Ink tack is a crucial aspect in the printing process that needs to be constantly monitored for quality control. When we talk about ink tack, it refers to the stickiness of ink or the force required in splitting an ink film from one surface to another. Proper ink tack needs to be achieved in order for the ink film to transfer from the ink train to the printing units and onto the printed substrate. It is important to understand the factors that affect ink tack and this will provide us better insight into tack-related problems and how they can be solved.
In offset lithography, inks are formulated to have a high degree of tack to avoid emulsification by the dampening solution. The vehicles used during ink formulation determine ink tack and viscosity. Offset inks should be as tacky as possible without picking the surface of the paper. It should also be noted that to some extreme, picking caused by high tack inks might even tear the substrate. To achieve proper ink trap in multi-colour printing, the first ink should have the highest tack value that won’t disrupt the substrate surface when printing. The succeeding inks should have progressively less tack. Increased or insufficient ink tack also influences the reproduction of solids, sharp lines and may cause improper dot gain.
Ink tack can be influenced by several factors: ink viscosity, press speed and ink film thickness. In flexographic and rotogravure printing, low-viscosity inks are used to accommodate high press speeds. If high-viscosity ink is used on a high-speed press, it will result in high ink tack and can cause picking. By maintaining optimal press speed and roller temperature, ink tack can be controlled.
Rub Resistance
Often during the shipping or handling process, printed products will be exposed to a degree of abrasion to its surface. Regardless of quality or perfection of the printed product, a scuffed, rubbed off or scratched surface would render it a print defect. Because commonly printed products such as magazines, newspapers and brochures are handled very often, the ink on the substrate needs to be formulated to have some degree of rub resistance. By adding components such as waxes and/or hard-drying oils, an ink’s rub resistance can be improved.
Waxes are often added to printing inks to improve rub and scuff resistance. Waxes can be Polyethylene or PTFE-based. The amount of wax added can affect the characteristics of the ink. If there is too little wax, little rub resistance will be obtained. If there is too much, it can soften the ink and lead to rub problems. Since lithographic inks need to emulsify with a certain amount of water, excessive wax content in ink can also render it too waterproof. This can cause problems during printing because the ink will begin to adhere to the non-image area, causing scumming or catch up.
Linseed oil is sometimes added to improve an ink’s drying property and bonding of pigments to paper. Linseed oil works both as a binder and a varnish and it creates a protective coating against scuffs that holds this pigment onto the substrate surface after drying. Linseed oil also has the ability to reduce ink tack. When using linseed oil, precautions should be taken because the amount added could affect the overall attributes of the ink.
We should also be cautious as to how much oil or waxes are added to the ink formulation. These additives may change the properties of the ink and may lead to other print-related problems. This is why it is crucial to understand how certain additives behave when they are added to printing inks. The right balance is needed to achieve high quality print as well as an abrasion resistant surface.