The Jetting and Dropping Characteristics and 3D Printing Performance of Needle-valve Inkjets

Abstract

A comprehensive literature review on the development of 3D inkjet printing technologies has revealed that inkjet printing is an effective method for additive manufacturing with advantages in specific applications such as microstructures and printable electronics. However, the knowledge of the flow development of the inkjet from the flow formation inside the nozzle to the dropping process is in severe dearth, so are the predictive models for the printing process.

A CFD (computational fluid dynamics) model is presented to study the jetting and dropping characteristics for needle-valve inkjets. The model is developed by using a dynamic mesh method and experimentally verified for both a low viscosity fluid typified by a distilled water, and a relatively high viscosity PEDOT:PSS polymer nano-particle containing ink. The model-calculations show a good agreement with the experimental data under the corresponding conditions for the both types of inks. The results from the model show that the parameter window for generating a single droplet from the low viscosity fluid is narrower than that using the high viscosity ink. It also reveals that the droplet size increases with the nozzle diameter and a droplet with a diameter slightly smaller than the nozzle diameter can be achieved with a low viscosity ink under a low dropping velocity.

The verified CFD model is then used to study the flow and dropping characteristics of an ink containing organic conductive polymer (PEDOT:PSS) nano-particles. It is revealed that three types of droplet defects can occur in the needle-valve ink jetting process, e.g. excess liquid suction, liquid accumulation, and satellite droplet formation. It is further found that an increase in the valve striking time, nozzle diameter or valve seat inclination results in a dramatic decrease in the droplet velocity, while an increase in the valve stroke or inlet air pressure causes an increase in the droplet velocity. The droplet volume and equilibrium deposition diameter increase with an increase in the valve stroke or inlet air pressure, but decrease with an increase in nozzle diameter or valve seat inclination. It is found that the droplet diameter is greater than the nozzle diameter and the droplet trajectory can be easily disturbed to become unstable when the droplet diameter is close to the nozzle diameter. A deposition diameter as small as 1.43 times the nozzle diameter has been achieved in this work.

An experimental study of the needle-valve inkjet printing process is carried out to assess the characteristics of dots and strips printed in a single-pass process. It is revealed that the voltage waveform plays a significant role in affecting the printing performance. The minimum to maximum strip width ratio for single layer printing can go as small as 0.6. Predictive models for the various relevant printing performance measures, such as the deposited drop diameter, strip width and strip width variation, have been developed, which provide a mathematical basis for the selection of process parameters to achieve the desired printed features in practice.