We are currently revisiting our dot gain compensation for a H102SM. After printing linear plates we are finding lots of gain, none more so than in the highlights. Subsequently when we build a dot gain table in the workflow software anything less than 5% we barely hold. We are facing the prospect of having a bump in the curve to print a highlight dot. Is this common? We are no strangers to bumps for flexo (we're jacks of all trades here) but seems strange for offset.
We are happy that our film (I should point out we are still old school CtF) and plates are producing linear results. There is no evidence of slur or doubling.
Taralot.
If I understand you correctly...basically...
In a CtF workflow the film is output linear (which you measure). When the film is contact exposed to the plate you will incur some dot gain in the midtone areas as well as some highlight and shadow dot loss. In most CtF workflows the plates are not measured.
I would start by testing your plate imaging using screen tint patches placed at at least 5 points on the plate (upper left/right, lower left/right and center). The patches should include 1, 2, 3, 4, 5, 10, 25, 50, 75, 90, 95, 96, 97, 98, 99, 100% tones. Expose with a 150 lpi AM screen, process and mount/strip up the film as you would with a normal job. Examine the processed film to determine whether dot loss/distortion is occurring during film imaging. Then Expose the plate in your vacuum frame.
Examine the processed plate at each of the 5 locations to determine whether dot loss/distortion is occurring during plate imaging. If you are exposing with negative film you should expect a midtone dot gain of about 3% at the 50% tone. You may also see a loss of the 1-2% and/or 98-99% dot. This is normal.
Expose 3 more plates using that film.
Then mount the 4 plates on press.
Ink up the first unit - come up to density and pull a few sheets.
Shut the first unit down and ink up the second unit - come up to density and pull a few sheets.
Do the same with units 3 and 4.
You now have 4 press sheets representing the same plate information but for C, M, Y and K inks.
Check the press sheets to determine it the tone values are preserved or lost or distorted the same on all 4 units. E.g. does the K unit hold the 2% dot that was on the plate but the M unit lose it? Your dot gains (i.e. from what was requested in the file) should be about K-22%, C-20%, M-20%, Y-18%.
By examining each unit this way you can determine where the loss is occurring and determine what press issue is causing the problem.
If all 4 units have the same loss e.g. 2% dots then you might look at your workflow to see if you have an XM type halftone screen available (e.g. Agfa :Sublima, Kodak Maxtone). This is similar to putting in a bump in your highlights but is a more sophisticated method and will give better results on sheetfed. (more info on this type of screening is here:
Quality In Print: Hybrid AM Screening/XM Screening )
hope this helps, gordon p
