Patent Publication Number: US-2005143239-A1

Title: Plastic sleeve for a blanket cylinder and a method for producing the plastic sleeve

Description:
FIELD OF THE INVENTION  
      This invention relates to a method for producing a plastic sleeve for use with a blanket cylinder in an electrophotographic process wherein the plastic sleeve requires no metal core.  
     BACKGROUND OF THE INVENTION  
      In electrophotographic processes requiring an image cylinder and a blanket cylinder to produce electrophotographic copies, the image cylinder typically receives a charge, an image and a toner coating on the image area and then transfers the toner image to a blanket cylinder. The blanket cylinder transfers the toner image to a substrate, such as paper or the like, which passes via a web between the blanket cylinder and a back pressure roller to transfer the toner image to the substrate with the substrate thereafter being fused, as well known to the art.  
      In such processes, the blanket cylinder is a cylinder that typically includes a mandrel, which may be of aluminum, steel or any other suitably durable metal or conductive plastic of a suitable thickness to produce a noncompliant member that may be about 10 millimeters (mm) in thickness. The mandrel may include reinforcing structure internally and includes a very smooth low out-of-round tolerance exterior. The blanket cylinder includes a mandrel and a sleeve positioned over the outside of the mandrel and is used for transfer of the images from the blanket cylinder to a substrate. The mandrel also includes bearings connected to each of its ends for positioning it in an electrophotographic copying machine and has an air inlet into an interior of the mandrel for an air discharge through a plurality of holes placed around one end of the mandrel near a tapered end of the mandrel.  
      The sleeves have been produced by use of a metal core, which is typically a noncompliant metal member, such as nickel or the like, which is produced by plating. The core must be seamless and must provide a very low variation surface outer diameter. The plastic layer is then positioned around the outside of the metal core, the metal core is mounted on a mandrel or the like, and the plastic layer is machined to a desired thickness. Additional exterior coatings have been applied by techniques such as ring coating and the like. The completed sleeve will have an internal diameter slightly less than the outer diameter of the mandrel upon which it is to be placed. This interference fit allows the sleeve to be firmly positioned on the outside of the mandrel after it is installed. The sleeve must have a smooth exterior and a closely controlled wall thickness.  
      The sleeve is typically installed by urging it toward and onto the tapered end section of the mandrel while air is ejected through the holes at the end of the mandrel near the tapered section. The air injection permits the positioning of the sleeve on the mandrel by an air bearing technique as known to those skilled in the art. The interference fit between the sleeve and the mandrel is accomplished and the sleeve is retained snugly and firmly in position on the outside of the mandrel. The outside of the mandrel, including the sleeve, must have an outside diameter variation within a range of about ±12.5 microns. This close tolerance is necessary to ensure accurate receipt of images from the image cylinder and transmission of the images to the substrate by the blanket cylinder.  
      There are various other specific requirements for the blanket cylinder and it has been previously considered necessary to meet these other requirements as well as those discussed above by the use of a metal core in the sleeve. This is a relatively expensive, time-consuming step and the cores are relatively expensive. As a result, a continued effort has been directed to the development of methods for producing sleeves more economically that will meet the demanding requirements for the blanket cylinder sleeves.  
     SUMMARY OF THE INVENTION  
      According to the present invention, plastic sleeves that do not include a metal core are produced for use with a blanket cylinder. Such sleeves are-produced by a method wherein a quantity of a liquid plastic or liquid plastic precursors are positioned in a mold having an inside and an outside and an inner diameter equal to the desired outer diameter of the sleeve, the quantity being a controlled amount sufficient to produce a sleeve on the inside of the mold having a selected wall thickness, rotating the mold at a velocity to produce a centrifugal force of at least about 5 times the force of gravity at the inside of the mold until the plastic has solidified, heating the mold to a temperature from about 20 to about 100° C. during rotation of the mold and removing the sleeve from the mold, the sleeve having a wall thickness from about 1 to about 20 mm.  
      The invention further includes a sleeve for use by positioning the sleeve over an outer diameter of a mandrel to form a blanket cylinder, the sleeve consisting essential 14 of a plastic sleeve having a wall thickness from about 1 to about 20 mm, an inner diameter smaller than the outer diameter of the mandrel, a Shore A hardness of about 60±5, and a wall thickness variation of no more than about ±12.5 microns from the average wall thickness. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  shows an embodiment of a process and system wherein a blanket cylinder according to present invention is used;  
       FIG. 2  is a schematic diagram of a mandrel and a sleeve, with the sleeve being positioned for installation on the mandrel;  
       FIG. 3  is a schematic diagram of a sleeve positioned on the mandrel; and  
       FIG. 4  is a schematic diagram of a mold positioned for rotation and injection of plastic material to form a plastic sleeve according to the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
      In the description of the figures, the same numbers will be used to refer to the same or similar components throughout in the discussion of the figures.  
      In  FIG. 1 , an electrophotographic process and system  10  are shown. The process includes a blanket cylinder  12  positioned in engagement with a blanket cylinder  14 , which is positioned in engagement with a web  16  and a back pressure roller  18 . A substrate  20 , which may be paper or the like, is passed along a web  16  between blanket cylinder  14  and back pressure roller  18 . The substrate, now bearing an image, is passed along web  16  to a fuser section  26  where it is fused as known to those skilled in the art. The direction of travel of the substrate is shown by arrow  22 . A sensor  24  is positioned to ensure that substrate  20  passes in contact with blanket cylinder  14  at a proper time so that the image is properly positioned on substrate  20 .  
      In the operation of the process, image cylinder  12  rotates in a direction shown by arrow  36  and blanket cylinder  14  rotates in a direction shown by arrow  42 . Back pressure roller  18  turns in a direction as shown by arrow  19 . A nip  38  is formed between image cylinder  12  and blanket cylinder  14 . The nip is typically about 4.5±1 mm in width. Similarly, a nip is formed between blanket cylinder  14  and back pressure roller  18 . This nip is about 4 to about 10 mm in width. A cleaning station  28  is positioned to contact the surface of image cylinder  12  after it passes nip  38 . The clean cylinder surface then passes a charger station  30 , a writer station  32  where an electrostatic image is placed on the surface of cylinder  12  and a toner section  34  that applies toner to the electrostatic image, which is then transferred at nip  38  to blanket cylinder  14 . Blanket cylinder  14  transfers the image to substrate  20  and is thereafter cleaned by a cleaner  40  to ensure that a clean surface is provided on blanket cylinder  14  for transfer of an additional image from image cylinder  12 .  
      Image cylinder  12  and blanket cylinder  14  are both of similar construction, although the materials and properties of their exteriors are different. As shown in  FIG. 2 , blanket cylinder  12  is a mandrel  50  that has side walls  54  and ends  52  and includes a tube  60  which supports a bearing  62  in operative engagement with one end of mandrel  50  and a shaft  64  which supports a second bearing  66  in operative engagement with the other end of mandrel  50 .  
      Tube  60  is adapted for the injection of air into mandrel  50 , which includes near one of its ends, a taper  56 , and a plurality of air holes  58 . These air holes are used for the ejection of air during the installation of a sleeve over mandrel  50 . Mandrel  50  has an outside diameter  68 , which is somewhat larger than the inner diameter  72  of a sleeve  70 . Sleeve  70  as shown, is a plastic sleeve according to the present invention. Its end  74  is urged into engagement with tapered section  56  of mandrel  50  and the sleeve is placed over an outside diameter  68  of mandrel  50  by an air step process using the ejection of air through holes  58 .  
      In  FIG. 3 , an installed sleeve, according to the present invention, is shown on mandrel  50 . The mandrel and sleeve that comprise blanket cylinder  14  are now assembled.  
      The blanket cylinder may have a diameter from about 2 cm to about 400 cm. While the diameter may vary widely, the variations in mandrel diameter or the out of round run out must be limited to ±12.5 microns. This is necessary to ensure that the proper nip is achieved with the blanket cylinder and the image cylinder and that good image transfer from the image cylinder to the blanket cylinder and from the blanket cylinder to the substrate is accomplished.  
      Desirably the outside of sleeve  70  has a Shore A hardness of about 60±5. The hardness is readily varied by changing the formulation of the plastic, as well known to those skilled in the art. The thickness of the sleeve wall may be from about 1 to about 20 mm. The sleeve wall is plastic and is rigid enough to handle. Further the plastic desirably has a conductivity of at least 10 8  to 10 14  ohms·cm. Generally the conductivity of the blanket cylinder sleeve is less than for the image cylinder sleeve, although the charge on the blanket cylinder is typically higher than that on the image cylinder. To produce an acceptable exterior surface on blanket cylinder  14 , it is necessary that the wall thickness of the sleeve be held to a thickness variation of ±12.5 microns. Generally the blanket cylinder sleeve exterior is more compliant than the exterior of the image cylinder sleeve.  
      Sleeves have been formed in the past by positioning the sleeves on a seamless metal core typically formed by plating. The metal core provided support for the positioning of the plastic around the metal core and then the plastic was machined to the required size. Both the requirement for the metal core and the requirement for machining represent expensive and time consuming operations that have been required to achieve the precision necessary to produce the sleeves for the blanket cylinder.  
      As well known to those skilled in the art, a thin hard release layer such as a sol-gel, a ceramer or a fluoropolymer may be placed on the outside of the sleeve for the blanket cylinder. This layer has been applied by processes such as ring coating. It is also known that inorganic or organic layers may be applied over the outside of the sleeve to modify surface properties such as surface energy. The use and application of such outer layers is not considered to constitute part of the present invention, which is directed to the production of a sleeve for a blanket cylinder meeting the exacting requirements for such a sleeve.  
      According to the present invention, such sleeves are readily produced by a method wherein a quantity of plastic precursors, a suitable liquid plastic, or liquefiable plastic are positioned in a mold with the mold then being heated by a heater  90  to a suitable temperature to result in the presence of the plastic in the mold in a liquid form. A suitable mold is shown schematically in  FIG. 4 . The mold has a wall  78  and ends  82 . Openings  84  are generally left in the ends. While the plastic could be introduced in a number of ways, it is shown as being introduced through a tube  86  that supplies a quantity of plastic suitable to form a sleeve of the desired thickness and of uniform thickness in the mold. The uniformity is achieved by spinning the mold while the plastic is heated and as the plastic moves through a molten phase. The molding may be done with a thermoplastic material, which after liquefying is allowed to cool back into a hardened phase. In any event, the formation of the solid sleeve is accomplished with rotation of the mold at a rate sufficient to produce a centrifugal force equal to at least five and preferably at least about 10 and desirably from about 5 to about 100 times the force of gravity at the inside of the mold until the plastic has solidified.  
      Suitable plastics include thermoplastic, thermosetting and elastomeric plastics and particularly polyurethanes are preferred. With polyurethanes, polymer precursors may be placed into the mold and allowed to polymerize, cross-link and otherwise react to form the desired plastics as the mold spins. Polysiloxanes may also be used. In any event, it has been found that when the inside of the mold is formed to have a surface within the required variations for the outside of the sleeve, that the inside of the sleeve and the sleeve wall thickness can be produced to sufficiently close tolerances by this method to enable its positioning over a mandrel and use on the blanket cylinder.  
      The heating and cooling of the mold may be at rates deemed suitable for the particular plastic used and desirably the rotation of the mold is continued until the plastic has reached a temperature of about 100° C. and thereafter until the plastic is cooled to a temperature selected for convenience in handling and the like. After cooling the sleeve typically releases from the inside of the mold by contraction of the plastic so that the sleeve is readily removed from the mold by simply removing one of ends  82  and removing the sleeve. The sleeve is then ready for use or for coating with additional materials that may be desired on its exterior. A heater  90  is shown but it will be understood that any suitable type of heater can be used.  
      By the process of the present invention, sleeves for the blanket cylinder can be produced much more economically and more efficiently than with previously used methods. The sleeves produced consist essentially of a plastic sleeve for use by positioning it over an outer diameter of a mandrel. The plastic sleeve has a wall thickness from about 1 to about 20 mm, an inner diameter smaller than the outer diameter of the mandrel, a Shore A hardness of about 60±5 and a wall thickness variation of no more than about ±12.5 microns from the average wall thickness. These sleeves are highly desirable as replacement sleeves around the outside of mandrels in blanket cylinders. These sleeves are also much more economically produced while providing sleeves of an equivalent or superior quality to those produced by prior art methods.  
      As well known to those skilled in the art, it is desirable in some instances to modify the surface energy properties of the blanket cylinder sleeve. This may be readily accomplished by coating a suitable surface energy modifying material onto the surface of the sleeve. This may readily be done by ring coating or the like.  
      While the present invention has been described by reference to certain of its preferred embodiments, it is pointed out that the embodiments described are illustrative rather than limiting in nature and that many variations and modifications are possible within the scope of the present invention. Many such variations and modifications may be considered obvious and desirable by those skilled in the art based upon a review of the foregoing description of preferred embodiments.