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

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PAGE

. 11

More important than how to

stop the dye is now to avoid

triggering the dyestuff. There

are two methods of achieving

such a product. First is by the

selection of a plasticizer that

is less likely to plasticize the

polyester fibers. Certain plas-

ticizers are literally used as

carriers for dyestuffs and they

can act as a conduit for the

dye once it is released from

the garment fibers. This fami-

ly of plasticizers should be

avoided. The second point is

to use the minimal amount of

plasticizer that the application

will permit and most of the

inks contain more than is

desirable.

Bleed resistance can also be

achieved with the use of

blowing agents comparable to

those used in puff inks.

These create a cell structure

that accomplishes two goals,

to keep the fibers below the

ink surface and to construct a

maze that the dye will have to

navigate to reach the surface.

Note that much of the bleed-

ing is from the top of the

garment.

Finally, there are manufactur-

ers that use cross linkers to

tie-up the ink film so that

the dye cannot pass through.

All of these adaptations tend

Inks

Migration is the permeation of the dyes through the

plasticizer due to the compatibility of the ink, dye and

garment. Plasticizers are literally used as carriers for

dyes in many cases also plasticizers invade the (poly-

ester) yarn structure and while it was once relatively

thermo set, it now becomes thermo plastic and flows

under the application of heat. The flowing causes it to

release the dyes into the inviting plasticizer of the ink.

Unlike sublimation, which is assumed to come from the

garment below the ink, triggered during the cure cycle,

migration can emanate from above or below the ink

film. Consider that the manufacturer uses bleaches that

decompose with heat. Your garment looks snow white

for a while and then begins to bleed. There is more than

a fair chance that the fibers on the top of the ink film

have been plasticized by the ink and released their dyes

into it. After all there are no bleaches left over to neu-

tralize the dye color.

Use a snow-white business card and punch a hole in it.

Compare this whiteness to that of the ink film immedi-

ately after it is printed. Then keep a continued eye on

the ink for any change. The comparison to the card will

allow you to detect the least change. As soon as you see

a shift toward the garment color, cut a slice in the test

sample and use a 10X loupe to observe if the dye

stemmed from the top or bottom of the print.

If it is from below you may not be totally fused,

recheck your settings. If the other extreme

occurred-over fusion, the bleach dies off and allows the

liquefied dyes to enter the plastisol, and bleeding occurs.

Be sure that if you are flashing then you check the dryer

exit temperature with a heat gun before entrance.

If the top of the ink film is stained with dye and the

internal segments are virgin, then your goods bled from

the top down. Your ink problem may be mat down.

You can select ink / mesh / blade combination that

mats the fibers better. Or you can print flash print, your

choice.

Testing For Migration

How To

Instructions

PAGE

. 12

to make ink that is more difficult to print and cure. Also they are traditionally

more finicky and exhibit less shelf stability. It is best to use bleed resistant ink

only when printing garments that contain synthetics such as polyester. The spe-

cific white that we selected has some inherent bleed resistance but is not recom-

mended for polyester printing.

xxyy CChhrroomm

aattiicciittyy CCoooorrddiinnaatteess

The xy-chromaticity coordinates indicate the position on the color locus and

gauge its hue as well as saturation. This model is most often used for process col-

ors but if you intend to use a white for underbasing process colors, you will want

to know its relative position from this model. For example if the white is exces-

sively cast to the blue side, it will drag every overprint color to the blue side,

which may be a consideration in your separation.

Chapter 3

Figure 3.8 The Chromaticity Diagram is the classical 1931 color locus. It is a plot of the hue and saturation

components of a color and was used for relative comparison of closely related colors. The x and y axes are used to

plot color. It has been updated to a more intuitive 1976 version where the distance between colors more closely

approximates their differences. Compliments of Photo Research.

PAGE

. 13

RRhheeoollooggiiccaall

The rheological properties of plastisol ink indicate how much force you need to

apply to get the ink moving, how the product will flow through the mesh once it

is moving and if and when it will stop flowing when printed on the garment or

on top of another ink.

Unfortunately, the standard viscosity instruments are not particularly informative

when it comes to on press performance. The equipment used for the readings

below included several types of laboratory instruments. If you are looking into a

low cost viscometer for in house QC efforts with plastisol inks, you may find that

only extreme differences are measurable. We recommend that you rely on data

from the manufacturers and subsequent press performance.

RReellaattiivvee VViissccoossiittyy

A viscometer measures the resistance to flow of the ink and is used to establish

thix index. It runs at one of very slow speeds between two and 20 rpm (revolu-

tions per minute). The spindle-type viscometer senses the resistance of the

spindle as it turns in the cup of ink  thicker ink means more resistance. The

measure of viscosity is centipoise (cps) and that is the scale that the resistance is

converted to. Relative viscosity is measured at a low rpm and accordingly has a

higher resistance and consequently a higher viscosity. Preferably the relative vis-

cosity (low shear stress viscosity) is high. Ink with a low relative viscosity would

tend to pour from the container and not hold the image once it is printed-howev-

er there is no assurance that it would flow through the screen mesh properly.

Inks

H-Cote 9000 Rheological Specifications

Relative

Viscosity

(cPs)

Yield

Stress

(

D/cm)

Plastic

Viscosity

(

cPs)

Shortness

Ratio

Thix

Index

Tack

Percent

Severs

(

cPs)

1,120,000

180,000

480,000

2.67

5.9

3059

Figure 3.9 This table lists the rheological properties of the H-Cote 9000 white ink. The relative viscosity is high, meas-

uring 1,120,000 cPs, which gives it a bodied consistency. The yield stress is low,measuring 180,000 dynes/cm, so the ink

begins to flow with little effort. The plastic viscosity measures near the midpoint, 480,000 cps, making it thin enough to flow

through the mesh, but able to recover its body quickly. The shortness ratio is also near the midpoint at 2.67, while the

thixotropic index is 5.9. The shortness ratio and thixotropic index here, describes a bodied ink that thins readily and yet

holds its shape under shear. The tack level is 20 percent, which is very low, and the Severs test indicated excellent fluidity at

100 psi.

PAGE

. 14

YYiieelldd SSttrreessss

Yield stress indicates the amount of force required to get the

ink to flow. It is not an indicator of viscosity, rather a device

response to initial resting viscosity. Since you want the ink to

have initial body but become fluid immediately with a gentle

push of the blade, you want a low-yield stress. Low yield stress

The in plant testing of plastisol

performance is difficult because

even the standard viscometers

used by most of the manufactur-

ers are not particularly representa-

tive of the performance of the

ink on press. There is a simple

and informative method of ink

testing that you should incorpo-

rate into your daily schedule.

Do not mix the ink before you

begin the test. At one edge of the

top of the ink pail, push a stain-

less steel spatula into the ink

about two inches in depth. Now

cut a channel into the ink, two

inches deep, the width of the

spatula across the diameter of the

pail and observe the spatula as it

crosses the pail.

If the spatula bends severely, the

relative viscosity and yield stress

are too high and you will need

excessive force to transfer the ink

through any screen mesh. If the

channel flows together immediate-

ly, the ink will transfer sufficiently,

but it will not hold its position on

the garment. It will perform poor-

ly when printed on an underbase.

If the ink remains in the shape of

the channel for a time, the results

will not be as bad, but caution

should be applied. The best ink

will cut the channel easily and

never allow it to fall or lose its

shape.

The test is subjective meaning that

the results are not transferable

from one person to the next. But

the impression gained about the

performance of the ink for each

individual is priceless.

Chapter 3

Quick Test To Check

Ink Flow Properties

Figure3.10 This is a macro view of the Rockport logo is a high-density T-

shirt. The relative viscosity of high-density inks is extreme, due to its high resin

and filler loading. If you are using high-density inks, we recommend the Turnabout

H.D. for complete low shear mixing. Printed garment compliments of Liberty

Screen Print.

Figure 3.11 This is a micro version of the Rockport T-shirt accentting the ter-

raced relief structure of the garment. The ink is less fluid than the standard plasti-

sol and maintains its shape quite well after it is printed. Garment compliments of

Liberty Screen Print.

PAGE

. 15

ink allows a minimal squeegee angle, speed and pressure to cause the ink to flow.

If the yield stress is high, it will require too much warm-up time getting the ink

to flow and this indicates you will have a color shift over the first few dozens

prints. Yield stress is a factor in the calculation of the shortness-ratio index.

PPllaassttiicc VViissccoossiittyy

Plastic viscosity is the fluid property of the ink at its thinnest point. This is where

the spindle type viscometers fall shortest of an accurate number. Ideally you want

the ink to be at its plastic viscosity and the plastic viscosity must be low enough

to allow the ink to transfer through the screen mesh.

If the plastic viscosity is too high you will be forced to run very high squeegee

pressure and a very low angle just to get the ink to clear the mesh. This will be

Inks

Ink Shortness Ratio Scenarios

Ink

Manufacturers

Problem

Ink

Characteristics

Effect

(one or multiple)

Short-Term

Solution

Low force,

high flow

(half-tone

extender)

Appears that the ink

should transfer, but

does not.

Use larger mesh

opening.

Deposit changes as ink

warms up.

Warm-up ink on

Turnabout, before

print run.

Excessively

High

Shortness

Ratio

High force,

high flow

(fluorescents)

Ink wont hold image.

Use thinner mesh.

Transfers mesh easily.

Use higher mesh

count.

Low force, low

flow (violet)

Ink wont hold image.

Lower shear rate.

Inadequately

Low Shortness

Ratio

High force, low

flow (aged-up

white)

Trouble transferring

ink.

Higher shear rate,

add reducer.

Low force is technically low yield stress, meaning it requires little effort to get the ink moving.

High flow is technically, low viscosity, meaning the ink is thin.

High force is technically high yield stress, meaning it requires more effort to get the ink moving.

Low flow is technically high viscosity, meaning the ink is thick.

Higher shear rate demands more squeegee pressure and lowering the blade angle.

Lower shear rate permits less squeegee pressure and raising the blade angle.

Figure 3.12 Use the table to verify ink problems. It offers short-term solutions, but more importantly, it gives you the

accurate launguage to the discuss the problem with your ink manufacturer. If you tolerate the type of problems listed above,

it will eventually affect your bottom line. Work with you ink manufacturer to solve the problem or find an ink that doesnt

exhibit these problems.

PAGE

. 16

particularly evident in the

center of the screen where

the blade contact pressure is

at its lowest.

If the plastic viscosity is too

low, as the squeegee injects

the ink into the mesh it wants

to splatter onto the substrate.

Such a flaw will be particular-

ly noticeable when printing

on an underbase. Ink with a

proper plastic viscosity trans-

fers the mesh cleanly at high

speed and still holds the

shape of the image.

SShhoorrttnneessss RRaattiioo

The shortness ratio is an

index that can offer insight

into the way that screen-printing inks will perform on press. It is a ratio of the

inks initial resistance to flow, and its final resistance to flow. Based on empirical

evidence, the shortness range for screen-printing inks should be between 1.5 and

3.5. A higher ratio would characterize inks that were not shear-thinning and a

lower ratio would profile inks that would not recover their body after the ink

had transferred. Either of these scenarios would not be conducive to quality or

high-speed printing.

There are four possible extreme, ink-shortness scenarios. Please review the table

shown in figure 3.12 (previous page). This aspect of the inks performance is a

delicate balance of force to cause the ink to flow and its viscosity once it is

flowing. To control shortness, the ink maker must accurately control or antici-

pate aging.

TThhiixxoottrrooppiicc IInnddeexx ((TThhiixx))

Thix index is a numeric scale that allows the ink formulator (or the printer) to

determine general tendency of the ink to be shear thinning. The operator will

run the ink at two different rpms on a spindle viscometer, a low rpm and one

that is 10 times as high. For example, two and 20 rpm measurements are taken

and the low rpm reading is divided by the high rpm reading. If it were 1,000,000

cps (centipose) at two rpm (revolutions per minute) and 200,000 cps at 20 rpm,

Chapter 3

To mat-down the surface of a T-shirt, you need to fol-

low these steps.

1. Select ink with a low-yield stress.

2. Select ink with a very high-relative viscosity.

3. Select ink with a moderate plastic viscosity.

4. Use a thin, flat screen mesh with a large opening.

5. Set tension to the required off-contact distance (refer

to page 4.4).

6. Use an 80-durometer blade with a sharp edge.

7. Set speed and angle for zero-mesh-lag and image

stretch.

Matting Down Surface Fibers

How To

Instructions

PAGE

. 17

you would divide the two to find the thix index.

1,000,000 / 200,000 =

5 as the thix index.

The two-rpm reading (low-rpm reading) indicates how the ink

will lay on top of the shirt. The 20-rpm reading (high-rpm

reading) indicates how it will transfer through the screen. The

thix index of five indicates the degree to which the product is

shear thinning (stir it and it gets thin). Generally, you will want

the highest thix index that the manufacturer can build. The

typical range of plastisol inks is from a low of three to a high

of 12.

If the thix index is too low it indicates one of two extreme

possibilities, a runny ink that flows too fast and wont hold an

image or print on an underbase. Or it is tar-like, stiff in the

container and wont transfer well through the mesh.

If the thix index is too high the ink may appear fluffy but con-

These accessories fit on the ends

of your squeegee blade and keep

the ink from spewing out from

around the ends of the blade. Of

course they are very convenient

on press, they allow you to run

longer without adding ink and will

save you the embarrassment of

poking an ink knife through a

screen during a production run.

One ripped screen more than pays

for End Caps for the entire press

(refer to figure 3.13 for details).

Here is the less apparent value:

they allow you to compact the ink

and take advantage of the fact

that in mass, the ink will not

react

as fast to heat. These attachments

allow you to put more ink

between the blades and the ink

viscosity will be more stable. If

you do a lot of flash curing these

will help keep the print colors

consistent and reduce the need to

stop and clean up or change inks.

End Caps work at any tension

level, on any M&R holder with

any blade at any angle or pressure.

Simply wash them off when you

need to change colors.

Inks

End Caps

Figure 3.13 The end caps shown here, compact the ink and by keeping a more dense area of

ink, reduce the impact of thermal-viscosity drop. If you are flashing, try using end caps to pre-

vent the inks low viscosity from plummeting and prevent premature gelation.

PAGE

. 18

vert from thick to thin and back, too fast to transfer through

the mesh.

TTaacckk PPeerrcceenntt

There are a variety of methods for the analysis of tack, which

may be the most important single variable of all. In all printing

and imaging processes tack is necessary so that the ink will

hold to itself at various times dictated by the process. The for-

mulator knows this property by the name visco-elasticity and can

typically measure it with a sophisticated viscometer. Generally

the lower the tack level, the faster you can print and the higher

the quality.

If the tack is too low the liquid and solid phases of the ink

will separate and build-up-mottling and wicking will result. If

the tack level is too high (and this is most often the case of

high solids dispersions such as plastisols)-the ink is difficult to

transfer at low shear rates. (The shear rate of the screen-print-

ing process includes the blade contact area, flexure and speed.

A low blade angle with a low contact area and high speed is

the highest shear rate). Most of the time excessively high tack

plastisols are a result of aging after the product is made. The

best analogy is honeychill it and it becomes high viscosity and

high tack,-warm it and it becomes low viscosity but remains

high tack. Thinning the ink tends to drop the viscosity, but

leave the tack level as it was (refer to page 3.28 for details on

ink tack).

SSeevveerrss VViissccoossiittyy

Severs is a capillary-tube viscometer with a specific orifice on

one end and air pressure on the other. When the tube is filled

with ink, the application of air pressure forces the ink through

the orifice at the other end of the tube. Viscosity is measured

by the time that it takes a given quantity of ink to flow

through the orifice, specific to the pressure applied. This test

method draws a high correlation between the lab and the

transfer of ink through a printing screen.

If the viscosity is too high, the ink will not transfer well at all.

If the viscosity is too low, there is a likelihood of phase sepa-

ration, which would result in build-up, mottling and wicking.

Chapter 3

After-flash tack is an ink and flash

issue, but there are other facets of

the process that you can pursue to

improve the condition.

Unfortunately a flat-coated screen

or capillary film is the worst possi-

ble surface, however we strongly

suggest that you look elsewhere

for answers and make the best

stencil possible. If you are run-

ning high strength twill-woven

mesh, you are creating more prob-

lems than you need for two rea-

sons. First, twill mesh has the

highest surface area so more mesh

is in contact with the hot ink at

any one time. Second, it has a

smaller opening so it forces you to

use a greater blade-contact area to

transfer the ink.

Review the press calibration and

flash settings sections of this

book and then use a plain-woven

screen mesh with a large opening.

Yes, continue to make a flat sten-

cil. Use a rigid blade, with a sharp

edge, set at the most vertical angle

possible.

If this is not enough, M&R offers

the Animister to cool the ink prior

to the next screen and eliminates

the need for a cooling station.

er-Flash T

PAGE

. 19

TThheerrmmaall

The thermal properties of plastisol inks have a serious impact on the optical and

rheological aspects. Generally as the ink is warmed, the plasticizer thins and even-

tually begins to solvate or fluxes the resin. Each of the significant plateaus of

thermal change is listed below for your evaluation. By understanding this time

temperature relationship you can use these properties to your advantage.

LLooww VViissccoossiittyy

Low viscosity is the first phase that the ink passes through, due to temperature

elevations. At temperatures below gel, but above room temperature, the plastisol

becomes very thin. This phase occurs in plastisols around 110° to 130°. The

point at which it occurs is less significant, than the viscosity the increase in tem-

perature produces. If there is a severe viscosity drop, you can be certain you will

have printing problems. These would be most apparent with dark-blue ink print-

ing on a white underbase. It is also this factor that causes a color shift during var-

ious stages of production. As the ink warms it fills the mesh cavity to a different

level. There can be a very noticeable color shift, particularly with transparent col-

ors. The high limit for this low point would imply that the viscosity of the ink

was unaffected by heat-but this is never the case.

GGeell PPooiinntt

The gel point of the ink is reached when the ink is no longer fluid at operating

temperatures. On press this is an indicator of how flash efficient the ink will be,

so in general, a lower number is better. In the real-world the ink is subjected to

infrared energy-the panel or tube temperature will be significantly higher and the

exposure much shorter than our test data.

If the gel point is too low, shelf stability is an issue; the ink will probably not ship

Inks

H-Cote 9000 Thermal Specifications

Low

Viscosity

Gel

Fusion

Re-melt

Peak

Wave-

length

Hot-Tack

Index

40,000 cPs

150° F

300° F

375° F

2.3m

Figure 3.14 This table lists the thermal properties of the H-Cote 9000 white ink. Low

viscosity indicates the ink, as do all plastisols, thins when they are warmed. The degree of thin-

ning is well within acceptable margins. Gel point is a typical 150°, fusion (cure) at 300° and

remelt at 375°. It has a low gelation point and sufficient distance between fusion and remelt to

be very user friendly. Its peak absorption wavelength is 2.3m, with a hot tack index of 3, on a

one to five scale. The hot tack is higher due to the smoothness of the print.

PAGE

. 20

or store well at all. If the gel

point is

oo high, you will

have a difficult time running

at high speeds due to a longer

flash time or higher flash

temperature.

FFuussiioonn

The fusion or cure tempera-

ture is the point at which the

performance properties of

the ink have reached their

maximum higher temperature

and longer retention time will

not improve the performance

of the ink. Elasticity, abrasion

resistance, light and color

fastness are at an optimum.

For standard plastisols this

occurs if and when the entire

ink film reaches 300° to 320°.

The variables that effect cur-

ing plastisol ink are extensive;

garment entrance tempera-

ture, ink fusion temperature,

ink deposit thickness, gar-

ment fabric mass, garment

specific heat, dryer tempera-

ture, and dryer retention time.

Thats a whole lot to control,

but the results will be worth

the effort.

Infrared dryers have more

variables, including the wave-

length and distance from the

garment to the panel or tube.

The emissivity of the ink and

the garment is a factor.

Chapter 3

Stop Gelling In The Screen by following these step:.

1. Calibrate the press and flash units.

2. Do not double stroke.

3. Run as-fast-as possible at all times.

4. Minimize the flash temperature.

5. Do not use flash additive.

6. Add curable reducer.

7. Add plasticizer, only as a last resort.

Eliminating Gel

In The Screen

How To

Instructions

Figure3.15 The photo shows an incremental gap in the squeegee and

floodbar. If your screen length and stroke adjust permit, you can invert

the floodbar and increase the spacing between the squeegee and floodbar.

For high tack-level inks, coarse meshes or long runs with a high coverage

area, this is a very helpful technique. The adaptation is very simple on

the Challenger II.

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