Bump not in midtones

janezl

Well-known member
Normally, you would expect highest dot gain in offset printing, to be in middle tones.
If printing to SID with linear plate and result shows bump in quarter tones - what could cause this effect on printing press?

Thanks, Janez
 
No expert here but the bump in the midtones is after you put a curve on the linear plate. Your gain on a linear plate will be anywhere. Once you compensate for the linear plates then the qualification plates should have the gain in the middle.
 
Normally, you would expect highest dot gain in offset printing, to be in middle tones.
If printing to SID with linear plate and result shows bump in quarter tones - what could cause this effect on printing press?

Thanks, Janez

1) Are you sure the plates are linear? Have you measured them?
2) If you are printing with a 175 lpi Am/Xm halftone screen then your max dot gain will occur between 40-50% tone on the plate.
3) make sure there is no doubling or slur.

best gordon p
 
Over ink emulsification increases mechanical dot gain which is more visible in the highlight areas. Try cleaning the unit and doing a re-run, it should improve.

Tarun Chopra
 
hi there,

you name it and it can affect dot gain, as gordo says expect to see max tvi in mid tones if not change blanket as i have had issues with midtones sharpening due to increased hardness.

Paul
 
Where is the 'bump'? Is it in a line laterally across the plate? Does it tend to be in the same position? Most importantly what yype of press / model?
Steve
 
Hi, I am back
Some data: Roland 705, built 2006, complete printing unit (rubber) rollers changed 4 weeks ago, readjusted 1 week ago. Printing blanket 1 week old, using torque wrench.
Fountain solution exchanged, Ph 5.2 - 5.4, 3.2% buffer aditive
Recycled GC2 carton MM Kromopak
Sun Chemical Diamond series ink
Exposed linear CTP plate.
Not very happy with results:
PU 1 (black) - high dot gain
PU 2 (cyan) and PU 3 (magenta) - very high dot gain
PU 4 yellow - normal dot gain

Find enclosed GMG Rapid check (nice tool!) result.

Makes me wonder...

Janez
 

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OK, no one else has jumped in so I guess I'll take a crack at it. %-))

You haven't said how you measure your plates to confirm that they're actually linear. Nor have you said what lpi or screening that your using - both of which may help understand these numbers.

I think you have more problems than just the highlight bump.

Your M and C solid densities are too low - yet their final tone values are much too high.
Your K solid density is also way too low - yet it's final tone value is close to target.
Your Y seems fine.

I'll assume 175 lpi AM
If I were you I would start by exposing 4 identical plates (or expose one plate only) using the black screen only with a test form made up of tone values from 100% to 0% in 10% increments and 1% increments from 1%-10%. Confirm that the plate(s) are in fact linear and that all the dots are there - i.e. 1% and 2% and 99% and 98%.
The test form should look something like this:
Testform-1.jpg

You should also include a print contrast target to measure.
You print this same plate on each individual unit (or mount 4 identical plates on press). Print the colors individually - do NOT over print the inks. Run the inks up to the appropriate SIDs - which I believe are:
Yellow 1.15
Magenta 1.50
Cyan 1.40
Black 1.70
Get the densities even across the width of the press sheet and around the cylinder.
Then increase the SIDs by about 5% or whatever your dry-back allowance is. You are using a polarized instrument which adds another variable - but such is life.
Run at least 500 sheets or until your press operator feels the press is stable.

Then pull press sheets for evaluation. Each press sheet will only have one color on it.

The idea is that:
1) the plates are identical - that eliminates any plate imaging and/or screening issues/variables
2) there is no overprinting - that that eliminates any overprinting/trapping issues/variables

Examine the halftone dots and solids for each color using a microscope of at least 25x. You're looking for sharp-edged dots and solids (including the solids in the dots). You are checking for doubling, slur, ink over emulsification, voids in the solids, etc. The halftone dots and solids for all four sheets/colors should appear very similar.
If the dots are OK then measure your dot gains and plot your curves.
They should all have a similar shape and their dot gains should be close to correct:
Yellow 18
Magenta 20
Cyan 20
Black 22:
If you still have the highlight bump with one color then there's likely a mechanical or chemical issue with that unit or that ink. If your dot gains are still way out of range for all colors then you likely have an ink and or chemistry issue.

hope this helps, best, gordon p
 
Hello Gordon,

Just for understanding:

By print contrast patch you mean a 75% along side 100% patch, to keep track of good print contrast in print? Presuming the fact that the TVI's are not correctly established at this point of time, do you think, checking print contrast would be of any consequence for the first run?

Regards
Tarun Chopra
 
Hello Gordon,

Just for understanding:

By print contrast patch you mean a 75% along side 100% patch, to keep track of good print contrast in print? Presuming the fact that the TVI's are not correctly established at this point of time, do you think, checking print contrast would be of any consequence for the first run?

Regards
Tarun Chopra

Yes. Something is terribly wrong with this shop. Print contrast might provide some information - it was not included in the original press runs. I'm just trying to reduce as many variables as possible and let the press tell the story. TVIs, by themselves, don't mean much. However, with linear plates, and good ink rheology, there is a typical response (in terms of dotgain/TVI). This shop is not getting anywhere near a typical response. I'm just trying to remove as many variables as possible in order to determine why they are getting the response they are getting.

gordon p
 
What screening are you running?

The graph of the K plate looks like it might be tinting/toning.

Have you looked for slurring or doubling?

Your ink trap between M & Y is too low.

Gordon, are you saying that they're using a polarized device because the measurements are Status E? Use of a polarized device would minimize the need to account for dryback, wouldn't it?
 
Gordon, are you saying that they're using a polarized device because the measurements are Status E? Use of a polarized device would minimize the need to account for dryback, wouldn't it?

I'm making assumptions 'cause the OP isn't providing the info. The bottom of the first page of the GMG report says polarizing filter which is quite typical of Euro settings. Yes, the intent of using a polarized device is intended to reduce the need to account for dryback, but it does introduce a variable.

The key info needed though remains:
1) what screening is used?
2) how are the plates measured to verify that they are linear?
3) what do the dots look like on the plate? (25x microscope)
4) what do the dots look like on the press sheet? (25x microscope)
5) why are the SIDs being run so low?

The SIDs (except for yellow) are too low which is the likely cause of the poor trap values. But it could also result in unstable ink performance, or ink breaking down which could result in the high TVI and the tinting/toning that you suggested.

best, gordo
 

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