Measures to Optimize Inkjet Print Accuracy
With the advancement of technology, scientists have taken measures to optimize the precision of inkjet printing. Inkjet printing is based on the formation, ejection, and deposition of each ink droplet. In every step of the printing process, the ink droplet experiences a sequence from being ejected from the nozzle, to impacting the substrate, and spreading across the surface of the material. To optimize the precision of inkjet printers, the first step is to improve the ink.
1. Optimizing the Chemical Composition of Ink
The resolution of inkjet printing depends on the deposited area of the ink droplet. When the droplet falls on the substrate, it will penetrate and spread across the surface. The larger the surface area, the lower the resolution. To reduce this area, the viscosity of the ink plays an essential role, as it limits the spreading of the ink. However, the viscosity should not be too high, as it may lead to nozzle clogging.
A test was designed using graphene nanosheets with an average thickness of 10 nm and an average diameter of 8 μm, including two formulations that differed by the presence or absence of commercial polymer dispersants. The experiment was conducted at various temperatures ranging from 15°C to 45°C.
The results showed that the viscosity of ink containing the dispersant was lower than that of the ink without dispersants. In both cases, higher temperatures corresponded to lower viscosity. This indicates that the viscosity of ink can be controlled by temperature.
2. Controlling the Structure of the Substrate
When the ink droplet penetrates the substrate, the deposited area reduces the resolution. Modifying the substrate structure can serve as a solution to this problem.
3. Suppressing the Coffee Ring Effect
3.1 Explanation of the Coffee Ring Effect
When the ink droplets printed on the substrate are allowed to dry, most of the particles aggregate at the edges of the droplet, forming a ring-like structure. This is known as the coffee ring effect. This phenomenon is caused by capillary flow within the droplet during the evaporation process. As the solvent of the ink evaporates, the edges of the droplet are pinned to the substrate, and the solvent flows from the center toward the edges. This causes the solute particles to accumulate at the periphery of the droplet. The coffee ring effect is one of the most detrimental factors affecting inkjet print quality.
3.2 Methods to Suppress the Coffee Ring Effect
To improve inkjet print accuracy, it is necessary to suppress the coffee ring effect. The pinning of the contact line and contact angle hysteresis play an important role in the formation of the coffee ring effect.
The coffee ring effect can be mitigated by using water-soluble polymer additives. These additives move the contact line through viscosity and the Marangoni effect. The viscosity provides resistance to the particles, reducing the number of particles that deposit at the edges, thus alleviating the pinning of the contact line.
The Marangoni effect causes the concentration of the polymer at the edge of the ink droplet to change during evaporation. This helps the contact line to move. Typically, the use of polyethylene glycol (PEG) can alter the coffee ring effect, breaking it and achieving nearly uniform particle deposition in the droplet area, resulting in a multi-ring pattern.
Microscopic images and videos are used to observe and analyze the movement and deposition of suspended particles in evaporating ink droplets. It was found that PEG alters the coffee ring effect by providing surface tension to the ink droplet and inducing centripetal Marangoni flow.
An experiment tested the effect of temperature on controlling the coffee ring. The movement of ink droplets at different temperatures was tested. They found that as the temperature increased, the solute in the droplet showed a greater transfer toward the edges. Using water as a solvent means that the dependence of surface tension on temperature does not need to be considered.
This is a detailed technical analysis of the measures to optimize inkjet print accuracy, focusing on improving ink properties, controlling substrate structure, and addressing the coffee ring effect.