Understanding ink tack is key to producing quality print, especially when overprinting a varnish. Following on from the scientific article last month on plasmas, and the wealth of advanced information that has flowed from the high tech world of drupa, this month’s article returns to a very basic function of paste inks, namely that of ink tack.
In recently talking with a good NPL customer, Fred Vale of DIC-Colortron, he remarked that the old misunderstanding about the relationship of ink tack to colour strength in four-colour process sets is still alive and well.
As I have commented in earlier articles in this series, it is quite astounding how problems and misunderstandings persist, and so now ink tack can be added to the list of those areas that seem to have defied comprehension.
I looked up the word tack in the dictionary, and was informed that tack meant a short light nail with a sharp point and a flat head, the position of a vessel relative to the trim of its sails, a loose temporary stich to add as an extra item, and the harness for a horse including the bridle and saddle, but not the definition that we seek that applies to inks and adhesives; the resistance of a film to being split.
Transfer of an offset ink down a roller train, then to the plate, to the blanket, and finally to the substrate is achieved by the splitting of the ink film at each point, and the ability of the ink film to divide many times, and so replenish the image, is a function of the rheological properties of the ink and largely dependant on the characteristic that we call tack.
Tack is a function of the external forces applied to the ink film, and internal characteristics of the ink layer itself.
External forces come from the speed of the machine, the thickness of the ink film, and in the case of the litho process, the amount of water emulsified into the ink. The internal ones come about as a result of the complex entanglement of the oil and resin chains of the ink vehicle and the solubility and compatibility of the ink components, (in which temperature plays a part). The ink vehicle solubility and compatibility aspect also reflects a property called tack stability, but that is a different subject relating to ink setting that we may undertake to discuss at another time.
The main tack-factors that directly affect the printing process, and hence lie within the printer’s area of control, are machine speed, ink film thickness, and water content.
As defined above, tack is a function of how easily the ink layer can split, one part following a new path, the remainder collecting a fresh deposit of ink on the next revolution of the press, and it does not take too much imagination to realise that a thick ink film has a lot more places inside the layer along which the film can split than a thin one – and this can be readily proven by taking one ink sample and testing its tack over a range of film thicknesses. Further, and quite obviously, the faster the film is torn apart, (machine speed), the greater the resistance to splitting will be, so slowing down reduces ink tack.
Tack is measured in the laboratory with a tackmeter, an instrument that usually has a set of rollers comprising of a temperature controlled drive roller, a distributor roller, and a rider roller that is connected to a force measuring device.
The near universal standard machine throughout the ink industry is the Thwing Albert Inkometer, where the rider roller is connected to a transducer (differential transformer) that registers an electrical signal proportional to the distance that the rider is displaced as the drive roller turns.
Simply the greater the tack of the ink, the further the rider will be pulled out of its equilibrium position, and the greater the number that will be displayed. The NPL has the latest model Inkometer, schematically shown in the diagram at the bottom left of this page.
The NPL also has the HydroScope, an instrument that has a basically similar tack measuring system, but also has an accurate pump that injects water or fountain solution into the ink, so that the differences in the influence of the water content may be measured. Clearly, a water film is very easy to split, and so as an ink becomes more emulsified the tack will decrease.
A new instrument is on order for delivery later this year, the Prüfbau Deltack, a machine that measures the tack of an ink as it is actually being applied to the substrate. A future article will be dedicated to the operation and performance of the Deltack machine.
So back to the original discussion; the subject that Fred raised related to a question from the industry as to why ink manufacturers do not tack-grade their process colours as was done in the past; each colour being at least a tack unit higher than the next one down.
The answer lies in the relationship we have explored between tack and film thickness. Inks are now graded in terms of colour strength, the strongest ink when applied to the specified density reading will have the thinnest film, and hence the highest tack, and so on for each other process colour. Thus tack will be automatically adjusted to ensure appropriate transfer, lay, inter-colour trapping, and avoidance of substrate pick. (This underlines the necessity to use densitometer control for accurate process colour printing, but then you all knew that already).
One other artefact of the tack to film thickness relationship relates to a recurring problem with machine applied overprinting varnishes.
There is often the thought in a printer’s mind that a coat of overprinted varnish will help prevent marking or scuffing, particularly on an abrasive stock like a matt-coated art, and indeed that is so. However, if a very thin lick of varnish is applied, the apparent tack as just explained will be huge, and so back trapping is almost inevitable. The worst thing is then to put reducer into the varnish, as that lowers the ability of the varnish to protect the print; it is correct to apply more varnish to bring the tack back to what the manufacturer intended - much easier to understand when you know how tack works.
So ink tack is not that complicated after all, and a bit of applied common sense will ensure that we do not saddle ourselves by sailing in the wrong direction with a loosely added on extra item, and instead get the point with not the trace of a flat head.
