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Prepress & Screen Making

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Frames come in two main varieties: static and retensionable. Of these two groups, the static or rigid frame—commonly known as a stretch-and-glue frame—is the simplest form. Rigid frames are available in many different materials and profiles. The most common materials are wood, tubular aluminum, and tubular steel. Wood frames are arguably the most popular due to their lower cost but are limited in size for strength reasons and generally not recommended for high-resolution, tight-tolerance work.

Frames come in two main varieties: static and retensionable. Of these two groups, the static or rigid frame—commonly known as a stretch-and-glue frame—is the simplest form. Rigid frames are available in many different materials and profiles. The most common materials are wood, tubular aluminum, and tubular steel. Wood frames are arguably the most popular due to their lower cost but are limited in size for strength reasons and generally not recommended for high-resolution, tight-tolerance work. Among the metal rigid-frame types, tubular aluminum is the most popular in the United States. Aluminum frames are lighter than steel frames but provide similar stability. Steel frames, epoxy coated or powder coated, are more popular overseas.

Tubular aluminum frames support a variety of gluing surfaces, including sandblasted, ground, or raw with no texture (Figure 1). Frames have a great surface to work with when they’re brand new. But the surface can become less than ideal once multiple layers of adhesive, fabric, and ink residue have built up. You can re-glue mesh over old adhesive numerous times before the frames must be reconditioned; however, many printers don’t take the time to prep the frame properly when they attempt to recondition it.

Sanding the glue off of an aluminum frame is a common mistake because it leaves the surface too smooth for the adhesive to bite into it effectively. Sandblasting is great for small frames but may be impractical for large frames. So this leaves mechanical brush grinders, angle grinders with flapper discs, or chemicals.

The chemical method is most commonly used with coated steel frames, but it is also used with aluminum frames. The chemicals come in two main varieties: gels and liquids. To ensure the best performance, the exact chemical makeup of the strippers should be recommended by the manufacturer of the adhesive you use.

The task of chemical adhesive removal can be performed in a few different ways. Some people dip their frames, if the size allows, in a stripper solution. Others will paint or spray the cleaning chemical onto the frame, wait a period of time, and then remove the residue with scrapers. To complete the process, excess stripper material and residue are cleaned from the frame with an appropriate solvent.

No matter which method of adhesive removal you use, once you’ve reconditioned a frame, you will need to treat it as if it were brand new. That means you’ll need to check the gluing surface and edges of the frame for nicks, cuts, or any sharp protrusions that could cut or puncture the mesh during the stretching and gluing process.

The use of a solvent to clean the gluing surface also is recommended for non-wood frames. The solvent should evaporate rapidly and leave little to no residue. Oily solvents, such as mineral spirits, lacquer thinners, or safety solvents, are not appropriate. Solvents such as acetone or isopropyl alcohol (99%) are very good choices and will leave the frame clean and free of any material that could compromise the adhesive bond.

Stretching and gluing with rigid frames
The two main categories of stretching devices are mechanical and pneumatic systems (Figure 2). Within the pneumatic category are air-bar and individual-clamp stretching systems. Air-bar systems tend to be less expensive, but they normally don’t stretch mesh as uniformly as a system based on individual pneumatic clamps. Systems based on individual pneumatic clamps can compensate for inconsistency in mesh loading, while air-bar systems can’t. Both types of pneumatic systems allow for the fabric to stabilize because constant pressure is put on the mesh during the stretching process. Another benefit to pneumatic systems is the fact that most can pre-bow the frame, resulting in even more consistent tensioning.

Mechanical stretching systems are most often used for low-volume stretching or where a tight budget is involved. Extremely large mechanical units also available for large-format screenmaking. Mechanical systems are highly effective in getting the job done, but generally they lack the finesse of a quality pneumatic system.

After the mesh is stretched, it’s time to adhere it to the frame. Consider the following points about adhesives:

• Adhesives are effective when using wood or metal frames. They’re the only option when using metal frames.
• Common adhesive formulations include two-part epoxies, two-part urethanes, and cyanoacrylates. All three systems can be called two-part; two feature a base material and a catalyst, and the other comes with an adhesive and an activator that is used to make the adhesive cure instantly.
• Urethane and epoxy adhesives consist of a base adhesive that requires a catalyst. The catalyst accelerates the curing and improves the hardness of the adhesive. The mixed adhesive and catalyst has a pot life, which will vary depending on the formulation and manufacturer. Cyanoacrylates consist of ethyl cyanoacrylates, which have a good strength characteristic, fair solvent resistance, and compatibility with most materials.
• Catalyzed adhesives are available, including formulations for just about any application, mesh count, frame type, and environment. Viscosity is important because it influences how well the adhesive penetrates the fabric to reach the frame.
• When mixing catalyzed systems, adding more catalyst will help with the chemical resistance but it will slow down the reaction time of the adhesive. Normally, the two-part epoxy systems are tack dry in 5-20 min but are not fully cured for 24 hr.
• Methods of adhesive application are personal choices. Use the one that’s best for you or suggested by the manufacturer.

Retensionable frames
The alternative to the stretch-and-glue process for attaching mesh to rigid frames is to use retensionable frames. These frames became extremely popular because standard polyester mesh fabrics were unstable in terms of maintaining consistent tension levels. With the advent of newer low-elongation (LE) fabrics, the benefit of retightening mesh is no longer as important, but these frames are still very useful. Retensionable frames area available in drawbar and roller configurations.

As their name implies, retensionable frames allow mesh to be retensioned. They use no glue, and they can be fitted with new mesh relatively quickly without the use of chemicals.

It takes some skill to get consistent tensions with these frames, and the process usually takes a little more labor to stabilize the fabric, but the results are usually worth it.

Drawbar frames have several configurations for locking the fabric into the frame. One is a stretch-and-glue version, but most consist of either one to two plastic rods or a metal bar that slide into grooves or channels on the frame sides and lock the mesh in place. Which configuration to use depends mainly personal preference, but also on the mesh type and thread count you use. For instance, printers working with stainless-steel mesh tend to use the single plastic rod or metal bar locking mechanisms as opposed to the two-rod systems since these meshes are stiffer and the single-component locking systems are easier to work with. Locking the fabric in the frame uniformly is fairly critical to ensure a consistent stretch with drawbar systems.

The act of stretching screens on a drawbar frame usually consists of either turning a series of bolts along the side of the frame or using a special tool on the corners to draw the bar back mechanically. In the first scenario, depending on how much you turn each individual bolt along the frame, you can influence the fabric in that general area to compensate for any inconsistencies in the way the mesh was loaded. In the second situation, the construction of the frame may cause issues because as you tension the mesh, the dimensions of the frame will increase. This is not a problem in some applications, but it’s something to keep in mind if you’re considering this frame type.

Roller frames are clearly the most popular of the retensionable frames. Once again, some skill is required to use these frames, but once they are mastered, roller frames provide a quick and easy method of making a screen. The principle behind the roller frame is that the fabric is locked in the frame with either a plastic strip or two round plastic rods. Then the rollers are turned away from each other to increase the tension. Just like the drawbar frame, the fabric can be retensioned to a degree to compensate for tension lost during printing. Also, pneumatic tensioning systems are offered for roller frames to simplify tensioning even more.

With all retensionable frames, you have to take care when reclaiming and handling due to the fact that there are many pockets in which cleaning chemicals can collect, only to drip out at the worst possible time. The other common issue with these frames is that oily reclaimers and ink washes tend to get trapped in the locking channels and can sometimes lead to the fabric slipping out of the channels. Liquids also can leak into the hollow tubes. This problem is quite common when using a dip tank in the screen-cleaning area. Everything works fine until the frame is placed under vacuum in the exposure unit, which causes chemicals trapped inside the tubes to be pulled out and across the screen, ruining the emulsion.

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