Effects of humidity and ageing on epoxy-based stereolithography materials
In recent years, the use of stereolithography (SL) to produce end-use parts has increased rapidly. However, only limited applications have been considered as there are several problems that impede the conversion of stereolithography to be used as a mainstream manufacturing process. One of the major problems is the instability of the SL parts post-build when subjected to environmental factors. This research included long-term material testing to investigate the effects of ageing, humidity and temperature on the mechanical properties of the SL parts. This work was carried out at a temperature range of -40°C to +100°C over differing ageing and humidity conditions (dry (10%RH), controlled (50%RH) and wet (100%RH)). The results indicated that the main degradation factor was humidity. It was therefore selected for further investigation in this research. This is the most comprehensive analysis and characterisation of materials data yet compiled for additively manufactured materials. Stereolithography materials tested have been shown to significantly degrade over time when subjected to a high level of relative humidity. Therefore, it was the primary aim of this research to identify the mechanism of hygroscopic degradation of epoxy-based SL materials. This was achieved using various techniques including mechanical tests, Differential Scanning Calorimetry (DSC) and Attenuated Total Reflectance (ATR). Modelling of water penetration into SL epoxy-based parts was undertaken and the type of diffusion was found to be anomalous (non-Fickian). Therefore a dual-Fickian model was developed to identify the diffusion coefficients of the investigated materials. Additional equations were developed to model the profiles of moisture concentration and Young's Modulus within a specimen and to predict the stiffuess of an SL material at a certain age. The methodology that has been developed in this work can now be further used to predict the mechanical properties of any future epoxy-based SL material providing the diffusion coefficient(s) of the material are known.