Technical Field

The present invention concerns a process for manufacturing a metering roll especially usable in flexographic printing presses, in varnish stations for offset presses and/or in short ink train dry offset presses for exampte, comprising a central cylindrical core and at least one layer of a hard material peripheral coating.

The present invention concerns also a metering roll, especially usable in flexographic printing presses, in varnish stations for offset presses and/or in short ink train dry offset presses for example, comprising a central cylindrical core and at least one layer of a hard material peripheral coating, obtained by this process.

Background Art

The metering rolls are widely used to bring a metered quantity of ink to the plate roll with which it is in contact. One type of metering rolls is provided with a peripheral coating of ceramic that is engraved, mainly by use of a laser beam. Another type of metering rolls has a metallic surface layer that is engraved either by mechanical tools or by use of a laser beam. The surface layer is more often made of a soft metal like copper or tin and is chrome plated after the engraving.

Another prior art method consists in a chemical etching of an image that is illuminated by a laser beam or similar.

The technical features which are required for metering rolls and especially for their superficial coating layer are the following : easiness of the engraving, high resistance to mechanical wear, protection against corrosion due for example to aggressive cleaning products. In particular a ceramic coating is sensitive to corrosive liquids because it is somewhat porous and though it has a great hardness the liquids may penetrate into the coating and reach the lower layers and the central core of the roll. One solution consists in filling up the pores with additional components mixed with the ceramic mixture. When the coating is made of a soft metal, it is necessary to chrome plate the upper layer to harden the surface and increase its resistance. Furthermore the scraper for metering the printing inks is applied with a high compression on the surface of the upper layer of the metering roll. Due to the hard material of the coating and the sharp edges of the structured surface, the scraper is submitted to an intensive wear. Disclosure of Invention

The process of the invention offers a new technology permitting to a manufacture metering roll having an improved resistance to mechanical wear, a better protection to corrosion at lower costs, and remaining easy to engrave. Therefore, the process of the present invention such as defined in the preamble is characterized in that the peripheral coating is made of an iron base containing at least one of the elements of the group consisting of chrome, nickel, boron, carbon, molybdenum, silicon, manganese, cobalt and/or at least one of the components of the group consisting of chrome carbide (Cr3C2); nickel chrome (NiCr); cobalt chrome (CoCr); tungsten carbide (WC).

According to one feature of the invention, the peripheral coating is provided by thermal spraying including at least one of the following methods: flame spraying, electrical arc spraying, plasma spraying, high speed flame spraying or cold gas spraying.

The proportion of chrome is advantageously comprised between 20 and 35%. According to one preferred variant, the proportion of chrome is at least approximately 32%.

According to another preferred variant, the proportion of chrome is at least approximately 27%

The proportion of nickel is advantageously comprised between 0 and 20%.

According to one preferred variant, the proportion of nickel is at least approximately 16%.

According to another preferred variant, the proportion of nickel is at least approximately 8%.

The proportion of boron is advantageously comprised between 0 and 8%.

According to one preferred variant the proportion of boron is at least approximately 4.6%.

The proportion of carbon is advantageously comprised between 0 and 3%.

According to one preferred variant the proportion of carbon is at least approximately 0.6%.

According to another preferred variant, the proportion of carbon is at least approximately 2%.

The proportion of molybdenum is advantageously comprised between 0 and 8%. According to one preferred variant, the proportion of molybdenum is at least approximately 4%.

The proportion of silicon is advantageously comprised between 0 and 2% and preferably between 1 and 3%.

According to one preferred variant, the proportion of silicon is at least approximately 1.2%. The proportion of manganese is advantageously comprised between 0 and 3%.

According to one variant, the proportion of manganese is at least approximately 0.7%. The metering roll of the present invention such as defined in the preamble is characterized in that the peripheral coating is made of an iron base containing at least one of the elements of the group consisting of chrome, nickel, boron, carbon, molybdenum, silicon, manganese, cobalt and/or at least one of the components of the group consisting of chrome carbide (C 3C2); nickel chrome (NiCr); cobalt chrome (CoCr); tungsten carbide (WC).

The thickness of the peripheral coating is comprised preferably between 50 and 400 micrometers. According to one preferred embodiment of the metering roll, the coating is advantageously composed of an iron base with 32% chrome, 8% nickel, 4.6% boron, 0.6% carbon of the total weight.

According to another preferred embodiment of the metering roll, the coating is composed of an iron base with 27% chrome, 16% nickel, 4% molybdenum, 2% carbon, 1.2% silicon, 0.7% manganese of the total weight. According to a first variant the coating may be composed of a WC-Ni base containing at least one of the following additional elements: Mo, Cr, C, and the amount of Ni may be at least approximately 8.5%. When the amount of Ni is at least approximately 8.5%, the amount of Mo is at least approximately 2.2%, the amount of Cr is at least approximately 2.2% and the amount of C is at least approximately 5.7%.

According to a second variant, the coating may be composed of a WC-Fe base containing at least one of the following additional elements: C, Cr, Al, and the amount of Fe may be at least approximately 10.8%.

When the amount of Fe is at least approximately 10.8%, the amount of C is at least approximately 5.6%, the amount of Cr is at least approximately 3.4% and the amount of Al is at least approximately 1 %.

Modes for Carrying Out the Invention

The process of the invention may be used for manufacturing new metering rolls having a central cylindrical metallic core. The coating may be provided by thermal spraying including at least one of the following methods: flame spraying, electrical arc spraying, plasma spraying, high velocity oxy flame spraying or cold gas spraying. After the application of the coating on the surface of the roll, it is first grinded then polished. Afterwards the coating is engraved with a laser beam or by mechanical tools. A further grinding and polishing operation may be necessary for withdrawing remaining protrusions.

It is to be noted that the plasma spraying provides a coating porosity that is higher than the porosity provided by high speed flame spraying The thickness of the coating is preferably comprised between 50 and 400 micrometers relating to the components that are used and the application planned for the rolls. The resulting corrosion resistance of the coating is higher than 200 hours according the (AASS-test). The hardness of the base alloys is preferably comprised between 750 and 1000 HV 0.1 , the hardness of carbides is comprised between 1000 and 1600 HV 0.1. It is to be noted that the hardness of the carbides is much higher than that of the base alloys. However the engraving is harder to do.

According to the invention, the materials that are used to realize the coatings of the rolls may be included in a first group of iron alloys and in a second group of carbides.

Group I : Cr 20-35%; Ni 0-20%; Mo 0-8%; B 0-8%; C 0-3%; Si 0-2%; Mn 0-3%; other elements < 0,1 %.

Variant 1 : Preferred composition: Fe Base, Cr 32%, Ni 8%, B 4,6%, C 0,6%.

Variant 2: Preferred composition: Fe Base, Cr 27%, Ni 16%, C 2%, Si 1 ,2%, Mn 0,7%.

Group II : Carbide materials (Cermets) WC; Cr3C2 in various matrixes with Ni, CoCr and Ni or Fe base alloys containing Fe 0-13%; Cr 0-5%; Ni 0-25%; C 0- 5%; Mo 0-4%; Al 0-2%; traces < 1 %, the rest being WC and/or CrcC2 or alloys.

Variant 3: WC-Ni-Base (Hastelloy-C) Ni 8,5%; Mo 2,2%; C 5,7% and traces of various elements < 1 %.

Variant 4: WC-Fe base : Fe 10,8%; C 5,6%; Cr 3,4%; Al 1 %.

The coatings of variant 1 and variant 2 are preferably applied by means of an atmospheric spraying or by means of a high velocity oxy flame spraying. Gas projection apparatus or liquid projection apparatus may be used for applying the said elements. Both groups of materials are particularly resistant to mechanical wear and corrosion so that the combination of structured rolls and the scraper will have a higher resistance and a longer working life as a ceramic surface. The coating surface may be structured directly by a laser beam without any further preparation like masking or etching.