Clarke Joe, Lynch Colleen C. M&R Professional Screen Printing Series

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If you can confidently answer yes to all of the above points, then adding some-

thing to reduce the tack level of the ink may be the only alternative. But before

you do, the ink should be put on a Turnabout (or other mixer) and thoroughly

mixed. The reason is simple-at rest the ink particles align and form a temporary

barrier. The barrier can be broken down easily with the application of shear for a

short time. If the ink has aged, then this false-body condition will exist, despite

the shearing force. If you add a liquid without stirring first, it causes the ink to

become far too thin and creates many other problems. The problems associated

with over reducing inks are dependent on the particulars of the ink, the quality

and type of reducing agent, but in general the results are as follows:

1. Loss of opacity.

2. The ink becomes harder to flash and fuse.

3. The ink becomes harder to overprint.

4. May visibly increase ink gloss level.

5. Assures the operator of after tack.

6. Leads to ink build-up, particularly due to absorption.

7. Likelihood of bleeding on polyester content garments.

8. Possibility of premature image fade.

If there is a need to adjust the ink, follow the manufacturers recommendations.

Do not exceed the limits without expecting a compromise in the performance of

the product.

BBuuiilldd-UUpp

Ink build-up, the accumulation of ink on the underside of subsequent screen, is

due to poorly made ink, which has lost plasticizer through either or both absorp-

tion or excess hydraulic force.

Although there are actions, which you can take to retard build-up, the real solu-

tion is the ink. The lists below suggest that you find out why you are having

build-up by observing its location on the underside of the screen. The first thing

to do if you are experiencing build-up is to watch closely to see if the ink begins

to build-up around the perimeter of the screen or the central areas. Build-up will

not occur if the ink does not lose its plasticizer, but it can lose it for more than

one reason.

Inks

PAGE

. 32

PPeerriimm

eetteerr BBuuiilldd-UUpp ((sshheeaarr rraattee))

If the ink build-up initiates or is worse around the perimeter,

then your build up is due to excess force. This type of build-

up is relatively insensitive to different garment types, brands or

constructions. The solutions to reduce the effect follow:

1. Raise the squeegee angle.

2. Slow the speed of the squeegee blade and

ensure that there is clearance between the

flood bar and screen mesh.

3. Reduce the squeegee pressure, unless the ink

is refusing to clear the central areas of the

screen.

4. Reduce the off-contact distance, unless you

have stroke direction image stretch.

5. Use a mesh with a higher flow rate and a

greater percentage of open mesh area.

6. Test a fresh batch of ink  one that has not

accumulated lint or been subjected to heat on

press.

7. Use a prescribed amount of a surfactant

added to the ink only after it has been thor-

oughly stirred on a Turnabout or other mixer.

8. If the press is running hot due to a lot of

flash curing, the viscosity of the ink may be

too low and lead to phase separation, where

the resin and the plastisizer break their bond.

Stop the press and allow it to cool.

Recalibrate the press before you begin print-

ing again.

9. Of course flashing or running the build-up

color last will totally resolve the problem.

10. Printing on an underbase will not help.

There are no real-world univer-

sal additives. Total compatibility

of one modifier with all plastisol

inks is a fallacy, so before you

adjust the properties of your ink,

talk to your supplier or directly to

the manufacturer of the ink.

There are two thinners for the

ink that has become too thick to

flow well and transfer cleanly

through the mesh. The two are

plasticizers and surfactants.

Plasticizers are the most common

and the most dangerous in the

hands of the press operator. They

are chemically compatible with the

ink but that is not the issue; they

are inefficient and at a level of

overflow. There is physically no

room for them in aged up ink.

When added they act as independ-

ent lubricants, just as if you put

one extra quart of oil in your

carit gets blown out after the

damage. The overload is easily

separated from the ink and then

the only option is build-up.

Surfactants do a far better job at

dealing with ink tack but dont

forgetonce plastisol ink has aged

up there is no way to reverse it.

The manufacturer can and must

prevent the aging. Surfactants are

very efficient wetting agents and

for mild tack on press they can

make it manageable. Also they

have a lesser affect than plasticiz-

ers on the thermal stability of the

ink. Consult with your supplier

and adhere to the limits on the

products that you are using and

stir thoroughly before adding any-

thing.

Chapter 3

Plasticizer

s vs.

Sur

factants

PAGE

. 33

CCeennttrraall BBuuiilldd-UUpp ((aabbssoorrppttiioonn))

If ink build-up initiates or is worse around the central areas of the screen, then

your build up is caused by phase separation, due to absorption. This type of

build-up is very sensitive to different garment types, brands or constructions. The

solutions to reduce the effect are follows:

1. Increase screen tension to a recommended level for the count

and thread diameter of low-elongation mesh.

2. Ensure that the off-contact distance is sufficient to eliminate

positive-screen lag (the lack of snap back).

3. Minimize the contact edge of the squeegee blade on the screen

mesh and be sure that you are not forcing ink onto the platen.

4. Use a higher mesh count; it will restrict the transfer of ink, pre-

venting it from absorbing into the garment as readily.

5. Speed up the printing blades

and reduce the dwell time

between print cycles, this

will reduce absorption time.

6. Under no circumstance

should you add any liquids

to the ink, they will not help

and can actually hurt the

absorption.

7. If the press is running hot

due to a lot of flash curing,

the viscosity of the ink may

be too low and lead to

accelerated absorption.

8. Of course flashing or run-

ning the build-up color last

will totally resolve the prob-

lem.

9. Printing on an underbase will

totally resolve this problem.

Inks

Figure 3.20 If there is a trace of ink build-up, all of

the detail is lost immediately. Shown is a five color 120-line

process print. Dropas is another HiRes AccuColor© T-

shirt print, printed by Target Graphics Ltd., artwork is by

Tim and Scott Etters.

PPeerriimmeetteerr aanndd CCeennttrraall AArreeaa BBuuiilldd-UUpp

If ink is building up initially, and equally, at the perimeter and the central areas of

the undersides of the subsequent screens, then you have an ink problem. Short

of flashing or running the color last, the only cure is another ink.

Before you blame the ink, be sure to consider that there are inherent properties

in various colors that may be a build-up problem. Altering the resin/plasticizer

ratio can definitely lead to build-up-as well as flashes, lint and a few other things.

1. Whites are dilatent (shear thickening) and tend to build up due to phase

separation. Of course to run at high speed is your goal but with whites,

particularly on thick thread mesh, they fight back. The harder you

push, the more the white resists. In this battle the white ultimately

breaks down and the solid phase (resin) and the liquid phase (plasticiz-

er) separate. In this state, the plasticizer can be absorbed into the gar-

ment or float on an under base, either way the result is build-up. Use a

thin fabric with a large mesh-opening, raise your squeegee angle and

you may have to reduce your speed.

2. Whites are often added to create pastel

colors or to improve the opacity of

another color. If white ink is added, it can

be a problem. If it is white pigment, the

problem may be worse (refer to number

one). If you are working with finished

inks, add a small amount of the strongest

of the color family to the white. For

example, if you want a light blue add

reflex to white. Similarly, if you are using

color concentrates, add a small amount of

dark blue pigment dispersion to the fin-

ished white ink.

3. Fluorescents are tacky and contain excess

plasticizer. They also require excess force

to transfer and are prone to rapid absorp-

tion into the garment. Use the recom-

mended surfactant (refer to the sidebar on

page 3.32 for details), on a thin mesh with

a large opening, and a near-vertical

squeegee at a moderate speed.

PAGE

. 34

Chapter 3

Figure3.21 The M&R KoolMist fits

between two heads; it is the ultimate solution

to after tack. It sprays a mist of water con-

taining 5% food grade silicone to prevent the

flashed ink from sticking to the next screen.

It maximizes color capacity on your press;

eliminates the need for a cooling station.

PAGE

. 35

4. Fluorescents are often used to punch-up another color, that is to

increase its saturation. If there is fluorescent and white in the color,

review numbers one and three and consider running this color last.

5. Black and blue pigments require higher shear rates to transfer and they

are loaded with plasticizer. These colors may separate phases (refer to

number one) and may absorb readily into the garment. Use a mesh

count with the largest possible opening for the hand and detail that

your image demands.

6. If the problem is related to the blue and or black ink, and not the pro-

cessing conditions, the two may exhibit a higher gloss level than other

colors at the same deposit. Your only alternatives are to run them last,

flash them, or talk to your ink supplier about the problem.

7. If you have added color concentrates there can be a problem. The pig-

ment dispersion contains between 22 and 80 percent plasticizer. So

when you add color concentrate between 1/5 and 4/5 of the addition

is plasticizer. The resin may not be able to accommodate the overload.

If so, the excess will separate from the ink and build-up is imminent.

Your only alternatives are to run it last, flash it or talk to your ink sup-

plier about the problem.

8. If you are using a base (unfinished ink) and its viscosity has increased

(aged-up), its use may lead to severe build-up. Run the channel test

(refer to side bar on page 3.14 for details), or ask your supplier for sta-

ble product. It is unlikely that anything will help hereother than

proper ink.

9. Is the (finished) ink aged-up (increased in viscosity) and requiring

excess force to transfer? Lay the squeegee more horizontal and speed

up the 80° -durometer blades. This increases the shear rate and pres-

sure on the ink.

10. Has the ink absorbed too much lint from use? Lint, in excess, will

absorb the plasticizer and virtually turn into glue. There is no way to

save the ink. Dispose of it properly and save your profits; put fresh

ink in the screen.

11. Is the ink too hot, thin and runny from a flash (refer to chapter five

for details)? If so, it is very prone to both separation and absorption

into the garment. Either case will cause build-up at an accelerating

Inks

PAGE

. 36

rate. Adjust your flashes as prescribed and properly dispose of the

ink. Once it has separated it cant be repaired.

12. Has the ink gone into gel state from continued exposure to heat? If

so, it must be disposed of. The cycle will thicken the ink, and you will

be forced to increase the shear rate. Build-up will be at the perimeter,

not through absorption into the garment.

13. Has the viscosity been altered, either with plasticizer or curable reduc-

er? There is no guarantee there was room in the ink for the amount of

plasticizer/reducer you added. If there was no vacancy, the added

plasticizer will move at a different rate than the balance of the ink,

causing phase separation. The excess plasticizer is prone to absorption

and will act as a wick to draw the original plasticizer into the garment.

14. Have you cross-mixed brands of ink? If so, there is no one that truly

knows what you have. There are no generic recipes that are market-

ed universally. There are fundamental and unique differences that can

actually cause a mixture to build-up; when in fact, neither of the two

virgin colors would have.

15. The purpose of this list is not to suggest that you tolerate ink build-

up but to help you properly identify its cause. If it is the ink, then you

must fix the problem, the cost of downtime is devastating and the

quality suffers on the way.

AAddhheessiioonn AAnndd SSuurrffaaccee EEnneerrggyy

Graphics screen printers are far more familiar with the concept of surface energy

than T-shirt printers, but to any of you who flash cure ink and then overprint it,

the issues are the same as for the graphics printer. The study of surface energy

pertains to the ability of ink to attach to a surface. That surface might be the raw

substrate or another ink. Before reading any further please read the sidebar,

Taking The Mystery Out Of Surface Energy.

Plastisols adhere to a T-shirt primarily through mechanical adhesion. The knit of

the shirt is a series of loops with a massive surface area onto which a plastisol

can fall. Once the ink is fused, it cannot be torn from the garment  its imbed-

ded into too many nooks and crannies. Plastisol does not penetrate into the indi-

vidual fibers, only dyes do that and plastisol is a coating.

Conversely, if you put that same plastisol onto a coated transfer paper, the ink

can be fused and still peeled cleanly from the surface without leaving a trace. The

Chapter 3

PAGE

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reason is the coating on the paper has a very low surface ener-

gy, and the surface is so smooth that there is little area for the

plasitsol to mechanically lock-on to. The wet plastisol having a

higher energy level; likes itself far more than it likes the sheet

of paper. If the energy of the liquid has a higher dyne level

than the substrate, the ink will not properly wet out the sub-

strate.

You have seen examples of this when you print on an under-

base. The slick glossy white ink looks great, then you try to

overprint it with Royal Blue. At worst, mottling, pinholes and

color shifts are obvious, even with a thicker deposit. The white

is tending to repel the blue- they dont like each other.

The following paragraphs explain that all surfaces have micro-

scopic pockets, no matter how smooth they seem. And that

wetting is the extent to which the inks fill those micro nooks

and crannies. Further, they detail the nature of the inks, how

to tell if this is a problem in your plant, and what to ask the

manufacturer for- to alleviate the problem.

SSuurrffaaccee TTeennssiioonn

This is a measure of the tendency of the ink to resist wetting.

Wetting at the point of initial contact is the degree to which

the liquids of the ink, change places with the gases in the

microscopic pockets on the surface of the flashed print.

Surface tension is measured in dynes per centimeter energy

units and is established on the ink while it is in its liquid form.

The surface tension of the ink is determined primarily by the

surface tension of the plasticizer that constitutes nearly half

the ink. If a low surface tension plasticizer is used, the ink will

tend to have a low surface tension and will wet out effectively.

If the plasticizer used has a high surface tension, the ink will

adopt this performance and not wet the surface below it well.

If the plasticizer chosen for the core of the ink has a high sur-

face tension, the ink manufacturer may select surfactants to

reduce the surface tension of the ink. If not, you are likely to

experience some of the wetting-out related problems covered in

this section.

Surface energy is the umbrella

term for the phenomena of sur-

face tension and critical surface

tension. The term surface tension

is used when measuring some-

thing in a liquid or fluid state.

Critical surface tension refers to

something measured in the solid

state. Both are measured in dynes-

per-centimeter. To avoid color

trapping, mottling and ink adhe-

sion problems, the surface tension

of the over-print ink (liquid) must

be a lower number than the criti-

cal surface tension of the flashed

and cooled ink film (solid) to be

printed on.

You must rely on the ink manu-

facturer to provide the surface

tension levels (liquid ink), but

dyne pens can be used in-house to

test for critical surface tension on

flashed ink film.

Inks

aking The Myst

Out Of

Sur

face Energy

PAGE

. 38

These are the worst-case colors,

those that tend to have high

surface tension: Whites, due

to the nature of the plasticiz-

ers used for anti-migration

properties and in the pig-

ments used for dispersion.

Blues and blacks use specific

plasticizers in their dispersion

state, and these are often

poor wetting agents.

Fluorescents are notorious for

poor flow and tend to have

higher-than-normal surface

tension and much of the time

they reduce the critical sur-

face tension of the gelled ink.

Laboratory equipment is

needed to measure the sur-

face tension of the inks

whether by critical angle or

with the use of a tensiometer.

You will want to leave this up

to the ink manufacturer but

ask for the results. You may

find that some of your trou-

bles are due to low surface

tension inks.

CCrriittiiccaall SSuurrffaaccee TTeennssiioonn

This half of the surface ener-

gy phenomena is a measure

of the tendency of the gelled ink (substrate) to be wet out by another color. The

best analogy is Teflon® (ptfe), it has a critical surface tension of 13.9 dynes per

centimeter. The reason that nothing sticks to it is that most common liquids,

foods, inks and chemicals have a surface tension that is higher than 13.9 dynes

per centimeter. If the surface tension of the liquid is higher than the critical sur-

face tension of the solid, then wetting will not occur.

Chapter 3

How to prevent surface energy problems:

1. Ask your supplier for the surface tension of

the inks that you are using.

2. Be sure you get surface tension levels on all

colors.

3. Remember, surface tension and critical sur-

face tension aren't the same (refer to side-

bar page 3.39).

4. Use dyne pens to test for critical surface

tension of flashed cured inks.

5. Six pens from 30 to 40 dynes per centime-

ter are sufficient.

6. Store the pens as recommended in a cool

dry area.

7. Do not keep a pen set longer than five

months.

8. Ask your supplier for proper surfactants to

adjust problematic colors.

9. Stick to the guidelines of the manufacturer

for limits of addition.

Preventing

Surface-Energy

Problems

How To

Instructions

PAGE

. 39

The critical surface tension of plastisol inks will typically range

from low to high thirties. At a critical surface tension in the

low thirties, not many ink colors will wet-it-out. Pinholes, color

shifts, mottling and smearing are likely issues with this as the

substrate. When critical surface tension reaches the high thir-

ties, the ink is a dream to overprint even with contrasting color

combinations such as Royal on White.

TTeessttiinngg TThhee CCrriittiiccaall SSuurrffaaccee TTeennssiioonn

There is a quick, low cost and simple test for the surface ener-

gy of the gelled (flash-cured) ink. It uses special felt-tip pens

that contain measured liquids at a specific surface tension. The

pens are swiped on the flash-cured ink and the critical surface

tension is related to the time, in seconds, it takes the pen solu-

tion to beads up.

Lets say that you want to evaluate the surface energy of your

white underbase. Print the underbase under normal conditions;

flash it and let it cool thoroughly. Hot inks will react differently

than cool and give you an erroneous reading.

Start with a pen at a dyne level of 30. Swipe the pen on a

clean, dry sheet of paper, three times. This action allows the

fluid in the pen to clean itself. Then swipe the pen on the

flashed, cooled underbase. The solution will probably flow out

and cover the underbase in a continuous line of fluid. In two

seconds, if the fluid has not beaded up, then you know that

the critical surface tension of the underbase is higher than 30

dynes per centimeter.

Now select a pen that has a reading of 40 dynes per centime-

ter. Swipe it on paper to clean the tip and then onto the same

flashed cooled under-base. It will probably bead up in two sec-

onds or less. If so, this means your under-base has a critical

surface tension of less than 40 dynes per centimeter. You now

know the critical surface tension is more than 30 and less than

40 dynes.

The pens are available in increments of two dynes: 30, 32, 34,

and so on. By testing one lower and one higher pen number,

you can establish the critical surface tension of the flashed

The following is a list of what you

will see if you have surface-enery

problems:

v White looks glossy, feels slick

and overprints poorly.

v Coverage over underbase

leaves pinholes.

v Some colors cover much bet-

ter than others.

v Blues and blacks tend to

mottle or reticulate on the

surface.

v Halftones cannot be printed

on an underbase.

v Halftones on an underbase

have erratic color shifts.

v Fluorescents do not flow out

onto an underbase.

v The ink climbs the squeegee

and flood bar.

v You cant print anything on

top of a metallic.

v Extreme color loss when

overprinting.

Inks

Visual Clues

Identifying Sur

face-

Energy Pr

oblems

PAGE

. 40

color. This will allow you to determine if the problem is due to an under printer

or an overprint color.

FFuussiioonn TTeessttiinngg

Testing for fusion or total cure is divided into two parts; a calibration stage that

allows you to set your standards and ensure results and second, a monitoring

stage. Once you have calibrated you can then use quick tests at press side and be

confident of the accuracy of those tests. The tests and how to use them are

described in detail below.

SSwwiippee tteesstt

Total cure or fusion is based

on the type of ink, its

deposit, entrance temperature

(as from a flash or not) and

the dryer parameters. The

possibilities are expansive.

Are you near the upper or

lower edge when you cure

your plastisol? If you do not

know, there is a quick test to

allow you to develop a sense

of position.

Take a scrap, which matches

your normal goods; tack the

cloth to the platen. Use

spatula and swipe a gradient

of white ink two-three inches

wide and six inches long,

from wafer thin to over 1/8-

inch thick.

Then put the scrap through

your dryer as usual. When it

exits you should see the fol-

lowing results: The thick end

of the swipe is still wet, or

nearly so, and indicates under-

cure. The thinnest end is

Chapter 3

Figure 3.22 Get Exotic (print compliments of Target Graphics) is a

65 Lpi halftone four-color-process print. There is no underbase and the

colors are deposited at one mil each. Thin film deposits are more likely to

exhibit problems with both the wet and dry crok test, but this shirt passed

the test with flying colors.

Clarke Joe, Lynch Colleen C. M&amp;R Professional Screen Printing Series

Подождите немного. Документ загружается.

Clarke Joe, Lynch Colleen C. M&R Professional Screen Printing Series