JPH07295431A - Double type image forming device - Google Patents

Abstract

PURPOSE:To obtain an image of high quality without the blur and turbidity of printing material by setting the heating temperature of a second fixing apparatus to the melting temperature of first printing material or lower and to the melting temperature of second printing material or higher, thereby reducing the melting of the first printing material fixed first on a sheet at the time of the second printing material being heated to melt. CONSTITUTION:A first unit 10 is provided with a first image forming unit 11, a first transfer apparatus 12 and a first fixing apparatus 13. A second unit 20 is provided with a second image forming unit 21, a second transfer apparatus 22 and a second fixing apparatus 23. First printing material is fixed on a sheet by the first image forming unit 11, and second printing material is fixed in the overlapped state on the sheet by the second image forming unit 21 to form an image. At this time, printing material melting at a higher temperature than the second printing material in the same condition is used as the first printing material, and the heating temperature of the second fixing apparatus 23 is set to the melting temperature of the first printing material or lower and to the melting temperature of the second printing material or higher in the fixing condition of the second fixing apparatus 23.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dual image forming apparatus,
That is, two image forming units are provided, and the first image forming unit fixes the first printing material on the sheet, and then the second image forming unit superimposes and fixes the second printing material on the sheet to form an image. Regarding the device.

[0002]

2. Description of the Related Art Conventionally, as an image forming apparatus, two sets of image forming units each having a toner image forming means for forming a toner image on a sheet and a fixing means for heating and melting the toner image to fix the toner image on the sheet are set. There is known a so-called double type in which the first toner is fixed on the sheet by the first group of image forming units, and then the second toner is fixed on the sheet by the second group of image forming units. ing.

[0003]

[Problems to be Solved by the Invention] However, the above 2
In the continuous image forming apparatus, when the melting temperatures of the first toner and the second toner are the same, the fixing means of the first set and the fixing unit of the second set have the same fixing conditions, for example, the fixing temperature and the toner at the time of fixing. If the applied pressures are set to be the same, the first toner that has already been fixed is melted again by the fixing unit of the second set to cause bleeding in the sheet, or the melted first toner and second toner are mixed and become cloudy. There was a problem of causing.

[0004]

SUMMARY OF THE INVENTION Therefore, the present invention has been made to solve the above-mentioned problems, and in a double image forming apparatus, the first printing material can be used under the same conditions as the first printing material. A printing material that melts at a temperature higher than that of the second printing material is used, and the heating temperature of the second fixing device is equal to or lower than the melting temperature of the first printing material under the fixing conditions of the second fixing device, It is set above the melting temperature.

[0005]

Embodiments of the present invention will be described below with reference to the accompanying drawings. In FIG. 1, a double image forming apparatus indicated by reference numeral 1 as a whole forms a two-color composite image on one side of a sheet such as ordinary paper or a film made of synthetic resin. And an electrophotographic second unit 10 for superimposing the toner image of the second color on the sheet on which the toner image of the first color is produced by the first unit 10. The unit 20 is provided.

In the image forming apparatus 1, the sheet is sent out from the sheet feeding device 30, for example, a well-known sheet feeding cassette type sheet feeding device, and the sheet conveying path 3 represented by a virtual line is shown.
1 is conveyed.

The first unit 10 is the first image forming unit 1
1, a first transfer device 12, and a first fixing device 13. The image forming unit 11 integrally includes a photoconductor, a charging device, a developing device, and the like, and exposes the image light output from an image exposure device (not shown) to the photoconductor based on a well-known electrophotographic method. To form a first electrostatic latent image, and the electrostatic latent image is developed by a developing device to form a first toner image. The first toner image is transferred by the second transfer device 12 to the sheet conveyed along the imaginary line. At this time, the first toner image is simply attached to the sheet by static electricity, and can be easily removed by applying a physical force. First fixing device 13
Is a pair of parallel fixing rollers, for example, a heating roller having a built-in heater and a pressure roller pressed against the heating roller at a predetermined fixing pressure. The sheet holding the first toner image is held by the two fixing rollers. The first toner image is melted and heated while being conveyed to be fixed on the sheet.

The second unit 20, like the first unit 10, includes a second image forming unit 21, a second transfer unit 22, and
The second fixing device 23 is provided. The image forming unit 21 is
Similar to the image forming unit 11, the image light output from an image exposure device (not shown) is exposed on the photoconductor to form a second electrostatic latent image, and the second electrostatic latent image is developed by a developing device. Second
Create a toner image. The second housed in this developing device
The toner has a color different from that of the first toner. For example, black toner is used for the first toner and red toner is used for the second toner. Next, the second toner image is transferred by the second transfer device 22 to the sheet conveyed along the imaginary line, and the second toner image is superposed on the first toner image. At this time, the first
The toner image is fixed on the sheet, but the second toner image is not fixed and can be easily removed by applying a physical force. The second fixing device 23 has the same configuration as the first fixing device 13,
The sheet is sandwiched and conveyed by a pair of parallel fixing rollers, that is, a heating roller and a pressure roller, and the second toner is melted and fixed on the sheet. Then, the sheet on which the first toner image and the second toner image are fixed is discharged to a discharge unit (not shown).

The first used in the image forming apparatus 1
The properties of the toner and the second toner will be described. As the first toner, as shown in FIG. 2, the outer diameter of the fixing roller is 2
When the length of the fixing roller is 0 mm, the length is 225 mm, and the pressure contact force is 6 kg, when the surface temperature of the fixing roller is 165 ° C. to 210 ° C. It is used. It should be noted that 210 ° C. is a temperature at which a so-called high temperature offset phenomenon, in which molten toner adheres to the surface of the fixing roller, starts to occur. Hereinafter, the temperature range in which the toner is fused and fixed with a fixing strength of 80 to 100% is referred to as "fixable temperature range". Further, the fixing strength indicates the degree of adhesion of the toner to the sheet and is represented by the following mathematical expression 1.

[0010]

[Equation 1] Fixing strength = (ID2 / ID1) · 100 (%) ID1: Original solid image reflection density ID2: Original solid image is rubbed with an eraser at a constant pressure for a fixed number of times, and a solid image reflection density is obtained.

On the other hand, as the second toner, as shown in FIG. 3, under the same condition, the temperature of the outer surface of the fixing roller is 14
A material that melts and fixes on a sheet with a fixing strength of 80% to 100% at 0 ° C to 175 ° C is used. Note that 175 ° C. is the temperature at which the high temperature offset phenomenon begins to occur.

To adjust the melting temperature of the toner, as a resin (for example, a polyester resin) which is the main component of the toner, a high molecular weight resin having an average molecular weight of about 5000 (a resin having a Gaussian distribution having a maximum peak at a position of the molecular weight of about 5000) Low molecular weight resin with a molecular weight of about 2000 (molecular weight of about 2000
It is possible by using a resin having a Gaussian distribution having a maximum peak at the position (1) and adjusting the compounding ratio thereof. Specifically, when the melting temperatures of the first toner and the second toner are adjusted as described above, the first toner mixes the high molecular weight resin and the low molecular weight resin at a ratio of 2: 1, and the second toner mixes them. Mix at a ratio of 1: 2.

Next, the temperature control of the fixing devices 13 and 23 will be described. As shown in FIG. 4, when the pressure contact force of the fixing roller is set to 6 kg, the temperature controller 40 causes the temperature of the outer peripheral surface of the heating roller in the first fixing device 13 to be the first temperature.
The temperature is adjusted to a temperature within the fixing possible temperature range (1165 to 210 ° C.) of the toner, that is, 180 ° C. On the other hand, in the second fixing device 23, the temperature of the outer peripheral surface of the heating roller is within the fixable temperature range (140 to 175 ° C.) of the second toner, that is, 16
Adjusted to 0 ° C.

Therefore, the first toner transferred to the sheet has a fixable temperature range of 165 to 210 ° C., so that when it is heated to 180 ° C. by the first fixing device 13, most of the toner is melted and fixed on the sheet. To do. Next, the first toner is transferred to the second fixing device 23 by superposing the second toner on the first toner, and is heated to 160 ° C. here. At this time, the second toner is
Since the fixable temperature range is 140 to 175 ° C., most of it melts and is fixed on the sheet. However, this temperature (160 ° C.) is the temperature range (165 to 165) at which the first toner can be fixed.
Since it is outside (210 ° C.), even when heated by the second fixing device 23, a considerable amount of the first toner is kept in a non-melted state. Therefore, the first toner is not remelted and bleeds on the sheet, and the second toner is not turbid.

Even if the toner has the same composition, the toner has different fixing properties depending on the diameter of the fixing roller and the fixing pressure. Therefore, it is necessary to adjust the temperature of the fixing device according to each condition. As a result of experiments conducted by the present inventors, in the case of the toner used in the above embodiment, the fixing roller diameter was 20.
mm and the fixing pressure is set to 4 kg, the feasible temperature range is 175 to 220 ° C. for the first toner and 1 for the second toner.
It was confirmed that the temperature was changed to 50 to 190 ° C. (see FIG. 5). It is considered that this is because the contact area (nip) between the fixing roller and the fixing roller changes according to the change in the pressure contact force. Therefore, when designing a fixing device under that condition,
The heating temperature of the first fixing device needs to be set in the range of 175 to 220 ° C, and the heating temperature of the second fixing device needs to be set in the range of 150 to 175 ° C.

When the fixing conditions are different between the first fixing device and the second fixing device, for example, a fixing roller having an outer diameter of 20 mm and a length of 225 mm is used for the first fixing device and is brought into pressure contact with a force of 6 kg. When a fixing roller having the same outer diameter (outer diameter 20 mm) is pressed against the fixing device with a force of 4 kg, the fixing temperature of the second fixing device is set in the range of 150 to 175 ° C.

In the above description, the first toner and the second toner are used.
Although the toner is fixed on the same surface of the sheet, these may be fixed on different surfaces. Also,
The present invention can be applied even when the colors of the first toner and the second toner are the same. Further, the printing material is not limited to the toner, and the present invention can be applied to any printing apparatus as long as it is an apparatus for melting and fixing by applying heat.

[0018]

As is apparent from the above description, according to the dual-type image forming apparatus of the present invention, the first printing material previously fixed on the sheet is melted when the second printing material is heated and melted. It is possible to obtain an image of high quality without bleeding or turbidity of the printing material.

[Brief description of drawings]

FIG. 1 is a diagram showing a schematic configuration of a dual image forming apparatus according to the present invention.

FIG. 2 is a diagram showing a fixable temperature range of the first toner.

FIG. 3 is a diagram showing a temperature range in which a second toner can be fixed.

[FIG. 4] The outer diameter of the fixing roller is 20 mm, and the pressure contact force is 6 k.
FIG. 7 is a diagram showing a fixable temperature range of the first toner and the second toner and a set temperature of the first fixing device and the second fixing device when g is set.

FIG. 5: The outer diameter of the fixing roller is 20 mm, and the pressure contact force is 4 k.
FIG. 7 is a diagram showing a fixable temperature range of the first toner and the second toner and a set temperature of the first fixing device and the second fixing device when g is set.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus, 10 ... 1st unit, 11 ... 1st image forming unit, 13 ... 1st fixing device, 20 ... 2nd unit,
22 ... 2nd image forming unit, 23 ... 2nd fixing device.

Claims (1)

[Claims] 1. A double image forming apparatus in which a first fixing device heats a first printing material to fix it on a sheet, and then a second fixing device heats a second printing material to superimpose and fix on the sheet. In the above, a printing material that melts at a temperature higher than that of the second printing material under the same conditions is used as the first printing material, and the heating temperature of the second fixing device is the first printing material under the fixing conditions of the second fixing device. A dual-type image forming apparatus, wherein the temperature is set to a melting temperature of the printing material or less and a melting temperature of the second printing material or more.