image length different

on my 2 col ryobi b3 press( 2yrs old). When putting the correct packing its not enough squeeze for 80 bond .so added more and now have different lenghts dispite returning to basic packing this problem was on 120 gsm i needed to pull the 1st unit to max to register it.since found more packing the shorter the print. any 1 experienced the same.thanks.

 

Indeed, increasing the diameter will shorten the image in relation to another unit's cylinder with less packing and vice a versa

You should try to have the same packing on all units

 

what ever packing you add to the blanket ..you need to take away from the plate...and vis versa what ever packing you take away from the blanket you need to add to the plate...that should keep the print length the same..... hope this helps

 

I ran a KBA 105 b1 10 col and never had problem.As for reducing the plate packing there is none under we are using polyester plates.Thanks for replying

 

perhaps the plate cylinder circumfrence from the factory is different. it doesnt take much to become visable on the print lenghth. i would set everything to factory specs and try printing a fine line grid on both units. if the plate cylinders print without packing from the factory you may find yourself needing to pack one plate cylinder to print as short as the other. and as 6colorpressman said you need to take away the same amount from the blanket in order to print with correct pressure. then you need to add to the back cylinder pressure to pull it all together. I would tend to disregard what the scales read and lean towards equal print lenghth and correct pressures. scales can usually be moved!!!! of course the assumption here is that the press is mechanically sound and that things like gripper tensions and gripper timings are all correct. on a press that size i would think that in order to show a measurable difference in print lenghth youd need to add app .005" to the plate cylinder to shorten the image. once youve established just how much packing you need to print the same you can purchase an adhesive backed packing that will stay on the cylinder.

 

Do yourself a favor and pack the blankets to spec. Then set your plate to blanket pressures and then blanket to impression cylinder pressures to spec. You have to zero your dials out and take the covers off and adjust the pressures with the jack bolts. Over packing is just a quick fix and should be done properly before you cause internal wear on your machine. After that you should print a target on each unit to check for fit. It should all fall into place at this point. The cylinders are matched at the factory to fall in specs. Also if you are squeezing to much on one unit verses the other you could be getting paper stretch. So set them all the same from the start. You can also add pressure or take away from a unit to get your fit as well. I have seen 4 colors run certain stock that by the time the sheet gets to the last unit the paper is stretched and operator compesate with adjusting pressure. What it basically does is stretch the blanket.

 

Is the stock short or long grain? This could also be affecting things. Also check you are not running too much water on one unit as this may leed to paper stretch also.