Introduction: JB Weld Casting

Picture of JB Weld Casting

If you need a small part and you have access to the original then use some JB weld to get 'er done.

JB weld did the trick on a missing piece for my airsoft gun.

UPDATE - Make sure you check out all the awesome comments about how to go even further and reinforce the epoxy.

Step 1: Playing With Playdough

Picture of Playing With Playdough

Take some kind of casting medium, I stole some playdough from my son. I think this would work even better if you used modeling clay or something made specifically for such an application. Who knows.

Step 2: Your Part

Picture of Your Part

Here was the part that I wanted to copy. It is the selector switch off of an airsoft gun. I'm sure you can buy these parts but I didn't feel like paying $10 + $10 shipping on ebay for such a silly part. Luckily I had a spare part from a buddy's gun that I could use to make the copy. Notice that there are some pretty finely machined areas of this part that were essential to fitting on the gun.

Step 3:

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This stuff isn't rocket science so I'm sure you could probably figure it out on your own. I just wanted to prove that it does in fact work. Or maybe you hadn't thought of it.

One trick I did learn was that for the really deep parts that it helped to fill them with the clay so that it would then adhere to the rest of the mold when you remove.

Also, try to make sure the top of your mold is even with the top of your piece to avoid too much excess.

Step 4: Mix It Up

Picture of Mix It Up

Mix up your jb weld and dump it into the mold. I had the quick setting type laying around. It would probably work better to use the slow setting, just leave it in the mold longer until it is hard. The original recipe is supposed to setup harder than the kwik version.

Step 5: Final Result

Picture of Final Result

Here you can see the final result. It turned out fairly well and most importantly it is fully functional. I can't wait to need to duplicate other things because I am really impressed with how it turned out. JB weld really does turn out pretty hard and durable.


PKM (author)2009-03-17

If only I'd known this when those igganunt fools in the drama department broke my prized Beretta all those years ago... do you know how strong the finished product is? I guess your switch isn't going to be under great mechanical strain, but could you cast a new trigger out of it? Or some part that takes a lot of local pressure?

jammin0 (author)PKM2009-03-17

I'm sure you could use this for a trigger as well. I used the quick setting stuff and it is probably about as hard as aluminum. If I gouge it with a screwdriver it will dent and scratch. However, I've heard that the regular JB weld is considerably stronger. Or do like kenbo suggests and add something. I'm thinking like metal shavings or something really fine. Dremel up some metal so that you get a fine metal powder to add.

sarkozykal (author)jammin02017-01-05

I'd suggest using one of the additives from West System Epoxy ( They're experts at this sort of thing. Offhand, it seems either West System 404 High Density Filler ( or West System 406 Colloidal Silica ( would do the job.

maruawe (author)jammin02009-03-17

You can take aluminum foil and grind it in a coffee grinder, it will take a while to get it to a fine powder , I needed aluminum powder to make thermite and this worked quite well. cut or tear foil into strips for best results Grind and regrind several times to get a 400 grit on the aluminum .

imthatguy1125 (author)maruawe2009-03-18

whats thermite

maruawe (author)imthatguy11252009-03-19

Thermite is a combination of aluminum powder and iron oxide usually a one to two mixture that will burn through most anything. It is dangerous to use , i have a hole in my patio as evidence , before using thermite do a lot of research.

Pazzerz (author)maruawe2009-04-08

Don't pressure contain it either. Makes a devastating bomb and throws incendiary particles everywhere that will burn through your hand, etc.

maruawe (author)Pazzerz2009-04-09

What do you mean by pressure contain it .I have been working with thermite for over a year and never ever thought about pressuring it. Are you referring to a powder compression in an explosive or air compression.. Please clarify

Pazzerz (author)maruawe2009-04-09

What's to clarify?? I suppose I should have added the extra ingredients on the last comment, but I really don't want to blatantly tell everyone how to make a bomb. Obviously you are working with it in the way that it is supposed to be worked.

maruawe (author)Pazzerz2009-04-10

Bombs are good in their place, But the components that make bombs are useful in many ways and I kinda like to have me around in case I need me. So I am very careful about how I use chemicals......having had the training in explosives that I have been fortunate to get through out my life give me the knowledge to work with chemicals . My motto is: if you don't know ask questions till you do know and then be real careful. (See above statement).
research is the key to longevity.........

imthatguy1125 (author)maruawe2009-03-19

where can i get iron oxide

PKM (author)imthatguy11252009-03-19

Leave some steel wool half submerged in slightly salty water, stir it every couple of days to break it up slightly. Add more steel wool when you can't see much shiny in the brown powder, and when you have enough rinse the salt out and crush with a pestle and mortar. You can then get residual iron out with a magnet.

Vermin (author)PKM2009-03-26

A quicker way to oxidise steel wool into Iron IIIOxide is by using Clorox bleach (or any bleach that contains NaOCl).

maruawe (author)Vermin2009-03-26

Thanks I'll try that .The way i do it is very time consuming

Stuff that burns at about 4,000 degrees Fahrenheit and was used to do welding until recently. They still use it to weld railroad tracks and large metal parts. The military uses it as an antitank weapon and for busting up locks (with the help of a little explosives). It is composed of Aluminum (400+ mesh) and iron oxide (rust). For every gram of aluminum you need 2 grams of iron oxide. You don't need to do anything other than mix these two powders.

maruawe (author)curecreator2009-03-19

In the tutorial he mentioned that you could mix in aluminum powder to strengthen the piece that you were molding . rather than buy the powder I usually make my own, true that when mixed with iron oxide it will burn,but you have to use magnesium to get it ignited or a very hot igniter. Thermite does not self ignite..

dll932 (author)maruawe2009-03-26

You can GET aluminum powder in a bottle-can't remember where, but my father used to use it to add to paint for painting signs.

maruawe (author)dll9322009-03-27

Yes you can, But that takes all the fun out of it . Skylighter has it on their website

rich_moe (author)jammin02009-03-19

I would advise against using any type of fiber or metal compound that can cause irratation or health/skin problems unles you seal the material REALLY well. Most of the strength from using composite structure (dry material mixed with a bonding agent) use the composite for strength. The less bonding agent used, the more strength the composite structure has. The same works for fiberglass cloth and resin. You want just enough to saturate the fibers, and not much more (somewhere in the neighborhood of 3%-5%). Some JBWeld components by themselves have metal shavings in them already; using more defeats the strength of the casting. I have used JBWeld with fiberglass cloth and a little heat to make the JBWeld flow better, and WOW! But, Caveat Emptor; too much heat and it flashes too quickly and does not 'wet' the material sufficiently to make the bond. The larger the component, the faster the epoxy hardens (the bonding agent creates heat in the process of curing, the laws of thermodynamics apply; more material, more heat, faster cure).

trebuchet03 (author)rich_moe2009-03-20

The less bonding agent used, the more strength the composite structure has. The same works for fiberglass cloth and resin. You want just enough to saturate the fibers, and not much more (somewhere in the neighborhood of 3%-5%).

o.0 That's more of 30-60% resin ratio. Depending on weave, of course... However, more resin will not reduce the strength of the structure - it will simply add weight and lower the strength to weight ratio. A factor sometimes overlooked is selecting the right binder. Ideally, your resin is weaker than your fiber such that the fiber carries the load and the resin acts to maintain geometry. If your resin is stronger than your fiber, the resin caries the load and you have interesting and less predictable rupture characteristics.

Lately, I've been doing a significant amount of empirical testing for a research report primarily on the use of composites in a vehicle (currently under construction). The sample in that picture reached over 800 psi of stress (<.1 strain) before it started getting pushed into the test rig (as seen). What's interesting is that this strength didn't come from the composite as much as it came from the relatively weak core working with the composite. I know it's a bit off on a tangent... But back on subject - this sample was laid up with approximately 50:50 resin to cloth by weight (probably a little more resin). Processing squeezed out a little resin making that ratio biased towards the cloth weight, but nowhere near 5% resin by weight. I could have poured more resin on and just let it cure without any squeeze, but that wouldn't make it weaker (I've got those samples too :p).

Now what I thought was cool (on another tangent), is that the sample recovered almost completely after about an hour. Damaged? Yes, but the geometry was mostly restored :)

Pazzerz (author)trebuchet032009-04-08

Who makes your testing apparatus? MTS?

trebuchet03 (author)Pazzerz2009-04-08

I think it's an MTS... It's either an MTS or Instron.

Pazzerz (author)trebuchet032009-04-08

I used to work for MTS in its nano division at Oak Ridge, TN. By the way: LOVED the apple pie.

rich_moe (author)trebuchet032009-03-25

Sorry, I should have made myself clearer; when I said 3%-5%, I meant by volume (i.e.- volume of the composite material, equal volume of resin/catylist +3%-5%). Most structural engineers (at least those I have met) do measure everything by weight. I apologize for any confusion.

ClayOgre (author)jammin02009-03-17

From the J B weld FAQ

Q: What is the tensile strength of J-B Weld and J-B Kwik?

A: J-B Weld has a tensile strength of 3,960 psi. J-B Kwik has a tensile strength of 2,424 psi. For more physical properties information, please see the "PRODUCTS" section of this website.

jtobako (author)ClayOgre2009-03-17

cast iron = 130 MPa = 18,000 psi
plastics = 12-43 MPa = 1,600-6,000 psi

aluminum bend = 15 MPa = 2,100 psi
break = 40 MPa = 6,000 psi

Powder isn't going to help, strands would. So mixing in steel wool would be better than aluminum powder : ) Think fiberglass or carbon fiber.

dll932 (author)jtobako2009-03-26

That's what I was thinking-you can get carbon tow (loose strands of carbon fiber) and use that to reinforce, but of course wearing rubber gloves is a must, so as not to get the splinters from it in you. Also, it must be fully contained in whatever results-quite an irritant. I believe you can get it from

Pazzerz (author)jtobako2009-03-18


Pazzerz (author)PKM2009-03-18

Strength? its been used to patch up engine blocks.

tvazquez2 (author)2015-11-05

JB Weld is Da Bomb! :D I normally use the steel version for repairs, but then I tried it for sculpting a small cockpit dashboard and a pilot seat to go with it. I used the 5-minute quick-set steel epoxy... I really enjoyed the challenge of "speed sculpting" for the objects as I had to get their shapes and details precise before the epoxy hardened. The pilot was made out of casting resin, and it measures 1 + 3/8 inches tall; so I had to make the seat (pic #1) and cockpit dashboard (pic #2) to scale with the pilot. The ship's body was made with Durham's water putty; after pressing the epoxy steel dashboard in place to get its shape I then used a small flat-head screwdriver to sculpt the detailed dashboard "panels". I still haven't finished the project since five years ago, I never thought of making any tutorials for it (I regret not having done so)...

llapdog (author)2014-10-22

I've been wondering about using JB weld to make a old-looking coin as a prop for a costume. My only worry is I don't how much it weighs. Could it be used to make a coin similar in weight to one made from iron?

DanTDM (author)2014-04-25

You can cast custom pipe fittings (like 4 inch to 1/8 inch bushings)

handyhippie65 (author)2009-05-25

this stuff is the best! i just repaired the handle of a 150.00$ goat milker with it, it works great, and only cost $5.99. i've used it to fix auto interior plastic panels, even filling in pitting on aluminum intake manifolds. if you want more strength, embed finishing nails.

meanbean (author)handyhippie652010-08-22

Goat milker?

FletchINKy (author)meanbean2013-09-16

Milks goats, I reckon.

handyhippie65 (author)meanbean2010-08-22

a plastic, hand operated vacuum pump, with a milk container, and a teat coupler, to milk small numbers of goats. kind of like a one man brake bleeder pump. squeeze the handle to create a vacuum, which draws milk from the teat to the container, through a tube. the handle broke at the pivot . easy fix for an expensive piece of equipment.

808create (author)2009-05-17

You could use JB in both steps, instead of PlayDoh, to get a better mold. Let it dry and spray with WD-40 to keep it from bonding.

wizard31 (author)808create2013-09-14

Using JB as the mold would be a great option, easily modified and mold could be tagged and put away for doing same mold again.

Capt. Kidd (author)2011-05-01

The only problem is, it makes a mirror copy of the original piece instead of an exact copy. I guess you could cast the copy to get the same thing as the original though. It just takes more JB Weld.

chocables (author)Capt. Kidd2011-05-12

The playdough makes an inverted copy and the JB weld then inverts it again so it becomes the original.

Capt. Kidd (author)chocables2011-07-06

hmm... right. DERP

jammin0 (author)Capt. Kidd2011-05-01

umm, it doesn't create a mirrored copy. That doesn't make any sense.

airsoftbeast40 (author)2011-04-20

YOUR A GENIUS!!! i serioously wouldve never thought that would work with an mp5 selector switch, cuz theyre so intricate

the mechanical engineer (author)2011-02-27

dude i believe i have that same airsoft gun. its firepower brand mp5 right?

syco123 (author)2010-02-09

I never go long distance hiking without JB weld. It's a lifesaver and worth it's weight, literally! I like this idea of casting with it. Hadn't thought of that.

ERCCRE123 (author)2009-03-18

is that an MP5?

Clayton H. (author)ERCCRE1232009-03-18

I think it might be a G3.

acidex (author)Clayton H.2009-08-13

Nope, its an MP5. G3's have a very different upper receiver. They do look similar though, because they are both originally made by H&K;.

jammin0 (author)ERCCRE1232009-03-18

You guessed it. Tokyo Marui

ERCCRE123 (author)jammin02009-03-19

My brother used to have one and the selector switch looked exacly the same

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