GB2100390A - Roller for use in conveying radiation-sensitive material - Google Patents

Abstract

A metallic body 1 having electrically conductive stub axles 3 is covered by a coating 4 of plastics material which defines an outer surface of the roller. In order to reduce the build-up of electrostatic charges caused by friction as the roller conveys the radiation-sensitive material (such as photographic film or X-ray film), the plastics material is chosen so as to be close to the radiation-sensitive material in the triboelectric series, suitable examples being polyurethane, polymethylmetacrylate, polycarbonate, polyamide 6,6 copolymer, polystyrene, polyethylene and polypropylene. In addition, the plastics material has a resistivity of between 10<9> and 10<2> ohm/cm<2> so that any static electrical charges built up may be conveyed to the stub axles 3. <IMAGE>

Description

SPECIFICATION Roller for use in conveying radiationsensitive materials This invention relates to a roller for use in the conveying of radiation-sensitive materials, such as photographic films or X-ray films.

Various processing apparatuses exist in which as yet undeveloped radiation-sensitive material is transported in a dry state. This applies, for instance, to input and feeder devices of developing machines and also to loading and unloading equipment for X-ray film cassettes, or to X-ray camera systems in which loose film sheets of different sizes are drawn-from respectively associated magazines, transported into a camera station and thence conveyed to a developing station. It may be assumed that the ambient air in the intake area of a developing machine is relatively moist whereas in the latter applications it is usually very dry because such equipment is normally set up in rooms where there are no moisture-generating apparatuses or appliances. In consequence, due to the dry atmosphere there is a far greater build-up of static charge in the film and rollers of the processing apparatus.However, the build-up of static charges in these two items entails the danger that at some point there will be a discharge of the electrostatic potential, producing a visible image of the discharge flash on the radiation-sensitive material and thus rendering it useless.

In the past it has been the practice to use high quality alloy steel rollers in order to avoid such a buiid-up of static charge. However, such rollers entail the risk that, due to their low coefficient of friction, there will be a comparatively large amount of relative movement between the roller and the film, which in turn will lead to higher static charges built up in the film. This, again, creates a high risk of discharge flashes occurring since the film can easily discharge through the highly conductive alloy steel roller.

In order to overcome these problems it has been proposed to use so-called anti-flash rubber rollers instead of alloy steel rollers. In these rollers, the rubber is embedded in an antistatic medium designed to reduce state charge build-up.

However, rubber has the disadvantage that due to its resilience a relatively large area of contact with the film is created which once again increases the risk of static charge build-up. German OS 2,939,473 on the other hand describes a conveyor roller in which a metallic powder is embedded by way of an antistatic additive in a plastics material. It is true that this kind of roller has certain advantages in-comparison with the rollers described previously, notably in regard to the discharging of the rollers to earth through the antistatic additive. However, for application in equipment for loading and unloading X-ray film cassettes where the film is transported at approximately ten times the speed of that used in developing machines, these rollers are far less suitable due to the much higher risk of charge build-up.

It is an object of the present invention to provide a roller for use in the conveying of radiation-sensitive materials which is able to reduce still further the risk of static charge buildup in the film and in the roller.

According to the present invention, such a roller has its outer surface formed of a plastics material which is close to the radiation-sensitive material in the triboelectric series and which has a conductivity of between 109 and 102 ohm/cm2, preferably 106 to 103 ohm/cm2.

By suitably choosing the plastics material to satisfy the criteria specified above, the danger of static charge building up is implicity reduced a priori. In addition, the choice of the conductivity range enables any residual charges to be continuously discharged by continuous flow without spark-over and thus without flash.

Desirably, the roller includes an electrically conductive axle or electrically conductive stub axles which is or are in electrical contact with the plastics material.

Suitable examples of the plastics material are polyurethane, polymethylmetacrylate, polycarbonate, polyamide 6,6'copolymer, polystyrene, polyethylene and (most preferably) polypropylene.

Conveniently, carbon particles are embedded in the plastics material.

Advantageously, the outer surface of the roller has a roughness coefficient of approximately 5 ym, and may additionally or alternatively have a coefficient of friction in excess of 0.3.

The invention will now be further described, by way of example only, with reference to the accompanying drawing, in which: Figure 1 is an axial sectional view of one embodiment of a conveying roller according to the present invention; and Figure 2 is an axial sectional view of a second embodiment of a conveying roller according to the present invention.

The roller shown in Figure 1 comprises a metallic body 1 in the form of a cylindrical sleeve, the body 1 having respective plugs 2 (preferably made of metal) inserted in its ends. Each plug 2 carries a respective stub axle 3 made of alloy steel.

The body 1 is provided with a coating 4 of plastics material which defines an outer surface of the roller, the thickness of the coating 4 being greater than the wall thickness of the body 1. The coating 4 is extended in the form of end coverings 4a over the axial end faces of the body 1 and right up to the stub axles 3.

The roller shown in Figure 2 is generally similar to this, but has a continuous axle 5 provided with a coating 6 of plastics material which is very thick in comparison with the diameter of the axle 5.

In both of these embodiments, the plastics material is chosen so that it is close in the triboelectric series to the radiation-sensitive film (such as photographic film or X-ray film) which it is to convey and so that it has a conductivity of between 109 and 102 ohm/cm2. Most preferably, the conductivity is chosen so as to be from 106 to 103 ohm/cm2. The relative positions of the plastics material and the radiation-sensitive material in the triboelectric series is vital, inasmuch as the charge potential caused by the frictional contact of two materials which are spaced widely apart in this series is quite considerable, whereas hardly any charge at all is generated if the two materials are close together in the series.It has been found that poiyurethane, polymethylmetacrylate, polycarbonate, polyamide 6,6 copolymer, polystyrene and polyethylene are all close to photographic film in the triboelectric series. A material which is also particularly well suited in respect of hardness and roughness characteristics is polypropylene, in which a conductivity of 103 ohm/cm2 falling within the required range can be obtained by the embedding therein of carbon particles in the form of soot. A particularly suitable polypropylene is sold under the trade name VESTOLEN.

In operation of the conveying roller, a certain charge will build up with each revolution despite the carefully selected material. However, due to the partially conductive nature of the plastics material this charge will become evenly distributed over the outer surface of the roller, thus avoiding high localised charges, and will be continuously discharged through the stub axles 3 or the axle 5 and the associated bearings. It is true that, in view of the moderate degree of conductivity chosen for the material in question, this kind of discharge is not really for very strong, but it is perfectly adequate for the relatively small build-up of charges which results from the appropriate pairing of materials. The presence of a certain amount of surface resistivity in the plastics coating of the roller further helps to prevent a sudden flash-discharge between the film and the roller.

Most of the plastics materials exemplified above are comparatively hard, so that rollers made from these materials are subjected to little deformation and have only a relatively small contact area with the film, thereby reducing the potential build-up of electrostatic charges. It is advantageous to make the surface of the material a little rough because this further reduces the area of mutual contact, and in this regard a preferred surface roughness characteristic is about 5 ,um. It is also of advantage to provide a relatively high friction coefficient y of greater than 0.3 : in the case of the above-mentioned type of propylene, the vale of this coefficient is 0.36. Again, this results in less relative movement between the film and the roller and therefore less friction to build up of static charges. The application pressure of the roller relative to the film may also be reduced due to the high friction coefficient.

Altogether, these factors contribute to avoiding friction and build-up of electrostatic charges between the roller and the radiation-sensitive material, while smoothly discharging any charges which are generated and yet allowing the radiation-sensitive material to be conveyed at high speed. However, practical tests have shown very clearly, amongst other things, how critical is the right choice of material. For example, a PVC roller was found to generate charges of 2000V and correspondingly cause spark-overs of flash-overs which could not be avoided even by application of the other physical provisions according to this invention. But then, PVC is a long way away from photographic film in the triboelectric series.

Claims (8)

1. A roller for use in the conveying of radiationsensitive materials, the roller having its outer surface formed of a plastics material which is close to said radiation-sensitive material in the triboelectric series and which has a conducitivity of between 109 and 102 ohm/cm2.

2. A roller as claimed in Claim 1, wherein the conductivity of the plastics material is between 106 and 103 ohm/cm2.

3. A roller as claimed in Claim 1 or 2, including an electrically conductive axle or electrically conductive axle stubs which is or are in electrical contact with the plastics material.

4. A roller as claimed in Claim 1, 2 or 3, wherein the plastics material is polyurethane, polymethylmetacrylate, polycarbonate, polya mide 6,6 copolymer, polystyrene, polyethylene or polypropylene.

5. A roller as claimed in any preceding Claim, wherein carbon particles are embedded in the plastics material.

6. A roller as claimed in any preceding Claim, wherein said outer surface has a roughness coefficient of approximately 5 ,um.

7. A roller as claimed in any preceding Claim, wherein said outer surface has a coefficient of friction which is greater than 0.3.

8. A roller for use in the conveying of radiationsensitive materials, substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying drawing.