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Printer's Guidebook, Part V

Frames/Stencils

13. Frame stability
Must be sufficient to maintain screen tension.
  Of utmost importance is that frames be stable in their corners. The corner is the weakest point of a frame and, unfortunately, that's where all the opposing forces are concentrated. A standard manual (wood) frame tensioned to 14 N/cm is withstanding more than 600 pounds of force. A standard automatic (metal) frame at 30 N/cm must withstand more than 1900 pounds of force. In both cases, all that force is concentrated at the corners.

This condition is most critical in the wooden frame corner, with its members cut at 45 degrees and held together with glue and corrugated fasteners. This inexpensive frame is usually used with manual presses where it can do the most harm in your shop. The constant pushing of the frames around the press weakens them so they don't register well. Swinging the screens around on a multi-color job causes them to twist out of register-unless your press has side clamps. But don't think about using your press as a clamp to straighten out the frame; as it straightens, its image (originally placed on a warped frame) will distort. The conclusion is that wooden frames should have overlapping corners (mortise-and-tenon) that are glued together. Welded metal frames don't have this problem. Retensionable frames can, however, and so must be tightened before they go to press or they will twist out of register.

Frame flex due to weak corners means image distortion. You can check for frame flex by measuring from opposite corners and comparing the dimensions. If they aren't the same, the frame is not square. You may test corner strength by pushing on the corners to feel for movement, or dropping the frame on its corner from six inches and listening for a dull sound, instead of the healthy ring of a solid frame. Do yourself a favor and get rid of your unstable frames.

Screen flatness

Frames must also be flat, a condition difficult to achieve in any frame. Wood frames can warp from poor corners, poor assembly or changes in their shape because of water absorption. They will flex one way as they absorb water, then change shape again as the water evaporates and the wood dries. You can protect them with a polyurethane coating to prevent them from absorbing water; this also makes them easier to clean.

Rigid-aluminum or -steel frames can warp when they are glued or welded. Glued frames should be clamped to a flat (not necessarily level, flat) surface until cured. Heat from the welding process will make even a clamped frame warped when it cools. Check them when you buy them and reject any that aren't flat.

Retensionable frames can be very frustrating if you don't have a flat surface on which to work. They will conform to the shape of your working surface and fool you-if your surface isn't flat-into thinking they are flat. Check this by pushing on opposite corners to try and rock the frame. If it does not rock, turn it 90 and check it again. If it still doesn't rock, you know you have a flat table, suitable for stretching.

You may carefully check flatness of any frame on the glass of your light table. Remember to lay the frame in the middle of the glass, though, because even glass is flexible and can sag.

 

14. Stencil thickness
The extent to which a direct-emulsion stencil is built up via multiple coats, or a film stencil via manufactured thickness.
 
It is important to remember that both capillary films and liquid emulsions are (in most shops) hand-applied, and therefore at the mercy of certain human inconsistencies. Although capillary films have generally been thought to produce more consistent stencil thicknesses than liquid emulsions (because they are available in pre-manufactured thicknesses), they still can be inconsistent in thickness if improperly applied.

In fact, a recent study by the Screen Printing Technical Foundation (SPTF) found that several factors related to stencil application can influence the final emulsion thickness of a capillary film. The use, for example, of various application techniques with 30-micron capillary film resulted in a final emulsion thickness variation of from 10 to 17.2 microns.

Of the many factors that can affect capillary-film stencil thickness, one of the most significant is the pressure exerted to remove excess water after film application. Therefore, even with factory-consistent products such as capillary film, a consistent procedure for application is imperative as well, in order to minimize variation in the emulsion thickness within the screen and from screen to screen.

Control of stencil thickness with direct emulsion is not as simple as with capillary film. Various techniques must be employed to create the thickness needed. Liquid emulsion is applied to both sides of the screen. Increasing the number of coats applied to the screen increases the stencil thickness. Due to the high proportion of water in the emulsion, a single coat on each side is likely to result in an inconsistent surface as it dries and shrinks around the mesh filaments. Additional wet-on-wet coats are necessary to compensate for the loss of water needed to achieve a uniform surface coating.

In addition, coating procedure is likely to vary between meshes of differing counts. For instance, a 255 mesh and a 355 mesh, coated two-by-two (two coats per side, applied wet-on-wet) will typically yield stencils with different emulsion buildup, because the lower count has greater open area between the mesh filaments, through which emulsion can flow.

Although it may sound contradictory, at the same coating pressure, with the same coating trough, the 255 might yield an 8-micron buildup when dry, while the 355 would build up just 4 microns. It is clear from this example that emulsion build-up sufficient to prevent the mesh from blocking ink flow onto the substrate cannot be consistently maintained by one single coating technique on all mesh counts. To achieve a similar buildup, additional coats would be required on the 355-two-by-four, for example, would yield 10 microns. Tests should be conducted under your own conditions to determine the proper coating procedure to use for each mesh in your inventory.

Emulsion buildup, then, is the critical factor in determining proper stencil thickness, and can be monitored using one of several commercially available, non-destructive electronic devices which measure the mesh and stencil thickness, producing the measurement of percent of emulsion over mesh, or emulsion-over-mesh ratio (EMR).

Thickness is primarily a function of the screen fabric. Therefore, emulsion thickness of the stencil can be determined according to recommended values for the mesh in question. For fine lines and halftones it is recommended to generate an 8 to 15 percent EMR. Generally, no stencil should exhibit greater than 20 to 25 percent EMR, because the increased ink-column height (the result of the increased overall thickness) tends to frustrate ink release from the screen, particularly in fine-detail lines and halftone dots, because the ink tends to adhere to the greater surface area of the stencil "wall" surrounding the stencil/mesh opening. Measurements should be taken prior to exposure and screens should be monitored from day to day to evaluate consistency of coating procedures.

In addition to benchmarking the overall emulsion thickness and EMR, it would be useful to measure thickness consistency on the micro level. Such measurement may be performed with a perthometer. This device moves a probe over the surface of the coated screen to measure its surface consistency-literally, its flatness.

Monitoring surface consistency is especially important with direct emulsions. Stencil films, applied in sheet form, tend to retain their manufactured-in flatness when applied to the mesh. This is not the case with liquid emulsions, which tend to conform to the uneven hills and valleys created by the knuckles of the mesh weave.

This can be mitigated by additional coating, wet-on-dry. For example, a screen coated two-by-three, wet-on-wet can be dried and then leveled by applying a face or fill coat to the print side with a sharp-edged scoop coater. This fills in the "lows" in the stencil's print side, improving the evenness of the coating without seriously increasing the general stencil thickness. The downside is that this operation requires additional coating and added (though brief) drying steps.

Liquid-emulsion coating thickness and consistency are best controlled with automated coating equipment which can be programmed for accurate, repeatable coating parameters.

Of course, coating equipment is expensive, as are the measuring devices mentioned above. Yet, if precise control of variables is the goal, their expense may be unavoidable.

The industry's general reluctance to invest in screen-making technology is ironic, given the amount of money so often poured into production equipment. In the near term, the industry could best spend its time not in development of the "perfect" stencil material, but instead teaching the screen printer to make a better stencil with the stencil materials-and the tools-at hand.

15-16. Stencil resolution
Resolution, definition, sensitivity and durability.
 
These characteristics are combined here because they are all important aspects of the actual stencil materials. We don't have any control over each manufacturer's choices in these areas, but we can measure finished product and decide which will work best for the jobs we print.

Resolution is the ability of the stencil to reproduce fine details, typically measured by the finest line or halftone dot it can hold on the mesh and still control the ink. We test resolution with halftone gray ramps and resolution targets. Most stencils will reproduce any fine line from a positive, but we must also take into account how such a line is supported by the mesh and how it resists wear from the squeegee-blade stroke.


stencil chartThe stencil characteristics of resolution and definition are critical factors that determine its ability to bridge the open spans of woven mesh. (The bottom illustration shows inadequate bridging.)


Definition is the accuracy of the microscopic stencil edge and the smoothness of the ink-well "walls." It describes the stencil's ability to reproduce the film positive's image outlines exactly. If the positive has a sharp edge, yet the stencil has a poor edge, ink could flow between stencil and substrate, resulting in bloated and misshaped halftone dots or distorted lines. Fortunately, as textile printers, this has little affect on our final print because the wicking or ink gain caused by substrate absorption is typically much greater than distortion caused by poor stencil edge definition.

The stencil's performance in these two characteristics also affects the stencil's ability to bridge the open span of the woven mesh, without being affected by the rugged surface of the mesh. Poor bridging produces the familiar "saw-tooth" or "stair-step" image edge.

Mesh bridging is naturally more difficult to achieve with coarse mesh because, with its larger openings and thicker threads, it simply soaks up the liquid emulsion. Capillary film, multiple face coats, and certain stencil characteristics such as high solids content produce the best mesh-bridging characteristics.

Sensitivity is a measure of speed-how fast the stencil is altered by the action of UV light. A stencil with high sensitivity will require less time to expose and would therefore be considered "fast." Unfortunately, there are no international standards for stencil speed as there are for photographic film for your camera (ASA, ISO and so forth), as stencil speed can also be affected by stencil thickness, lamp power, lamp UV output, moisture content and the type of sensitizer formulated into the emulsion.

Diazo sensitizers are the most popular. They replaced the evil carcinogenic bichromate sensitizers in the 1980s. They are stable in both solvent and water-based stencils but are relatively low in sensitivity when compared to modern photopolymer-sensitized emulsions which can be very fast. On the other hand, the weakness of pure photopolymer emulsions is that they are not very resistant to water-based inks. To increase water-resistance, diazo and photopolymer sensitizers have been combined into a third category, known as "dual-cure" emulsions.

Durability refers to the stencil's ability to hold up under the stress of blade pressure, off-contact distance distortion, inks and chemicals. If the stencil decays while you are printing, it causes bad prints and huge downtime if you have to replace it during the run. Actual stencil performance also depends on how well the screen maker controls the variables of the stencil-making process such as optimum exposure, complete drying of the stencil before exposure and the way in which the stencil is handled immediately after washout. Sitting in the sun to dry, for example, makes stencils brittle, while rushing them to the press before they are completely dry gives them little abrasion resistance.

When you discuss what you need with a sales rep, he will tell you there are four types of stencil systems, each with a general set of "built-in" potentials for resolution, definition, sensitivity and durability. Generally, those with the greatest resolution and definition potential tend to be less durable, and vice-versa. The screen printer is left to find a compromise. And the final result, of course, is always in hands of the screen maker and printer to make sure they don't prematurely age the stencil with printing abuse.


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