Patent Application
20120148274
This invention relates to maintaining a consistent image gloss when continuously printing on receivers of mixed basis weights.
In typical commercial electrostatographic reproduction apparatus (copier/duplicators, printers, or the like), a latent image charge pattern is formed on a uniformly charged charge-retentive or photo-conductive member having dielectric characteristics (hereinafter referred to as the dielectric support member). Pigmented marking particles are attracted to the latent image charge pattern to develop such image on the dielectric support member. The developed image is transferred to a receiver member, such as a sheet of paper, transparency or other medium, in an electric field. After transfer, the receiver member bearing the transferred image is transported away from the dielectric support member, and the image is fixed (fused) to the receiver member by heat and pressure to form a permanent reproduction thereon.
Some reproduction apparatus have been designed to produce multi-color copies. In such reproduction apparatus, multiple color separation images are respectively developed with complementary colored marking particles, and then transferred in superposition to a receiver member. Fixing of multi-color marking particle images to a receiver member requires substantially different operating parameters than fixing standard black marking particle images to a receiver member. Moreover, the respective operating parameters can in fact be in contradistinction. That is, multi-color images require a high degree of glossiness for a full, rich depth of color reproduction; on the other hand, since glossiness for black marking particle images can significantly impair legibility, a matte finish is preferred.
Additionally, in the multi-color copy market, many different types of receiver members are used. The receiver members vary in surface finish gloss, weight, and thickness. Since the market demands a high quality appearance for finished copies, the image gloss should match the gloss of the selected receiver member. The reproduction apparatus is used to print receivers that will be collected and bound together as a book. In such a book, it is common for the cover and some insert sheets to be made of receivers that are of a different basis weight (typically measured in grams per square meter, or gsm) than the sheets that make up the body of the book. It is convenient for the receiver sheets in the book to be printed in one pass through the printing apparatus, instead of sheets of different weight separately prior to collating them into the finished book. When a reproduction apparatus is used to print on multiple types or weights of receiver sheets in a single print job, this is referred to as printing with mixed media.
It is known that the glossiness of a marking particle image is, at least in part, dependent upon the marking particle melting characteristics in the fixing process and the characteristics of the receiver members. In general, the fixing apparatus serves to soften or at least partially melt the marking particles, enabling the marking particles to permeate into the fibers of the receiver member so that the marking particles are fixed to the receiver member. For example, the fixing apparatus can include a heated roller which contacts the marking particles and the receiver member under pressure.
It is also known that the glossiness of the marking particle image is, at least in part, dependent on the basis weight of the receiver sheets. The fixing apparatus delivers heat to the marking particles in order to fix the particles to the sheet and to soften the particles sufficiently for them to change their shape and form a glossy surface. Heat from the fixing apparatus that is absorbed by the receiver sheet is no longer available to transform the marking particles and, in general, receiver sheets of higher basis weight have greater capacity to absorb heat. Therefore, receiver sheets of higher basis weight typically require more heat from the fixing apparatus to attain a given level of image glossiness.
In a fixing apparatus that uses heated members, such as rollers or belts, to contact the receiver sheet there are typically three parameters that are used to regulate the amount of heat that is delivered by the fixing members to the receiver sheets. First, speed of the receiver sheets through the nip between the fixing members is regulated. When the speed is slower, the receiver sheets are in the nip for a longer time, permitting more heat to flow from the fixing members to the receiver sheet. Second, the temperature of the fixing members is regulated. When the fixing member is at a higher temperature, more heat will flow to the receiver sheet. Third, the force in the nip between the fixing members is regulated. Higher pressure in the nip forces more intimate contact between the fixing member and the receiver sheet, permitting higher heat transfer. Also, the higher pressure causes the fixing members to deflect so that the nip between the fixing members is wider. The wider nip causes the receiver sheet to be in contact with the fixing members for a longer time, permitting more heat to flow from the fixing members to the receiver sheet. U.S. Pat. No. 5,956,543 describes a fixing apparatus wherein speed, temperature, and pressure are controlled according to the type and basis weight of the receiver sheets to obtain a desired gloss level on multiple receiver types.
The three methods of regulating the amount of heat that is delivered by a fixing apparatus to a receiver sheet have drawbacks that can make the use of these methods inconvenient or uneconomical. For example, it is uneconomical or impractical to design an imaging system that can operate at multiple speeds. It can also take an excessive amount of time to change the temperature of the fixing members, so that using a temperature change to regulate the heat delivery can cause an unacceptable loss of productivity for the reproductive apparatus. The mechanism for changing the force in the nip between the fixing members is prohibitively expensive. These limitations can cause the designers of the reproductive apparatus to provide another way of varying the heat that is delivered to the receiver sheet by the fixing apparatus.
When it is desired to have marking particle images on both sides of the receiver sheet, the typical reproduction apparatus will form and fix an image on one side of the sheet then transfer the sheet to a duplex path that will return the sheet to the point where it receives the marking particle image, but this time with the opposite side of the receiver sheet presented to receive the image. It is known that a fixing apparatus that fixes marking particle images on one side of the sheet at a time will often produce a higher gloss level on the image that has passed twice through the fixing apparatus. U.S. Pat. No. 5,581,339 describes a method of coping with the tendency for the fixing apparatus to give a higher gloss on the image that has passed twice through the fixing apparatus. U.S. Pat. No. 6,078,760 describes a fixing apparatus that uses multiple passes of the receiver sheet through the fixing apparatus to obtain the desired level of gloss on the image.
A method of fixing marking particles to sheets in a continuous stream of receiver sheets wherein some sheets have a lighter basis weight and at least one sheet has a heavier basis weight, comprising:
a) providing two fixing members which engage each other to form a nip, at least one of the members being heated;
b) adjusting at least one parameter including temperature applied to a sheet as it passes through the nip, or speed of the sheet as it passes through the nip, or pressure in the fixing nip applied to the sheet, which will provide a predetermined gloss upon the sheet(s) after they have passed through the nip;
c) feeding the sheets of the higher basis weight media through the nip before marking particles are formed on them so that the higher basis weight sheet(s) is fed more times through the nip than the other sheet(s) and these sheets are heated in preparation for receiving marking particles; and
d) applying marking particles to the prepared heavier basis weight sheets and passing them through the nip so that the gloss formed by the marking particles is substantially the same for the heavier basis weight sheets as for the lighter basis weight sheets.
In one effective embodiment of the invention, a controller of the reproduction apparatus can set the parameters of the fixing apparatus to provide the desired fixing level and gloss level of the image on the lightest basis weight receiver sheets in one pass through the fixing apparatus. If it then becomes necessary to create an image on a receiver sheet with a heavier basis weight, such as for a book cover or for an insert sheet, rather than change the parameters of the fixing apparatus, the controller will pass the heavier receiver sheet multiple times through the fixing apparatus using the duplex path of the reproduction apparatus to return the sheet to the entrance of the fixing apparatus before the marking particle image is transferred to the receiver sheet. These multiple passes through the fixing apparatus will change the temperature and moisture content of the receiver sheet so that when it finally passes through the fixing apparatus bearing an image, the image will attain the desired fixing level and gloss level in one pass through the fixing apparatus.
A feature of the present invention is that it avoids the negative consequences of the other ways of regulating the heat delivered to the receiver sheets, the cost of mechanisms to permit for multiple speeds in the reproduction apparatus or for multiple nip force levels in the fixing apparatus, or the loss of productivity while waiting for a change in the temperature of the fixing members, or damage to the image in the return path.
The response of a receiver sheet during the second pass through a fixing apparatus is different than during the first pass because the nature of the receiver sheet has been changed by the first pass. First, the receiver sheet will begin the second pass through the fixing apparatus at a higher temperature than the first pass because of the heat that was delivered to the sheet during the first pass. Second, if the receiver sheet is made of a hygroscopic material such as paper, the first pass through the fixing apparatus will drive moisture out of the receiver sheet so that on the second pass through the fixing apparatus, the heat capacity of the sheet will be less because of the reduced moisture content. For both reasons fixing the image on the second pass through the fixing apparatus will require less heat than did the fixing of the image on the first pass through the fixing apparatus. For high quality images, particularly photo-quality images, it is undesirable to pass the printed image through the duplex path as there is increased risk that the image will become damaged by the nip rollers or areas where the receiver undergoes a sharp change in direction as is common in compact printing devices. Additionally, for single sided printing such as book covers, it is further desirable that the receiver is heated and returned without additional structure to maintain the image side facing the fixing member as this lowers overall cost. The book covers are made of a higher basis weight sheet than are the body sheets of a book.
Referring to
The control unit 62 then operates the reproduction apparatus 41 with fixing parameters set at the selected levels (Block 76). Block 78 is a decision point for each receiver sheet in the print job in which the control unit 62 asks whether the next receiver sheet is the lightest basis weight receiver. If the answer is yes, Block 80 shows that the receiver sheets of the lightest basis weight are processed by conventional methods, with each receiver sheet passing through the image transfer station 51 and fixing apparatus 44 one time, or twice if the receiver sheet is to receive images on both sides. After Block 80 the control unit 62 will print the next sheet (Block 82). If the answer is no, Block 84 shows that the control unit 62 delays the formation of the image while the heavier basis weight receiver sheet, according to Block 86, is passed through the fixing apparatus 44 and the duplex receiver path 43 the number of times that the control unit 62 has determined is necessary. The control unit 62 also controls the heat applied to the sheets and the speed of the sheets as they pass through the nip. When the control unit 62 determines that the heavier basis weight receiver sheet has been sufficiently prepared, the control unit, according to Block 88, permits the image to be formed and transferred to the receiver sheet before the receiver sheet's final pass (or passes, if there is to be image on both sides) through the fixing apparatus 44. After Block 88 is completed the control unit 62 will print the next sheet (Block 82). This process repeats until all of the receiver sheets that are in the print job have been printed.
In the simplest embodiment of this invention, the control unit 62 determines the necessary number of passes of the receiver sheet through the fixing apparatus 44 based on rules that had been programmed into the logic of the control unit 62. In another embodiment of this invention, there would be placed electronic sensors in the duplex receiver path 43 to detect the temperature or the moisture content of the receiver sheet. The control unit 62 would then decide the necessary number of passes of the receiver sheet through the fixing apparatus 44 based on the receiver sheet's temperature and moisture content response to earlier passes through the fixing apparatus 44.
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention
