Experimenting with the offset process Ottawa area

[Deleted member 16349]

We gang jobs of the same color and they need different densities to match what has been done previously. We place them across the sheet to accommodate the color densities required. We have been doing this for over 15 years on various presses. Thank you for the information.

OK. I understand your example. I worked for Tetra Pak Canada and in the late 1980's we ran ganged jobs on our web press (Chambon EB inks). If you know of the Tetra Pak drink boxes, you will get an idea of what the print is. In North America, they ran a lot of this product with offset but in the mid 1990's they dropped offset and went back to Flexo.

Anyhow, we did not have standard densities for the packaging and even if the production group would have liked to run standard densities, the sales and graphic departments wanted to give the customer the most impact with the print. This meant that different jobs were run at different target densities.

This was a difficult problem when ganging jobs on an offset press especially because the bands of printing the carton material had to match when folded around when it was sealed in the back. So the densities between these bands of different jobs had to be nearly the same even thought the front of the pack was to be different. So densities had to go up and down across the web to suit the different products.

Having standard density target for all products would have made life easier for the press operators but packaging does not tend to follow standards but follows customer wants.

 

[gordo]

That sounds more like a problem with a particular ink and not what should be the case.

When you say over emulsifying, what do you mean? How does this condition show up in the print? Curious.

Over emulsifying would result in the ink being overcome in those areas and no longer being able to accept ink.
E.g. 1pt rules in that color would not accept ink and would fade away in the presswork.

 

[Deleted member 16349]

Over emulsifying would result in the ink being overcome in those areas and no longer being able to accept ink.
E.g. 1pt rules in that color would not accept ink and would fade away in the presswork.

What you are describing in my opinion is not a problem of over emulsification but of the poor control of ink feed at low coverage conditions. Washout in general is not an over emulsification problem but an ink feed problem. This is even for a small solid area.

OK, you may not believe that but that is my view and that is how I have seen things in tests with positive ink feed. Put as much water on the plate as you want with the dampening unit and it will not wash out the print even if it is low image coverage.

Although I think washout at the edges of print is related to too much water on the plate in the non image area.

Your point about 1pt rules, I think would have problems with too much water even if it is in line with high coverage. Too much water on the plate does affect edges of print. But who actually runs so much water to give such a problem. Probably operators that have a problem with ink water balancing, which was the goal of the ITB.

I think it is hard to get an accurate idea of what is actually happening when one is dealing with equipment that is not capable of controlling the primary variables in the process. It would be ideal if testing of many of these kinds of problems could be done after the ink feed is made to be positive and independent of water etc. so one can get a better idea of how things are working. Blaming emulsification for many problems is easy to do but does not really explain much.

In the future I would be interested in what could be done with different ink formulations and fountain solution formulations when the mechanical causes of some of these problems are corrected.

Gordon, in your experience with newspaper printing, have you had any experience with printing done on presses with the digital inkers that are sometime used? They in principle are positive ink feed devices.

 

[gordo]

Gordon, in your experience with newspaper printing, have you had any experience with printing done on presses with the digital inkers that are sometime used? They in principle are positive ink feed devices.

Nope :-(

They have a hard enough time just getting good plates out to the press and getting basic measurable targets included on the page.

 

[Green Printer]

After reading this thread the ITB appears to be a problem for many types of work that require different densities. If the itb can be turned on and off it could have merit.

 

[Deleted member 16349]

After reading this thread the ITB appears to be a problem for many types of work that require different densities. If the itb can be turned on and off it could have merit.

I don't agree. Can you explain why you think it can not be used to run works which require different densities?

 

[Green Printer]

If you are thinking of different densities for different jobs on the press at the same time, one could run different densities as long as they are not too different and too close together.

Because you said so.

 

[Deleted member 16349]

Because you said so.

That statement applies to conventional press just as much as it does to one with an ITB. The ITB would be more stable in printing different densities across the press.

 

[Green Printer]

Doesn't the ITB try to maintain a set ink film thickness across the entire sheet? Don't you have to change a setting on the ITB to change ink film thickness? If the ink film thinkness you require is less than the ITB gap setting what good does it do?

 

[Deleted member 16349]

Doesn't the ITB try to maintain a set ink film thickness across the entire sheet? Don't you have to change a setting on the ITB to change ink film thickness? If the ink film thinkness you require is less than the ITB gap setting what good does it do?

It is clear you don't understand how the ITB works.

Yes, there is a gap between the ink fountain roller and the ITB tip. This gap is required because ink keys do not touch the ink fountain roller and completely stop the feed of ink onto the ink fountain roller. The gap of the ITB between the ink fountain roller and the ITB tip is meant to be the same as the zero set point of the ink key. In practice, one would set the zero set point of the ink key so there is no ink collected at the ITB tip that would go into the press.

If the ink key could completely close so that no ink was fed onto the ink fountain roller, then the ITB gap could be zero as if it was a simple scrapper blade. If one had a positive ink feed , such as the DigiRail used on some Goss presses or with QuadTech's positive inker, the ITB could be used to improve their performance by transferring the ink into the press more quickly, which would improve their response time. Those systems tend to be have slow response especially for low coverage. I won''t explain why here since it is a bit too complicated.

So the gap between the ink fountain roller and the ITB tip is needed so one does not feed ink into the roller train when no ink is required.

But to make things easier to understand, we can think of the ITB as a simple scrapper blade. It is then much easier to see that the ink which the ink key feeds onto the ink fountain roller, will be the amount of ink that is transferred into the press roller train. The ITB is an ink transfer device and not an ink metering device. The ink keys meter the ink and the ITB transfers that ink independent of changing conditions in the roller train such as changes in water, temperature and press speed.

On the ITB set up, there is a gap between the ITB tip and the pickup roller. This has no function except that it should not be so close as to scrape ink off the pickup roller. It is a clearance gap.

The gap between the ink fountain roller and the pickup roller has no influence on the average ink feed rate. That gap is used to shear the globs of collected ink that come from the ITB tip so that the ink that goes into the roller train is not going into the roller train as globs of ink but a sheared ink films. The roller train continues to even these out before it gets to the plate.

In conventional presses with ductors, only about 1/4 of the ink that the ink keys meter to the ink fountain roller actually goes to the roller train due to the intermittent and ink film splitting action. Therefore with the ITB, one needs to run the ink fountain roller about 1/4 of the speed as one would with the conventional ductor configuration for a similar ink key profile height.

I hope this explains the ITB more.

 

[Green Printer]

I have looked at your patent drawings. I understand the concept. On light coverage jobs I already run my ductor
to duct anywhere from every other press revolution up to every 4th revolution with the ink ball running at 1 or 2 percent of speed. There is no way I could control light ink coverage with the ITB.

Press is a Komori sheetfed 4 color

Patent US6857366 - Printing press ink transfer mechanism and employment of same - Google Patents

 

[Deleted member 16349]

I have looked at your patent drawings. I understand the concept. On light coverage jobs I already run my ductor
to duct anywhere from every other press revolution up to every 4th revolution with the ink ball running at 1 or 2 percent of speed. There is no way I could control light ink coverage with the ITB.

Press is a Komori sheetfed 4 color

Patent US6857366 - Printing press ink transfer mechanism and employment of same - Google Patents

Sorry that you still can't understand the concept. Light coverage is easier with the ITB.

If you have high coverage (80%) on one side of the press unit and light coverage (4%) on the another side. What you describe above can not be done. With the ITB, high and light coverage can be run with control at the same time.

 

[Green Printer]

You said that you must cut you ball speed to 1/4 of what you normally run. I am already at 1 percent of ink ball speed and ducting every 4th press revolution.

The job I am describing is just 4 or 5 lines of 8 point text on a 20 x 26 15 point c1s board. There are no color bars or any other images to remove ink. This job as been ran for over 10 years.

 

[Deleted member 16349]

You said that you must cut you ball speed to 1/4 of what you normally run. I am already at 1 percent of ink ball speed and ducting every 4th press revolution.

The job I am describing is just 4 or 5 lines of 8 point text on a 20 x 26 15 point c1s board. There are no color bars or any other images to remove ink. This job as been ran for over 10 years.

Again, it seems you don't understand. With the ITB, the ink fountain roller speed needs to be reduced to about 1/4 for all print coverage. It is not related to print coverage but to the fact that much more ink is transferred into the roller train with the ITB than with a ductor per each revolution of the ink fountain roller.

The problem with controlling low coverage on conventional presses is that ink is transferred forward to the roller train or backward to the ink fountain all along the length of the ink fountain roller. This makes low coverage difficult to control. With the ITB, ink only goes into the press where it is needed and the feed rate is much more consistent and therefore is easier to control.

There is no point in my responding to your comments because you argue about things you did not understand and when I try to clarify things, you don't understand the explanation. So I will not respond to your comments.

 

[Green Printer]

I am 1% of ink ball speed now I cannot run it any slower. If i would cut it to 1/4 of 1% it would stop rotating 1% is as slow as the ink ball rotates.

How would I turn the ITB on and off during a press run to only supply ink every 4th press revolution, or any number of set press revolutions?

 

[Deleted member 16349]

I am 1% of ink ball speed now I cannot run it any slower. If i would cut it to 1/4 of 1% it would stop rotating 1% is as slow as the ink ball rotates.

How would I turn the ITB on and off during a press run to only supply ink every 4th press revolution, or any number of set press revolutions?

Sorry, you are not able to understand.

 

[Green Printer]

How do you start and stop ink feed using the ITB. Do you stop and start the ink ball roller or is there a mechanism that engages and disengages the ITB.?

 

[Deleted member 16349]

How do you start and stop ink feed using the ITB. Do you stop and start the ink ball roller or is there a mechanism that engages and disengages the ITB.?

One engages and disengages the ITB as stated in the patent. In the testing at the packaging printer, the electrical signal that was normally used to start and stop the ductor action, was used to activate a solenoid air valve to engage and disengage the ITB.

 

[Green Printer]

Can the ITB be engaged at press speed of 8,000 rpm and higher.

 

[Deleted member 16349]

Can the ITB be engaged at press speed of 8,000 rpm and higher.

Yes. If you start the sheets feeding and printing at that speed, then one should engage the ITB. If you stop the sheet feeding, then disengage the ITB.