Preventing Screen Damage
Most of the damage to the screen mesh occurs during tensioning due to insufficient tensioning and stabilization times, incorrect tensioning values, and stressed screen corners.
Polyester screen fabrics are one of the most expensive supply items in screenmaking. Besides being costly, they are also delicate and easy to damage. Careless handling, poorly designed storage, and accidents on the press account for 20-30% of ruined screens. Although some of these problems are preventable, they also represent the unavoidable cost of doing business.
The majority of screens, however, are damaged during screenmaking, which is a problem that can be avoided and corrected by training the screenmakers properly and providing them with necessary instrumentation and equipment. The leading causes of screen damage are poor tensioning practices, lack of instruments tools and properly maintained tensioning devices, and poorly prepared frames. Most of the damage to the screen mesh occurs during tensioning due to insufficient tensioning and stabilization times, incorrect tensioning values, and stressed screen corners.
Although some meshes, notably those under 100 threads/in., can be tensioned to their maximum tension level in one stretching step, the safest approach is to stretch screens incrementally. The screen should first 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 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 be significantly larger than they should be, and the registration of subsequent colors will be difficult to maintain.
The initial and final allowable tension of the mesh depend 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). As shown in Table 1, the specific cross section can be calculated if the mesh count and thread diameter are known (these values can be found on mesh-specification charts).
The chart clearly indicates that the mesh count alone cannot be used for determining the strength of the fabric. A 420-thread/in. mesh with a 35-μm thread diameter (420/35), for example, is stronger than 196/48, 230/40, and 280/40 mesh and, therefore, can be stretched to higher values. The chart also shows that the strength of the same nominal mesh counts can vary significantly depending on the thread diameter—for example, a 196/48 mesh cannot be tensioned as high as a 196/55 mesh.
The SCS determines the applicable force to the fabric and the appropriate initial and final tensioning values. SCS data are the only reliable sources of information that can be used for this purpose.
Screen corners 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 90° to one another, and since at the corner of the frames there is very little material between the fabric clamps, the tension increases very rapidly to the breaking point of the fabric. To eliminate this problem the corners must be softened by loosening the fabric prior to or during the stretching procedure.
Instruments, tools, 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 for manually stretched wooden frames. A measuring microscope is also necessary to verify mesh counts and thread diameters, as well as to evaluate the quality of the prepared screens. Self-tensioning 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, they 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 on these clamps when they suddenly exert extra force on the fabric as the pressure equalizes. Even more frequently, the clamps are set up incorrectly so that they do not travel in a straight line during tensioning. This problem also causes sticking and sudden motion of the clamps during stretching.
The drying of the prepared (cleaned and coated) screens should be done in a screen-drying cabinet at temperatures not exceeding 100°F. At temperatures higher than 100°F, 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 break the screens 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 due to repeated abrading. Self-tensioning frames can have nicks and burrs that may seem inconsequential initially, but which 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 one-foot drop on the corner of the frame will shatter a well- tensioned screen nearly every time. To reduce the number of wasted screens, screenmakers should memorize the three most important steps in screenmaking:
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.