G7 - Solids Are Good - Overprints Are Not

[Bill W]

I run P2P qualifications on flexo presses and our new HP digital 6600 press. I can get the solid CMYK to meet the specs of ISO 12647-2, but quite often my overprints are out of spec. Not all of them sometimes just 1, sometime 2. CMY is the one that is the most trouble.

What tool does one use to figure out how to get the overprints into spec?

 

[meddington]

As I understand itr, in ISO 12647-2 the RGB overprints are subordinate to the primaries, and thus aren't beholden to any delta E value. That said, in the Litho world, there are tools that allow you to more easily determine the ideal densities needed to hit the secondary overprints (Tucanna PrintControl comes to mind), but often to the detriment of the primaries. not sure this would be relevant to Flexo...certainly not to Digital, in which case you could use ICC based transforms to meet the criteria. If you referring to a 100CMY overprint, I wouldn't lose any sleep over that.

 

[rich apollo]

Solid CMY doesn't have a normative value, does it?

I find that the overprints are more important than the solids. I always have to give a little on the solids to get the overprints to hit.

PrintControl has some very clever targets for determining optimal densities.

 

[Bill W]

A G7 expert from one of our ink companies was the person that got me thinking of trying to get the secondaries to be closer, as I was concentrating on only the primaries.

My question rephrased, is: If one does not have a tool such as Print Control, does one just use their knowledge (SWAG) to adjust the solid densities. For instance if I see the Blue is not quite green enough do I decrease the C density hoping as it goes greener that the Blue will also go greener? Unless of couse I have both a need for more green and blue, in which case I would lower the magenta density.

 

[gordo]

A G7 expert from one of our ink companies was the person that got me thinking of trying to get the secondaries to be closer, as I was concentrating on only the primaries.

My question rephrased, is: If one does not have a tool such as Print Control, does one just use their knowledge (SWAG) to adjust the solid densities. For instance if I see the Blue is not quite green enough do I decrease the C density hoping as it goes greener that the Blue will also go greener? Unless of couse I have both a need for more green and blue, in which case I would lower the magenta density.

IMHO, you need to first find out what the cause is of the poor ink trapping - then correct that rather than fiddling with SIDs.

Some causes of poor ink trapping include:
• Poor ink/water balance (the most likely cause)
• Wrong tack sequence
• Improperly balanced ink strengths
• Improper ink viscosities
• An additive in the ink that prevents succeeding ink layers from adhering.
• Poor ink release from the blanket

best gordo

 

[Bill W]

Thanks Gordo, but I am dealing with flexo and digital presses, not litho presses. As you know a whole different set of challenges in terms of ink trapping.

 

[gordo]

Thanks Gordo, but I am dealing with flexo and digital presses, not litho presses. As you know a whole different set of challenges in terms of ink trapping.

Oooops! Sorry.

G

 

[kewlbigdan]

Thanks Gordo, but I am dealing with flexo and digital presses, not litho presses. As you know a whole different set of challenges in terms of ink trapping.

Long time Flexo person. Why would Gordo's answers not be applicable to Flexo? I am scratching my head here.
Exchange 'Blanket' for cyrel or appropriate photo-polymer material and the other suggestions are all valid as written.

 

[gordo]

Thanks Gordo, but I am dealing with flexo and digital presses, not litho presses. As you know a whole different set of challenges in terms of ink trapping.

Just curious...have you measured the flexo trap values using a densitometer? What values are you getting?

gordo

 

[Bill W]

G7 - Solids Are Good - Overprints Are Not

Gordo,

I have attached a PDF file that contains measurements, (densities, Labch, dE-Lab) of overprints and primaries. A bit of background.

All of our jobs have bearers that include, among other graphic elements, primary solid and tint blocks, and secondary overprint blocks. When I run a profile target, I measure both the primaries in the bearer and the target, because the only QC that can be checked on a production run is the elements in the bearer. If I have to compromise, I will lean toward insuring the target values are closest to the values I am trying to match.

I have in the past used only one press run to predict the curve and the curved profile. In such a case I run only a profile target, not a P2P, due to stock "real estate" and then use Excel, Photoshop, ColorThink, Colorlab and IDEALink to discover the gray balance curve and create the final profile. I have checked this procedure against running a P2P to use to discover the gray balance curve and both procedures produce results that are very close. Additionally in most cases the proof using the "predicted profile" matches a press piece run with the curve.

Sometimes I do two press runs, a CV and a PB, and the data in the attached PDF file contains measurements from both a CV and PB run on the same press. CV denotes the target had correct values, eg 1=1, 10=10, 20- 20, etc. The PB denotes the target has values that were pulled back to match the gray value curve.I measure both bearers to insure even impression across the web is being maintained. In the attached file I only enter measurements from one bearer.

-Bill-

 

[Louis Dery]

Hello Bill,
for troubleshooting:
have you ever compared solid (100%) against 95% tint?
I’ve already seen in flexo, that 95% renders higher density than 100%.
Also, what about the sticky back? I already seen that changing the hardness of sticky back changed the rendered density!

Louis

 

[Bill W]

Greetings Lewis,

Thanks for your interest. You are correct in stating that sticky back can play a much larger than expected role in ink density. However we have optimized sticky back with the specific plate material we are using.

In regards to comparing 95% to 100%, the targets measured were on semi gloss with water based inks which generally does not exhibit what you have experienced. The 95% on these targets measures 1.32 density. My experience with what you have seen applies more to solvent based ink on plastic type materials such as BOPP.

-Bill-

 

[gordo]

Gordo,

I have attached a PDF file that contains measurements, (densities, Labch, dE-Lab) of overprints and primaries.

My math is failing me as I'm not getting numbers that make any sense for trap values - or something's very wrong.

So maybe others on this forum could use your numbers and calculate the resulting ink trap values. The spec for flexo calls for a minimum of 80%. The formula is:

((D1+2)-D1)/D2 x 100 = %trap

Where D1+2 = the overprint density = e.g. Red (M+Y)
D1 = the density of the first down color (e.g. M)
D2 = the density of the second down color (e.g. Y)

I'm assuming a KCMY ink sequence.

The reason ink trap values are important (vs Lab values) is that they may indicate a mechanical ink adherence issue that needs to be corrected for before doing any color characterizations/corrections on press.

best, gordo

 

[Deleted member 16349]

My math is failing me as I'm not getting numbers that make any sense for trap values - or something's very wrong.

So maybe others on this forum could use your numbers and calculate the resulting ink trap values. The spec for flexo calls for a minimum of 80%. The formula is:

((D1+2)-D1)/D2 x 100 = %trap

Where D1+2 = the overprint density = e.g. Red (M+Y)
D1 = the density of the first down color (e.g. M)
D2 = the density of the second down color (e.g. Y)

I'm assuming a KCMY ink sequence.

The reason ink trap values are important (vs Lab values) is that they may indicate a mechanical ink adherence issue that needs to be corrected for before doing any color characterizations/corrections on press.

best, gordo

I suspect that specific density measurement values are needed and not just any density values. I suspect that all measurements need to be from values that use the same filter.

I am sure that there are some instructions somewhere on how to measure the values for that calculation.

 

[gordo]

I suspect that specific density measurement values are needed and not just any density values.

All i've got is what the OP Bill provided.

G

 

[Deleted member 16349]

All i've got is what the OP Bill provided.

G

For an engineer, that would not be an acceptable excuse.

That is the problem with data and formulas. There needs to be a clear description of what the data is and an understanding of how formulas are used. Can't just grab a formula and throw numbers into it and expect a valid result. A formula is a tool and requires some skill in its use. Just like any other tool.

 

[meddington]

Gordo's formula is correct, but Erik's point is critical. The density values to calculate trap should come from the complimentary filter of the second down ink. Bill would have to remeasure to accurately calculate the trap values. There is likely a "trap" option on the spectro-densitometer Bill is using.

 

[Deleted member 16349]

Gordo's formula is correct, but Erik's point is critical. The density values to calculate trap should come from the complimentary filter of the second down ink. Bill would have to remeasure to accurately calculate the trap values. There is likely a "trap" option on the spectro-densitometer Bill is using.

I would add to this that the densities should be relative to the paper since the formula is based on the idea that densities are additive.

The formula also assumes that that the densities are linearly related to ink film thicknesses, which they are not, so there will be some error due to that factor.

 

[meddington]

I would add to this that the densities should be relative to the paper since the formula is based on the idea that densities are additive.

Good point. I believe most spectrodens will require a paper measurement to subtract out substrate density when measuring for trap.

 

[gordo]

For an engineer, that would not be an acceptable excuse.

That is the problem with data and formulas. There needs to be a clear description of what the data is and an understanding of how formulas are used. Can't just grab a formula and throw numbers into it and expect a valid result. A formula is a tool and requires some skill in its use. Just like any other tool.

You're missing the issue. Likely my fault.

Best gordo