Plastic Casting: The Next Step For Homebuilt Firearms?

Hognose at his home website takes a look at the fascinating process of plastic casting utilizing a 3D printed plastic pattern to create, over and over again, firearms receivers that hopefully should last longer than existing 3D printed lowers:

There are still several technologies open to us, like metal injection molding (which probably made the fiddly bits inside your carry handgun, unless you’re old school). But even that has some complexities, even though it shows signs of integrating really well with 3D printing. Basically, you can do the prep work, but someone with an industrial setup needs to do the actual MIM for you.

How about plastic casting? There are plastics that are a pain to 3D print, but that can be cast at room temperature and atmospheric pressure. Fosscad experimenter FP gave it a shot, and produced some gratifying results: plastic AR lowers that appear to be superior in strength to 3D printed versions.

The difference between plastic casting and plastic injection molding which is how your Smith or Glock frame is made is that injection molding is done under pressure, and casting is done in atmospheric conditions. That means that casting will usually be less dense and will be done with materials that are poured and set at lower temperatures, as a rule of thumb. Molding is commonly used on Hollywood sets and props, for example, but it also has architectural and industrial applications. Both the silicon “rubber” for the mold and the plastic for the castings come in parts that react and solidify when mixed.

Mold silicon and casting plastic

This page on imgur walks you through two complete batches of plastic casting multiple AR lowers using two different molds, one contained in a see-through plastic box and one in a wooden box.

The sequence of events is:

  1. Print and prepare (i.e. strip off support material, acetone-vapor treat, etc.) your lower or other master part (called a “pattern” in casting).cast plastic -- 3DP pattern
  2. Prepare a mold box and place your pattern in it. Include some material to form a pouring inlet, runners or sprues if needed, and an air release hole.cast plastic lower - mold box 1
  3. Prepare and mix the mold RTV and pour it into the mold box. Let it cure. Beware of exothermic reactions.cast plastic lower - mold box
  4. Once the mold has fully set, remove it from the box, and carefully cut the silicone away from the pattern, taking care to neither damage the pattern (you may want another mold; they don’t last forever) nor, especially, the mold.  Cut apart, the mold will have four parts: left, right, bottom, and core. cast plastic lower - pattern out of mold
  5. Reassemble the mold in the mold box.
  6. Mix and pour casting plastic; let it set.
  7. Open the mold and remove the cast lower. cast lower - out of the chrysalis
  8. Repeat as needed.

Initial testing suggests that these lowers are stronger than printed lowers, and there are stronger, more exotic casting plastics available. Some of this testing has already begun.

Hognose sure knows his oddball manufacturing techniques. Polymer casting is a natural conclusion for anyone with a 101-level knowledgebase in manufacturing, but recognizing something is an interesting idea and getting it to work well are two entirely different things. It may be that polymer casting doesn’t work, or that it works only as a way to produce better components with cheaper 3D printers and media, but if it does shake out, that could be the push the home-grown polymer AR lower market needs to become truly practical.

At WeaponsMan, commenter Ratus adds to the discussion, posting a couple of polymer casting videos from the YouTube channel Tested, both embedded below:

Nathaniel F

Nathaniel is a history enthusiast and firearms hobbyist whose primary interest lies in military small arms technological developments beginning with the smokeless powder era. In addition to contributing to The Firearm Blog, he runs 196,800 Revolutions Per Minute, a blog devoted to modern small arms design and theory. He is also the author of the original web serial Heartblood, which is being updated and edited regularly. He can be reached via email at


  • SimpleStudent

    Interesting that they have gone about this process in this manner. There is another, mirrored process that they could also have tried. We have some guys at universities doing some very similar things, so it could be worth a look into.

    Instead of 3D printing the pattern for the mold, maybe you could try 3D printing the mold itself. Initial issues with the 3D printed molds failing to produce results have been frustrating, but by plating the molds in thin (we are talking like <100 atoms thick) Copper layers we are starting to see the results we are looking for. We are working towards the stage where these are not used for casting, but for injection moulding!
    For a complex part such as a lower you could probably even make more sectioned mold to allow for easier removal of the product.

    I'm interested to see the results of these 'Home-brew' gunsmiths, even if they aren't battle-rated.

    • Paul Epstein

      Out of curiosity, how are you plating plastic molds with copper?

      • marathag

        Vacuum plating is one way, but I never thought of that as a home process

    • Tim Bridge

      You might want to look into 3d print metal systems for moldmaking…
      Quite efficient and capable of printing high stress items. Including firearms themselves. 🙂

      • Boris

        Printers that can do that now are very expensive and take long time.

        To contrast, you can cast plastic lowers in a parking lot, 10min time, most of it spent drinking and waiting for the plastic to cure. You can do it now.

        • Tim Bridge

          I’m also thinking in terms of production house quantity work, and business applications, though.

    • Boris

      That last bit about “home-brew” gunsmith’s wants me to say something nasty to you, but I will not.

      Aren’t battle-rated??? What does constitute battle ready? Which battle are you talking about? Receivers cast this way will handle multiple of US Army combat loads of ammo without a problem, source: my own experience. If you have done your research and can back up your hollow claim to not be “combat-ready”, habeas numbers!

      The reason why molds are not 3D printed is like making appendix surgery through anus. It’s overly complicated and not needed. This process is pretty fool proof and would produce acceptable results from both actual lowers and printed lowers. You do no grasp the reason why molds are made of RTV rubber.

      Fruity ARs can be made in large numbers, cheap, untraceable. Av. cost of one receiver is $3. This is the battle and these receivers are winning it.

    • Curtis Thompson

      Try printing the mold with the exterior sides open exposing the internal structure. Then fill it with epoxy, or another strong resin. GoEngineer found that they could get a printed/resin-filled part that could withstand up to 18ksi. They use it in the sheet metal forming processes such as hydro-forming and vacuum forming.

  • Griz

    How about doing this with brass, or smelter pop cans? Also with the above process, you could reinforce the weak point of the lower with steel, Do a loop of spring steel that goes around the part of the lower that the buffer tube screws into.

    • Southpaw89

      I’ve often wondered about casting an aluminum lower, especially after reading on a forum where someone claimed that the army just wanted rifles that they could make out of old beer cans, ridiculous as it sounds it got me thinking. Using brass would produce a very heavy lower by AR standards, but I’m guessing that it would look pretty cool, and it would certainly draw a lot of attention at the range.

      • Boris

        You may make wax receivers just like plastic, same mold. I’ve used lost wax to make zink and al lowers.

        In addition, there are low temp metal alloys that could be cast in RTV. They are pretty heavy though.

        • Southpaw89

          Interesting, how do the cast AL lowers hold up? I hear as a material it can be pretty brittle.

          • Boris

            I don’t know, I have trouble buying ammo just to feed the plastic ones. They don’t break easy.

            Cast aluminum is more brittle than forged or softer virgin and may have additional problems like porosity which isn’t visible.

            With price point on 80% being this low and many methods already developed to process 80% into working receivers, this is just academic pursuit. This is not the way to make better aluminum receiver, starting from 90% forged or billet is.

          • Southpaw89

            No argument there, just so long as its not likely to blow up in your face, maybe I’ll try it with a 10-22 first, that little rifle seems like a good candidate for the process too.

        • The Brigadier

          Boris, its spelled zinc.

      • The Brigadier

        Aluminum casting requires very high temperatures. Not quite as high as melting gold but close.

    • Cymond

      From what I’ve been told, melting & casting brass at home does significant environmental damage across a wide area. Unfortunately, I don’t know what to Google to learn more.

      • Griz

        Someone made a brass lower.

        • tts

          Sheeesh don’t link stormfront even if you see something cool there. Tooo many insane conspiracy theorists/racists. That place is toxic man.

          • griz

            I googled brass lower and it was the only one I found, never been there prior

          • tts

            Just use imgur to post pics from unsavory places. Its not like anyone really wants to see anything else anyways.

      • Boris

        There are plenty of people who cast bronze. It’s not hard. It could be done using charcoal even. Most people who do use a sort of waste oil burners. A well running burner is clean and produces just CO2. Many use propane too or combo of preheat propane, then WO. There is nothing crazy about this process. It’s been done since ancient times. Lost wax techniques has been used in India since … really long time.

        • iksnilol

          bronze and brass are two different things last time I checked.

    • Boris

      Casting brass is much harder than bronze, silicone bronze. In brass, zink has a tendency to burn out fast and it does not flow well.

      Yes, there is a way to make reinforcement. Also, another company that does injection molding of plastic AR receivers uses aluminium piece to reinforce the rear section. It works well.

  • Vhyrus

    Can you use any AR lower as the pattern or do you need a special one made? If this works out I could see kits being sold with an AR master and all the jugs. The ATF couldn’t lay a finger on it.

    • You would most likely want to acquire one of the 3D-printed specific lower models, because the regular aluminum lower would not be a good choice for plastic casting.

      • Boris

        Not true. I just got back from a range, 700 rounds through a plastic receiver that was molded from a regular AR receiver.

        • FP

          Gotta throw the 3D printing in there to make it extra scary. 😀 I don’t even own a “real” AR lower.

        • They can work, but are much less durable.

    • Boris

      I’ve cast AR fruity lowers from existing receivers, they work fine. Billet receivers are better because they generally have thicker sides and some have preformed trigger guard.

      You can also use clay to add heft to places where you want.

      The biggest issue with plastic receivers, isn’t so much plastic, but reinforcement of the receiver in the strategic places. Receivers brake in select few places. There is a thread about it on arfcom. Come and read.

  • 6ShotsOr5?

    Awesome. Everybody doesn’t want to make a mold though. If it is legal to sell the molds, I just want to buy one of those.

    • Boris

      There was a co in CA that made molds 3 years ago or so. They went for $150 but it looks like they boned their customers and stop making them. This summer they came back with price being $600. I can see that they list inventory as 0 for some time now. I’m not sure what the deal is.

      Making mold is easy and much cheaper.

    • noob

      I wonder if anybody on 3dhubs would be willing to do that for you?

  • lowell houser

    You might want to try a 3D printed lost-pattern metal casting. Can be done in your backyard with scrap aluminum. Even steel.

    • Boris

      probably not steel. People cast iron, which is a big step from doing aluminum, but I don’t know of any backyard metalcasters who can cast steel, like actual steel, not grey or white cast iron.

      In the industry, many things like props get cast with steel, but those are very expensive setups with colloidal suspension to cover wax and burn out preheat ovens. Not exactly your typical backyard foundry setup.

      One big issue with any type of casting are defects. It’s not as simple as just pouring molted metal in hole. Risers and sprues need to be design around the mold.

      • The Brigadier

        Casting? Heaven’s no. Get a block of structural aluminum and a CNC cutter and cut one out. Even the small ones can finish one in a couple of hours.

    • FP

      This can be done on the kitchen table, and my neighbours don’t wonder WTF I’m trying to do in my backyard.

      • crysys

        Sheit, I need to come back to the fold and hang out in the channel more often. Have you considered trying to get your hands on the brass inserts used by TAC for their lowers?

        • FP

          It might be nice to extend the life of threads, but it isn’t needed for strength. Some fiberglass cloth embedded in the plastic and it’s damn near unbreakable.

  • Boris

    you also need dies, those are very expensive and you need some experience handling the material as well.

    Mean arms make injection molded lower that’s rear aluminum reinforced, the usual breaking point.

  • Boris

    Also, one thing not mentioned, these are very light receivers. Mine was a 3rd of it’s aluminum original even with steel reinforcement piece.

    For states like MA where a pistol must be 50oz or less, this is the way to go.

  • patrickiv

    You’d need to CNC machine or purchase the dies, and you might as well get an FFL at that point.

  • Roy G Bunting

    I always assumed that the true value of 3d printing for receivers was to create a complex positive, that you then create a negative mold from, as above. You use that negative mold to create many wax positives that you use with more traditional Lost Wax sand casting, melted aluminum cans being your casting medium. After some cleanup, you now have a metal receiver. It doesn’t have to be an AR receiver, any design where the bolt locks into the barrel, would probably work well with a aluminum receiver. With lower pressure cartridges you could probably use this to create trunions or other designs where the barrel doesn’t lock to the bolt.

    Personally, casting looks like it could solve the handgun frame complexity problem for 0% homebuilds.

  • Tassiebush

    It’s understandable that this is mostly being discussed in context of lower reciever for AR15 but another area that this approach has significant promise for might be magazines. Odd shaped drum magazines that ordinarily would need complex stampings or milling come to mind.

    • The Brigadier

      Not hard to make your own magazines at all. A simple bender and some plate steel and you can easily bend one into shape. The bottom will have to cut out of the same steel. There is a guy who has drawn up plans for two sub-machine guns. The first is made from plate steel, and the other is made out of British plumbing parts. He makes his own magazines and the 40 round 9mm mags are very cool indeed. He calls the second gun a 9mm BPS (British Plumbing System). I see no reason why you can’t make one out of American plumbing parts. You’d have to convert from metric to the imperial system, but that should be no obstacle.

      • Tassiebush

        I think the fellow may be P A Luty may he R.I.P. Yeah those methods are pretty well established and certainly seem very accessible. I guess I’m thinking of cottage scale aftermarket magazines and novel designs. You know drums, helical that sort of thing.Maybe more options for rimfires too.

        • The Brigadier

          Yes it was Luty. I didn’t know he died. I have the plans for both of his designed 9mm sub machine guns. I love the bolt he made from washers. I was amazed at his ingenuity for making everything.

          Yeah drums will have to be pressed. If you can find them, you will probably save more then building them yourself. Even if you can find a suitable hydraulic press you still have to make the mold, and they have to be very heavy duty. That also means very expensive. Of course if you want to make thousands of them then the cost of the molds goes down for each one you sell. Let me know what you find out. Try big cities and look for metal fabricators. They will be your best bet for having a press and be willing to contract with you.

          • Tassiebush

            Sorry mate I meant I was thinking of it more in the hypothetical sense than the doing sense. I’m a gunna as in I’m gunna get round to that eventually someday. I live in Australia too so options for big magazines are pretty limited and the legalities of diy even of lawful mag types for personal use are a tad perilous :(. Not to say that one couldn’t get the dealers licence and experiment away though.
            BTW I didn’t realize that Luty designed a 40 round mag!
            Getting back to this technology in the article it seems to have the merit of not requiring sophisticated equipment to use it. It’s basically mix and pour to make the mold and the object once the pattern piece is made.
            Like your ideas BTW!

          • The Brigadier

            I think I read that Tasmania has more lenient gun laws than Australia and NZ. If that’s true you might want to vacation there and see if you can find someone to help you with it.

          • Tassiebush

            Tasmania (my home) used to be but it’s been much the same as the rest of Australia since 1996 due to our national gun laws agreement. New Zealand certainly enjoys more gun related freedoms but they have some restrictions. They just seem far more shooter friendly and they’re a hunters paradise from what I can see.

  • Giolli Joker

    Rather than with AR lowers, this could be great to have customized frames on new modular pistols with serialized metallic chassis as Sig P320 or Beretta APX, Nano and Pico.
    Some experiments in strengthening the polymers by loading them with fibres (glass, carbon, kevlar) could be interesting and the industry practice could suggest the best recipes.

  • MrEllis

    How much does a lower cost of comparable quality or better?

  • Joshua

    One day, when things like LSAT replace standard cartridge designs and 3D printers hit their prime. When they can make sturdy and durable polymers into real physical items that are just as strong as modern polymer items, then the sky will be the limit.

    At that point a large majority of the LSAT weapon would be made from polymer. If the polymer LSAT case is strong enough to absorb most of the pressure you can then design a firearm out of polymer.

    It’s coming, not now but give it 10-20 years and just wait.

    • Joel Thompson

      What about carbon fiber? Right now we can print the hollow internal structure of a part, then cover it in a composite material like carbon fiber. We can then dissolve out the 3D printed support filament with water, and leave us with a single strong part without any seams, and an intact internal matrix.

      Or how about investment casting? Print a part in FDM, cover it in a ceramic cast, harden it. Burn/melt out the 3d printed part, and you now have a sacrificial mold to cast a metal part into.

      Don’t discount the ability to print custom jigs, fixtures, and drill guides that remove all the guess work, measuring, and human error out of machining a part with simple power tools, like a drill press and router.

      You are thinking 2 dimensionally by considering 3D parts as the final end user part, and not as a manufacturing tool to create cheaper, better parts, in a more timely fashion, and with less technical knowledge and skill.

      • The Brigadier


        Carbon fiber has to be coated with Urethane to be strong. Even graphene that makes carbon fiber look like uncooked spaghetti needs a film of urethane to bond two layers. Each layer is one mm thick and to make a foot wide thick plate of graphene will require a lot of layers. That adds weight. Furthermore, how these carbon forms handle heat is not too well known. Carbon fiber yes, but graphene no. The stuff is very expensive right now, about $200 an ounce, but a grad student at UTEP just discovered how to make it from used oil for about 4 cents an ounce. As a lower it will be fantastic and nigh unbreakable. As an upper probably not so bueno. Metals are still the king until we can make a polymer than take the heat and the pressure. We might never figure it out.

  • nova3930

    Wonder how well fiberglass or carbon fiber would integrate into the liquid polymer as a reinforcing medium..

  • Phil Hsueh

    A better option would be a pressure pot(?) for your molds, this helps to eliminate any bubbles in your castings. I know a number of guys who make their own props at home and they always mention either getting or using a pressure pot for clean, bubble free castings. I don’t know what this would do for gun castings but no bubble holes can’t be a bad thing.

  • Tassiebush

    I really like comments like this. They are really good for folks like myself who wouldn’t have a clue such innovations were occurring otherwise!

  • Scars-2-Prove-It

    With all this DIY stuff going on–using these newfangled plastic-ma-bobs and all–I can report my own findings:

    Play-Doh does NOT work!

  • The Brigadier

    Or you can get a relatively cheap CnC cutter and a block of 6061 or 7075 aluminum. It will require you get a laser scanner, an original high quality lower to scan and a block of nylon. First you scan the lower into your computer and once its done with the CNC software, you place the cutters in the CnC machine and position the nylon block. Then the cutters make the lower in nylon. That will give you a permanent source in case your software model gets corrupted.

    Once the nylon is made and you verify its true, then you position the aluminum block and install the metal cutting blades and let it make your lower. You can do the same with the the upper, but a word of caution here. You can make one of each for own personal use, but if you make them for sale even for friends, you are subject to massive fines and a long prison sentence. To make them for sale, you must have a firearm manufacturer’s licence. The ATF will come after with a vengeance and most judges will not be merciful. You have been warned.

    Now that we have unpleasantness out of the way, why you want to deal with noxious chemicals to make an inferior lower? No way is it going to be stronger than the two types of structural aluminum and its a hell of lot more work to boot. Lowers are so cheap anyway, just buy one and save yourself a boatload of money unless you just gotta have the satisfaction of making your own.