EP0639451B1 - Ferroelectric printing process - Google Patents

Description

The invention relates to methods for the reproduction of an image by means of a printing form which has a layer of a ferroelectric material on the surface.

DE 38 35 091 C2 already discloses a method for reproducing a ferroelectric image template, in which electrically charged toner particles are used. The ferroelectric material can be polarized differently in very narrow areas, so that prints with very high resolution are possible with single-color toners and that when using two colors with differently charged particles, i. that is, one color contains positive and the other color contains negatively charged toner particles, two colors can be applied at the same time with one print, so that low numbers of passes are necessary for colored prints. The printing form is suitable for the use of dry toners as well as toners dissolved in fountain solutions as carriers. At which temperatures the printing form is polarized is not disclosed.

On the other hand, it is already known from US Pat. No. 3,899,969 to polarize pyroelectric materials, in particular ferroelectrics, at very high temperatures, for example at 150 ° C., using an electric field. For this purpose, the material to be polarized must be placed in a bath of hot oil, for example.

From DT 25 30 290 A1, the one-time design of an external electric field on a ferroelectric material after a polarization process to produce a latent image on the Surface of the ferroelectric, but only as in a capacitor, a proportional to the field strength of the applied field and therefore limited amount of charges is applied to the surface of the ferroelectric through the field. Since the charges are also released when the toner image is delivered to a printing material, only a limited number of copies can be made of the latent image on the ferroelectric until all the free charges generated by the external field have been used up. The same applies to the application of the pyroelectric or the piezoelectric effect by heating the ferroelectric or applying pressure to the ferroelectric. Therefore, the process known from DT 25 30 290 A1 is not a continuous printing process, but rather a copying process for producing a limited number of copies.

It is the object of the present invention to provide printing processes in which the print quality, i.e. the contrast is improved compared to the known printing processes. This object is, as stated in claim 1, solved.

Advantageous further developments result from the subclaims. According to claim 15, a method for imaging a printing form is created, which also improves the contrast when printing with the printing form thus imaged.

In contrast to the prior art, in the method according to the invention, new charge carriers are continuously applied to the printing form to increase the contrast, so that more can be stored at the polarized points, which toner is released to the print carrier when the toner image is released.

Fig. 1

eine Hystereseschleife,

Fig. 2

eine Vorrichtung zum Drucken mit einem Ferroelektrikum, wobei der Formzylinder auf seiner Mantelfläche mit einer Schicht aus einem ferroelektrischen Material beschichtet ist und wobei seitlich des Formzylinders Ladungsquellen angeordnet sind,

Fig. 3

die Druckvorrichtung gemäß Fig. 2, wobei die Schicht durch eine Heizung beheizt wird, und

Fig. 4

die Druckvorrichtung gemäß Fig. 2, wobei eine Tonerauftragwalze zum Auftragen des Toners auf den Formzylinder gegen diesen gedrückt wird.

The invention is explained in more detail in exemplary embodiments with reference to the drawings. Show it:

Fig. 1

a hysteresis loop,

Fig. 2

a device for printing with a ferroelectric, the forme cylinder being coated on its outer surface with a layer of a ferroelectric material and wherein charge sources are arranged on the side of the forme cylinder,

Fig. 3

2, wherein the layer is heated by a heater, and

Fig. 4

the printing device of FIG. 2, wherein a toner application roller for applying the toner to the forme cylinder is pressed against it.

Ferroelectric material is characterized in that its micro building blocks, i. H. its unit cells have a stable electrical dipole moment that can be aligned in an electrical field. Ferroelectrics are, for example, inorganic ceramic materials with an asymmetrical perovskite structure, such as. B. barium titanate, lead zirconate and mixed structures thereof or organic substances, such as. B. polyvinylidene fluoride with C-F chains as elementary dipoles. The inorganic ferroelectrics have structures in which the unit cells are so asymmetrically arranged that two energetically equivalent and structurally identical modifications, i. H. enantiomorphic modifications exist which can only be transferred from one state to the other by the supply of energy, e.g. B. by the action of external forces, for example an electrical field, or by thermal energy.

If this energy is supplied by an electric field, above a certain material-dependent field strength, the so-called coercive field strength, the states that are not in the field direction change in the field direction and remain in this state even after the electrical field has been switched off. This process is called polarization of the ferroelectric.

If this supply of energy takes place through heat, as a result of the thermal lattice vibrations from a certain temperature, the Curie temperature, both modifications are equally probable and lose the alignment generated in an external electric field completely after switching off the electric field. The ferroelectric therefore changes into the paraelectric state above the Curie temperature. When cooling from this state to the ferroelectric state without an external field, statistically distributed regions are formed, the domains, whose field effects cancel each other out, so that a macroscopically neutral non-polar state is created.

If the ferroelectric is polarized below its Curie temperature, the electrical field generated by the alignment of its dipoles cannot continue on the material surface; Since there are no self-contained electric field lines, but always end at charges, a layer of surface charges forms on both surfaces of the ferroelectric layer, which stabilize the field inside the ferroelectric. A polarized ferroelectric plate is therefore, even after removal of electrodes that may be used for polarization, similar to an electrical capacitor, on the electrodes of which there are surface charges that are bound in a fixed manner by the internal electrical field.

These surface charges shield most of the inner field from the outside. However, this shielding is not complete, rather a residual field sufficient for the printing process acts on the outside and is able to remove electrically charged particles, e.g. an electrostatic toner. The polarity of the image is achieved by different alignment of the image and background areas (DE 38 35 091 C2).

1 shows a hysteresis curve of a ferroelectric. The surface charge density of the electrical charge flowing onto the surface of the ferroelectric is as Function of the electric field E present in the interior of the ferroelectric is shown. When the polarity of a ferroelectric, the randomly distributed in a macroscopically neutral state, in the positively polarized state that is called. Virgin curve 1 from a point 0 through to a point A _1. After the electrical field has been switched off, the material remains in a stable, polarized state P ₁ . By applying an opposing field, the curve is traversed from the point P ₁ via a point A ₂ to a point P ₂ . This process is reversible and can be repeated any number of times. After polarity, the pixels of the ferroelectric are, for example, in a state P ₁ , while the background areas, ie the non-image areas, are in the state P ₂ . Reverse polarization is also possible. It is also possible that only the image areas are polarized positively or negatively and the non-image areas are neutral.

2 shows a device for printing on a printing material web 2 with a form cylinder 3, the outer surface of which is surrounded by a printing form 30, which consists either entirely or at least in its outer layer of a ferroelectric material. The printing form 30 receives toner from a toner application roller 4, which the toner application roller 4 in turn receives from a toner reservoir 5. The toner reservoir 5 contains a toner supply 50, which is kept at a certain, fixed level via a toner feed 51. Via a toner discharge 52, toner not taken up by the toner application roller 4 is fed to a filter arrangement, not shown here, and returns to the toner reservoir 5 via the toner feed 51. Toner particles, which are imaged on the form cylinder 3 on the surface of the printing form 30, are transferred via a transfer cylinder 6 to the printing material web 2, the transfer cylinder 6 pressing the printing material web 2 against a printing cylinder 7.

However, before the printing form 30 can be used for printing by means of the toner, it must pass through an imaging unit 8 Polarize as described above. Then the number of superficially available free charges is electrically increased for printing with the toner. For this purpose, charge sources 11, 12 are arranged next to the forme cylinder 3 and charge the surface of the printing form 3 either positively or negatively. As charge sources such. B. corona discharges, contacting dielectric or weakly electrically conductive layers or pixel-separated individual electrodes possible. In this case, the charge sources 11, 12 are either the same with which the printing form 30 was previously polarized imagewise, or, as shown in FIG. 4, other charge sources 11, 12.

auf. Durch diesen Potentialunterschied ergibt sich ein Kontrast zwischen den beiden Bereichen, der beispielsweise um einen Faktor 100 größer ist als der ursprüngliche Kontrast bei dem polarisierten, aber ungeladenen Ferroelektrikum. Entsprechend der Aufladung mit positiven Ladungsträgern, wie hier dargestellt, läßt sich die Druckform 30 als ganze auch mit negativen Ladungsträgern aufladen. Diese bildmäßig einmal gepolte und aufgeladene ferroelektrische Druckform 30 übersteht eine Vielzahl von Druckvorgängen. Es zeigt sich allerdings, daß die ortfeste Bindung der in dem Punkt B₁ befindlichen Ladungsträger größer ist als die der im Punkt B₂ befindlichen Ladungsträger, da Ladungen im Punkt B₁ eine Verstärkung, im Punkt B₂ eine Schwächung des Feldes in der ferroelektrischen Schicht bewirken. Diese werden abgegeben, und die Druckform wird im Bereich der negativen Polarisation von dem früheren Polarisationszustand P₂ auf einen Polarisationszustand P₂ teilweise depolarisiert. Um diese Depolarisierung wieder rückgängig zu machen, wird die Druckform 30 mit negativen Ladungsträgern beaufschlagt, dann durchläuft das Ferroelektrikum von dem Punkt P₂' zu dem Punkt A₂ eine Polarisierungs-Kurve 15. Nachdem einmal wieder der Punkt A₂ erreicht ist, kann das Ferroelektrikum wieder positiv bis zum Erreichen des Punktes B₂ aufgeladen werden. Entsprechendes gilt in schwächerem Umfang für den Punkt B₁. Die unipolare Aufladung der Druckform 30, z. B. nur mit positiven Ladungsträgern, bewirkt einen Kontrast ${\text{ΔE=E}}_{\text{1}}{\text{-E}}_{\text{2}}$

mit positivem Potential auf Bild- und Nichtbildstellen. Durch Einstellen des Potentials der Tonerauftragwalze 5 auf ein Niveau zwischen E₁ und E₂ wird eine für die Tonerpartikel an- bzw. abstoßende Wirkung erzeugt.After the imaging process has ended, ie after the electrodes of the imaging unit 8 are again at ground potential, for example the pixels are in the polarization state P ₁ (cf. FIG. 1) and the non-pixels are in the polarization state P ₂ . By renewed, but the entire surface in this case, charging of the printing plate 30 with a defined amount of charge, for example, .DELTA.P, the points which previously had the polarization state P _1, raised to an electrical potential E. ₁ The points that were previously in the polarization state P ₂ are raised to a potential E ₂ . Without the amount of charge ΔP now applied to the printing form 30, there was only the relatively small potential difference generated by the residual field between the positively and negatively polarized regions P ₁ and P ₂ . Now they have a potential difference ${\text{ΔE = E}}_{\text{1}}{\text{-E}}_{\text{2nd}}$

on. This difference in potential results in a contrast between the two areas, which is, for example, a factor 100 greater than the original contrast in the polarized but uncharged ferroelectric. Corresponding to the charging with positive charge carriers, as shown here, the printing form 30 as a whole can also be charged with negative charge carriers. This ferroelectric printing form 30, which is once polarized and charged imagewise, survives a large number of printing processes. It turns out, however, that the fixed binding of the charge carriers located in point B ₁ is greater than that the charge carriers located in point B ₂ , since charges in point B ₁ cause an amplification and in point B ₂ a weakening of the field in the ferroelectric layer. These are released and the printing form is partially depolarized in the region of the negative polarization from the previous polarization state P ₂ to a polarization state P ₂ . In order to reverse this depolarization, the printing form 30 is charged with negative charge carriers, then the ferroelectric traverses a polarization curve 15 from the point P ₂ 'to the point A _2. Once the point A _{2 has been} reached again, this can be done Ferroelectric can be charged positively again until point B ₂ is reached. The same applies to a lesser extent for the point _B1. The unipolar charging of the printing form 30, for. B. only with positive charge carriers, causes a contrast ${\text{ΔE = E}}_{\text{1}}{\text{-E}}_{\text{2nd}}$

with positive potential on image and non-image areas. By setting the potential of the toner application roller 5 to a level between E ₁ and E ₂ , an effect which is repulsive or repulsive for the toner particles is produced.

For this reason, in the case of a long-term, continuous printing process, it is necessary to charge, for example, a positively charged ferroelectric with negative charge carriers at certain time intervals. As a result, both polarization states are completely regenerated again.

haben.In another embodiment of this method, the contrast increase is effected in connection with the image generation. First, the printing form 30 is negatively polarized on its entire surface by a first electrode and additionally negatively charged with negative charge carriers (electrons) by an amount ΔP and brought to a potential E ₃ (point B ₃ ). The image areas on the surface of the printing form 30 are then positively polarized by a second electrode and charged positively by removing electrons by the amount P to the potential E ₁ (point B ₁ ), so that the pixels B ₁ become the non-pixels B ₃ a potential difference ${\text{ΔE '= E}}_{\text{1}}{\text{-E}}_{\text{3rd}}$

to have.

In addition to this method, the number of free charges on the surface of the printing form can be increased by heat. First, the printing form is imaged at a temperature T ₁ , for example at 20 ° C. In order to maintain this temperature, the entire printing device is preferably thermostatted at this temperature. For this purpose, it is arranged, for example, in a closed room.

After the polarization has ended and before the printing process begins, the printing form 30 (FIG. 3) is brought from its outer surface to a higher temperature T ₂ , for example 25 ° C., by a heater 9 and is kept at this temperature. It should be noted that this temperature T _{2 is} below the Curie temperature of the respective ferroelectric. The increased temperature increases the number of surface charges on the surface of printing form 30. The charge required to compensate for the internal electric field according to FIG. 1 is temperature-dependent, ie less compensation charge is required at higher temperatures. If polarization was carried out at low temperature, the compensation charge that is not required is released when the temperature rises. So there are free positive charges in the positively polarized area, free negative charges in the negatively polarized area. Since the increased temperature increases the number of free surface charges on the surface of the printing form, the contrast, ie the potential difference between the positively and negatively polarized areas, increases. As a result, if the particles are positively charged, for example, more toner particles are deposited on negatively charged image areas and background tones are avoided. The increased contrast voltage between the image and background areas thus produces a better optical contrast between the image and background areas, ie a denser toner layer in the print areas with a toner-free background. This effect can be used for a large number of printing processes, for example 1000 prints. However, it can not be completely prevented that parts of the Surface charge on the printing form 30 are entrained by toner particles on the transfer cylinder 6 and from this onto the printing material web 2. Therefore, a cooling device 10 is preferably arranged on the side of the forme cylinder 3. This cools the printing form 30 either before or after the toner is released onto the transfer cylinder 6. When cooling before the toner is released, the free surface charge is bound again as a compensation charge due to the reversible pyroelectric effect. When cooling after the toner has been released, the required compensation charge is transferred from the surrounding medium to the surface and bound in place.

The compensation charge required for this pyroelectric effect is transferred from the surrounding medium, for example the air, to the surface and bound there in a stationary manner. Due to the cooling device 10, the printing form 30 assumes a temperature T ₃ which is below the temperature T ₂ . Subsequently, the printing form 30 is heated up again by the heater 9 to the temperature T ₂ , and there is again an excess charge on the surface, which causes the above-described effect of contrast enhancement. The cooling process can either take place continuously or at certain time intervals after a predetermined number of prints if the number of free surface charges has decreased accordingly. With continuous cooling it is also achieved that the amount of heat supplied by the heater can be removed again.

Instead of applying the toner 50 to the printing form 30 with the printing roller 4, any other supply device for applying the toner 50, e.g. a tape, find use.

If a liquid toner is used instead of the dry toner 50, the cooling of the printing form 30 caused by the evaporation of the heat of evaporation may also be sufficient.

Instead of the heater 9, it is also possible to heat the surface of the printing form 30 by immersing it in a toner bath of a liquid toner, this toner liquid being heated to a temperature T ₂ .

In connection with the method of contrast enhancement by applying a charge carrier source or also in connection with the method of increasing the temperature, the number of available charges on the surface of the printing form 30 can also be increased by exerting a mechanical force thereon. This takes place, for example, in that the toner application roller 4 is pressed against the forme cylinder 3 with a specific, predetermined pressure p (FIG. 3). The free surface charge is created here by the piezoelectric effect effective in ferroelectric materials.

The devices according to FIGS. 2 to 3 can advantageously be used if toner and removal electrodes 13, 14 as shown in FIG. 4 are also present. These electrodes 13, 14 are located at a certain distance from the surface of the printing form 30 and influence the toner acceptance on the surface of the printing form 30. For example, a negatively charged electrode 13 repels negatively charged toner particles from the electrode 13 and makes them stronger and faster accepted positively charged image areas on the printing form 30. The same applies vice versa for the electrode 14 which, if it is positively charged, the better prevents the accumulation of the negatively charged toner particles in non-image areas which are also negatively charged. This also increases the contrast between image and non-image areas and avoids background toner.

According to the invention, methods are created by means of which the number of charges available on the surface of a printing form 30 with a ferroelectric layer can be increased, ie the potential difference between image and non-image areas is increased. To do this, either the temperature on the surface of the Printing form 30 is increased compared to the temperature at which it was polarized, or a mechanical load is exerted on the form cylinder 3 for toner transfer, or charge carriers are applied over the entire area to the printing form 30, as a result of which a potential difference between positively and negatively polarized regions arises, leads to an increase in contrast between image and non-image areas.

Claims (15)

1. Process for reproducing an image original by means of a printing forme (30) which has a layer of a ferro-electric material, in which the layer is polarised at a first temperature (T₁) according to an image to be printed, and in which electrically charged particles are deposited on the surface of the ferro-electric material, characterised in that after it has been polarised the printing forme (30) is additionally charged with electrical charges from a charge carrier source for the image-wise increase in the number of surface charges on the layer.
2. Process according to claim 1, characterised in that at least one positive (11) or one negative electrode (12) is arranged near the layer so as to transfer charges onto its surface.
3. Process according to claim 1, characterised in that the charge is transmitted from an electrically non-conductive dielectric layer.
4. Process according to claim 1, characterised in that charges are applied to the layer by corona-discharge or by an electrode (11, 12) contacting the surface of the layer.
5. Process according to one of claims 1 to 4, characterised in that to compensate for charge carrier densities of different strengths in free surface charges in the image and non-image areas, the image and non-image areas are oppositely charged and then charged again with the electrical charges provided.
6. Process according to claim 5, characterised in that the charge is generated on the dielectric layer by friction, contact with the electrodes (11, 12) or corona-charging.
7. Process according to one of claims 1 to 6, characterised in that after it has been polarised the printing forme (30) is heated to a second, higher temperature (T₂) below the Curie temperature, and in that the particles are deposited on the surface of the layer at this temperature and then transferred to a material (2) to be printed.
8. Process according to claim 7, characterised in that the printing forme (30) together with the layer is tempered at the second temperature (T₂).
9. Process according to claim 7, characterised in that after deposition of the particles on the layer surface the layer is cooled to a third temperature (T₃) which is lower than the second temperature (T₂) at which the particles were deposited on the layer, and in that the layer is heated again to the second temperature (T₂) to receive the particles again.
10. Process according to claim 7, characterised in that the charged particles are contained in a liquid, in that after deposition of the particles the layer is cooled, by evaporation of the liquid drawn by its surface, to a temperature (T₃) which is lower than the second temperature (T₂) at which the particles were deposited, and in that the layer is heated again to the second temperature (T₂) to receive the particles again.
11. Process according to claim 10, characterised in that the layer is heated by immersion into a toner bath tempered at the second temperature (T₂).
12. Process according to one of claims 1 to 9, charcterised in that for the image-wise increase in the number of surface charges on the layer, a mechanical force is exerted on the surface of the layer during deposition of the particles.
13. Process according to claim 12, characterised in that the printing forme (30) is applied to the casing surface of a forme cylinder (3), in that the toner is applied to the forme cylinder (3) by means of a feeding device, in particular by means of a toner application roller (4), and in that the toner application roller (4) is pressed against the forme cylinder (3) and hence against the layer.
14. Process according to one of claims 1 to 13, characterised in that a toner application and/or toner removal electrode (13, 14) for increasing toner application in the image areas and reducing toner application in the non-image areas are arranged near the surface of the layer and according to its charge remove the toner from the surface of the printing forme (30) or press it firmly on the surface.
15. Process for the imaging of a printing forme (30) which has a layer of a ferro-electric material, characterised in that the entire surface of the printing forme is polarised by a first electrode (11) in a first polarisation direction (ΔP, E₃) and is additionally electrically charged, and in that the image areas are then polarised by a second electrode (12) in the opposite polarisation direction and additionally electrically (ΔP, E₁) charged in the same direction.

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