Reducing Screen Casualties
Unravel the causes of ruptured screens, and find out how to keep the problem from repeating.
At roughly $10-40 per linear yard, polyester screen fabrics are one of the most expensive supply items in screenmaking. Unfortunately, they are also delicate and easy to damage. A shop that wastes $100-200 a week in mesh (2-4 medium-size screens) may end up increasing its annual production costs by as much as $8000-10,000!
Careless handling, poorly designed storage, and mishaps on the press account for about 20-30% of ruined screens. Although these problems are occasionally preventable, accidents do happen, and you have to expect some screen replacement as an unavoidable cost of doing business.
The majority of screens, however, are damaged during screenmaking or as a direct result of screenmaking. Ranked by order of importance, the leading causes of screen damage from screenmaking in-clude the following:
* poor tensioning practices
* lack of proper test instruments, well-maintained tensioning devices, and other tools
* poorly prepared frames
These are problems that you can avoid if you properly train screenmakers and have a correctly equipped screenmaking department. You may have to invest in new tools for your screenroom, and training will bring its own expenses, but the total cost represented by these improvements is significantly cheaper than the cost of wasted mesh.
Most damage to screen mesh occurs during the tensioning process or as a direct result of how screens are tensioned. The three primary factors that lead to screen failure include
* insufficient tensioning and stabilization times
* incorrect tensioning values
* stressed screen corners
The tensioning procedure discussed in the passages that follow are based on what I would call "traditional" methods. During the early 1990's, an alternative--the rapid-tensioning method--was developed by the Screen Printing Technical Foundation, primarily to increase the efficiency of screen production. Using this method, the screen was tensioned in a single step to just below its breaking point, eliminating the tedious job of retensioning in several steps. While this method gained some followers, it never replaced the traditional procedure, primarily, because it was a less forgiving process that often resulted in broken screens.
Although some meshes, notably those under 100 threads/in., can be tensioned to their maximum tension level in one stretching step, the traditional approach is to stretch screens in 3-4 small increments. The screen first should be stretched to its initial tension level, which is a fraction of its final tension. The mesh should then rest for 20 min. before the next tensioning step. Each subsequent stretching should increase the tension level by 1/3-1/4 of the difference between the initial and final tensions. For example, if the initial tension is 15 N/cm and the final (maximum) tension is 24 N/cm, then the next three tensioning steps should increase the tension by 3 N/cm each, with 20-min. rest periods in between. (These rest periods should be not less than 15 min and not more than 30 min, since there seems to be no practical advantage for longer rest periods.)
Once the screen is stretched to its maximum tension, it should be left to stabilize for a minimum of 24 hours and preferably for 48 hours. During this time, the screen will lose 3-6 N/cm of tension due to "cold-flow," which is normal for polyester material. If the screen is processed without this stabilization period and used for printing, the images will become significantly larger than they should be, and the registration of subsequent colors will be difficult to maintain.
|Table 1 Specific Cross Section of Polyester Meshes|
|Mesh count (threads/in.)||Mesh count (threads/cm)||Thread diameter (microns)|| Specific cross section |
(in. x 10-3)
| Specific cross section |
(cm x 10-3)
|Specific cross section is a better indicator of a screen mesh's relative strength than its thread count alone. The higher the value of the specific cross section, the stronger the material and the more tension it will withstand.|
The initial and final allowable tension of a screen mesh depends on the fabric's specific cross section (SCS) and its inherent strength. The SCS of a mesh is simply the sum of the cross-sectional areas of each thread within a unit length (1 in. or 1 cm). You can calculate the specific cross section if you know the mesh count (Mc) and thread diameter (D) of the fabric, which can be found on virtually any mesh-specification charts. The formula is SCS = / x Mc x D2 ÷ 4. Some charts, such as the "Comparative Screen-Fabric Chart" from Screen Printing magazine, provide the calculated values for SCS.
Since the inherent strength of normal polyester is relatively constant--although slightly higher in low-elangation (LE) fabrics than regular polyester-- the SCS determines what level of force the fabric will support and the appropriate initial and final tensioning values. Specific cross section is the only reliable data that can be used to guage the strength of mesh.
To illustrate the point, I have selected various plain-weave meshes and sorted them on the basis of their specific cross section (Table 1). The chart clearly indicates that the mesh count alone cannot be used for determining the strength of the fabric (yet, this is the primary method suggested by many suppliers). A 420- thread/in. mesh with a 35-micron thread diameter (420/35) has a greater specific cross section than 196/48, 230/40, or 280/40 mesh and thus can be tensioned to higher values. The chart also shows that the cross section of the same nominal mesh counts can vary significantly depending on the thread diameter (e.g., a 196/48 mesh cannot be tensioned as high as a 196/55 fabric).
Based on empirical evidence and studies by various suppliers (Stretch Devices, SaatiPrint, NBC), I was able to relate the specific cross section of the mesh to its initial and maximum tensioning values. This relationship is illustrated in the screen-tensioning chart shown in Figure 1. Note that this chart was prepared based on results with medium to large screens (60 in. wide or more). Because of the "corner effect," large screens are difficult, if not impossible, to stretch to the same values as small screens.
The corners of a 10 x 10-in. screen are so close together that the entire screen is easily stretched to the same value as the corners. However, as the corners move further and further apart, the difference between the corner tension and tension in the middle of larger screens becomes more pronounced. While the tension limits shown in Figure 1 may be too low for smaller screens, they will definitely be on the safe side, regardless of the screen size you use.
With this chart, you can determine the tensioning values for any screen fabric if the fabric's specific cross section is known (or established through calculation). For example, if you use an 86/100 mesh, its specific cross section is 0.00104 in. (or 1.04 x 10-3 in.). If you draw a vertical line through the X axis of the chart at 1.04 and note where the line crosses the initial and maximum tensioning curves, you can read off the corresponding values on the Y axis of the chart (21 and 38 N/cm). A 280/40 or a 196/48 mesh will give you 16 and 22 N/cm for initial and final tensions using the same chart (the tensions are the same for both meshes because their specific cross sections are nearly identical).
I recommend that you find the specific cross section of all your frequently used meshes and mark their value on a copy of the tensioning chart in Figure 1. Draw vertical lines at the appropriate points on the X axis, using a different color for every mesh you commonly use. Wherever these lines cross the initial and maximum tensioning curves, you will have the appropriate tensioning values to use. When you're finished, post this chart in the screenmaking area.
Also keep in mind that the maximum tension values will not be the same as the values the screens maintain at the time of printing. Over 24-48 hours, the tension values will drop 3-6 N/cm below the maximum value attained during stretching. Finally, note that the tensioning chart in Figure 1 applies to LE mesh as well as regular fabric. The only differences are that LE mesh allows initial tension to be 2-3 N/cm higher and that the fabric will retain its tension under use for a longer period than regular fabric.
Regardless of the method of screen stretching (i.e., self-tensioning frames, stretch-and-glue, etc.) or the type of fabric used, the mesh in the corners of the frame must have as little tension as possible. Since the stretching forces act perpendicular to one another, and since at the corner of the frames there is very little material between stretching clamps, the tension increases very rapidly to the breaking point of the fabric (Figure 2). To eliminate this problem, the corners must be "softened" by loosening the fabric prior to or during the stretching procedure .
Instruments, tools, and tensioning devices
Screenmakers must have the necessary instruments and equipment to perform their jobs without mistakes. The use of a tension meter is essential in all cases, except when manually stretching screens on wooden frames. A measuring microscope or high-power magnifier is also necessary to verify mesh counts and thread diameters, as well as to evaluate the quality of the prepared screens. Retensionable frames require the appropriate wrenches, and pneumatic stretchers need the necessary controls that allow smooth, jerk-free adjustment of the tensioning force.
When using pneumatic clamps for stretching, the clamps should be evaluated on a regular basis for smooth operation. When the clamp is under tension, it should be tapped lightly with a rubber mallet to see if it is sticking. If the clamp moves (jerks) as a result of tapping, it should be cleaned, realigned, or replaced. Unexplainable screen breakage often occurs with pneumatic clamps when they suddenly exert extra force on the fabric as the pressure driving them equalizes. Even more frequently, the clamps are set up incorrectly so that they do not travel in a straight line during tensioning (Figure 3). This problem also causes sticking and sudden, jerky movement of the clamps during stretching.
Unravel the causes of ruptured screens, and find out how to keep the problem from repeating.
The drying of the prepared (cleaned and coated) screens should be done in a screen-dryer cabinet at temperatures not exceeding 100°F (38°C). At temperatures higher than this, you may run the risk of rupturing highly tensioned fine meshes. The expansion rate of the frame and fabric vary substantially (aluminum expands at almost twice the rate of polyester), and this difference can cause the screens to rupture during, or right after, the drying process.
The last important cause of screen breakage is the poor preparation of frames for stretching. Metal frames should have no sharp comers or edges that are in contact with tensioned fabric. Stretch-and-glue frames that are sanded or ground after each use often have sharp points or edges caused by repeated abrading. Retensionable frames can have nicks and burrs that may seem inconsequential initially, but will pop the screen under high tension.
Finally, tensioned screens should be handled with care and stored in an accident-proof area. You should never allow the screen frames to drop on their sides and especially on their corners. A 1-ft drop onto the corner of the frame will shatter a well-tensioned screen nearly every time.
To reduce the number of faulty screens and the amount of wasted mesh, screenmakers should memorize three basic rules of the screenroom:
1. Proceed slowly when stretching screens.
2. Know what the tensioning limit is for all the meshes used.
3. Take good care of tools, equipment, and materials.heeding these recommendations and approaching the screenmaking process in a deliberate and repeatable manner, your screenroom staff will realize new mileage from the printing screens they produce. At the same time, your company will save hundreds, if not thousands, of dollars annually in mesh-replacement costs, money you can use to improve other areas of your operation.