The proper use of measuring devices is essential in controlling quality and consistency. Only through factual data, not subjective opinions, can consistency be controlled throughout the production process. Basic measuring devices needed to obtain these important facts include screen tension meter, electronic thickness gauge (deltascope), Rz meter, radiometer and densitometer. Screen tension meters are used for verifying screen mesh tension, both warp and weft.
An electronic thickness gauge is an important instrument for ensuring consistent, precise stencil coating by allowing us to measure the thickness of the emulsion over mesh. The Rz meter can determine the surface smoothness and/or flatness of a stencil. For UV shops, a radiometer is a fundamental tool in ascertaining the millijoule distribution of curing units. As for four-color process work, densitometers are essential in controlling the consistency of printed densities.
The data collected from these instruments can help in evaluating and pinpointing problems. Sericol technical representatives can help you select the proper instruments and give guidance on how to use them.
There is no problem with the addition of opaque black and/or white in a color match for solvent-based inks, however, this practice is not recommended with UV inks. Since UV ink has to be completely penetrated by UV energy to cure properly, the use of opaque colors can create an overload of pigment in a color known as “windowing.” Windowing occurs when several opaque colors are mixed together in a ratio that will not allow sufficient UV energy to penetrate through the ink film to the substrate surface.
Summer has arrived, and as the days get warmer, the technical phone calls increase with questions concerning the performance of UV inks on vinyl banner and static cling substrates. It is not uncommon to hear that the inks “have changed” and the same ink that used to be glossy and dry now appears matte and tacky or wet. The problem is not that the ink has changed, it’s that the warmer weather conditions are causing changes to the substrates.
Most plastic materials contain plasticizers to make them flexible. While these plasticizers make the material softer and more flexible, they also can cause screen printers many difficulties. Plasticizers migrate to the surface of the material and change the surface which the ink must adhere to. When the material is exposed to higher temperatures, the migration process occurs more quickly. Since static cling and vinyl banner materials contain high levels of plasticizers, storing them in a hot warehouse, transporting in a hot truck, or curing through a hot unit will quickly cause plasticizers to migrate to the substrate surface, potentially causing ink adhesion and/or blocking problems.
Some vinyl banner manufacturers now offer top-coated vinyl banner materials which prevent any plasticizer from contaminating the surface of the material. This top-coating eliminates the plasticizer migration problems, but it adds cost and may change the UV ink chemistry needed to adhere to the banner. Printers should weigh the higher material costs against the potential production down time in making their substrate decision.
Another solution to the problems of plasticizer migration may be in determining the best UV ink to use, given your choice of vinyl material, your shop environment, and your specific processing equipment. If the ink film is not flexible enough, it can simply flake off when installed or handled. Intentionally undercuring a harder ink to try to keep it flexible is definitely not the answer as it can cause blocking and/or adhesion problems.
A less flexible ink may appear flexible today when under cured, but will post-cure and embrittle with time and cause problems later. Select an ink which can be thoroughly cured and provides the required level of flexibility. Curing speed is also an issue to consider. Typically, the more flexible an ink system, the more curing energy is required to achieve a proper cure. When the UV energy is increased through higher power settings or slower belt speeds, more heat is generated which will accelerate plasticizer migration and possible problems with adhesion or blocking. High levels of heat will also cause distortion problems with substrates, especially with static cling. The substrate should be processed as cool as possible and still achieve proper cure, so your decision on which ink system to use will in part depend on the type and condition of your curing equipment.
The effects of plasticizer migration on a printed ink film will depend on the ink system being used. The more flexible/slower ink systems are more susceptible to the plasticizer rising through the soft ink film during the curing process, which can cause a greasy or matte surface of the ink and can contribute to blocking problems. On the other hand, an ink which is faster curing can actually help to block some of this plasticizer migration to the surface of the ink film.
The choice of inks depends on your ability to control the issues which cause plasticizer migration. If you are using vinyl which you and your supplier store and transport under controlled conditions, and you have the equipment to cure a slower ink system, then you can look to the most flexible ink system available. If your material is not so well controlled, you may need to compromise somewhat on ink flexibility.
To allow you to use the best product for your situation, Sericol offers a number of UV and Solvent based inks which are very flexible and adhere well to vinyl materials. To make the best decision, it is important to understand how substrates can change and how to control the variables which cause these changes. As always, Sericol offers the technical assistance to help you identify key issues in making your ink decisions.
When analyzing the thousands of technical and customer service calls Sericol receives during the year, a vast number of these calls deal with problems relating to the proper deposit of UV inks. UV ink technology provides many advantages over solvent-based ink systems, including: lower cost in use, faster production, more efficient use of space, better print definition and being more environmentally safe.
These advantages lead to higher quality products in less time, which ultimately results in higher profits for the printer. However, due to the uniqueness of the technology, greater control in the process of application is necessary to fully achieve these advantages. It does not take very long for a printer with years of experience and expertise in printing solvent-based inks to realize that the majority of techniques they have learned are not entirely applicable to UV technology.
Sericol has identified six main areas that, with a little more careful attention and fine-tuning, can assist you in maximizing your UV investment return. In this issue we will address the first two areas, which are mesh and stencil selection. Mesh-Finer Is Better One of the biggest differences between solvent-based and UV inks is that UV ink is 100% solids.
A solvent-based ink is generally 60%-75% solvent, so when it dries and the solvent evaporates, you are left with only solids, which are 25%-40% of the actual ink. Since UV ink is 100% solids, when it is cured you are left with what you printed. Therefore, UV inks require less wet ink deposit which leads us to the logical conclusion of a much finer mesh.
In fact, even with a 390 mesh screen, the cured film deposit of UV ink will exceed that of a typical solvent-based ink printed through a 230 mesh screen. Because UV inks do not dry in the screen, screen preparation becomes a critical part of your process, even the finest pinholes will show up on your substrate. To achieve quality lines and dot definition, we recommend that printers use UV ink through a 355 to 390 monofilament mesh, attempting to achieve a 0.5 mil deposit of ink.
You can also use plain weave mesh instead of twill, which will result in even thinner deposits of wet ink film. All Sericol UV inks are manufactured to perform within these particular parameters. Stencil Selection-Thin Is In Whatever stencil type you are most comfortable using, always think less instead of more. A stencil that is too thick will result in poor curing and print quality, especially on those projects where small type and halftones are involved.
When using conventional direct emulsions or a diazo-photopolymer emulsion, such as Dirasol 914, 911, or 905 Rapid, we recommend a thin film coating that would consist of 2-8 microns thicker than the mesh. You may also use a thin capillary film of about 18 microns. When using a direct emulsion, it is critical that the print surface is uniform and level.
An uneven coating with low and high spots will produce changes in color density and will also affect proper curing. The ability to achieve stencil consistency using direct emulsions is affected by several key variables, including the type of emulsion, its chemical properties, number of coatings, coating technique, and the position of the screen during drying.
When printing UV ink, two additional variables that are occasionally overlooked are the squeegee and ink viscosity. Squeegee Keep it sharp. The properly selected mesh, appropriate stencil build and superior ink cannot compensate for an improperly adjusted, worn out or poorly-maintained squeegee. There are several squeegee considerations in applying a proper, uniform deposit of ink: Determine that the blade you are using is resistant to UV formulations. If a blade is not universal it could work very well with solvent-based inks, but could swell or deteriorate very quickly when introduced to UV ink.
This could drastically alter the uniformity of the ink deposit. Make sure that your squeegee is sharp and well-maintained. Ink cannot be transferred through mesh consistently with a dull or nicked blade. Slight imperfections in the blade can negatively affect the print quality when using UV ink. Sharpen your blade only after it has been out of the ink a sufficient time for it to fully recover. Do not use a solvent wipe to remove dust after sharpening, as it will swell the blade. Any solvent absorption could cause the blade to swell.
Develop a squeegee rotation program that designates three squeegees to each press. Use the first today, the second tomorrow, and the third the day after. This rotation will allow each squeegee two days between printings so it can fully “recover,” and be properly sharpened before use. Adjust the squeegee angle and pressure to allow the blade to remain at the correct printing angle. Too much pressure or angle will allow the blade to bend severely and result in poor print quality.
Ink Viscosity The viscosity of a UV ink can affect the amount of ink deposited and therefore, the cure energy required and color density may both vary. While the majority of Sericol’s UV inks are formulated to be press ready, variations in screen mesh, substrate, climate, and individual variations in presses can lead to the need for slight modifications. For instance, UV inks may need to be thinned 2%-5% when printing with a cylinder press, as opposed to using without thinning on a clamshell or a four post semi-automatic press.
Sericol’s technical personnel are available for on-site assistance in recommending the proper print viscosity for each of our formulations based on your equipment, substrate, and other variables. Another consideration when talking about viscosity is temperature. Temperature can have a tremendous effect on the viscosity of the ink itself, which can lead to printing inconsistencies. As ink cools it becomes thicker. For instance, if a can of ink comes off a truck in Minnesota, during February, and sits on a dock all weekend, it will most likely need time to warm up to the normal printing temperature before using.
Although it will probably print immediately, the ink deposit will change drastically due to the inconsistent viscosity. This same principle applies to warm climates as well. If the ink is warmer than usual, it will effect the viscosity, and therefore the ink deposit through the screen. Remember, even though there are many variables in printing UV ink that need to be monitored and controlled, the benefits far outweigh the liabilities.
You will find many technical pointers including how to set proper screen tensions, as well as key information regarding curing and adhesion. Press Set-up If screens are processed properly for UV printing, many of the potential problems are already under control. We are often asked what are the optimal screen tensions for printing UV ink. As always, consistency is the key. It is preferable to have tensions of at least 20 newtons, however this will depend on frame choice, mesh count, stretching device, etc.
Consistency at a level of 20 newtons is much more important than inconsistency in the 30+ range. If consistency at high tensions can be achieved, this will help minimize off-contact and will assist in keeping ink deposits optimal, not heavy. Flood stroke should be consistent and tight to the screen. When the press is not actually printing, keep the screen covered to reduce exposure to light sources, since over the course of a long production run ink can begin to cure in the screen.
Exposing UV inks to light, especially daylight, is a potentially expensive proposition because you could lose the ink in the screen as well as the screen itself. A few minutes of exposure is all it takes for a wet UV ink to start cross linking in the screen, blocking open areas and increasing ink deposit due to build up on the squeegee side of the screen. Ideally, UV filters should be installed above the press to minimize UV exposure to the ink in the screen.
Finally, good housekeeping is essential when printing UV ink. Keep your press and the area around it clean and free of foreign particles that can easily clog a fine mesh screen and lead to pinholes in the print itself. Substrates – Test, test, test The slightest variations in substrate stock can greatly affect the printability and adhesion of UV inks. It is always recommended to test the substrate prior to full production. Test under the exact conditions in which the full production run will be produced.
It is extremely critical when printing with coated plastics to test the number of colors that will actually be printed. Sometimes top coatings will deteriorate under exposure to UV energy. Under certain conditions, this could be the reason that the first color printed will adhere extremely well, while the fourth, fifth or sixth color printed will not. If printability is in question, make the same print on a stock that you are certain is compatible with the ink.
If the ink prints fine, it may be that your substrate/ink choice is not compatible. Determining proper cure of a UV ink is critical to adhesion. A properly cured UV ink is completely dry to the touch and has a uniform gloss. To test cure, first try the thumb test. Place your thumb on the ink, apply high downward pressure and twist. If you cannot separate the ink and it feels dry, you have acceptable cure. Next, check adhesion using the cross-hatch test.
Cut completely through the printed ink several times at opposite angles using a cross-hatch tool or razor blade. Then, using an aggressive adhesive tape, rub it over the cut lines, apply pressure then lift with a rapid motion. If a significant amount of ink comes off during this test, allow some time to pass and check again later to see if post cure has taken place.
If the ink still comes off after another cross-hatch test, it is likely that the ink is not going to adhere to that particular substrate. The American Society for Testing and Materials (ASTM) has an excellent brochure for details concerning this test. The address is 1916 Race Street, Philadelphia, Pa., 19103 or call 215-299-5400.
Yes, a U.V. ink under 70 degrees F (21 degrees C) will be more susceptible to flow problems and curing inconsistencies.
To correct this problem, thoroughly mix the U.V. ink with a power mixer.
When considering the issue of color matching, there are a number of topics that could be addressed to help improve efficiency. Some topics take longer than others to learn, while others are very simple to grasp and easy to implement if only we take the time to do the process correctly.
A large amount of time is consumed in color matching, so whatever can be done to make this time more productive, the better off you are. This article will share four fundamentals of color matching. If you do not practice these fundamentals, try them for a few months and you will see the benefit. If you are already doing these things, use this article to refresh your memory and rekindle your commitment to this process.
The first fundamental in color matching is to weigh everything. It is a common practice to slightly “tweak” a color or add thinner or some other additive without weighing it. This makes it virtually impossible to reproduce the exact color at a later time. The use of an accurate scale that is regularly checked for proper calibration is imperative. A general rule of thumb is to use a scale that weighs at least .1% of your total batch size. For instance, if you are weighing a batch that is 1,000 grams, your scale should weigh down to one full gram. If you are only weighing up 100 grams, it is essential to have a scale that will weigh one tenth of a gram.
The only way to perfectly reproduce a color time after time is to know the precise weights used for the original match. The second fundamental in color matching is to document everything. Every time you add a component to a color match you should document it immediately. This discipline will prevent the confusion that sometimes occurs when you get side-tracked. Properly documented color match data is critical in reproducing the color at a later date. Fundamental three is probably the one that gets the least attention, but is just as critical as the others. Fundamental three is: re-weigh every match.
When most technicians begin a color match, they weigh-up an estimate of what that specific color formula will be. They will make a print, and then add something to adjust the color. This procedure may take several attempts before the desired color is precisely matched. If this process takes place using only the original estimated weigh-up, their final formula may still be slightly off. Even in a color match lab environment, each time you flood and make a print, you lose approximately three to seven grams of ink, even if you’re careful.
That is why you should never make more than two adds to a specific ink sample. After that, re-weigh the formula you have up to that point and correct it. The last step to actually match a color is to re-weigh a sample of the finished formula. This will insure that your finished formula is correct. The last fundamental of proper color matching is to accurately file each color match. This does not mean to just write down the formula on a loose piece of paper. It is generally suggested to use a 3×5 or 5×7 card file or a computer database system to record all of your finished color matches.
On this card or computer database system there is much more information that needs to be recorded besides the formula itself. This card should contain: the date matched, screen mesh, screen tension, squeegee durometer, any thinner or additive that was added, substrate matched on, DE if a color computer was used, what light source the match was done under, work order or job reference number, customer reference, emulsion build, any press-specific adjustments and a small color chip of the final match. This information is critical if the need arises to repeat a color match.
Practicing these four color match fundamentals will guarantee consistent color matching:
- Weigh everything;
- Write everything down;
- Re-weigh every match and
- Accurately file each color match.
Correct exposure times can be easily determined using a specifically designed calculator, such as Sericol’s Dirasol exposure calculator. This useful tool will help to set the optimum exposure time required to maximize reproduction quality while at the same time minimize pinholes and other exposure related stencil problems. Instructions for use are included within the package.
The calculator process should be repeated monthly where an exposure lamp has no built-in integrator because lamp power output, and therefore exposure time, deteriorates over time. It must also be recalculated every time the lamp is replaced, the exposure distance is altered or the emulsion is changed
When setting or checking for proper performance of UV equipment, wattage and belt speeds are only a reference point and can not be used in determining actual energy being produced. A radiometer should be used to obtain and document accurate readings for the purpose of lamp maintenance and settings for proper ink performances. All equipment is NOT equal and will not always produce the same energy even when the same parameters are set.
Degradation of lamps and reflectors will also have a tremendous effect on energy levels achieved. Therefore, reference only to wattage and belt speed could be misleading and actual reading of energy, in millijoules, must also be monitored. This can only be accomplished with the use of a radiometer. So if you own UV curing equipment, then you should also own a radiometer.
As we approach the summer, with its rapid weather changes and frequent high humidity, it is important to remember how critical your screen making process is, and how the weather can affect it. It is extremely important for your stencil to be completely dry before exposure. Also, be sure to calculate the proper exposure times, so that your stencils will achieve the optimum durability and performance. If these areas are not continuously monitored, it will eventually lead to stencil failure and ultimately downtime.
Emulsions must be throughly dried for optimum exposure. Springtime’s high humidity is just around the corner creating possible difficulties in stencil making. Thorough drying of the emulsion coating is essential for maintaining exposure speed and producing durable stencils.
Here are tips for accelerating emulsion drying speeds:
- Use a drying cabinet. This will limit dust and minimize fogging due to exposure to stray light. Dry emulsions at temperatures below 95° F (35° C). Higher temperatures will cause the Diazo to react and possibly fog screens.
- Good ventilation or a dehumidifier will reduce moisture buildup, resulting in a drier emulsion coating and reduced drying times.
- Dry the coated screen in the printing position (squeegee side up) to maintain stencil build.
Today’s screen printers are asked to print on various substrates specified by customers who may not understand the processing limitations of various plastic materials. This article will introduce some of the testing methods used for identifying common plastic substrates used for screen printing.
Initial testing for an unidentified plastic material begins with observation, and then only if necessary, chemical testing using solvents and flames. It is possible to identify many plastics by their handling characteristics, without the necessity of further testing by chemical means. For quick and easy Solvent Testing, place a drop or two of the test solvent on the material and let it sit for 30 seconds, then rub the droplet with a gloved fingertip to check for plastic softening or swelling.
Common solvents used in the testing described here are cyclohexanone, and an aromatic solvent such as a high-flash naphtha like SC-100 (Hysol 10) or SC-150. (The numbers 100 and 150 refer to the flash points of the solvents in this instance).
Most thermoplastics are generally sensitive to some solvents, but in screen printing we commonly use several solvent-resistant materials including polyethylene, polypropylene, and polyester. For Flame Testing or Burn Testing, use a clean flame from a Bunsen burner, propane torch, or alcohol lamp. Use a pair of tongs or tweezers to place a SMALL piece of plastic into the flame, and notice the color and appearance of the flame from the burning plastic. The flame color is an indicator of the plastic type.
Remove the plastic from the torch, and notice whether the material continues to burn on its own, or if the burning plastic flame extinguishes itself. Once the plastic is extinguished, lightly smell the plastic to assess the odor. Do not inhale deeply! Some plastics have unpleasant, strong, or acrid smells. Do not use a match! The sulfurous smell will interfere with the smell of the plastic. The chart below shows some identifying characteristics of the more common plastics that a screen printer may encounter.
Proper substrate identification will allow the screen printer to select the correct ink for the application. For additional information on identifying plastics or for help selecting an ink for a particular plastic, call our Technical Services team at 1-800-SERICOL.
Styrene brittle, distinctive shattering or tearing; “metallic” sound when shaken; very sensitive to solvents; burns with an orange sputtery flame and dense sooty smoke.
Polypropylene (includes Coroplast?) cuts cleanly, floats on water, bends repeatedly without tearing; unaffected by solvents; burns with yellowish-blue flame with little smoke, and continues to burn when removed from the flame.
Polyethylene waxy feel; very solvent resistant, cuts cleanly, floats on water, will stretch before tearing, burns similar to polypropylene.
Vinyl softened by cyclohexanone but not by aromatics; burns with distinctive bright green-tinged flame and dirty sooty smoke.
Polycarbonate solvent testing with cyclohexanone is the easiest test as it will soften polycarbonate and Polyester within 15 to 30 seconds; polycarbonate is difficult to ignite and burns with a sputtery orange flame and smelly smoke; Polyester film has a metallic sound when it is shaken, and is difficult to tear; it burns rapidly with bluish-yellowish flame, and continues to burn when removed from the flame source.
Graphic overlays have become part of everyday appliances and machinery. Most microwave ovens, membrane switches and OEM labels incorporate some form of graphic overlay. Traditionally, these overlays have been printed on polycarbonate and polyester films with solvent-based inks. The demand for more productivity through increased processing speeds is fueling the demand for UV inks which can be successfully used in this challenging application.
This article will address many of the issues the printer will face as they move to UV inks for this application. Graphic overlays consist of a polycarbonate or polyester film of various thickness ranging from 5 to 15 mils. All of the printing is done second surface to provide protection from harsh environments, strong chemicals or cleaning solvents that the overlay may be exposed to in everyday use. The most difficult challenge for the screen printer will be the degradation of the polycarbonate and polyester films which result from exposure to UV energy.
Exposing these films to short wavelength UV light (200-260 nm) results in a degraded surface layer which frequently may result in ink adhesion problems. If the ink is not formulated correctly and cannot successfully penetrate the degraded film layer to fuse with the underlying virgin layer, the resulting adhesion will be poor. The printer should insure that the UV ink they use for the printing on polycarbonate and polyester films has been formulated to successfully overcome this issue. Most graphic overlays will incorporate multiple colors (sometimes as many as 12 to 15) in a single job.
Therefore, the ink’s intercoat adhesion is absolutely critical. Since the typical overlay requires multiple colors, the first layers of the ink applied will see multiple exposures to the UV curing lamps, which can lead to a hard ink surface that cannot be etched by the next ink layer. Again the printer should ensure that the UV ink they are using has been formulated to provide an adequate processing window so that multiple passes under the UV lamps will not cause an intercoat adhesion failure. After printing, the overlay is laminated to the part (such as an instrument panel) with a pressure sensitive adhesive (PSA).
The PSA used is typically very aggressive and is designed to prevent the overlay from being stripped off the finished part. The ink must perform as a functional part of the lamination since it is the bonding layer between the PSA and the part. Therefore, the ink’s adhesion to the overlay film is paramount, and can not be affected by exposure to extremes of temperatures or humidity. Ink delamination is a common cause of failure due to an improperly designed or applied ink. The overlay may require embossing for use as an actuation or membrane touch switch. This will require that the UV ink have a high degree of flexibility and formability.
Typically, the end user of the actuation panel will require that it be able to pass a specified number of actuations (as much as 1 million or more). If you are producing actuation panels you should ensure that the UV ink being used has been tested to meet or exceed your customer’s specification (we recommend ASTM F1578-95).
The printer should work closely with their ink supplier to develop a full conversion plan when switching to UV inks to ensure all necessary issues are addressed. It is strongly recommended that the printer conduct a complete analysis to assess what areas need to be addressed prior to making a conversion to UV ink. Sericol’s Needs Assessment Survey provides an excellent format for conducting such an analysis. You can contact Sericol’s technical support team direct at 1-800-SERICOL to discuss the Needs Assessment Survey in greater detail.
At Sericol we continually monitor the trends in the screen printing marketplace. One such trend that has come under debate is the increased use of specialty inks rather than general purpose inks. In the following article, Mitch Bode, marketing director for Sericol, discusses the reasons behind this trend and how your particular screen printing business can benefit. There has indeed been a trend away from “general purpose” inks to ink formulations that perform better on a more narrow band of substrates or applications, particularly within the specialty category (including: compact disc, roll label, container decorating, etc.).
The driving force behind this trend is the specific product performance requirements demanded by printers in certain markets. As an example, compact discs are printed on highly automated equipment at a very rapid rate (50 to 70 discs per minute), so ink cure speeds must be very fast. This rapid cure could pose difficulties in general graphic printing, because cure controls are less precise. In addition, CD inks must be somewhat thixotropic so they don’t flow through the mesh when the CD decorator is idle, yet they must flow out smoothly, once on the disc.
This would not allow them to be printed easily on more conventional graphic-printing equipment. Conversely, many general graphic inks are not suitable for CD applications as many graphic formulations contain raw materials that adversely affect the polycarbonate disc and may cause distortion of the digitized data on the disc. And, as mentioned earlier, the viscosities of the graphic inks are different. They typically are not “short” enough to prevent drip-through on the automated CD printer, resulting in a mess that consumes valuable production time to clean up.
Another good example: Different weathering or light fastness requirements exist in the long-term decal (O.E.M. and fleet) market as compared to the P.O.P. market. Obviously, P.O.P. inks do not require the same weathering capability, since most applications are short-term exterior and/or interior. Obviously, the higher grade pigments used in a long-term decal ink are more costly than those pigments normally used in a P.O.P. ink, and most P.O.P. printers would not want to bear this increased cost. Even within applications such as P.O.P., there are a variety of substrates ranging from rigid plastics, like fluted polyolefins and styrene, to highly flexible materials like static cling vinyl and vinyl banners.
With the raw materials currently available to the screen-ink manufacturing community, it is not possible to formulate a single ink that will adhere to this broad band of stocks and provide the performance criteria desired. Different resins are required to gain adhesion to certain substrates which, in turn, require different solvents to solubilize these resins and result in multiple specialized formulations. In summary, I think the best analogy to this situation is the woodworking tool – The Shopsmith. According to ads, the Shopsmith is a drill press, it’s a lathe, it’s a table saw, and yes, it is even a sander.
And although I am confident the ads are true and the Shopsmith performs all these functions, I am equally confident that the discerning woodworker would prefer to use an individual tool designed for a specific function rather than a “multi-purpose” tool that may compromise the quality of the end product. Like the discerning woodworker, screen printers are becoming more specialized and demanding of ink and process performance. This demand will continue to drive the trend toward specialty inks for specific applications.
In this article we will clarify one of the more difficult aspects of using U.V. inks – distinguishing between U.V. ink that does not adhere due to inadequate cure versus ink that does not adhere due to ink/substrate incompatibility. The easier of these scenarios to troubleshoot is ink/substrate incompatibility. No one ink system is universal to all substrates or applications.
Varying performance objectives prevent the formulation of a “Universal Ink.” The first cut at ascertaining ink/substrate compatibility is to check the ink manufacturer’s recommendations or to consult with a service technician to determine if you are using the proper ink for your substrate. With the wide range and grades of substrates in use today, it may be necessary to forward your substrate sample to your ink supplier’s technical service team for their recommendation.
When time constraints do not allow for testing by the ink manufacturer, you will need to make this call yourself. Typically, if adhesion failure is the result of incompatibility between the ink and substrate, the cured ink film will have a tendency to flake or pop off the substrate with minimal mechanical abrasion or tape test. When this occurs, there is normally little or no ink residue left on the surface of the stock. The entire ink film releases from the substrate demonstrating no “bite” into the material.
Often these ink flakes will have a dry appearance and will crumble when rubbed between your fingers rather than roll up into a gelatinous lump. To eliminate the affect of postcure, allow the printed ink to remain untouched on the substrate for up to two hours and re-test. If the ink still releases cleanly and in relatively large flakes or sheets, it is likely that you will need to evaluate other inks for this application. However, before you throw in the towel on the ink/substrate combination you are evaluating, you may want to check with the manufacturer one more time to determine if there is a recommended adhesion modifier that may be added to a given ink line to improve adhesion characteristics on the most difficult substrates.
Care should be taken in that most additives have potential trade-offs of pot life, a postcure requirement, or loss of ink film flexibility. Once satisfied that an adhesion failure is not due to lack of ink/substrate compatibility, direct your attention to the most frequently encountered technical issue with U.V. inks – adequacy of cure. Thoroughness of cure and adhesion are closely related. The portion of ink film that actually provides adhesion to the substrate, called the contact layer, is the last layer to receive the crosslinking dosage of ultraviolet energy.
If U.V. energy does not reach this contact layer due to excessive ink deposit or inadequate energy exposure, the contact layer remains un-cured or under cured, resulting in adhesion failure. In this case, when you put pressure on the ink film with your thumb and twist, the undercured contact layer will appear soft and gelatinous. Often the cured surface of the ink may slide or move over the liquid layer beneath. Frequently, when an ink exhibits adhesion to a material but is grossly undercured, there will be an ink residue on the substrate surface indicating the contact layer is attempting to bite into the substrate.
Seldom will the ink layer completely release in the form of ink flakes or large pieces from the substrate as it did with ink substrate incompatibility. In less severe cases of undercure, the thumb twist test would indicate that the ink film is at least partially cured at the contact layer by not readily twisting. At this time a cross-hatch and tape test (described in Sericol Solutions Number Three late last year) should be used to confirm the level of adhesion.
If a significant portion of the ink film releases with the tape, but the ink film appears to be splitting in thickness with a portion of the contact layer remaining on the substrate, it indicates that the ink is exhibiting adhesion characteristics but is undercured. The portion of the contact layer remaining on the substrate will appear soft, indicating a lack of ink film through cure. To resolve this marginal cure we need to either decrease ink film deposit or increase U.V. energy exposure.
The last three issues of Sericol Solutions reviewed the variables that affect ink film deposit and suggested methods controlling the ink deposit including: mesh, stencil, squeegee, ink viscosity, press setup and substrate influences. Most U.V. inks require an exposure of 350 to 400 millijoules/cm squared to facilitate cure. If, after passing a U.V. energy monitor or “light bug” through the curing reactor at these energy levels we still have not achieved cure, we need to review these deposit variables.
If millijoule levels are below this range, first check lamp use hours, cleanliness of the lamp/reflector and maintenance history of the curing unit. Provided these issues are all satisfactory, we must either slow down the conveyor speed or increase lamp wattage output to bring millijoule readings into that 350 to 400 range.
Care must be exercised while increasing U.V. energy because this may increase the substrate temperature which can cause the substrate to distort. However, by experimenting with belt speed and lamp energy output, you should be able to strike a balance between heat related problems and ink film through cure.
For more help distinguishing between adhesion failure due to ink/substrate incompatibility versus undercure, contact Sericol’s Technical Service Department or the Screen Printing Technical Foundation (S.P.T.F.).
Pinholing is a result of uneven ink coverage or poor ink flow. After the ink is transferred through the mesh onto the substrate, it should flow evenly and completely cover the image. There are many causes of pinholing, which include:
- Low surface tension levels
- Contamination on substrate
- Screen closed or contaminated
- Age of substrate
- Uneven application of topcoats or adhesives on pressure sensitive films
- Inks not thoroughly mixed or not compatible with the substrate
Because screen tension can affect many other variables, such as squeegee pressure, off contact, ink deposit, localized moire, and registration, it is an area of the process that MUST be controlled. Control does not necessarily mean higher tension. Though it cannot be disputed that higher tension can and will produce higher quality, there is not a standard tension level, except one that can be maintained consistently throughout production on a day-to-day basis.
Generally, a 355-390 plain weave would be the best mesh choice. Plain weave deposits some 20% less wet ink film than twill. Plain weave is therefore recommended, for most fine line and halftone applications.
Proper flood pressure is related to ink viscosity, mesh count, mesh tension and press mechanics. Generally speaking, sufficient pressure should be used to provide a thin, uniform flood coat while minimizing screen deflection during the flood stroke. Too much flood pressure will force ink through the mesh resulting in image slur and excess ink deposit. Insufficient or uneven flood pressure will result in a correspondingly uneven ink deposit. This will be particularly evident with transparent inks or halftone printing.
A key component of consistent ink deposits is a sharp squeegee to shear the ink, allowing it to release cleanly from the mesh and emulsion. Therefore, a squeegee maintenance program should be implemented in every shop. Standard procedure should include a 12-24 hour “rest” for each squeegee following eight hours of use.
This rotation plan allows all residual solvents or monomers to evaporate out of the squeegee, returning it to its original durometer and dimensions. Sharpening squeegee blades after this rest period minimizes damage and the amount of squeegee blade removed. Remember, never use solvents to clean a squeegee blade after sharpening.
Perhaps one of the most important elements often overlooked or misused in the production environment is documentation. You should establish systems that track all movements in each department, analyze the information and create targets that will be tracked by your system.
If you keep the documentation simple and consistently follow through, you will benefit by reduced down time, procedural control and increased production. Each shop must customize the items they need to document.
The following list will give you a starting point:
- Mesh / Screen Tension
- Emulsion / Stencil
- Ink: Formula / Color Density / Gloss / Modifiers
- Squeegee Durometer
- Off Contact and Peel
- Drying Temperature / Cure Rate
Many production problems arise when making modifications to ink if additions of thinners are made by volume rather than weight. Colors within an ink range vary in weight per gallon resulting in additions of thinners/modifiers by volume having significantly different effect on each color. A digital scale should be used to properly weigh any additions made to ink. This is particularly important when weighing and adding very small percentages of thinners or catalysts to an ink. These modifications must be properly combined by weight if the ink is to perform consistently.
If you have experienced UV energy in your UV reactor diminishing over time, it is not unusual. In fact, you can expect it. As with any machinery, maintenance is easier and cheaper than repairs. Maintaining your UV reactor properly will extend its life.
The most significant variable in maintaining high UV energy is to clean the reflector and bulb often. It’s easy to see when they are dirty. Clean the reflector and bulb with a clean cotton cloth and alcohol. NEVER TOUCH THE REFLECTOR OR BULB WITH YOUR FINGERS AT ANY TIME. This should provide the longest life for your bulb and reactor.
Thorough, high speed mechanical stirring for one to two minutes is key to optimizing the performance of UV inks. Most UV inks contain between 10 and 20 raw materials, each with a different molecular weight. Likewise, each has a very specific and important function in the performance of the ink. These various components can “settle” in the container and must be mixed to homogenize the ingredients to obtain consistency of color, printability and cure.