US2243706A - Method of printing on transparent flexible foil material - Google Patents

Description

May 27, 1941. H. HILLE METHOD OF PRINTING 0N TRANSPARENT FLEXIBLE FOIL MATERIAL Filed July 2, 1938 /n Mentor I -HE/N/2/CH H/LLE Attorneys Patented May 21, 1941 METHOD OF PRINTING N TRANSPARENT FLEXIBLE FOIL MATERIAL Heinrich Hille, Berlin, Germany, assignor to Transart Aktiebolag, Gothenburg, Sweden, a corporation of Sweden Application July 2, 1938, Serial No. 217,128 In Germany July 5, 1937 9 Claims.

This invention relates to the printing on sheets, webs or foils of thin flexible transparent ma-- terial, more particular of cellulose film or transparent cellulosic materials such as those known on the market under, the trade names of Zellglas, Ultraphane, Cellophane, "Kodapack and the like, in the case of which, accurate register plays a most important part. In the case of materials of the above-mentioned type, particularly of acetate foils, such register is much more difiicult to obtain than with paper, because the stretching of such foils during the printing operation plays a not altogether unimportant part as compared with the case of paper. When printing on such transparent foils, a great deal depends on maintaining in accurate registenthe position of the printed image, because when printing on both sides of the sheets, the imageon one side must be in perfect register with that on the other side; on this is based the so-called Transart process. In ordinary rotary printing machines, the transport of the web of material is effected substantially through the printing units themselves and by means of the spindles mounted on and driven by the machine. When printing on transparent cellulosic material, more particular on cellulose acetate stock, as Zellglas or like material, this guiding through the printing units is rendered even more difficult for the reason that the pressure applied by the pressure exerting means of the printing units finds its limitation in the comparatively light coating with a plurality of superposed layers, such as those required with the Transart" process, and for which layers it is first necessary to provide a suitable supporting and adhering base for the colours to be printed for the time being, and for which no provision has to be made when printing with printing inks on paper capable in itself of easily absorbing them. This is precisely why the register question is twice as important for the prevention of ground edges.

A further difficulty resides in the fact that when using rotary printing machines with a plurality of successively aligned printing units, and more especially when they are of different types (such as letterpress printing units, photogravure printing units and in some instances aniline printing units) the tension strain of the web to be printed on by the different printing units is altogether different. Consequently, in the case off-combined letterpress and photogravure printii'lE, for example, there is present with a material having a greater extensibility than paper an increasing non-registration of the top surface in relation to the bottom surface. While letter press printing units hold the web to the size of the image to be printed, the guiding of the web,

of material when perfecting by printing the opposite side is effected, for instance, in photogravure units, over the whole width of the printing cylinder. Even if the registration of the colours amongst themselves, both in the letterpress printing units per se and also in the photogravure sections per se were-brought about, there occurs through the described differential strains of Zellglas or as the case may be, acetate webs in the different printing units of the machine, variations leading to the defects alluded to, more especially when the top side is to be printed by means of photogravure cylinders for example, and the underside by means of letterpress cylinders.

In order to overcome these disadvantages, the

invention aims at printing on transparent cellulose material, more particular on cellulose acetate material of the above mentioned kind, e. g. the material known under the trade names Ultraphane, Zellglas or the like in a stretched condition. For this purpose the tension of the stretched sheet or web may suitably be so calculated that during the printing operation, no substantial additional stretching can take place by the action of the printing mechanisms.

In the carrying out of the process, the printing units and the driven spindles do not therefore take an essential part in the conveyance of the web. 0n the contrary, the transport may suitably be carried out by a powerful tension shaft which is provided in front of the cutting cylinder and which pulls the web without any slipping under the necessary amount of tension from the correspondingly braked material reel. The pressure applied by the printing units themselves, can therefore exert no unwanted stretching action on the material, as the latter, from the first photogravure impression, for example, up to the last letterpress impression will be pulled along uniformly and under such powerful tension through the machine, that deviations in the impression pressures prevailing in the different printing units will have no influence on the material itself.

In other words, the invention is based upon the following facts discovered by the inventor: With increasing load of the cellulosic material foil its stretching at first gradually increases. Before it reaches the tearing point, however, a

. stage is reached where the extended material has on the onehand adequate solidity to withstand all the strains occurring in the course of operation, and on the other hand to bring about no further additional detrimental extension or stretching of the material by the additional stretching strains occurring in the different printing mechanisms.

If a cellulosic material foil extended in this manner is pulled through the machine, the printing units merely act as intermediate stations but not as elements takingan important part in the guiding of the web. The tension of the web is therefore perfectly uniform in all the machine parts from the braked material reel to the pulling spindle. The register and the guiding are therefore also safeguarded when the pressure of the pressure exerting-means of the printing mechanism which no longer takes an active part in the transport, is comparatively small as desirable for the above-mentioned reasons in the case of the Transart process for example.

In order to ensure that the whole web will be kept continuously taut, it is necessary for the pulling spindle located between the last printing unit and the cutting cylinder to bear on the web with such a high pressing force, that notwithstanding the smooth surface no slipping will occur. Consequently, this spindle may suitably be fitted with an especially good gripping surface by using for instance one or more roller bodies composed alternately of adjacently located cloth and india rubber discs. It is advisable to have the roller bodies travel on unprinted parts of the material and it has been proved to be sufficient to grip the material adjacent the edges thereof.

The tension to be applied under which printing takes place, must naturally be adapted to prevailing operative conditions. Thus the smaller the pressure applied by the pressure exerting means of the printing units, the smaller can be the operative tension given to the foil, and inversely as the pressure applied by the pressure exerting means of the printing units rises, so must the preliminary tension also be increased. The operative tension is obviously also dependent on the character of the foil material used for the time being. 7

An attempt has been made to give in the following example, the tension ratios resulting during the printing of a cellulosic sheet material known commercially under the name of Ultraphan G No. 60. If a web of this material is printed in a machine equipped with a plurality of successively aligned printing units of different types, such as those illustrated in the drawing, the pulling tension preferably adopted for the purpose of ensuring perfect register is so great, that a paper 60 grams/m in weight would no longer be able to withstand the strain, but would tear. The acetate foil pulled down from the braked reel and which moreover already possesses according to the supplying firm (Lonza Werke in Weil) a certain initial stretching of the magnitude of 1%, is once more extended by a powerful pulling spindle by about 1% in the longitudinal direction and is printed in this stretched condition, whereby absolutely perfect register is obtained. By slackening the tension, that is, after leaving the machine, the stretching brought about by the pulling spindle substantially disappears, owing to the elasticity of the material. It is evident that, in this case, the appropriate tension for printing lies in the neighbourhood of the limit of elasticity. For, when reaching this value some considerable force must be exerted in order to provoke further stretch- -ing, which force being obviously so large, in comparison with that exerted upon the web in the course of printing in the printing mechanisms, that the latter forces will no longer detrimentally make their appearance. When exercising the above large force the limit of the elasticity will soon be exceeded. If the same material is stretched considerably more, say from 3% to 5%, the stretched material, after the pulling is slackened, no longer retums,.whereas a rupture will only occur with a considerably higher stretching, namely an expansion of over 20% in length, corresponding to a pull of about 750 kilos per square centimetre.

One constructional form of the invention will now be described, by reference to the accompanying diagrammatic drawing. The reel of material I is equipped with an adjustable brake 2, for example a band brake with an adjustable clamp lock 2a on one side of the band brake. In the construction illustrated by way of example, the machine is made up of two photogravure printing units 3 and I for printing on one side of the web, two further photogravure printing units 5 and 6 for printing on the opposite side and four letterpress units 1 to H! for printing again on the first side of the web. Before the machine is started up, the web from the reel is guided, over the usual guiding and conveying rollers, through the printing units 3 to III, which are preliminarily set to inoperative position and tripped (released from pressure). Then the web is guided through the bite formed by a pulling spindle H and a cooperating pressure roller strongly urging the web without any slip against the positively driven pulling spindle, whence the web is fed to the usual cutting cylinder [2. The pulling spindle is driven at a peripheral speed appropriate to ,the necessary operative stretching of the web which speed is therefore greater than that of the reel of material, whereas the form or, as the case may be, the copper cylinders, located in between, are all driven with the same normal operative speed. After the whole of the length of the web has been stretched to the above indicated degree by tightening the pulling spindle, the respective impression cylinders or, as the case may be, pressing mechanisms, are adjusted, and the printing of the web is then effected continuously in the usual man ner. The cutting cylinder cuts the printed web into sheets, which are delivered in the known manner.

The illustration in the drawing is only intended to explain the principle of the invention, which is not restricted to the constructional form shown. For instance, between the reel of material I, and the photogravure printing unit 3, a further photogravure printing unit could be arranged.

All rollers engaging the web, i. e. the pulling spindle II, and all rollers cooperating with the web in the printing units, are cooperatively connected with each other and are positively driven by gears with the same circumferential speed. The web reel lis not power-driven and is subjected to a braking action to such a degree that the circumferential speed of said reel is smaller than the circumferential speed of the other rollers an amount corresponding to the stretching effect exerted on the web by the pull of the spindle In other words, the reactionary pull exerted on the web by the web reel 1 against the action of the pull spindle II is, by suitable adjustment of the brake 2 rendered so powerful that the forces exerted on the web by the printing units appear negligible compared therewith and cannot produce any appreciable additional strain in the web which would affect the accuracy of the register. The adjustment of the brake is to be adapted to the material printed upon, so that for instance the substitution of a material of other qualities (e. g. in respect of elasticity) for the material used previously requires a corresponding change in the adjustment of the brake to compensate the changes in the qualities or reactions of the material relative to the stresses exercised. The stress produced in the material is always many times larger than the stressesexercised on the material by the printing units, so that said last-named stresses do not make themselves noticeable and do not detrimentally afiect the register.

In the Patent No. 1,867,405 to Irving Gurwick issued on July 12, 1932, it has been proposed prior to the present invention to print on transparent sheets of a cellulose material and to pull the web for this purpose from the supply roller by a feed or drawing roller firmly pressed upon the web and rotating at the same peripheral speed as the printing cylinders. In this prior process the printing members engaged the web at so slight a pressure as to avoid any participation in the feed of the web, the feed being efiected independently of the printing members and solely by the feed roller. Experience has shown, however, that the mere tautening of the web in this manner is not sufficient to obtain a true register and that particularly wave formation in the web cannot be avoided. If considerable waves are formed, the feed roller must be temporarily disengaged from the web.

I have discovered that I can obtain an accurate register in the printing operation by the above described invention in which the web is not only kept taut and fed past the printing rollers in this condition but in which the web is fed through the printing units in stretched extended condition. In this condition no waves or corrugations will be formed and while the permissible pressure of engagement of the printing. roller with the web was limited in the prior process, I may adjust the printing pressure to any desired suitable degree which is not subject to the hmitations present in the prior process. The only requirement to be observed is that of such an extension of the web thatthe stress set up therein is many times larger than the forces liable to be exerted on the web by the printing units and tending to produce an additional strain therein. If I use a drawing spindle pulling the web from a braked supply roller and thus feeding the web through the printing units which do not participate in the feeding operation, the braking force exerted on the supply roller must be so chosen by suitable adjustment of the brake as to prevent the tension set up in the stressed web and determined by the braking efiect from being materially affected by the additional stress produced in the web by the printing rollers.

What I claim is:

1. A process for printing on a transparent flexible foil consisting in-bringing the foil into a stretched and extended condition and subjecting it to the printing operation while maintaining said stretched condition.

2. A process for printing on a transparent flexible foil by means of a plurality of printing elements consisting in introducing the web into printing position while keeping the printing elements out of operative engagement with the web, exerting a pull on the web to bring it into a stretched condition and bringing the printing elements into operative engagement with the web and causing them to print upon the web while niiaintaining the latter in said stretched condit on.

3. A process for printing on a transparent flexible foil by means of a plurality of printing elements consisting in introducing the web into printing position while keeping the printing elements out of operative engagement with the web, exerting a pull on the web to bring it into a stretched condition and bringing the printing elements into operative engagement with the web and causing them to print upon the web while maintaining the latter in said stretched condition, the tension of the web being larger than that at which any substantial additional stretching could occur by the action of the printing elements on the web.

4. A process for printing on a transparent flexible foil by means of a plurality of printing elements consisting in introducing the web into printing position while keeping the printing elements out of operative engagement with the web, exerting a pull on the web to bring it into a stretched condition approaching the limit of its elasticity and bringing the printing elements into operative engagement with the web and causing them to print upon the web while maintaining the latter in said stretched condition.

5. A process for printing on a transparent flexible foil by means of a plurality of printing elements comprising introducing the web into printing position while keeping the printing elements out of operative engagement with the web, subjecting the web to an advancing pull in the feeding direction beyond the printing elements and to a braking action in advance of the printing elements and exerting thereby a. pull on the web to bring it into an elongated condition but within its elastic limit, and bringing the printing elements into operative engagement with the web and causing them to print upon the web while maintaining the latter in the stretched condition.

6. In the operation of machines having printing couples for printing on a transparent flexible foil, the step which comprises subjecting the foil to such a degree of tension during the printing operation as to produce and maintain a strain in the foil which is many times larger than that at which any substantial additional stretching could occur by the action of the printing elements on the foil.

7. In the operation of machines for printing on a transparent flexible foil web and having a web reel, at least one printing unit and a positively driven pulling spindle, in tandem relationship, and a brake cooperating with said reel, the step which comprises applying said brake to a degree producing a pull on the web which brings it into a stretched condition approaching the limit of its elasticity.

8. In the operation of printing machines comprising a web reel carrying a supply roll of a transparent flexible foil web, at least one rotary printing couple, a pulling spindle adapted to pull said web from said supply reel through said printing couple, and a brake cooperating with said reel, the steps which comprise driving the pulling spindle and the printing couple at the same peripheral speed and applying said brake to a degree producing a pull in the web which is many times larger than that at which any substantial additional stretching could occur by the action of the printing elements on the web.

9. In the operation of printing machines comprising a web reel carrying a supply roll 01 a transparent flexible foil web, a plurality of difl'erent printing couples, a pulling spindle adapted to pull the web from said supply roller in succession through said printing couples, means

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