Variables. - Garment-screen-printing... by Professor Samuel Hoff

Transcription

1 - Garment-screen-printing... Variables by Professor Samuel Hoff rnagine yourself reading yet another article in your favorite garmentprinting magazine. You rnust, declares that month s expert, control the variables. Good advice, yes, but then comes that creeping realization that, if asked to do so, you might find it difficult to even sit down and list those variables. Perhaps you even scratch your head and wonder (not out loud, of course) what exactly is a variable, anyway? Wonder no longer... Garment screen printing is often spoken of as a simple process: a visual image is duplicated via forcing ink through an image-bearing screen by means of a squeegee onto a substrate - in this case a garment - laid over a flat surface or platen, after which the ink is dried and/or cured. A screen-print variable is (again, simply) any component of the above process that may vary or change. Successful screen printing involves preventing or limiting such changes, establishing what we call control of the variables. And that s where we begin to appreciate the complexities of this simple business. I ve come up with 49 possible variables, and present them roughly in the order you ll meet them in production. The image The first three concern the positive image carrier, usually film or vellum paper generated via camera or computer (although other methods of producing this positive image have recently emerged; see Printwear, April 93, Page 16). #1 Image Density concerns the dark positive image residing on the clear or translucent base. It must successfully block any burn-through of light during stencil exposure. This is D-Max, or the maximum density of the film, and may vary due to incorrect camera exposure, changes in photochemistry, the quality (or lack of same) of available laser-imaging equipment, etc. #2 Film-base density describes how easily light passes through non-imaged areas to expose and harden the stencil. Less dense (optically clear) carriers block less light, and therefore possess the minimum density (D-Min) necessary to optimize variables #s 22 & 23. #3 Image resolution refers to the quality or sharpness of image edges on the film, whether produced electronically or on camera. This, along with both stencil-system and exposure quality help achieve optimal results in #22. The ink #4 Pigment grind. The size of particles added to color the ink vary. A coarse or fine grind can contribute to the net result in #s 6 & 7. #5 Pigment dispersal describes the degree (usually expressed in percentages) to which pigments have been dispersed in inks, controlling color intensity and opacity. Over pigmentation can degrade abrasion resistance and washfastness. #6 Viscosity - often described with terms like creamy or peanut butter - refers to the ink s degree of fluidity or internal resistance to flow. It may vary from high (stiff, hard to stir) to low (thin, even runny). #7 Flow characteristics refer to the degree viscosity changes (ordinarily, from high to low) when ink is sheared (agitated), especially by the squeegee during the print stroke. Numbers 6 and 7 determine the ink s relative ability (or lack of same) to flow through mesh openings 16 PRINTWEAR MAGAZINE JULY 1993

2 , The image: No matter how carefully other variables are controlled, the print on the garment won't be any better than the art with which you start. The ink: Each of the 49 variables, in one way or another, ultimately has its way with the component that goes out the door as product. JULY 1993 PRINTWEAR MAGAZINE 17

3 and then form a smooth, uniform ink deposit on the substrate. Both are preset at manufacture, but viscosity may be adjusted through the use of additives. #8 Tack indicates the degree of stickiness when wet, and is aggravated by heat (as in flash curing). Excessive tack contributes to pickup on successive screens and may loosen the garment from the platen, causing spoilage through misregistration. #9 Adhesiveproperties of a given ink, when dry, must be compatible with variables inherent within its substrate destination. An ink designed to adhere well to a cotton knit, for example, may peel easily from a nylon jacket. (See #'s 36 & 37.) #10 Dry/cure rates among textile inks range widely. Some plastisols heat-cure in seconds, while water-based inks must both dry and cure in a lengthier heat process or (less common) air-dry over a period of hours or days. (See #'s 45 & 46.) The screen The above image is transferred to this ink-metering image carrier, composed of an image stencil mounted on the familiar mesh and frame. Underscoring the screen's key role in the process are its sixteen variables. #11 Mesh fiber composition (originally silk and later, organdy, nylon and polyester) determines mesh strength, moisture resistance and elasticity. #12 Thread structure is either multifilament, where multiple strands are wound together, or monofilament (single-strand). These first two are excellent examples, by the way, of one way to control variables - by eliminating them. Garment printers have, for the most part, adopted monofilament polyester as an industry standard. #13 Thread diameter determines mesh thickness in cross section (slightly less than twice the diameter). Today's monofilament meshes usually come in three types - a thin S, a medium T and HD or heavy duty thread - but are often listed more precisely today with actual diameter measurements. #14 Mesh count denotes the number of threads per inch in the mesh weave. Together #'s 13 and 14 combine to control our next variable. The screen: These extremes - static wood and retensionable metal - illustrate the range of choices within but one of the sixteen variables that control the preparation of this printing process namesake. JULY 1993 PRINTWEAR MAGAZINE 19

4 IULY 1993 PRINTWEAR MAGAZINE 21 #15 The mesh opening is the space between mesh threads through which ink may pass. Relatively small openings restrict ink flow (a problem, for instance, with large-particled metallic inks) and vice versa. #16 Weave structure may be plain, where each thread goes under one cross-running thread and over the next or (in some high-count meshes) twill, following an under-one-over-two pattern, a distinction important in fine-line and halftone work, because twill weave tends to interfere with image edge definition in the print. #17 Mesh color. White (natural) polyester fibers tend to transmit or bounce light along their lengths during exposure. Meshes colored with light-opaque yellow, orange or red dyes reduce such light scatter (known as halation) and resulting image undercutting, yet require slightly lengthier exposures to optimize # s 22 & 23. #18 Screen tension. This most writtenabout variable needs no further introduction except to say that the printer must control not only the amount of tension, but also its consistency, both within each screen and between screens for a particular design. #19 Frame stability must be sufficient to maintain #18. Frame materials must be strong enough to resist side deflection, a cause of uneven tension, and warpage, a cause of uneven ink deposit. #20 Mesh Preparation. An oft-neglected variable, the degree to which mesh is properly abraded and degreased directly affects stencil adhesion. Poor mesh prep, or lack of it, seriously shortens stencil life expectancy (see #23). #21 Stencil thickness (along with #13) determines the profiie or height of the ink column (a factor in ink-film thickness). Printers control emulsion thickness with a variety of hand-coating practices or select from among various thicknesses of premanufactured stencil films. #22 Stencil resolution, definition and sensitivity are inherent characteristics of the emulsion or film product itself (which is why I don t treat them separately) determining its capacity for fineness of detail, edge definition and its optimum exposure requirements, respectively. #23 Stencil durability. Stencils vary widely in the maximum number of impressions they can deliver before wear or breakdown cause image loss. Paper stencils, for instance, may render as few as a dozen prints, while direct emulsions can endure in excess of 100,000. #24 Stencil moisture content must be minimized in photostencils by thorough drying prior to and after stencil application. Moisture can drastically reduce optimum performance in # s 22 & 23. #25 Exposure intensity varies according to the type of exposure lamp selected (fluorescent, mercury vapor, metal halide, etc.). Each emit UV-light at frequencies and output capacities significantly different than the others. Complicating matters, output in any lamp degrades with dge. The light integrator clnd exposure calculator are typical means to monitor and correct for lamp variables. #26 Exposure distance between lamp and emulsion must be balanced between competing needs for optimum results: if increased, image undercut is reduced (assuming a point light source), but so is the lamp s effective power, prolonging exposure time; if decreased, vice-versa. #27 Stencil processing: the temperature and velocity of the water used to develop an exposed photoemulsion must be controlled. If either is too high, Stege door: The crux of the screen print drama is played out on this microscopic stage, where microns of change in mesh and stencil make major differences in the final print. washout may be speedier, but stencil edge definition can degrade. The Squeegee The five variables associated with the print stroke are the most recognizable and most directly affect print quality. (For more on the squeegee, see also Glenn Shull s comments in this month s On Target, page XX.) #28 Blade durometer or hardness and #29 Blade shape both affect the amount of ink deposit and the sharpness of the images printed. Sharper, harder blades reduce smearing, dot gain and other problems, especially in fine-lined or halftoned art, while soft, round blades improve coverage in open area,. #30 Blade angle helps control the gross amount and velocity of ink delivered to the mesh openings. Held near vertically, ink is cut clean, most is scraped away over the openings. A low angle produces a V -shaped funnel in which ink gains velocity as it is introduced into the mesh opening. #31 Stroke speed determines the amount of time the ink will have to flow over (flood stroke) or through (print stroke) the mesh/stencil openings. #32 Squeegee pressure (downward) and screen tension operate as opposing

5 99 vou are, J raking care of #I is the top priority it Outdoor Cap. Our commitment s to provide the highest level of :ustomer service along with a full ine of competetively priced, high luality headwear to our customers. OUTDOOR P.O. BOX210 BENTONVILLE, AR ORDERS PHONED IN BY 2:OO PM CST WILL BE SHIPPED THE SAME DAY forces that determine ink placement. For example, high tension with low pressure tends to lay ink on the garment surface. Lower tension or greater pressure has the effect of driving the ink into the garment fabric. The downside, of course, is that low tensiodhigh pressure creates mesh elongation resulting in image distortion. #33 Flood bar edge shape, #34 Flood bar stroke speed and #35 Flood bar pressure control ink flow over the image area when this separate tool is employed to account for the different settings necessary in # s to accomplish flood coat action on automated presses. The substrate Some of the most significant variables are those associated with the garment itself: #36 Surface texture varies widely in garments, and places limits on the degree of image detail that can be achieved with the ink deposit. As with screen mesh, the garment s thread count, composition and method of weave or knit determine texture. #37 Substrate porosity determines garment capacity for ink absorption (often a key factor in #9). Cotton fibers have high porosity, while nylon and polyester resist absorption. Knits tend to be more porous than wovens. #38 Garment color varies widely and has significant impact on other variables: Dark colors force changes in printing techniques, such as the use of underbasing to prevent show-through, Moreover, dark dyes may migrate when heated, requiring the use of low-bleed inks. Dark and light colors also absorb radiant heat at different rates. This and #39 may dictate changes in # s 45 & 46. #39 Garment moisture content, especially in heavy knits, can prolong or even prevent cure by keeping inks cool until water boils off. (Effects are similar to those described in last month s discussion of water-based curing.) The platen This component keeps the substrate in proper alignment with the screen, a relationship controlled by a number of on-press mechanical adjustments. #40 Off-contact distance prevents The squeegee: It s in-screen variables - angle, pressure, speed, hardness and shape - adjust the quantity and velocity of ink delivered to the substrate as well as its penetration of the substrate surface. stencil and substrate contact except at the point at which the squeegee edge forces them to meet. Benefits include reduced smear and improved cut or shear of the ink as the squeegee passes, but involve a trade off for the image elongation involved in screen deflection. Off-contact, while critical, should therefore be kept to a minimum to avoid appreciable image distortion and misregistration. #41 Press bed evenness. The printing platen (or in the case of belt printing, the belt) must be flat to ensure consistent squeegee pressure within the image area. #42 Platen-to-screen parallelism must be maintained, that is, both screen and platen must be level left-to-right and front-to-back before off-contact distance will remain constant at any point between screen and platen surface. #43 Platen-to-platen plane. For multi-color work, all platens must be set at the same elevation, that is, with any given screen, all platens must show identical off-contact. #44 Color sequence is a critical variable in multi-color work as well. The order in which screens are printed has a profound effect on the appearance of the final print, due to many factors such as loss of color intensity or color Circle Reader Service No. 394 I 22 PRINTWEAR MAGAZINE JULY 1993

6 inter-mixing due to ink pick-up on successive screens. The platen: From a plywood shirtboard on a one-color home-built press to the Sl0,OOO belt on this 50-foot monster, the imperative is the same - flat and level. The dry/cure: Only two of 49 variables concern keeping the ink on the garment once it s been printed. But neglect of washfastness and abrasion Issues can turn a profitable run into a pile of shop rags. The dry/cure Once the ink is on the garment, the following variables determine whether it will stay on. During printing these factors also play a part in preventing unintended consequences associated with #44 by means of flash-cure equipment. #45 Curing temperature must meet or exceed that necessary to dry andlor cure the ink in use, without damaging the garment. 1 nadeq ua te tempera tu re reduces durability (adhesion, abrasion resistance and washfastness) of the ink deposit. #46 Curing speed and temperature are typically brought under the printer s control via conveyorized heat-cure equipment, with overall curing speed determined by a balance of chamber heat and belt speed adjustments appropriate for the ink in question (see #lo). The shop Yes, as if the above weren t enough, three final and often neglected variables concern environmental factors: #47 Airborne contamination such as dust or lint can clog screen openings and/or leave streaks in print image areas (not to mention playing hob in screcn room and art department). #48 Ambient temperature and #49 Ambient humidity may alter screen prep variables such as stencil moisture content and drying times. Major swings in either during printing may shift on-press variables like ink viscosity and wood frame stability. And that brings us to a convenient point at which to stop. But don t assume that s because the list is necessarily complete. Lists of variables tend to... well, vary. You could argue, for example, that if stencil durability is on the list, so should be press durability. And how about squeegee wear or, for that matter, the voltage supply coming to an infrared dryer? You d be right, of course. The variables one might attempt to control could make our list endless - and the job impossibly expensive and difficult. Suffice it to say, those who learn to control those mentioned here are likely to be way ahead of their competition PRINTWEAR MAGAZINE JULY 1993