Our group was a little delayed on running the 3D printing simulations in the provided Cura software due to difficulties with the 3D scanning of the foot that we experienced. Now that we have a solid .STL file to work with, the optimization of the 3D print can be performed. The model was scaled down slightly in length so that it would fit on the print bed in the simulation (see image below). Moving forward, the insole will either have to be split into two parts so that it will fit on the platform, or slightly scaled down but the simulations were done with the slightly smaller model. In future posts about print tests, we will most likely try different strategies, and it will be addressed in future posts which strategies worked best. With the splitting of the insole into two parts, we figure we can attach the two parts with some form of adhesive, since they will be in a shoe and unlikely to shift around. Also, the PLA support will be adhesively attached to the NinjaFlex material that we use for the same reason. It is logical that the insole would be laid down flat on the build surface to minimize supports needed (see image below), so simulations were run in this orientation. These simulations were conducted using PLA as the the material, but the actual prints will be done with the NinjaFlex material. Recommended printing parameters were found on http://3dprinting-blog.com/tag/ninjaflex-print-settings/. After a couple test prints, the printing parameters for this material required a slightly lower nozzle temperature (than the PLA prints) of 240 C, a lower bed temperature of 50 C, and slower extrusion speed of 10 mm/s to work well on the Wanhao printer.
When looking at layer height, the smaller the layer height and greater the infill density, the higher the quality of the part that will be produced. Greater infill density will require more material to be used but a greater overall strength will result. Also, the smaller the layer height, the longer the print time required print time. As I stated earlier, the simulations were conducted for the one piece, smaller scale model for simplicity when looking at printing parameters. The part will be build in the orientation shown in the image above because it is the most logical way to print it and it would be very challenging to try and build it in the other direction.
Since the part is large and has two areas in contact with the build platform, a simple brim structure will be used on the base of the insole. Since the part will need to support the weight of a normal human, we will assume that an infill density of at least 30% will be needed to be strong enough to handle the load. We learned in class that materials are always stronger in compression, and therefore we are confident that the NinjaFlex will have sufficient strength to support the feet. The first simulation ran with 30% infill density, 0.2 mm layer height, and shell thickness of 0.8 mm gave an estimated build time of 10 hours and 2 minutes, requiring 43 grams (14.44 meters) of material. Increasing the infill density to 50% only increased the build time by 6 minutes with only 3 more grams (about 1 meter) more material. This is because the insole is so thin (about 5 mm) that the shell thickness on the top and bottom of the insole make up most of the print, and there is not much infill in the part at all.
Changing the layer height of the part will affect the part quality and build time. Increasing the layer height from 0.2 mm to 0.25 mm reduces the build time from 10 hours 2 minutes to 8 hours and 21 minutes with about the same amount of material required. Decreasing the layer height to 0.1 mm will give a higher quality print but the print time is greatly increased to almost 19 hours. Since there is not any major features with minute details on the surface, a layer height of 0.2 mm should suffice to achieve the surface quality we desire in our print.
The last consideration when printing is the build direction in each layer. As we learned in class, having shorter FDM path lengths often results in higher tensile strength of the part as the temperature gradient across the part is less and the material has more time to fuse together to the warm, newly added material. However, since our insole will not be subject to tensile loads, but more compressive loads in the vertical direction, we will allow the program to generate the g-code with random layer orientations (X-Y movements in each layer plane).
In the labs moving forward, we will be conducting our initial prints and will report back on progress and optimization decisions based on the success and quality of the prints. We will also be optimizing the PLA support based on different loading experienced by the foot, and will combine those results with the current model to give a complete insole design.