Spots in K

[Prepper]

On the second side pass, our K, which is in unit #1, is getting lots of white spots in the print on the sheet. Pressman says its spray powder and they have to stop and clean the blanket and plate on that unit and it clears up for a while then comes back, so seems to be that's what it is.

My question is, we are printing 4 color work with some spot varnish and aqueous coating over all, how much powder is really needed? If starting back thru on second side and he has to stop and clean that unit from just making ready on second side, is he using way too much powder on the first side you think?

Its a Heidelberg 72SP if anyone has any exact setting suggestions for that press?

Thanks

 

[turbotom1052]

on the average job if you are using an inline aqeous coating my feeling is that you should be able to run the powder pretty low. I wont even suggest a number on your powder unit because there are so many variables that influence just how much powder actually makes it to the sheet on a first pass through the press. I can say that if you cant back up at least 3000 sheets with an inline aqueous coating your probably running too much powder. Is it possible that your pressman needs to run that much powder because of circumstances beyond his control. For instance are your rich black builds exceeding the recomended percentages??? Or perhaps the powder unit is not delivering an even dusting of powder across the sheet? If a pressman can not have confidence in his powder unit then the tendancy is to run more than usually needed to avoid offsetting.
My suggestion would be to go over the powder unit and make sure its delivering an even powder across the sheet. As impostant as even powdering there needs to be repeatable powdering. Your pressman needs to be confident that setting his powder unit on 15% for example will deliver predictable powdering of the sheet. Only then can you begin to establish what powder settings are required for any given coverage or stock. Id also suggest that once your able to gain confidence in the powder unit that you work towards running the smallest powder size you can run. Also dont underestimate the damage that wedges in the load can do to a job. If you need to run wedges to deliver a job then try to be sure you use them sensibly in areas where the gripper coverage is light. Most commercial jobs on coated paper would require a powder size no more than 28 microns. It should also be mentioned that the timing of the powdering needs to be accurate. It needs to start just before the sheet reaches the powder nozzles and end when the tail of the sheet has passed the nozzles. You also need to be sure that the shadow of the delivery grippers are not blocking the powder from getting on the very gripper of the sheet.
On a final note i think it needs to be mentioned that the right combination and settings for your drying unit needs to be estalished. I wont go into that here because its been talked about on this forum often. Do a searchand you will find a wealth of info on the proper use of your presses drying unt.

 

[Prepper]

Thanks

Thanks

We do use a heavy K because of a high GCR amount but I cut the rich blacks down to 50-40-40-100 to help with that. Curious as to more info on your mention of wedges cause they are using those pretty heavy I see today?

Pressman doesn't pay attention to detail, ran for who knows how long before some jobs stuck together and we found out powder unit was plugged up and not working at all. So just trying to help troubleshoot a little.

 

[turbotom1052]

If I must use wedges to aid in delivery I try to be very conscious of just when they are safe to use and even then try to use the wider plastic wedges. By using the wider wedges you are distributing the weight of the wet printing above the wedge over a greater surface area. I also try to avoid using wedges in the corners if there is a heavy build bleeding off the gripper. This is dangerous because no matter how well engineered the delivery of a press is there will always be a little shadow area right behind the delivery grippers that just won't get that full hit of powder. Couple that shadow area behind the grippers with the inevitable air turbulence and your looking at a lack of powder onthe gripper.

 

[spigot]

Pressman doesn't pay attention to detail, ran for who knows how long before some jobs stuck together and we found out powder unit was plugged up and not working at all. So just trying to help troubleshoot a little.

He could be preventing this error from repeating but overcompensating due to angry managers or other masters of the universe. What powder are you using? what is the micron size?

Try printing a black dry solid with the current powder levels and drop them back until there is a very light but importantly even covering all over the sheet. Its important the "feeling" of the fresh sheet loses its "stickiness" and you can only just register the powder being there with your fingertips.

The final option is smaller stacks, put some angles in there and run 3 tiers on a stack, or whatever you can manage before you get some set off on the heavy areas.

For a really heavy coverage with coating i always am resigned to cleaning the blankets every 5000 sheets, running 2x2500 on a trolley.
I dont really care what the "quotation" department have to say about time, the jobs are always mark free and on spec.

 

[Prepper]

Thanks for the response, I don't know the micron size of the powder and also haven't felt the fresh sheet myself to know its "stickiness" but do know that the finished sheets at the cutter feel at though they are covered in powder, lots there, but that may not be a bad thing necessarily on the second pass, I'll check some of the first pass sheets to see.

 

[turbotom1052]

The micron size of the powder should be printed right on the box. As far as not being a bad thing to have too much powder on the last pass through the press i have to disagree. Ive seen customers reject jobs because the sheet felt like "sandpaper". Too much powder also makes a mess of your press delivery and finishing equiptment. Lastly too much powder can cause other issues in finishing.
The best way i know to get the powder settings right is to take a look at the 1st printing unit blanket during various intervals while backing up a job. Id check it first at about 1000 sheets. At 1000 sheets you should be able to notice a light and EVEN dusting of powder on the first blanket. I stress the word even because if the powder seems heavier in spots its usually an indication that either your powder unit needs maintenance, adjustment, or repair. If all looks good at 1000 sheets then just keep printing till 2000 sheets, then stop and check the 1st unit blanket again. By 2000 sheets you should be able to definitely see some powder on the blanket. If you dont id think that either you need to run a little more or than your unit was in need of maintenance, or repair. If all looks good at 2000 then id run until the buildup got so bad that you needed to clean the blanket. Hopefully you wont have to do that until at least 3000 sheets have been run. If your having to clean the blanket before 3000 sheets, and when you do clean it you see that the buildup is even across the entire blanket then its just a matter of turning down the powder dial a bit. Of course there will be that occasional job that will require more frequent cleaning (for instance black halftones or black screens) but for the most part you should be able to go at least 3000 sheets without having to stop to clean.
Remember that if the buildup is not even across the full sized sheet you need to remedy that issue. Keep in mind that the sheet is turned over and the area of light powder, or heavy powder, as the case may be will now show up on the opposite side of the blanket when backing up!!!
Once again i cant stress enough the importance of the proper drying combination, and a smooth and well jogged delivery of the sheets on the first pass in all attempts to getting your spray powder use down.
On a final note id like to say that you can have state of the art presses, the very best prepress, a top quality sheet of paper, and the most beautiful images, go to waste in the very last operation of a final printed sheet by faulty powdering. Powdering is in my opinion, definitly not the area to be looking to save a few bucks, because all you do before the sheet hits the powder nozzles can easily be negated by this seemingly insignificant and "low tech" part of the equation.

 

[jonprintz]

TurboTom has already nailed the powder application and I agree with his logic.

One thing that can be done when there is an occasional job with too much powder due to heavy density is to make a "powder pull"....ie. run the job through the press with impressions on but without printing. This will get most of the powder off so it won't be necessary to stop during the print run to wipe the powder off the blanket. This is not something that you want to do regularly but will work in a pinch.

Too much powder is worse than not enough. Depending on finishing it can require a reprint, especially if film over-laminating will be a finishing step.