Importance of physical properties of paper during printing
Physical Properties of Paper
During production and printing, there are certain physical properties that paper producers and users should consider. Physical properties such as basis weight, thickness, humidity, Cobb value, tensile strength, surface smoothness, porosity value, optical characteristics, surface ph value, grain direction are all important for consistency of paper with the ink and the printing machine.

Basis Weight
It is weight of the paper in grams in 1m2 area. Some physical characteristics of paper differ according to basis weight. Therefore, to avoid negative effect of print quality, basis weight should be checked machine horizontal directions. Basis weight check must be performed by professional equipment in laboratories.

Thickness
It is the distance between front and back sides of a paper sheet in terms of microns. The thickness of paper should be uniform in the same sheet and the preceding sheets as the variations in thickness causes color differences. Thickness is also important for blanket balance and ink pressure.

Humidity
It is the water amount contained within the paper. High or low humidity may cause negative effect on print quality. As the humidity rate decreases, the paper becomes more fragile. Excessive humidity delays ink drying process and negatively effects the print quality.  Apart from the humidity rate after production, humidity rate during storage is also important.  As paper has a hydrophilic structure, it is likely to absorb or bleed moisture when stored at different humidity conditions than itself.  Therefore the humidity rate at storage room should in accordance with the paper. Any inconstancy causes humidity traffic that result in fluctuations on profile and curling problems. The optimum relative humidity rate is 55% in storage conditions.

COBB Value
It is the resistance or absorbance of paper surface to water. In other words it is the water absorbed in 1 m2 at a certain time. According the international standards, it is 120 seconds in label and cardboard types and 60 seconds in offset, photocopy and coated papers. Cobb value should be at the optimum range. Lower or higher value of Cobb effects the print quality during contact of ink with paper. In offset printing, water used with the ink contacts paper by blanket. Print problems will occur unless the water is transferred under control. 

TENSILE STRENGTH
Tensile strength is the resistance to break per unit width before the paper breaks. It is measured in machine direction (MD) and transverse direction (TD). Both before and after print elongation values are important. For instance, after 4 color printing, elongation value should increase by at least 28%. Unless this increase is observed there should be inconsistency with the paper and the ink. Increase in tensile strength value can be observed up to 100%.  

SURFACE SMOOTHNESS
As the surface smoothness increases ink use to cover surface decreases, the print quality and brightness increases. Two methods are used in measuring surface smoothness: Bendtsen Medhod and Bekk Methods. With Bendtsen Method air transmission speed between a paper sheet and a flat surface is measured in terms of ml/min. As the value degreases the surface is considered smoother. With Bekk Method, air transmission period from a paper sheet through a flat surface is measured in terms of seconds. As the value increases, the surface is considered smoother.
There are optimum smoothness values according to paper types and use areas. For instance, higher surface smoothness is not preferred in photocopy papers as it may cause problems during photocopying. However, higher surface smoothness is required in coated papers.     

POROSITY
It is the amount of air flown though unit area at a unit time under stable pressure. It is measured as ml/min. As the porosity value increases, air permeability increases. Porosity value should be at the optimum level according to paper type and usage area. In printing papers, higher porosity value causes problems during ink contact with paper and negatively affects the print quality.

Optical properties :
a- Brightness
Brightness is the ratio of light reflected by a material to light reflected by a totally reflecting material. It is value in R457 wave length. Brightness value may differ with measurement tool’s UV filter being on or off. For instance brightness value when the UV fitler is on is 86,0 and 93,0 when the UV filter is off.
Higher brightness value is an indicator of higher brightness of paper. Both faces of paper should have the same brightness value. In order to increase the visible brightness, indigo and optical brighteners are used.     

b- CIE
It is the visible brightness of paper containing or not any whiting material under D65 daylight source. Higher value indicates higher visible brightness.

c- OPACITY
It is the light transmission of paper. It is used to check visibility rate of a paper with same types of paper below. Opacity is expressed in percentage and important for printing papers and envelope papers and low basis weight papers. For instance, the print may be visible from the back if opacity is not high.  

d- Surface Ph Value
Ph value is concentration of acid or base in water. It can be measured by Ph meter or litmus paper. Numerically it is between 1 and 14. Ph value is less than 7 means acid concentration of water is high and when the Ph value is higher than 7 means base concentration is high and 7 pH value means neutral.
The ink chosen and surface of print paper should be in conformity. Conditions during paper production must be compared when choosing acidic or basic inks. In order to prevent problems such as corrosion of ink, color variations and drying problems, the pH value of water used in reservoir during printing should be under control.    

e- Grain direction
It is the array of fibers forming the paper in a page. During production, machine direction of paper defined as grain direction and transverse direction is defined as reverse grain direction.
Generally, the second value of paper size is the grain direction. For instance, grain direction of a sheet 64 x 90cm is parallel to 90 cm side. 

Offset printing Techniques

Sheet fed offset printing technique
Offset printing technique is based the principle that oil based print ink do not contaminate with water. The image is transferred on mould by photographic technique. Printing and non printing areas are differentiated by chemical way. There should be not thickness variation on the plate. In all units of sheet fed offset printing machines, mould cylinder, blanket, pressure cylinder ink and water application systems are present.

Web Offset Printing Technique
4 color printing machines with high print quality became widespread by late 50s and early 60s. Web offset printing has similarities with offset printing with better print configuration. With this new system both sides of paper can be printed at the same time. Today, the most common offset printing machines can print at a single run using all color units(4 or higher). These machines contain from 4 up to 8 printing units. Outputs are bundled after passing through drying unit.

In case the paper used in Web offset print machine is uncoated, drying process in not necessary as the ink will dry by absorbance and evaporation of solvent in its structure. Printing speed is high.

Properties of coated paper used in offset printing

Coated paper use in offset printing has been increasing. The coating holds the ink on paper surface. Since the ink provides the best brightness on surface of paper Very bright print quality can be obtained with coated paper.
Due to the structure of offset printing inks viscosity is higher and as a result, they are sticky.  
Despite the pull strength of print ink on paper, the paper should be resistant, should not swell and its coating should not tear off.  
Homogeneity of thickness, humidity and coating amount when measured by all length and width is essential for printability in offset printing machines. Brightness and surface smoothness affect the print quality.

In offset printing;
The pull force on paper surface applied is also a function of print force and print speed. Excessive increase of print force or print speed or irregularity of print force though machine width may cause swelling problems.
As the viscosity and stickiness of printing ink are high, the pull force applied during printing is high as well. On the other hand, this property of printing ink is related to temperature and when the temperature decreases, viscosity and as a result, the pull force applied on paper surface increases rapidly. 
In order t decrease viscosity and stickiness of the printing ink at a certain temperature, softeners and thinners are used. In case any swelling problem will occur, it can be solved by these additives.

Rotogravure Printing

Rotogravure printing machines are designed to make high speed, roll fed rotating printing. Printing is performed through copper cylinders or print plates fixed to plate cylinders. For multiple printing jobs, chrome coated plates are used. Design is transferred to plate by photographic method. Rotogravure printing had been used top print single color additions of newspapers on weekends, and today it is used to print multi color magazines, packaging paper and catalogs.

Properties of coated paper in rotogravure printing
During rotogravure printing, the paper absorbs the ink by going through concave parts in the plates. The paper should have a smooth surface and should be flexible to provide printing when passing through concave parts. During high speed printing, the plate will not contact wavy parts and therefore these parts will absorb less or no ink.
In order to get the best printing result, the paper must go through calendaring process before printing.
Opacity of paper must be at optimum level to avoid visibility of pritning a the back and the paper should be free of any rigid material which may damage the printing plate.

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