How analog copiers work

How analog photocopiers work.

Basically, analog photocopiers use many components and follow the principle below in producing a copy or an output of the document:

1) Drum (Organic Photoconductor Drum)

          This is where the actual image of the document being copied is produced.  The drum is a hollow aluminum alloy cylinder coated with a light sensitive organic photoconductor.  This makes the drum a very fragile part of the photocopier.  Any damage on the drum will appear on the output or the copied image.  Even small scratches or damages on the drum surface will also be visible in the output.  Different models have different drum diameters and lengths.  Usually, bigger and high end models also have a larger drum.

2) Drum Charging

          A charging corona assembly located and mounted on the top of the drum evenly deposits a strong negative charge on the entire surface of the drum.  This makes the drum surface more sensitive to light.  The charging assembly may have the same length as the drum and it also comprises of a wire (or an electrode plate) and a grid (or a screen) housed together.  The charge runs along the wire while the grid evenly deposits the charge on the surface of the drum.  Most copiers supply the assembly with a handle that cleans the wire easily when pulled. 

3) Erasure (LED exposure)

          After charging, any area of the drum which will not be used for producing an image will be neutralized.  Most analog copiers use LEDs (Light Emitting Diodes) to do this job.  To further illustrate, if your analog photocopier can photocopy documents up to A3 size (297x420mm) and you are photocopying an A4 size (210x297mm) document only, all areas outside the size of the document will be neutralized.  This is done to conserve toner usage.  Also, these LEDs are placed in parallel and after alongside the drum charge corona assembly.

4) Light Exposure

          This process is done by the scanner section.  As you put the document on the scanner table or on the document feeder, light from the scanner lamp (sometimes called the exposure lamp) is directed towards the drum through a series of mirrors and lens.  The lens and some mirrors on the scanner unit move in accordance to the zooming ratio setting that you input before making copies.
           The reflected light from the scanned document strikes the surface of the drum reducing the amount of negative charge deposited by the charge corona assembly.  This process creates an invisible mirror image of the scanned document and it will be seen in the next step.  
The amount of light reflected to the drum from the scanner unit is regulated through the combination of the movement of the lens and mirrors in accordance to the zooming (reduction or enlargement) ratio that you input before scanning the document.  Also, the intensity of the light given off by the scanner lamp is automatically controlled and adjusted through the help of the exposure sensor located alongside one of the mirrors to help maintain the quality of the output regardless of the quality of the document being scanned.

5) Toner Supply and Developing

Before the actual developing of the image is done, toner is mixed with the developer inside the developing unit through coil springs that continuously mix the toner and developer together creating a perfect ratio needed to supply the right amount of toner to the developing roller and onto the drum.  This mix ratio is monitored by a sensor.  If the amount of toner inside the developing unit decreases, a toner replenishing process is done.

The developing roller which is constantly in contact with the drum supplies positively charged toner on the drum.  Only areas on the surface of the drum that have been exposed to the light from the scanner will be filled with toner.  This forms the visible mirror image of the scanned document on the surface of the drum. 

6) Paper Feeding

            Paper is then supplied from the trays (cassettes) or the bypass tray of the photocopier.  But before reaching the developing area, right margin (registration) and skew control must be ensured so that an exact copy of the document being scanned is achieved, meaning that when you place the original document and the copied output side by side, its margins are just the same.  This is done by registration rollers (sometimes called the synchronizing rollers).  Some models have 3 trays including the bypass but some have an optional tray accessory that adds two or three more trays giving more paper sources and lessening the amount of time supplying paper to the machine. 

7) Transfer and Separation

            As the paper enters the developing area and goes underneath the drum, the mirror image created on the surface of the drum is then transferred to the surface of the paper.  This creates an exact duplicate image of the scanned document and is done through the image transfer assembly installed underneath the drum.  The transfer corona assembly emits a negative charge that attracts the positively charged toner from the drum surface.     

          As the image is transferred, the leading edge of the paper is then separated or peeled off from the drum surface.  This is done by the separation corona assembly which emits a charge to neutralize the paper naturally separating it from the drum.  Also, separation projections (also called claws) are installed underneath the drum to help to this job.

          Some photocopiers house the transfer and separation separately while others put the two assemblies together.  Some models include a cleaning mechanism for this unit that can be easily accessed by users.

8) Drum Cleaning and Toner Recycling

          Remaining untransferred toner on the surface of the drum from the developing process is physically scraped off by a rubber blade.  This rubber blade is sometimes called a cleaning blade or simply a blade.  It is constantly in contact with the drum to ensure that every unused toner is cleaned off.  A spring coil mechanism installed underneath the blade then transports the scraped off toner back to the developing unit and then to the developing roller.  However, some models don’t have a recycling mechanism and disposes the untransferred toner as waste. 

9) Drum Neutralizing

         After cleaning the drum, an array of small lamps light up to neutralize the cleaned area ensuring that any remaining charges are gone.  This part of the drum is ready for the charging process to repeat again. 
         These small lamps are located almost above the drum next to the cleaning blade and before the charging corona assembly.

10) Paper Transport, Fusing and Exit

            The paper separated from the drum is then transported by rollers and/or belts to the fusing unit where the image transferred to the surface of the paper is permanently fused.  Inside the fusing unit, fusing rollers apply heat and pressure to do this process.  This is like asphalt being applied on the surface of the road and then being steam rolled for a smoother finish.  Temperature is controlled by combination of thermistor and thermostat installed near the rollers.  Depending on the model of the copier, some may have up to 4 thermistors and 2 thermostats.

    After the fusing process, the paper is then transported out to the receiving trays or the sorter trays.  Receiving trays may just be an ordinary tray placed on the side of the copier or complicated mechanical sorter with up to 20 trays or bins where automatic sorting, stapling and folding is done.

        This whole process takes place continuously until the last document scanned is copied and produced.  Since the drum circumference is not enough to produce 1 whole page length of the document being scanned, it is continuously charged, exposed to light, supplied with toner, cleaned and neutralized until the entire image of the whole document is produced. 

         Also for analog photocopiers, the scanner lamp lit up continuously per page and per set of the document being scanned.  It means that the scanner lamp will light up equivalent to the total number of pages of the whole document multiplied by the number of sets that you input.  This process had been eliminated in the introduction of a memory or buffer in digital photocopiers.
         
        Some of the most common brands of analog copiers are: Canon, Konica, Minolta, Ricoh and Xerox.