Dry Offset Dot Gain

[claytonperry]

I am new to setting-up art for Dry Offset Printing. Does anyone know what standard dot gain curve values would be for:

2-4-6-8-10-15-20-30-40-45-50-55-60-70-80-85-90-92-94-96-98-100%

I am eager to learn anything I can about Dry Offset.

Clayton

[danremaley]

The typical dot gain for commerical color separations, is around 20% (a 50% in file =70% at press). Call me offline and I can get you the entire tone scale.
Dan 412.889.7643

[Erik Nikkanen]

I am new to setting-up art for Dry Offset Printing. Does anyone know what standard dot gain curve values would be for:

2-4-6-8-10-15-20-30-40-45-50-55-60-70-80-85-90-92-94-96-98-100%

I am eager to learn anything I can about Dry Offset.

Clayton

Clayton,

Maybe you could clarify some issues.

When you say Dry Offset do you really mean Dry Offset which has a raised flexo type plate or are you referring to Waterless Offset? There is a difference.

Also your application. Is this Dry Offset on a conventional type press or is it on a common blanket type press? This has a big difference in the requirements for prepress.

[Marc De Mey]

It is an ardent and often misinterpreted issue: you can not change the "physical" Dot Gain of a press. A press, every press, has its own Dot Gain and DGC (Dot Gain Compensation Curves) are Look Up Tables that change x% input into y% output.
Therefore Standards have been developed to which every printer should work, such as ISO, SWOP, GraCol.
To elaborate on the ISO (google for ISO 12647): (valid for screen rulings between 150 and 175 lpi) using a Reflectance Densitometer, and paper type 1 & 2 (white) 50% on the digital file (Photoshop, Indesign, Illustrator…) prints 64.3% for CMY and 67 for K , using Murray Davis formula, on X-Rite/Gretag densitometers it is named Dot Area, which is misleading.
A densitometer measures a reflected amount of light (which is compared to the amount of light that was projected). In standard Offset the 64% printed in fact comes close to 50-55% surface covered @150-175 lpi. For lower rulings one has to aim for less than 64%, for higher rulings (240+) as a rule of thumb you can add 5%.
The main point is to get the tone scale, images and tint panels "perceptually" equal to the standard.
If you want to go through the effort of testing: a very same 50% checkerboard (white, black repeated squares) looks darker if you use smaller squares. It stops when your eyes can no longer see it is made of black and white elements and see it as continuous grey (the resolving power is 210 lpi, single colour and 240 multicolor, higher is percepted as "photograph" instead of screened.)
For paper of lesser quality (type 3 to 5) the dot gain increases. But the philosophy remains: the 50% printed 64% on glossy white paper looks the same as the 75% on paper type 5.

[Erik Nikkanen]

A densitometer measures a reflected amount of light (which is compared to the amount of light that was projected).

Marc,

I am pretty sure in your rush to post that you took a short cut on the description.

Densitometers measure reflected light only. They compare the reflected light coming off say a screen with the reflected light coming off a white tile or the non printed paper.

Densitometers do not compare the reflected light with the amount of projected light. Not all the projected light will be reflected up to the sensor.

The M-D equation uses the reflection from the paper, the solid and the screen to calculate Dot Area as a fraction or percent. That fact that it does not provide an accurate value is due to the fact that paper tends to diffuse light and causes optical dot gain and due to the fact that the dot itself does not necessarily have the same ink density as the solid, which is used in the equation to provide a value for the dot.

With all the imperfections in the measurement capability, the densitometer is sure a practical device.

[Lukas Engqvist]

It is an ardent and often misinterpreted issue: you can not change the "physical" Dot Gain of a press. A press, every press, has its own Dot Gain and DGC (Dot Gain Compensation Curves) are Look Up Tables that change x% input into y% output.

There are variables that change the physical dot gain! The resulting TVI (tone increase value) can be compensated but it is a large assuption saying that the "pysical" dot gain is a constant for a given press. Ink viscosity, blankets, pressure, paper these all affect the "physical" dot gain, dont they? (I don't know flexo enough to say it is the same there)

[Marc De Mey]

That's very right. All these settings and parameters must be optimized and/or checked on beforehand.
For the amount of ink it is not up to the printer, while producing, to change this to make up for something wrong or overlooked in prepress.
As happens so often, print buyers are present to give their OK to print and ask the press man: can you crank up a little this or reduce a bit of that. And after a few rounds they put their signature on the print. I am very unsure those final settings are standard and can be repeated run after run, or last a full print run.
So the answer to your question: Can a printer change the physical dot gain of his press ?
Yes he can.
Should he change the physical dot gain? Maybe. But only for the right reasons.
To start a print run: in case the settings and parameters are not as the agreed standard.
To test new: ink, blankets,....
But definitely NOT when the press settings must be pushed beyond the tolerances.
Although I understand a small touch by the master boosts one's ego and the good relationship with a customer.

[Lukas Engqvist]

Thanks Marc. Then I am in agreement, the press should not be where to make colour correction (after all how does one visually forecast in wet ink what dry product will be). Also if that "constant" changes it is important that the pressman knows the cause of the change.