Web Growth or Web Creep Training?

[Thexx]

I work for a company that has a off-set web press and a CPT system with Kodak Prinergy for the workflow.
We are doing a recalibration on the press because of blanket changes and just the whole thing being out of sorts.
Long story short. One guy who got trained and somewhat understood web growth trained another who didn't get it who trained me.
Both of them quit and I am left to fix it. I get it for the most part but I am looking for any training manuals of videos.
We have just been using bustle wheels to solve it for now but it would be nice to have it fix on the plates.
Our foremen is making things a bit difficult with it and not letting us do trial and error to really figure out what needs to be done.
Measurements and that are easy, its the finer points and the little details that we are trying to figure out.
Any help would be greatly appreciated. Also, if any further detail is required I will do my best to provide it.
Thanks.

 

[gordo]

Bustle wheels can help, but they may cause slur and/or web breaks since they push against the web and stretch the paper.
Your imposition software may have web growth compensation included in the package.
Assuming everything presswise is in order, the usual process is to include register marks in the center and each side of the web.
Run the press at normal speed and register the center marks. Use a loupe that has a measurement reticule in it to measure the amount of the web growth. Usually the last color down is the key image that the other colors are incrementally stretched to fit since the printed image gets larger as it travels from one printing unit to the next. The measurements that you made are put into your web growth compensation software. The compensation distortion of each channel of the CMYK flat is then usually applied to the PDF before screening, but can also be applied to the 1 bit screened flats (workflow/software dependent).

hope this helps, gordo

 

[Thexx]

We do run CMYK, in that order and cyan is the furthest out.
Followed by magenta then yellow and then black is the tightest in. We register it all to the black.
What we have to compensate for it is an xml file that is coded for every 4 inches of the web. 16 inches in total.
I guess the basic questions we are having are:

1. If we make a move does it shift the whole plate or is there a break at the 4" marks and at the center? Or just the half of the plate, from zero, that we are moving.

2. If going from the 10 side to the 13 side do we need to reverse the order?

3. Is the shift a gradual build up or a solid 4" shift?

It has been hard trying to do testing with what it really does. So I was mainly just looking for a manual.
We are running Prinergy 2.5 if I remember correctly.
Before we had the plate room being ran by a separate team that handled all of this type of thing.
Now that they are all gone we are trying to pick up the pieces.
We know that the software can handle what we need it to do, we just don't fully know how to make it do it.

 

[Deleted member 16349]

Usually the last color down is the key image that the other colors are incrementally stretched to fit since the printed image gets larger as it travels from one printing unit to the next.


hope this helps, gordo

I am not exactly sure what you mean since it could be a understood in different ways.

My view is that in an offset press, the later image width gets narrower relative to the earlier units, as one goes through the press to some degree. The paper gets wider mostly due to water absorption and a wider image (prepress) of the later units is required to maintain the fit.

On a gravure or flexo press, the image width printed by the later units would tend to get wider relative to the earlier units, as it goes through the press. This is for gravure and flexo presses that have inter station drying. Drying tends to shrink the paper width. In this case the image needs to be made narrower for later units, in prepress to maintain a fit.

For an offset press that has very small blanket gaps (no paper slip at the gap), theoretically the fit in the machine direction should not change. The image length is some percentage of the print repeat and the print repeat on average will always be the same for each print unit and is independent of tension or permanent stretch between units. Misregister due to variations in infeed tension is a different issue and one has to be careful it does not mislead the understanding of what is happening.

I tend to think of the first unit as the master unit and the other units follow and are relative to the first. I guess for prepress is does not matter which is picked as the key unit, since it is the absolute image width and the relative fit that is important.

Print repeat length can be adjusted by changes in infeed web tension settings. A higher infeed tension setting will result in a shorter print repeat length. To minimize variations in register requires a very steady infeed tension control system.

My views are based on some practical and theoretical research I have done on the subject and the paper I presented at a TAGA conference.

 

[gordo]

@Thexx,

Not sure what you mean. You don't just shift the plate - although some press operators do so to average out the misregistration. You apply an asymmetrical distortion to the individual channels. You probably don't need to do dedicated testing - you just need to do the measurements of live production runs.

best, gordo

 

[gordo]

@Erik Nikkanen. Yup, agreed.

The reason that you use the last down ink as the key is because it hasn't been distorted by web growth as the previous colors. So, in a CMYK ink sequence K would be the undistorted image and C the most distorted. So you would shrink the width of the C image so that as it expands due to web growth it would end up the same size as the K image. M would be scaled less and Y least of all.

best, gordo

 

[Thexx]

You apply an asymmetrical distortion to the individual channels.

It's the fine art of applying the asymmetrical distortion that we need help with. The ins and outs of it.
We will figure it out. You have pointed me in the right direction.
Come Monday I will talk to the boss about this and see what he says.
Thanks again.

 

[Alois Senefelder]

Misregister on Web Offset Presses

Misregister on Web Offset Presses

Hello Thxx and fellow Lithographers,


I urge you to read ALL of the following thread/PDFs

http://printplanet.com/forums/computer-plate/16499-registration-doopid-pressmen

having read the the information, you will notice that I'm at variance with Erik Nikkanen views

I also caution you against relying on a Software programme to compensate for Mis-register

on Web Offset Presses! Remember you are printing on a Unstable Material


Regards, Alois

 

[Deleted member 16349]

@Erik Nikkanen. Yup, agreed.

The reason that you use the last down ink as the key is because it hasn't been distorted by web growth as the previous colors. So, in a CMYK ink sequence K would be the undistorted image and C the most distorted. So you would shrink the width of the C image so that as it expands due to web growth it would end up the same size as the K image. M would be scaled less and Y least of all.

best, gordo

Seems reasonable.

My main experience with web dimension issues is with a process running 5 colours KSpotCMY. With this particular operation, when it was first started here in Ontario, the process had to print on pre printed and laminated material. (later changed back to the normal method of printing first on board)

The pre printed material had an existing print repeat length that had to be matched by the press. The press had a system (insetting) where a mark on this material provided the repeat length information. The mark was scanned and the infeed tension was adjusted on the run in order to stretch the web so the print repeat length of the web would match the print repeat of the press for the first unit. The rest of the units just followed in the normal way.

This was coated board and the tension ranges were from 200 DAN (Deca Newtons force) to about 450 DAN. A DAN is a metric measure of force but one can think of it in terms of Kilograms of mass. One DAN is almost the same as a Kg in value.

This was for a web that was about 1 m wide. It was also precreased and laminated in many layers with Polyethylene and alu foil.

At the press we tended to think of the first black unit as the master but also since this was for a packaging system, the final dimensions of the product were important and that determined what the initial dimensions should be as a starting point in the process. So in a way we also worked from back to front.

So during that time, I did a lot of testing and developing theory in order to make predictable decisions. Such as the affect on the repeat length for different squeeze values and different tension values on either side of the blanket impression nip, different blankets and different elasticity values of the web, different infeed tension values, etc. This was back in the mid 1980's.

 

[Drake]

I see a lot of replies on this thread pointing to prepress and I am forced to defend myself. While web growth may pose a problem, it is highly dependant on the presses which you output to. Without calibration across the board and a matching profile between press and pre-press, press operators will always struggle and pre-press will always come up short.

 

[Thexx]

We got it all figured out. The boss was not in so we were able to do some serious testing.
Started out with make some very over exaggerated moves in the web growth profile on just the cyan with some test grid plates.
Was able to see what was really going on with it after that. A lot of our assumptions were wrong to begin with.
By the end we had 2 towers so close that if tension and water curve are ran just right you could almost fire it up and sell.
Little more fine tuning and we will be golden.
Thanks for all the replies. I am gonna poke around a bit and see what else I can learn from you guys.