Apparatus for collecting ablated material

#This invention relates to the formation of images directly from electronically composed digital sources. More particularly, the invention is concerned with imaging methods wherein particles of an imaging layer are ablatively removed from an imaging member during exposure of the member to imaging radiation. Such methods are especially applicable, for example, to the preparation of lithographic printing plates.

#Lithographic printing is a process of printing from surfaces which have been prepared in such a way that certain areas are capable of accepting ink (oleophilic areas), whereas other areas will not accept ink (oleophobic areas). The oleophilic areas form the printing areas while the oleophobic areas form the background areas.

#Plates for use in lithographic printing processes may be prepared using a photographic material that is made imagewise receptive or repellent to ink upon photo-exposure of the photographic material and subsequent chemical treatment. However, this method of preparation, which is based on photographic processing techniques, involves several steps, and therefore requires a considerable amount of time, effort and expense.

#The invention will now be illustrated, though without limitation, by reference to the attached drawings, in which:

#FIG. 1 shows a front view of an apparatus according to the present invention, used for collecting ablation debris generated on imagewise laser exposure of an imaging member by means of an imaging device having an internal drum configuration.

#FIGS. 2(a)-(c) show, respectively, front elevation, side elevation and end elevation views of a reverse flow gas cyclone, useful for the collection, according to the method of the present invention, of ablated debris generated on imagewise laser exposure of an imaging member.

#In operation, the apparatus according to the present invention may be employed in combination with various imaging devices as detailed above, but is particularly suitable for use in combination with an imagesetter or platesetter which may be of either internal or external drum configuration.

#Thus, referring to FIG. 1, there is shown an imaging member 2 including an ablatable imaging layer, said imaging member being mounted to the plate bed 1 of an internal drum platesetter. Typically, said imaging member may comprise a lithographic printing plate based on metal ablation technology, such as the Silverlith® SDT plate produced by Agfa-Gevaert Ltd., which comprises an ablatable silver layer coated on a grained and anodised aluminium substrate. Imaging of such a member occurs when the metal surface is struck by an IR laser beam of sufficient energy and power density, and for a sufficiently lengthy duration, to cause heating and melting of the metal layer prior to its expulsion from the plate surface.

#The ablation debris which is generated on imaging the imaging member 2 is picked up by the vacuum head 3, from whence it passes to the duct 4, which causes the debris to be directed tangentially into the gas cyclone 5. As a consequence of the action of the cyclone, particles having a diameter of greater than about 5 μm are thrown outwards against the cylindrical wall of the vessel, collecting thereafter in a hopper 6 at the base of the cyclone. On the other hand, debris particles having a diameter less than about 5 μm, which is the case with ablated material from the Silverlith® SDT plate, collect by impacting on the internal walls of the cyclone chamber. It is found that most of the ablation debris is thereby collected by means of the operation of the cyclone 5. In any event, improved efficiency of collection of particles of diameter less than 5 μm may be achieved by employing a cyclone of reduced diameter.