Optics: measuring and testing – By monitoring of webs or thread
Reexamination Certificate
2000-01-12
2001-04-17
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By monitoring of webs or thread
Reexamination Certificate
active
06219141
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and an apparatus for measuring waviness of paper. In particular it provides a quantitative measurement of such waviness which may be due to calendering or other operations used in the production, converting or printing of paper products.
2. Description of the Prior Art
It is well known that during the manufacture of printing paper, the latter is subjected to calendering, i.e. the paper is pressed between a series of rolls in order to improve its surface printing quality. When calendering is very intense, such as soft calendering or supercalendering, where a major reduction of the paper thickness is produced, and particularly when a small number of calendering nips is used, this may result in some waviness in the transverse direction of the paper sheet. Some of this waviness, which can also be called fluting, puckering or cockling, results in waves, wrinkles or compression lines that are permanent and will appear in the printed product where they affect the quality of the paper product by interfering with the reflection of light. This is unsatisfactory, since one of the most important features of printing paper is its gloss, namely its capacity to reflect low incidence light. Moreover, some printing processes, such as heatset offset, may aggravate the waviness of the paper because of application of heat and water during the printing operation.
Examination of various samples of soft calendered and supercalendered papers has shown various degrees of waviness, from light to heavy. The analysis of manufacturing conditions of such samples suggests a relationship between the intensity of calendering and the level of waviness. Thus, in order to better understand this phenomenon and quantify the calendering and other processing parameters, the need for a suitable method and apparatus for measuring such waviness has become apparent.
Several prior art references disclose methods and/or apparatus to measure crimp frequency in crimped material, warp measurement of surfaces such as corrugated webs or sheets, fabric extensibility, and roughness on a web of paper and the like.
Thus, U.S. Pat. No. 4,274,746 of Jun. 23, 1981 provides for the measurement of crimp frequency of crimped material by utilizing the pattern of light reflections produced by the waviness of the material.
U.S. Pat. No. 5,339,534 of Aug. 23, 1994 discloses a device to measure warpage of a surface where a dial indicator is mounted in the middle for measuring deflection of a resiliently flexible band which deflects to conform to a warped surface when placed against it.
Canadian Patent No. 1,331,702 of Aug. 30, 1994 refers to an apparatus for measuring fabric extensibility where the fabric is mounted between two clamps on a base with an arm pivotally mounted intermediate the two ends of the base for rotation about a horizontal axis. When a tensile force is applied to the fabric, it produces an angular deflection of the arm about its axis, which is measured to indicate the extension of the piece of fabric.
Also, Canadian Patent Application No. 2,111,842 laid open Jan. 1, 1993 provides for a roughness detector on a travelling surface of a web, such as paper, by directing a beam of polarized light at an angle onto the web, focused by a focal lens to illuminate a spot on the surface and then collecting a specularly reflected component of the light through a second lens focused on the spot. A detector is aligned to receive a portion of the specularly reflected collimated light and generates a signal depending on the intensity of the light to provide an indication of the roughness or smoothness of the surface.
None of these prior art references deal with the measurement of waviness in paper after calendering or the like, as is done pursuant to the present invention, nor can they be adapted for such purpose.
OBJECTS AND SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a method and an apparatus for measuring waviness or fluting of paper resulting from calendering, particularly soft calendering and supercalendering, and other operations in the production, converting or printing of paper products.
Another object is to provide a simple and efficient method and apparatus for carrying out the aforesaid task that provides a single number to quantify the fluting intensity.
Other objects and advantages of the invention will become apparent from the following description thereof.
The waviness or fluting measured in accordance with the present invention refers to a deformation of the paper web, essentially in the cross-direction, resulting from calendering, particularly soft calendering and supercalendering, and other operations in the production, converting or printing of paper products. A portion of this deformation is permanent and some other portion may be of a temporary nature. Studies have also shown than waviness may be amplified once again by the printing process. In this state, a sample of fluted paper acts like a spring that can be extended and contracted. Thus, the sum of waves in such sample, i.e. their number and amplitude, is equal to the extension required to make the paper sample fully straight. In other words, it corresponds to the stretch required to reach full sample unwaviness. The present invention relies on the finding that the waviness produced as described above and also the straightness or unwaviness of the paper can be readily visible by means of a low incidence light projected thereon. Such low incidence light is normally projected at an angle not exceeding 20° and preferably at an angle of between 5° and 10°. The angle of incidence may also be adjustable to optimize the visibility. The method of the present invention therefore comprises stretching a sample piece of paper until it is fully straight while observing it with the help of the low incidence light, and once it is fully straight with no waviness or fluting of any kind remaining therein, measuring the elongation by means of a suitable instrument, such as a micrometer, a dial indicator or a linearly variable displacement transducer (LVDT).
It should be noted that the low incidence light used pursuant to the present invention is not employed for determining the waviness or crimp frequency as in the case of U.S. Pat. No. 4,274,746 or the roughness as in the case of Canadian Patent Application No. 2,111,842, but rather to observe that all waviness has disappeared and the paper is completely straight, at which point the elongation is measured, thus providing the exact measure of the total waviness or fluting in the sample of paper being tested.
The apparatus of the present invention comprises:
(a) a supporting table for supporting a paper sample;
(b) a holding clamp at one end of the table for clamping and holding one end of the paper sample;
(c) a tensioning clamp at the opposite end of the table for clamping and holding the other end of the paper sample;
(d) means for pulling the tensioning clamp to produce stretching of the paper sample;
(e) measuring means for measuring elongation of the paper sample produced by the stretching of said sample; and
(e) a source of low incidence light for projecting such light onto the paper sample during the stretching of said sample to the point where waviness disappears and the paper is fully straight, said low incidence light being such as to facilitate visibility of the waviness in the sample and the point of its disappearance.
The size of the table should be such as to enable measurement of a long enough paper sample to provide a satisfactory evaluation of the waviness and also such that the total elongation is sufficient to obtain a precise measurement of such waviness. The measuring means may consist of a micrometer connected to the tensioning clamp, thereby providing a direct reading of the elongation of the paper sample, for instance as a percentage of the length of said sample. Such length of the sample and the size of the supporting table are, therefore, preferably chosen so that the m
Alliance Forest Products Inc.
Font Frank G.
Primak George J.
Stafira Michael P.
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