Offset printing : how does it work ?

Technically, how does offset printing work ? There are two main processes of offset printing :  conventional “wet’ printing process and “waterless” printing process.

Conventional “wet” offset printing

Historically, this process comes form lithography. The printing process is based on the principle of water - oil repulsion ; In other words, water and oil cannot mix : it makes an emulsion between the moistening solution and the oil ink.

An offset printing takes place this way, step by step :

• Step 1 - Preparing the plate The printing file is engraved on a metal plate with a ComputerToPlate (a peripheral allowing to directly make the plates, destined to the presses).
• Step 2 - Inserting the metal plate in the printing process : A metal plate is inserted in the first machine of the offset printing process. The offset printing process consists of several machines : each machine matches with one color. Usually, there are 4 of them (Cyan Magenta Yellow Black)
• Step 3 - Placing the paper : Then, the paper is placed in the machine, which checks with air blast if the paper is not stuck or that an only sheet will be sent every time. 

When everything is ready, the machines are turned on and the printing process is started.

• Step 4 : Moistering : The plate goes into the first machine. It is assembled on a rotary cylinder. The plate is then in contact with moistering rolls that wet the whole plate.
• Step 5 - Applying the ink The ink rollers then put the ink and spread it on the plate.
• Step 6 - Copying the image : It is the specificity of printing process : the blanket. This second to last rubber cylinder allows the inked image to be inversely traced.
• Step 7 - Printing The paper sways between the blanket and the printing cylinder to progressively put the reversed image on the paper.

The plate and paper carry on in the following press for a new color. After having been through all the presses, there’s only drying step left ! The paper is then dried with hot air and piled before being sent ! 

Waterless offset printing :

Waterless offset printing use the same principles as conventional offset printing, the only major difference being it doesn’t use water ! There is no moistering system : water is replaced by silicone to avoid the inking of the plate.

Offset printing : for what uses ? 

As told above, offset printing can be used for a wide range of products. Among which can be found the production of newspapers, books, catalogs, banknotes, screen-printed transfers etc. More generally, several different matters can be treated : cardboard, paper, plastic, metal, etc.  

At PrintMyTransfer, we use it for printing our screen-printed transfers  !

Offset printing : advantages

Here are the main advantages of offset printing :

• Starting the process is easy
• Adaptability and flexibility for the production of products and quantities
• The costs are mastered and low compared to other printing processes
• Drying is quick
• The ink is well stabilized and printing has a photo quality !

That’s the perfect printing process for CMYK printing 

Offset printing : disadvantages

• The consumable cost : the inks and plates must be frequently changed.
• The maintenance : important and regular controls are needed on the machines to avoid gaps and errors.
• Possible paper loss during the settling