JP2005227322A - Fixing device and image forming apparatus equipped with the fixing device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To eliminate faulty fixing by varying the abutting force of a belt tension member on a heating roller in accordance with paper thickness. <P>SOLUTION: The fixing device is equipped with the heating roller 1401, a pressure roller 1411 pressed against the heating roller, the belt tension member 1421 arranged on an upstream side in a paper transporting direction with respect to the pressure roller and placed on the pressure roller through a tension spring 1704, a belt 1431 wound around the outer circumferences of the pressure roller and the belt tension member and moving while forming a fixing nip with the heating roller in between, and a pre-load spring 1801 making the belt tension member abut on the heating roller, and the urging force of the pre-load spring is varied in accordance with the paper thickness. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention provides a heating roll, a pressure roll that presses against the heating roll, a belt that is attached to the outer periphery of the pressure roll and is sandwiched and moved between the heating roll, and the belt is stretched. The present invention relates to a fixing device that includes a belt stretching member and fixes an unfixed toner image formed on a sheet material, and an image forming apparatus equipped with the fixing device.

  In image forming apparatuses such as copying machines, printers, and facsimiles, as a heating roll type fixing apparatus that heats and fixes an unfixed toner image on a transfer material, the surface is covered with an elastic body and can be rotated with a built-in heating source. A heating roll, a belt stretched by a plurality of support rolls, and a belt is wound around the heating roll by a predetermined angle to form a nip region, and at the outlet of the nip region, a pressure higher than other portions is locally applied. In addition, there has been proposed a fixing device (see Patent Document 1) that is provided with a pressure unit that causes distortion on the elastic body on the surface of the heat fixing roll, and facilitates the discharge of the sheet material from the nip portion.

  In this conventional fixing device, since the surface of the heat fixing roll has distortion in advance due to the presence of pressure means, the toner is in contact with the surface of the heat fixing roll at the exit of the nip region. The surface distortion is instantaneously released. For this reason, when the sheet material is discharged from the nip portion, the adhesion force between the toner and the heat fixing roll is reduced to prevent the sheet material from being wound around the heat fixing roll. Can be easily peeled off. In this way, the above-described apparatus eliminates the peeling claw that has been conventionally required.

  However, in the fixing device structure of Patent Document 1, a movable belt stretched around a plurality of support rolls is wound around a heating roll by an angle capable of forming a nip by pressure means, and locally at the outlet of the nip region. Since it is driven by applying a large pressure, a plurality of support rolls and their rotary bearings are required. Furthermore, the perimeter of the heat-resistant belt is increased, and the fixing device is not only complicated and large, but also expensive. The complicated, large and expensive construction of the fixing device inevitably makes the image forming apparatus equipped with the fixing device complicated, large and expensive.

  In addition, the belt is heated at a nip portion with a rotatable heat fixing roll with a built-in heating source, but in a configuration in which the belt is stretched by a plurality of support rolls to increase the peripheral length, the belt moves along a predetermined path. Sometimes, heat energy is deprived by a plurality of support rolls, and natural heat dissipation increases according to the length of the circumference. For this reason, it takes a long time to reach a predetermined temperature, and a so-called warm-up time from when the power is turned on to when fixing is required is not preferable.

  Further, in the configuration in which the belt is wound around the heating roll at an angle capable of forming a nip, and a pressure greater than that of other portions is locally applied at the exit of the nip portion to form distortion in the elastic layer of the heat fixing roll, the sheet material is heated Although it is suitable for suppressing wrapping around the roll, the sheet material discharged along the strain of the elastic layer may curl following this strain or generate wrinkles due to local high pressure. Bring deformation.

  In view of this, the present applicant disclosed in Patent Document 2 that the structure of the heat roll type fixing device can be simplified, reduced in size and cost can be reduced, the warm-up time can be shortened, the stress on the sheet material can be reduced, and the occurrence of curling We have proposed a fixing device that can suppress the deformation of discharged sheet material such as the generation of wrinkles and wrinkles.

This will be described with reference to FIG. The heating roller 1401 is coated with an elastic layer 1402 on the outer periphery of the cored bar 1403 and has a heating source 1405 inside, and rotates in the direction A in the figure. A belt 1431 supported by a pressure roller 1411 and a belt stretching roller 1421 is in contact with the surface layer 1404 of the heating roller 1401. The pressure roll 1411 is coated with an elastic layer 1413 on the outer periphery of the core bar 1412 and rotates in the B direction in the drawing. A transfer material such as paper enters from the C direction and is discharged in the D direction.
Patent No. 3084692 JP2004-4234

  In FIG. 14, the heating roller 1401 is brought into contact with the pressure roller 1411 via a belt 1431, and the belt 1431 is wound around the heating roller 1401 by a belt stretching roller 1421. The belt stretching roller 1421 is in contact with the heating roller 1401 via the belt 1431. A tension force is applied to the belt tension roller 1421 in the arrow G direction with respect to the pressure roller 1411.

  While the transfer material enters from the direction of arrow C and is discharged in the direction of arrow D, the toner on the transfer material is fixed by heat and pressure. Heat is supplied from a heating roller 1401, a belt 1431, and a pressure roller 1411 that have become higher in temperature than the heating roller heat source 1405. The pressure is supplied by the contact force of the belt stretching roller 1421 to the heating roller 1401, the squeezing force of the belt 1431 to the heating roller 1401, and the contact force of the heating roller 1401 and the pressure roller 1411.

FIG. 15 is an explanatory view (cross-sectional view) of a contact method between the heating roller 1401 and the pressure roller 1411. 16 is a top view seen from the direction of arrow H in FIG. In FIG. 15 and FIG. 16, the belt stretching roller 1421 is omitted.
Since the pressure roller 1411 needs to be driven to rotate without fluctuation, the pressure roller shaft 1502 formed at both ends of the pressure roller is fixedly supported by the pressure roller fixed side plate 1505 via the bearing 1510. Has been. Note that the pressure roller 1411 can be rotated around the rotation axis by the bearing 1510. Further, the intersection point of the rotating shaft and the pressure roller fixing side plate 1505 is a point P2. Reference numeral 1509 denotes a heating roller movable gap.

On the other hand, the heating roller shaft 1501 is fixedly supported by a heating roller movable support side plate 1503 via a bearing 1507. Note that the heating roller 1401 can be rotated around the rotation axis by the bearing 1507. Further, the intersection point of the rotating shaft and the heating roller movable support side plate 1503 is a point P1. The heating roller movable support side plate 1503 is rotatably supported around the heating roller movable support side plate rotating shaft 1504.
The heating roller movable support side plate 1503 is connected to the pressure roller fixed side plate 1505 by a pressure load spring 1506. One end of the pressure load spring 1506 is fixed to the heating roller movable support side plate 1503 at a spring end fixing point 1508, and the other end is fixed to the pressure roller fixing side plate 1505 at a spring end fixing point 1511. The pressure load spring 1506 is extended by X3 (m) from the free length and fixed at both ends. At this time, the pressure load direction is an arrow E parallel to the spring.

    Here, a straight line passing through the heating roller movable support side plate rotating shaft 1504 and the spring end fixing point 1508 is defined as a line L2, and an angle formed by the line L2 and the arrow E is defined as θ1 (rad). A straight line passing through the points P1 and P2 is defined as a line L1, and an angle formed by the lines L1 and L2 is defined as θ2 (rad). Further, the distance between the heating roller movable support side plate rotating shaft 1504 and the point P1 on the line L2 is X1 (m), and the distance between the heating roller movable support side plate rotating shaft 1504 and the spring end fixing point 1508 is X2 (m).

If the load by the spring is Q1 (N), the moment M1 (Nm) applied to the spring end fixing point 1508 is
M1 ＝ Q1 × sin (θ1) × X2 (1)
On the other hand, if the pressure load applied to the heating roller 1401 and the pressure roller 1411 acting along the line L1 is Q2 (N), the moment M2 (Nm) applied to the point P1 is
M2 ＝ Q2 × sin (θ2) × X1 (2)
Here, M1 = M2 from the lever principle, so from equations (1) and (2)
Q2 ＝ Q1 × sin (θ1) / sin (θ2) × X2 / X1 (3)
Therefore, since the arrangement of the heating roller 1401 and the pressure roller 1411 is determined, in order to obtain the desired pressure load Q2, it is sufficient to apply the spring load Q1 obtained by the equation (2).
If the spring constant is K1 (N / m),
Q1 ＝ K1 × X3 (4)
Since the pressure load Q2 is applied to both ends of the heating roller 1401 and the pressure roller 1411, the pressure load applied to the entire heating roller 1401 and the pressure roller 1411 is 2 × Q2.

FIG. 17 shows an explanatory diagram (cross-sectional view) of a method of stretching the belt 1431 using the pressure roller 1411 and the belt stretching roller 1421. FIG. 18 is a top view of the pressure roller 1411, the belt stretching roller 1421 and the belt 1431 and their peripheral members, omitting the heating roller 1401 and its peripheral members in FIG.
A pressure roller tension bearing 1701 is rotatably fitted to the pressure roller shaft 1502. The pressure roller tension bearing 1701 is made of a material having a low friction coefficient, and the pressure roller 1411 is driven to rotate around the rotation axis P3.

    On the other hand, a belt tension roller tension bearing 1703 is rotatably fitted on the belt tension roller shaft 1702. The belt tension roller tension bearing 1703 is made of a material having a low friction coefficient, and the belt tension roller 1421 is supported rotatably around the rotation axis P4. Further, the pressure roller tension bearing 1701 and the belt tension roller bearing 1703 are provided with protrusions t, and both ends of the tension spring 1704 are supported by the protrusions t.

The tension spring 1704 is contracted by X4 (m) from the free length and is supported at both ends. Take it. The direction of arrow G is the same as arrow G in FIG. Here, if the spring constant of the tension spring 1704 is K2 (N / m), the tension QT (N) is
QT = K2 × X4 (5)
In FIG. 19, assuming that the direction of the pressure applied by the belt stretching roller 1421 to the heating roller 1401 and the angle formed by the force F2 in the opposite direction of the arrow J2 and the tension QT (N) is θ4, the tension QT (N) The force F2 (N) exerted on the pressurization of
F2 ＝ QT × cos (θ4) (6)
Since the tension QT is applied to both ends of the pressure roller 1411 and the belt stretching roller 1421, the total effect on the pressure is 2 × F2.

FIG. 20 is an explanatory view (cross-sectional view) of a method for contacting the belt stretching roller 1421 to the heating roller 1401 via the belt 1431.
The belt tension roller shaft 1702 is fitted with a belt tension roller tension bearing 1703. The belt tension roller tension bearing 1703 is made of a material having a low friction coefficient, and the belt tension roller 1421 is supported so as to be rotatable around the rotation axis P4. Further, the belt tension roller tension bearing 1703 is newly provided with a projection t ′ different from the projection t shown in FIGS. 17 and 18, and one end of the pressurizing spring 1801 is supported. The other end of the pressurizing spring 1801 is supported by a spring end support plate 1802. The spring end support plate 1802 is fixed and does not move.

The pressurizing spring 1801 is contracted by X5 (m) from the free length and supported at both ends, and the pressurizing direction is applied in the direction of arrow J1 parallel to the pressurizing spring 1801. On the other hand, if the direction of the line passing through the rotation axis P1 of the heating roller 1401 and the rotation axis P4 of the belt stretching roller 1421 is an arrow J2, the pressure applied to the heating roller 1401 by the belt stretching roller 1421 is the same as the arrow J2 direction. is there.
When the pressurized spring load Q3 (N), the pressurized pressure F1 (N), and the angle θ3 formed by the arrows J1 and J2,
F1 ＝ Q3 × cos (θ3) (7)
Here, if the spring constant of the pressurized spring 1801 is K3 (N / m), the pressurized spring load Q3 (N) is
Q3 ＝ K3 × X5 (8)
From equations (7) and (8)
F1 ＝ K3 × X5 × cos (θ3) (9)
Since the pressurizing force F1 is applied to both ends of the heating roller 1401 and the belt stretching roller 1421, the pressurizing force applied to the entire heating roller 1401 and belt stretching roller 1421 is 2 × F1.

FIG. 21 shows an explanatory diagram (cross-sectional view) of the influence of the rotational driving force. The force Q5 (N) is transmitted to the heating roller 1401 through the belt 1431 by the rotation driving moment M3 (Nm) of the pressure roller 1411.
If the radius of the pressure roller 1411 is X6 (m),
Q4 = M3 / X6 (10)
Here, since the rotation shaft P3 of the pressure roller 1411 is fixed, a moment M4 (Nm) in the arrow T direction with the line segment P3P4 as an arm is applied to the belt stretching roller 1421 via the belt 1431.
If the distance between the line P3P4, that is, the rotation axis P3 of the pressure roller 1411 and the rotation axis P4 of the belt stretching roller 1421 is X7 (m), the force Q5 (N) applied to the belt stretching roller is
Q5 ＝ M4 ／ X7 (11)
Because M3 = M4 from the lever principle,
Q5 ＝ Q4 × X6 / X7 (12)
Assuming that the angle formed by the arrow J2 and the arrow T in the same direction as the pressurization in FIG. 21 is θ5, the force F3 (N) exerted on the pressurization of the rotational drive moment M3 (Nm) is
F3 = Q5 × cos (θ5) (13)
Note that the rotational drive moment M3 (Nm) does not need to be doubled because it is applied to the entire rotational axis P3 of the pressure roller 1411.

FIG. 22 shows an explanatory diagram (cross-sectional view) of the influence of gravity acting on the belt tensioning roller 1421. If the weight of the belt tension roller 1421 is MR (kg), the working gravity Q6 (N) is
Q6 ＝ g × MR (14)
g: Gravity acceleration (9.8 m / s ² )
If the angle formed by the arrow J2 and the gravity Q6 (N) in the same direction as the pressurization in FIG. 22 is θ6, the force F4 (N) exerted on the pressurization of the gravity Q6 (N) is
F4 = Q6 × cos (θ6) (15)
Note that gravity Q6 (N) is applied to the entire rotation axis P4 of the belt stretching roller 1421 and need not be doubled.

Summary of Force Relationship As a summary so far, FIG. 23 shows all the forces applied to the rotation axis P4 of the belt stretching roller 1421 in the J2 direction.
The contact force F5 (N) of the belt stretching roller 1421 to the heating roller 1401 via the belt 1431 is
F5 = 2 × F1-2 × F2 + F3-F4 (16)
Table 1 shows the correspondence between design parameters and each force.

FIG. 24 shows an enlarged view of the nip portion of the fixing device shown in FIG.
The nip is a nip region Ns where the belt stretching roller 1421 and the heating roller 1401 are in contact via the belt 1431, a region Nb where the belt 1431 is only caught on the heating roller 1401, and a pressure roller via the belt 1431. It consists of a nip region Nr where 1411 and the heating roller 1401 are in contact.

Schematic diagrams of the behavior of the toner layer when the transfer material (paper) passes through the nip are shown in FIGS.
FIG. 25 shows immediately after the paper enters the nip region Ns, and FIG. 26 shows the paper passing through the nip region Ns. In FIG. 25, since the belt stretching roller 1421 is in contact with the heating roller 1401 with the contact force F5 (N) via the belt 1431, the heating roller surface layer 1404 follows the unevenness of the unfixed toner 2601.
As a result, the contact area between the heating roller surface layer 1404 and the unfixed toner 2601 increases, and in FIG. 26, the amount of heat 2702 flowing into the unfixed toner 2601 from the heating roller heat source 1405 via the heating roller 1401 increases. Due to the preheating effect, the unfixed toner 2601 becomes soft and develops viscoelasticity (a state in which it can be easily deformed when pressure is applied), resulting in a viscoelasticity expressing toner 2701. While the paper 2602 passes through the nip region Nb, heat is further transferred to the viscoelastic toner 2701 and the viscosity becomes stronger than the elasticity (deformation is possible even with a lower pressure).
FIG. 27 shows a state in which the paper is approaching the exit of the nip region Nr. The pressure load of the pressure roller 1411 and the heating roller 1401 through the belt 1431 is applied to the viscoelastic toner 2701 together with the heat 2802, and is pressed into the irregularities of the paper fibers to become a melt-bonded toner 2801.

Description of Paper Stiffness When fixing paper passes, the paper is along the heating roller 1401 in the Nb region of FIG. Therefore, due to the stiffness of the paper, a force F6 (N) that reduces the contact force F5 (N) of the belt stretching roller 1421 to the heating roller 1401 via the belt 1431 acts (see FIG. 28).
According to the formula (4.58) on page 47 of the reference "Contemporary Materials Mechanics / Toshikazu Shibuya, Hiroomi Honma, Kenji Saito / Asakura Shoten / First Edition August 20, 1986" Conceivable.
1 / ρ = M / (E × I) (17)
ρ (m): Radius of heating roller 1401, M (Nm): Bending moment, E (Pa): Elastic modulus of paper, I (m ⁴ ): Secondary moment of paper section Bending moment M (Nm) is If the length of the nip region Nb in FIG. 24 is X8 (m), the relationship with the force F6 (N) due to the stiffness of the paper in FIG.
M = F6 × X8 (18)
Also, if the paper cross-sectional secondary moment I (m ⁴ ) is paper width b (m) and paper thickness h (m), the equation (4.41) on page 41 of the above-mentioned reference
^{I = bh 3/12 ··· (} 19)

また、一般的に加熱ローラ1401サイズはプリンタのサイズに応じてφ20〜φ 60である。ニップ領域Nbは5〜50(mm)である。
したがって、表3に示したように紙のコシにより当接力F5(N)を減ずる力F6 (N)は、最もコシの弱い紙と最もコシの強い紙で1000倍の開きがあり、かつ、 加熱ローラ1401の径、ニップ領域Nbが小さいほどF6(N)が大きくなる。

On the other hand, the elastic modulus E (Pa) of the paper is measured by the method shown in FIG. That is, the paper 3003 is placed on the horizontal table 3001, and the sheet is pressed firmly from above so as to be in close contact with the horizontal table 3001 with the holding plate 3002. Next, the paper is projected from the end of the horizontal table 3001. The protrusion length L (m) at that time and the deflection amount W (m) due to the weight of the paper are measured.
When the weight per protrusion length L (m) of paper 3003 x paper width b (m) is Mp (kg), the evenly distributed load q0 (N / m) 3004 due to its own weight is
q0 ＝ Mp × g / L (20)
g: Gravity acceleration (9.8 m / s ² )
From the equation (4.84) on page 57 of the above reference,
W = q0 × L ⁴ / (8 × E × I) (21)
Since W (m), q0 (N / m), L (m), and I (m ⁴ ) are already known, E (Pa) is obtained.
Here, cardboard, thin paper, plain paper, high quality paper, recycled paper, paper fiber direction vertically, the paper fiber direction transverse, humidity paper, was measured E of the paper, such as high wet paper ^{(Pa), 1.5 × 10 9} ~ There was an opening 10 times as large as 2 × 10 ¹⁰ (Pa).
On the other hand, the paper width b is 3 times larger than 100 to 300 (mm) with an A3 size printer. The paper thickness h is slightly more than three times as large as 70 to 220 (um). As shown in Table 2, the paper with the weakest stiffness and the paper with the strongest stiffness are 1000 times wider.

In general, the size of the heating roller 1401 is φ20 to φ60 depending on the size of the printer. The nip region Nb is 5 to 50 (mm).
Therefore, as shown in Table 3, the force F6 (N) that reduces the contact force F5 (N) due to the stiffness of the paper has a 1000 times difference between the weakest paper and the strongest paper, and the heating F6 (N) increases as the diameter of the roller 1401 and the nip region Nb decrease.

Description of the problem To reduce the size of the printer, reduce the diameter of the heating roller 1401, and reduce the paper curl to shorten the Nb area to the extent that the fixing strength can be secured, as described above, F6 (N) Becomes larger. On the other hand, F6 (N) opens 1000 times depending on the stiffness of the paper.
Therefore, when fixing strong paper such as thick paper, high-quality paper, high elastic modulus, and wide paper width, the heating roller 1401 of the belt tension roller 1421 via the belt 1431 in the nip Ns region As a result, the contact force F5 (N) to the surface decreases, resulting in problems such as a decrease in gloss and poor fixing.

Schematic diagrams of the toner layer behavior at that time are shown in FIGS. FIG. 30 shows that the paper has just entered the nip area Ns, and FIG. 31 shows that the paper is passing through the nip area Ns. In FIG. 30, since the contact force F5 (N) of the belt stretching roller 1421 to the heating roller 1401 is small, the heating roller surface layer 1404 does not follow the unevenness of the unfixed toner 2601. As a result, the contact area between the heating roller surface layer 1404 and the unfixed toner 2601 is reduced, and in FIG. 31, the amount of heat flowing from the heating roller heat source 1405 to the unfixed toner 2601 through the heating roller 1401 decreases. Since the preheating effect is small, the unfixed toner 2601 becomes a toner 3101 having elasticity higher than viscosity.
While the paper 2602 passes through the nip region Nb, heat is further transferred to the toner 3101 having higher elasticity than viscosity. However, since the belt 1431 is thin and has a small heat capacity, and the contact force of the belt 1431 to the heating roller 1401 is small, the amount of heat transferred is small. Therefore, the state of the toner 3101 which is stronger than the viscosity is not changed.
FIG. 32 shows a state where the paper is approaching the exit of the nip region Nr. The pressure load of the pressure roller 1411 and the heating roller 1401 via the belt 1431 is applied to the toner 3101 having higher elasticity than viscosity with heat. However, due to the low viscosity, the toner deformation amount is small, resulting in toner 3201 with unevenness on the surface, resulting in a decrease in gloss, or poor fixing due to insufficient pressing into the fiber unevenness of the paper. Become.

  The present invention solves the above-described problem, and controls a contact force of a belt stretching member with respect to a heating roller in accordance with the type of paper, and a fixing device capable of eliminating a fixing defect and the fixing device. An object of the present invention is to provide a mounted image forming apparatus.

Therefore, the fixing device of the present invention and the image forming apparatus equipped with the fixing device include a heating roller 1401, a pressure roller 1411 pressed against the heating roller, and an upstream side in the paper conveying direction with respect to the pressure roller. A fixing nip is formed between the belt tensioning member 1421 disposed on the pressure roller via a tension spring 1704 and the heating roller and attached to the outer periphery of the pressure roller and the belt tensioning member. The moving belt 1431 and a preload spring 1801 for bringing the belt stretching member into contact with the heating roller are provided, and the urging force of the preload spring is made variable according to the paper thickness.
Further, the belt tension member is a belt tension roller.
In addition, the belt stretching member is a sliding member having a semicircular cross section.
Further, the biasing force of the preload spring is made variable according to the paper thickness and the paper width.

  According to the present invention, the contact force of the belt stretching member with respect to the heating roller can be varied according to the paper thickness, and the fixing failure can be eliminated.

  In addition, since the belt is wound around the heating roll by the cooperation of the pressure roll and the belt stretching member, a nip length can be easily formed, and the nip length can be easily increased, and the structure is simple, small and inexpensive. Can be. Further, when a belt sliding member is used as the belt stretching member, a bearing or the like is not necessary, and the support structure is simplified. In addition, when the belt stretching member is formed in a half-moon shape, the half-moon notch direction is arranged on the pressure roll side, and the arrangement close to the pressure roll is made possible to the limit. This also makes it possible to configure the belt with a reduced circumferential length. Therefore, the fixing device can be made simple and can be made small and inexpensive. Furthermore, since the belt moves in the minimum necessary path, the belt heated in the nip portion with the heating source built in and rotatable can minimize the heat energy lost when moving in the predetermined path. In addition to being able to be suppressed, since the perimeter is short, there is little temperature drop due to natural heat dissipation, and so-called warming time from when the power is turned on until it reaches a desired temperature and can be fixed can be shortened.

  Further, in order to stably fix an unfixed toner image formed on a sheet material, it is essential to sufficiently melt and fix the unfixed toner image, and a desired temperature and melting time are required. However, in the configuration according to the present invention, there is no need for a means to lengthen the nip length by greatly distorting the elastic body coated on the surface of the heat fixing roll in order to increase the nip length. It is configurable. In addition, it is not necessary to set the pressure contact pressure of the pressure roll to be large in order to distort the elastic body, and the sheet material carrying the unfixed toner image is applied to the sheet material that passes when passing between the heating roll and the belt. Since the stress is small, deformation of the sheet material such as wrinkles on the sheet material discharged after fixing the unfixed toner image is suppressed. Therefore, not only the mechanical rigidity of the fixing device need not be increased, but also the heating roll can be made thinner and the heating speed for heating the belt from the heating source is improved. Similarly, the pressure roll can also be made thin and thick, and the heat capacity can be made small, so that the heat energy absorption from the belt is small, so-called until the desired temperature is reached after the power is turned on and can be fixed. It is possible to shorten the warm-up time.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. 1 and 2 are sectional views showing an embodiment of a fixing device of the present invention. FIG. 1 shows a state when paper stiffness is weak, and FIG. 2 shows a state when paper stiffness is strong. Yes.

In this embodiment, the outer diameter of the heating roller 1401 is 25 (mm), the thickness of the heating roller elastic layer 1402 is 0.8 (mm), the material is silicone rubber, the thickness of the heating roller surface layer 1404 is 30 (um), and the material is PFA. Heat roller core metal 1403 thickness 1 (mm), iron material, pressure roller 1411 outer diameter 25 (mm), pressure roller elastic layer 1413 thickness 0.8 (mm), material silicone rubber Yes, pressure roller mandrel 1412 thickness 1 (mm), material iron, belt tension roller 1421 outer diameter 20 (mm), thickness 1 (mm), material iron, belt 1431 outer diameter 39 In this configuration, a PFA surface layer of 30 (um) is provided on a polyimide layer of (mm) and thickness 90 (um). Table 4 shows the design values of the force relationship described in Table 1.

  The steps so far are the same as those of the conventional fixing device described with reference to FIGS. 14 to 19 and FIGS. 21 to 23. In the following description, the same components may be given the same reference numerals and description thereof may be omitted. . The present invention differs from FIG. 20 in that the pressurization is variably controlled.

FIG. 1 shows a state where the stiffness of the paper is weak. The end of the pressurizing spring 1801 is supported by a movable spring end support plate 101, and the movable spring end support plate 101 is movable in the direction of arrow J3. An eccentric cam 102 is in contact with the movable spring end support plate 101 on the side opposite to the pressurizing spring 1801.
In FIG. 1, the eccentric cam 102 is in contact with the movable spring end support plate 101 with a short diameter X9 (m), and the shortened length from the free length of the pressurizing spring 1801 at that time is X5 (m). is there. The angle between the direction of arrow J3 parallel to the pressure roller 1801 and the arrow J2 of the pressure direction of the belt tension roller 1421 to the heating roller 1401 is θ3, the pressure spring load Q3 (N), the pressure F1 (N) Then,
F1 ＝ Q3 × cos (θ3) (22)
Here, if the spring constant of the pressurized spring 1801 is K3 (N / m), the pressurized spring load Q3 (N) is
Q3 ＝ K3 × X5 (23)
From equations (22) and (23)
F1 ＝ K3 × X5 × cos (θ3) (24)
The eccentric cam rotating shaft 103 is connected to a stepping motor (not shown) and its driving circuit, and rotates in the direction of the arrow U1 to transition to the state shown in FIG.

Fig. 2 shows the condition when the stiffness of the paper is strong. The eccentric cam 102 is in contact with the movable spring end support plate 101 with a long diameter X10 (m), and the shortened length from the free length of the pressurizing spring 1801 at that time is X5 + X10−X9 (m).
From equation (24), the pressure F12 (N) at this time is
F12 = K3 × (X5 + X10−X9) × cos (θ3) (25)
When the eccentric cam rotation shaft 103 rotates in the direction of the arrow U2, the state shifts to the state shown in FIG. As a result, the urging force of the preload spring 1801 can be increased.
Table 5 shows force-related design values to be added to Table 4.

(17)、(18)式より加熱ローラ1401の半径ρ(m)、ニップ領域Nbの長さX8(m) が決定していれば、紙のコシによる力F6(N)はEI(Nm²)に比例する。 Next, variables that reflect the stiffness of paper will be described. Table 6 shows a list of measured values for the elastic modulus and thickness of various papers.

If the radius ρ (m) of the heating roller 1401 and the length X8 (m) of the nip area Nb are determined from the equations (17) and (18), the force F6 (N) due to the stiffness of the paper is EI (Nm ² ).

FIG. 3 plots the relationship between EI (10 ⁻⁵ Nm ² ) and paper elastic modulus E (0.1 GPa) or paper thickness (um) for the paper in Table 6. As shown in FIG. 3, the relationship between the paper stiffness EI (10 ⁻⁵ Nm ² ) and the paper thickness (um) is smaller than the relationship between the paper elastic modulus E (0.1 GPa). This is because, from Equation (19), the paper stiffness EI (10 ^-5 Nm ² ) is proportional to the cube of the paper thickness (um), so the effect of paper thickness variation is greater than other variables. is there. Therefore, the paper thickness was selected as a variable that reflects the stiffness of the paper.
The paper thickness 120 (um) or less indicated by the point P5 has a small paper stiffness EI (10 ⁻⁵ Nm ² ) variation with respect to the paper thickness (um) variation, but when the paper thickness exceeds 120 (um), the paper thickness ( um) The paper stiffness EI (10 ^-5 Nm ² ) fluctuates with respect to fluctuations.
Therefore, when the paper thickness is 120 (um) or less, the fluctuation in influence on the contact force F5 (N) of the belt belt 1421 to the heating roller 1401 via the belt 1431 is small with respect to the paper thickness fluctuation.
That is, there is no difference in gloss at any paper thickness. However, when the paper thickness exceeds 120 (um), the fluctuation of the influence exerted on the contact force F5 (N) with respect to the fluctuation of the paper thickness increases, and the gloss fluctuation increases. Therefore, controlling the pressurization is most effective in the range where the paper thickness exceeds 120 (um).

Next, paper thickness and pressurization control will be described. Fig. 4 shows a schematic diagram of a circuit that detects the paper thickness and controls the pressurization. The paper thickness detection apparatus uses a known method such as a method using a magnetic flux change disclosed in JP-A-5-8900, a method using transmitted light according to JP2003-186264, or a method using reflected light disclosed in JP2003-329407.
When the paper thickness detection signal is received and the paper thickness is 180 (um) or less, the pressurization control circuit sends a signal to the stepping motor drive circuit, and the eccentric cam 102 is rotated by the stepping motor, as shown in FIG. To control.
On the contrary, when the paper thickness exceeds 180 (um), the pressurization control circuit sends a signal to the stepping motor drive circuit, and the eccentric cam 102 is rotated by the stepping motor to control to the state shown in FIG.

    FIG. 5 shows the gloss value when the paper of Table 6 is fixed. The gloss was measured by using a gloss meter (GM-26D FOR 75 ° / manufactured by Murakami Color Research Laboratory) with black solid printing on A4 size paper. FIG. 5 also shows a comparative example in which fixing is performed in the state of FIG. 1 regardless of the paper thickness.

Next, another embodiment of the present invention will be described with reference to FIGS. In the present embodiment, the same effect can be obtained by using a sliding member 601 having a semicircular cross section as shown in FIG. 6 instead of the belt stretching roller 1421. In the present invention, the belt tension roller 1421 and the sliding member 601 are collectively defined as a belt tension member.
As shown in FIG. 7, sliding member tension spring end support protrusions 701 are provided at both ends of the sliding member 601 to support one end of the tension spring 1704 as shown in FIG.

On the other hand, how to apply the pressurizing spring 1801 is shown in FIGS. FIG. 8 shows when paper stiffness is weak, and FIG. 9 shows when paper stiffness is strong. One end of the pressurizing spring 1801 is supported by sliding member pressurizing spring end support protrusions 801 provided at both ends of the sliding member 601. Compared with FIGS. 17, 1, and 2 where the belt tension roller shaft 1702 and the belt tension roller tension bearing 1703 are rubbed, FIG. 7 is slid with the sliding member 301 (601). Member tension Spring end support projection 701 is directly connected. 8 and 9, the sliding member 301 and the sliding member pressure spring end support protrusion 801 are directly connected.
Therefore, the tension spring 1704 does not vibrate due to vibration, dirt, wear, or the like of the belt tension roller shaft 1702 or the belt tension roller tension bearing 1703, and the tension QT (N) does not change in vibration. Therefore, there is no vibration change of the influence F2 (N) on the contact force F5 (N) of the belt stretching roller 1421 to the heating roller 1401 via the belt 1431 due to the vibration. Further, in FIGS. 17, 1 and 2, there was a case where some gloss unevenness appeared in the image, but uniform gross images were obtained with the structures of FIGS.

Next, another embodiment of the present invention will be described with reference to FIGS.
As described in equation (19), the stiffness of the paper is also affected by the paper width. Therefore, as shown in FIG. 10, in addition to the paper thickness detection device, the paper width was also detected by the paper width detection device to control the pressurization. The paper width detection device can use a known method such as detection from a paper size designation signal from the user or detection by detecting the position of the paper width fixing member 1102 of the paper feed cassette 1101 (see FIG. 11). In FIG. 11, reference numeral 1103 denotes a paper length fixing member, 1104 denotes a paper feed roller, 1105 denotes a cover, and 1106 denotes paper.

When a paper thickness detection signal is received and the paper thickness is 120 (um) or less, the pressurization control circuit sends a signal to the stepping motor drive circuit, and the eccentric cam 102 is rotated by the stepping motor, as shown in FIG. To control.
When the paper thickness exceeds 120 (um) and is 180 (um) or less and the paper width is 210 (mm) or less, the pressurization control circuit sends a signal to the stepping motor drive circuit and the stepping motor decenters it. The cam 102 is rotated to control the state shown in FIG.
When the paper thickness exceeds 120 (um) and is 180 (um) or less, and the paper width exceeds 210 (mm), the pressurization control circuit sends a signal to the stepping motor drive circuit and is biased by the stepping motor. The lead cam 102 is rotated and controlled to the state shown in FIG.
When the paper thickness exceeds 180 (um), the pressurization control circuit sends a signal to the stepping motor drive circuit, and the eccentric cam 102 is rotated by the stepping motor to control to the state of FIG.
The pressurization was controlled in the above four cases.

    FIG. 12 shows gloss values when fixing is performed on the paper of Table 6. The gloss was measured using a gloss meter (GM-26D FOR 75 ° / manufactured by Murakami Color Research Laboratory) with black solid printing on A3 size or postcard size paper. FIG. 12 also shows a comparative example in which fixing is performed in the state shown in FIG. 1 regardless of the paper thickness and paper size. According to FIG. 12, in the example, the variation of the gloss value is small and a good result is obtained.

  FIG. 13 is a schematic cross-sectional view of the overall configuration showing an embodiment of an image forming apparatus according to the present invention. In the figure, 10 is an image forming apparatus, 10a is a housing, 10b is a door, 11 is a paper transport unit, 15 is a cleaning means, 17 is an image carrier, 18 is an image transfer transport means, 20 is a developing means, and 21 is a scanner. Means, 30 is a paper feeding unit, 40 is fixing means, W is an exposure unit, and D is an image forming unit. The fixing unit 40 includes the heating roller 1, the pressure roller 2, and the belt sliding member 3 which are the above-described fixing device of the present invention.

  The image forming apparatus 10 of the present embodiment includes a housing 10a, a paper discharge tray 10c formed on the top of the housing 10a, and a door body 10b that is attached to the front surface of the housing 10a so as to be openable and closable. 1 includes an exposure unit (exposure means) W, an image forming unit D, a transfer belt unit 29 having an image transfer and conveyance means 18, and a paper feed unit 30, and a paper conveyance unit 11 is arranged in the door body 10b. ing. Each unit has a configuration that can be attached to and detached from the main body, and can be removed and repaired or replaced integrally during maintenance or the like.

  The image forming unit D includes image forming stations Y (for yellow), M (for magenta), C (for cyan), and K (for black) that form a plurality (four in this embodiment) of different color images. I have. In each of the image forming stations Y, M, C, and K, an image carrier 17 that is a photosensitive drum, a charging unit 19 that is a corona charging unit and a developing unit that are disposed around the image carrier 17 are provided. Means 20 are included. These image forming stations Y, M, C, and K are arranged in parallel on the lower side of the transfer belt unit 29 so that the image carrier 17 faces upward along an oblique arched line. The order of arrangement of the image forming stations Y, M, C, and K is arbitrary.

  The transfer belt unit 29 is disposed below the housing 10a and is driven to rotate by a drive source (not shown), a driven roll 13 disposed obliquely above the drive roll 12, a tension roll 14, An image transfer conveying means 18 composed of an intermediate transfer belt that is stretched between these three and at least two rolls and driven to circulate in the direction of the arrow in the figure, and a cleaning means 15 that contacts the surface of the image transfer conveying means 18. I have. The driven roll 13, the tension roll 14, and the image transfer / conveying means 18 are arranged in a direction inclined to the left in the drawing with respect to the drive roll 12, and thereby the belt conveying direction when driving the image transfer / conveying means 18 is downward. The belt surface 18a is positioned below, and the belt surface 18b whose transport direction is upward is positioned above.

  Accordingly, the image forming stations Y, M, C, and K are also arranged in a direction inclined to the left in the drawing with respect to the drive roll 12. Then, the image carrier 17 is brought into contact with the belt surface 18a facing downward in the transport direction of the image transfer transport unit 18 along the arched line, and is rotationally driven in the transport direction of the image transfer transport unit 18 as indicated by the arrows in the drawing. The The flexible endless sleeve-shaped image transfer / conveyance means 18 is brought into contact with the image carrier 17 from substantially the same wrapping angle so as to be covered from above, so that the image carrier 17 and the image transfer / conveyance means 18 are in contact with each other. The contact pressure and the nip width between them can be adjusted by controlling the tension applied to the image transfer / conveying means 18 by the tension roll 14, the arrangement interval of the image carrier 17, the winding angle (arch curvature), and the like. .

The drive roll 12 also serves as a backup roll for the secondary transfer roll 39. A rubber layer having a thickness of, for example, about 3 mm and a volume resistivity of 10 ⁵ Ω · cm or less is formed on the peripheral surface of the drive roll 12, and the secondary transfer roll is grounded through a metal shaft. This is a conductive path for the secondary transfer bias supplied via the line 39. Thus, by providing the drive roll 12 with a rubber layer having high friction and shock absorption, it is difficult for the impact when the sheet material enters the secondary transfer portion to be transmitted to the image transfer conveying means 18, and the image quality is deteriorated. Can be prevented. Further, by making the diameter of the driving roll 12 smaller than the diameter of the driven roll 13 and the backup roll 14, the sheet material after the secondary transfer can be easily peeled off by the elastic force of the sheet material itself. Further, the driven roll 13 is also used as a backup roll for the cleaning means 15 described later.

  The image transfer / conveying means 18 is arranged in a direction inclined to the right in the drawing with respect to the drive roll 12, and correspondingly, each of the image forming stations Y, M, C, K is also illustrated with respect to the drive roll 12. Then, it may be arranged along the oblique arch shape in the direction inclined to the right side, that is, on the left and right objects in FIG.

  Suitable materials for the image transfer / conveyance means include PC resin, PET resin, polyimide resin, urethane resin, silicon resin, polyether resin, polyester resin, etc. Of course, conductivity, rigidity, etc. For the purpose of setting the degree, friction coefficient and the like to desired characteristics, a corresponding additive or the like may be added, and the rigidity can be set to a desired rigidity by setting the thickness.

  In this embodiment, the image transfer / conveying means is formed of urethane resin and polyether resin which are relatively small in rigidity and have permanent distortion and no creep, and the tension is 40 N by the urging force of the roll, and the winding angle of the image carrier is 4 °. The contact pressure acting on the nip portion was set to about 2.8 N (= 40 N × sin 4 °), and stable transfer conditions were set. However, in consideration of the above-mentioned materials, the desired transfer conditions can be set by setting a combination of a tension of 10N to 100N and a winding angle of the image carrier of 0.5 ° to 15 ° by the urging force of the roll. It was confirmed that it was possible.

  The primary transfer member 16 is disposed at a position in contact with the inner side of the image transfer conveying means as a transfer bias applying means for forming an image by sequentially superimposing and transferring the toner images, but applying the contact pressure as described above. Therefore, it is not necessary to apply a pressing force for forming the transfer nip. Simply contact with the image transfer / conveying means as a means capable of ensuring energization, for example, a conductive roll or rigid contactor rotating in contact with the image transfer / conveying means, or a conductive elastic member such as a leaf spring, resin A conductive brush formed by a group of fibers such as can also be configured. Accordingly, the sliding resistance with the image transfer / conveying means is small, and not only can the lifetime of each other be improved, but also it can be constructed at low cost.

  As described above, in the image forming apparatus according to the present embodiment, a plurality of image carriers 17 are arranged in parallel, and the endless sleeve-like shape is flexible with a posture having substantially the same winding angle with respect to each image carrier 17. The image transfer / conveying means 18 is placed in contact and stretched around at least two rolls 12 and 13 and rotated. The image transfer / conveying means 18 is applied with tension by any of the rolls 12 and 13 to carry an image. The toner image on the body 17 is configured to be transferred in a superimposed manner. In this way, substantially the same nip is easily formed at the contact portion between the image carrier 17 and the image transfer conveying means 18 according to substantially the same winding angle, and the contact pressure at the contact portion is also substantially the same. Composed.

  On the other hand, in the image transfer member 17 and the image transfer / conveying means 18 driven in contact therewith, it is preferable that the moving peripheral speeds of the contact portions coincide with each other. It is not realistic to set the speed completely at a constant speed due to variations in the outer diameter of 17 or eccentricity or the eccentricity of the driving means, the diameter of the driving roll 12 of the image transfer / conveying means 18, or variations in the driving means.

  Therefore, when these variations are taken into consideration, the moving speed of the image transfer / conveying means 18 becomes relatively fast or slow with respect to the moving speed of the image carrier 17, and the transfer conditions are set. This is not preferable. Rather, it is preferable to provide a relative speed difference that is shifted to one of the image carrier 17 and the relative speed. However, if an extreme speed difference is provided, the toner image transported by the image carrier 17 is transferred to the image transfer transport means 18, and the position of the toner image is shifted to cause image disturbance. It is preferable to provide a speed difference.

  When the speed difference caused by the above contents is set to a relative speed difference shifted to one of the plurality of image carriers 17, the speed difference is calculated in consideration of the ability from mass production and the limit of image disturbance. The speed of the image transfer / conveying means 18 with respect to the moving speed of the image carrier 17 is preferably about ± (direction) 3 ± (variation) 2%.

  Further, when the moving speed of the image carrier 17 and the moving speed of the image transfer / conveying means 18 are constant, the toner image is transferred by the electric energy action of the transfer bias, but when the above speed difference is provided. Since the mechanical scraping action is added in addition to the electric energy action to improve the transfer efficiency, the process of cleaning the transfer residual toner on the image carrier 17 can be eliminated or simplified.

  Further, if a relative speed difference is provided between the moving speed of the image carrier 17 and the moving speed of the image transfer / conveyance means 18, the flexible image transfer / conveyance means 18 is placed between the drive rollers 12 or to the image carrier 17. It is not preferable because slack occurs between the contact nips. Therefore, when the speed of the image transfer / conveyance means 18 is shifted in the fast direction with respect to the image carrier 17, the drive roll 12 of the image transfer / conveyance means 18 is disposed on the downstream side, and When shifting the speed of the transfer / conveyance means 18 in the slow direction, if the drive roll 12 of the image transfer / conveyance means 18 is arranged on the upstream side, the occurrence of the slack can be prevented, and a preferable transfer condition can be set.

  The cleaning unit 15 is provided on the side of the belt surface 18a facing downward in the transport direction, and a cleaning blade 15a that removes toner remaining on the surface of the image transfer transport unit 18 after the secondary transfer, and a toner transport that transports the collected toner A member 15b is provided. The cleaning blade 15 a is in contact with the image transfer / conveyance means 18 at a portion where the image transfer / conveyance means 18 is wound around the driven roll 13. Further, a primary transfer member 16 is brought into contact with the back surface of the image transfer conveying unit 18 so as to face an image carrier 17 of each of the image forming stations Y, M, C, and K described later. A transfer bias is applied.

  The exposure means W is disposed in a space formed obliquely below the image forming unit D disposed in the oblique direction. A paper feeding unit 30 is disposed below the exposure unit W and at the bottom of the housing 10a. The exposure means W is entirely housed in a case, and the case is disposed in a space formed obliquely below the belt surface facing downward in the transport direction. A single scanner means 21 comprising a polygon mirror motor 21a and a polygon mirror (rotating polygon mirror) 21b is horizontally disposed at the bottom of the case, and a plurality of laser light sources 23 modulated by image signals of respective colors. In the optical system B that reflects and scans the laser beam by the polygon mirror 21b and deflects and scans the image carrier, the single f-θ lens 22 and the scanning light path of each color are not parallel to the image carrier 17 respectively. A plurality of reflecting mirrors 24 are arranged so as to be folded.

  In the exposure means W configured as described above, an image signal corresponding to each color is emitted from the polygon mirror 21b with a laser beam modulated and formed based on a common data clock frequency, and the f-θ lens 22 and the reflection mirror 24 are passed through. After that, the image carrier 17 of each image forming station Y, M, C, K is irradiated to form a latent image. By providing the reflection mirror 24, the scanning optical path can be bent, the height of the case can be lowered, and the optical system can be made compact. In addition, the reflection mirror 24 is arranged so that the scanning optical path lengths to the image carrier 17 of the image forming stations Y, M, C, and K are the same. In this way, the length of the optical path from the polygon mirror 21b of the exposure means W to the image carrier 17 to the image carrier 17 (optical path length) for each image forming unit D is configured to be substantially the same, thereby scanning in each optical path. The scanning widths of the emitted light beams are also substantially the same, and no special configuration is required for image signal formation. Therefore, the laser light source can be modulated and formed based on a common data clock frequency even though it is modulated corresponding to images of different colors by different image signals, and uses a common reflecting surface. Color misregistration caused by a relative difference in the sub-scanning direction can be prevented, and a simple and inexpensive color image forming apparatus can be configured.

  Further, in this embodiment, by arranging the scanning optical system below the apparatus, the vibration of the scanning optical system due to the vibration that the drive system of the image forming means gives to the frame that supports the apparatus can be minimized. Degradation of image quality can be prevented. In particular, by arranging the scanner means 21 at the bottom of the case, the vibration that the polygon motor 21a itself gives to the entire case can be minimized, and deterioration of the image quality can be prevented. Further, by making the number of polygon motors 21a, which are vibration sources, one, vibration applied to the entire case can be minimized.

  The sheet feeding unit 30 includes a sheet feeding cassette 35 in which sheet materials are stacked and held, and a pickup roll 36 that feeds sheet materials from the sheet feeding cassette 35 one by one. The paper transport unit 11 includes a pair of gate rolls 37 (one roll is provided on the housing 10a side) that regulates the timing of feeding the sheet material to the secondary transfer unit, the drive roll 12 and the image transfer transport means 18. A secondary transfer roll 39 serving as a secondary transfer unit pressed against the main recording medium, a main recording medium conveyance path 38, a fixing unit 40, a discharge roll pair 41, and a duplex printing conveyance path 42.

  The secondary image (unfixed toner image) secondarily transferred to the sheet material is fixed at a predetermined temperature at the nip portion formed by the fixing unit 40. In the present embodiment, the fixing unit 40 is disposed in a space formed obliquely above the belt surface 18b facing upward in the transfer belt conveyance direction, in other words, in a space opposite to the image forming station with respect to the transfer belt. Therefore, heat transfer to the exposure unit W, the image transfer conveyance unit 18, and the image formation unit can be reduced, and the frequency of performing the color misregistration correction operation for each color can be reduced. In particular, the exposure means W is located farthest from the fixing means 40, and the displacement of the scanning optical system components due to heat can be minimized, thereby preventing color misregistration.

  In the present embodiment, the image transfer / conveyance means 18 is arranged in a direction inclined with respect to the drive roll 12, so that a wide space is created in the right side space in the drawing, and the fixing means 40 can be arranged in that space. In addition, downsizing can be realized, and heat generated by the fixing unit 40 is transmitted to the exposure unit W, the image transfer conveyance unit 18 and the image forming stations Y, M, C, and K located on the left side. Can be prevented. Further, since the exposure unit W can be arranged in the space on the lower left side of the image forming unit D, the vibration of the scanning optical system of the exposure unit W due to the vibration applied to the housing 10a by the drive system of the image forming means is minimized. The image quality can be suppressed and deterioration of the image quality can be prevented.

  In this embodiment, the use of the spherical toner increases the primary transfer efficiency (approximately 100%), and each image carrier 17 is provided with a cleaning means for collecting the primary transfer residual toner. Not done. Thereby, it becomes possible to arrange each image carrier 17 composed of a photosensitive drum having a diameter of 30 mm or less close to each other, and the apparatus can be miniaturized.

  Further, the corona charging means 19 is adopted as the charging means in accordance with the absence of the cleaning means. When the charging unit is a roll, a small amount of primary transfer residual toner that is present on the image carrier 17 accumulates on the roll to cause a charging failure, but the corona charging unit 19 that is a non-contact charging unit. Can prevent toner from adhering and can prevent charging failure.

  In the above-described embodiment, the intermediate transfer belt is configured to come into contact with the image carrier 17 as the image transfer conveyance unit 18. However, the sheet material is adsorbed and conveyed to the surface, and a toner image is formed on the surface of the sheet material. A sheet material conveyance belt that forms and conveys an image by sequentially superimposing and transferring the image may be configured to contact the image carrier 17 as the image transfer conveyance means 18. In this case, the difference from each of the above embodiments is that the belt conveyance direction of the sheet material conveyance belt which is the image transfer conveyance means 18 is upward in the opposite direction on the lower surface in contact with the image carrier 17.

The outline of the operation of the entire image forming apparatus as described above is as follows.
(1) When a print command signal (image forming signal) from a host computer or the like (not shown) is input to the control unit of the image forming apparatus 10, the image of each image forming station Y, M, C, K is displayed. The carrier 17, each roll of the developing unit 20, and the image transfer / conveying unit 18 are driven to rotate.
(2) The outer peripheral surface of the image carrier 17 is uniformly charged by the charging means 19.
(3) The exposure unit W selectively exposes the outer peripheral surface of the image carrier 17 that is uniformly charged in each image forming station Y, M, C, K according to the image information of each color. An electrostatic latent image is formed.
(4) The toner image is developed by the developing means 20 from the electrostatic latent image formed on each image carrier 17.
(5) The primary transfer member 16 of the image transfer conveying means 18 is applied with a primary transfer voltage having a polarity opposite to the charged polarity of the toner, so that the toner image formed on the image carrier 17 is transferred to the primary transfer portion. As the conveying means 18 moves, the images are sequentially transferred onto the image transfer conveying means 18.
(6) In synchronism with the movement of the image transfer conveying means 18 that primarily transferred the primary image, the sheet material stored in the paper feed cassette 35 is fed to the secondary transfer roll 39 through the resist roll pair 37. Is done.
(7) The primary transfer image is a secondary transfer roll 39 that is synchronously joined with the sheet material at the secondary transfer portion and pressed toward the drive roll 12 of the image transfer conveyance unit 18 by a pressing mechanism (not shown). A bias having a polarity opposite to that of the next transfer image is applied, and the primary transfer image formed on the image transfer conveyance unit 18 is secondarily transferred to the sheet material fed in synchronization.
(8) The transfer residual toner in the secondary transfer is conveyed in the direction of the driven roll 13 and scraped off by the cleaning means 15 disposed opposite to the roll 13, and the image transfer conveyance means 18 is It is refreshed and the cycle can be repeated again.
(9) When the sheet material passes through the fixing unit 40, the toner image on the sheet material is fixed, and then the sheet material is directed to a predetermined position (if it is not double-sided printing, the double-sided printing is directed to the discharge tray 10c). In this case, it is conveyed toward the conveyance path 42 for double-sided printing).

  Although the embodiment of the present invention has been described above, the present invention is not limited to this, and a conventionally known or well-known technique can be replaced or added as necessary.

1 is a cross-sectional view showing an embodiment of a fixing device of the present invention. It is sectional drawing which shows the different state of FIG. It is a figure which shows the relationship between the stiffness of paper and the elastic modulus of paper. It is a block diagram of the control system of this invention. It is a figure which shows an experimental result. It is sectional drawing which shows other embodiment of this invention. FIG. 7 is a partial cross-sectional view of FIG. 6. It is sectional drawing which shows the detail of FIG. It is sectional drawing which shows the different state of FIG. It is a block diagram of the control system which shows other embodiment of this invention. It is a figure for demonstrating the paper width detection of FIG. It is a figure which shows the experimental result of FIG. 1 is a schematic cross-sectional view showing an embodiment of an image forming apparatus of the present invention. It is a figure for demonstrating the conventional fixing device. It is a figure for demonstrating the subject of this invention. It is a figure for demonstrating the subject of this invention. It is a figure for demonstrating the subject of this invention. It is a figure for demonstrating the subject of this invention. It is a figure for demonstrating the subject of this invention. It is a figure for demonstrating the subject of this invention. It is a figure for demonstrating the subject of this invention. It is a figure for demonstrating the subject of this invention. It is a figure for demonstrating the subject of this invention. It is a figure for demonstrating the subject of this invention. It is a figure for demonstrating the subject of this invention. It is a figure for demonstrating the subject of this invention. It is a figure for demonstrating the subject of this invention. It is a figure for demonstrating the subject of this invention. It is a figure for demonstrating the detection method of a paper stiffness. It is a figure for demonstrating the subject of this invention. It is a figure for demonstrating the subject of this invention. It is a figure for demonstrating the subject of this invention.

Explanation of symbols

1401 ... Heating roller, 1411 ... Pressure roller, 1421 ... Belt tension member, 1431 ... Belt
1704 ... Tension spring, 1801 ... Preload spring

Claims (5)

A heating roller, a pressure roller pressed against the heating roller, a belt stretching member disposed on the upstream side of the pressure roller in the paper conveyance direction, and disposed on the pressure roller via a tension spring; A belt that is attached to the outer periphery of the pressure roller and the belt stretching member and moves with a fixing nip formed between the heating roller and a preload spring that causes the belt stretching member to contact the heating roller. And a fixing device, wherein the biasing force of the preload spring is variable according to the paper thickness. The fixing device according to claim 1, wherein the belt stretching member is a belt stretching roller. The fixing device according to claim 1, wherein the belt stretching member is a sliding member having a half-moon cross section. 2. The fixing device according to claim 1, wherein the biasing force of the preload spring is made variable in accordance with a paper thickness and a paper width. An image forming apparatus comprising the fixing device according to claim 1.

Images