• Offset plates do not have raised images
• Image is created so that certain areas of the plates accept water (hydrophilic) and other areas will "reject" water (hydrophobic)
• The non-print area is hydrophilic and therefore attracts water. The print area is hydrophobic and therefore attracts the ink to be printed
• Ink is mixed with water (emulsified). This mix is transferred via blanket to plate, and ink-water particles are split to the respective surfaces
• The wetting of the plate depends on its surface energy and surface tension. Different areas of the plates have different surface energies, and  this either allows the water to form a continuous film, or makes it bead up and move away.
• The ink has to function with water, blanket, plate, and material
• This process is sensitive to changes in water, ink, temperature, and humidity

These parameters need to be understood and controlled to achieve consistent print quality
The success of the offset process derives from the fact that oil and water do not mix. Traditional offset inks are based on different oil-bound resins, however, the same principle applies in UV-offset, except that the ink is based on acrylate resins.The water spreads out well in the non-image area of the plate, and much less in the image/print area. The interfacial surface tension between the water and the plate is much greater in the image area.

Dampening system
The dampening system of the printing press keeps the non-image area of the plates coated evenly with water. When the particles of water and the ink (traditional oil) hold each other in place, it is called emulsification. However, an ink can become "waterlogged" or "over-emulsified" as the amount of water increases. Once over-emulsification is reached, there is no other way to recover the ink, than shutting down the press and cleaning the ink system completely.

Fountain solution
A plate's water receptivity decreases as the plate runs on the press. The fountain solution is a mixture of chemicals that help to maintain a plate's receptivity to water in the non-image area. The fountain solution has three tasks:

• Lowering surface tension (γ) (water is 79dynes/cm)
• Increasing the plate's receptivity to water
• Maintaining a plate's water receptivity

Most narrow-web fountain solutions are "neutral". Reverse osmosis systems are typically used to purify water and remove minerals, and are easiest to control. The fountain solution should be monitored during printing by measuring pH and conductivity (however, most fountain solutions are buffered today, which prevents pH change during the run)

Printing ink
A good offset ink will maintain a fine and stable emulsion irrespective of changes in other parameters, and has a so called large operating window. This means better productivity, and stable print quality for printers. The pigment level of the ink will effect the ink / water balance and the only way to control this, is by monitoring the density reading during a press run. By maintaining a set density, the least amount of ink is always being applied. Basically, an optimum ink / water balance is achieved, when the least amount of ink and the least amount of water is used. This generates the "least amount of problems"
Less is best……!

Optimizing offset quality through process standardization
XSYS Print Solutions has been working together with Nilpeter trying to develop a "road map" for beginners in UV-offset in order to get some fundamental principles correct, but also to obtain good offset quality quickly. We also investigated all parameters involved trying to determine how to improve the quality of UV-offset printing. Initially, all parameters required from a narrow-web UV-offset ink were identified. Secondly, the parameters which will effect the UV-offset performance properties were listed and key print performance variables were identified and studied.

Parameters required from a narrow-web UV-offset ink:

• Colour strength
• Dot gain (paper, film, over screen ink)
• Cure speed
• Operating window (water tolerance settings)
• Lithographic properties / emulsification (no tinting at extremities)
• Tolerance of water fluctuations
• Adhesion to synthetic substrates
• Scratch resistance
• Chemical resistance
• Hot foil and transfer printability
• Odour and low migration (dependant on the end use)
• Duct behaviour

Parameters which will effect the UV-offset print performance:

• Repro
• Dot gain compensation
• Plate type (positive / negative)
• Plate process (baked / un-baked)
• Blanket (type, shore hardness, compressible)
• Ink
• Water type (hardness, conductivity, pH) and amount
• Fountain solution type (buffer capacity, conductivity)
• Amount
• IPA / alcohol
• Print speed
• Humidity
• Substrate

It is obvious that many more variables in UV-offset printing are to be considered compared to UV-flexo printing for example. In order to improve the overall quality, all the potential variables need to be mapped and understood. The intention with the "road map" is to develop a program to help printers print "right from the start". However, only a few key ink parameters specifically affect offset print performance!

The Key printing performance criteria are identified as being:

• Colour strength
• Dot gain (paper, film, over screen ink)
• Cure speed
• Operating window (water tolerance settings)
• Lithographic properties / emulsification (no tinting at extremities)
• Tolerance of water fluctuations
• Adhesion to synthetic substrates
• Scratch resistance
• Chemical resistance
• Hot foil and transfer printability
• Odour
• Duct behavior

UV-offset road map – by looking at basic ingredients, fine tuning.

Repro

• The quality of the final print is determined by the quality of the plate, related to the original picture
• Correct fingerprinting and plate correction should be completed
• Standardized guidelines are available and should be followed when possible
  

Plate type

• Most plates are generated by ctp process
• Plates can be either positive or negative
  

Blanket

• Quality of top coat (good chemical stability, without swelling with UV-inks)
• EPDM is commonly used
  

Humidity

• Should be controlled to avoid fluctuations
• Will effect fount if humidity is too low or too high (ideal 50-60%)
• EPDM rollers that are compatible with UV must be used
• Should be set with a minimum of bounce to the plates to avoid scum lines
  

Dot gain/compensation

• Where dot gain curves are understood, the repro should be compensated for allowing this
• Dot gain is dependant on substrate and fount conditions used
  

Plate process

• Our experience is that a baked plate is easier to work with and has a longer lifetime
  

Print speed

• Will effect fount levels required to maintain a clean print
• Automated fount dosing on press?
  

Substrate

• Will effect stability of fount solution, synthetics are typically hardest

The key to high quality consistent UV-offset printing is to map and understand all the variables within the process. When changes are made, the effect downstream on the printing process and the end result should / need to be understood.

But once all variables are set, the key to maintaining a high quality UV-offset print result is to maintain an optimum ink / water balance in the press. This is described as "The relationship between the mixture of ink and water that provides the best printing outcome whilst using the least amount of both materials". Basically, the least amount of ink and the least amount of water will lead to the least amount of problems. Or….. Less is best!

"XSYS Print Solutions has recently introduced a dedicated and truly tailor-made UV-offset ink for narrow-web presses", claims global brand manager, Niklas Olsson, XSYS Print Solutions. Lithocure 3G is the new generation of UV-offset developed from the combined experience and expertise by Flint Group and XSYS Print Solution. Lithocure 3G is the result of intense development work to optimize ink and water balance on the specially developed UV-offset presses for narrow-web applications. Niklas Olsson is delighted with the end result. “Leading converters across the industry and across Europe have been involved in the test phase, and tell us that Lithocure 3G offers excellent litho print properties and is very stable on press, irrespective of press settings and fluctuations". They have commented on the printability and colour strength across a variety of print substrates, from matt paper to synthetic films – a real advantage for today's multi-capability narrow-web presses. Overall, we are confident that Lithocure 3G is now delivering the best performance in narrow-web UV-offset.