JP2004029563A - Fixing device and image forming device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fixing device which properly corrects curling of a sheet material in both-surface image formation and multiple-image formation. <P>SOLUTION: The device is equipped with a driving part 57 which drives a heat roller and a pressure roller independently of each other, a setting part which sets a peripheral speed difference between the heat roller and the pressure roller, an image information detecting means 30 of detecting image information of an image formed on the sheet material, and a control part 56 which controls the driving part 57, and the control part 56 controls the driving part 57 according to the detection result of the image information detected by the image information detecting means 30 and an inputted image formation mode to rotate the heat roller and the pressure roller at the speed of revolution satisfying the peripheral speed difference set by the setting part. <P>COPYRIGHT: (C)2004,JPO

Description

【００８３】
【表２】

【００８４】
【表３】

【００８５】
このとき、各線速差は０≦Ｘ１≦Ｘ２≦Ｘ３の関係にあり、Ｘ１は可能な限り０に近づけ、カール補正がほとんどない用紙搬送状態であり、Ｘ２、Ｘ３の順番にカール補正力が強まる。
【００８６】
以上の構成において、トナー量（ΔＷ）に閾値（Ｗ１、Ｗ２、Ｗ３）を設け、前述の紙厚値（Ｔ）との組み合わせに対する最適な線速差Ｘを、図２のようなイメージで設定された線速差（Ｘ）設定テーブルより読み取ることにより、カール量に応じた適正なカール補正能力の制御を行うことが可能となる。
【００８７】
図２の線速差（Ｘ）設定テーブルをカール補正量算出手段５５内のメモリに記憶させておく。カール補正量算出手段５５は紙厚センサー３２より送られてくる紙厚値信号（Ｔ）と、トナー量検出手段３７によって検出された１面目と２面目の単位面積当りのトナー量差ΔＷより、最適な線速差（Ｘ）を記憶している線速差設定テーブルに基づいて決定する。
【００８８】
例えば、両面に画像形成されたあるコピーシートの紙厚がＴＡ（Ｔａ＞ＴＡ＞Ｔ１）、トナー量差がΔＷＡ（Ｗ２＜ΔＷＡ＜Ｗ３）の場合の最適線速差はＸ３であるとカール補正量算出手段５５は判断する。
【００８９】
以上の線速差制御は、普通紙に対するものであり、ＯＨＰ用紙等の樹脂フィルムのような、元々、吸湿やトナー収縮によるカールの影響を受けにくく、カール補正を必要としないシート材は、判別された時点で図２のテーブルを適用することなく、線速差０に設定すればよい。
【００９０】
周囲の温湿度環境により、カール量が変化する場合はトナー量の閾値（Ｗ１、Ｗ２、Ｗ３）を検出される温湿度に関する変数として扱い、例えば高温高湿下では、カールが大きくなる（カールし易い）ため、トナー量の閾値（横軸全体）を図２のマイナス側へ、低温低湿下では、カールが小さくなる（カールし難い）ため、その逆に図２のプラス側へ、それぞれシフトさせることにより、いかなる環境下においても適正なカール補正制御が可能となる。
【００９１】
上記表１〜表３のトナー量閾値の替りに、原稿の読取画像を感光体への書込レーザ光へ変換する際に、レーザ制御部で計数されるドット数として置換しても同様である。上記表１〜表３の線速差設定例では、加圧ローラ線速は４段階（加熱ローラ１００％に対して、加圧ローラを９８．５％〜１００％とする）に切換えているが、例えば紙厚とトナー量（またはドット数）の２個のパラメータから決まる任意の線速差を達成できるように切換えることもできる。
【００９２】
〔実施形態２〕
実施形態１では、両面記録用紙の正カール修正のみについて説明したが、両面記録がなされたシート材３は例えば１面目（排出時下面）のトナー量が２面目（排出時上面）のトナー量より多い場合には、１面目のトナーの収縮量がより大きいために、逆カール状態で排出される場合がある。
【００９３】
シート材３の逆カールは正カールに比べてシート材自体の自重によってならされるため、後処理装置や用紙搬送系における弊害は比較的に少ないが多数枚積載された時など、積載の不揃いやステイプルやパンチ穴のずれ等の不整合を生じる場合がある。そこで、実施形態２では、シート材３の逆カールに対しても適切なカール修正を行うことを可能にした。
【００９４】
すなわち、シート材３のカールが正カールの場合は実施形態１と同様に、加圧ローラ２０の線速を、算出されたカール補正量に応じて切りかえ、加熱ローラ１９の線速よりも減速方向に駆動させることによりカール補正を行う。シート材３のカールが逆カールの場合（１面目のトナー量＞２面目のトナー量）には、加熱ローラ１９の線速を算出されたカール補正量に応じて切換え、加圧ローラ２０の線速よりも減速する方向へ駆動させることによりカール補正を行うことができる（上記の図１（ａ）を用いた説明に相当する）。
【００９５】
以上説明したように、加熱ローラ１９と加圧ローラ２０を独立的に駆動させ、その線速差の切換機構を配置することにより、装置自体を大型化することなく、シート材３の正逆双方向のカール補正が可能な定着装置を構成でき、後処理装置におけるシート材３の整合性が更に向上する。
【００９６】
本実施形態の画像形成装置では両面印字が可能であり、さらに片面印字の場合でもフェースアップ出力または用紙反転によってフェースダウン出力が選択できる。従って、Ａ面とＢ面との明確な定義付けは難しいが、反転せずにそのまま排出された状態における画像面（上面）をＡ面（表面）、その反対面をＢ面（裏面）としている。
【００９７】
そのため、片面印刷が反転されて出力した場合にはＢ面（裏面）に画像情報が存在しており、また、両面印刷で最後に反転せずに出力された場合には１回目の転写画像がＢ面（裏面）、２回目の転写画像がＡ面（表面）に現れることとなる。
【００９８】
【表４】

【００９９】
表４は、以上のように定義されたＡ面及びＢ面に対する画像量に基づいたカールの発生状況を示す。画像量とカールの関係については、トナーの転写されたシート材３が熱定着されると、シート材３の冷却時にトナーが大きく収縮し、シート材３はトナーが転写された面を内側にしてカールする。そのため、画像量が多い場合（印字されるトナー量が多い場合）には、その量が多い面を内側にして収縮する。
【０１００】
表４のＡ面、Ｂ面の表欄における数値は、最大濃度を１００としたときの画像量の比率（画像量比）を示している。例えば、Ａ面の画像量比よりも、Ｂ面の画像量比が多い場合には、シート材先端及び後端が下方へ垂れ下がった（上に凸）ダウンカール（逆カール）状態なので、この場合にはシート材先端及び後端が上方に曲がった（下に凸）フェイスカール（正カール）方向となるようにカール補正を施すことになる。
【０１０１】
片面印字の場合には、シート材３の反転の有無に応じて表４の一行目または一列目のカール発生状況になる。すなわち、一行目が反転を伴わない場合で、一列目が反転を伴う場合である。表４では、ダウンカールよりもフェイスカールの発生状況を重視している。
【０１０２】
これは、端部が下側に曲がったダウンカールはシート材自重によりカールが補正される方向にあるが、端部が上側に跳ねたフェイスカールは自ら補正できないこと、また、後処理装置はフェイスカールに弱いこと等の理由により、下向きの補正（フェイスカールをダウンカール方向に向ける修正）を施すため、フェイスカールの発生状況を重視している。なお、表４中の記号で、Ｆはフェイスカール（正カール）を表し、Ｄはダウンカール（逆カール）を表す。そして、後の数値はカール量（高さ：ｍｍ）を表す。
【０１０３】
【表５】

【０１０４】
表５は、表４で得られたそれぞれのカールの発生状況に対して、そのカールを打ち消すために、定着ローラ対１９，２０への線速差設定例である。正カールの場合、逆カールの場合ともに、加熱ローラ１９の線速は１００％に固定し、加圧ローラ２０の線速を切換えている。正カールの場合は、下向きのカール補正を行うべく、上記図１（ｂ）に相当する定着ローラ対１９，２０の線速差の設定を行うとともに、加熱ローラ線速よりも加圧ローラ線速を減速することで、加圧ローラ２０からシート材３へ付与する熱量を増加し、加熱ローラ１９からシート材３へ付与する熱量との差を詰める方向にしている。
【０１０５】
一方、逆カールの場合は、上向きのカール補正を行うべく、上記図１（ａ）に相当する定着ローラ対１９，２０の線速差の設定を行うとともに、加熱ローラ線速よりも加圧ローラ線速を増速することで、加熱ローラ１９からシート材３へ付与する熱量を増加し、加熱ローラ１９に接するシート材面の収縮を促進する方向である。本例では、線速制御の簡易化のために、全ての状況で、加熱ローラ線速を１００％に固定しているが、逆カールの補正で、加圧ローラ線速を１００％とし、加熱ローラ線速を減速する線速差の設定を行うこともできる。
【０１０６】
【発明の効果】
以上の説明から明らかなように、本発明は、以下のような効果を奏する。
【０１０７】
（１）画像形成モードと画像情報とに基づいて発生するであろうカール量を予測し、そのカール量に対応して設定された適切な周速度差を満たす回転数で、定着ローラ対の駆動速度（回転線速）を制御するので、シート材に発生するカール方向及びカール量に対応した適切なカール補正を定着工程でタイミングよく行うことができ、コピー後のシート材の搬送性、積載性が向上し、シート材の後処理作業を容易かつ的確に行うことができる。
【０１０８】
（２）第１面目の画像情報をメモリに格納し、第２面目の画像情報と比較した上で、カール補正量を決定する手段を設けたので、カール除去工程の増設等によって装置構成を複雑化することなく、両面に画像が記録されたシート材に対しても、カール量及びカール方向に応じた適切なカール補正を行うことが可能となり、シート材の後処理装置やシート材搬送経路におけるシート材搬送性が向上し、ソート、ステイプル、パンチ穴あけ、製本等の後処理を安定して行えるようになる。
【０１０９】
また、両面印字を行なわない場合でも、シート材の反転を伴わずに排出（フェイスアップ排出）させるか、あるいは反転を行なって排出（フェイスダウン排出）させるかで、シート材の搬送性や後処理に及ぼす不具合の程度が異なるので、片面印字の場合でも反転パス内においてシート材がフラットな状態に維持されるため、搬送性が向上する。
【０１１０】
（３）画像濃度が高く、シート材へ転写されるトナー量が多くなるほど、シート材の排出後のトナー収縮がシート材に与える影響が大きく、カール量が増大するため、画像情報として画像濃度を反映することによって、適切なカール補正を行なうことができる。
【０１１１】
（４）シート材両面のトナー量を比較考量してカール補正を行なうので、両面印字の場合に適切なカール補正を行なうことができる。
【０１１２】
（５）両面印字が可能とされる場合、シート材の表面及び裏面での画像量差、すなわち画像形成に使用されるトナー量差によって、トナー量の多い面を内側にしてカールが発生するため、このカール方向の逆方向にシート材を湾曲させることによって、適切なカール補正を行なうことができる。
【０１１３】
（６）像露光後の感光体電位を測定することによって、予め求まる感光体電位と感光体へのトナー付着量との関係に基づいて、２値画像を形成する場合のカール発生量をより正確に予測することができ、シート材の搬送、後処理装置等に不具合を与えることのない適切なカール補正を施すことができる。
【０１１４】
（７）画素データから画素数を計数することによって、予め求まる１画素のレーザ照射光量とトナー付着量との関係に基づいて、多値画像を形成する場合のカール発生量をより正確に予測することができるので、シート材の搬送、後処理装置等に不具合を与えることのない適切なカール補正を施すことができる。
【図面の簡単な説明】
【図１】本発明の実施形態に係る定着装置のカール発生原理の説明図である。
【図２】同定着ローラ対間の線速差制御のための線速差設定テーブルである。
【図３】同定着ローラ対を駆動制御するための制御系統ブロック図である。
【図４】同別の制御系統ブロック図である。
【図５】同カール発生原理を別の観点から説明するための説明図である。
【図６】同画像形成装置の構成説明図である。
【符号の説明】
１３−定着装置
１９−加熱ローラ
２０−加圧ローラ
３０−画像情報検出手段
３１〜３４−種類情報検出手段
５６−制御部，制御手段
５７−駆動部[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a fixing device provided in an image forming apparatus such as a printer, a copying machine, and the like, and more particularly, to a fixing device which is generated on a sheet material by forming a plurality of (multiple) images on the back surface (both surfaces) or one surface of the sheet material. The present invention relates to a fixing device capable of effectively performing curl correction and an image forming apparatus including the fixing device.
[0002]
[Prior art]
2. Description of the Related Art In an image forming apparatus such as an electrophotographic copying machine, a sheet material (transfer paper, recording paper) is used, a toner image on a drum surface is transferred to a sheet material surface via a photoreceptor drum, and a heating roller type fixing is performed. In general, a method of heating and melting a toner image transferred by an apparatus to fix the toner image, and then discharging the toner image is performed. For such an image forming apparatus, there is also provided an apparatus for automating an operation in which a device for executing post-processing such as stapling, punching, and sorting (collating) is connected.
[0003]
When these post-processing devices are used, the curl state generated in the sheet material has a great influence on the quality of the sheet alignment in the conveyance jam or the sorter device, the quality of the post-processing performance, and the like. The sheet material after the transferred toner image is heat-fixed to form an image is easily curled due to contraction of the toner and expansion due to moisture absorption of the sheet material itself. A special curl correction device or a curl correction step may be provided between them.
[0004]
Regarding the cause of the curl, since the toner transferred onto the sheet material is made of resin or the like, the shrinkage rate of the attached toner becomes larger than the shrinkage rate of the sheet material itself after fixing. For this reason, there is a difference in the shrinkage ratio between the front and back sides of the sheet material, and as a result, the sheet material is curled on the side (front side) where the toner is attached.
[0005]
In the above-described curl correction device or curl correction process, in order to always perform appropriate curl correction on the sheet material, the curl correction amount is multi-stepped according to changes in various conditions detected in the image forming apparatus. It is configured to switch. For example, Japanese Patent Application Laid-Open No. H4-251067 proposes a technique in which the curl correction amount is adjusted by detecting humidity, moisture content of a sheet material, type of paper, and paper thickness.
[0006]
Further, the toner once transferred is heated when it is fixed, and shrinks with a decrease in temperature when the toner is discharged, and since this shrinkage greatly affects the curl of the sheet material, for example, JP-A-8-217313. Japanese Patent Application Laid-Open Publication No. H11-163873 proposes a configuration in which a curl correction device that adjusts a curl correction amount according to a detection result of an image density or a toner amount is provided externally between an image forming apparatus and a post-processing apparatus. .
[0007]
In recent years, in order to make effective use of the sheet material, the mounting rate of a double-sided / multiple device capable of forming an image multiple times (multiple) on the back surface (both sides) or one side of the sheet material has increased, and the back surface of the sheet material has been increased. Opportunities to use (both sides) are also increasing. There is an advantage that various image information can be formed on a sheet material by combining an image forming apparatus capable of forming not only a monochrome (black and white) image but also a color image, and a double-sided / multiplex apparatus. The use of the image forming apparatus has become widespread by adding a sheet post-processing apparatus.
[0008]
[Problems to be solved by the invention]
However, in the conventional image forming apparatus, the control of the curl correction amount for the sheet material is performed based only on the image information recorded only on one side. The curl correction is still insufficient. That is, since the curl of the sheet material recorded on both sides is mainly caused by the difference in the amount of toner contraction between the front surface and the back surface, the curl of the sheet material used on the front surface and the back surface is determined to determine the overall curl correction amount. It is necessary to individually compare the amount of toner to be applied.
[0009]
In addition, as in the above configuration, in order to arrange the curl correction device or the curl correction process for exclusively removing the curl between the fixing device and the paper discharge tray, it is necessary to secure an installation space, and the size of the device is reduced. There was also a problem that was disadvantageous for miniaturization. Further, in the above configuration, after the sheet material is curled by the heat of the fixing device, the sheet material is conveyed to the curl removing device, and after the temperature is lowered and the curl is fixed, the curl is removed. Has become.
[0010]
Therefore, there is a problem in that the timing at the time of curl removal is inappropriate, and a sufficient effect cannot always be obtained. That is, regarding the occurrence of curl of the sheet material, the curl amount is small without the toner shrinking immediately after the sheet material passes through the fixing device, but after a while, the curl amount is reduced when the toner shrinks. Becomes large and is fixed.
[0011]
When a color image is formed using a plurality of different toners, the amount of image information (toner amount) is larger than that of a general monochrome (black and white) image. Even in the formation, the curl amount after passing through the fixing unit increases. Therefore, when a color image is formed (for example, a small number of color catalogs are created) in combination with a double-sided / multiple device and a sheet post-processing device, the sheet material cannot be transported to the intermediate tray or re-supplied. In addition, the sheet material may not be able to be adsorbed on the sheet material supporting member, and a paper jam (jam) may occur, thereby significantly reducing workability, and making it difficult to use both sides of the sheet material and post-processing the sheet. . In particular, in a color image forming apparatus, there are three or four colors of toner, and the amount of curl increases because the amount of toner used is three to four times as large as that of a monochrome image.
[0012]
The present invention has been made in view of such circumstances, and provides a fixing device and an image forming apparatus that can appropriately correct curl generated in a sheet material even during double-sided image formation or multiple image formation. Aim.
[0013]
[Means for Solving the Problems]
According to the present invention, means for solving the above-described problem are configured as follows.
[0014]
(1) A fixing device including a fixing roller pair including a heating roller and a pressure roller, wherein the fixing device heats and fuses the toner image transferred to the sheet material and fixes the toner image on the sheet material.
A drive unit for independently rotating the heating roller and the pressure roller, a setting unit for setting a peripheral speed difference between the heating roller and the pressure roller, and detecting image information of an image formed on the sheet material Image information detecting means, and a control unit for controlling the driving unit,
The control unit, based on the detection result of the image information detected by the image information detection unit and the input image forming mode, based on the rotation speed that satisfies the peripheral speed difference set by the setting unit, The driving unit is controlled to rotate the heating roller and the pressure roller.
[0015]
In this configuration, the image forming mode and the image information are input to the control device (the amount of curl to be generated is predicted), and the rotation that satisfies the set appropriate peripheral speed difference (based on the prediction) is performed. By controlling the driving speed (linear rotational speed) of the fixing roller pair by the number, appropriate curl correction corresponding to the curl direction and the curl amount generated in the sheet material can be performed in the fixing step with good timing.
[0016]
Therefore, while the toner is not solidified, appropriate curl correction can be performed in accordance with the direction and degree of curl generated in the sheet material, so that the transportability and stackability of the copied sheet material are improved, and the post-processing work of the sheet material is improved. Can be easily and accurately performed.
[0017]
(2) Image forming means for forming an image on both sides of the sheet material, type information detecting means for detecting type information on the type of the sheet material, and information on an image formed on the sheet material by the image forming means , A fixing device that heats and fuses the toner image transferred to the sheet material and fixes the toner image on the sheet material, and independently rotates a heating roller and a pressure roller of the fixing device. A driving unit, and control means for controlling the driving unit,
The control unit controls the roller of the fixing device based on the type information of the sheet material detected by the type information detection unit and the image information of the first surface and the second surface detected by the image information detection unit. It is characterized in that the linear velocity difference between pairs is changed in a plurality of steps or in multiple steps.
[0018]
In this configuration, the image information of the first surface is stored in a memory and compared with the image information of the second surface, and then the curl correction amount is determined. It is possible to perform appropriate curl correction according to the curl amount and the curl direction even for a sheet material on which images are recorded on both sides without using a sheet material. The transportability is improved, and post-processing such as sorting, stapling, punching, bookbinding, and the like can be stably performed.
[0019]
In addition, even when two-sided printing is not performed, the sheet material is transported and post-processed depending on whether the sheet material is discharged without reversing (face-up discharging) or reversed and discharged (face-down discharging). In this configuration, the sheet material is maintained in a flat state in the reversing pass even in the case of single-sided printing, so that the transportability is improved and there is no problem in post-processing.
[0020]
(3) The image information is information relating to an image density of an image formed on the sheet material.
[0021]
The higher the image density and the greater the amount of toner transferred to the sheet material, the greater the effect of toner contraction after the sheet material is discharged, and the greater the curl amount. By reflecting the reflection, appropriate curl correction can be performed.
[0022]
(4) The control unit is configured to perform the fixing based on a difference between an image density of an image formed on the first surface of the sheet material and an image density of an image formed on the second surface of the sheet material. The apparatus is characterized in that the setting of the linear velocity difference between the roller pairs of the apparatus is changed.
[0023]
When a two-sided device is added to the image forming apparatus and two-sided printing is enabled, the difference in image amount between the front and back surfaces of the sheet material, that is, the difference in the amount of toner used for image formation, causes the surface having a large amount of toner to be removed. Curling occurs on the inside. That is, the curl amount and the curl direction change depending on the amount of toner on both sides of the sheet material.
[0024]
In this configuration, curl correction is performed by comparing the amount of toner on the front surface and the amount of toner on the back surface of the sheet material, so that appropriate curl correction can be performed in double-sided printing.
[0025]
(5) Among the first and second surfaces of the sheet material, the sheet material is curved such that the sheet surface having a high image density is located outside and the sheet surface having a low image density is located inside. It is characterized by the following.
[0026]
When double-sided printing is enabled, a curl occurs with the surface having a large amount of toner inside due to a difference in image amount between the front and back surfaces of the sheet material, that is, a difference in toner amount used for image formation.
[0027]
In this configuration, an effective curl correction can be performed by curving the sheet material in a direction opposite to the curl direction (reverse curl).
[0028]
(6) The image information detecting means measures a toner amount by measuring a potential on the photoconductor, and detects image information based on the toner amount.
[0029]
In this configuration, by measuring the photoconductor potential after image exposure, the curl generation amount in forming a binary image is determined based on the relationship between the photoconductor potential and the toner adhesion amount to the photoconductor determined in advance. More accurate prediction can be performed, and appropriate curl correction can be performed without giving any trouble to the conveyance of the sheet material and the post-processing device.
[0030]
(7) The image forming unit includes a laser beam transmitting unit that transmits a laser beam based on pixel data in order to form a latent image on the photoconductor, and the image information detecting unit includes a pixel in the pixel data. The number is counted, and image information corresponding to the counting result is detected.
[0031]
In this configuration, by counting the number of pixels from the pixel data, the amount of curl generated when forming a multi-valued image can be more accurately determined based on the relationship between the amount of laser irradiation of one pixel and the amount of adhered toner, which is obtained in advance. Since the prediction can be made, it is possible to perform appropriate curl correction without giving any trouble to the conveyance of the sheet material and the post-processing device.
[0032]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, a fixing device and an image forming apparatus according to an embodiment of the present invention will be described in detail with reference to the drawings.
[0033]
(Overview of image forming apparatus)
FIG. 6 shows the overall structure of the image forming apparatus. In FIG. 6, reference numeral 1 denotes a housing, and one side of the housing 1 has a sheet material (transfer paper, recording paper) stacked in a paper feed tray 2. A paper feed unit 5 for feeding paper (paper) 3 by a paper feed roller 4 is provided. On the downstream side of the paper feed unit 5, a separation roller 7 that separates the sheet material 3 fed by the paper feed roller 4 into sheets one by one, and a pair of registration rollers 8 that determine the leading end position of the sheet material 3 by a nip portion , An image forming unit as an image forming unit is provided.
[0034]
Photoconductors 9B, 9Y, 9M, and 9C as image carriers are sequentially arranged in the image forming unit, and developing units 10B, 10Y, and 10B for developing toner images of different colors are provided on the respective photoconductors. 10M and 10C are provided. Immediately below each of the photoconductors, a transfer belt 11 that rotates along the direction in which the photoconductors 9B, 9Y, 9M, and 9C are arranged, and the photoconductors 9B and 9Y , 9M, and 9C, and transfer units 12B, 12Y, 12M, and 12C for transferring the toner images.
[0035]
The photoreceptor, the developing unit and the transfer unit are provided for each color. Hereinafter, representatively, the photoreceptor is denoted by 9, the developing unit is denoted by 10, and the transfer unit is denoted by reference numeral 12. The developing unit 10 omits the image forming means and shows only an outline of the developing device. The developing unit 10 charges the outer periphery of the photoconductor 9 and irradiates the charged portion with laser light to form an electrostatic latent image. This is a known device that visualizes the electrostatic latent image with toner.
[0036]
On the downstream side of the transfer belt 11, a fixing device (fixing unit, fixing device) 13 and a gate 15 as a conveyance direction switching means for switching the conveyance direction of the sheet material 3 by a rotational movement are arranged. On the other side of the housing 1, a paper discharge receiver 6 is provided. Further, below the belt conveyor 11, there is provided an intermediate tray 16 for temporarily storing the sheet material 3 guided by the gate 15 after the sheet material 3 is inverted without passing through the S-shaped path, and is discharged. A re-feeding path 17 for guiding the sheet material 3 from the intermediate tray 16 to the upstream side of the registration roller 8 is provided.
[0037]
The re-feeding path 17 is provided with a pre-heating means 18 for pre-heating the non-transfer-side surface (the second surface which is the lower surface in FIG. 6) of the sheet material 3 re-fed from the intermediate tray 16. I have. The fixing device 13 includes a heating roller (fixing roller) 19 in which the outer periphery of a cylindrical core having a built-in heater is covered with silicon rubber, and the outer periphery of a cylindrical core having a built-in heater is formed of silicon rubber or fluorine. The pressure roller 20 covered with rubber is configured to be elastically urged and contacted.
[0038]
In such a configuration, in the double-sided printing mode, the sheet material 3 initially fed from the paper feeding unit 5 is transported by the registration roller 8 and the transfer belt 11 that rotate in synchronization with the operation of the developing unit 10, and the transport is performed. In the process, the toner image on the photoreceptor 9 is transferred onto the first surface of the sheet material 3 by the transfer unit 12, and subsequently, is fixed by heating by the fixing unit 13. The sheet material 3 that has passed through the fixing device 13 is changed in direction by the gate 15, conveyed along an S-shaped path, and temporarily stored in the intermediate tray 16.
[0039]
In this state, the surface on the transfer side transferred to the first surface is directed upward in FIG. 6, and the surface on the untransferred side (second surface) is directed downward. Then, before the sheet material 3 of the intermediate tray 16 is preheated by the preheating means 18 and reaches the registration roller 8 via the re-feeding path 17, the next toner image is developed on the outer peripheral surface of the photoconductor 9. .
[0040]
In the sheet material 3 re-fed by the registration rollers 8 and the transfer belt 11, the transfer surface (first surface) on which the image was previously transferred is turned downward, and the surface on the non-transfer side (second surface) is turned upward. Therefore, the toner image on the photoreceptor 9 is transferred to the second surface and fixed by the fixing unit 13 in the same manner as the previous time, and the gate 15 has been rotated clockwise by this time. After the second printing, the sheet material 3 is discharged to the discharge receiver 6.
[0041]
In the fixing process of such an image forming apparatus, the sheet material 3 is heated and contracted. However, as described above, the sheet material 3 is degraded due to the difference between the contraction rate of the sheet material itself and the contraction rate of the attached toner. The first surface (image surface) is curled (normal curl) so as to be concave. In particular, in the case of a color image, different toners are superimposed, so that the toner layer becomes thicker, and the effect of shrinkage increases, which tends to cause curling.
[0042]
Therefore, in the fixing device 13 of the present embodiment, by setting the peripheral speed difference between the fixing roller pairs 19 and 20 as described below, the shearing action generated in the sheet material 3 and the both sides of the sheet material 3 The curl of the sheet material 3 is effectively corrected by the difference in the amount of heat generated in the above. Hereinafter, a configuration for correcting curl will be described with reference to the drawings.
[0043]
(Effect of shear force)
FIGS. 1A and 1B are partially enlarged views for explaining the principle of curl removal of the fixing device 13. As shown in the figure, the fixing device 13 sandwiches the sheet material 3 between the outer peripheral surface 19a of the heating roller 19 having the radius R2 of the cross section and the outer peripheral surface 20a of the pressure roller 20 having the radius R3 of the cross section. Is configured. The upper surface S2 of the sheet material 3 shown in FIGS. 1A and 1B is an image surface to which the toner has been transferred, and the lower surface S3 on the opposite side is a back surface without an image.
[0044]
In the above configuration, the heating roller 19 and the pressing roller 20 are independently driven to rotate at a peripheral speed V2 and a peripheral speed V3 by a driving unit (not shown), and are sandwiched between the rollers 19 and 20. The sheet material 3 receives the force of F2 from the heating roller 19 on the upper surface S2 and receives the force of F3 from the pressing roller 20 on the lower surface S3, and advances at a speed V5.
[0045]
The operation for removing the curl of the fixing device 13 will be described below with reference to FIGS. 1 (a) and 1 (b) and FIGS. 5 (a) and 5 (b). FIG. 1A shows a case where the peripheral speed V2 of the heating roller 19 is set to be lower than the peripheral speed V3 of the pressing roller 20. In this case, the speed V5 at which the sheet material 3 is conveyed is set to be equal to the sheet conveying speed of another sheet conveying mechanism. Further, the peripheral speeds V2 and V3 and the transport speed V5 are set so as to satisfy the relationship of V3> V5 ≒ V2.
[0046]
Under such conditions, the frictional force acting between the sheet material 3 and the heating roller 19 has a relationship of the conveying speed V5 of the sheet material 3３the peripheral speed V2, and therefore, the frictional force F2 shown in FIG. The static friction force is as follows. On the other hand, since the frictional force acting between the sheet material 3 and the pressure roller 20 has a relationship of conveying speed V5 <peripheral speed V3, dynamic frictional force such as frictional force F3 shown in FIG. It has become.
[0047]
For this reason, a frictional force acts on the sheet material 3 in different directions on the upper surface and the lower surface, respectively, and a shearing force is applied to the sheet material 3. Due to the action of the shearing force, the sheet material 3 is wound around the heating roller 19 that generates a frictional force in the direction opposite to the conveying direction, and the curl toward the upper surface side is promoted. Such curl toward the recording surface is referred to as face curl (normal curl).
[0048]
FIG. 1B shows a case where the peripheral speed V2 of the heating roller 19 is set to be higher than the peripheral speed V3 of the pressure roller 3. In this case, the speed V5 at which the sheet material 3 is conveyed is set so as to be equal to the sheet conveying speed of the other sheet conveying mechanism, and the peripheral speeds V2 and V3 and the conveying speed V5 satisfy the relationship of V3 <V2 ≒ V5. It is set to meet.
[0049]
Under such conditions, the frictional force acting between the sheet material 3 and the heating roller 19 has a relationship of conveying speed V5 ≒ peripheral speed V2. Therefore, the frictional force shown in FIG. It is like F2. On the other hand, the frictional force acting between the sheet material 3 and the pressure roller 20 has a relationship of conveying speed V5> peripheral speed V3, so that the frictional force of the frictional force F3 shown in FIG. It has become.
[0050]
Therefore, a frictional force acts on the sheet material 3 in different directions on the upper surface and the lower surface, and a shearing force is applied to the sheet material 3. By the action of the shearing force, the sheet material 3 is wound around the pressure roller 20 that generates a frictional force in the direction opposite to the conveying direction, and the curl toward the lower surface side is promoted. Such curl toward the surface opposite to the recording surface is called back curl (reverse curl).
[0051]
In addition, the principle of the curl as described above can be described from another viewpoint as shown in FIGS. 5A and 5B (however, in the description using FIG. 1, the upper and lower roller pairs 19, Attention is paid only to the speed difference 20 and the deformation of the pressure roller 20 is not considered). Also in this case, the relationship of the heating roller peripheral speed V2 ≒ the pressure roller peripheral speed V3 is established.
[0052]
FIG. 5A is a diagram illustrating the same situation as FIG. 1A, and shows a state in which the pressure roller 20 is more advanced than the heating roller 19. In this case, regarding the influence of the nip on the curl, in addition to the fact that the surface of the pressure roller urged in the direction of the heating roller is concavely deformed, the sheet material 3 is further pushed up in the nip portion in the direction of the heating roller. Since the paper is conveyed while being squeezed, the curl (normal curl) increases along the nip shape direction. This tendency becomes stronger as the transport speed becomes slower.
[0053]
Regarding the influence of the frictional force on the curl, the pressure roller 20 is set to have a lower surface friction coefficient (for example, covers the PFA tube) than the heating roller 19, and the toner adhering surface (electrostatic surface) is The heat roller 19 is dominant in conveying the sheet material 3 because the sheet material 3 faces the heat roller side. However, the pressure roller 20 slips (the state is slower than the advance of the heat roller 19). A frictional force acts on the sheet material 3 so that the sheet material 3 is wound around the pressure roller 20 (reverse curl). The greater the driven load of the pressure roller 20, the more the heat roller 19 is advanced, so that the effect of winding the sheet material 3 around the pressure roller 20 is enhanced.
[0054]
FIG. 5B is an explanatory view of the same situation as FIG. 1B. In this case, the heating roller 19 is more advanced than the pressing roller 20, and the sheet material 3 is pulled out. Curl along the nip shape direction is weakened.
[0055]
As is apparent from the above description, when the sheet material 3 passes between the pair of rollers 19 and 20 having different peripheral speeds, the transport speed of the sheet material 3 depends on the speed of the roller having the higher peripheral speed. Then, it is wound around the roller having the lower peripheral speed. Therefore, in a situation where the curl originally generated by the nip effect exists as a normal curl, the peripheral speed difference between the pair of fixing rollers 19 and 20 is appropriately adjusted, and the difference between the rollers 19 and 20 and the sheet material 3 is adjusted. By controlling the relationship of the frictional force between them, the reverse curl is appropriately promoted, and the corrective operation of the normal curl can be performed.
[0056]
(Additional heat difference on both sides of sheet material)
When a fixing operation in which the upper and lower fixing roller pairs 19 and 20 are rotated at the same peripheral speed is performed, the difference in the amount of water evaporation between both surfaces of the sheet material 3 is large because the heating roller 19 has a higher temperature than the pressing roller 20. Therefore, the sheet material 3 is curled. That is, since the temperature of the heating roller 19 containing the heating means is higher than that of the pressure roller 20, the amount of water evaporation from the sheet surface in contact with the heating roller 19 is smaller than the amount of water evaporation from the sheet surface in contact with the pressure roller 20. Therefore, there is a difference in the amount of fiber shrinkage of the sheet material 3 on both the front and back sides, and the sheet material 3 curls toward the heating roller 19 having a large amount of shrinkage.
[0057]
Therefore, in order to correct the curl in the direction of the heating roller, the difference in the amount of water evaporation on both surfaces of the sheet material 3 is reduced, and the difference in the amount of fiber shrinkage on both surfaces of the sheet material is reduced. In order to increase the amount of heat applied to the sheet material 3, the peripheral speed of the pressure roller 20 may be set to be lower than the peripheral speed of the heating roller 19.
[0058]
In a conventional fixing device, since a toner image is only fixed on a sheet material and conveyed, the sheet material is curled. On the other hand, in the fixing device 13 according to the present invention, by setting the peripheral speed difference between the fixing roller pair 19 and 20 together with the fixing operation as described above, the force acting in the reverse curl direction (on both sides of the sheet material). (In terms of both generated shear force balance and thermal energy balance), curling can be effectively suppressed or mitigated.
[0059]
(Control system block diagram)
3 and 4 are block diagrams of a control system for controlling the drive unit of the fixing device 13. The control system includes a curl information detecting unit 30 as an image information detecting unit, a curl correction unit, as shown in FIG. Amount calculation means 55, a drive control device (setting unit, control unit, control means) 56, and a drive unit (heating roller drive unit, pressure roller drive unit) for independently rotating heating roller 19 and pressure roller 20 The curl correction amount calculating means 55 and the drive control device 56 include an arithmetic processing unit (CPU) and storage devices (ROM, RAM).
[0060]
The image information detecting unit 30 includes, for example, a paper type sensor 31, a paper thickness sensor 32, and a paper size sensor 33 as a sheet type detecting unit or a sheet type transmitting unit. 5 (see FIG. 6) can be used to obtain information such as paper type detection (discrimination between plain paper and OHP paper), paper thickness detection, and paper size detection. it can. For detection of temperature and humidity, detection information of an environment sensor (temperature / humidity sensor) 34 as environment information transmitting means provided in the image forming apparatus may be used.
[0061]
As for the toner amount detecting means in the image information detecting means 30, for example, as shown in FIG. 4, the potential sensor 37 measures the potential on the photosensitive drum on both the front and back sides of the sheet material 3 and detects both. The configuration may be such that the amount of toner (comparison value) as image information is calculated by comparison, and, for example, as shown in FIG. A configuration in which the amount of toner (comparison value) is predicted from the number of dots (video count value) counted by the counter 35 may be employed.
[0062]
(Humidity detection means)
A typical method for detecting humidity will be described below, but the present invention is not limited to this, and other appropriate detection methods can be applied.
[0063]
1) Humidity sensor
For example, a humidity sensor for detecting the humidity in the paper storage unit may be provided in the paper storage unit (the paper supply unit 5) in the image forming apparatus main body. Alternatively, a hygrometer may be arranged at an arbitrary position in the image forming apparatus main body, and information obtained from the hygrometer may be input to the control device.
[0064]
2) Moisture content of sheet material (or paper moisture content)
The humidity in the paper storage unit and the water content (or paper moisture content) of the sheet material show a positive correlation. Therefore, it is possible to detect the paper moisture content (or paper moisture content) and replace it with the humidity in the paper storage unit. The paper moisture detecting means is capable of detecting in a short time, that is, changes in electric resistance due to moisture contained in the sheet material, absorption of infrared rays, microwaves, etc. (The amount of attenuation is detected in accordance with the rate).
[0065]
The position of the paper moisture detection means is set in a sheet feeding and conveying portion of the sheet material (for example, a registration roller pair is formed of a conductive member and is set as a part of a circuit constituting the paper moisture detection means), The moisture content of the sheet material passing therethrough may be measured. Further, the moisture content of the sheet material in a stacked state may be measured by disposing the sheet material in the vicinity of a sheet cassette containing a sheet material or a sheet feeding tray.
[0066]
[Embodiment 1]
In the present embodiment, for example, an image forming apparatus including an intermediate tray 16 as shown in FIG. In such an image forming apparatus, although an internal detailed illustration is omitted, first, an image of a document placed on the document image reading device is simultaneously transmitted via an optical reading sensor such as a CCD at the same time as an image forming start signal is turned ON. It is read into the image memory as pixel data and is temporarily stored.
[0067]
The image information of the document stored in the image memory is transferred to the sheet material 3 in accordance with the timing at which the sheet material 3 passes through the image forming section, so that the image information is transferred to the sheet material 3 as an image writing device or a laser beam transmitting unit. Is written on the photosensitive drum 9 by a laser optical system that emits a writing laser beam to the photosensitive drum 9 based on the pixel data.
[0068]
By such an operation, the sheet material 3 on which the image has been transferred is sent to the fixing device 13 along the paper transport path and is thermally fixed by the fixing device 13, and at the time of single-sided recording, the sheet material 3 is fixed and then discharged. The paper is discharged to a paper receiving tray 6 by a paper roller.
[0069]
At the time of double-sided recording, the sheet material 3 enters the branch portion by switching the gate (transport direction switching unit) 15 of the sheet transport path by an actuator (not shown). Sent to Then, the reversing roller is reversed and the reversed sheet material 3 is guided to the intermediate tray 16 and is stacked on the intermediate tray 16 with the image surface facing upward. The sheet material 3 stacked on the intermediate tray 16 is fed again by the pickup roller, and after the image is transferred and fixed by the same process as in the single-sided recording, the sheet material 3 is discharged to the discharge tray 6.
[0070]
In the image forming apparatus described above, the sheet material 3 on which single-sided recording has been performed is often discharged in a normal curl state in which the recording surface side contracts. Such a normal curl generated in the sheet material 3 causes problems such as conveyance failure in the post-processing device, stacking irregularities, misalignment such as displacement of staples and punch holes, and a reduction in the number of stacked sheets due to curl.
[0071]
The normal curl is mainly caused by the fact that the toner heated and melted in the fixing rollers 19 and 20 contracts due to air cooling after discharge. The amount of curl varies depending on the image density (toner amount), the type of paper (material, rigidity, thickness, size, direction of gap, etc.) and the surrounding temperature and humidity environment. There is a correlation between them. For example, as the image density increases and the amount of toner transferred to the sheet material 3 increases, the effect of toner contraction after discharge on the sheet material 3 increases, and the curl amount increases.
[0072]
On the other hand, the higher the rigidity of the sheet material 3 is, the less the influence of the toner contraction force is, and the curl amount is reduced. Further, even in the same sheet material 3, the higher the humidity (the higher the humidity), the more the sheet material 3 elongates due to re-absorption of moisture after the moisture is once released by the fixing rollers 19 and 20. In particular, the elongation due to moisture absorption on the non-recording surface side becomes larger than that on the recording surface side where moisture hardly penetrates due to toner fixing, and the occurrence of regular curls is increased.
[0073]
In the present embodiment, by detecting these change factors in advance as curl information, the amount of curl generated in the sheet material 3 is accurately predicted, and the fixing device 13 is used to constantly control the post-processing device and the sheet conveyance. Therefore, the curl can be corrected to a level that does not hinder the curl. That is, the setting of the linear velocity difference (peripheral velocity difference) between the pair of fixing rollers 19 and 20 for adjusting the curl correction function in the fixing device 13 is based on the sheet thickness and the toner amount (the toner adhesion amount per unit area, Curve correction is performed according to the state and degree of toner adhesion by switching over multiple steps or multiple steps based on the paper thickness and the number of dots (video count value). We are trying to do it properly.
[0074]
More specifically, when double-sided printing is selected or instructed via the operation panel of the apparatus or a terminal device connected to the outside, for example, as shown in FIG. 3 or FIG. The amount of toner (on the surface side of the material 3) (the measured potential of the photoconductor 9 measured by the potential sensor 37 or the converted value based on the dot count result by the dot counter 35) is stored in the memory 38 or 36, and the second surface By providing a sequence for comparing with the toner amount of the (back side of the sheet material 3), the curl amount is accurately predicted by the curl correction amount calculating unit 55, and the drive control device as the control unit is used to eliminate the curl. By controlling the driving of the pair of fixing rollers 19 and 20 by the control 56, appropriate curl correction can be performed as in the case of single-sided recording.
[0075]
(Example of linear velocity setting)
Regarding the drive control of the fixing roller pairs 19 and 20, for example, the linear speed of the heating roller is fixed at 100%, and the linear speed of the pressing roller is switched at a lower speed side in multiple stages or in multiple stages. A linear velocity difference can be generated between the intervals 19 and 20. FIG. 2 shows an example of a linear velocity difference setting table for controlling the linear velocity difference between the fixing roller pairs 19 and 20.
[0076]
In FIG. 2, the vertical axis indicates the sheet thickness (T). The paper thickness measured by the paper thickness sensor or the like is classified according to the threshold value, and is stored in the memory in the curl correction amount calculating means 55. Ta is the minimum paper thickness, and Tb is the maximum paper thickness. The horizontal axis indicates the toner amount. The toner amount here is a toner amount per unit area, and corresponds to an average image density.
[0077]
Therefore, when WT is the total toner amount used for image recording calculated from the video count value and the potential control value on the photosensitive drum surface, and A is the area of the entire recording area of the paper calculated from the paper size detection. The toner amount W is represented by W = WT / A.
[0078]
Further, when double-sided recording is performed, a comparison is made between the toner amount of the first surface (lower surface at the time of discharge): WD = WTD / A and the toner amount of the second surface (upper surface at the time of discharge): WU = WTU / A. Therefore, the toner amount difference ΔW, which is the final condition for determining the curl correction amount, is expressed by the following equation (1).
[0079]
ΔW = WU−WD = (WTU−WTD) / A (1)
Wa is the maximum toner amount (image density) that can be achieved, and 0 ≦ WU ≦ Wa and 0 ≦ WD ≦ Wa. Therefore, the range of the toner amount difference ΔW is expressed by the following equation (2). You.
[0080]
−Wa ≦ ΔW ≦ Wa (2)
In the case of single-sided recording, the amount of toner on the lower surface at the time of paper discharge and WD = 0 may be considered in the above, and ΔW = WU (the amount of toner on the recording surface side which is the upper surface at the time of discharge), and 0 ≦ ΔW ≦ Wa. You just have to use the range.
[0081]
The setting of the switching of the linear velocity difference X between the fixing roller pair 19 and 20 as a means for adjusting the curl correction function in the fixing device 13 is performed in multiple stages as shown in Tables 1 to 3 below, for example. Is preferred.
[0082]
[Table 1]

[0083]
[Table 2]

[0084]
[Table 3]

[0085]
At this time, each linear velocity difference has a relationship of 0 ≦ X1 ≦ X2 ≦ X3, X1 is as close to 0 as possible, the paper is in a paper transport state with little curl correction, and the curl correction force increases in the order of X2 and X3. .
[0086]
In the above configuration, threshold values (W1, W2, W3) are provided for the toner amount (ΔW), and an optimum linear velocity difference X for the combination with the above-described paper thickness value (T) is set in an image as shown in FIG. By reading from the set linear velocity difference (X) setting table, it is possible to appropriately control the curl correction capability according to the curl amount.
[0087]
The linear velocity difference (X) setting table of FIG. 2 is stored in the memory of the curl correction amount calculation means 55. The curl correction amount calculating means 55 calculates the paper thickness value signal (T) sent from the paper thickness sensor 32 and the toner amount difference ΔW per unit area of the first and second surfaces detected by the toner amount detecting means 37. The optimum linear velocity difference (X) is determined based on a linear velocity difference setting table that stores the linear velocity difference (X).
[0088]
For example, if the paper thickness of a copy sheet on which images are formed on both sides is TA (Ta>TA> T1) and the toner amount difference is .DELTA.WA (W2 <.DELTA.WA <W3), the optimal linear velocity difference is X3 and the curl correction is performed. The amount calculation means 55 makes a determination.
[0089]
The linear velocity difference control described above is for plain paper, and sheet materials that are not originally susceptible to curl due to moisture absorption or toner shrinkage and do not require curl correction, such as resin films such as OHP paper, are identified. At this point, the linear velocity difference may be set to 0 without applying the table of FIG.
[0090]
If the curl amount changes due to the surrounding temperature and humidity environment, the toner amount thresholds (W1, W2, W3) are treated as variables relating to the detected temperature and humidity. For example, under high temperature and high humidity, the curl increases (curl increases). 2, the threshold value of the toner amount (entire horizontal axis) is shifted to the minus side in FIG. 2. Under low temperature and low humidity, the curl is reduced (it is difficult to curl), and conversely, the threshold value is shifted to the plus side in FIG. 2. This allows proper curl correction control under any environment.
[0091]
In place of the toner amount threshold values in Tables 1 to 3, when the read image of the document is converted into the laser light to be written on the photosensitive member, the same applies even when the dot number is counted by the laser control unit. . In the linear velocity difference setting examples in Tables 1 to 3, the linear velocity of the pressure roller is switched to four stages (the pressure roller is 98.5% to 100% with respect to the heating roller of 100%). For example, switching can be performed so that an arbitrary linear velocity difference determined by two parameters of the paper thickness and the toner amount (or the number of dots) can be achieved.
[0092]
[Embodiment 2]
In the first embodiment, only the correction of the normal curl of the double-sided recording sheet has been described. If the amount is large, the toner may be discharged in a reverse curl state because the amount of contraction of the toner on the first surface is large.
[0093]
Since the reverse curl of the sheet material 3 is smoothed by the weight of the sheet material itself as compared with the normal curl, the adverse effects in the post-processing device and the paper transport system are relatively small, but when a large number of sheets are stacked, irregularities in the stacking may occur. Misalignment such as displacement of staples and punch holes may occur. Therefore, in the second embodiment, it is possible to perform appropriate curl correction even for the reverse curl of the sheet material 3.
[0094]
That is, when the curl of the sheet material 3 is a normal curl, the linear velocity of the pressure roller 20 is switched in accordance with the calculated curl correction amount, and the linear velocity of the heating roller 19 is reduced more in the deceleration direction than in the first embodiment. To perform the curl correction. When the curl of the sheet material 3 is reverse curl (the amount of toner on the first surface> the amount of toner on the second surface), the linear velocity of the heating roller 19 is switched according to the calculated curl correction amount, and the linear velocity of the pressure roller 20 is changed. The curl correction can be performed by driving in a direction in which the curl is slower than the speed (corresponding to the description using FIG. 1A).
[0095]
As described above, by independently driving the heating roller 19 and the pressure roller 20 and arranging the switching mechanism for the linear velocity difference, both forward and reverse of the sheet material 3 can be performed without increasing the size of the apparatus itself. A fixing device capable of correcting the curl in the direction can be configured, and the consistency of the sheet material 3 in the post-processing device is further improved.
[0096]
The image forming apparatus of the present embodiment can perform double-sided printing, and can select face-down output by face-up output or sheet reversal even in single-sided printing. Therefore, it is difficult to clearly define the A surface and the B surface, but the image surface (upper surface) in a state where the image is discharged without being inverted is referred to as the A surface (front surface), and the opposite surface is referred to as the B surface (back surface). .
[0097]
Therefore, when one-sided printing is inverted and output, image information is present on the B side (back side). When the two-sided printing is output without inverting last, the first transfer image is output. The surface B (back surface) and the second transfer image appear on the surface A (front surface).
[0098]
[Table 4]

[0099]
Table 4 shows the state of curl generation based on the image amounts for the side A and the side B defined as described above. Regarding the relationship between the image amount and the curl, when the sheet material 3 to which the toner has been transferred is thermally fixed, the toner shrinks greatly when the sheet material 3 is cooled, and the sheet material 3 is set with the toner-transferred surface inside. Curls. Therefore, when the image amount is large (when the amount of toner to be printed is large), the image is contracted with the surface having the large amount inside.
[0100]
Numerical values in the table columns of the surface A and the surface B in Table 4 indicate a ratio of an image amount when the maximum density is set to 100 (image amount ratio). For example, when the image amount ratio of the surface B is larger than the image amount ratio of the surface A, the leading end and the rear end of the sheet material are down-curved (convex upward) and are in a down-curl (reverse curl) state. The curl correction is performed so that the front end and the rear end of the sheet material are in a face curl (normal curl) direction in which the front end and the rear end are bent upward (convex downward).
[0101]
In the case of single-sided printing, the curl occurrence status in the first row or the first column of Table 4 is set according to the presence or absence of reversal of the sheet material 3. That is, the first row does not involve inversion and the first column involves inversion. In Table 4, the occurrence of face curl is more important than down curl.
[0102]
This is because the down curl whose end is bent downward is in the direction in which the curl is corrected by the weight of the sheet material, but the face curl whose end is bounced upward cannot be corrected by itself. In order to perform downward correction (correction of the face curl in the down curl direction) for reasons such as being vulnerable to curl, emphasis is placed on the occurrence of face curl. In the symbols in Table 4, F represents face curl (normal curl), and D represents down curl (reverse curl). The latter numerical value indicates the curl amount (height: mm).
[0103]
[Table 5]

[0104]
Table 5 is an example of setting a linear velocity difference between the fixing roller pair 19 and 20 in order to cancel the curl in each of the curl occurrence states obtained in Table 4. In both the case of the forward curl and the case of the reverse curl, the linear velocity of the heating roller 19 is fixed at 100%, and the linear velocity of the pressure roller 20 is switched. In the case of normal curl, in order to perform downward curl correction, the linear velocity difference between the fixing roller pair 19 and 20 corresponding to FIG. , The amount of heat applied from the pressure roller 20 to the sheet material 3 is increased, and the difference from the amount of heat applied from the heating roller 19 to the sheet material 3 is reduced.
[0105]
On the other hand, in the case of reverse curl, in order to perform upward curl correction, the linear velocity difference between the fixing roller pair 19 and 20 corresponding to FIG. Increasing the linear velocity increases the amount of heat applied from the heating roller 19 to the sheet material 3 and promotes contraction of the sheet material surface in contact with the heating roller 19. In this example, the linear velocity of the heating roller is fixed at 100% in all situations to simplify the linear velocity control. It is also possible to set a linear speed difference for reducing the roller linear speed.
[0106]
【The invention's effect】
As is clear from the above description, the present invention has the following effects.
[0107]
(1) The amount of curl that is likely to occur based on the image forming mode and image information is predicted, and the driving of the fixing roller pair is performed at a rotational speed that satisfies an appropriate peripheral speed difference set according to the curl amount. Since the speed (rotational linear speed) is controlled, appropriate curl correction corresponding to the curl direction and amount of curl generated in the sheet material can be performed in a proper timing in the fixing process, and the transportability and stackability of the sheet material after copying. And post-processing work of the sheet material can be performed easily and accurately.
[0108]
(2) Since the image information of the first surface is stored in the memory and compared with the image information of the second surface, a means for determining the amount of curl correction is provided. It is possible to perform appropriate curl correction according to the curl amount and the curl direction even for a sheet material on which images are recorded on both sides without causing the sheet material to be curled. The sheet material transportability is improved, and post-processing such as sorting, stapling, punching, bookbinding, and the like can be stably performed.
[0109]
In addition, even when two-sided printing is not performed, the sheet material is transported and post-processed depending on whether the sheet material is discharged without reversing (face-up discharging) or reversed and discharged (face-down discharging). In this case, the sheet material is maintained in a flat state in the reversal path even in the case of single-sided printing, so that transportability is improved.
[0110]
(3) As the image density is higher and the amount of toner transferred to the sheet material is larger, the influence of toner shrinkage after discharging the sheet material on the sheet material is greater, and the curl amount is increased. By reflecting the information, appropriate curl correction can be performed.
[0111]
(4) Since the curl correction is performed by comparing the toner amounts on both sides of the sheet material, appropriate curl correction can be performed in the case of double-sided printing.
[0112]
(5) When double-sided printing is enabled, curling occurs with the surface with a large amount of toner inside due to the difference in image amount between the front and back surfaces of the sheet material, that is, the difference in the amount of toner used for image formation. By curving the sheet material in a direction opposite to the curl direction, appropriate curl correction can be performed.
[0113]
(6) By measuring the photoconductor potential after image exposure, the curl generation amount in forming a binary image can be more accurately determined based on the relationship between the photoconductor potential determined in advance and the amount of toner adhering to the photoconductor. Therefore, it is possible to perform appropriate curl correction without giving any trouble to the conveyance of the sheet material and the post-processing device.
[0114]
(7) By counting the number of pixels from the pixel data, the amount of curl generation when forming a multi-valued image is more accurately predicted based on the relationship between the amount of laser irradiation of one pixel and the amount of toner adherence determined in advance. Therefore, it is possible to perform appropriate curl correction without giving any trouble to the conveyance of the sheet material, the post-processing device and the like.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram of a curl generation principle of a fixing device according to an embodiment of the present invention.
FIG. 2 is a drawing illustrating a linear velocity difference setting table for controlling a linear velocity difference between a pair of identified landing rollers.
FIG. 3 is a block diagram of a control system for controlling the drive of an identified landing roller pair.
FIG. 4 is a block diagram of another control system.
FIG. 5 is an explanatory diagram for explaining the curl generation principle from another viewpoint.
FIG. 6 is an explanatory diagram of a configuration of the image forming apparatus.
[Explanation of symbols]
13-Fixing device
19-heating roller
20-pressure roller
30-Image information detecting means
31-34-type information detecting means
56-Control Unit, Control Means
57-drive unit

Claims (7)

A fixing device comprising a fixing roller pair including a heating roller and a pressure roller, wherein the fixing device fixes the toner image transferred to the sheet material by heating and melting the toner image,
A drive unit that independently rotates the heating roller and the pressure roller,
A setting unit for setting a peripheral speed difference between the heating roller and the pressure roller,
Image information detecting means for detecting image information of an image formed on the sheet material,
A control unit that controls the driving unit,
The control unit includes:
The heating roller and the heating roller are rotated at a rotation speed that satisfies the peripheral speed difference set by the setting unit based on a detection result of the image information detected by the image information detection unit and an input image forming mode. And a control unit that controls the driving unit to rotate the pressure roller. Image forming means for forming an image on both sides of the sheet material,
Type information detecting means for detecting type information on the type of the sheet material,
Image information detecting means for detecting information about an image formed on the sheet material by the image forming means,
A fixing device for heating and melting the toner image transferred to the sheet material and fixing the toner image to the sheet material;
A driving unit that independently rotates the heating roller and the pressure roller of the fixing device,
Control means for controlling the driving unit,
The control means includes:
A linear velocity between a pair of rollers of the fixing device, based on the type information of the sheet material detected by the type information detecting unit and the image information of the first surface and the second surface detected by the image information detecting unit; An image forming apparatus characterized in that the difference is changed in a plurality of stages or in multiple stages. The image forming apparatus according to claim 2, wherein the image information is information on an image density of an image formed on the sheet material. The control unit controls a roller of the fixing device based on a difference between an image density of an image formed on a first surface of the sheet material and an image density of an image formed on a second surface of the sheet material. 3. The image forming apparatus according to claim 2, wherein the setting of the linear velocity difference between the pair is changed. The sheet material is curved such that a sheet surface having a high image density is located outside and a sheet surface having a low image density is located inside the first surface and the second surface of the sheet material. The image forming apparatus according to claim 2, wherein The image forming apparatus according to claim 2, wherein the image information detecting unit measures a toner amount by measuring a potential on the photoconductor, and detects image information based on the toner amount. The image forming unit has a laser beam transmitting unit that transmits a laser beam based on pixel data to form a latent image on the photoconductor,
The image forming apparatus according to claim 2, wherein the image information detection unit counts the number of pixels in the pixel data, and detects image information according to the counting result.

Images