Colour changing on sheets every 4th or 5th sheet

We are running a full size sheet with pattern screen and about every 4th or 5th sheet the ink colour changes for about 2 sheets and then prints correctly. I wonder if this is build up of ink on the blanket not transfering properly or the ink tack not correct. Also we just replace the ink rollers, so perhaps we should recalibrate the rip as the rollers may be carring more ink now.

Would appreciate any suggestions

Thanks

a couple of suggestions
sheet distortion.
make sure your in feed brushes on the feeder are down far enough to keep the sheet from flapping around. the feeder set up air/vacuum has alot to do with blended screens to make solids. ink ductor on full coverage should have at least twice as much movement then what usually comes up on your ink profile. you can always close the keys more. but some screen will do that on this press. the form rollers are small and it doesnt recover fast enough with full coverage.

Hi,

The impression cylinder grippers may need cleaning or adjusting, that may be the issue. Because there are 4 sets of grippers if 1 set is not maybe set correctly this will present the sheet in a different position thus giving a different colour on the sheet, and then it will clean up then do it again.

Regards.

Regards.

curious

Just out of curiosity is the colour a green?

Could the grain direction of the paper be giving you a problem? Paper is fanning out?

djicewolf said:

We are running a full size sheet with pattern screen and about every 4th or 5th sheet the ink colour changes for about 2 sheets and then prints correctly. I wonder if this is build up of ink on the blanket not transfering properly or the ink tack not correct. Also we just replace the ink rollers, so perhaps we should recalibrate the rip as the rollers may be carring more ink now.

Would appreciate any suggestions

Thanks

Click to expand...

You might have a gripper transfer problem on a unit

What you are looking at is a classic issue with the QMDI. Any time you produce a color with screen builds that involve black and doubling occurs you will get color shifts. It is typically the black that doubles.

The main thing that cause this is the infeed rollers. The settings on these are never to be touched and will seriously effect the way a sheet prints. The first set of infeed wheels must touch down at between 260 and 270 degrees and lift up at exactly 6 dergrees. The 2nd set of wheels must touchdown at 4 degrees and lift up at 65 degrees. This allows the sheet to be held until the grippers have it. These settings can only be achieved correctly by using dial indicators. I highly recommend you have a technician set these for you.

Adjustments with sheet arrival will not produce reliable results unless the following four items have been checked and adjusted if necessary:

1. Infeed wheel timing.
2. Sheet to gripper timing. Should be 57.5 on Classics, between 57.5 and 59.8 on Plus machines.
3. Pile height mechanism is working properly.
4. The sheet guides are square to the press.
5. Infeed brushes are in good condition.

Once these have been checked then you can start making adjustments to your sheet arrival timing with the control knob on the feeder of the press. Move it slightly and then make a pull to check if you are still doubling.

Also changing the speed of the press helps.

If it is a screen that uses only three colors, put the color you are not using in the 1st unit and move the other 3 colors into the remaining units. This typically will get you through the job.

All of the other suggestions with the gripper bars are also important. Make sure all of your grippers are properly lubricated.

If you send me yor e-mail address I have some photos I took while working on a customers press in the field from a few years back depicting the issue you are seeing and showing what was done to correct it.

several years ago i was running a qmdi classic and had colour variations, it turned out to be the duct roller clutch not holding the duct roller when the ductor roller made contact.
jet screamer

kev c

djicewolf said:

We are running a full size sheet with pattern screen and about every 4th or 5th sheet the ink colour changes for about 2 sheets and then prints correctly. I wonder if this is build up of ink on the blanket not transfering properly or the ink tack not correct. Also we just replace the ink rollers, so perhaps we should recalibrate the rip as the rollers may be carring more ink now.

Would appreciate any suggestions

Thanks

Click to expand...

DI printing is a lot more difficult in my opinion than normal damper litho printing.The biggest obstacle is the fact that your printing portrait instead of landscape.Grain direction of paper is so important ie (long grain) for DI up to 200gsm..Tack is also very important especially on register on heavy coverage..Assuming you have no clutch slippage on plate mechanisms and no transfere problems then you have to contend with paper grain and tack of ink problems. Assuming the press is mechanically sound and set correctly then run 500 sheets of scrap through press onto job stock then re evaluate your problems. Tack on DI is a big problem till you pull out the ink..