Stretching Your Own Screens, Part II: Making Sense of Frames
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SignLab from CADlink


Stretching Your Own Screens, Part II: Making Sense of Frames

Stretching your own screens means working with an outside source, the frame suppliers. Keep the following essentials in mind when placing your orders to ensure you get what you need.

By Bill Stephens

One of the challenges in setting up your own screen stretching operation is finding a source of frames.

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  • As we mentioned in the previous article, in the past many shops built their own frames. These frames were made of wood, strong, light, easily cut and easily assembled. Wood frames could be quickly built in-house in a wide variety of sizes, allowing screenprinters to custom-build frames to fit the jobs at hand. Wood, however, is also a relatively weak frame material. It deteriorates rapidly in wet conditions such as the repeated dousings frames undergo in stencilmaking and during the reclaiming process. This has led the industry to a growing reliance on frames made of metal, especially aluminum. The downside of this switch in frame materials is that few screenprinting shops have the either the equipment necessary to cut and weld aluminum. This has led to fewer frames being assembled in-house and a growing reliance on frames ordered from specialized outside suppliers.

    What this means to you is that when you've got your own screen stretching operation set up, you too, will more than likely find yourself ordering your frames from an outside source. Keep in mind that whenever you rely on an outside supplier for key materials, two risks arise: unanticipated delays in shipments and the possibility of misunderstood orders.

    Given enough time these problems are not really problems. Given enough breathing space, any mix-up or delay can be corrected. But what if you don't have enough time? What happens when you need to stretch new frames to meet a looming deadline? One way of buying time is to stockpile frames in your most popular sizes. Not only does an inventory of frames allow you to react more quickly to urgent jobs, larger orders placed with your frame supplier may qualify for volume discounts that could lower your cost per unit. Shipping costs could also be lower than the costs of shipping for one or two frames at a time. However, there are also costs associated with carrying inventory, and you need to do some careful calculation to determine the most cost-effective approach. To some extent this will depend on the reliability of your frame supplier.

    Nothing takes more time than getting orders mixed up. If you work on a just-in-time basis sending back the wrong frames and reordering the right ones can put you well over your deadline. To reduce the chance of error in orders, whenever possible place your orders in writing. Faxing or e-mailing order takes a little additional time, but improves accuracy and leaves both you and your supplier with hard copies of the order. One of the key pieces of information is the dimension of the frame. So let’s begin with some standard terminology used to describe frame dimensions.

    Frame dimensions
    One area of possible confusion is whether the dimensions you've given your supplier apply to the inside or the outside of the frame. This may so fundamental that no one could ever possibly get it wrong. Nevertheless, getting the two confused has led to more than one botched order. To eliminate any possible confusion always mark dimensions either “I.D.” for the inside of the frame or “O.D." for outside measurements.

    Mostly, you'll be concerned with getting the inside dimension (I.D.) right, since the outside dimension depend on the inside dimension plus the width of the frame member. As we'll see later, the critical limits to a frame's outside dimension are mostly determined by the limitations of the press.

    The inside dimension of the frame is determined principally by the print area of the screen, roughly the extent of the final print that will be covered by ink. However, screens must also contain a certain amount of nonprinting space. This so-called free mesh area, refers to the open mesh between the image in the stencil and the inside edge of the frame.

    Some free mesh area must be allowed on every side of the print area. This includes open spaces at both the top and bottom of the squeegee’s limit of travel. These open spaces are important because the serve as ink wells, that is holding areas for excess ink dragged along by the squeegee.

    Additional free mesh areas must be allowed on either side of the area that will come into contact with the squeegee. The squeegee will, of course, overlap the open areas of the stencil by several inches on either side, and roughly equal amounts of free mesh must be allowed between those limits and the frame. It is difficult to exaggerate the importance of allowing an adequate amount of free mesh. These areas allow the mesh to flex downward and upward as the squeegee passes. This movement is what makes the screenprinting process possible. Allowing a substantial amount of free mesh not only helps produce good prints, it makes for easy printing. Mesh too close to the frame lacks this flexibility, so and inadequate free mesh allowance can lead to distorted prints and excessive wear and tear on mesh and squeegee.

    The size of free mesh area is roughly proportional to the size of the screen. It should increase as frames grow larger. For example, the free mesh allowance in a 16” X20” (40 cm X 50 cm) screen may be no more than 4” (10 cm) to either side of the print area and 6” (15 cm) on both ends. A larger frame, 36” (91 cm) x 40” (101 cm), might have a free mesh area of 8” (20 cm) to 10” (25 cm) on either side and 12” (30 cm) or more the ends. Opinions vary about how much free mesh to allow, but the effects of allowing too little has the potential to cause many more problems than allowing too much, so it's better to err on the generous side. Unfortunately, the more free mesh you allow the larger the screen you'll need and larger screens cost more. This has led some screenprinters to try to get by with the smallest free mesh area possible, surely false economy when you consider the list of potential printing problems, ruined substrate, and wear on screens and other equipment. You'll be d oing yourself a favor when you order your frames if you make sure what you're ordering is big enough to do the best possible job.

    It’s time for your profile
    After the dimensions width and length have been dealt with, it’s time to consider another important frame measurement: profile. This refers to the height and width of the frame members, as opposed to the dimensions of the frame itself. It really just means dimensions of the frame member seen in cross-section. The size of the frame profile plays a vital role in the strength of the frame. Generally-speaking, the larger the profile indicates the stronger the frame.

    As we discussed in the previous article, frames must withstand not only the tremendous forces exerted upon them by tensioned mesh but the repeated stresses applied by the squeegee during printing and the general knocking around screens are subject to on the shop floor. But the most serious stress comes from the tension of the mesh. The effects can often be seen in the inward bowing of some larger frames. For the most part, this has little effect on the screen’s printing ability, but excessive bowing is a clear sign that a more substantial frame profile or a stronger frame material should have been used.

    In addition to the overall size of the frame, the choice of a profile can also be influenced by other factors like the intended use of the frame and the anticipated mesh tension. The other thing to consider about frame profile is the clamping mechanism of the press. Other factors become meaningless if the frame is too large to fit into the clamping mechanism. And, since frame profile also has a direct bearing on the outside dimension of the frame, you also need to keep in mind how large a frame your press can accommodate.

    Your frame supplier should be able to suggest an appropriate profile to meet your needs if you can supply the inside dimension and the intended use for your screen. Frame manufacturers usually have a variety of profiles on hand, and of profiles not only of different sizes, but of different shapes. The two principal shapes will be either square or rectangular. Square profiles tend to be used only for smaller frames, possibly those under four feet in length (122 cm), for example.

    Larger frames tend to require the increased strength provided by a rectangular profile. Common sizes range from 1” X1” (2.45 cm X 2.45 cm) square profile to 2” X 6” (5.08 cm X 15.24 cm) rectangular.

    An unusual shape is the slope profile. This profile is wider at the bottom edge (where the mesh attaches) than the top. The inside edge of this profile is angled, giving the slope profile its name. Most often found in frames six feet or longer, the slope profile is more in length. Advocates believe the slope profile provides superior strength, however, it is relatively rare and few screen-stretching shops will ever encounter one.

    Another thing to keep in mind about frame dimensions, including profile, is that as they increase in size, so does the weight of the frame. Weight is a factor worth considering, because handled frequently and the lighter the frame the easier it is to lift and to position. To help keep frame weight down most metal frames are of tubular construction, which means they have hollow interiors. Their strength therefore depends on another critical dimension of the profile: wall thickness. This, of course, refers to the thickness of the metal and there are various wall thicknesses available. Aluminum frame profiles, for example, may feature wall thickness that range from 1/16” (0.625) to 1/8” (.125). Obviously, the greater the wall thickness the more durable the frame is. It's really a trade off of weight versus strength. The frame manufacturer should be able to recommend an appropriate wall thickness depending on the size of the frame and its intended use.

    Re-tensionable frames
    Up till now we have been focusing on fixed, or rigid frames, but retensionable frames have become an increasingly important part of the screenprinting scene. Retensionable frames are made up of moveable parts that allow the frame to serve as its own self-contained stretching device. In addition to allowing the printer to stretch their own screens, retensionable frames give them the ability to make periodic adjustments to mesh tension. This insures that retensionable frames can always be maintained at optimum printing tension.

    One bonus to this technology is extended life for screen mesh. As mesh wears it stretches, which means falling screen tensions. At some point the mesh tension will drop so low, print quality will be affected. With a rigid frame your only alternative is to replace the mesh, but if you’re using a retensionable frame that lost tension can quickly be restored.

    Of course, this comes at a higher price tag. Retensionable frames cost more, but they also eliminate the need for an expensive external stretching system. This offers an attractive option for a small screen printer, operating with a small number of frames who would like to get into stretching his own screens. Retensionable frames have become fairly common in T-shirt shops, especially those that specialize in four-color process printing where mesh tension is critical.

    Today, they are beginning to make inroads into other screenprinting shops as well. One reason for their increasing popularity is their ability to sustain the extremely high mesh tensions favored by some screenprinters. We’ll take another look at retensionable frames later on when we shift our focus to stretching equipment.

    Next time, though, we’ll turn our attention to the second major screen component: mesh, or fabric. We’ll also consider our first piece of screen stretching equipment, a key device you should have on hand even if you never intend to stretch a single screen.

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