Where could these lines be coming from?

[robbg439]

On certain halftone images, I am seeing very faint light lines (as in slightly less ink in these areas) down the length of the print. The lines correspond to the grippers on the impression cylinder. What would be the cause of this? And more importantly, is there anything I can do to remedy it?

I'm running a hamada 611CD duplicator press, laserplates.

Thanks!

 

[Al Ferrari]

Let's see if we can figure this one out:

First do you have or can you get a NON laser halftone or screened plate, or plates from a different source? If they do not show up on different source plates, then it's the laser plates.

If it's the laser plates, are they noticeable on the plate when they are showing up on the print? Do they show up right away with a new halftone or screened plate, or does it take 100 or more impressions before they show up?

If they are noticeable on the plate AND they show up right away, then it' the plate making process.

If they ere NOT noticeable on the plate when they are showing up on the print, then they are being caused on the press somehow.

Post some answers before we go further.

Al

 

[gordo]

Adding to Al's excellent suggestions....

There is a phenomenon in the pressroom known as 'Grind-lines' which will take on the appearance of what is called 'banding' and will completely have all the visual attributes of 'banding'. Plates can certainly appear band-free before going to press so it's worth checking for banding on the used, inked up plate.

Depending on the compounds used in conjunction with the solvents in the inks, washes and dampening solutions the surface anomalies in the form of line/streaking from manufacturing will begin to show in screen and solids. This streaking can also show up in dead-rollers, these are rollers that may still have the required durometer but the compound under the surface has lost its ability to rebound. In the dampening systems the chrome rollers will take on these lines and transfer the lines to the work (really shows up on older presses).

Blankets, will often have the same wear attributes of rollers. However, paper, packing, torque requirements and back cylinder pressures may accentuate the problem.

best, gordo

 

[Deleted member 16349]

There is a phenomenon in the pressroom known as 'Grind-lines' which will take on the appearance of what is called 'banding' and will completely have all the visual attributes of 'banding'.

Are "Grind-lines" the same thing as "ribbing" or is it different?

Ribbing has the lines that are very close together and tends to be due to the way thicker ink films are splitting just after the nip. The ribbing lines are so narrow and close together that I would not call them bands but that is just me.

 

[Deleted member 16349]

On certain halftone images, I am seeing very faint light lines (as in slightly less ink in these areas) down the length of the print. The lines correspond to the grippers on the impression cylinder. What would be the cause of this? And more importantly, is there anything I can do to remedy it?

I'm running a hamada 611CD duplicator press, laserplates.

Thanks!

This is a long shot but maybe related.

When you said that these faint lines correspond to the grippers, I started to think of a phenomena that some researchers call Mechanical Ghosting. It is not the normal mechanical ghosting that we think of that is due to the form rollers and plate interaction but these researchers use the term mechanical ghosting for a phenomena in a blanket to blanket web press, where the image of the one blanket will affect the image of the other blanket with a ghost.

For interest here is a link: Gordon might like a new reference to this term.

www.fpc.fi/uploads/files/Ghosting_eng_1.pdf


So how might this be related to your problem?

I think the mechanical ghosting on the web press is due to differences in the rate or force of release of the web from the blanket surface due to image coverage and web wrap geometry. Anyhow that is my best guess so far.

With your problem, it may be that the rate and/or force of release of the sheet from the blanket is slightly different in line with the grippers. This them might make the bands, which is somewhat like the ghost in the web press but without an image doing it.

If this is related to the problem, then what can you do about it? Not sure but maybe stiffer paper, or less stiff paper. Lower tack ink. Lower speed. Different blanket.

Is your problem also related to the type of image you are printing? Lots of coverage etc.?

As I said, its a long shot. Good luck.

 

[gordo]

@ Erik

Are "Grind-lines" the same thing as "ribbing" or is it different?

I don't know.

best, gordo

 

[turbotom1052]

If you have narrowed this problem down to the press and ruled out as per Als suggestion any plate problem i would look at the following.... In what direction of the sheet do these lines run? are they gripper to tail or do they run across the sheet? If they are gripper to tail i would be looking for something on the press that may be mechanically disturbing the surface of the sheet. Ive seen the slightest disturbance of certain stocks surface cause the ink to lay down a little different along the length of the that "rub" It may be undetectable when printing type or even solids but will show itself more in screened areas. If that turns out to be the problem then you will need to find a way to eliminate the "rub" by either moving things around a bit or by easing up on the pressure of the offending press part. Im not at all familiar with hamada presses so i can't tell you where to even begin looking but a little trial and error while moving things around will hopefully help you narrow it down.

 

[robbg439]

I occasionally get metal plates made and I have never noticed the banding when I use them. But its also a specific type of image that will be affected by the banding. Specifically, I notice it on large halftone images where coverage is light (say 30-0%). It would be worth it to get a metal plate made with a specifically bothersome image just for testing purposes, thanks for that tip. I will try it and let you know.

The banding is not visible on the plates when they come out of the platemaker nor when they are inked up. It does appear on the initial prints though, not something that develops over time.

Thanks for all the advice!