Patent Publication Number: US-6907220-B2

Title: Cooling device for cooling recording sheet

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a recording sheet, and specifically to a cooling device that cools a recording sheet after the recording sheet was thermally fixed with a toner image. 
   2. Related Art 
   An electrophotographic printing device performs a developing process for developing visible images using colored particles on a surface of a recording sheet and a fixing process for fixing the visible images onto the surface of the recording sheet. Usually, toner in powder form designed to be suitable for electrophotographic printing devices is used as such colored particles. The toner fuses upon heating and fixes upon cooling. The electrophotographic printing device fixes toner images onto a recording sheet by thermal fusion by utilizing such property of the toner in the fixing process. 
   One type of conventional fixing device used in such an electrophotographic printing device includes a heat roller and a backup roller pressed against the heat roller. The heat roller and the backup roller are collectively referred to as fixing rollers. The heat roller is for generating heat and includes a metal body, which is a hollow tube formed of aluminum, and a heater housed in the metal body. Usually, a halogen lamp is used as the heater. The backup roller serves as a supporting roller and includes a metal shaft coated with a resilient layer, which deforms when pressed against the heat roller, thereby forming a nip portion. 
   When a recording sheet with a toner image formed thereon passes through the nip portion between the heat roller and the backup roller, pressure and heat are applied to toner forming the image, fusing and deforming the toner. As a result, the toner is fixed onto the surface of the recording sheet. This fixing method is called a heat roller fixing method. Here, only at least one of the two fixing rollers needs to generate heat. 
   When the recording sheet passes through the nip portion, the toner image contacts the heat roller. Accordingly, there is a danger that fused toner clings onto the surface of the heat roller, which is called offset phenomenon. If offset phenomenon occurs, toner clinging on the surface of the heat roller may be transferred onto a recording sheet during a subsequent fixing process, adversely affecting the printing result. 
   In order to overcome such a problem, the metal body of the heat roller is usually coated with a mold-releasing layer formed of fluoric resin, fluorine-containing rubber, or silicon rubber. Especially, the fluoric resin is well known in its excellent performance as a mold-releasing member, and so polytetrafluoroethylene (PTFE), perfluoroalkoxy (PFA), and the like are well used. Also, an exfoliation claw is attached to the heat roller for stripping the recording sheet off the heat roller. 
   The recording sheet and the toner fixed onto the recording sheet have substantially the same temperature as the heat roller when the recording sheet is discharged from the nip portion between the fixing rollers. Thereafter, the recording sheet gets cooled down while being transported through a sheet feed path, and then the recording sheet is discharged onto a sheet stacker. However, if the temperature of the recording sheet has not decreased to a glass transition temperature (Tg) before the recording sheet reaches the sheet stacker, then a toner-stick problem occurs. 
   That is, the fused toner is not completely fixed at a temperature higher than a glass transition temperature of the toner. If recording sheets are stacked one on the other in this condition, then the unfixed toner on a front surface of a recording sheet will stick to a rear surface of another recording sheet stacked thereon. As a result, unnecessary images may be formed on the rear surface of the adjacent sheet, or a part of image may be lost from the recording sheet, due to transfer of toner from the recording sheet to the adjacent sheet. In this manner, image quality may be degraded. 
   This problem occurs more likely in a high-speed printing device in which sufficient time is not always secured for allowing the recording sheet to cool down after being discharged before being discharged onto the sheet stacker. 
   In order to overcome such a tone-stick problem, there has been proposed to cool a recording sheet by providing a fan that blows air to the recording sheet. There has been also proposed to dispose a blower for lowering the internal temperature of the sheet stacker by supplying cool outside air into the sheet stacker. However, the fan and blower are disadvantageous in generating noise. 
   Japanese Patent-Application Publication No. HEI-4-260065 proposes to cool a recording sheet by brining the recording sheet into contact with a cooling roller on a downstream side of the fixing rollers. A heat pipe is used as the cooling roller. A recording sheet may pass through a nip portion between the cooling roller and a resilient support roller resiliently pressed against the cooling roller. Alternatively, the recording sheet may pass through a nip portion between the cooling roller and a belt in contact with the cooling roller. 
   Since moisture contained in a recording sheet evaporates when the recording sheet is heated, the humidity in the recording device increases when the fixing operation is performed to thermally fix the toner image. Using the cooling roller maintained at a cool temperature makes easier to effectively cool the recording sheet. However, using the cooling roller in such a high-humid environment causes dew condensation on the surface of the cooling roller. This causes errors in sheet transport operations and printing operations. 
   In order to prevent dew condensation on the cooling roller, Japanese Patent-Application Publication (Kokai) No. HEI-11-15308 proposes to dispose an duct above a cooling roller for ventilating a printing device by supplying outside air to the cooling roller. However, this necessitates a space for disposing a large duct and also a blower for generating air current through the duct, increasing the size of the printing device. 
   SUMMARY OF THE INVENTION 
   In view of the foregoing, it is an object of the present invention to overcome the above problems, and also to provide a compact-sized cooling device for cooling recording sheets while preventing dew condensation. 
   In order to attain the above and other objects, the present invention provides a cooling device used in an image forming device that forms images on a recording medium and thermally fixes the images on the recording medium. The cooling device includes a cooling roller and a support member that contacts the cooling roller to define a nip portion between the cooling roller and the support member. The cooling roller cools a recording medium at the nip portion sandwiched between the cooling roller and the support member. The temperature of the cooling roller is set to equal to or greater than 85° C. 
   There is also provided a cooling device used in an image forming device that forms images on a recording medium and thermally fixes the images on the recording medium. The cooling device includes a cooling roller and a support member that contacts the cooling roller to define a nip portion between the cooling roller and the support member. The cooling roller cools a recording medium at the nip portion between the cooling roller and the support member. A temperature coefficient of the cooling roller is set to 0.73 or greater, the temperature coefficient being equal to (T in −T out )/(T in −T c ) wherein T in  is a temperature of the recording medium when entering the nip portion, T out  is a temperature of the recording medium when leaving the nip portion, and T c  is the temperature of the cooling roller. 
   There is also provided an image forming device including an image forming unit that forms images on a recording medium, a fixing unit that thermally fixes the images on the recording medium, and a cooling device that cools the recording medium. The cooling device includes a cooling roller and a support member that contacts the cooing roller to define a nip portion between the cooling roller and the support member. The cooling roller cools a recording medium at the nip portion sandwiched between the cooling roller and the support member. The temperature of the cooling roller is set to equal to or greater than 85° C. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
       FIG. 1  is a plan view of an electrophotographic printing device according to a first embodiment of the present invention; 
       FIG. 2  is a graph showing change in temperature of a recording sheet after being discharged from fixing rollers according to the first embodiment of the present invention. 
       FIG. 3  is a perspective view of components of a cooling device according to a second embodiment of the present invention. 
   

   PREFERRED EMBODIMENT OF THE PRESENT INVENTION 
   Next, an embodiment of the present invention will be described with reference to the accompanying drawings. 
     FIG. 1  shows an electrophotographic printing device that uses a cooling device according to an embodiment of the present invention. As shown in  FIG. 1 , an electrophotographic printing device  1  includes an image forming unit  10 , a pair of fixing rollers  20 , a cooling device  30 , and a sheet stacker  40 , disposed from an upstream side to a downstream side in this order with respect to a sheet feed direction in which a recording sheet  15  is transported. 
   The image forming unit  10  includes a photosensitive drum  11 , a charging unit  12 , a developing unit  14 , a transfer unit  16 , a cleaner  18 , and a light source  19 . The charging unit  12  uniformly charges a surface of the photosensitive drum  11  to a uniform charge. The light source  19  includes a semiconductor laser and a light system controlled by a control unit, such as a laser driver (not shown). A light output from the light source  19  forms an electrostatic latent image on the surface of the photosensitive drum  11 . When the electrostatic latent image comes into confrontation with the developing unit  14 , then the developing unit  14  selectively supplies toner  13  to the surface of the photosensitive drum  11 , thereby forming a visible toner image corresponding to the electrostatic latent image. When the toner image reaches a transfer position where the photosensitive drum  11  confronts the transfer unit  16  via the recording sheet  15 , the toner image is transferred onto the recording sheet  15 . The cleaner  18  is for removing residual toner from the photosensitive drum  11  after the toner image has been transferred onto the recording sheet  15 . 
   The recording sheet  15  with the toner image transferred thereon is supplied to a nip portion defined between the pair of fixing rollers  20 , whereby the toner image is thermally fixed onto the recording sheet  15 . Here, one of the pair of fixing rollers  20  is a heat roller that generates heat and the other is a pressure roller that presses against the heat roller to generate the nip portion therebetween. 
   Afterwards, the recording sheet  15  is supplied into the cooling device  30 . The cooling device  30  includes a cooling roller  31  and a backup belt  32 , together defining a nip portion  33  therebetween. The cooling device  30  cools the recording sheet  15  as the recording sheet  15  passes through the nip portion  33 . After discharged from the cooling device  30 , the recording sheet  15  is discharged into the sheet stacker  40 . In this manner, an image forming process completes. 
   The toner  13  used in this embodiment has a glass transition temperature (Tg) of approximately 60° C. Therefore, it is necessary to decrease the temperature of the recording sheet  15  to less than 60° C. before the recording sheet  15  is discharged into the sheet stacker  40 . 
   The recording sheet  15  used in this embodiment is a A4-size recording sheet having a ream weight of 55 kg, which is a recording sheet having a minimum heat capacity among various recording sheets that the electrophotographic printing device  1  of the present embodiment can print on. In order to lower the temperature of the recording sheet  15  to 60° C. or less before the recording sheet  15  is stacked in the sheet stacker  40 , it is necessary to remove 130W of heat from the recording sheet  15  after the recording sheet  15  is discharged from the fixing rollers  20  before reaching the sheet stacker  40 . In this embodiment, the cooling roller  31  is configured to have a cooling capacity of 150W to have some leeway. 
     FIG. 2  shows a graph showing change in the temperature of the recording sheet  15  after being discharged from the fixing rollers  20 . The vertical axis represents an average temperature of the recording sheet  15  in its thickness direction, and the horizontal axis represents time. As shown, the recording sheet  15  has a temperature T o  immediately after the fixing operation was performed by the fixing rollers  20 , a temperature T in  when entering the nip portion  33  between the cooling roller  31  and the backup belt  32 , a temperature T out  when leaving the nip portion  33 , and a temperature T s  when discharged into the sheet stacker  40 . The cooling roller  31  is maintained at a temperature T c . 
   In the present embodiment, the recording sheet  15  has the temperature T o  of 130° C. and the temperature T in  of 126° C. The cooling roller  31  is set to the temperature T c  of 102° C. The cooling roller  31  cools the recording sheet  15  by 15° C., so that the recording sheet  15  has the temperature T out  of 111° C. As a result, at the time of reaching the sheet stacker  40 , the recording sheet  15  has the temperature T s  of 57° C., which is less than the glass transition temperature of 60° C. of the recording sheet  15 . Therefore, there is no danger of the toner stick problem. 
   With the above configuration, dew condensation does not occur on the surface of the cooling roller  31  for the following reason. 
   Dew condensation occurs when the humidity in the electrophotographic printing device  1  is higher than the saturated vapor amount for the temperature T c  of the cooling roller  31 . 
   In this embodiment, the temperature T out  is greater than the temperature T c  and less than the temperature T in  as described above (T in &gt;T out &gt;T c ). Atmospheric temperature T r  surrounding the cooling roller  31  is affected by the temperature of the recording sheet  15 , and is greater than the temperature T out  and lower than the temperature T r  (T in &gt;T r &gt;T out ). Therefore, the atmospheric temperature T r  is greater than the temperature T c  of the cooling device  30  (T c &lt;T r ). Accordingly, the moisture content of the ambient air surrounding the cooling roller  31  is less than the saturated vapor amount of the atmospheric temperature T r . 
   If the moisture content of the ambient air is less than the saturated vapor amount of the temperature T c  of the cooling roller  31 , then no dew condensation occurs. That is, if the temperature T c  of the cooling roller  31  is higher, then the dew condensation occurs less likely. Also, because a pressure surrounding the cooling roller  31  will never exceed the atmospheric pressure (1013.25 hPa), if the temperature T c  of the cooling roller  31  is 100° C. or greater, then the saturated vapor pressure is maintained greater than the atmospheric pressure, and thus no dew condensation occurs. 
   Because the temperature T c  of the cooling roller  31  is set to 120° C. in this embodiment, there is no fear of dew condensation. Accordingly, it is possible to decrease the temperature of the recording sheet  15  to 60° C. or less before the recording sheet  15  reaches the sheet stacker  40  while preventing dew condensation on the cooling roller  31  without providing a ventilation system to the cooling device  30 . 
   Next, a cooling device according to a second embodiment of the present invention will be described. It should be noted that the components of the electrophotographic printing device  1  of the present embodiment identical to that of the first embodiment will be assigned with the same numberings and detailed description thereof will be omitted. 
   In this embodiment, the recording sheet  15  is transported faster than in the first embodiment. Therefore, the time duration to transport the recording sheet  15  from the fixing rollers  20  to the sheet stacker  40  is shorter than that of the first embodiment. Accordingly, it is necessary for the cooling device  30  to remove a greater amount of heat from the recording sheet  15  in order to achieve the temperature of 60° C. or less before the recording sheet  15  reaches the sheet stacker  40 . 
   In this embodiment, it is necessary to remove 260 W heat from the A4-size recording sheet  15  having the ream weight of 55 kg after the recording sheet  15  is discharged from the fixing rollers  20  before reaching the sheet stacker  40 . The cooling device  30  of the present embodiment is configured to have a cooling capacity of 300W to have some leeway. 
   The cooling device  30  of the present embodiment is similar to that of the first embodiment but differs in that the cooling device  30  of the present embodiment further includes a ventilation system S shown in FIG.  3 . The ventilation system S does not include a large-scale ventilation device for taking in outside air, but includes a ventilation fan  34 , radiation fins  35 , and a casing  36  defining a radiation room  37 . The ventilation fan  34  is disposed to the side of the cooling roller  31  to take air into the radiation room  37  from a sheet-pass area, through which the recording sheet  15  is transported. 
   The ventilation fan  34  blows air to the radiation fins  35 , which are housed inside the radiation room  37  and attached to the cooling roller  31 . In this manner, the ventilation system S facilitates the cooling roller  31  to radiate heat so as to maintain the cooling roller  31  at a low temperature. 
   Here, the amount of heat Q 1  into the cooling roller  31  from one piece of recording sheet  15  is expressed by the formula:
 
 Q   1 = W×l×h ( T   in   −T   c )× L/v   (1)
 
   wherein, W is a width of the recording sheet  15  with respect to a widthwise direction perpendicular to the sheet feed direction; 
   l is a length of a contact area of the cooling roller  31  (width of the nip portion  33 ) which the recording sheet  15  contacts with respect to the sheet feed direction; 
   h is a cooling capacity (W/(m 2 ·° C.)) of the cooling roller  31 ; 
   T in  is a temperature of the recording sheet  15  when entering the nip portion  33 ; 
   T c  is a temperature of the cooling roller  31  during the time when the fixing is operated; 
   L is a length of the recording sheet  15  with respect to the sheet feed direction; and 
   v is a peripheral velocity of the cooling roller  31 . 
   Here (h(T in −T c )) indicates thermal flux from the recording sheet  15  to the cooling roller  31 , which is defined in the same manner as the thermal conductivity. 
   On the other hand, the amount of heat Q 2  removed from the recording sheet  15  is calculated based on the temperature decrease by the formula:
 
 Q   2 =ρ× C×δ×L×W ( T   in   −T   out )  (2)
 
   wherein ρ is a density (kg/m 3 ) of the recording sheet  15 ; 
   C is a specific heat ((J/kg·° C.)) of the recording sheet  15 ; 
   δ is a thickness of the recording sheet  15 ; 
   L is the length of the recording sheet  15  in the sheet feed direction; 
   W is the width of the recording sheet  15  with respect to a direction perpendicular to the sheet feed direction; 
   T in  is a temperature of the recording sheet  15  when entering the nip portion  33 ; and 
   T out  is a temperature of the recording sheet  15  when discharged from the nip portion  33 . 
   Because Q 1  is equal to Q 2 , the following equation is derived from the above equations (1) and (2):
 
( T   in   −T   out )/( T   in   −T   c )=( l×h )/(ρ× C×δ×v )  (3)
 
   Here, T in &gt;T out &gt;T c . 
   Dimensionless temperature coefficient for evaluating the performance of the cooling roller  31  is set to (T in −T out )/(T in −T c ), and the temperature coefficient is defined as θ, that is, θ=(T in −T out )/(T in −T c ). 
   Temperature T r  of atmosphere surrounding the cooling roller  31  (hereinafter referred to as “atmosphere temperature T r  of the cooling roller  31 ”) is ruled by the temperature of the recording sheet  15  and falls between the temperatures T in  and T out  (T in &gt;T r &gt;T out ). The moisture content of the atmosphere surrounding the cooling roller  31  is less than the saturated vapor amount for the temperature T r . If the moisture content of the atmosphere surrounding the cooling roller  31  is less than the saturated vapor amount for the temperature T c  of the cooling roller  31 , then dew condensation does not occur on the cooling roller  31 . That is, dew condensation occurs less likely if the temperature T c  is higher. Since increasing the temperature T c  decreases the cooling capacity of the cooling roller  31 , it is desirable to set the temperature T c  high while keeping a proper balance with the cooling capacity of the cooling roller  31 . 
   As described above, if the temperature T c  is equal to or greater than 100° C., then the saturated vapor pressure is greater than the atmospheric pressure, and thus no dew condensation occurs. However, it is possible to prevent dew condensation even when the temperature T c  is set less than 100° C. That is, the moisture content of the recording sheet  15  is several percents at most. Even if the moisture evaporates during the fixing operation, vapor amount in the atmosphere does not increase to the saturated vapor amount for the temperature T r , but to a saturated vapor amount for a certain temperature T w , which is less than the temperature T r . Therefore, it is possible to set the temperature T c  of the cooling roller  31  lower than the atmospheric temperature T r  as long as the temperature T c  is higher than the temperature T w . Because the temperature T w  is found 85° C. through experiments in this embodiment, the temperature T c  of the cooling roller  31  is set to 85° C. 
   In this embodiment, the recording sheet  15  has the temperature T o  of 130° C. immediately after being discharged from the fixing rollers  20 , and the temperature T in  of 126° C. The cooling roller  31  maintained at the temperature T c  of 85° C. decreases the temperature of the recording sheet  15  by 30° C. to have the temperature T out  of 96° C. This enables the recording sheet  15  to have a temperature T s  of 57° C. when reaching the sheet stacker  40 . Because 57° C. is lower than the glass transition temperature 60° C. of the toner, the toner stick problem can be prevented. 
   Here, according to the above formula (3), the temperature coefficient θ is 0.73 in this example. 
   As described above, according to the present embodiment, it is possible to make the temperature T s  of the recording sheet  15  less than the glass transition temperature of 60° C. Therefore, there is no danger of toner stick. Also, because the temperature T c  of the cooling roller  31  is set equal to or greater than the temperature T w  of 85° C., it is possible to prevent dew condensation on the cooling roller  31 . Further, there is no need to provide a mechanism to supply the cooling roller  31  with outside air so as to maintain the cooling roller  31  at relatively high temperature T c  of 85° C., but only the ventilation fan  34  is used. Accordingly, a compact-sized cooling device  30  can be provided. 
   Setting the temperature coefficient θ of the above equation (3) to a higher value enables to set the temperature T c  of the cooling roller  31  to a higher temperature, giving leeway regarding dew condensation, although it is necessary in this case to decrease the sheet transport speed v, to use a recording sheet with less density ρ, specific heat C, or thickness δ, or to increase the parameter l or h. Increasing the width l of the nip portion  33  is practical. 
   As described above, according to the present embodiment, dew condensation and toner stick problem are both prevented if the temperature T c  of the cooling roller  31  is set higher than the temperature T w  and also if the temperature coefficient of the cooling roller is set to 0.73 or greater. Because the temperature T w  differs depending on the specifications of the image forming device, it is necessary to obtain the temperature T w  beforehand through experiments. 
   While some exemplary embodiments of this invention have been described in detail, those skilled in the art will recognize that there are many possible modifications and variations which may be made in these exemplary embodiments while yet retaining many of the novel features and advantages of the invention.