PHYSICAL PROPERTIES OF PAPER
There are some physical properties to which the paper manufacturers and paper users should pay attention, during both the production and printing. The physical parameters such as the paper grammage, thickness, the Cobb value, strength, surface smoothness and porosity values, optical properties, surface pH, water way are extremely important for the integration of paper- ink-printing press in the printing stage.
The grammage is the weight of 1 m2 of paper in grams. Some physical characteristics of the paper show change depending on the grammage. Therefore, it must be controlled in the machine width and machine height directions during the production in order to prevent the influence on the quality of printing and use. The grammage is measured in laboratories with sensitive electronic scales.
It is the measurement result of the distance between lower and upper surface of a single paper piece in microns. The thickness of paper must be homogeneous both in the same layer and in the successive layers. Because the differencies that may occur between the thickness values lead to the creases during printing and different ink intake and therefore the formation of tonal differencies. The thickness value is also important in terms of blanket setting and the adjustment of the ink pressure to be given.
It is the amount of water included in the paper. The high or low humidity causes negative consequences in terms of quality. The decrease in paper humidity causes paper fragility. The high paper humidity affects the printing quality because it delays the physical and chemical drying of the ink on paper. The humidity value in the storage conditions is important for paper as much as the humidity in the production output. The paper has a tendency to dehumidify in the environment of which humidity is higher than itself and in the contrary case to lose humidity due to its hydrophilic structure. Therefore the environment humidity should be in equilibrium with the paper humidity. If this balance is not present, humidity exchange occurs between the environment and paper until the humidity is balanced, this affects the flat
standing of paper and may cause the undulation and curling problems that affect the printability and processability of the paper. The ideal relative humidity is 55% in the storage conditions.
It is a measurement of the resistance of the paper surface to the water or its water acceptance. It is the value of water amount in gr/m2 absorbed by paper of 1 m2 within a certain period and this period is 120 seconds in label and carton papers, 60 seconds in photocopy and coated papers, in world standards. Cobb value is required to be kept at optimum range. The low or high Cobb value affects the printing quality significantly at the contact of ink with paper. The water used with printing ink in the ofset printing technique, contacts the surface of the paper by printing rubber during printing. If this water used is not taken controlledly by the paper surface, it may lead various printing problems.
The strength is the largest breaking strength per unit width resisted by the paper before breaking. It is controlled in two ways as machine direction (length) and the reverse machine direction (width). The strength value after the printing is also important as much as the strength before printing. After the four-colour printing, at least 28 % of increase is expected in the strength value. If this increase does not occur, this means that there is a paper and ink imbalance. The increase in the strength value can go up to 100 %.
Surface smoothness is known as finishing in the printing industry. As the finishing improves, the need for adequate print ink decreases and printing quality and printing brightness increases. The surface smoothness is measured by two different methods. The air speed between paper layer and a flat surface is measured in the measurement method with Bendtsen Method. Ml/ min is used as unit and as the numerical value decreases, it is understood that the surface is smoother. The transition period of the air between paper layer and a flat surface is measured in the measurement method with Bekk Method.
Second is used as unit and as the numerical value increases, it is understood that the surface is smoother. There are optimum smoothness values according to the type and usage of paper. For example, in the photocopy papers, very high surface smoothness value is not required for that cause problems during photocopy. In contrast, the coated paper is required to be good in surface smoothness values.
It is the average airflow amount through the unit area under the unit pressure difference per unit of time. The air permeability is stated as ml/ min. As the porosity value increases numerically, it is understood that the air permeability of paper is high. The porosity value should be at the optimum level according to the type and usage of the paper. The high porosity value in the printing papers affects adversely the printing quality because it creates problem during the penetration of ink to the paper.
Whiteness is an indicator of blue light reflected from the paper surface. It is measured at R457 wavelength. The whiteness value changes depending on whether the UV filter of the measuring device is on or off. For example, as a result of the measurement made with a device with a closed UV filter, 86.0 measured whiteness value can be measured with the same device while the UV filter is open.
The numerical increase of the whiteness value indicates that the whiteness of the paper is high. The whiteness value of both surfaces of the paper is desired to be the same. Optical brighteners and indigo dye are used to increase the visible whiteness.
The values obtained under the D65 daylight light source correspond to the visible whiteness of the paper with or without fluorescent whitening agents. The numerical increase of the value is expressed as the increase of the visible whiteness in the paper.
It is the light transmittance of the paper. It is used to determine the a paper’s degree of displayin the same type papers under it and it is expressed in percent. It is an important feature for printing and envelope papers. It comes to the forefront in the low-weight papers particularly. The opacity must be high because the paper’s thinness increases the risk of print to appear on the reverse side.
SURFACE pH VALUE
pH value is the unit of measure describing the acidity or alkalinity of a solution. It can be measured with a pH meter or litmus paper. It is valued between 1 and 14 numerically. A pH value of less than 7 indicates that the acid concentration in the solution is high, a higher than 7 indicates that the base concentration is high, and a 7 indicates that the solution is neutral.
The sequences of the fibers forming the paper in the sheet form the water way of the paper. The flow direction of the paper in the paper machine during the manufacturing process is called Water Way, the direction in the machine width is called Reverse Water Way. The changes in some physical properties of the paper such as standing flat, strength, tearing are further observed in the water direction which is the sequence direction of the fiber, therefore the Water Way concept is a property to be considered in the subjects such as creasing line, curling, etc.
The second number of the paper size shows the water way as a general rule in our country. For example, the water way of a 64*90 size paper is parallel to the 90 cm length edge. Also, the water way of a 100*70size paper is parallel to the 70 cm length edge.
World Digital Technology Developments Symposium
OFFSET: The usage rate will remain the same.
ROTOGRAVURE: The usage rate will remain the same.
FLEXO: The usage rate will increase.
INK-JET: The usage rate will increase.
TIPO: The usage rate will decrease.
Reference: The research results of 2004 of the DOW company have been taken as a basis.
OFFSET: Pigment based, high density ink,
ROTOGRAVURE: Solvent/ water and pigment based, low density ink,
FLEXO: Solvent, pigment based ink.
TIPO: Pigment based, high density inks are used.
Sheet Offset Printing Technique
The offset printing technique is based on the principle that oil-based printing ink and water do not mix. The image is transferred onto the mold by photogrophic methods. The difference between the areas to be printed and not to be printed is obtained by chemical means and height difference is not created on the plate surface. Each unit of the sheet offset printing machines has a die cylinder, blanket, pressure roller, ink and water application systems.
Web Offset Printing Technique
The ofset printing machines that are capable of high quality four color printing were become widespread at the end of the 50s and at the beginning of the 60s. Web ofset printing is similar to the sheet ofset printing as principle and the printing configuration has been developed. In this new system, both sides of the paper can be printed simultaneously. Today the most common
offset machines are the machines on which four or more printing units are present and which are capable of printing through single pass between these units to the paper. Printing units from four units up to eight units are present in some of these machines. It is automatically handled after the drying process. If the paper used in the web ofset printing machines is uncoated, an extra drying process is not necessary because that the drying of the ink will be through absorption. Drying of the ink is realised by evaporation of the solvent in its composition. Printing speed is high.
Properties of Coated Paper to be Used in Offset Printing
A large increase in the use of coated paper in ofset printing systems is observed. The coated paper holds the ink on the surface of the paper and the ink provides the highest brightness on the surface of the paper, thus high gloss prints can be obtained with coated papers.
The viscosity and accordingly the viscidness of the ofset printing inks are extremely high due to their structure.
During printing, the paper should show sufficient strength against the tensile force by the printing ink on the paper surface, it should not blister and the coated paper should not be pulled up.
The homogeneous grammage of the thickness, humidity and coating in the transverse and longitudinal profiles of the paper is particularly important in ofset printing for operability. The homogeneity of the brightness and smoothness affects significantly the printing quality.
In the Offset Printing;
The tensile force applied to the paper surface during printing is a function of printing force and printing speed at the same time. The excessive increase of the speed and force or different force in the machine width may cause blistering problems in printing. Due to the high viscosity and stickiness of the offset printing ink, the tensile force applied to the paper surface during the printing is high. On
the other hand, this feature of the printing ink is highly dependent on the temperature and with the temperature decrease, the viscosity and accordingly the tensile force applied on the paper surface increases in large quantities.
In order to reduce the viscosity and stickiness of the printing ink at a certain temperature, the plasticizers and solvents are used. When the blistering problem occured during printing for any reason, the solution of the problem is possible with the use of these additives.
The rotogravure printing has been developed as high speed, roll-feed, to be suited to the rotary printing. The printing is made from copper cylinders or copper plates fixed to the plate cylinders. Chrome coated plates are used for multi-printing works. The design is taken on to the printing plate through photograph. The rotogravure printing was used in the monochromic printing of the Sunday supplement of the newspapers but today it is used in polychrome and bright printing of the magazines, packaging papers and the catalogues additionally.
The Properties of Coated Paper Suitable for the Rotogravure Printing
When the printing is done with rotogravure process, paper enters into the holes of the plates and gets the ink contained there. But the paper must have good surface smoothness and must be soft and flexible to allow printing on the plate holes. During printing, while high speed printing, in the areas where the paper surface is not smooth enough, some areas will get less ink or will not get any ink because the paper and the plate will not come into contact. In order to get a very good printing result, the paper need to go through the calendering process. The paper should be sufficiently opaque for the printing on the back side of the paper not to be seen, but it does not contain hard filler substances that may scratch the glossy surface of the printing plate.
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