FIELD OF THE INVENTION

The present invention relates to curing printing plates having photo-curable material thereon using LED light. In particular the invention relates to a device for automatically carrying out a rear-face pre-exposure process and a main exposure process.

BACKGROUND OF THE INVENTION

The preparation of flexographic printing plates coated with photopolymer material is being more and more automated. A typical process of preparing such printing plates includes starting with a plate that has an ablatable material thereon, imaging in a digital imager to ablate the ablatable material according to imaging data, and a curing process of the exposed plate that involves exposure to light energy, e.g., ultraviolet (UV) light energy. Automation often includes inline exposure to cure the plate inline after imaging. The introduction of UV light emitting diodes (LEDs) has created new alternatives for curing some UV sensitive materials. LED technology offers several advantages over the traditional mercury-arc technology. LED sources can also be turned on and off as required because LEDs do not require the warm-up and cool-down periods common in mercury-arc lamp systems. Some LED curing systems may implement driver circuits to control the current supplied to the LEDs. These circuits typically use a closed-loop system to monitor and control the output power of the LEDs, by controlling the drive current, to provide a stable and reliable UV source. These circuits may also define different curing cycles for different photosensitive materials, such as emitting a specific output power for a specific length of time.

The production of flexographic printing plates is performed using a multi-step process using appropriate devices for each process step. First, a mask is written into the imageable layer. The flexographic printing is then exposed to UV radiation. The photopolymerizable layer is polymerized in the areas not covered by a mask while in the areas covered no polymerization occurs. After exposure, the remnants of the mask and the unpolymerized parts of the photopolymerizable layer are removed. This can be done using one or more solvents or thermally. When using solvents for developing the plate a drying step is typically needed.

Often, the surface of the obtained flexographic printing plate is post-treated after drying, for example by exposing the surface with UVA and/or UVC radiation. For this, in turn corresponding exposure apparatuses are used. The devices for carrying out the respective steps normally required human intervention. After carrying out process step the processed flexographic printing element of the device has to be manually taken out and fed to the respective subsequent process step. Because of repeated manual intervention the production of flexographic printing plates is expensive.

Therefore, devices have been suggested, which combine the implementation of different process steps in an apparatus and thus reduce the number of manual interventions. WO2015/044437 discloses a device for automatically carrying out a rear-face preexposure process and a main exposure process and for developing flexographic printing elements, which can be digitally imaged and to a method for producing flexographic printing plates on the basis of digitally imaged flexographic printing elements using said device.

An object of the present invention is to provide an improved apparatus for in-line exposure of flexographic printing plates.

SUMMARY OF THE INVENTION

The present invention provides an apparatus for producing a flexographic printing plate comprising:

a housing providing a closed environment for producing the plate, said housing having an inlet for entering untreated plate, and an outlet for treated plates;

a transport device based on a roller system for transporting the flexographic printing plate through the apparatus, said transport device being configured with an inlet part with roller elements for introducing the plate from outside the housing and through the inlet towards a treatment part having at least three roller elements arranged in a circle arc along the transport path of the plate, and an outlet part for transporting the treated plate out of the housing;

a first gripping device for attachment to a first end of the plate, and a second gripping device for attachment to a second end of the plate, said gripping device being able to transport the plate over the at least three roller elements arranged in a circle arc; a UV LED exposure unit for rear side pre-exposure arranged below the at least three roller elements arranged in a circle arc;

a UV LED exposure unit for main exposure the of the plate arranged above the at least three roller elements arranged in a circle arc;

· a control unit for regulating the power of the exposure units, the performance of the exposure unit for the main exposure and the transport speed of the transport device, including the speed by which the gripping devices are moved.

The present inventors have surprisingly found that when the plate is processed while bended in the circle arc a significant improvement over existing treatment devices is achieved as far as quality and cleanliness is concerned. Moreover, the entire treatment in one or few treatment cycles is much faster than existing apparatuses.

In a preferred embodiment the UV LED source exposure unit for rear side pre-exposure and the UV LED exposure unit for main exposure is a single UV LED unit movable between a position below and above the at least three roller elements arranged in a circle arc to ensure sequential rear side pre-exposure and main exposure the of the plate. In this embodiment the rear side pre-exposure and main exposure are performed as separate steps and thus the plate has to be moved back and forth along the circle arc, which is ensured by the control unit. The distance between the plate and the UV

LED exposure unit is preferably adjustable.

It is also preferred that the first and second gripping devices each comprise an arm that is hinged in a pivot point at the center of the circle arc. This is a simple way to ensure that the force exerted by the gripping arms and the roller elements on the plate is evenly distributed.

In an alternative embodiment a cleaning unit for removing un-polymerized parts of the plate is provided. Such a cleaning unit may solvents and/or infrared light for the removal of non-polymerized parts. If solvents are used that may be sprayed onto the plate through nozzles, and removed parts and solvent may be collected in a container below the treatment part of the apparatus.

The first gripping device and/or the second gripping device is/are preferably provided with a pin bar having a plurality of protrusions for clamping the flexographic plate to the pin bar(s), said protrusions arranged orthogonally along the length of the bar and each of said protrusions being configured as a pin with a rounded upper part and with a cylindrical stem having a cavity or groove on at least one side of the pin. A drying unit is provided inside the housing to dry the treated plate.

BRIEF DESCRIPTION OF THE DRAWING Figure 1 shows a schematic representation of an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION Throughout the specification and claims, it is to be understood that by exposing a plate is meant exposing a plate that has a photo-curable material thereon to cure the material thereon using UV radiation. Plates as described herein are plates that have a photo- curable material thereon, and include polymer plates. Such a polymer plate may be a plate for flexography made of a photopolymer material that can be cured by exposure to UV. Thus the present invention covers the exposing of any plate that has photo-curable material thereon, e.g., a photopolymer. The present invention is thus applicable to all printing plates on which curing by exposure to UV radiation is applicable. The term printing plate is understood to mean such a plate herein. Figure 1 shows a schematic representation of one embodiment of apparatus according to the invention. The apparatus comprises a transport device with which the flexographic printing element to be processed is transported by the modules of the apparatus. The already digitally imaged flexographic printing plate is inserted into the inlet of the housing and the processed flexographic printing elements can be removed at the outlet. Using the device according to the invention the processing can be carried out automatically. Specifically Figure 1 shows a housing (1) providing a closed environment for producing the plate. The housing has an inlet (2) for entering untreated plate, and an outlet (3) for treated plates. Also shown is a transport device based on a roller system for transporting the flexographic printing plate through the apparatus. This transport device has an inlet part with roller elements (4) for introducing the plate from outside the housing and through the inlet towards a treatment part having multiple, and at least three roller elements (5), arranged in a circle arc along the transport path of the plate, and an outlet part with roller elements (6) for transporting the treated plate out of the housing (1). The apparatus is further provided with a first gripping device (7) for attachment to a first end of the plate, and a second gripping device (8) for attachment to a second end of the plate. These gripping devices are able to transport the plate over the roller elements (5) arranged in the circle arc.

In order to ensure rear side pre-exposure a UV LED exposure unit (9) is arranged below the roller elements (5). Also, in order to perform main exposure of the plate another UV

LED exposure unit (10) is arranged above the elements (5); alternatively only one UV LED exposure unit is used for both kinds of exposures, whereby such a universal exposure unit is movable between positions below and above the elements (5). A control unit is also required for regulating the power of the exposure units, the performance of the exposure unit for the main exposure and the transport speed of the transport device, including the speed by which the gripping devices are moved.