Patent Publication Number: US-2022229389-A1

Title: Image formation apparatus and control method therefor

Description:
The entire disclosure of Japanese patent Application No. 2020-110301, filed on Jun. 26, 2020, is incorporated herein by reference in its entirety. 
     BACKGROUND 
     Technological Field 
     The present invention relates to an image formation apparatus and a control method therefor, and, in particular, to a technique for forming an image suitable for an envelope when the envelope is a recording medium subjected to image formation. 
     Description of the Related Art 
     Conventionally, there is known an image formation apparatus such as a printer or a multifunction peripheral (MFP) that measures thickness of a sheet of printing paper, or the like, and performs setting for the sheet on the basis of the measured thickness to form an image (For example, Japanese Patent Application Laid-Open No. 2018-10056). An image formation apparatus capable of forming an image on an envelope also conventionally known (for example, Japanese Patent Application Laid-Open No. 2017-211634). 
     In recent years, there are increasing types of envelopes used for being subjected to image formation in an image formation apparatus, and there are envelopes of various thicknesses depending on the types of the envelopes. In a case of being used as a recording medium subjected to image formation by an image formation apparatus, an envelope has a problem of being susceptible to wrinkling when passing a fuser, as compared with a sheet such as ordinary printing paper. Therefore, an image formation apparatus is formed to reduce wrinkling on an envelope as a recording medium subjected to image formation by performing control different from control performed when a sheet is subjected to the image formation. 
     However, in a case where a recording medium is an envelope, a conventional image formation apparatus uniformly applies a predetermined setting value for envelope to form an image That is, the conventional image formation apparatus applies the same setting value to form an image both when a thin envelope is used and when a thick envelope is used. 
     However, in general, a thin envelope is characterized by being more susceptible than a thick envelope to wrinkling when passing a fuser. Therefore, the conventional image formation apparatus has a problem that a thin envelope as a recording medium subjected to image formation may still be wrinkled, resulting in deterioration in quality of a printed material. 
     SUMMARY 
     The present invention has been made to solve the above-described problem, and an object of the present invention is to provide an image formation apparatus and a control method therefor that are capable of appropriately forming an image according to a type of an envelope. 
     To achieve the abovementioned object, according to an aspect of the present invention, an image formation apparatus reflecting one aspect of the present invention comprises: an image former that forms an image on a recording medium delivered along a predetermined delivery path and outputs the recording medium; a first hardware processor that judges whether or not the recording medium is an envelope; a second hardware processor that acquires a basis weight of the recording medium; a third hardware processor that determines control for when an image is formed on the recording medium on the basis of a judgment result from the first hardware processor and a basis weight acquired by the second hardware processor; and a fourth hardware processor that controls operation of the image former on the basis of a determination result from the third hardware processor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The advantages and features provided by one or more embodiments of the invention will become more fully understood from the detailed description given hereinbelow and the appended drawings which are given by way of illustration only, and thus are not intended as a definition of the limits of the present invention: 
         FIG. 1  is a diagram showing an example of an external configuration of an image formation apparatus; 
         FIG. 2  is a diagram showing an example of an internal structure of an image formation unit; 
         FIGS. 3A and 3B  are diagrams showing a configuration example of an envelope detection sensor; 
         FIG. 4  is a diagram showing a configuration example of a basis weight detection sensor; 
         FIG. 5  is a diagram showing a configuration example of a loop detection sensor; 
         FIG. 6  is a block diagram showing a configuration example of a control mechanism of the image formation apparatus; 
         FIGS. 7A to 7D  are diagrams describing operation of the envelope detection sensor and the basis weight detection sensor; 
         FIGS. 8A and 8B  are diagrams exemplifying an aspect of delivering an envelope; 
         FIG. 9  is a diagram showing an example of reference information; 
         FIG. 10  is a flowchart showing an example of a processing procedure performed by a controller; 
         FIG. 11  is a flowchart showing an example of a detailed processing procedure of envelope control determination processing; 
         FIGS. 12A to 12C  are diagrams showing operation screens displayed in an operation panel; and 
         FIG. 13  is a diagram showing a configuration example of an image formation apparatus according to a third embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, one or more embodiments of the present invention will be described with reference to the drawings. However, the scope of the invention is not limited to the disclosed embodiments. In the embodiments described below, components common to each other are provided with the same reference signs, so that repeated description of these components is omitted. 
     First Embodiment 
       FIG. 1  is a diagram showing an image formation apparatus  1  according to an embodiment of the present invention. The image formation apparatus  1  shown in  FIG. 1  is formed as a multifunction peripheral (MFP) including a plurality of functions such as a copy function, a scanning function, and a print function, for example. Focusing on the print function, the image formation apparatus  1  is a type of apparatus that forms an image with an electrophotographic system, and is an apparatus capable of forming a color image by a tandem system. The image formation apparatus  1  includes an image formation unit  2  in a central portion of a main body of the apparatus. A paper feed tray  3  provided below the image formation unit  2  can accommodate recording medium such as a sheet or an envelope in advance. The image formation unit  2  includes an openable and closable manual feed tray  4  on a side surface of the main body of the apparatus. The image formation apparatus  1  feeds a recording medium set in the paper feed tray  3  or on the manual feed tray  4  one by one, forms an image on the recording medium, and ejects the recording medium to a paper ejection tray  5  provided above. 
     The image formation apparatus  1  includes an image reading unit  6  in an upper part of the main body of the apparatus. The image reading unit  6  is for reading an image of a document in a case where, for example, the copy function or the scanning function is selected. That is, the image reading unit  6  optically reads a document set by a user to generate image data. 
     Further, the image formation apparatus  1  includes an operation panel  7  serving as a user interface on a front side of the main body of the apparatus. The image formation apparatus  1  displays various operation screens that can be operated by the user on the operation panel  7 , and receives operation by the user. 
       FIG. 2  is a diagram showing an example of an internal structure of an image formation unit  2  in the image formation apparatus  1 .  FIG. 2  shows a state where the manual feed tray  4  is opened. As shown in  FIG. 2 , the paper feed tray  3  can accommodate a plurality of recording media  9 . A plurality of recording media  9  can also be set on the manual feed tray  4 . The image formation unit  2  is formed to deliver a recording medium  9  accommodated in the paper feed tray  3  or a recording medium  9  set on the manual feed tray  4 , form an image on the recording medium  9  when the recording medium  9  passes a predetermined position, fix the image, and then eject the recording medium  9  to the paper ejection tray  5 . As the recording medium  9 , a sheet such as printing paper can be used. An envelope  15  can also be used as the recording medium  9 . 
     The image formation apparatus  1  includes a controller  8  comprehensively controls image forming operation inside of the main body of the apparatus. The controller  8  control image forming operation on a recording medium  9  by controlling operation of each unit provided in the image formation unit  2 . 
     The image formation unit  2  has an image forming unit  10  that forms an image and transfer the image onto a recording medium  9 , a delivery mechanism  11  that delivers the recording medium  9 , and a fixing unit  12  that fixes the image, which is transferred onto the recording medium  9 , onto the recording medium  9 . 
     The image forming unit  10  includes a plurality of toner image generation units  20  provided to generate a toner image of each of color components of yellow (Y), magenta (M), cyan (C), and black (K), a plurality of exposure units  21  to expose each of image carriers (photoreceptor drums) provided on the respective toner image generation units  20 , an intermediate transfer belt  22  onto which toner images generated in the toner image generation units  20  are primarily transferred, and a secondary transfer roller  23  that secondarily transfers the toner images primarily transferred by the intermediate transfer belt  22  onto a recording medium  9 . 
     The image forming unit  10  forms electrostatic latent images corresponding to the respective color components on the image carriers of the respective toner image generation units  20  by driving the plurality of toner image generation units  20  and the plurality of the exposure units  21  on the basis of image data to be printed. Then, the image forming unit  10  generates a toner image on a surface of each of the image carriers by developing the electrostatic latent images with toner of the respective color components. The image forming unit  10  primarily transfers the toner images of the respective color components sequentially onto the intermediate transfer belt  22  that circularly moves in a direction of an arrow F 1 , the color components being generated in the plurality of respective toner image generation units  20 . Thus, on a surface of the intermediate transfer belt  22 , the image forming unit  10  generates a color image obtained by superimposing toner of each of the color components. Then, when the color image on the intermediate transfer belt  22  passes a position of the secondary transfer roller  23 , the image forming unit  10  secondarily transfers the color image onto a surface of a recording medium  9  delivered by the delivery mechanism  11 . 
     Ina case where the image data to be printed is a monochrome image, the image forming unit  10  causes a toner image generation unit  20  corresponding to color K to operate alone. In this case, the image forming unit  10  primarily transfers the monochrome image onto the intermediate transfer belt  22 , and then secondarily transfers the monochrome image onto a recording medium  9  when the recording medium  9  passes the position of the secondary transfer roller  23 . 
     The delivery mechanism  11  feeds a recording medium  9  from either the paper feed tray  3  or the manual feed tray  4 , and delivers the recording medium  9  in a direction of an arrow F 2  along a predetermined delivery path  33 . The delivery mechanism  11  includes a pickup roller  31   a  that sends a recording medium  9  from the paper feed tray  3  toward the delivery path  33 , and a paper feed roller  32   a  that feeds the recording medium  9  sent out by the pickup roller  31   a  to the delivery path  33 . The paper feed roller  32   a  has a function of feeding one piece of recording medium  9  at a top to the delivery path  33  on a downstream side when a plurality of recording media  9  is sent out by the pickup roller  31   a . The delivery mechanism  11  includes a pickup roller  31   b  that sends a recording medium  9  from the manual feed tray  4  toward the delivery path  33 , and a paper feed roller  32   b  that feeds the recording medium  9  sent out by the pickup roller  31   b  to the delivery path  33 . The paper feed roller  32   b  has a function of feeding one piece of recording medium  9  at a top to the delivery path  33  on a downstream side when a plurality of recording media  9  is sent out by the pickup roller  31   b.    
     The delivery mechanism  11  includes a resist roller  34  upstream of the secondary transfer roller  23 . The resist roller  34  temporarily stops delivery of recording media  9  with tips of the recording media aligned, the recording media  9  being fed by the paper feed rollers  32   a  and  32   b . Then, the resist roller  34  supplies a recording medium  9  toward the position of the secondary transfer roller  23  at a timing when an image (color image or monochrome image) formed on the intermediate transfer belt  22  by the image forming unit  10  moves to the position of the secondary transfer roller  23 . 
     The delivery mechanism  11  includes an ejection roller  37  that ejects a recording medium  9  onto the paper ejection tray  5 , the recording medium  9  being subjected to fixing processing of an image in the fixing unit  12 . From an ejection opening  38  onto the paper ejection tray  5 , the ejection roller  37  ejects a recording medium  9  on which an image is formed. 
     The fixing unit  12  is provided downstream of a position of the secondary transfer roller  23  on which an image is secondarily transferred onto a recording medium  9 . The fixing unit  12  performs fixing processing for fixing an image, which is transferred onto the recording medium  9 , onto the recording medium  9 . The fixing unit  12  includes a pair of fixing rollers  35  and  36 . Of the pair of fixing rollers  35  and  36 , one fixing roller  35  is a roller heated by a heater, which will be described later, and heats a recording medium  9 . The another fixing roller  36  is disposed facing the fixing roller  35 , and is pressure-contacted against a surface of the fixing roller  35  to pressurize a recording medium  9 . Pressure contact force of the fixing roller  36  against the fixing roller  35  is adjustable. In this way, the fixing unit  12  fixes an image onto a recording medium  9  by heating and pressurizing the recording medium  9  when the recording medium  9  passes a nip of the pair of fixing rollers  35  and  36 . 
     Further, the image formation apparatus  1  includes an envelope detection sensor  40 , a basis weight detection sensor  50 , and a loop detection sensor  60  on the delivery path  33  through which a recording medium  9  is delivered. 
     The envelope detection sensor  40  is provided between the paper feed rollers  32   a  and  32   b  and resist roller  34  on the delivery path  33 , and detects whether or not a recording medium  9  fed by the paper feed rollers  32   a  and  32   b  is an envelope  15 . 
       FIGS. 3A and 3B  are diagrams showing a configuration example of the envelope detection sensor  40 . For example, the envelope detection sensor  40  includes an ultrasonic sensor. As shown in  FIG. 3A , the envelope detection sensor  40  includes an ultrasonic wave transmission unit  41  and an ultrasonic wave reception unit  42  provided at positions facing each other with the delivery path  33  for a recording medium  9  interposed therebetween. The ultrasonic wave transmission unit  41  transmits an ultrasonic wave having a predetermined frequency toward the ultrasonic wave reception unit  42 . In a case where a recording medium  9  delivered along the delivery path  33  is an envelope  15 , the envelope  15  has a hollow  16  inside a portion where two sheets overlap. Therefore, the ultrasonic wave transmitted from the ultrasonic wave transmission unit  41  attenuates in the hollow  16  of the envelope  15  and is received by the ultrasonic wave reception unit  42 . Meanwhile, in a case where the recording medium  9  is not an envelope  15  but one piece of sheet, an amount of attenuation of the ultrasonic wave transmitted from the ultrasonic wave transmission unit  41  is smaller than an amount of attenuation in a case where the recording medium  9  is an envelope  15 . Therefore, the envelope detection sensor  40  can detect whether or not the recording medium  9  is an envelope  15  by detecting whether or not the amount of attenuation of the ultrasonic wave transmitted from the ultrasonic wave transmission unit  41  is equal to or greater than a predetermined value. 
       FIG. 3B  shows a detailed configuration example of the envelope detection sensor  40 . The ultrasonic wave transmission unit  41  has a drive circuit  43  and a transmission device  44 . The drive circuit  43  drives the transmission device  44  to cause the transmission device  44  to transmit an ultrasonic wave having a predetermined frequency (for example, 300 kHz). The ultrasonic wave is received by the ultrasonic wave reception unit  42  via a recording medium  9  delivered through the delivery path  33 . 
     The ultrasonic wave reception unit  42  has a reception device  45 , a resonance circuit  46 , an amplification circuit  47 , and a rectification circuit  48 . The reception device  45  is a device that receives an ultrasonic wave and outputs an electric signal corresponding to the received ultrasonic wave. The resonance circuit  46  filters the electric signal output from the reception device  45  with a predetermined frequency (for example, 300 kHz) to extract a signal component of an ultrasonic wave transmitted from the ultrasonic wave transmission unit  41 . The amplification circuit  47  amplifies the signal component of the ultrasonic wave and outputs the amplified signal component to the rectification circuit  48 . The rectification circuit  48  rectifies the amplified signal to generate and output an analog voltage signal corresponding to intensity of the ultrasonic wave received by the reception device  45 . The analog voltage signal has a value smaller than a predetermined value if a recording medium  9  is an envelope  15 , and has a value equal to or greater than the predetermined value if the recording medium  9  is not an envelope  15 . Therefore, the analog voltage signal output from the rectification circuit  48  is a signal indicating whether or not the recording medium  9  is an envelope  15 . 
     The envelope detection sensor  40  also functions as a flap detecting unit for detecting a flap of the envelope  15  in a case where a recording medium  9  is an envelope  15 . The envelope  15  has a portion where two sheets overlap and a flap where the two sheets do not overlap. In a case where the flap is positioned on a head end side or rear end side of a direction of delivering a recording medium  9 , the envelope detection sensor  40  can detect a flap before or after the portion where the two sheets overlap. 
     The basis weight detection sensor  50  is provided between the paper feed rollers  32   a  and  32   b  and resist roller  34  on the delivery path  33 , and detects a basis weight of a recording medium  9  fed by the paper feed rollers  32   a  and  32   b . The basis weight detection sensor  50  according to the present embodiment is provided downstream of the envelope detection sensor  40  in the delivery path  33 . 
       FIG. 4  is a diagram showing a configuration example of the basis weight detection sensor  50 . The basis weight detection sensor  50  includes a light transmissive sensor and a light reflective sensor. That is, the basis weight detection sensor  50  includes a first light source  51  that emits first light toward the delivery path  33 , a second light source  52  that emits second light toward the delivery path  33 , and a light receiving element  53 . The first light source  51  is a light source of the light reflective sensor. The second light source  52  is a light source of the light transmissive sensor. The light receiving element  53  serves as a light receiving element of both the light transmissive sensor and the light reflective sensor. 
     The first light source  51  is disposed on the same side as the light receiving element  53  with respect to the delivery path  33 , and emits the first light toward the delivery path  33 . When a recording medium  9  passes an area where the first light is emitted, the first light is reflected on a surface of the recording medium  9 , and the reflected light is incident on and detected by the light receiving element  53 . 
     The second light source  52  is disposed on an opposite side of the first light source  51  and the light receiving element  53  with the delivery path  33  interposed therebetween, and emits the second light toward the delivery path  33 . When a recording medium  9  passes an area where the second light is emitted, the second light is transmitted through the recording medium  9 , and the transmitted light is detected by the light receiving element  53 . 
     The basis weight (weight per unit area) of a recording medium  9  varies depending on thickness of the recording medium  9 . Therefore, the larger the basis weight of the recording medium  9 , the smaller transmission amount of light emitted from the second light source  52 . Further, the transmission amount of light emitted from the second light source  52  also varies depending on a tint of the recording medium  9 . Therefore, the basis weight detection sensor  50  detects a basis weight of the recording medium  9  by measuring an amount of transmitted light emitted from the second light source  52 , and detects the tint of the recording medium  9  by measuring reflected light of light emitted from the first light source  51 . Then, by correcting a basis weight based on an amount of transmitted light according to the tint of the recording medium  9 , the basis weight detection sensor  50  detects a basis weight of the recording medium  9  from which influence of the tint is excluded. 
     The loop detection sensor  60  is disposed between a position where an image is secondarily transferred onto a recording medium  9  and the fixing rollers  35  and  36 . The loop detection sensor  60  is a sensor that detects a loop of a recording medium  9 , which is formed by the recording medium  9  bending at a part between the position of the secondary transfer roller  23  and the fixing rollers  35  and  36 . 
       FIG. 5  is a diagram showing a configuration example of the loop detection sensor  60 . In a case where no loop is formed on a recording medium  9  at the part between the position of the secondary transfer roller  23  and the fixing rollers  35  and  36 , the recording medium  9  passes a path L 1  shown by a broken line in  FIG. 5 . Meanwhile, if a loop is formed on the recording medium  9  at the part between the position of the secondary transfer roller  23  and the fixing rollers  35  and  36 , the recording medium  9  passes a path L 2  shown by a solid line in  FIG. 5 . The loop detection sensor  60  can detect a size of the loop. The loop of the recording medium  9  is formed in one direction in which the loop detection sensor  60  is disposed. For example, the loop detection sensor  60  includes an actuator  61  that displaces by coming into contact with a loop of a recording medium  9  as the loop is formed. The loop detection sensor  60  is formed so as to detect a size of a loop of a recording medium  9  according to a position of the actuator  61 . 
     Next, a control mechanism of the image formation apparatus  1  will be described.  FIG. 6  is a block diagram showing a configuration example of the control mechanism of the image formation apparatus  1 . As shown in  FIG. 6 , the image formation apparatus  1  is formed such that the controller  8  controls operation of the image formation unit  2  described above. The controller  8  includes a central processing unit (CPU)  70  and a memory  71 , and causes the image formation unit  2  to operate by the CPU  70  reading and executing a program  78  stored in the memory  71 . The operation panel  7  and a communication unit  72  are connected to the controller  8 . 
     The operation panel  7  includes a display unit  7   a  and an operation unit  7   b . The display unit  7   a  includes, for example, a color liquid crystal display, and displays various operation screens that can be operated by the user. The operation unit  7   b  includes a touch panel key, or the like, and receives operation by the user. The controller  8  can acquire information input by the user via the operation panel  7 , and can also control the image formation unit  2  on the basis of the information input by the user. 
     The communication unit  72  communicates with an external apparatus via a network such as a local area network (LAN). For example, the controller  8  can acquire image data to be printed from an external apparatus via the communication unit  72 . Via the communication unit  72 , the controller  8  can also acquire information for controlling the image formation unit  2 . 
     The image formation unit  2  includes a delivery roller drive unit  73 , a fixing roller drive unit  74 , a fixing roller pressure contact force adjustment unit  75 , and a heater  76 . 
     The delivery roller drive unit  73  includes, for example, a motor, or the like that rotationally drives each of the rollers for delivering a recording medium  9  along the delivery path  33 . The delivery roller drive unit  73  drives a plurality of rollers including the pickup rollers  31   a  and  31   b , the paper feed rollers  32   a  and  32   b , the resist roller  34 , the secondary transfer roller  23 , and the ejection roller  37 . However, drive force of the delivery roller drive unit  73  is formed so as to be transmitted to each of the rollers via an electromagnetic clutch. Therefore, for example, it is possible to supply a recording medium  9  to the secondary transfer roller  23  at an appropriate timing by stopping drive of the resist roller  34  at a timing when a tip of the recording medium  9  reaches the resist roller  34  and driving the resist roller  34  again at a timing when an image primarily transferred onto the intermediate transfer belt  22  reaches a predetermined position. 
     When starting paper feeding operation of a recording medium  9 , the delivery roller drive unit  73  sets system velocity designated by the controller  8  to drive each of the rollers. The system velocity is delivery velocity when a recording medium  9  is delivered along the delivery path  33 . The delivery roller drive unit  73  can change velocity of delivering a recording medium  9  according to system velocity if the controller  8  gives an instruction to change the system velocity after the delivery roller drive unit  73  starts delivery of the recording medium  9 . For example, the delivery roller drive unit  73  can change the system velocity according to thickness of the recording medium  9  detected by the controller  8 . 
     The fixing roller drive unit  74  is a drive unit such as a motor that rotationally drives the fixing rollers  35  and  36 . That is, the fixing rollers  35  and  36  are rotationally driven by a drive source different from each of the rollers for delivering a recording medium  9  along the delivery path  33 . Therefore, velocity of delivering a recording medium  9  by the fixing rollers  35  and  36  can be set to the same velocity as velocity of delivering the recording medium  9  by each of the rollers, or to velocity different from the velocity of delivering the recording medium  9  by each of the rollers. 
     The fixing roller pressure contact force adjustment unit  75  adjusts pressure contact force of the fixing roller  36  on the fixing roller  35  in the fixing unit  12 . For example, the fixing roller pressure contact force adjustment unit  75  presses and energizes the fixing roller  36  toward the fixing roller  35 , and adjusts the pressure contact force by adjusting the press force. 
     The heater  76  is a heat source for heating the fixing roller  35  of the fixing unit  12 . The heater  76  is driven by the controller  8  and is formed to heat the fixing roller  35  to maintain a predetermined fixing temperature. The fixing temperature differs depending on a type of a recording medium  9 . 
     The controller  8  is formed to control each of the delivery roller drive unit  73 , the fixing roller drive unit  74 , the fixing roller pressure contact force adjustment unit  75 , and the heater  76  on the basis of a detection result from each of the envelope detection sensor  40 , the basis weight detection sensor  50 , and the loop detection sensor  60 , which are included in the image formation unit  2 . Although not shown in  FIG. 6 , the controller  8  controls operation of the above-described image forming unit  10 , needless to say. A detailed configuration and operation of the controller  8  will be described below. 
     As shown in  FIG. 6 , the CPU  70  of the controller  8  functions as an envelope judgment unit  81 , a basis weight acquisition unit  82 , a determination unit  83 , and a print control unit  84  by executing the program  78 . 
     The print control unit  84  drives the image formation unit  2  to control image forming operation on a recording medium  9 . The print control unit  84  drives the image formation unit  2  to start printing operation for forming an image on a recording medium  9  in a case where an instruction on execution of a print job is given via the communication unit  72 , or in a case where an instruction on execution of a print job is given via the operation panel  7 . With the start of the printing operation, the print control unit  84  drives the delivery roller drive unit  73 , feeds a recording medium  9  from a tray among the paper feed tray  3  and the manual feed tray  4 , whichever is designated by the user, and delivers the recording medium  9  along the delivery path  33 . At this time, the print control unit  84  starts the delivery of the recording medium  9  by setting system velocity corresponding to plain paper on assumption that the recording medium  9  is plain paper, for example. Further, the print control unit  84  starts operation of generating a toner image in the image forming unit  10  with the start of delivering the recording medium  9 . Thereafter, on the basis of a determination result from the determination unit  83 , the print control unit  84  can appropriately change control for when forming an image on the recording medium  9 . 
     The envelope judgment unit  81  is a processing unit that judges whether or not a recording medium  9  delivered along the delivery path  33  is an envelope. When the print control unit  84  starts delivering a recording medium  9 , the envelope judgment unit  81  drives the envelope detection sensor  40 , and causes the envelope detection sensor  40  to detect whether or not the recording medium  9  is an envelope  15  when the recording medium  9  passes a position of the envelope detection sensor  40 . Then, the envelope judgment unit  81  acquires a detection result output from the envelope detection sensor  40  and judges whether or not the recording medium  9  is an envelope on the basis of the detection result. 
     The basis weight acquisition unit  82  is a processing unit that acquires a basis weight of a recording medium  9  delivered along the delivery path  33 . When the print control unit  84  starts delivering a recording medium  9 , the basis weight acquisition unit  82  drives the basis weight detection sensor  50 , and causes the basis weight detection sensor  50  to detect a basis weight of the recording medium  9  when the recording medium  9  passes a position of the basis weight detection sensor  50 . Then, the basis weight acquisition unit  82  acquires a basis weight detection result output from the basis weight detection sensor  50 . For example, in a case where the envelope judgment unit  81  judges that the recording medium  9  is an envelope  15 , the basis weight acquisition unit  82  acquires a basis weight of the envelope  15  detected by the basis weight detection sensor  50 . Further, in a case where the envelope judgment unit  81  judges that the recording medium  9  is not an envelope  15  but one piece of sheet, the basis weight acquisition unit  82  acquires a basis weight of the one piece of sheet detected by the basis weight detection sensor  50 . 
     In a case where the recording medium  9  is an envelope  15 , a basis weight that the basis weight detection sensor  50  detects at a portion where two sheets overlap and a basis weight that the basis weight detection sensor  50  detects at a flap where two sheets do not overlap are different. This is because an amount of transmitted light of light emitted from the second light source  52  may be lowered at the portion where the two sheets overlap than at a flap, tint of the portion where the two sheets overlap may be slightly different from tint of the flap, and an amount of reflected light of light emitted from the first light source  51  may vary. Therefore, the basis weight acquisition unit  82  drives the basis weight detection sensor  50  so as to detect basis weight at either the portion where the two sheets overlap or the flap. 
       FIGS. 7A to 7D  are diagrams describing operation of the envelope detection sensor  40  and the basis weight detection sensor  50 . On the delivery path  33  through which a recording medium  9  is delivered, the envelope detection sensor  40  is disposed upstream of the basis weight detection sensor  50 . Therefore, as shown in  FIG. 7A , a recording medium  9  delivered in a direction of an arrow F 2  along the delivery path  33  first passes the position of the envelope detection sensor  40 , and then passes the position of the basis weight detection sensor  50 . Velocity of delivering of the recording medium  9  is known, and a distance between the envelope detection sensor  40  and the basis weight detection sensor  50  is also known. Therefore, it is possible to predict a timing when the same portion as the portion detected by the envelope detection sensor  40  passes the position of the basis weight detection sensor  50 . For example, as shown in  FIG. 7B , a portion  17  where two sheets of the envelope  15  overlap passes the position of the basis weight detection sensor  50  after a predetermined time from a time when the envelope detection sensor  40  detects the same overlapping portion  17 . Therefore, the basis weight acquisition unit  82  can cause the basis weight detection sensor  50  to detect a basis weight of the portion  17  where two sheets of the envelope  15  overlap as shown in  FIG. 7C  by causing the basis weight detection sensor  50  to detect the basis weight after a predetermined time from a time when the envelope detection sensor  40  detects the portion  17  where two sheets of the envelope  15  overlap. 
     Further, as shown in  FIG. 7C , a flap  18  of the envelope  15  passes the position of the basis weight detection sensor  50  after a predetermined time from a time when the envelope detection sensor  40  detects the flap  18 . Therefore, the basis weight acquisition unit  82  can cause the basis weight detection sensor  50  to detect a basis weight of the flap  18  of the envelope  15  as shown in  FIG. 7D  by causing the basis weight detection sensor  50  to detect the basis weight after a predetermined time from a time when the envelope detection sensor  40  detects the flap  18  of the envelope  15 . 
     The basis weight detection sensor  50  can detect a basis weight of a flap  18  of an envelope  15  in a similar manner to a case where a recording medium  9  is one piece of sheet. However, in a case where the basis weight detection sensor  50  detects a basis weight of a portion  17  where two sheets of the envelope  15  overlap, an amount of transmitted light of light emitted from the second light source  52  may be lowered, and an amount of reflected light of light emitted from the first light source  51  may vary. Therefore, in this case, the basis weight may not be able to be detected properly. Therefore, it is preferable that the basis weight acquisition unit  82  cause the basis weight detection sensor  50  to detect a basis weight of a portion  17  where two sheets of the envelope  15  overlap by using a method different from a basis weight detection method used in a case where a recording medium  9  is one piece of sheet. That is, the basis weight acquisition unit  82  sets a basis weight detection method by the basis weight detection sensor  50  to a first method in a case where the envelope judgment unit  81  judges the recording medium  9  not to be an envelope  15 , and to a second method in a case where the envelope judgment unit  81  judges the recording medium  9  to be an envelope  15 , the second method being different from the first method. Specifically, in a case where the envelope judgment unit  81  judges the recording medium  9  is an envelope  15 , the basis weight acquisition unit  82  uses the second method to detect the basis weight. For the second method, an amount of light or sensitivity of the light transmissive sensor or light reflective sensor in the basis weight detection sensor  50  is increased as compared to the first method used in a case where the recording medium  9  is not an envelope  15 . Thus, it is possible to reduce influence of a decrease in an amount of transmitted light of light emitted from the second light source  52  or of a change in an amount of reflected light of light emitted from the first light source  51 , achieving proper detection of a basis weight of the envelope  15 . 
     In a case where the basis weight detection sensor  50  detects a basis weight of a flap  18  of an envelope  15 , it is not necessary to switch the basis weight detection method as described above. Therefore, processing load on the controller  8  is reduced. However, in a case where the recording medium  9  is an envelope  15 , depending on a type or orientation of the envelope  15  delivered through the delivery path  33 , it may be difficult to detect a basis weight by the flap  18 .  FIGS. 8A and 8B  are diagrams exemplifying a plurality of types of envelopes  15  delivered in a delivery direction F 2 . For example, as shown in  FIG. 8A , in a case where a flap  18  of an envelope  15  is on a rear end side of the delivery direction F 2 , the flap  18  passes a position detected by the basis weight detection sensor  50 . Therefore, in a case where an envelope  15  is delivered in an orientation shown in  FIG. 8A , the basis weight detection sensor  50  can detect a basis weight of a flap  18  of the envelope  15 . Similarly, in a case where a flap  18  of an envelope  15  is on a head end side of the delivery direction F 2 , the flap  18  passes a position detected by the basis weight detection sensor  50 . Therefore, the basis weight detection sensor  50  can detect a basis weight of the flap  18  of the envelope  15 . 
     Meanwhile, as shown in  FIG. 8B , in a case where a flap  18  of an envelope  15  is at an end of a direction orthogonal to the delivery direction F 2 , the flap  18  does not necessarily pass a position detected by the basis weight detection sensor  50 . Therefore, in a case where an envelope  15  is delivered in an orientation shown in  FIG. 8B , there may be a case where the basis weight detection sensor  50  cannot detect a basis weight of a flap  18  of the envelope  15 . In such a case, it is preferable that the basis weight acquisition unit  82  cause the basis weight detection sensor  50  to detect a basis weight of a portion  17  where two sheets of the envelope  15  overlap, because the envelope detection sensor  40  also cannot detect the flap  18  of the envelope  15 . 
     The determination unit  83  is a processing unit that determines control for when an image is formed on a recording medium  9  on the basis of a judgment result from the envelope judgment unit  81  and a basis weight acquired by the basis weight acquisition unit  82 . The determination unit  83  reads reference information  79  stored in advance in the memory  71 , and determines the control on the basis of reference information  79 . 
       FIG. 9  is a diagram showing an example of the reference information  79 . The reference information  79  is information in which system velocity Vs, fixing roller velocity Vt, whether or not loop control is performed, fixing temperature T, and fixing roller pressure contact force FP are defined in advance for each type of the recording media  9 . 
     For example, in a case where the recording medium  9  is a sheet and the sheet is plain paper, the reference information  79  defines first velocity V 1  as system velocity Vs. The first velocity V 1  is, for example, 200 mm/s. In addition, it is defined that the fixing roller velocity Vt should be set to equal to the system velocity Vs, the loop control should be set to “performed”, the fixing temperature T should be set to first temperature T 1 , and the fixing roller pressure contact force FP should be set to first pressure contact force FP 1 . The loop control is a control by which a recording medium  9  is supplied to the fixing unit  12  with a loop of the recording medium  9  formed upstream of the fixing unit  12 . The first temperature T 1  is, for example, 170° C. The first pressure contact force FP 1  is a standard pressure contact force (normal pressure contact force) for when an image is formed on a recording medium  9  in the image formation apparatus  1 . 
     In a case where the recording medium  9  is a sheet and the sheet is thin paper, the reference information  79  defines first velocity V 1  as system velocity Vs, as in a case of plain paper. In addition, it is defined that the fixing roller velocity Vt should be set to equal to the system velocity Vs, the loop control should be set to “performed”, the fixing temperature T should be set to second temperature T 2 , and the fixing roller pressure contact force FP should be set to second pressure contact force FP 2 . The second temperature T 2  is, for example, 150° C. The second pressure contact force FP 2  is pressure contact force smaller than pressure contact force of the first pressure contact force FP 1 . That is, because thin paper is thinner than plain paper, the fixing temperature T can be lowered and pressure contact force FP of the fixing roller  36  can be reduced. 
     In a case where the recording medium  9  is a sheet and the sheet is thick paper, the reference information  79  defines second velocity V 2  as system velocity Vs, the second velocity V 2  being lower than the first velocity V 1 . In addition, it is defined that the fixing roller velocity Vt should be set to equal to the system velocity Vs, the loop control should be set to “performed”, the fixing temperature T should be set to second temperature T 2 , and the fixing roller pressure contact force FP should be set to third pressure contact force FP 3 . The second velocity V 2  is, for example, 100 mm/s. The third pressure contact force FP 3  is pressure contact force greater than pressure contact force of the first pressure contact force FP 1 . That is, for thick paper, system velocity Vs is lowered so that an image is properly transferred onto a sheet. Therefore, fixing temperature T can be lowered as in a case of thin paper. However, in order to properly fix an image onto thick paper, it is preferable that pressure contact force FP of the fixing roller  36  be greater than the normal pressure contact force. 
     Meanwhile, in a case where a recording medium  9  is an envelope  15  and the envelope is a thin envelope, the reference information  79  defines first velocity V 1  as system velocity Vs. In addition, it is defined that the fixing roller velocity Vt should be set to about a few percent faster than the system velocity Vs, the loop control should be set to “not performed”, the fixing temperature T should be set to lower than the first temperature T 1 , and the fixing roller pressure contact force FP should be set to smaller than the first pressure contact force FP 1 . 
     A thin envelope is susceptible to wrinkling when passing the fixing unit  12  if a loop is formed upstream of the fixing unit  12 . Especially when the thin envelope is skewed, wrinkling is noticeable. This phenomenon occurs when a bent generated in a portion  17  where two sheets of the envelope  15  overlap is fixed as is in the fixing unit  12 . Therefore, it is defined that, for a thin envelope, velocity (fixing roller velocity Vt) of delivery by the fixing rollers  35  and  36  should be set to velocity slightly faster than system velocity Vs so that a loop is not formed upstream of the fixing unit  12 . However, if the fixing rollers  35  and  36  pull the thin envelope too much, an image secondarily transferred onto the thin envelope by the secondary transfer roller  23  may be disturbed. Therefore, difference in velocity between the fixing roller velocity Vt and the system velocity Vs is preferably kept within a few percent. If the loop is not formed upstream of the fixing unit  12 , the fixing roller velocity Vt may be set to velocity equal to the system velocity Vs. 
     Further, because a thin envelope is thicker than thin paper, the fixing temperature T is higher than fixing temperature of the thin paper. However, the fixing roller pressure contact force FP may be pressure contact force similar to a case of thin paper. 
     In a case where the recording medium  9  is an envelope  15  and the envelope is a thick envelope, the reference information  79  defines second velocity V 2  as system velocity Vs. In addition, it is defined that the fixing roller velocity Vt should be set to equal to the system velocity Vs, the loop control should be set to “performed”, the fixing temperature T should be set to lower than the first temperature T 1 , and the fixing roller pressure contact force FP should be set to normal pressure contact force that is the same as the first pressure contact force FP 1 . 
     For a thick envelope, system velocity Vs is lowered so that an image is properly transferred onto a sheet. Therefore, fixing temperature T can be lowered as in a case of a thin envelope. However, fixing temperature T for a thick envelope may be lower than fixing temperature T for a thin envelope. In order to properly fix an image on a thick envelope, it is preferable that pressure contact force FP of the fixing roller  36  be greater than pressure contact force FP for a thin envelope. Therefore, for a thick envelope, it is defined that pressure contact force FP of the fixing roller  36  should be set to the same as normal pressure contact force. 
     In the examples in  FIG. 9 , only two types of envelopes, a thin envelope and a thick envelope, are shown as examples of an envelope  15 . However, reference information  79  may be used, the reference information  79  defining specific control for three or more types of envelopes according to thickness of the envelope  15 . 
     The determination unit  83  determines control for when an image is formed on a recording medium  9  on the basis of the reference information  79  as described above. For example, when the envelope judgment unit  81  judges that the recording medium  9  is an envelope  15 , the determination unit  83  identifies thickness of the envelope  15  on the basis of a basis weight of the envelope  15  acquired by the basis weight acquisition unit  82 , and judges whether the envelope  15  is a thin envelope or a thick envelope. In a case where the envelope  15  is a thin envelope as a result, the determination unit  83  refers to a field of thin envelope in the reference information  79 , and determines system velocity Vs, fixing roller velocity Vt, loop control “not performed”, fixing temperature T, and fixing roller pressure contact force FP, which are suitable for the thin envelope. In a case where the envelope judgment unit  81  judges that the envelope  15  is a thick envelope, the determination unit  83  refers to a field of thick envelope in the reference information  79 , and determines system velocity Vs, fixing roller velocity Vt, loop control “performed”, fixing temperature T, and fixing roller pressure contact force FP, which are suitable for the thick envelope. Then, the determination unit  83  outputs the determination result to the print control unit  84 . 
     In a case where the envelope judgment unit  81  has judged that the recording medium  9  is one piece of sheet, the determination unit  83  determines control corresponding to a type of sheet on the basis of the reference information  79 , and notifies the print control unit  84  of the determined control. 
     Determination by the determination unit  83  is made, at latest, before a tip of the recording medium  9  is sent downstream of the resist roller  34 . That is, the determination unit  83  determines the control while delivery of the recording medium  9  by the resist roller  34  is temporarily stopped before or after the tip of the recording medium  9  reaches the resist roller  34 . 
     After starting delivery of the recording medium  9 , the print control unit  84  temporarily stops the delivery of the recording medium  9  at a timing when the tip of the recording medium  9  reaches the resist roller  34 . Thereafter, the print control unit  84  drives the resist roller  34  again and sends the tip of the recording medium  9  toward the secondary transfer roller  23  at a timing when an image primarily transferred onto the intermediate transfer belt  22  by the image forming unit  10  reaches the position of the secondary transfer roller  23 . At this time, the print control unit  84  performs control that reflects the determination result from the determination unit  83 . For example, in a case where the determination unit  83  has determined that system velocity Vs is to be reduced, the print control unit  84  reduces the system velocity Vs when the resist roller  34  is driven again, and reduces velocity of delivering the recording medium  9 . The print control unit  84  sets the velocity of delivering the recording medium  9  by the fixing rollers  35  and  36  to a fixing roller velocity Vt determined by the determination unit  83 , and rotationally drives the fixing rollers  35  and  36 . The print control unit  84  performs control to form or not to form a loop of the recording medium  9  upstream of the fixing unit  12  on the basis of whether or not loop control is performed determined by the determination unit  83 . For example, for a thin envelope, the loop control is determined to be “not performed”. Therefore, the print control unit  84  adjusts system velocity Vs and fixing roller velocity Vt so that the loop detection sensor  60  does not detect a loop of the recording medium  9 . By driving the heater  76 , the print control unit  84  sets fixing temperature T in the fixing unit  12  to a temperature determined by the determination unit  83 . By driving the fixing roller pressure contact force adjustment unit  75 , the print control unit  84  adjusts pressure contact force of the fixing roller  36  to a value determined by the determination unit  83 . 
     For example, in a case where the recording medium  9  is an envelope  15 , the print control unit  84  can switch control for when an image is formed on the envelope  15  according to thickness of the envelope  15 . In other words, the print control unit  84  selects an optimum control parameter corresponding to the thickness of the envelope  15  and performs control so that the image is formed on the envelope  15 . Thus, when a thick envelope passes the fixing unit  12 , image formation suitable for a thick envelope is performed by a first control parameter suitable for a thick envelope being applied. Meanwhile, when a thin envelope passes the fixing unit  12 , image formation suitable for a thin envelope is performed by a second control parameter suitable for a thin envelope being applied, the second control parameter being a control parameter different from a control parameter for a thick envelope. Therefore, even in a case where an image is formed on a thin envelope  15  as a recording medium  9  for example, the conventional image formation apparatus  1  according to the present embodiment can reduce wrinkling of an envelope  15 , and therefore deterioration of quality of a printed material can be reduced. 
     Next,  FIG. 10  is a flowchart showing an example of a processing procedure performed by the controller  8 . The processing is performed by the CPU  70  executing the program  78 . In addition, the processing is processing started in a case where the user gives an instruction on execution of a print job. 
     When the processing is started, the controller  8  starts a print job (step S 1 ). Accordingly, the controller  8  sets the system velocity Vs to the first velocity V 1  (step S 2 ). That is, the controller  8  sets the system velocity Vs to the first velocity V 1  on assumption that the recording medium  9  is plain paper, because whether or not the recording medium  9  is the envelope  15  is unknown at this point. Then, the controller  8  starts operation of supplying and delivering the recording medium  9  to the delivery path  33  from either the paper feed tray  3  or the manual feed tray  4  by driving the delivery roller drive unit  73  (step S 3 ). 
     When delivery of the recording medium  9  is started, the controller  8  judges whether or not the envelope detection sensor  40  has detected the recording medium  9  (step S 4 ). If the envelope detection sensor  40  detects the recording medium  9  (YES in step S 4 ), the controller  8  judges whether or not the recording medium  9  is the envelope  15  on the basis of detection result of the envelope detection sensor  40  (step S 5 ). 
     In a case where the recording medium  9  is the envelope  15  (YES in step S 5 ), the controller  8  sets the second method as a method for detection by the basis weight detection sensor  50  (step S 6 ). For example, as described above, the controller  8  specifies a setting so that basis weight can be detected at a portion  17  where two sheets of the envelope  15  overlap by increasing an amount of light of the first light source  51  and second light source  52  to higher than a normal amount of light (amount of light when detecting basis weight of one piece of sheet), or by increasing sensitivity of the light receiving element  53  to higher than a normal sensitivity (sensitivity when detecting basis weight of one piece of sheet). Thereafter, the controller  8  drives the basis weight detection sensor  50  to acquire basis weight of the envelope  15  from the basis weight detection sensor  50  (step S 7 ). After being able to acquire basis weight of the envelope  15 , the controller  8  executes envelope control determination processing (step S 8 ). 
       FIG. 11  is a flowchart showing an example of a detailed processing procedure of the envelope control determination processing (step S 8 ). When starting the processing, the controller  8  judges whether or not the basis weight of the envelope  15  detected by the basis weight detection sensor  50  is equal to or more than a predetermined value (step S 20 ). As a result, if the basis weight of the envelope  15  is equal to or greater than the predetermined value (YES in step S 20 ), the controller  8  decides that the envelope is a thick envelope (step S 21 ). If deciding that the envelope is a thick envelope, the controller  8  changes a setting of the system velocity Vs on the basis of the reference information  79  (step S 22 ). In addition, the controller  8  changes a setting of fixing roller velocity Vt (step S 23 ) and changes a setting of fixing temperature T on the basis of the reference information  79  (step S 24 ). 
     Meanwhile, if the basis weight of the envelope  15  is less than the predetermined value (NO in step S 20 ), the controller  8  decides that the envelope is a thin envelope (step S 25 ). If deciding that the envelope is a thin envelope, the controller  8  changes a setting of the fixing roller velocity Vt on the basis of the reference information  79  (step S 26 ). That is, in a case where the basis weight of the envelope  15  is less than a predetermined value, the controller  8  sets velocity of delivering the envelope  15  by the fixing rollers  35  and  36  to higher than normal delivery velocity (first velocity v 1 ). In addition, the controller  8  changes a setting so as not to perform loop control (step S 27 ). Further, the controller  8  changes a setting of the fixing temperature T on the basis of the reference information  79  (step S 28 ). That is, in a case where the basis weight of the envelope  15  is less than a predetermined value, the controller  8  changes a setting of the fixing temperature T of the fixing roller  35  to a temperature lower than a normal fixing temperature (first temperature T 1 ). In addition, the controller  8  changes a setting of pressure contact force of the fixing roller  36  (step S 29 ). That is, in a case where the basis weight of the envelope  15  is less than a predetermined value, the controller  8  changes a setting of pressure contact force FP of the fixing roller  36  to a force weaker than a normal pressure contact force (first pressure contact force FP 1 ). This completes the envelope control determination processing. 
     Returning to the flowchart in  FIG. 10 , the controller  8  switches a control of subsequent image forming operation to a control determined in the envelope control determination processing (step S 8 ) (step S 9 ). Then, the controller  8  applies the switched control to control the subsequent image forming operation. As a result, if the recording medium  9  is the envelope  15 , control corresponding to the thickness of the envelope  15  is performed, by which deterioration of quality of a printed material can be reduced. 
     In a case where the recording medium  9  is not the envelope  15 , but one piece of sheet (NO in step S 5 ), the controller  8  sets the first method as a method for detection by the basis weight detection sensor  50  (step S 10 ). For example, by setting an amount of light of the first light source  51  and second light source  52  to a normal amount of light, and setting sensitivity of the light receiving element  53  to normal sensitivity, the controller  8  specifies a setting so that a basis weight of one piece of sheet can be properly detected. Thereafter, the controller  8  drives the basis weight detection sensor  50  to acquire basis weight of the sheet from the basis weight detection sensor  50  (step S 11 ). After being able to acquire basis weight of the envelope  15 , the controller  8  executes sheet control determination processing (step S 12 ). Similarly to the envelope control determination processing, the sheet control determination processing is processing for determining control when image is formed on the sheet on the basis of the reference information  79 . When the sheet control determination processing is performed, the controller  8  judges whether or not it is necessary to switch control of the image forming operation (step S 13 ). At a start of a print job, plain paper is set as a default. Therefore, when the sheet is thin paper or thick paper other than plain paper, it is necessary to switch the control. If judging that it is necessary to switch the control of the image formation operation (YES in step S 13 ), the controller  8  switches a control of subsequent image forming operation to a control determined in the sheet control determination processing (step S 12 ) (step S 14 ). Then, the controller  8  applies the switched control to control the subsequent image forming operation. As a result, the image formation apparatus  1  performs image forming suitable for thin paper or thick paper. 
     Meanwhile, if judging that it is not necessary to switch the control of the image formation operation (NO in step S 13 ), the controller  8  controls the subsequent image forming operation without switching the control of the image forming operation. As a result, the image formation apparatus  1  performs image forming suitable for plain paper. 
     Thereafter, the controller  8  waits until the print job ends (step S 15 ). Then, when the print job ends (YES in step S 15 ), all the processing by the controller  8  ends. 
     As described above, the image formation apparatus  1  according to the present embodiment includes the image formation unit  2  that forms an image on a recording medium  9  delivered along a predetermined delivery path  33  and outputs the recording medium  9 , the envelope judgment unit  81  that judges whether or not the recording medium  9  is an envelope  15 , the basis weight acquisition unit  82  that acquires a basis weight of the recording medium  9 , the determination unit  83  that determines control for when an image is formed on the recording medium  9  on the basis of a judgment result from the envelope judgment unit  81  and a basis weight acquired by the basis weight acquisition unit  82 , and the print control unit  84  that controls operation of the image formation unit  2  on the basis of a determination result from the determination unit  83 . Therefore, even in a case where the image formation apparatus  1  uses an envelope  15  as a recording medium  9  to form an image, the image formation apparatus  1  can form an image with optimum operation according to a basis weight of the envelope  15 . Specifically, when forming an image on a thick envelope, image formation with optimum operation according to the thick envelope can be performed, meanwhile when forming an image on a thin envelope, image formation with optimum operation corresponding to the thin envelope can be performed. Thus, it is possible to output a high-quality printed material while reducing wrinkling of a thin envelope. 
     The present embodiment exemplifies the case where the basis weight acquisition unit  82  causes the basis weight detection sensor  50  to detect a basis weight in both cases where a recording medium  9  is judged to be an envelope  15  and where the recording medium  9  is judged not to be an envelope  15 . However, not limited to this, a configuration may be employed in which the basis weight acquisition unit  82  causes the basis weight detection sensor  50  to detect a basis weight of an envelope  15  exclusively in a case where a recording medium  9  is judged to be an envelope  15 . By detecting a basis weight of an envelope  15  when a recording medium  9  is the envelope  15 , the image formation apparatus  1  can perform image formation with optimum operation according to the basis weight of the envelope  15 . 
     The flowchart shown in  FIG. 10  exemplifies a case where a basis weight detection method by the basis weight detection sensor  50  is set to the first method in a case where the recording medium  9  is judged not to be an envelope  15 , and to the second method in a case where the recording medium  9  is judged to be an envelope  15 , the second method being different from the first method. However, this is processing on a premise that a basis weight is detected at a portion  17  where two sheets of the envelope  15  overlap. Meanwhile, if the basis weight detection sensor  50  is caused to detect a basis weight at a flap  18  of an envelope  15  in a case where the recording medium  9  is judged to be the envelope  15 , a method for detecting a basis weight by the basis weight detection sensor  50  is not necessary to be set to the second method, and it is only required to detect the basis weight by adopting the first method as in a case where the recording medium  9  is a sheet. Therefore, time required to switch a method for detection by the basis weight detection sensor  50  can be saved, and therefore efficient processing can be achieved. 
     The present embodiment exemplifies a case where the determination unit  83  determines a plurality of control parameters including velocity of delivery by the fixing rollers  35  and  36  that fix an image onto a recording medium  9 , whether or not a loop of the recording medium  9  is formed between a position where an image is transferred onto the recording medium  9  and the fixing rollers  35  and  36 , fixing temperature by the fixing roller  35 , and pressure contact force by the fixing roller  36 , on the basis of a judgment result from the envelope judgment unit  81  and a basis weight acquired by the basis weight acquisition unit  82 . However, not limited to this, the determination unit  83  is only required to determine at least one of the above-described plurality of control parameters. 
     The present embodiment exemplifies a case where the basis weight detection sensor  50  includes a light transmissive sensor and a light reflective sensor. However, it is possible to detect a basis weight of a recording medium  9  by using either the light transmissive sensor alone or the light reflective sensor alone. Therefore, the basis weight detection sensor  50  is not necessarily limited to a basis weight detection sensor including both the light transmitting type sensor and the light reflective sensor, and may include either the light transmissive sensor alone or the light reflective sensor alone. 
     Second Embodiment 
     Next, a second embodiment of the present invention will be described. The above-described first embodiment exemplifies a case where the envelope judgment unit  81  judges whether or not a recording medium  9  is an envelope  15  on the basis of a detection result from the envelope detection sensor  40 , and the basis weight acquisition unit  82  acquires a basis weight detected by the basis weight detection sensor  50 . Meanwhile, in the present embodiment, there will be described an aspect in which a user inputs information about a type of a recording medium  9  or a basis weight of the recording medium  9  to an operation panel  7 . Note that a configuration of an image formation apparatus  1  according to the present embodiment is similar to the configuration of the image formation apparatus  1  described in the first embodiment. 
     In the present embodiment, an envelope judgment unit  81  and a basis weight acquisition unit  82  function in a controller  8  before the image formation apparatus  1  starts a print job. That is, each of the envelope judgment unit  81  and basis weight acquisition unit  82  functions while the user operates the operation panel  7  to perform a setting operation related to the print job. Then, on the operation panel  7 , the controller  8  displays an operation screen on which the user can input a type of a recording medium  9  or a basis weight of the recording medium  9 . 
       FIGS. 12A to 12C  are diagrams showing operation screens displayed in a display unit  7   a  of the operation panel  7 .  FIG. 12A  shows a screen G 1  for selecting a type of a recording medium  9 . For example, the envelope judgment unit  81  displays the screen G 1  shown in  FIG. 12A  in the display unit  7   a  of the operation panel  7 , and receives operation by the user. In the screen G 1 , a plurality of buttons B 1 , B 2 , B 3 , and B 4  corresponding to a type of a recording medium  9  is displayed. While performing setting operation of a print job with respect to the operation panel  7 , the user selects one button from among the plurality of buttons B 1  to B 4  for operation when the screen G 1  is displayed. Thus, the user can designate a type of the recording medium  9  with respect to the image formation apparatus  1 .  FIG. 12A  exemplifies a case where the button B 4  for envelope is selected by the user. 
     The envelope judgment unit  81  according to the present embodiment receives selection operation by the user with respect to the screen G 1  as described above. Then, in a case where the user selects the button B 4  for envelope on the screen G 1 , the envelope judgment unit  81  judges that the recording medium  9  is an envelope. Therefore, in the present embodiment, it is possible to identify whether or not a recording medium  9  is an envelope before a print job is started in the image formation apparatus  1 . 
       FIG. 12B  shows a screen G 2  that is displayed when the user selects an envelope. The screen G 2  is a screen for inputting a basis weight of an envelope. The screen G 2  is provided with a basis weight display field R 1  for displaying a basis weight of the envelope, and a button group B 5  for a user to input a numerical value is displayed at a position below the basis weight display field R 1 . When selecting an envelope as the recording medium  9 , the user can manually input a basis weight of the envelope used for image formation by operating the screen G 2 . 
     The basis weight acquisition unit  82  according to the present embodiment receives operation by the user with respect to the screen G 2  as described above. Then, the basis weight acquisition unit  82  acquires the basis weight input to the screen G 2  by the user as the basis weight of the envelope used for image formation. Therefore, in the present embodiment, it is also possible to acquire a basis weight of an envelope before a print job is started in the image formation apparatus  1 . 
     However, it may be difficult for the user to grasp a basis weight of the envelope used as the recording medium  9 . Therefore, the controller  8  may display a screen G 3  as shown in  FIG. 12C  on the display unit  7   a  instead of the screen G 2  in  FIG. 12B . The screen G 3  shown in  FIG. 12C  is a screen for displaying in a list format of a plurality of types of envelope images (thumbnail images) B 7 , B 8 , B 9 , and B 10  registered in advance in the image formation apparatus  1  Each of the envelope images B 7  to B 10  includes a function as a button with which the user can perform selection operation. Therefore, even in a case where the user does not know the basis weight of the envelope used for image formation, the user can identify and select the envelope used for the image formation from the plurality of envelope images B 7  to B 10  displayed in the screen G 3 . For example, a memory  71  of the controller  8  stores envelope information including envelope images and corresponding envelope basis weights, the envelope information being registered in the image formation apparatus  1  in advance. Then, when the user selects one envelope image, the basis weight corresponding to the envelope image can be acquired. That is, the basis weight acquisition unit  82  acquires a basis weight of an envelope used for image formation by receiving operation on the screen G 3  by the user and referring to envelope information on the basis of the envelope image selected by the user. In this case as well, it is possible to acquire a basis weight of an envelope before a print job is started in the image formation apparatus  1 . 
     As described above, the image formation apparatus  1  according to the present embodiment can judge whether or not a recording medium  9  is an envelope on the basis of information input by a user before starting a print job, and, if the recording medium  9  is an envelope, can acquire a basis weight of the envelope. Therefore, with the image formation apparatus  1 , various control parameters corresponding to basis weights of envelopes can be set by default for a print job executed on the basis of an instruction from the user. Therefore, it is not necessary to change system velocity Vs, or the like, after starting delivery operation of an envelope as a recording medium  9 . 
     Further, in a case where a recording medium  9  is an envelope, the image formation apparatus  1  according to the present embodiment can perform image formation corresponding to a basis weight of the envelope as in the first embodiment. Therefore, even if the recording medium  9  is a thin envelope, it is possible to perform image formation while reducing wrinkling of the envelope. 
     The points other than those described above in the present embodiment are similar to the points described in the first embodiment. For example, as described in the first embodiment, the image formation apparatus  1  may have a configuration in which an envelope detection sensor  40  and a basis weight detection sensor  50  are disposed along a delivery path  33  for a recording medium  9 . Therefore, in a case where the user does not designate a type of a recording medium  9  or does not designate a basis weight of an envelope before executing a print job, the image formation apparatus  1  performs operation described in the first embodiment. Thus, even if the recording medium  9  is an envelope, the image formation apparatus  1  can perform appropriate image forming operation corresponding to a basis weight of the envelope. 
     Further, the image formation apparatus  1  according to the present embodiment may not include an envelope detection sensor  40  and a basis weight detection sensor  50 . 
     Third Embodiment 
     Next, a third embodiment of the present invention will be described. In the present embodiment, there will be described an aspect in which an image formation apparatus  1  acquires information about a recording medium  9  and a basis weight thereof from an external apparatus. Note that a configuration of an image formation apparatus  1  according to the present embodiment is similar to the configuration of the image formation apparatus  1  described in the first embodiment. 
       FIG. 13  is a diagram showing a configuration example of the image formation apparatus  1  according to the third embodiment. The image formation apparatus  1  is connected to an external information processing apparatus  101  via a network  100  such as a LAN. The information processing apparatus  101  includes, for example, a personal computer (PC) or the like, and can communicate with the image formation apparatus  1  via the network  100 . In the present embodiment, an image formation system is constructed by the image formation apparatus  1  and the information processing apparatus  101 . The image formation system may include an apparatus other than the image formation apparatus  1  and the information processing apparatus  101 . 
     The information processing apparatus  101  includes a recording medium measurement apparatus  110 . The recording medium measurement apparatus  110  has a substantially U-shaped outer shape in which an upper housing  111  and a lower housing  112  are disposed so as to face each other at a predetermined distance in a vertical direction. A gap between the upper housing  111  and the lower housing  112  is an insertion space  113  into which a recording medium  9  can be inserted. The upper housing  111  and the lower housing  112  have configurations similar to the configurations of the envelope detection sensor  40  and the basis weight detection sensor  50  described in the first embodiment. That is, there is described in the first embodiment an example in which the envelope detection sensor  40  and the basis weight detection sensor  50  are disposed so as to sandwich a delivery path  33  for a recording medium  9 . Meanwhile, in the present embodiment, each of an envelope detection sensor  40  and a basis weight detection sensor  50  is disposed so as to sandwich the insertion space  113 . Therefore, by causing the envelope detection sensor  40  and the basis weight detection sensor  50  to operate with a recording medium  9  inserted in the insertion space  113 , the recording medium measurement apparatus  110  can detect whether or not the recording medium  9  is an envelope, and also can detect a basis weight of the recording medium  9 . 
     When a user gives an instruction on start of detection by operating the information processing apparatus  101  with a recording medium  9  to be used in the image formation apparatus  1  inserted in the insertion space  113  of the recording medium measurement apparatus  110 , measurement operation of a recording medium  9  starts in the recording medium measurement apparatus  110 . That is, the recording medium measurement apparatus  110  starts operation to detect whether or not the recording medium  9  inserted in the insertion space  113  is an envelope and detect a basis weight of the recording medium  9 . When the measurement operation is completed, the recording medium measurement apparatus  110  outputs, to the information processing apparatus  101 , a detection result of whether or not the recording medium  9  is an envelope and the basis weight of the recording medium  9 . 
     When the information processing apparatus  101  receives the detection result of whether or not the recording medium  9  is an envelope and the basis weight of the recording medium  9  from the recording medium measurement apparatus  110 , the information processing apparatus  101  generates information about the recording medium  9  on the basis of the information. At this time, if it is detected that the recording medium  9  is an envelope, the information processing apparatus  101  converts the basis weight of the recording medium  9  into a basis weight of an envelope. Then, the information processing apparatus  101  transmits information about the recording medium  9  to the image formation apparatus  1 . 
     When receiving the information about the recording medium  9  via a communication unit  72 , the image formation apparatus  1  saves the information in the memory  71  as information about the recording medium  9  newly set in a paper feed tray  3  or a manual feed tray  4 . Therefore, the image formation apparatus  1  can register the information about the recording medium  9  before starting a print job. 
     Further, the recording medium measurement apparatus  110  may not have an envelope detection sensor  40  built in. In this case, the information processing apparatus  101  can grasp that a recording medium  9  is an envelope by receiving envelope designation operation by the user, and can convert a basis weight of the recording medium  9  into a basis weight of the envelope. 
     Therefore, an envelope judgment unit  81  according to the present embodiment can judge whether or not the recording medium  9  is an envelope on the basis of information about the recording medium  9 , the information being received from outside via the communication unit  72 . The basis weight acquisition unit  82  according to the present embodiment can acquire a basis weight of the recording medium  9  on the basis of the information about the recording medium  9 , the information being received from outside via the communication unit  72 . Therefore, in a case where a recording medium  9  is an envelope, the image formation apparatus  1  can perform image formation corresponding to a basis weight of the envelope as in the second embodiment. Therefore, even if the recording medium  9  is a thin envelope, it is possible to perform image formation while reducing wrinkling of the envelope. 
     The above description exemplifies the aspect in which the recording medium measurement apparatus  110  transmits information about a recording medium  9  to the image formation apparatus  1  via the information processing apparatus  101 . However, not limited to this, in a case where the recording medium measurement apparatus  110  can directly communicate with the image formation apparatus  1 , a configuration may be employed in which the recording medium measurement apparatus  110  directly transmits information about a recording medium  9  to the image formation apparatus  1  without using an information processing apparatus  101 . 
     The points other than those described above in the present embodiment are similar to the points described in the first embodiment. For example, as described in the first embodiment, the image formation apparatus  1  may have a configuration in which an envelope detection sensor  40  and a basis weight detection sensor  50  are disposed along a delivery path  33  for a recording medium  9 . Therefore, in a case where information about a recording medium  9  is not saved in the memory  71  before a print job, the image formation apparatus  1  performs operation described in the first embodiment. Thus, even if the recording medium  9  is an envelope, the image formation apparatus  1  can perform appropriate image forming operation corresponding to a basis weight of the envelope. 
     Further, the image formation apparatus  1  according to the present embodiment may not include an envelope detection sensor  40  and a basis weight detection sensor  50 . 
     (Modifications) 
     Although embodiments of the present invention have been described and illustrated in detail, the disclosed embodiments are made for purposes of illustration and example only and not limitation. The scope of the present invention should be interpreted by terms of the appended claims, and various modifications can be applied. 
     For example, the above embodiments exemplify a case where an image formation apparatus  1  is formed as an MFP. However, the image formation apparatus  1  is not limited to an image formation apparatus formed as an MFP. That is, the image formation apparatus  1  may be a printer including a printing function exclusively. 
     Further, the above embodiments exemplify a case where a program  78  executed by a controller  8  of an image formation apparatus  1  is stored in a memory  71  in advance. However, the program  78  may be installed or updated in the image formation apparatus  1  by being downloading from outside via a communication unit  72 . In this case, the program  78  is provided so as to be downloadable on the image formation apparatus  1  via a network. Further, the program  78  may be provided recorded on a computer-readable recording medium such as a CD-ROM or a USB memory.