AP projects 2015
With the ‘Science Fair’ approaching, our priority was to optimize our chicken breast and to try to produce a few successful samples to demonstrate to the world. We started by testing different variables, for example by changing the infill patterns of the prints or by trying to print without needle.
Friday further print experiments were made, trying out various infill methods and speeds or extrusion as we did earlier on (see weblog 20-10-2015). The main problem we encountered was with the extrusion, especially with creating a constant flow. Even with the first few prints we noticed that the material started clogging from time to time or stopped extruding, probably also due to clogs formed in the material. We expected that the clogs in the material were probably caused by the temperature of the material or other variables, such as the thin needle. However, we are still uncertain as to what causes the clogs to occur.
Also the extruder itself showed some mechanical issues. Firstly, the belt that pushes the syringe down to extrude the material kept tangling up and didn’t tighten around the syringe anymore which put the printing to a halt. We solved this by untangling it, fastening it with zip ties and checking it from time to time. Secondly, the motor started acting up and didn’t deliver enough force to keep the gears turning throughout the entire print. We realized that this wasn’t caused by the motor itself but by the electronics that drives the motor (Stepper Driver). That’s why on Monday we replaced the stepper driver and the motor worked fine after that. Finally, the last problem we faced was that one of the gears on the printer gradually came loose due to all the vibrations caused by the movements of the printer. To solve this we fixed the gear by gluing it and fastening the nuts and bolts with lock tight.
Furthermore, we tried printing without the needle to see if this would improve the extrusion. Using a bigger nozzle led to a faster printing time, but the structure became less similar to that of a real chicken breast due to the thicker lines. An interesting finding was that when extruding with a bigger nozzle it is possible to spray the salt-water solution on the filling before it touches the previous layer, which made it possible to immediately extrude a solid line in mid-air (similar to the printing of plastic).
To conclude, we decided to go for the thinner needle which gives a more accurate fiber structure to the samples, even though it will increase the printing time. However, in the future we could see printing with a bigger nozzle as a cheaper alternative to printing with a thinner needle.
Monday we continued trying to optimize the extrusion, by making as many prints as possible and trying to change the printer settings with every print to find the best print speed, infill method and speed for our final chicken breast models. Instead of printing life size chicken breast we chose to print smaller samples that would demonstrate sufficiently the different print methods without taking too long to print. Once we found in our opinion the best combination of settings that we could achieve we went on to print our first successful full sized breast!
By the end of the project all the documentation, settings and models will be published.
We noticed that the first few prints of the day, when the infill was still warm, the material would extrude smoothly without clogging. When the infill cooled down, the clogging started to happen more frequently. This is when we tried to use a heating element to warm up the infill to see if it was the temperature that caused the clogs to occur during the prints. For this we used a heat gun to warm up the material close to the nozzle and extruding right after and while applying heat. The result was that it didn’t change much; it might be a small factor but not a significant one. The conclusion that can be made regarding the clogs in the extrusion is that having a well premixed mixture with no clumps in it is the main thing when trying to avoid clogs. Having a heated mixture might help, but the difference isn’t significant enough to be a relevant factor. Another element that minimizes clogging significantly is the proper use of retraction. At first we had looked over a line in our layer change G-code. It would go to a stop position, extrude a bit and then continue on to print the next layer. However, we did not notice that after extruding at its stopping position, it would retract that very same distance back effectively leaving us with a loose belt and very little tension for extrusion. Some of the salt-water solution might have gotten into the nozzle, hardening our mixture and clogging the nozzle or simply leaving us with half empty layers. So by disabling the retraction every time it switched layers we prevented many of our clogging problems. In fact ever since we did this modification our results have been overwhelmingly positive.