TONER IMAGE FIXING APPARATUS FOR AN IMAGE FORMING MACHINE

Abstract: A toner image fixing apparatus (39) for an image forming machine (11) such as a copying machine is disclosed. In the toner image fixing apparatus (39), a toner image transferred on a paper (28) is heated at a predetermined fixing temperature so as to fix it thereon. The fixing temperature is set to be larger than or equal to the melting temperature plus 10 [°C], and smaller than or equal to the melting temperature plus 20 [°C] in order to prevent rumples from forming on the paper.

Description: The present invention relates to a toner image fixing apparatus for an electrostatic transfer type image forming machine such as a copying machine, a printing machine or the like. Conventionally, electrostatic transfer type copying machines have been widely used as one of many types of business machine. In the electrostatic transfer type copying machine, a document is illuminated by a light source, and the light image reflected from the document is focused onto a surface of a cylindrical photoconductive drum by an optical system so as to form an image of the document thereon. When the light image of the document is projected onto the previously charged surface of the photoconductive drum an electrostatic latent image is formed thereon corresponding to the light image, and thereafter, the electrostatic latent image is developed into a toner image by a developing unit. Then, the toner image is transferred onto copying paper running at a predetermined transfer speed VP by a transfer unit. The copying paper onto which the toner image has been transferred is fed to a fixing unit at a predetermined fixing speed VH in order to thermally fix the toner image, and then the copying paper is discharged outside of the copying machine. Fig. 1 is a schematic longitudinal cross sectional view showing a portion of a conventional typical electrostatic transfer type copying machine 1. Referring to Fig. 1, after a toner image is transferred onto copying paper running at the transfer speed VP by the aforementioned transfer unit (not shown), the copying paper is fed to a fixing unit 4 with the aid of a guide member 3. The fixing unit 4 comprises an upper heating roller 5 and a lower heating roller 6. The upper heating roller 5 constitutes a metal cylinder arranged rotatably so that its axial direction thereof is parallel to a horizontal direction. The upper heating roller 5 has a heating lamp 7 used as a heating source located inside it, and a coating layer of a resin such as Teflon® formed on its outer surface. On the other hand, the lower heating roller 6 is made of a material such as silicon rubber, having a hardness of 30° or less which is prescribed in the Japanese Industrial Standard (referred to as JIS hereinafter) K6301 - 1975. In the conventional copying machine 1, the temperatures HT of the surfaces of the upper and lower heating rollers 5 and 6 (referred to as a fixing temperature HT) are set so as to satisfy the following inequality (1): TT + 30 ≦ HT ≦ TT + 40 [°C] wherein TT is a melting temperature of the toner used as a developer in the copying machine 1. The rollers being arranged such that the upper heating roller 5, heated by the heating lamp 7, and the lower heating roller 6 are pressed together by a mechanical means (not shown). Therefore, the lower heating roller 6 is heated by the upper heating roller 5, resulting in that the surface temperature of the lower heating roller 6 becomes substantially the same as that of the upper heating roller 5. Furthermore, the fixing speed VH corresponding to the rotation speeds of the heating rollers 5 and 6 is set so as to satisfy the following inequality (2): 0.99 VP ≦ VH ≦ 1.0 VP wherein VP is the transfer speed as described above. If the fixing speed VH at which copying paper 2 is fed by the fixing unit 4 is larger than the aforementioned transfer speed VP for example, a tensile force is applied to the copying paper 2 while a toner image is transferred onto the copying paper and it is possible that the toner image may not be properly transferred onto the copying paper when there is considerable tensile force. In the conventional electrostatic transfer type copying machine 1, the distance ℓH between the edge of the aforementioned guide member 3 on the side of the fixing unit 4 and an intersection at which an imaginary line 9 extending in a longitudinal direction of the guide member 3 intersects either of the heating rollers 5 and 6 is set so as to satisfy the following inequality (3): 0.1 DH ≦ ℓH ≦ 0.15 DH wherein DH is a diameter of the upper heating roller 5. If the aforementioned transfer speed VP increases, a relatively large force is applied to the copying paper 2, resulting in that rumples are apt to be caused on the copying paper 2. In order to suppress the occurrence of the rumples, it is desirable to strongly squeeze the copying paper 2 by the guide member 3. In order to improve the squeezing effect, the end portion 8 of the aforementioned guide member 3 is positioned at a position relatively near to the upper heating roller 5. Recently, the aforementioned electrostatic transfer type copying machine 1 has been improved considerably by reducing its size and weight. Generally, speaking, it is recognized experimentally that the transfer speed VP becomes slower as the size of the copying machine becomes smaller. Thus, when the transfer speed VP becomes slower, the copying paper 2 is heated excessively if the fixing temperature HT is set to satisfy the inequality (1) as in the conventional copying machine 1. Then, snaking patterns are generated on the copying paper 2 discharged onto a discharging tray 10 by a cooperative pressing action between the heating rollers 5 and 6, and also rumples are caused on the copying paper 2 if the snaking patterns become too large. The fixing temperature HT is set so as to satisfy the inequality (1) because of the following reasons. In the case when the fixing temperature HT is too low, the toner stuck on the copying paper 2 transported to the fixing unit 4 is not melted sufficiently, resulting in a portion of the toner which has not melted being stuck to the aforementioned heating roller 5, and then, the remaining toner might get stuck onto to the next copying roller 2 thereby causing a deterioration in the quality of the next toner image. It is generally referred to as a low temperature offset phenomenon. On the other hand, in the case when the fixing temperature HT is too high, the toner on the copying paper 2 is melted excessively, resulting in that part of the toner being melted excessively adhering to the upper heating roller 5. This is generally referred to as a high temperature offset phenomenon. Furthermore, when the transfer speed decreases as described above, the fixing speed VH, set so as to satisfy the inequality (2), also decreases, resulting in that the copying paper 2 is heated excessively as described above. Furthermore, when the aforementioned distance ℓ is set so as to satisfy the inequality (3), the end portion 8 of the aforementioned guide member 3 is positioned at a position substantially near to the upper heating roller 5. That is, the copying paper 2 is heated by the fixing unit 4 as described above, however, radiant heat from the fixing unit 4 is also applied to the copying paper 2 whilst it is positioned in the vicinity of the guide member 3. Therefore, the heat applied to the copying paper 2 is comprised of not only the transfer heat which is transferred from the heating rollers 5 and 6 but also the aforementioned radiant heat. In order to prevent such a situation, the contact distance LH between the heating rollers 5 and 6 may be made as small as possible. Such a situation can be achieved by increasing the distance between the centre points of the heating rollers 5 and 6. However, since the conventional lower heating roller 6 is made of a relatively soft silicon rubber material, the pressing force between the heating rollers 5 and 6 becomes weak when the distance between the heating rollers 5 and 6 is increased, resulting in that the fixing process can not be performed properly. Furthermore, when the aforementioned fixing speed VH decreases, the copying paper 2 is warped prior to entering the fixing unit 4, and thereby the running path of the copying paper 2 becomes long while heat is applied to the copying paper 2 from the guide member 3 which is heated by the fixing unit 4, resulting in the copying paper 2 being heated excessively as described above. Furthermore, in order to prevent the rumples from occurring in the copying paper 2 as described above, the paper 2 should be strongly squeezed by the guide member 3 so as to make the surface of the copying paper 2 smooth. However, the aforementioned squeezing effect does not occur since the copying paper 2 is warped as described above. An essential object of the present invention is to provide a toner image fixing apparatus for an image forming machine which is able to transfer and fix a toner image onto a sheet of paper with an excellent transfer performance without causing rumples on the paper. According to one aspect of the present invention, there is provided a toner image fixing apparatus for an image forming machine which is operable to transfer a toner image onto a sheet medium from an image carrier with the sheet medium running at a transfer speed of 60mm/sec or less, said fixing apparatus being operable to heat the transferred toner image so as to fix it on the sheet medium with said sheet medium running at a fixing speed which is slightly less than the transfer speed, characterised in that the fixing temperature HT of the fixing apparatus, in use, is given by TT + 10 ≦ HT ≦ TT + 20 (°C) where TT is the melting temperature of the toner. According to another aspect of the present invention, there is provided a toner image fixing apparatus for an image forming machine which is operable to transfer a toner image onto a sheet medium from an image carrier with the sheet medium running at a transfer speed of 60mm/sec or less, said fixing apparatus being operable to heat the transferred toner image so as to fix it on the sheet medium and comprising a pair of opposed rollers for pressing the sheet medium in the nip therebetween, characterised in that the surface hardness of one of said rollers is set at a value of at least 40° and in that the fixing length (LH), measured through the nip between the rollers, along which the medium is pressed by the rollers is set at a value no greater than 2.5mm. According to a further aspect of the present invention, there is provided a toner image fixing apparatus for an image forming machine which is operable to transfer a toner image onto a sheet medium from an image carrier with the sheet medium running at a predetermined transfer speed VP, said fixing apparatus being operable to heat the transferred toner image so as to fix it on said sheet medium with said sheet medium running at a predetermined fixing speed VH, characterised in that the value of said fixing speed satisfies the inequality 1.005 x VP ≦ VH ≦ 1.02 x VP. According to a still further aspect of the present invention, there is provided a toner image fixing apparatus for an image forming machine which is operable to transfer a toner image onto a sheet medium from an image carrier with the sheet medium running at a transfer speed of 60mm/sec or less, said fixing apparatus being operable to heat the transferred toner image so as to fix it on the sheet medium with said sheet medium running at a fixing speed which is slightly less than the transfer speed, said fixing means comprising a pair of opposed rollers for pressing the sheet medium in the nip therebetween, and guide means being provided for guiding the toner image-bearing sheet medium toward said rollers, characterised in that the distance measured along the path of travel of the leading edge of the sheet medium guided by said guide means, between an edge of said guide means facing said rollers on the one hand and the rollers on the other hand is set to be greater than or equal to 0.2 x DH where DH is the diameter of one of said rollers. In a preferred aspect of the invention the image carrier from which the toner image is transferred is a photoconductive image carrier and image formation means are provided to form an electrostatic image corresponding to the image of a document on the carrier, and preferentially the sheet medium is of paper. These and other objects and features of the present invention will become clear from the following description taken in conjunction with the preferred embodiments thereof with reference to the accompanying drawings, in which: Fig. 1 is a schematic longitudinal cross sectional view showing a portion of a conventional typical electrostatic transfer type copying machine; Fig. 2 is a schematic longitudinal cross sectional view showing the whole composition of an electrostatic transfer type copying machine of a preferred embodiment according to the present invention; and Figs. 3 and 4 are schematic longitudinal cross sectional views showing a portion of the copying machine shown in Fig. 2, respectively. The preferred embodiments according to the present invention will be described hereinafter, referring to the attached drawings. Fig. 2 is a schematic longitudinal cross sectional view showing the whole composition of an electrostatic transfer type copying machine 11 of a first preferred embodiment according to the present invention. Referring to Fig. 2, a document table 20 for arranging a document 21 thereon is arranged movably in a horizontal direction on the upper surface of a body 12 of the copying machine 11, and a document cover 13 for covering and pressing down the document 21 on the document table 20 is arranged on the document table 20. Under the document table 20, there is arranged an exposure optical system comprising a light source 22 for illuminating the document 21 arranged on the document table 20, and a lens 23 such as a Selfoc™ lens for focusing light reflected by the document 21 onto an exposure area 25 of a surface of a photoconductive drum 24 arranged under the lens 23 so as to form an electrostatic latent image corresponding to the image of the document 21 thereon. The photoconductive drum 24 is rotated in a direction indicated by an arrow R as shown in Fig. 2 by a driving unit (not shown), synchronous with the aforementioned movement of the document table 20, i.e., the scan operation for scanning the document 21. Around the photoconductive drum 24, in the rotation direction thereof indicated by the arrow R, there are sequentially arranged, a corona charger 30 for electrifying the surface of the photoconductive drum 24, a development unit 26 comprising a magnetic brush 26a for supplying a toner to the surface of the photoconductive drum 24 so as to develop the electrostatic latent image in a visible toner image therewith, a transfer charger 27 for electrifying the surface of the photoconductive drum 24 and a copying paper 28 so as to transfer to visible toner image formed on the surface of the photoconductive drum 24 onto the copying paper 28, a separating nail member 36 for separating the copying paper 28 on which the toner image is transferred from the photoconductive drum 24, and a cleaning unit 29 for removing the toner remaining on the surface of the photoconductive drum 24 after the toner image is transferred on the copying paper 28. A handle 18 for carrying the body 12 is arranged on a side portion 14b of the body 12, and a paper feeding tray 31 for accommodating the copying papers 28 is arranged under the handle 18. A paper drawing roller 32 for drawing the copying paper 28 loaded on the paper feeding tray 31 one by one is arranged above a left end portion of the tray 31. A paper feeding roller 33 and friction plate 34 for ensuring that only one sheet of copying paper 28 is fed at a time are arranged on the left side of a paper pick up roller 32. A pair of resist rollers 35 for transporting the copying paper 28 to a transfer area TA positioned between the photoconductive drum 24 and the transfer charger 27 are arranged on the left side of the paper feeding roller 33. On the left side of the separating nail member 36, there are arranged transportation rollers 37 for transporting the copying paper 28 separated from the photoconductive drum 24 to a guide member 38, and the guide member 38 for guiding the copying paper 28 transported by the transportation rollers 37 to a fixing unit 39 which is arranged on the left side thereof. The fixing unit 39 for fixing the toner image transferred onto the copying paper 28 comprises an upper heating roller 40 and a lower heating roller 42. The upper heating roller 40 comprises a metallic cylinder, and is arranged rotatably in a horizontal direction. The upper heating roller 40 further comprises a heating lamp 41 used as a heating source inside thereof, and a coating layer of a resin such as Teflon® formed on the outer surface of the upper heating roller 40. On the other hand, the lower heating roller 42 has a coating layer made of a material such as silicon rubber, having a hardness of approximately 40° which is prescribed by the aforementioned JIS K6301 - 1975. The temperature of the upper heating roller 40 is measured by a thermistor 43 which is arranged above the roller 40 in contact therewith, and the temperature thereof is controlled by a control unit (not shown) so as to keep it at a predetermined room temperature. A paper discharging opening 15 is formed in an opposite side portion 14a to the side portion 14b of the body 12, and a paper discharging tray 16 for receiving the copying paper 28 after the toner image is fixed by the fixing unit 39 is arranged detachable from the paper discharging opening 15. An operation of the electrostatic transfer type copying machine 11 will be described hereinafter. The document 21 arranged on the document table 20 is illuminated by a light source 22, and light reflected by the document 21 is transmitted onto the exposure area 25 of the surface of the photoconductive drum 24 through the lens 23 so that the image of the document 21 is formed on the exposure area 25 of the surface of the photoconductive drum 24. Contemporaneously, the surface of the photoconductive drum 24 is electrified by the corona charger 30 arranged on the upper stream side of the aforementioned exposure area 25, and the photoconductive drum 24 is rotated in the direction indicated by the arrow R and the light reflected by the document 21 is exposed to the exposure area 25 as described above, so that an electrostatic latent image corresponding to the image of the document 21 is formed thereon. The electrostatic latent image is developed in a visible toner image with a toner supplied by the magnetic brush 26a of the development unit 26, and the visible toner image is transferred onto the copying paper 28 transported from the paper feeding tray 31 through the paper pick up roller 32, the paper feeding roller 33, and a pair of resist rollers 35. Thereafter, the toner remaining on the surface of the photoconductive drum 24 is removed by the cleaning unit 29. After the toner image is transferred onto the copying paper 28, the copying paper 28 is separated from the photoconductive drum 24 by the separating nail member 36, and is transported to the fixing unit 39 through the guide member 38 by the transportation roller 37. Thereafter, the toner image transferred on the copying paper 28 is fixed by the fixing unit 39, and the copying paper 28 is sent to the paper exhausting tray 16. Next, the manner for setting the fixing temperature HT which is defined as the surface temperature of the heating roller 40 of the fixing unit 39 will be described. In the present preferred embodiment, the fixing temperature HT is set to be considerably lower than that of the conventional copying machine. Namely, the melting temperature TT of the toner such as 140°C is predetermined according to the kind of the toner to be used, and the fixing temperature HT is set so as to satisfy the following inequality (4): TT + 10 ≦ HT ≦ TT + 20 [°C] As described above, snaking patterns are formed on the copying paper 2 when it is heated excessively upon fixing the toner image. Therefore, in order to judge the degree of excessive heating, the difference between the moisture contents of the copying paper before and after the fixing process (referred to as a moisture content difference hereinafter) was measured for various fixing temperatures HT and transfer speeds VP. The result of this experiment is shown in Table 1. Furthermore, the moisture content difference was measured in the conventional copying machine having a relatively high transfer speed Vp for comparison, and the result of this experiment is shown in Table 2. These experiments were performed under the following conditions: (1) Unitary mass of the copying paper : 62 grms/m² (2) Initial moisture content of the copying paper : 5% (3) Melting temperature TT of the toner used : 140°C (4) Fixing distance LH of the portion where the heating rollers 40 and 42 are in contact with each other : 3 mm In Table 1, a symbol "O" represents that no rumple is caused on the copying paper upon fixing the toner image, and a symbol "x" represents that rumples are caused. On the other hand, in the case of the conventional copying machine having a high transfer speed as shown in Table 2, it was confirmed that no rumple is caused. According to the result of the experiment shown in Table 2, it was found that the moisture content difference was approximately 2%. According to the result of the experiment shown in Table 1, the moisture content difference when no rumple pattern is caused is approximately 2.54% at the maximum. Therefore, if the copying paper has an initial moisture content of approximately 5%, it is desirable to control the moisture content difference so as not to exceed approximately 2.5% in order to prevent rumples from occurring. The dependency of the moisture content difference upon the fixing temperature HT was confirmed from the results shown in Tables 1 and 2, as follows. Namely, as shown in Table 2, in the case of a transfer speed VP of 70 mm/sec, no rumples are caused on the copying paper at a fixing temperature HT of 180°C. However, as shown in Table 1, in the case of a transfer speed VP of 60 mm/sec, rumple patterns are caused thereon at a fixing temperature of 170°C. Accordingly, in the copying machine 11 having a transfer speed VP of 60 mm/sec or less, the fixing temperature HT is preferably set so as to satisfy the above inequality (4). When the fixing temperature HT is smaller than (TT + 10) [°C], the toner image may not be fixed properly because of insufficient melting heat. When the fixing temperature HT is set as described above, the toner image can be reliably fixed onto the copying paper 28 by the fixing unit 39, and also it is confirmed that the inequality (4) is a proper range in which the toner image is properly fixed onto the copying paper 28. Furthermore, it is confirmed that rumple patterns can be prevented from occurring in the copying paper 28. In the aforementioned first preferred embodiment, the fixing temperature HT is set to be a lower value than that of the conventional example, in order to solve the problems as described in the description of the related art. However, in a second preferred embodiment, the aforementioned fixing distance LH is set to be a smaller value than that of the conventional example. The composition of the copying machine of the present preferred embodiment has essentially the same technical problems as that of the aforementioned embodiment which are caused by heating the copying paper 28 excessively, and therefore, it is supposed that the above problems can be solved by decreasing the heating amount to be applied from the fixing unit 39 to the copying paper 28. Generally, in this copying machine 11, the aforementioned transfer speed VP is determined first of all. Accordingly, a fixing time HS for which the leading end of the copying paper 28 runs through the fixing distance LH is expressed by the following equation (5): HS = LH / VP According to the above equation (5), it is understood that the fixing time HS can be decreased by decreasing the fixing distance LH in order to solve the aforementioned technical problems. As described in the aforementioned first preferred embodiment, the aforementioned fixing distance LH has a length of approximately 3 mm because the upper heating roller 40 pinches the lower heating roller 42 made of silicon rubber. Accordingly, the following two methods for decreasing the aforementioned fixing distance LH as much as possible can be considered: (1) To make the hardness of the surface of the lower heating roller 42 higher. (2) To make the fixing force WH between the upper and lower rollers 40 and 42 lower in place of making the hardness of the surface of the lower heating roller 42 harder. In order to evaluate these methods, the present inventor not only measured the fixing distance LH and the fixing time HS but also checked the occurrence of rumple patterns by varying the hardness of the surface of the lower heating roller 42 stepwise. Furthermore, the above measurement and check operation were performed by varying the fixing pressure. The result of above measurement and check operation are shown in Tables 3 and 4. The results of the above measurement and check operation shown in Table 3 were obtained under the condition of a transfer speed VP of 60 mm/sec, a fixing force WH of 78.4532 N and a fixing temperature HT of 170°C. In the items of the rumple patterns of Tables 3 and 4, a symbol "O" represents that no rumple patterns are caused in the copying paper 28, and a symbol "x" represents that rumple patterns are caused. The present inventors confirmed that the fixing time HS becomes 50 msec in a conventional copying machine having a transfer speed VP of 70 mm/sec and a fixing distance LH of 3.5 mm, and assumed that the maximum fixing time HS being capable of preventing rumple patterns from occurring was 50 msec. According to the results of the measurement and check operation shown in Tables 3 and 4, rumple patterns are observed on the copying paper 28 in the case of hardness of 30° although the fixing time HS is shorter than the maximum assumed (HS = 43 msec). Further, as show in Table 4, in spite that the fixing time HS is shorter than 43 msec, rumple patterns are caused. These results are considered to be due to the fact that the total amount of heat applied to the copying paper 28 is different in the case when the fixing velocity VH has been set at a relatively large value and when the fixing length LH has been set at a relative small value. Namely, since the fixing time VH calculated from the fixing distance LH is the time interval for which the copying paper 28 is actually pressed between the upper and lower heating rollers 40 and 42, it reflects only the amount of heat transmitted to the copying paper 28 directly from the heating rollers 40 and 42. In practice, even before or after the copying paper 28 is pressed between the heating rollers 40 and 42, the copying paper 28 receives radiant heat from the heating rollers 40 and 42. These exceptions contrary to the above assumption may be caused due to this fact. Tables 3 and 4 shows that various results were obtained by varying only the fixing force WH while keeping other conditions unchanged. This is because of the fact that the copying paper 28 is contracted and corrugated easily if the fixing force WH to be applied to the copying paper 28 is very small. Therefore, it is considered that rumple patterns may be caused even though the fixing time HS is set to be a relatively small value. Accordingly, in the copying machine 11 having a relatively slow transfer speed VP of an order of 60 mm/sec or less, in order to transfer the toner image on the copying paper 28 at a proper melting temperature thereof and also prevent rumple patterns from occurring therein, the fixing force WH is preferably set to be 29.41995 N or more and the coating layer of the lower heating roller 42 is preferably made of a silicon rubber having a hardness of 40° or more, so that the fixing distance LH is set to be 2.5 mm or less. The copying machine of the present preferred embodiment composed thus has the same effect as that of the aforementioned first preferred embodiments. In the aforementioned second preferred embodiment, when the transfer speed VP and the fixing speed VH are set so as to satisfy the aforementioned inequality (2) and the aforementioned transfer speed VP is set to be 60 mm/sec or less, as shown in Fig. 1 referred to in the description of the related art, the copying paper 2 is warped at a position prior to the fixing unit 4, and then, the copying paper 28 is close to the heating rollers 5 and 6 beyond a predetermined preferable fixing distance LH, and the total heat amount applied to the copying paper 2 due to direct heat transmission and heat radiation becomes too large. Furthermore, the reason why the aforementioned rumple pattern is caused by the warp of the copying paper 2 is that the copying paper 2 is not pressed on the guide member 3 properly, and thereby, the aforementioned "squeezing effect" of the guide member 3 is lowered. In order to solve this problem, in a third preferred embodiment, the fixing speed VH is set so as to satisfy the following inequality (6): 1.005 VP ≦ VH ≦ 1.02 VP. If the fixing speed VH is set so, the fixing speed VH becomes higher than the transfer speed VP, and the copying paper 28 is tensed between the fixing unit 39 and the vicinity of the transfer charger 27 in the running direction thereof. Accordingly, the aforementioned two problems due to the warp of the copying paper 28 can be solved. Next, an experiment for determining respective coefficients of the transfer speed VP to be used in the aforementioned inequality (6) for determining the range of the fixing speed VH will be described. In the experiment, occurrence of rumple patterns and mistransfer of toner image were checked while increasing the fixing speed VH relative to the transfer speed VP stepwise. The result is shown in Table 5, wherein IR denotes the rate of increase of VH relative to VP as defined by the following equation (7). IR = VH - VP VH x 100 [%] The experiment was done under the conditions of a transfer speed VP of 60 mm/sec, a fixing temperature HT of 170°C, and a transfer force WH of 78.4532 N. According to Table 5, it is confirmed that the aforementioned increasing rate IR of the fixing speed VH is preferably in the range from 0.5% to 2.0% in order to prevent rumple patterns and mistransfer of toner image effectively. As described above, in the conventional copying machine, the distance ℓH which is the length of the imaginary line extending from the end portion of the guide member 38 positioned on the side of the fixing unit 39 to the outer surface of the fixing unit 39, is set to be approximately in the range from 0.1DH to 0.15DH, wherein DH is the diameter of the upper heating roller 40. In the conventional copying machine composed thus, there are various technical problems as described above. In a fourth preferred embodiment, the aforementioned distance ℓH is set to be a larger value, such as 0.2HD, than that of the conventional copying machine. Then, the end portion 46 of the guide member 38 is further than the position of the conventional copying machine from the outer surface of the upper heating roller 40, and therefore, the heat amount to be applied to the copying paper 28 by the heating rollers 40 and 42 is decreased. The sliding angle ϑH as shown in the drawings is set to be approximately 8°. Namely, the guide member 38 is inclined at a larger angle than that of the conventional copying machine so that the end portion 46 is positioned at an upper position so that the end portion 46 is positioned at an upper position than that of the conventional copying machine, as shown in Fig. 3. Accordingly, even though the end portion 46 of the guide member 38 is further than that of the conventional copying machine from the outer surface of the upper heating roller 40, the guide member 39 has essentially the same squeezing effect on the copying paper 28 as that of the conventional copying machine. In order to determine the relationship between the aforementioned distance ℓH and the diameter DH of the upper heating roller 40, occurrence of rumple patterns and/or real rumples formed by folding the copying paper partially were checked while varying the ratio of the distance ℓH to the distance DH. The result of this experiment is shown in Table 6. The experiment was performed under the condition of a transfer speed VP of 60 mm/sec, a fixing temperature HT of 170°C, a sliding angle ϑH of 8°, and the diameter DH of the upper heating roller 40 of 25 mm. According to the result of the experiment shown in Table 6, the distance ℓH is in the following range in order to prevent rumple patterns and real rumples from forming: 0.2DH ≦ ℓH ≦ 0.4DH However, it is supposed that the above range varies when various values for the sliding angle ϑH are set. Accordingly, in the copying machine 11 having a transfer speed VP of 60 mm/sec which is used in respective above preferred embodiments, the distance ℓH is set preferably to be 0.2DH or more. In the copying machine composed thus, an essentially similar effect can be obtained as that of the aforementioned preferred embodiments. The present inventor made various copying machines 11 compound as described above, having the parameters of the following range, under the condition of a transfer speed VP of 50 mm/sec, a melting temperature TT of the toner of 140°C, the fixing temperature HT of 160°C, a fixing force WH of 78.4532 N, and a diameter DH of the upper heating roller 40 of 25 mm. (a) The distance ℓH : 0.22DH (b) The fixing speed VH : 1.01VP (c) The sliding angle ϑH : approximately 8° (d) The thickness of the coating layer of the upper heating roller 40 : 0.9mm (e) The diameter of the lower heating roller 42 : 18mm (f) The thickness of the coating layer of the lower heating roller 42 : 4 mm (g) The hardness of the surface of the lower heating roller 42 : 45° to 55° (h) The fixing distance LH : approximately 2 mm In various copying machines 11 made as described above, the present inventor confirmed that various kinds of aforementioned effects could be obtained. In respective aforementioned preferred embodiments, the copying machines having an independent composition are described, respectively. However, the copying machine having a proper combination of parameters set in respective aforementioned preferred embodiments may be used.

Claims: A toner image fixing apparatus (39) for an image forming machine which is operable to transfer a toner image onto a sheet medium from an image carrier (24) with the sheet medium running at a transfer speed of 60mm/sec or less, said fixing apparatus (39) being operable to heat the transferred toner image so as to fix it on the sheet medium with said sheet medium running at a fixing speed which is slightly less than the transfer speed, characterised in that the fixing temperature HT of the fixing apparatus, in use, is given by TT + 10 ≦ HT ≦ TT + 20 (°C) where TT is the melting temperature of the toner. A toner image fixing apparatus (39) for an image forming machine which is operable to transfer a toner image onto a sheet medium from an image carrier (24) with the sheet medium running at a transfer speed of 60mm/sec or less, said fixing apparatus (39) being operable to heat the transferred toner image so as to fix it on the sheet medium and comprising a pair of opposed rollers (40, 42) for pressing the sheet medium in the nip therebetween, characterised in that the surface hardness of one of said rollers (40, 42) is set at a value of at least 40° and in that the fixing length (LH), measured through the nip between the rollers (40, 42), along which the medium is pressed by the rollers (40, 42) is set at a value no greater than 2.5mm. A toner image fixing apparatus (39) for an image forming machine which is operable to transfer a toner image onto a sheet medium from an image carrier (24) with the sheet medium running at a predetermined transfer speed VP, said fixing apparatus (39) being operable to heat the transferred toner image so as to fix it on said sheet medium with said sheet medium running at a predetermined fixing speed VH, characterised in that the value of said fixing speed satisfies the inequality 1.005 x VP ≦ VH ≦ 1.02 x VP. A toner image fixing apparatus (39) for an image forming machine which is operable to transfer a toner image onto a sheet medium from an image carrier (24) with the sheet medium running at a transfer speed of 60mm/sec or less, said fixing apparatus (39) being operable to heat the transferred toner image so as to fix it on the sheet medium with said sheet medium running at a fixing speed which is slightly less than the transfer speed, said fixing means (39) comprising a pair of opposed rollers (40, 42) for pressing the sheet medium in the nip therebetween, and guide means (38) being provided for guiding the toner image-bearing sheet medium toward said rollers (40, 42), characterised in that the distance measured along the path of travel of the leading edge of the sheet medium guided by said guide means (38), between an edge of said guide means (38) facing said rollers (40, 42) on the one hand and the rollers (40, 42) on the other hand is set to be greater than or equal to 0.2 x DH where DH is the diameter of one of said rollers (40, 42). A toner image fixing apparatus according to any one of claims 1 to 4, wherein the image carrier from which the toner image is transferred is a photoconductive image carrier and image formation means are provided to form an electrostatic image corresponding to the image of a document on said carrier. A toner image fixing apparatus according to any one of claims 1 to 5, wherein the sheet medium is of paper.

Inventors: KIDA, HIROSHI
Applicants: SHARP KABUSHIKI KAISHA
Application year: 1989
Main classification: G03G15
IPC codes: G03G15