|Demands on the T-shirt printer have continually increased, and apparel professionals have responded with quality in kind. (Image courtesy the MIDI PRINT Co.)|
Twenty-five years ago, digital imaging was predicted to “completely eliminate screen printing within 10 years.” This same decade of fate was predicted again 20, 15, 10 and five years ago… eventually they may be right, but history says otherwise! Our trade has only gotten more sophisticated with each threat.
In 1960, a quality T-shirt print needed to be spelled correctly, less than five years later it needed to pass the wash test as well. Forward 50 years and we’ve become far more discriminating. The consumer has combined comfort along with color and contrast and has very rigid ideas as to their combination—the image has to look and feel just right. Demands on the T-shirt printer have continually increased, and apparel professionals have responded with quality in kind.
Throughout this series on the methodology that explores those sophistications, we’ve presented parameters for best print practices in order to keep setting a higher standard for screen prints. Here are the highlights from our progression.
Controlling the variables
All imaging forms and methodologies have at their essence ink and a pump. In the case of T-shirt screen printing, there are four types of inks—transparent, colorant, opaque and textured.
Ink goes through the screen in one of two ways—either by ejection or extrusion. An ejectable ink deposits more ink when printed at higher speeds while the deposit of an extrudable ink decreases with an increase in print speed.
|Opaque white ink is by far the most difficult to pump while colored prints print best on a calibrated press. (Image courtesy the MIDI PRINT Co.)|
There are three primary components of ink transfer—screen mesh, stencil and blades. These components should be altered according to the type of ink in use. Transparent prints, for example, require a blade which has moderate flex and compression. And, the most critical selection in printing water-based inks, in another example, is the screen mesh. It should have a thin thread—the limiting factor for highlight dots—and a large opening, which limits the shadow dots, resulting in a low mesh count. Opaque white ink is by far the most difficult to pump while colored prints print best on a calibrated press. When we understand the characteristics of inks and their relationship to the components of ink transfer, the result is consistent quality.
Screen mesh looks so simple; a few elastic yarns per centimeter, bonded at right angles, intertwined in a sinusoidal pattern which must maintain a relationship between thickness, wavelength, amplitude and orifice two meters wide for 5,000 to 6,000 meters running lengths…that’s right; it’s not simple at all. Screen mesh is a textile engineering feat but its selection is often as misunderstood as its sophistication. Choose a mesh that will allow ink to flow with minimal restriction, at a volume that we can maintain without interruption and at a speed that will efficiently fill the cells of the mesh.
Both water-based and plastisol inks each have charming characteristics that mature to be true margin killers. That is, water-based inks often dry in the screen and plastisol inks usually build-up on the underside of them. Interestingly, both phenomena (drying and building) are due to phase separation in that takes place in the screen. When inks are shaken, stirred or squeegeed, the liquids and solids contents move at different speeds which results in those dried and built up problems; this phase separation. The key to reducing phase separation once on press is to minimize blade pressure and optimize shear-rate. Here, we begin to see why print speed can have a dramatic effect on ink transfer. This also reveals why the quality of the ink system—and all raw materials at that—is so much more important than the price tag.
|Cause & Effect when Printing White Inks: This table indicates a few options to regulate the coverage and finish of white inks with the flood bar and squeegee blade. By selecting the proper pre-load, print speed, footprint and funnel, you can achieve the finish and coverage desired for a given design.|
White ink is the most prolific and the most problematic for screen printers. If the goal is a satin smooth finish, maximize pressure between the blade and the mesh (NOT between the blade and the platen) and be certain to print at top speed. If the desired result is to matte down the surface, pre-load the mesh with a blade that won’t harm the screen mesh. If it’s opacity you’re after, a beveled edge on the back side of the hinge is best. A sensible selection of the core materials; mesh, stencil and blades can control white ink, no matter if it is discharge, water-based or plastisol.
Value versus cost
There are three ways to improve profitability: raise the selling price, cuti the costs or some combination of both. Many screen print businesses believe cost cutting is the path to profitability, but the numbers prove otherwise. Calculating the value of raw materials provides a more precise picture of true cost.
|The raw material costs (RMCs) account for a total of just two percent of the cost of producing a 500-piece, three-color order of garments in this example; since the RMCs are not the real problem it doesn’t make sense to consider them a real part of the solution.|
In the business of printing apparel, anything that slows down the press is bad and anything that makes it run faster and maintains or improves quality is good. It is a simple axiom but one which is most often neglected. The reason is simple: It’s a lot easier to find RMCs (raw material costs) by simply looking at an invoice from a supplier. But it takes a bit of work to evaluate the cost of conversion.
Leveraging RMCs against other contributing costs—direct labor, general and administrative costs, fixed costs and the price of substrates—reveals that RMCs account for a minimal portion of the total price of operating a business.
Methodology Terms at a Glance
- Durometer—the metric which gauges the hardness of the blade (based on the Shore-A scale for elastomers).
- Ejection—one method of transferring low viscosity inks through mesh.
- EOM (emulsion over mesh)—stencil thickness as determined by the amount of emulsion used to expose a stencil.
- Extrusion—another method, opposite of ejection, of transferring high viscosity inks through mesh.
- Footprint—the amount of blade in contact with the mesh during the print stroke.
- Funnel—the shape of the divergent tunnel created between the blade and mesh during the print stroke.
- Mesh count—a result of the diameter of the thread and the dimension of the opening.
- Shear (thinning)—describing the viscosity of inks and how easily they can build momentum to move through mesh.
Choosing the Right Mesh
Pick a mesh that will support your process while remembering three facts—thicker ink films equal tougher printing, the stencil can be used to exaggerate fine details and there is not a lot of percentage in running very large solid areas in thick film unless you like bulletproof prints.
The optimal screen-mesh choices for thick-film are:
- Mesh number three for extreme thick ink printing with an ultra-thick stencil; well-suited for manual printing.
- Number five is well-suited for a thick print with the fewest (but long dwell) flashes and cooling, and a surface-enhancing high print speed.
- Number seven works well as a base coat or for a multitude of layers (although you may want to go thinner as you add layers).
- Mesh number 14 is great for detail-enhancement with thick stencils. Higher counts than our number 14 may be usable, but pretest for suitability and be sure the ink will go the distance at a reasonable speed under such high restriction.