WO1991017889A1

WO1991017889A1 - A printing ink drying apparatus and method in flexographic or rotogravure printing machine

Info

Publication number: WO1991017889A1
Application number: PCT/IT1991/000040
Authority: WO
Inventors: Roberto Macchetta
Original assignee: Sama Engineering S.A.S. Di Roberto Macchetta & C.
Priority date: 1990-05-14
Filing date: 1991-05-14
Publication date: 1991-11-28
Also published as: IT9020301A1; IT1239913B; IT9020301A0; AU7855891A

Abstract

The printing ink drying apparatus includes a plurality of microwave emitters (4) located within a substantially closed drying chamber (1), having the printed support (2) led therethrough. For withdrawing the vapours generated by microwave heating, one or more suction ports (6) are provided on top of the chamber, as well as, possibly, a slit nozzle (7) for ejecting a 'blade' of pressurized air against the printed support.

Description

" A PRINTING INK DRYING APPARATUS AND METHOD IN

FLEXOGRAPHIC OR ROTOGRAVURE PRINTING MACHINE "

This invention concerns an apparatus and a method for printing ink drying in flexographic and rotogravure printing machines, and in particular the use of microwaves for endogenous heating and vaporization of the solvents contained in printing inks.

At present, the printed support in flexographic or rotogravure continuous machines, comprising an endless web of plastic film or paper is led through an enclosed chamber wherein, while being supported by a number of rollers, it is impinged upon by a number of laminar air jets, at a temperature of approximately 40-70°C, speeding up the vaporization of the solvent or diluent means carried by the ink. However, due to the low temperature of the air used, relative to the solvent or diluent vaporization temperature (usually ranging from 80 to 100°C), in order to obtain such vaporization an enormously large amount of air is required, with the associated heavy problems of balanced distribution, throttling and suction.

On the other hand, any additional heating, even a moderate one, of the blown air or of said printing support when the latter is a thin plastic film, would result in heavy thermal expansion and deformation problems and therefore problems in keeping the register between the various printing colours.

At present, the drying air is exhausted through special stacks, and it is dispersed through the open atmosphere, causing enormous amounts of solvents to be discharged into the environment, often exceeding the concentration limits tolerated by the local emission control laws. On the other hand, the emission reduction methods presently in use are destructive type methods, and result in high plant hardware and energy consumption costs, as well as in high heat losses .

A high cost is caused as well by the heat required for heating the enormously large amount of air used for the drying operations.

Therefore, the object of this invention is to provide an apparatus and a method as much as possible free from the drawbacks of the present state of the art.

The apparatus according to this invention substantially comprises a continuous drying chamber provided with several electro- magnetic wave emitters having a suitable wavelength (microwaves), one or more suction ports for exhausting solvent vapours, and possibily one or more suitably directed slots for blowing in on the surface of the ink- bearing film, a laminar flow of pressurized air. This invention will be described in the following for examplary and non limiting purposes, reference being made to the attached drawings, wherein:

Figure 1 shows a schematic, longitudinal section of the drying chamber of this invention, used in a flexographic or rotogravure printing machine; and

Figure 2 shows a schematic cross section along line II-II of Figure 1.

As it is shown in the Figures, drying chamber 1 is a substantially closed chamber, there being provided at the longitudinal ends thereof two slot passages 3 laterally extending for a span corresponding at least to the width of printed film 2 which is led therethrough, and having a maximum width of about 1 cm, just enough for film 2 to get in and come out, while preventing microwaves from scattering cut and reducing the admission of air from outside to a minimum. A certain number of microwave emitters 4 are provided in predetermined positions, laterally of drying chamber 1, and their power is proportioned to the machine size, the film speed, the printed area, and therefore to the amount of ink solvents or diluents to be dried up.

The frequency, range and power of the microwaves generated by emitters 4 are calibrated according to the type and amount of solvents to be vaporized in the unit time.

For the purpose of heating up and vaporizing only the specific solvents and diluents, without heating nor in any way supplying energy to the printing support, in order to avoid thermal expansion thereof and therefore problems of registry in multi-color printing, microwave emitters 4 should be exactly calibrated at the frequencies included within the range of maximum absorption by the solvent or the solvent mixture to be removed.

A further important parameter for optimizing operations of the apparatus according to this invention is the provisicn ofwaveguides or other devices adpated to concentrate or uniformly scatter the radiations on the surfaces to be treated.

The solvent or diluent vapours issuing from the inks applied onto support 2 are withdrawn through one or more suction ports 6 connected to a suction exhauster (not shown). In order to provide an easy with- drawal of the solvents from the printing surface and their disposal through suction ports 6, one or more transverse slit nozzles 7 may be provided through which air "blade" jets are directed towards the ink-bearing surface. Since the microwaves thoroughly flood the environment of chamber 1, the molecules of solvent are continuously impinged upon and stimulated by the microwaves, during all the time they remain within the drying chamber. A powerful suction system withdraws the vapours out of the chamber, said vapours bearing a high solvent concentration, and sends them to a condenser for recovery.

By using the microwave drying chamber according to this invention in printing machines it is possible to do away with a large number of support rollers for the printed film (in practice one for each "blade" jet of air blown in) whereby the structure of said machine is substantially simplified. The fact that the apparatus of this invention makes it possible to influence only the solvents and diluents, prevents a large amount of. air to be heated, with a corresponding energy saving.

The high concentration of vapours relative to the air and therefore the small amount of polluted air, relative to the conventional systems, involves the possibility of recovering said solvents by condensation, whereby the down- stream emissions to the environment are a strongly reduced amount, almost pollutant-free. Therefore, the solvent recovery as well provides large plant operation savings.

This invention will be described in more detail in the following, by means of two examples referred to an application on a six colors flexographic printing machine for printing on a 1468 mm wide polyethylene film fed at a speed of 250 m/min and ink-coated by 95%.

Taking into account that the amount of ink normally used is 6 g/m², and that the solvents comprise about 70% of the ink, based on the above data the resulting solvent amount to be vaporized is 87.38 kg/h.

Example 1

A drying chamber as described above is provided with four microwave generators, with an emitting frequency of about 2.4GHz and a power of 10kW for each generator applied on the chamber side, each of them cooperating with its own waveguide. Two suction ports connected to an exhauster providing a flowrate of about 2600 m³ /h draw both the air coming from the chamber slots and from a pressing air blade, and the vapours issuing from the ink and comprising 30% ethyl alchool, 65% ethyl acetate, and 5% propylene glycol.

Taking into account a 75% efficiency of the generators, a heat loss to the environment of 15%, an average absorption value of 70%, and that the energy required for vaporizing 87.38 kg/h of solvent is 17.5 kW/h the total energy used for vaporization is 39.2 kWh, while the power consumption for the pressure and suction fans is 7 kw/hand heating of incoming air requires about 10 kW/h.

Therefore the total energy consumption is 56.2 kWh.

Example 2 (Comparative)

A suction chamber constructed according to the conventional art and applied to the same flexographic printing machine uses 3200 m³ /h of hot air at 70°C for the printing group, and an equal volume of hot air at 70 °C for the drying chamber, with a heating power consumption of 117.2 kW/h, while the power consumption of the fans is 15 kW/h. Therefore the total power used is 132.2 kW/h.

Comparison of the power consumptions in the Examples results in a total energy saving higher than 70 kW/h for the apparatus of Example 1.

As it has been pointed out before, the number of micro- wave emitters, the power thereof, as well as the microwave frequency bands must be calibrated on the size and the operating speed of the flexographic or rotogravure printing machine wherein they are provided, and on the amount and type of organic or inorganic solvents to be vaporized in the unit time. Thus, for instance, plural shielded drying chambers may also be provided in a single printing machine, with a varying number of microwave emitters, while their power and microwave frequency must be manually or automatically controllable according to the production speed of the machine and to the amount and type of solvent carried by the inks, either organic or water.

Further parameters to be taken into account when designing the drying chambers of this invention are both the absorption spectra of the substances to be vaporized, eitherpure or in mixtures, and the absorption spectra of the other substances or materials wich are contacted by the radiations.

Therefore, it should be apparent that the microwave power and frequency must be such as to ensure the highest absorption by the solvents or diluents to be vaporized, and at the same time a minimum involvement of the other materials impinged upon by radiation, like the drying chamber walls and the flexible ink-bearing substrate.

The automatic control mentioned above is put into operation based on a program stored in a computer, which accounts for the frequency bands of the various materials used as ink solvents or diluents, for the number and power of the emitters, as well as for the printing machine operating speed.

º= º= º= º= º= º= º= º= º= º= º= º= º= º= º= º= º= º= º= º= º= º= º= º= º= º= º= º

Claims

1. An ink drying apparatus for flexographic or rotogravure printing machines comprising a chamber for vaporizing and exhausting the solvents or diluents of printing inks applied onto an endless flexible support moving therethrough, characterized in that said drying chamber (1) is provided with one or more microwave emitters (4) adapted to vaporize, by means of dielectric loss selective heating, the solvents or diluents carried by the inks applied onto said flexible support (2).

2. The apparatus according to claim 1, characterized in that the power of the individual emitters is manually controlled according to the operation mode of the printing machine.

3. The apparatus according to claim 1 , characterized in that the power of microwave emitters (4) is automatically controlled based on the operating speed of the printing machine.

4. An apparatus according to any of the above claims, characterized in that the microwave frequencies generated by said emitters (4) are automatically set, based on the absorption spectra of the substances to be vaporized.

5. An apparatus according to any of the preceding claims, characterized in that said microwave emitters (4) are provided with devices (5) adapted to concentrate or scatter the radiation onto the ink-bearing surface of said flexible support (2).

º= º= º= º= º= º= º= º= º= º= º= º= º=º= º= º= º= º= º= º= º= º= º= º= º= º= º= º= º