To achieve optimal print quality, special pad printing inks must be used. These inks display very highly concentrated pigmentation, as only very small quantities
of ink are transferred in the pad printing process.
The properties of the ink should, as far as possible, include:
- easy workability
- long pot life in wells/cups
- non-hazardousness to health
- absence of environmental risks
- ease of cleaning
- best possible adhesion to all objects and material without need of pre-treatment or subsequent treatment
- capability to serve as a universal ink system for all substrates
Depending on the area of use, the requirements placed upon an ink vary greatly. For simple marking or labeling, the demands are low, for decoration, very high.
COMPOSITION
Inks consist of binders, pigments, solvents and additives. These ingredients give the individual ink systems their particular properties.
BINDER of a pad printing ink is composed of one or more resins. Since the resins are, in most cases, obtained in the form of granulates or powders, they must be dissolved in suitable solvents or solvent mixtures. The selection and combination of the resins determine the ink’s later area of utilization and the resulting properties, such as adhesion to various substrates, grade of gloss and resistance to chemicals.
SOLVENTS differ, above all, in their evaporation speeds and solvent strengths. The solvent content of a pad printing ink is the deciding factor for its drying behavior and thereby, printing speed on and adhesion to moderately soluble substrates.
Solvents are broken down into THINNERS and RETARDERS. Retarders have almost no significance in pad printing, as the output of the prints is usually very fast. Retarders are merely necessary when printing slowly and when using very fast drying ink systems. A retarder which evaporates too slowly can have a negative influence on the prints. Thinners consist of solvents as contained in the corresponding ink systems. Mixing them with the ink in the correct proportion provides the desired viscosity, which is prerequisite to the ink transfer. For the pad to transfer the ink, part of the thinner must have already evaporated during the transfer process, leaving a tacky ink film on the pad.
PIGMENTS provide the color tone of the ink and determine its hiding power. A distinction is made between organic and inorganic pigments. Pigments containing heavy metals are no longer utilized.
ADDITIVES are substances normally used in small quantities. Their function is to fine tune the ink properties, such as flow, viscosity, or characteristics of the surface. Additives include, for example, leveling agents, thickeners, or waxes.
PROPERTIES
The most varied requirements are placed upon the finished print, regardless of the substrate:
- the motif is to be matte, satin, glossy, or high gloss
- the ink is to be resistant to chemicals, dishwasher safe, resistant to UV light, scratchproof, or resistant to saltwater, saliva, or perspiration
- the print is to cover (hide) the surface or be transparent
The variety of partially contradictory properties demonstrates that a wide range of inks is absolutely necessary.
INK SYSTEMS
Pad printing inks are broken down into different categories based on their drying behavior:
- physically drying inks (one-component inks)
- chemically curing inks (two-component inks)
- heat curing inks
- UV curing inks
SOLVENT BASED INKS
SINGLE COMPONENT INKS are given the desired viscosity by using thinners. Their drying is induced by the physical evaporation process of the solvents they contain. At the same time, substrates of thermoplastic compounds, such as polystyrene, polycarbonate, or PVC are partially dissolved. This “slight dissolution” of the printing surface results in a direct bond between the ink and the substrate. A high degree of scratch proofing and good adhesion of the ink usually pose no problem in this case.
TWO COMPONENT INKS display very high resistance to chemicals along with good bonding and scratch proofing properties, especially when used on problematic substrates. A hardener which reacts chemically with the binder must be added to the ink. Here, it is of critical importance for the hardener to be added in the correct proportion. It should not be added until shortly before the printing run, as the ink is only workable for a short time thereafter. The complete drying and bonding of this ink system is dependent on the ambient temperature; at room temperature (approx. 20 °C / 68 °F), this takes several days. In this case, the mistake is often made of undertaking bonding and scratchproofing tests much too soon.
HEAT CURING INKS display the same properties as two-component inks. The chemical reaction of this ink system is a cross-linking of the binder which does not take place at room temperature, but only under the influence of high temperatures.
OXIDIZING INKS dry/cure as a result of their reaction with atmospheric oxygen. This is a relatively slow process, whereby the final state is reached in one to two days at room temperature.
UV CURING INKS
In pad printing, the transfer of ink is usually dependent on the evaporation of the solvents. This evaporation induces a tackiness on the surface of the ink film which modifies the adhesive behavior of the ink. The ink must be transferred as a film to achieve optimal results with respect to hiding power and sharpness. This transfer is contradictory to UV technology. Due to the absence of a solvent in UV inks, the surface of the ink film is not altered. The transfer takes place in a “wet state”, which makes it more difficult.
The primary area of application for UV curing inks is found in high-output industrial pad printing.
The ADVANTAGES of UV inks are:
- no drying out on/in cliche
- guaranteed reliability in production with uniform grade of quality
- far less wear and tear on cliche and doctoring mechanism
- immediate drying on contact with UV light -> no time lag in further processing
- no emissions from solvents.
On the other hand, the DISADVANTAGES are:
- restricted hiding power and dependency on performance of available UV curing unit
- partial transfer of shadows of doctor blade by pad
- restricted possibility to clean pads with tape, as ink on pads does not develop tackiness as do conventional inks
- precision of transfer for the high demands placed on printed motif does not attain quality level of solvent inks.
WATER BASED INKS
The use of water based inks has not established itself in pad printing. This is due to the very slow rate of output. The necessary properties of quick drying, tackiness and adhesion cannot be attained using water as a solvent, as the evaporation speed of water is considerably slower than that of the usual solvents.
PROCESSING
The ink is made ready for printing by adding thinners and, the need arising, hardeners. Because the difference of only a few grams in the small quantities required to fill the cups or wells can lead to a considerable deviation, the individual components must be weighed carefully.
However, even after the ink has been prepared, further steps may be necessary to make the printing process possible at all. This is due to the fact that some substrates, e. g., certain plastics, can only be printed after pre-treating, or they require a subsequent treatment to guarantee the adhesion of the ink.
Substrates which are soiled with an oil film or with silicone cannot be printed without pre-cleaning. Some materials are so sensitive that even perspiration from the fingers can impair the final results or the adhesion of the ink. The simplest method to clean a substrate is to wipe it with alcohol.
Plastics such as polypropylene or polyethylene can be pre-treated using corona (electrical) discharge or flaming. Corona discharge units operate with a high frequency / high tension discharge in the neighborhood of 20,000 volts. Flaming units produce an open flame over the substrate. The intensity and duration of the flame are adjustable. Both units function the same way; the surface tension is increased to anchor the ink in the substrate. Another option to achieve better adhesion is treatment with primers. For polypropylene, ink systems which make pre-treatment unnecessary are available.
With some plastics, there is an occurrence of very strong static electric charges which result in the appearance of dust inclusions or spiderwebs on the edges of the printed motif. These manifestations can, for the most part, be avoided by using an ionizing unit.
At an accelerated cycle time, for multi color prints, for materials which do not readily pick up the ink and for slow drying inks, cold or hot air blowers provide good support to speed up the drying process. The pad can be ventilated on its way to the front and in the forward position. Alternatively, the parts or material can be ventilated so that the ink film dries more quickly. Care must be taken, however, not to mount the blower directly above the ink well, as this will cause the thinner to evaporate too quickly and more will have to be added very frequently. Subsequent treatment of hard to print materials with hot air can improve the adhesion of the ink. Subsequent treatment with hot air or flame drying is the only way to achieve durable print on some materials. The temperature and duration of these treatments can be found in Pröll`s Technical Information.