Finally, I’ve made the step up to PA6-CF. Where I live it’s humid as fuck. That’s why I was totally set on using PA612… then I found out about PA12… and then I read here and saw someone mentioning PPA-CF / PAHT-CF.
Question: I know PA12-CF is quite a bit less hygroscopic… but does it still have enough strength anyway for a 5.56 lower? Same question for PPA-CF.
And by the way: if some of my posts sound weird or poorly worded, it’s because I’m using the “translation method” I just learned in the stealth section. Just to be safe.
People really need to stop worrying so much about PA6-CF moisture absorption. Unless you’re going to store your build underwater, it’s just not a problem. At 5.3% moisture content (which you’re unlikely to reach even if you live on the gulf coast and store everything outdoors) the bending modulus of Fiberon PA6-CF is still 2286 MPa. That’s only 18% less than Polylite PLA Pro and 16% more than eSun PLA+.
PA612-CF, PA12-CF and PPA-CF all have more than enough impact strength for an AR lower and will get you a little more stiffness. with PA612-CF and PA12-CF you may need to anneal to mitigate creep issues. PPA-CF needs higher print temps (up to 320°C) to achieve optimal strength which rules it out for some people. Builds in any of these materials will be slightly more susceptible to breakage due to drops or rough handling vs. moisture conditioned PA6-CF (though if that’s a serious concern, a $40 Anderson lower – if you can still find one – is probably a better choice than printing).
My only worry is the fcg pin becoming loose. I’m not that worried about it. I have a couple test prints that I haven’t even annealed yet and they’re solid as hell still.
Thanks for the advice on the m5 screw/nut. Makes me feel much more confident in my decision. Probably gonna go that route instead of just tapping the threads into the lower itself.
Pins loosen due to creep, which is a result of how the polymer crystalizes and has nothing to do with moisture content. Polymaker’s PA6 has additives that slow crystalization resulting in very low creep. Some brands crystalize faster which leads to more creep and may require annealing.
Oh okay. For some reason I thought whenever the polymer becomes saturated. It allowed for more wiggle room in the holes. Now that you mention it. I think I remember Hoffman showing on an orca, even once the pa6 was saturated. It fired fine. The only problem was when he shouldered it really tight . For some reason it wouldn’t fire. I imagine it caused the fcg to shift somehow. If he shouldered a tad softer, it functioned just fine.
That Hoffman video was a huge setback to 3D firearm material knowledge. He’s a smart guy with excellent design skills, but when you see statements like “PLA Pro … is not known to have any significant levels of creep …” which are provably incorrect, you have to be very skeptical of everything else he claims. The paper he wrote to accompany the article “looks” like an academic research paper but would be laughed out of a real academic peer review.
The AR lower failure he demonstrates was caused by buffer tower flex, which leads to the buffer tube becoming misaligned with respect to the upper. When that happens the BCG binds up as it travels rearward. Imaging trying to push a straight bar through a bent tube. In his video, you can actually see the BCG getting stuck rearward. Hoffman does a piss-poor job of explaining that in the video.
When it comes to creep, you can see huge differences across different brands of the same polymer. For example, Polymaker PA12-CF is known to be very high creep, while Sunlu PA12-CF exhibits even less creep than most PA6-CF brands. There are factors other than base polymer chemistry (in this case rate of crystallization) that can dramatically affect performance. For reasons unknown, Hoffman doesn’t get into any of that and that’s one more reason why his results should probably not be informing your material selection.
Yea I watched that video when I first started printing. Haven’t wanted to watch any of his stuff because I’m too dumb for his delivery I guess. Do the sunlu pa6 variants as a whole have less creep? Or just the pa12? I’ve been using sun lu pa6 to practice and was going to use polymaker pa612 for the final. Now I’m conflicted. Also, the brass bushings Hoffman uses on his fcg pins. Are those bullshit? Or do they actually help with strength? I always thought they helped but I’ve seen alot of people say they’re a money grab. What do you think about his hose clamp and printed parts for the take down pins compared to the ubar? I really liked the metal buffer tower on the Apple pie but the printed part for the take down pins made me nervous so I went with the ubar
Creep test results I’ve seen show little difference between Sunlu PA6 and PA12 (both are quite good). I don’t have any trusted creep data for Polymaker PA612 (but I haven’t looked that hard).
But you’re really overthinking it. If you’re getting good prints with Sunlu PA6-CF, use it - it’s more than good enough. Printed lowers aren’t going to last decades or 10s of thousands of rounds no matter what you do. There’s this “dream” that with the right material you can get a premium-quality lower. The reality is that the absolute best you can do is still miles below what you’d get with a $39 Anderson “poverty pony” (yeah, they’re $59 now that Anderson sold to Ruger - still cheaper than 1kg of Polymaker PA612).
My recommendation is print your first one in PLA Pro and run it till it fails. When it does, reprint in something better (if you loved it), or print a different design in PLA Pro to see what that one is like. To mitigate creep skip the Hoffman bushings and get a set of anti-walk pins with the “dogbone” connectors (https://www.amazon.com/GLKTRS-Prevent-Rotation-Precision-Motorcycle/dp/B0D9XBCTQG). Or use a drop-in cartridge trigger (cheap milspec triggers suck and you’ll eventually want to upgrade anyway). Don’t be afraid to iterate. That’s the true joy of 3D2A … being to try out lots of interesting stuff without a huge expense.
Can you help me understand the anti walk pins? I keep hearing If you have a mid spec trigger, theyd stop the pin from rotating which could cause problems etc. A lot of people say it functions fine with them. Sadly the link you sent is out of stock. All the others on Amazon have bad reviews. So I’m just gonna grab some different ones.
Anti-walk (really, anti-rotation) pins are of dubious value for a milspec receiver. The idea is that you want the hammer and trigger to rotate on their pins, but friction can cause the whole pin to rotate. That puts wear on the pin holes in the receiver, which really can’t be fixed without going to oversize pins. But this is really a tens of thousands of rounds with no lube kind of problem, so not relevant for most recreational shooters.
But for a printed receiver, the stamped steel “dogbone” plate takes some of the stress off the receiver pin holes. The hammer spring is constantly pushing the hammer pin away from the trigger pin. Tying them together with a metal plate helps prevent creep (which is the result of a constant stress over an extended period of time).
None of the Amazon offerings are particularly high quality. But most 3D printed guns aren’t duty-grade or precision competition guns. I’ve used an assortment of Chinese anti-walk pins and they’ve all been “good enough”.