FR2696978A1 - Control process for print head in thermal printer - involves application of two different pulse patterns to lessen overheating and wear of print head - Google Patents

Abstract

The process involves application of a series of current pulses to the print head, following a specific cycle for a given number of print lines (N), The current cycle is then inverted for the next group of print lines (from N+1). The number N is determined experimentally for each type of print head. The changing pattern of heating applied to the print head prevents excessive heating or wear of the print head. ADVANTAGE - Allows more rapid printing with thermal print head than can be achieved if pulses are sent sufficiently slowly to allow cooling.

Description

-1

  THERMAL TRANSFER PRINTING PROCESS

  The invention relates to a method of printing by

Thermal transfer.

  The invention applies to thermal printing on any support and in particular on plastic supports

such as electronic cards.

  It will be recalled that thermal transfer printing machines use a printing head which includes a line of heating points obtained by electrical resistances. The heating of the dots is caused by the passage of an electric current in the

  resistances When the heating points are applied

  ques on a thermosensitive support or an ink ribbon, several lines of parallel dots are successively printed and allow to give the final motif of the impression. An electronic device comprising a current pulse generator and a generator control program is capable of supplying current pulses

  required level and following a programmed heating cycle

  me. However, there is a phenomenon of thermal persistence between two lines of printed dots. Indeed, the energy required for a good print causes the temperature of the heating points to rise after a few printed lines, which causes premature wear of the head as well. that a bad impression (a blur, and even sometimes a tearing of the ribbon) We have represented in figure l A, the curve of the heating current obtained by a conventional electronic control and in figure l B, the energy curve corresponding. To solve this problem, various solutions are proposed. A first solution consists in inserting a sufficiently long time between two pulses (greater than the time proposed by the manufacturers) to allow the heating points to cool between two printed lines The current curve according to this solution

  is shown in Figure 2 A Figure 2 B shows

  feel the corresponding energy curve Now it turns out that this solution has the drawback of considerably slowing down the printing time, which is harmful to

machine performance.

  Another solution consists in using several heating pulses as shown in FIG. 3 The pulses have a shorter duration which avoids the rise in temperature but which presents in

  however the disadvantage of reducing the speed of printing

  sion and the lifetime of the head which is given for

a maximum number of pulses.

  A third solution consists in using a thermistor which measures the temperature of the head as a whole and which acts on the duration of

the heating pulse.

  The thermal inertia of the thermistor which is placed at a good distance from the heating points for

  mechanical reasons, does not allow sufficient correction

  fast enough to avoid overheating of the heating points and the energy required for printing

  of an isolated point becomes insufficient.

  There is also another solution which consists in using a history head which takes into account the previously heated points. This technique gives good results but requires electronics.

  sophisticated and very expensive control system.

  The present invention solves this problem.

  problem without having the disadvantages of the solutions

  known to date It relates to a method of im-

  pressure by thermal transfer from a print head comprising a line of heating points, the heating of the points being effected by application of heating pulses, mainly characterized in that it consists in applying the heating pulses according to a heating cycle determined during the first N printing lines and to reverse the cycle regularly The cycle is preferably reversed

every N lines.

  Other features and advantages of the invention

  will appear on reading the description which is

  made by way of illustrative and nonlimiting example with regard to the drawing in which: FIG. 1A, represents a heating cycle for printing a line according to a first state of the art, FIG. 1B, represents the corresponding energy curve; FIG. 2 A, represents a heating cycle for printing a line according to a second state of the art, FIG. 2 B, represents the energy curve for a series of pulses according to FIG. 2 A, FIG. 3, represents a heating cycle for printing a line according to a third state of the art, FIG. 4, represents an example of heating cycles on a series of printing lines, in accordance with the process object of the invention, FIG. 5, represents another example of heating cycles on a series of printing lines,

  according to the method of the invention.

  Figures l A, l B, 2 A, 2 B and 3 relate to the state

  of the technique and have already been described.

  The method according to the invention consists in applying current pulses according to a heating cycle on a predetermined number of lines, then in reversing this cycle on a determined number of lines as shown in the example given in FIG. 4 Indeed , a heating cycle, as given in this first example, is applied to each printing line and this on a predetermined number N of lines The number N

  is determined experimentally for each head of im-

  pressure, the number selected being that which gives the best printing results According to the method, the heating cycle is regularly reversed. Thus, the cycle is reversed for the next N lines or for a number of lines substantially equal to N. In FIG. 4 , the cycle starts again on line N + 1 and according to the example taken, it begins with a followed cooling time

  a heating pulse and so on.

  According to the invention, the heating pulses recommended by the manufacturer but applied are used.

  in the way just indicated.

  The alternation of heating and cooling times

  This allows the heating points to be cooled every N lines (N depending on the characteristics of the head) which avoids overheating and this very quickly but keeping the same level for all points

printed (even insulated).

  The technical solution proposed makes it possible to maintain a constant and rapid printing speed without overheating the heating points for a very low price The print quality is thus improved without having to use expensive heads or sophisticated electronics as in the state of the art. For example, for a head of type KT 2002 C 1 from the company ROHM, energy pulses of 0.5 W and duration Tl equal to 500 microseconds have been chosen for a

total duration T 2 of 3 mms.

  A heating cycle is applied on N = 4 lines then reversed on the next 4 lines and so on. In practice, the method is implemented by the electronic control device for the head. For this, the control program of the existing pulse generator has been modified so as to reverse the cycle of

  heating planned, every N lines.

  Of course, the method applies to any kind of heating cycle and in particular to heating cycles

  as shown in Figure 3.

  In fact, if it proves necessary to use such a cycle, the method then applies

as shown in Figure 5.

  For example, for the same print head as in the previous example, we choose: t 1 = 400 gs t 2 = 400 gs t 3 = 100 Is t 4 = 300 Ms

t = time for a line.

Claims (2)

  1 Printing process by thermal transfer from a printing head comprising a line of heating points, the heating of the points being done by application of heating pulses, characterized in that it consists in applying the pulses of heating according to a heating cycle determined during the first N printing lines and to reverse the cycle regularly. 2 thermal transfer printing method according to claim 1, characterized in that the cycle is inverted every N lines.