What is the repeat length issue in flexo printing

When printing, have you ever experienced varying repeat lengths?  It is true that you measure the repeat length when you stop the press; however, when you convert the roll, unacceptable variations or shifts in repeat length occur.

If we print on substrates that are “stretchy”, or if our equipment is unable to control web tension or web speed relative to plate speed due to its properties, the repeat length can vary.

Print repeat length in flexo printing is influenced by three main factors:

  • Tensioning of the material during the entire process
  • Structural stability of the machine
  • Temperature of the air used in drying.

Tensioning of the material during the entire process

Poor tension control often leads to variations in repeat length.  It is almost certain that tension variation throughout the printed roll will occur even under the best tension control conditions because the unwinding and rewinding rolls have simultaneously and inversely changing diameters.  There is no major problem with this with many substrates.  It can be challenging to control repeat length when printing on light gauge low-density polyethylene webs or turning heavy gauge materials through the presses with old, cranky equipment.

In spite of the material’s extensibility, polymer film and other extensible materials can present printing challenges. Although polymer films are not considered elastomeric materials, they can stretch and deform when stressed.

Structural stability of the machine

As the material tensions during the printing process, the structural stability of the machine will determine the repeat length, and therefore the tensioning of the material. It is important to keep in mind many different aspects, since stability is achieved by balancing the electronic and mechanical parts of the printer.

An anilox roller and sleeve plate cylinder structure are utilized, which enable simple, flexible, convenient storage, high accuracy, and ‘rapid version change’ functions for the upper and lower plates.

Temperature of the air used in drying

As a result of the temperature of the air used to dry the ink, the repeat length of a printed design can be affected. There will obviously be variations in the variables associated with the drying process based on the type of ink. Solvent-based inks dry much faster than water-based inks, which is why water-based inks are slower.

Ensure that the material does not suffer from these problems by setting the temperature inside the drying hoods.

Conclusions

The concept of obtaining high quality ink can be achieved provided the material is of high quality, that the tension is correct at all times during printing, that the machine is structurally stable, and that the drying process is suitable for the type of ink and material being used. It is essential to have all of these factors in place to ensure a high quality, defect-free printed reel.