Understanding Screen-Tension Loss

Learn how to identify problems in screenmaking that most often lead to reduced screen tension and how to anticipate and minimize tension loss.

Gradual screen-tension loss is inevitable, even when you control your screenmaking and press-setup workflows carefully. A variety of factors contribute to screen-tension loss, and several can occur simultaneously, making the culprit hard to pinpoint.

Source vs. cause
Low screen tension is a leading contributor to poor image quality. Improper flooding, poor snap-off, smears, and loss of detail are some of the problems associated with low tension. In addition, inconsistency in screen tension in a multicolor job can lead to misregistered colors, moiré, mesh marks, and image defects.

You can trace screen-tension loss to two primary sources and several secondary causes. Every screen consists of only two principal physical elements: the mesh fabric and the frame. In every case, tension loss stems from one or both of these sources. External factors that lead to weakening or failure of either of the two elements are considered causes.

Wood frames
A properly designed and constructed wood frame will yield an entirely satisfactory print, but wood frames are more likely to cause tension loss than their metal counterparts. Here are some reasons why.

Reclaiming The way wood frames are processed for future use is one of the primary causes of screen-tension loss. Wood absorbs the water and cleaning chemicals used in reclaiming. As a result, the frame is less able to withstand the stress imposed by a tightly tensioned screen.

Beam deflection This condition refers to the tendency of the frame’s longer sides to bend inward. Beam deflection is worsened by the wood frame’s inability to hold its proper shape under stress. Beam deflection occurs to some degree in all frame types, including rigid and retensionable metal frames, but the fact that wood frames absorb moisture makes them most susceptible to the problem. If the wood bends while it’s wet, the frame won’t likely return to its original shape—even after the mesh fabric is removed.

Heat Screens are often dried in heated cabinets or in rooms that are equipped with dehumidifiers, which also create heat. The wood frame’s tendency to bend inward becomes even more pronounced when the frame is hot. Combining moisture and heat further compounds the problem.

Frame dimensions Wood is an acceptable material for screen-frame construction, but some of these frames are built of wood that is not sized appropriately for the job. Frames often are built with typical lumber-yard stock, sold as two-by-fours, two-by-twos, etc. A 23 x 32-in. frame, for example, built with 1.5 x 1.5-in. wood (sold in a lumber yard as a two-by-two) can hold high tension, but its ability to maintain a high level of tension diminishes dramatically when the frame is exposed to moisture and/or heat. Wood frames that are engineered, rather than built of convenient materials, are the best choices for jobs that require high levels of screen tension.

Metal frames
Wood isn’t to blame for all of the problems that lead to tension loss. Rigid and retensionable metal frames present their own opportunities for reduced tension. For example, tension loss can be traced to mesh that isn’t properly affixed to the frame. Use an adhesive that is formulated specifically for screenmaking, and use only as much adhesive as necessary to affix the mesh fabric to the frame. Note that the frame must be thoroughly clean and dry when you’re ready to affix mesh to it.

When using retensionable frames, take a moment to inspect the mesh-capturing system—often a strip or rod that locks the mesh into grooves on each side of the frame—to ensure that it will grip the mesh properly and prevent the mesh from slipping.

Mesh fabric
The type of frame you use can play a part in screen-tension loss, but mesh often is the source of the majority of screen-tension problems.

Mesh elasticity If you stretch any mesh material—even steel—beyond its elastic limit, it will not return to its original shape. Material elongation below this stress-limit point is call elastic deformation. Any material stretched below its elastic limit will snap back to its original size. If stretched above the stress-limit point, the material experiences plastic deformation, which causes tension loss. You can attain a stable tension level by tensioning the mesh to the desired level, leaving it at that level for a short time, and retensioning to the desired level after it loses some of its tension. You can repeat the process until the mesh reaches a stable level of tension. Using low-elongation mesh, which is engineered to resist tension loss, is an option you may wish to explore.

Heat Polyester mesh fabric softens when it’s exposed to heat and, consequently, stretches more easily. In many cases, the heat to which screens are exposed in a typical production environment falls far below the material’s limits. However, if your production workflow involves the use of flash or inline curing, the press’s bed or shuttle system may pick up enough heat to affect the mesh.

On press Most screen-printing applications involve off-contact printing, where the screen is held slightly above the item being printed. As a result, the squeegee must push the mesh down and into contact with the substrate to transfer ink. The squeegee travels the length of the screen, pushing down and stretching every part of the screen in its path; therefore, mesh tension will drop over the course of thousands of squeegee strokes in a print run. You should set up your press to provide enough off-contact to force screen snap-off immediately behind the squeegee.

Off-contact
Use the least possible off-contact and the maximum tension that the particular mesh fabric will hold throughout a long run. Mesh manufacturers can provide you with the suggested tension level for the mesh you use. You also need to ensure that off-contact distance is consistent over the entire print surface and the path of the squeegee during the print stroke is parallel to the press bed or platen. If either of these conditions isn’t satisfied, it’s likely that you’ll have to use excessive squeegee pressure, which puts extra stress on the screen and hastens tension loss.

Squeegee pressure
You must control squeegee pressure carefully to maintain screen-tension levels. As a general rule, using minimal squeegee pressure helps preserve the useful life of the mesh fabric and produces a superior print. If the press or press operator is forced to use a lot of pressure, then something else is going wrong. Platens may be warped or out of parallel with the plane of the screen. The squeegee may be dull, or its contact surface may not be perfectly straight.
 

View more from this Screen Printing issue