Partial printing of a substrate

Details Partial printing of a substrate Partial printing of a substrate

1998-09-28

2001-04-03

Krynski, William (Department: 1774)

Stock material or miscellaneous articles

Structurally defined web or sheet

Nonplanar uniform thickness material

C428S204000, C428S195100, C428S198000

Reexamination Certificate

active

06210776

ABSTRACT:

This application is the national phase of international application PCT/GB96/02600 filed Oct. 24, 1996 which designated the U.S.
This invention relates to the partial printing of a substrate with a plurality of layers, at least one layer being applied to the substrate with inexact registration in relation to another layer.
There are a number of visual and other functional benefits in printing only part of the surface area of a substrate. For example, it is common to partially print a substrate with one or more colours to reveal the substrate exposed to form part of the required design. Such methods may also be used in the printing of printed circuits, membrane switches and backlit display panels in which superimposed layers must be in exact registration or one layer must overlap another layer, for example to achieve an insulating layer over a conductive layer of ink.
White is the most common colour of substrate to be printed over part of its area and revealed in other parts, firstly because it is easiest to achieve the desired perceived colour of other colours if they are printed on white, especially if such colours are formed by transparent or translucent inks. Secondly, white forms a good contrast to many other colours and enables easily visible graphic designs. Thirdly, white commonly forms a significantly high percentage of many designs. Fourthly, the mass processing of white substrates provides economy and efficiency in production, by standardization of the base colour, if not the material specification. Fifthly, white forms the normal background to four colour process printing, in which four colours (black, cyan, magenta and yellow) are typically printed in “half tone” dot patterns onto a white background, the size of the dots of each colour being typically printed in varying size according to “colour separations” to be replicated or by digital printing techniques utilising Raster Image Processing (RIP). From above a minimum distance, the eye cannot resolve the individual coloured dots but the coloured dots merge to give a combined perceived colour at any position on the printed product.
Conventional printing processes all suffer inexact registration, owing to
i) printing machine error or “tolerance” in delivering ink or other marking material,
ii) the dimensional instability of a liquid ink or other marking material in liquid state on a substrate,
iii) the dimensional instability of a substrate through temperature and humidity changes between printing “passes” (printing of individual layers), and
(iv) the error or “tolerance” in delivery of a substrate into the printing position.
For many products, this lack of registration, or lack of being able to print ink on a substrate exactly where intended, is not important. However, there are a number of products which can be adversely affected by such lack of registration, one example being unidirectional or other vision control products, such as those disclosed in British Patent No. 2165292, which includes methods of printing with substantially exact registration and methods of overcoming the limitations of registration error of conventional printing methods. Such products typically comprise the partial printing of a transparent substrate with a fine pattern in the form of dots or lines with surrounding or intermediate transparent areas or of a grid pattern surrounding transparent areas.
A cross-section taken through such partially printed substrates will be in the form of a continuous substrate material on which are superimposed alternate printed portions and unprinted portions. When the cross-sectional dimensions of the printed portions of such a printed product are small and it is desired to superimpose more than one layer on such printed portions, the registration error of conventional printing processes can severely prejudice the achievement of the desired visual or other performance criteria. The critical factor is the registration error or tolerance of the printing process compared to the crosssectional dimensions of the printed portions.
In the case of conventional four colour process printing (sometimes referred to as four colour half-tone printing) or digital four colour process printing, the size of the individual dots of colour are very small in relation to the background substrate, which is typically white and made of paper, card or plastic materials. Substantial lack of registration in the printing of individual dots is normally acceptable as the individual dots of one colour are not perceived as individual dots, but are combined with differently coloured dots to provide the required overall impression. Lack of registration between the dots of various colours is only generally perceived as a lack of sharpness of design boundaries within the design, such as the edges of insignia seen against a background colour. The observer sees what is printed. Only if the observer knows that the desired degree of edge clarity is different to that observed, or if the lack of registration is such that colour “halos” are seen at colour boundaries, is the lack of registration recognisable.
However, if the requirement is to print a relatively fine pattern or background colour, such as white dots, then superimpose one or more other uniform colours or four colour process colours on some or all of these dots, the lack of registration of the printing process can have a significantly deleterious effect on the functional performance compared to that intended. For example, the perceived colours of an image or design will vary over the area of the substrate from the desired colours owing to the visual interaction of the unregistered layers. If a pattern of 1 mm sided square white dots are intended to be covered with 1 mm sided square dots of a different colour, but there is a registration error of 0.2 mm in two orthogonal directions on plan, as in
FIG. 1
of the accompanying drawings, then 36% of the desired area will appear white and have a corresponding effect of 0.36 mm
2
white on the overall printed area of 1.36 mm
2
. If the substrate is black and the different colour is formed by transparent ink, the different colour will be substantially invisible against the black substrate and the 0.36 mm
2
of white will be seen in combination with the 0.64 mm
2
area of the different colour, which will appear consequently “whitened” in this area. Such alteration from the desired perceived colour will be most noticeable compared to other individual squares making up the pattern where the error in registration differs and compared to any squares in which the different colour substantially covers the white. If the different colour was intended to appear uniform over an area of panel, it will instead appear to be shaded.
If the substrate is transparent, such lack of registration will be typically visible from the other side of the substrate as well, the overlapping different colour in the above example being visible as well as the white square. There is another problem that undesirable perception or colour can be caused by lack of opacity of individual ink layers. In the above example, if the white and different colour were printed on a transparent substrate, when the white is observed from the other side of the substrate, this could be modified by the different colour, which could be exacerbated by the illumination condition behind the substrate.
From the printed side of the panel, the different colour covering the white area would be perceived as being a whitened or a lighter colour tone of the different colour. It is common in printing to overcome such lack of opacity by printing more than one layer of a colour, to achieve the desired or necessary degree of opacity. However, if the registration error is relatively large compared to the cross-sectional dimensions of the printed portions being printed, the lack of registration will result in yet further areas of different perceived colour where the edges of the desired shape overlap through lack of registration.
The purpose of this invention is to overcome the above-mentioned problems in the part

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