GB2208813A - Metering liquid coatings - Google Patents

Abstract

Liquid metering assembly comprising a roller (47) for receiving a coating of a liquid, a doctor blade (49) for metering the liquid on the roller to provide a uniform coating of the required thickness prior to transfer of liquid from the roller to a receiving member, and a holder (50) for the blade extending generally parallel with the axis of the roller. The holder (50) is supported between end members (61, 62) which also provide bearing members (64) in which the roller is journaled for rotation. This ensures accurate spacing and loading of the blade relative to the roller and makes the assembly especially useful for metering a thin layer of release oil onto the fuser rollers of a xerographic copying machine. <IMAGE>

Description

Liquid Metering Assembly This invention relates generally to a liquid metering assembly which is particularly, although not exclusively, suitable for use in the fuser of an electrostatographic recording machine such as, for example, a xerographic copier. The assembly is of the kind comprising a roller for receiving a coating of a liquid, a doctor blade for metering the liquid on the roller to provide a uniform coating of the required thickness prior to transfer of liquid from the roller to a receiving member, and a holder for the blade extending generally parallel with the axis of the roller.

In a xerographic copier a light Image of an original document to be reproduced is recorded in the form of a latent electrostatic image on a photosensitive member. The latent image is rendered visible by the application of a resin-based powder known as toner. The visual toner image is transferred electrostatically from the photosensitive member on to sheets of paper or other substrates. The toner image Is then fixed or "fused", for example by applying heat and pressure, which causes the toner material to become soft and tacky whereby it is able to flow into the fibres or pores of the substrate or otherwise upon the surface thereof. Thereafter, as the toner material cools, it solidifies and is bonded firmly to the substrate. In the electrostatographic art generally the use of thermal energy and pressure for fixing toner images on to a substrate is well known.

It has long been recognised that one of the fastest and most positive methods of applying both heat and pressure for fusing the toner image to the substrate is by direct contact of the resin-based toner image with a hot surface such as a heat roller which also applies pressure to the substrate One approach is to pass the substrate with the toner image thereon between a pair of opposed rollers forming a nip, at least one of the rollers being internally heated. The actual temperature and pressure ranges will of course vary depending upon the softening range of the particular resin used in the toner. Typically, however, it will be necessary to heat the toner powder above 1800C. Temperatures of 200 C or even higher are not uncommon in commercial fusers.

A problem with this kind of fuser is that, as the toner becomes tacky, it can stick to the surface of the fuser roller which is undesirable because some of the toner on the fuser roller can then be transferred to subsequent substrates being fused. This effect, known as "offset", clearly impairs copy quality. Furthermore, if the rollers are rotated when there is no substrate present in the nip therebetween, toner may also be transferred from the fuser roller to the backup roller so that when a substrate subsequently passes through the nip some of the toner may be transferred to the reverse side thereof. Also, stripping failures may occur because substrates may stick to the rollers Instead of simply passing between them.

An arrangement for minimising the problem of offset has been to provide a fuser roller with an outer surface or covering of, for example, polytetrafloroethene known by the trade name Teflon, to which a liquid release agent such as silicone oil is applied. The thickness of the Teflon is typically of the order of tens of microns and the thickness of the oil is less than 1 micron. Silicone based oils, for example polydimethylsiloxane, which possess a relatively low surface energy, have been found to be suitable for use in the heated fuser roller environment where Teflon constitutes the outer surface of the fuser roller. In practice, a thin layer of silicone oil is applied to the surface of the heated roller to form an interface between the roller surface and the toner images carried on the substrate.Thus, a low surface energy layer is presented to the toner as it passes through the fuser nip thereby preventing toner from offsetting to the fuser roller surface.

In attempts to improve the quality of the image fused by a heat roller fuser, such rollers have been provided with conformable surfaces comprising silicone rubber or Viton (Trademark of E I Du Pont for a series of fluoroelastomers based on the copolymer of vinyladinefluoride and hexafluoropropylene). As in the case of the Teflon coated fuser roller, release fluids such as silicone based oils are applied to the surface of the silicone rubber or Viton to both minimise offsetting and to facilitate stripping. When the fuser system is one which provides for applying silicone oil to silicone rubber or Viton, a low viscosity silicone oil (i.e. in the order of 100 to 1000 centistokes) has most commonly been employed, although liquids of relatively high viscosity, for example 12,000 to 60,000 centistokes and higher, have also been used.

Various forms of applicator have been employed to supply the liquid release agent to the surface of the fuser roller. Thus, for example, US Patent No. 4 231 653 discloses an applicator comprising an elongate trough for containing a supply of release oil. A wick which is partially immersed in the release oil supply draws the oil up from the trough for application to the fuser via a pair of cooperating rollers in pressure contact, namely an oil application roller and an oil supply roller. The wick is in engagement with the oil supply roller and thus applies the release oil directly to the surface thereof. Release oil is transferred from the oil supply roller to the oil application roller, and from the application roller to the fuser roller.

In release oil applicators of the kind described above, various ways have been used to meter the oil on the rollers. One way was to use a rubber blade in contact with one of the rollers to ensure a thin uniform layer of oil on that roller. Careful control of the load applied to the blade, and the attack angle of the blade, were needed to ensure a uniform, reproduceable oil supply. The use of springs or weights to load the blade was hampered by space constraints, and offered poor seviceability. Although it is possible to use the blade itself as a leaf spring to supply the loading, the problem arises that unless the blade mounting arrangements are within very close dimensional tolerances, the cumulative effect of the tolerances involved gives rise to inadequate positional accuracy.

It is an object of the present invention to provide a liquid metering assembly using a doctor blade which overcomes this problem, and which provides an inexpensive, readily assembled and easily maintained unit.

Accordingly, the present invention provides a liquid metering assembly of the kind specified which is characterised in that the holder is supported between end members which also provide bearing members in which the roller Is journaled for rotation.

The end members are shaped to accommodate the ends of the blade holder, and to provide bearing surfaces for the journals of the roller. The spacing of the blade holder from the bearing surface in each end member is such as to load the blade against the roller.

Preferably the end members are of conductive material, in which case any static electricity generated by the engagement of the blade with the roller will be dissipated A metering assembly in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a schematic cross section of a xerographic copier incorporating a fusing apparatus which includes a liquid metering assembly in accordance with the invention, Figure 2 is an enlarged cross section of the fusing apparatus incorporated in the copier of Figure 1, Figure 3 is a perspective view from above showing an applicator trough of the fusing apparatus into which the metering assembly fits, and Figure 4 is an exploded, perspective view of part of the metering assembly.

Referring first to Figure 1, there is shown schematically a xerographic copying machine incorporating the present invention The machine includes an endless flexible photoreceptor belt 1 mounted for rotation (in the clockwise direction as shown in Figure 1) about support rollers la and ib to carry the photosensitive imaging surface of the belt 1 sequentially through a series of xerographic processing stations, namely a charging station 2, an imaging station 3, a development station 4, a transfer station 5, and a cleaning station 6.

The charging station 2 comprises a corotron 2a which deposits a uniform electrostatic charge on the photoreceptor belt 1.

An original document D to be reproduced is positioned on a platen 13 and is illuminated in known manner a narrow strip at a time by a light source comprising a tungsten halogen lamp 14. Light from the lamp is concentrated by an elliptical reflector 15 to cast a narrow strip of light on to the side of the original document D facing the platen 13.

Document D thus exposed is imaged on to the photoreceptor 1 via a system of mirrors M1 to M6 and a focussing lens 18. The optical image selectively discharges the photoreceptor in image configuration, whereby an electrostatic latent image of the original document is laid down on th'e belt surface at imaging station 3. In order to copy the whole original document the lamp 14, the reflector 15, and mirror M1 are mounted on a full rate carriage (not shown) which travels lateraliy at a given speed directly below the platen and thereby scans the whole document. The mirrors M2 and M3 are mounted on another carriage (not shown) which travels laterally at half the speed of the full rate carriage in order to maintain the optical path length constant.The photoreceptor 1 is also in motion whereby the image is laid down strip by strip to reproduce the whole of the original document as an image on the photoreceptor.

At the development station 4, a magnetic brush developer system 20 develops the electrostatic latent image Into visible form. Here, toner is dispensed from a hopper (not shown) into developer housing 23 which contains a two-component developer mixture comprising a magnetically attractable carrier and the toner, which is deposited on the charged areas of belt 1 by a developer roller 24.

The developed image is transferred at transfer station 5 from the belt to a sheet of copy paper which is delivered into contact with the belt in synchronous relation to the image from a paper supply system 25 In which a stack of paper copy sheets 26 is stored on a tray 27.

The top sheet of the stack in the tray is brought, as required, into feeding engagement with a top sheet separator/feeder 28. Sheet feeder 28 feeds the top copy sheet of the stack towards the photoreceptor around a 1800 path via two sets of nip roller pairs 29 and 30. The path followed by the copy sheets is denoted by a broken line in Figure 1. At the transfer station 5 a transfer corotron 7 provides an electric field to assist in the transfer of the toner particles to a copy sheet.

The copy sheet bearing the developed image is then stripped from the belt 1 and subsequently conveyed to a fusing station 10 which comprises a heated roller fuser to which release oil is applied as described in more detail below. The image is fixed to the copy sheet by the heat and pressure in the nip between the two rollers 10a and 10b of the fuser. The final copy is fed by the fuser rollers into catch tray 32 via two further nip roller pairs 31a and 31b.

After transfer of the developed image from the belt some toner particles usually remain on the surface of the belt, and these are removed at the cleaning station 6 by a doctor blade 34 which scrapes residual toner from the belt. The toner particles thus removed fall into a receptacle 35 below. Also, any electrostatic charges remaining on the belt are discharged by exposure to an erase lamp 11 which provides an even distribution of light across the photoreceptor surface. The photoreceptor is then ready to be charged again by the charging corotron 2a as the first step in the next copy cycle.

The photoreceptor belt 1, the charge corotron 2a, the developer system 20, the transfer'corotron 7, and the cleaning station 6 may all be incorporated in a process unit 12 adapted to be removably mounted in the main assembly 100 of the xerographic copier.

As shown in more detail in Figure 2, the fuser 10 comprises a driven heat roller 10a made for example of a steel cylinder coated in Viton (Trademark) and having a 1 KW tungsten filament lamp 10c disposed along Its axis A pressure roller lOb which may also comprise a steel cylinder with a Viton coating is urged against the heat roller 10a, for example by springs (not shown) suitably applying a force of approximately 68 kg, thereby forming a nip between the two rollers 1 0a and 10b where fusing takes place.

The path of a copy sheet through the fuser is represented by a broken-line arrow in Figure 2. In order to prevent toner offset and to aid stripping the copy sheet from the heat roller 10a, a silicone lubricating oil is applied to the surface roller 1 Oa by an applicator 40 The oil applicator 40 comprises an elongate trough 41 which is also shown in Figure 3. The release oil 42 is introduced into the trough 41 from a storage reservoir (not shown) mounted above an overflow tank 52, the oil enters the trough 41 at an inlet 43 at one end and flows along a channel 44 at the base of the trough towards the opposite end thereof. A wick 45 is retained internally adjacent the side of the trough by a castellated wall 46 extending upwardly from the base of the trough.It is noted that, for the sake of clarity, the wick is not shown in the perspective view of the trough in Figure 3. Release oil is able to flow through the gaps 46a in the wall 46 to reach the wick 45 which draws the oil up and applies it to the surface of a metering roller 47 against which the wick 45 engages. The metering roller 47, in the form of a tube made for example of stainless steel is journaled in end members 61, 62 (Figure 4) which are supported by recesses 46b and 46c formed in the end walls of the trough 41. The manner in which the metering arrangement operates is described in detail below.

The metering roller 47 applies the release oil to a donor roller 48 with which it is in contact and the donor roller 48 transfers a controlled amount of oil to the surface of the heat roller 10a. The donor roller 48 may be in the form of a tube made of for example aluminium coated with silicone rubber. The direction of rotation of all the rollers is shown by short solid-line arrows in Figure 2, but it is noted that only the heat roller 10a is directly driven. The pressure roller 10b, the donor roller 48 and metering roller 47 are all driven by the heat roller 10a.

A metering blade 49 which may be made for example of an elastomer such as Viton (trade mark) is fixed in a holder 50 which is set by end members 61, 62 at a predetermined distance from the surface of the metering roller 47 thus controlling the loading of the blade on the roller 47. in this manner the blade removes surplus oil from the roller 47 in a cutting tool fashion to leave thereon a coating of a predetermined thickness.

The metering blade 49 is arranged such that the surplus oil removed from the roller 47 will find its way under gravity back to channel 44 in the base of trough 41. A series of three similar ramps Sla, Slob, 51c are disposed in saw-tooth configuration along the full length of the jive wall of the trough directly below the metering blade 49. Oil which is removed from roller 47 by the blade 49 falls onto the ramps 51a, 51b, 51c and fills the space between the ramps and the roller 47. The direction of rotation of roller 47 tends to prevent the oil falling directly back into the channel 44 at the bottom of the trough. Instead the oil flows down the ramps under gravity before spilling over the edge back into the channel 44 at the bottom of the trough.This arrangement ensures rapid and effective distribution of the release oil along the full length of the trough.

In order to set up a complete continuous circulation system the channel 44 at the base of the trough 41 may slope gently downwards from the end adjacent ramp Sic to the end of the trough adjacent input 43. Any excess oil may then be collected in an overflow tank 52 adjacent input 43 and the level of supply oil in the trough may be set at a desired limit by providing a weir 53 at the entrance to the reservoir at a predetermined height so that only when the oil level exceeds the desired level will it spill over the dam into the overflow tank 52.

Resilient blade-like stnpper fingers 80 are provided at intervals along the length of the fuser system to ensure the stripping of the copy sheet paper from the heated fuser roller.

To this end the remote end of the fingers 80 bears against the heat roller surface on the exit side of the fuser as shown in Figure 2.

Referring now to Figure 4, the liquid metering assembly in accordance with the invention will be described in more detail.

The metering roll 47 has at its ends axially projecting journals 63. These are received in bearing surfaces 64 formed in generally sieeve-like portions 65 of the end members 61 and 62. Also received by the end members 61 and 62, in cup-like portions 66, are the ends 67 of the blade holder 50. Holder 50, which may be an aluminium alloy extrusion, is formed with a longitudinal groove 68 adapted to receive as an easy sliding fit the blade 49. The interior corners 69 of groove 68 are extended as shown to provide small channels which accommodate the corners of the blade and prevent distortion when the blade is pushed into the groove by virtue of the rotation of the metering roller 47 with which the blade is engaged. The blade holder has its upper region shaped with a generally triangular cross section, the apex of which is formed with a ridge 70.Further ridges 71 are provided along the lower edges of the outside surface of the blade holder, the three ridges together ensuring an accurate location of the ends of the blade holder within the end members 61 and 62 (see also Figure 2).

Interior ledges 72 and 73 within the cup like portions 66 of the end members provide accurate location of the blade holder relative to the bearing surfaces 64 of the end members. This in turn ensures accurate spacing of the blade holder 50 from the axis of the metering roller 47.

The blade holder 50 is thus accurately located and maintained at a fixed spacing trom the axis of the roller 47, with the blade 49 resiliently urged against the roller in the configuration shown in Figure 2. The blade is trapped in the groove 68 by the rotation of the roller 47, which pushes the blade into the groove.

The entire assembly is easily assembled and dismantled (for example for servicing} without the need for any bolts, screws or other fasteners. The blade slides into the groove 68 in the holder 50, the holder 50 and roller 47 are push fitted into the end members 61 and 62.

and the sleeve-like portions 65 of the end members locate in the recesses 46b and 46c.

Claims (7)

Claims:

1. Liquid metering assembly comprising a roller for receiving a coating of a liquid, a doctor blade for metering the liquid on the roller to provide a uniform coating of the required thickness prior to transfer of liquid from the roller to a receiving member, and a holder for the blade extending generally paraliel with the axis of the roller, characterised in that the holder is supported between end members which also provide bearing members in which the roller is journaled for rotation.

2. The assembly of claim 1 wherein the blade is of a resilient material, and is urged by the holder into contact with the roller.

3. The assembly of claim 2 wherein the blade is a sliding fit into a groove in the holder, the blade being urged into and retained in the groove by the reaction of the resilient blade against the roller.

4. The assembly of any one of claims 1 to 3 wherein the blade holder is of uniform cross-section, and the end members are adapted to receive and locate the ends of the holder.

5. The assembly of any one of claims 1 to 4 wherein each end member is a one-piece moulding of a plastics material.

6. The assembly of any one of claims 1 to 5 wherein the end members are electrically conductive.

7. A fuser apparatus for an electrostatographic copying machine including a pair of fuser rollers, and applicator means for applying a liquid release agent to one of the fuser rollers1 the applicator means including a liquid metering assembly according to any one of claims 1 to 6.