AQ coating and surface scratches

We are having trouble coating over solid coverage ink without scratching the ink through the AQ.
Air knives, IR, coating viscosity all at recommended levels. Any suggestions!!!

What is your TIC/TAC at?

Where is the scratching happening? When the sheets land in the delivery? I've seen pictures of how the coating, aqueous, is not a continuous film, like UV. It lets the ink continue to dry underneath. It's a quick handling fix. If the big solids were builds, like the question in the post above, the water emulsified in the various ink layers could soften or slow the drying of the aqueous coating.

John Lind
Cranberry Township, PA
724-776-4718

eliminate the obvious

eliminate the obvious

of course you should run the job without coating and look for scratches, this should eliminate areas of transfer. Coating brings in the last unit into play and you might look more closely at the pull off the coating impression cylinder since when on impression it alters the sheet path since the sheet is under blanket tension. Be sure you are looking at scratches and not coating streaks which usually come from roller coaters (rarely from anilox chamber coaters) and the streaking might be reduced by watching the viscosity and keeping the coating in suspension (stirring). good drying in the extension should help minimize scratching in the delivery since the coating should have good rub resistence.
A single unit solid is different than a screen build solid and so you might have to go back into all the units to look for a point of marking after an earlier unit. Try progressive pulls to look for the offending unit. Make sure stock is not being scratched in the infeed, perhaps the paper coating is scratched prior to print and it shows through the transparent inks??
good luck

jlind said:

Where is the scratching happening? When the sheets land in the delivery? I've seen pictures of how the coating, aqueous, is not a continuous film, like UV. It lets the ink continue to dry underneath. It's a quick handling fix. If the big solids were builds, like the question in the post above, the water emulsified in the various ink layers could soften or slow the drying of the aqueous coating.

John Lind
Cranberry Township, PA
724-776-4718

Click to expand...

You have it John the scratches are happening in the delivery. For the most part heavy coverage solids, not built colors. As we pull sheets out of the delivery to scan the pulled sheet scratches the internal sheet in the load.
Coating Gloss is important to our customer so we can't do to much tampering. Viscosity is good and the coating does fine on most jobs, only heavy solid ink coverage scratches are a problem.
Ken

I have run into this a few times in the past. Some things to try if you have not already.
Slow press speed down to give the coating more time under the air knives,and more dry time before it hits the delivery pile.
Make sure pressman is running lowest possible ink/water balance. If possible have ink reformulated stronger,so that a lighter ink film thickness can be run.
I have found in some situations that turning the airknife temp. and volume,as well as cold air up,and lowering IR can help. If sheet is to hot from excessive IR,it tends to keep the coating soft.
Make sure that delivery air evacuation system is working properly.
This may be a long shot,but figure it can't hurt to mention. On my Heidelberg,the delivery air evacuation draws the hot ,humid, air out. Ambient air is then drawn in from a series of grates and louvers on top of the delivery. Make sure these are not blocked!
I used to run a 4 color with aqueous coater that had a short delivery. I noticed that the coating was not drying like it used to,so I had to constantly turn the speed down. After a few weeks of racking my brain I noticed one day that my 2nd man had started storing spare blankets up on the delivery,covering most of the ambient air louvers. I thought nothing of it till then. When the blankets were removed,the dryer worked like it should again.

Water evaporation is the key. Hot air knives up, IR lamps down. Moisture extraction must work well as 67drake suggests. Set the delivery to jog well without movement. Even though the coating dries well, the solid still needs to set if conventional. Not so much a problem with UV.

i agree lower your ir temp. and go up to a higher percentage on your air knife.
with coating the air knife plays a huge roll in taking the moisture from the sheet.
be careful not to go down to much on ir since that solid ink still needs to setup, may want to try adding some dryer to the solid inks.
do not go below 90 degrees on your load temp.
another trick worth trying is to cut coating viscocity to where you normally run it with 50/50mix of water and alcohol this will help coating dry a little bit.