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SCREEN PRINTING

Graphics, Textile andIndustrial applications

by Dawn Hohl, SPTF R&D Engineer/Screen Printing

O f the many press variables in the screenprinting process, off-contact is one of themost important to standardize and control.

Print quality and registration problems can, inmany cases, be traced to off-contact or someother variable poorly set-up on press to com-pensate for inappropriate off-contact.

Control requires a good understanding ofthe variable you want to control, and how itaffects your print results. Next, a method ofmeasurement must be available; and third,sound practices and standards must be imple-mented. This article will deal with each ofthese issues to help you print better, faster, andmore consistently.

What Is Off-Contact?

Off-contact distance, also called snap-off dis-tance, is the small distance between the bottom

of the screen and the substrate being printed.This small space is necessary during printing tokeep the screen from adhering to the substrate.Adhesion can cause severe separation problemsthat damage print quality. Off-contact distanceeliminates this problem by allowing the mesh tocome in contact with the substrate only at thesqueegee edge during the print stroke. Oncethe squeegee passes the area, there is a zone ofpeel where the mesh and substrate separate,leaving a crisp print behind.

It is important to clearly define peel-off, aswell. Some presses have this option — and it isdifferent from off-contact.

Peel-off refers to a separate upward move-ment of the frame that happens gradually dur-ing the print stroke. It starts at the end wherethe stroke is started, and only occurs when thesqueegee is in motion during the printing cycle.

Controlling Off-Contact

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As the name implies, it helps with theseparation (or peel) of the mesh fromthe substrate. If the press is set to oper-ate this way, the peel distance changesduring the print stroke… in addition tothe off-contact distance.

Off-contact distances range from“near contact” to as much as 3/8 or 1/2inch (9.5 to 12.7 mm) in someinstances; but ranges from 1/16 to 1/8inch (1.5 to 3.0 mm) are more commonand acceptable. Excessive off-contactswill cause a number of problems inprint quality and registration. In mostsituations, the best off-contact is the low-est off-contact distance possible. Whenprinting products with tight tolerances,the allowable amount of screen distortionwill dictate the off-contact distance. Inelectronics printing, stainless steel meshis often used making it possible to usevery low off-contact distances — like 6–8mils (0.1 to 0.2 mm).

Why Measure Off-Contact?

Whether you are printing graphic ortextile products, off-contact is one of

the critical press parameters thatdeserves attention. Incorrect or incon-sistent off-contact distances can causemyriad problems that end up costingyou money.

We’ve touched on some of theseproblems before, but they warrant acloser look.

Registration

In multi color and four-colorprocess printing, registration must bemaintained. Off-contact plays a vitalrole in consistent print registration.By definition, it is understood that off-contact printing cannot produce atrue 1:1 reproduction of the originalimage. This is due to the deflectionand elasticity of the fabric during theprint stroke. The higher the off-con-tact the more the squeegee elongatesthe mesh (Figure 1), resulting in a dis-placement of the image on the sub-strate. Peel-off further compoundsthis problem. To minimize this distor-tion, the lowest possible off-contactdistance should be used. Recom-

mended off-contact is 1/16" (1.5 mm)for tight tolerance printing, but nomore than 1/8" (3.0 mm).

As off-contact distance increases,several things happen. First, thesqueegee pressure must be progressivelyincreased to create contact between themesh and substrate for ink transfer. Theextra force exerted on the screen is mag-nified at the edges of the squeegeeblade. Longer squeegees, relative to theframe size, will make this effect evenmore problematic (Figure 2). The resultis not only increased image distortion atthe edges but poor print quality, as well.

Second, the integrity of the imagegets more skewed and elongated. Thiscondition can be attributed partly tothe effect just mentioned; but it alsowill vary with image placement,squeegee length, and stroke direction.The size and locations of these irregu-larities are not defined and can be diffi-cult to predict and manage.

Substrate shrinkage and expansioncan also be a primary part of the problem.

Overall Effect of Off-Contact Printing

Image Shift

squeegee shift

minimal to no image shift

Image Shift

Off Contact

An elongated print is the final result. The greater the off-contact —the greater the image shift.

FIGURE 1

Registration Shift with Off-contact

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Off-contact uniformity is an issue. Ifit is not the same front-to-back or side-to-side, registration will be off in a local-ized area. In such a situation, when thenext color is put down, it is highlyunlikely to register with the first — nomatter what you try. If this registrationproblem becomes aggravated, the wholejob could be lost.

It is worth mentioning that somedistortion can be compensated for inthe artwork with calculations, oftendone in electronic printing. In addi-tion, some equipment manufacturers

offer an optional compensating featureon certain types of presses to deal withsuch problems.

Print Quality

Incorrect off-contact distance canhave adverse effects on print quality inthat it can require squeegee pressurechanges. Examples of problems it cancreate include: poor reproduction accu-racy, smearing, bubbles and voids, dou-ble images, ink strings, loss of detail,blocked edges in image, mesh marksand incomplete images.

Several things occur with high off-contact conditions to create theseproblems. High squeegee pressuremust be used to force the screen intocontact with the substrate, and thesehigh squeegee pressures push ink tothe print side of the screen beforeprinting. Heavy amounts of ink willruin image detail. Excessive squeegeeforce will also crush the image, creat-ing smearing and dot gain.

With high off-contact, the screenmay snap back too quickly, not allow-ing ink to fully transfer to the substrate.

Regular Squeegee Length

Long Squeegee Length

High Stress Areas:both squeegee and mesh are distorted

HighOff-Contact

HighOff-Contact

High Squeegee Pressure

High Squeegee Pressure

More stress is present in these areas resulting in excessive squeegee and mesh distortion

FIGURE 2

Effect of Excess Force on Ends of Squeegee

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Resulting print defects can be meshmarks, incomplete images, ink strings,bubbles, voids, and blocked edges inimage.

Yet another cause of poor imagequality results from low tensionedscreens printed with low off-contactdistances. In this case, the squeegee

produces a ripple in themesh during printing…throwing off registrationas well as causing suchprint quality issues asdouble images.

Ink Deposit/Color

Ink deposit inconsis-tencies can develop fromseveral things: (1) Poorparallelism between thescreen and the platen.(2) Unbalanced levels ofoff-contact, squeegee

pressure and tension. (3) Squeegeedistortion resulting from high off-con-tact and squeegee pressure causing theedge area of the print to distort andhave more ink deposited. Each ofthese can cause a non-uniform depositthat will show up as unacceptablecolor variation.

A low off-contact distance will alsoproduce a uniform deposit, whereas ahigh distance results in an unevencoating from uneven pressure along the

squeegee blade. The high pressure atthe squeegee ends caused by theupward force of fabric tension createsnegative contact pressure in the centerof the blade, producing poor deposituniformity.

Mesh, Stencil, and Squeegee Durability

Screen printing mesh, stencils andsqueegees are resilient by nature —and must be — to work in the process.These properties can be overextendedand lost with excessive repetitiveforce, ultimately leading to short lifeand even breakdown. High off-con-tact distances necessitate highsqueegee pressures, creating suchforces. Durability of the mesh, stencil,and squeegee will be compromisedunder these conditions.

Production Speeds

High off-contact will slow produc-tion speeds. Instead of having thesqueegee’s transfer force directed toprinting the ink, it must be used toovercome the screen tension to makecontact with the substrate duringtransfer. Diverting the force in thismanner makes for inefficient printing.In the case of manual printing, theoperator must exert more force toprint… leading to fatigue and workrelated injuries.

Off-Contact on Textiles

There are some specific issues intextile printing when discussing off-contact. Due to the absorbent natureof textile substrates, off-contact dis-tances are generally lower on textilesthan in graphic applications.

Optimum distances are between1/32" to 1/16" (0.8 mm to 1.6 mm) fortight tolerance work, but again, will besomewhat dependent on screen ten-sion, screen size, and ink being print-ed. Lower off-contacts on textiles alsominimize the ink build up on the bot-tom of the screens.

Since textiles are printed with multi-ple platens and print heads, consistencyin off-contact between all componentsis a key to getting good registration andink deposit (color) uniformity. Textilejobs sometimes call for several types ofgarments with the same image. Thethicknesses of these garments change,so adjustments to off-contact must bemade accordingly.

How to Minimize Off-Contact

Screen tension should be the firstarea improved in the pursuit of loweroff-contact distances. Higher tensionson your screens will allow the off-con-tact to be reduced and printing speedsto be increased. There are, of course,limits on how high operating tensions

Control and

standardization

are absolutely

essential

to achieve

the excellence

customers are

now demanding.

FIGURE 3

Mechanical Off-contact Gauge SPTF Electronic Off-contact Gauge

FIGURE 4

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can get, and there are other issueswith high tensions that are out of thescope of this article. But the bottomline is that screen tensions should bethe maximum level for your capabilityand printing application.

Keep in mind that larger screens aremore difficult to stretch to higher ten-sions; so consider both screen size andtension when establishing proper off-contact distances. As a general recom-mendation, a tension between 18-25N/cm should be selected. Just increasingscreen tension levels can still leavesome problems unresolved. Tensionsmust be consistent screen to screen, aswell. In fact, such consistency is moreimportant than tension value. Then,not only is registration maintainedcolor to color, but relative printing forcealso remains the same.

After tension is dealt with, theother area to be looked at is the ink.Some inks are simply tackier than oth-ers. Ink tack is a characteristic thatresults from such factors as ink rheolo-gy, ink ingredients, pigment size andload, surface tension and age.Modifying the ink, or finding anotherink that prints better, are options forreducing ink tack. Lower ink tack willreduce the needed off-contact distanceof a well-tensioned screen.

The geometry of mesh is also anissue in ink transfer capacity. A meshwith lower surface area (for examplemesh with 27-31 micron threads) willneed less force to separate from the inkand substrate than a mesh with moresurface area. The less force that isneeded, the lower the off-contact dis-tance can be.

The substrate has a lot of influenceon ink transfer and peel, too. Absorbentsubstrates will have a much easierrelease than non-porous ones. Textilesubstrates are very absorbent and haveno trouble peeling from the mesh. Thisis why off-contact distances are smallerfor textiles than many graphic applica-tions. Substrate surface tensions, asthey relate to the ink surface energy,also will have varying effects on peel.This area will soon be investigatedusing SPTF’s Screen Printing TackTester.

Other variables that can influenceoff-contact include frame size, squeegeelength in relation to frame size, imagearea in relation to frame size, and imagetype (large spot color areas and reverseimages may require more off-contactdue to increased surface area in contactwith one substrate).

While some of these variables arepossible to change, others are not.

Any given job has its own unique setof conditions under which it must beprinted. It is important to realize thatoff-contact may have to be adjustedsomewhat for each job to compensatefor different variables. Success in min-imizing these adjustments to anacceptable range will occur as eacharea of the process is adjusted to pro-duce predictable results through stan-dardizing procedures and testingincoming materials.

Measurement

The potential printing problems wehave just examined plead their owncase. Control and standardization areabsolutely essential to achieve the excel-lence customers are now demanding.

Current Measurement Methods

There are two ways to create andregulate off-contact. Many automatic,and some manual, screen printingpresses incorporate some adjustmentto vary off-contact distance. Thetrouble is that many of these adjust-ments do not have good precision andonly the higher end presses actuallyinclude a readable gauge. Those press-es without a specific feature to set off-contact distance require the operatorto shim the screen frame to varysnap-off.

FIGURE 5a FIGURE 5b

In the zero mode, the instrument measures the substrate (Figure5a), or in the case of a textile situation, the shirt and the neoprene (Figure 5b).

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A number of methods and trickshave been used by screen printers tomeasure and control actual off-contactdistance. One of the simplest methodsrelies on set-up spacers, like washers orquarters. These spacers are placed onthe four corners of the platen and thescreen moved to just touch each one.Different spacers are used to create vari-ous off-contact distances. Calibratedwedges can also be made for this pur-pose, and provide a greater degree ofaccuracy and set-up versatility.Another simple method is to use astraight ruler placed against the bottomof the screen frame to visually gauge thedistance from it to the top of the platenor vacuum table.

Should you be fortunate enough tohave a press with an actual readout totrack off-contact distance, this is anobvious method of measurement.However, as stated before, poor preci-sion may be an issue (more on that inthe next section). A final optionavailable to the screen printer is amechanical off-contact gauge suspend-ed over the screen by some means(Figure 3). The gauge is zeroed on themesh surface, and then depressed untilthe screen contacts the substrate. Thegauge is read at this point, and themesh/stencil thickness is subtracted toget the actual off-contact distance atthat spot. Measurements from thissystem are fairly accurate and repeat-able when used correctly. The down-sides to these devices are that thegauge must be reset every reading, andthe gauge has limited movementaround the screen. It is also somewhatawkward and difficult to use on somepresses. Despite these inconveniences,they have been the only stand-alonedevices available to the industry up tothis point.

Electronic Off-Contact Gauge

The Screen Printing TechnicalFoundation (SPTF) has introduced anew concept and device for measuringoff-contact more effectively. Instead ofmeasuring mechanically as has beenpracticed, an electronic gauge (Figure 4)has been developed to allow quick andaccurate measurements anywhere onthe screen. The gauge is easy to use,making it appropriate for the press oper-ator, and readily shows off-contact and

FIGURE 6a

FIGURE 6b

Off-contact can then be measured by setting the probe inside the screen (Figures 6a and 6b).

level inconsistencies during press set-up. The new electronic gauge works on a

principle of eddy current, and has a cus-tomized probe design to accommodate alarger measuring range than many ofthe currently available electronic thick-ness gauges. One limitation is that itrequires an aluminum platen. In thecase of textile presses, an aluminumplaten that has a layer of neoprene ontop will work as well.

To read off-contact with the SPTFgauge, a substrate is first placed on the

vacuum table/platen. In the zeromode, the instrument measures thesubstrate (Figure5a), or in the case oftextiles, the shirt and the neoprene(Figure 5b). This reading is thenstored in the device. Off-contact canthen be measured by setting the probeinside the screen (Figures 6a and 6b).The instrument takes this reading andsubtracts the stored substrate/neoprenethickness to correctly reflect actualoff-contact distance (except for themesh/stencil thickness). The probe

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can be moved anywhere on the screeneasily, and measurements takeninstantly.

Testing at SPTF has proven this newdevice to be very revealing during pressset-up. Several presses were measuredusing the electronic gauge, and readingsshowed significant differences in front-to-back and side-to-side off-contactthat were not evident from the press’soff-contact gauge.

In addition, some had such looseadjustment precision that, even whentheir off-contact gauge indicated thesame point as a previous set-up, theactual distance was quite different.Therefore, it is possible that even if apress has an off-contact gauge it maynot reflect reality.

SPTF Research on Off-Contact

In the past year SPTF completed a

limited study of the relationships of ten-sion, off-contact, and squeegee pressureon ink deposit. Using the powerfultechnique of experimental design, theexperiment raised some interestingquestions. Unfortunately, due to itslimited nature, few solid conclusionswere gained. A more comprehensivestudy has been designed and may beconducted in the future. It is hopedthat the new study will offer a greaterunderstanding of the interactions ofthese three variables.

A Final Word

It is important to realize that screenprinting variables are often extremelyinter-related, and that problems withone translates into problems with all ofthem. For this reason, press problems areoften difficult to accurately diagnose andremedy. All the more reason to elimi-nate the problems before they happenthrough standardization and control.While this takes time, the payoff of lesstime-consuming problems on press willmore than compensate for the difference.

For more information on SPTF’s newelectronic off contact gauge, see the adon page 4 of this Journal.

Bibliography

“Off-Contact & Peel-Off:Understanding the Influence toImprove Print Quality,” Mike Young,SGIA Technical Guidebook.

“Understanding the On-PressDistortion Factors,” Mike Young,SGIA Technical Guidebook.

“Off-Contact & Squeegee Pressure,”Tamas Frecska, Screen Printing,February 1987.

“Problems & Ideas: Tension, Distance,Force and Other Things,” TamasFrecska, Screen Printing, January 1994.

“Image Distortion: Diagnosing thePrinted Image Part I,” Tamas Frecska,SITE, April 1987.

“Force Dynamics of Screen Printing,”Mark Coudray, Impressions, February1995.

“Printing with Plastisol Part II,” GlennShull, Printwear Magazine, June1994. ■

Procedures to Implement

What practices and procedures can you implement to increase your control of off-contactdistance? Here are some to get you started.

1. Set up tension level standards for your plant. Using LE mesh, higher tensions canbe maintained on all screens. This can only be accomplished if standard tensioning pro-cedures using a tensiometer are used and screens are tracked. At the very least, makesure that your screens used for any job all have the same tension.

2. Check your frames for warpage. Any irregularities on the screen plane will bereflected in your print.

3. Re-level your presses. It will be difficult, if not impossible, to achieve good parallelismin off-contact if the press itself is out of whack.

4. When working with textile presses, make sure the print surfaces of all thescreens and platens are set to the same plane or height (not to be confused withlevel). Also be aware that platens warp and bow from the heat of flashing.

5. Buy an instrument to measure off-contact on press, and train your press operatorsto use it correctly.

6. Design a procedure for setting up off-contact on each press (it may vary with presstype) that is easy and effective. Remember that the gap you need to measure isbetween the mesh and the top of the substrate. Change substrates, and this distancewill change.

7. Measure off-contact distance in all four corners of a screen and ensure that allfour readings are close to each other. How close they need to be will depend on thedifficulty of the printing application. This will ensure front-to-back and side-to-side uni-formity, creating a good parallelism between the screen and the print bed.

8. Do some simple testing to determine the lower limits of off-contact you can achieveunder your printing conditions. Begin to establish a standard off-contact for printingbased on the data you gather from testing and production runs. To minimize registra-tion problems, make it a policy that, during a multi color job, the off-contact distance isthe last thing to be changed.

9. Document what you do. Keep a running record of the off-contact distances used oneach job that is printed. An easy way to do this is to keep a chart by each press. Then,look at and use the data. Measurements mean nothing if you do not use the informa-tion to learn about your process. Another important place to reference the off-contactis on the actual job or engineering sheet.

10. Understand that other press variables changed will have an effect on ink trans-fer as well and are inter-related to the off-contact distance. A great example of this issqueegee pressure, which ranks with off-contact as one of the variables most oftenchanged on press. While a change in squeegee pressure will not affect off-contact dis-tance, a change in off-contact is likely to require a change in squeegee pressure. Thisrelationship dictates that off-contact be established before final squeegee pressure is set.These two variables are so linked that optimum setting exists for each, in relation to theother, where the quality of the printed product is at its maximum.