CROSS-REFERENCE TO RELATED APPLICATION

This present application is the US national phase of International Patent Application No. PCT/EP2019/059508, filed Apr. 12, 2019, which claims priority to European Application No. 18171293.6, filed May 8, 2018. The priority application, EP 18171293.6, is hereby incorporated by reference in its entirety.

TECHNICAL FIELD OF THE INVENTION

The present invention is in the field of the thermal printing mechanisms, such as thermal printers. They are widely used for handheld payment terminals where compactness and cost are the main factors of improvements.

PRIOR ART

A thermal printing mechanism is usually composed of the following elements: thermal printhead, which is made of a ceramic substrate, having a rectangle shape with the width of the paper to be printed on, and a plurality of thermal dots installed on the ceramic substrate; a platen roller, to drive a thermally sensitive paper against said thermal dotline, said platen roller being driven in rotation through a gearing system by a motor, which is in the most cases is a stepper motor, plus urging means to create compression on the thermal paper between the thermal printhead and the platen roller.

All the above-mentioned elements are held directly by a chassis on which can be mounted intermediary holding parts that are integral to the chassis, said chassis being a plastic molded frame of a folded metal sheet. In most of the cases, the thermal printhead is mounted on a thermal head support, which is rotatively hinged onto the chassis, and is spring loaded against the chassis. It is widely known that in almost all thermal print mechanisms the platen roller is detachable from the thermal print mechanism in order to give access to the thermal paper roll reservoir, thus easing the loading and unloading of the paper roll.

One possible arrangement to reduce the size of the mechanism is to mount the thermal printhead directly onto the chassis. The distance between the back side of the mechanism to the paper inlet position defines the thickness of the thermal print mechanism, and it is a critical compactness characteristic. Here and below in the text “back side” means the side of the chassis that is reverse to the side where the thermal printhead is mounted. As in the most of the arrangements the motor is laid below the thermal printhead with its rotation shaft parallel to the thermal printhead dotline, the paper path has to go around the shape of the motor thus following an arc, increasing the mechanism thickness to at least the stepper motor diameter.

One way to reduce this thickness is to mount the motor vertically, out of the paper path. If the motor diameter is small enough this affects in a very minor way the overall width of the mechanism and allows having the thickness of the thermal print mechanism composed only by the thermal printhead plus the chassis thicknesses. In such arrangement the paper path becomes almost straight. Such arrangement is disclosed in the patent application EP15168283.8 with patent publication number EP 3095608 A1 (Compact platen roller motion system for thermal printing mechanism). FIG. 16 of said application, shows such an arrangement, wherein the paper path is completely flat. In this arrangement the pressure on the thermal paper is exerted by two lateral pressure means onto the platen roller, which are out of the paper path and because of this, the back side of the flat chassis gets bended and this leads to an arc deformation of the chassis, that lowers the pressure between the paper and the rubber of the platen roller in the central area, causing a pale printout in the middle of the thermal paper. The stiffness of the mechanism disclosed in document EP3095608A1 is not satisfying.

SUMMARY OF THE INVENTION

The aim of the present invention is to provide a thermal printer chassis with a reinforcing structure able to improve the stiffness of the central area of a thermal print mechanism, neither complicating its construction, nor increasing the manufacturing costs or its size.

The above-mentioned aim is achieved by a thermal printing mechanism according to the present invention that comprises:a printer chassis comprising first flat area for mounting a thermal printhead, comprising a dotline, the first flat area having two opposite ends parallel to the printhead dotline, a top end where the paper goes out of the printer chassis when printing and a bottom end where the paper enters the chassis when printing;said thermal printhead being fixedly mounted close to the top end of said first flat area of the printer chassis;a flexible circuit for electrically connecting the thermal printheada motor for putting on rotation a platen roller;a platen roller to put into pressure the thermal paper against the thermal printhead dotline;two lateral pressure means to urge the platen roller against the printhead;at least one reinforcing element for the first flat area of the printer chassis, parallel to the thermal printhead dotline.

According to the present invention, at least one reinforcing element is arranged at the bottom end of the first flat area where the printer chassis has at least one fold forming a second flat area parallel to the first flat area of the printer chassis.

According to a preferred embodiment the reinforcing element comprises a U-shaped fold, so that the second flat area partially overlaps the first flat area of the printer chassis.

Advantageously in the above preferred embodiment the second flat area does not overlap an interconnection area of the thermal printhead with the flexible circuit cable.

Preferably the longitudinal gap created by the fold on one side and the thermal printhead on the other side is closed with closing material. Advantageously said closing material is made of resin material. According to a variant of the present invention the resin material covers also the interconnection area of the thermal printhead for connecting the flexible circuit cable.

According to another preferred embodiment the reinforcing element comprises two successive folds of the printer chassis wherein in each fold the printer chassis is folded at substantially right angle and wherein the second flat area is formed after second fold and does not overlap the first flat area.

Preferably the second flat area of the reinforcing element forms a paper guide.

Advantageously the printer chassis is made of a sheet metal.

Advantageously in the reinforcing element a longitudinal slot is provided as a passage for the flexible circuit cable. Preferably said longitudinal slot is closed on both ends with material of the printer chassis

Preferably there is another reinforcing fold at the top end of the first flat area of the printer chassis where the thermal printhead is mounted.

The main advantage achieved by the present invention is to rigidify the central chassis structure of a flat thermal printing mechanism providing a very stable and uniform pressure distribution across the paper width and keeping its thickness minimal.

