This invention relates to franking machines and to printing means thereof and in particular to thermal printing means in which ink is transferred from a thermal transfer ink ribbon to a print receiving surface of a mail item.
Franking machines include accounting and control means usually comprising a microprocessor operable to carry out accounting in respect of values of postage charges to be printed on mail item and to decrement a stored value of credit by an amount equal to the value of the postage charge. The microprocessor controls operation of feed means to feed the mail item past a print head and at the same time controls the print head to print a franking impression on the mail item, the franking impression including an indication of the value of the postage charge in respect of that mail item. Previously the print head has been implemented as a rotatable print drum carrying print dies and print wheels, the print dies being utilised to print an invariable part of the franking impression, and a slogan if desired, and the print wheels being settable to print variable parts of the impression comprising the value of postage charge and date. More recently it has been proposed to used a thermal print head to print the franking impression and slogan. The thermal print head includes a plurality of thermal printing elements disposed in a line extending transversely to the direction of feed of the mail item. A thermal transfer ink ribbon is interposed between the thermal printing elements and the mail item with an ink layer of the ribbon in contact with the mail item. As the mail item is fed by the feeding means past the line of thermal printing elements, the contact between the ribbon and the mail item causes the ribbon to adhere to the mail item and thereby to be drawn with mail item past the print head. The thermal printing elements are selectively energised by the control means in each of a plurality of printing cycles so as, in each printing cycle, to heat areas of the ink layer to cause transfer of ink from those areas to the mail item to form dots printed at selected positions on the mail item. Repeated selection and energisation of selected thermal printing elements in a series of printing cycles causes printing of dots to form a required printed impression in a line by line manner on the mail item.
The thermal transfer ink ribbon is supplied wound on a spool (supply spool) and is drawn from the supply spool by the feeding of the mail item past the print head due to the adhesion between the ink layer of the ribbon and the mail item. After passing the print head the used thermal transfer ink ribbon is peeled from the mail item and is wound onto a take-up spool. A motor drive is coupled to the take up spool to rotate the take-up spool so to wind the ribbon onto the take-up spool and to apply, to the used ribbon downstream of the print head, sufficient tension to the used ribbon to effect peeling of the ribbon from the mail item. It will be appreciated that sufficient torque must be applied to the take-up spool to overcome the adhesion between the ink layer and the mail item however the torque must not be so great as to cause the ribbon to be drawn at a higher speed than the speed at which the mail item is fed past the print head. If the ribbon moves at a higher speed past the print head than the mail item, slipping occurs between the ink layer of the ribbon and the surface of the mail item with the result that smudging of the printing impression occurs or is likely to occur.