cmyk values

The press operator can only control densities on the press not L*a*b*.

 

Why?
The press operator must know Lab values measure!
First step always the best Lab values approach...

 

Lets see if I can do this in a non offensive manner.... The mechanical, and sometimes chemical aspects of a good clean printing impression happen at the printing unit. They do NOT occur in estimating. They do NOT occur in pre press. They do NOT occur in the bindery. I realize that sometimes egos and self importance, have people wanting to think that the printing process revolves around their particular job, but lets be honest here. "PRINTING" by most reasonable peoples definition is about images on paper. If we can agree upon that, then it stands to reason that its the printing unit where the "rubber meets the road" Everything that precedes the actual impression, and everything after the actual impression is there to support the actual impression. All that being said it would stand to reason that the starting point for a good impression should be at the press. Adjustments to the process need to begin at the press and work their way backward. This is often the time where pre press guys feel like their pee pee has been stepped on. This is often the time where words like Lab values and such get thrown around. If there is a pre press employee reading this right now, then I would urge you to acknowledge what I'm saying, and recognize that your role, as important as you think it is, in reality just a lead up to the pressroom. Nothing more than a support role the the actual presswork. The average customer will NOT look at a printed image and have the first impression that "wow this is some really good pre press work".
This all brings me to this point.... When I talk about working back from the press it needs to be done with the understanding that the best images will ALWAYS occur when the actual printing impression is done with the expectation that all variables are managed. The paper needs to be of a known quality. The ink needs to be consistent in strength, printability, drying, ect. The press crew needs to be experienced enough to keep a tight control on other variables as they pop up. The INK FILM THICKNESS needs to be kept consistent from job to job, with the only exception being an adjustment for the various substrates.
All of the pre press lingo as often thrown around regarding density, and Lab, and curves FM screening... the list goes on and on, exist as a control, to provide the press with what they need to deliver consistently good images. To think of this process any other way is to put the cart before the horse.
This rant is not intended to make small what pre press guys do, but to put things in proper perspective. It never ceases to amaze me the way that many companies management, have gotten to a point where it would be unthinkable to not hire the best and brightest pre press managers, but will often look to cheap out, by not having a fully competent pressroom manager, or to promote press staff off the broom just to save 5 bucks an hour. This point is further driven home by the fact that by a wide margin, in just about every company I've worked for the Pressroom is the highest cost center in the company!!!

 

Thank you for sharing such a piece of important information. Printing ink density is important in terms of maintaining print quality. Can you provide some more links on how one should check density??

 

Density is measured with a densitometer of course, but your missing the biggest point im trying to make. A densitometer measures, based on a color bars ability to reflect light back into the unit, where through various filters the machine converts this reflection and assigns a number. This number is USUALLY, but not always a 3 digit number. For example 1.85 or 1.40. In the case of yellow, and depending on its calibration standard, the number just might be a 2 digit number like .95. This assigned number does NOT measure the actual ink film thickness of the printing ink on the rollers. Different ink companies will vary the strength, or amount of pigment in their inks. They may even vary the pigment load from one series of their inks to another. This variation in pigment load will have the effect of making the printing ink read stronger or weaker at a given ink film thickness.
When I spoke of working back from the press, when setting up pre press curves, I was pointing out that in order for any given ink to perform well on press, there MUST be a proper, consistent, and repeatable ink film thickness on every printing unit of the press. This ink film thickness is measurable, just as a densitometer will measure densities. The thickness of the ink films must be adequate to provide hard drying, be proper thickness to provide a stable emulsion, of the proper thickness to manage on press dot gain, thick enough to keep roller train and paper contaminants in check ect, the list is long.
It is very possible to have densitometer readings that hit the often asked about, and suggested "CORRECT" readings, and at the same time have an ink film thickness on your rollers thats a far enough deviation to cause a multitude of various problems. This is the reason that its strongly suggested to start the fingerprinting process with an ink that can suit your needs, whatever they might be, running at a manageable ink film thickness. Once you are locked into this thickness for all units, then, and ONLY THEN, should the the densitometer read the color bars for that particular ink set. These reading should be your standard densities. Pre press needs to adjust the dot structure to work within these readings, which are derived from a press with the proper ink film thickness on ALL units, with variation for different substrates.
Ive found from close to 4 decades of running multicolor presses and managing pressrooms, that this concept is one that is often outside of the understanding of many upper management personal, to include even some of the brightest Pre Press managers. This concept is also beyond the grasp of many supposed Press operators. This is often the reason why when a job goes bad on press, the conclusion drawn when its time for a reprimand, is aimed at the press operator, when in effect "the fish often stinks from the head." A companies upper management that places little to no credence on all this that im outlining, is doing a disservice to every person in the chain of delivery, to include your customers.
If what im describing is YOUR company, or if your a front line manager in either Pre Press or the Pressroom, to ignore what im saying is to be part of an operation thats just crossing their fingers and saying a Hail Mary that Murphys law doesn't rear its ugly head!!!

 

Hey turbotom1052,
I have seen in some of your postings that you talk about ink film thickness being important to the print process. I am one of the pressroom managers that you speak of above that treats density like gold. I would like to get a better understanding of ink film thickness and was wondering if you could point me in the right direction. I want to make sure that I have all the tools necessary to be effective in my job. My go to, due to the limited training I received when I first started running press, is to drop my waters first thing in the morning til the sheet starts to scum a little and then achieve density on my color bar and raise waters to remove any scumming that is left. I am sure this is not the best way, but it is the way I was trained. When talking about ink film thickness, should I be trying to hit a specific mil of thickness or is it more important to just be even across the rollers. The reason I am looking into this more is because I am having an issue with my yellow ink quickly becoming dirty on jobs that have low ink coverage for that color. I was looking into lowering the tack of the yellow ink and then I saw your post here and it has me thinking that I need to learn more about this. Thank you in advance for any help you can provide on this.

- Kris

 

Actually Kris, the practice of dropping your waters down a bunch when you start up in the morning, and then looking for an even scum line on the plates gripper is IMO the "best way" When it comes to ink film thickness here are a few things ive learned over time... Even the most successful and widely sought out ink brands at times have quality control issues that arise. They are no different than any other manufacturing business. One of the issues that arises, as you might well imagine, is inaccuracies when measuring. What do you suppose would happen if an ink mixer were to measure wrong and add 20% more than the required amount of magenta pigment to its ink? You would have an ink that would still be able to arrive at your desired density measurements, but would do so with a 20% thinner ink film. An ink with a 20% thinner ink film would perform differently on press at the very same ink density. Now suppose that your running a long run and everything is going fine running your go to ink set. After 20,000 impressions you wind up opening a can of magenta ink, from the same series, from the very same manufacturer. The only difference is that this can of magenta ink is from a different lot number where the ink mixer didn't measure correctly and added that 20% more pigment. Now your still feeding the same amount of ink from the ink fountain, into the inker, but the just added ink is 20% stronger. Now the pressman sees a jump in the magenta ink density, and either manually reacts by dropping the magenta ink sweep, or even allows the presses closed loop inking to do its thing. After running a couple hundred sheets and getting back to proper density things seem to be "under control." The press continues on its run for awhile at proper density until things start to go wrong. The thinner ink film your now running to arrive at the desired density, will begin to catch up, with the potential for numerous problems. The desired density will become harder to keep consistent as fountain solution begins to wreak instabilities in your inks emulsion. Hickeys will start to show up because you now have a thinner ink film on the rollers, to keep them in check. As a result of the thinner ink film you might begin to encounter trapping and dot gain changes, because we all know that as ink film gets thinner, ink tack increases. As ink tack increases dot gain decreases. That very same 1.40 magenta densitometer reading that your company worked so hard at achieving as your coated stock "standard", begins to not work so well, as a result of everything I mentioned above.
There is no current technology that I know of that in addition to either density control or spectrometer control that accounts for variations of ink film thickness as the press runs. This is why when I was running presses I never missed on an opportunity to give a visual inspection to the number 4 ink form roller every time I opened a units safety gate . Between the visual inspection for ink film thickness, and inspection of the plates scum line every single time you stop the press and go into a unit, a much higher level of awareness is created. This visual inspection does not come with an assigned number, but in time a seasoned pressman begins to know what the proper ink film looks like on the rollers. This practice becomes second nature and should only require a quick glance. What I described regarding the magenta ink applies to all inks, as well as PMS and ink unit applied varnishes. It applies to thicker ink films as well as thinner ink films with only the problems from thicker ink films being different. My suggestion would be that if your press crews have yet to master this visual inspection, that they should be taught to. You can purchase an ink film thickness gauge to do just this. Its also known as a Peto gauge. Im hesitant to list the best numbers to shoot for just as Im hesitant to list the "best" ink densities. The addition of a strict adherence to ink film thickness as well as ink density, will give the best results.

 

This is very enlightening and very helpful. Thank you very much for taking the time provide such a detailed response. I have to imagine that adding cold ink to a warm fountain can also affect the ink in this way as well. I will definitely look into this and start training my guys to look for this consistency as well. Thanks again for the help and the info!

 

You're very welcome. Please pass the word as adherence to these principals will make many peoples jobs easier.

 

for wet ink the density should be 0.05 more?

 

Agree georgbob , Current international standard have moving forward to Fogra 51 & 52 or G7 .
All primary colour are base on L.a.b target value.
For density guideline is uncertain value , much depending on your process colour pigment from ink supplies.
Our internal un-coated paper density reference value as K-1.10 , C- 0.95 , M- 0.95 , Y- 0.90 base on L.a.b Fogra 52 guideline. Ink type Heidelberg Saphira process ink.

 

Dear JeffLim Ah Chua, exactly, this is the basis of the standard.

 

Thank you

 

Density should stay at 0.05+/- tolerance control.

 

Ive found that on many jobs plus or minus .10 is tolerable if your pre press curves are spot on and everything else is spot on with your press. When I say plus or minus .10 ,I'm assuming that the other 3 colors are held to a closer tolerance. If your printing a heavy green image, and your minus .10 on the yellow, and plus .10 on the cyan things will look pretty bad. Of course "tolerable" is a relative term. What's most important, as far as Im concerned is that "tolerable" is defined by the printing company, and that the standard does not waver from client to client. Its when a company allows it clients to dictate the standard that things start becoming a problem!!!

 

Here are the density values and preferred dot gain values to print to grey balance.

 

W0w turbotom, this is very interesting. Can you give a rough guide for the ink film thickness so I can know what kind of gauge to buy until I can master the visual inspection. I am attaching a photo of the type of gauge I think you mean. If it is right I will just need to know which measurement range to buy it for.

 

Devon as I mentioned in my previous post, to assign an actual number will not account for many of the variables we encounter. I would suggest you ask your ink supplier for the best thickness number, being sure to give them the required information, particularly the stock you will be printing on!!!

 

I understand. What would the range of instrument be? Is one that goes up to 25μm ok or does it need to be a wider range. I have actually been printing for 20 years. I taught myself. I would have loved to have apprenticed to someone like you. I just want a rough idea to start with. I can get the press going well and have figured a lot of things out the hard way (my background is mechanics). I even ran a 9 unit harris newspaper press for a few years and did all the mechanics on it. But I am still an amateur. My printing is as a volunteer, putting on paper what I think is important.
I do struggle at startup though to know when I have enough ink on the rollers (or too much). I print infrequently and am trying to pass it on. If I have a window to shoot for I can fine tune and then pass that on.

 

Devon you are forcing me to put on my thinking cap. I seem to remember from many years ago a range of between 1 to 2 microns. A Peto gauge that covers that range should get you in the ballpark. What's most important when using the gauge is that you do not read the thickness off of a rubber roller. You want to read it off of of a rislan or teflon coated, or even off of a stainless or copperized roller. It must be hard ,so as to not offer any deflection. Your ink manufacturer should be able to provide you with the optimum thickness for a particular ink set. I would caution you to be aware that an inks pigment load can alter these thicknesses up to approximately 20% depending upon just how the ink is formulated. Once you are able to arrive at the proper thickness, it is of most importance that you ingrain into your mind just what that measurement translates into as far as a visual clue arrived at by "looking" at the rollers. With experience a pressman should be able to take a quick peak at these visual clues, and know if they are running within the recommended range. Its my experience that many pressman haven't a clue as to what im talking about here. Some have learned the trade in a way that has them living and dying by densitometer readings. Now dont get me wrong... a densitometer is a very useful tool when it comes to getting and keeping color through a press run, but a working knowledge of ink film thickness, ALONG with a well calibrated densitometer will elevate your skills to a superpower level!!! If you desire a more in depth discussion on this topic I would suggest reaching out to me via private message where we could set up a phone discussion free of charge. My mouth runs a lot faster than my fingers, and a conversation about my lifes work allows me to still print vicariously through others while in my retirement.