SD Warren Bulletin Wet Ink Trapping

7/29/2019 SD Warren Bulletin Wet Ink Trapping

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BULLETIN NO. 4

Wet InkTrapping


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WARRENHow will it print?

This bulletin is issued by S.D. W arrenCompany to aid the graphic artscommun ity in dealing with thecomplexities of the printing process. Information contained hereincombines the findings of scientistsand the observations of exp eriencedcraftspeople.No scientist will claim that existingknowledge is complete, and nosincere craftsperson will pose as afinal authority. The text of this bulletin,therefore, represents merely theconsidered opinions of experiencedand thoug htful analysts.


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WetInkTrapping

You've adjusted ink for the third time and still haven'tgotten your flesh tones. Not only are they too pink,they're uneven, mottled, and splotchy, which seemsimpossible when the color proof looked so good.The same inks are being run on the same paper. Andwith your last try, you even increased the amount ofink. Water balance is O.K., as are cylinder pressureand register.So what's wrong? Are you going to have to make newplates?Perhaps not. Your problem may have nothing to dowith plates or with any of the other conditions thatyou've checked and corrected for. It may simply beinadequate wet ink trap.Most problems with wet ink trapping were solvedwhen we learned that inks must be laid down in propertack sequence if they are to trap, that is, to adhere toone another and produce the colors we want. The inkwith the highest tack goes down first. The others areprinted in descending tack order. The rule wasobvious and easy to follow.But processes and materials change, and today we'veadopted some that simplify certain phases of our workyet complicate others. High speed presses anduniform tack inks are two such innovations. Byincreasing our productivity, they have reintroduced aprobleminadequate we t trapthat hasn't botheredus for years.In this booklet, we'll cover the principles of wet inktrapping. Special emphasis will be placed on therelationships among tack, absorption, and speed;which concern ink, paper, and press respectively.You'll discover how trap is measured, whatprecautions to take in using various kinds of ink, whypaper shouldn't be relied upon to do the whole job oftrapping, and when press speed is a factor to considerin planning a job.Then by following a few simple rules to keep systemsin balance, you can again file wet ink trapping withproblems safe to forget!


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A close lookatwet trap

Figure 1A Figure 1BWHAT IS TRAPPING?

Trapping is the ability of a wet ink film tograsp and hold a second film that is printedover it. It's like putting the second coat ofpaint on a wall. If it doesn't stick, the resultswill be disappointing.Ideally, you can trap one ink with anotheras efficiently as you can trap that ink withpaper. Good multicolor printing depends onit. For unless inks overprint properly andcombine their films in just the rightproportions, your colors will be off.We evaluate or measure trap in terms ofpercentage. A high percentage is "good"because it gives the desired color (Figure1A). A low percentage, w hich gives unevenor off color, is "poor" (Figure 1B).


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Figure 2SOME SAMPLES

Here's what different percentages of trapook like. Both samples were printed withFirst you see 100% trap (Figure 2). Its coloren. The photomicrographic in

lustrating 30 % trap,muddy (Figure 3). And in the"poor" magenta over yellow trap.

Is the problem wet trap?


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Measuringtrap

Figure 4WHAT YO U'LL NEED

How do you determine if it ispoor trap that iscausing your problem? The answer is to runa set of color bars, like the GATF ColorReproduction Guide, then take densitometerreadings, and apply a simple formula.What this procedure actually yields is acomparison, by percentage, of 1) the densityof an ink film when it is printed over andtrapped by another film, and 2) the density ofthat top film printed directly on paper.How well is the ink trapping? Is it trappingas well as paper (100%)? Almost as well(88%)? Or isn't itholding on to much of thesecond color at all (30%)?

MAGENTA CYAN GATF COMPACT COLOR TEST STRIP

Figure 5

HOW TO PROCEEDHere is an example of what a complete setof color bars looks like (Figure 4). It showsthe overprinting of all combinations of colors.Basic bars simply show ink density; theyaren't designed to measure percent of trap(Figure 5). A com plete set, therefore, shouldbe run with every m ulticolor job.Let's now assume that you're concernedwith the trapping ofmagenta on cyan.You would take three readings with adensitometer through the appropriate filterfor the color on top. (In this case the greenfilter for magenta.) (Figure 6): the solid cyan(Figure 7), the solid magenta (Figure 8), andthe overlap of the two (Figure 9).The percentage of trap is then found byapplying this formula (Figures 10, 11, 12):

Figure 6

Density of over lap(Magenta + Cyan) Density of bot tom color(Cyan)

(Figures 10, 11Density of top color(Magenta)

12)

x 100 = % Trap


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Figure 7 Figure 10

Figure 8

MAGENTA1.4

9::;,;::;i;.; ::::, i : ;, ;.: : ^MW- l f ; : i ;vSI ^i l ;

!; l ; i ' ; i

CYAN +MAGENTA

1.3


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Figure 13Thethree keys totrapping

A high percentage of trap requires theright balance of three factors: Tack,Absorption, and Time. Tack, as you'llsee, is primarily a matter of ink.Absorption depends on paper. Time ispress related (Figure 13).It is important to keep these factorsin mind, for when a trapping problemoccurs, it can usually be traced to oneor more of them. In the following pages,we'll look at trapping from these threepoints of view. By understanding theeffects of each, you will be able tomake decisions that can help assuremore satisfactory printing results.


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Tack/Ink

TACK-WHAT IS IT?Tack is the "stickiness" of ink, whichcan range in consistency from molasses tosoft butter (Figure 14). Tack is a measureof the force needed to split an ink filmbetween two rotating rollers at a predetermined speed and temperature. You coulddetermine the tack of honey, for example,by measuring the force needed to pullapart the two pieces of bread in a honeysandwich.

When you buy ink, it is tack rated. That is,all inks are formulated to specific tacklevels, which are indicated on their cansand referred to as "ca n tac k." But whenyou remove ink from the can and begin towork it, tack cha nges. It may increase 2 or3 points by the time it runs through thepress and is applied to paper. The newtack level, at the moment of im pression,is an ink's "effective tack", and it isthis, not "can tack," that you're reallyconcerned with in trapping.


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Figure 15INK MAKE-UP

Why the tack gain? Generally speaking, inare composed of pigments that providecolor, solvents (vehicle) that promotesetting, and resins that hold pigments on paper's surface and give ink its particulardegree of gloss (Figure 15).When the ink comes in contact first with blanket and then with the paper, solventsdrain out of it and resins begin to solidify,making the ink tackier.


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Figure 16TACK SEQUENCE

Tack is a major factor that enables inks toadhere to, or trap, one another; it is tacksequence that enables us to control andachieve quality trapping. In the propersequence, your first color down must havethe highest tack. Subsequent colors havelower effective tacks, with a descending 2-to4-point spread between them. Every ink film,therefore, will trap the one that follows.

SOME EXAMPLESLet's simulate some more p rinting and seehow this works in practice. To keep it simple,we'll use just two colors. In the first example,Cyan (with an effective tack of 14) wasprinted first. Then came Magenta (with aneffective tack of 11). The sequence wascorrect and the trap is 1 00% (Figure 16).In the second e xample, the order waschanged. It was decided to run Cyan last.But then the tack sequen ce is wrong and thetrap is only 40% (Figure 17). If you stillwant your Cyan last, you m ust either run atackier Magenta or a less tacky Cyan. Inexample three, you see the latter and itworks! (Figure 18)It's essential to remember that when youalter your normal color sequence to meetspecial printing objectives, you m ust stillmaintain proper tack sequence .

Figure 17

TACK 11

1 I40% TRAP TACK 14

| |Figure 18

TACK 11 100% TRAP TACK 8


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INK TYPES INK TYPES

CONVENTIONls SKHCK SETTING'" SU pE R S E T T iH f i ' ' UC

Inks are available today in great variety.There are conventional inks, which arerelatively slow setting. These were once thestandard of the printing industry because oftheir simplicity and the consistency of theresults they produced. In recent years, fastesetting inksquick setting, super setting,and acrylicshave come into widespreaduse (Figure 19). They are designed to takeadvantage of the production capabilities of new high speed presses and quick back-up.These different types of ink come in twoforms: graded and uniform.

GRADED TACK INKSThe examples you saw on page 12 wereprinted with graded tack inks.As you saw in Figure 18, it is sometimesnecessary to have an inventory of certaininks with m ore than one tack value.For example:In Figure 16, when cyan wa s printed firs t, atack of 14 was required. When m agenta waprinted first, however, the tack requirementfor the cyan was 9.

UNIFORM TACK INKSUniform tack inks are formulated in sets wita single can tack. Your Magenta, Cyan,Yellow, and Black might all have a tack of12. This benefits the printer by reducinginventory (no longer must you stock eachcolor in a variety of tacks) and ad dingflexibility (since such inks may be run inwhatever sequence you desire) (Figure 21).


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Figure 22But if good trapping depends on proper tacksequence, how can you trap with inks that allhave the same tack. Two of the factorswhich allow uniform tack inks to work are theabsorptivity of the paper and the press whichgives adequate time for the tack to build.

EXAMPLESHere's what happens. Your first ink downmight have a tack of 12 (Figure 22), but bythe time it reaches the second printing unit,its tack has increased to 14 (Figure 23).This has occurred because the ink releasessome of its solvents into the paper, and thetime between impressions allows the ink togain in tack. Now it can easily trap thesecond ink, whose tack is still only 12. Inlike manner the second ink will trap the thirdand the third traps the fourth.It's important to remember, however, thatthe process only works with a paper andpress combination allowing sufficient time forsolvents to drain and tack to build betweenprinting units. When this fa ils, the inks will nottrap properly, as you see in the third example(Figure 24).

Figure 23


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Figure 25INK STABILITY

or w hichinkometer is used to express tack stability

EXAMPLESa set of

18, 16, 14, and 12. The rate of

10 tack units while the other three have

Figure 26

22

20

M 18

14

12 1 2 3 4 5 6 7 8 9 1TIME (m inu tes)22

20

M 18

K14

12

^

^ *

/

-r^ t *

Xw^^ . L --

- I^

^

^

^ ^

^ *

1 2 3 4 5 6 7 8 9 1TIME (m inu tes)


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Absorption / Paper

PAPER STRUCTURE?We've seen that trapping often requiressolvents to be absorbed from the ink so that itcan build tack. Absorption is primarily the taskof paper. How quickly it performs this task, orits degree of absorption, is a function of itssurface and structure.You see here photomicrographs of thecoating structure of two different papers.(These cross-sections a re shown at a 4 5angle, necessitated by equipment used totake photom icrographs. All that is visible

is coating structure. No cellulose fiberappears.) The first, which is more ope n, hasa higher degree of absorption becausesolvents can easily drain through its pores(Figure 27A). The s econd, with a closedstructure, will provide better ink hold-outbut also slower tack build, for solvents mustflow around its flat, closely packed platelets(Figure 27B). The balance between hold-outand absorption is difficult to achieve, whichis why papers differ considerably in theirmake-up.

The more absorbent the paper the greatethe increase in tack. Recognition of thisinteraction enables the printer to makeproper ink adjustments.


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28A Figure 28B

UNIFORM TACK INK EXAMPLES

ic ure 29A Figure 29B

GRADED TACK INKk A

^ r OPEN \v / CLOSED ^ B

You seehere twoprints that demonstratehow trap isaffected bydifferentcombinations of inks and papers. First,uniform tack inks are run on the two papersone open and one closed.With the open sheet, uniform tack inks trapsatisfactorily (Figure 28A). But on the closesheet, which does notpermit rapid drainageof solvents, thepercentage of trap is lowand you get an off-color, mottled print(Figure 28B).Consistently good results on any paperare easier toachieve with graded inks. Asyou see here (Figures 29A & B), trap isequally good on both the open and closedpapers. Maintain proper tack sequence, usinks with good stability, and success ispractically assured.


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Time/Press

PRESS TYPE STime is a function of press speed, whichhas increased steadily over the years sothat today some sheet-fed presses can runat an average of 10,000 impressions perhour. While this yields tremendou s production, it can also cause trouble, particularlywith trapping. For now there is less dwelltime (time between printing units) for tackto build.These are the two most commonconfigurations of mo dern m ulti-colorpresses (Figures 30 & 31). The first is anin-line or unitized press. The second, acommon impression cylinder press. On aunitized press, paper travels from unit tounit where e ach color is applie d Althoughthe interval between units is very short, itdoes provide adequate dwell time for mostinks and papers to build tack for trapping.The common impression cylinder press,with two printing units working off oneimpression cylinder, provides less dwelltime. It is estimated to be 1/8 of a secondas opposed to 1 second on a unitizedpress. (These calculations are based on apress speed of 7,600 iph.)

Figure 30


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Figure 32B

GRADED TACK INKk Ar

NITIZEDI COM MO N IMPRESSION

^

EXAMPLES

33A Figure 33B

Observe how quality of trap is affectedby the dwell time that these two pressesprovidefirst using graded inks, which arein proper tack seq uence; and second usinguniform tack inks, which must have time torelease solvents before gaining the tackneeded to trap. The same paper is usedfor all prints.With graded inks, both prints aresatisfactoryon the un itized press an dthe common impression cylinder press(Figure 32A&B) .But with uniform tack inks, in this case,you would get an acceptable print onlyon the unitized press. On the co mmo nimpression cylinder press, there simplywas n't sufficient time for the tack tobuild (Figure 33A&B) .If we had used a different paper, the resultsmight have been different. For on an opensheet, uniform tack inks could have trappedeven with the short dw ell time afforded bythe comm on impression press. The paperwould have absorbed the inks' solventsrapidly enough for tack to build. M atchingpaper with uniform inks can be difficult andlimiting.


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Summary

Satisfactory wet ink trap isn't difficult toachieve if you simply run your inks in theproper tack sequence. Start with the ink thathas the highest tack and keep a 3- to 5-pointspread between colors.And remember that it's the effective tack ofan ink film that counts, for tack in the canjust isn't the same as tack at the time ofimpression.Achieving this proper effective tacksequence involves three factors: Tack,Absorption, and Time.

TACK ABSORPTIONThe main determinant of tack is ink. Withgraded tack inks, tack value is built in. Butwith uniform tack inks, it must be built upand, consequently, is as much a function ofpaper absorption and press time as it is ofthe ink itself. Trapping problems often occurwhen it is assumed that uniform tack inkswill trap "autom atically" and the paper andpress influences are ignored.It's well to remember also that tack can beaffected by temperature, fountain solution,and ink film thickness.

Absorption is mainly a function of paper. surface and overall structure permit thesolvents to drain from an ink, allowing thetack building process to begin.But paper has additional jobs to do, suchproviding a smooth, tight surface for highhold-out. Graded tack inks trap well on awide range of papers with varied absorptcharacteristics. Uniform tack inks, howevwhich depend on rapid drainage in order develop tack, usually require a paper withsurface that is more open.Note that blankets play a role here, too; aas they vary in the rate at which they abssolvents during a run, each will affect thetrapping process differently.


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TIMETime is a press factor. With increasingspeeds, particularly on comm on impressioncylinder presses, there is often inadequatetime for uniform tack inks to build sufficienttack for good trapping. These inks are runmore successfully on a unitized press, whichprovide a full second or more of dw ell time incontrast with the 1/8th second provided bythe common impression cylinder press.The time factor can explain why theproduction run often fails to m atch thequality of proof sheets, even when run withthe same inks on the same paper. If the typeof press or the speed at which it is runvaries, so might your results.

Conclusion

When your colors are off, mo ttled, orsplotchy, you may be looking at a problemcaused by poor wet trap. How can you knowfor sure? Always run a complete set ofcolor bars on your press sheet. Then, byfollowing the procedures outlined in thisbooklet, you can even determine percentof trap.To help insure against poor wet trap, us egraded tack inks. Because they provideproper tack sequence for a high percentof trap, they minimize the influence of allthe other factors w e've discussed, such asvariation in paper and presses.You've seen that absorption qualitiesvary from paper to paper (and frommanufacturer to manufacturer). And thatdwell time varies with the type of press youhave and the speed at which it's run. Butwith stable graded tack inks and a clearunderstanding of the principles of wet inktrapping, every printer can p roduce job afterjob that is a source of pride to himself andto his customers.


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Cover printed onLUSTRO GLOSS COVER,basis 20 x2 6 100/200MText pages printed onLUSTRO GLOSS,basis 25 x 3880/160M 1988, S.D. Warren CompanyA Subsidiary of Scott Paper Company.


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S.D.WARRENCOMPANY, A SUBSIDIARY OF SCOTT PAPER COMPANY, BOSTON, MASSACHUSETTS

SD Warren Bulletin Wet Ink Trapping

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