I went some informatiom about printing

[alhabtoor]

Hi Everybody,

I Am A New Member In This Forum. And I Would Like To Know More Information About:
1. How To Producing Lithography`s Plate.

Because I Am Studing Now Positive And Negative Plate, And How To Make Them.also, The Lithography Platemaking System.

So Anybody Have Any Details About Them , Let Them Told Me This Details


Best Regard,
Alhabtoor

 

[alhabtoor]

why nobody response me??!

i must have this information this week please

 

[Danger]

Sorry, dude. I didn't respond because I know nothing about it. If you'd asked about screen printing, I could help.

 

[brewnog]

Ok, suppose I'm the resident Lithography specialist here too then!

Offset Lithography is by far the most widely used printing process, and works on the basic premise that water and oil don't mix. The inks used are generally oil based (many are rubber based but the principle is the same). The desired image to be printed is put onto a lithographic plate, which is then dampened with water (or commercially, a water or alcohol based damping fluid), and then by the ink. The ink is picked up by the area where the image is to be formed, and the water is picked up by the remainder of the plate. The image is then transferred (by rollers) onto a 'blanket', and then onto the paper.

- The first step of production of a lithographic plate is to obtain the graphic image which you are wishing to print. This may be done by hand (pen & ink) or printed out off a computer, it doesn't really matter.

- The next step is to produce negatives of these images. This is usually done onto film, using the photographic process (although the cameras used are much bigger!).

- Then, the image is formed on the plate. The plates are light-sensitive, so the image from the film negatives is simply projected onto the plate in a dark environment for a prescribed length of time. Here, a photochemical reaction occurs (as with the photographic process) which forms a latent image on the plate. This image is receptive to ink. We generally use paper plates today because they're cheaper, but aluminium plates are still used for high-volume work because they last longer, and can be stored for long periods of time.

- Obviously for multi-colour work, a separate plate is made for each colour, and the paper then run through the process as many times as required, for each colour.

Any more detail you need, let us know.

 

[alhabtoor]

Thank you,

but I want a method to produce the negative or positive plate

 

[brewnog]

There are a number of processes used to make plates, but as I said, I'm most familiar with the silver halide process (similar to black & white photography). Basically, the plate is supplied with a silver halide emulsion, which is kept in the dark until use. The image to be printed is then projected (I believe now a laser image can be plotted instead) onto the plate, when the silver halide (usually silver bromide, I believe) grains are oxidised. The photochemical reaction is Ag+Br- crystals + hv (radiation) --> Ag+ + Br + e-. In short, a free Halogen atom is released when light hits the silver halide crystal. The silver ion can then combine with the electron to produce a silver atom. The free silver is what's referred to as the 'latent image'; it is this which is developed to make the visible, stable image we need.


The image then needs developing. Paraphenylendiamine (I think, check this if you need to know) is washed over the plate for a predetermined length of time in order to reduce the silver ions back to free silver, but not long enough to allow the unexposed silver halide grains to reduce too (the ions reduce more quickly than the grains).

In order to halt the development process to produce a stable image, the image must then be fixed. Sodium Thiosulphate is used, which through a series of reactions, produces a stable, visible image on the plate.

The rest is as per my previous post.


All the reactions can be found at:
http://www.cheresources.com/photochem.shtml

Information about other plate-making technologies can be found at:
http://www.internationalpaper.com/PDF/PDFs for Papers/Offset Plate Technology.pdf