Method for impressing directly on paper holograms, kinetic holograms, diffraction patterns or microengravings producing other optical effects

A method for impression microengravings, which reproduce holograms, kinetic holograms, or diffraction patterns, directly on paper through an embossing process. Paper is subjected to a humidification step to give it a degree of humidity between 60 and 80% of relative humidity. The humidified paper is then passed through an embossing group, consisting of an embossing cylinder carrying the microengravings and a counterthrust cylinder, at a temperature and a pressure on the paper within the ranges of 90-220.degree. C. and 20-120 kg/mm.sup.2, respectively.

The present invention relates to a method for impressing directly on paper 
holograms, kinetic holograms, diffraction patterns or microengravings 
producing other optical effects. 
In order to permanently transfer optical microengravings (i.e. 
microengravings of the above-mentioned type) from a type plate to paper 
sheets it is usually necessary to provide a pretreatment of the paper 
consisting of a coating of the paper with a film of a material (such as 
PVC, polyester, polypropylene, polythene, coating, etc.) suitable to 
receive and permanently retain the microengravings present (in low relief 
and/or high relief) on the plate. 
In fact, it is usually excluded that it is possible to transfer directly 
said microengravings onto common paper (i.e. not pre-treated as above) due 
both to the deformability (at micrometric level) of paper in time and to 
the surface micro-roughness usually found in paper sheets, caused by the 
fibrous structure of paper, which roughness is not compatible with the 
degree of resolution of the microengravings to be transferred, which can 
range from about 120 lines/mm to about 5000 lines/mm. 
The present invention is aimed at overcoming said prejudice on the 
impossibility of impressing microengravings on common untreated paper by 
providing a method for impressing microengravings on common paper, which 
not only is simpler and cheaper than the conventional methods for 
impressing microengravings on a pre-treated paper substrate but also gives 
further advantages such as making available: 
common paper which is personalized or decorated with the above-mentioned 
type of microengravings; 
personalized or decorated paper as above which lends itself to be used as 
writing paper and printing paper (for offset, copperplate, typographic, 
serigraphic, rotogravure, xerographic, etc., printing); 
personalized or decorated paper as above for documents which are to be 
protected from forgery or guaranteed for originality. 
According to the method of the present invention for impressing directly on 
paper holograms, kinetic holograms, diffraction patterns or 
microengravings producing other optical effects, the impression of said 
microengravings is achieved by means of an embossing process wherein the 
paper is passed between a pair of embossing cylinders, namely an embossing 
cylinder, carrying said microengravings, and a counterthrust cylinder, 
said process being carried out, after humidification of the paper, at a 
temperature and a pressure on the paper passing between said cylinders 
respectively within the ranges of 90.degree.-220.degree. C. and 20-120 
kg/mm.sup.2, preferably 130.degree.-170.degree. C. and 40-100 kg/mm.sup.2.

FIG. 1 shows an example of a machine for embossing paper in reels according 
to the present invention. 
The structural and functional characteristics of the machine follow those 
of a conventional embossing machine with substantial differences regarding 
the conditions (temperature and pressure) under which the embossing 
process is carried out and regarding the presence of a paper humidifier 
upstream from the embossing group consisting of the embossing cylinder and 
the relevant counterthrust cylinder. 
In FIG. 1, the meaning of the various members is the following: 
1: reel of common paper for feeding the machine. 
2: spare reel of common paper. 
3, 4, 5, 6, 7: idle rollers. 
8: roller, preferably coated with teflon or rubber, for driving and 
pre-heating the paper strip. 
9: roller, also preferably coated with teflon or rubber, for driving and 
cooling the paper strip. 
10, 11, 12: idle rollers elastically biased against rollers 8 and 9 
respectively in order to assure the friction driving of the paper strip by 
rollers 8 and 9. 
13, 14: idle rollers for tensioning the paper strip, which are capable of 
shifting their axis so as to keep the passing strip in conditions of 
proper tension. 
15: humidifying group. 
16: cooling roller. 
17, 18: embossing group which consists of an embossing cylinder 17 carrying 
on its cylindrical surface the matrix of the microengravings to be 
impressed on the paper (the microengravings may be engraved directly on 
the embossing cylinder or on a thin type plate fixed onto the cylinder), 
and of an idle counterthrust cylinder 18 exerting on cylinder 17 an 
adequate pressure. 
19: pre-heating unit, which is provided with infrared lamps or operated by 
gas and capable of shifting vertically for moving closer/farther with 
respect to the passing strip in order to adjust the pre-heating 
temperature of the strip. 
20: strip winding reel. 
21: arm which supports reel 20 and is capable of oscillating around its 
lower end, the angle of inclination thereof being a function of the 
diameter D of the winding reel (which diameter is smallest at the 
beginning of the process and largest at the end). The operation of the 
machine can be described as follows. 
During the operation, the paper strip unwinds from reel 1 and passes first 
on rollers 3, 13, 11 and then on cylinder 8 where it undergoes a first 
pre-heating and a humidification by means of group 15, which gives paper a 
degree of humidity around 60-80% of relative humidity (in particular a 
degree of humidity equal to 70% according to a preferred embodiment). 
After coming out from roller 8, the paper strip passes under unit 19 where 
it undergoes a further pre-heating and then passes between the two 
cylinders of the embossing group where it receives the impression of the 
microengravings present on the embossing cylinder while undergoing a 
strong stretching action produced by both the high pressure exerted by 
cylinder 18 and the temperature of the two cylinders 17, 18. After coming 
out from the embossing group, the strip undergoes a first cooling by 
passing on cylinder 16 and then, after passing on rollers 5, 6, 7, 14 and 
12, a last cooling on cylinder 9 to a temperature substantially equal to 
room temperature, after which it is wound on reel 20 abutting against said 
roller 9. 
Cylinder 17 must have the characteristics usually required for embossing 
cylinders, namely it must be made from ground and lapped hardened steel of 
high surface resilience, with a very high degree of surface finish, such 
that the cylinder surface has a degree of linearity (i.e. of approximation 
of a straight line), along any generatrix of the cylindrical surface, 
close to the order of magnitude of the resolution of the microengravings 
on the plate. An acceptable value of said degree of linearity could be, 
for example, around 2000 lines per mm. 
Cylinder 18, also made from ground and lapped hardened steel, is idle 
around its axis so that, when operating, it can be driven by the rotation 
of the upper cylinder 17. It exerts against cylinder 17 a pressure of a 
quite high value, to be selected in the range from 20 kg/mm.sup.2 to 120 
kg/mm.sup.2 (said pressure is selected around 50 kg/mm.sup.2 according to 
a preferred embodiment). 
When operating, both cylinder 17 and 18 are heated at a temperature such as 
to assure that the paper of the document passing under the plate reaches a 
temperature between 90 and 220.degree. C. (said temperature is selected 
around 150.degree. C. according to a preferred embodiment). 
Said temperature of cylinders 17, 18 (as well as that of cylinder 8 and 
unit 19) shall be selected according to the operating speed (strip moving 
speed), in that higher speeds require higher temperatures of the cylinders 
(and of unit 19) in order to achieve the same paper temperature. 
For a paper temperature of about 150.degree. C. and an operating speed 
around 60 m/min, the temperature of cylinders 17, 18 should be around 
150.degree. C. 
The heating of cylinders 8, 17 and 18 may be obtained by means of a fluid 
circulating therein or through an electric resistor. 
Also the cooling of cylinders 9 and 16 may be obtained by means of a fluid 
(e.g. water) circulating therein. 
Fig. 2 shows an enlargement of group 15 of FIG. 1. This group has 
structural and functional characteristics like those of similar units used 
in other technological fields, for example in offset printing. It is made 
up of a water tank 22 wherein a continuously rotating roller 23 is 
partially immersed. Idle roller 24 has an oscillating axis such that it is 
taken from a position of temporary contact with roller 23 (e.g. for a 
portion of a revolution of 24) to a position of longer contact with roller 
25 (e.g. for a complete revolution). The axis of roller 25, which is also 
idle, is in turn provided with a slight oscillating motion along a plane 
parallel to the plane passing through the axes of rollers 26 and 27 (which 
are also idle and permanently in contact with the passing strip), so that 
roller 25 is alternately in contact with said rollers 26 and 27 and can 
evenly distribute thereon the humidity received from 24. 
The surface of the rollers of group 15 is preferably coated with a material 
easily soaked up with water, for example a velvet-like cotton fabric. 
The correct humidity of the paper strip passing is adjusted by means of a 
humidity detector (not shown) positioned in contact with the strip coming 
out from group 15, which adjusts the duration of the temporary contact of 
roller 24 on roller 23 according to the detected humidity. 
At the end of the process, the strip wound on reel 20 retains the impressed 
microengravings transferred from the embossing cylinder. The surprising 
possibility of permanent transfer of the microengravings on the paper 
strip can be explained by the fact that due to the humidification process 
and the subsequent heating under a high pressure, the paper undergoes a 
compacting and stretching process which results in a considerable 
reduction of its surface irregularity and deformability. 
It may be desirable that the microengravings transferred on the paper (and 
consequently their optical effect) can not resist (or on the contrary can 
resist, according to needs) possible alterations or counterfeiting of the 
paper (humidification, erasing, etc.). 
Whenever it is required that the impressed microengravings can resist 
possible humidifications, it will be necessary to use a refined paper 
charged with glue such as the paper employed in the manufacturing of kraft 
paper, of paper for nautical handbooks, etc. When it is desirable, on the 
contrary, to have microengravings of opposite characteristic, it will be 
necessary to use common paper (of hygroscopic nature). 
As far as the possibility of resisting possible erasures is concerned, it 
should be considered that the printing matrix can be made with all 
engravings in low-relief or in high-relief or with engravings partly in 
low-relief and partly in high-relief possibly distributed in different 
areas (i.e. some portions of the plate have only high-relief engravings 
and other portions only low-relief engravings). 
The above-mentioned possibility allows the manufacturing of microengraved 
paper which can (or can not) resist erasures. In fact, the microengravings 
impressed on paper in low-relief can resist erasures and the surface wear 
which paper undergoes in time, whereas those impressed in high-relief are 
erased at any counterfeiting attempt, this latter characteristic being 
very important whenever the paper is to be used for issuing documents 
whose originality is to be assured and protected from forgery (such as for 
example paper securities, bank cheques, etc.). 
FIG. 3 shows an example of an embossing machine for paper in sheets 
according to the present method. 
In this figure the meaning of the various members is the following: 
30, 31: inlet and outlet sheet trays. 
32: tube for picking and feeding the sheets, provided with suction mouths, 
which is part of a sheet-feeder device (not shown in the figure) quite 
similar to a suction sheet-feeder device for conventional offset printing. 
33, 49: rotating rollers for feeding incoming and outgoing sheets 
respectively. 
34: planes for sheet sliding. 
35, 36: a pair of rollers for humidifying and feeding the paper, with 35 
idle and driven into rotation by the underlying roller 36, and 36 which 
provides not only the sheet feeding but its humidification as well. To 
this purpose, 36 is coated with a fabric (e.g. cotton) which is soaked up 
with water by means of a humidification group 15' quite similar to, in 
structure and operation, group 15 of FIG. 1. 
37/38, 39/140: pairs of rollers for pre-heating and feeding the paper, said 
rollers being preferably coated with a rubber or teflon layer and provided 
with a rotating motion. 
41/42, 43/144, 45/146: pairs of rollers for cooling and feeding the paper, 
also preferably coated with a rubber or teflon layer. 
47, 48: embossing group consisting of an embossing cylinder 47 carrying on 
its cylindrical surface the matrix of the microengravings to be impressed 
on the paper, and of an idle counterthrust cylinder 48 exerting on 
cylinder 47 an adequate pressure. 
50: plane for sheets sliding and pre-eating, with heating obtained by means 
of infrared lamps or gas. 
Likewise in the case of the machine of FIG. 1, the heating of cylinders 
37-40 and 47, 48 may be achieved through a fluid circulating therein or by 
means of electric resistors. Also the cooling of cylinders 41-46 may be 
achieved through a circulating fluid. 
The machine operation is quite similar to that of the machine of FIG.1, 
also with respect to the conditions of paper humidification and the 
temperature and pressure on the paper when it passes through the embossing 
group.