Flexographic Printing: A Process Description

Flexography is used predominately in the packaging sector with flexible packaging, labels, corrugated post print and pre print all using the flexo process. The flexo process is relatively simple and operates by transferring ink from a soft rubber transfer roller to the anilox roll; this roll is filled with billions upon billions of tiny cells. Once they are filled, a doctor blade is used to scrape away any excess ink from the surface of the roller; this meters the exact quantity of ink to reach the printing plate. The plate then transfers the image to the substrate.
 
 

Printing Plates

Printing plates are made from photopolymer and are normally supplied in flat sheets of un-reacted polymer. They are then processed by specialist pre press houses. The quality of the print is dependent directly on the quality of the plate manufacture as it is the raised areas of the plate that transfer the ink. Typically the relief may be up to 0.8mm in height while the total thickness of the plate is usually around 1.5 mm. See the image below.
 
  There are six steps in the conventional platemaking process.
 
1. Expose the plate on its reverse side in order to form the back of the plate.
2. Reverse the plate with the negative film then exposed the plate.
3. This generates the dot/line image as the exposed areas become hardened.
4. The plate is washed out to remove the un-hardened photopolymer
5. The plate is then dried.
6. Final hardening is achieved by post exposure that also removes any plate tackiness.
 
More recently Computer to Plate applications are more popular, with the removal of the negative and the possible errors that could arise in transferring the image from the film to the plate.
 
Once allowed to dry, the plate is ready to mount onto a plate cylinder. This is normally done using a double sided adhesive tape. The thickness and hardness of the tape must be suitable for the images being printed with harder tapes being used for solid areas of colours whilst softer tapes are used for halftone images.
 
Registration marks ensure accurate mounting of all the printing plates onto the plate cylinders for perfect dot-on-dot registration. The plate cylinder is then mounted in the press. 
  

Anilox Roll and Metering

The purpose of the anilox and doctor blade is to meter an even amount of ink to the printing plate.
 
 
 
The ink is held in the millions of anilox cells while the doctor blade runs over the surface of the anilox removing any excess ink. The anilox is normally manufactured from a ceramic compound whose hardness ensures long life and is not prone to abrasion from the doctor blade. The anilox roll is manufactured to release a certain ink weight from the laser engraved cells, so the size of the cells and resolution are important to the colour on the final printed image.
 
There are two main types of ink supply and doctoring system used in flexography. In open systems the anilox rotates in an open bath of ink and the doctor blade meters the ink just prior to the ink being transferred to the plate cylinder.
 
The Chamber system (on the right) encloses the ink using two blades within a chamber. The retaining blade at the bottom acts as a seal while the doctor blade at the top performs the ink metering. The ink is pumped through the system to maintaining a constant supply of ink to the anilox surface. The advantage of the closed doctoring system is that there can be less ink required to "charge" the system and there is no release of VOC’s as the system is contained. Their prime disadvantage is the complexity of the cleaning required between ink colour changes.
 
 

Flexo Ink

The ink used in flexography is generally of low viscosity and must be kept consistent during a print run in order to maintain print quality. The main source of variation of the ink is an increase in the viscosity as the solvent or water evaporates from the ink. With this in mind, many modern presses incorporate automatic viscosity control units, which maintain the viscosity of the ink by the addition of exact quantities of solvent to the ink, reducing the viscosity. Alternatively the press minder maintains viscosity by manually measuring its viscosity and then adding more solvent. Both water based and solvent-based inks are used in flexography. Solvent-based inks are easier to dry and generally posses better transfer characteristics on low surface energy substrates such as packaging films.
 
UV cured inks are increasingly important and have the advantage of reducing VOC’s, allowing immediate curing of the ink and maintain a constant viscosity through the production run, as no solvent is lost.
 
 

Types of Flexographic Printing Presses

Sleeve technology has greatly improved performance of modern flexographic presses. The sleeves are light weight can be used for both the anilox and plate cylinders. The print jobs are prepared off the press with the plates being mounted and correct anilox for the image collected. The make ready time is then greatly reduced and the components can all be loaded by hand in many cases.
 
A flexographic printing press usually comprises from two to eight stations. These can be laid out in up to three main configurations, central impression, in-line and the stack press, there are advantages and disadvantages associated with each type of press.
 
 
CI Press
The web passes around the central impression cylinder with each colour being printed in turn. Inter-station driers are used to cure the ink between prints stations to ensure wet on dry printing.
 
A distinct advantage of this type of press is that registration is excellent, as the web is held over the CI drum between print stations. There is less chance of the substrate being extended between the print stations. The press speed of central impression presses can generally be increased beyond that of the other press types. The main disadvantage of this type of press is that it is not generally possible to print on both sides of the substrate without a second CI station. This type of press is predominantly used for flexible packaging printing.
  
In-line press
The in line press is a combination of individual units with a small distance between the print heads. It is commonly used in the narrow web sector for labels.
Each colour is printed on the web fed substrate. Each colour has an individual impression cylinder against which the plate is pressed. An advantage of the in line press is that other printing technologies (such as rotary screen or gravure) and additional colours / processes can be incorporated by simply adding another station. Additional processes that can be added include embossing and foiling.
 
These machines often use UV inks which are dried between each print station. The major disadvantage of the in-line press is the cylinder size is limited which limits the image size and repeat length.
 
 
Corrugated Press
 
This is a sheet fed press machine that prints onto the substrate after it has been manufactured into flat boards.
 
The corrugated sheet flows through the press in one straight line. Between the printing decks the sheet are held in place by rollers. Corrugated presses can range from older type presses that need an engineer to change the anilox rolls to newer presses that are robust and give excellent register with fine tone-work. One of the features of this type of press is that die-cutting and converting can be accomplished in line after the printing section.
 
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