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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


PS Printing color scale

Schematic representation of a screen printing process line

The variances in tonal values, plotted as a curve, result in the socalled process line:

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.