Print contrast

[Colornumber9]

Print contrast is a hot topic with my boss. He describes it as a measure of the difference between a solid and 75% screen. I've also been told it's the best way to tell if my press is balanced and that good print contrast will add POP! to an image that would otherwise look flat. From what I've experienced with this ink set and chemistry, maintaining good dot gain and print contrast is dicey. I routinely see big swings from makeready to end of job. 50s and 75s gain significantly from throughout run, but this seems to be inherent in UV ink.

Any of the printing wizards on this forum care to school a young operator? What can I expect in terms of print contrast stability with a full UV ink set?

Thanks.

 

[Lukas Engqvist]

The trick is that your contrast is affected by your dotgain. So you will need to plot where your contrast is high and stable, (by plotting I mean doing a contrast/density curve used to find target density) most probably this will be at your target density acording to the regional standard wich should coincide where you get the correct Lab values for that ink. (As long as all these coincide then the job is easy, the tricky part is trying to find the compromise when things don't match up).
If this is not the case it would be an idea to doubble check all else that affects dot gain. Blankets, fountain solution, etc.
Once that is done, do a TVI curve to get meet the regional ISO standard, and you should find your contrast will be correct and stable. The contrast must be at the "stable plateau" where the contrast is relatively constant at the target value. That your target density is where the process is stable is primaty importance. The contrast value will be acheived when you have the correct TVI curve.

Having the wrong shape on the TVI curve can give you tepmorarily a satisfactory contrast value, but an unstable process. If your TVI curve is wrong and you are fiddling with other parameters to get the right contrast you will have an unstable contrast and problems with gray balance.

What I am trying to say is that the TVI and Density are some of the factors that affect your contrast. So if you use contrast to verify target density and your TVI is not correct you may end up going the wrong direction.

 

[rich apollo]

If you're seeing a lot of fluctuation in the dot gain and print contrast, then the press is running in an unstable condition. I don't know of any reasons why UV printing should be any more unstable than any other process. In fact, I would think that it could be the opposite since you are exerting additional control over drying/curing.

With CTP you can play with the plates to get whatever print contrast you want.

 

[Colornumber9]

Full UV inks gain more and have a very tight water window compared to conventional and hybrid UV inks. It could very well be my inexperience but from what I hear from other UV printers as well as our ink techs, this is an inherent characteristic of UV printing. The curing process has nothing to do with ink/water balance.

I'm still trying to get a tangible overview of print contrast. What is it measuring, what can I do with that measurement, what are the limitations, what information ISN'T important? I know what the books tell me and what my bosses tell me; what are other press operators doing about print contrast, particularly in a full UV environment?

 

[Bret Hesler]

You are right about the water. We run conventional and hybrid UV as needed, and even the hybrid UV is more sensitive to water balance. I read a post on Ray Prince's "ask an expert" blog on UV, and his recommendation was to use an air bar to help drive the water out of the ink train.

As for print contrast, it is simply a measure of shadow detail. It is only one point on a curve and it is of little use in analyzing your overall tone reproduction. It is actually opposite of how a color person views contrast. A quick and dirty contrast move in Photoshop is an S curve, which adds to the 75% and takes away from the 25%.

You definitely need to address the stability before you can address the tone curve, and that is more than just the 75%. You should really be working with prepress to match the tonality of proof to the press (or press to proof) or better yet, a specification like Gracol.

Bret

 

[Alois Senefelder]

re - Print Contrast

re - Print Contrast

Hello fellow Lithographers and "Smudgers of Ink on Paper"
Hopefully the the PDFs will be of interest !

But first -- What practical information can the measurement of Print Contrast obtained by density measurements provide?

#1 Determination of optimum ink film thickness (measured as Ink Density) # 2 Examination of the effect of hard or soft packing # 3 Determination of best Impression setting
# 4 Checking Blanket Quality # 5 Determination of Tonal Value reproduction characteristics of Printing Inks # 6 Determination of the reduction in Ink Density Value due ink dryback


Also a good indicator of "INK/WATER BALANCE" during the Press Run ! examine -- the 40% 80% dots on the Print Control Strip, if PrePress decide "Image one onto the Plate"

Regards, Alois

 

[Alois Senefelder]

Oh - yes

Oh - yes

Pre Press, Should provide the Press Room "WITH" production compliant plates !!!!! and some more "Information"

Regards, Alois

 

[Colornumber9]

Excellent information, Bret and Alois! I really appreciate it.

Now, the pdf you provided had standard print contrasts of K 40% C 36% M 36% Y 30% as acceptable or even desired numbers. Our shop standard is K 45-50% C 42%+ M 41%+ Y 32%+ and we get scrutinized when we can't achieve this on press. Are those numbers too high an expectation? It seems very difficult to achieve and, especially, maintain this level of print contrast. The numbers your pdf provided seem much more manageable.

Thanks again for your help.

 

[Dr Who]

As already pointed out relative print contrast is a measure of your dot density compared to your solid ink density. So you need to know your TVI for each unit and the shop or ISO SID values you are running. Once you have got these you can find out your contrast values thus:

Get your dot density via DD= -log [1- (1- 10 ^ –SID ) x DS/100] where DS equals the screen patch 75% or 80% plus dot gain at that point.

Then find the contrast value via

Contrast = SID – DD / SID

Example

SID = 1.55
TVI = 9 %

So if measuring an 80% patch DS = 89%

Dot density = -log [1- (1- 10 ^ –1.55 ) x 89/100]

= 0.87

Then contrast is found

Contrast = 1.55- 0.87 / 1.55

Contrast = 0.44 or 44

If you do these calculations using your shop values then you will be able to see if the print contrast values are achieveable, your boss should be able to provide them for you because he would have had to work them out to set the contrast limits as they are a function of the SID and Dotgain.

they were a big thing a number of years ago but are not used that much these days.

 

[Dr Who]

Oh and what can you use it for well a few things like monitoring your press [but frankly there are better indicators] mainly to ensure you are printing nice and open and not losing any detail, so you can use it to decide how often to wash your blankets

 

[vaithi47]

the optimum density to get the maximum contrast is the value at which you get the L a*b*value of that color correct.does anyone agree?

 

[GJOHN]

The print contrast number is an indicator of dot gain. Are you seeing a gain in 50% screen as well? Ink, fountain solution and blanket play a big role in dot gain. The UV ink chemistry is notorious for tight ink /water balance. Some UV inks are too heavy and weak in pigment strength and you have to push a lot of ink to get your solid ink density. Perhaps that is why you are plugging the 75% screen. Hope your prepress is all calibrated and on your plate a 50% dot is a true 50% dot and 75% dot is a true 75% dot. Do you have a curve on your CTP or are you running linear? One would think that these days it will be easier to manipulate your dots in prepress with CTP. What press are you running the UV on? What ink and fountain solution, plate and blanket are you using? If you are using the same CTP for your conventional press and youare only having issues with the UV press, I would say look at the ink/fount/and blanket onyour UV press.
George John
direst 269-216-8088.