Roller Electrode for Use in Apparatus for Treating Plastic Film With High Voltage Corona Discharge

This invention relates generally to rollers of the type that serve as the ground electrode in apparatus for treating plastic film and the like with high voltage corona discharge, and particularly to an improved roller electrode that is economically manufactured and may be disposed of in the event of failure.

It is well known that the surface treatment of various kinds of plastic film by exposure to a high voltage corona discharge improves the adhesion characteristics of the film surface so as to render it receptive to adherends such as printing ink.

The typical corona discharge treating apparatus for this purpose includes a stationary electrode connected to a high voltage alternating current source and positioned close to a backing roller about which the flim is passed and which serves as the ground electrode.

In existing apparatus the roller electrode comprises a steel or aluminum core, either solid or tubular, coated with a dielectric material that serves as an insulative barrier between the core and the corona discharge. The core is ground and polished to a very accurate dimensional tolerance preparatory to receiving the dielectric coating whose outer surface must also be accurately ground. Such rollers are both expensive to manufacture and to recoat. Recoating is necessary whenever the dielectric barrier breaks down causing arcing between the electrodes and consequent damage to the film being processed. Such breakdowns occur frequently.

Roller electrodes in accordance with the present invention can be fabricated for a small fraction of the cost of manufacturing existing rollers and for less than the cost of recoating. Because it is so economical, the roller of the invention may be disposed of in the event of failure. Despite its economy, the roller electrode of the present invention is exceedingly durable and provides a uniform corona dischage along its entire working length.

Pursuant to an exemplary embodiment of the invention, a roller electrode is provided that includes a tubular outer casing of dielectric material and a shaft positioned coaxially of the casing. The casing and shaft define between them an annular cavity that is filled with an electrically conductive composition comprising a base material in which conductive particles are dispersed, the composition being in intimate contact with the casing and the shaft. The base material is preferably of the type that is cast in place in a plastic condition. For example, the conductive composition may comprise a cementitious matrix of plaster or the like in which conductive particles, such as powdered carbon or metal, are bound.

Other objects, features and advantages of the invention will become evident from a reading of the detailed description below in conjunction with the accompanying drawings in which:

FIGS. 1 and 2 are end and front elevation views, respectively, partly in cross-section, of portions of an apparatus for treating plastic film with high voltage corona discharge and including a roller electrode pursuant to the invention.

Referring now to the drawings, there is shown portions of a corona discharge film treating appparatus including a rotatable grounded roller electrode 10 and a fixed bar electrode 12 positioned above and in close proximity to the outer surface of the roller 10. A plastic film 14 having a surface 16 to be subjected to corona discharge treatment is drawn over the roller 10 for travel in the direction indicated by the arrows. The film 14 is transported by appropriate drive and tensioning mechanisms that are not shown but are well known in the art.

The fixed electrode 12 is connected to a high voltage alternating current source. By way of example, a voltage of 20 kv at a frequency of 10 kHz may be employed. The resulting corona discharge generated in the gap 18 between the electrodes 10 and 12 is shown schematically by a series of broken lines in FIG. 1.

The grounded roller electrode 10 that is the subject of the invention includes an outer tubular casing 20 of an appropriate dielectric material. Fiberglass, ceramic, hard rubber or the like is suitable. In accordance with a pracical example of the invention, the casing 20 is made of a length of reinforced fiberglass pipe having a smooth interior surface 4"(10.16 cm) in diameter. The casing is sealed by end plugs 22 and 24.

A steel shaft 26 positioned coaxially of the casing 20 and having end portions 28 and 30 projecting from the end plugs 22, 24 serves to support the roller 10. The shaft 26 is coupled to an appropriate ground connection.

The inner surface of the casing 20 and the outer surface of the shaft 26 define an annular cavity that is filled with a conductive composition 32 in intimate contact with the surfaces of the casing and the shaft. Broadly, the composition 32 comprises a base material in which conductive particles are dispersed. The base material preferably consists of a substance cast in place in a plastic condition. A cementitious matrix such as ordinary casting plaster within which conductive particles of carbon are bound is a specific example of the composition 32. Alternatively, the particles may consist of a powdered metal such as aluminum.

In large rollers, the composition 32 may be reinforced with embedded wire mesh 34 or the like.

The proportioning of the ingredients of composition 32 is such as to provide the required electrical and mechanical properties. Where the composition comprises casting plaster and finely divided carbon, 1:1 and 2:3 parts by weight plaster to carbon are examples of practical mixtures. To this is added a quantity of water sufficient to render the resulting mixture pourable but short of an amount that would undesirably impair the strength of the hardened composition. It will be obvious to those skilled in the art that various admixtures may be included in the mix to achieve desired mechanical or other characteristics. For example, agents such as asbestos fiber may be used to fill voids and to minimize shrinkage of the composition during setting.

The roller electrode 10 is conveniently manufactured by capping off one end of the casing 20 with one of the end plugs 22 or 24, inserting the shaft 26 through the end plug and, with the assembly standing on end and the shaft position stabilized, slowly pouring the plaster/carbon/water mixture into the annular cavity between the casing and the shaft. To minimize air entrapment and assure intimate contact of the composition 32 with the casing and shaft, pouring is best accomplished in stages while vibrating the casing with a vibrator appparatus. Upon completion of the filling process the other end plug is inserted. The plugs 22 and 24 may simply be held in place by an interference fit with the casing. Last, after the composition 32 sets the outside of the casing 20 is machined to provide both a true, smooth outer surface and the required casing thickness, for example, 0.1" (2.54 mm).