Fused filament fabrication of PETG :Investigation of the mechanical properties through the parameter optimization

Abstract

Additive manufacturing methods are in increasing demand every year due to their low cost, production of complex parts, rapid prototyping possibilities, and accessibility, and they can be preferred over other traditional methods (casting, forging). Additive manufacturing; is used effectively in many fields, especially in aviation. In addition, it is available in the literature that wax patterns (wax-patterns) used in precision casting, with its rapid prototyping feature, are obtained by the Fused Filament Fabrication (FFF) method. However, it is seen that the choice of polymer used here is very important. The polymer having high strength, low thermal expansion coefficient, and no causing shrinkage and warping during production are the desired properties. There are wax models produced with PLA and ABS in the literature. It is seen that parts produced with FFF are used not only in prototyping, but also in unmanned aerial vehicles. Additive manufacturing methods are classified according to the type of material as metal, ceramic and polymer-based. According to the ISO/ASTM 52900:2015 standard, material types are also divided into their sub-headings. The basic working principle of additive manufacturing is based on the principle that the feeder (it can be a powder or polymer filament) is melted with the help of a melting source and reassembled on a table at the desired dimensions. First of all, the CAD (Computer Aided Design) models of the part are created as .STL (Standard Triangle Language) file format and it is combined with the parameter information to be used in the printer with the help of a slicer and the g-code is created. This generated g-code is uploaded to the printer and the processes are started. Parameter selections play an important role in determining the mechanical properties of the polymer parts. The most important parameters used in the FFF method are as follows; the infill ratio, the layer height, the layer thickness, the width of the raster, the infill pattern, the air gap ratio, the raster orientation, the build direction, the printer speed, the printer temperature and the nozzle diameter. The choice of polymer type is another important parameter. In this study, PETG polymer was used because of its high resistance to chemicals, fatigue resistance, high toughness, and low shrinkage during production compared to other polymers and its easy production. This study aimed to examine the effects of the negative air gap, selected infill pattern and tensile sample standard, annealing heat treatment temperature and time on tensile properties (Ultimate Tensile Strength (UTS) and Elastic Modulus (E)). For the first parameter set, 60 samples were produced. 20 of these samples were concentric and produced by ASTM D638 Type IV standard. The remaining 20 samples were also concentric ASTM D3039. To examine the infill pattern difference in the last 20 samples, they were produced in rectilinear infill in accordance with the ASTM D3039 standard. All 5 samples were produced to have a 0%, 10%, 15%, and 20% negative air gap. As a result of the comparison of the infill patterns, it was seen that the concentric filling resulted in 29,65-50,54% higher results in E and 33,06%-47,88% higher results in UTS than the rectilinear infill. Another comparison was made between samples produced by ASTM D638 Type IV with concentric infill and 0%, 10%, 15% and 20% negative air gap, and samples produced according to ASTM D3039. According to the comparison of the different infill patterns, the concentric infill samples produced according to ASTM D638 Type IV showed the highest properties of 16,33% in E and 20,69% - 48,16% in UTS compared to those produced according to ASTM D3039. The effect of increased negative air gap was also investigated in both concentric (ASTM D638 Type IV and ASTM D3039) and rectilinear (ASTMD3039) samples. In all comparisons, the samples with a 0% negative air gap were compared with the samples produced with 10%, 15%, and 20% air gaps. As the negative air gap ratio increased, ASTM D638 Type IV concentric samples showed an increase of 11,38% - 31,54% in E and 37,18% - 63,89% in UTS. The effect of the air gap was found to be negative in the concentric filling produced according to the ASTM D3039 standard (as the gap increased, there was a decrease between 2,84% and 10,20% in E, while a decrease between 4,9% and 8,14% in UTS was observed. The effect of the air gap was found to be positive in the rectilinear filling produced according to the ASTM D3039 standard (when the negative air gap increased, there was a decrease between 2,51% and 32,44% in E, and an increase between 6,24% and 17,45% in UTS). According to all these results, the parameter set that gave the best results was obtained in the sample with ASTM D638 Type IV, concentric infill, and 15% negative air gap (E: 1.87 Gpa and UTS: 41,84 Mpa). Another aim of this study is to examine the post-process effects. To examine the effects of annealing heat treatment, 20 samples of ASTM D638 Type IV, concentric and 15% negative air gap were produced. This study was planned for two annealing temperatures and two selected times. The selection of the tensile test specimen is still a controversial issue, and the effect of the two standards was examined and discussed in this study. In this study, the importance of the effect of heat treatment temperature and time on mechanical properties was emphasized. The effects of two temperatures, 80°C, and 55°C, were investigated. At these temperatures, each sample was held in the furnace for 1 hour and 4 hours. Samples that were heat treated at 80°C were first compared with those that were heat treated at 55 °C. The tensile test results of the samples annealed at 55°C for 1 hour are higher 17,94% in E and 13,73% in UTS than the samples kept at 80°C for 1 hour. In the same way, the tensile test results of samples that were heat treated at 55 °C kept for 4 hours, are higher 17,10% in E and 13,67% in UTS than compared to 80°C. In order to see the effect of the time, the temperature was kept constant and the samples were held for 1 hour and 4 hours. According to the results obtained, there was no high increase in E and UTS as the holding time increased. All results were compared with non-heat-treated concentric specimens produced with 15% air gap and treated according to ASTM D638 Type IV. As a result of this comparison, while a 14,32% decrease was observed in E in the samples kept at 80°C 1 hour, this decrease was recorded as 2,16% in UTS. In the samples kept at 55°C 1 hour, it increased up to 4,42% in E and up to 13,41% in UTS. These results were also compared with the data in the literature, and the results were also compatible with the literature. In the samples processed at 80°C for 4 hours, a decrease of 13,77% was observed in E, while this decrease was recorded as 0,39% in UTS. In samples processed at 55°C for 4 hours, it increased by 4,01% in E and 15,38% in UTS. In the literature, 7% increase in E and 6% increase in UTS were obtained in the heat treatment of line infill samples produced with ASTM D638 Type I, 100% infill, and held at 55°C for 1 hour. The reason for the higher increase in literature compared to the samples produced with 100% infill is the effects of the negative air gap. The mechanical properties of samples produced with FFF are always lower than those obtained by injection molding, due to molding defects (like voids) and anisotropy. It is known that due to the nature of the FFF method, there are many voids inside the structure in the parts printed with 100% infill ratio. All the results obtained in this thesis were also compared with the mechanical properties obtained by injection molding. As a result of this comparison, it was observed that the highest difference was in the rectilinear produced samples (57,76%-44,06% in E, 68,96%-61,21% in UTS). In concentric samples produced according to ASTM D3039, this difference was between 23,31% and 14,60% in E and 36,91%-42,05% in UTS. Samples produced according to ASTM D638 Type IV it was found to be lower in 10,78-32,18% in E; 14,14% and 47,6% in UTS compared to the samples produced by injection. It was determined that the results of the samples produced by injection were approached with the annealing heat treatment at 55°C at most. The difference was recorded as 6,84% in E; 5,09% in UTS for 1 hour and 7,21% in E; 3,44% in UTS for 4 hours. The novel approach of this study is that reach the injected molded part results with appropriate parameter optimization studies. After the tensile tests, the fracture surfaces of the samples were also examined, and it was observed that 2 of the 60 samples were fractured in the GAT (G: Failure Type A: Failure Area T: Failure Location) rupture mode. It was observed that 20 samples produced according to ASTM D638 Type IV were broken from the inner narrow length, except for 2 of them. In addition, PETG is an advantageous polymer; no delamination and shrinkage problems were encountered compared to other polymers. In this study, it has been seen that the effects of infill, tensile specimen standard selection and negative air GAP, heat treatment, time, and selected temperature have significant effects on mechanical properties.