JPH0569218B2 - - Google Patents

Description

[Detailed description of the invention]

TECHNICAL FIELD The present invention relates to an electrophotographic apparatus, and particularly to a fixing device that presses a recording material between a pair of rollers. BACKGROUND ART Conventionally, in this type of fixing device, when the pressure contact force between the rollers is large, in order to soften the shock when the recording material rushes between the rollers, the recording material is moved obliquely to the roller-to-roller nip portion. I was letting them in. However, since the recording material gradually straddles the roller-to-nip portion in a direction perpendicular to the roller-to-nip portion, there is a drawback that the trailing edge of the recording material deviates more in the width direction than the leading edge. The inventors of the present invention have discovered that this problem appears as an image shift near the trailing edge of the recording material when the distance between the image transfer position and the roller-to-nip portion is shorter than the length of the recording material. This problem is important because in recent years, electrophotographic devices and the like have become smaller, and the distance between the image transfer position and the roller nip tends to become shorter. OBJECT OF THE INVENTION An object of the present invention is to reduce the above-mentioned image shift at the rear end. Summary of the Invention An image forming apparatus that transfers an image onto a recording material includes a first roller that is provided downstream of an image transfer position with respect to the recording material conveyance direction, and that has a substantially truncated conical shape; a second roller having a truncated conical shape and having a function of contacting the first roller so that the smaller diameter side thereof is the smaller diameter side of the first roller and at least feeding the recording material at the corresponding contact portion; There is provided an image forming apparatus in which the intersection point of the generatrix of the roller and the extension line of the line of contact between the rollers is located near a straight line perpendicular to the direction in which the recording material travels through the image transfer section. Image displacement at the rear end can be reduced. Embodiments Before describing embodiments of the present invention, problems with known techniques discovered by the inventors of the present invention will be explained.
FIG. 1, FIG. 2, and FIG. 3 are explanations of conventional examples. FIG. 1 is a schematic top view showing only the roller configuration of a fixing device including rigid fixing rollers 1 and 2. FIG. A high load is applied to the rollers 1 and 2 from both ends, and when a line S is perpendicular to the paper traveling direction, the rollers 1 and 2 are arranged so as to intersect at a constant angle β to this line. The pressure applied between the two is made to have little variation over almost the entire area. At this time, the upper and lower roller pairs are both moved in opposite directions to the line S.
Since they are crossed, the abutting portions of the rollers (upper and lower nip portions N) coincide with the center line and the line S. At this time, since the leading end of the paper P and the nip portion N are parallel, when the paper P enters between the rollers, a large load is applied to the drive source of the rollers at once. Therefore, in this case, the toner image carried on the paper P is smeared by the shock, and furthermore, if the paper P is present in the transfer section at that time, there is a problem that transfer misalignment occurs. Furthermore, in order to tolerate this impact, it was necessary to increase the leeway of the drive source, which led to an increase in the size of the device. In order to eliminate these inconveniences, the axes of the rollers 1 and 2 are tilted by θ as shown in FIG.
Since they intersect at an angle of α/2=θ, when the paper P enters the roller nip section, the edge of the paper P gradually enters in the width direction, so it will not receive a large impact. It's gone. However, in this case, a new problem is the fact that the paper P is ejected from the nip portion N in a direction of θ with respect to the traveling direction of the arrow. FIG. 3 is a perspective view of these fixing rollers 1 and 2, and the diameters of the roller ends satisfy the relationships r ₁ =r' ₁ and r ₂ =r' ₂ , respectively. FIG. 4 explains this point in detail.
To simplify the explanation, it is assumed here that the pair of rollers do not intersect, and that the nip is inclined at an angle θ with respect to the direction perpendicular to the direction in which the paper travels. In order to see the movement of the paper P while it is being held in the nip, at any position on the paper P,
Consider a straight line X3-Y3 drawn parallel to the leading edge of paper P. FIG. 4 shows a state in which the straight line X3-Y3 on the paper P has just reached the nip line N (on the right side in the drawing). From this, after a certain amount of time, the straight line X3-Y
3 is the position indicated by the straight line X4-Y4. Since the conveyance force of the paper P by this pair of rollers is perpendicular to the nip line N, the straight line connecting points X3 and X4 is perpendicular to the nip line N. Similarly, after a certain period of time has elapsed, the line reaches straight line X5-Y5. X5 and Y5 are on extensions of straight line X3-X4 and straight line Y3-Y4, respectively. On the other hand, at a certain time before the straight line X3-Y3 reaches the nip, the straight line
Y3 is located at the position indicated by X2-Y2. Straight line X3-X2 and straight line Y3-Y2 are perpendicular to nip line N. Even earlier, X1-
It was in Y1. Here, for the sake of understanding the movement of the paper P, the straight line X3-Y3, the straight line X4-Y4, and the straight line X5-Y5 correspond to the movement of the leading edge of the paper P.
Straight line X1-Y1 and straight line X2-Y2 do not correspond to the movement of the leading edge of paper P. This is because in the latter case, the leading edge of the paper P has not yet reached the nip portion, so the paper P is not affected by this pair of rollers. However, this latter corresponds to the movement of the trailing edge of the paper P after the leading edge of the paper P reaches the nip portion. In this way, after the paper P is sandwiched between the pair of rollers,
It is sent in a direction perpendicular to the nip line N. Next, we will use this roller pair as an actual electrophotographic device.
For example, the case of use in a transfer type electrophotographic copying machine will be discussed using FIG. 5. In this figure, 5 is a photosensitive drum, from which a developed image is transferred onto paper P. The nip line N of the roller pair is a straight line M that passes through the image transfer section and is perpendicular to the recording material traveling direction (hereinafter referred to as the transfer section extension line).
Therefore, the nip line N intersects the transfer section extension line M at a point O. In the figure, the paper P indicated by a solid line shows the state in which it is passing through the pair of rollers, and the paper P shown by a dotted line shows the state of the paper P before reaching the nip portion. Here, in order to see the state of image transfer with this configuration, the image transferred from the photosensitive drum to the paper P at one point Q on the photosensitive drum, that is, the image is copied as a straight line in a direction perpendicular to the traveling direction of the paper P. Think about the image you want. When the leading edge of the paper P is not held between the pair of rollers, the movement of the paper P is not affected by the pair of rollers, so the movement of the paper P
As shown by the dotted line, the image moves in a direction perpendicular to the transfer section extension line M, and the image transferred to the paper P becomes a straight line L that is perpendicular to the transfer section extension line M. Next, after the leading end of the paper P reaches the nip line N of the roller pair 1, the paper P is fed diagonally as explained in FIG. 4. Therefore, the image transferred at point Q becomes an inclined straight line as shown by straight line BQ. Point B is a point that is transferred at point Q when the leading edge of paper P reaches the nip portion. Therefore, the point transferred to the rear edge of paper P is straight line BQ
This is the intersection point C between the extension line and the rear edge. In this way, as shown in image Figure 6, which should be recorded as straight line L and its extension, the trailing edge is recorded as a straight line BQ with an inclination, and the amount of deviation at the trailing edge is determined by the extension of straight line L. When the intersection of the line and the trailing edge of the paper P is defined as H. In addition,
The angle between the extension of the straight line L and the straight line BQ is equal to the angle θ between the transfer section extension M and the nip line N. The above analysis also applies to a pair of rollers having a crossing angle, if the nip line N described above is made to correspond to the nip line N in FIG. Next, examples of the present invention will be described. FIG. 7 shows an embodiment of the invention. As shown in the figure, the outer diameters of the rollers are asymmetrical, each roller has a truncated conical shape, and has an intersecting angle.
As is clear from the figure, r ₄ <r' ₄ and r ₃ <r' ₃ . The movement of the paper P when this roller is used will be explained in detail using FIG. 8. For convenience of explanation, a case in which the roller pairs do not intersect will be described first. The nip line N is perpendicular to the traveling direction of the paper P. Let O be the intersection of the extension of the generatrix of the truncated conical roller and the axis of the roller. In order to see the movement of the paper P while it is being held in the nip, at any position on the paper P,
Consider a straight line X3-Y3 drawn parallel to the leading edge of paper P. FIG. 4 shows a state in which this straight line X3-Y3 on the paper P has just reached the nip portion (on the right side in the drawing). From this, after a certain period of time has passed, the straight line X3-Y3 will be at the position indicated by the straight line X4-Y4. The conveyance force of the paper P by this pair of rollers is perpendicular to the nip line N at each point on the nip line N, but the conveyance amount is larger for the roller with the larger diameter. because,
This is because the conveyance amount is proportional to the distance from point O.
Therefore, straight line X4-Y4 is on a straight line passing through point O, and point X4 is on a circular arc with point O as the center and the radius is the distance between point O and point
is on an arc centered on point O and whose radius is the distance between point O and point Y3. Similarly, after a certain period of time has elapsed, the line reaches straight line X5-Y5. Point X5,
Y5 is on the extension of each arc. On the other hand, at a certain time before the straight line X3-Y3 reaches the nip, the straight line X3-Y3 becomes
It was in the position shown by 2. Straight line X2-Y
2 is also on a straight line passing through point O, and as shown, point X2 and point Y2 are on extensions of the respective arcs. Furthermore, before a certain time, X1-
It was in Y1. Here, in order to understand the movement of the paper P, the straight line X3-Y3, the straight line X4-Y4, and the straight line X5-Y5 correspond to the movement of the leading edge of the paper P.
Straight line X1-Y1 and straight line X2-Y2 do not correspond to the movement of the leading edge of paper P. This is because in the latter case, the leading edge of the paper P has not yet reached the nip portion, so the paper P is not affected by this pair of rollers. However, this latter corresponds to the movement of the trailing edge of the paper P after the leading edge of the paper P reaches the nip portion. In this way, after the paper P is sandwiched between the pair of rollers,
It is sent in a direction perpendicular to the nip line N. Next, a case where the roller pair 3, 4 is used in an actual electrophotographic apparatus, such as a transfer type electrophotographic copying machine, will be discussed with reference to FIG. In this figure, 5 is a photosensitive drum, around which a known image forming means is provided to form a developed image on the drum. A developed image is transferred from the drum 5 onto the paper P. The nip line N is inclined at an angle θ with respect to the axis M of the drum. According to the present invention, the intersection point O between the generatrix and extension of the roller and the nip line N
is placed near the transfer part extension line M,
Here, it is assumed that it is placed on the transfer section extension line M.
In the figure, the paper P indicated by a solid line shows the state in which it is passing through the pair of rollers, and the paper P shown by a dotted line shows the state of the paper P before reaching the nip portion. Here, in order to see the state of image transfer with this configuration, the image transferred from the photosensitive drum to the paper P at one point Q on the photosensitive drum, that is, the image is copied as a straight line in a direction perpendicular to the traveling direction of the paper P. Think about the image you want. When the leading edge of the paper P is not held between the pair of rollers, the movement of the paper P is not affected by the pair of rollers, so the movement of the paper P
As shown by the dotted line, the image moves in a direction perpendicular to the transfer section extension line M, and the image transferred to the paper P becomes a straight line L that is perpendicular to the transfer section extension line M. Next, after the leading edge of the paper P reaches the nip line N of the pair of rollers 3 and 4, it is fed as explained in FIG. 8. Therefore, the image transferred at point Q becomes an arc of radius OQ, as shown by curve BQ. Point B is a point that is transferred at point Q when the leading edge of paper P reaches the nip portion. Therefore, the point to be transferred to the trailing edge of the paper P is the intersection C between the extension of the arc BQ and the trailing edge. In this way, the rear edge of the image that should be recorded as the straight line L and its extension is recorded as a circular arc BQ, as shown in FIG. When the intersection with the trailing edge of the paper P is H. In addition, the arc
Since the point O is on the transfer section extension line M, the shape of BQ is such that it touches the straight line L at the point B. The amount of deviation changes depending on the radius of the arc and the position of point O. Since the radius of the arc is substantially determined by the truncated cone-shaped taper that can be used in the recording device, the amount of deviation is determined by the position of point O. However, the above analysis is purely theoretical, and actually changes depending on the restraint state of the paper P at the transfer position, the elasticity of the paper P, etc.
If a truncated cone-shaped taper can be used in a normal recording device, that is, the length of the conical generatrix obtained by extending the truncated cone-shaped generatrix is 8 to 25 m.
In the case of a taper such that It has been experimentally found that it is preferable to place the distance within about 1/4 of the distance of . In the transfer section, the transfer operation is performed not only in a straight line, but also over a certain width (in the direction of movement of the paper P).
Considering that it is within the range of , it is preferable that the width is within the range of extension of this width. Furthermore, in FIG. 10, the arc BC touches the straight line L at the point B, but this is because the point O is placed on the extension of the transfer section extension line M, and if the point O is brought closer to the roller pair, the point B Since the tangent line of the arc in is inclined to extend from the upper left to the lower right in FIG. Since the tangent to the arc at point B is inclined so as to extend from the upper right to the lower left in FIG. 10, the amount of deviation or the average value of the amount of deviation at the rear end decreases. Therefore, it is preferable to position the point O further from the roller pair than the transfer section extension line M. The above analysis and explanation also assumes a virtual truncated cone shape with the same taper amount and having the nip line N as the center line, which corresponds to the nip line N shown in FIG. If so, it applies as is. Therefore, such a case is included in addition to the case where the present invention does not have the intersection angle. Therefore, in the case where there is a crossing angle, the generating line in the claims is the generating line of this virtual truncated cone. Next, specific examples of the present invention will be explained. In this specific example, the roller pair was made of S45C tool steel that was induction hardened and had a diameter of 36 mm and had its surface treated with hard chrome.
That is, in Fig. 11, α=1.8°, θ=
0.9°, r ₃ = r ₄ = 35.29 mm, r′ ₃ ratio ₄ = 36 mm, length
It was set to 270mm. As a result, even when paper P of 257 x 364 mm was fed, the amount of deviation in the width direction at the trailing edge could be suppressed to within 1 mm. As is clear from the figure,
The distance from the leading edge of the recording material to the nip portion N is l ₁ < l ₂ ,
The small diameter _r4 side is located on the _l1 side. This value is 1/10 to 1/12 compared to when the roller shape is not tapered.
The amount of deviation was very large. The experimental results are shown below.

[Table] Sample images 1, 2, and 3 in the above table represent three-point measurements, and the average of the straight roller shows only the results because there was almost no difference between the three-point measurements. Also, the "roller of the present invention" in the table
is just an example, and the minus sign in the table indicates the deviation in the direction shown in Figures 12 and 13 by straight line BH.
This is for times that are determined to be positive. Here, perpendicularity indicates how much the parallelism deviates at the trailing edge of the paper with respect to a straight line parallel to the leading edge of the paper, and linearity indicates the straightness of a line perpendicular to the leading edge of the paper. Yes, and the difference between A and B is linearity,
A is when considering the entire length of the paper, and B is when considering the distance from which the paper passes through the transfer and reaches the fixing unit as a straight line.
This is a value when considering only the deviation after that. Also, e and f are the 12th
The straight roller is shown in FIG. 13. Regarding the straight roller, the reason why the inclination changes as shown by e and f is that when the leading edge of the paper enters the nip, the paper does not immediately incline as described above, but gradually inclines at right angles to the nip. This is to get closer to the desired direction. Furthermore, in addition to correcting the deflection of the rollers themselves by changing the intersection angle of the rollers to compensate for the unevenness of the applied pressure between the rollers, we have also made the rollers into an inverted crown shape to ensure uniform pressure at the nip and improve conveyance. was able to improve. In other words, by making the roller diameter asymmetrical with a taper shape, it is possible to correct the negative effects caused by skewing the paper P, and by making it into an inverted crown shape, it is possible to equalize the pressure distribution and make paper wrinkles less likely to occur. It became. A specific example of this is shown below. The above diameter is at both ends
35.29mm and 36.00mm tapered rollers from both ends
When we created an inverted crown shape by drawing a hyperboloid with a diameter smaller than 23μ at the center in a 72mm area, the average variation in pressure distribution was 12mm.
1.5 in 95% area when applying a load of Kg/cm
Kg/cm or less, and the frequency of paper P wrinkles improved from 0.01% to 0.005% compared to the case without an inverted crown. The present invention is not limited to a truncated conical roller in the strict sense, but may be a roller with a crown or an inverted crown, as long as the roller diameter as a whole decreases or increases from one side to the other, i.e. The effect of the present invention can be obtained as long as it has a substantially truncated conical shape. Although this embodiment has been described using a pressure fixing device as an example, the present invention can also be applied to a heat fixing device. Furthermore, the present invention is intended to prevent an adverse effect on the transfer section due to the conveyance force of a pair of rollers provided downstream of the transfer section, and is therefore not limited to fixing devices; Any pair of rollers having conveying force can be used. Effects of the Invention As explained above, according to the present invention, the outer diameters of the left and right rollers are different, the circumferential speed is different, and the position of the intersection of the generatrix of the roller and the extension line of the contact line between the rollers is set to the extension line of the transfer section. By placing it near the extension line of M, it is possible to reduce the deviation of the image on the recording material.

[Brief explanation of the drawing]

1 to 3 are explanatory diagrams of the conventional example, FIG. 4 is a plan view illustrating the movement of the recording material when the nip line N of the roller pair is not perpendicular to the traveling direction of the recording material, and FIG.
The figure is a plan view illustrating the state of image transfer when the roller pair of Figure 4 is used in a recording device, and Figure 6 is a plan view of the image transfer state when the roller pair of Figure 4 is used in a recording device.
Plan view showing the image obtained by the device shown in Figure 7.
The figures show a pair of rollers used in the present invention, FIG. 8 is a plan view illustrating the movement of the recording material when a truncated conical roller is used, and FIG. 9 is a diagram showing the pair of rollers shown in FIG. 8 used in a recording device. FIG. 10 is a plan view showing an image obtained by the apparatus shown in FIG. 9, and FIG. 11 is a plan view explaining a specific example of the present invention. FIGS. 12 and 13 are plan views of recording materials for explaining data of experimental results. Explanation of symbols: 3: first roller, 4: second roller, 6: roller pair, 5: photoreceptor, P: recording material.

Claims (1)

[Scope of Claims] 1. An image forming apparatus that transfers an image onto a recording material, comprising: a substantially truncated conical first roller provided downstream of the image transfer position with respect to the recording material conveyance direction; is in the shape of a truncated cone, and its small diameter side is connected to the first
a second roller that contacts the first roller so as to be on the small diameter side of the roller and has a function of at least feeding the recording material at the corresponding contact portion, and between the generatrix of the roller and the roller. An image forming apparatus characterized in that an intersection point of an extension line of the line of contact with the line of contact is located near a straight line that is perpendicular to the direction in which the recording material travels through the image transfer section.