In WO 00/59727 we describe improved gravure printing rollers comprising a body or shell moulded from syntactic compositions comprising microbeads of a plastic glass or ceramic material in an e. g. polyurethane matrix. The shell is moulded to an approximate size and then machined to the exact size required and a copper coating is deposited on the machined surface and chrome treated. An electrically conductive coating of a conductive agent such as finely divided metal is however first deposited on the machined surface. The body or shell is provided on a tubular core which can be adapted for mounting in a printing machine.

This method of preparation enables gravure printing rollers to be produced by mou ! ding in a number of stock sizes which are then adapted to the customers needs by machining the moulded shell. In practice this means "fine tuning"by shaving off a few millimetres of thickness to ensure a perfectly smooth cylindrical surface without moulding flaws, and then preparing the printing surface. Thus it is necessary, as with the general practise of the gravure printing industry using steel rollers, to hold large stocks of rollers to satisfy the many variations in length and diameter which are required. This requires considerable space, and ties up a considerable amount of capital.

Using steel rollers, or the moulded rollers described above, a new set of rollers is required whenever a different diameter is specified and it is not economic to satisfy one off or bespoke orders for non-standard lengths and/ or diameters. Any requirement for rollers according to a new specification takes several days to satisfy as the manufacture of a steel roller is a slow process, and to produce moulded rollers, the mould must first be constructed and machined.

It is an object of the invention therefore, to provide a method for the manufacture of gravure printing rollers which will enable such rollers to be manufactured economically to any required dimensions of length and diameter, including non-standard sizes, and thus enable differing requirements to be met quickly.

According to the invention there is provided a method for the manufacture of gravure printing rollers comprising rotating a core, applying a <BR> <BR> <BR> <BR> potyurethane material to the core from a dispensing head, moving the core and the dispensing head relative to one another in the direction of the longitudinal axis of the core to apply said material to the core along a selected part of the length of the core, in one or more passes to build up a body of said material on said core to a selected thickness, and machining the body thus produced to a selected diameter.

A variety of materials may be used in producing the core, which is preferably of steel, but could for example be of any other suitable metal, or of any of a range of synthetic plastics and other synthetic materials such as high density polyethylene, polypropylene or other synthetic materials such as high density. polyethylene, polypropylene or other polyalkene, polycarbonate, rigid expanded or unexpanded polyurethane, PVC, glass, reinforced plastics, timber, cardboard, or indeed any other material which has a rigidity or other properties necessary to support a roll in a printing machine.

The core may be tubular or solid without any central bore or passage and in the latter case the end trunnions used for mounting the roll in a machine may be formed integrally with the core.

One embodiment of roll may for example comprise a core formed from a cardboard tube with a low density syntactic polyurethane sheath formed therein by the method of the invention to form a light-weight roller.

The surface of the body is then preferably treated to apply a chrome treated copper printing layer, and this may include the steps of rendering the surface of the polyurethane body conductive by either an electrochemical process or application of a conductive coating to the surface, followed by electro-deposition of copper on the conductive surface or coating, engraving and chrome treating. The steel core may or may not have been used previously as a printing roller.

The polyurethane material may be a syntactic polyurethane as defined below, or may be non-syntactic-e. g. a homogenous solid, non-expanded material. It is however possible that dense cellular (partly expanded) materials could be used.

For the purpose of this description, a syntactic polyurethane is defined as being a matrix of polyurethane polymer including pre-formed hollow beads or microbeads providing discrete bubbles. Such a composition is capable of providing a uniform cellular construction of homogenous and consistent density which can be machined without leaving a porous or void containing surface.

The syntactic polyurethane composition may preferably comprise preformed plastic beadles each comprising an individual shell surrounding an interior void and bonded in a matrix of a polyurethane composition. The beadles may be of sizes in the range of 10-150 microns weight average diameter. Alternatively the hollow beadlets may be of polymers such as polyurethane, PVDC or glass or a ceramic material.

Non-syntactic polyurethanes may be high or low density as desired.

The material of the polyurethane matrix may as is usual be formed by mixing an isocyanate component, containing one or more diisocyanates, and a polyol component including one or more polyesters and/or polyols.

The mixture is preferably formed by mixing immediately before application in the dispensing head of the isocyanate and polyol components, and the polyol component preferably contains fast-reacting compounds.

These may comprise one or more primary, secondary or hindered amines which react rapidly with the diisocyanate (s) in the other component to produce a partially gelled material with sufficient viscosity to stay on the rotating roller. Further reactions of the remaining compounds in the polyol component (typical hydroxyl group bearing molecules) with the remaining isocyanates then produces the fully cured matrix encapsulating the beadles to form* the syntactic polyurethane material.

In a variant, the fast reacting amine or amine mixture may be added as a third component. This may be necessary when the microspheres, which are preferably contained in the polyol component are made from a plastic which is unstable in the presence of amines. This may be carried out using a three-component mixing and dispensing machine. A further advantage of this variant is that the amount of amine component can be varied which could be of advantage if the flow characteristics need to be slightly altered for instance to accommodate variations between large diameter and small diameter substrates.

The polyurethane used in the composition can be manufactured from a range of ingredients. For example, polyether or polyester polyols may be used, with any of the common isocyanates, including methylene diphenyl diisocyanate (MDI), toluene diisocyanate (TDI), naphthalene diisocyanate, tetramethyl xylidene diisocyanate, isophorone diisocyanate, phenylen diisocyanate, cyclohexyl diisocyanate, xylidene diisocyanate, hexamethylene diisocyanate, as well as biuret, allophanate and prepolymeric adducts of these. Diol, triol, polyol or polyamine crosslinkers may be used, together with amine or metal catalysts. Additionally reactive amines may be used, together with amine or metal catalysts. Additionally reactive amines may be incorporated to provide the initial viscosity to provide"non-slumping"characteristics which are important for the rotational casting process. These amines may be aliphatic, aromatic or hindered polyamines.

The preparation of the gravure printing roller may be completed by coating the machined surface of the body with a conductive coating, for example by depositing a layer of chrome treated copper on the machined surface to provide a printing face.

The invention also provides a gravure printing roller made by the method according to the invention, and apparatus for carrying out the method according to the invention including a dispensing head capable of mixing two or more components to produce a single output, means for holding a core and for rotating the core, and means for moving the dispensing head and the core relatively to each other in the axial direction of the core.

The apparatus may include adjustable means for limiting the relative <BR> <BR> <BR> <BR> movement of the dispenser head and the core, to thereby define the iength <BR> <BR> ; of the roller to be produced, and preferably also further adjustable means for radial movement of the head to enable rolls of pre-set diameters to be produced.

A preferred method and apparatus, and gravure printing roller produced thereby according to the invention will now be further described by way of example, with reference to the accompanying drawings, wherein :- Fig. 1 is a diagram illustrating apparatus for carrying out the method of the invention; Fig. 2. shows a cross-section showing application of polyurethane syntactic material, to a core; Fig. 3 is a view similar to Fig. 2 showing a machining step, Fig. 4 is a diagram, much magnified, of a syntactic polyurethane material used on the method of the invention; Fig. 5 is an axial sectional view of a variant embodiment of roll produced according to the invention, and Fig. 6 is a similar view of a second variant of roll.

In a method according to the invention a gravure printing roller is formed by building up a body of syntactic polymeric material on a tubular steel core 10. The material is dispensed from a mixing and dispensing head 11 which is mounted on a carriage 12 to be moved along a rail 13 between limit switches MS-1 and MS-2 which are adjustable in position along the rail 13 to alter the length of the core 10 which is covered with material to build up a roll body 14. the head 11 is movable radially to and from the core 10 on guides 15 which may include an adjustable limit switch (not shown) to limit outward travel to thereby set a (variable) diameter for the body 14 built up in the core 10.

An isocyanate component may be fed to the head 11 by a line A, and a polyol component may be fed to the head 11 by a line B each controlled by a flow control valve to finely control the relative proportions of the components. A third component comprising a fast-reacting amine component may if required be fed by a third line C shown in broken lines. If no separate amine component is supplied, the rapid reacting amines may be added to the polyol component fed through line B. This component will normally contain microspheres for inclusion in the syntactic polyurethane material.

As suggested by Fig. 2, the body 14 of the polyurethane material may be built up on the core 10 over two or more rotations to build up the body to the required diameter, at the same time the dispensing head being moved b. ack and forth along the core 10.

The core 10 is mounted on trunnions 15, mounted on bearings in a frame 16, and one incorporating a clutch connected to a drive motor 17.

After building up the body 14, the surface is machined e. g. using a blade such as 18, to exactly the required diameter.

Fig. 4 shows diagrammatically the structure of a syntactic polymer such as used in the body 14. The polymer comprises a matrix 20 of a polyurethane material formulated according to the properties desired, and encapsulates a multitude of hollow microspheres or beadles 21, each comprising a shell and an interior void. These microspheres can be of polyurethane or PVDC, but can be of glass, ceramic or another polymeric material.

After machining, the surface of the roll produced is treated to render it conductive by an electrochemical process or application of a conductive coating, then copper is electro-deposited thereon engraved and chrome treated.

The roller is then ready for processing for use as a gravure printing roller in the known manner.

Two variant embodiments of roll produced by the inventions are shown in Figs. 5 and 6.

In Fig. 5, a solid high density polyurethane core 30 is provided with a syntactic polyurethane sheath 31 by the method disclosed above. The core 30 has integral trunnions 32 for mounting the roll in a printing machine.

Fig. 6 however shows a tubular core 40 of cardboard, with low- density polyethylene end plugs 41 with trunnions, 42. The core. 40 is provided with a low-density syntactic polyurethane sheath 43 which incorporates hollow glass microspheres.

Where a metal core is used this may be of metal other than steel, or example aluminium alloy tube where a light-weight roll is required, and the interior of the core may be packed with material of higher specific gravity, such as iead, where a high pressure roll is required.

The materials used may be as set out above, and variations to the apparatus may be made within the scope of the invention, particularly with regard to the way in which the core is mounted and rotated, the dispenser mounted and traversed and fed with the components of the material, and provision made for varying the dimensions of rollers which can be made.

This latter capability means that the apparatus may be quickly set up to produce rollers of different length and/or diameter between batches, and short runs and one off production contemplated together with production of non-standard, bespoke unique examples.