Abstract:
The optimum layered double hydroxide (LDH) loading in poly(lactic acid) (PLA) filament, layer height, nozzle temperature and infill density levels were determined by statistically maximising the ultimate tensile stress of printed parts. Fused deposition modelling (FDM) is a material extrusion additive manufacturing (AM) method. It allows the printing of complex parts with simple and relatively cheap equipment. After stereolithography, it is the most popular AM method. FDM parameters that influence a printed artefact most are layer height, nozzle temperature and infill density. PLA is a compostable polymer which can be synthesised from renewable sources. It is the most used polymer in FDM and is projected to continue dominating the 3D printing landscape. LDH is an anionic clay with a brucite-like structure. It contains carbonate anions in its interlayer, which can be exchanged with other substances, making it extremely versatile for various applications.
From a systematic literature review following preferred reporting items for systematic reviews and meta-analyses (PRISMA) guidelines it was shown that only one paper describes acrylonitrile butadiene styrene filament filled with LDH. This showed that filaments containing LDH for FDM purposes have scope for research. With PLA being the most popular FDM polymer, a second review using PRISMA guidelines were completed on PLA filled with LDH. No other review on LDH in PLA was found in the 87 articles considered.
A 24−1 fractional factorial experiment was used to screen the four factors, and was augmented to fit quadratic models for the respective responses. These included ultimate tensile stress and load, elongation at break, Young’s modulus and impact energy. A central composite design was used to verify the optimum conditions predicted by the derived models. The statistical design of experiments (DoE) considered the following ranges for LDH loading, layer height, nozzle temperature and infill density: 0 % to 10 %, 0.18 mm to 0.42 mm, 190 ◦C to 220 ◦C and 10 % to 100 % respectively. Analysis of variance (ANOVA) was used to derive models and analyse the factors that affect the responses.
For tensile properties the optimum combination of factors were at lower levels of layer height, nozzle temperature and LDH loadings at 0.18 mm, 190 ◦C and between 0 % and 4 % respectively. Infill densities between 80 % and 100 % also yielded the maximum tensile properties. Impact properties did not vary statistically in this region either. Inconclusive results were observed for Young’s modulus, and it is expected that another material extrusion parameter affects this response. Even though inferior in strength, parts could be printed with filament containing up to 10 % LDH.
It was shown that LDH PLA filament can be made and that artefacts can be printed with FDM. Up to 4 % LDH can be printed without negatively affecting mechanical properties compared to pure PLA, and printing is still possible with LDH loadings as high as 10 %. Because substances can be intercalated into LDH for specific purposes, a legion of applications including medical, environmental and flame retardance applications are theoretically possible. The combination of the benefits of FDM and LDH can lead to tremendous advancements in a variety of fields. Since this is the first work reported on LDH PLA filaments, further research and work is recommended. Development should focus on limiting degradation of PLA when making filled filament, achieving the required
strengths for specific applications and testing the efficacy of intercalated substances after printing.
The proper use of statistics in research is highly recommended. It was shown that resources are wasted because statistics are not fully exploited, especially in the nature science and engineering establishments. Specifically, it is not necessary to have five or more replications for each experimental point in a statistically designed experimental programme, especially in the screening stage of a DoE. More research in the effect of slicing software on printed parts are also required. It was found that the fracture locations on printed tensile parts depended on the Ultimaker Cura slicer settings instead of the material.
