The purpose of printing doctors is to strip the excess ink off a rotating form cylinder. They normally comprise a thin strip of steel sheet which is clamped along one edge in a holder while its free edge bears resiliently against the cylinder. A very narrow end face, which bears against the cylinder surface, of the doctor plate is provided at the free edge, the width of which end face (measured transversely to its longitudinal extent) lies in the order of magnitude of 0.1 mm or less. EP-A-709 183 shows a typical example. During use, the end face of the doctor becomes worn, a fact which limits the service life of the doctor. To extend the service life, it is known to coat the end face of the doctor body, which is formed by the strip of steel sheet, with a hard material which is applied by physical vapour deposition (PVD) or plasma-activated chemical vapour deposition (PA-CVD). Examples are to be found in DE-A 40 24 514 and in Japan Patent Abstract 8197711. In the PVD process, atoms or particles are removed from a target by sputtering or using the arc process and are conveyed in the plasma onto the surface which is to be treated. In the PA-CVD process, the layer deposition takes place by means of the plasma activation of a hydrocarbon-containing gas. The hard material is preferably DLC (diamond-like carbon), a layer of carbon or a carbon-rich layer which is in part essentially characterized by diamond crystal structures and has corresponding resistance to abrasion and good sliding properties. However, it is also possible to use other hard material or mixtures of DLC with other substances, in particular metal. In this way, that surface of the doctor which is subjected to load caused by friction against the form cylinder is provided with an increased wear resistance and good sliding properties. Examples of suitable hard coatings are disclosed by GB-A 2 128 551, WO 86/07309, DE-C 37 14 327, EP-B 087 836, DE-A 32 46 361. It is also known from Japan Patent Abstract 4296556 to apply ink-repelling materials to doctor surfaces using the CVD process.
The layer of hard material is brittle. There is therefore a risk of impacts or temperature changes causing cracks which impair the cohesion within the layer of hard material or its adhesion to the doctor body. Then, under frictional loading, parts of the coating may become detached or splinter off. This not only impairs the service life of the doctor but also that of the surface of the impression cylinder, owing to the fact that the sharp edges of the coating which remain at the site of the defects have an abrasive action and may cause strips on the printed product. Therefore, the abovementioned hard coatings of the end face of doctors have hitherto not been able to gain widespread acceptance in practice.
USPTO Backgrounds
USPTO Backgrounds