. 
     It should be understood that the multi-servo control scheme could be based on other error data than the one produced by the cameras in  FIG. 2 , for instance using direct laser measurements on the stampers themselves in between pressing cycles. 
     The present disclosure is not restricted to the above described examples and may be varied in different ways within the scope of the appended claims.