Fixing device for fixing a powder image on a receiving sheet

Abstract: Fixing device for fixing a powder image on a receiving sheet (11) in a fixing nip (10) formed by a fixing roller (1) and a pressure roller (8) which has a lower flexural stiffness than the fixing roller (1), which rollers (1 and 8) are pressed against one another by a sta­ tionary cushion (22) filled with compressed air and pressing against the pressure roller (8) in a pressure zone (24) situated opposite the fixing nip (10). When the pressure roller (8) and the fixing roller (1) are driven, tangentially directed frictional forces are exerted on the pressure roller (8) and cause the pressure roller to deflect against a curved support surface (25, 26, 27) which limits the deflection. The pressure roller (8) deflected in the operative position causes a receiving sheet (11) fed through the fixing nip (10) to be stretched in a direc­ tion transversely of the direction of conveyance, so that any bubbles or corrugations in the receiving sheet (11) are eliminated.

Description: The invention relates to a fixing device for fixing a powder image on a receiving sheet, comprising a fixing roller and a pressure roller together forming a first zone which forms a fixing nip, and a biasing member which presses against the pressure roller in a second zone of the pressure roller situated substantially on the opposite side of the pressure roller with respect to the side containing the fixing nip, a tangentially directed frictional force being exerted on the pressure roller in at least one of said zones on applying a rotational force to the rollers. A fixing device of this kind is known from US Patent 4 553 826, which describes a fixing device in which the biasing member has a cylindrically curved surface which is in contact with a large part of the circumference of the pressure roller. If, in this known fixing device, a receiving sheet passing through the fixing nip has more intensive expansion locally as a result of temperature or moisture content differences in the receiving sheet, so that bubbles or creases form at those places, the receiving sheet may easily not be completely flat when it leaves the fixing nip but still have bubbles or corrugations, or else creases form due to the bubbles and/or corrugations being flattened in the fixing nip. This type of irregularity occurs particularly in polyester foils with uneven temperature distribution and in paper sheets with uneven moisture distribution. The object of this invention is to provide a fixing device of the kind referred to in the preamble in which any bubbles and/or corrugations present in the receiving sheet can be removed during fixing. According to the invention this object is attained in that the flexural stiffness of the pressure roller is so much less than the flexural stiffnes of the fixing roller that the pressure roller is deflected because of the rotational force applied to the pressure roller which in turn acts against the frictional force exerted on the pressure roller and in that deflection-limiting means are provided which limit the deflection of the pressure roller in the direction of the frictional force. As a result, the receiving sheet fed through the fixing nip in a direction parallel to the direction in which the frictional force acts is subjected in the fixing nip to forces which, as considered in the longitudinal direction of the rollers, extend from the middle to the ends of the fixing nip, which forces stretch the receiving sheet to such an extent that any bubbles and corrugations present disappear. This effect is particularly effective in processing receiving sheets of polyester in a fixing device in which a powder image transferred to the fixing roller from a photoconductive support is transferred and fixed under pressure and heat on a heated polyester sheet. With such a combined transfer and fixing device the receiving material is preheated to a temperature of 100°C before it reaches the fixing nip. If, as a result of slight temperature differences present in the polyester sheet during and after this preheating the polyester sheet is subjected to greater expansion at the hotter places, a bubble or corrugation will form at those places and readily enlarges as a result of the low flexural stiffness of polyester at high temperature. Since the temperature in the fixing nip is also high, a part of the polyester sheet in the fixing nip stays hot, so that the sheet can easily be stretched in the fixing nip at the places where there are no bubbles or corrugations, so that bubbles and corrugations present at other places disappear. Preferably, the flexural stiffness of the pressure roller is so small that the deflection of the pressure roller in the middle is at a minimum 0.5 mm per metre length of the pressure roller. Consequently, shortly after entering the fixing nip a sheet is sufficiently stretched for any bubbles and corrugations to disappear. In one embodiment of the fixing device according to the invention, the deflection-limiting means comprise a leaf spring, which bears by a free edge against the periphery of the pressure roller, and a fixed support is provided which has a curved support surface with which the leaf spring comes into contact on deflection of the pressure roller. As a result, the amount of deflection can be made independent of the flexural stiffness of the pressure roller and of the frictional forces exerted on the pressure roller to provide the deflection. Another effect of this is that the abutment can also be used as a scraper to scrape off developing powder transferred to the pressure roller and paper dust remained on the pressure roller. In the activated state of the fixing device the leaf spring operating as a scraper always remains in contact with the pressure roller over the entire length thereof irrespective of the amount of deflection thereof, so that no developing powder or paper dust can pass between the scraper and the pressure roller and thus stick on the pressure roller. In one advantageous embodiment of the fixing device according to the invention, to produce the frictional force required on the pressure roller for the deflection thereof, the pressure roller is driven directly and the biasing member occupies a fixed position in the activated state of the fixing device. Consequently, the amount of deflection of the pressure roller is determined practically completely by the drive torque and the friction between the biasing member and the pressure roller and only to a slight degree by the friction in the fixing nip. If the frictional force required for deflection of the pressure roller is produced solely by driving the fixing roller, and a counter-acting torque is exerted on the pressure roller in combination therewith, a relatively high frictional force occurs in the fixing nip so that fixing of a powder image on a receiving sheet fed through the fixing nip may be adversely affected. The invention will be explained in detail hereinafter with reference to the accompanying drawings wherein: Fig. 1 is a side elevation of a fixing device according to the invention, and Fig. 2 is a cross-section on the line II-II in Fig. 1. The fixing device represented in the Figures comprises a fixing roller 1 consisting of a steel cylinder of a diameter of 100 mm and a length of 900 mm, the cylinder being covered with a 1.75 mm thick layer of silicone rubber 2. A heating element 3 is provided inside the cylinder to heat the layer of silicone rubber 2 to a temperature of about 110°C. At the ends the fixing roller 1 is provided with journals 4 each passing through an elongate hole 5, the holes being formed in arms 6 fixed to a frame 7. The fixing roller 1 can be driven via a drive shaft (not shown) mounted at a fixed place in the frame 7. Between the drive shaft and the fixing roller 1 a coupling is provided, e.g. a Schmidt coupling, which allows the fixing roller 1 to be freely movable within specific limits in the longitudinal direction of the holes 5 to enable the fixing roller 1 to be positioned in the said direction as will be explained hereinafter. The fixing device also comprises a pressure roller 8 consisting of an aluminium cylinder having a diameter of 25 mm and a length of 900 mm again, the cylinder being covered with an 0.6 mm thick layer 9 of wear-resistant fluoro-ethylene propylene provided in the form of a shrunk covering on the metal cylinder. Another material which is wear-resistant and non-adhesive to developing powder can also be used. The pressure roller 8 is constructed in the form of a heat pipe to obtain a uniform temperature at the roller surface. A photoconductive drum 12 with a diameter of 200 mm is mounted rotatably on a fixed place above the fixing roller 1 in arms 13 secured to the frame 7. The photoconductive drum 12 can be brought into contact with the fixing roller 1 in a zone 14 of the fixing roller 1 which forms a transfer nip and which is substantially straight opposite the fixing nip 10. In the operative position of the fixing device, the photoconductive drum 12, fixing roller 1 and pressure roller 8 are pressed against one another by the means to be described hereinafter, for the purpose, on the one hand, of forming the transfer nip 14 in which a powder image applied to the photoconductive drum 12 is transferred to the fixing roller 1 and, on the other hand, forming a fixing nip 10 through which a receiving sheet 11 can be passed for the transfer and fixing thereon of the powder image. For good movement of a receiving sheet, preheated between heated plates 32, through the fixing nip 10, the diameters of the pressure roller 8 and the fixing roller 1 are constant within narrow limits over the entire length of the rollers. At its ends the pressure roller 8 is provided with journals 16 each passing through an elongate hole 17, the holes 17 being formed in arms 18 secured to the frame 7. The longitudinal direction of the holes 17 forms an angle of about 90° with the longitudinal direction of the holes 5. The holes 17 extend to the plane passing through the axes of rotation of the fixing roller 1 and the photoconductive drum 12, where the journals 16 fit into the holes 17 vertically with ample clearance. At the other ends of the elongate holes 17 the journals 16 fit vertically with little clearance into the holes 17. In the inoperative position of the fixing device the pressure roller 8 is held by springs 19 in the position represented in broken line in Fig. 1, in which position the pressure roller 8 is practically unable to move in the direction of the fixing roller 1 and is free from the fixing roller 1 resting on the bottom part of the elongate holes 5, this position of the fixing roller 1 being represented in broken line 20 in Fig. 1. A rectangular gutter 21 is secured to the frame 7 some distance beneath the fixing roller 1. The gutter 21 contains an elongate rubber cushion 22 filled with compressed air. At the open top the gutter 21 is covered by a strip 23 consisting of a mohair-covered fabric which lies on the rubber cushion 22 without tension but is secured to the gutter 21 on either side. In the inoperative position represented in broken line in Fig. 1, and in the operative position represented in solid line, and in each intermediate position, the pressure roller 8 presses on the mohair surface of the strip 23, and in so doing compresses the rubber cushion 22. In the operative position of the device the strip 23 is in contact with a peripheral zone 24 of the pressure roller, which zone 24 forms a segment of the pressure roller which includes a segment angle of 30°. With an excess pressure of 0.8 bar in the rubber cushion 22 the pressure roller 8 presses against the fixing roller 1 at the fixing nip 10 with a force of 1000 N per metre, the fixing roller 1 pressing against the photoconductive drum 12 in the zone 14 with the same pressure. A leaf spring 25 projecting freely upwards is fixed to the gutter 21 and consists of 0.6 mm thick spring steel. In the operative position of the fixing device the free top edge of the leaf spring 25 is in pressure contact with pressure roller 8 along a contact line on the pressure roller 8 situated substantially in the middle between the facing pressure zone 24 and the fixing zone 10. The leaf spring 25 includes an angle of some tens of degrees with the tangent to the pressure roller through the contact line. A rigid bar 26 is provided on that side of the leaf spring 25 which is remote from the pressure roller 8 and is immovably connected to the frame 7. Bar 26 has a symmetrically curved support surface 27 on the side facing the leaf spring 25 as will be seen from Fig. 2. In the inoperative position of the fixing device in which the pressure roller 8 occupies the position shown in broken line and is therefore free of the leaf spring 25, only the ends 27a and 27b (Fig. 2) of the support surface 27 are in contact with the leaf spring 25. The pressure roller 8 is adapted to be driven via a drive shaft 28 disposed at a fixed place (see Fig. 2), through the medium of a coupling 29 which allows a variable distance between the fixed drive shaft 28 and the axis of rotation of the pressure roller 8. When, starting from the inoperative position of the fixing device, the drive for the drive shaft 28 is switched on, for rotation of the fixing roller in the direction indicated by an arrow in Fig. 1, the pressure roller rolls over the strip 23 on pressure cushion 22, the pressure roller 8 performing both a rotational movement and a translational movement in the direction of leaf spring 25, and in so doing presses the freely rotatable fixing roller 1 up against the photoconductor drum 12. On coming into contact with the leaf spring 25, the pressure roller 8 deflects in the middle, the leaf spring 25 also being deflected there. The curved shape of the support surface 27 limits the deflection of the leaf spring 25 and hence of the pressure roller 8. When the drive is switched off the pressure roller 8 is again pulled back into the broken-line position by springs 19. This pulling back can also be obtained by driving the pressure roller 8 in the reverse direction. For the fixing operation, the drive for the fixing roller 1 is then switched on, the fixing roler 1 being rotated at a speed such that the surface speed of the fixing roller 1 outside the transfer zone 14 and the fixing zone 10 is somewhat higher than the surface speed of the pressure roller 8 outside the fixing zone 10. A frictional force is exerted in the fixing nip 10 as a result of this somewhat higher speed and in combination with the frictional force exerted by strip 23 on pressure roller 8 presses the pressure roller 8 with a force of 200 N/m against the leaf spring 25 which is at its maximum deflection. This force is required to scrape the surface of the pressure roller 8 clean by means of the leaf spring 25. The radius of curvature of the support surface 27 is 100 000 to 200 000 mm. This means that the pressure roller 8 can deflect 0.5 to 1 mm in the middle. The curvature of the pressure roller 8 causes a speed component in the axial direction, such component extending from the middle of the pressure roller 8 towards the ends thereof. As a result, a receiving sheet present between the rollers 1 and 8 is stretched axially and any bubbles and corrugations are smoothed flat. The axial speed with the said curvatures is about 0.3% of the transit speed. With a transit speed of 3 m/min these axial speeds are then 0.15 mm/sec. The stretching of the receiving sheet 11 in the fixing nip 10 is apparent from the faint wrinkles 30 and 31 extending respectively just in front of and just after the fixing nip 10 in a direction substantially parallel to this nip in the receiving sheet. These wrinkles 30 and 31 remain at the same place during conveyance of the receiving sheet and have no effect on the flatness of the receiving sheet. Once a wrinkle has formed it no longer changes, because there is equilibrium between the stretching force applied and the tension thus formed in the receiving sheet. If the stretching force continues, the pressure roller will slip axially over the receiving sheet 11. The speed difference applied by driving the fixing roller 1 and the pressure roller 8 is also used to provide a relative movement between the receiving sheet 11 and the powder image in the fixing nip 10, in order thus to compensate for the elongation of the image which occurs in the fixing nip 10 as a result of the powder image being stretched out on the pressed-in silicone rubber layer 2. The reason for this is that the friction between the shrunk covering layer 9 and a receiving sheet 11 results in practically no slip, at the applied nip pressure, between the pressure roller 8 and a receiving sheet 11, and thus slip occurs only between the receiving sheet 11 and the powder image fed over the surface of the fixing roller 1. Tests have shown that the fixing device according to the invention is particularly suitable for fixing powder images on hot polyester material which, as a result of temperature differences, is fed with bubbles into the fixing nip. These bubbles are eliminated in the fixing nip. The above-described fixing device is not dependent on a specific maximum length of the rollers for good operation. The rollers are substantially unable to deflect in a direction extending through image transfer zones 10 and 14, which would interfere with the image transfer. The fixing device is therefore extremely suitable for processing large-format receiving material.

Claims: A fixing device for fixing a powder image on a receiving sheet, comprising a fixing roller (1) and a pressure roller (8) together forming a first zone which forms a fixing nip (10), and a biasing member (22, 23) which presses against the pressure roller (8) in a second zone (24) of the pressure roller (8) situated substantially on the opposite side of the pressure roller (8) with respect to the side containing the fixing nip (10), a tangentially directed frictional force being exerted on the pressure roller (8) in at least one of said zones (24) on applying a rotational force to the rollers, characterised in that the flexural stiffness of the pressure roller (8) is so much less than the flexural stiffness of the fixing roller (1) that the pressure roller (8) is deflected because of the rotational force applied to the pressure roller (8) which in turn acts against the frictional force exerted on the pressure roller (8) and in that deflection-limiting means (25, 26, 27) are provided which limit the deflection of the pressure roller (8) in the direction of the frictional force. A fixing device according to claim 1, characterised in that the flexural stiffness of the pressure roller (8) is so small that the deflection of the pressure roller (8) in the middle is at a minimum 0.5 mm per metre length of the pressure roller (8) A fixing device according to claim 1 or 2, characterised in that the deflection-limiting means (25, 26, 27) comprise a leaf spring (25) which bears by a free edge against the periphery of the pressure roller (8), and in that a fixed support (26) is provided which has a curved support surface (27) with which the leaf spring (25) comes into contact on deflection of the pressure roller (8). A fixing device according to any one of claim 1 to 3, characterised in that the pressure roller (8) is directly drivable and in that the biasing member (22, 23) in the active state of the fixing device is a stationary element to generate the frictional force in the zone (24) where the biasing member (22, 23) presses `against the pressure roller (8). A fixing device according to claim 4, characterised in that the fixing roller (1) is directly drivable at a circumferential speed higher than the circumferential speed of the pressure roller (8) in order also to generate the frictional force in the zone (10) forming the fixing nip. A fixing device according to any one of the preceding claims, characterised in that the biasing member (22, 23) is stationary and is disposed at a fixed place, and in that the pressure roller (8) is rollable over the biasing member (22, 23) between a first position in which the pressure roller (8) is not in contact with the fixing roller (1) and a second position in which the pressure roller (8) is in pressure contact with the fixing roller (1). A fixing device according to claim 6, characterised in that the fixing roller (1) is movable between a first extreme position (20) occupied by the fixing roller in the first position of the pressure roller (8) and a second position occupied by the fixing roller (1) in the second position of the pressure roller (8). A fixing device according to any one of the preceding claims, characterised in that the biasing member (22, 23) comprises a cushion (22) filled with compressed air.

Inventors: GERAETS JACQUES JOSEPH MARIE, GRASWINCKEL JULIUS VITRINGA CO, VAN DER STERREN MARTIN LEONARD, GERAETS, JACQUES, JOSEPH, MARIE, GRASWINCKEL, JULIUS, VITRINGA, CORNELIS, VAN DER STERREN, MARTIN, LEONARD
Applicants: OCE NEDERLAND BV, OCE-NEDERLAND B.V.
Application year: 1989
Main classification: G03G15
IPC codes: G03G15