Misregistered prints, damaged mesh, excessive squeegee wear, and production inefficiency are all symptoms of improperly tensioned screens. Learn how to eliminate these problems from your production process by developing standard procedures for screen
By David J. Lee
While virtually every screen printer recognizes the importance of properly preparing screens, many fail to apply correct techniques that will allow them to produce quality screens on a consistent basis. One of the most common stumbling blocks that printers face in screen prep is properly tensioning their screens and maintaining correct tension levels from screen to screen and job to job. Proper tensioning techniques (and retensioning techniques, when applicable) are essential to the entire production process, as is careful monitoring of screen-tension levels.
In this article, we'll explore the pitfalls of going to production with screens that are undertensioned. We'll look at the impact that poorly tensioned screens can have on press and consider how compensating for low screen tension through press adjustments can lead to print errors, as well as damage to the mesh and squeegee. Finally, we'll review correct screen-preparation pro-cedures and learn how they can help eliminate troublesome press setups and lead to quality prints.
Frame types and tension measurement
Before you can control tension levels on your screens, you have to understand the impact of the frame system and the importance of reliable tension-measurement tools.
Retensionable frames Retensionable frames are designed so that screens can be stretched to the desired tension level without requiring a separate stretching device. More importantly, they not only can be used for initially tensioning screens, but also, as the name implies, for retensioning screens before and after a print run.
The problem is that some shops fail to take advantage of the retension ability of screens mounted on these frames. Screens will be stretched to their production-ready tension level, coated and exposed, and used for a print run. Then they'll be reclaimed and sent back through the production process for re-use, but without being retensioned correctly or at all. This usually occurs in shops where retensionable frames have just been introduced to replace rigid frames or where there is a lack of training in the correct use of retensionable frames. Frequently, it occurs when employees cut corners in order to keep production flowing.
Often, time constraints placed on screen printers cause poor habits to develop. One common bad habit is that printers allow screens to return to production on the assumption that the next job will have less critical registration or content and that tension will not be a factor. Later in this article, you'll discover why this is not true.
Even with proper retensioning techniques in place, you must always be aware that screen tension will slowly drop over time. This means that tension levels must be continually monitored to ensure they're suitable for the application. The same requirement exists if you are using fixed frames.
Fixed frames With fixed frames, retensioning of the mesh after it's been attached to the frame is not possible. So problems can occur if the screens aren't properly tensioned to begin with or if proper techniques are not applied to maintain the correct tension level as long as possible.
Fixed-frame screens will slowly lose tension during the printing process and subsequent cleaning and reclaiming procedures. Over time, the screen will lose enough tension that achieving consistently good prints using proper setup techniques no longer will be possible. Tension loss can be most pronounced during large production runs, when the forces acting against the screen are prolonged.
Many printers are reluctant to retire a stretched screen on a fixed frame. Shop employees may find it difficult to justify material waste from stripping undertensioned fabric from frames or the time required to start the whole screenmaking process from scratch. As a result, many develop the poor habit of looking for less important jobs on which to use the screens.
Tension meter Without using a tension meter, it is impossible to verify that your screen-tension levels are appropriate and that tensions are consistent on all your screens. But in order to rely on tension meters, you need to ensure that the tools are correctly calibrated.
I once witnessed a situation in which the tension meter had not been calibrated correctly. Screens were being stretched to what was thought to be 20 N/cm, when in reality the tension levels were exceeding 32 N/cm. This excessive tension created problems on press as the screens became more prone to tearing.
Things only got worse after the problem was discovered but not correctly solved. In an effort to produce screens at a correct tension level, the shop started to undertension its screens. But because the tension meter was inaccurate, it was impossible to achieve tensions that were suitably high and consistent from screen to screen.
When a tension meter becomes inaccurate, the only solution is to send the tool back to the manufacturer for recalibration. Otherwise, the result will be improper tension levels and unpredictable printing results.
The impact of low screen tension on press setup
The negative impact of a poorly tensioned screen can be felt throughout the print-production department. Low tension decreases press-setup efficiency and accuracy, reduces productivity and print quality, and promotes damage to the mesh and squeegee.
Warning signs of improperly tensioned screens are readily apparent during both press setup and operation. If a press has been set up correctly and the operator still finds the need to increase the screen's off-contact height, then a problem with screen tension probably exists. Poor tension also is a likely culprit if a press operator needs to adjust a press's peel settings beyond the level normally considered to be correct.
A poorly tensioned screen will not snap off from the substrate correctly during the print stroke. Instead, the screen will stick to the substrate after the squeegee passes. Screen tension that's too low also will be apparent in the print, which will exhibit poor edge definition or irregular ink coverage. Once the operator can rule ink viscosity and squeegee/floodbar pressure out as causes, it's generally safe to assume that the screen doesn't have enough tension. To compensate, the operator must increase the off-contact, which creates the potential for many other problems to occur. Here are a few of the more common ones we see when low screen tension is combined with high off-contact:
Damage to the mesh The combination of adequate screen tension and off-contact leads to sharp, well-defined prints because you add tension to the mesh where the squeegee comes into contact with the screen and substrate during the print stroke. This added tension allows the mesh to snap off of the substrate immediately after the squeegee passes and preserve the edge definition of the printed image. Consider that at a tension level of 20 N/cm, a printing screen experiences approximately 100 lbs. of force acting against the frame and through the mesh. If a screen tensioned to 20 N/cm is used with an off-contact of 1/4 in., the tension level can jump to more than 80 N/cm at the point of squeegee contact during the print stroke. That is more than 400 lbs. of total force.
For most mesh counts, 20 N/cm is the minimum safe tension level to use. At this level, good snap off can be achieved with minimal off-contact. With lower tension levels, off-contact must be increased to provide the desired snap-off characteristics. But this introduces the potential for a problem known as screen stretch. Using low-tension screens with high off-contact increases the distance that the squeegee has to move the mesh so that the stencil will make contact with the substrate. Extra force on the squeegee is required to ensure this contact. As the squeegee moves across the screen, this excessive force and screen displacement can cause the mesh--and the printed image--to distort, making it impossible to register subsequent colors. More on this in the section that follows.
The repeated application of this excessive squeegee force, particularly during longer print runs, also can weaken the mesh threads and lead to complete screen breakdown. Even if the screen survives these conditions, the threads are likely to be weakened. For users of retensionable frames, the result can be that the mesh won't support retensioning--it either fails to achieve an adequate tension level upon restretching or tears during the process.
With fixed frames, the consequence of low screen tension and high off-contact is extremely short screen life. This means that more screens will have to be stretched more often to support production. Besides the waste in mesh that this causes, the extra time required to stretch additional screens also can be costly to screen shops. Working with undertensioned screens in this manner can be an easy trap to fall into, and if it goes unchecked long enough, it could become adopted procedure.
In general, the best way to guarantee quality printed results without risking screen stretch or complete screen failure is to use the highest possible screen tension with the lowest possible off-contact. This approach requires the least overall squeegee pressure and prolongs the life of the screen.
Misregistration As mentioned previously, print misregistration is a serious problem that may emerge as a result of improperly tensioned screens being stretched excessively by the squeegee during printing to overcome high off-contact. The mesh will have a tendency to stretch in the direction of squeegee movement (Figures 1A, 1B). This causes prints to become larger than the target size. When this occurs during a print run on any of the colors being applied, the chance of all the colors aligning properly decreases dramatically.
Image skewing Image skewing is another phenomenon that results from screen stretch caused by low mesh tension. If the squeegee stroke itself is not controlled to remain parallel with screen edges in the stroke direction, the stretching of the mesh during the stroke can be inconsistent across the screen's width, resulting in a skewed or twisted image. When skewing occurs with any screen in a multicolor job, registration problems are certain to arise. Improperly aligning the artwork on the screen or the screen on the press only makes the problem worse.
Bleeding Smearing or bleeding of the printed image is another common defect caused by poor mesh tension. Remember, lower tension necessitates higher off-contact, which leads to greater mesh displacement during the print stroke. The mesh displacement significantly reduces the contact angle between the mesh and squeegee (Figures 2A, 2B). The result is similar to what happens when a squeegee flexes under excessive pressure.
As the mesh is forced downward at a greater angle, the squeegee begins to double as a flood bar, actually pushing more ink into the stencil and causing too much to be transferred to the substrate. The excess ink is squeezed beyond the stencil edges in the stroke direction and creates heavy, irregular coverage. The same phenomenon contributes to dot gain with undertensioned screens. Once again, using sufficiently tensioned screens (at least 20 N/cm) along with minimal off-contact distance and squeegee pressure will allow you to maintain a contact angle as close to the initial setup angle as possible.
Squeegee wear Mesh itself acts like a fine sandpaper on the squeegee edge. The abrasive nature of the mesh is most obvious when you use screens with lower thread counts, which have larger thread diameters and are therefore more coarse. The problem also tends to be more pronounced when screens feature stencils made from capillary films rather than liquid emulsion. The capillary stencils are applied to the print side of the screen and generally don't encapsulate the threads on the squeegee side as completely as stencils created by emulsion coating. Consequently, the squeegee side of a screen featuring a capillary-film stencil is more rough and damaging to the squeegee. The abrasive effect is enhanced by aggressive inks and cleaning agents, which weaken the squeegee material and promote breakdown of the squeegee edge.
As mentioned previously, the increased off-contact distance you need to print with low-tension screens requires you to increase squeegee pressure. The extra force on the squeegee not only causes the contact angle between squeegee and mesh to decrease, but it also increases the friction between the squeegee edge and the mesh, even when ink is present to serve as a lubricant.
Squeegee wear from low-tension screens is most noticeable during long runs, especially those involving UV inks. The degradation of the squeegee edge shows up as a decrease in printed image quality during the course of the run. The accelerated squeegee wear caused by low screen tension leads to lost time and increased material costs because blades must be changed more often.
Adopting the right procedures
Now that you've considered the primary causes of undertensioned screens and the drawbacks they bring to production, it's time to eliminate the problems they cause by developing effective and repeatable procedures. These procedures should identify techniques and tolerances to use in screen preparation, as well as on press.
Screen preparation Proper screen-preparation techniques are essential for ensuring efficient production and high-quality prints. Becoming proficient in these techniques requires time and training, as well as a clearly defined procedure that employees can refer to when questions arise. Here are some tips that can help any shop produce quality screens:
1. Develop a set of procedures and write them down in a step-by-step format. Make sure that employees understand the procedures and stick to them whenever screens are prepared. This way, everyone in the operation will approach screen preparation in the same manner, and the characteristics of the resulting screens will be consistent.
2. Maintain an accurate and up-to-date chart that displays correct tension levels for the types of mesh commonly used in your operation, and post the chart where employees can easily find it in the screen department. Keep the chart simple and list only the mesh counts you use. This will make it easier for any employee to determine exactly to what level the screen should be tensioned. A sample chart is shown in Table 1. Make sure your staff stays up to date with the latest tensioning procedures because new types of mesh and refined tensioning techniques are constantly introduced.
|Table 1 Sample Mesh-Tensioning Guide|
|Mesh count (threads/in.)||1st stretch (N/cm)||2nd stretch ( N/cm)||3rd stretch (N/cm)||Retension after 1st run (N/cm)||Retension after 2nd run (N/cm)||Retension after 3rd run (N/cm)|
|Chart developed using Sefar PECAP LE mesh on Newman Roller Frames from Stretch Devices.|
3. Any time you tension (or retension) a screen, record its tension level and the time and date when it was tensioned on the screen frame. This will help guarantee that any screenroom employee can maintain the screen's tension at the correct level and that the tensions will remain consistent from screen to screen.
4. Thoroughly train employees to carefully follow procedures and use tensioning records as a guide. It may be possible that some employees require more training on certain procedures. Don't hesitate to give it to them. Not providing adequate training can prove to be far more costly in the long run. Remember that a well-trained employee will be a confident employee who will produce quality screens at a much faster pace. Keep an updated training record for each of your employees so that you know who is qualified to work on each piece of equipment. Do not grant anyone else access to the equipment without proper training.
On press Screens that are consistently tensioned to the correct level are only one piece of the puzzle for ensuring good print quality. Proper press setup is important as well, not only to provide accurate prints, but also to maintain long screen life. The following list identifies the primary concerns that need to be addressed during press setup:
1. The levelness of the press bed or platen, screens, and other press components has a substantial influence on print quality. Levelness should be checked prior to running any job. It's not a bad idea to make levels part of each press's setup toolkit. Operators can use the levels to check every main component of the press, including the print table, squeegee, floodbar, and screen. This will ensure that the entire press is completely level during the print run. It also will keep squeegee and floodbar pressures consistent across the screen, decreasing wear on the squeegee and damaging stress on the mesh.
2. Proper squeegee selection and use also are critical for achieving quality prints. Every detail about the squeegee must be considered. First, the squeegee blade must be level in the holder. Second, the squeegee must be level relative to the screen and press bed. Third, the correct blade durometer must be chosen for the task at hand. And fourth, the level of squeegee pressure during the print stroke must be correctly adjusted. Usually, the first three requirements are easy to satisfy. The fourth, however, is frequently overlooked or carried out incorrectly.
Many press operators are quick to increase squeegee pressure during a print run to overcome various problems in the print. This is not always the best solution. Squeegee pressure can be over-applied very easily, particularly with screens at higher tension levels. Besides degrading overall print quality, excessive squeegee pressure produces extra wear on the mesh and the edge of the squeegee.
To make matters worse, press operators sometimes set up jobs so that the squeegee will pass over the edges of substrates and substrate-positioning guides on the press bed. It's impossible to miss the impressions left on the screen and stencil by the substrate edges and guides when too much squeegee pressure is applied. If the pressures are to high, you can bet that these areas of the screen will have pinholing problems and will be more prone to tearing.
When a properly tensioned screen is in place, squeegee pressure should be minimized to the point where the screen just "kisses" the substrate as the squeegee passes. During setup, squeegee pressure should be increased incrementally and evenly on both ends of the blade until a complete print can be seen on the substrate. If print coverage starts looking too thin or too heavy as the run progresses, you can resort to adjustments other than squeegee pressure to eliminate the problem, such as changing squeegee angle, floodbar angle, and blade durometer. If, for example, the ink deposit becomes too thin, then slightly decreasing the angle between the squeegee blade and sustrate may be all that is required. Alternately, a slight increase in floodbar pressure to help prefill the mesh may be a solution. Sometimes merely changing to a softer blade durometer will do the trick.
3. Earlier in the article, you learned how proper off-contact distance between the screen and substrate not only improves image quality, but also helps prolong screen and squeegee life. For most applications, if the screen tension is adequate, the highest level of off-contact needed will be 1/8 in. Greater off-contact will only increase the chance for errors during the print run.
High-tension screens support low off-contact distances and require less squeegee pressure to transfer the image to the substrate. Under these conditions, halftones are much less susceptible to dot gain, and fine lines or patterns in a print are less likely to exhibit problems like bleeding. The lower off-contact heights also allow printers to increase squeegee and floodbar speeds for greater productivity. To ensure proper snap off with larger images, the press's peel settings may need to be adjusted. Increasing the stroke length also may help. But the point is that the problem can be corrected without increasing the off-contact distance.
Another reason that low off-contact distance is so important is that it keeps more of the screen usable for the image area. As the off-contact increases, so does the mesh-deflection angle, which increases pressure on the corners at each end of the squeegee and makes distortion of the print more likely (Figures 3A, 3B). This distortion only can be avoided by creating a smaller deflection angle, either by reducing the width of the image and squeegee relative to the screen or reducing the off-contact height.
High levels of off-contact with a low-tension screen are bad enough. But when higher tension levels are combined with high off-contact distances, then total screen failure is right around the corner. However, once high-tension screens are being produced consistently, printing with low off-contact becomes easy and many potential problems in the screen-printing process can be avoided or completely eliminated.
Give screen tension the attention it deserves
Using proper screen-preparation techniques, combined with proper setup methods on press, is the only way to guarantee accurate and predictable results from the screen-printing process. There is no room for shortcuts if success is your goal.
To ensure this success, you need to develop and write down comprehensive screenmaking and press-setup pro-cedures, then make the procedures easily accessible to every employee in your operation. Insist that the procedures be followed and take steps to enforce your policy. If the procedures you introduce are new, you may experience an initial decrease in productivity as employees get used to them. But this will only make the rewards sweeter when they do emerge. Over time, as the procedures are adopted by employees and become a part of everyday practice, you'll see productivity increase along with print quality and consistency.
Be prepared to invest both time and money in order to develop and maintain correct procedures. Invest in tensioning equipment that will allow you to tension screens in a repeatable manner. If you work with retensionable frames, make sure to use a torque wrench when adjusting mesh tension so that you provide uniform tension across the entire mesh and frame and avoid problems such as roller slipping.
Once you have the proper equipment for screen tensioning and press setup, make sure to maintain it correctly. Have your tension meters calibrated regularly, and keep a record of the calibration dates. If any of your equipment becomes faulty or unreliable, stop using it. Fix the problem or replace the tool, then continue.
The most important step you can take to establish reliable screenmaking and press-setup procedures is to put these processes in the hands of qualified personnel. Simply having good equipment does not mean that correct procedures will fall into place. You need well-trained employees who understand why the procedures exist and what can happen if the procedures are ignored. Once you have skilled individuals backing up the procedures, a consistent stream of high-quality screens and high-quality prints is certain to follow.
About the author
David J. Lee is head of graphics printing at Deluxe Engraving and Screen Print Service, a Rio Rancho, NM-based screen shop specializing in signage, promotional products, awards, and decorated garments. His background includes both production and management positions in graphics, industrial, and textile screen-printing environments. Lee's experience encompasses screen preparation, as well as print production using both solvent-based and UV inks.
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