Simulation of Polymer Crystal Growth in Fused Deposition Modelling

Abstract

Fused deposition modelling (FDM) has been recognized as an important part of 3D printing technique as it has several key advantages to be an efficient rapid prototyping method. Poly(lactic acid) (PLA) is a commonly used polymeric material in FDM. The great advantage of PLA is that it is a bio-degradable polymer which is environmentally safe. In this work, an attempt was made to simulate the crystallization behavior of the material after being extruded from the 3D printing nozzle. Based on the theoretical knowledge, a Matlab code was developed to numerically compute PLA crystallization in respect to time and temperature. The Kolmogoroff-Avrami model was used to simulate the spherulitic development during solidification, and the Hoffman-Lauritzen (H-L) theory was adopted for calculating the crystal growth rate during crystallization. The crystallization process in the printed PLA was simulated under non-isothermal conditions. The temperature decreasing rate, properties of PLA, nucleation density and the molecular parameters related to the H-L theory were used from the previous literatures. The crystallization simulation was conducted with a standard 0.4-mm nozzle at 20 mm s-1 feed rate. Then, the effect of different nozzle size on PLA crystallization was compared. The modelling results showed that the degree of PLA crystallinity in a printed line is less than 1%. This modelling work focused only on the crystallization in a single line, hence it is expected that the temperature decrease rate is faster than that of printing of multi-lines and layers. Non-isothermal crystallization in a short time might limit crystal development. Moreover, a previous study has indicated that PLA cold crystallization is higher than melt crystallization. The effect of reheating from new layer deposition leads to cold crystallization on the previous layers, but this effect is absent in the model developed in this work. Off the different nozzle diameters, 0.15 mm nozzle size had the highest degree of PLA crystallinity, whereas 0.8 mm nozzle size had the lowest degree of PLA crystallinity. This is because in the proposed model printing by 0.15 mm nozzle size formed the maximal number of PLA nuclei. Based on this information, it can be concluded that PLA crystallization is low when it occurs in a single line, and using a smaller nozzle size could promote the degree of crystallinity.

Date of Award	1 Aug 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Wajira Mirihanage (Supervisor), Anura Fernando (Supervisor) & Chamil Abeykoon (Supervisor)