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Requirements to the printing paper quality


  • Sufficient mechanical strength one way or other ensuring normal conditions of a printing process (the continuity of the paper web at printing, minimal sheet jams and double sheet feeding, prolonged use of printed materials without visible destruction).
  • Absence of pollution, characterized by the potential number of 0.1-0.5 mm² specks on the area of 1 square meter of paper.
  • Thickness, density, structure and other properties of the paper are respective and uniform not only of sheets of the same batch, but also of any sheet inside.
  • The moisture content of the paper shall be in the range of 6-8%.
  • Paper sheets must be strictly rectangular. Not-rectangularity of the sheet shall not exceed 0.2%. Corrugation of sheets in piles when unpacking is not allowed.

Basic printing properties of the paper:

  • whiteness;
  • smoothness;
  • elasticity;
  • plasticity;
  • absorption;
  • opacity;
  • absence of pollution;
  • surface layer strength;
  • flatness.

Paper whiteness

The high level of printing paper whiteness is highly desirable because the clarity, readability of publications depend on the contrast of printed and non-printed areas of the prints. The level of the paper whiteness depends on its possibly deeper and more uniform reflection of rays of various lengths across the spectrum of the scattered fluorescent white light. In order to increase the whiteness of paper and eliminate the possible yellowish tint, the paper during manufacture is added blue and violet dyes or injected with optical brighteners.

The brightness of some types of paper:

  • coated with optical brighteners - 84%
  • coated without optical brighteners - 78%
  • wood-free printing paper with optical brighteners - 83%
  • wood-free printing paper without optical brighteners- 78%
  • printing paper with a white wood pulp - 72%
  • newsprint paper - 65%

Paper smoothness

Depends on micro-geometry of the paper surface or its relief created by ridges and depressions between the plant fibers and filler particles.Very smooth paper will beglossy, while the rough paper - matted.

Resiliency-Elasticity

Under the action of the paper machine working body is deformed, but depending on the degree of mechanical stress and production operations objectives appear every possible deformation character resilient, elastic and plastic.

The property of a material to change its shape and size instantly under the relevant load and after the termination of its action to restore instantly is called- resiliency. Thus, the resilient deformation–is an instantly nascent and entirely reversible one. Elasticity –is a property of a material to change shape and dimensions under a load during a certain time and gradually completely reconstruct the original shape and dimensions after termination of the action.The property of a material to retain the acquired deformation after termination of the mechanical action causing it is called - plasticity.

Resilient-elastic properties thoroughly affect the printing process, but they are often inadequate for the necessary paper surface smoothing during the printing process and irregularities elimination both of the printing plate and the paper itself. Plastic deformations of paper are technologically necessary when folding, creasing and embossing.

Absorbency

Necessary absorbency of paper –is a significant absolute condition for timely and secure embedding of paint. The absorbency of paper is especially important for those printing and coating methods, where the paint and varnish embedding is mainly determined by absorption (for example, gravure printing, flexographic printing on paper, coating with dispersion varnishes. This property is especially important for illustrative, multi-color printing.Strength of the paper surface increases with the use of well-designed long-fiber pulp, its sizing with urea resin and starch.

Flatness

For the stable operation of a sheet printing machine the impeccable flatness of the paper is essential. Flatness of the paper is largely determined by climatic conditions of its packaging, transportation and storage.Paper must not be adversely affected by moisture and temperature fluctuations. If paper is held in the room with a certain relative humidity without packaging that protects it from atmospheric influences, it absorbs the moisture from the ambient air or will give out moisture long until the equilibrium moisture content has been reached. At absorption of moisture the paper edges form pronounced undulations. When paper gives out moisture, its edges become bent.


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