US3517164A

US3517164A - Image fusing assembly

Info

Publication number: US3517164A
Application number: US746456A
Authority: US
Inventors: Ralph T Huggins; Frederick D Meller
Original assignee: Multigraphics Inc
Current assignee: AB Dick Co
Priority date: 1968-07-22
Filing date: 1968-07-22
Publication date: 1970-06-23
Anticipated expiration: 1987-06-23
Also published as: FR2014674A1; DE1937039A1; NL6911190A; GB1275921A; BE736363A

Description

June 23, 1970 R, T, HUGGlNs -ET AL 3,517,164

IMAGE FUSING ASSEMBLY Filed July 22, 1958 United States Patent O 3,517,164 IMAGE FUSIN G ASSEMBLY Ralph T. Huggins, Glen Ellyn, and Frederick D. Meller,

Lombard, Ill., assignors to Addressograph-Multigraph Corporation, Mount Prospect, Ill., a corporation of Delaware Filed July 22, 1968, Ser. No. 746,456 Int. Cl. H05b 1/00; F26b 3/04; G03d 3/12 U.S. Cl. 219-388 6 Claims ABSTRACT F THE DISCLOSURE rThe application discloses a unit for fusing powder images on copy material using conduction of heat through the material. The unit includes a vacuum chest having a thermally conductive wall over which the back of the copy material slides and to which heat is supplied by electrical heaters carried thereon. The walls are provided with spaced openings so that the moving copy material is held in intimate engagement with the wall, and any air lm between the wall and the copy material is removed to permit an efficient heat transfer to the powder image by conduction from the wall and through the material.

BACKGROUND OF THE INVENTION This invention relates to copy making apparatus of the type using fusible powder images and, more particularly, to a new and improved method of and apparatus for lixing or fusing powder images.

Photoelectrostatic machines for making one or more copies of graphic material on sheet or web material form an electrostatic image corresponding to the graphic material which is then placed in visible form by the application of a developer such as a toner material or electroscopic powder. This toner material can be applied in various manners, as by a magnetic brush, cascade techniques, powder cloud generators, or from liquid systems, and is selectively attracted or repelled by the electronic image to provide the visible image. Frequently, the toner material comprises electroscopic particles or powder formed of a thermoplastic resinous material. After the powder or toner image has been formed, it is lixed or placed in permanent form, as by fusion or solvent removal, incident to or after transfer to sheet or web copy material.

-One common approach to lixing the powder or particle image relies on the application of heat, and this heat is frequently applied to the image bearing surface using radiation or convection techniques. Since the quantum of heat required for proper fusion tends to be constant for a given thermoplastic toner material, it becomes difficult to attain adequate fixing as the speed at which copies are made is increased without operating the heating units at an undesirably high level or increasing the the length of the travel path during which the image is exposed to heat. This increases the size of the machine and introduces the possibility of damaging the copy.

SUMMARY Accordingly, one object of the present invention is to provide a photoelectrostatic copy machine including new and improved llixing or fusing means.

Another object is to provide an apparatus for fusing powder images that is compact and operable at acceptable temperature levels and is also capable of high speed image fusing.

A further object is to provide a powder image fusing apparatus in which heat is transferred to the image largely by conduction.

3,517,164 Patented June 23, 1970 lv ce A further object is to provide a powder image fusing apparatus including a vacuum chamber having a thermally conductive wall against which the back of the image bearing copy material is held and to which heat for fusing the image is applied.

Another object is to provide a new and improved method of fusing powder images on copy material in which the back of the copy material is held in sliding contact with a heated and thermally conductive member by a pressure differential.

In accordance with these and many other objects, an embodiment of the invention comprises a vacuum chest or chamber having a thermally conductive wall to which are afxed one or more electrical heating elements. The wall is also provided with spaced perforations adjacent the heating elements. The back of a copy sheet containing a powder image is moved over the thermally conductive wall in sliding engagement therewith, and the back of the sheet is held in intimate contact with the thermally conductive wall by the pressure differential communicated to the back of the sheet through the apertures. In this way, heat from the electrical heating elements is transmitted by conduction through the thermally conductive wall and the back of the sheet to fuse the powder image. Using this method and apparatus, copy material speeds in the range of 15 to 50 feet per minute have been attained using thermoplastic powders having softening points ranging from 70-190 C. It will be appreciated that even greater copy material speeds can be realized by operating the system at the maximum temperature difference between the wall and softening point of the powder. For example, copy material speeds of 65 feet per minute and greater can be realized when using a powder that softens at about 70 C. and maintaining the wall at a temperature of 150 C.

BRIEF DESCRIPTION OF THE DRAWINGS Many other objects and advantages of the present invention will become apparent from considering the following detailed description in conjunction with the drawings in which:

FIG. 1 is a top elevational View in partial section of a fusing assembly embodying the present invention;

` and DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now more specifically to the drawings, therein is illustrated a Ifixing or fusing unit or assembly which is indicated generally as 10 and which is capable of effecting the relatively high speed fusing of powder images carried on web or sheet copy material 12. The powder image can be formed directly on the material 12 or transferred thereto, and in a preferred embodiment is produced on a photoconductive member and transferred to the upwardly facing surface of the material 12 (FIG. 2) by an apparatus of the general type shown and described in detail in the copending application of Loren E. Shelffo, Ser. No. 632,819, filed Apr. 21, 1967, which application is assigned to the same assignee as the present applica- Vacuum chest or chamber indicated generally as 18. The vacuum chamber 18 is illustrated as Irbeing formed of a single sheet of folded metal sealed along an edge 18A Y and closed at its opposite ends by the

side walls

14 and 16 on which it is supported. The wall 14 is provided with an opening 20 .by which the interior of the vacuum chamber 18 is coupled to an air moving means or vacuum pump 22 by a section of duct 24. Although the illustrated vacuum chamber 18 is illustrated as fbeing formed of sheet metal, other suitable materials could be used so long as a wall 18B or a suitable portion thereof is formed of a material having good thermal conductivity. The wall 18B is formed with a plurality of rows of openings or

apertures

26, 28, and 30 that are spaced from each other in the direction of movement of the sheet or web material 12 relative to the unit 10.

To provide a source of heat for fusing the powder image, the vacuum chest or chamber 18 is provided with electrical heating means including two electrical heating units or

assemblies

32 and 34. These two units are generally elongated and are secured to the lower surface of the wall 18B disposed between and extending generally parallel to the

rows

26, 28, and 30- of openings or perforations. The

heating units

32 and 34 can be of conventional construction including a sole plate brazed or welded to the wall 18B and carrying resistance elements on the opposite surfaces thereof, or can comprise units in which the resistance elements are cast into the sole plates included in the

heating units

32 and 34. These units are energized from a suitable source of alternating current energy during periods of use of the copying machine of which the assembly forms a part and preferably are provided with adjustable temperature regulating means of known construction. In one unit 10 constructed in accordance with the present invention, the temperature control means can 'be regulated to energize the

units

32 and 34 to maintain the temperature of the thermally conductive wall 18B at a temperature in the range of 120- 150 C.

To provide means for feeding the sheet or web copy material 12 relative to the wall 18B of the vacuum chamber or chest 18, two sets of

rollers

36 and 38 mounted on shafts 40 and 42, respectively, are provided. The sets of

rollers

36 and 38 are rotated in synchronism so as to advance the material 12 over the heated wall 18B of the vacuum chamber 18 at the desired speed and with the powder image facing outwardly or upwardly away from this wall. In one fusing unit 10 constructed in accordance with the present invention, a variable speed drive (not shown) coupled to the shafts 40 and 42 can be adjusted to advance the material 12 at a speed in a range of 15-50 feet per minute. As illustrated in FIG. 2 of the drawing, the sets of

rollers

36 and 38 are offset vertically from each other so that the sheet or web material 12 is held in engagement with the curved and somewhat up wardly inclined wall 18B of the vacuum chest 18.

When the fusing or fixing unit 10 is placed in operation, a control circuit (not shown) associated with the unit 10 or with the copying machine of which the unit 10 is a part is energized to initiate the operation of the vacuum pump or air moving means 22 so that air is withdrawn from the vacu-um chamber 18 through the opening 20 and the duct 24. Air is removed by the moving means 22 at a greater rate than air can enter through the rows of

openings

26, 28, and 30 so that a pressure differential is maintained across the wall 18B. The energization of the control circuit also initiates the energization of the

electrical heating units

32 and 34 so that the thermally conductive wall 18B or the thermally conductive portion thereof is heated to the desired temperature under the control of the temperature regulating means. This temperature can be adjusted in accordance with the type of toner material to be fused by the unit 10. If desired, the control circuit for the

electrical heating units

32 and 34 can lbe provided with a control which maintains the wall 18B at a temperature somewhat lower than that re quired for proper fusing during standby periods and is elevated to the proper fusing temperature when the unit 10 is actually placed in use to x copies.

Powdered images are then produced on the copy material 12 either directly or, in the preferred embodiment, by transfer to the material 12 in a machine of the type described in the above-identified copending application of Shelffo et al. In this embodiment, the copy material 12 preferably comprises plain or untreated paper, and the material 12 is fed to the nip between the inlet rollers 36 with the powder image facing upwardly (FIG. 2) or away from the wall 18B. The rollers 36 which were placed in operation together with the rollers 38 when the unit 10 was placed in operation feed the leading edge of the copy material 12 against the upwardly inclined wall 18B and advance the leading edge over the wall 18B to be received within the nip between the rollers 38, which in turn feed the copy material to a discharge point.

As the copy material 12 moves over the wall 18B, it closes off communication between the atmosphere and the interior of the vacuum chamber 18` through the

rows

26, 28, and 30 of perforations. This establishes a predetermined pressure differential across the copy material 12 and holds it in intimate sliding engagement with the wall 18B. This permits the heat stored in the wall 18B and in the heat sinks or sole plates associated with the

electrical heating units

32 and 34 to be transmitted by conduction through these thermally conductive parts and from the .back of the copy material 12 to the top or front surface on which the powder image is disposed. The use of the vacuum chest 18 producing the pressure differential across the copy material 12 removes any film of air that exists between the back surface of the copy material 12 and the wall 18B and thus provides a good overall coeicient of thermal conductivity to the thermoplastic resin forming the toner particles. These particles are thus fused to fix or place in permanent form the image carried on the front or outwardly facing surface of the copy material 12.

In one fixing or fusing unit 10 constructed in accordance with the present invention using a 'vacuum in the chest 18 on the order of 3-6 inches of water, a toner material that fuses in the temperature range 'between 70-115 C., a copy material feed rate in the range of 15-50 feet per minute, a plate 18B temperature in the range of 1Z0-150io C., it was observed that the toner material was fully fused to the copy material 12 and that there was no evidence of discoloration of the copy material 12. The length of the wall 18B in this unit is approximately 3-5 inches, and thus the fusing unit 10I affords a very small or compact fusing unit with a relatively short path of travel during which heat is applied but which is capable of attaining substantial paper feed rates.

Although the phenomena is not completely understood, it is believed that the improved transfer of heat to the powder image achieved with the unit 10 results from the removal of any air film or boundary layer of air from the interface between the back of the copy material 12 and the adjacent surface of the wall 18B. It appears to be true that the thermal resistance R (resistance to transfer of heat from the wall 18B to the copy material 12) approaches the relationship where dx is the thickness of the boundary layer or film of air between the copy material 12 and the thermally conductive wall .18B affording the heat source, K is a constant for thermoconducti-vity of air which at one atmosphere varies from .014 to .017 with air temperatures between 32 F. and 200 F., and A is the area of the copy material 12. From this relationship, it can be seen that if the thickness of the air film can be reduced to the greatest extent possible, a corresponding reduction in the thermal resistance is effected, and the efiiciency of the heat transfer from the thermally conductive wall 18B to the copy material 12 is increased.

It will be appreciated that the significant advance made in the technique of fusing powder images by conductive heat transfer is the reduction in the air layer between the heating element and the image carrying support. The description of the invention sets forth temperature ranges for the wall 18B and the softening point of the powder material comprising the images and the speeds with which the fusing can occur. The speeds of fusing may vary widely in the instant invention depending on the softening point of the thermoplastic powders, the thickness of the copy paper or material, and the temperature of the wall 18B under the inuence of the air suction system that tends to decrease the air layer.

Without the benefit of exhausting the air from between the copy material and the wall 181B, the speed of fusion is greatly decreased, and the temperature at which the wall 18B must be maintained is just below the scorching point of the copy material. Thus, the fusing system without the vacuum means is insensitive to changes in the melting point of the toner and the power input to the wall 18B in the sense that it is not possible to use a lower melting point toner and obtain an accompanying reduction in the heating power input or the temperature at which the wall 18B is held. This system lacking the vacuum means thus operates in a narrow range of speed and heat le'vel below which there is no fusion (only at very slow speeds) and above which further increases in temperature cause scorching or burning of the copy material. The system incorporating the present invention is sensitive in the sense that changes in copy feeding speeds and temperature level are related and the use of lower melting point toners permits a reduction in operating temperature or an increase in copy speed. Accordingly, the present invention results in a highly eicient system permitting both the use of low wattage inputs with high speed fusion at temperatures well below the scorch point of paper and the use of thermoplastic materials having a wide range of softening temperatures.

Although the present invention has been described with reference to a single illustrative embodiment thereof, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the principles of this invention.

What is claimed and desired to be secured by Letters Patent of the United States is:

1. Apparatus for fixing thermoplastic powder images carried on one surface of web or sheet material comprising:

stationary heating means for slidably :receiving said sheet material thereon and applying heat by conduction to the surface opposite said image carrying surface,

transport means for slidably moving said sheet material across said stationary heating means, and

means for removing the air between said heating means and the material so that said sheet or web material is held in intimate contact with said heating means as the material is transported thereacross.

2. An apparatus for fixing powder images carried on one surface of web or sheet material comprising:

an enclosure having a stationary wall portion of thermally conductive material and spaced apertures through said wall portion,

heating means for applying heat to said wall portion,

an air moving means coupled to said enclosure for moving air into said enclosure through said apertures and for withdrawing air from said enclosure, thereby to maintain a pressure differential along said wall portion,

and feeding means for feeding web or sheet material over said wall with said one surface facing away from said wall, said sheet oir web material being held in intimate sliding contact with said wall by said air moving means so that heat from said heating means is transferred to said powder image by conduction through the wall and the web or sheet material as the latter is moved across said wall.

3. An apparatus for fusing powder images carried on one surface of web or sheet material comprising:

an enclosure having a stationary wall of thermally conductive material, said lwall having spaced openings therein,

electrical heating means carried on an interior surface of said wall for supplying heat to said wall,

feeding means for feeding web or sheet material over an outer surface of said stationary wall with said one surface of the sheet or web material facing away from said outer surface of said wall, and

air moving means coupled to the enclosure for producing a pressure differential across the wall and material to hold the material in intimate sliding contact with the wall as the material is transported thereacross.

4. An apparatus for fusing toner images on one surface of web or sheet material comprising:

a vacuum chamber having a stationary material engaging structure formed of thermally conductive material, said structure having spaced openings therein,

electrical heating means coupled tothe structure for supplying heat to the structure7 and material feeding means for feeding said material relative to the structure, said material being held against said structure during movement by the vacuum in the chamber so that heat is transmitted by conduction through the structure and the material to the toner image. f

5. The apparatus set forth in claim 4 in which the structure includes la wall having spaced lines of openings,

and the electrical heating means are carried on the wall in the areas between the spaced lines of openings.

6. The apparatus set forth in claim 5 in which the 5 electrical heating means includes spaced and elongated heating units extending generally transverse to the direction of movement of the material relative to the structure.

References Cited UNITED STATES PATENTS 1,575,366 3/1926 Johnson 34-233 2,775,677 12/1956 sennetze 219-445 2,807,703 9/1957 Rennen 219-343 X 2,807,704 9/1957 Auen et al. 219-343 2,821,612 1/1958 schuetze 219-445 3,123,700 3/1964r Snyder e1 a1. 219-388 3,199,223 s/1965 Carlsen 34-155 3,222,800 12/1965 Siegel et a1 34-212 3,397,303 8/1968 smith 219-388X VOLODYMYR Y. MAYEWSKY, Primary Examiner U.S. Cl. X.R.