One of the great advantages of printing to standards is having a known target to aim for. ISO 12647 indicates precise L*a*b* values for offset CMYK solids when using ISO compliant inks with ISO compliant paper.
But what do we do to establish proper ink density targets when no standard exists? Printing onto unusual substrates such as plastics or synthetics, printing to “G7 Extreme”, or printing using unusual techniques are all examples of important non-standard printing proceses. Without an established guide to optimum ink levels, it is up to the printer to determine proper target densities.
In many printing processes, ink reaches a point where increasing the amount of ink on the substrate yields little gain in terms of density or increased color. Ideal ink film thickness will be levels that yield maximum density while retaining control over reproduction.
Additivity Failure in Dye Sublimation
A good example of this is the dye sublimation process, When the polyester receiver has reached its point of maximum dye absorption, “additivity failure” sets in, and increased ink film thickness no longer contributes to increased density on the final product. The result is wasted ink and impairment of the ability to control the process.
The best method for determining optimum ink film thickness is to run a test form of solid horizontal ink stripes with ink keys on each color unit set from low to high across the sheet.
The result will be a press sheet with densities going from too light to too dark across the sheet, with optimum levels somewhere in the middle. The task is to find the point where near-maximum densities have been achieved but additivity failure has not yes set in.
Finding the Sweet Spot
Using the Techkon SpectroDens, select the “Mini Expresso” option. Starting from left to right, take density readings at each ink zone. Mini Expresso will record each reading and display final density values in both numeric and graphic forms. Typically, final density values will rise from left to right as ink film thickness of the donor sheet increases.
However, at a certain point, final density values will reach their maximum absorbency level and additivity failure sets in. At this point, increasing ink film thickness on the donor no longer results in increased density on the final receiver substrate.
In the illustration above, taken from a dye sublimation press test, maximum density is reached at 1.72, and no increased density is reached after this point, even with increased ink loads. There is in fact a serious downside to exceeding maximum practical inkloads, as this will lead to a loss of shadow detail that cannot be corrected with curves.
So in this case, optimum ink load is reached at approximately 1.64: near the practical limit while still retaining control over shadow detail leaving a cushion for normal process variation. Using SpectroDens MiniExpresso makes it easy to find the area of maximum practical density.
By Glenn Andrews, Color Clarity