The Fundamentals of Screenmaking, Part 2
Screen Stretching and Tension Measurement
Tensioning is one of the most critical aspects of the screenmaking process. The tensioning procedure you use prescribes the speed, stretching intervals, and stabilization time of the stretching process. The proper stretching method is one that satisfies all of the quality requirements of the image you will print -- but not all applications have the same quality requirements. The scope of our discussion here will include coverage of some of the major techniques currently used in the industry and a discussion of reliable ways to measure tension. Let's start with tension stabilization.
Tensioning mesh serves two functions. First, the fabric must be stiffened by tension so the stencil can be applied and maintained in terms of dimensional stability. Second, the mesh must be stretched to a tension level where it can provide the necessary recovery, or snap back, to transfer ink during the printing process. Properly stretched screens have elastic memory -- the ability to return to their former state (or tension) after deflection during the print stroke.
The basic goal of a good tensioning procedure is to minimize tension loss caused by both cold-flow elongation of mesh threads and thread realignment in the fabric before printing (see "Mesh Movement and Its Impact on Screen Tension" by Dr. John Anderson, Screen Printing, March 2003, page 44). Mesh is an unstable, woven structure that must be manipulated. We use stretching equipment not only to apply tension to the mesh, but also to begin the stabilization process. The more stable the mesh, the greater the opportunity to achieve superior results when printing.
Mesh-stabilization time describes the period in which the mesh is allowed to rest or align once the desired target tension is reached. This applies with retensionable frames, as well as with stretch-and-glue frames before the mesh has been affixed. With mechanical stretching systems, including retensionables, one or two readjustments to the target tension are needed during this stabilization phase to compensate for tension loss due to thread elongation and realignment. Pneumatic stretchers usually do not require this adjustment because the constant force from the clamps automatically takes up the slack as the threads stabilize.
Some manufacturers recommend a 15- to 30-minute stabilization time. Waiting 30 minutes will result in less tension loss. For very large screens, longer stabilization times may allow the forces to equalize from one end of the mesh to the other. Stabilization times that exceed one hour generally do not provide enough benefit to justify the extra time. Whatever time you allow, make sure it's the same for all screens you stretch.
A second concept that relates to stabilization is waiting at least 24-48 hours after the mesh is glued to the frame (or final retensioning in the case of retensionable frames) before applying a stencil. The greatest tension loss occurs in the first 24-48 hours after final stretching. Letting the screen relax before putting it in production will prevent this tension loss from occurring on press, where it can distort the stencil image, change the ink deposit and color, and lead to loss of registration. This is especially true when printing four-color-process work or other fine-detail applications.
Current tensioning procedures
Three tensioning procedures prevail in the screen-printing industry: staged, rapid, and pulsed tensioning. Some shops mix these procedures together to form a hybrid tensioning process.
One ground rule that stands, no matter what the method or equipment used, is to always stretch screens the same way. This includes using the same target tension, speed, stretching intervals, and stabilization time. Measurement intervals should also be done at the same time in the process so they can be accurately compared.
Staged tensioning was one of the original methods used in the screen-printing industry and remains a common technique. The mesh is brought up to the final target tension in stages to allow the mesh to stabilize along the way. Job requirements, time limitations, and production needs dictate the necessary length of stabilization and number of retensionings. While effective, this method is more complicated and time consuming than others. Here is a general procedure to use for staged tensioning:
1. Insert the mesh in stretching system, taking extra care to ensure that the fabric is properly aligned and corners are adequately softened.
2. Tension the warp and weft directions to the desired target tension value.
3. Allow the fabric to align for one minute.
4. Increase the tension 2-4 N/cm in both directions.
5. Let the fabric align for one minute.
6. Repeat steps 4 and 5 until desired tension is reached.
7. For pneumatic systems, let the mesh sit for 30 minutes to stabilize. With mechanical stretching devices, allow 15 minutes for stabilization, then retension to the target value. Wait another 15 minutes and retension to the target again.
8. Check and record tension with a tension meter using the five- or nine-point-measurement method. (See "Multipoint Measurement" on page 48 for more information).
9. Glue the fabric to frame.
Rapid tensioning is a term that was first applied to a screen-stretching procedure developed by the Screen Printing Technical Foundation. Rapid tensioning generally requires fewer than five minutes for the entire stretching process and will go from zero tension to the target or final tension in less than a minute. The screen is allowed to stabilize for just five minutes before it is glued to the frame. This is in sharp contrast to staged tensioning, in which some approaches may require up to 45 minutes to complete. Staged tensioning actually increases the degree of elongation, thereby causing the screen to stabilize at lower tensions than the rapid method.
Rapid tensioning tends to be easier to implement, and its simplicity and speed contribute to the process's consistency. Other benefits of rapid tensioning include reduced time to construct a screen, more consistent tensions in the plant, and ease of training. Rapid tensioning compares favorably to other, slower, methods of stretching—specifically in the areas of overall screen quality and tension loss after printing. Studies by SPTF and others have shown that there is no apparent gain realized from staging the fabric up to the target tension.
Rapid tensioning can be done on any stretching system, but individual pneumatic clamps seem to produce the best results. In theory, it should be possible to increase the stretching speed of any known tensioning system. However, not all devices lend themselves to the quick ramp-up speeds and four-way-pulling possibilities as easily as the many pneumatic systems that are now available.
Although most of the original work on rapid tensioning was done on a "single circuit" pneumatic tensioning system (both warp and weft cylinders pulling with the same force at the same time), field testing has been successful on both mechanical stretching devices and retensionable roller frames. Keep in mind that rapid tensioning may not make sense for every application. For example, it is physically impossible to rapidly stretch screens on grand-format retensionable frames by hand.
Extra monitoring is initially necessary when you implement a rapid-tensioning program. You must make sure the corners are adjusted properly and that warp and weft tensions are even. One word of caution when doing things faster: The process is much less forgiving when you make a mistake. Any poor work habits, such as leaving rough edges on the frames, misaligning the fabric, and failing to soften corners, will manifest themselves quickly in the form of torn screens. Even small errors will surface quickly when rapid tensioning.
Here are some guidelines to follow:
1. Determine and record what air-pressure settings (or what distance to move the retensionable bars) will produce the target screen tension for a given mesh and frame size. If warp and weft controls must be independently adjusted, such as on unequal frame sizes, a set point must be determined on each one. Smaller frames can be set up to be controlled by one regulator. Therefore, they have one set point.
2. Determine and record the correct distance to soften the corners for the final target tension.
3. When using a pneumatic system, adjust the bleed valve so that the full, regulated pressure is applied to the clamps at the approximate rate of 1 N/cm/sec.
4. Set the pressure regulators to the proper values determined in Step 1. Turn the air pressure off with the master switch. (Most systems have a single master switch that will turn off pressure without using the pressure regulator.)
5. Insert the mesh, taking extra care to ensure that the fabric is properly aligned and corners are softened.
6. Activate the switch to apply the pressure to the clamps and reach your final target tension. Use a tension meter to confirm the results.
7. Allow the screen to align for approximately five minutes.
8. Readjust to the target tension if needed.
9. Check and record tension with the five- or nine-point-method.
10. Glue the fabric to the frame.
Pulsed tensioning is an advanced form of rapid tensioning that can reduce tension loss. Pulsed tensioning, as the name implies, applies force to the fabric in pulses. The mesh is tensioned to its target tension, held at that tension for approximately one minute, and then released from the force completely until the fabric returns to zero tension. The process is repeated at least four more times before the mesh is glued to the frame. This method requires the use of a pneumatic stretching system to produce the pulsed effect.
A simple procedure for applying the pulsed-tensioning method follows:
1. Set the pressure regulators to the appropriate levels. Turn the air pressure off with the master switch. (Most systems have a single master switch that will do this without using the pressure regulator.)
2. Insert the mesh, taking extra care to ensure fabric is properly aligned and corners are softened.
3. Throw the switch to apply the pressure to the clamps, then use a tension meter to confirm that you've reached your final target tension.
4. Allow the fabric to align for one minute.
5. Turn off the air pressure switch until all the tension is gone.
6. Immediately switch the air pressure switch back on so the mesh returns to the target tension.
7. Repeat steps 4 through 6 four more times (you can repeat nine times for even better results).
8. Check and record tension with the five- or nine-point-measurement method.
9. Glue the fabric to the frame.
A certain amount of staging is built into the process of manually stretching retensionable frames because all four sides of the frame can't be adjusted at once. Some retensionable-frame manufacturers recommend a series of tensioning stages to bring the fabric up to the final tension value, with anywhere from 30 minutes to four hours between each one. This allows an extended period of time for the fabric to align and stabilize.
You can apply rapid tensioning by first determining the distance to move each side to obtain a certain tension. You then move each side the appropriate distance so time is saved in this initial tensioning phase. Several retensionings can then take place to compensate for tension loss. Mesh retensioning should be an ongoing practice if you want to realize the full benefit of retensionable frames. The purpose of retensioning is to improve the consistency and continuity of your screens from one to the other and from job to job.
When properly retensioned, the screens will stabilize after several uses in a process known as work hardening. Screens that are work hardened may not actually need to be retensioned every time. However, you should still check them on a regular basis. Here are the steps involved with work hardening a mesh:
1. Use the method of your choice to tension the mesh to the desired target tension.
2. Perform at least two retensionings on the new mesh over a period of four to eight hours to compensate for the tension loss.
3. Use the screen to print a low-tolerance job, such as simple line artwork without tight registration requirements.
4. Reclaim the screen.
5. Retension the screen to the target tension. The screen should not have a stencil on it when you perform this operation.
6. Use the screen for another low-tolerance job.
7. Reclaim the screen again, and retension a second time.
8. The screen is now ready to be used for close-tolerance work.
9. For best results, retension the mesh to the target tension after each reclamation until tension loss no longer occurs.
The stall-point method is another advanced stretching technique. Manufacturers generally do not encourage the practice, but it deserves explanation. When using this method, the fabric is tensioned until the stall point is reached. The mesh's stall point is reached when additional force applied to the fabric will not result in a sustainable increase in screen tension. This is seen when the tension meter's readings either do not increase or only increase slightly and then slowly fall back as more force is applied to the screen.
The stall tension on static frames is usually very high, so when the screen relaxes, the resulting tension is higher than what you'd achieve with other methods. If you use retensionable frames, the work-hardening effect will enable you to reach the stall point each time you re-tension to a higher level. If you exceed the stall point during tensioning, the mesh will break. Taking mesh to its stall point puts the screen in a very vulnerable state—to the point where if you use too much squeegee pressure, the mesh will break on press. For this reason, it's safer to tension the mesh to 1-3 N/cm below the actual stall point. To do this, you must tension a test screen all the way to the stall point and record the final tension.
Tensioning large-format frames
The appropriate tensioning technique for large-format frames depends on the equipment. Frame profile has a huge impact on tension, with losses as much as 50% possible with weak frames. Processes for tensioning large-format screens on mechanical and pneumatic systems follow.
For a mechanical stretching system:
1. Insert the mesh, taking extra care to ensure fabric is aligned and corners are properly softened.
2. Tension the warp direction to half the target tension.
3. Allow the fabric to align for two minutes, and adjust the tension to where it was if needed.
4. Tension the weft direction to half the target tension, adjusting the stretcher so both warp and weft are at the same level. (Remember that stretching in one direction creates tension in the other.)
5. Allow the fabric to align for two minutes.
6. Gradually tension the warp and weft until you reach the target tension desired.
7. Allow the fabric to align for 10 minutes.
8. Readjust to the target tension if needed.
9. Use a tension meter and the nine-point-measurement method to check tension. Be sure to record your results.
10. Glue the fabric to the frame.
For a pneumatic stretching system:
1. Insert the mesh, taking extra care to ensure fabric is aligned and corners are properly softened.
2. Tension the mesh at approximately 1 N/cm/sec directly to the target tension level in both warp and weft. Confirm results with a tension meter.
3. Allow the fabric to align for 15 minutes.
4. Readjust to the target tension if needed.
5. Use a tension meter and the nine-point-measurement method to check tension. Be sure to record your results.
6. Glue the fabric to the frame.
Tensioning extremely long frames
Preparing screens for use on long frames can present a challenge. When tensioning large-format frames that are twice as long as they are wide, you'll notice that the warp direction of the mesh bolt is typically oriented lengthwise. The additional slack in this direction can cause problems unless you use a stretching device that can compensate for it. A pneumatic system with dual control can help produce more consistent results by tensioning both directions of the mesh to within 2 N/cm of each other.
If you use a stretch-and-glue tensioning system that can't pre-bow the frame, you can compensate for the tension loss by tensioning the mesh 1-2 N/cm higher in the mesh direction that spans the two long sides of the frame. The following steps explain how to use such a system to stretch screens for long frames. Figure 1 illustrates part of the process.
1. Insert the mesh, and clamp the fabric only on the short end clamps. Take extra care to ensure that fabric is aligned and corners are properly softened.
2. Tension the direction that is clamped to roughly half of the final air pressure needed to reach the target tension for that mesh (you must determine target tension beforehand).
3. Close in the long direction clamps.
4. Tension the newly clamped side to half of the final air pressure.
5. Adjust both directions to the desired final tension. Confirm results with a tension meter.
6. Use a tension meter and the nine-point-measurement method to check tension. Be sure to record your results.
7. Glue the fabric to the frame.
Angling mesh on a frame
Angling mesh on a frame (mounting at a bias) can improve print quality in some types of printing. Angled mesh can reduce sawtoothing in artwork that includes thin, straight, parallel, and perpendicular lines. Angled mesh can also help battle certain types of moiré in halftones. Other benefits include longer mesh durability, improved ink flow, and faster printing speeds. However, if your prints don't incorporate fine lines or moiré prone patterns, the increased cost of mesh waste isn't worth the benefits of this method.
The best procedure for angling mesh on a frame is to rotate the frame under mesh that has been stretched normally. A stretch-and-glue system that allows the frame to be angled is required for this method. Rotating the frame provides the best angle accuracy and most consistent mesh geometry and stretch. This method also makes it possible to affix the mesh to multiple smaller-format frames.
Angling the mesh in the stretching unit itself tends to increase mesh elongation, creating inconsistent mesh stress and skewed mesh geometry. These effects can lead to image distortion or moiré. Mesh also should not be inserted at an angle on retensionable frames. Common angles for mounting stretched mesh are 7° for halftone and process-color work, and 15° or 22.5° for line work.
Determining Proper Screen Tension
Recommended screen tensions are available from mesh manufacturers and distributors, and the tension values vary with mesh type, count, and thread diameter. These suggested tensions can be used as target values during the stretching process. A tension level should be chosen that can be consistently achieved and maintained under your shop conditions.
Often tension specifications are provided in a range or in three levels dictated by the printing application, frame size, tensioning device, and screenmaking expertise. The lowest level of the tension range should be made the target tension if you don't have state-of-the-art tensioning equipment or if you have minimal screenmaking experience. This level also is appropriate if you stretch screens for large-format frames (one side measuring at least 50 in. or 1270 mm). However, proper procedures can allow you to reach higher tensions on large-format screens.
The middle and highest parts of the tension range are reserved for advanced and expert screenmakers who use state-of-the-art equipment and stable frames. Proper tensioning procedures are critical for achieving mid to upper tension levels.
The impact of improper tension
Improper tension conditions fall into three categories: too little, too much, and inconsistent tension. All of these can, and will, create problems in a screen-printing operation. Let's examine each of them further.
Insufficient tension Some of the effects of low tension are poor print detail and edge definition, slower printing speeds, excessive image elongation, poor registration, and variation in the flood coat. Inconsistent ink thickness also results from low or inconsistent screen tension and can lead to color variation. With halftones, moiré and excessive dot gain are often worsened or created from poorly tensioned screens. Too little tension results in poor snap-off or peel and a reduction in ink flow through the mesh openings. It can also increase the incidence of mesh marks that normally disappear at higher tensions.
Off-contact distance must be increased on press to compensate for low screen tension. However, high off-contact requires the squeegee to force the mesh down to the substrate, elongating and distorting the image and creating severe registration difficulties. The force required to overcome the off-contact distances also results in excessive wear on the mesh, stencil, and squeegee.
Too much tension Although many screen printers—especially garment printers—tout the advantages of high tension, everything has its limit. The tension limit for a particular fabric is its yield point. The yield point describes the tension at which the mesh's elastic memory breaks down and the threads reach the point of plastic deformation where they are permanently damaged. When the mesh is tensioned beyond the yield point, it will not maintain stable tension during the print run—nor will it snap back correctly. The result is poor registration and print quality. Additionally, tension loss will become excessive over the remaining short life of the screen, and the screen will be more prone to break at some point in the process.
The fabric's yield point comes before its breaking point, but there are no practical shop tests that will help you determine that point. In addition, each mesh is different. You can avoid tensioning a mesh beyond the yield point by following—and not exceeding—the manufacturer's recommendations for maximum tension.
Inconsistent tension The squeegee cannot consistently transfer ink when variations in tension exist on a screen. You won't have control over the printing process until you can guarantee consistent tension across the screen. Tension variations among different screens can result in inaccurate registration of multicolor images, as well as different printing effects on the image. Furthermore, you'll also have to adjust the press for each screen, which increases set-up time. Consistent tension is especially important when printing process-color work or any high-tolerance job.
Measuring screen tension
Tension meters are a must. These devices measure screen tension based on the application of direct pressure against the fabric. A tension meter measures screen deflection and then converts it to units of Newtons/cm, which is simply a measure of force over the distance that it is exerted (the US equivalent is lb/in.).
Tension meters are readily available in mechanical and electronic models (Figure 2). Prices range from $250-1000. The electronic ones are more accurate and easier to read, but they're more expensive. Mechanical meters are the most common in the industry.
Most tension meters are highly reliable and repeatable within their own calibration standards; however, a lack of official calibration standards means that a meter from one manufacturer often won't agree with readings taken from another manufacturer's meter. You also might notice variations in readings from the same manufacturer's instruments if the devices don't share the same scale. For example, a 0- to 50-N/cm instrument may not agree on the tension values measured by a 0- to 130-N/cm unit over the whole range of tensions between 0-50 N/cm. For absolute reliability, you should stick with one instrument manufacturer and the same scale on the dial.
Using a tension meter
The tension meter is a quality-control tool that we use to monitor screens during their useful life. You should use a tension meter on every screen. This includes when you originally stretch the mesh, adjust the tension, or re-stencil a screen. Measuring screens before and after a print run also can be beneficial. Measuring tension ensures achievement of target tension during stretching, consistent tension within a screen during the tensioning process, and tension consistency from screen to screen. Measuring tension also enables you to make adjustments that prevent screen breakage and other problems on press.
When the meter's measurement probe responds to tension, a small, movable foot at the end of a lever arm, located below and between two fixed pads at each end of the instrument, presses against the fabric and measures the amount of deflection on the mesh. The more deflection, the lower the tension. That deflection is then compared internally against the standard originally used to calibrate the instrument. This results in a number that represents the mesh tension. To measure the mesh tension, position the gauge on the tensioned mesh so the long edge of the bottom measurement foot is parallel to the thread direction. In this position, the face of the dial on the meter is also parallel to the direction being read (Figure 3). If the measurement foot is aligned with the warp mesh direction, the warp tension is displayed.
Measure the weft direction by rotating the gauge 90° so it is parallel to the weft. You should measure warp and weft directions independently. Angling the meter at 45° to the threads will produce an average measurement, but it won't tell you whether the tension is uniform in both directions.
Sliding the meter around on the mesh is not a good practice. Tension meters should be lifted off the screen and placed back on the mesh for the next reading. The hard, metal feet can actually damage the surface of the mesh. If the meter's needle is jumpy, you can give the screen a light tap to get the meter to register the correct tension. Tension meters should not be positioned closer than 2.5 in. (64 mm) from the frame for accurate readings. Never leave the tension meter on the mesh while the mesh is relaxing (the weight of the meter may cause the mesh to tension unevenly). And remove the meter after you've taken your measurements.
Preventing tension headaches
Improperly tensioned screens can cause a host of problems in the printing process. You can keep these issues from intruding on your next job by using the appropriate stretching equipment and procedures to tension your mesh, checking your work with a calibrated tension meter, and documenting and comparing your results each time you tension screens.
Once your screen reaches its target tension, you should take measurements in at least five places on the screen to assure consistency in both the stretching and mounting procedures. This is known as a five-point-measurement method.
To establish your measurement locations, divide the screen into four equally spaced quadrants (four rectangles), each with a corner joining at the center of the usable mesh area. Take one measurement at the center of the screen just as you would at the beginning stages of tensioning and one measurement at the center of each of the four quadrants, for a total of five tension readings in both the warp and weft directions. All points should fall within the image area (see photo). Measurements for larger screens (where one side is more than 40 in. or 1016 mm long) should be expanded to include nine points.
You can create a template that has the measurement points marked to fit inside the screen. This is useful as a training aid to ensure that measurements are taken in the same place each time.
About the author
Dawn Hohl is a technical training manager for the Screen Printing Technical Foundation (SPTF) in Fairfax, VA. She oversees SPTF's workshop programs, develops training resources, and conducts research into the screen-printing process. Hohl is a frequent author and speaker at industry events, and she's a member of the Academy of Screen Printing Technology. She can be reached at firstname.lastname@example.org.