IMAGE FORMING APPARATUS

An image forming apparatus includes: a belt that is capable of circulating for conveying a recording medium from an upstream side to a downstream side; a fixing pad that presses the belt from inside the belt toward at least a first side; a support member that supports the fixing pad; and a roller. The fixing pad is used in a first mode and a second mode. In the first mode, a first type recording medium passes between the belt and the roller. In the second mode, a second type recording medium passes between the belt and the roller. Assuming that a first point is defined on an upstream side of the fixing pad and a second point is defined on a downstream side, an inclination of a straight line that connects the first point and the second point is different between the first mode and the second mode.

The entire disclosure of Japanese Patent Application No. 2019-009448, filed on Jan. 23, 2019, is incorporated herein by reference in its entirety.

BACKGROUND

Technological Field

Description of the Related Art

An image forming apparatus including a fixing device has been known. In recent years, a fixing device adopting a system using a pad has sometimes been employed in terms of energy saving and cost reduction. This system allows the pad having a high degree of freedom in shape. As a result of freely designing pads in consideration of improvement in fixability and the like, some pads may have a complicated shape. When a recording medium like an envelope structured of a plurality of overlapping sheets of paper is conveyed to pass over a pad having such a complicated shape, the conveyance amount is more likely to differ between the sheet of paper on the front side and the sheet of paper on the back side. As a result, the sheets of paper may be creased.

Japanese Laid-Open Patent Publication No. 2017-120398 discloses a technique for reducing the difference in amount of conveyance between the sheet of paper on the front side and the sheet of paper on the back side by reducing the area of contact between a pad and an envelope when the envelop passes over the pad.

SUMMARY

According to the technique disclosed in Japanese Laid-Open Patent Publication No. 2017-120398, the area of contact between the envelope surface and the pad is reduced, which may lead to unsuccessful conveyance such as slipping. There are two peaks in a pressure distribution in a fixing nip portion when an envelope passes over the fixing nip portion. This pressure distribution does not allow excellent fixability.

Thus, an object of the present invention is to provide an image forming apparatus that is capable of preventing creases that may occur during a fixing operation due to a difference in type of a recording medium.

To achieve at least one of the above-mentioned objects, according to an aspect of the present invention, an image forming apparatus reflecting one aspect of the present invention comprises: a belt that has an endless shape and is capable of circulating for conveying a recording medium from an upstream side to a downstream side; a fixing pad that presses the belt from inside the belt toward at least a first side; a support member that supports the fixing pad from a side opposite to the belt; and a roller. The belt is sandwiched between the roller and the fixing pad. The fixing pad is used in a first mode and a second mode. In the first mode, a first type recording medium passes between the belt and the roller. In the second mode, a second type recording medium passes between the belt and the roller. Assuming that a first point is defined on an upstream side of the fixing pad and a second point is defined on a downstream side of the fixing pad, an inclination of a straight line that connects the first point and the second point is different between the first mode and the second mode.

DETAILED DESCRIPTION OF EMBODIMENTS

First Embodiment

Referring toFIGS. 1 to 8, an image forming apparatus in the first embodiment according to the present invention will be hereinafter described.

FIG. 1is a conceptual diagram of an image forming apparatus801in the present embodiment. Image forming apparatus801includes a housing1and a cassette40that is disposed in a lower portion of housing1. Housing1includes therein a photoconductor44, a charging unit46, a transfer belt47, an image exposure device48, a developing roller45, a primary transfer roller42, a secondary transfer roller43, a fixing device101, and the like. One or more recording media2are housed in cassette40. Recording medium2may be a sheet of paper, for example, or may be a medium other than a sheet of paper. Transfer belt47has an endless shape and is disposed so as to move in a circulating manner.

In image forming apparatus801, the surface of photoconductor44is charged by charging unit46so as to uniformly carry a prescribed electric potential. Over this charged area, an image is exposed from image exposure device48in accordance with the document image. Thereby, an electrostatic latent image is formed on the surface of photoconductor44. This electrostatic latent image is developed by developing roller45having a development bias applied thereto, and then formed as a visible toner image. A bias for attracting toner is applied to primary transfer roller42. Thus, the visible toner image on the surface of photoconductor44is transferred onto transfer belt47.

On the one hand, recording media2are taken out one by one from cassette40by paper feeding roller41and conveyed to secondary transfer roller43. A voltage is applied also to secondary transfer roller43as with primary transfer roller42. Transfer belt47is sandwiched between secondary transfer roller43and pressing roller49so as to constitute a secondary transfer nip portion38. When recording medium2passes over secondary transfer nip portion38, recording medium2and transfer belt47are sandwiched and pressed between secondary transfer roller43and pressing roller49. The visible toner image carried by transfer belt47is transferred onto recording medium2in secondary transfer nip portion38. Recording medium2having the visible toner image carried thereon is fed to fixing device101. The detailed structure of fixing device101will be described later. Fixing device101performs heating and pressurization so as to fix the toner onto recording medium2. Thus, recording medium2on which an image has been formed is discharged from an exit39.

FIG. 2shows fixing device101alone. Fixing device101includes a belt51having an endless shape, a roller58for pressurization, and a fixing pad52disposed fixedly on the inside of belt51. Fixing pad52and roller58are positioned to sandwich belt51therebetween so as to constitute a fixing nip portion50. Fixing pad52defines a path of belt51. Belt51is conveyed along the shape of fixing pad52.

Fixing pad52is supported by a support member54. A heat source56is disposed inside a heating member57. Belt51is tensioned by heating member57and fixing pad52. Roller58is rotated by a motor (not shown) at a prescribed rotation speed. Thereby, belt51is conveyed.

Fixing pad52may be formed of a resin such as polyphenylene sulfide, polyimide, and a liquid crystal polymer, for example. Also, fixing pad52may be formed of metal such as aluminum and iron, ceramics, and the like in place of a resin. Fixing pad52may include a fixing member made of silicone rubber, fluorine rubber, and the like. Fixing pad52may be formed of a combination of two or more components.

Heating member57is made of metal such as aluminum or stainless steel (SUS) and formed in an approximately cylindrical shape. Roller58has a three-layer structure including a cored bar/an elastic layer/a release layer. It is preferable that the elastic layer is made of a material such as silicone rubber or fluorine rubber having high heat resistance. It is preferable that the cored bar is made of metal such as aluminum or iron. The cored bar may be formed in a pipe shape or may be formed in a solid shape. It is preferable that the release layer is formed of a fluorine tube or has a configuration with releasability by a fluorine-based coating and the like.

Heat source56may be a halogen heater. Alternatively, heat source56may be a system of applying heat to heating member57and/or belt51with induction heating (IH). Further alternatively, heating member57and/or belt51may be formed of a resistance heating element so as to generate heat therefrom.

Recording medium2having a toner image transferred thereto is conveyed to fixing nip portion50in the state where its surface having the toner image transferred thereto faces heated belt51. Then, recording medium2passes over fixing nip portion50. Recording medium2that passes over fixing nip portion50is sandwiched between belt51and roller58so as to be pressed and heated therebetween. Thereby, the toner image is fixed onto recording medium2.

FIG. 3shows an enlarged view of fixing pad52and a region therearound. In this figure, recording medium2is conveyed from below to above as indicated by arrows91and92, in which the lower side corresponds to an upstream side while the upper side corresponds to a downstream side.

Recording medium2conveyed to fixing nip portion50is properly guided to fixing nip portion50as indicated by arrow91. Recording medium2having passed over fixing nip portion50is discharged from fixing nip portion50as indicated by arrow92.

Referring toFIGS. 4 to 6, the process of attaching fixing pad52to support member54will be hereinafter described. Support member54may be formed of a metal sheet. As shown inFIGS. 4 and 5, support member54is provided with a hole54athrough which fixing pad52is attached to support member54. As shown inFIG. 5, a pin52bfor attachment to support member54is provided on the surface of fixing pad52that faces support member54. By inserting pin52binto hole54a,fixing pad52can be attached to support member54.FIG. 6shows a cross-sectional view of the state where attachment is completed.

A combination of fixing pad52and support member54(which will be hereinafter referred to as a “fixing pad structure body”) can be rotated by a prescribed angle as indicated by an arrow96inFIG. 7. This rotation can be caused using a motor and the like. InFIG. 7, the central line of this rotation is shown by a dashed-dotted line. This rotation changes the posture of the fixing pad structure body as shown inFIG. 8. InFIG. 8, arrow92shows the direction in which recording medium2is discharged from fixing nip portion50. The direction of arrow92is set depending on the posture of fixing pad structure body60. Assuming that recording medium2is discharged in the direction indicated by arrow92when fixing pad structure body60is in the posture shown by a solid line inFIG. 8, recording medium2is discharged in the direction indicated by an arrow92ewhen fixing pad structure body60is in the posture shown by a chain double-dashed line.

It should be noted that fixing pad structure body60is not necessarily rotated about the central line that is accurately fixed. Fixing pad structure body60may be displaced such that the angle of fixing pad52is changed while the central line of the rotation changes. At least the angle of fixing pad52may be changed.

The configuration of image forming apparatus801in the present embodiment may be described as follows.

Image forming apparatus801includes: belt51that has an endless shape and is capable of circulating so as to convey recording medium2from the upstream side to the downstream side; a fixing pad52that presses belt51from inside belt51at least toward a first side95; a support member54that supports fixing pad52from the side opposite to belt51; and a roller58. Roller58serves to apply pressure. Belt51is sandwiched between roller58and fixing pad52. Fixing pad52may be used in the first mode and the second mode. In the first mode, a first type recording medium passes between belt51and roller58. In the second mode, a second type recording medium passes between belt51and roller58. Assuming that the first point is defined on the upstream side of fixing pad52and the second point is defined on the downstream side of fixing pad52, the inclination of the straight line that connects the first point and the second point is different between the first mode and the second mode.

As shown inFIG. 8, fixing pad52protrudes particularly at its end on the downstream side such that the outline of fixing pad52partially has a shape of a nearly straight line. This portion will be hereinafter referred to as a “fixation finishing portion”. Fixing pad52has a fixation finishing portion52f.The first point and the second point are set on the outline of fixation finishing portion52fwhen seen in a cross-sectional view of fixing pad52. AlthoughFIG. 8does not explicitly show the first point and the second point, the first point may be arbitrarily set at an appropriate position in the vicinity of the end of fixation finishing portion52fon the upstream side. The second point may be arbitrarily set at an appropriate position in the vicinity of the end of fixation finishing portion52fon the downstream side.

In the present embodiment, fixing pad52can be used in the first mode in which the first type recording medium passes thereover, and in the second mode in which the second type recording medium passes thereover. The posture of fixing pad52differs depending on the mode in which fixing pad52is used. Thereby, the fixing operation can be performed while properly preventing occurrence of creases in accordance with the type of recording medium2. This consequently can prevent any crease that may occur during the fixing operation due to the difference in type of the recording medium.

In the present embodiment, assuming that the first type recording medium is a sheet of paper, the second type recording medium is an envelope, and the posture of fixing pad52in the first mode is defined as the first posture, in the second mode, fixing pad52is preferably changed from the first posture to the second posture in which the second point is rotated about the first point in the direction away from roller58.FIG. 8does not show roller58, but this roller58is located on the right side in the figure and arranged to apply pressure to fixing pad52. In other words, when the second mode is selected in the case of an envelope, and when fixing pad52is changed from the first posture to the second posture for switching into the second mode, it is preferable that the posture of fixing pad52is changed such that the downstream side (that is, an upper portion in the figure) of fixing pad52is displaced toward the left side in the figure.

The following is an explanation with reference toFIG. 9about the reason why it is preferable to change the posture as described above in the case of an envelope. InFIG. 9, the angles are shown in an exaggerated manner so as to allow easy understanding of the explanation. In fixing nip portion50, the conveyance path of the recording medium does not necessarily have a shape of a straight line, but the conveyance path in fixation finishing portion52fcan be approximated to a straight line. Thus, the conveyance path in fixation finishing portion52fis shown in a shape approximated to a straight line in this case.

The direction in which support member54presses fixing pad52is defined as an X-axis while the direction perpendicular to this X-axis and toward the downstream side is defined as a Y-axis. The Y-axis may correspond to the vertical direction, but does not necessarily correspond to the vertical direction. A point in fixation finishing portion52fon the upstream side of the conveyance path in the recording medium is defined as a point A. The surface of fixation finishing portion52finclines. In other words, fixation finishing portion52fhas a shape protruding more largely toward the downstream side. Accordingly, the recording medium discharged after passing over fixing nip portion50is not conveyed from point A to a point D in parallel with the Y-axis, but conveyed from point A to a point B, for example. An angle DAB is set at an angle θ1. Like this angle θ1, the angle formed between the Y-axis and the direction in which the recording medium is discharged will be hereinafter referred to as a “paper discharge angle”. The conveyance distance of the recording medium in this case corresponds to a length L1of a line segment AB. In the case where a recording medium is a plain paper, the recording medium is conveyed at paper discharge angle θ1in this way and discharged in the desired direction.

In the case where a recording medium is an envelope, the longer conveyance distance is more likely to cause a difference in amount of conveyance between the sheet of paper on the front side and the sheet of paper on the back side. Consequently, creases are more likely to occur. In the case where a recording medium is an envelope, it is preferable to shorten the conveyance distance to the possible extent for preventing creases. The paper discharge angle is changed from θ1to θ2that is smaller than θ1, to thereby change the conveyance path from line segment AB to a line segment AC, which results in the conveying distance of a length L2. Since L2<L1, it can be said that creases are less likely to occur when the conveyance path of the recording medium extends along line segment AC. Assuming that line segment AB corresponds to the first mode and line segment AC corresponds to the second mode, the conveyance path is changed between the first mode and the second mode in accordance with the type of the recording medium. Thereby, occurrence of creases can be suppressed as appropriate.

However, the paper discharge angle does not always need to be kept set at θ2. A small paper discharge angle may deteriorate separativeness, for example, in the case of thin sheets of paper. The term “separativeness” used herein means the degree as to how smoothly the recording medium separates from belt51after it has passed over the fixing nip portion. With deteriorated separativeness, the sheet of paper having passed over fixing nip portion50is not conveyed to be discharged in the originally intended direction after it separates from belt51, but is more likely to be discharged in an unintended direction behind the timing of separating from belt51. With reference toFIG. 3, there is an increasing possibility to cause a failure that a sheet of paper that should originally be discharged in the direction indicated by arrow92may be discharged more leftward in the figure. Thus, for sheets of paper such as thin paper, the paper discharge angle needs to be large for ensuring separativeness. In the case of an envelope, which has a relatively large basis weight, excellent separativeness is achieved even when the paper discharge angle is relatively small.

Thus, as described above, assuming that the first type recording medium is a sheet of paper, the second type recording medium is an envelope, and the posture of fixing pad52in the first mode is defined as the first posture, in the second mode, fixing pad52is preferably changed from the first posture to the second posture in which the second point is rotated about the first point in the direction away from roller58. In other words, it is preferable that the paper discharge angle is set to be smaller in the second mode for an envelope than in the first mode.

For example, it is conceivable that the paper discharge angle is set at 6° or more in the first mode in the case of printing on a commonly used recording medium; and that the paper discharge angle is set at 4° or less in the second mode in the case of printing on an envelope. By changing the paper discharge angle in this way, printing on different types of recording media can be done without any problem.

The section in which roller58and belt51are in close contact with each other will be hereinafter referred to as fixing nip portion50. It is preferable to change the posture of fixing pad52between the first mode and the second mode such that the peak position in the pressure distribution from the upstream side to the downstream side inside fixing nip portion50does not change between the first mode and the second mode.FIG. 10shows an example of the pressure distribution. The horizontal axis shows the position inside fixing nip portion50. The numerals of 0 to 12 indicating positions are arbitrarily set for indicating the relative positions inside fixation finishing portion52fof fixing pad52. On the horizontal axis, the right side corresponds to the entrance side while the left side corresponds to the exit side. The vertical axis shows acting pressure. In the graph inFIG. 10, a solid line shows the pressure distribution at a paper discharge angle of 6° while a broken line shows the pressure distribution at a paper discharge angle of 4°. The paper discharge angle is changed between 4° and 6°, thereby changing the pressure distribution. In each case, the pressure reaches a peak at a position close to the exit. In other words, the peak position in the pressure distribution does not change between the first mode and the second mode. In order to excellently perform the fixing operation, the highest pressure needs to act at the position close to the exit. As shown inFIG. 10, when the posture of fixing pad52is changed between the first mode and the second mode so as to prevent the peak position in the pressure distribution from changing between the first mode and the second mode, the fixing operation can be excellently performed both in the first mode and the second mode.

It should be noted that an envelope is more likely to be creased as the basis weight of the envelope is smaller. The second type recording medium is limited to an envelope having a basis weight equal to or less than a prescribed value. In the case of an envelope having a basis weight greater than the prescribed value, it is preferable that fixing pad52is used in the first mode. Thereby, the separativeness can be excellently maintained. An envelope having a basis weight greater than the prescribed value is essentially less likely to be creased when it passes over fixing nip portion50. Thus, this envelope may be used in the first mode.

Also, it is preferable that the prescribed value is 80 g/m2. In other words, it is preferable that the envelope having a basis weight equal to or less than 80 g/m2is used in the second mode, and the envelope having a basis weight greater than 80 g/m2and recording media other than an envelope are used in the first mode.

The present inventor has conducted an experiment under the conditions that the paper discharge angle was changed at four angles of 3°, 4°, 6°, and 9° in fixing device101shown inFIG. 2, and, under each of these angle conditions, the fixing temperature was changed at five temperatures of 120° C. to 160° C. Then, the inventor examined how many envelopes were creased among five envelopes in each of combinations of the paper discharge angles and the fixing temperatures. The results are shown in Table 1.

At a fixing temperature equal to or greater than 150° C., creases occurred irrespective of the paper discharge angle. At a fixing temperature equal to or less than 140° C., creases occurred at paper discharge angles of 6° and 9°, but no crease occurred at a paper discharge angle of 4° or less. This shows that occurrence of creases in an envelope can be suppressed at a relatively small paper discharge angle. For example, the paper discharge angle may be set at 6° in the first mode while the paper discharge angle may be set at 4° in the second mode.

Among the above-described embodiments, a plurality of embodiments may be employed in an appropriate combination.