Screen Printing - December 2012/January 2013 - (Page 12)

SOLUTION SOURCE BOOK UNDERSTANDING SCREEN-TENSION LOSS radual 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. The following discussion identifies problems in screenmaking that most often lead to reduced screen tension and explains how to anticipate and minimize tension loss. 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 12 SCREENPRINTING G 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.

Table of Contents for the Digital Edition of Screen Printing - December 2012/January 2013

Screen Printing - December 2012/January 2013
Prepress for Special-Effect Garment Printing
Tips for Optimum Screen Exposure
Maximizing Image Size and Position
Understanding Screen-Tension Loss
A Look at Screen Lift-Off
Staging Garment-Printing Jobs
How to Control Dye Migration
Fine Tuning for Fine Details
Application-Specific Considerations for Graphics Printers
Upping the Ante at SGIA 2012
Statement of Ownership
U.S. & Canadian Directory
Opportunity Exchange
Advertising Index

Screen Printing - December 2012/January 2013