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Are you in love with 3D printing, but tired of flimsy plastic parts? Are you ready to take your work to the next level?! So was I.

While studying Electrical and Explosives Engineering at New Mexico Tech, I learned a technique called lost wax casting, which I used to make a nice bronze necklace pendant for my wife. But carving machinable wax with a CNC milling machine was too much work, so I began experimenting with lost polymer casting. It's basically the same thing as lost wax casting, except that it allows you to make metal parts directly with your 3D printer!

I was absolutely amazed at how easy it was to do, and how well it works. I can't believe that this process isn't common knowledge within the maker community! It opens up all sorts of possibilities for home manufacture of durable, mechanical objects. The weaponeer community will be especially interested in this. Virtually everyone agrees that plastic guns were a horrible idea. With lost polymer casting, they don't have to be plastic anymore.

This 12-step process I've developed is by no means an exhaustive treatise on the subject, but it should provide a great jumping off point for your own experimentation. In my assessment, lost polymer casting is literally the greatest thing since 3D printing itself. I can't wait to see what you do with it!

Step 1: Design Object

This isn't a CAD tutorial (there are plenty of those), but I recommend Trimble SketchUp for its price tag (free) and ease of use. OpenSCAD is also free, but is perhaps a bit harder to use (unless you're a programmer like me, in which case, you'll love it!)

Step 2: Add Sprue

This is basically just a conical piece that attaches to your part somewhere. Length, width, and flare dimensions don't have to be precise, but it should be large enough in diameter to accommodate pouring liquid metal through.

Sometimes, with larger objects, it can be beneficial to add more than one sprue, or air vents through which air can escape from the mold as the metal is poured. If a pocket of air is allowed to form during the casting process, the result will be a hole in your part!

Step 3: Enlarge by 2%

Scaling the part by 2% along all axes will create a volume increase of about 6% (1.02 ^3 = ~1.06). This compensates for the shrinkage the metal part will undergo as the metal cools from ~230C to room temperature.

Step 4: Print Object W/ Sprue

If you're experienced with 3D printing, this step should be fairly straightforward. In most cases, your sprue will probably be facing upward, but I've had success in the past with laying it horizontally atop the build platform. If you need to print your sprue horizontally, you may want to design it as a rectangular prismatic frustum rather than a cone.

Step 5: Embed in Plaster With Sprue Sticking Out

Make sure the object is fully submerged, but not touching the bottom or sides of the vessel. Also make sure that at least a small piece of the sprue is sticking out of the plaster. You may want to rig something to hold it in place while it dries.

Step 6: Let Plaster Harden and Dry

Ideally, you would wait at least 24 hours prior to doing the burn out. This can be sped up somewhat by using techniques such as the above: sun drying (Fig. 1), and accelerated heat drying (Fig. 2). If you decide to accelerate the drying process by applying heat, take care not to apply too much heat. Doing so can flash trapped moisture to steam, causing the plaster to swell and crack.

Only proceed to the next step once the plaster has fully dried!

Step 7: Burn Out Polymer

While wearing flame-resistant gloves (oven gloves are great), carefully place the mold upside-down in a fire. I usually cook it for an hour or more to get all the polymer out.

If your part was printed in PLA, the resulting fumes shouldn't be too toxic.

If your part was printed in ABS, it will smell horrible, and if you breathe in too much of the smoke, you will die.

Step 8: Pour Metal Into Sprue Hole

Experience has taught me that you only get one shot at this. Do not attempt to pour the metal back out of the mold! Once the metal is poured, you're done.

For this example, I used pewter (an alloy of tin), heated with a Hot Pot 2 electric melting pot. I would recommend them. Last I checked, they were something like $40 on Amazon.

Safety goggles, long pants, closed-toe shoes, and thermal gloves (e.g. oven gloves) are strongly recommended for this step.

Any metal that spills will harden into a coin-like object, and can be easily recycled.

Step 9: Wait for Metal to Cool

I recommend waiting 2 or 3 hours, or until the metal is cold to the touch. Remember that plaster is an excellent thermal insulator, so it will keep the metal hot for much longer than you might expect. Smash it out too early, and your part will look like somebody stomped on a gummy bear.

Step 10: Smash Plaster; Remove Metal Part

This is the fun part. It shouldn't require much explanation. If your part is fragile, you might consider whacking it many times softly (from different angles), as opposed to one big smash.

Step 11: Cut Off Sprue With Hacksaw

Saw the sprue off as close to the model as you can without accidentally cutting into the model itself. Soft metals like pewter shouldn't take more than a couple minutes with a sharp hacksaw blade.

Step 12: File Sprue Attachment Point

Optionally, you can also file off all the ugly gold oxidation, revealing the smooth, shiny metal beneath. Just don't get carried away, or you'll find there's nothing left of the part you just made!

Use your newfound metal manufacturing skills only for good. If you manufacture weapons, use them only to defend the innocent from harm. Also, stay on the right side of the law. The ability to manufacture metal parts at home puts you on the same playing field as companies like Glock, Smith & Wesson, and Ruger. But remember that these companies have entire armies of lawyers to keep them out of trouble. It is much easier for an individual hobbyist to trip up and do something verboten without even knowing it, so be careful.

If you enjoy my work, please subscribe to my YouTube channel, and I promise to keep it coming!

Cheers,
~ Peter

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<p>I'm sorry, but no.</p><p><em>&quot;The weaponeer community will be especially interested in this. Virtually everyone agrees that plastic guns were a horrible idea. With lost polymer casting, they don't have to be plastic anymore.&quot;</em></p><p>Plastic guns were a bad idea, but not because they were flimsy. You should not encourage 3D printing metal guns, that could be used to harm people. Shame on you.</p><p>Otherwise, wonderful idea and instrucatble.</p>
<p>Shame on me for what? For being an amateur gunsmith?? It's fun - you should try it! I'm not encouraging anyone to harm anyone here. I'm encouraging experimentation. I don't know where you're located, but here in America, everyone owns guns. It's considered normal. Like the 99.999% of Muslims who aren't terrorists, 99.999% of gun owners aren't psycho killers. It's really unfair to say that I've done anything wrong by encouraging my fellow weaponeering hobbyists. I'm glad you liked the instructable otherwise. Cheers.</p>
<p>It is still legal for an amateur gunsmith to make a complete weapon, barrel and all, so long as it is NOT for fullly automatic operation, as stated above. (With some exceptions)<br><br>Until they repeal the Second Amendment, If you;re anti gun, then don;t buy or own one. Don't try and infringe on my right to own my own. The author, <a href="https://www.instructables.com/member/lyratron">lyratron</a>, kept it legal.</p>
<p>Amen and kudos for showing a good method for repairing or replacing parts that may not be available any longer. Try finding a sliding block for the trigger assembly for an old (pre-1900) revolver. They ain't out there...</p>
<p>I am a retired police officer, and there is nothing illegal about making<br> a replacement part for a weapon, so long as said part does not vary <br>from the original design AND that it is not for a full auto weapon, or <br>for modifying a weapon to fully automatic operation. These restrictions <br>are not applicable to those who have a Class 3 FFL, or a legal document<br> from ATF called a Stamp, allowing manufacture of a replacement seer <br>for full auto.</p>
<p>EXCELLENT!<br><br>Question: Is there any estimable percentage difference due to plastic shrink or plaster shrink or expansion? Do the part(s) end up the exact dimensions you drew them in CAD?</p>
<p>Encouraging video! Thanks. It looks like you are using the Hot Pot 2 to melt aluminum. I thought that only got hot enough to melt lead (big temperature difference). Can you confirm I saw what I thought I saw? Other thing's i've heard. Don't use plaster directly but add 50% sand. Glad to see this work. I've been researching sand casting but this is a heck of a lot less hassle. </p>
<p>Very useful instructable and excuse me if someone else has suggested this already, but I did not read all the comments. Instead of burning out the plastic, couldn't you use the water soluble material (PVA, I think) when creating the 3D print and then just wash out the original? Or is the heating process also intended to bake the plaster?</p>
<p>that's an excellent suggestion! no, i don't recall anyone suggesting that before. what a great idea! i don't have any PVA on hand, but if i ever get some, i'll be sure to try it. melting out the plastic usually isn't the problem. PLA melts out easy. ABS is trickier but still works. what you suggest may or may not work, because the plaster might not like being rehydrated. you still need to use molten metal, though, which will result in part shrinkage as the object cools. thanks for the idea! cheers, ~P</p>
<p>P.S.: oh yeah; the plaster is still wet when you pour it, meaning the moisture might start degrading the PVA immediately... not sure how that works, as i've never used PVA. worth trying, though!</p>
<p>Do you get bubles in the plaster mold when you pour the molten metal or when using different types of metals?</p>
<p>Bubbles usually form during the mixing phase. You should try to get them out before the plaster dries, or it will be too late. You can use an electric (vibrating) toothbrush, a back massager; whatever is handy. :-)</p>
<p>Thanks but i meant bubles in the metal peice that you cast after pouring</p>
Nice idea and love your writing!
<p>Thank you! I aim to entertain. ;-)</p>
<p>a fantastic, simple idea that dramatically extends the utility of the more inexpensive 3D printers available today. Bravo!</p><p> Amazing how the best ideas look obvious in retrospect.</p>
<p>Thank you! I was actually reluctant to publish this, because I figured everybody must already be doing this. Then I remembered that I thought the same thing about my Wifi Hacking method, and I was surprised that almost nobody knew about it. It makes me wonder how many good ideas get cast aside every day because people just assume they're already common knowledge. I'm learning to just follow my intuition and pay no attention to what others are doing. ;-)</p>
<p>This looks like a sear and I'm curious as to what it is designed to work with.</p><p>You should research the shrinkage of the metal you intend to use and scale your part accordingly. Many casting metals shrink more or less than the suggested amount above. </p><p>I also suggest submerging your part slowly, and with a point facing down so you don't trap bubbles against your part.</p><p>Thank you for the 'ible. It certainly shows that there is more than one way to cast a metal object that is easily accessable!</p>
<p>Quite right: it's an updated version of the trigger from my dart pistol: <a href="http://www.peteralaric.com/DartPistol/" rel="nofollow">http://www.peteralaric.com/DartPistol/</a></p><p>Although it's actually intended for a completely new pistol design... I would like to say more, but for now I'll just say that if you're really curious, subscribe to my YouTube channel and you'll see. ;-)</p><p>Thanks for the kind words!</p>
<p>I found a great filament that burns out even better :-) </p><p>It is a fillament wax...</p><p><a href="http://www.machinablewax.com/product.php?product=52" rel="nofollow">http://www.machinablewax.com/product.php?product=5...</a></p>
<p>Ha! I actually used machinable wax from that company to make that necklace pendant in Spring 2013. Didn't know they were making 3D printer filament now, but I guess it doesn't surprise me. From my experience though, PLA works just fine, and if you don't mind the odor, ABS can work well too. The part I made for this Instructable used ABS. Cheers!</p>
so simple yet excellent
<p>Thanks! :-)</p>
This is one of the coolest ideas ive seen! Im so excited to actually try this now.
<p>Thank you! It really is quite amazing how well this process works. Good luck with your experiments, and feel free to drop me a line! :-)</p>

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Bio: Lyratron was founded in August of 2011 after four years of research into the development of gesture-based electronic musical instruments. Originally called Light Harp Industries ... More »
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