General Guidelines for Screen Printing On Flat Glass

Transcription

1 General Guidelines for Screen Printing On Flat Glass Typical applications cover appliance, automotive and architectural glass Key ARTWORK Film Positive Quality Film must be clean, void of marks, greasy stains, scratches or other blemishes such as static spots Poor artwork is the single largest cause of poor print quality Image Image output must be solid of the correct density, complete image(s) intact, geometrically accurate in representation with no appearance of distortion, moiré or banding Printing can never better the original only equal it at best Edge Resolution If necessary, re-output the film at a higher DPI if edge quality of image is not smooth or there is evidence of moiré/banding (see above) Check with imagesetter supplier for further resolution and density recommendations Accuracy If printing more than one color, check registration accuracy between the set of films Films shrink due to environment changes which can cause registration problems later ALTERNATIVE IMAGING TECHNIQUES Computerization CTS and Direct Projection A variety of Computer-To-Screen and Direct Projection systems offer alternative imaging techniques that do not require film positives and perhaps other processing steps employed in conventional screen making. Follow system s guidelines closely for optimal error-free results. Check image for unacceptable serrated edges, as well as moiré with monotone or process work and banding with vignettes (dot gradations). SCREEN Frame Type Stretch & Glue and Self-tensioning are the two main types of frame systems commonly used for glass printing Stretch and glue frames are lighter, more cost effective and practical for auto/architectural glass Self-tensioning frames are a more practical consideration for appliance/white goods due to close-tolerance multicolor registration requirements

2 SCREEN Frame Size Image size should ideally not exceed 65% of the inside of the frame for architectural glass 50% for automotive glass 40% for appliance and other smaller format sizes Percentage recommendations may not be possible in all situations Allow as much space as possible from squeegee to inside frame for good ink well (see separate entry) Consider equipment and other possible limitations Ink Well Ensure there is a healthy space large enough for the squeegee and floodbar to clear the image at either end to prevent ink from seeping back into the open image area Ink seepage due to insufficient clearance will cause potential rejects particularly with etched or frosted coatings Tubing Tubing/section size should be based on overall frame size and desired tension levels Consult with fabric and screen supplier for optimum recommendations Construction For extra large screens for architectural applications, consider having corner gussets and prebowed Gussets reduce corner warpage and prevents cracking Prebow reduces inward deflection under high tensions and helps maintain tension Flatness All four corners should lay flat on the print table without any signs of warpage Do not use if the frame doesn t clamp down flat as the image may distort as well as other problems Squareness Corners squared 90º An out of square frame can potentially lead to registration and distortion problems 2

3 SCREEN Frame Standardization Keep frame sizes to a minimum to reduce adjustments and set up times, particularly with architectural applications Consider a universal master frame holder to accept various frame sizes for architectural applications Determine one frame size (maximum) for appliance and automotive jobs and no more than three for architectural (S, M and L for example) A master frame holder can significantly reduce setup/changeover times (having two sets: one on press and the other prepared offline for next job) Fabric / Mesh Type Monofilament polyester Stainless steel Nylon Low elongation polyester is the most common mesh type used for glass printing Stainless steel is good for conductivity purposes Typical mesh counts used for glass printing with inorganic enamels are: Mesh Count Effects ink consumption, ink deposit, opacity, screen tension, screen life among other factors Consider particle size when selecting, particularly for fine line images and characters (see Aperture on next page) Higher mesh counts (TPI/TPC) provides better image detail to be reproduced - Appliances: Threads Per Inch (TPI)/ Threads Per Centimeter (TPC) - Automotive: TPI/ TPC - Architectural: TPI/ TPC Printing mediums other than ceramic frit, such as other types of inorganic, organic, elastomeric, solvent-/water-based or UV (ultra-violet) may require different mesh counts, processing and handling refer specifically to supplier s mesh recommendations and processing procedures as well as those of the ink coating manufacturer 3

4 SCREEN Fabric / Mesh Thread Diameter Effects how ink flows to edge of stencil Best to use thinnest thread possible for small dots, fine lines and graduations Thinner thread results in less resistance to print Thicker thread is prevalent for glass printing due to the aggressiveness of the ceramic frit Thinner threads typically deposit more enamel and provide better resolution and overall print detail Aperture Aperture or physical opening size should ideally be at least 3 4 times greater in size than the largest particles contained in the ink Aperture can be increased or decreased in size by selecting a thinner or thicker thread diameter POA (Percentage Open Area) POA is a useful guide to determine how receptive ink will transfer to the glass surface in comparison to other fabric grades Higher POA requires less squeegee pressure and wear while permitting higher printing speed POA is determined by thread count and diameter Mesh Color (dyed) As opposed to white, dyed mesh impacts quality and edge sharpness of the stencil image Yellow or orange mesh provides better resolution and definition than white White mesh is cost effective for nondemanding applications such as flood/open screens Dyed fabrics are best to use for all photographically reproduced images regardless of pattern 4

5 SCREEN Fabric / Mesh Tension Screens should have enough tension to allow for a clean separation from the glass surface immediately behind the squeegee during the print stroke with minimum offcontact Follow fabric supplier s recommended tension level for each fabric grade, usually around level I for automotive and architectural and level II for white goods/small format intricate jobs Smaller screens withstand higher tension levels than their larger cousins Tension should be uniform within the image area, checked in five places (center and four corners) in both directions with small screens, and consider up to nine places for very large architectural screens While each fabric grade has its own desirable tension level, how straightforward or demanding the job is needs to be taken into consideration too Higher tension screens will increase productivity, quality and yield If screen making is outsourced, establish minimum shipping tension for each fabric type based on recommendations, as opposed to when it is received, sometimes referred to as shipping or transit loss Consistent print results and superior coating uniformity can only come about with close tension consistency regardless of tension level or screen size Stretching Follow stretching equipment s procedures for rapid tension Fabric stretched at a bias angle for glass printing is generally not required so fabric ought to be glued linearly better than 1% angle to the screen frame Fabric should ideally be held pneumatically at the desired tension level consistently for 30 minutes prior to gluing If stretching is outsourced, discuss methodology to gain best results for assured quality and consistency Methodology behind the 30-minute rule is to prevent screens from weakening too quickly from repeated use, thereby extending their durability and useful life 5

6 SCREEN Fabric / Mesh Preparation All newly stretched fabric needs to be degreased so that it s surface is clean and slightly abraded to promote better adhesion of the stencil Cleaned screen should ideally be dried in an enclosed area or cabinet void of collecting dust particles and must be completely dried/moisture-free before being coated Procedure and treatment techniques involved are according to film or emulsion used If stretching screens for storage purposes, clean and degrease screens within 24 hours of coating and exposing Test Screens Screens made for samples and testing purposes should have the exact same fabric grade and tension level that is used in actual production regardless of what they may be Production can better match printed samples, deposit thickness, color and opacity/translucency when both screens are made to the same specifications except for size Emulsion or Stencil Film Rest Period Type When stretching has been completed, allow at least 24 hours - 48 hours before coating for reorientation of the fibers (this is when the long side of large screens are forced to bow in the center) While there are numerous types available, they basically comprise of two groups according to enamel s chemistry: solventresistant or water-resistant Rest period will help to prevent lines or dots at the outer edge of the image developing a curve, if the frame bows along its length, as well as improving closetolerance registration Select emulsion type based on type of ink system to be used, being mindful that the screen may not be used with all types of printing ink Application For capillary film application, follow manufacturer s processing recommendations For emulsion coating, one or more coating strokes are required to encapsulate the mesh Full encapsulation is necessary to provide integrity for maximum durability 6

7 SCREEN Emulsion or Stencil Film Thickness EOM (emulsion over mesh) on the substrate side should be kept below 10 microns with coating recipes preprogrammed if using an automatic coating machine Following supplier s recommendation to attain mesh encapsulation, which may require several test screens to achieve the objective For superior quality image detail, EOM should ideally be between 4 8 microns depending on emulsion type Thinner EOM is recommended for fine dots and lines as it reduces the unsightly thickedge appearance Thickness has no effect on ink deposit in open areas except at the outer edges Surface Roughness Substrate side of screen should provide a good gasket for ink transfer and image integrity If there are any doubts about obtaining the most desirable R z value (measure of roughness) with fine images, have test screens made to determine optimal value Screens with a R z value of 8 12 is recommended for glass surfaces If stencil is too smooth (lower than 8 R z), static may damage the printed image as the screen separates If R z value is too high, edge resolution will be lost Drying Screens coated with emulsion or capillary film applied should be dried horizontally with squeegee side uppermost in a clean dust-free environment Allowing screens to dry in a vertical position will render coating or stencil to be uneven and will lead to print quality issues 7

8 SCREEN Emulsion or Stencil Film Exposure Determine correct distance of light source according to screen/image size Determine correct exposure time, via a light integrator, with the aid of an exposure calculator Each fabric grade and emulsion thickness will have its own unique exposure time that should be programmed into the procedures for consistency For very fine line resolution requirements, consider the secondary-exposing technique to improve definition and coating integrity more prevalent for appliance applications Keep exposure glass, top and bottom, continuously clean of particle matter, tape and greasy finger marks Suppliers will recommend optimum lamp system according to screen/image size but higher Kw lamps provides superior image reproduction Obtain a full vacuum seal between blanket and exposure glass as it is critical to prevent image distortion and loss of detail Change lamps at the recommended life, usually 1000 hours regardless of a light integrator employed Keep a record of each screen made, emulsion applied, image (job #) and exposure time for tracking potential problems Washout Establish acceptable procedures to washout the emulsion, including spray consistency and warm water temperature 95º - 113º F (35º - 45º C), if used and highly recommended Do not overly washout as it will irreparably damage the emulsion/stencil properties Blockout and Tape Blockout areas outside the emulsion/stencil and tape according to set procedures It is better for screen makers to finish the screen completely and have ready for printing than to allow printers to do this part of the job 8

9 SCREEN Emulsion or Stencil Film Final QC If outsourced, check maker s QC inspection sheet upon receiving screens and keep on file Prior to placing into production, inspect the whole screen to ensure stated tension is correct, the complete image is in tact (such as half a dot missing), line integrity, edge resolution acceptable (with the aid of a loupe) and no appearance of banding, moiré, etc If necessary, keep a log of screens recording tension of fabric each time it has been used before and after Proper documentation will map longevity of screens and may indicate potential problems from reoccurring Custom Taping Mark up screen prior to production, indicating where the tape will go when processing various custom sizes with the same screen/image Pre-marking will significantly improve accuracy and productivity Cleaning Washbooth For professional-looking clean screens, use a backlite washbooth designed for this purpose including a high-pressure washer with suitable PSI Remove tape, adhesive residuals and any particles of ink, including those at inner edges of the frame Consult washbooth suppliers as well as state, city and local ordinances regarding drainage and waste disposal Consider two separate systems, or a combination, when processing water and solvent-based inks PRINT TABLE and PRESS SETUP Squeegee Material Urethane is the most common squeegee material and make sure the grade selected holds up to aggressive frit and solvents used Single durometer straight edge squeegees (90º corners) are the most commonly used for glass printing Dual-durometer blades are gaining in popularity for fine detail applications, particularly for smaller format printing and demanding results High quality, more expensive squeegee blades are generally more cost-effective as they do a better job of printing, have more resistance to aggressive frit/solvent cleaning and last much longer 9

10 PRINT TABLE and PRESS SETUP Squeegee Durometer (hardness) Affects ink deposit and edge resolution Durometer ranging from 65º 85º are widely used for glass printing Continuous cleaning as well as wear and tear will increase durometer over time and those increasing in hardness by 5º should be discarded for demanding print quality Softer squeegees yield more deposit but reduce edge definition while the reverse occurs with harder blades Check durometer periodically to insure hardness has not changed due to solvents or when incrementally wearing down Always refer to durometer by its hardness number not by color - when troubleshooting since a common colorcode system does not exist between manufacturers Edge Quality Most critical variable of the squeegee: keep clean and sharp, let it rest after cleaning and sharpening (see separate entry) Solvents and chemicals weaken the edge; so does printing too soon after sharpening (see separate entry) Follow manufacturer s recommendation for proper squeegee handling and care For long production runs, rotate squeegees frequently, as determined by experience, to allow their edges to regain integrity (reduce swelling) Mechanical Angle Print Angle Set mechanically fixed angle on the squeegee assembly bridge 10º - 15º from the vertical position Large depth squeegee blades will require less of a fixed angle as opposed to those shorter in depth Actual print angle is an important variable that needs to be constantly attended to in order to render suitable print performance Angle decided by several variables: durometer, mechanical fixed angle, depth of squeegee from holder, pressure, offcontact and peel-off settings, mesh, image and more Never preset more than 20º angle mechanically under any circumstances for glass printing Print angle should not vary more than 5º 10º from fixed mechanical angle and back off pressure slightly when reducing angle or print speed Less angle improves edge quality/thinner deposit while the opposite occurs with a greater angle 10

11 PRINT TABLE and PRESS SETUP Squeegee Pressure Pressure needed is in direct relation to screen tension, fabric grade, off contact, peel-off, image size, copy characteristics, durometer and frit size to complete ink transfer The least amount of force is ideally required to transfer ink through the screen, which under suitable conditions, will reduce overall stress in the process Keep pressure at a minimum to reduce unnecessary squeegee and screen wear as well as image distortion and possibly registration problems Best way to keep pressure low is by using higher screen tension and lower off-contact distance Speed Ideal speed is determined by transferability of ink (ink flow) and shear rate according to mesh grade and reproduction of image detail Lower viscosity generally permits faster print speed Speed should ideally be fixed with the same ink type and viscosity range for all jobs Length Probably the most overlooked variable for consistency in deposit uniformity Fit the squeegee to suit the image, not the frame Squeegee length should be no longer than 1 (25 mm) past the image width on either side A blade that is too long will collapse in the center, resulting in excessive pressure, premature screen wear, image distortion and uneven ink deposit Snowplow Consider snowplowing the squeegee a few degrees, on either axis when printing lines and halftones Snowplow helps to improve edge definition of lines by reducing the sawtoothing effect as well as helping to eliminate moiré Dripless Use feature when printing acid-etch coatings or any ultra thin/transparent-type coatings The anti-drip device prevents ink dripping off the squeegee blade onto the open screen during the flood cycle, which will ruin the next print 11

12 PRINT TABLE and PRESS SETUP Squeegee Sharpening A good quality squeegee sharpener is imperative as the number one support tool for screen printing onto any clear substrate, such as glass and in particular for large sizes Prepare a detailed procedure to ensure sharpening is regularly carried out after squeegees have been printing and cleaned, rested before sharpening and then rested again before reuse Frequent sharpening is perhaps the most critical pre-press process on the production floor but about the least considered Follow sharpener s recommended procedures very carefully and modify them to suit as required Ideally cross-train in its operation, rotate but designate one person per shift/per month to handle the job to yield consistent results Management Care Uncoil material when received, cut to size if practical, lay flat, store away from inks, solvents and bright lights at 68º 77º F (20º 25º C) Clean only with water or press-wash for solvent-based inks and do not leave ink to dry on them With solvent-based inks and aggressive cleaning solvents, rest blades ideally for 24 hours or overnight before reuse or sharpening Rest again for 24 hours or overnight after sharpening (see comments) Have several sets of squeegees in the loop of the same size so they can be rotated during production Squeegee blades are the cheapest item for printing yet they can be the most expensive in the process if they are not treated properly Superior blade material will always last longer, resist solvents better and significantly improve overall print performance Some sophisticated sharpeners create hardly any heat from friction if used correctly, so squeegees could be reused immediately after sharpening Using aggressive solvents for cleaning will unnecessarily make the edges swell and be completely unusable until properly rested Variables Variables listed in this section are some of the more critical operational variables. Please be aware that there are many more variables associated with the squeegee Consult your supplier 12

13 PRINT TABLE and PRESS SETUP Floodbar Role The real purpose of the floodbar (floodcoater or scraper blade) is not to return the ink back to commence the next print cycle but to determine how much the squeegee will deposit Squeegee can only transfer what the floodbar has left in and on the screen regardless of its conditions Type Those that are mechanically angled at 0º from vertical are ideal for applying heavy deposits and are sometimes referred to as a standard floodbar Those pre-angled at 25º - 45º degrees are better for obtaining thinner deposits and improved image resolution/edge quality. Sometimes are referred to as a halftone or special floodbar If the dynamics of the screen permitted more ink coating to be transferred than desirable, switch to an angle (halftone) floodbar If ink deposit is insufficient due to the dynamics of the screen, switch to a standard floodbar Set Up Should have enough pressure to leave an even amount of ink across the surface along its entire length Floodbar adjustments are independent of the squeegees, which can be very advantageous Adjust floodbar pressure for controlling ink deposit before changing other variables Edge Edge must be straight, level, nick and burrfree Any nicks will print as an unsightly line Never lay a floodbar on its edge but rather on its back and place a protective cover over the edge Controls Keep control settings consistent, such as speed, angles, pressure, etc. Oftentimes, the flooding action has more influence on the print than the squeegee so be mindful of its formidable power 13

14 PRINT TABLE and PRESS SETUP Floodbar Length Extremely important that its length should be paired with the squeegee A floodbar that is too long will leave more ink coating in the middle of the screen thus permitting the squeegee to print an uneven deposit Print Modes Should ink dry too quickly in the screen between print cycles, consider using the flood/print mode, if available, as it keeps the screen flooded during the pause between cycles Conversely, if too much ink seeps through the screen between print cycles, switch to the print/flood mode of operating, which is generally considered the normal way of operating Variables Variables listed in this section are some of the more critical operational variables. Please be aware that there are many more variables associated with the floodbar Consult your supplier Make-Ready Before setting up a press, check that it is in a useable state from the last job and safe to commence make-ready - especially at the start of a shift and in particular when a formal handshake (handover) was not conducted between the previous operator Sometimes the most simplest things forgotten create unnecessarily long downtimes between changeovers Mechanical Off-Contact Screen tension alone determines distance to use Set as low as possible to allow for a clean release of screen from the substrate during the printing stroke Since larger screens typically have lower tension, they would require higher offcontact Too much off contact will result in excessive squeegee pressure, image distortion, poor registration, reduced screen life, loss of image detail and inconsistent ink deposit Higher tension screens will always allow a lower off-contact distance corresponding with less squeegee pressure; setting up the ideal rheology for successful printing 14

15 PRINT TABLE and PRESS SETUP Mechanical Peel-Off Shear rate of ink determines the amount of peel-off to use, if required, to prevent a large area of the fabric to remain in contact with the glass behind the squeegee Peel should be set high enough to allow the screen to release from the substrate to provide a clean looking print Increase peel-off rather than off-contact if high tension screens do not create a clean separation from the glass surface High tension screens will always reduce the dependence of peel-off and excessive squeegee pressure Nesting To reduce wear on the screen and yield a better-looking print, consider using nesting supports to prevent the squeegee from jumping when it runs on and off from the glass surface May also be used to register the glass panel In many instances where good printing techniques are used, nesting is not always required but generally used to provide a safety net Nesting material should be the same thickness as the substrate Teardown Upon finishing a production run, it is a good working practice to leave the printing machine in a neutral safe state that is zero back registration adjustments and cancel any excessive settings or adjustments On the last print of a job, set the machine so the print cycle finishes at the end of the print stroke, thereby making it easier to clean the screen As screen printing equipment can be rather complex, insist on having handshakes (an update of the printing machine s state and current/finished job) when switching operators Drying or Curing Ensure heat settings configuration is correct for the ink type, color, deposit thickness and conveyor speed Ensure correct lamp configuration, voltage power settings and conveyor speed with UV curing If cooling is used, ensure the air is not restricted, too warm and that the exhaust is working properly In general terms, water- or solvent-based inks require the longest possible resident time in the heating section that will keep up with productivity, which will then afford the lowest temperature setting for a more efficient way of operating and reducing glass exit temperature Usually the reverse with UV, ink is exposed for the shortest possible time to crosslink and not over cure 15

16 PRINT TABLE and PRESS SETUP Mechanical SOP Each of the printing equipment s settings and conditions should be properly recorded for each type of print need and ink medium used (standard and transparency etch) so that they can be repeated by any operator Generate a full working set of operating procedures (SOPs) according to practices and techniques employed when results produce good-looking prints with minimal hassle in make-ready and production Some form of standardized procedures become necessity when mechanizing an artistic printing process into one that is more controllable, repeatable and manageable by everyone involved ENVIRONMENT Conditions Print room Consider building a separate room around the printing operation to keep factory dust and dirt away To keep the print room continuously clean, minimize traffic by making it Authorized Personnel Only and, as necessary, supply operators with cleanroom outerwear (coats, hats, bootees, etc) Consistency to maintain actual or practical working conditions, regardless of what they are, is more important than the ideal values stated if they cannot be continuously met Cost to provide some form of a separate print room is greatly outweighed by spectacular print results and superior production performance Temperature Ideal range 68º 72º F (20º 22º C) Humidity Pressure Ideal range throughout the year 50% - 60% R/H Atmosphere slight over pressure, 1¾ in (45 mm) over water column Consider using an industry-recognized viscometer Consistency in viscosity promotes quicker changeovers for short custom jobs and makes it easier to track problems as they arise Air Quality Quality filtered air with at least four changes/hour although six should be considered 16

17 ENVIRONMENT Variables Characteristics Guidelines Conditions Static Reduce the effects of static electricity by using antistatic mats and coats as well as consider separately grounding equipment as necessary Comments Enamel (ink) Access Viscosity Ensure access doors open outwards to prevent dragging factory dust particles from entering Enamels, referred to as ink or paint, should be thinned and/or mixed prior to use within the supplier s viscosity range Establishing a workable range will enable operators to use the same machine settings Recovery Always filter the ink before used, particularly if ink recovery is practiced, to prevent hard dried pieces from being printed Dried up, hard ink can not only ruin an otherwise good print, but can also pop the screen Material Storage Screens As above If materials cannot be stored in this type of environment, they should be brought into the printing environment at least 24 hours prior to being used and allowed to acclimate to the print room-type conditions Enamels As above As above, but recommend 48 hours prior to use Squeegee Floodbar and Consider hanging them on the wall or provide a rack for each size including paired floodbars As with floodbars, squeegees should never be rested on their edges but left laying on their backs blade uppermost Others Air Side Use a tin side meter to ensure the glass air side surface is uppermost for architectural and appliance glass printing When printing on the tin side of glass, the ceramic frit will react with the tin and may cause discoloration or mottling. This could give the appearance of a defect or poor print quality 17

18 ENVIRONMENT Others Tin Side Use a tin side meter to ensure the glass tin side surface is uppermost for automotive glass printing The tin side is desirable for automotive applications, due to the reaction with the tin and the silver conductive ceramic enamel Antistatic Consider using antistatic devices particularly as the glass enters the print room from the washer Refer to suppliers recommendations Ink Feeding Consider suitable access for operators to pour ink into the screen at the floodbar end, according to size of the press and layout Work with OEM initial recommendations Inspection Consider need to inspect print immediately after printing before it enters the dryer Highly recommended to allocate space for such checking since it will enable operators to make print adjustments as necessary For information on additional reference resources including the GANA Glazing Manual and other Glass Informational Bulletins visit GANA website:. The Glass Association of North America (GANA) has produced this document solely to provide general information as to general guidelines for screen printing on flat glass. The document does not purport to state that any one particular type of screen printing process or procedure should be used in all applications or even in any specific application. The user of this document has the responsibility to ensure that architectural glass constructions meet building code and other specific project requirements, GANA disclaims any responsibility for any specific results related to the use of this document, for any errors or omissions contained in the document, and for any liability for loss or damage of any kind arising out of the use of this document. This document was developed and approved by the GANA Decorative Division Printing on Glass Task Group and approved by the Decorative Division membership and GANA Board of Directors. This is the original version of the document as approved and published in January