Poly plate stability & consistency

[Nate]

Hello fellow prepressers.

I've got a couple questions for y'all about poly plates.

1) What is the lightest screen you can hold stable on poly plates?

For the average job we normally run a fresh set of plates for every 10,000 impressions, but on jobs with especially light screens (around 2% - 4%) we often wind up running new plates much sooner—sometimes under 1,000 sheets. We don't even attempt to hit 1% screens—those are gone by the time the press is up to color.

2) How often do you calibrate plate exposure?

I had a tech tell me that the exposure should stay pretty consistent, and that I should only need bump the exposure up by a hair every now and then as dust accumulates on the laser (and then back down once the laser is cleaned).

In my experience however, exposure is much less consistent than I would expect. Sometimes it seems like it's due to an inconsistency in plate media, but other times it seems to change somewhere in the middle of the roll—which makes me suspect that chemistry may be to blame. My plan now is to start checking exposure at the beginning, middle, and end of each new roll we load to try and get a better picture of what's causing this.

3) Do you think both of these problems are related?

Makes sense that exposure problems could cause issues with light screens, but I want to make sure this is actually feasible with poly plates. I don't want to chase my tail trying to print a 1% screen if it's just beyond the capabilities of the material.

Thanks,

Nate

[Joe]

I don't think there has been a plate material invented that will reliably hold a 1% dot.

[Nate]

Even metal plates can't hold a 1% dot? That's definitely good to know.

What would you consider the smallest dot that's reasonable to expect to hold?

[Joe]

We use metal plates. Reliably we don't count on anything below a 4%. Yeah you might get a 2% or 3% to print. And you might not. It's like Murphy's law...if you want a 3% to print it won't but if you have a 2% you don't want to print it will.

[Nate]

Awesome--that's good news to me. I knew 1% was a stretch, but I was never sure about that 2-4 range. If even metal plates can't reliably hold a 3% dot, it sounds like our poly plates are actually doing surprisingly well.

Thanks Joe! :-)

[born2print]

Industry standard is +/- 2% as well...

The only trouble we had with poly is that the cost came out about the same as metal over time so we ditched it
(Which got rid of a whole separate rip, imager, processor, materials, maintenance and calibration)

[Ear]

And if you are concerned with 1 and 2% dots, you should probably be looking at metal plates. Joe is correct tho, and even if the plate will hold it, good luck with the press.

We have a local small shop (1 GTO) and they run poly plates for basic jobs. They have me burn metal plates, for anything critical, which tells me their poly plates are fair but not great quality.