Charging

The charging unit is kept in a dark location, and a high voltage (usually several thousand volts) is applied to the charged electrodes, creating a corona discharge, which creates an even positive charge on the photoreceptor drum.

Whether a positive charge is generated evenly on the drum or not determines the quality of the image.

On the photoreceptor drum, only those locations that have been exposed to light will become conductive, discharging when earthed. In common dry electrophotography, the charge on the drum is usually negative, but in this machine the charge is positive, so special attention must be given to the photoreceptor drum.

Writing the image

Each LED in the 600 dpi LED array can be turned on and off at high speed by a corresponding semi-conductor switch. When light is shone on the charged photoreceptor drum, only the exposed locations lose their charge, creating an electrostatic latent image.

For example, when black lettering is recorded, the locations on the receptor drum corresponding to the black lettering are not exposed, and so the positive charge is retained in those locations, while other areas are exposed to the light and so lose their charge.

Developing

The liquid, consisting of resin and dyestuff dispersed in a solvent, is usually called toner. This resin and dyestuff constitute a single body that is designed to carry a negative charge.

Thus, because the toner and the positive charge mutually attract, when the photoreceptor drum carrying the positive latent image and the negatively charged toner come together, the resin and the dyestuff are drawn to the surface of the photoreceptor drum, forming the image.

Toner used in electrophotography is commonly either of powdered (dry) or liquid (wet) form. In addition, among dry toners, there is the two-component type that includes an iron component, and the one-component type that does not. The main component of all the different types of toner is pigment.

The toner used in this system is a wet, one-component type, the main component being dyestuff. This is the first toner in the world for which the main component is dyestuff.

Aside from its electrostatic properties, a number of other problems can also be expected during the development of this toner, including those concerning its transfer to the substrate and the cleaning of the photoreceptor drum.

The squeeze roller

The image created (the toner attached to the photoreceptor) contains a large amount of solvent, and so if it is transferred to the substrate in this state, there is the high possibility that it will bleed, damaging the image.

To avoid this, a roller system is required that presses the solvent, making it suitable for transfer.

If the hardness, the speed of revolution and the direction of the roller are not set appropriately, a good image cannot be obtained.

Image transfer

The image consisting of toner on the photoreceptor drum must be transferred through some means to the substrate (be it paper or fabric).

Aside from pressing together the photoreceptor drum and the substrate, an electric field applied from the reverse side of the substrate will also serve to transfer the toner from the photoreceptor to the substrate.

If the peripheral velocity of the photoreceptor and the feed rate of the substrate (fabric, etc) are different, the image will not be transferred properly and if more than one color is used, they may be unaligned. To avoid this, a high-degree of precision is necessary, with every effort being made to reduce any alterations of speed, and machinery allowing a high degree of speed control is required.

Through the process outlined above, an image can be formed on the substrate, be it paper or fabric.

Cleaning

In order to form the next image, the photoreceptor drum must be returned to its original state. During the transfer, not all the toner will necessarily be transferred, and a certain amount of toner may remain on the photoreceptor. In addition, there may be residual charge from the latent image.

The remaining toner is wetted with solvent and removed with a cleaning blade. If the blade which removes toner is not flush with the surface of the photoreceptor, some locations will not be totally cleaned of toner, damaging the quality of the following image.

However, if the blade is applied too firmly, the surface of the photoreceptor may be scratched, which can damage the quality of the image through the creation of lines. Establishing the correct conditions here can be very difficult.

In the manufacture of the cleaning blade, sufficient knowledge and experience is necessary in order to select a material that can be used to produce an edge of the required straightness and hardness with sufficient resistance to the solvent.

Quenching lamp

Any remaining charge can be removed by exposing the photoreceptor to a quenching lamp.

To ensure that the charge is removed, the wavelength of the light source should be matched to the wavelength of the optimum light absorption of the photoreceptor.

Through this process, the photoreceptor can be returned to its original state, ready to record the new image.

The electrophotographic unit mounted in this new printing machine as outlined above is created through the integration of a wide range of technologies including those of optics, electronics, chemistry, mechanical engineering, electrical engineering and software engineering. Comprehensive technological skills organically linking these different disciplines are necessary if the unit is to be produced successfully.

Specifications of the new printing machine based on electrostatic photography

1. Outline of the specifications of an output sample

1. Substrate	Polyester
2. No. of colors	One: magenta (although YCK are also possible)
3. No. of lines in image	> 50
4. Dye types	Disperse dyestuff, reactive dyestuff
5. Overprint density	>1.4 (necessary), >1.6 (ideal)
6. Color difference	Undetermined
7. Lowest image density	<10%
8. Hand	No remaining resin
9. Density uniformity	Discussed elsewhere
10. Defective images	None

2. Imaging engine and unit

1. Imaging method	Wet-type electrophotography
2. Imaging light source	LED array
3. Image resolution	600dpi
4. Imaging speed	>150mm/s (>9m/s)
5. Maximum image width	>305mm
6. Maximum image length	Discussed elsewhere
7. Image density	>0.5 (necessary), >0.8 (ideal) before developing (provisional)
8. Copied colors	One: magenta
9. Up/down mechanism	Yes
10. Substrate	Polyester
11. Toner	Special M toner
12. Unit size	See machine diagram
13. Unit weight	>15kg
14. System I/F	Discussed elsewhere
15. Required power supply	Three-phase AC200V

3. Feed

1. Distance between winding axes	2.5m
2. Height of print belt	900mm
3. Print belt width	>500mm
4. Print unit support	4 locations (YMCK)
5. Unit spacing	Discussed elsewhere
6. Transfer roller support	4 locations (YMCK)
7. Feed rate	5~15m/min (alterable)
8. Uniformity of feed rate	Discussed elsewhere
9. Substrate status signal	Yes
10. Fabric fastening	Adhesive roll
11. Fabric adhesion operational width	>500mm
12. Fabric pre-processing apparatus	None
13. Automatic belt cleaning	None (manual cleaning)
14. Required power supply	Three-phase AC200V

Note: The items that are not prescribed in these specifications will be decided after further discussion.

Direct Textile Printer: Test model design

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Diagram of monochromatic printing unit

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