There are various forms of process and apparatus for printing on material in web form using a screen printing stencil. In such processes, it is advantageous for the web of material to be moved away from the screen printing stencil at an acute angle directly after the printing ink has been applied to the material. The resulting rapid separation of the screen printing stencil and the printed web of material has the effect of enhancing the quality of the printed image on the material. If the support or impression cylinder for supporting the material during the printing operation is displaced relative to the web of material as printing takes place, it is necessary that the portion of the web of material, which passes into the printing mechanism on to the impression cylinder by way of a guide roller, and the portion of the web of material which moves away from the impression cylinder as the material leaves the printing mechanism, with the web of material passing around the impression cylinder over a part of the periphery thereof, extend parallel to each other at least in the region within which the impression cylinder is displaced during the printing operation, and that the printing mechanism which comprises a squeegee and impression cylinder as well as a guide roller is moved during the printing operation parallel to the portion of the web of material on which the printing is produced during the printing operation. That operating procedure takes account of the fact that, in the event of defective parallelism between the movement of the impression cylinder during the printing operation and the path of movement of the portions of the web of material which are adjacent to the printing mechanism, displacement of the impression cylinder would result in a change in the relative speeds as between the screen printing stencil and the web of material, which would have an adverse effect on the quality of the printed image. For that reason the movement of the impression cylinder and the movement of the associated guide roller are parallel to the path of movement of the web of material between the guide elements which guide the material upstream and downstream of the printing mechanism.
When the screen printing stencil is arranged in a horizontal position, then under the above-indicated conditions the web of material must be introduced into the printing mechanism from above and discharged therefrom again in a downward direction, as is the case for example with the process disclosed in European patent specification No. 0 003 983 and U.S. Pat. No. 4,425,554. However it is not always possible for the web of material to be guided in that way in the printing station because for that purpose there must be a suitable amount of space in the area beneath the printing station, for the web of material to be passed therethrough. When using a screen printing stencil which is of considerable length in the printing direction, it is also difficult for the material to be fed into the printing mechanism at an inclined angle from above as in that situation the screen printing stencil tends to be in the way.
When dealing with a multiple printing process, for example when producing a printed image which is made up of a plurality of different colours or inks, a plurality of individual printed images have to be successively applied to the web of material. In many cases, once such an individual printed image has been applied to the web of material, that image has to be dried by the application of heat thereto before the next individual printed image can be applied to the material. In that case it is frequently necessary for the web of material to be passed through the screen printing machine two or more times. If for example the printed image to be produced on the web of material is made up of six colours and the machine comprises three printing stations, then in a single pass through the machine, three individual printed images of different colours can be successively applied to the material. The three further printed images which are necessary in order to complete the finished image, of different colours, are then applied in a second pass through the machine, using screen printing stencils carrying the printed images associated with the respective inks in question.
An important condition for producing a satisfactory printed image which meets all reasonable requirements in respect of quality is that the individual components which make up the finished printed image, that is to say the individual printed images as referred to above, which are each applied using a screen printing stencil, are in precise registry with each other. Meeting that requirement does not give rise to any particular difficulty at any event when the web to which the printing is to be applied does not experience a change in its dimensions between the individual printing operations as, when using the technical equipment which is available nowadays, the web on which the printing is to be produced can be aligned relative to the individual screen printing stencils with such a degree of accuracy that the contours of the individual printed images precisely register with each other and are therefore congruent. However it is entirely possible and even probable when the web to which the printing is to be applied consists of certain materials that between the individual printing operations the web of material may undergo a change in length, for example it may shrink or stretch somewhat in its longitudinal direction. That may be caused for example due to the application of heat for the purposes of drying the printing ink between the individual printing operations, or by virtue of the moisture which is applied with the printing ink to the web of material in the printing operations. However such changes in length may also be caused by the moisture content of the ambient atmosphere, in particular when the web of material has to be put into intermediate storage over a prolonged period of time, which may be for example up to several days, for the purposes of applying a corresponding number of individual printed images which therefore go to make up the finished printed image, between two or more passes of the material through the printing apparatus. Intermediate storage in that situation is practically inevitable as the second and possibly further passes through the machine normally require the use of different screen printing stencils so that for reasons of economy the obvious procedure to be adopted involves firstly applying to all the web of material the initial individual printed images which are to be produced in the first pass through the equipment, and then, after suitable conversion of the machine and the fitting thereof with the appropriate different stencils, applying the remaining individual printed images to the entire web of material.
It may be appreciated that the extent of the changes in length which are caused by the above-indicated factors and in particular moisture is normally very slight and may be for example of the order of magnitude of 0.1 to 1.0 mm per 1000 mm of length of web. However, even such minor deviations can no longer be tolerated with the requirements which are now made on a screen printing process, in regard to the degree of accuracy which is to be observed therewith, so that it is necessary for the individual printed images which are to be successively applied to the web of material to be accurately matched to each other in regard to the length thereof, with the individual printed image which is applied in a first working operation generally defining the desired or reference length for all printed images which are to be subsequently applied. The high levels of requirement in respect of accuracy with which the individual printed images are applied to the web of material arise out of the fact that in many cases those printed images are later cut out of the web of material, for example by a stamping operation. If the individual printed images which go to make up the finished image are of different lengths, then, when the printed image is cut out of the web of material, either a narrow strip is cut off a printed image which is of excessive length or, in the case of a printed image which is too short, a narrow strip of material which does not have any printing thereon is not cut off, and thus becomes a part of the finished printed image. It will be readily appreciated that that is unacceptable in many situations. A typical example in that respect are printed circuits although the above-described problem is in no way limited thereto but also arises in connection with printed images which are used in different ways and for different purposes, for example as labels.
It would be theoretically possible, by suitably adjusting the tension of the web of material, to match the length of the web of material and therewith the individual printed image which has already been produced thereon to the reference length which is defined by the length of the image carried by the screen printing stencil, in the respective subsequent pass through the machine, by subjecting the web of material to an appropriate longitudinally acting force which therefore stretches the material to the appropriate length. In practice however that tends to involve very serious difficulties as for example the modulus of elasticity of the material of the web may vary over the length of the web. Added to that is the fact that adjusting the printed image by suitably stretching the web of material as indicated above will normally only over provide for an increase in the length of an individual printed image which is already on the web of material and which has turned out to be too short due to the effect of the above-mentioned factors such as moisture and the like. It will be appreciated that it would be theoretically possible for the web to be pre-stressed and thus increased in length in the first pass through the printing machine so that, in subsequent passes, the length of the web and therewith also the individual printed images already produced thereon could be reduced by reducing the level of stretching of the web. However that would make the entire procedure even more complicated so that in practice it could only be carried into effect, if at all, at a very high level of expenditure.
It will also be appreciated that the web of material may experience a change in its dimension which is transverse to its longitudinal direction, that is to say its width, under the effect of the above-mentioned factors which tend to influence the web of material. However that change is negligible in view of the fact that the width of the web is very small in relation to its longitudinal extent, especially as the extent or the dimension of the printed image in the transverse direction of the web of material is normally considerably shorter than in the longitudinal direction of the web.