30 April 2016

AR15 Mold Update

Marv's Blueberry Muffin has suffered our first failure with the Freedom-15 lower.

I don't think this from the stress of firing.  I think this is from the mold maker putting the rear holes so they're too fucking tight on the upper and you have to hammer the take-down pin in and out.

The good news is the thread spacer method of capturing the buffer retainer worked flawlessly.

As you can see from combining these three shots is the crack is almost all the way through and the stock is about to come off.

We decided to see how much effort it would take to finish it off.  Marv says "moderate" effort from cracked to this:

The take down pin, detent and spring all went flying!  Amazingly we were able to find all three parts after a short search.

No matter the precautions we've taken, there is no avoiding bubbles and voids with this material and mold.  The set time doesn't allow for the bubbles to escape before they're permanently part of the part, and the route the material has to take to fill the mold is not simple, so there's lots of places for air to get trapped too.


  1. Can you use a vacuum chamber? I've read about that method for casting bubble free lenses.

  2. or vibration like it is done in concrete....maybe a brass tumbler.

    1. Vibration has failed. Neither a personal massager or a "lifelike" 9" rubber dildo provided enough vibration to matter.

      We'd like to try a vacuum chamber, but the way the mold is made you'd have to encapsulate the entire thing and we don't have that kind of stuff handy.

  3. Based on my materials science courses, if I had to guess it's less the bubbles that are a problem than the too-tight takedown pin. One of my profs relayed a story on using plastic to mold/3d print stuff, and it turned out great! Held up to everything, they assembled the device, tested it, everything was golden. Came back the next day and everywhere the thing was held in continuous tension by the bolts and screws it had cracked apart almost exactly like this lower did.

    You mentioned cleaning up the hole on one of the other lowers you made for a looser fit, which should help immensely.


Try to remember you are a guest here when you comment. Inappropriate comments will be deleted without mention. Amnesty period is expired.