is there a way to check if printing is overly applied or starving with water? any special control strip i can attach with my color bar? thanks...
Ink is designed to take up a certain amount of fountain solution (water).
Too much water - solid ink densities drop, solids become orange peel looking, dot edges or other hard ink/non ink transitions become soft or fuzzy, small areas of ink ma wash away/go blind, trapping is poor
Not enough water - the non-image area of the plate will start to take ink resulting in scumming in the presswork.
Solids and dots in your color bar or live image areas as well as non-printing areas are what you use to check ink/water balance.
It's a bit more complicated since there are 9 different combinations possible:
1 Not enough ink, not enough water
2 Not enough ink, water just right
3 Not enough ink, too much water
4 Ink just right, not enough water
5 Ink just right, water just right
6 Ink just right, too much water
7 Too much ink, too much water
8 Too much ink, water just right
9 Too much ink, not enough water
fogra had come up with a test form, I originally found this info on heidelbergs website in their "heidelberg news" section
That test form will only help to show you that all your roller settings are correct..
Your damp feed rate will be dependent on a few things.
1) % coverage of ink on the sheet (more coverage = higher damp feed rate / less coverage = lower damp feed rate)
2) Speed that your running the machine - Faster you run the press the higher the damp feed rate needs to be.
3) Heat in and around the machine will also effect where you need your damp feed rate.
4) Certain PMS colours will need Higher/Lower damp feed rate.
One simple way is to run a certain % screens in certain areas of your colour bar strips.
Check along the lead /tail of the plate there should be a 1mm strip of ink on the plate at the bend of the cyclinder after printing the job, this tells you that you have your fount feed in a good area.
When starting off with a new ink that you havn't used always start off with the minimal fount feed and also ink feed and gradualy bring them up in unison untill both correct density has been achieved and there is a smear free print
Also if you have inked up and find you have had to suddenly raise your damp feeed rate a lot higher, chances are you have put too much ink on, and from then on you will struggle to achive a crisp sharp print without mottled orange peal poor trapping looking print,
Finally your aim should be to print every job with just enough fount to prevent scumming up.
There was a test image in the far past made by GATF with the innocuous name of "the QC strip." Don't know if it's around or in a digital form. It was small but let me describe it. There was a solid bar parallel to the cylinder, and then narrow fingers of lines and spaces coming out of the solid at an angle, like 45 degrees. It was important that the lines and spaces were equal width, about 4 mils each. Not enough water, and the fingers would fill in. Too much water and they would look snow-flaked. Probably no place for this target today, but it could be in a color bar.
A star target, the old optical resolution target, could be used in the same way, and this is often on a color bar, maybe the best visual target ever invented for the press operator.
The wise old press operators of the past, probably all laid off by now or retired, would start up the press and cut back the water until there was just visible scumming on the lead edge of the plate. Then the water would be increased by 5%. That should be the running point.
When a plate manufacturer does a press trial for a new product, they establish this low water mark (sic, no pun intended) and then increase the water feed until the density of a target solid density drops 10 points, calling that the wash out point. Sometimes the wash out point never shows up, but the low water point is very distinct.
Cranberry Township, PA
this is the method i usually use.
Originally Posted by jlind
ar17, like said already, if you have good balance the best visual is a small (about 1mm) band of ink at the bend at the gripper edge of the plate that is even all the way across.
A method taught by "John Davies" the head printer and trainer at a place where I worked, was used on web presses. It might be also useful on a sheetfed press.
Originally Posted by ar17
The idea was to take the water feed off and count how long it takes to start to scum the plate. Since there is extra water on the roller train, it takes a short amount of time for that water to be used up and then the plate scums. For that press, I think the target time was about 4 seconds. I don't remember for sure. If the plate scummed up in more than the 4 seconds, then too much water was being applied. If the plate scummed up in less than the 4 seconds, then not enough water was applied.
For your press, when you think the water setting is good, stop the water feed and count until it scums. You might have a count of x seconds. Then see if you can use that method to maintain water setting for other runs by stopping the water feed and seeing if you are more or less than the x seconds. Of course, put the water feed on as soon as scumming starts to avoid waste.
The comment on having a scum line on the leading edge of a plate might be OK for sheetfed presses but it is not good for web presses. One does not want a line on the printed web like that.
I hope this works for you. When I heard of this technique, I thought it was simple and brilliant. And much safer than touching the running rollers with your finger.
Your best bet is when starting a Make ready start out with low water and ink . It is easier to add ink and water than to get ink and water out. Take water out till you scum and then add the water back till the scum cleans up.
The tint line one the lead edge of the plate works well as an indicator. Also if you are picking up the press from another operator and the job is in process of running. Look at all of your plates while running. If your plates look wet and shiny you have to much water. The plates should look dull and flat with no shine in your non image area while you are running.
The 3 golden rules of offset Lithography
Minimum amount of ink
minimum amount of water
minimum amount of pressure for good ink transfer.
Start water low so that small amount of scum is present, Adjust ink to get to required density, increase water slowly till scum dissapears, add one more click for luck and now you are looking at minimum ink and minimum water.
If you observe scum on one side of the sheet and not the other then look for mechanical issues Rollers/dampners etc.
If you follow the golden rules and still observe Scumming/Tinting look for possible Plate sensitivity issues, ink issues or temperature issues.
Its the mystery of ink and water balance that has long prevented Litho presses from being switched to Auto Pilot
My 0.02c worth
It is not a mystery. It is just that the print industry does not want to believe in the explanation that was published over ten years ago. All offset presses do not properly control ink feed. Fix that problem and you will not have the balancing problem between ink and water. Ink will be set independent of water setting.
Originally Posted by arfamoe