Print length problems on sm74 5p

Hi guys
I have a problem on my sm74 5 p (5 years old) 2 over 3. Units 3 and 4 are printing longer than 1 and 2, with 4 being slightly longer than 3. On stocks up to 250 gsm i stretch 1 and 2 without any problem,but on thicker stocks ,especially 350 and 400gsm i run into more problems.as the problem is magnified.When a job has fine screens to make a solid colour,a screen clash occurs barring across the sheet 3/4 of the way up.if i overpack the plates to shrink 3 and 4,the same thing happens 1/2 way up the sheet.Where else can i go.This problem needs to be resolved. All blankets, transfers are packed correctly,bearers clean,cam followers good,grippers and pads have been cleaned and checked.Does anyone have any ideas,pincer bar maybe,many thanks Thomas

 

Hi Thomas, That doesn't sound good to me. Heres my input, We have a 2006 2 over 3 cd74. We have issues with very heavy stocks pulling out of the grippers by upto a 1mm at the perfecting transfer due to grain or just the single size clyinder, we just run slower on those jobs and have first units in different position to ones after perfector. We seem to have small length issues on our 4th unit for some jobs but we can easily control with a tug on the back egde of plate. I have been told that as plate clamps get older they can cause some issues with fit at back edge maybe this could be part of your problem but why only some stocks i'm not sure. adding packings under plates doesn't seem the correct thing to be doing on a press like this. Add this or repost in heidelberg see if those guys can be of assistance. The one thing we have to do 90percent of time is nudge the back corner of unit 2(our black) to get good fit on jobs, Good luck Daz

 

Thanks for reply DWF,
I have an engineer coming in tomorrow,hopefully he we enlighten me.I agree overpacking plates on a press of this stature
should not be needed,but its a fix on most jobs, But not one i am happy with. Plate clamp theory sounds interesting,i shall ask for these to be checked.The fit is out by a couple of lines nothing like a 1mm.It is on all stocks, but magnified on the thickest (350/ 400 gsm.Cheers Thomas

 

when you overpack your plates to print shorter are you also removing an equal amount of packing from under the blanket? and then adding an equal amount to your impression cylinder pressure setting?
ive encounterd this very problem on a few presses ive run. My fix was to spend the time to adust all units to print at the same length. The process i used would require 2 pages of typing to list here. If interested pm me and ill set up a time to talk with you on the phone about it.

 

Typically print length issues on presses with a perfector are caused by the perfector or jackets. Most mechanics will want to give the perfecting cylinder a tune up assuming the jackets are ok. A very common cause is the cam followers on the reversing drum. Even though they may look and feel ok they still can be bad. Ususally cam followers will last around 30-50 million impressions on the reversing drum depending on how will they are maintained. They need to be greased once a month per 8 hour shift. Even a little wear at the cam follower will result in alot of change at the gripper tip. Once new cam followers are installed fixed gripper height will need to be varified and the grippers set. Also if you are using perfect jackets (black jackets), they all need to be the same size. you have a choice of 4 different sizes. If you ever had 1 jacket replaced it needs to be replaced with the same size that was removed. this also will result in a fit issue. It is also possible to have fit issues when running 1 new jacket with 2 old jackets all of the correct size.

 

Not sure about the 74 although very thick stock can ruin the perfector grippers but you say youre getting stretch which is not mis register. Id check which way youre board gain is. Have you backed off the impression cylinders and increased pressure to obtain a dry solid with minimum pressure? If so, use one test plate on unit one and print some sheets (lines and/or register marks), then swap the same plate to unit 2 and register image to pre printed sheets altering the blanket packing only to obtain print length. Do the same on units 3 & 4. Check dry solids again once completed.

 

THOMAS
I've seen that issuse too many times, start with the camfollowers, to me it is the number one cause of print lenght problems on perfectors.
FSA

 

Thanks for your interest turbotom,since my last post i have had cam followers replaced on the reversing drum and gripper height reset. I have now changed blankets from sava to kinyo (electro dot).Since doing this we are using less back pressure and the fit is improving all the time,although still not entirely happy.My next thing is take off immpression jackets to check the thickness.Would a thicker jacket help print shorter?I may yet take up your kind offer of a chat.

 

Thanks for your suggestions ,this is a work in progress and have used some of your ideas,

 

i could see a change in jacket size having an effect of print but my feeling is that the "print length" will be determined by the circumference of the plate. By the time the actual impression is happening on that unit the final print length is already on the blanket!!! This is not meant to say that a worn jacket wont adversely effect print but if your plates do not all share the same "rollout" you will never print in perfect register from gripper to tail.