Half-tone printing process line
Measuring percentage coverage to produce a screen printing process line
Percentage coverage is easily measurable using a commercially available reflection densitometer. The instrument only needs calibrating with the null (printing substrate) and full tone references, then it can be switched to measure tonal values.
Should an instrument be incapable of automatic conversion, the formula is:
A process line traces the variance in tone values, plotted as a curve, between those on the film (or output device) and those in the printed image. The screen printing process line allows a repro expert to identify the dot gain (positive or negative), in order to calculate a compensation guideline for the printed image.
The screen printer initially needs no measuring instrument. A half-tone step wedge with at least 10 tone values should be printed alongside the artwork. The repro specialist uses a transmission densitometer to measure the half-tone values on the positive film, and a reflection densitometer on the print. The results are then compared on a chart.
Example of a printing process line
Film tone value | Printed tone value | Dot gain/loss | ||||
% | % | |||||
99 | 100 | + 1 | 100 | |||
91 | 98 | + 7 | 90 | |||
83 | 94 | + 11 | 80 | |||
76 | 89 | + 13 | 75 | |||
69 | 84 | + 15 | 70 | |||
63 | 76 | + 13 | 65 | |||
53 | 59 | + 8 | 60 | |||
47 | 53 | + 6 | 50 | |||
40 | 44 | + 4 | 40 | |||
32 | 35 | + 3 | 30 | |||
22 | 22 | 0 | 20 | |||
17 | 15 | – 2 | 15 | |||
11 | 7 | – 4 | 10 | |||
6 | 2 | – 4 | 5 | |||
1 | 0 | – 1 | 0 | |||
Every process line must also specify the following operational parameters:
– Half-tone
lines/cm, type
– Mesh
type, threads/cm, tension N/cm
– Type of stencil
emulsion, capillary film, indirect film
– Stencil thickness
specified in μm
– Surface roughness
Rz value in μm
– Ink
type, manufacturer, composition, viscosity
– Machine
type, manufacturer
– Squeegee
hardness, thickness, clearance, angle, pressure, speed
– Printing stock
exact description, e.g. type of paper, etc.
The process line can be significantly influenced by a change in any single item in the list above.