The present invention relates to an intermediate sleeve which is adapted for use in flexographic or gravure printing systems, and more particularly to a bridge mandrel which is adapted to be mounted onto a printing cylinder and adapted to receive replaceable printing sleeves in flexographic or gravure printing systems.
In a typical flexographic printing process, a flexographic printing plate is attached to a cylinder, and as the cylinder rotates, the inked plate provides an image onto a substrate carried on an impression drum. The art conventionally provides the printing plate in the form of a printing sleeve which is expandable by air pressure for mounting and demounting onto the print cylinder. Typical flexography presses operate at high speeds, sometimes printing over 600 linear feet of paper per minute. These high printing speeds require that the print cylinders and sleeves also rotate at high speeds. The construction of the printing cylinders and printing sleeves can vary, and different constructions have been used to attempt to optimize their printing performance.
As known in the art, the diameter of the inner surface of an air-mounted printing sleeve must be slightly smaller than the diameter of the outer surface of the printing cylinder. The difference in these diameters is a dimension known as the interference fit. Moreover, the diameter of the inner surface of the printing sleeve must be expandable by the provision of pressurized air between the opposed surfaces of the sleeve and the printing cylinder in order to mount such printing sleeves onto the printing cylinders as well as remove the sleeves therefrom.
Typically, a printing job will involve an xe2x80x9cimage repeat,xe2x80x9d which is the circumferential length of the text and graphics that are to be printed one or more times on the substrate with each revolution of the printing sleeve. The circumference of the printing sleeve must be large enough to contain at least one image repeat. The sleeve repeat, which is equivalent to the sleeve""s circumference (including the printing plate mounted on the sleeve), can contain a number of image repeats. Different printing jobs involve image repeats that differ in size, and different printing jobs require sleeve repeats that differ in size. The larger sleeve repeat sizes require printing sleeves with larger circumferences, which means larger outer diameters. When a xe2x80x9cconverter,xe2x80x9d i.e., the operator of the machinery that uses a printing sleeve, orders a printing sleeve that is set up with the printing plates for a job that demands a given sleeve repeat size, the inner diameter of that printing sleeve is determined based on the outer diameter of the printing cylinders on hand in that converter""s inventory. This is because the printing sleeve must be mounted on a printing cylinder that has a commensurate outer diameter.
To perform a job that requires a large sleeve repeat size, the diameter of the outer surface of the printing sleeve must be large enough to yield the large sleeve repeat size. This requires printing cylinders with larger outer diameters to support thin printing sleeves. However, new printing cylinders are expensive. As one alternative to incurring this expense, thicker printing sleeves resulting from multiple layers are used instead of the single layer, so-called xe2x80x9cthinxe2x80x9d sleeves. Thompson et al (U.S. Pat. No. 5,544,584) and Maslin et al (U.S. Pat. No. 4,583,460) describe multi-layer printing sleeves that can be mounted on relatively smaller diameter printing cylinders. Such multi-layer printing sleeves have the effect of reducing the inner diameter of the sleeve so that the sleeve can be mounted on a smaller diameter printing cylinder that is already available in the converter""s inventory. Multi-layer sleeves are less expensive than printing cylinders, but more expensive than thin sleeves.
Moreover, it is more costly in labor to change printing cylinders on the printing machinery than it is to change printing sleeves on a printing cylinder. However, this solution has lead to a proliferation of multi-layer printing sleeves, which are more costly than the thin sleeves.
In other sleeve-mounting systems, larger repeat sizes can be printed using a thin sleeve mounted on an intermediate sleeve, also known as a bridge mandrel, that can be provided with pressurized air to mount and dismount the thin printing sleeve. In one such bridge mandrel system, as described in Rossini, U.S. Pat. No. 5,819,657, the mandrel is provided with internal xe2x80x9cplumbingxe2x80x9d in the form of air inlets, fittings, and passageways so that air may be supplied to its outer surface. One major disadvantage of this type of bridge mandrel construction is that it must have a relatively thick wall to accommodate the xe2x80x9cplumbing.xe2x80x9d This makes the bridge mandrel relatively heavy as well as increasing its cost to manufacture. Nelson, U.S. Pat. No. 5,904,095, also describes a similar mandrel construction which includes internal air passages.
Another type of bridge mandrel simply provides a relatively thin spacer sleeve open at both ends and equipped with air holes such as the sleeve described in Rossini, U.S. Pat. No. 5,782,181. However, in order for pressurized air to be supplied, the mandrel must be fitted with plugs at either end to seal those ends, or, the air hole pattern on the mandrel must be carefully aligned with the air hole pattern on an underlying print cylinder. However, as there are no standard air hole patterns in the art, it becomes problematic to achieve proper air hole alignment in all cases.
Accordingly, there remains a need in this art for a bridge mandrel construction which is simple to manufacture, light weight, and easy to mount and dismount from underlying printing cylinders in flexographic and gravure printing systems.
The present invention meets that need by providing a bridge mandrel construction which is simple to manufacture, light weight, and easy to mount and dismount from underlying printing cylinders in flexographic and gravure printing systems. According to one aspect of the present invention, a bridge mandrel is provided and includes a generally hollow, cylindrically-shaped tube adapted to fit over a print cylinder. The tube has an inner surface and an outer surface, a first end and a second end. A channel extends substantially around the circumference of the inner surface of the tube, and a plurality of orifices extends generally radially outwardly from the channel to the outer surface of the tube. The channel and orifices permit pressurized air to be provided from the interior of the mandrel to its surface for the mounting of a print sleeve onto the mandrel.
In a preferred embodiment, the channel is located adjacent the first end of the tube. The bridge mandrel preferably comprises a base layer, an intermediate layer, and a surface layer. The base layer preferably comprises a metal or a rigid polymer, the intermediate layer preferably comprises a foamed polymeric material (which may be either rigid or compressible), and the surface layer preferably comprises a rigid polymer. The intermediate layer of foamed polymeric material makes the mandrel light in weight, yet the rigid inner and outer layers provide a sturdy construction.
The channel preferably has a depth of between about 0.05 to about 0.5 mm and a width of from between about 1 to about 20 mm. The orifices preferably have a diameter of between about 1.0 to about 2.5 mm. Because the channel extends substantially about the circumference of the inner surface of the tube, there is no need to align the orifices on the mandrel with corresponding air holes on the print cylinder. Air under pressure from the interior of the print cylinder escapes into the channel and finds its way out of the orifices. Thus, there is no need, as in the prior art, for any alignment of the orifices on the mandrel with those on the print cylinder. Nor is there any escape of pressurized air out of the channel. The present invention eliminates the need for tedious alignment of bridge mandrel and print cylinder orifices, or the provision for standard orifice location and spacing on various print cylinders and bridge mandrels.
In accordance with another aspect of the present invention, the bridge mandrel includes a notch on the inner surface of the tube, with the notch adapted to engage a corresponding pin on the print cylinder. Thus, when the bridge mandrel is mounted onto the print cylinder, it may be locked thereto so that there is no movement between the mandrel and print cylinder surfaces. In a preferred embodiment, the notch is generally C-shaped such that the mandrel and print cylinder are locked together by a simple twist of the mandrel. The mandrel may be readily unlocked and removed by simply reversing the procedure. Thus, the invention includes, in combination, a print cylinder and a bridge mandrel assembly, the bridge mandrel including a locking mechanism adapted to releasably secure the bridge mandrel to the print cylinder. The mandrel is readily removable from the print cylinder, and another mandrel having a different outer diameter can easily replace it.
In use, the print cylinder and bridge mandrel assembly are designed so that a print sleeve having at least a radially expandable inner surface may be mounted onto the bridge mandrel by the application of air supplied under pressure through the orifices in the print cylinder and the tube. The print sleeve typically will have either raised (flexographic) or depressed (gravure) areas on its surface to carry ink in a printing process. Once a printing job has been completed, the print sleeve is easily removed by the use of pressurized air.
Accordingly, it is a feature of the present invention to provide a bridge mandrel construction which is simple to manufacture, light weight, and easy to mount and dismount from underlying printing cylinders in flexographic and gravure printing systems. This, and other features and advantages of the present invention, will become apparent from the following detailed description, the accompanying drawings, and the appended claims.