Most gradual tension loss is due to a phenomenon known as cold flow.  Cold flow occurs when the synthetic threads of the fabric are brought under constant stress and they continue to slacken over time.  This effect is also known as mesh relaxation.  This type of tension loss is natural and should be expected, regardless of what tensioning system is being used.  Tension loss from cold flow occurs primarily 24-48 hours after the mesh is stretched and during the early part of a print run. 

　　Since the advent of low elongation mesh, this tension loss is not near as dramatic as it used to be. But there are still some recommendations to manage and compensate for the tension loss caused from the fabric’s relaxation property. 

　　1.  Always observe sufficient mesh stabilization time during the stretching process, both on retensionables and stretch and glue. 

　　2.  Allow the screen to rest a minimum of 24 hours before applying a stencil.  Never use a new screen immediately after stretching for high tolerance work. Within the first 24-48 hours on a newly stretched screen 10-25% of tension can be lost from cold flow. 

　　3.  Don’t use a new screen on tight registration jobs for the first 2 print runs. The printing forces can cause a new screen to lose an additional 3-5 N/cm in the first 500 impressions of the first print run.  This tension shift can cause registration changes and ink deposit variations.  Long print runs, and reclaiming a screen several times will produce additional losses of tension.

　　4.  Use a working screen tension that will remain consistent throughout the print run.  Be aware that over the course of long runs, thousands of squeegee strokes will affect screen tension.  If you overdo the tension, this drop will be more pronounced. 

　　5.  Work harden the fabric when using retensionable frames.   Work hardening is the term given to the process of training the molecules of the polyester to be in a new tensioned position.  The repeated print stroke cycle puts consistent force on the mesh to help align these molecules.  Initially this cycle will cause a loss in tension during the print run.  Then the mesh is retensioned to the original target tension, the molecules are in a more stable place, and less tension is lost on the next print run.

　　To work harden mesh on a retensionable frame, you simply use a new screen on a few print jobs that do not have close tolerance requirements, and re-tension the mesh to the desired level after each print run.

　　Once a screen has been work hardened it maintains a consistent and stable tension throughout a print run which ensures registration and color consistency. It is also said that work hardened screens will be useful for a longer period of time and retain tension longer. However, work hardened mesh, especially to high tensions, can create a brittle screen as the thread molecules become more aligned and less elastic.

　　Static frames can be work hardened to some degree, but there is no control over the tension they stabilize to.  After several print runs and reclaims, a stretch and glue frame will become stable (under consistent operating conditions).

　　Other Issues Causing Tension Loss

　　There are a number of mesh mishandling practices that result in tension loss.  These include incorrect insertion of the fabric in clamps, uneven pulling of stretching clamps, insufficient stabilization time before gluing and exposing the screen to large temperature changes.  Correct procedures for stretching will be covered in future feature articles.

　　Another cause of tension loss is a weak frame.  Frames that have insufficient profile will create tension loss.  Wood frames will change as they are processed with the water and chemicals they are exposed to.  The frame change will translate into tension loss.  Heat will also cause changes in wood frames.  Wood frames are often not constructed with the proper profile to hold adequate mesh tension, and this can be an issue as well.

　　Even with a good frame profile, tension loss can be seen if the frame is not pre-bowed on a stretch and glue systems.  Without pre-bowing, the deflection of the sides once the mesh is attached to the frame will drop the tension of the mesh after it is removed from the stretcher.  Generally this will be greater across the longest side of the frame. 

　　Improper press set up conditions surrounding a carefully prepared polyester screen can be the cause of screen tension loss during the production run. The biggest danger areas are:  excessive or uneven off-contact distance and excessive or uneven squeegee pressure.

　　Both squeegee pressure and off-contact distance should be optimized at the minimum value needed to obtain an acceptable quality print. This becomes increasingly more important at screen tensions of 20 N/cm and above. As tension increases the force required to deflect the mesh at the center of the screen also increases directly.  However, as we move farther away from the middle of the screen it grows exponentially. Because of this effect a small off-contact increase will produce a magnified result at the outer edges of the screen. Excessive off contact distance requires excessive squeegee pressure, causing unacceptable levels of drag and screen distortion.

　　Managing screen tension is vital to print quality and productivity.  Shortcuts in the screen room will only cut profits when the presses must stop to deal from a screen problem.  Give due diligence to monitoring screen tension and instituting practices to keep your screens in tip top shape so the presses keep running. 

[时间：2007-05-31  作者：Dawn Hohl  来源：信息中心]