The uniformity of paper refers to the uniformity of the distribution of fibers and fillers in a certain area, mainly manifested in the change of quantitative and thickness on the paper web. For printing paper, it is not only required to have uniform fiber distribution, consistent thickness, and small quantitative difference, but also requires the entire paper web to be flat and smooth with little fluctuation. For example, cement bags are subjected to external forces in all directions during filling and transportation, which requires that the cement bag paper be able to share the external forces evenly when subjected to external forces. If the fiber distribution of this paper is not uniform, it will break during the filling and transportation of cement bags.
(1) Test principle
The index for evaluating the uniformity of paper is evenness. The test principle of paper evenness is to use light transmittance to evaluate, that is, to scan a given area of the tested paper surface with a beam of light intensity and light spot, and accept the light intensity changes of the transmitted paper surface point by point to obtain the average light transmittance And the deviation value, and then multiplied by the appropriate weighting coefficient, namely:
(2) Test equipment
The test instruments for paper evenness include the American m/k evenness test system and the evenness meter of Shanghai Paper Institute. The American m/k formation test system consists of probe head, data processing and output. Figure 2-4 is the optical path of the American m/k uniformity testing system. The sample is installed on a borosilicate glass rotating cylinder. A light bulb is installed on the axis of the cylinder, and a photocell is installed outside the sample at a position perpendicular to the light bulb. , the two parts move linearly along the axis parallel to the axis with the rotation of the cylinder, and the scanning line that the sample touches is helical on the sample. After the light is emitted from the bulb, it is transformed into parallel light by the lens, and then irradiates the sample in parallel, and the diameter of the spot is about 7mm. After the light passes through the sample vertically, it is focused by the lens, passes through a grating with a diameter of 1mm (the distance between the grating and the sample is 7cm), and is received by the photocell, which is converted into an electrical signal by the photocell, and is finally displayed by the amplifier, A/D conversion and calculation. uniformity index.
(3) Formation test method
Now take the American m/k evenness test system as an example to illustrate the test method for paper evenness. The specific test steps are as follows.
①Sample collection and processing. Sampling according to GB450, cut 5 samples of 280mm×220mm, and carry out temperature and humidity pretreatment on the samples according to the requirements of GB 10739.
②Connect the printer, turn on the power, turn on the main switch, and let the instrument warm up for a period of time.
③Choose and install the receiving light bar, most papers use A light bar, uneven paper uses C light bar, and cellophane uses B light bar for measurement. If it is natural color paper or cardboard with higher weight, there is no light bar.
④ Select the control switch according to the paper type, 1# is suitable for general paper and cardboard, 2# and 3# are suitable for thin paper. The rules for using the control switch are: start using the lowest gear, and require the evenness F value to be greater than 2. If the F value is less than or equal to 2, use the next higher gear.
⑤ Put the standard sample on the drum of the Tester, measure its formation index, and check whether the result is consistent with the standard value. The standard sample shall be composed of wood-free paper and the supplied dark-colored plastic sheet.
⑥ Place the sample on the drum of the Tester and fix it with four rubber rollers. The sample should be attached to the glass cylinder, and the light bulb and photocell should be placed at the right end of the glass cylinder.
⑦Select the long (or short) measurement coverage distance, press the start button, the glass cylinder will start to rotate, and the bulb and photocell will move linearly along the axis of the cylinder. After reaching the set distance, the motor stops automatically.
⑧ After the glass cylinder stops rotating for 1 second, the instrument displays the formation index F, and prints out the sample number and the formation index at the same time. If necessary, press the switch of the tracer dispenser to display the frequency of the highest point of optical density change and the average optical density value, and print out the frequency distribution data.
⑨Remove the tested sample and prepare for the next test.
The test results are expressed by the average evenness index of all the measured values. The larger the evenness index, the better the evenness of the paper. When F≤5, the reproducibility is ±0.5; if the F value is between 5 and 10, the reproducibility is ±1. This shows that the reproducibility decreases as the formation index increases.
