FTA (изд-во). Flexography: Principles And Practices. Vol.1-6

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PLATES 55

Appendix A

CRUSHED TYPE FORM

CRACKS I N THE COATING SURFACE AFTER BEING MOLDED

BLISTERING

COATING OF MATRIX MATERIAL PULLS OFF AND STICKS TO MASTERS OR TYPE FORMS

RIDGING OR PILING-UP OF COATING SURFACES AROUND TYPE CHARACTER EDGES

1. Excessive temperature

2. Excessive preheat

3. Too fast a press close

4. Mold cavity too deep

1. Check for correct press temperature

2. Reduce preheat time

3. Close mold slowly on the work and allow material to

flow and displace gradually

4. Raise bearer height and mold to a thicker floor

depth

1. Insufficient cure

2. Press temperature too low

3. Undercut originals

1. Cure matrix at prescribed time (at least 10 minutes)

2. Check molding press temperature.

3. Check shoulders of master; reject very badly under-

cut originals, otherwise use graphite on master and

matrix board to make a release liner

1. Excessive moisture in matrix material

2. Too high press temperature

1. Store raw stock properly; keep it away from mois-

ture exposure and high humidity. Precondition

(dry out) matrix board before using

2. Check molding press temperature

1. Improper cutting or damage to matrix material

2. Too long a preheat

1. Cut the matrix cleanly; use a sharp cutter, which will

not drag along the edges. Avoid shattering or break-

ing the coating surface

2. The matrix board’s elastic coating may be partially

set-up before being displaced. Reduce preheat cycle

time. Close platens slowly on the work

1. Improper heat

2. Excessive pressure

3. Improper preheat

4. Too fast a close on the work

5. Too deep a molded floor and/or wrong bearers

1. Heat upper and lower platens to 300° F to 310° F

2. Reduce molding pressure; apply only the amount of

pressure to keep the bearers tight

3. Check for optimum preheat time to ensure easy

press closing and material displacement

4. Slow down initial approach to work and application

of molding pressure

5. Mold a shallower floor; recalculate the bearer height

so the mold is deep enough for matrix thickness

PROBLEM/PROBABLE CAUSE CORRECTIVE ACTION

MATRIX MOLDING PROBLEMS AND CORRECTIVE ACTIONS

56 FLEXOGRAPHY: PRINCIPLES & PRACTICES

Appendix B

PROBLEM CORRECTIVE ACTION

TROUBLESHOOTING GUIDE FOR RUBBER PLATES

RUBBER STICKS TO MOLD: PLATE IS SOFT AND GUMMY ON REMOVAL

RUBBER STICKS TO MOLD: PLATE IS BRITTLE AND TEARS ON REMOVAL

RUBBER STICKS TO MOLD: TEMPERATURE AND CURE TIME OK

RUBBER TEARS ON REMOVAL FROM MOLD

PLATES CURL – WILL NOT LAY FLAT

CONT’D ON FOLLOWING PAGE

1. Rubber compound is old and set-up

2. Excessive preheat

3. Excessive pressure

4. Press temperature too high

5. Press closure too slow

6. Nonuniform heat distribution

7. Undercured

1. Check age of rubber and storage conditions; temper-

ature should be 45° F to 55° F. Rotate stock to turn

over inventory

2. Reduce preheat cycle, especially for older stock

3. Apply only enough pressure to bring platens tight to

bearers. Reduce rubber charge

4. Check platen tempratures for even heat distribution

or set to 307° F

5. Sets up rubber, increase rate of close

6. Check both plates for even heat distribution. Preheat

molds before laoding with rubber

7. Fully cure rubber for prescribed time at 307° F

1. Improper removal

2. Mold too deep

3. Undercut engraving

4. Press closing too far

5. Powder density in mold

6. Wrong type of rubber for mold format

1. Do not tug at stuck rubber. Start separation in one

corner and pull slowly from mold with an even pull

2. Allow a least 0.030" for background rubber (more for

thick plates). Use correct matrix board. Reduce depth

of magnesium original

3. Check etching for undercut condition. Remake mas-

ter engraving, if necessary

4. Check bearer height and raise, if needed. Reduce

molding pressure

5. Make new powder mold using enough power and

pressure to increase density

6. Consult manufacturer for plate gum with high, hot

tear strength

Poor release Check board coating. Use a spray release agent on

matrix. Dust rubber and matrix with talc and place

talc-side of rubber to mold

Overcured compound Check platen temperature, if too high, reduce to

307° F, or reduce cure time

Uncured compound Check platen temperature. If too low, raise to 307° F,

or increase cure time

PLATES 57

PROBLEM/PROBABLE CAUSE CORRECTIVE ACTION

B: TROUBLESHOOTING GUIDE

CONT’D

DISTORTED LETTERS AND RULES

PLATES HAS SKIPS AND NONFILLED AREAS

PLATE BLISTERS OR BUBBLES

UNEVEN PLATES

1. High centers, too much rubber

2. Insufficient pressure

3. Excessive heat

4. Press platens not parallel

1. Reduce rubber charge on mold, especially in middle

2. Increase molding pressure. Select free-flowing gum

for large molds

3. Reduce preheat cycle, especially for old stock

4. Have platens readjusted

1. Air is trapped in rubber

2. Too rapid a press close

3. Press temperature too high

4. Insufficient talc or dusting on rubber

1. Slow the press closing after proper preheat

2. Bump mold by quickly releasing pressure momentar-

ily after closing press

3. Check platen temperature for even heating; adjust as

needed

4. Check rubber for dust and lightly powder with talc if

needed

1. Inadequate loading and pressure

2. Air tapped in mold cavitities

3. Press temperature too high

4. Excessive preheat time

5. Insufficient preheat

6. Rubber is old and set-up

1. Increase loading – should be 90% to 100% of

desired plate thickness. Strip in rubber where addi-

tional fill is needed

2. Check rubber or adequate dusting. Use spray release

or lightly powder deep molds. “Bump” molds by

releasing pressure momentarily after closing press

3. Check plate temperature; adjust to 307° F. Check for

even heating of both platens

4. Decrease or elininate preheat cycle – especially with

older stock

5. Increase preheat cycle – especially if stock is fresh

6. Store rubber at 45° F– 55° F. Rotate stock to turn

over inventory

1. Inadequate pressure

2. Slow cure

3. Damaged mold

1. Check bearers (too thick) and or compound loading

(too little). Adjust as needed

2. Use faster curing rubber. Increase preheat cycle

3. Inspect mold; remake if necessary

58 FLEXOGRAPHY: PRINCIPLES & PRACTICES

Appendix C

REVERSES FILL IN

CAN NOT WASH DOWN TO FLOOR

LINES WAVY

FINE DOTS OR FINE TYPE WASHES OFF

PLATE TOO HARD

LETTERS OR SOLIDS CRACK WHEN FLEXED

CONT’D ON FOLLOWING PAGE

1. Too much face exposure

2. Too much postexposure

3. Too much chlorinating

4. Not enough back or face exposure

5. Incompatible ink or wash solvents

1. Decrease face exposure

2. Decrease post exposure

3. Decrease amount of light finishing

4. Check UV lamp intensity

5. Use only compatible ink or wash solvents

1. Over-exposed 1. Reduce face exposure and/or post exposure time

1. Not enough face exposure

2. Not enough back exposure

3. Over-brushing or too much brush pressure

4. Artwork exceeds material capabilities

1. Increase face exposure

2. Increase back exposure

3. Make sure time is set correctly or pull pressure

drum away from brushes

4. Redo artwork or compensate by increasing face

exposure

1. Not enough face exposure

2. Not enough back exposure

3. Not enough drying

4. Saturated solvent

5. Artwork exceeds material capabilities

1. Increase face exposure

2. Increase back exposure

3. Increase drying time –but not temperature or leave

for 5 hours at room temperature

4. Refill reservoir with clean solvent

5. Redo artwork or compensate with more face

exposure

1. Too much back exposure

2. Negative not dense enough

1. Decrease back exposure

2. Make another negative

1. Too much face exposure (especially with metal-

backed plates)

1. Reduce amount of time for face exposure or mask

this area and make the plate again

PROBLEM/PROBABLE CAUSE CORRECTIVE ACTION

C: TROUBLESHOOTING GUIDE FOR PHOTOPOLYMER PLATES

PLATES 59

SMALL HOLES AND DEPRESSIONS IN SURFACE OF SOLIDS

KINKS IN POLYESTER CARRIER SHEET

TACKY FEELING PLATES

1. Insufficient light finishing 1. Check finishing times and lamp output

1. Plate curls in brush unit of falls out of drum clamp

in the processor

1. Refer to the mechanical troubleshooting section

1. Poor housekeeping during platemaking

2. Improperly made negative: Over- or under-exposed

film, insufficient density, pinholes or kinks in nega-

tive

3. Low face exposure

4. Faulty material

1. Provide positive room ventilation. Get rid of dirt and

lint: Clean room periodically, clean exposure bed

daily, change vacuum sheet

2. Redo negatives that have scrapes or kinks in the

printing areas; touch up pinholes

3. Increase face exposure

4. Return sample to manufacturer for quality

evaluation

PROBLEM/

PROBABLE CAUSE CORRECTIVE ACTION

C: TROUBLESHOOTING GUIDE FOR PHOTOPOLYMER PLATES

CONT’D

CHAPTER 2

Mounting and Proofing

62 FLEXOGRAPHY: PRINCIPLES AND PRACTICES

ACKNOWLEDGEMENTS

Author/Editor: Howard B. Vreeland, Jr., Anderson and Vreeland

Contributors: Anthony Foley, Edward Graphics, Inc.

Steve Utschig, Fox Valley Technical College

MOUNTING AND PROOFING 63

n the early days of rubber plate print-

ing, “aniline” presses were usually one-

color, homemade, “tail-end” printers

positioned on the end of bag-making

machines. The hand-engraved rubber

plates, which often had caliper vari-

ances of 0.063" to 0.0312", were either nailed

to a wooden cylinder or, at best, glued to a

steel cylinder of dubious concentricity. The

accumulated variations in rippled plates and

irregular cylinders were compensated for by

forcing the plate cylinder and impression

cylinder more tightly together, adding more

impression to the plates.

Most print jobs of the day were single-color

and were run with just one plate mounted on

the cylinder. The major requirement was to

print a reasonably aligned reproduction of

the plate image somewhere on the face of the

bag. When the job called for two colors, the

press operator lined up the horizontal and

vertical center-scribe lines on the face of the

rubber plate with the horizontal and vertical

grid lines engraved in the surface of the plate

cylinder. Alignment was assisted by “sight-

holes” punched through the rubber plate cen-

ter scribe lines.

If each plate was mounted straight and was

fairly well centered on the plate cylinder, the

press operator could bring the two colors

into register by moving the two cylinders cir-

cumferentially and sideways in the press in a

sort of trial-and-error fitting exercise. This

simple, on-press plate-mounting procedure

sufficed, as long as jobs were run “one-up”

(only one design repeat per plate cylinder),

and print quality and press downtime was of

no great concern or importance.

In the early 1940s, with the advent of cel-

lophane, molded-rubber printing plates and

wider multicolor presses, the nature of print-

ing with rubber plates took a giant step for-

ward and flexographic printing technology

became more specialized. The hard, nonab-

sorbent surface of cellophane was almost

impossible to print by conventional letter-

press or gravure processes, so the lowly rub-

ber plate, which indeed could do the job,

came into its own.

Clear cellophane required white to be

printed as a base or a back-up color on

almost every job. This requirement resulted

in a demand for presses capable of printing

three and four colors and wide enough to run

several design repeats across the web simul-

taneously. Multiple images could then be slit

into separate rolls, and wound for placing on

wrapping or bag-making machines.

Demand for more sophisticated printing

caused presses to become more expensive.

Lost press time became correspondingly a

more critical concern, creating a need to

reduce press downtime. A method or

machine capable of accurate mounting and

proofing of rubber plates off-press devel-

oped. It became an essential component of

the prepress functions.

DEVELOPMENT OF MOUNTING

AND PROOFING EQUIPMENT

The first commercial “machine” for accu-

rately mounting and proofing rubber printing

plates was developed by Franklin Moss,

founder of the Mosstype Corporation in, what

most people believe to have been, the early

1940s. The well-known letterpress “line-up

table” was adapted for rotary use by mount-

Introduction

64 FLEXOGRAPHY: PRINCIPLES AND PRACTICES

ing a calibrated straightedge bar exactly over

the press plate cylinder axis. The press cylin-

der was held in alignment by resting the shaft

bearings in V-blocks.

Circumferential divisions and/or spacing of

the printing plates was achieved by mounting

a “dividing head” (similar to that used on a

lathe) to the end of the plate cylinder shaft.

Plates were aligned to the straightedge and

fixed to the cylinder using a paint-on rubber

adhesive. A movable impression cylinder

mounted in front of the plate cylinder allowed

proofing of various-sized plate cylinders and

various thicknesses of printing plate.

In 1945, Earle Harley, president of E.L.

Harley, Inc., then associated with the press

supplier H. H. Heinrich, adapted another let-

terpress plate-positioning device for rotary

rubber-plate mounting. He adapted the

“Taylor Regiscope” principle, which uses a

slanted, transparent, reflecting mirror to

superimpose the image of a rubber plate to

be mounted over the proofed image of a pre-

viously accurately mounted “key” plate. This

resulted in his patent for mounting plates

optically. In this equipment, the proofing or

impression cylinder is positioned above the

plate-mounting cylinder.

In Europe, during the late 1940s, Bieffebi,

Inc., developed a mounting and proofing

machine which also used the transparent,

reflecting mirror principle. Over the years,

each of these three basic machine types

have been refined and improved, some with

the addition of electronic devices such as

digital readouts and computer aids.

The growing importance for highly accu-

rate and more expedient “off-press” mount-

ing and proofing, created other unique

devices and approaches, which will be dis-

cussed later in this chapter. This pressroom

preparatory tool is one of the most impor-

tant links in the chain of improvements in

the flexographic printing industry and has

lead the way to the high quality, high speed

flexographic printing seen today.

THE PURPOSE OF

MOUNTING AND PROOFING

The purpose of mounting and proofing is

to prove that the job which is to be printed is

press-ready. A determination must be made

that it is properly laid out and positioned to

conform to end-use specifications; that it

has the correct copy mounted on an

approved color cylinder in appropriate regis-

ter to one another; and that its plates, cylin-

ders, gears and bearings are sufficiently

mechanically accurate to perform, on the

press, within acceptable standards.

To accomplish this, two things must occur:

• The mounting and proofing procedure

must be performed correctly and with

precision, according to clearly defined

and accepted practices.

• The equipment used must be manufac-

tured and maintained within stringent

mechanical tolerances.

To be a successful printer demands a good

mounting department, good mounting knowl-

edge and techniques and good documenta-

tion procedures. If they are not in place, the

same errors can be made again and again.

Each time a job is run, more should be

learned about it, so for the next press run, it

can run more easily and efficiently. A one-

page sheet of documentation should be com-

pleted by each department involved in the

printing process and placed in a job ticket to

be reviewed by all departments. Table 14

details the minimum information that should

be included for review.

For example, consider a print job that is

mounted, with the conditions above noted,

and sent to press. If there is a problem with

makeready or taping, then the next time the

job is mounted, the mounting technician can

look at how the job was mounted before. By

looking at the problem and solution section

of the press condition sheet, a determination

can be made if there is something else that

can be done in mounting that would help the

job run better on press this time. There usu-

ally is a better way, but without the proper

documentation the same erroneous mount-

ing procedures could happen again. With the

proper mounting techniques, the proper doc-

umentation and communication, print jobs

will, for the most part, run with fewer prob-

lems and with better quality.

MOUNTING AND PROOFING 65

Table 14

1. Plate type, caliper and plate condition

2. Cylinder ID and TIR

(total indicated runout) of each cylinder

3. Mounting tape used for each color

5. Any makeready done for each color

6. Problems encountered and solutions

CHECKLIST

Documenting a Print Job