The most common, if not the first or even only question we ask our mesh suppliers is how much does it cost, and the second question is usually how long does it last. Ironically the answers to these two questions define the worst possible fabric for quality and productivity. The generic goal of screen printing is to transfer a maximum quantity of ink in a minimum film thickness with minimal force at maximum speeds. To allow us to differentiate between good quality mesh and merely quintessential gauze, we will first take a cursory look at inks and garments.
Thin thread mesh allows for immaculate detail and opacity even with water based inks. (Image courtesy Forward Printing Inc.)
White and blue
Screen printing is a pumping process. If we want to gauge the efficacy of the pump, we should test it with the two trickiest colors which must pass through our pump—the hard-to-push white and the hard-to fit-blue.
White pigment is a very dense microsphere. It is un-absorbent and often contaminates the ink at a ratio between 15 and 25 percent. White causes the flow properties of the ink’s viscosity to increase—it resists flow and, the harder we push, the more resistant it becomes. As such, it doesn’t have much fluid momentum. When we stop pushing, it stops flowing. So, the best mesh to transfer white ink will have a minimal fabric thickness (so the ink doesn’t have to travel very far) but a high ink volume so the white can be opaque.
Blue pigment is virtually the opposite of white pigment; it is very lightweight, very long and thin, is highly absorbent and may contaminate the ink at ratios between 3 and 8 percent. Blue causes the flow properties of the ink to be very thixotropic—it thins rapidly when force is applied and continues to thin and flow after the force is removed. So, the best mesh to fit the gigantic blue pigment will have a maximum mesh opening and a maximum percentage of open area. Generic blues won’t allow thick deposits as they will tend to sag but they will copy every flaw and report it as a mesh mark or pinhole.
Curiously, even though the properties of white and blue pigments are diametrically opposed, similar advice for mesh selection for the white inks applies to the mesh selection for the blue inks, although for very different reasons. But, before we select a type of mesh let’s look at the tackiness of the various types of inks.
There are many variables in achieving highly-detailed, precise screen prints. Though durability is compromised with thinner threads, using low count, thin thread mesh is key to quality. (Image courtesy Motion Company)
The intrinsic tack level of the ink is difficult to quantify because absolute tack levels are so dependent on pump and process. Screen printing, unlike virtually all other traditional printing processes, pumps the ink so that high tack is not a necessity, but a disadvantage. For our purposes, we will define our impression of ink tack as…
1) How easily will it begin to flow once pushed (yield)
2) How thin it gets once pushed (flow), and 3) How fast it will attach to the garment or another ink (wetting).
Following the ink from the blade into the cells of the mesh and then onto the shirt, the movement of the blade creates fluid pressure in the ink, causing it to flow into the cells of the mesh. Once inside the mesh, you’ll note the middle of the mesh is skinny (the narrowest part of the hourglass), so there is an inevitable fluid volume drop as we squeeze ink through the mesh. Finally, the reduced volume either surrounds the mesh (soft hand inks) or fills the cells sufficiently to contact the garment (high opacity inks). Thus, for soft-hand inks, we want the thinnest deposit with minimum loss of volume and a large percentage of open area. For high opacity inks, we want maximum volume with the thinnest tunnel and largest open area percentage possible. (See Table 1)
Table 1 lists four generic types of inks soft hand and high opacity plastisol and water based inks. From the top, a soft hand plastisol begins flowing with little effort, it becomes very thin when shearing forces are applied (stirring, shaking or squeegeeing) and it attaches to the fabric or ink surface at a good speed. Make it higher opacity and it gets harder to push, not nearly as thin and doesn’t wet as fast, aka has a higher tack. Jump down to row three and you’ll see water based flows very easily and gets very thin but water systems have a high pH (to prevent molding), making them characteristically poor wetters which, in turn, may come at the expense of low tack. Once again, the opaque versions suffer in the tack department. Note: plastisol inks are, by comparison, perfectly stable on press—the tack level of a plastisol is not likely to increase. When selecting a mesh for water based inks, be sure to force them to evaporate and tack up first, before testing the results with your preferred screen mesh.
Garments violate the first two axioms for quality printable substrates—they don’t have much substance and aren’t really flat/smooth. Regardless of this pair of shortcomings, folks like to wear them. So…
To matte down the low fabric-mass surface with ink, it is logical to hope for a thin thread with the largest opening possible. Fortunately, such a mesh will permit high print speeds; the key to matte down on press. This mesh additionally deposits a thin and smooth ink film with a far superior hand.
To deal with the hilly terrain of the T-shirt, we want as much fluid momentum as possible. Fluid momentum relies on a flat surface (found in a thin thread, low count mesh) and a short tunnel in order for the ink to remain fluid as long as possible and penetrate further into the garment.
So what’s the down and low?
Let’s take our six results for white and blue colored inks (all other colors fall between these extremes), our low and high tack inks and our worst-case garments and see which type of fabric geometry will be best suited.
• Whites work best with thin thread, low count mesh using maximum blade volume.
• Blues print best with a low count, thin thread mesh at maximum blade speed.
• Soft hand inks should be printed through thin thread, low count at maximum blade speed.
• High opacity inks print best with a low count, thin thread using maximum blade volume.
• Garments with a low fabric mass need a low count, thin thread printed at maximum blade speed.
• Garments with an irregular surface print best with thin thread, low count mesh using maximum blade volume.
Recognize a pattern? Thin threads and low mesh counts are best suited for colors, fluids, fabrics… and customers.
Some other axioms to keep in mind in regard to mesh selection:
• Keep the average mesh opening as large as possible and the thread diameter as low as practical. A larger open area permits all types of ink to transfer with less volume drop. Fabric thickness is the primary determinant of maximum thickness only.
• Tension (N/cm²) is the mesh’s average resistance to deflection. Higher ratings indicate a more robust fabric. However, stronger fabric will compromise quality and print speed.
• For efficient transfer, the distance the ink has to transfer should be kept to a minimum. The pitch of the fabric should also be minimized for matte down and a smoother surface. The capacity of the fabric is the key to minimum thickness and maximum volume. (See Table 2)
Table 2 lists nominal information—the data is not a firm specification according to upper and lower limits. Instead, the comparison is of four meshes with the same count of roughly 135 threads and openings in either direction, woven with varied thread diameters. Note that these nominal data points are prior to stretching; most change significantly when the mesh is stretched. The data on the left is copied from manufacturer’s published specs. Data on the right in blue is extrapolated from those specs. Optimum metrics per category are highlighted in yellow.
Testing the waters
There is an irrefutable fact that more durable mesh comes at the expense of quality and print speed. As such, the print professional’s goal is to balance longevity with on-press performance. If the proceeding argument has you convinced or at least compelled to action, test the water.
Take the mesh count and thread currently in use, maintain the thread and drop by one count for high opacity inks. For soft hand inks, maintain the count and reduce the thread diameter by one. This will quickly indicate whether or not your shop is a candidate for higher quality and productivity without blowing up screens all day. Or use this quick test: if you ask yourself am I abusing the mesh, you are a candidate; if your query is why is this mesh so delicate, you are not!
Mesh count is nominal. Do not make a decision on the mesh count alone without being aware of its specific geometry.
Mesh opening averages warp and weft counts and will enlarge as the fabric is tensioned. As the mesh opening increases, the blade must go faster to mitigate the increase in fluid pressure drop.
Thread diameter is the constraint to lines and highlight dots and, along with the stencil, limits edge acuity. The reduction in thread diameter during tensioning is minimal; tension levels less than 35N/cm² are sufficient for high quality and high speed.
Percentage of open area is the secret to dealing with high tack inks, high opacity colors and soft-hand/high opacity water-based inks. A larger open area percentage within the same mesh count will reduce the volume drop of the ink during transfer.
Fabric thickness is the basis for ink deposit and should be kept as thin as possible. Thin threads enable white inks to travel a minimal distance to the substrate and all inks to recover their body quickly to provide a sharper image.
Newtons per square centimeter is a measure of the average resistance to deflection. A higher recommended level of tension indicates a more robust mesh. The printing tension of the screen rarely needs to exceed 35N/cm² for optimum transfer and dimensional accuracy.
Cross-section of the mesh is an indicator of its comparative strength per modulus (the yarn’s resistance to elongation). Cross section is a composite of the thread diameter and the number of threads per linear measure at equal warp and weft counts.
Ink transfer distance—the distance from the top of an upper knuckle to the bottom of an adjacent lower knuckle—is the minimum constraint for soft-hand fluid momentum and high opacity fluid volume.
The pitch of the mesh determines the texture of the printed image and acts to limit the stroke speed. The rough results of high pitch are more noticeable with anti-sag inks. A low pitch mesh requires higher print speeds in order to mitigate the fluid pressure drop.
The capacity of the mesh for soft hand inks defines how long the fluid momentum must be sustained. For paste-like, high opacity inks the capacity defines how much fluid volume must be sustained. The effect will be more dramatic with high-opacity inks than with soft-hand inks.