Solving the Mystery of Screen-Tension Loss
Rosson offers advice on tracking your tension culprit.
I'm often confronted with the question, "What causes screens to lose tension?" While this question appears simple, the answer is not. The truth is that a variety of factors contribute to tension loss. More importantly, several of these factors can occur simultaneously, obscuring the reason for the drop in tension. In the following pages, I'll identify the material and screenmaking problems that most often lead to reduced screen tension and shed some light on how you can anticipate and minimize tension loss on your screens.
Tension loss: Source vs. cause
Low screen tension is a leading reason for poor image quality in screen-printed work. Common problems associated with low-tension screens include improper flooding, poor snap-off during printing, an uneven printed ink film, and image flaws such as smears and loss of detail. Maintaining a high and consistent tension level among screens used on a multicolor job (e.g., process-color work) is also important, otherwise you risk misregistered colors, moiré, mesh marks, and related image defects.
You can trace all screen-tension loss to two primary sources and numerous secondary causes. Note the distinction I've made between the words "source" and "cause." Every printing screen consists of only two principal physical elements: the frame and the mesh fabric. In every case, tension loss stems from one or both of these sources. External factors that lead to weakening or failure of either of these two elements are considered causes.
For example, if a truck ran over your frame, the frame might become too weak to hold screen tension. In this scenario, physical damage to the frame is the cause of tension loss, while the frame itself is the source of the problem.
The most widely used frame material is wood. A well-designed and constructed wood frame will yield an entirely satisfactory print. But wood frames are more likely to cause problems than metal frames.
Reclaiming: The main cause of tension loss with wood frames stems from the way the frames are recycled for future use. Frames are generally reclaimed and processed with water and various cleaning solutions to strip off the old emulsion. In some cases, the frames are even placed in soaking tanks to soften the emulsion to the point where it can be easily sprayed off of the screen. The problem is that wood absorbs the water and cleaning liquids, which can reduce the frame's ability to withstand the stress imposed by a tightly stretched screen.
All frame types will bend inward to some degree under stress from a tensioned screen, which will cause the tension to drop. With wood frames, if the sidebars have too small a cross section, or if the frame is exposed to moisture and/or heat, the chance of deflection increases. Under stress from the screen, the wood (especially along the frame's longer sides) tends to bend inward. This condition, known as beam deflection, occurs to some degree with all frames, including rigid and retensionable metal types. But the tendency of wood frames to absorb water makes them most susceptible to this problem.
The more a frame deflects, the more relaxed the mesh will become. So a screen that was perfectly acceptable when affixed to a dry wood frame may become unacceptable after the screen goes through processing, cleaning, or reclaiming. And if the wood bends while it's wet, the frame rarely returns to its original shape, even if the screen fabric is removed.
Heat: Screens are often dried in heated cabinets or rooms that have dehumidifiers, which also create heat. The problem is that hot wood is not as strong as cool wood. Consequently, the frame's tendency to bend inward can become even more serious when it is hot. You can see evidence of this phenomena by observing old farm buildings with sway-backed roofs. Sagging roof lines are especially prevalent when the roof trusses are sheathed with tin. The reason is obvious--it is hotter under the tin roof. With wood screen frames, the presence of moisture only compounds the problem.
Frame dimensions: Do the previous concerns mean that wood is an unsatisfactory material for constructing frames? No, they do not. But the problem with many wood frames is they are built of wood that is not sized appropriately for the job. Frames traditionally are built with typical lumber-yard stock, sold as two-by-fours, two-by-twos, etc. However, these dimensional labels are really used as a matter of convenience rather than to accurately define the lumber's true width and depth. For example, lumber that we call "two-by-two" actually measures 11/2 x 11/2 in.
In my experience, I have found that a 23 x 31-in. frame constructed with 11/2 x 11/2-in. wood can hold screen tensions in the neighborhood of 35 N/cm, provided that the frames are kept dry and at a consistent room temperature. The frames' capacity to maintain this tension level will decrease substantially if the frames are wet or too warm.
The best wood frames are engineered rather than just built of convenient materials. While 11/2 x 11/2-in. boards are adequate for constructing 23 x 31-in. manual press frames, they are out of the question for building automatic press frames of 23 x 31 in. or for 25 x 36-in. frames (manual or automatic). These frames should have sides that are a minimum of 11/2 in. deep by at least 21/8 in. wide. Such frames have more than twice the strength of frames with a conventional 11/2 x 11/2 -in. cross section and are less likely to bend inward when tensioned mesh is affixed.
Screen attachment and deflection
Some factors that can bring about tension loss are not reserved for wood frames alone. Although metal frame types, including rigid and retensionable frames, do provide a more stable platform for the screen, they can also present opportunities for reduced tension. The two sections that follow detail a few things to keep in mind regardless of the frame system you're working with.
Screen attachment: An obvious step you can take to reduce tension loss is to make sure the screen fabric is fixed securely to the frame. Wood frames offer several fabric attachment possibilities. One approach is the rope and groove method, which is as antiquated as a buggy whip and not worthy of much comment.
Some printers argue in favor of using staples to attach pretensioned mesh to a wood frame. But staples are not cheaper than suitable adhesives--not when you consider time, damaged fabric, frame damage, and the tedious job of pulling all those staples when renewing fabric. If your screen maker uses staples, do him or her a favor--hide the staple gun and forbid the purchase of a new one. Staples are good for attaching garage sale signs to trees and poles. They are not suitable for making printing screens.
With both wood and rigid metal frames, you can attach screen fabric reliably using adhesives designed specifically for screenmaking. Cyano acrylate (CA) types are the most popular and are sold under brand names such as Frame Fast, Pacer, Mesh Bond, etc. Other available adhesives include epoxy, urethane, and polyester. CA types are much faster than these alternatives, but they are rather expensive.
Most screen makers use too much adhesive by covering the whole frame surface. A 1/2-in.-wide strip along the outer edge of the frame is entirely adequate. The frame must be thoroughly dry and thoroughly clean when adhering fabric. Correctly adhered fabric will not slip.
Retensionable frames have a fabric capturing system that usually consists of some sort of plastic strip or rod that locks the mesh into grooves on each side of the frame. The frames are characteristically aluminum, which is not affected by moisture or heat. On rare occasions, the capturing strip will allow the fabric to slip. This is most likely to occur when you use a fine fabric, such as a 390-thread/in. mesh, but almost never happens with lower mesh counts of 110, 155, or 230 thread/in.
Deflection: As mentioned previously, under the load imposed by tensioned fabrics, all frame types will deflect or bend inward to some degree. However, this deflection is only debilitating if it occurs after the tensioned fabric is attached to the frame. As a general rule, you can calculate the acceptable deflection for the longest sidebar on your frame as 0.3% of the length of the longest sidebar.
Your ability to control beam deflection is largely dependent on the type of screen stretcher you use. One variety simply stretches the mesh, holding the desired tension level until the mesh is attached to the frame. When the mesh is cut loose from the stretcher, the tension load is transferred to the frame, which can bend under the stress and allow the center of the screen to relax.
Retensionable frames also fall into this category. However, with retensionable frames, the frame itself serves as the tensioning mechanism. So the frame is subject to deflection throughout the tensioning process. In most cases, retensionable frames only exhibit severe deflection when the mesh is tensioned far in excess of normal tension levels or when the frame is extremely large.
The other stretcher type pre-bends the frame as it stretches the mesh. This way, when the mesh is affixed to the frame and cut loose from the stretcher, the frame will not bend further, allowing it to maintain the screen tension level provided by the stretcher.
Although the framing system you use can play a part in screen tension loss, the majority of tension problems occur due to the mesh fabric itself. Mesh elasticity: All materials have limited ability to stretch and return to their original shape and size. That includes polyester, rubber, and steel--yes, even steel will stretch when enough force is applied. If you stretch any of these materials beyond their elastic limit, they will not return to their original shapes. Material elongation below this stress-limit point is called elastic deformation. Any material stretched below its elastic limit will snap back to original size. But if stretched above the stress-limit point--a condition known as plastic deformation--the material will not return entirely to its original shape.
To illustrate this concept, consider a balloon. If you blow it up, hold it in its inflated state for a few minutes, and then release the air, you'll notice that the uninflated balloon is now larger than it was before you inflated it. It has been stretched beyond its elastic limit and cannot return to its original size. The same holds true for mesh threads tensioned to typical screen-printing levels.
Screen printers exceed the elastic limit of polyester fabric all the time. And as the fabric threads stretch, tension drops. This condition is most pronounced when the fabric is stretched the first time. However, after initial tensioning, polyester threads actually get a little stronger and resist excessive stretching--in other words, their elastic limit increases.
To get the most stable tension level, mesh fabric should be tightened to the desired tension level and then left for a few minutes. Over a short period, the fabric will lose some of its tension. The fabric can then be retensioned and left to relax once more. Again, the mesh will lose some tension over time. But this time, the loss will be significantly smaller. This process can be repeated until the mesh reaches stability at the tension level you desire. This procedure can be easily accomplished using most mechanical stretchers and retensionable frames.
For screen makers who use pneumatic stretchers, the tensioning procedure is a little different. Pneumatic stretching devices continually adjust for tension losses until the desired tension level is achieved. Therefore, it's usually a good idea to leave the fabric in the pneumatic stretcher for a few minutes before bonding it to the frame. If the mesh is bonded immediately, more severe relaxation of the fabric will probably occur.
Regardless of the stretching system you use, giving the mesh time to relax and adjust to new tension levels is important. And while it may take longer to produce a printing screen in this way, the extra time you spend in stretching will help you avoid screen remakes and other costly problems on press.
Incidentally, printers used to believe that polyester fabric had to be stretched and held for several hours before it was adhered to the frame. But research conducted by the Screen Printing Technical Foundation disclosed that fabrics performed almost as well when given only a few minutes to adjust to new tension levels before they're affixed to frames.
Heat: As mentioned previously, heat makes wood frames more prone to beam deflection. Heat is also debilitating to polyester fabric. When exposed to extreme heat, polyester gets softer, and consequently, stretches more easily. In most cases, the heat screens are exposed to in a typical production environment falls far below polyester's temperature limit. However, if your production system involves flash or inline curing, the bed or shuttle system on your press may pick up enough heat to affect the mesh.
On press: Most screen-printing applications involve printing off-contact, where the screen is held slightly above the item being printed. As a result, the squeegee must push the fabric down into contact with the part in order for ink to transfer. The squeegee is pulled along the length of the screen, which means it eventually pushes down and stretches every part of the screen within its path. As you might have guessed, over the course of thousands of squeegee strokes during a print run, the screen's tension will drop.
The dilemma is that the screen should be as tight as possible since it will perform better under high tension. But if you overdo the tension, it won't hold for an extended print run. Additionally, the press should be set up to provide enough off contact to force the screen to snap off the print surface immediately behind the squeegee.
To meet all these criteria, you must compromise. Use the least possible off-contact that will permit instant snap off and use the maximum tension that a particular fabric will hold over a long run. The suggested tension level for your particular mesh will be specified by the manufacturer.
Also make sure 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 one of these conditions is not 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 is another parameter you must carefully control to maintain tension levels on your screens. As a general rule, use minimal squeegee pressure, which not only helps preserve the life of the fabric but produces a superior print. If the press or press operator is forced to use a lot of pressure, it indicates that something else is wrong. Platens may be warped or out of parallel with the plane of the screen. The squeegee may be hopelessly dull. Or the squeegee contact surface may not be perfectly straight.
Even if you carefully control your screenmaking and press setup, your screens will lose tension. The fact is, gradual tension loss is a natural occurrence. However, you can improve the overall performance of your screens and their ability to resist tension loss by using low elongation (LE) fabrics. These fabrics, available from all mesh manufacturers, cost slightly more than standard fabrics, but the improved performance they provide more than offsets the price difference. LE fabrics also tolerate slightly higher tensions than conventional mesh.
Now that we've identified frames and mesh as the two main sources of tension loss and pointed out specific causes of this phenomenon, you can see that controlling screen tension is not a hopeless challenge. You are now ready to pinpoint factors leading to tension loss in your own shop by taking a closer look at the materials and methods you employ.
Scrutinize your wood frames. If any of them are waterlogged, retire them for a while. And educate employees to avoid exposing the frames to water or heat. With rigid and retensionable metal frames, take care to ensure that the mesh is properly affixed to the screen. Tension your fabrics to the highest levels recommended and allow them to relax for a period before you affix them to frames. Finally, make sure your press operators use minimum off-contact distance and squeegee pressure. Follow these recommendations, and you'll realize better print quality, less downtime, and longer screen life.