Both tack and shear-thinning properties require lab equipment to quantify but it is equipment every ink maker should have. It is most certainly "the hard way" but we gauge tack [~how the white adheres to iteslf VS how it adheres to something else] every day on press. Ink which has excessive tack won't bridge or matte-down easily, it requires a lot of squeegee angle and / or pressure, doesn't clear the mesh well and tends to leave a rough surface. Shear thinning is a ratio of two viscosities [~the friction between two fluid layers of ink]; flooded viscosity and transferring viscosity. We want the tack ratio to be low as possible and to a limit we want the shear-thinnning ratio to be as high as possible.
So share thinning, is how easily the ink inside the mesh funnel, separates from the top layer of itself, as the squeegee's edge passes over the mesh opening? If this is interpreted this way, if the ink tack ratio is low, one of the attributes of the ink would be that the shear-thinning ratio would be (on the positive side of things) high? So one measurement does not work independently of the other- right? Like you said a high tack ink won't separate from itself well.
And what causes the ink to be high tack in the first place?
Your conclusion is absolutely correct! The “splitting” you refer to is a result of several interrelated factors; 1) the lower limit or “plastic viscosity” of the shear-thinning in [how thin does it get when placed under a shearing force] 2) the tack level at that lower viscosity limit 3) the [fluid] pressure differential created by the blade and 4) the fluid momentum of the ink as it leaves the blade to enter the cells of the mesh.
The relationship between shear-thinning and tack is not conspicuous on press. However an ink that is not highly shear-thinning and / or high tack will not clear or clean at elevated print stroke speeds. We use a calibrated test image on press to gauge the speed limit, in the lab it is easier to isolate the two attributes of a white ink.
We use different pieces of equipment to analyze the shear-thinning nature of an ink independent of its tack level. The goal is to offer an ink which responds favorably to both low and high shear forces [specifically shear-stress which is perpendicular to the screen and shear-rate which is parallel to the screen]. The target is to achieve the widest latitude possible so the product works under the widest range of conditions—highly shear-thinning AND low tack. In practice, press one to press two, side one to side two, first shift to second shift ETC.
The “speed limit” is a result of the transfer-rate of the mesh divided by the tack level of the ink multiplied by the [fluid] pressure differential caused by the blade. So a shear-thinning ink will offer the widest latitude but if the mesh and blade don’t restrict it too much it will allow printing at top speed with superior bridging and matte-down. This is how Alan802 can run at 30”/second [as fast as his press will stroke] with merely 18 PSI of pressure on the blade and get exceptional matte-down at top speed.
As for “why is it tacky” Colin is correct. When the two primary ingredients of an ink are intermixed the liquid phase has an affinity for and will alter the solid phase. The moment this happens “tack” occurs as a result. We screen-printers need tack for one reason; if there was no tack the phases would separate during the press run. Any more tack than this requisite level only gets in the way of transfer and quality. With white ink there are some formulators who build tack into the ink to keep it from penetrating the garment, their ink tends to be high viscosity AND high tack requiring a lot of blade “pressure” [downward force]. Others take the same approach to stop penetration but then the set up a delicate structure in the ink which is subject to “thinning out” during a longer press run which incorporates flashing. There are others who try not to build tack into the mix and rely on shear-thinning properties to avoid penetration.
