Thread: slurring/color shift

Our color is shifting thru-out the run. We notice that the black type is slurring and also the halftone dots on all colors seem to be slurring. It might go several sheets before any shift shows up and then it starts over again. When the color is correct there is not a slur. Does this sound like a gripper issue?

yes Id say grippers ....

We washed up our black and put in new without any reducer and it solved the issue?

I was running a CD where the black would doulble and slur on lightweight stocks 70# mostly but it was where ever we ran the black so it was deff a transfer gripper proplem...how low of a tack k are you using? I use gel when needed but never saw it cause a slur? what kind of press is this?
Steve

We backed off pressure and saw that the black type was not slurring so we replaced the blanket and have not had the doubling of black text since. It seems odd that backing off that little of pressure (due to the new blanket requiring less squeeze) would solve the problem but it did.

Doubling in the first unit on the QMDI could be an issue if the following items are not performed:

1. Use a #1 grade paper.
2. Use long grain text, short grain cover. This is extremely important on the Classic and Plus machines.
3. Make sure the paper is cut square.
4. Perform registration tests once per month with the QMTEST file that came with the press computer and the RIP. This test has 85% coverage and railroad tracks that go around the outside edge for registration purposes. It is run on a full size 13 3/8 x 18 1/8 100lb text coated stock. If this job has been deleted from the press and RIP one can be made to substitute by your prepress department.
5. Do not move register from job to job. Ink coverage and paper thickness affect register. For instance; if a job with moderate coverage on a text weight paper is imaged and run without registration problems, then followed by a business card job on card stock with very little coverage, more than likely the business cards may have a slight fit issue. A register move is made to correct the problem and the job is run. Then another job is imaged with moderate coverage on text weight, now this one is out register because the register was never moved back from the business card job. We call this "chasing register". If step #4 was performed most jobs should register without problem, in most instances the change between text and cover stock results in a register change that is barely noticeable. If a move should be made make sure to write down the amount of the move and move it back before imaging a new job. Most service calls concerning registration are due to this situation.
6. Make sure all blankets are packed to bearer height. .090 in.
7. Blankets must be tensioned between 17 and 20 ft. lbs.
8. Do not use excessive printing pressure.
9. Move the position of the infeed wheels until the end result is best. These positions will vary due to sheet thickness and size. DO NOT adjust the wheel pressure as this affects lift off and touchdown timing. Typically having the first set of wheels outside of the second set produce the best results.
10. Make sure infeed brushes behind the 2nd set of infeed wheels are in good working condition. These are designed to hold the sheet against the impression cylinder, if they are damaged or worn it will severely affect registration.
11. Too much or too little sheet advance. Start at "0" with the brushes off the sheet. Run 50 sheets and check for doubling. Next move the sheet advance to - 1 and run 50 more. Do this until your doubling is the least noticeable and take note of what the sheet advance was at to produce this. Next start slowly moving the brushes down at until the sheets no longer show signs of doubling.
12. Guide springs have been modified by pressman applying too much squeeze on sheet. Remove them and to run the job. (This can only be done if the sheet has been cut on all 4 sides. Also only applies to Classic and Plus machines.)
13. Machine speed can affect register.
14. Grease the impression cylinder gripper bars every 3 months.
By performing the above, most registration problems should be alleviated.