Method and apparatus for ambient temperature pressure fixing of toners
United States Patent [191 Pedersen METHOD AND APPARATUS FOR AMBIENT TEMPERATURE PRESSURE FIXING OF TONERS [75] Inventor: Peder M. Pedersen, Elmhurst, 111.  
 [73] Assignee: Addressograph-Mul tigraph Corporation, Cleveland, Ohio [22] Filed: Oct. 27, 1972 [21] Appl. No.: 301,463  
 [52] US. Cl l17/l7.5, 96/1 SD, 117/21,  
 117/111 R, 118/116, 118/637, 355/3 DD&#39; [51] Int. Cl. G03g 13/20, G03g 15/20 [58] Field of Search 117/175, 21, 111 R; 118/114, 116,117,637, 641; 219/216, 388;  
 355/3 DD; 96/1 SD [451 Apr. 1, 1975 Primary Examine rMichael Sofocleous Attorney, Agent, or Firm-Howard L. Weinshenker; Sol L, Goldstein; Russell L. Root [57] ABSTRACT Improvement as defined in the title of the application wherein the working surface of at least one of a pair of toner-fixing pressure rolls is formed of nylon-6 and undergoes recovery from deformation at ambient temperatures while remaining in service. Method of ef- [56] References Cited f d f 1 UNITED STATES PATENTS gegctigfegn-stream recovery rom e ormatron 1s a so 3,094,429 6/1963 Howell 1l7/17.5 3,269,626 8/1966 Albrecht 220/177 4 Claims, 1 Drawing Flgure 094F615? iai fg PEI/HOPE? rem 3m f smmw I METHOD AND APPARATUS FOR AMBIENT TEMPERATURE PRESSURE FIXING OF TONERS This invention relates to improvement in the ambient temperature pressure-fixing of toners. The invention has particular utility in electrostatic copying apparatus.  
  Electrostatic copying methods and apparatus for accomplishing the same are well known in the art. Typically, electrostatic copying involves providing a photoconductive surface, establishing a uniform electric charge on the surface, and projecting on the charge surface a light and shadow image of the subject matter to be reproduced, thereby discharging portions of the photoconductive surface in accordance with the intensity of radiation reaching the surface. This creates an electrostatic latent image which is developed by exposing it to finely divided toner particles which are attracted by electrostatic forces to those portions of the photoconductive surface which have retained their charge. The image so developed is then made permanent by a fixing or fusing operation.  
  Fusing generally refers to a method conducted at elevated temperatures involving a coalescene of the particles to each other and an adhesion of the resulting mass to a substrate. The term fixing is generally applied to a procedure whereby coalescence is accomplished at ambient temperatures either through the use of pressure or a solvent system.  
  In some electrostatic copying arrangements, the photoconductive surface is an integral portion of the copy. In other systems, the photoconductive surface is formed on an intermediary member such as a platen or cylinder, from which a developed toner image is transferred to a sheet for subsequent fusing or fixing to provide the final copy. The present invention is applicable to either of these arrangements because it is addressed to that portion of the electrostatic copying method which is concerned with giving permanency to the toner image once developed on or transferred to the copy sheet.  
  Until recently, most commercial embodiments of electrostatic copying machines employed elevated temperature fusing devices because they were most consistent in producing high quality copies. This, notwithstanding their disadvantages of high initial and maintenance costs and the ever present danger of fire. However, a recent discovery in ambient temperature pressure-fixing, described and claimed in copending application Ser. No. 51,089, now abandoned, assigned to the assignee of the present application, has removed the principal road block standing between ambient temperature pressure-fixing and commercial success. The problem solved in the co-pending application is embodied in a method and apparatus which provides a pressure roll with a somewhat compliant surface which accommodates itself through deformation to minor differences in the thickness of the sheet material and toner fed between the pressure members. By loading the pressure members sufficiently to provide a calculated peak nip pressure in excess of the threshold pressure for the toner, and by selecting a roll which will deform an amount in excess of the thickness variations of the sheet and toner, uniform fixing of the toner across the entire width of the sheet can be effected.  
  As further described in the co-pending application, conventional toners can be fixed uniformly by employing as the working surface on the deformable roll a material which has a calculated modulus of elasticity in compression within the range of about 100,000 psi and about 2,000,000 psi, and which when loaded to provide a calculated peak nip pressure averaging between 5,000 and 10,000 psi, deforms an amount to which exceeds the thickness variations of the copy sheet and toner. As a specific example, the co-pending application discloses the use of a cotton filled roll having a Shore D hardness of about and a calculated modulus of elasticity in compression of about 200,000 psi. Other suitable materials for forming the compliant roll are said to include hard rubber, paper fill rolls, rolls formed from corn cobs or abestos, or any of various resilient synthetic resinous materials such as nylon and acetal resin.  
  The thickness variations accommodated by the compliant roll disclosed in co-pending application Ser. No. 51,089, now abandoned are small, and of the type encountered during normal operation of the pressure fixing device. The deformation the roll undergoes is, perhaps, on the order of 0.0004 inches, which is well within the elastic limit of the material forming the working surface of the roll.  
  There are situations, however, when during abnormal operating conditions the roll is forced to undergo a much more severe level of deformation which exceeds the elastic limit of the material forming the working face of the roll, but yet is within the plastic limit for the material. Deformation of this severe character may be encountered where, for example, the sheet material is misfed and becomes folded upon itself; or a defect in the sheet material produces a lump having a thickness many times that of the nominal thickness of the sheet; or the sheet is fed between the pressure rolls with a paper clip or some other similar structure. For purposes of this discussion such abnormal deformation will be referred to as aberrant deformation. The term is intended to embrace levels of deformation which exceed the usual level of deformation encountered during normal operation of the copier. More specifically, aberrant deformation is that level of deformation caused by localized pressure contact of a pressure roll with a material substantially thicker than the combined thickness of the sheet material and toner.  
  Heretofore the occurrence of aberrant deformation was handled in one of two ways. Either the defect was tolerated, or the deformed roll was replaced. ln elevated temperature fusing devices some occurrence of aberrant deformation could be tolerated without too much difficulty since in many instances there was enough radiant heat energy in the vicinity of the deformation to effect at least a degree of fusion even though portions of the roller were out of contact with the toned sheet, inhibiting heat transfer by conduction.  
  The problem is more severe in an ambient temperature pressure-fixing device since no level of fixing will take place at the deformed locations.  
  It is also known that aberrant deformation in rolls having working surfaces formed of certain thermoplastic materials can be healed by applying heat to the deformed areas thereby causing the thermoplastic materials to expand, flow and thus relieve the stresses created by the deformation. Since the rolls used in a pressure-fixing device are unheated, correction of aberrant deformation of such rolls would still require that they be taken out of service and repaired.  
  As previously noted, aberrant deformation of pressure rolls in an electrostatic copier embodying an ambient temperature pressure-fixing device has a particularly serious impact. During the time that the deformed roll is retained in service, less than satisfactory copies will be produced since toner opposing the deformed portions of the roll will remain unfixed and readily removable from the copy sheet. Further, even where such fixing devices employ rolls having thermoplastic working surfaces which can be restored at elevated temperatures, these will have to be taken out of service to be repaired, since the operating environment of the device does not generate sufficient heat to effect in-service restoration of the roll. Removing a roll from service, replacing it with a new or repaired roll and repairing or discarding the deformed roll is an expensive and time consuming proposition.  
  It is therefore apparent that there is a need for an improved pressure-fixing device for electrostatic copy apparatus wherein aberrant deformation of rolls can be corrected while the roll remains in service. The present invention is addressed to filling this need.  
  In accordance with one aspect of the invention there is provided, in electrostatic copying apparatus having a device for the ambient temperature fixing of an image on sheets, comprising a pair of unheated pressure rolls at least one of which has a smooth, deformable surface which is susceptible to failure due to aberrant deformation, means for applying pressure to the roll sufficient to provide aberrant deformation and means for feeding individual sheets to the rolls at spaced intervals, the improvement comprising in combination, a working surface on at least one of the rolls, consisting essentially of nylon-6 and a working surface on the other roll which is nylon-6 or a material at least as hard as nylon- 6, the nylon-6 working surface having a thickness of at least about 0.25 inches, sufficient to permit it to recover aberrant deformation at ambient temperature, while remaining in service.  
  In accordance with another aspect of the present invention there is provided a method of effecting onstream, ambient temperature correction of aberrant deformation of pressure rolls operative to pressure fix toner deposited on a sheet advanced therebetween, which comprises providing a working surface consisting essentially of nylon-6 on at least one ofthe rolls and a working surface on the other roll which is nylon-6 or a material at least as hard as nylon-6, advancing toned sheets between the rolls at spaced intervals, and maintaining the rolls in direct rolling contact with each other when out of contact with successive advancing sheets, whereby aberrant deformation developed during the advancement of sheets between the rolls are corrected when the rolls are in direct rolling contact with each other.  
  Thus, I have discovered that where at least one pressure roll in a pair of such rolls is provided with a working surface of nylon-6 and the other roll has a working surface of nylon-6 or a material at least as hard as the working surface on the nylon-6 roll, any aberrant deformations which develop during the advancement of tone sheets between the rolls, can be corrected at ambient temperatures when the rolls are in direct rolling contact with other, i.e., when they are out of contact with successive advancing sheets.  
  Surprisingly, of the four major classes of polyamide resins, only those made by the polymerization of lactams, and more specifically only nylon-6, made by the polymerization of e-caprolactam possesses the necessary combination of properties which permits ambient temperature correction of aberrant deformation by direct rolling contact between the deformed roll and its mating pressure roll. Polyamide resins falling within the other three classes, i.e., those made by the condensation of a diamine with a dibasic acid, as exemplified by nylon-6,6 and nylon-6,10; those made by selfcondensation of amino acids, as exemplified by nylonl l; and vegetable oil-based polyamide resins as exemplified by the Versamid polyamide resins, are not useful for purposes of the present invention.  
  It is therefore an object of the present invention to provide an improvement in ambient temperature pressure-fixing of toner images in electrostatic copying.  
  A further object of the present invention is to provide in electrostatic copying apparatus, an improved sheet fed, ambient temperature pressure-fixing device having at least one pressure roll which is capable of recovering from aberrant deformation at ambient temperatures while remaining in service.  
  Another object of the invention is to provide an improved ambient temperature pressure-fixing device of the type described in which at least one of the pressure rolls has a working surface formed of nylon-6.  
  Yet another object of the invention is to provide a method of effecting on-steam recovery from aberrant deformation of a pressure roll in an ambient temperature pressure-fixing device in an electrostatic copier.  
  These and other objects and advantages of the invention will become apparent from the following detailed description of the invention when taken in conjunction with the accompanying drawing in which:  
  The sole FIG. is a schematic representation of an electrostatic copier in which the pressure-fixing device is pictorially illustrated.  
  Referring to the drawing, it will be seen that the conventional portions of an electrostatic copier are depicted in block diagram form. These include charger A, for establishing a uniform electric on the surface of a photoconductive member; exposure station B, where a light and shadow image of the subject matter to be reproduced is projected on the charged surface of the photoconductive member; developer station C, where contact is effected between the charged portions of the photoconductive member and a finely divided particulate toner; and optionally, transfer station D, where the toner image is transferred from the photoconductive member to a copy sheet. in certain embodiments of electrostatic copying equipment the photoconductive member constitutes the copy sheet, and in such arrangements, transfer station D is omitted.  
  The remaining portions of the drawing depict a feed station designated generally as E and a pressure fixing station designated generally as F.  
  The means comprising feed station E include endless belt 10, reeved about drive roll 12, driven by means not illustrated, and idler rolls I4, 16, 18. With belt 10 moving in the direction illustrated by the arrows, copy sheets 20 with their unfused toner images 22, are advanced in spaced relationship, from left to right (as the drawing is viewed) toward pressure fixing station F.  
  The pressure fixing station is defined by a pressure fixing device comprising stationary upper frame member 24 upon which are mounted for rotation, ends 26 of upper unheated roll 28; and movable lower frame member 30, upon which are mounted for rotation ends 32 of lower unheated roll 34.  
  The pressure-fixing device further comprises separate means for applying pressure to the ends of rolls 28, 34 each of which in the embodiment illustrated comprises bolt 36 mounted in aligned apertures in upper frame member 24 and lower frame member 30, compression spring 38, washer 40, adjusting nut 42 and locking nut 44. To increase the pressure on rolls 28, 34, adjusting nut 42 is turned in thedirection which forces washer 40 against compression spring 38 causing the spring to compress further. This in turn causes movable lower frame member 30 and roll 34 to move upwardly toward stationary upper frame member 24 and roll &#39;28. The setting is maintained by snugging locking nut 44 against adjusting nu&#39;t42,  
  The pressure applied to sheet 20 by rolls 28, 34 is such that upon passage through the unheated rolls, unfixed toner image 22 is converted to fixed toner image 46 at ambient temperatures.  
  In the preferred form of the invention illustrated, lower roll 34, i.e., the roll which is out of contact with the toner image is designated as being more deformable or compliant than .its mating upper roll 28. Thus, in the preferred form of the invention, it is lower roll 34 which is provided .with a working surface illustrated as sheath 48, consisting essentially of nylon-6, while upper roll 28 is preferably formed of steel.  
  However, it is contemplated that both rolls may have the identical degree of compliancy or deformability, in which event each would have working surfaces of the identical nylon-6 resin. In a still further modification, it is contemplated that both rolls may be compliant but that the upper roll is somewhat less compliant than the lower roll. In this embodiment it is contemplated that the lower roll would have a working surface of nylon-6, while the upper roll would have a working surface which is slightly harder than the working surface of the lower roll. Such a surface may either be a harder form of nylon-6 or some other deformable material which is relatively less compliant than the nylon-6 working surface of the lower roll.  
  Where a nylon-6 working surface is provided it should be of a thickness sufficient to permit the surface to recover from aberrant deformation at ambient temperature. In general, it has been found that thickness of at least about 0.25 inches is satisfactory for this purpose. However, it will be appreciated that working surfaces of substantially greater thickness may be employed, and indeed, the entire roll may be formed of nylon-6.  
  As referred to herein, nylon-6 denotes the polymerization product of e-caprolactam. In general, polymerization of e-caprolactam is conducted by heating the monomer to a temperature above about 250C in the presence of a catalyst for example water, or phosphoric acid or alkali metals; and a chain terminator, such as a lower aliphatic acid, for example acetic acid.  
  As obtained from a continuouos reactor, the mixture is approximately 90 percent polymer and 10 percent monomeric e-caprolactam. The latter can be removed in a number of ways including leaching with hot water and vacuum distillation. The polymer can then be pelletized for subsequent use as feed material to an extruder. I  
  Various conventional additives may be incorporated in nylon-6, for example heat and light stabilizers, ultraviolet inhibitors and colorants.  
  Reference may be had to U.S. Pat. Nos. 3,017,391; 3,017,392; 2,018,273; and 3,028,369 for discussion of prior art processes for polymerization of e-caprolactam to nylon-6 which may be used in the practice of the present invention. Of course nylon-6 is commercially available from a number of sources, for example from Allied Chemical Corporation, under the name Caprolan.  
 To form relatively thin working surfaces of nylon-6,  
  for example on the order of 1 inch to 1 inch thick, it  
 minator and the mixture is poured into the mole.  
  Typical properties for nylon-6 as reported in Floyd Polymide Resins, Second Edition, Reinhold Publishing Corporation, 1966, are set forth in the table below.  
 TABLE I ASTM PROPERTIES UNl&#39;l&#39;S METHOD NYLON-6 Density D792-5 7T 1.14 Tensile strength at 73F psi D638-52T 12,000 at 170F psi D638-52T 7,300 Elongation at 73 F D638-52T 50-90 at 170F 7r D638-52T -200 Modulus of elasticity at 73F psi D638-52T 400.000 Rockwell hardness D785-5l R1 18 210W temperature F D569-48 419 oefficient of linear thennal expansion F D696-44 4.8 X 10&#39; Thermal conductivity Btu/hr/ft&#34;/ 1.2  
  F/in. Specific heat 0.4 Dielectric strength Volts/mil Dl49-55T 480 (short time) Dielectric constant 1000 cycles Dl50-54T 4.8 Water absorption 24 hr D57057T 2.3 Volume resistivity ohm-cm D257-54T 4.5 X 10 Dielectric constant at 10&#34; cycles Dl50-54T 4.8 Power factor at 10 cycles Dl50-54T 0.05 Water absorption D570-64T 1.6  
  Referring again to FIG. 1, it will be noted that sheets 20 are fed at spaced intervals to pressure fixing station F. When rolls 28, 34 are out of contact with a sheet 20, the rolls are in pressure contact with each other. It is believed that the direct rolling contact of rolls 28, 34 with each other contribute importantly to the ability of the nylon-6 sheath 48 on roll 34 to recover from aberrant deformation at ambient temperatures while in service.  
  As previously noted upper roll 28 should be formed of nylon-6 or a material which is at least as hard as nylon-6. This willensure that only a roll having a working surface of nylon-6 will suffer aberrant deformation. Thus if upper roll 28 is a steel roll only lower roll 34 will undergo aberrant deformation. If both rolls have working surfaces of nylon-6 they will both suffer aberrant deformation but each will be capable of being restored at ambient temperatures while remaining in service.  
 Having thus described my invention, I claim:  
  1. Apparatus for providing a permanent image on a sheet wherein the image is a fixed image of toner powder, comprising means for creating an electrostatic latent image; means for depositing toner powder upon said sheet to form a toner powder image pattern in conformity with said latent image; and means for fixing the toner powder image to said sheet, said last mentioned means including two unheated pressure rolls at least one of which has a smooth, deformable surface which is susceptible to failure due to aberrant deformation, said deformation exceeding the elastic limit for the surface but being within the plastic limit for the surface, said deformation being caused by localized pressure contact with a material substantially thicker than the combined thickness of said sheet and said toner powder; means for applying pressure to said rolls sufficient to fix said toner powder image and to produce aberrant deformation of said deformable surface; means for feeding individual sheets to said pressure rolls at spaced intervals; a working surface on said at least one roll, consisting essentially of nylon-6 and a working surface on the other roll which is at least as hard as said nylon- 6, said nylon-6 working surface having a thickness of at least about 0.25 inches, sufficient to permit it to recover from aberrant deformation at ambient temperature, while remaining in service.  
  2. The apparatus as defined .in claim 1, wherein each of said rolls is provided with a working surface consisting essentially of nylon-6.  
  3. The apparatus as defined in claim 1, wherein said other roll has a steel working surface.  
  4. The method of providing a permanent image on a sheet wherein the image is a fixed image of toner powder comprising the steps of creating an electrostatic latent image; depositing toner powder upon said sheet to form a toner powder image pattern in conformity with said latent image; and fixing the toner powder image to said sheet, said last mentioned step including providing two unheated pressure rolls, at least one having a working surface consisting essentially of nylon-6, and the other having a working surface which is at least as hard as said nylon-6; and advancing sheets, bearing toner powder images between said rolls at spaced intervals, thereby fixing said toner images and maintaining said rolls in direct rolling contact with each other when out of contact with successive advancing sheets, whereby aberrant deformations which exceed the elastic limit for the material forming the working surface of the rolls but are within the plastic limit for said material developed during the advancement of sheets between said rolls, are corrected when the rolls are in direct rolling contact with each other.