Cura was used to look at part and process optimization to prepare our part for the first prototype. Cura is an open source pre-processing software that gives approximations on build time based on the desired process parameters. In order to consider optimization of the build process we mainly considered density, layer height, and infill. We used the parameters to optimize our process such that we could obtain the desired strength for our part, as well as minimize the build time required to produce an adequate build. Two parts would need to be printed, the main holder that would attach to the wheelchair along with a lid. The lid and holder will be attached with a hinge that can either be incorporated into the print to have both parts print together or print the parts separate and use a pin to complete the hinge.
Two other additive manufacturing techniques other than FDM were also considered. We considered the use of SLS and SLA methods to print the parts. SLA would be able to produce a part that would look better, but the strength would be lower than using FDM or SLS. We don’t know exactly how strong the connection between the part and the wheelchair will need to be, but the strongest method would be our best option.
SLS would produce the strongest part and would also have a good resolution. This method could be used for our part as there are no internal features that would be problematic for SLS. We would be able to expect properties from SLS that would be comparable to creating the part with injection molding. This option would be more expensive, but if we can’t get high enough strength from FDM then SLS would be our next option. We need the added strength from the SLS process in order to ensure that any load applied to the assembly from Jake’s hand when he reaches for his cards wouldn’t damage the assembly. This was the main motivation for using SLS rather than FDM. SLS would also increase our strength in the T-slot which was another big concern of ours in the design of the assembly.
We will be printing with FDM using ABS for our first prototype. Our initial settings of 0.1mm layer height and 90% fill density the part was estimated to take 58 hours to print. We would like to have a high fill density to create the strongest part possible. With the part oriented as shown below, the only support structures needed would be under the T shape attachment to the wheelchair and under the curved lid. I also selected a “touching buildplate” support setting and a brim platform adhesion type. Both parts would be printed simultaneously, but the hinge will have to be decided upon before printing.
Printing the hinge provided a design challenge for our team as we were unsure of the best method to print the hinge. One option considered was to print a living hinge, which is achievable using the SLS print method. We ultimately decided against the living hinge as we didn’t feel it would be very practical for the end use once attached to the wheelchair. After deciding to print a regular hinge on our final part, we had to decide on whether to print it separately or as an assembled piece. We decided to print it as an assembled piece because the SLS technology allows us to print everything as one piece. The main advantage to using SLS to print it as one assembled piece is that we could print a pin in to connect the hinge that would allow for movement of the pin without bonding everything together. One concern for this method of printing the hinge is the possibility of the pin breaking, however we used tolerances allowed by the process so that if it did break a new one could easily be printed or replaced with any conventional pin.
We will try to use different settings to lower the printing time. 0.1mm layer height might not be necessary so that was the first parameter to change. By changing the layer height to 0.2mm, the printing time was cut in half down to 29 hours. If this doesn’t provide a good resolution the part could be sanded and painted to look better. If this does provide a good resolution we could increase the layer height even more to lower the printing time. A layer height of 0.3mm would take 20 hours and 46 minutes to print. Other settings that could be changed are the printing speed and fill density. The initial settings of 50mm/s print speed with 90% fill density will be used for our first prototype. These settings will produce a part weighing 458 grams. If this produces a satisfactory part we could change the settings to lower the print time even further. The settings shown below will be used for the first prototype.
In terms of resolution we were mainly concerned with the T-slot. We really needed a quality resolution in this part of the assembly to make sure that there was adequate strength within the slot. This was one of the larger concerns for our design as it would have the greatest impact on the functionality of the part. If this were to break due to any structural inefficiency it would render the whole wallet attachment useless as it couldn’t connect to the wheelchair anymore. Surface finish was not a great concern for our part. Because most of the assembly wouldn’t benefit from the additional surface quality the only added benefit from a higher surface quality would be any aesthetic improvements.