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All you'll ever need to know about paper
Hints
There are a number of properties that make papers suitable for particular jobs. Among them are:
- Whiteness
- Opacity
- Smoothness
- Stiffness
The less obvious grain orientation ('long' grain and 'short 'grain) can also affect the appearance and function of paper products. The properties can be classified under the general headings of:
- Runnability
- Printability
- Finishing
RUNNABILITY
Stiffness
Stiffness is governed by a number of factors, the most significant being the thickness (caliper) of the sheet. Thickness and stiffness is also related to the type of wood pulp used in making the paper. Some woods have thicker fibres, making them more suited to certain papers and applicatioapplications. Fibre density and orientation also affects stiffness, as do coatings. Stiffness is important to ensure the smooth flow of paper through mechanical devices like printers and photocopiers as well as in making products that will be folded to stand.
Stiffness in the cross direction -across the fibres -has high influence on runnability and "paper feel". The paper stiffness should be high to run in fast machines. Stiffness in the machine direction -along the fibres -has no influence on runnability since most copy and laser machines are fed lengthwise.
Smoothness
Smoothness is also another factor in runability as it relates to the ease with which paper passes through a machine. We will cover smoothness in more depth later in this article.
PRINTABILITY
Whiteness
Whiteness is usually defined as a colour measurement to get a value that corresponds to the whiteness perception of the eye. The eye is extremely sensitive to changes in light and colour, but humans are less good in describing what they see or how they see it. As a result,when looking at a 'white 'piece of paper, someone could place another sheet next to it that makes it look creamy, or bluish. Perceived whiteness can also be affected by the type of lighting the sheet is viewed under, reflected colours in the room and other external factors.
For practical purposes, the selection of papers for whiteness should relate to the function of the paper product. For general business requirements, our Symbio range of papers provides the optimum contrast to inkjet and laser print, to maximise legibility.
Whiteness is also an important factor when considering what stock to use for reproducing photography and other graphical images.
Smoothness
Smoothness is a surface property that not only affects reflectivity, but also can affect receptivity of inks and toners. As a general rule,uncoated papers are less smooth than coated papers. Since toners melt into the surface, uneven surfaces can aid the bonding of the toner to the paper. Conversely, uneven surfaces can cause liquid inks to appear slightly blurred.
Smoothness is determined by two principal factors, the fibre mix and the calendaring method. To meet the needs of electro-static printing and the conflicting requirement for smoothness and stiffness, Thermo soft calendering was introduced in the mid 1990's. Here, two soft heated calenders are used to produce papers with an even surface yet without compromising the bulk of the paper.
One tool we use to measure surface structure is profilometry. This maps the paper 's surface via a fine stylus or a laser beam providing an exact surface profile that can be correlated more accurately with print quality. This type of characterisation is especially important for higher resolution printers. Lower resolution printers tend to be more forgiving on rougher papers,as their toner particles are larger and less likely to become lost in the valleys or pores of a paper surface.
Opacity
Opacity,the degree to which print shows through a sheet of paper, is governed by the thickness of the sheet, the density of the fibres and the reflectivity of the finish.
Papers with high opacity are desirable when printing double-sided (duplex), or using images that require high ink or toner saturations.
FINISHING
Grain direction
This refers to long grain and short grain paper. Grain describes the orientation of the fibres in a sheet. In the papermaking process, the fibres tend to orientate lengthwise along the web of paper. Paper is usually cut so that paper positioned 'portrait 'has the grain running vertically. In ordinary business use, grain is usually not an issue, though papers fed short-grain into machines may be slightly more prone to jamming. Also, when binding documents, the grain orientation should be the same throughout the document. This is because when the humidity changes, paper fibres swell and shrink in their cross-direction (i.e., they get fatter, not longer). Documents bound with long and short grain sections will swell and shrink in different directions, resulting in a condition known as luffing.
For products that will be folded (brochures, leaflets, booklets, etc.), printing should be carried out so that the fold is along the fibre direction.
Other factors that affect finishing include the choice of coated papers for suitability in printing device selected. For example, papers with too glossy a coating may present problems with friction paper feeders.
Good runability, printability and finishing depend on keeping the paper conditioned, thereby avoiding the common problems of curl, wavy edges, tight edges and static.
Storage
Following a few guidelines will help ensure that you have trouble-free runs with any electrostatic printing process.
Storage hints
- Always store boxes and reams of paper off the floor to avoid damage
- Don 't place anything on top of paper boxes or reams.
- Your print room should be conditioned to a relative humidity of 40%,and kept to a temperature between 19-23 °.
- Don 't store paper near a radiator,air conditioner or in direct sunlight.
- Keep the protective (waterproof) wrapping on until you are ready to use the paper.
- Partly used reams should be put back into their moisture-proof wraps between runs.
- Always apply the first-in,first-out principle to your paper storage.
Glossary
Bulk
The term used to describe the degree of thickness of a paper, measured by caliper, volume or pages per inch.
Chlorine-Free
Paper made from pulp bleached without the use of chlorine.
Coated
Usually a term used to describe plain (base) paper given a coating, to give an enhanced surface for detail and colour reproduction. Alternatively, the coating may be formulated to impart a particular property, eg resistance to moisture, or the coating may be part of a specific process, to make a particular kind of paper.
Fibres
Cellulose fibres, hollow hair-like structures, the raw material of paper making - usually obtained from wood (for volume paper production) but may be obtained from cotton, straw etc.
Furnish
The consituents of the paper being made, fibres, fillers, additives.
gsm or g/m2
Grammes per square metre.
Grain Direction
The direction in which the majority of fibres will be inclined to lie in a sheet, which will be the direction of travel of the paper machine.
Inkjet Printing
A method of printing, the images being created by minute jets of ink being directed at the paper.
Ink-Rub
Smearing caused by an ink film being rubbed before completely dry.
Long Grain
When the grain direction/machine direction is parallel with the long dimension of a sheet.
Micron or micrometre
A thousandth-of-a-millimetre the unit of measurement of thickness or caliper of paper and board.
Short Grain
When the grain direction/machine direction is parallel with the short dimension of the sheet.
Uncoated
Plain paper, not coated
Woodfree
Paper made from chemically produced pulp, ie free from mechanically produced (wood-containing) pulp
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