Dutch patent application No. 72,03312, filed on Mar. 13, 1972 and published on Sept. 17, 1973, discloses a printing apparatus for applying markings to a plurality of packed articles, passing the apparatus on a conveyor, comprising a stamp, which is laterally expulsed from the apparatus upon the passing of an article by a vertical displacement of a driving rod connected with the stamp by means of an articulated rod system, so that the article is hit by the stamp, the said stamp consisting of a holder to which a printing head is attached by means of at least one spring and of which the lateral movement is interrupted by an abutment arranged in the path of the driving rod before the printing head has reached the article, after which the printing head is shot further outwardly by the kinetic energy, hits the article and is retracted by the spring, the stamp being displaced by the movement of the driving rod from a vertical rest position in which the printing head engages an ink pad to a horizontal operating position.
A printing apparatus of this kind may be used e.g. to apply a date or a price to a plurality of articles. The apparatus is actuated by the proximity of an article, which may be sensed in any desired manner e.g. by means of a mechanical sensor engaging the article, a photocell or a pneumatic sensor responsive to the interruption of an air current. After the sensor has signaled the presence of an article, the printing apparatus is actuated with a suitable delay, whereby the stamp shoots outwardly and applies the desired marking to the article. The printing head is inked each time when the stamp occupies the rest position. The known apparatus has the disadvantage that the ink does not always supply the same amount of ink to the printing head during the inking process, because it is not possible to control the ink supply to the ink pad with a sufficient accuracy. In the known apparatus the ink pad, to which the ink is directly supplied, is made of felt and the ink seeps through the ink pad from the top to the bottom. However, since the ink always contains small pigment particles, which are captured by the ink pad on their way down, the porosity of the ink pad decreases after some time, so that an insufficient amount of ink is supplied to the ink pad. A further disadvantage of the known apparatus is that the ink pad is subject to heavy wear due to the impact of the printing head, so that it has to be replaced at frequent intervals.
U.S. Pat. No. 3,797,390 (Marozzi et al.) discloses an ink cartridge for a film marking apparatus, comprising an ink container having a foam-like filler spaced from a closed end and exposed to a marking device at an open end through a porous ink pad, which is made of felt. The filler is compressed when the printing head engages the ink pad, and expands again when the printing head is removed, whereby a pumping action is obtained to cause a flow of ink through the filler to the ink pad. Both the ink pad and the filler are subject to heavy wear due to the impact of the printing head.