The effect of the relief plate shoulder angle on flexographic dot gain

103 Pages·2016·1.4 MB·English

by  Bruce Brier

Rochester Institute of Technology
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Theses Thesis/Dissertation Collections
9-1-1986
The effect of the relief plate shoulder angle on
flexographic dot gain
Bruce Brier
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Brier, Bruce, "The effect of the relief plate shoulder angle on flexographic dot gain" (1986). Thesis. Rochester Institute of Technology.
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THE EFFECT OF THE
RELIEF PLATE SHOULDER ANGLE
ON FLEXOGRAPHIC DOT GAIN
by
Bruce Brier
A thesis submitted in partial fulfillment
of the requirements for the degree of
Master of Science
in the School of Printing
in the College of Graphic Arts and Photography
of the Rochester Institute of Technology
April 1986
Thesis Advisor: Professor Walter Home
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ABSTRACT
The mechanism of image-formation in photopolymer
flexographic relief printing plates is reviewed. In
addition to the identified factors, it was hypothesized
that the level of exposure could influence the plate
shoulder angle. It was further hypothesized that an
optimization of shoulder angle with respect to halftone
dot area may reduce the dot gain of the printing process,
An experiment was conducted using a novel method of
plate exposure. Continuous-tone masks were placed in
contact with the plate back and the halftone negative
used on the plate face. The resulting shoulder angles
were measured and analyzed for significant fit to a lin
ear mathematical model. For many dot sizes the face expo
sure level was significant in determining shoulder angle.
The plates were printed on a flexographic press, and
prints were analyzed to identify any benefits in dot gain
reduction. Regression analysis showed very inconsistent
correlations between shoulder angle and dot gain for indiv
idual dot sizes printed at an optimized level of impression,
11
Certificate of Approval -- Master's Thesis
School of Printing
Rochester Institute of Techo1ogy
Rochester, New York
CERTIFICATE OF APPROVAL
MASTER'S THESIS
This is to certify that the Master's Thesis of
Bruce Brier
With a major in Printing Technology has been
approved by the Thesis Committee as satisfactory
for the thesis requirement for the Master of
Science degree at the convocation of
September 1986
Walter Horne
Thesis Committee:
Thesis Advisor
Joseph L. Noga
Graduate Program Coordinator
Miles Southworth
Director or Designate
TABLE OF CONTENTS
Page
ABSTRACT.. 11
O9O*OO49OOOOO099OO
INTRODUCTION
Effect of Pressure on Flexographic Printing Plates. 3
Consistency of Tonality in Halftone
Process Flexographic Printing.
.....,......,,. .4
Elastomeric Printing Plates 6
,..,..,,,..,....,.,,,...
Light Theory 10
......... .............................
Light Propagation Model 11
...........................
Refraction.
oooaooooooevoooo ..12
Dispersion
Looo0oo*ooeo99o ..13
Particle Properties of Light
o o o o o ..15
Introduction to Photopolymerization.
. . ..16
Unsaturated Organic Compounds
' o o o oaoooo* O 9 9 o o ...17
Photopolymerization,
...... ooaaooo ...18
Dot Gain.
..... o o o o ooo*oooooo ...25
REVIEW OF LITERATURE
Bennett and Webster..
o o o o o ...26
urawrorQo
ooo.oooo.o ...28
Plambeck and Crawford
...31
Halpern. 34
oooooooooooooo a o o o o o <-S~
Moore. 37
....... ooooooooo O O O 9 O O O o''
Moore
oooooocoooooooooo oooocooooa ...38
Newberry
oooooooooooooo oo*ooa*oooo**o ...40
Smith
O O 0 O oooooooooooooo*o 0 0 0 0 0*0 ...43
HYPOTHESIS
Hypothesis 46
O0000000O000O0O0OQ0000OO90OOOOO**0O*
The Problem Proper 47
o*oooooaooo*o*oooooo o o ~ r
METHODOLOGY
Test Object.
. ooooooooo ..48
Exposure.
o*o*ooaooo*o ..50
Plate Processing.
o o o o o o o oooooooo ?ooo ..51
Determination of Shoulder Angle.
oooooooooooa ..51
Press Operation.
................ oooa*soooo ...52
Measurement of Prints 53
........... o**o*ooooaoo o o >J+J
111
TABLE OF CONTENTS (cont.)
Page
RESULTS ........... jj
L1\J-LjO*o*ao0000*00*0***0**000*0*1**9a'3***0**<'9***^''*M
BIBLIOGRAPHY. 64
....,.,.,..,,..,..
njTrLDUJi^V*O00O9*00O0904*9**409?a?9?a*09?')'3*'^''
Dafa 71
A^ClL*Ct> OO0O*O*'OaOO9OO****Oao*O*O*BO99O*9'3O9O9*9O'
UlUbbdiyn oocoo oosoooooo i o o o oo**ooo*'
IV
LIST OF FIGURES
Page
1 The Shoulder Angle 9 8
2 Characteristic Dot Profiles 8
3 Refraction of Light 13
4 An Example of Addition Polymerization 20
5 Critical Angle of Exposure 22
6 Sidewall Exposure in Shadow Tones 24
7 Characteristic Relief Profile of Laser-
Engraved Printing Element 27
8 Compound Curve Relief Profile. 32
..,,.....
9 The Cross-Section of Layer During Exposure. .. .35
10 Characteristic Dot Profile of the
Hercules Capped Plate 41
11 Taking Measurements of a Halftone or
Line Image in a Relief Plate 52
12 Prints Showing Kiss Impression. 88
v
LIST OF TABLES
Page
1 Dot Gain Curve for Average Plate. 60
.....,..,
2 Dot Gain Curves for Individual Inks 61
. ............
3 Face Exposure vs. Theta for 6.87, dot on film...
,71
4 Face Exposure vs. Theta for 16.17o dot on film..
.72
5 Face Exposure vs. Theta for 27.2% dot on film... 73
6 Face Exposure vs. Theta for 35,2% dot on film..
.74
7 Face Exposure vs. Theta for 51,3% dot on film... 75
8 Face Exposure vs. Theta for 56.3% dot on film..
.76
9 Face Exposure vs. Theta for 68.87, dot on film... 77
10 Face Exposure vs. Theta for 82.5% dot on film... 78
11 Face Exposure vs. Theta for 91.2% dot on film. ..79
12 Back Exposure vs. Theta for 6.8% dot on film.... 80
13 Back Exposure vs. Theta for 16.1% dot on film... 81
14 Back Exposure vs. Theta for 27.4% dot on film... 82
15 Back Exposure vs. Theta for 35.27, dot on film., .83
16 Back Exposure vs. Theta for 51.3% dot on film.
..84
17 Back Exposure vs. Theta for 68.87, dot on film... 85
18 Back Exposure vs. Theta for 82.5% dot on film.
..86
19 Back Exposure vs. Theta for 91.2% dot on film.
..87
20 Regressions Categorized by Printing Ink.. 89
21 Experimental Exposure Levels 90
.
22 Film Dot Area vs. Theta--Summary. . . ............ .91
vi
Chapter One
INTRODUCTION
Flexography
Flexography is a form of relief printing in which
the resiliant element of the press configuration is the
plate itself. The inks used in flexography are low
viscosity fluid inks. The ink metering roll, called an
anilox roll, is essentially a gravure cylinder which
prints an overall solid. The anilox roll consists of
a copper cylinder engraved with a fine mesh of intaglio
cells, then heavily chrome plated.
Consistent metering of the amount of ink available
to the plate is accomplished by covering the surface with
ink, and then shearing the ink at the extreme surface. The
excess ink is then ducted away (doctored) leaving the cells
full of ink.
The shearing function is performed by either a dif
ferentially rotating rubber-covered fountain roll or by
a reverse-angle doctor blade. At higher speeds the rubber-
covered fountain roll may deflect with the high. hydro
static pressure in the shearing nip. The reverse-angle
doctor blade, however, is held more tightly to the anilox
roll as speed increases.
The term "reverse-angle" stems from the fact that its