Factors affecting interface bonding in multi-material additive manufacturing

A 3D-printing system by E3D (London, United Kingdom) was used for this study which is equipped with four different tools/printing heads and a tool changer, which allows the tool changeover to produce multi-material parts in one printing cycle. Figure 2 shows the assembled E3D printer. The PrusaSlicer software was the slicer software utilised throughout this study. Machine calibration was carried out to enable the fabrication of prints with the desired dimensional accuracy and specifications. Calibrating the E3D printer consisted of homing and tool change configurations, PID tuning of the heaters, bed levelling and automatic bed mesh levelling, and adjusting the Z-, X- and Y-offsets. In addition, custom G-codes were incorporated into the setting of the printer to carry out prime lines and purging after the completion of a tool change. E-step calibration followed by extrusion multiplier calibration was also completed to modify the actual amount being extruded out of the nozzle of the printing heads.

PC and PMMA are both hygroscopic polymeric materials and hence it was ensured that filament drying was completed before initiating the prints [12, 13]. Two eBOX Lite filament dryers by eSUN (Shenzhen, China) were used to pre-heat and dry both filaments before and during 3D printing with the following parameters: 6 h at 80 °C for PC and 6 h at 75 °C for PMMA filament. Issues regarding adhesion of the polymer layers to the printing bed were present; therefore, after cleaning the printer bed with isopropyl alcohol, a thin layer of an adhesive polyvinyl acetate glue was added to the bed before each print.

Since the interface bonding strength of the chosen PC and PMMA material combination for FFF MMAM was unknown, two specimen designs were created and named as Design A and Design B. Design A was mainly to investigate the influence of the printing approach in terms of the movement of the nozzle and was created based on the dimensions of the ASTM D3163 standard, as shown in Fig. 3. This is a standard test method for determining strength of adhesively bonded rigid plastic lap-shear joints in shear by tension loading [14]. Whilst the overlap length was increased for a higher contact/overlapping area of the inter layers, the overall length of the specimen was reduced to decrease the printing time. The dimensions of Design A are shown in Fig. 4.

Design B was created also based on the ASTM D3163 standard, as shown in Fig. 5, with such modifications that the second material will be printed directly on the overlapping area where shear would be tested. The design also resulted in the second material having a lower volume to be printed due to the removal of the tab from the original standard lap-shear sample and does not consist of overhung material. Design B was used to conduct the design of experiments and to investigate the effect of overlapping/contact area for the processing parameters and printing order which resulted in the highest bonding strength. For the latter test, the overlapping area of Design B was gradually decreased.

Eight factors were analysed to determine which parameters are to be utilised in the design of experiments (DOE). Layer thickness and raster width were selected whilst the other parameters were given constant values throughout all the prints as shown in Table 2. The levels for each factor were then determined.

Layer thickness Layer thickness, also referred to as layer height, describes the thickness or height of each deposited layer along the z-axis of the printed item. The layer thicknesses were selected to be less than the nozzle diameter, ranging from 25 to 80% of the nozzle diameter or half the nozzle diameter [15]. The E3D printer’s nozzle used was 0.4 mm in diameter [16]. As a result, the chosen layer thicknesses were 0.1 mm (25% of the nozzle diameter) as the minimum level value, and 0.2 mm (50% of the nozzle diameter), as the maximum level value of this factor.

Raster width and raster angle The raster width, also known as the infill line width, is typically set at the nozzle diameter; however, in this case, greater interface bonding required taking 120% of the nozzle diameter, resulting in a width of 0.48 mm ≃ 0.5 mm [17]. Therefore, 0.5 mm and 0.4 mm were chosen as the maximum and minimum levels of this factor. The raster angle of 0° was taken at a constant value to be able to test the highest expected strength during the lap-shear testing. With a raster angle of 0°, the extruded strands would be aligned parallel to the direction of loading during lap-shear testing, and hence would maximise their ability to resist the applied force.

The printing order in multi-material specimens was also an important factor and was taken into consideration by the generation of two DOEs for the two printing orders: PC followed by PMMA (order designation PC/PMMA) and PMMA followed by PC (order designation PMMA/PC). A 1/2 2-level fractional factorial resolution IV design was used for each printing order, resulting in 8 runs. The factors and their respective levels and the DOEs for each run of both printing orders are listed in Tables 3 and 4, respectively. Each run was repeated three times to obtain accurate results and reduce overall experimental errors.

A Python file was developed to enable the printing of multiple specimens with the same factors and levels using a single G-code. With this file, each lap-shear specimen may be completed utilising both materials before proceeding on to the next specimen.

Mechanical testing was conducted using the Testometric M350-20CT (TestometricTM, Lancashire, England) tensile testing machine with a 20 kN load cell and steady rate of tensile force was applied. The EN ISO 527-1:2012 [18] for determination of tensile polymer properties was followed where a test speed of 2 mm/min was selected, which was the same speed utilised in another study [7].

A customised tensile puller was created to carry out the lap-shear tests of the new specimen. The puller was designed in such a way so as to allow symmetrical pulling of the specimen from the centre of the part, i.e. the interface between the first and second material. The puller was made to accommodate the machine and replace the standard tensile grippers. Clearances and semi-circular holes were incorporated into the design to account for any warpage as well as to address any potential dimensional errors that could occur whilst printing the specimens. To ensure symmetrical testing, an additional aluminium block was machined and attached to the other side of the specimens during testing to compensate for the thickness of the specimen. Figure 6 depicts the design and assembly of the puller, the block, and the testing specimen during the lap-shear testing.

The different runs produced different values of force, elongation, and strength, depending on the process parameters used and dimensional accuracies achieved. Interface failure and layer-tear failure modes were experienced throughout all the specimens of the printing order PC/PMMA. It was concluded that the shear strength of this printing order was lower than the tensile strength of PC, which according to its material datasheet is 54.88 MPa, since the major failure mode was interface failure [20]. The highest shear strength was achieved by the specimens of run 7 with an average shear strength of 7.573 ± 4.957 MPa, i.e. only 13.8% of PC’s tensile strength. These specimens were printed using the same printing parameters for both materials i.e. the lowest layer thickness (0.1 mm) and highest raster width (0.5 mm). This highest interface bonding resulted from the highest contact area created by the same and highest raster width and accompanied by the same and lowest layer thickness. As stated in Li et al. [5] and Rajpurohit and Dave [6], a lower layer thickness resulted in greater bonding strength due to the high amount of heat which aided the melting and fusing of the deposited layer to the previous layer. However, the lowest shear strength was obtained using different printing parameters for each material, i.e. where PC was printed with a layer thickness of 0.2 mm and a low raster width of 0.4 mm, followed by a layer thickness of 0.1 mm and a raster width of 0.5 mm for PMMA. This difference in printing parameters resulted in less contact area at the interface due to the difference in raster widths.

A Pareto chart was plotted to determine the most influencing factors of the PC/PMMA specimens in shear strength as shown in Fig. 11. It was concluded that all the factors could be considered equally significant since none of the factors exceeded the reference line at 3.182, which is the t-quantile of the t-distribution, identified using the Minitab software.

The mean response value was applied to the maximum and minimum levels of each factor in the main effects plot to highlight the relationship between the factors and the shear strength. Figure 12 shows that the PC layer thickness was a significant factor for the PC/PMMA specimens. In addition, the shear strength was enhanced by lower PC layer thickness values and greater PC raster width and PMMA layer thickness values. The shear strength was barely affected by the variations in raster width of PMMA. The ideal combination of factors for this printing order would be 0.1 mm and 0.2 mm for PC and PMMA layer thickness, respectively, and 0.5 mm for both PC and PMMA raster width.

Figure 13 shows the generated interaction plot for the PC/PMMA specimens, where the influence of one factor is dependent on the level of another factor. The raster width of PC had strong interactions with both the layer thickness and raster width of PMMA. Further examination of the interaction plot between PC raster width and PMMA layer thickness indicated that shear strength was greatest when raster width was kept high and layer thickness was kept low. The interaction plot between the raster widths of PC and PMMA revealed that shear strength was greatest when both raster widths were at their maximum. There were no interactions found between the other factors.

Most of the specimens of runs 1, 2, 3, 4, 5, and 8, exhibited interface failure. This type of failure was characterised by a clean cut at the interface area. However, some of the specimens had the materials remaining in contact at the corners. The occurrence of this failure mode could be attributed to the print quality or the influence of the process parameters which fabricated these runs. However, run 6 displayed diverse failure patterns. Some specimens experienced interface failure, similar to the aforementioned runs, whilst others exhibited layer-tear failure. Run 7 consisted of specimens A and C experiencing layer-tear failure with some PC residue at the PMMA corner, whilst specimen B had most of the PMMA sheared off along with the PC as shown in Fig. 14. Despite variations in shear strength values, run 7 specimen B had the highest average shear strength amongst all runs. The lower force and strength in specimens A and C could be attributed to interface defects caused by PC residue. Greater overall thicknesses correlated with better interface bonding and increased shear strength.

All the specimens of the PMMA/PC printing order experienced stock-break failure, i.e. the PMMA/PC interfacial bonding strength was greater than the tensile strength of PMMA as shown in Fig. 15. This is because the PC’s higher extrusion temperature of 250 °C and PMMA’s lower glass transition temperature T_g of 90 °C, aided in polymer chain diffusion and entanglement. The PC’s molecular chains remained mobile and could easily diffuse into the PMMA during printing because of its greater extrusion temperature, whereas PMMA’s lower T_g enabled the PMMA macromolecules to become mobile whilst the first PC layer was being printed.

Although all the specimens failed in stock-break failure mode, different ultimate tensile strength values were achieved due to the process parameters which fabricated the PMMA in the different runs. Greater ultimate tensile strength values were achieved when the specimens were fabricated with a layer thickness of 0.1 mm such as ones from runs 2, 7, and 8. Compression of the layers occurred since more layers were deposited which reduced voids or air gaps between the layers. These runs also attained the greatest average thicknesses, which may have contributed to obtaining high tensile strength due to the additional mechanical adhesion and interlocking at the interface. The lowest and highest tensile strength obtained were 5.445 MPa and 27.9 Mpa, respectively, which means 8.4% and 42.9% of the tensile strength of PMMA, which according to its material datasheet is 65 MPa [21].