The main objective of this project is to obtain the mechanical characterization of the printed specimens. The specimens can be divided into two groups: pure nylon specimens and nylon reinforced with carbon. Both nylon and nylon reinforced specimens will be printed in the exactly the same ISO dimension. Therefore, the results of both groups can be compared.
The types of the printing specimens are selected after deciding the experimental tests and the mechanical properties. At the beginning the proposal of the tests which was going to do were tensile test, 3-point bending test (flexural test), Charpy impact test, density and optical microscopy. Table 10 shows the specimens printed for this project.
Table 10-Final Experimental Plan.
Number of Specimens
Tests ISO Standard Nylon MF Nylon MF + Carbon Fiber MF
Tensile (dog-bone shape) ISO 527-2 6 6
3-point bending ISO 178 6 6
Charpy ISO 179 6 -1
Nº of specimens 18 12
All specimens are printed using the same printer. The printer provides several printing parameters. The printing pattern is better in +45º/-45º as reviewed before in the chapter of state of art. This pattern makes the specimen more isotropic [52]. The specimens will be printed with 100% density. F. Van der Klift et al. (2016) [56] have already done a similar research using The Mark One printer basing on this article some of the parameter settings of this project are learnt from here. In the following paragraphs, the printing procedures will be explained in more detail.
1 The Mark One printer was not able to print with carbon fibers due to the dimension was small. More details will be explained in the following paragraphs.
Nylon MF Specimens
Firstly, the geometry of the specimen is obtained in a CAD file. Secondly, the CAD file is converted into STL file. The STL file is imported into the Eiger Software. At the part view of the sample, there are general options for printing it. In the case of pure nylon samples, there is no need to add the fiber option, neither opening the internal view of the sample. The general setting for printing the tensile part and the flexural part is the same and it is shown in Table 11.
The dimension of the tensile part is 150 x 20 x 4 mm, but the actual section for testing is 10x4mm (Figure 6). The dimension of the first version of flexural part is 80 x 10 x 4mm2.
Table 11-Setting options to print Nylon specimen.
Setting option for Nylon specimen
Value
Layer height 0,2 mm
Fill Density 100%
Fill Pattern Triangular Fill
Roof and Floor Layers 10
Wall Layer 2
Others Use Brim (Tensile Parts)
The Mark One printer has its limitation while printing. The aim is to print the specimen with 100% density of pure nylon in 45º pattern, which means one layer printing 45º and the next layer -45º. Although the fill density is 100%, the triangular fill (Figure 16) cannot fill 100%
material to the specimen. As roof and floor layers are printed in 45º and with full density, the roof and floor layers have been set to 10 layers in order to get 20 layers filled fully with +45/-45º pattern. The total height of the two parts is 4mm, as each layer is 0,2mm high, the total layers of the part will be 20 layers where 10 will be the roof layers and other 10 will be floor layers. As a consequence, the triangular fill setting is not affecting the printing. The use of the brim is to avoid the warping problem of nylon as the first printed specimens presented warping problem. Since nylon is a material which is easy to warp with high temperature, while printing the difference of the temperature of each layer can lead to warp, for this reason the use of the brim is necessary avoid this problem. Before starting to print, it is important to apply glue on the printing bed so as to fix the nylon on the bed. Figure 12 shows the three nylon tensile specimens with brims. Figure 13 shows the Mark One printing the six first version flexural
2 Due to the printing limitation to print fibers with this dimension, the dimension of the flexural part has been changed to fulfill the printing requirement which will be explained in detail later.
specimens. Moreover, it has also printed six specimens of Charpy impact specimens which have the dimension 50 x 6 x 4 mm (Figure 14).
Figure 12-Nylon Tensile Specimens printed by Mark One.
Figure 13-The Mark One printing Flexural specimens.
Figure 14-The Mark One printing Nylon Charpy impact specimens.
Nylon reinforced with Carbon Fibers specimens
The same as the case before, the CAD file of the ISO specimens are converted into STL files and imported into Eiger afterwards. Once imported in Eiger, at the interface of the part view (Figure 10), the general settings can be done to fill the carbon fibers. Table 12 shows the setting to print carbon fiber reinforced tensile nylon specimen.
Table 12-Setting options to print Carbon fiber reinforced Nylon specimen.
Setting for Nylon Matrix Value
Layer height 0,125 mm
Fill Density 100%
Fill Pattern Triangular Fill
Roof and Floor Layers 10
Wall Layer 2
Others Use Brim
Settings for Carbon Fiber
Reinforcement Value
Fiber Fill Type Concentric Fiber3
Fiber Layers 4
Concentric Fiber Rings 34
In the case of tensile specimen, the matrix setting is almost the same as the case before except the layer height which is 0,125mm as this is the fibers maximum height. As a result, the total layer for the 4mm height specimen is 32 layers instead of 20 layers as the previous case. In this 32 layers, 20 layers are printed as roof and floor layers (printed in 45º full density) and the rest of the layers are printed in the triangular fill pattern (Figure 16). For the setting of the fibers, it has been selected the concentric fiber option because the Mark One printer only allows this option for carbon fibers. However, the Mark Two printer [57] allows the isotropic fiber option for the carbon fiber which is better to get isotropic characteristic for the reinforced part. In this case, using the Mark One and concentric printing option, the prediction is that the test result of the final part will not be optimum as desired. Moreover, due to the small width (20mm) of the center part of the specimen, the maximum number of the concentric fiber rings are 3. And the number of the fiber layer selected is 4 which is not too much but enough to see whether there
3 Among the options “Concentric Fiber” and “Isotropic Fiber”, the Mark One only has the option “Concentric Fiber” for Carbon Fibers.
4 Due to the width of the specimen is the maximum concentric fiber rings in this case is 3.
will be any effect on the final reinforced parts. The 4 fiber layers are situated at the layer 11, 12, 21 and 22 as shows Figure 15 at the lower part of the figure, the blue part is the fiber layers.
In this internal view, it can be seen the geometry that the fiber has been printed. It is shown in 2D but it is also possible to see in 3D the distribution of the fiber layers. Figure 17 shows the Mark One printing the six carbon reinforced tensile specimens.
Figure 15-Internal view of the carbon reinforced Nylon tensile specimen in 2D.
Figure 16-Triangular Fill Pattern.
Figure 17-The Mark One printing carbon reinforced Nylon tensile specimens.
In the case of flexural specimen, there were some problems while printing with fibers. The dimension of the specimen was 80 x 10 x 4 mm, the problem was that the width of 10mm was not wide enough to add concentric fiber rings. The Eiger software was failing to fill fibers. Due to this problem, the dimension of the flexural part was changed to a bigger size but with the
same ISO standard. The final dimension of the flexural part is 120 x 15 x 6 mm (Figure 7).
After using this dimension the software Eiger was able to fill the concentric fibers inside the part. As a consequence, the six pure nylon flexural specimens printed previously should be printed again with new dimensions in order to compare the test results between nylon specimens and carbon reinforced specimens. The settings to print the six pure nylon flexural specimens are the same as shown in Table 11. Figure 18 shows the internal view of the carbon reinforced nylon flexural specimen. As the layer height is 0,125mm and the total height of the specimen is 6mm, there are 48 layers in total where layers 11, 12, 37 and 38 are the ones with carbon fiber. Figure 19 shows the Mark One printing the six carbon reinforced flexural specimens.
Figure 18-Internal view of the carbon reinforced Nylon flexural specimen in 2D.
Figure 19-The Mark One printing carbon reinforced Nylon flexural specimens.
In the case of reinforced Charpy impact specimens, it happened the same as for the flexural parts. The width of the specimen was not enough to fill the carbon fibers using concentric fill.
According to ISO 179, there are no bigger enough dimension of the Charpy impact specimens.
Also due to the time limitation, there was no time to change the ISO and restarted the printing from the beginning, therefore the Charpy impact tests were not able to be fulfilled at the end.
For future work, one of the solutions could be machine small parts from a large sample printed with carbon fiber reinforcement. Figure 20 shows all the tensile specimens printed to be tested.
Each specimen has a central line and two side lines at 25mm each side of the central line. The strain gauge will be situated at these two side lines in order to measure the elongation of the specimens while making the tensile test using MTS 810 (Figure 8). Figure 21 shows the final printed 12 flexural specimens before testing.
Figure 20-Nylon and Carbon reinforced Tensile specimens.
Figure 21-Nylon and Carbon reinforced Flexural specimens.
4 Results and Discussion
The current project involved mechanical characterization of 3D printed pure nylon and carbon reinforced nylon specimens so as to compare the mechanical properties improvement due to the reinforcement. In order to obtain comparable data, all tensile specimens and flexural specimens were produced according to the same ISO standard as shown previously. The purpose of this research is to analyse the dimensional accuracy and the mechanical properties of the 3D printed specimens. The mechanical properties can be obtained from two tests, the tensile test and the three point bending test.
In the following sections, the results from dimensional inspection, tensile test and flexural test will be explained in detail.