controlling dotgain

[ronson]

Hi to all,

been looking for web offset related forum for a long time
I have some strange problems on my press.

On our press it looks like the dot gain has a mind of its own, one day the gain on 50% area is 15%, next day the gain on the same area and the same paper is 25%. (similar job)
Ink roller temp.(77F) and dampening temp.(52F) don't deviate very much.
Dots on the plates are always the same.

What are the most important things to look for when controlling dot gain?

Running Goss M600 4 unit press with Sun Chemical universal inks.

Thanks in advance.

 

[cjwworld]

Is the blanket keeping guage? I mean, if they are underpak it may change

 

[Bob Peterson]

Is it the same operator? If not are they running the waters the same? Are the dot gains fluctuating morning to afternoon? I would also recommend that you track ink batch numbers daily to keep track of whether or not you have consistancy issues.

 

[dimitri]

What I've found as the most determining factor in decreasing dot gain variation is having a robust maintenance program for the rollers.
Apart from that, packing, water settings, craftmanship, speed, fountain solution, solid density levels, pretty much everything affects dot gain.

 

[cjwworld]

What I've found as the most determining factor in decreasing dot gain variation is having a robust maintenance program for the rollers.
Apart from that, packing, water settings, craftmanship, speed, fountain solution, solid density levels, pretty much everything affects dot gain.

Well, sure but Ronson wants a solution

Maybe its the fountain solution.. might be making the ink emulsify... Just a thought

 

[ronson]

Thank You all for responding

Is the blanket keeping guage? I mean, if they are underpak it may change

packing seems to be OK. maybe very slightly over packed.

in both of these cases the circumstances were similar, press was warm and the operator also same.

I have checked the paper on witch the jobs were printed, paper is the same but the manufacturing dates are different.
Could it be the paper?

 

[cjwworld]

I doubt it. Does it change within the same roll.. I think it is more the blanket issue.. perhaps its not the right canvas for your press?? wish I knew and I know it can be frustrating.

 

[dimitri]

Dummy question: are you running the same density?
Are you during makeready? if you stop and start the press trying to figure out what's wrong, the gain will tend to stay high. I wish you were sheetfed to run some book
Then again, check the stripes of your rollers.
How was the makeready done? Is it likely that the operator put too much water to begin with? That would make you start chasing it too...
There are so many variables it's tough to nail it down.

 

[jwilson@tracypress]

If you're burning the plates rather than coming off a ctp.

Check the plate burner. If I burn my plates to a solid 6 we have a 15% dot gain, if we burn the plate to a solid 5 we're down to a 5% dot gain.

If your ctp check the calibration of the unit to be sure it's not the problem.

 

[gordo]

Doubling? Slur?

gordo

 

[qaos]

Check your dot meter

Check your dot meter

Ensure your Dot Meter (Densitometer) calibrated.
Check Dot shape.
Blanket underpacking. Paper.......

 

[SpeedKing]

are you sure its not down to your plates? are the blankets too soft? are the torqued up right? just some ideas.

 

[schrademo]

Dot Gain 15-25%

Dot Gain 15-25%

All I have to say is that if your dot gain swings that much, we must be working at the same place!

 

[sveibl]

It's the SUN!

It's the SUN!

We use Sun Chemicals also. Ink is inconstitant from batch to batch. We have Sun ink tech here full time. Mixing & ordering inks.

Keep a -5% & -10% plate curve & use to adjust for what color is off for the week-month etc.

Few plate remakes required

 

[dimitri]

why would you have to deal with this?

 

[sveibl]

Why we deal with it

Why we deal with it

Cost of Ink is good enough that Company overlooks short comings.

Not a horrible issue, If printing doesn't match proof pressroom adjusts, if they can't get it we plate curve.

We use massive amounts of Sheetfed & Web Inks

 

[tmason]

Gordo has the first idea for you to check into. Make sure the paper is not slurring or doubling. If the paper is solid, move to your plates. Make some linear plates and measure the dots. Record these numbers and put on press to see where it's at. Typical gain for a web is 20-30% in the midtone depending on the paper. If you are getting swings of 10%, and are not slurring, you may have a mechanical problem. If you suspect the ink, you should check the LAB values of your ink at standard density and compare this to the Gracol standard. You should document these numbers for future reference. You will be able to tell instantly if ink is the culprit. If you are buying cheap ink that is ok as long as it is consistent, but you need to set the benchmark to know if the ink has changed. Density alone will not give the reading to verify the ink.

 

[David Ingram]

another consideration, did the stock stay unwrapped overnight for the next shift? unwrapped paper begins to absorb moisture like a sponge!, If the stock sat unwrapped all night, that Could be part of your answer.

Though, i have seen my thermal trendsetter, way overexpose a plate for no reason, then come back to normal the next plating. We are now putting water control / grey balance bars on the left and right of our sheets, using an equal value of grey 30-40% on all 4 seps. Kinda a pain to modifiy all the preps templates, but its helpful with the water. I think moisture or water is part of your issue.

 

[ronson]

Thank You for responding

got lots of new thoughts from here

here`s what I have done so far...

1. checked blanket packing - is 0.01 mm. over recommended value
2. checked RIP dot gain curves - are made to reduce 3% in 50% area
3. checked plates - dots were 1-2% lower than needed (with gretag d19c)
4. there is little slur on 2nd (cyan) unit, gain is 3-4% higher there
5. roller stripes OK

although paper grammage written on reel wrapping is 80g. second batch seems to be 0.01 mm thicker,
could this affect gain?

Another question: do you use different dot gain curves, for example on 65g and 110g coated paper?

 

[ronson]

problem solved

10% gain difference was caused by a plate developing unit.
we changed the developer circulator pump, as it stalled from time to time.