Coating processes – Electrical product produced – Integrated circuit – printed circuit – or circuit board
Patent
1988-09-30
1990-05-22
Lawrence, Evan
Coating processes
Electrical product produced
Integrated circuit, printed circuit, or circuit board
427259, 427264, 427265, 427272, 427288, B05D 306, B05D 132, B05D 302, B05D 500
Patent
active
049276638
DESCRIPTION:
BRIEF SUMMARY
This invention relates to printing and in particular to screen printing (commonly known as silk screen printing). As is well known, the silk screen printing process involves the use of a screen of silk, polyester, nylon, metal gauze or any other suitable mesh-like or perforated material. For convenience (and for consistency with the terminology historically employed in this art) the term "silk screen printing" will be used herein and it will be appreciated that no restriction to "silk" material is thereby intended.
Although silk screen printing is a very old technique, it still has numerous problems and/or limitations associated with it. For example, where several large-size multi-color sheets are to be silk screen printed for, say, a poster to be pasted up on a hoarding, large quantities of printing inks are needed which are expensive.
The printing inks required for screen printing are quite different from those to be used for other forms of printing. The screen printing inks being generally used at present are organic solvent-based and usually contain ethyl or nitro cellulose resins. They are dried in warm air tunnels by driving off the organic solvents, and are thus potentially hazardous to the printing operatives. Another problem which arises with such organic solvent-based screen printing inks, relates to solvent loss from the ink on the printing screen whereby the ink increases in viscosity and after a certain period of time will not properly pass through the open areas of the printing mesh. This "drying in" results in loss of printed image and is described as screen stability, i.e. inks with poor screen stability "dry in" rapidly.
Recent developments over the last ten years have seen the introduction of ultraviolet-curing screen process inks.
Here the formulations use raw materials which do not increase in their viscosity on the printing screen but instead dry by exposure to intense ultraviolet (UV) light. This means that this type of ink has almost unlimited screen stability compared to non-UV curing, organic solvent-based, screen printing inks. However, such UV-curing inks are expensive, and leave a very thick print coat on the paper surface -- particularly where multi-color, e.g. four-color, silk screen printing is effected.
Such thick or heavy print images on the surface of a sheet of poster paper often necessitate that for mounting the poster to its hoarding, a special adhesive -- which is other than an inexpensive starch-based adhesive -- must be used if efficient bonding is to be achieved. A similar problem often arises with the older type of organic solvent-based silk screen printing inks. This adhesion problem is common where the individual sheets overlap, e.g. at overlapping marginal edges of adjacent poster sheets. The back of the top sheet does not have sufficient adhesion to the ink dries out, the top sheet of the overlap often curls up and away from the sheet underneath. The problem is particularly severe when the paper is completely covered with several layers of ink. Posters printed with offset litho inks have less tendency to curl when they dry out compared to those produced with screen printing inks. As the screen printing process achieves its high quality print appearance by a much higher ink deposit, the curl problem is pronounced with screen printing inks. Therefore the adhesives used with screen printed posters need to have better adhesion properties to prevent overlap curl.
Mixtures of one or more starch adhesives and one or more polyvinyl alchol (P.V.A.) adhesives were investigated by the applicants, and these were found to give a much improved adhesion at the overlapping edges of superimposed sheets as compared to a 100% starch-based adhesive product. However, mixtures of starch and P.V.A. adhesives have the disadvantage of being more expensive and the resultant improved adhesion makes the removal of the total poster from the hoarding more difficult. Thus they are not likely to find favour with bill-posters, i.e. those responsible for putting up the posters on the hoardings.
REFERENCES:
patent: 3470054 (1969-09-01), Tyrrell
patent: 3499781 (1970-03-01), Krueckel
patent: 3574617 (1971-04-01), Skoultchi
patent: 4113895 (1978-09-01), Watt et al.
Barrow Lascelle A.
Dix Barry M.
Patel Dakshesh S.
Small Richard F.
Lawrence Evan
Small Products Limited
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