GB2035901A - Fixing toner images in a copying machine - Google Patents

Abstract

A support member 15, such as a sheet of paper, carrying toner images 16 on a surface thereof is passed through a nip between a first heated roll 1 and a second roll 4 in pressure contact with the first roll 1, so that the toner images 16 are fused on the support member 15. In order to minimise adherence of the toner images 16 to the first roll 1 the electrostatic charges on at least one of the rolls and/or the toner images are modified, for example by means of a corotron 17, so as to increase the electrostatic forces acting on the toner images in the direction of the support member. This can be achieved, for example, by decreasing the electrostatic attraction or increasing the electrostatic repellancy between the first roll and the toner images, and/ or by decreasing the electrostatic repellancy or increasing the electrostatic attraction between the second roll and the toner images. <IMAGE>

Description

SPECIFICATION Method of fixing toner images in a copying machine The invention relates to a method of fixing toner images in a copying machine.

Herotofore, it has been known that toner images may be formed on a support member using an electrophotographic process or the like. The toner images thus formed are, if necessary, then fixed on the surface of the support member. The toner used is generally in the form of electroattractive fine particles of a size of 0.1 to 50ju composed of a thermoplastic resin admixed with coloring agents such as carbon black. The toner images may be formed on the support member using a variety of developing methods. The toner images are fused or dissolved on the support member by application of heat, pressure or vaporized solvent for permanent fixing.

Fixation by the vaporized solvent method is more efficient than the heat fixing method but involves problems of odour and sanitation due to dispersion of the vapor. While fixation by pressure requires only limited energy and permits quick-start and high speed operations, fixing performance is worse than either of the other fixing methods. Moreover this method has a drawback in that pressure-sensitive toner is expensive because of its complicated production process. Thus, pressure fixation is not widely used in practice.

For these reasons, heating is most generally and widely employed for fixing the toner images.

One heat fixation system includes a pair of rolls, one of which is heated, in pressure contact with each other to form a nip through which a support member carrying unfused toner images is moved for effecting fixation. This system is advantageous in comparison with other fixation systems in that it requires but little electrical energy and there is little danger of fire due to jamming of paper in the fixing zone. Such systems are shown, for example, in U.S. Patent Nos.

3,268,351 (Dern) issued August 23, 1966 and in U.S. Patent No. 3,666,247 (Banks) issued May 30, 1972.

For the heated roll (hereinafter referred to as the first roll) of this heat fixation system, there is preferably employed a roll coated with a heat resistant, non-sticking material such as tetrafluoroethylene, HTV silicone rubber, RTV silicone rubber and the like. Rolls of this type are disclosed in U.S. Patent No. 3,795,033 (Donnelly et al) issued March 5, 1 974. The other, pressure roll (hereinafter referred to as the second roll) is formed of heat resistant elastic material so that when it is brought into pressure contact with the first roll a contact area is formed at the nip.

Thus, when a supporting member carrying toner images thereon is passed through the nip formed by the first and the second rolls, the toner is softened by the heat generated from the surface of the first roll and is integrally fused. The fused toner may then penetrate into the fibres of the supporting member which is subsequently cooled and solidified to form permanent fixed images.

In this fixing system, the heated surface of the first roll comes into direct contact with the toner images so that sticky materials such as toner have a tendency to adhere to the surface of the first roll. This phenomenon is referred to as offset. To prevent offset, it is necessary to provide an oil feeding device for continuously applying a mould release agent such as a silicone oil, for example methyl silicone oil. The silicone oil applied may form a thin layer over the surface of the first roll, so that it provides a boundary layer between the surface of the first roll and the toner images on the supporting member. The layer of low surface energy is transferred to the toner when it passes through the nip thereby preventing offset of the toner to the first roll.

Even using silicone oil, a nearly invisible offset can still occur caused by fine particles. The device which feeds oil to the surface of the first roll may gradually become clogged by such an offset, resulting in failure to supply the oil smoothly and causes stains and oil spots on the copies.

To overcome such fine particle offset, attempts have been made to use various types of cleaning devices, such as cleaning blades and cleaning rolls. Although it is possible to clean the offset particles by the use of such cleaning devices, a problem still remains in that the cleaning performance becomes unstable over a long service period so that the cleaning device needs to be cleaned or replaced after every several thousand to several tens of thousands copying operations. It is important to minimize the occurrence of offset in order to reduce the load on the cleaning device as well as for the maintenance thereof.

It has hitherto been known that the amount of offset varies greatly depending upon the surface temperature of the first roll. That is, if the heating of the roll surface is not sufficient, a portion of the toner particles fail to be integrally fused and remain in particle form. Since the fused toner in the vicinity of the surface of the roll is sticky in nature, considerable offset occurs.

On the other hand, if the surface of the roll is excessively heated, the agreegation force by which the fused toner is maintained in an integral form becomes lower than the sticking force, again causing toner offset. Although the majority of contact heat-type fixing devices are provided with a temperature controller to control the surface temperature within appropriate temperature ranges, such devices cannot prevent the occurrence of fine particle offset.

The amount of offset is also known to be influenced by many other parameters such as the type of resin of the toner, the roll quality, the surface temperature of the second roll, the pressure in the nip, the amount of oil supplied, and the time required for passing through the nip. All of these parameters are related to the attraction force between the toner and the roll and the aggregation force of the toner.

According to experiments conducted by the inventors, it has been found that the amount of offset is, to a large extent, subject to the influence of electrostatic charges carried on the first roll, the second roll, the supporting member and the toner in addition to the above described parameters.

Figure 1 of the accompanying drawings depicts schematically a general and conventional contact heat-type fixing device in which a first roll 1 including an aluminium core 3 has a coating 2 of heat resistant, non-sticking material such as TEFLON (Registered Trade Mark), HTV silicone rubber or RTV silicone rubber, for the purpose of preventing offset. Reference numeral 4 designates a second roll having a metallic core 6 coated with silicone rubber 5 as a heat resistant elastic material. The second roll 4 operates in association with the first roll 1 to fix toner images 1 6 on a supporting member 1 5 by fusing the images by the application of heat and pressure.An oil feed device is associated with the first roll 1 and includes a wick 7 which supplies a quantity of oil 10 by capillary action to the surface of the first roll 1, the amount of oil thus supplied being adjusted by means of a blade 8. A cleaning blade 9 is provided to scrape offset toner from the first roll 1. Cleaning of the second roll 4 is performed by means of a cleaning roll 11. An infrared quartz lamp 1 2 is disposed within the first roll 1, and a sensor 1 3 is provided on the exterior of the first roll 1 for controlling the temperature thereof. An exfoliating blade 14 is also provided for separating the supporting member 1 5 from the first roll 1.

When the supporting member 1 5 carrying thereon toner images 1 6 passes through a nip formed by the first and second rolls, negative electrostatic charges are established on the surface of both of the rolls by friction charging, while a positive electrostatic charge is formed on the supporting member 1 5. As the most generally employed coating materials for the first and second rolls are TEFLON (Registered Trade Mark) and silicone, the polarity of the charges and surface potential established by contact with the supporting member can be determined from the charging properties of the roll material and the supporting member.By the inventors' measurements, the surface potential has been found to be typically minus several hundred volts to minus several thousand volts for the first roll, minus several thousand volts to minus several tens of thousands volts for the second roll and plus several hundred volts to plus several thousand volts for the supporting member. Further, it has been found that these potentials can affect the offset phenomenon.

The present invention makes use of these findings.

According to the present invention there is provided a method of fixing toner images in a copying machine, comprising passing a support member carrying said toner images thereon through a nip formed between a heated first roll and a second roll in pressure contact with the first roll to fuse the toner images on the support member, and modifying the electrostatic charges on the first roll, the second roll and/or the support member so as to increase the electrostatic forces acting on said toner images in the direction of the support member.

Also according to the present invention there is provided a method of fixing toner images in a copying machine, comprising passing a support member carrying said toner images on a surface thereof through a nip formed between a heated first roll and a second roll in pressure contact with the first roll such that the first roll contacts said surface of the support member, thereby fusing the toner images on the support member, and modifying the electrostatic charges on at least one of the rolls and/or the toner images so as to increase the electrostatic forces acting on said toner images in the direction of the support member.

In this way, the amount of offset is reduced by suitably changing both the polarity and magnitude of the electrostatic charges on the surfaces of the rolls and on the toner images. This can be accomplished by making the polarities of the first roll surface and the toner equal or making both the first roll and the toner non-polar, or by making the second roll surface and the toner opposite in polarity or making both the second roll and the toner non-polar. Namely, by imparting electrostatic repellency between the first roll and the toner or by imparting electrostatic attraction between the second roll and the toner, a very significant reduction in offset is achieved.

Embodiments of the present invention will now be described, by way of example, with reference to Figs. 2, 3 and 4 of the accompanying drawings, which are cross-sectional views illustrating schematically three embodiments of toner image fixing devices which make use of the present invention. In these Figures, the reference numerals of Fig. 1 are used to designate the same components.

The invention will be described by way of three specific examples.

Example 1 The fixing device shown in Fig. 2 was used, including a first roll 1 having an aluminium core 3 with a diameter of 50 mm, a length of 380 mm and a thickness of 10 mm, and a heat resistant non-sticking layer 2 formed of TEFLON (Registered Trade Mark) and coated over the outer surface of the core 3. As a heating element, a tubular infrared quartz lamp 1 2 having rating of 100 V and 900 W was employed. A second, elastic roll 4 was used having a diameter of 50 mm and a length of 370 mm, the roll 4 being constructed with a steel core 6 and a heat resistant elastic member 5 having a thickness of 12 mm coating the core 6. The elastic member 5 was formed of KE-550 U silicone rubber (produced by Shinetsu Kagaku K.K.). KE-96 silicone oil (also produced by Shinetsu Kagaku K.K.) of 100 cs was used as an offset preventing oil 10.The oil 10 was continuously supplied to the first roll 1 by means of a wool felt wick 7 through the capillary action thereof. While keeping the surface temperature of the first roll 1 at 165"C, a supporting member 15 of M-2 paper (produced by Fuji Xerox K.K.) carrying thereon toner images formed with the use of a 4000 toner (produced by Fuji Xerox K.K.) was passed through the nip at a rate of 200 mm/sec. The nip had a width of 10 mm. Since the second roll 4 has a silicone rubber surface, negative charges were established on its surface, as will be apparent from the charging rank of triboelectric series of the materials, when the roll 4 was contacted with the supporting member 1 5. The surface potential was found to be minus several thousand to several tens of thousands volts.This surface potential served to deposit the negatively charged toner from the supporting member 1 5 onto the surface of the first roll 1.

A corotron 1 7 was disposed facing the peripheral surface of the second roll 4 at a point opposite the said nip. A high DC voltage of 6KV was impressed upon the corotron 1 7 so that the surface potential of the second roll 4 was charged from negative to positive. The second roll was then found to have a surface potential of plus several KV.

In order to compare the amount of the offset between the case where a high voltage was impressed on the corotron 1 7 and the case where no voltage was applied thereto, a cotton gauze was pressed with a load of 3 kg against the surface of the first roll 1 in a 2 X 2 cm area at the discharge portion of the nip. After 50 copying operations, the gauze was released therefrom to observe the degree of stain. From this test, it was found that the degree of stain in the case of impressing a positive high voltage was about 20% of that when no such voltage was used, thereby demonstrating the effectiveness of the present invention.

Example 2 The same contact heat-type fixing device as used in Example 1 was employed. As shown in Fig. 3, the corotron 1 7 was disposed facing the outer peripheral surface of the first roll 1 and a high DC voltage of minus 5 KV was impressed thereon, thereby charging the first roll 1 to a surface potential of about minus 400V. The degree of offset was observed in the same manner as in Example 1. The degree of stain due to offset in this case was found too be about 80% of that in the case where no such voltage was applied, again showing the effectiveness of the present invention.

Example 3 The same contact heat-type fixing device as used in Example 1 was employed. As shown in Fig. 4, the corotron 1 7 was disposed so as to impress a high DC voltage of either plus 5 KV or minus 5 KV between the surface of the first roll 1 and the toner images 1 6 on the supporting member 15, so as to make the surface potential of the first roll 1 about plus 400 V or minus 400 V and, at the same time, to increase the charge on the toner images 1 6. The degree of offset was observed in the same manner as in Example 1, revealing that in either case (negative or positive) the stain due to offset was about 50% of that when no potential was impressed, thereby further proving the advantages of the present invention.

Thus, it is possible to reduce offset to an appreciable extent by providing a charging device, a discharging device or a voltage applying device on the first roll 1, the toner images 1 6 and/or the second roll 4 so as to make the surface of the first roll 1 and the toner images 1 6 of equal polarity, to render both the first roll 1 and the toner images 1 6 unpolarized, to make the surface of the second roll 4 and the toner images 1 6 opposite in potential, or to render both the second roll 4 and the toner images 1 6 unpolarized.

For the case where the supporting member 1 5 for carrying toner images 1 6 is paper, it is preferred that the support member 1 5 has an opposite polarity to that of the toner, although the influence of the surface potential is small because the paper has a greater electrical conductivity than the toner, and the first and second rolls 1 and 4.

Additional embodiments are illustrated in the following table together with the above examples where the amount of the reduction in offset for other combinations of the polarity between the roll surfaces and the toner images are summarized.

Table

I isI o I I 0 c o" 43$1, W"i + , < u Application Application Provision of < Application Application Application Application Impression Provision of DC plus of AC to discharge of DC plus of DC minus of DC plis of DC minus of AC to of positive of positive to corotron corotron on cross on to corotron to corotron to both to both' both potential bias roll on 2nd roll 2nd roll 2nd roll on 1st roll corotron on corotron on corotron on by corotron on 2nd roll 1st roll Int roll 1st roll to toner and toner and toner and toner images images image Polarity E ç É S ç | | | of 1st roll Polarity of toner U: U C45 e~g O + + R o $ o W 8 ~ .g roll e | X r o i40qw c Q c H 4 U H + + I X two w qO 04 bt O C4 UN 4 > oeH u W o & &commat;} 84 B e e S e ~ U C4 I + (5) tw0 X io; cH t

.............. Offset is reduced by over 70 %.

.............. " by 40 - 50 %.

.............. " by 10 - 30 %.

.............. Offset is not reduced or is increased.

While in the illustrated embodiments charging was mainly performed by means of a corotron, the present invention is of course not limited thereto. Voltage impressing devices such as a bias roll and discharging devices such as a discharging cross were also found to be effective.

The toner offset was particularly reduced in Examples 1 and 3. An experiment was further conducted using these embodiments in combination, revealing that the toner offset was yet further reduced. In this case, it was found that the amount of offset was very low even when silicone oil serving as a releasing agent was not supplied. In this case, the oil feed device and cleaning device can be omitted.

Further, since the first roll 1 and the toner images have the same polarity or are both unpolarized, there is no electrostatic force that will affect the toner image-carrying supporting member as it is wound around the first roll. Rather, there is provided a repelling force therebetween so that the supporting member 1 5 can be exfoliated easily from the nip.

Moreover, since the fused toner in the nip region is attracted towards the supporting member by the action of the electrostatic forces, the fused toner is easily wettable to fibers of the supporting member. This feature is itself advantageous with regard to the fixing operation.

Additionally, by providing an AC corotron in the outlet portion of the nip for applying charges to both the second roll 4 and the supporting member 1 5 carrying the fixed images, not only is the offset reduced but also the charge on the supporting member is neutralized. This is advantageous in that the collection efficiency of the copies is improved.

Claims (11)

1. A method of fixing toner images in a copying machine, comprising passing a support member carrying said toner images thereon through a nip formed between a heated first roll and a second roll in pressure contact with the first roll to fuse the toner images on the support member, and modifying the electrostatic charges on the first roll, the second roll and/or the support member so as to increase the electrostatic forces acting on said toner images in the direction of the support member.

2. A method of fixing toner images in a copying machine, comprising passing a support member carrying said toner images on a surface thereof through a nip formed between a heated first roll and a second roll in pressure contact with the first roll such that the first roll contacts said surface of the support member, thereby fusing the toner images on the support member, and modifying the electrostatic charges on at least one of the rolls and/or the toner images so as to increase the electrostatic forces acting on said toner images in the direction of the support member.

3. A method as claimed in Claim 2, wherein said electrostatic charges are modified so as to impart an electrostatic repellancy between the first roll and the toner images and/or an electrostatic attraction between the second roll and the toner images.

4. A method as claimed in Claim 1 or 2, wherein said electrostatic charges are modified so as to increase the electrostatic repellancy or decrease the electrostatic attraction between said toner images and the first roll.

5. A method as claimed in Claim 4, wherein the surface of the first roll and said toner images are made of equal polarity.

6. A method as claimed in Claim 4, wherein the surface of the first roll and said toner images are made non-polar.

7. A method as claimed in Claim 1 or 2, wherein said electrostatic charges are modified so as to increase the electrostatic attraction or decrease the electrostatic repellancy between said toner images and the second roll.

8. A method as claimed in Claim 7, wherein the surface of the second roll and said toner images are made of opposite polarity.

9. A method as claimed in Claim 7, wherein the surface of the second roll and said toner images are made non-polar.

10. A method as claimed in Claim 1 or 2, wherein said electrostatic charges are modified so as to increase the electrostatic repellancy or decrease the electrostatic attraction between said toner images and the first roll, and so as to increase the electrostatic attraction or decrease the electrostatic repellancy between said toner images and the second roll.

11. A method of fixing toner images in a copying machine, substantially as hereinbefore described with reference to Fig. 2, Fig. 3 or Fig. 4 of the accompanying drawings.