Coating Technology
The Benefits of Coated Paper:
In recent decades, print media have had to meet increasingly high demands, in terms of visual aspects and in terms of printability. To meet these demands, coated papers were introduced many years ago.
Coating a paper enhances its optical and tactile characteristics – whiteness and shade, gloss and smoothness – but it also improves its printing behaviour, allowing the use of very fine screens, yielding more colour in thinner ink layers and producing more contrast in printed images. When paper is coated, a covering layer of pigments, binding agents and process materials is applied to the surface.
Coating Machine:
A primary reel of base paper to which a coating of any kind has been applied is used. It has an unwinding system, designed for use of a flying splice. Next, the paper is coated. First on one side, followed by drying. Then the other side is coated and dried. For drying purposes, infrared dryers, airfoils and drying cylinders are used.
The heart of the coating machine, however, is the coating unit with the integrated coating aggregate. Under each coating unit, a workstation pumps up the coating mass from the preparation tanks, where the colour is mixed. Automatic control systems continuously monitor and adjust coating quantity and humidity of the coating.
At Shree Krishna, different methods and techniques of coating are used. The cavities of the paper are filled with coating and the fibre backs remain nearly uncovered.
Coating Preparation:
Coatings mainly consist of pigments (chalk, clay or talcum). They need, binding agents to provide adherence to each other and to the paper. Depending on the intended use of the paper and the type and structure of the pigments used, different quantities of binding agents are required. Binding agents can have a natural basis (casein or starch) or a synthetic composition (synthetic dispersions).
Process materials add specific properties to the coating. One commonly used process material is optical whitener. It converts invisible, ultraviolet light into visible bluish white light, giving an impression of true whiteness. In the so-called “coating kitchen”, these individual components, taken form large storage silos, are mixed in stainless steel tanks. Each specific coating has its own recipe, exactly prescribing the quantities of each component. To preserve consistency from preparation to preparation, the whole process of coating production is fully automated.
Finishing
Calender:
Calenders are used to make the paper surface extra smooth and glossy. A calender consists of a number of rolls, where pressure and heat is applied to the passing paper.
Depending on type, the paper is ready after it leaves the coating machine or the calender.
Rewinder:
The function of the rewinder is to rewind the reels from one tambour to another tambour. Here, the web run can be changed, from the outer to the inner side, the reel edges may be cut and deficiencies in the paper can be removed.
Slitter Rewinder:
The finished paper, which on the tambour still has the full machine width, is cut to smaller reels on the slitter rewinder.
Sheet Cutter:
In a cross cutter, the smaller reels that have been cut to size from tambours by the slitter rewinder, are cut to sheets of a specified size. Several reels can be processed simultaneously, depending on the design of the Sheet cutter and the “cutting weight” of the paper. The important thing here is to produce sheets with clean cutting edges, in other words, to prevent cutting dust from clinging to the edges, since this would cause problems in the printing process. One important aspect is that the cutting process must be perfectly synchronised to produce the exactly right size and squareness.
Guillotine:
Guillotine type cutters are used for cutting relatively small quantities of paper in special sizes. Guillotines are also used for the so-called four-sided trim which is necessary for certain print jobs.
Packing and Storage
Finally, the paper is packaged for transport to the customer. The packing is important to avoid transport damages and to provide protection against moisture. Transport methods and means determine the type of packing.
Sheet size paper can be packed in reams or delivered as bulk-packed goods, with pallet packing only. Reams can contain 100, 250 or 500 sheets. In the case of smaller orders or special sizes, ream wrapping is carried out manually.
The packing material is designed according to customers’ requests. Protection against damage and moisture is the main concern, but ream wrapping can be used as an advertising medium as well.
Paper Properties
The data sheets list the most important quality characteristics of the paper. These include:
Basis Weight:
The basis weight of a paper means the weight in grams per square meter (g/m2) under conditioned circumstances.
Brightness:
The brightness (ISO) is a measure for the brightness degree of the paper expressed in percent compared with the brightness standard (magnesium oxide = 100%). The higher the brightness value, the brighter the paper is.
Gloss:
The gloss figure in the data sheets indicates the percentage of reflected light with a defined angle of incidence. A higher gloss leads to stronger light reflections and higher gloss values.
Opacity:
The opacity is a measure for the opacity degree of the paper, expressed in percent in relation to the reflected light. Paper which lets a lot of light through is transparent; paper that lets little light through is opaque. The higher the value, the more opaque the paper is.
Relative Humidity:
At a given temperature, there is a maximum to the amount of water vapour that the air can absorb. Relative humidity indicates the percentage of this maximum which is actually in the air.
pH Value:
The value in the data sheets defines the pH value of the surface. Papers should have a pH close to the neutral point in order to meet ideal requirements for printing and further treatment.
Specific Volume:
Paper thickness is expressed in micrometer (µm). To compare the thickness of papers with different basis weights, specific volume is used.
