Method and device for determining the topography of a material surface

A method and a device for determining a topography under load of the surface of a material, wherein a test piece (40) of the material intended to be determined is subjected to a compression with a determined load between a first and a second clamping surface (7, 27), after which, in a compressed state, at least one representation is made of surface portions of the material that are in contact with at least one of said clamping surfaces (7, 27), and that the representation is evaluated. The compression is controlled in respect of its speed for obtaining said predetermined load, and said at least one representation is made at a chosen point in time or chosen points in time during this process.

FIELD OF THE INVENTION

The invention concerns a method and a device for determining the topography of a material surface according to the preambles of the respective independent claims.

BACKGROUND OF THE INVENTION

In most commercial printing processes (for example flexography, intaglio printing and lithography), printing ink is transferred to a paper surface through direct mechanical contact. Since the printed image is formed by a number of small screen dots having diameters of 5-50 μm, it is important to achieve good contact over the entire printing surface. If at all places on the printing area of the paper such a contact is not obtained, ink dots not coming into contact with the paper surface will be missing on the printed surface (“missing dots”) which causes blank surface portions or, sometimes in case of multi-colour printing, tinting.

A number of different solutions are known in order to investigate properties of different paper surfaces with regard to the “topography” of the paper in order to assign a value for the paper's ability to correctly receive prints without the occurrence of “missing dots”. With the topography of a material surface is here intended the roughness which is caused by small differences in height in the material surface.

The most common methods for measuring the roughness of a paper can be grouped in the following manner:

Test printing in a commercial printing press or a printer in laboratory outfit, where a printing ink is used in order to evaluate the number of missing printing points.

Profilometry, where a stylus is drawn in a straight line over the surface and the topography is recorded.

Air leakage, where a measuring ring is pressed against the test surface and the air leakage between the inside and the outside of the ring is measured.

Optical non-contacting measurement methods, using for example a confocal microscope or can be calculated from a shadow of light which incides at the surface at a small angle.

“The Chapman method”, wherein a paper with the aid of a pressuring body is pressed against a flat glass prism (Chapman prism).

During a determination according to the last mentioned method, the paper will lie with larger or smaller portions pressed against the glass of the prism. At points where the contact surface of the prism lacks contact with the test surface, obliquely collimated light inciding against the contact surface will be reflected inside the glass body of the prism and these points thereby appear as dark surfaces in the picture registered by the camera.

In the portions of the contact surface against which the paper is in contact, the light will refract through the contacting surface of the prism and illuminate underlying portions of the paper. These illuminated portions will thereby be light. A picture taken through the prism perpendicular to the clamping surface, will therefore have light and dark portions. This picture is subsequently evaluated to determine the topography of the paper according to the above.

All established measuring methods have one or more drawbacks, since on the one hand they may be time-consuming to use, on the other hand they give insufficient detail information about the properties of the contact surface. Some measuring methods use printing inks which affect the measuring result. Most of them lack pressurizing load during measurement.

AIM AND MOST IMPORTANT FEATURES OF THE INVENTION

The aim of the present invention is to provide a method and a device wherein the drawbacks of the so far known techniques are avoided or at least reduced.

Concerning the Chapman method, the contact surface between a paper surface and a clamping surface of the prism increases with increased pressure force. It has further been found that the contact surface also increases at a constant load as a function of the time when a pressure force is supplied following increased compression of the paper surface over time. For that reason the Chapman method gives an erroneous value of the contact surface, since it compresses the measuring area during an essentially longer time compared to the conditions in a printing press.

In an ordinary printing process, a paper web is transferred at great velocity through a printing nip with a high nip load with which the paper becomes compressed. A point on the paper web normally remains in the printing nip during a very short time, typically 10-50 milliseconds (ms).

The invention offers the possibility of performing quality control of the printing properties of a material and in particular of a paper, with a more relevant measuring method, which can be performed fast and simple in connection with the production by ordinary, not specially trained, operating personnel. With the method suggested here, a single measurement only takes a few seconds to carry through. The method according to this invention gives an image of the paper in a condition which well corresponds to the conditions that prevail at the very printing event.

Further there can be used an adjustable test load in order to reproduce the pressure forces which appear in a printing press during different printing methods with such different conditions as for example flexography (2.5 MPa) or intaglio printing (10 MPa). The suggested method therefore gives an exact and direct quality measurement for the test surface in relation to the selected printing method.

An apparatus with illumination and image capturing such as for the Chapman method as described above is preferably used.

A paper test piece is hereby introduced between an upper, rigidly mounted glass body in the form of said prism with a flat underside and a lower, flat compression body, which is movable in the direction to and from the prism. The compression body is quickly pressed against the lower side of the glass body until a predetermined load has been reached and the paper thus has been pressed between the glass body and the compression body corresponding to the provisions that prevails in a printing nip.

At a determined point in time during the compression, an image of the paper is captured, which image can thereafter be evaluated. It is also within the scope of the invention that a plurality of images are captured at different points in time during the compression process in order to control the dynamic compressibility of the topography of the paper as a function of time during the process and make it possible to evaluate how the paper performs during different phases of a printing process.

There are many possibilities of varying the invention. The system can thus be adjusted in order to correspond to different printing methods when it comes to:nip load, in order to simulate the compression in the printing nip,acceleration, in order to simulate different web speeds in a printing press,the compression time during set maximum pressuring force,the point(s) in time at which one or more images shall be registered by the camera.

It is important for the invention to simulate the true conditions the paper is exposed to in a real printing process and to synchronize the registration such that it occurs at desired point(s) in time during the very short compression of the test specimen.

“Dynamic topography” refers to the topography of a material surface during a dynamic process, in particular according to the invention in a fast process such as in a real printing process.

With reference to the feature that at least one representation is created from surface portions of the specimen which are in contact with at least one of said clamping surfaces according to the invention, also includes the reverse, namely representations of surface portions of the specimens that are not in contact with at least one of said clamping surfaces, since also this can be consider a form of representations of surface portions of the material which are in such a contact. Essential is that the topography is determined through said representation.

DETAILED DESCRIPTION OF EMBODIMENTS

With reference numeral1is generally indicated a device according to the invention for determining the dynamic topography of a material surface and in particular the surface of a paper, in relation to its ability to receive a print through mechanical contact.

The device1includes an illuminating and representation unit2and a compression unit3. The unit2includes a prism4which can be of the Chapman prism type, an illumination device5and a camera6with associated lens system. The lower surface of the prism4, as seen on the image, is a clamping surface7. The beam path from the illumination device5is indicated with broken lines, wherein with8is indicated light inciding against the clamping surface7of the prism and with9reflected light from the clamping surface on the prism where there is no contact with the surface of the test piece.

In the portions of the contact surface against which the paper come into contact, according to the above, the inciding light8will be refracted through the clamping surface7of the prism and illuminate underlying portions of the paper. These illuminated portions will thereby become light. Light from these illuminated portions that reaches the camera6are in the figure indicated with10. See in this connection alsoFIG. 1b. The positions where (only shown for the sake of clarity) light beams are refracted because of prevailing contact are marked with rings.

A test piece to be evaluated is indicated with40, and it can be held pressed together, according to the device in the shown example, pressed against the clamping surface7of the prism with the aid of a piston-formed compression body11being included in the compression unit3. This compression body11is in the shown example provided with a clamping surface27directed against the test piece40having the area “a” whereas its lower side, which is affected by a load (as described below) has the area “A”.

The compression body11is movable up and down in a cylinder housing12and is sealed against the inner surface of the cylinder housing through a sealing ring13. Whereas the compression body has a small mass, damage to the lower side of the glass body is avoided. Thereby a compressed position can be reached in a very short time.

In the figure is for clarity reasons the parts4and11shown separated so far that there is shown distances also between the respective clamping surface7and27and the test piece40. In compressed position, of course, the test piece is lying squeezed between the clamping surfaces7and27.

A pressure conduit20is connected to the cylinder housing12and is also connected to a valve16, which is connected to a control unit18.19indicates a computer unit which is connected to a display screen22, which on the one hand can be a touch screen for the actuation of the device, on the other hand be a screen which is connected to the camera6for showing images captured by the camera6during a test process.

In order to obtain pressure forces exceeding what is directly possible by the aid of available pressure air, the invention has been provided with a pressure rising unit T14, wherein normal system air pressure (for example 6 bar) can be stepped-up to a required, higher pressure (for example 15 bar), which can be stored in an accumulator tank A15.

InFIG. 2is shown the load as a function of time for a pressuring process performed by the device according toFIG. 1. At the time t0an application load Pais applied onto the compression body11inFIG. 1in order to firstly come close to the clamping surface7of the prism4and thereby avoid dynamic forces in the following step, secondly thereby to ensure parallelism between the clamping surface27of the compression body11and the clamping surface7of the prism. Actuating the cylinder housing12with load of the comparatively low load level Pacan be made in different ways. InFIG. 1is shown a regulator R21, which after signal from the control unit18provides this pressurization. By applying a low initial application load Pathis way, the compression body11will move in the direction of the glass prism and lie against and hold the test piece in position. The compression body11will thereby be positioned parallel to the lower side of the prism.

At the time t1inFIG. 2a control signal is emitted from the control unit18to displace the valve16into a position A, where the pressure in the pressure tank15is transferred to the cylinder housing12. Hereby a quick pressure increase takes place in the cylinder housing12up to the pressure level Pmaxat the time t2. The pressure level Pmaxcan be chosen such that it corresponds to a chosen printing process, for which the paper or the like is to be evaluated.

When the pressure Pmaxhas been reached, one or more images are captured, for example at the points in time t3and t4. It can also be the case that images are captured during pressure build-up between the points in time t1and t2. The time point t5indicates release of the pressure inside the cylinder housing12, whereby the valve16is moved to a position V which connects the cylinder housing12to an under-pressure17.

It is also possible through construction of the valve and the valve actuating device17to alter the characteristic of the curve inFIG. 2for example such that it can assume different non-linear paths or be made more or less flattened. The progress of the pressure increase between the pressure Paoch Pmaxcan also be controlled as desired.

Through the shown construction of the compression body, a relatively low pressure can be used for pressurizing the chamber such that a higher load is obtained against the test piece through the relation between the areas: A/a.

FIG. 3illustrates a method sequence according to an embodiment of the invention, wherein:Position30indicates the start of the sequence.Position31indicates the insertion of a paper test piece in the slot between the two clamping surfaces and choice of parameter values such as maximum load level, pressure build-up characteristics and points in time for image registration for the test in question.Position32indicates initiation of a low application load Paonto the compression body.Position33indicates initiation of a pressure pulse Pmaxonto the compression body.Position34indicates capturing of one or more representations of the compressed test piece with one or more exposures, by letting the control unit synchronize the image registration either at a certain point in time after initiation of the pressure pulse, at a certain pressure level or at a certain time after reaching a certain pressure level.Position35indicates evaluation of said image (images) or representation (representations).Position36indicates the termination of the sequence.

The invention can be modified within the scope of the following claims. The illumination and representation unit can thus be constructed differently. For example it could be that illumination is made differently, which is indicated inFIG. 4through a transparent compression body11′, which presses against a quickly reacting pressure sensitive foil24, which in turn lies in contact against a test piece40′, which is pressed against a rigid flat body4′. At contact areas of the foil24it is arranged to for example change colour in order for local variations of the load distribution of the surface to be registered through the camera6′.

In an alternative embodiment a representation can also be established from surface portions of the specimen which are not in contact with the prism. This can most simply be understood such, with reference toFIGS. 1 and 1b, that a camera6or the like is positioned for registration of the reflected beams9. This method can be used in order to obtain a corresponding or essentially reverse representation to the embodiment which is appearing fromFIG. 1.

Compression using a compression body can also be made differently than through the shown pneumatic method, for example purely mechanical by way of a mechanical or hydraulic actuation device or otherwise. Variants of the invention can use other means for obtaining pressure even if the pneumatic method, which is shown inFIG. 1, is preferred.

Further the surface of the compression body can be constructed differently or have different properties. On the one hand the clamping surface can be rigid. On the other hand it can be elastic such as provided with a rubber blanket, whereby the advantage is obtained that conditions can be simulated in a direction of printing with a rubber coated offset cylinder, even if the very rubber blanket will act on a non printing receiving surface of the test piece, since the elastic blanket will partially exert an increased impression of the test piece against the prism or the corresponding contact surface.

In a practical embodiment of the invention, the distance is small between the mutually movable surfaces that compress the test piece by the compression body firstly being applied with a lower load from a lower pressure of for example 0.3 bar during a short while in order to set the compression body in parallel with the glass body. This application force is typically held between about 4 and 25% of said predetermined load depending of the printing method to be simulated and is preferably less than 10% of the maximum pressuring force in order not to affect the test result.

The space under the movable compression body is thereafter loaded with a pressure air pulse set such that the paper is compressed with the desired load in a very short while between the two compressing flat surfaces. After having reached a set test load after a predetermined time, for example after initiated process, (typically after 1-100 milliseconds) one or more images of the contact surface are captured through the upper glass body. At terminated testing, the movable compression body is lowered by means of an under-pressure which is applied to the chamber of the cylinder housing such that the test piece can be removed.

Registered images are analyzed and evaluated in respect of the likelihood of “missing dots”, which is presented as a quality measurement.

The test piece40,40′ can also be evaluated in respect of the dynamic compressibility of the topography as a function of time starting out from a plurality of representations made at different points in time. This can be of great value in order to fully predict the suitability of the material in different printing processes, since it can be essential, for example to get an image of the behaviour of the material in processes of different duration. The means for carrying out this evaluation is suitably provided within the computer unit19.

Other test materials than paper can be examined with a method according to the invention such as for example different textiles or plastic materials.

The load levels that can be appropriate are those that correspond to different printing processes, for example: 2.5 MPa in order to simulate flexography, 5.0 MPa in order to simulate offset printing and between 7.5 MPa and 15 MPa in order to simulate different types of typographic printing methods. In general terms it could be said that a normal load range within which the invention is used is between about 1 MPa and 20 MPa. Other load values can, however, come into question.