Patent Application
20060130688
1. Field of the Invention
The present invention relates to a method and apparatus for the drying of the printing plates for flexography.
2. Description of the Prior Art
As known, flexography is a printing technique using flexible printing plates preferably made of a photopolimeric material like rubber or plastic. The inked plates with a slightly raised image are rotated on a cylinder which transfers the image to the substrate. Flexography is a high-speed print process, can print on many types of absorbent and non-absorbent materials, and can print continuous patterns (such as for giftwrap and wallpaper).
Some typical applications for flexography are paper and plastic bags, milk cartons, disposable cups, and candy bar wrappers. Flexography printing may also be used for envelopes, labels, and newspapers. It is frequently used for printing on plastic, foil, acetate film, brown paper, and other materials used in packaging.
One of the steps for the preparation of printing plates for flexography is the plate drying, to allow the solvent, penetrated inside the plate in the previous phases for eliminating the non-etched parts of the plate, to evaporate.
The state of the art for the printing plate drying step, as described for example in U.S. Pat. No. 4,478,931, or U.S. Pat. No. 4,806,506, or U.S. Pat. No. 5,085,976, is to create a flow of hot air, at a velocity of about 1-1.5 m/s, above the plates, in a plate chamber, to allow the solvent going out of the plate to evaporate due to the plate heating. The temperature of the air is controlled to not go beyond a threshold of 60° C., to avoid permanent deformation of the plates.
The total time needed for completing the drying step depends on the plate thickness, and is around one hour for every millimeter of plate thickness. As in the practice the normal plate thickness available on the market ranges from 1.14 to 7.00 mm, the total time needed for completing the drying step is 1 to 7 hours. Additional air drying overnight (sixteen hours or even more) is common. This long time is therefore a problem.
The long time is needed for the fact that, due to the low thermal conductivity of the material of the plate, the use of hot air for the plate heating renders the temperature of the plate surface higher than inside. Therefore also the tension (pressure) of the solvent is higher on the plate surface than inside, hindering the migration of the solvent from inside the plate to the outside.
On the other hand, the drying step must ensure a complete evaporation of the solvent before the exposure of the plate to UVC and UVA rays in the following step for preparing the printing plate, otherwise that exposure to UVC rays creates the effect of forming a sort of layer (film) on the surface of the plate, hindering a following evaporation of the solvent.
Another known technique for drying the printing plates for flexography is to irradiate the plates by infrared rays, typically using an infrared owen, as described for example in U.S. Pat. No. 4,478,931, or U.S. Pat. No. 4,806,506, or US 2004/0081908. This kind of technique creates similar (or even worse) problems as the hot air flow described above. In fact the infrared rays tend to remain on the surface of the plate and not penetrate the inside, with the effect of heating much more the surface than the inside of the plate. This creates difficulties in the control of the temperature on the surface of the plate.
Therefore in view of the above mentioned problems, it is the main purpose of the present invention to provide a method and apparatus for the drying step of the printing plates for flexography, which reduces drastically the total time needed, basically by heating the inside of the plate at a temperature equal to or higher than that of the surface, to allow the solvent to migrate more rapidly, and also rendering the solvent recovering much easier and cheaper.
The basic idea of the present invention is to heat the printing plates by irradiation of visible light, preferably in the range of 400-650 nm wavelength.
The visible light penetrates the plate which is transparent at these wavelengths, and is reflected by the base of the plate. The light is refracted inside the plate, heating it inside as well as on the surface. The solvent particles change to the gaseous state and migrate toward the surface rapidly. The migration velocity is such that the plate thickness does not affect the total time needed for drying. Typically a total time of 15/30 min is sufficient for taking all the solvent away, independently from the plate thickness, and also keeping into account that the temperature is controlled not to go beyond the threshold of 60° C., thus ensuring the stability of the material making up the plate. For example in the use in the packaging sector (printing on corrugated boards) the typical plate thickness is 3 to 7 mm, therefore the saving of time and energy is extremely high.
A particular subject of the present invention is a method for drying printing plates for flexography, so as to let a solvent present inside the plates to evaporate, wherein it comprises the step of heating the printing plates by irradiation of visible light wavelengths.
Another subject of the present invention is an apparatus for drying printing plates for flexography, so as to let a solvent present inside the plates to evaporate, wherein it comprises:
The apparatus also comprises means for eliminating an unwanted parasitic emission of infrared rays.
These and further objects are achieved by means of a method and apparatus for the drying step of the printing plates for flexography, as described in the attached claims, which are considered an integral part of the present description.
The invention will become fully clear from the following detailed description, given by way of a mere exemplifying and non limiting example, to be read with reference to the attached drawing figures, wherein:
The same reference numerals and letters in the figures designate the same elements.
The method for drying printing plates for flexography subject of the invention comprises the step of heating the printing plates by irradiation of visible light wavelengths.
Preferably the irradiation of visible light wavelengths is in the range of 400-650 nm wavelengths. In fact, as it will be described below, with reference to the tests performed on some samples of printing plates, the range of wavelengths in which the effect of migration of the solvent to the outside of the plate is more efficient and quick is just that one, while above about 650 nm, the migration effect decreases, and the corresponding irradiation energy becomes more and more useless as the wavelength increases, and should be eliminated.
Preferably the method also comprises the step of letting a flow of air pass over the plates, for taking the solvent vapour emanating from the plates away.
Preferably the method also provides for bringing the flow of air passing over the plates to an air purification system to recover the solvent, so as to reduce pollution as much as possible.
As shown in
Suitable fan systems 3 and 10 are also provided for the air circulation, as described below.
The apparatus is divided in three layers, L1, L2 and L3.
The lower layer L1 forms a chamber for hosting the printing plates to be dried. An hollow space above the plates 8 is created possibly having the glass sheet 6 as a cap: an air flow is created by air inlet 11 and air outlet where a fan system 10 is located. The arrow shows the air flow direction.
As explained above, a light flow of low velocity room temperature air is created, preferably in the range of 0.1 to 0.5 m/s, for taking the solvent vapour emanating from the plates away. The air is someway heated by the hot temperature developed over the plates, around 60° C. Due to the low velocity of the air, the temperature gradient inside the plate chamber is also low, and this is an advantage for the better uniformity of temperature inside the chamber.
A preferred scheme of air flow in the chamber L1 is as shown in
An air purification system, not shown in the figures, is connected to the air outlets 10, and can be dimensioned according to the average air flow in the chamber L1. With a drying time of 20 min, a total air flow of 10 m3 is enough.
The air purification system can be of any known type, preferably of the refrigeratory type. This because the solvent includes a volatile component (aliphatic alchool) that can also be completely recovered through a system like this.
The whole of the solvent can be recovered this way, by air condensation. As an example, for a plate of 2 m2, a quantity of about 50 cc of solvent is recovered for each millimeter of plate thickness.
The upper layer L3 forms a chamber for hosting the lighting system 1. The glass sheet 5 is the floor of the chamber, while the cap is a tank 2 for air depression.
The lighting system 1 must be suitable to the purpose of creating a uniform irradiation of visible light wavelengths towards the plates located in the lower layer L1, as all the points of the plate surface must receive the same quantity of light rays, namely the same energy and temperature. The difference in the level of irradiation in the different points of the plate surface 8 has to remain within a range of 6-7%.
An embodiment of the lighting system 1 is shown in
Each lamp irradiates with an intensity decreasing more or less linearly with the angle of irradiation; putting the lamps at a relative distance equal to that with respect to the plates, in the different points of the plates the total irradiation is given by the sum of the contributions of two contiguous lamps with a mutual compensation, so giving a uniform level of irradiation in the different points.
Each lamp has a series of lighting sectors 12 with an individual length equal to the distance between them and equal to the distance between the different lamps, so as to form a complete two-dimensional grid of lighting sectors.
In a non limiting example this common distance is 40 mm.
To ensure a uniform lighting level even at the boundaries of the surface to be lighted, the dimensions of the grid of lighting sectors of the lamps exceeds that of the plate surface 8 to be lighted by a value equal to the common distance. In the specific example if the maximum dimension of the plates is 1320×2000 mm, the dimensions of the grid of lighting sectors will be 1400×2080 mm.
The power supply voltage of the quartz lamps depends on the total length of the lighting sectors of the lamp. Therefore the lamps are so designed as to be supplied by normal existing power supply voltage, for example a three-phase 400 V. In some installations with a considerable total length to be lighted, it is possible to put two or more lamps connected in series, so as each lamp can be still powered by available voltage.
By means of the tank 2 for air depression, a cooling system for the lamps is obtained. A clean room-temperature air, which does not need any purification action, enters the chamber L3 from air inlets 4, makes a cooling action passing through the lamps, and is taken away from the air outlet 3, by a suitable fan system on the cap 2.
The intermediate layer L2 in between the two glass sheets 5 and 6 is created to eliminate the effects of the parasitic infrared emission by the lighting system 1, and transparent to the wanted visible light wavelengths, which pass through and reach the lower layer L1. The sheet 5 nearer to the lamps reflects a part of infrared emission back towards the lamps; a remaining part of infrared wavelengths is refracted towards the other sheet 6 and reflected back. This remaining part reflects and bounces in the interspace 7 between the two sheets: the relating energy is dissipated by generating heating which is taken away by a suitable air flow. This is a clean room-temperature air which does not need any purification action.
A second embodiment of the apparatus for drying the printing plates is shown in
It provides for a lighting system having normal incandescent lamps 23, preferably of linear filament shape disposed parallel over a printing plate 26 to be dried, so as to generate heating. An unavoidable parasitic emission in the range of infrared wavelengths is also present, to be possibly eliminated as described below. The plate is put on a base element 25 preferably in an extractable box.
Preferably a reflector 22 is present above the lamps 23, so as to concentrate the most of the irradiation on the direction of the plate 26, and to ensure a uniform lighting level in all the points of the surface to be lighted. The reflector 22 can be made of a surface of smoothed aluminium, to get the maximum reflection efficiency.
In the first variant of
The flow of air needed is a strong one, with a typical value of about 350 m3 per hour. This way the negative effect of the parasitic emission of infrared rays from the lamps is eliminated, as the flow of air cools mainly the surface of the plate, just where the most of the heating caused by the infrared rays is generated.
This first variant is simple and cheap to reach the main purpose of the present invention. On the other hand, due to the huge flow of air needed, in practice it is hard to realize a recovery system for the solvent vapour emanating from the plate.
In the second variant of
A number of openings 30 is obtained in the walls of the lower part of the body in the lower layer L21, and a fan system 31 is present aside and just above the plate, so as to create a second flow of air through the openings 30, passing over the plate and to the purpose of both cooling the plate, keeping it a a temperature below 60° C., and taking the solvent vapour emanating from the heated plate away. The second flow of air is a light flow of low velocity room temperature air, making it possible to implement a recovery system for the solvent vapour emanating from the plate, for example of the kind described above.
Typical values for the two flows of air are about 350 m3 per hour for the first flow, and 20 m3 per hour for the second flow.
The experimental tests described in the following have shown the effectiveness of the method and apparatus subject of the invention.
The tests have been performed using printing plates for flexography having different thickness, for example 1.74 mm and 7 mm. For each thickness a squared sample has been taken of 10 cm×10 cm wide.
First, the sample has been exposed to UVA rays for the polymerization step. Then it has been weighted with a very precise balance and thereafter processed with solvent for the normal time needed depending on the thickness and type of the plate. Then it has been weighted again, and dried with the incandescent lamps of the second embodiment, having a spectral emission as shown in
In the first case (
In both cases after 30 or 35 minutes drying, the sample plate has reached the original weight again, that's all the solvent has been taken away.
This is the proof of the effectiveness of the method of the present invention. The time necessary for the drying step does not depend on the plate thickness and is very short as compared with the drying time needed with the methods known in the art.
The lighting system of the second embodiment of the apparatus for drying the printing plates shows a much lower level of emission of parasitic infrared rays than the first one, so the presence of the tempered glass sheet may not be necessary in the most of the practical applications.
To control the temperature of the printing plate so as not to go beyond the threshold of 60° C., some normal temperature probes not shown in the figures are put near to the plates on the base element 9 or 25, so as to regulate the switching (duty-cycle) of the lamps by a suitable state-of-the-art circuitry.
Further implementation details will not be described, as the man skilled in the art is able to carry out the invention starting from the teaching of the above description.
Many changes, modifications, variations and other uses and applications of the subject invention will become apparent to those skilled in the art after considering the specification and the accompanying drawings which disclose preferred embodiments thereof. All such changes, modifications, variations and other uses and applications which do not depart from the spirit and scope of the invention are deemed to be covered by this invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 04102647.7 | Jun 2004 | EP | regional |
