Characterization Curves

[tambo]

New in the long perfector world - is it common to have different characterization plate curves for each side of the sheet when perfecting? I am getting slight variations in color from side to side, understandably because each side is printing on complete different units on a perfecting press.

 

[gordo]

New in the long perfector world - is it common to have different characterization plate curves for each side of the sheet when perfecting? I am getting slight variations in color from side to side, understandably because each side is printing on complete different units on a perfecting press.

If you mean different tone reproduction/plate curves then I would speculate that's it's not common if it's done at all. I've visited hundreds of printshops and have never heard of it being done. Heck, it's not even that common for printers to use different curves to deal the difference between gloss coated and uncoated book stock let alone the difference between one side of the sheet and the other. Paper is two sided (wire and felt) and depending on the paper mill/grade of paper there will often be a difference between and how they react on press - but the difference is typically "in the noise" i.e. not enough to be an issue for the job.

On a somewhat dotted-line related issue, I have seen printers that have experienced "wire-side moiré" where the AM screen they're using harmonically beats with the extremely subtle pattern on the wire side of the press sheet causing a faint moiré to appear in single color flat tints. (does not happen with FM screening.)

 

[aqazi81]

How old is your press? If it's old than you might need to check your Impression cylinder jackets that might be picking the ink from the printed side resulting in difference between both sides.

 

[Mike Herndon]

New in the long perfector world - is it common to have different characterization plate curves for each side of the sheet when perfecting? I am getting slight variations in color from side to side, understandably because each side is printing on complete different units on a perfecting press.

Yes, depends on the job, paper, and image area, coverage. The first 4 units print on chrome impression cylinders, last 4 print on jacketed impression cylinders; there is a difference in how the dot lays down on the sheets. On our 8 color perfector we will use 2 sets of curves for presswork with crossovers and what not to make sure everything matches.

We run two test forms, one for the first 4 units and another for the 4 units after the perfector, from there we adjust accordingly. Our press stays in perfecting 4/4 90% of the time, it's worth the effort to check first 4 units to the last 4 units.

Aqazi is correct, inspect your impression cylinder jackets; if they have lost their ability to repel ink they will need to be changed. Our shop pushes the jacket life a bit and averages 4 jackets every 15-16 months/ 50-55 million impressions. Also, using a high VOC wash or an mrc type cleaning solvent will kill the jackets pretty fast. Good luck

Mike

 

[Alois Senefelder]

Comment from Alois.

From my many years on Web Offset Presses, we never used different - Tone Reproduction Curves

for each side of the web, just the correct Halftone screen (Round Dot) rulings for different substrate grades.

These presses are Blanket to Blanket perfectors, running at 30/40, 000 IPH.

Paper.

The universal introduction of - Twin Forming paper making machines as eliminated the Two Sidedness of

paper used for the body stock of coated paper.


Regards, Alois