Plate Blinding

Plate blinding in offset printing is a situation when printing plate’s non-image areas (the areas that are meant to remain ink-free) become covered or clogged with ink, causing them to inadvertently accept ink.

 This results in unwanted marks or faded images being printed in areas where they should not be.

 As result we have poor print quality, often manifested as “ghosting” or unwanted printing in areas that should be blank.

Causes of Plate Blinding

Inadequate Balance between Ink and Dampening Solution

If the dampening solution and ink balance are not properly maintained, then the non-image areas of the printing plate may absorb some amount of ink, and that will lead to unwanted printing.

Over-inking

 Too much ink on the printing plate can cause it ink to spill over into the non-image areas, leading to plate blinding.

Poor Plate Maintenance

 If the plate is not properly cleaned or if some contaminants like dust or ink are accumulate on plate, the non-image areas may begin to accept ink

Improperly Developed Plates

 When the plate exposure or development process is not done correctly, it will result in poor definition of the non-image areas, causing ink to stick to those areas.

Damaged or Wear Printing Plate

Worn or damaged plates may lose their ability to reject ink in the non-image areas, leading to plate blinding.

Prevention of Plate Blinding

• Adjust fountain solution to proper pH, temperature and conductivity

• Ensure that you have proper ink-dampening solution balance maintained during the printing process.

• Regularly clean the printing plates and rollers to prevent contamination.

• Properly expose and develop plates to ensure clean non-image areas.

• Adjust correct ink levels to prevent over-inking the plate.

Emulsification

Emulsification in the offset printing process refers to the mixing of ink and dampening solution on the printing plate.

In the process of offset printing, there are two main components: ink (which adheres to the image areas of the plate) and dampening solution (which adheres to the non-image areas).

Proper emulsification is necessary if we want these two components to work effectively together during the printing.

How Emulsification Works

Ink and Water Interaction

The dampening solution is applied to the non-image areas of the plate, while the ink is applied to the image areas.

When the ink and water meet, emulsification occurs, forming a mixture of both substances in the non-image areas.

Causes of Emulsification

Excessive Dampening Solution

Emulsification helps maintain a proper balance between ink and water, ensuring that the non-image areas do not attract ink while the image areas do.

The proportion of water (dampening solution) in a “stabile emulsion” is around 20 %, so the right balance is crucial.

If it is higher, then the ink’s splitting properties are reduced and the ink flow is interrupted.

As a result, the ink “settles” and “emulsifies”.

Incorrect Ink Viscosity

If the ink is too thin, will be be more susceptible to emulsification, and this will affect the quality of the print.

Effects of Emulsification

Poor Print Quality

 Too much emulsification can cause the ink to lose its strength, and this will result in weak or dull colors and blurred images.

Ghosting or Ink Smearing

Emulsified ink can spread beyond its intended area, leading to ghosting (faint, unintended images) or smearing on the printed page.

Ink Repellency Issues

 Non-image areas may begin to attract ink, leading to poor plate performance and unwanted marks in areas that should remain clean.

Prevention and Solution for Unwanted Emulsification

Set the ink and dampening balance correctly, and check on it continuously throughout the printing process.

In case of emulsified ink, wash the rollers, and re-establish the smearing limit.

Check the dampening solution, and change it regularly (the ideal dampening solution has a water hardness from 8 to 12° dH, a pH-value from 4.8 to 5.5, and a temperature from 10 °C to 15 °C (50 °F to 59 °F).

On an Alcolor dampening unit – separate the intermediate roller from the inking unit; install cross-flow ventilators.

Avoid frequent phases of pre- and post-dampening (during make-ready or stoppages).

Inspect and optimize the adjustment of the inking and dampening rollers.

Where there is less ink taking, an additional ink stripe may be introduced to stabilize the balance of ink and water.

Lags in Drying Time

If the ink dries too slowly in the stack (paper or cardboard), then we can speak of a lag in drying time.

It is critical part of the printing process because this may lead to an ink to rub-off of the print during additional post print operations like die-cutting, folding gluing…  

An incorrect interaction between ink, additives and dampening solution is usually the main culprit for lags in drying time.

For example, if the pH value of the dampening solution falls below 4.7, then longer drying times can certainly be expected.

Incorrectly dosed additives (drying accelerators), too low a stack temperature or overly emulsified ink also slow down drying.

 In general, the heavier the ink layer, the slower the drying process.

Other reasons for lags in drying time during printing can be:High humidity,paper/cardboard type,over-inking,insufficient airflow in the printing machine or high printing speed.