Nerf Rocket Injection Mold

[pyronaught]

For Christmas I got my kid one of those Nerf dart guns that has the single pump rocket launcher built into it. It only came with two rockets and it only took my two year old about three weeks to find and destroy them. So I thought I would try and make a mold for creating unlimited Nerf rockets

I haven't actually tried this out yet because I'm still trying to figure out the best foam to use. Some of that two-part foam on the market would cost more than just buying new Nerf darts, so ideally I need something cheap like Great Stuff to make this cost effective. The foam would be injected into the hole in the top, or sprayed into each half before closing it. The cylindrical mold is then held shut with pipe clamps until it sets. I believe the trick with Great Stuff is that it needs moisture in order to cure, so either the mold release must be water based or a liner of wet tissue paper must be used. The rocket is hollow up the middle where it slides onto a spindle in the Nerf gun, which is what the removable pin is for. The pin has a collar that fits into a slot at the bottom, holding it in place during the casting. Then it is used to help remove the finish rocket. The real nerf rockets do not include the fins in the mold, but rather the fins are glued on afterwards. I've actually included the fins in this mold though.

nerf_bomb.jpg (385.41 KiB) Viewed 11167 times

[benavery]

How toxic is that insulation foam stuff ? If your two-year-old is anything like mine, anything soft and foamy and bitable will ultimately end in their mouth, most likely with chunks bitten into it before you catch them. I found this: http://energy.ces.ncsu.edu/wp-content/u ... safety.pdf that claims

Once cured, GREAT STUFF™ foam forms a nonhazardous
solid and is nontoxic if ingested by pets. However, it could
block the intestines. If signs of distress are exhibited, seek
medical or veterinary care

So maybe its safe (enough).

Though earlier it claims:

GREAT STUFF™ Insulating Foam Sealants
contain dangerous chemicals and should not be
used by or around children.

But I assume that is when in un-cured form.

[pyronaught]

I thought about dipping the cured foam rocket in Plasti-Dip just to put a better skin on it. That might make it hard to bite a piece off too. My kid is getting past the stage of putting stuff in his mouth, but he does like to tear stuff up and will use his teeth as a tool if necessary. I think that's how he tore up the original Nerf rockets.

[jimc]

the great stuff is a polyurethane so once its 100% cured its ok. it damn sure doesnt kill the mice around here. you can close up that mold and just inject it in the end of the mold with that tube they supply with the can. you dont need water with it. it will cure in a deep hole like that no problem. just let it sit for 24hrs and you should be fine. you only need to fill it half way. it will expand to eventually fill the entire mold. that is a hard foam when its done. nothing like nerf like. the other thing is be sure you really mold release the crap out of that thing. lots of mold release wax or pray a couple good coats of pva in it. yes the same as the filament we use . polyurethane foam is extremely sticky so if it only bonds slightly youll never get the rocket out without breaking it. you can get soft spongy foams from smooth on. you only need the smallest size available if you go with a real light density and youll be able to do a bunch of them. just a thought.

[pyronaught]

I saw the smooth-on stuff but it seemed expensive for the quantity you get. I guess it all comes down to how many rockets a given amount will make.

I was going to use Vaseline as the mold release. I've heard WD-40 works too, but Vaseline can be applied in a thicker layer so I was going to just go with that as long as it doesn't damage the plastic. Apparently hair spray works too, as unintuitive as that sounds. It might have to be used over top of wax though, not sure. I'll have to test on some scrap ABS to find what works before using the real mold. It takes 10 hours to print that thing, I don't want to make another one.

I figured the DIY Nerf rockets would be harder than the original, but this is for serious Nerf warfare. I was thinking about embedding a weight in the tip also to see if I could get a longer range than the original rockets.

[jsc]

You can get 3 lbs of Smooth-On FlexFoam-iT!® III for $28. It claims a 15-18x expansion (depending on which part of the web page you're looking at), and says it comes in at 400-450 cu.in./lb. For 3 lbs, that's a cube a yard on each side. Based on your photo, that's... a lot of rockets.

One suggestion on your mold: from my previous attempts at mold making, I've found it very handy to model in registration keys with .2mm clearance on the holes. It is super annoying to try to clamp two smooth faces together and keep everything aligned.

[jsc]

Also, try a google search on "casting with great stuff foam". There's some youtube videos, some RPF forum links, good stuff. No, great stuff!

[jimc]

lmao....serious nerf warfare. man that sounds like my kid all the way! i can confirm that vaseline does work.

[jimc]

something i just noticed with your mold. your fins being attached. they enter or should i say exit the bomb body at a 45 deg angle. how do you plan on removing the bomb using a rigid foam like great stuff without breaking the fins? the one on the left of the pic will be a problem

[pyronaught]

something i just noticed with your mold. your fins being attached. they enter or should i say exit the bomb body at a 45 deg angle. how do you plan on removing the bomb using a rigid foam like great stuff without breaking the fins? the one on the left of the pic will be a problem

There's three fins, so yeah on the one mold two of them are angled such that if the foam is not flexible then they will break off. In the videos I've seen the Great Stuff appears to be flexible, but my own personal experience with it is that it is not. It might bend a little but then it just snaps. There's some other copy-cat foams on the market though, maybe one of them is flexible. It looks like the smooth-on foam is flexible, so I can use that if Great Stuff doesn't work out. If the fins become a big PITA I can always just fill the slots with something and glue them on after casting like what Nerf does. Another way would be to make it a four fin design, with two of the fins split across the mold line and the other two being a straight lift out of the mold.