Printing

Details Printing Printing

2000-07-18

2003-06-17

Hess, Bruce H. (Department: 1774)

Stock material or miscellaneous articles

Structurally defined web or sheet

Discontinuous or differential coating, impregnation or bond

C428S297400, C428S298100, C428S299100, C428S323000, C428S429000, C428S447000, C428S448000

Reexamination Certificate

active

06579599

ABSTRACT:

The invention relates to printing and particularly, although not exclusively, relates to a printing member, and a method of preparing a printing member. Preferred embodiments relate to planographic, especially lithographic, printing.
Lithographic processes involve establishing image (printing) and non-image (non-printing) areas on a substrate, substantially on a common plane. When such processes are used in printing industries, non-image areas and image areas are arranged to have different affinities for printing ink. For example, non-image areas may be generally hydrophilic or oleophobic and image areas may be oleophilic. In “wet” lithographic printing, a dampening or fountain (water-based) liquid is applied initially to a plate prior to application of ink so that it adheres to the non-image areas and repels oil based inks therefrom. In “dry” printing, ink is repelled from non-image areas due to their release property.
One of the most common substrates used in lithographic printing comprises an aluminium base layer which is treated to make it suitable for use. In general, the aluminium layer comprises high quality aluminium, for example 1050 alloy which is at least 99.5% pure. For preparation of a substrate, the aluminium is roughened, for example by electrograining, anodised and then conditioned by chemical means, for example by treatment with water, a solution of phosphate or silicate salt, or a polycarboxylic acid.
Aluminium has many of the properties required for making a good printing plate. For example, 1050 alloy has a high Young's Modulus (tension) of about 70 GPa; an ultimate tensile strength (before baking) in the range 150-200 MPa; a fatigue limit of about 50 MPa (at 5×10
8
cycles —R.R. Moore-type test); and a hardness of about 38 HB (measured using 50 Kg load, 10 mm ball, 30 seconds duration).
However, aluminium's high thermal conductivity can cause problems in certain types of printing plates. For example, printing plates are known (see e.g PCT/GB98/00266 and/or U.S. Pat. No. 5,339,737) wherein areas of a radiation sensitive layer are ablated by heat delivered by a laser in order to define ink-accepting and non-ink-accepting areas. However, if a layer to be ablated is contiguous with or close to an aluminium layer, the aluminium may conduct heat away from imaged areas, thereby reducing the amount of heat energy available to effect ablation. Thus, more heat may need to be delivered to effect ablation compared to a case wherein a less thermally conductive material is used instead of aluminium. Similarly, thermal imaging systems are known which image a plate by direct physical contact of a heated body, such as a heat stylus (or the like), with the plate. Heat is conducted from such a body to a heat sensitive layer to effect a change in the layer, for example involving increasing its solubility in a developer. However, it is found that aluminium is a poor support for such thermally imageable plates, since it conducts too much heat away from a heat sensitive layer and thereby increases the power needed to image the plate. Another disadvantage associated with aluminium plates is that more energy may be required to dry such plates and/or to dry coatings used in the manufacture of the plates.
Furthermore, it is a practice, particularly in the U.S, to bake plates at about 280° C. for up to 2 minutes after development in order to cross-link the photocoat and thereby increase the run length of the plate. However, such baking risks excessively annealing the aluminium and, in any event, its tensile strength may be reduced by more than 30 MPa, as a result of the baking process. Consequently, there is a risk that the aluminium may crack and fail in use, resulting in possible damage to the press and subsequent press down-time.
Another disadvantage of aluminium is that aqueous chemicals used in developer fluids or fount solutions may dissolve areas of the aluminium leading to contamination of the fluids and possible clogging of presses and/or development apparatus. Additionally, contaminated fluids must be disposed of in an environmentally acceptable manner.
Further disadvantages of aluminium include the need to clean it before use to remove grease which is applied by the manufacturers to restrict corrosion and, additionally, its sharp edges mean it must be handled with extreme care.
Despite the highlighted disadvantages, aluminium plates are widely used, especially for long print runs of 500,000 or more.
The use of polyester as a support of a lithographic printing plate is widely described in patent literature and some such plates are commercially available. However, polyester plates are generally significantly inferior to aluminium plates due to the properties of polyester. For example, polyester has a low tear resistance. Additionally, the Young's modulus (tension) of polyester may be in the range 3-7 GPa. This leads to one major disadvantage of polyester which is the fact that polyester has inadequate dimensional stability and tends to stretch on-press which means that there is a registration problem if polyester plates are used in colour printing. As a result, polyester plates are generally only used for black and white or spot colour printing or for colour printing in small formats.
An additional problem with polyester is the difficulty in causing a hydrophilic layer to adhere strongly to the polyester. Consequently, the hydrophilic layer may wear quickly and/or delaminate. As a result, polyester plates fail relatively quickly and are, therefore, only used for relatively short run lengths, typically of up to 20,000 impressions.
Compared to aluminium, polyester has a very low thermal conductivity and, therefore, does not suffer from the same disadvantages in thermal imaging techniques as for aluminium discussed above. However, there is a restriction on the power that can be supplied to polyester based thermally-imageable plates since too much power may heat the polyester to such an extent that there is a risk that it may distort. It is found in practice that polyester cannot be used as a support for plates imaged by heated bodies because sufficient power cannot be supplied to image the plates without affecting the polyester.
Another support which has been proposed for printing plates is paper. However, the physical properties of the papers used are such that they can only be used for extremely short run length plates of only a few thousand impressions. One major drawback is that paper is water absorbent and, therefore, has a tendency to absorb fount solution used in “wet” lithographic printing. Another problem with paper is that its tear resistance and its Young's Modulus (tension) are of a similar magnitude to that of polyester, discussed above.
One advantage of polyester over aluminium is the potential use of widely available film image setters to digitally image polyester-based plates. Such apparatus requires relatively light and flexible plates, usually having a Young's modulus (tension) of less than 45 GPa. Thus, polyester can be used in such situations, but aluminum cannot. Digital imaging of aluminium plates, therefore, tends to be carried out using relatively expensive purpose-designed plate setters. Another advantage of polyester is that it can be formed into small coils and used in, for example, cassettes for image-setters used in direct to press applications.
It is an object of the present invention to address problems associated with substrates of printing members.
According to a first aspect of the present invention, there is provided a printing member which includes:
a first layer comprising a fibrous material and a binder material; and
an image-defining means.
Said printing member could be for any type of printing, for example flexographic or planographic printing. Preferably, said printing member is a planographic printing member. Whilst said printing member could be a roller (or the like), it is preferably a printing plate.
Said fibrous material may be a naturally-occurring fibrous material or a synthetic fibrous material. It

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