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

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Screens

published specifications are useful for one purposecomparison. For example, if

you have one mesh with a 45-micron thread and another of the same count but a

larger mesh opening, it is fair to plan on improved ink flow and, perhaps, higher

elongation. A problem arises if and when specification names are taken as preci-

sion measurements. Below we explain the utility of each of the most frequently

used specification names.

Mesh count  is nominal and defines the target number of threads and open-

ings per linear inch in either direction. In the real world, the count is not too

often exactly on target and rarely is the count the same in both directions. This is

not an inherent problem. Rather, the manufacturers have a specification (or toler-

ance range) within which they work and, at times, that range may be inadequate

for our purposes. Also, as the fabric is stretched, the count literally drops, based

on the amount of elongation. The on-press mesh count is the only one that really

matters to you. Thus, mesh count should never be relied upon as a singular entity

for the reasons above and those to follow.

Thread diameter  is nominal and defines the target diameter of the filament

as they arrive from the yarn spinner; there is no guarantee that your bolt was

composed of precisely the named thread diameter. Once the fabric is woven then

finished, the threads are oval in shape. They do not significantly change diameters

in the tensioning process, particularly with low-elongation fabric. The thread

diameter sets the range for fabric thickness and, other than that, only connects

the stencil to the frame. If there was another way to secure the stencil to the

frame, you would want to eliminate the mesh, because the threads only get in the

way. Thinner threads offer far less interference in the forms of printed resolu-

tion, ink passage, moiré and artifacts, but are not as strong as thicker threads.

Virgin Thread Diameter

Figure 1.1 Shows a

line drawing of a fabric

thread in its cross sec-

tional form. The yarn

(thread) is spun in a

cylindrical form. In the

manufacturing and fin-

ishing processes it

becomes ovaled. The

thread diameter is

given as though the yarn

was still cylindrical in

form

PAGE

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Chapter 1

Mesh opening  is theoretical and defines the averagenot the minimum

distance between adjacent threads. It is not often square as indicated by the fact

that the mesh count is not usually equal, warp to weft. Further, it becomes

enlarged as the fabric is stretched. You should monitor the stretched distance

(percentage of elongation) of your fabric in order to know just how un-square

the mesh opening has become. When the mesh opening is large enough, it allows

the ink to pass with a minimum of effort. There are three ways to enhance this:

coarser mesh, higher elongation, or thinner, stronger threads. We recommend the

last of these options.

Percent open area  is theoretical and, unfortunately, is based on a calculation

that uses a thread diameter that is round and a mesh opening that is square.

Again, as a comparative factor used prior to in-plant testing, this may have some

utility. Generally, we recommend the highest percentage of open area per mesh

count. While it does have a proportional relationship to the flow rate and pres-

sure drop of the mesh, as a single entity this measurement offers little more than

comparative value.

Theoretical ink volume  is, again, very theoretical, as its name suggests. This

specification is best used for general comparison of ink volume within a single

count of differing diameters. Any more creative comparisons become less

accurate.

Fabric thickness  is offered in two forms, nominal and measured. Naturally,

the measured form should be far more accurate (depending on the accuracy of

the test methods versus the variance from nominal data used to established the

Figure 1.2 This a

photomicrograph of a

thin thread 300 S fab-

ric compliments of

Murakami Screen US

Inc. The dark areas of

the photo represent the

mesh opening. Even

though this fabric would

print very well, you can

readily see that the

opening is quite small.

Just imagine the aper-

ture if the thread was a

T or HD, ink passage

becomes very restricted.

Thread

Opening

300s Mesh

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Screens

target). Fabric thickness roughly describes the length of the tunnel through

which you will have to pump ink. This is of considerable significance to the tee

shirt printer for a few reasons; white constitutes approximately 45% of the inks

used. It must be deposited in a sufficient film thickness to be opaque. It has a

dilatent (shear thickening) flow property so the harder you push it the less it

flows. Whites like a large opening and a thin tunnel.

Weave type 

options include plain and twill weaves, as well as multi-filament,

which is infrequently used today. Plain woven is the only fabric that should be

used for high quality screen-printing. Most of the manufacturers and or dealers

have at one time or another swapped twill weave fabric for plain and not

announced this to the public. The attitude may have been that the screen printer

will never know the difference. But the fact is the printer will notice a difference,

but identifying the mistaken twill mesh as the culprit, can be time consuming and

difficult. You should be aware of the problems twill weave fabric causes and

develop a system for verification of your incoming materials. Generally it is pre-

cisely the wrong type of geometry, a small opening and thick threads. Blue Jeans

should be twill woven for strength, but for printability screen mesh should not.

Properties Of Test Screen Fabric

Now let us build on the basic information we have identified. We offer a model

(refer to figures 1.4 and 1.5) based on information of the theoretical category

measured SS curves, and measured fabric thickness. The model is not intended as

an absolute, but to give you a variety of perspectives to help illustrate the cause-

and-effect relationship of the mesh specifications.

The table in figure 1.4 lists comparative factors of four test mesh counts used in

Figure 1.3 This a cross sectional view of plain-woven screen mesh. From the left, the warp yarns are separated and a

weft yarn is shot between for the width of the bolt and held under tension. Next the warp yarns are again separated and are

wrapped around the weft yarn. Next the process is shown as it continues. Drawings courtesy of Printwear Magazine.

Plain Woven Mesh Construction

End View of Weft Threads

PAGE

. 8

Chapter 1

this manual (and explained on pages 1.8 through 1.14). The

data below is computed after the noted percent elongation and

resulting tension level. Some of the criteria used were a 65-lpi

for modeling halftone equivalents and 10-ips (inches per sec-

ond) squeegee speed for modeling pressure drop and flow rate.

The SS information was based on 45 dyed mesh.

AApppplliiccaattiioonnss

The four fabrics are 80/71, 150/45, 230/40 and 305/35

thread. These were chosen for a variety of reasons, including

that they address a good cross section of printing needs and

offer good mesh geometry. For inventory and real-world con-

trol, you will want to keep some distance between mesh counts

and limit the number of counts you use.

The mesh manufacturers usually use a single thread to weave

four mesh counts; it is economical and provides a wide range

of products from which you may choose. The thread diame-

ters are nominalthat is, given a name. These are not

absolute dimensions nor do they need to be. The tried and

untrue mesh count is just that. It is rare that a published

count is the actual number of threads, in either direction.

Further, the count is not usually the same in warp and weft

directions. If you are ordering strictly by the count of the

mesh, you are probably getting a fabric with a thick thread and

a small openingsuch is the least expensive for the weaver to

manufacture. Last but definitely not least, the warp direction

may not build tension as fast as the weft, and no tension meter

will report that reality. All in all, you may not know what you

are buying, particularly if your reference is the manufacturers

catalog. We will, however, try to help you get to the bottom of

the mesh.

The 80/71 can be used for athletic printing, high-density

graphics, and metallic and puff-ink designs. It will deposit a

thick-but-smooth layer of even the tackiest of inks with a min-

imum of squeegee force.

The 150/45, in the context of these four fabrics, is the work-

horse; it will print most wet-on-wet applications with a reason-

We selected fabrics that were low-

elongation, whenever possible,

and obtained SS data to support

our choices. We selected fabric

that had the selvage edge on both

sides so we knew our bolt was not

cut from a larger woven width.

The filament in all possible cases

were colored to reduce light scat-

ter (halation) during exposure.

The threads were thin as was the

fabric thickness, and they all had a

proportionately large mesh open-

ing. Such geometry allows the fast

and unrestricted transfer of the

thickest plastisols. The fabrics

were all relatively flat to permit

fast squeegee speeds. Finally, we

published the model of the fab-

rics once they were tensioned, in

order to make the information as

real world as possible.

How We Developed

The Mesh Model

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able hand and drape. The print speed on this fabric is excellent. It is fine for the

brightest of underbases and clears well on both fleece and jersey-knit T's.

The 230/40 mesh offers finer detail, softer hand but higher print speed only with

the proper ink, intended for high-speed, automatic printing. Naturally, the ink

mileage on this fabric is excellent and it performs well for overprinted colors.

The 305/35 is traditionally intended for halftones and process color, but even

serves well as an underbase for high-detail work. Its flow rate and pressure drop

allow it to fill the knit in the garment at high speeds and still provide details for

65-lpi and above.

Many of the specifications in the tables below are defined as nominal. These

are target dimensions and, per manufacturer, are to be within a published or

unpublished range. The use of this data allows us to anticipate results of certain

printing conditions with a minimum of testing prior to production. From this

data we have constructed models of printing performance. The model is a simu-

lation of what can be expected from a fabric with particular properties. We

encourage you to test for yourself and determine the best product for your needs.

Meanwhile, use the tables below to jump start your process.

Following is a detailed explanation of the variables for the first six columnsthe

mesh specifications of Part 1.

Mesh Model Part 1

Mesh Count

Thread

Diameter

N/cm²

Percent

Elongation

Actual

Mesh Count

Fabric

Color

Fabric

Thickness

Mesh

Opening.

Open Area

Ink Height

Flow Rate

Pressure

Drop

80S

71m

3.9ST

White

119

247

60%

4.0

.223

3.5%

150S

45m

5.6LE

141

Amber

124

54%

2.5

.126

10.3%

230S

40m

5.0LE

216

Amber

41%

1.5

.102

19.8%

305T

35m

6.1LE

291

Amber

34%

1.0

.070

27.6%

Figure 1.4 This table lists the properties of our four sample screens (details follow in the text). The 80S for example has

a 71 micron thread diameter, was tensioned to 25 Newtons per square centimeter, the ST elongation fabric was taken to

3.9% elongation. The actual count as woven and finished was a 77 average mesh. The cloth was white with a thickness of

119 microns, a mesh opening of 247 microns and an open area of 60%. Its nominal ink height is 0.004 or four mils,

with a flow rate of 0.223 L/cm2/S and a pressure drop of 3.5%. Note that the flow rate and pressure drop can be used

at this stage for comparative purposes. They are used more appropriately after the mesh is tensioned.

PAGE

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Chapter 1

SSppeecciiffiiccaattiioonnss

These are the terms applied to the mesh that you buy. As we described above,

many of them are nominalor simply namesand should not be mistaken

for dimensions or specifications. Depending on the manufacturer, you may be

able to get actual measurementsor even a Certificate of Analysis (C of A) to

specify the measurements on the actual bolt of material.

Mesh count  means the number of threads and openings in a linear inch in

either direction. It is nominal because the fabric is frequently of a different count

than labeled. For example, the 305 mesh may have a weft count equal to 301 and

a warp count equal to 306 so that the stress strain can be balanced. (refer to page

1.16 for explanation). The letter designation next to the nominal count is a rela-

tive diameter of the thread and should be used as a reference only; for real-world

comparisons use the thread diameter data provided in microns instead.

Thread diameter  refers to a nominal thread diameter in microns. You should

realize that this name is based on a cylindrical (unwoven) filament. The actual fila-

ment become ovaled during the weaving process. So, if you realize that the thread

acts to obstruct details and ink passage, then coarse-thread fabrics are even worse

than this data might suggest. Further, there is little change in thread diameter at

any tension level.

N/cm² (Newton per square centimeter)  is a measure of the average ten-

sion of the test screen in either direction, based on the percent elongation to

keep a balanced SS relationship (refer to page 1.15). This tension level was a

result of a percentage of elongation, which allowed us to maintain a square mesh

Mesh Model Part 2

Actual

Mesh

Count

Stress-

Strain

Stress-

Strain

Variance

Cross

Section

Actual

Mesh

Opening

Flatness-

Angle

Open Dot

Tonal-

Range

Alpha-

Angle

Moiré-

Angle

Moiré

Frequency

Ideal Lpi

6.4

< 1.0%

120

272

11.3°

----

84.3°

----

.033

7.6

141

4.1

< 0.5%

145

12.9°

----

19%

79.8°

27.5°

.016

24.9

216

5.0

< 0.5%

106

17.4°

50.5

62%

75.3°

17.5°

.013

54.9

291

3.9

< 0.5%

110

19.8°

37.2

77%

72.0°

12.9°

.012

89.8

Figure 1.5 This is a list of the performance properties of the four sample mesh counts. Row Three shows a 216 mesh

as tensioned. The stress-strain variance was less than 1/2%, which is agreeable. It has a cross section of 106 with an actu-

al open area of 83 microns with a flatness angle of 17.4°. The open dot area for a 65-line halftone is 50.5%, substantial-

ly greater than the 41% open area. Its geometry represents a tonal range of 62% (excluding the stencil) and the optimal

angle of intersection is 75.3°. Symmetry is found at 17.5° to forestall radial moiré and its tendency to cause moiré based on

the 65 Lpi is 0.013. The ideal Lpi for ink transfer and symmetrical balance is 54.9 lines per inch.

PAGE

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Screens

opening as well

as a balance of

strain on both

the warp and

weft.

Percent. elon-

gation  is the

percentage of

elongation for

warp and weft

threads,

required to take

the mesh to a

specified ten-

sion level. If

the fabric is bal-

anced, this per-

centage is the

same in both the warp and weft directions. Otherwise, you compromise the shape

of the opening or the tension between stroke and non-stroke directions (refer to

figure 1.24, on page 1.30 for details.)

Figure 1.6 This is a photomicrograph compliments of Murakami Screen US Inc. It is an

enlargement of a 420 S mesh count. The count should be given in both the warp and weft

directions for two reasons; the mesh count from all manufacturers is nominal, it is rarely a per-

fect 420. Secondly the warp and weft do not always match identically. You want the manufac-

turer to tell you both actual, not published, mesh counts. If these are too different or move from

the target too far, you may have problems with moiré and registration.

a. Virgin-before

weaving.

b. Finished-

after weaving

and finishing.

c. Tensioned-after

stretching.

Figure 1.7 This is a cross sectional illustration

of the changes in the thread diameter from vir-

gin to finished to tensioned. The top illustration

shows the relationship when the threads are cylin-

drical, in a virgin state. The middle illustration

shows the mesh as finished in manufacturing. The

threads have been ovaled, there fore the opening is

smaller. The bottom illustration shows the results

of the mesh once it is tensioned. The opening has

increased in size. Note that the theoretical data

does not show such differences but when used as

intended it is very representative of the relation-

ships between one mesh and another related mesh.

PAGE

. 12

Chapter 1

Low elongation is noted in the same column and indicates the modulus of

the fabricwe refer to it colloquially as low elongation. Note that it is less a

specification than a tendency, and each manufacturers LE properties differ. LE

fabric actually has a higher modulus of elasticity; that is, it develops tension at a

higher rate. This is the only type of fabric that you should use for high-quality

printing. Not only does it get to tension faster, but it retains that tension level

better than traditional (lower modulus) mesh. With low-elongation fabric, the

purpose of retensioning became less of a factor in a high quality print and the

concept of work hardening screen mesh applied to conventional mesh much

more than low elongation (refer to figure 1.8 below for details).

Actual mesh count  is based on the nominal count, and percentage of elon-

gation. For example, if the fabric starts at 230 and is elongated five percent, the

resultant count would be 218.5. As elongation increases, the count continues to

drop. This is critical information when you are aligning images specific to the

mesh in order to eliminate radial moiré or fluctuations in printed-line thickness

(see figure 1.36 on page).

Fabric color  is most often a factor with finer mesh counts (that have dyed fil-

ament), while the coarser counts are typically white (undyed). The colored fila-

ment inhibit light scatter or halation from costing detail in the image. The manu-

Figure 1.8 This is a graph of four

relative possibilities of percentage of

elongation. The vertical axis indicates

tension while the horizontal axis indi-

cates stretch or elongation as a percentage.

From left to right, the first mesh is a low

elongation fabric at a relatively high-ten-

sion level. The next is a low elongation

fabric taken to a low-tension level.

Number three is a fabric taken to high

tension that required a great deal of

stretch. And the last fabric is a high

elongation fabric that was never taken to

high tension. For stable on press geometry

and consistent openings, one or two are

best suited. The level of tension should be

to support the proper off-contact distance

refer to page ___ for details. Graph com-

pliments of Printwear Magazine.

PAGE

. 13

Screens

facturer logic is that the dying allows you a greater window of exposure latitude

at a longer exposure time. Additional manufacturer logic is that no one (so they

think) wants finer detail on lower mesh counts. Be wary of both. Some fabrics

lose their SS balance because theyve been dyed as woven material; and many of

the white fabrics are not production friendly when it comes to detail.

Fabric width (not listed in chart)  may, if changed, also affect your SS balance.

Once you set your standards, do not arbitrarily switch widths without correspon-

ding data on the balance of that width. Also, you want to be sure the manufactur-

er does not cut your cloth from a wider bolt or you will never know what you

have.

IInnkk TTrraannssffeerr

The next six elements deal with the ability of the tensioned mesh to transfer ink

both at a high rate of speed and with minimal obstructions. Note that there is an

ink height but no theoretical ink volume listed. The reason is that the specific vol-

ume is greatly affected by the fabric mass of the garment.

Fabric thickness  is the thickness of the woven cloth. Depending on the

manufacturer it may be a measured (versus computed) number. This aspect has a

primary influence on the ink-deposit height and smoothness of the print. You

want the fabric to be as thin as possible so that it is easier to pump ink through

the tunnel of the mesh, as measured by its thickness. This is most important

when the ink in question is white. White ink is shear thickeningits viscosity

actually increases under shearand does not travel well or far on its own. Thus,

the solution for white ink is as thin a mesh as possible.

Ovaled Thread

Diameter

Mesh Opening

Fabric

Thickness

Figure 1.9 This is a

cross sectional illustra-

tion of the basic geome-

try of the fabric.

Labeled is the ovaled

thread diameters the

mesh opening and the

fabric thickness. Since

the fabric thickness is

based on the initial

(cylindrical) thread

diameter and the crimp

angle, it may wise to

have an actual measure-

ment taken from time to

time.

PAGE

. 14

Chapter 1

Mesh open  is the average dimension from thread to thread prior to tension-

ing. Once the fabric is stretched, the mesh opening becomes enlarged. The open-

ing increases in direct proportion to the elongation, but it is not proportional to

the measured tension level. A larger opening allows larger-particle and higher

tack-level inks to transfer.

Open area  is a percentage of mesh area. As a single entity, it has little rele-

vance to the printer other than the rule of thumb that bigger is better in virtually

all cases. In our context, it allows a comparison to the Open dot area, below.

The open dot area is calculated for the 230 and 305 fabrics only. You may note

from the table that the percentage open area runs nearly parallel to pressure drop

(but it is not a linear relationship).

Nominal ink height  is the nominal ink height in mils, deposited by the ten-

sioned mesh. The substrates, the inks themselves and the press settings also affect

the ink height. The numbers shown are reasonable and can be used for compari-

son, but are not intended for precise mileage calculations. Actual thickness is

quite specific to the ink used.

Flow rate  is expressed in liters per square centimeter per second and is a key

factor in the printing process. It is the rate that a quantity of ink can transfer

through the mesh, and is bounded by the geometry of the tensioned mesh. Our

model is based on a blade speed of 10 inches per second. You want to select a

mesh with as high a flow rate as possible, per actual fabric thickness.

Pressure drop  lists the percentage of change in hydrostatic pressure on the

ink due to the mesh geometry and based on a squeegee interface greater than

1/MC.Graphically, the mesh is shaped like an hourglass, wide open on the top

and on the bottom with a narrow passage in the middle. It is the particular shape

of the hourglass that must be dealt with. Either by using ink that is very thin to

accommodate the hourglass or by better mesh selection. It is preferable to have a

lower pressure-drop number. This number can be used to compare various mesh

Mesh Model Part 2

Actual

Mesh

Count

Stress-

Strain

Stress-

Strain

Variance

Cross

Section

Actual

Mesh

Opening

Flatness-

Angle

Open Dot

Tonal-

Range

Alpha-

Angle

Moiré-

Angle

Moiré

Frequency

Ideal Lpi

6.4

< 1.0%

120

272

11.3°

----

84.3°

----

.033

7.6

141

4.1

< 0.5%

145

12.9°

----

19%

79.8°

27.5°

.016

24.9

216

5.0

< 0.5%

106

17.4°

50.5

62%

75.3°

17.5°

.013

54.9

291

3.9

< 0.5%

110

19.8°

37.2

77%

72.0°

12.9°

.012

89.8

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