4/c process color bars

[gman]

running a 4/c gto 52 can't seem to run my numbers where they should be. this has happened ever since we went ctp. numbers are k-90, c-50, m-50, y-70. at these numbers my colors go up and down throughout the run. happens to both coated and offset papers. making me look foolish. keep telling the owner the lower my inks the more it varies.

[Jackie]

There are so many things that could be causing colour shift but I think in order to help you we need more information. Are those numbers referring to solid ink density? Did you change plates when you went to CTP? Was your CTP calibrated to your specific plate and is your prepress department using compensation curves?

[gman]

Jackie, Nothing was calibrated, bought used ctp system. It was ink solids I'm talking about. Right now the computer operater and I are trying different curves so I'll be able to run more ink on the press sheet. Running low ink everything looks flat. We're accomplishing alittle with the curves. Press gain is around 10%, however we're trying to control plate gain which is 15-20%. Gary

[Jackie]

15-20 percent plate gain seems bizarre from a CTP device, something is definitely wrong with how it is set up. I'm guessing you are running your ink densities low to compensate for the huge gain on the plates, that is probably why you are having ink fluctuations and a flat image as it's throwing your ink and water balance on press out, too little ink.

I think you need your CTP machine calibrated properly, if you get your plates back to linear then you can look at applying a standard SWOP or Gracol plate curve and get good results running normal ink densities.

There might be an independent service provider in your area or you could look at contacting the original equipment company to see if they have a tech near where you guys are.

[turbotom1052]

normaly i would say that you need to bring your press sheet up to full ink density and then have pre press adjust their curves and proofing to be able to run at full ink density. But in your case id have to say that theres not much point in that because you are talking about curves that are not even in the ballpark. This to me indicates something very wrong in your prepress dept. My first thought would be to get who ever sold you your ctp to come over and get things more inline. How do the proofs come out? This might lead you in the right direction. If the proofs print as flat looking as the press sheet then you should be able to figure things out from there.

[gman]

turbotom, we have an older ebson proofer that doesn't print with a dot. proofs seem to be o.k. or within reason. we included a scale on the plate but outside the image so not to be in the way of the work. i'm reproducing the same size dot the plate is giving me, however the 35% screen looks like a 50% and the 50 looks like a 60-70%. therefor i print light inks not by my choice. we're working on things making little progress.

[gman]

jackie, yes we did change plates to thermal kodak. i don't believe the machine was set up correct. printed a scale on the side of a job and i'm reproducing the dot i'm given no gain,however my 35% looks like a 50% and the 50 looks 60-70%.

[Lukas Engqvist]

@gman when you are saying "looks like" what are you refering to? 50% in computer should have 67% ink coverage on the surface on a coated paper (total dotgain from DTP software to paper being 17%@50)

There is advice going left right and centre. I would look into getting the G7 methodology and work though it one step at a time. You will need to start by making sure all your equipment is in order and printing consistently. If not then you need to figure out why not. After that look at the Lab values of primaries and secondaries to see that they meet the regional standard. Then move on to imaging a plate with no correction, document the raster on the plate and measure the patches on standard paper when you have your target densities met. From these you can build your TVI curve.

Now if after this you get fluctuations and you may want to do a contrast test by printing at various densities (like a bracket of exposures on a camera) from bellow the target value to above the target value. Plotting the contrast at 80% you should find that your contrast is relatively unchanging at the target density, if it is not but rather changing with slight variation in density, then examine the curve to see if there is another target you could choose that would not give a large variance in contrast with a slight variance in density, alternatively you will need to find another ink. Usually this is in your in suppliers intrest to see that you can achieve satisfactory results with their product.

I would strongly advice getting a solid understanding of the G7/ISO/FOGRA methodology because it will be worth all the effort.

[gman]

Lucas, I appreciate your advice, however we as a small shop struggling to survive don't have all the tools we need. Our desitometer does not measure dot gain. Everything is visual. We are in need of tools but cannot afford. The steps we take have to be slow and sure and cost affective for the company. Regards, gman

[Lukas Engqvist]

Did you consider getting an Xrite Munkey? you shoud be able to get all the measuring you need, with the advantage of it being objective. It is an investment.
It is possible to do a calibration visually, but there is a breakpoint where experimenting to get desired results is more costly than measuring and doing it right. Also one sleeps better at night if you can trust your flow and know that what you see on screen or proof is what you are going to get.

Dotgain can be calculated from density. As print contrast. though I don't know the math.