Cast Mold Your Own AR Lower Receiver

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80% lowers are all the rage for the do-it-yourself types or those wanting to avoid the prying eyes of the US Government. The lowers are effective, but always require a machine to be able to finish (including the polymer lower receivers). Out to change that requirement and lower the barrier to entry to non-technical folk is AR15mold.com, which has created a home casting kit for their polymer receivers.

The base kit retails for $329.99 which includes enough material to manufacture 5 lower receivers. It includes:

  • The base reusable mold halves
  • The small parts kits (inserts)
  • Mag hold block
  • Fire control group mold block.
  • Nuts & bolts
  • Polymer Resin

The receivers can be customized to a myriad of colors including black, blue, brown, red, orange, green, violet, & yellow. Conceivably, one could do any color with various combinations of the dyes. Additional resin mold material is about $119 for 5 additional receivers. Colors can likewise be purchased as a kit or individually.

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To ensure long-term usability, the receiver has some of the usual reinforcements found on other polymer receivers. The buffer tube tower is reinforced, the trigger guard is integral, and base receiver has struts.

For those interested in the kit, check out their website and some videos below on how to use the kit with its various inserts.


Nathan S.

One of TFB’s resident Jarheads, Nathan now works within the firearms industry in weapon design, operations, and sales. A consecutive Marine rifle and pistol expert, he enjoys local 3-gun, NFA, gunsmithing, MSR’s, & high-speed gear. Nathan has traveled to over 30 countries working with US DoD & foreign MoDs.

Nathan can be reached at Nathan.S@TheFirearmBlog.com


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  • Theo Braunohler

    Does it have the fiberglass reinforcement that commercial poly lowers have?

    • JumpIf NotZero

      No. It’s already very questionable with a 1 inlet 1 outlet for a complicated mold, it is possible that the part-A resin has glass beads but does not at all look like it from the video.

      With a no-pressure casting like this, you couldn’t expect any equal dispersement of a fiber filled post/resin. There is a reason that real injection mold and low pressure castings have multiple inlets and outlets. And glass filled nylon casting isn’t really a DIY thing.

      This I’m afraid is a really amateur product. Just buy 5x Anderson aluminum lowers for $250.

      • Jwedel1231

        Good analysis, I have to agree on all fronts.

      • Budogunner

        Also, for the DIY folks, you can get 3d printer kits for less than this.

    • Geoffry K

      No. But I did email Matt and suggested shredded Kevlar threads if he can figure out hour to mix it. He replied that it sounds like a good idea, whether or not he tries, who knows?

      • JumpIf NotZero

        shredded kevlar

        That’s a a red flag to me as a dangerous misunderstanding of why fiber reinforced composites have strength. Shredding some fibers and tossing them in would likely do more harm than good.

        • Ramsey

          No reason someone couldn’t use an out of autoclave prepreg carbon/Kevlar and vacuum bag it. There is lots of room for extra material on a lower without interfering with spec parts, too.

          • JumpIf NotZero

            An aluminum lower is $50. Don’t try and make a $400 polymer.

          • Ramsey

            To me it strikes the same fancy as building my own speakers, or skateboard, or shed. The making of a thing matters. I can take some fancy fabric and liquid chemicals and turn it into something useful. If I could cast my own barrels that cheap I would be all over it.

          • American Spartan

            Its the cost of the mold, with this you just set it and forget, come back and a fully down lower, this could change the game and its amazing, if you want to drill out an 80% lower, great, but some do not want to, or do not have the tools for it.

        • Mcameron

          umm….not really……

          works in the same principal as chopped mat fiberglass…….it creates a matrix of long, very tough fibers, allowing the resin to have something to bond to.

          almost like UHMWPE……where the super long polymer molecular strains make the material incredibly tough.

          the chopped fiberglass help to transfer load and serve as a binder to the resin, increasing its strength.

          while not as strong as woven kevlar impregnated with resin……chopped mat is easier to form.

  • JumpIf NotZero

    Bad product. Run away.

    Just realized that they aren’t controlling for CO2 bubbles (as the resin and catalyst cook off), which you would absolutely need for no-pressure casting a part.

    They need to be running a flame over the top of that mold at the minimum. But really, even the cheapest nonsense polymer AR lower mfg is going to pull a vacuum at the out, and pressurize the inlet, there will still be bubbles but they’ll be compressed to much smaller size.

    Not even mentioning that if your polymer AR lower has the same geometry as an aluminum one, that’s a joke. Now take away the fiber reinforced nylon, add bubbles, gravity feed the resin…. lol. Whoever buys into this stuff needs a non-firearm related hobby.

    • Geoffry K

      Properly mixed resin will not generate gas bubbles, only if it gets too hot.
      www dot crosslinktech dot com/support/tips-and-tricks/epoxy-casting-tips.html

      • JumpIf NotZero

        Properly mixed resin will not generate gas bubbles

        Yes. It will. It’s an exothermic reaction and the amount of bubbles has no specific correlation to the amount of heat. It WILL make bubbles, whether they’re pea sized or 1/4 a grain of sand size, they’ll be there.

        Low pressure casting is common for a reason.

        • Ramsey

          I feel that you are doing a disservice to the community by spreading disinformation. I have both a BS in chemical engineering and a PhD in chemistry, and I can assure you that there is no correlation between reaction thermodynamics (enthalpy change or heat) and reaction stoichiometry (atomic ratios, the only possible source for gas bubbles). This product may or may not work, but it won’t be because of that. Wikipedia has excellent articles on many aspect of polymer chemistry.

          • JumpIf NotZero

            Uh huh… And as someone who has worked with resins, epoxies, catalysts, casting, injection molding, and etc for 20 years – I don’t care what your degree says.

            You mix a resin and a hardner like this, pour in like this, you get bubbles in the mix. That’s all there is to it. You understand why you’d run a flame over the inlet right? Because you’re degree is pretty terrible if you don’t even get that far.

          • Ramsey

            Not to be pedantic, but I never said there wouldn’t be bubbles. I said there would not be bubbles from heat or chemical reactions. There is a reason that vacuum is used to degas resins after mixing, you get air mixed in when mixing the resin and the catalyst and any additives or filler. If the resin has a low viscosity and any bubbles are large, they will simply rise to the surface and be trapped in the sprue and the affected region cut of when deflashing. If the bubbles are small, fix your mixing technique.

            As far as running a flame over the inlet, I honestly have no idea what you are talking about. There is no mention of what type of resin is used, only that it is a polymer. It is likely epoxy or polyester, neither of which form any gas as a product of their polymerization. Tight temperature control is needed for the final part to have the desired properties (autoclaves the size of small buildings are often held to within 1˚C over their entire volume). You can’t do that with an open flame. I have never seen it done, and I have worked
            with resins in applications from garage woodworking to aircraft and wind
            turbines.

            If the resin is offgassing hardener or toxic gasses (maybe you are confusing isocyanate with hydrogen cyanide?), you picked the wrong resin/mix for the application. If you are worried about water from the atmosphere interfering with the polymerization reaction, again only the sprue or flash will be affected. Running a flame near a large volume of flammable organic chemicals that
            potentially release poisonous gasses with combustion strikes me as
            potentially lethal folly.

          • JumpIf NotZero

            So what you’re telling me is you’ve never cast molded anything.

            You run a flame over the inlet. The ambient O2 burns up, causes the bubbles to rise from the mold, then breach the surface. It’s often a pretty dramatic effect.

            About as dangerous as getting out of bed in the morning.

          • Ramsey

            Nothing you have said has any basis in chemistry or physics. You might as well be invoking magic. If anyone went near an active polymer casting in my lab or my shop with an open flame, they would no longer be welcome. MEKP catalyst is literally an explosive. Cobalt accelerator is deadly toxic and flammable. You are dangerously wrong. I hope nobody is injured because of it.

          • Cymond

            It’s pretty funny to hear you promote self-learning over formal training.

  • Geoffry K

    Also he won’t sell the mold and parts separately either so no experimenting with different resins.

    • hking

      Or buying the exact same resin he is selling for 10x cheaper somewhere else……

  • iksnilol

    Hmm, could you use the mould with aluminum?

    • George

      No.

      A premade foam core shape with pin inserts could be sold, however, which you’d just need to dip in plaster and dry and then put in a can or bucket of sand and cast into.

      Fabbing a steel or iron permanent mold for Al will probably run prices out of reasonable range. The single use foam could be $5…

      • allannon

        Since you seem to have some familiarity with the process: what about using a mould such as this to create you own sacrificial aluminum moulds as desired?

        • JumpIf NotZero

          He’s discussing “investment casting” and while it’s OK to do with foam, wax is where it’s at.

          Home lost-wax casting a lower is only about a a dozen times dumber than just paying $50 for a real aluminum lower.

          • George

            Lost foam casting probably cast the block in your car’s engine. Not needing to burn out the wax has its advantages. One quick dip in non-structural ceramic-ish surface coat for surface quality, green sand for bulk structural support, and you cast right into the foam (which vaporizes).

            The home 2-part foams have less person-healthy outgassing when poured into than a commercial styrofoam mold. But that requires much more industrial steam/pressure hardware to form.

            $50 commercial Al lowers make homebrew an exercise in independence, but that doesn’t mean we should ignore that exercise.

  • Hudson

    When we do resin casting in rubber molds we put the filled mold in a pressure pot and and raise the pressure a few psi, it doesn’t take much to get rid of all the bubbles and get a good fill on the mold. I know of people that use a cooking pressure cooker and a bicycle pump.

  • Twilight sparkle

    Would be cool to try to use the mobile to cast a wax lower for a lost wax casting.

    • hking

      It would have to be a %80 lower if you want to do lost wax. No way using even super fine greensand are you going to get the threads and pin holes clean enough for use.

      • Ramsey

        Use a plaster like jewelers use. Now I am all excited to spin cast aluminum!

    • Chris

      No. The wax and metal both shrink as they cool, so you would end up with an aluminum receiver shaped paperweight that is about 2% smaller than a functional receiver.

      • Twilight sparkle

        Well that would be a cool paperweight… but it wouldn’t really be the desired result

      • American Spartan

        What if you took said shrinkage into account?

  • LV-426

    I just wanna know where I can find that awesome sweet soundtrack in the video??

  • Gryzon

    What in the world do they think they’re getting a patent on with this?

  • Don Ward

    “80% lowers are all the rage for the do-it-yourself types or those wanting to avoid the prying eyes of the US Government.”

    Which is silly since most people are probably paying for this with credit cards and leaving a digital footprint for the lower and all of the parts which they have to buy to go with it. Then they’ll post selfies of themselves getting over on the Feds with their “untraceable” lowers on Facebook.

  • bull

    you should check out Boris thread on ar15 com . its called fruity ghost.

  • Geoffry K

    I did a lot of searching on casting resins, but most are 1:1 mix, I could not find a high strength 2:1 resin with the same pot time and mold time.
    I did come across a 1:1 resin with 11,000 PSI composite strength and 7800 PSI Tensile strength that is a bit faster. Pot life 7 minutes and demold 1 hour. It is called Task9 (Reg. Trademark)

  • Southpaw89

    Hmmm, an AR-15 jello mold, an interesting idea, but I’m going to have to see it to believe it, especially since it isn’t saving you any money, there are some poly 80% lowers out there that are cheap enough you could buy as many as this makes for the same price or less, at least last time I checked. Don’t get me wrong, I like the idea, its the execution that I’m concerned about.

  • Chadd

    This seems like it would be fantastic for a pistol caliber lower. But knowing what I do about FRPs, I wouldn’t trust just straight resin with anything that had too much recoil.

  • jerry young

    I’ve never been around a polymer AR and am not sure I even want to, I just see to much going wrong with a plastic AR

    • Cymond

      So you have no experience with the subject, but you’ve already made up your mind …
      Fwiw, there’s basically nothing that can go seriously wrong with a polymer lower. The most common failure is the stock breaks off, ruining the lower but hurting no one.

      • jerry young

        well I see you have your opinion about plastic and already pointed out a potential failure, what about the trigger and hammer pin hole wearing over extended use or the whole receiver flexing during heavy use? I just stated I see problems and don’t like plastic but that’s just my opinion, I prefer metal for guns, aluminium is good but steel is better for some applications, I do own polymer framed handguns but still like my all metal framed guns much more

        • Cymond

          Pin-hole wear is probably an issue as well. There’s definitely a lot more potential for wear, but that would be over a life of many, many thousands of rounds. Many would consider that a fair trade for lighter weight and lower cost.

          As for flexing, polymer has a great reputation for taking that kind of stress/abuse and returning to normal. In some cases, certain polymer receivers have survived trauma that has destroyed aluminum AR-15s. Unless you’re trying to build a longe range precision rifle, the flex in a polymer AR receiver is a non issue.

          Fwiw, I really like the feeling of an all-steel handgun, too. I had a full size 9mm 1911 that was a sweet shooter.

          Maybe I misunderstood you. Your phrase “I’ve never been around a polymer AR and am not sure I even want to” sounded like you were afraid of them for safety reasons.