DESCRIPTION

Field of application

The present invention relates to a head-carrier bar for digital printing device, as well as to a digital printing device comprising said head-carrier bar and a method for controlling the positioning of print heads assembled on a head-carrier bar.

The invention may in particular find useful application in many technological fields where industrial printing on large surfaces with high printing precision is required.

The printing surfaces may be, for instance, paper, corrugated cardboard, cardboard, plastic film, thermoplastic sheets; the digital printing device can be set up, for instance, for printing labels, packaging, signs, or even ceramic coatings, glassy materials, wooden or metal supports.

Prior art

Industrial printing devices generally comprise one or more printing modules equipped with print heads, designed to deposit ink drops above a printing support that proceeds below the printing modules themselves.

The above printing devices adopt a plurality of printing modules, typically one for each color with which the image may be printed.

Each printing module includes a head-carrier bar, or a support base made of aluminum or another metal, which one or more print heads are associated with. The print heads are arranged, generally alternated on two rows, so as to cover an entire longitudinal printing area below which a printing support slides. The placement of the print heads is of considerable importance in the quality of the print obtained through the digital printing device. Both the absolute positioning of the heads, to allow the alignment with the printing support and with the other color modules, and the related positioning among the different heads that make up the single module, to avoid the generation of graphic defects at the interface between successive heads along the printing area, must be ensured with high accuracy.

For very high resolution digital prints, micrometric accuracy may be required in positioning the print heads along the printing area.

However, it has been found that maintaining print quality over time may be difficult in the presence of considerable dimensions of the printing area, due to the thermal expansion of aluminum or another metal along the longitudinal development direction of the head-carrier bar.

To avoid misalignments and printing defects, in these cases a strict control of the temperature of the environment in which the printing device is inserted is maintained.

However, the control of the ambient temperature is not always easy and cheap to implement, for example in the case of large buildings and / or with poor thermal insulation. It should also be considered that the same print heads heat up in operating conditions and can therefore change the temperature locally even where the ambient temperature is kept constant.

A different approach may be represented by the choice of materials with low thermal expansion coefficient for the head-carrier bar. However, the alternative materials considered to date are highly expensive, as in the case of titanium, or significantly higher machining difficulties than metal, as in the case of ceramics.

Prior art documents US 2018/311956 Al, US 2017/274647 Al and US 2018/281399 Al disclose printers with heating means embedded in their print heads. Therefore, the technical problem underlying the present invention is to solve the above-described drawbacks, and in particular to ensure a high and constant printing accuracy over time without resorting to expensive or difficult to process materials for the head-carrier bar.

Summary of the invention

The previously identified technical problem is solved by a device and a method according to the appended claims.

The solution is represented by the adoption of heating means which are arranged directly on the head-carrier bar. The heating means make it possible to perform local control of the temperature directly on the headcarrier bar, thus preventing thermal deformations of the bar which might negatively impact printing accuracy.

It is observed that the head-carrier bar is the structural body which supports the print heads; it can be conveniently made of metal, for instance aluminum. Therefore, according to the invention the heating means are directly attached to said structural body; they are not integrated in the print heads.

In other words, the head-carrier bar is a plate, preferably a metal plate, which spaces and supports the print heads. The heating means are directly applied or attached to said plate.

Head-carrier bars are employed in industrial printers which, has mentioned in the prior art description, feature an extended printing area which means a considerable longitudinal length of the head-carrier bar, well above that of an A4-size paper. For example, the head-carrier bar can have a longitudinal size above 1 meter.

Further features and advantages will become more apparent from the following detailed description of a preferred, but not exclusive, embodiment of the present invention, with reference to the enclosed figures given by way of example and not for limiting purposes. Brief description of the drawings

Figura 1 schematically shows a digital printing device according to the present invention; figure 2 schematically shows a bottom view of a head-carrier bar according to the present invention.

Detailed description

With reference to the enclosed figure 1, reference number 1 globally indicates a digital printing device according to the present invention.

Said digital printing device 1 is schematically illustrated in the above figure, with the omission of some constructive elements which are per se known and graphical simplification of the elements represented.

In the following of the present description, terms such as “above”, “below”, “lower”, “upper”, or derivatives thereof are to be intended with reference to the normal use configuration of the digital printing device 1 illustrated in the above figures.

The digital printing device 1 comprises, in a known manner, a fixed frame 10, below which advancing means 11 are provided for moving a not represented printing support forward along a printing plane S. The printing support thus moves in an advancement direction A.

The digital printing device 1 further comprises a plurality of printing modules 3, cantilever supported above the printing area S by a support portion 2. The printing modules 3 follow each other along the advancement direction A and are substantially side by side to each other.

Each printing module 3 extends along a longitudinal direction Y, substantially orthogonal to the advancement direction A. It has a lower portion that develops close to the printing area S and that houses a plurality of print heads 4. Said print heads 4 naturally face with the nozzles in the direction of the printing area; the operating surface of the nozzles is identified below as the nozzle plate.

The print heads 4 are assembled on a head-carrier bar 5, which has a metal base, preferably of aluminum, extended along the longitudinal direction Y and which defines a bottom element of the printing module 3.

The print heads 4 are uninterruptedly assembled to the head-carrier bar 5 along an entire printing area extended along the longitudinal direction Y ; in other terms the print heads 4 are positioned along the head-carrier bar so that, at the longitudinal end of a print head 4, a subsequent print head 4 is longitudinally juxtaposed.

To avoid interference between subsequent print heads 4, the print heads are positioned in an alternated manner on two longitudinal rows, in a per se known manner.

At the level of each print head 4 a longitudinal section S of the headcarrier bar 5 is identified.

Each longitudinal section S comprises, on the two sides of the headcarrier bar, as many heating means 6 represented by electric resistances of the adhesive or cartridge type.

Between the two electric resistances, in an alternated position with respect to the print heads 4, a temperature sensor 7 is also provided, in particular a NTC sensor.

A control unit is arranged to acquire the signals of the temperature sensors 7 and to operate the respective heating means 6, so as to control, preferably through a PID control, the temperature at each single longitudinal section S of the head-carrier bar.

In an alternative embodiment, not represented in the enclosed figures, the heating means 6 with the related temperature sensors 7 may be alternated with a different rate with respect to that of the print heads 4. In other terms, each longitudinal section S may comprise a number of print heads 4 which is different from one and is not necessarily an integer number.

Preferably, the temperature of all the longitudinal sections S is kept constant over time, at a value indicatively comprised between 35°C and 50°C.

Obviously, a skilled person can make several changes and variants to the above described invention, in order to meet contingent and specific needs, all of them by the way contained in the scope of protection of the invention as defined by the following claims.