Density difference gripper to tail

[Prepper]

What do you find is the average/acceptable difference in density from gripper to tail of sheet? The reason I'm asking is we moved our colorbars from the tail to the gripper but left some marks at the tail that I read with the spectro to check Lab colors of inks and we found that cyan was 21 points lower at the tail, magenta 17 points, but black and yellow was only 2-3 points lower.

Is this normal or is something not right with cyan and magenta units?

Thanks,
Terry

 

[jlind]

Re: Density difference gripper to tail

Prepper:
The 21 points is pretty excessive. On a new press, data by my side, it can be 0.02-0.04, over 50 consecutive sheets. Press manufacturers will allow 0.05. The highest reading should be in the center of the sheet, suggesting these numbers came from a test form.... The oscillators can be adjusted to put the highest density near the center of the sheet.
Reasons for the big point spread are certainly mechanical. If it's good for two of the colors and off for two others, then probably not the ink. Could be water feed. This leads back to rollers and rollers settings, blankets and blanket tension, back cylinder settings, gripper adjustments. Time for some maintenance. Yes, I could be overlooking a few things, you need to be there and see how things are done. By the way, the press above was an LS40.
Yours truly,
John Lind
Cranberry Township, PA
724-776-4718

 

[papa v2.0]

Re: Density difference gripper to tail

you measure lab in points

points of what
what instrument do you have for measuring

 

[Prepper]

Re: Density difference gripper to tail

No, don't read lab in points, the points I was referring to was density, I was just saying why I had some patches at the tail while the colorbar was at the gripper

Thanks

 

[Prepper]

Re: Density difference gripper to tail

Thanks John, that's what I was thinking since 2 colors were way off and 2 were okay. The next job on the press was some black solids and they found they couldn't print them without going thru units 2 and 3 and doing a thorough cleaning and roller setting. Guess what? That's the 2 units the cyan and magenta were in on the previous runs.

Terry

 

[papa v2.0]

Re: Density difference gripper to tail

sorry got confused when you said

I read with the spectro to check Lab colors of inks

i didn't realise you meant Density

 

[Gerhard]

Re: Density difference gripper to tail

Definitely this is a mechanical problem. Good you found it. Not all of us are measuring front - end of the sheet.
Check your blanket and the under packing and make sure that the packing covers the whole cylinder. I have seen that sometimes the packing was too short and this will affect the pressure and the end of the sheet.

 

[Jay Toler]

Re: Density difference gripper to tail

What type of press is it and please describe your ink train and condition of the plate and blanket cylinders. Its sounds more like ink roller setting.
Test each print couple by printing a screen on each cylinder and check the dot condition from gripper to the tail.

 

[Mindlabyrinth]

Re: Density difference gripper to tail

It will probably have little to do with blanket and more to do with roller settings. Your not carrying enough ink in the roller train.
With blankets you would be having dot gain unless your using secastic

 

[Prepper]

Re: Density difference gripper to tail

It's a Heidelberg 72SP, running a 19" sheet on this particular job, on the gripper to tail density difference that was measured those 2 sets of patches were 17.75" apart. We haven't figured it out yet, going to put some patches around the cylinder in the middle of some work on the next job and see what we measure today.

I guess the size of the press would have been important to mention as this is a 4up press and not an 8up. Also, the job we measured this on didn't have much coverage where we were measuring the difference. Could that contribute to the big difference?

Thanks,
Terry

 

[Mindlabyrinth]

Re: Density difference gripper to tail

If all roller settings are good move away from it. What does your overall blanket and packing mic at. and what do you mean by patch. do you have a mic that will gauge the blanket while still on the press if so check the blanket at gripper and tail. I have run into some of the larger format blankets being inconsistent in there
measurement from head to tail in the past.

 

[fxpress]

Re: Density difference gripper to tail

Try Change your oscillation start point. Open the the door on the drive side and there should be the oscillation control arm. Stop the machine when it at its lowest point. Open the bolt and inch the machine roung halt a turn and lock up the bolt. See if that helps. The film of ink from head to tail is controlled by oscillation. You can even set it so you get more at the tail than the head. I used to a heidelberg data sheet to tell the diffrience for all the numbers marked on the oscillation wheel.

Terry

 

[Nickm]

Re: Density difference gripper to tail

It sounds like an adjustment of the *ink charge* is needed, which is done on the drive side of most presses. What this setting does is sets the timing of when the ink ductor hits the fountain roller therefore.......(replenishing) the ink supply. You can set the timing point to anywhere along the plate you like......including the plate gap. We use this setting quite often. It coincides with the ink ocillation that Terry mentioned. Proper setting techniques should be found in your Operators manual or can be received from the press manufacturer.

 

[Prepper]

Re: Density difference gripper to tail

Thanks for all the great info and help, it really is great because we are a small in-house shop, no one here including pressman has ever worked in printing outside of our place, so no real world, round the clock, all kinds of jobs, experience.

Ok, some further info on this problem from a job ran yesterday. It seems magenta is high in the first 1-2 inches of the sheet and then drops off from there to about 10 points low in the middle (8" back) and 3-6 lower even than that at the tail, 19 inch sheet.

Magenta seems to stay about 10-16 points lower gripper to tail. The other colors KCY 2-6 points different at the most. Then we read a sheet later in the run and we have the same thing in black and cyan on that sheet, a 10-13 point difference gripper to middle of the sheet!

Now I've measured differences from the same run where it is the same, gripper to tail, in the black and cyan and then 10 points different from gripper to middle of sheet?

Are we just loading up at the gripper for some reason? Would it come and go? Could it be water balance? This was a 25,000 run, so not small but not all day either.

Any more input?

Thanks again!

Terry Walker

 

[Satusr1]

Re: Density difference gripper to tail

First off, I don't think its even possible to be 21 points off from gripper to tail... unless something "large" in between the two points is taking the ink off. The only real test to determine how much is to print a blank form with a gripper color bar and a tail color bar.... maybe one in the middle for reference as well. Then see if its really that far off. All presses have a gripper to tail ink variation. Its determined by the ductor timing of the press, and the form roller circumferences and spacing. Now if someone has some rollers not touching in the ink train... and has basically incapacitated one or more of the form rollers by starving them of ink... it might be possible.

Also your "test" needs to be conducted one unit at a time, as the possibility exists that the unit is putting the ink down and successive units are back-trapping it off, or mottling the solid patches of the color bar. So when it mottles up... and you start reading the paper = lower ink density.

I have heard the argument for years that the colors bars belong on the tail... but for no real reason. Yes they may read slightly different on the tail than on the gripper, but it makes no difference as long as you determine what the standard densities are for your shop. If you run them the same density every time, you will be printing the same every time.

 

[Nickm]

Re: Density difference gripper to tail

When the ink form rollers have reached the plate gap, no ink is being removed from the rollers at that point. As soon as the ink form rollers hit the leading edge of the plate, the ink that was built up on the rollers due to the gap in the cylinder begins to ink the leading edge of the plate with an over abundance of ink. Therefore, the timing of the ductor is an important factor in leveling the inks from gripper to tail.
Nick

 

[Prepper]

Re: Density difference gripper to tail

Well, I think its 10-16 points different, not 21, but anyway, yes it is possible, don't know why its happening but it is.

We did a test by running a colorbar around the cylinder, from gripper to tail on the the sheet, in the middle of the sheet, nothing in line with it, turned the sheet sideways and scanned the color bar and the patches at the gripper are reading 10-15 points higher and it drops off dramatically 1-2 inches from the gripper going towards the tail.

Its around 10 points higher 1" from the gripper edge of the sheet to the middle of the sheet, only 8" from the gripper edge and 4-6 points lower 9" farther back at the tail depending on the color. It seems the same for all four colors, KCMY sequence, first 1-2" are loaded up but evens out after that.

Why all four colors? You think that the ink ductor timing is set wrong and its just been set the same on all the units?

This is a Heidelberg Speedmaster 72SP.

Thanks again for your input,

Terry

 

[Indexx Printing]

First of all, are you familiar with how to adjust the lateral distribution on this machine? It sounds like you are charging at the front of the sheet instead of the middle. We run a SM74 which is very similar and have had this problem. e-mail me at:[email protected] and we can have a discussion about this if you need to. It would also help to know your Ink/Cemistry.

Todd

 

[Indexx Printing]

The gear side door opened roll the press to where the arm is at it's lowest point. Pay attention to where the numbers on the disc are. The circled 1 is the default setting. We have found that running it to around 2.5 is where it runs best. To accomplish this, with the arm at the lowest point loosen the bolt on the center, then inch to press to where the number is at the lowest point too. Tighten the bolt and try it. You may need to experiment.

 

[PSchw]

Check the oscillation roller timing. Usually this can be changed 360 degrees this will influence your ink density front to back. You should run a solid and measure every 2" front and back should be the same and the center will be a little higher that is the neutral setting. Press manufacturer give you the option to move your higher ink charge to the from or to the back depending on your print form.