TITLE

PREHEATING OVEN FOR THE PREPARATION OF PRINTING PLATES

DESCRIPTION Field of the Invention The present invention relates to the field of printing equipment, and in particular it concerns a preheating oven to be built in with, or associated to, developer machines for printing plates such as photopolymer presensitized offset printing plates (according to the technology known as Violet CTP) , and negative thermal plates.

Background of the Invention

As is well known to any man skilled in the art, the development of printing plates such as those mentioned above require a step in which the plates are preheated.

Such operation is carried out by ovens, physically separate from the actual developer machine and located immediately upstream of it, or 'built in with the machine itself. The second solution, in order to attain the structural integration, requires a drastic reduction of the size of the oven, and thus worsens the plate heating efficiency (above all in terms of uniformity) , and therefore the quality of the development . It is in fact used, although with qualitative results which are not always entirely satisfactory, for plates - such as the photopolymer presensitized offset plates (Violet CTP) - in which the needs for heating efficiency are less severe.

When, on the other hand, it is of prime importance that the heating attains high quality standards, as in the case of the development of negative thermal plates, the first solution (i.e. that of an oven separate from the developer machine) represents the only possible choice,

which nevertheless involves a considerable increase in the size of the system and in its productive costs. Summary of the invention

The object of the present invention is to provide a preheating oven for the preparation to the development of printing plates, which permits to achieve a heating with fully satisfactory overall homogeneity and efficiency, even in case of "critical" plates such as the negative thermal plates, with limited size which, if required, permits its integration into a developer machine.

A particular object of the present invention is that of providing an oven of the aforementioned type with a system for transporting the plates which is structurally simple but fully functional as far as the specific quality maintenance needs of the plates are concerned.

Such objects are achieved with the preheating oven according to the present invention, the essential characteristics of which are defined by the first of the appended claims. Brief description of the drawings

The characteristics and advantages of the preheating oven according to the present invention will be , apparent from the following description of one of its embodiments, given as exemplifying and not limitative, with reference to the attached drawings, in which: figures 1 and 2 show the oven in respective axonometric views, from opposite sides and partially broken; figure 3 represents a cross-section of the oven, i.e. conducted orthogonally to the progress direction of the plates; and figure 4 is a section of the oven taken along

lines IV-IV of figure 3.

Description of the preferred embodiment With reference to the above figures, the oven according to the present invention comprises a box-like frame 1 of generically parallelepiped shape, defining on two opposite vertical walls, respectively, an entrance i side Ia and an exit side Ib of the plates to be heated

(not represented) . The walls of the entrance and exit sides are in practice orthogonal with respect to a motion direction X along which the plates, lying horizontally, translate and move in the oven, as will be seen shortly. Two brackets 2, 3 for supporting the plates, respectively at the entrance and at the exit, are mounted on the sides Ia and Ib of the frame 1, at about half the height thereof. Openings 23 formed in the upper cover wall of the frame

(figure 2) are used for the installation of suction means for carrying out an action of heat removal and consequent maintenance of the temperature of the outer frame 1 within acceptable values. The box-like frame 1 encloses the actual heating chamber, in turn having a parallelepiped box-like shape, indicated at 4. The heating chamber 4 is divided, by means of two horizontal separating walls 9, 10, into three, sections, from top to bottom: a section 41 for the passage of the plates, cut by the horizontal transfer plane of the same plates, indicated at a., in correspondence to which entrance and exit openings 4b, 4c are formed in the chamber (figures 1 and 2); a heat generation section 42, housing a plurality of resistance heating bars 5 incorporated in the upper part of a finned heatsink 6, fed by electric power circuitry of known type which is not shown; and a thermal convection*• propulsion section 43, in

W

- 4 - which, by means of a centrifugal fan 7 driven by a motor projecting outwards from the box-like frame 1 within a covering Ic, the thermal flow is drawn from the heatsink 6

(to this purpose, the lower separating wall 9 being permeable to the air in correspondence with the fan 7) and recirculated towards the plate passage section 41, as can be seen in figure 3.

With particular reference to figure 3, the forced convective flow mainly develops in the transverse direction with respect to the direction of motion X, i.e. orthogonally to the lateral walls 4a of the chamber 4,

(that is, the walls orthogonal to the entrance and exit sides Ia, Ib of the body 1) . _, On the other hand, the v communication between the various sections of the chamber 4 occurs according to a vertical passageway, in proximity to the aforesaid lateral walls 4a.

More precisely, while the lower, propulsion section of the flow 43 is directly plugged by the lateral walls 4a, the plate passage section 41 and the heat generation section 42 are separated from the walls 4a by means of counter-walls 11 rising from the edges of the lower separating wall 9, which in fact stops at a certain distance from the lateral walls themselves. The air passages 12 thus defined between the walls 4a and the counter-walls 11 conduct upwards, in particular towards the plate passage section 41, th _j e flow previously directed horizontally and outwards by the fan 7 in the propulsion section 43.

The upper separating wall 10, extending between the counter-walls 11, stops in turn at a certain distance from the same counter-walls, so that gaps 13 communicating the plates passage section 41 and the heat generation section

42 are formed. Through the gaps 13, the heat flow is vertically drawn within the heatsink 6 in the section 42, and within this section returns horizontally inwards, where the fan 7 recirculates it, as mentioned, in the propulsion section 43.

Returning to the plate passage section 41, it was mentioned that this section received the heat flow from the air passages 12, expelling it from the gaps 13. Inside v section 41, therefore, two currents cross with each other horizontally, along the two external -internal and internal-external directions, transverse with respect to the direction of motion of the plates X. In fact, a plurality of air outflow pipes 14 are arranged in two sets respectively above and below the plane a along which the plates are transferred and move. This arrangement is the basis of a substantial doubling of the oven efficiency without increasing the overall size thereof. Each set comprises a plurality of pipes 14 arranged with parallel axes which extend between the two counter-walls 11, opening at the ends on the air passages 12. The circulation of air within the pipes ^' 14 therefore extends in the external-internal transverse direction.

As shown in figure 4, slits 14a are formed respectively in the pipes 14, along generatrices facing the transfer plane a and in the immediate proximity to it . The slits 14a generate an air jet ^' which exits radially from the pipes so as to hit both faces of the plates during their passage. The flow then takes the internal- ^' external transverse direction, outside the pipes 14, so as to exit from the section 41 through the gaps 13.

Still inside the section 41, a transport system of the plates is arranged which - in addition to a plurality

of shafts with guide rollers 15 interposed between the pipes 14 of the upper set, and to two additional distributions of guide roller^ 16, 17 respectively in proximity to the entrance and exit brackets 2, 3 - makes use of two pairs of thrust rollers 18, 19, respectively upstream and downstream of the pipe sets 14, i.e. on the entrance and exit side. The two pairs of thrust rollers, made of silicon material, extend parallel to the pipes 14, between the lateral walls of the chamber 4, each pair consisting of two rollers which are mutually tangent along a common generatrix lying on the plane a, in order to achieve the gripping on the plates .

Each pair comprises a motor roller 18a, 19a and an idle roller 18b, 19b. The motor roller 18a of the pair of entrance rollers 18 and the motor roller 19a of the exit rollers 19 are driven through respective geared pinions 20, 21, mounted coaxially with an end of the shaft of the relevant roller projecting outside of the body 1. The two pinions 20, 21, which are therefore arranged in a mutually coplanar manner on the same lateral side of the body 1, are engaged with a gearing chain, not shown, in turn driven by a motor, not shown.

As shown in figure 2, the driving pinion 21 of the exit rollers has a lower number of teeth than the entrance pinion 20, so that the pair of exit rollers 19 achieves an advancement speed of the upper plate which is higher than the entrance rollers 18. With this expedient, the plate is always maintained taut during the transport, thus avoiding i possible deformations, favoured by the heating, and consequent damage .

In order to avoid that the different speeds of the rollers 18, 19 may cause anomalous stress on the motor,

the pinions 20, 21 are advantageously mounted on the related shafts through so-called "free-wheel" bearings, i.e. which make the gear-shaft fitting idle when the chain is not in traction. It can finally be noted from figures 3 and 4 how rings 22 are arranged around the pipes 14 of the lower set, spaced with regular distribution along the axis of the pipes so as to support the sliding of the plates and to distance the plates from the lower pipes, also avoiding the possibility of a direct contact between plates and pipes .

With the above-described configuration of the oven, the plate is hit with hot air under pressure on both of its faces, the air being supplied very close to the faces of the plate during its passage. Due to this, an extremely high heating efficiency and a considerable uniformity can be achieved, which improves the quality of the development with a considerable savings of absorbed electrical power.

It is also to be noted that the oven, still due to the heating effectiveness, and in particular the effectiveness of the double set of superimposed pipes, permits reducing the number of heating elements arranged in succession in a set, and : _v can consequently have a compact longitudinal extension (i.e. measured along the transport direction X of the plates) , thus assisting its incorporation into a single unit with the developer machine. The compactness along with the high efficiency of the heating permits using the integrated solution (oven built in with the developer) even for the negative thermal plates, which as said is not allowed by the known devices. All of the above, together with the transport system which avoids crimping and, more in general, deformations

of the plate, ensures that the preheating oven according to the invention represents a substantial improvement of the solutions according to the prior art.

Variations and/or modifications can be brought to the preheating oven for the preparation of printing plates according to the present invention without departing from the protective scope of the same as defined by the appended claims. In particular, the number of pipes 14, which could obviously have any cross-section shape, can vary, as can the shape and width of the related slits 14a. Also, the development of the heat flow in the two lower sections of the chamber 4 can be different from that of the depicted embodiment .