Humidity and fit problems

[Brer]

I have been having problems fitting color to color. My black print too wide compared to the CMY. Only on light weight papers and stock running grain short. I have replaced packing and blankets to try to keep this to a minimum.
Press is a Heidelberg PM74
Ink sequence KCMY
We are in the Southwest and humidity in our shop is regularly under 20%. Today it is raining and I have no fit problems. As always there is more than one variable between today and the last time I saw this problem.
Has anyone seen humidity fix this type of problem?
Thanks

 

[DotBox]

The cause of the problem is the short-grain. I don't know for sure if humidity can make the problem better or worse, but it wouldn't surprise me either way. Ordinarily, with "long-grain" stock (i.e., the grain direction is running across the press, side-to-side), the sheet flares out as it is printed on each unit, but then snaps back. Each unit does it the same way and the fit is thus o.k. With short-grain stock, the sheet flares out but doesn't snap back nearly as much...because the grains actually start to break. You will often be able to see the effect by examining targets near the tail of the sheet. It is always worse on lighter-weight sheets. You will usually be able to see the targets separate left-to-right in the exact color sequence run on a left-rear target, and the right-rear target will be separated in the opposite order...often k-c-m-y on the left, and y-m-c-k on the right.

For proof, cut some stock so that the grain is "correct", and run it under the exact same conditions. Problem will either disappear or improve every time, no matter where you are or what the weather is.

 

[Cornishpastythighs]

See if these people can help you if the problem is costing you time,money and quality.

Industrial Humidification, humidity control, humidity control systems, humidification for manufacturing

 

[johnnyone]

Sbsolutly, I have seen humidity cause a great deal of print problems. I work in the southeast and I wrote a paper a while back describing our winter season as "THE MEAN SEASON. I call it that because the heat goes on in the winter and causes the plant to dry out. I have seen the humidity go down to 17%. It is impossible to try to run a long grain paper in that type of RH but, to try to run a short grain paper through a multi color [press and expect to fit unit to unit is crazy. I am surprised you don't get wrinkles along with the fit problems.
When the rain comes it will raise the RH closer to what it should be in printing 50%. TOO much RH causes waves on the edges of the paper. TOO little RH causes the paper to shrink especially on the edges of the stock called tight edge corners.

 

[Brer]

Thanks DotBox. It is not only on grain Short. Sometimes it happens on grain long on 70# gloss.
Cornish, Johnnyone I will look into it thanks

 

[Brer]

Would humidity help other problems, like cracking on the folder?

 

[KPetrochko]

I have seen similar problems that had nothing to do with humidity (although grain is a possibility, you say both grain directions give you problems?) I would seriously look into the condition of the grippers. Gripper tip wear or mis-adjustment, along with gripper timing (overlap) can be something that can ruin a lot of days in the pressroom. Especially with those tiny multicolor folios out in the corners of the sheets!

Gripper tip work is not for the average joe though. Leave that to a good mechanic.

-Kevin

I have been having problems fitting color to color. My black print too wide compared to the CMY. Only on light weight papers and stock running grain short. I have replaced packing and blankets to try to keep this to a minimum.
Press is a Heidelberg PM74
Ink sequence KCMY
We are in the Southwest and humidity in our shop is regularly under 20%. Today it is raining and I have no fit problems. As always there is more than one variable between today and the last time I saw this problem.
Has anyone seen humidity fix this type of problem?
Thanks

 

[DotBox]

Sorry then, Brer. You wrote "Only on light weight papers and stock running grain short." I took that to mean it only happened when the grain is short. My bad, I guess.

I still feel comfortable with saying that fit problems will always improve if the stock is run grain-long as opposed to grain-short, but perhaps you have something else going on too. I work in an environment where the humidity is usually constant so can't help you much in that regard. Good luck!

 

[Brer]

I still feel comfortable with saying that fit problems will always improve if the stock is run grain-long as opposed to grain-short,

I agree 100%. Unfortunately some jobs require we cut a 28*40 sheet to 20*28. We try to avoid this but it is not always possible.
We have moved a swamp cooler into the press room to test out this theory. I don't know if it will be enough to bring the humidity up to a good level. I will report back when I can say if it helped or not.

 

[DuaneBryant]

Try this first:
1)Back off the Black/first-unit back-cylinder/impression-cylinder pressure until print becomes mottled, then increase just to the kiss-point.

If that does not sufficiently improve the situation, and more progress is required, then move along to #2 below...

2)Remove .005" of packing underneath the Black/first-unit blanket, and see if your situation improves (may have to slightly increase back-cylinder/impression cylinder to compensate for less blanket-packing.


You are saying that the first color printed, in this case black, is the one that sees all or most of the paper distortion, because it is the first blanket that "shocks" the paper...therefore try "shocking" the paper before printing any colors, by trying the following:

3)If you have a six color press and are printing only 4 or 5 colors, just dry-hit the first unit to cause all or the stretch in the paper. Put the blanket cylinder on impression when the sheet runs through, even though you are not printing anything..

If all else fails:

4)Lastly, Yellow is the least visible of the process colors. You could print yellow first, knowing it will receive the most distortion, but be less noticeable to the eye than the other a darker color like C, M, or K.

 

[Cornishpastythighs]

Duane, surely if you remove .005" from under the blanket and not put it under the plate you will seriously effect your plate/blanket squeeze.

 

[Brer]

We use a packing gauge to ensure we are packed to a .004 squeeze to the plate (manufacturer recommendations). I may over pack by .0015 but no more. And we are printing just above our breakaway point. I dont think shocking the paper is the issue.
The black prints the widest and the other colors get narrower as you go down the sequence. Sorry I didn't include that info in the original post

 

[Bama12]

Humidity or lack of it are always a factor with paper and printing. I am a southeastern printer and we run long and short grain light weight stocks daily. The RH of the paper is more of a problem during seasonal changes here but there are a few things you can do to help. If you don't have a RH meter I recommend you purchase one.
1. Bring your paper into the pressroom for at least 24 hours before running.
2. Always keep your paper wrapped until you run it and if it is going to be printed on the second side and is going to sit for any amount of time rewrap the paper.
3. Check the RH of the paper before and after you print it. RH between 40 and 60 % work very well for us. This RH range should help you with any other bindery processes the paper may have to go through as well.
4. Make sure your IR dryers are not too hot. Load tempatures between 90-95 degree works best for us. As the ink dries the load tempature will raise slightly but shouldn't be a problem.
Hope this helps.

 

[DotBox]

Some good ideas above, I've done each before and they do work. These days, we generally don't have the excess time necessary to go through all these extra steps, and then get the press back to normal setup for the next normal job. Running yellow first and black last works well and is SOP in many shops, BTW.

One additional thing: if you can get your hands on a mostly blank plate without significant images on it, mount it on the first unit with the water engaged and turn the dry-hit into a wet one. That works even better for pre-shocking the sheet. Squeeze that sheet with unit 1 as much as you're willing to go, too.

"The black prints the widest and the other colors get narrower as you go down the sequence. Sorry I didn't include that info in the original post". That sounds like a truly CLASSIC case of running the grain wrong/short. It is the primary reason why press ops don't like running short grain paper, in addition to feeding and delivery problems related to sheet curl. All these problems are much less noticeable on heavier sheets. Cover stocks will usually run in either grain direction with no noticeable difference, but that isn't the case with lighter-weight sheets.

 

[RGPW17100]

If you are running a 5 color and you depowder on unit 1 try using the damps on unit one to plate. This will cause the stretch on the dead unit. You can also try packing the black plate with either 2 or 3 thousands packing. That will help the black unit print shorter

 

[Bama12]

I understand completely about not having any extra time but if you can take good care of your paper you should get quicker make readies, faster runs speeds, less feeder trip offs and less reprints which all should help you gain some of the time back. As a former pressman my experience has been that if you are having fit problems with your printing it takes a lot longer to makeready a job to a point where you can tolerate it and hope it sells but you never feel good about anything you are putting off of the press.

 

[Alienjones]

Fifty years ago - yes. I'm still in business! I ran a purpose built plotter with pens that needed to be filled with Indian ink and cleaned out afterwards) to record pulses from probes that were fed deep into the ground by putting them down artesian bores. The humidity was always in the 5% to 15% range. Desert country was 2% or less. The plotters used rolls of 60 gsm, short grain paper. When it rained (95%) inside the logging truck, the plotters would stop working because the paper expanded in width by as much as 1/4" overnight. This meant that the first few feet of paper would be so wide, adjusting for accuracy was near impossible. We cured the problem by fitting a small auto air conditioner to the generator this lab ran off. Problem solved. It took 18 hours or so to normalize the humidity but we left the power on continuously anyway.

I'm not an (inkjet) and digital printer. I don't get that problem the way you 'real' printers do but if I did, I would seriously consider air condition in dehumidify mode the print room and store my paper in that same room for a at least a week before using it. Of course it wouldn't be any use doing this in large iron clad building with 4 or 5 presses running but for a small operation, very practical. I Hope my experience didn't bore you all but if you find help in my tale, I've done my good deed for the day. I still use a platen press for wedding invitations so I suppose I rank qualified enough to offer help now and then.

 

[BeauchampT]

If the problem is predictable enough, and depending on your prepress setup, a lot of CTP programs allow for working in a web growth profile. This is pretty typical on a web press where there is consistent change unit to unit, but the principal can apply to sheet fed too.

I have used Kodak Prinergy, and the web growth compensation program for that can allow you to adjust images in every which way imaginable (gripper end too big? make it smaller; black too small? make it bigger).

If you have a pattern, keep some samples and talk with your prepress/platemaker...see what you can come up with together.

 

[DuaneBryant]

Cornish Pasty-Thighs: Our theory is that part of the problem being experienced, is caused by Over-Packing under the blanket. So let's perform a test. Simply removing .005" is a reasonable test to see if it helps. If it does not help or improve the problem, simply put the packing back to the original thickness.

 

[Cornishpastythighs]

But if you cannot produce good quality product (weak print/poor fit) after taking out packing then what good is the test. I would go back to the OP's comments about when it rains he does not have an issue, so if this was me I would place a humidity chart recorder and log what the humidity is and do I have a problem when the humidity reaches a certain level, that way at least I could go to the bosses and maybe justify getting some humidity control systems in place.