Bad plates or a print mechanical problem brain teazer??

[Pressroom Specialist]

I was called to a customer to look into print quality issues on a 5/C 25" press running CTP plates. I was shown a 4/C job that had a brown solid created by 4/C tints. The sheet had streaks and "hot" areas in the magenta and overall looked like crap. Sequence was KCMY. I was shown a 2/C pull of the black and magenta plates and everything looked great. The cyan unit was off for that pull. The cyan plate was moved to the magenta unit and everything looked good. when the K,C, M and Y plates were on it looked like crap. Singles on all 4 colors looked good. I noticed there were "pink blankets on units 1,2 and 3.(K was in 2 dusting on 1).Blue blankets on magenta and yellow blankets. I had the pressman remove the magenta blanket so I could confirm the packing. They were about .002 over factory specs. and running compressible blankets. I checked bearer pressure on the magenta unit with the blanket off. no issues. Checked roller pressure to the plates and found no "major" issues. In broken English the pressman told me they did not have this problem when they ran film(a while back). Only recently with the CTP. WHAT WAS THE PROBLEM??

 

[Pressmanscooter]

My guess, backtrap mottle. The 4 ink films print fine seperatly or a few together. But all together, there is not enough decrease in tack from unit to unit for the ink film being printed to stick to the previous ink films. Did they change fountain solution? If there is a ink/chemistry issue that would cause this. Any increase in the amount of fountain solution over previous plates would make this problem worse.

 

[chad.trent]

My guess, backtrap mottle. The 4 ink films print fine seperatly or a few together. But all together, there is not enough decrease in tack from unit to unit for the ink film being printed to stick to the previous ink films. Did they change fountain solution? If there is a ink/chemistry issue that would cause this. Any increase in the amount of fountain solution over previous plates would make this problem worse.

I'm going to agree. I say that because I had a job do virtually the same thing to me. We ended up switching it to a spot color and ran it as 5 colors to fix the problem, which may or may not be an option for you.

 

[Alois Senefelder]

Start with Plate Imaging.


1) Screen ruling - LPI ? 2) % of tones KCMY that make the Brown Solid ? 3) Screen Angles ?

4) Round Dots or ? 5) Are the plate imaging control strips OK ? 6) Are the plates, Chemistry Free or O.P.D ?

On Press.

1) Check ph and Conductivity of F.S. ? 2) Are they running - TOO much Damp !

3) My guess, Secondary Plate Scumming


Regards, Alois

 

[Pressroom Specialist]

Alois is correct. The black and magenta screen angles were both 45 degrees. Someone had changed the parameters in prepress and for a few weeks had been making plates this way.