Scum line on lead edge of plate on Ryobi

[egorus]

Ryobi 662PFH.
pH 5, 850-900 mS.
Water t° 12-13°C.
IPA 12-14%.
Huber inks.
Prisco and Varn Supreme.

At the beginning of print (morning) its OK.
Water is 2.2-2.3 (Ryobi unit of measure).

Strip on lead edge of paper appears with increasing temperature in pressroom. (24-25°C and higher).
After 1-1,5 hours.
I add water (2.5-2.7) - the line disappears.
After 15-20 minutes is necessary to add more water (2.9-3.0).
And so...
In the evening water is 5.5-6.0.
And emulsified inks.

 

[egorus]

more pictures...

 

[R Rabbit]

The scum line is caused by form rollers not being set properly, the form to oscillator pressure must be greater than the form to plate. The form rollers are a slave they can be driven by the oscillators or by the plate if they are driven by the plate you will get the scum line.

 

[egorus]

I set:
to oscillator -> 4 mm
to plate -> 3,5-4 mm

 

[D Ink Man]

Also question; what is the residence time of your ink in the fountain? Are take offs, coverage generally light, medium or heavy on average?

We are basically trying to get the correct answer, with two questions targeting the same means.

D

 

[Alois Senefelder]

Scum Line

Scum Line

Hello egorus,


A thin scum line at the lead edge of the plate - on any Press, 1-2 mm wide,

IS evidence that you have correct - Ink/Water Balance !


1) Provisio: NO "catch-up or scumming"


Regards, Alois

 

[egorus]

I made a mistake in the thread title.
Blurry strip on lead edge of PAPER (not Plate).
Scum line on plate -> 1 mm or less (see photo).
Its normal (normal?)

I'm sorry for my bad English. "residence time" in other words - is"..."?
And how to measure it?

 

[R Rabbit]

Try lightening the form roller to plate pressure especially on the last form and see if the scum line goes away.

 

[keith1]

Hi Egorus,

Previous posters are correct in asking you to check pressures. However I think you answered your own question when you said it works fine in the morning and gets worse as the shop heats up. Heat & humidity create problems, as does a shop that is too cold.
The heat is thinning the ink and heating the water, which makes it more susceptible to toning as you've described.

There's a couple ways you can fight this.
1) Add isopropyl alcohol to the fountain solution. A lot of shop owners have banished it from the shops, as with other volatile chemicals - but it works.

2) Add a body gum varnish to your ink as the day progresses. It will stiffen the ink that has been made more malleable by the heat in your shop. The downside is, a stiff ink doesn't lay as nice on the paper.

This is one of the reasons large presses have water cooling devices. In a perfect world, your shop would be climate controlled.
I once worked in a large shop where the owner turned off the heat overnight. It would be noon before we could get the inks to run properly. We'd have to work the ink every morning and add reducers. Plus the ink wouldn't dry because it was too cold. Guess he saved on his heating bill though.

Keith

 

[keith1]

Another thought is to try a different type of plate. If you're using a deep etch metal, try a smoother one. These require less water to keep clean.