Pad Printing Inks: Viscosity, Pigment, & Adhesion
Press Release from ITW IDS Division (ITW CER, ITW Morlock, Trans Tech, United Silicone)
It is common for those unfamiliar with the pad printing process to refer to pad printing inks as “paint”. The truth is that these inks have very little in common with paint or even other inks, including screen printing inks. The Trans Tech inks that we use today are designed specifically for the pad printing process. A full line of inks are available for printing on different substrates.
Unlike other decorative processes, ink viscosity plays a crucial role in the image transfer process. Most manufacturers recommend a ratio of 15 to 20% of thinner to ink, by weight, with the ratio adjustable to environmental conditions. It is the evaporation of the thinner that creates ink tackiness. This enables the image transfer to occur. Without the creation of tack, the transfer pad would be unable to properly function. The ink would either be too wet or too dry, preventing complete transfer.
Often, seasonal changes will affect the actual ratio used. For example, the thinner may flash quicker in winter months, due to the dry air, than in summer when the air is full of moisture. Most printing problems that are reported in the summer relate directly to high levels of heat or humidity. To minimize the effects of weather or other environmental factors, Trans Tech recommends printing in an air-conditioned area.
Given this discussions of thinners in the ink and ever present environmental concerns, Trans Tech is often asked about the availability of water-based inks. Water-based pad printing inks have not been successful due to their inability to create sufficient tack within the cycle time of the machine. Conventional thinners are much faster and better suited to the process.
To fully understand pad printing, it is helpful to know how the ink interacts with the other components - pad , cliché , and machine . The initial stage of each pad printing cycle begins with the cliché immersed in ink. As the cycle begins, the doctor blade or ink cup cleans the cliché surface and leaves the ink in the image exposed to the air. Upon exposure to air, the thinner in the top half of the image begins to evaporate or “flash off” and becomes tacky. The transfer pad then compresses onto the cliché image and draws the ink from the etch. The portion of ink that was located at the bottom of the etch is now exposed to air for the first time. As this occurs, the thinner on the bottom half begins to evaporate and the tack process is complete. The image is now ready to transfer.
There are two important conclusions to draw from this description. First, the speed in which thinner evaporates plays a critical role in the process. If the thinner does not evaporate, complete image transfer cannot occur. Secondly, ink cohesion is crucial to remove entirely the ink deposited in the etch.
This is the main reason that etch depths rarely exceed 25 microns. It is certainly possible to etch steel or other materials deeper; however, the ink will not remain as a cohesive unit but instead will separate resulting in an incomplete image transfer.
The standard cliché is 25 microns or the equivalent of .001”. From this depth, a layer of ink approximately .0008" of an inch thick is transferred to the part. To maintain good image opacity, the pigment in the ink must be small enough to avoid removal in the plate doctoring cycle.
If you have ever pad printed with a screen printing ink and experienced poor opacity or distorted color (i.e., yellow image on black appears greenish), opacity is the reason. Pigments in screen printing inks are typically not milled to the fine density required to achieve proper opacity for the pad printing process. By contrast, in the screen printing process, opacity is gained through the thickness of ink applied.
How do machines affect ink? Machines do not affect ink as much as they affect thinner evaporation. A typical open ink well machine with a cycle rate of 1,000 parts per hour will affect thinner evaporation differently than a machine that uses a sealed ink cup that runs at a rate up to 3,600 parts per hour. It is safe to assume that the open ink-well machine will require more operator intervention to maintain ink viscosity because the thinner is continuously evaporating. In a closed cup system, the ink and thinner are encapsulated. This retards evaporation.
Speed is another consideration. A machine with a faster cycle rate will not allow as much time for the thinner to evaporate as a machine with a slower cycle rate. This could result in incomplete image transfer due to the lack of time required to create the proper tack. To counter this phenomenon, thinners are available that provide different evaporation rates. A fast evaporating thinner should be used on fast cycling machines, for example.
Pad printing inks are available in two basic formats: single-component and two-component. When we refer to two-component inks, we are referring to hardener as the second component.
The task of choosing between the two formats depends on the requirements of the image and the substrate being printed. Single-component ink provides the benefit of extended pot life. Often, single-component ink is left in the machine overnight or in preparation for the next work shift. A heat cure is often not necessary because the single-component ink will cure at ambient temperature in 24 to 48 hours. Although they are easier to use, single-component inks do not provide the same level of resistance to mechanical abrasion or thinners as their two-component counterparts.
Two-component ink is better suited to applications that require a specified performance. The addition of hardener enables the ink to resist mechanical abrasion and most thinners. It is also necessary to give two-component ink a heat cure. The recommended level of cure is dependent upon the amount of heat applied. A general rule of thumb is 350F for fifteen seconds to two minutes, depending on the application.
Generally speaking, the phrase “the higher the heat, the better the cure” does apply. The limiting factor in most applications is the melt temperature of the material. Most pigments will remain stable up to 650F. Two-component ink will cure at ambient temperature in five to seven days; but, if the goal is to maximize performance, a heat cure is recommended.
It is important to note the difference between the terms “dry” and “cure” as they have two different meanings and should not be confused with each other. “Dry” simply means dry to the touch. It does not mean cured. “Cure” means that an ink has gone through a chemical reaction and has reached its maximum level of performance.
It is a common misconception that the thinner in the ink is the component that promotes adhesion to the substrate surface. This is not true. The thinner does serve to soften the surface, but the ability to adhere to the surface is strictly a trait of the ink’s chemistry. It was earlier stated that the amount of ink transfer is .0008". It is also important to note that once the ink is fully cured, the thickness is reduced to approximately .0004". The loss or difference is from thinner evaporation.
Various inks are formulated to allow adhesion to different substrates. The ink used to print on polycarbonate may be different from one that is used to print on styrene.
An ink’s ability to adhere to the substrate is a product of two factors: the ability of the ink to cross-link with the intended substrate and surface tension. When an ink is properly matched to a substrate, a chemical cross-link occurs that promotes adhesion. However, in case of polyolefins, the second factor of surface tension becomes present.
Surface tension, measured in dynes per centimeter, is a function of the “wet-ability” of a substrate surface. Materials that have low surface tension do not allow the chemical cross-link to occur. In these instances, pre-treatment is required. Pre-treatment does add cost to the decorating operation in terms of equipment and labor.
