Introduction
Flexography is a high speed printing technology for various printing substrates mainly used in packaging industry.
The greatest changes in flexo printing occurred due to computer-to-plate (Ctp) making technology which has improved the reproduction quality with a wider tonal range and in more detail, particularly in the highlight area.
These evolutionary changes were continuously developed with some fine tuning which can enhance the basic technology. The most important trend in the evolution of the flexoprepress is the creation of different top dot shapes on the printing plate, round and flat. There are three impression levels at round top dot depending on printing pressure which can lead to significant dot deformation (dot gain) compared to one impression level at flat top dot.
Analog Dots -Film Analog process
- Analog plates are made in a vacuum with a film negative.
- Set the image using scattered UVA light resulting in a quickly growing dot
- Dot forms broad shoulders and flat top
- Requires a heavy compensation curve to compensate for major dot gain.
- With scattered UVA dot is 15% larger than film image requiring heavy compensation curves
Analog Dots – Film Analog process- Print Results
- Shallow dot shoulders prevent plate floor depth if dots are too close to each other = low LPI
- LPI is restricted dependent on the plate thickness
- Corrugated 3.94mm plates used 85 LPI
- Flexible 2.84mm plates used 120 LPI
- Labels 1.70mmplates used 150 LPI
Analog Dots – Film Analog process- Other Considerations
- Minimum plate highlight dot restricted to approx. 3-5% by the film imaging resolution
- Shadow tones “filled in” due to limited plate relief Dot Gain a serious restriction when printing highlights and mid-tones
- Flat Top print area produced “cupping” from cylinder size and mounting
Digital Dots (round top dots)
- Presence of oxygen in standard digital workflow slows photo polymerization
- Results in smoothing of the plate surface & a reduction of the dot size (sharpening)
- Sharpening of dot offsets press dot gain, increases tonal range and ‘opens up’ shadows and reverses
- Leads to 50% less dot gain on press, increasing the tonal range and opening up reverses.
Digital Dots- Digital Imaging Printing results
- Steep dot shoulders allows for greater plate floor depth even when dots are close to each other
- LPI is NOT restricted by the plate thickness.
- Corrugated 3.94mm plates uses 120/150 LPI Flexible 2.84mm plates uses 150/175 LPI
- Labels1.70mm plates uses 200/250 LPI
Digital Dots- Digital Imaging Other Considerations
- New Plate types to strengthen the highlight dots with steep shoulders
- Solvent, Water and Thermal Plate processing Ink pigmentation stronger for fine highlight dots
- Better image quality from better color control Press control of impression improved.
- Press registration advanced
Digital Imaging today
- Small screening dots
Midtones and dot closure areas kept open
Text, Linework and Barcodes
High Definition Imaging for Flexo
High Definition Imaging for Flexo
- Screens are in the solids and screened dots
- Finest structures are not destroyed
- Fade out towards fine screening dots
High Definition Imaging for Flexo
Improved ink transfer due to plate screens
Reticulation patterns of the printing ink are destroyed = smooth ink laydown
- HD Flexo delivers best compromise between good solids and color accuracy – FTA
- Perfect solid appearance with only moderate SID increased (+0.1 … +0.2)
- SID +0,1 25% more ink usage on press
- SID +0,3 100% (!) more ink usage on press
- Solids easily print inside ISO standards
- Pantone inks can be emulated (< 3 ∆E) by using C,M,Y,K
Flat top digital dot
- Absence of O2 produces flat top & ‘hybrid’ shoulders
- Capable of nearly exact 1:1 reproduction
- Bump curve reduced or eliminated but some dot gain compensation may be needed
Digital Dots Geometries
Flat top digital dot – (image preparation)
- Advanced screening tools well suited to flexible packaging
- Can further push the boundaries of quality
- Requires press profile for new set of tools
- Minimum printing dot of 2% – 5% required to fade to zero
- Requires good printing conditions, fine anilox and careful impression settings
- Setting a minimum dot ‘limiter’ can prevent a harsh break
- Carefully set FM®AM transition points
- Be realistic about printing conditions If high quality expectations, the press
- should have a profile
- Carefully prepare images to align with those press capabilities
- A well-done 120 LPI screen is preferable to a poorly done 150
Flat top digital dot – Film Imaging + Plate Lamination
- Imaging technology producing a Flat Top dot onto a film then laminating film onto the plate surface restricts O2 and makes a dot 1-3% bigger than the image exposed – this is an analog workflow.
- Achieving 1:1 is a critical process with many variables for success like UV scattered light
Flat top digital dot – Selecting Proper Minimum Dot
- The right min dot is the smallest correctly formed dot that can stand up to the pressure of printing.
- Standard workflow digital dots sharpen (shrink) due to effects of O2 during plate exposure and require a compensation (bump) curve. The bump curve enlarges the mask opening to ensure that minimum dots form at full print height.
- Flat top digital dots are 1:1 so make sure you can hold & print small enough min dot at chosen screen ruling
Flat top digital dot – Creating Curves
- Regardless of dot profile, curves are needed to compensate for optical and mechanical press gain.
- The effect of oxygen on digital dots provides a natural cutback for dot gain.
- The 1:1 reproduction of flat top digital dots requires a greater compensation curve than standard digital to keep from printing “too dark”.
- Curves are generated by printing tone scales and comparing the predicted result with the actual result.
About the Author:
Mohammed Abdul Haleem
Email: packagingprinting@consultant.com
With almost 30 years of experience in the Printing industry, he offers proactive recommendations, Flexographic Printing machine, Central Impression,Stack, or Narrow web, anilox roll specifications,flexographic printing sleeves,testing, inventory management, flexographic, Lamination Solventless, or solvent based,training, representation from experience in technical marketing,production,technical service,parts and supplies.
His experience covers the spectrum of flexographic markets including: pre-press,wide web flexible packaging , narrow web, tag & labels,foil, folding carton, corrugated packaging applications.