Stretching For Success
A look at screen-tensioning devices
When you consider all of the variables in the screen-printing process, those that occur early tend to have the greatest impact on the final printed results. The screen is a perfect example. If the screen isn't prepared in such a way that it allows proper transfer of ink to substrate, it becomes impossible to compensate for the shortcomings by adjusting other variables later in the process. On the other hand, if a job begins with a screen of excellent quality, subsequent steps in the process will work more smoothly.
The screen itself represents a collection of variables that you must be concerned with. Among the most critical screen variables are frame quality and mesh tension. The frame is important because once a screen is stretched, it's up to the frame to maintain the desired tension. A quality frame is one that holds its shape without distortion during the printing process. To this end, the best frames for most applications have a high degree of rigidity and allow for even tensioning of mesh.
Tension is a more difficult variable to control. The challenge stems from the fact that you must not only achieve a suitably high tension level but also make sure that tension is consistent across the entire screen.
If a screen is tensioned to a lower level in one orientation than in the other (different tension levels between warp and weft mesh threads), it becomes impossible to hold even mesh tension across the entire surface of the screen. And if the mesh has lower tension in one area, the print will also be uneven. In other words, a screen with uneven mesh tension will not hold registration.
To achieve adequate and even tension levels on your screens, a number of stretching tools are available, ranging from manual devices to sophisticated automatic stretchers. This article will explore each category of stretching devices, pointing out benefits, drawbacks, and usage issues.
Types of screen stretchers
Stretching systems run the gamut, from inexpensive handheld tools to complex microprocessor-controlled pneumatic devices. Depending on your applications (resolution, number of colors, etc.), simple, less-expensive stretching devices may be more than adequate for your needs. On the other hand, if you're doing close-tolerance industrial work or high-line-count process-color jobs, you may find that high-end stretching systems offer a level of control and accuracy that you can't do without.
Pliers At the lowest level of stretching technology you'll find mesh pliers. Resembling standard pliers with a gripping head several inches wide, these devices are used to grab and manually stretch mesh over wooden frames. The screenmaker simply staples the mesh to the frame when he feels he stretched it to an acceptable tension. These tools and the inexact stretching method they support lead to arbitrary and inconsistent tensions. Consequently plier stretchers should be avoided by any shop that's serious about printing quality and will be excluded from the remainder of this discussion.
Manual roll tensioners Manual roll tensioners were the first improvement on the plier method. Roll tensioners utilize four rollers in a rectangular configuration (one roller on each side). The rollers are housed in an adjustable frame and include a ratcheting fixture that prevents the rollers from unwinding during the stretching process.
Each roller also includes a track or groove that holds the mesh while it is tensioned. When mesh is attached, the rollers are manually rotated to raise its tension. The key to making these systems work is evenly loading the mesh onto the rollers and having a strong arm to turn the rollers.
You can achieve excellent tensioning results from manual roll tensioners. They can put a significant amount of tension on the mesh, and they work without any outside power or air supply. Their main drawback is that they are very slow and require a highly skilled operator.
Bar tensioners Bar tensioners are one of the two main types of screen-stretching systems commonly used today. Bar tensioners utilize a long single clamping bar on each side of the screen. The screen fabric is locked into the bar on each side, then manual or pneumatic pressure is applied to tension the mesh.
Automatic bar-tensioning systems can work with as few as one pneumatic air cylinder on each side, and manual models typically rely on screws that are tightened by the user until the proper tension is achieved. They are highly economical and can be loaded quickly.
Retensionable frames as stretchers for rigid printing frames Many smaller shops have found that an oversized retensionable frame can work just fine as a way to stretch the mesh for their rigid frames. Instead of investing in a whole shop full of retensionable frames, they buy one or two that are several inches larger than their rigid frames for printing.
In this application, the retensionable frames function in much the same way as the manual roll tensioners discussed previously. The main difference is that the manual tensioners use some sort of ratcheting device to hold the rollers, while the retensionable frames are locked into position with end bolts on each roller. It takes a little more dexterity to tighten the roller frame, but it can work quite nicely in lower-volume production environments.
Retensionable frames used in printing screens Using retensionable frames to stretch the mesh and then serve as the printing screen is another popular option. They provide the added benefit, as the name implies, of allowing the mesh to be retensioned before reuse. Much higher sustained levels of mesh tension are possible using retensionable frames.
Multiclamp tensioners Multiclamp tensioners are the type that most people think of when stretching systems are mentioned. They involve a series of clamps along each side of the screen. Each one of these clamps, which vary in width from 6-12 in. (150-300 mm), is individually locked onto the mesh.
While manual versions are offered, on most higher end multiclamp systems the clamps are fitted on pneumatic pistons that simultaneously draw the screen taut. Some units feature a vertical axis of adjustment that pushes the screen frame up into contact with the mesh. On high-end systems, the clamps may be tied into a central microprocessor control that allows the user to preset the stretching sequence and store the data for recall whenever a similar screen is needed.
Proponents of multiclamp systems feel that they provide the most accurate mesh tensioning. The main reasoning is that with individual clamps, it is much easier to correctly align the mesh when it is loaded into the stretching system. Some also feel that individual clamps distribute tension over the screen more evenly than other stretching systems.
Which system is the best?
All of the stretching systems mentioned above can be effective in screen tensioning. But a system that works well for one shop may be cumbersome or impractical for another. The following discussion will attempt to guide you in deciding which system is right for your operation.
Several important factors must be considered in comparing stretching systems. These factors include the following: frame-size capability size adjustability frame bow-in (pre-tensioning of the frame) ease of mesh loading (speed) mesh capture (does it hold all thickness of mesh without modification) independent X-Y tension adjustment Z-axis adjustment (screen contact with the mesh) evenness of tension (across the whole screen area) price
Now, let's look more closely at how these factors impact stretching-system performance.
Frame-size capability Manual roll stretchers and bar-type stretchers are best used on frames with sides of less than 36 in. (914 mm) wide. On bars or rollers longer than 36 in., it's difficult to line the mesh up accurately. The multiclamp tensioners work best in larger applications because the small individual clamps make accurate mesh loading much easier.
Another factor that comes into play as the sides of the screen get longer is that trying to pull evenly with just one or two pneumatic pistons is very difficult. The multiclamp pneumatic stretching systems spread the tension load out over multiple pistons and make it possible to achieve a much more even tension along the entire length of the mesh.
Frame-size adjustability Most screen-printing operations use several different screen sizes. Each press format seems to demand its own frame dimensions, so having a stretching system that is adaptable to the various screen sizes is a plus. Only a very large operation can justify having separate stretching devices for each of its screen sizes.
A shop that needs to stretch 1-20 screens/day can accomplish its tensioning tasks with a single adjustable stretching system. In shops that stretch more than 20 screens/day, having more than one stretching system just makes good sense.
Frame bowing Frame bowing (or prebowing, as it is sometimes called) is a little bit controversial. Those that favor the practice feel that bowing the frame before the mesh is glued down can help to compensate for tension loss as the mesh relaxes after it is affixed to the frame. Those that do not like the practice feel that it puts unnecessary stress on the frame and can lead to "racking" of the frame. Racking is the phenomenon of torquing the frame out of level or plane. Once a rigid frame has a significant amount of racking, it is very difficult to ever get it back into plane again.
Racked frames create their own problems. They are difficult to load in most printing machines, they mess up most registration systems, and they tend to push themselves out of registration during a print run.
My position on frame bowing is that a little can be beneficial, but too much can lead to screen failure. Only bar type and multiclamp systems allow you to bow frames. Multiclamp systems spread the load out over the entire frame length, while the bar clamps tend to push only in the middle of each side of the frame.
Ease of mesh loading The speed at which mesh can be loaded is the factor that determines the overall productivity you can achieve with any given stretching system. But loading speed is only part of the equation. If loading fast means loading unevenly, what good is it?
In order for proper mesh tensioning, fabric must be locked in evenly on each side. Any unevenness or skewing of the fabric can lead to uneven screen tension and, worse yet, can cause the mesh to fail before it can be glued down. A very high percentage of mesh failures that occur on a stretching device can be tied directly to uneven loading.
A personal example occurred recently on a visit I made to a large-format textile transfer operation. During a discussion, the production manager mentioned that they were having a high rate of screen-mesh failure. As we discussed the situation, it became apparent that the failure was happening before the mesh was even glued down. I asked if I could see their stretching operation.
They were using an excellent quality multiclamp stretching system. But as I watched the operator, it became quite obvious what the problem was. The stretching table was pushed up against a wall, and the employee had to reach across to clamp the mesh on the side nearest the wall. Consequently, there was no way for him to keep the mesh aligned accurately on that side. We pulled the table out and reclamped the misaligned side. That ended the company's screen-failure problem.
Mesh capture Mesh capture is really a part of the previous step, but it deserves its own review because of the variety of systems available. Most of the wear-and-tear in a stretching system occurs at the point where the mesh is locked, clamped, or otherwise affixed to the stretching mechanism. Most clamps and rollers are covered with a soft resilient material such as rubber or polyurethane. The reason for using resilient materials is twofold. First, the flexibility helps the clamp properly grip a wide range of mesh sizes. The second reason is that the resilient material helps to protect the mesh from tearing. A rough or nicked surface or edge is the fastest way to destroy a perfectly good (and expensive) piece of mesh.
Independent X-Y tension adjustment The X and Y dimensions are the warp and weft of the fabric. Most good stretching systems allow for independent tension adjustment in the two dimensions. This is an important feature because with most mesh fabrics, the warp and weft threads require a different amount of pressure to equalize the final screen tension. Pneumatic systems that only have one air valve are very difficult to adjust evenly. Screens produced with different tension levels on the X and Y axes are very difficult to keep in registration.
Z-axis adjustment The Z-axis is the vertical dimension when a screen is laid flat for stretching. Certain high-end stretching systems have the capability to exert pressure upward on the frame to help it make good contact with the tensioned mesh prior to gluing. This feature is very valuable in insuring a good bond, especially with the higher mesh counts.
Evenness of tension This is really a function of how well the opera-tor loaded the mesh. If the mesh is loaded unevenly, it is very difficult to tension it properly. Therefore, this quality is closely related to ease of loading.
Price The cost of the system will be the ultimate determining factor for anyone investing in new stretching equipment. Like most good things, quality costs money. The money you save on a lower end stretching system can quickly loose its attractiveness if the system produces poor-quality screens, leads to a high level of mesh failure, or eats up labor time because it's difficult to use.
Controlling tension quality
A lot of work has been done in recent years on improving the useful life of screens. A screen that continues to lose tension each time it is used quickly becomes useless. The higher the initial tension, and even the way the tension is reached, can have a huge impact on the useful life you can expect from a screen.
The goal of a good tensioning system is to provide screens that are stable and evenly tensioned. If the goal is to have screens that stabilize at a tension of 20 N/cm, then the initial tension put on the mesh before it is glued must be well above the 20-N/cm level.
Staged vs. Rapid Tensioning Staged tensioning is the stretching method most screen printers are familiar with. In staged tensioning, the mesh is brought up to its ultimate tension in slow staged steps over the course of 15 min- 1 hr. Rapid tensioning, on the other hand, is the practice of taking the tension directly from no tension to its final tension in one step.
Rapid tensioning is not for the novice stretcher. The operator must be practiced in loading the mesh and in making sure that there are no mistakes in the setup. If there are any flaws, the mesh will usually fail immediately. However, if everything was set up correctly, then the mesh will take the jolt of rapid tensioning.
Studies conducted by the Screen Printing Technical Foundation and at least one mesh manufacturer indicate that rapid tensioning of the mesh rather than staged tensioning can lead to screens that have higher stability at greater tension levels. Other tests have shown that rapid tensioning can have some very beneficial effects on the physical properties of the polyester monofilament that the mesh is made of, which explains why a rapid-tensioned mesh is more stable and predictable and maintains a higher final tension than a stage-tensioned screen.
Pulsed tensioning A recent variation on the concept of rapid tensioning is pulsed tensioning. At least one mesh manufacturer has done tests indicating that pulsed tensioning can produce highly stable, high-tension screens.
Pulsed tensioning uses a pneumatic stretching system that exerts a rapid-fire series of pulls against the mesh that culminate at the final tension level. The method sounds like a winner, but does require special equipment. It is certainly worthy of further investigation.
Whatever system you ultimately decide on, it must be the one that works best in your shop with your staff. As mentioned earlier in the pricing section, you should try to buy the best one your shop can afford. Screens are the last place that a "screen" printer should cut corners. It only takes a couple of poorly tensioned screens to lower a shop's productivity to the point where the cost of a more dependable, higher priced stretching system is justifiable.