Sorry Gordo, I need to re-word the question. If I change the LPI on my RIP from 100 to 90 (Newsprint) is there any ink saving benefit.
As I see it there is not.
I would imagine LPI would impact ink usage. How much is the million dollar question. But again, why sacrifice print quality when there are ink optimization solutions out there?
Exactly the opposite. As you increase lpi (e.g. go from 90 lpi to 100 lpi) you decrease ink usage. going from 90 lpi to 100 lpi is such a small increase that you probably won't see a measurable effect though.
Here's what happens using 150 lpi as the example...
In printing there is an effect called “dot gain” (a.k.a. Tone Value Increase). This is the natural spread of the dot of ink as it is squeezed and transferred from plate to blanket and finally to substrate. Dot gain happens at the perimeter of the dot. And small dots have a greater perimeter to area ratio. Hence more initial dot gain. Dot gain is of two kinds - the mechanical - which is the physical spread of ink, and optical which is the shadow of the dot in the substrate.
So, as you increase lpi dot gain increases. The dot gain must be “normalized” by using a tone reproduction curve. The result is that to get the same final tone on press that, for example, a 50% dot at 150 lpi gives - you might only need a 42% dot with a 300 lpi screen. Using a 42% size dot rather than a 50% size dot means that you are using less ink to get the same final tone on press. Also, more of the dot gain of the 300 lpi screen comes from optical rather than mechanical (ink) gain. The same thing happens if you use an FM screen rather than a high lpi AM screen.
Also, you cannot build as thick a film of ink on a small dot as you can with a large dot - so high lpi screens have thinner ink films for the same final tone on press.
Here is a photo comparing the same tone - on the left an AM screen and on the right an FM screen:
The AM screen is at 175 lpi and the FM screen is equivalent to about 320 lpi (my local newspaper uses this FM screen)
Here is the image projected in 3D to show the relative difference in ink film thickness:
The result of the use of a cut-back curve to normalize tone reproduction, thinner film of inks in the mid to quarter tones, and optical dot gain comprising a greater proportion of the final tone value results in the use of less ink for the same final presswork appearance. Typically about 10-15% depending on image content.
Here's a chart that illustrates the sources of ink savings and where they occur in the tone range:
So if you know your current consumption of ink (typically 3-4% of the printer’s gross) - just by changing to a very high lpi or FM screen you can take about ten percent of that cost and put the money back in your pocket. As an example for a $25 million dollar printer -ink cost will be about $750,000 per annum so savings will be about $75,000.
If you combine that with high GCR reseparations you could easily double the reduction in ink usage.
