Method and apparatus for measuring waviness of paper

The invention provides a simple and efficient method and apparatus for carrying out a quantitative measurement of waviness in a sample of paper. This is done by stretching the paper sample to the point where all waviness is removed and measuring the elongation produced at that point. The observation of waviness and its removal is facilitated by illumination of the paper sample with low incidence light while the sample is being stretched.

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.degree. and preferably at
 an angle of between 5.degree. and 10.degree.. 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
 micrometer reading would give a direct percent measurement without
 necessity of providing some corrective factor.
 The holding clamp may comprise a lower and an upper plate as well as a bolt
 and a nut, such as a butterfly-nut, interconnecting the two plates. One
 end of the paper sample is inserted between the two plates and the bolt is
 tightened by the nut to clamp said end. The holding clamp may be provided
 with an adjusting screw in order to slightly move the holding clamp
 horizontally and properly position the paper sample flat on the table and
 remove any warpage or the like, that may exist without, however, affecting
 the waviness or fluting to be measured.
 The tensioning clamp may again comprise a lower and an upper plate as well
 as a bolt and a nut, such as a butterfly-nut, interconnecting the two
 plates. It is mounted at the opposite end of the table so as to clamp the
 other end of the paper sample which is inserted between the plates and
 clamped by tightening the butterfly-nut. Other clamping devices can be
 used as the holding clamp and tensioning clamp, for instance spring loaded
 clamps, pneumatically loaded clamps and the like. In all cases, the inner
 surfaces of the clamps should provide sufficient friction to prevent any
 sample slippage. For example, such surfaces could have a knurled pattern
 or be covered with a suitable rubber or like lining. The tensioning clamp
 is connected to the measuring means for measuring the elongation produced
 by the waviness after stretching of the paper sample has been completed.
 If the measuring means is a micrometer, it can be connected to the
 tensioning plate by means of a U-shaped bracket and a bolt. The bracket is
 mounted so that it can slide horizontally; for instance its two parallel
 side arms may be placed on a sliding surface.
 The measuring means may be a standard micrometer the spindle of which is
 connected to the bottom part of the U-shaped bracket in such a way that it
 can pull the bracket and with it the tensioning clamp in which one end of
 the paper sample is clamped. This can be done by means of a collar mounted
 on the spindle, preferably with a nylon disc placed between the collar and
 the bottom of the bracket to reduce friction. The micrometer can be held
 in position, for instance by means of a C-shaped bracket, so that its head
 is supported vertically on a suitable surface.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 The preferred embodiment of the invention is illustrated, but not limited,
 by the appended drawings where the same reference numbers are used to
 designate the same parts in both figures.
 Referring to FIG. 1, it illustrates both the method and the apparatus of
 the present invention for measuring waviness of paper. This is done by
 placing a sample piece of paper 10 having waves therein, flat onto a
 supporting table 12.
 The paper sample has a length A, such as to provide a representative
 measurement of the waviness of the paper. In this example, the length is
 10 inches (25.4 cm) which has been found satisfactory for this purpose.
 The paper sample 10 is clamped at one end by means of holding clamp 14
 which comprises plate 16 and plate 18 interconnected by means of a bolt
 (not shown) that may be tightened by a butterfly-nut 20. One end of the
 paper sample 10 is inserted between plates 16 and 18 and the nut 20 is
 thereafter tightened until said end is firmly clamped and held between
 said plates. If the paper sample 10 is not positioned entirely flat on the
 table 12 but shows some warpage or the like, an adjusting bolt-nut
 combination 22 is provided to enable movement of plate 18 to such extent
 as to remove any such warpage, but without affecting the waviness of the
 paper sample 10 to be measured.
 The opposite end of the paper sample 10 is likewise clamped between plates
 24 and 26 interconnected by a bolt with a tightening butterfly-nut 28
 forming a tensioning clamp 30. The lower plate 26 of this tensioning clamp
 30 is connected via bracket 32 to the micrometer 34 so that by turning
 thimble 36 of the micrometer 34, the paper sample 10 may be stretched
 until all waves or wrinkles present therein disappear and the paper is
 completely straight. This point is readily observed due to the
 illumination of the paper sample by means of a source of low incidence
 light 38 placed at a suitable angle to the paper sample 10, in this case
 at an angle of 10.degree.. This is one of the basic features of the
 present invention, since it is important to provide just enough tensioning
 of the paper sample 10 to remove all waves or wrinkles from the paper and
 render the paper sample straight without, however, further stretching the
 paper. It has been found that this can be well observed with the use of
 the low incidence light projected onto the paper sample 10 as described
 above. The angle of incidence can be adjusted to optimize the visibility
 of waviness by the operator.
 The head 38 of the micrometer 34 is held in a vertical position by clamp 40
 on the base 42. As shown in FIG. 2, clamp 40 is a C-shaped clamp having a
 tightening bolt 39 which allows to clamp the micrometer head 38 against a
 block 44 provided for this purpose. The bracket 32 is a U-shaped bracket
 connected to the clamp 30 by means of bolt 46 at its top or open end,
 while its bottom end is connected to the spindle 37 of the micrometer 34
 by means of collar 48 and a nylon disc placed between this collar 48 and
 the bottom of the U-shaped bracket 32. This bracket 32 is slidably
 supported on blocks 41, 43, 44 and 45.
 The measuring operation proceeds as follows:
 At the start of the operation, the gauge of the micrometer 34 is set to
 "zero", while making sure that the tensioning clamp 30 abuts the end of
 the supporting table 12 and also that the collar 48 provides a good
 connection between the bracket 32 and the spindle 37 of the micrometer 34.
 Then, the paper sample 10 is cut to an appropriate size. With table 12
 being 10 inches (25.4 cm) long, the paper sample is cut to have a length
 of 11 inches (28 cm) and a width of 2 inches (5.1 cm). At least three such
 samples should be provided and tested to have a representative
 measurement.
 Then the holding clamp 14 is opened and the end of the paper sample 10 is
 inserted between plates 16 and 18 thereof. Thereafter, the tensioning
 clamp 30 is opened and the other end of the paper sample 10 is inserted
 between plates 24 and 26 thereof. The paper sample 10 is then properly
 centered between the two clamps 14 and 30 and the holding clamp 14 is
 tightened by means of the butterfly-nut 20 to clamp the end of said paper
 sample 10 therein. Making sure that the paper sample 10 is lying flat on
 the table 12, the tensioning clamp 30 is tightened at the other end of the
 table 12 by means of the butterfly-nut 28 to clamp said end therein. The
 sample 10 should lie flat on the table 12 to get an accurate reading. If
 there is some warpage, looseness or the like, one can use the adjusting
 bolt-nut 22 to slightly move the holding clamp 14 horizontally until full
 flatness of the paper sample 10 on the table 12 is achieved without,
 however, overstretching the paper sample and thereby removing some of the
 waviness to be measured.
 Once the paper sample 10 is properly in place on the table 12 and clamped
 between clamps 14 and 30, a low incidence light is projected thereon from
 the light source 38 which is positioned above the holding clamp 14 so that
 the light is directed toward the tensioning end of the paper sample. This
 could be achieved using any suitable light source, including a flashlight.
 The angle at which the light is so projected is such as to permit good
 observation of the waviness of the paper sample 10 as well as of the point
 of disappearance of said waviness when the sample has been sufficiently
 stretched. Normally, it is between 0.degree. and 20.degree. and preferably
 between 5.degree. and 10.degree., and is adjustable, if desired.
 The thimble 36 of the micrometer is then turned so as to pull the spindle
 37 and thereby stretch the paper sample 10 until a point is reached when
 all waviness has disappeared and stopping at this precise point. Using a
 micrometer that has dimensions from 0-1 inch (0-2.54 cm), this would
 produce a certain gauge reading, for instance 0.057 inch. This
 displacement gives a direct reading of an extension of the paper sample of
 0.57%. This corresponds to the percentage of the waviness of the sample.
 Such measurement will allow the operator to determine whether the
 calendering or other operation in the manufacture of the paper needs
 adjustments due to excess waviness.
 It should be understood that the invention is not limited to the specific
 embodiments described above, but that many modifications obvious to those
 skilled in the art can be made without departing from the spirit of the
 invention and the scope of the following claims.