The ScreenFrame: Foundation of the Process
A look at frame performance and maintenance
In many companies, a visit to the screenroom is reminiscent of a trip through Sherwood Forest: Resembling an arsenal of long bows for Robin Hood and his merry men, screen frames with distorted sides are apparent throughout the department. But unlike bows that launch their arrows true to target, these distorted frames will send a shop's image registration--and profitability--far off course.
The screen frame is the most fundamental element of the screen-printing process. Our "printing plate" is mesh imaged with a stencil, but unless the mesh is mounted on a stable frame, it is virtually useless.
Screen frames come in many sizes, shapes, and profiles. They are built from several different materials, including wood, aluminum, magnesium, and steel. But regardless of their composition, shape, or style, all screen frames are required to perform the same vital functions in the screen-printing process. Together, these functions include the following: providing a means for mounting the mesh withstanding the force of the tensioned mesh standing up to additional forces applied during printing remaining flat and square being light enough to handle easily
Printers expect more of the frame than perhaps any other element of the process. To appreciate the job it performs, consider how much energy it must resist. First, consider the tension of the screen, measured in Newtons per centimeter (N/cm). A tension of 1 N/cm is equivalent to 0.22480894 lbs of force/cm.
Now, assume that a 40 x 40-in. screen has been tensioned to 20 N/cm. This puts a load of approximately 455 lbs along each side or beam of the frame (40 in. x 2.54 cm/in. x 20 N/cm x 0.22480894 lbs/N = 456.8 lbs). As frame sizes increase, the loads increase proportionately. A 60 x 80-in. screen tensioned to 20 N/cm will have to withstand almost 915 lbs of force on its longer beams and 685 lbs on its narrower beams.
If these forces can be controlled, you will have a firm foundation for screen printing. When they cannot be controlled, they can destroy the process. This article will consider the importance of a frame's structural integrity, what can go wrong when frames distort, things you should consider when buying frames, and when you should get rid of frames that have outlived their usefulness.
More than just a frame
In the earliest days of commercial screen printing, no real concern was given to the construction or material makeup of screen frames. Most were wood, and in spite of the developments in metal frames, many still are wood. Although the newer seasoned wood frames are an improvement over the easily bowed natural wood frames of the past, the inherent instability of wood interjects a very challenging variable into the screen-printing process, particularly when mesh tensions are high and registration is critical. Although many screen printers get good results with their wooden frames in less demanding applications, this article will focus only on metal frames.
Once tension is applied to the mesh and the mesh is affixed to the frame, the flatness of the frame becomes a critical concern. Mesh tension is the "hidden energy" of the screen-printing process, and the frame's job is to resist the forces of that tension. A suitable frame will resist deforming under the force of tension and remain flat and square for as long as it's needed.
Forms of frame deformation
Inadequate screen frames become distorted under mesh tension. This distortion manifests in two ways: bowing and twisting.
Bowing Also known as beam deflection, this problem occurs when mesh tension pulls beams inward and creates a frame that is "out of square." A variety of frame shapes that can result from bowing are shown in Figure 1. The examples are exaggerated, but these types of distortion are regularly observed in screen shops.
Bowing can occur with both retensionable and rigid (static) screen frames. The question to ask yourself is, "How much distortion is acceptable?" If you are trying to print close-tolerance work, and you can visually detect frame distortion, the screen is likely to be unacceptable.
Among other things, distortion of the frame causes mesh threads to follow the same (curved) direction as the beams, resulting in mesh distortion at the edges of the screen. This drastically reduces the screen's "sweet spot" or maximum print area. It also leads to tension inconsistencies that further complicate your ability to achieve good registration from color to color.
Twisting This form of distortion causes the frame to pull upward at one or more corners and results in beams that bow up or down relative to the plane of the mesh (Figure 2). Twisting also occurs with both rigid and retensionable frames and will cause a frame to rock when it's placed on a flat surface.
A twisted frame affects off-contact settings by creating both tension inconsistencies across the mesh and poor parallelism between the screen and press bed. Where the curvature of the beams is most severe, twisting toward the center of the screen occurs. And once the frame is twisted, it will no longer lay flat.
The effects of poor frame flatness may first appear during prepress operations, especially if the process involves screen coating with an automatic coating system or direct screen imaging with a computer-to-screen system. The distance between the screen and the coating trough of an automatic coater or the imaging head of a CTS unit is a key factor in the quality of the stencil image.
Non-flat screens can lead to inconsistent stencil thickness when emulsion is applied with an automatic coater. And on a CTS system, such frames can cause the printhead to make contact with the mesh, thus destroying image quality (Figure 3). On both devices, the contact caused by a severely distorted frame can even lead to equipment damage. If you are investigating automatic coating or CTS imaging for your operation, make sure you carefully monitor for correct frame flatness.
Data collected by Anton Hurtz GmbH, a manufacturer of rigid frames, indicates that when a beam twists, the corresponding deflection or bowing it experiences is approximately five times greater (Table 1). In other words, twisting is 20% of the bowing distance. The company has also found that twisting distance should be no more than 10% of the off-contact height, particularly when printing high-resolution, process-color graphics. If twisting exceeds 10% of off-contact or is so severe that off-contact must be raised substantially in order to maintain this ratio, the results can be disastrous. Among other things, these conditions can affect the ink deposit, alter the size and accuracy of the printed image, and over-stress the mesh.
Printers have been known to say, "Don't worry! The screen will flatten out when we clamp it in the machine." Many printers also routinely put warped frames into their vacuum frames, expecting them to flatten out during exposure. These practices are unwise.
|Table 1: Frame Distortion Comparison|
|Frame profile; wall thicknesses (mm)||Frame OD (mm)||Tension level||Probable deflection (mm)*||Probable twist (mm)||Minimum off-contact (mm)|
|30 x 30; 3.0, 1.8||500 x 500||30 N/cm||0.20||0.04||0.40|
|Slope 40/30 x 30||500 x 500||30 N/cm||0.15||0.03||0.30|
|40 x 40; 2.8, 2.0||500 x 500||30 N/cm||0.10||0.02||0.20|
|40 x 40; 2.8, 2.0||1000 x 1000||20 N/cm||1.00||0.20||2.00|
|Slope 45/35 x 40||1000 x 1000||20 N/cm||0.90||0.18||1.80|
|50 x 40; 3.2 x 2.0||1000 x 1000||20 N/cm||0.50||0.10||1.00|
|Slope 55/45 x 40||1016 x 1524||20 N/cm||4.50||0.90||9.00|
|60 x 40; 3.0, 2.0||1016 x 1524||20 N/cm||3.50||0.70||7.00|
|Slope 65/55 x 40||1016 x 1524||20 N/cm||2.50||0.50||5.00|
|*Deflection given for longest beam on each frame. This table was generated from data provided by Anton Hurtz GmbH, Nettetal, Germany, a manufacturer of precision metal screen-printing frames. Based on evidence collected from customers using its aluminum frames, the company has determined a relationship between deflection (bowing) and twisting, where twisting is 20% of the amount of deflection and should not exceed 10% of the off-contact distance.|
In the first case, severely distorted frames can distort the mounting rails of the press. And the effect during stencil exposure is twofold: If the frame remains twisted under the vacuum blanket, the mesh will be skewed by the blanket, and the image will become distorted. Furthermore, if the frame is flattened in the vacuum frame, the image will be created while the frame is flat, but the frame will snap back to its distorted shape when the screen is removed from the vacuum frame. If you place the film positive on the processed stencil, you will see clearly how much the image is distorted.
Maintaining frame flatness to better than 10% of the off-contact requires care and attention, particularly in frame selection and handling. Although no frame is perfect, thoughtful selection will help keep your frames within acceptable tolerances.
Selecting a rigid screen frame
A rigid frame is typically a single structure of metal beams welded together. It is also known as a static or fixed frame and requires the mesh to be affixed with glue. In selecting a rigid frame, several factors deserve consideration: size of frame and image image resolution target mesh tension off-contact distance mesh type angle of mesh frame weight
The frame size is an obvious factor, since it needs to both fit the machine and accommodate the required image size. You also want a frame that leaves enough free mesh area around the image to promote accurate image transfer. The greater the free-mesh area, the less stress placed on the mesh--and consequently the frame--during printing.
Image resolution will affect the thread count you need, which will subsequently influence the tension you'll use. A good frame should be able to resist the screen tension you require, including extra tension during the print stroke to overcome the off-contact height. It should also resist extra tension that may be exerted during the stretching process to compensate for mesh relaxation later. The type of fabric selected (e.g., low-elongation polyester) is based on the maximum tension expected at any point during the stretching or printing process.
Circumstances rarely occur when it's necessary to angle or bias mesh relative to the frame. However, when biasing is necessary, keep in mind that it puts non-uniform stress on the frame and increases the potential for frame distortion.
From a handling perspective, frame weight is also important for transporting screens around the shop. Additionally, excess frame weight on some machines, particularly cylinder presses, will reduce print speeds, affect registration, and wear down machine components more quickly.
A variety of rigid-metal frame profiles are available, and some are illustrated in Figure 4. With the exception of solid-metal CD-printing frames and similar frames used in the electronics industry, screen frames are manufactured from hollow beams extruded from various materials. Aluminum is very common. Steel is also used and is less costly than aluminum, but it is heavier and, if it is not stainless, is likely to corrode.
Optimizing rigid frame performance
Screen frames do not last forever, no matter how good they are when new. From an accounting point of view, they should be treated as consumables in the process, yet they are often seen as capital equipment. Capitalization is a major cause of frames' being forced to perform long after their usefulness has expired. Shop managers are constrained from dumping poor frames because they are listed on the books as capital assets.
Let's adopt the principle of avoiding problems rather than solving them after they occur. Frames should be discarded just before they go outside the accepted flatness and stability limits. But in many shops, printers use bowed frames for non-critical work and reserve the flat ones for more challenging jobs. In such situations, defining the "best" frames becomes subjective, which makes the frame inventory difficult to manage.
One way to deal with this is to log key characteristics of frames each time they are used. Record tension levels after printing, and if the frames are failing to hold satisfactory tension, restretch new screens for them. Based on your records, you can determine the approximate number of redressings that frames will support before they become unreliable. Set a limit slightly below this value for the maximum number of redressings allowed before a frame must be discarded and replaced.reducing the number of times that frames are reused (increasing the number of new screens required), you will increase your screenmaking costs. However, that sacrifice will more than pay for itself given the improvements you will see in other areas of production.
Initial frame selection is important because you want a frame that is suitably flat before the mesh is attached. Affixing the mesh to an uneven frame surface will result in gaps or weakly bonded areas that will allow the mesh to pull free of the frame. Using layer upon layer of adhesive to compensate for an uneven surface is no solution, either, because it creates the need for excessive frame-preparation time.
New frames often come with a preroughened surface to improve adhesion when the mesh is attached. When used screens are redressed, the mesh is removed and the adhesive residue is typically ground off to provide a flat yet roughened surface similar to the frame's original condition. However, when too much metal is removed in the redressing process, the entire frame weakens.
Every time you abrade the frame, you reduce its strength. Even if you remove very minimal amounts of the frame surface, the cumulative effects of multiple redressings will eventually catch up and reduce the beam thickness enough to decrease its resistance to bowing and twisting.
A simple method of determining how much material has been removed is to weigh the frame before and after sanding or grinding. For example, on a rectangular beam with dimensions of 2.4 x 1.6 in. (60 x 40 mm) beam and wall thicknesses of 0.12 in (3 mm) top and bottom and 0.24 in. (6 mm) on the sides, a 10% reduction in weight after sanding will translate to a 46% reduction in thickness of the beam surfaces.
Grinders are often used for frame preparation, but they carry the most risk of removing too much of the frame material itself. And unless the grinding is carefully controlled, it can create a serrated frame surface that will actually tear new mesh while it's being applied. A better tool to use for redressing is a rotary belt sander, which is less aggressive and removes less metal.
In recent years, automatic grinding equipment has been introduced for the sole purpose of preparing metal frame surfaces. Where large volumes of frames are used, such automatic systems can be useful.
After a frame is redressed, always clean it well with a fast-evaporating solvent. This will remove particles and contaminants that might otherwise interfere with adhesion when the mesh is attached.
In addition to preparing and maintaining frames properly, it is important to realize the effect of temperature variations on screen frames. Tensioned screens can be subjected to wide temperature variations in storage, shipping, exposure, and the press room. The frame expands and contracts and tension variations follow. While the screenroom temperature may be 68°F (20°C) , the temperature at the vacuum frame can exceed 130°F (54°C).
In one recent experience, a temperature variation of 24°F (13°C) caused a 4 N/cm tension variation on a large batch of screens that were required to be within ±1 N/cm of each other. To avoid this situation, make sure screens will be prepared and used under similar temperatures.
Some words of retensionable frames
Retensionable frames are used most commonly in garment-printing applications, but many graphics printers also find them useful. Retensionable frames come in three varieties:
Roller These frames feature a tubular extrusion into which the mesh is inserted. Tension is applied by turning the extrusion at the corners using a special wrench.
Square This style of retensionable frame has side bars to which the mesh is attached. The screen expands in all directions by means of special tensioning jacks provided by the manufacturer.
Draw bar Draw bar frames are also square, but the mesh is attached to a bar located inside a master frame. Tension is achieved by tightening a series of screws (typically Allen screws) that move the bar outward.
Like a rigid screen frame, a retensionable frame needs to resist the forces of the tensioned mesh. For a retensionable frame to do this, it must be substantially more robust than a rigid frame, especially when applying some of the tension levels being discussed these days--up to and beyond 50 N/cm. At 50 N/cm, a frame with a beam length of 40 in. will be resisting a load of approximately 1124 lbs. Consequently, you must be confident that the engineering and materials that go into making your retensionable frames are suited to the conditions they will face.
Retensionable frames can be managed in much the same way as rigid frames. Since the mesh is not glued to the frame and the frame edge doesn't have to be sanded or ground, frame degradation due to redressing becomes a non issue. However, more attention must be paid to guarantee consistent tension from screen to screen and prevent arbitrary retensionings that can offset color-to-color registration.
Signs of inadequate frames
Unstable and unreliable screens can wreak havoc on the screen-printing process in a number of ways. Among the problems the create are the following: varying tension levels within the same screen altered thread geometry, particularly closest to the frame reduced "sweet spot" or workable printing area variations in off-contact distance the need for increased squeegee pressure, which can damage both the mesh and squeegee blade reduced life expectancy of the mesh irregular emulsion coating, which leads to inconsistent image reproduction throughout the print image distortion and poor color registration poor adhesion of the mesh to the frame inability to hold required tension levels inability to use automatic-coating and CTS-imaging equipment reduced production rates increased rejects, machine downtime, and stencil costs lost profits
Each of these is a costly problem, and when you use badly bowed or twisted frames, it's unlikely that you will experience just one or two of them. They tend to travel in groups!
Successful screen printing is about controlling the variables. Having adequate mesh tension and stability is the foundation of the process and the key to achieving control. Your screens must be controlled and predictable, or you risk making the printing process unnecessarily complicated.
Whether you use fixed or retensionable frames is a matter of personal preference. What is essential is that they are capable of maintaining their structural integrity with little or no distortion under the tension levels you require. Finding the right frames and using them properly can add to your costs. But the smoother, more accurate production and reduced downtime that will result is a tradeoff you can easily accept.