Pressure Spring For A Thermoprinter

Abstract

A thermoprinter for use, for example, in electronic table calculators and curve recorders, in which a thermosensitive recording strip is advanced step by step across a printing plate having a line of type of an electric resistance material, which are selectively energizable by heating current, and is pressed into contact with the printing plate during the printing of each line by a pressing device including a leaf spring extending across the printing plate with such a curvature in its unloaded condition that a convex lateral surface of the spring faces the printing plate, said leaf spring being relieved from the pressure contact when the carrier is to be advanced by an actuator forming part of the advancing mechanism and applying local bias forces in the direction away from the printing plate in points on the leaf spring situated at a short distance from either end thereof.

Description

The present invention relates to a thermoprinter of the type in which a printing plate with controllable circuits actuates a thermosensitive recording material line by line, said recording material being advanced step by step across the printing plate by means of an advancing mechanism and being pressed against the printing plate during the recording process by a pressing means.

Thermoprinters can be utilized in connection with many different devises, for example, for writing out measuring and calculating results. With the present-day continuous technological advances, ever-increasing demands are made with respect to the speed of printing and the uniformity of the blackening of the printed characters while, at the same time, increasing demands for a reduction in the noise level are also put forward.

It is known that the printing speed is increased and the uniformity of the blackening is improved if, in the writing out process, the total number of figures or other individual characters or symbols in a line are written out in two or more stages in stead of being printed one by one, by employing types for the recording which are composed of a few uniform type segments that are constructed as electric resistors, e.g., by thin film technology, having a shape corresponding to the type segments desired. The individual types on the printing plate may be composed identically of the same number of type segments and the printing of a symbol in the form of a given figure is then carried out by actuating the segments of the type in question corresponding to the shape of this figure.

When writing out line by line in the above-mentioned manner energization will first take place of those resistors in all the types located on a line on the printing plate which, in each character, cause specific, uniform elements to be written and, subsequently other resistors are energized which, in each type, cause other, specific type elements to be written and so forth, until every single character in the line has been built up and recorded. Since the electric signals necessary for scanning the character elements are fed into a store, the scanning of the characters in a line is carried out electronically and at such a fast rate that no significant loss of time occurs. Moreover, by arranging the writing out process in such a way that -- in successive stages within the individual types -- type segments are energized with the longest intervals feasible, a good heat distribution in the printing plate is attained during the recording process.

As the recording is effected by means of heat transfer from the electric resistors on the printing plate to the thermosensitive recording material, it is desirable to have a direct mechanical contact between the printing plate and the recording material, the electric resistors being, however, covered with a wear -- resistant layer such as quartz, which can be applied by a sputtering process, in order to reduce wear and tear.

The thermosensitive recording material normally consists of paper which is provided on one side with a thermosensitive coating which, in case of local heat transfer contact with a type segment constructed as an electric resistor, melts or evaporates on the spot in question. In order to obviate that the thermosensitive coating material is hereby deposited on the printing plate which -- in the course of time may result in a substantial non-uniformity of the blackening due to the differences in the frequency with which the individual types on the printing plate are used for recording, provision is normally made for the thermosensitive coating of the recording paper to face away from the printing plate. As a matter of fact, it is possible to obtain a satisfactory recording by the action of heat from the back of the paper when using a suitably thin recording paper material.

It has been found, however, that even when taking such precautionary measures, there is nevertheless a tendency for disparities to occur in the blackening and it is the solution of this problem which is the subject-matter of the present invention.

The invention is based on the recognition of the fact that the problem is chiefly to be sought in the pressing means which, in prior art thermoprinters, is constituted of a rigid rail member of a pivotable, frame-shaped holder which, precisely with a view to obtaining a good and uniform contact with the printing plate is of strong construction and is actuated by a relatively powerful spring, against the spring force of which the frame has to move when the pressure on the paper is relieved in order to advance the paper.

The thermoprinter according to the invention is characteristic in that the pressing means is a leaf spring supported at its ends in a holder, said spring having in its unloaded condition a curvature with the convex lateral surface facing towards the printing plate and being adapted for being biased in the direction away from the printing plate at a short distance from the supporting elements.

It has been found that by this measure the above-mentioned drawback of contrast differences disappears and simultaneously the necessary secure and uninterrupted contact with the printing plate is achieved during the recording process. In the thermoprinter according to the invention, the relief and springing movements are left solely to the pressing means proper which is self-adjusting in its longitudinal direction. Thus, only the leaf spring is moved during the operation. The result of this is that the masses to be accelerated and retarded become significantly smaller, which is also reflected in a substantial noise level reduction.

In an expedient embodiment of the thermoprinter according to the invention the leaf spring is fitted in the holder so as to be rotatable a small angle around its longitudinal axis. Hereby it becomes self-adjusting not only in the longitudinal direction but also in the transverse direction. In addition, the leaf spring is preferably detachably mounted so that it can be easily replaced. This is particularly important if the spring is coated with a softer material that is susceptible to damage or ageing. Such damage may occur, for instance, if the thermoprinter is operated without any recording material so that the soft material, e.g., felt or foam plastic, burns or melts when coming into contact with the heated printing plate or as a consequence of contact between said soft material and the prepared side of the recording material.

In another expedient embodiment of the thermoprinter according to the invention the leaf spring is adapted for actuation by an electromagnetic activator.

As the recording material is released to be advanced by a slight movement of the leaf spring it is preferred according to the invention that the actuating mechanism be adapted to be controlled in such a way that the pressing means is relieved as soon as recording has been effected on the thermosensitive material. Moreover, this results in the advantage that the softer, pressure-equalizing material on the leaf spring is automatically relieved when the thermoprinter is inoperative so that a permanent deformation of this material during prolonged rest periods is avoided.

The invention will now be explained in greater detail with reference to the accompanying drawings, in which

FIG. 1 is a perspective view of the parts of an embodiment of a thermoprinter according to the invention which are necessary for understanding the invention,

FIGS. 2, 3 and 4 show the interaction between a printing plate and a pressing means for the recording material in FIG. 1 in a purely schematic manner,

FIG. 5 is a schematic sectional view illustrating an actuating mechanism for advancing the recording material and relieving the pressing means,

FIG. 6 shows a drive means for the actuating mechanism in FIG. 5,

FIG. 7 shows a supporting element for the pressing means, and

FIG. 8 shows a modification of the actuating mechanism in FIG. 5.

The recording process in the thermoprinter shown in FIG. 1 which is intended to be employed in an electronic table calculator, is carried out in that a thermosensitive recording material -- which is not shown in FIG. 1 -- while being advanced step by step, is kept in contact line by line with a printing plate 1 having a plurality of character types arranged thereon in alignment, as shown with dotted lines at 2. Each character type in the embodiment shown consists of seven rectilinear elements of electric resistance material which, in combination, form two squares located above one another so that the character type has the shape of a square figure eight whereby it becomes possible, with such a type, to print any figure in the decimal system by supplying current to different combinations of the rectilinear type elements. The recording material may, for example, be paper which is prepared on one side with a material that evaporates locally when it is exposed by current being supplied to a type element to heating at the point in question, whereby a blackening of the material takes place.

In order to avoid to the greatest extent possible contrast differences stemming from the varying background heating of the printing plate, which are caused by the fact that a different number of type elements are utilized for printing different figures, the printing process may be divided into two or more stages as mentioned in the foregoing in each of which current is supplied to resistor elements forming identical type elements of the types. By way of example, those type elements located in upper half of all types which are intended to be printed may be energized in a first stage and, in a subsequ3nt stage, energization of the type elements intended for printing which are located in the lower half of the types can take place together with energization of one out of a number of additional resistor elements (not shown) located between the character types so as to produce a decimal comma. Thereby, a substantially inform heating of the printing plate is achieved during said recording stages.

As mentioned in the foregoing, the printing plate is constructed by employing known thin film technology and is provided, in addition to the character elements, with controllable electric circuits for selectively energizing the character elements. The character elements and said circuits can be applied to a supporting plate by means of a vapour deposition or sputtering process. In order to protect the resistance elements and to reduce wear and tear, the printing plate may be provided with a wear-resistant surface layer consisting of, for example, a quartz layer applied by sputtering.

In the embodiment shown of the thermoprinter the printing plate is in contact during the recording process with the unprepared side of the recording paper so that the atomic components of the preparation evaporated during the printing process cannot attack the printing plate.

In order to obtain a good contact between the recording paper and printing plate 1 the thermoprinter is equipped with a pressing means which, according to the invention, is constituted by a leaf spring 3 which is supported at its ends in a frame-shaped holder 4 and is adapted to be actuated during paper advancement in a direction away from pressing plate 1 at a short distance from the supporting elements.

The interaction between printing plate 1 and pressure spring 3 appears from FIGS. 2-4. FIG. 2 shows the pressure spring in its unloaded condition, in which holding frame 4 is pivoted away from the printing plate, said holding frame being pivotably connected by means of a shaft 5 to the side plates of the thermoprinter as shown at 6, and being locked in said unloaded condition relative to said side plates with the aid of snap means, as shown with dotted lines at 7, engaging locking flanges which project perpendicularly from the side plates as shown with dotted lines at 8. It appears from FIG. 2 that the pressure spring is shaped so as to have, in said unloaded condition, a curvature with the convex lateral surface facing towards the printing plate.

In FIG. 3, pressure spring 3 is shown in contact with printing plate 1. In order to ensure pressure equalization over the entire printing width, the spring is provided on the side facing towards printing plate 1 with a coating 9 of a soft material, e.g., a felt or velvet-like textile material.

In the condition illustrated in FIG. 4 pressure spring 3 is subjected to biasing forces in the direction away from the printing plate at a short distance from each of its ends supported in holder 4 by means of actuating arms 10 forming parts of an actuating mechansim described in the following in order to effect paper advancement. As a consequence of this biasing, the spring acquires a curvature with the concave lateral surface facing towards printing plate 1.

As shown in FIG. 5 each of actuating arms 10 constitutes one arm of a two-armed lever which is rotatably supported on a through shaft 11 upon which a paper advancing roller 12 is mounted. The other arm 13 of the lever which forms an angle of approximately 90.degree. to arm 10 is provided of its outer end with a flange 14 acting as a cam follower by being engaged by a cam disc 15 which is rigidly mounted on a drive shaft 16 which can be turned by a driving means described in the following through an angle of approximately 75.degree. between the positions shown in solid and dotted lines, respectively, in FIG. 5.

The circumference of cam disc 15 describes approximately a quarter ellipse limited by a minor axis half and a major axis half. In the position shown in solid lines, cam follower flange 14 is located opposite the portion of the cam disc circumference at the minor axis so that pressure spring 3 is not biased by actuating arm 10. In the position shown in dotted lines, cam follower flange 14 is resting on the portion of the cam disc circumference at the major axis, whereby actuating arm 10 biases the pressure spring in the direction away from the printing plate as shown in FIG. 4.

In addition, a pivot arm 17 is rigidly connected with cam disc 15, said arm constituting one part of a two-armed crank connection, the second part of which consists of an arm 18 which is connected in one end with the free end of pivot arm 17 and engages in its opposite end one end 19a of a coil spring 19 wound around advancing roller shaft 11. By turning cam disc 15 from the position shown in solid lines to the position shown in dotted lines, spring end 19a is moved to the right-hand side of arm 18 in FIG. 5, whereby spring 19 is tightened and turned together with shaft 11. The rotation is restricted by the other end 19b of spring 19 impinging upon a stop flange 20 projecting from side plate 5. By returning cam disc 15 to the position drawn in solid lined, spring 19 is again relieved from driving engagement with shaft 11 and turned back into the starting position without participation of shaft 11 in this return movement due to the fact that roller 12 is subjected through the recording paper, to pressure from a plurality of resilient paper pressing elements 21 mounted on shaft 5 in holder 4.

As illustrated in FIG. 6, the driving means for cam disc shaft 16 comprises two electromagnetic activators 22 and 23 having U-shaped, lamellated soft iron yokes 24 and 25 the legs of which carry series-connected coils 26, 27 and 28, 29, respectively, and at the ends of which legs pole pieces are formed with circular recesses matching the terminal faces of two lamellated soft iron armatures 30 and 31, respectively, which are rigidly secured on shaft 16. Armatures 30 and 31 are mounted in such a way on shaft 16 that their planes of symmetry through the shaft are mutually displaced by an angle of approximately 75.degree. so that, in the position shown in which armature 30 has its terminal faces located in recesses in yoke 24, armature 31 assumes a position in which a small portion of its terminal face extends into the recess in 25.

The position shown in FIG. 6 corresponds to the solid line position of the actuating mechansim shown in FIG. 5 in which pressure spring 3 presses the recording paper against printing plate 1. When the actuating mechanism in FIG. 5 is to be activated for paper advancement and for biasing pressure spring 3 in the direction away from printing plate 1, the coils 28, 29 on yoke 25 are supplied with current while coils 26, 27 are without current whereby armature 31, under the influence of the magnetic field produced, is turned in the direction of the arrow into the position which, in the figure, is shown for the other armature 30, and hereby shaft 16 and cam discs 15 are turned through an angle of approximately 75.degree. into the position shown by dotted lines in FIG. 5. When a recording is once more to be made, current is supplied to coils 26, 27 while coils 28, 29 are de-energized, whereby the position shown in FIG. 6 is reassumed. When no current is supplied to either coils 26, 27 or coils 28, 29, armatures 30 and 31 and, thereby, shaft 16, assume a position determined by that activator 22 and 23 which was energized last.

According to the invention, the energization of activators 22 and 23 may advantageously be effected in such a way that, in the inoperative periods of the device, pressure spring 3 will invariably assume the condition shown in FIG. 4 so that deformation of the recording paper during a prolonged rest period is avoided, which means that the individual components of the actuating mechanism will -- as a rest position -- assume the position shown in dotted lines in FIG. 5. This can be achieved by arranging the operating cycle of the recording process in such a manner that, prior to the actual recording period where the printing elements are actuated, there comes a period in which activator 22 is energized so that the actuating mechanism is made to assume the position shown in solid lines in FIG. 5 whereby pressure spring 3 is brought into the position shown in FIG. 3 and, immediately subsequent to the recording period, there follows a period in which activator 23 is energized so as to bring pressure spring 3 into the position shown in FIG. 4 and to turn roller 12 one step.

With respect to the working speed of the thermoprinter it has been found that 20 msec are available for each of the said actuation periods since the remaining part of the total working cycle is taken up by the recording process proper. In this connection, it is a significant advantage of the thermoprinter according to the invention that only small masses have to be moved and retarded when removing the pressing means from the printing plate and returning it into contact with the plate, that is to say, the masses of the pressure spring 3 and the elements of the actuating mechanism which may be constructed chiefly of light-weight materials, whereas the movement of the relatively heavy holder 4 is avoided.

Since the coating 9 of pressure spring 3 is of soft material which, in the course of time, may age and is also exposed to damage due to the contact with the prepared side of the recording paper, the spring in embodiment shown is mounted to be easily removable from holder 4 in order to be replaceable in that its ends are supported in nylon bearings 32 as shown in FIG. 7, which bearings are inserted into recesses in the frame-shaped holder 4 as shown in FIG. 1. Each of bearings 32 has an insertion aperture 33 the width of which is greater than that of the leaf spring, said aperture having in the central part of one of its longitudinal lateral faces a projection 34. Thereby, it is achieved that the spring 3 mounted in bearings 32 is rotatable through a small angle around its longitudinal axis for ensuring pressure equalization in the transverse direction of the spring.

FIG. 8 shows purely schematically a modification of the actuating mechansim in FIG. 5, in which actuating arm 10' serving to bias pressure spring 3 in the direction away from printing plate 1, and a corresponding actuating arm on the opposite side of the thermoprinter, are interconnected via a through flange portion 35 and are pivotably journalled on a shaft 36 which is mounted in the side walls of the thermoprinter. Like the embodiment shown in FIG. 5, actuating arm 10' in FIG. 8 constitutes one arm of a two-armed lever, the other arm 13' of which is constructed with a flange 14' functioning as a cam follower by being engaged by a cam disc 15' which is rigidly mounted on a drive shaft 16' which may be driven, for example, by electromagnetic activators of a structure as shown in FIG. 6. The shape of cam disc 15' in FIG. 8 corresponds to that of the cam disc in FIG. 5, and by turning cam disc 15' counter-clockwise from the position shown in FIG. 8, actuating arm 10' will be actuated to move pressure spring 3 away from printing plate 1 and simultaneously cam disc 15' rotates advancing roller 11 through a crank and spring transmission in the same way as shown in FIG. 5.

Compared to the embodiment in FIG. 5, the modification shown in FIG. 8 has the advantage that cam disc 15' and lever 13 are present only at one side of the thermoprinter the actuating arm at the opposite side being actuated via through flange 36. It has been found that this embodiment of the actuating mecahnism results in reduced wear and tear of the movable parts, particularly the actuating arms and their supporting elements as compared to the construction shown in FIG. 5.

Even if the thermoprinter has been described in the foregoing with reference to its utilization in electronic table calculators, it will be apparent that the present invention is applicable also to thermoprinters intended for curve recorders etc. Moreover, it will be possible within the scope of the invention to bias the pressure spring in other ways than by means of the actuating mechansims described.

Claims

What I claim is:

1. A thermoprinter comprising

a supporting frame;

a printing plate mounted in said supporting frame and including a line of typing means of an electric resistance material for printing line by line on a thermosensitive recording strip and means for selectively supplying heating current to said typing means;

pressing means for pressing said strip into contact with said printing plate during the printing of each line, said pressing means including a holder connected with said supporting frame, a leaf spring supported at its end in said holder in operative relation to the printing plate and extending in the line direction thereof with such a curvature in its unloaded condition that a convex lateral surface of the leaf spring faces the printing plate, and means for locking said holder to said supporting frame in a position in which said leaf spring is pressed against the printing plate with a substantially uniformly distributed pressure in the length direction of the leaf spring;

and an advancing mechanism for stepwise advancing said strip across the printing plate, said mechanism including actuator means for applying local bias forces in the direction away from the printing plate in points on said leaf spring situated a short distance from either end thereof supported in said holder, whereby the leaf spring is relieved from pressure contact with the printing plate when said strip is to be advanced.

2. A thermoprinter as claimed in claim 1, wherein the lateral surface of the leaf spring facing the printing plate is provided with a layer of a pressure-distributing material the extension of which in the longitudinal direction of the leaf spring corresponds to at least the maximum line length of the printer.

3. A thermoprunter as claimed in claim 2, wherein said pressure distributing material is a felt-like textile material.

4. A thermoprinter as claimed in claim 2, wherein said pressure distributing material is a velvet-like textile material.

5. A thermoprinter as claimed in claim 1, wherein the leaf spring is supported in said holder so as to be pivotable through a small angle about its longitudinal axis.

6. A thermoprinter as claimed in claim 1, wherein said holder is provided with bearings for detachably mounting the ends of said leaf spring.

7. A thermoprinter as claimed in claim 1, wherein said advancing mechanism comprises an electromagnetic actuator.

8. A thermoprinter as claimed in claim 7, wherein means are provided for energizing said actuator for advancing said strip and relieving said leaf spring from pressure contact with the printing plate immediately subsequent to the printing of each line on the strip.