US3474223A

US3474223A - Selective flash fusing

Info

Publication number: US3474223A
Application number: US598811A
Authority: US
Inventors: Algird G Leiga; Charles F Gallo; Roy A Walder
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1966-12-02
Filing date: 1966-12-02
Publication date: 1969-10-21
Anticipated expiration: 1986-10-21
Also published as: GB1208770A; NL6716366A; DE1597898A1

Description

' Oct. 21, 1969 LE|GA ET AL SELECTIVE FLASH FUSING 2 Sheets-Sheet 1 Filed Dec 2, 1966' ABSORBTIVITY (PERCENT) L WAVELENGTH (m FIG.

PULSE DURATlON (SE0) DEGREE OF SELECTI VITY INVENTORS RD 6. LEIGI ES F. GALLC Y A. WALDEF 16 4 f FIG. 2

,4 1;, A TTORNEV Oct. 21, 1969 Filed Dec. 2, 1966 A. G. LEIGA ET AL 3,474,223

sELEcnviFLAsn'Fusme 2 Sheets-Sheet 2 ,4 /4 5 gg I6 V I 0 I v v w L DC POWER R HIGH VOLTAGE SUPPLY P4 PULSE .24 TRIGGER CIRCUIT 20 INVENTORS ALGIRD G. LEIG CHARLES F. GALL' BY ROY A. WALDE! A "OR/var United States Patent SELECTIVE FLASH FUSING Algird G. Leiga, Pittsford, Charles F. Gallo, Penfield, and Roy A. Walder, Rochester, N.Y., assignors to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed Dec. 2, 1966, Ser. No. 598,811 Int. Cl. H05!) 1/02 US. Cl. 219216 13 Claims ABSTRACT OF THE DISCLOSURE Method and apparatus for achieving selective flash fusing of an electroscopic toner image by applyinga pulse of radiation for a duration of time long compared to the time constant for heat loss from isolated background particles but short compared to the time constant for heat loss from the toner image so that the net heat added is sufficient to fuse only the image.

In general, the present invention relates to techniques of fusing and more specifically to a technique for selectively flash fusing and the associated apparatus.

In the process of xerography, for example, as disclosed in US. Patent 2,297,691, issued Oct. 6, 1942, a xerographic plate comprising a layer of photoconductive insulating material on a conductive backing is given a uniform electrostatic charge over its surface and is then exposed to the subject matter to be reproduced, usually by conventional projection techniques. This exposure discharges the plate areas in accordance with the radiation intensity that reaches them and thereby creates an electrostatic latent image on or in the photoconductive layer. Development of the latent image is effected by an electrostatically charged, finely divided material such as an electroscopic powder that is brought into surface contact with the photoconductive layer and is held thereon electrostatically in a xerographic powder image pattern corresponding to the electrostatic latent image. Thereafter, the developed xerographic powder image is usually transferred to a support surface to which it usually is affixed.

Typical forms of electrostatic or electroscopic powder or toner compositions used for developing are usually of a pigmented resin such as disclosed in US. Patents Nos. 2,788,288, and 2,892,794 and US. Reissue 25,136.

A common method by which a powder image is fixed is by the process of heat fusing, that is, by the application of heat in which case the powder image or its support must be formed of a thermoresponsive material, such as a heat fusible resin, which flows without image distortion when heated and which coalesces and adheres to the surface when cooled to ambient temperature.

In order to fuse resinous powder images, it is necessary to heat the powder'and the paper to which it is to be fused to a relatively high temperature. For given materials a temperature range exists in which fusing will be produced. Below that temperature range the resinous powder will not properly adhere to the support surface. If the temperature is too high, there is a tendency for the support material to discolor or scorch and in some cases for the toner to explode or be vaporized.

Various techniques have been developed for fusing in prior art. Among these are oven fusing, hot air fusing, radiant fusing, hot and cold pressure roll fixing and fusing, and flash fusing. Each of these techniques by itself has suffered from limitations and deficiencies which made them inapplicable for certain specific fusing jobs which are required in xerographic technology. In general, it has been difficult to achieve an entirely satisfactory design of heat fusers with regard to short warm-up time, low electric current requirements, adequate heat insulation 3,474,223 Patented Oct. 21, 1969 and uniform heat distribution. Specifically, hot air and oven systems tend to be slow and involve high power consumption. Hot and cold pressure systems have presented problems of offsetting, resolution degradation, poor fixing and limited quality.

Flash fusing has been desirable for some time since it is very efficient at slow or intermittent reproduction speeds but still suitable for high speed copying. A major problem with flash fusing as known in the prior art has been that it was not selective. Since the term selective has been used in various ways in connection with fusing processes in the past, it should be clearly understood that it is herein referred to as the preferential fusing of dense image areas leaving low density or background areas unfused. The undesirable, unfused background can then be wiped off or otherwise removed to yield a cleaner, more readable copy. Thus, since in the past it has been believed that flash fusing could not be selective fusing in the sense employed herein, this problem is one of the major problems toward which the present invention is directed.

Accordingly, it is an object of this invention to provide a new and useful, highly effective, and elficient selective flash fusing technique and apparatus which overcome the deficiency of the prior art as described above.

It is a further object of this invention to achieve both flash and selective fusing by the same process.

It is an additional object of this invention to achieve a high class selective fixing of toner images with minimum total energy in comparison with existing selective fusing systems.

It is also an object of this invention to avoid producing document warpage by the flash process.

Yet another object of this invention is to eliminate long warm-up time required prior to fusing.

An additional object of the present invention is to provide for selective fusing at high speeds.

Other objects and fuller understanding of the invention may be had by referring to the following description and claims taken in con unction with the accompanying drawings.

The present invention overcomes the deficiencies of the prior art and achieves its objectives by tailoring the pulse duration and energy density delivered to the toner particles so as to provide selective flash fusing. In order to facilitate understanding of this invention, reference will now be made to the appended drawings of a preferred embodiment of the present invention. The drawings should not be construed as limiting the invention but are exemplary only.

In the drawings;

FIGURE 1 is a representation of the absorptivity of paper and toner as a function of wavelength.

FIGURE 2 is a representation of the degree of selectivity for given pulse durations in flash fusing.

FIGURE 3 is a schematic representation of the heat loss phenomenon involved in selective flash fusing.

FIGURE 4 is a schematic representation of the apparatus of the present invention.

While it is not intended to limit the invention to any specific theory of operation, it is presently believed that the following analysis explains the relevant criteria in tailoring the pulse duration in selective flash fusing to achieve the desired objectives of the present invention.

When toner particles 14 are placed upon a sheet of material 16 to form an image in a xerographic or electrostatic printing process, a majority of the toner particles are accumulated in the area of the image pattern. Scattered about in areas not intended to make up a part of the image, are single isolated toner particles or small clumps of such particles which are referred to as the background. This light background around the image area is not to be fused. In selective fusing it is desired to fuse only the dense image areas.

It is known that the selectivity of radiant fusing is enhanced if the spectral output of the light source is such that the wavelength absorptivity of the tone 14 is a maximum and if simultaneously the wavelength absorp tivity of the paper 16 is a minimum. It may be seen in FIGURE 1' that this is in fact the case for the selectivity or absorptivity of the paper in and around the area of the visible region of light. The spectral output of xenon flash lamps is particularly desirable for selective fusings although other lamps may be utilized. It should be understood that by achieving the above spectral control in terms of absorptivity for a given wavelength output by the lamp, the effect is to heat the toner and not to heat the paper. If the paper were heated to sufficient temperature, all of the toner including the background would become fused to it. It is this type of situation which the present invention seeks to avoid. Among the conditions and parameters utilized in the present invention to obtain selective flash fusing is the control of the spectral output to optimize the transfer of energy, thus obtaining selective flash fusing. However, the control of spectral output alone has not been found to be sufficient to provide selective flash fusing.

A prior art model for the toner fusing phenomenon is that of a single layer of spherical toner particles 14 whose mean separation determines the optical density of the image or background. However, in such a model the energy absorbed per unit area is determined merely by the mean separation of the toner particles. Thus, while the energy absorbed per unit area of the paper surface will increase as the surface density of the toner particles increases, it is also clear from the model shown in FIG- URE 3 that the energy absorbed per toner particle remains essentially the same and is independent of whether a toner particle is completely isolated or whether it is surrounded by other toner particles resting on the paper surface. Since the energy absorbed per toner particle determines toner temperature, if the effect of heat losses from the toner particles is ignored, such a model would lead to the conclusion that a uniform toner temperature is achieved throughout the sheet independent of optical density. Based upon such reasoning and unsuccessful experiment efforts, the prior art concluded that flash fusing could not be made selective.

However, when the heat loss from each toner particle 14 is considered as shown in FIGURE 3, selectivity may be produced by using the tailored, pulsed output of a xenon flash lamp 12 with reflecting element which produces an output which is partially in the visible region and satisfies the spectral requirements for achieving selectively as indicated in FIGURE 1. It has been discovered that the flash duration is a critical parameter in achieving selectivity. If the flash duration is less than approximately a millisecond as shown in FIGURE 2, little or no selectivity is achieved. If the flash duration is greater than a millisecond, selective flash fusing is achieved. A prime reason for this selectivity with longer pulses is that the net rate of heat loss by a single toner particle 14 is decreased by the presence of adjacent toner particles 14 as shown in FIGURE 3. Since any adjacent toner particles decrease the effective surface area of a toner particle available for the heat loss by conduction, convention, or radiation, any heat flow between adjacent toner particles essentially conserves heat in the toner system and thus clumps of toner particles get hotter initially and stay hotter longer.

The reason for the above described behavior can be understood in terms of the following discussion based upon the time constants of heat loss from the toner particles, where the time constant (T,,) is the time required for the temperature of the toner particles (n) to decrease to some arbitrary fraction, such as, for example, l/e of its initial value, where e is the base of the natural logarithms and has a value of 2.71828 As noted above and shown in FIGURE 3, the rate of heat loss from an isolated toner particle is greater than the rate of heat loss from a toner particle in a clump of toner particles. In different terminology, the time constant (T for heat loss from an isolated or background toner particle is less than the time constant (T for heat loss from a toner particle in a clump forming a part of the image pattern. That is, T T If the optical pulse duration (T is long compared to the time constant (T of the background but short compared to the time constant (T of the image areas, then selectivity of fusing will be achieved. Schematically, the duration (T of the optical pulse should be for selective flash fusing.

It has been determined by experimentation that the time constant for the rate of heat loss by a single isolated toner particle is on the order of a millisecond; thus, if the flash duration is less than one millisecond, a single isolated toner particle gets and remains as hot as a toner particle which is adjacent to other toner particles. If, however, the flash duration is longer than a millisecond with a measured input intensity, an isolated toner particle will not become very hot even when clumps of toner fuse because an appreciable amount of heat is allowed to leak away by the large surface area of that single toner particle in that time interval. However, the toner particles which make up a clump or form part of the image retain the heat better and stay hotter longer. Thus, the image toner is selectively fused while the other isolated and randomly scattered particles are not. In an idealized manner this explanation accounts for selective fusing of toner particles. However, it should be realized that in actual practice the dense image area may contain two or more layers of toner particles and that the toner particles may consist optically of two phases: a carbon particle acting much like a black body and a toner polymer which is relatively transparent. Also, in practice the possibility of multiple reflections and absorptions may be taken into account. It should be noted that in practice the spectral characteristics of the system, the time of pulse duration and the energy density received at the fusing surface are variables which may be adjusted to achieve the desired results. The relevant considerations concerning spectral characteristics and pulse duration have been discussed above. In general, the required energy densities and the intensity of the flash required to fuse toners commercially available are well known in the prior art for conventional systems. In any case, with the pulse duration established as described above and held constant, the intensity of the flash may be adjusted in a given system until the dense image areas are fused as desired. As indicated in FIGURE 2, if it is desired to fuse all toner including the background on a document, it is more efficient to employ flashes ofthe required energy with durations less than one milisecond so that the toner does not have sufficient time to lose heat. If the flash duration of fusing energy is too short, however, the instantaneous toner temperature will rise so quickly as to cause vaporization and decomposition. If one wishes to achieve selective flash fusing, the duration of the flash should be greater than approximately one millisecond and generally, in the range of 10- to 10' seconds. It should be noted from the above discussion that the selective flash fusing phenomenon is highly dependent on the pulse duration. Also, for flash fusing the rise of time of the pulse should be as gradual as possible to help to avoid excessive toner temperatures.

For the sake of clarity for the disclosure and simplicity of explanation the invention has been herein described above in terms of the theory of operation as presently understood although it is to be clearly understood that the theory is illustrative only and is not intended to be interpreted in the limitation of the scope of the invention.

As indicated by the above analysis, the production of selective flash fusing involves the transfer of the proper amount of energy with the proper pulse duration to transfer a sufficiently high intensity of radiation while simultaneously avoiding any deleterious effects to the substrate on which the toner particles are to be fused.

A preferred embodiment of the present invention for achieving this tailored pulse duration is shown in FIG- URE 4. A DC power supply indicated at 20 is connected across a capacitor 24 which may be grounded on one side at 22. This storage capacitor 24 typically has a value on the order of 150 microfarads and serves to build up from 2,000 to 5,000 volts maximum in stored condition for use when the flash lamp 28 is to be pulsed. The storage condenser 24 is connected to the flash lamp 28 through a variable inductor 26 which is typically in range of 150 microhenrys to 3 millihenrys and determines the pulse duration produced by the flash lamp.

The flash lamp 28 consists of an envelope containing xenon gas and a pair of electrodes at each end which are not electrically connected to each other. Surrounding the glass envelope of the flash lamp 28 is a coil 30 which is connected to a high voltage pulse trigger circuit 34. Upon pulsing trigger circuit 34 a surge of current at approximately 20,000 to 30,000 volts passes through the coil 30. The flow of current through coil 30 couples with the electrodes of flash lamp 28 causing a gas breakdown and pulsing of the flash lamp 28 which results in a flash of suitable duration as determined by inductance of 26.

While reference has been made throughout to such elements as the xenon tube as the preferred embodiment, it is obvious that any other suitable flash tube may be used. Any suitable power supply and pulser may be used and any other equivalent electrical circuitry may be utilized to produce a tube pulse of suitable duration for selective flash fusing. In a similar manner, alternating significantly the geometry of the light path or the total energy transferred may suggest alternations in the pulse durations and energy transfers required to bring about selective flash fusing. Toners other than the typical electroscopic toner compositions referred to above may be utilized. Specific modifications of papers and toners consisting of different chemical composition than commonly employed within the term toner may also be employed with suitable alternations in the tailoring of the pulse duration in accord with the principles set forth in this invention. In operation the bringing of a sheet with toner into position may automatically trigger the switch resulting in appropriate pulse of the flash lamp source.

As described above in the discussion of the principles of this invention, time durations greater than approximately 1 millisecond are utilized to achieve selective flash fusing. So long as a suitable energy density in a given period of time is applied to produce the optimum selected flash fusing, which consists of not fusing the background and not leaving the image unfused, the apparatus will be suflicient to accomplish the purposes of the present invention. Longer pulse durations than the one millisecond pulse duration expressed as the duration of the preferred embodiment in this invention may be achieved by the use of large inductors, by storing energy at low voltages and high capacity, the use of special flash lamps, and special pulse forming circuit designs.

Using the above apparatus and applying the above theory, the selective fusion of line copy toner images has been accomplished with a single flash of a six-inch long cylindrical design flash tube which was placed along the focal axis of an aluminum sheet reflecting, parabolic cylinder. With the copy resting on an aluminum plate 6 /2 inches below, the light input energy was determined to be approximately 600 joules with a pulseduration of three milliseconds. Thus, in operationa pulse of suitable energy input is created by a flash lamp having the tailored time or pulse duration suitable for the particular toner and paper and distance configurations. This tailored pulse is utilized to produce selective flash fusing.

Although a specific preferred embodiment of the present invention has been described in the detailed description, the description is not intended to limit the invention to the particular forms or embodiments disclosed herein since they are to be recognized as illustrative rather than restrictive. It will be obvious to those skilled in the art that the invention is not so limited. The invention is declared to cover all changes and modifications of the specific examples of the invention herein disclosed for purposes of illustration which do not constitute departures from the spirit and scope of the invention.

What is claimed is:

1. A method of achieving selective fusing of an electroscopic toner image by an optical pulse of suitable intensity onto a substrate for supporting said image while leaving the isolated background particles in an unfused condition comprising:

(1) pulsing a flash lamp,

(2) applying said optical pulse for a duration of time long compared to the time constant for heat loss from said isolated background particles but short compared to the time constant for heat loss from said electroscopic toner image, said pulse being of a suflicient intensity so that the net heat added to said toner image is suflicient to melt said toner but the net heat added to said isolated background particles is insuflicient to melt them.

2. The method of claim 1 wherein said duration of time is not less than one millisecond.

3. The method of claim 2 wherein said duration of time is between 10* and 10- seconds.

4. The method of claim 3 wherein applying said or tical pulse produces an input energy of approximately 600 joules over a pulse duration of approximately three milliseconds.

5. A device for use in a xerographic apparatus to selectively fuse an electroscopic toner image by an optical pulse of suitable intensity onto a substrate for supporting said image while leaving the isolated background particles in an unfused condition comprising:

(1) a flash lamp, and

(2) means to produce a pulse of light of said suitable intensity for a duration of time long compared to the time constant for heat loss from said isolated background particles but short compared to the time constant for heat loss from said electroscopic toner image, said pulse intensity being such that the net heat added to said toner image is suflicient to melt said toner but the not heat added to said isolated background particles is insuflicient to melt them.

6. The device of claim 5 wherein said means to produce a pulse of light of said suitable intensity for a duration of time long compared to the time constant for heat loss from said isolated background particles but short compared to the time constant for heat loss from said electroscopic toner image comprises:

(a) means to energize said flash lamp, and

(b) means to apply an increasing amount of energy mput to said toner particles for a duration of time of not less than one millisecond.

7. The device of claim 6 wherein said duration of time is between 10- and 10- seconds.

8. A device for use in a xerographic apparatus to selectively fuse an electroscopic toner image by an optical pulse of suitable intensity onto a substrate for supportmg said image while leaving the isolated background particles in an unfused condition comprising:

(1) a flash lamp,

(2) electrical means to energize said flash lamp,

(3) means to supply electrical power to said flash lamp to produce illumination of sufliicent intensity to cause fusing of toner particles, and

(4) electrical control means to produce a pulse of sufficient intensity light for a duration of time long compared to the time constant for heat loss from said isolated background particles but short compared to time constant for heat loss from said electroscopic toner image, said pulse intensity being such that the net heat added to said toner image is sufficient to melt said toner but the net heat added to said isolated background particles is insuflicient to melt them.

9. The device of claim 8 wherein said duration of time is not less than one millisecond.

10. The device of claim 9 wherein said duration of time is between 10* and 10 seconds.

11. A device for use in a xerographic apparatus to selectively fuse an electroscopic toner image by an optical pulse of suitable intensity onto a substrate for supporting said image while leaving the isolated background particles in an unfused condition comprising:

(1) a flash lamp, and

(2) means to pulse said flash lamp at said suitable intensity for a predetermined duration of time such that the rate of heat transfer to said electroscopic toner during said pulse is less than the rate of heat loss by said isolated background particles but greater than the rate of heat loss from said electroscopic toner image areas, said pulse intensity being such that the net heat added to said toner image is sufficient to melt said toner but the net heat added to said isolated background particles is insuflicient to melt them.

12. The device of claim 11 wherein said means to pulse said flash lamp at said suitable intensity for a predetermined duration of time such that the rate of heat transfer to said electroscopic toner during said pulse is less than the rate of heat loss by said isolated background particles but greater than the I rate of heat loss from said electroscopic toner image areas comprises:

(1) means to energize said flash lamp, and

(2) means to apply an increasing amount of energy input to said toner particles for a period of not less than one millisecond.

13. A device for use in a Xerographic apparatus to selectively fuse an electroscopic toner image by an optical pulse of suitable intensity onto a substrate for supporting said image while leaving the isolated background particles in an unfused condition comprising:

(1) a flash lamp,

(2) electrical means to energize said flash lamp,

(3) means to supply electrical power to said flash lamp to produce illumination of sufficient intensity to cause fusing of toner particles, and

(4) electrical control means to produce a pulse of said suflicient intensity light having a duration of between 10- and 10- seconds, whereby said image will be fused w-hile said background will remain unfused.

References Cited UNITED STATES PATENTS 2,807,703 9/1957 Roshon 219-3'88 x 2,844,733 7/1958 Miller et al 25-o 3,163,755 12/1964 Kotz et a1. 250L-65 3,187,162 6/1965 Hojo et al 219-3ss JOSEPH V. TRUHE, Primary Examiner C. L. ALBRI'ITON, Assistant Examiner US. Cl. X.R. 250-65