Buren joined the airforce at the beginning of this year and spent most of the year away doing training but he is back in Brisbane now and keen to get back into rocketry. We had lunch yesterday and talked about a new project we could work on. I have basically no money at the mount so a big project is out of the question but we came up with a few ideas for smaller projects which haven't been done before, mainly neighbourhood of 3D printing.
Call me a mechanical engineer but in the past I have been pretty sceptical about the usefulness of plastic 3D printing. I never really saw any application for printed parts as structural components. In the company I was exposed 3D printing fanatics and I think I caught the bug. They got me thinking about printing high load bearing parts from plastic. Something I thong would be really cool is to print an entire hybrid motor (tank and all) in one piece.
This isn't as crazy as it might initially seem. Porosity is probably the biggest issue but airtightness can be achieved by sealing with epoxy or partially dissolving it with a solvent depending on what sort of plastic you're using. Strength was my main issue but even if you assume the strength between layers is 5% (papers say 5-20%) you can just make parts thicker. They will be heavy but there isn't any reason why they shouldn't hold up. Heat is obviously a major issue but for a hybrid I think it would be possible with a clever injector design. So I think printing an entire hybrid from plastic would be a really interesting challenge and not too costly either as I have pretty much everything I would need.
I figure the hardest bit will probably be the pressure vessel. The other day I made a simple test vessel. Its pretty basic, just a 20mm ID sphere. I don't have my own 3D printer but we have one I can use at work. Unfortunately it only can print in PLA. The wall thickness is 10mm. Assuming a inter-layer strength 10% of low strength PLA (difficult to tell what I have) it should be good to 850PSI (leaks will be the main issue) I think there will be much better materials than PLA (have ben doing a bunch of research) but for now I will have to put up with it.
The first attempt was printed at a density of 20% and it had a bunch of leaks. It skipped a layer half way which probably didn't help so for the second attempt I printed at 100% density. I made the hole for the fitting slightly smaller so I could drill and tap it. In the end I just epoxied it in because I didn't want it flying out. I would like to do some tests on tapped threads in the future. I am not sure I will ever be able to print a thread using a hobbyist printer but thats something to explore too! I tested it with compressed air @120 PSI and there was a really small leak. I was pretty happy with this as the printer I am using wasn't set up well and there are pretty big gaps between layers. I should be able to improve this with tweaking. I ended up pouring a bit of super glue in which plugged the hole. Now it doesn't leak at 120PSI. Unfortunately I don't have any source of higher pressure to test it currently. The startup is currently in a large industrial complex owned by a university and there are a few gas cylinders....around. One night I went out to try a few but unfortunately they are all empty....I've been thinking about buying a high pressure dive compressor for a while now, this would be a good excuse!
I think I will also buy a 3D printer just for experimentation. I also want to get a bunch of different types of plastic to test with. There are some nylon ones with a fairly high UTS which look promising. More to come!
