Positive vs. Negative CTP

[roller61]

What if any benefits re there to imaging a posi vs. a neg CTP plate? Obviously the differences in a film based workflow, do not apply, so besides the specific chemical development, are there any advantages in producing the dot one way over the other? I've heard many opinions on this, all of which sound more like opinions to me rather than science. I'd appreciate any thoughts on the subject! Thanks!

 

[ajr]

Neg CTP plates wont have a big solid bar on the grip edge where the plate is clamped in the drum, can be importantt when running web presses.

A

 

[roller61]

Clamp Shadow

Clamp Shadow

Caused by non-image area held by platesetter clamps which are used to hold plate in place while imaging or spinning. Otherwise known as "clamp shadow"; most if not all users can bury this in the cylinder gaps, and it is a non-factor; if not this creates some issues with ink buildup in the gaps. Thanks!

 

[ajr]

On our smaller presses this was a major problem! So we changed to a neg working system when we upgraded. Also can be quicker to image sometimes as on some systems it only needs to expose the image area not the whole plate.

A

 

[DavidMa]

you can do double burns on a negative plate, but not a positive plate.

 

[roller61]

Are you referring to conventional film burns on a CTP plate, or actually in the platesetter?

 

[roller61]

To be clear, was the problem the ink buildup in the gap from the buildup caused by the 100% solids peeking out of the plate bends?? That's a mechanical issue that could only be avoided by imaging a longer plate and trimming (which some folks do) or using a negative plate, as you are doing.

 

[TigerPress]

Negative Advantage

Negative Advantage

In a negative working plate workflow, the laser writes the image. This has a reliability advantage - if the laser misses writing a dot either from dirt on the plate or a laser malfunction, you get a tiny white spot in the middle of image area. This is generally not noticeable in the printed product. Assuming a 20% ink coverage, there's also an 80% chance it won't show up at all.

 

[KevinC@EFI]

There were huge differences between pos/neg with conventional plates - such as the impact of dust... (much more likely to result in a spot on a pos plate than a neg plate, due to ink coverage averages). In the digital world, you have to look a bit further than just the pos/neg aspect, and see what each plate system offers.

No clamp shadow, as mentioned above, is a benefit for some - especially if you have a press with narrow gripper margins on the plate, or if you image your plates rotated compared to how they go on press (for faster throughput, etc.). It can also speed up imaging if you don't have to expose the full plate (which isn't possible on pos plates).

Negative plates tend to give you some benefits on processing chemistry too. Negative chemical systems often are not sensitive to air exposure, reducing the anti-ox replenishment usage and extending the life of the bath. They can also have lower pH, and lower overall replenishment - but that varies from system to system.

Our new Trillian SP plate is a negative-working plate, and has significant chemical benefits as mentioned above (pH-neutral, very low chemical usage, etc.).

However, in the digital world, it's not always so clear cut. Even our positive-working Sword Excel plate uses a "negative-type" developer, with the pH and anti-ox benefits noted above. You have to compare each plate system against your business and environmental needs, instead of generalizing on pos/neg.


Kevin.

 

[KevinC@EFI]

Are you referring to conventional film burns on a CTP plate, or actually in the platesetter?

Technically, there's no problem with double-exposing a digital negative plate either. I'd ask the question of WHY you'd do that, but it's certainly possible. (that's not to say there's no watch-outs if you were going to try - such as the effect of reciprocity failure, or leakage light exposure, etc.)

Kevin.

 

[gordo]

Technically, there's no problem with double-exposing a digital negative plate either. I'd ask the question of WHY you'd do that, but it's certainly possible.

With some workflows it is the only way to do mixed screening (e.g. FM plus AM).

gordon p

 

[KevinC@EFI]

With some workflows it is the only way to do mixed screening (e.g. FM plus AM).

gordon p

Good point, Gordo - I know we've done mixed screening many times in the past for comparison purposes (and printers may do the same to demonstrate their own capabilities), but is that something that printers would actually do for production work?

Double-burning seems like a kludgy way to accomplish just about anything in a digital world, but I guess you sometimes have to do extraordinary things to accomplish extraordinary results.

Kevin.

 

[gordo]

Good point, Gordo - I know we've done mixed screening many times in the past for comparison purposes (and printers may do the same to demonstrate their own capabilities), but is that something that printers would actually do for production work?

You've answered your own question.

As one example, the printshop of the poster who noted double burns as a benefit, does a great deal of award winning production presswork on a regular basis using, comparing, and contrasting, exotic and custom halftone screening techniques.

I guess it's a feature worth mentioning if it is more reliable with one plating system than another, even though I doubt that it's common practice. But, in this industry, you never know what printers will do.

best, gordon p

 

[John W]

We have a very simple software command that allows merging of 1bit tif plate images for a positive plate system. We use it for every black reprint to merge a current annotation without risk of damage to our ROOM workflow, ie, the job plate image integrity is never lost.
John W

 

[John W]

We have tried both methods. For years I was a negative plate supporter but now I am a convert. The plate image on a positive plate is the factory emulsion while the area exposed is the unwanted and blown away portion with laser and processing. We simply watch conductivity and our plate 50% has an accuracy of plus or minus one percent throughout the batch cycle (about 2500 plates per batch). That movement of the 50 is always very slow (slowly, over many days) so a set of plates will be within a half a percent at the 50 or even better! It is true that if either the laser or the chemistry moves off the mark that some sharpening can occur although we never see it (unless we want to deliberately sharpen a plate!).
John W

 

[Suhail Musarsa]

i have a jetplate System Machine that works with Epson 7880 Stylus. I have been looking for Marking fluid for a long time. do you an idea where or how to convert to other solution. I really need help thanks

 

[Laxmi Kant Kuthiala]

I think it may also have to do with laser life. A neg plate will need to expose a lot less area of the plate so conserve laser minutes. I t may also end up being faster, specially when the image size is smaller than the max print area.

 

[DavidMa]

You've answered your own question.

As one example, the printshop of the poster who noted double burns as a benefit, does a great deal of award winning production presswork on a regular basis using, comparing, and contrasting, exotic and custom halftone screening techniques.

I guess it's a feature worth mentioning if it is more reliable with one plating system than another, even though I doubt that it's common practice. But, in this industry, you never know what printers will do.

best, gordon p

thank you Gordo!

We use the double burn feature for quite a few things.

 

[roller61]

There were huge differences between pos/neg with conventional plates - such as the impact of dust... (much more likely to result in a spot on a pos plate than a neg plate, due to ink coverage averages). In the digital world, you have to look a bit further than just the pos/neg aspect, and see what each plate system offers.

No clamp shadow, as mentioned above, is a benefit for some - especially if you have a press with narrow gripper margins on the plate, or if you image your plates rotated compared to how they go on press (for faster throughput, etc.). It can also speed up imaging if you don't have to expose the full plate (which isn't possible on pos plates).

Negative plates tend to give you some benefits on processing chemistry too. Negative chemical systems often are not sensitive to air exposure, reducing the anti-ox replenishment usage and extending the life of the bath. They can also have lower pH, and lower overall replenishment - but that varies from system to system.

Our new Trillian SP plate is a negative-working plate, and has significant chemical benefits as mentioned above (pH-neutral, very low chemical usage, etc.).

However, in the digital world, it's not always so clear cut. Even our positive-working Sword Excel plate uses a "negative-type" developer, with the pH and anti-ox benefits noted above. You have to compare each plate system against your business and environmental needs, instead of generalizing on pos/neg.


Kevin.

Kevin,
Thanks, that's a great response. How does that negative developer stack up with VOC's? Regarding the PH, which can be treated. I'm interested in the environmental aspects of dumping the chemistry. Thanks!

 

[KevinC@EFI]

Kevin,
Thanks, that's a great response. How does that negative developer stack up with VOC's? Regarding the PH, which can be treated. I'm interested in the environmental aspects of dumping the chemistry. Thanks!

Hi Roller -

Great question... it's something we've been working hard on for our full line of chemistry because we too are committed to environmentally-responsible product design.

The difficulty today is that the test methodology for measuring VOC's in aqueous solutions is essentially broken. The test methods are designed for things like inks/etc. with high solid contents. When you get to anything that's largely water, you end up with negative VOC readings.

So - the result is that the data we can provide (or that any vendor can provide) isn't very useful. The testing we've done in real-customer sites show negligable or no actual volitilized organic compounds for our chemicals.

Being the conservative company Kodak is though, what we have published on our MSDS sheets is the total volume of ANY carbon-containing component in the solution. Essentially that would be the VOC emitted only if you let the entire bottle of chemistry evaporate into the air - which isn't a real-world situation. Other vendors use different methods... so it's not useful at this point to compare.

It's a problem that needs to be solved, with industry agreement on a testing methodology that's consistent and meaningful to the actual application of these types of materials.


EDIT: FYI, the test method mentioned above is EPA Test Method 24: DETERMINATION OF VOLATILE MATTER CONTENT,WATER CONTENT, DENSITY, VOLUME SOLIDS, AND WEIGHT SOLIDS OF SURFACE COATINGS. This is the method specified for things like developer, washes, etc. too - even though they're not "surface coatings"

Kevin.