So I started this project about a year, mess up some steps and put it aside, and recently decided to actually finish it. Now I loved Arkham Asylum when it came out, and from looking at a bunch of other peoples work on the internet, figured I could make my own Batarang. I didn't take photos originally so I had to 'recreate' some similar steps on some scrap wood, but you should be able to make your own at the end of this.

I used a bunch of materials for this project, which I'll list here (note some materials are my countries version of Smoothon/Bondo/etc):

5mm thick balsa wood from a craft store. You can also use any thin wood (plywood, chipboard, etc)
Car Filler and Spak Filler to fill in the wood grains/imperfections.
Spray Paints (Black), primer filler or spray putty to fill in pin hole bubbles, silver acrylic paint and some paint brushes
Thin pole/pipe for the hinge. I used the plastic from a Cotton tip ear cleaner, was thin/small enough for me.
Dremel (Drill bit, sanding drum, cutting tool), in order to cut out the design, drill in holes and sand down the blades edge.
Molding supplies. I use something called PinkySil, basically the same as SmoothOn Rebound. It's non-stick so you don't need a release agent for casts, but you will need one for the 2-part mold (I used petroleum jelly) so it doesn't stick to itself.
Casting Supplies. I use a 2 part resin called EasyCast, mixes in equal volume and can be tinted (normally cures white). Equivalent is SmoothOn 300.
Sandpaper. I used a variety of grits for this, but usually 80, 180, 240 and 400. I also wetsanded at 800 and 1500 before molding it.
Clay of some kind. I sue Monster Maker clay to use in the 2 part mold, but any clay should do.

Step 1: Design and Rough Cut

So I kinda cheated here, there are multiple other guides to make this particular replica online and I basically took a side on image of a couple and made a stencil/outline of one side of it. I traced it onto the wood and it is about 19cm from tip to the middle corner, and poke small dots where the holes will eventually be. Cut out a rough shape using a dremel or a hacksaw and start to cut out the stencil just outside of the traced line, this is so you can sand out the blades edge. Swap to a sanding drum and lightly sand along the edge on an angle, making multiple passes until you're happy with the angle. Flip it over and do the same until you get yourself a nice even edge. Also with the drum sand the rest of the outline until it just touches the traced lines.

Now I was afraid to split the wood with a full size drill bit, so I used the small drill bit of the dremel to do small guide holes in 5 places, and then used a much thicker bit to make the holes larger. For this version make sure you leave the middle-most hole small, as it will be filled in later as a hinge. Once all the cutting it done you can do a light sanding of everything (edges, faces, and the holes), I did a pass at 180 and 240 grits. You'll notice there is a woodgrain along the faces of the batarang, so I used some car filler to fill it in (just mix it according to it's instructions and smear on thin layers), let it cure and sand it back. I did a couple of passes and sprayed it with a layer of filler primer so I could see the smaller imperfections. I use some quick dry SpakFiller (very lightweight filler) on it again, sanded that back and used the primer again.

Step 2: Refinements and Materials for Molding/Casting

So here is where I originally messed up. The white batarang is the one made from balsa wood, and you can see it was not done as well as it could be, with noticeable wood grain, small half-holes and a kinda broken tip. What I did originally was mold and cast that copy, fixed up that cast and used that as the 'master' for the final molding process. If you do it properly the first time with the wood version you can skip that step and save some money (I wasted so much resin doing it improperly -_-). The grey one above is the 'master' I used for the next step.

So you can see that it is all smooth and ready to mold above, but it's not yet complete. I cut the plastic part of a Q-tip out and slid it into the small hole to act as a hinge part, and held it in place with some clay. Now its ready.

The materials I used for the next step are shown above, but you should research your countries version of them. The PinkySil Silicone is used for the mold, petroleum jelly is used to keep the 2 halves apart, latex-free gloves are to reduce mess (and latex-free won't mess with the silicone or resin reactions), EasyCast is the casting resin, and the black tint is so the resin cures 'dark grey' (if the final piece gets scratched it just shows grey instead of bright white).

Step 3: Molding and Casting a Prototype

Once everything is ready, you can start making a 2 part mold. First lay out a clay bed for the batarang and shift the clay along the edges of it. I used an exact-o blade to make sure the clay is perpendicular the batarang as it makes the 2 parts easier to join together later. Flatten out the rest of the clay and set up some walls, I had some foam core sheets lying around I cut into shape, but more clay would work fine. Add some registration keys in the clay so the 2 parts can rejoin in place for every new casting, I just took a brush handle and stabbed the clay so there were lots of small 'holes' for the silicone to fill, and added a spout to pour resin in and some escape holes for resin to come out for the tips (to avoid trapped air bubbles). Now you can mix up you silicone (according to the instructions) and pour it in, I did a thin detail coat to make sure there were no bubbles on the actual model. You can use a brush to spread it around thinly, and once that cures you can make a larger batch and dump that in as well, making sure the model is completely covered.

Let that set for as long as needed (mine was fine in 10min, others may need longer), and start taking off the walls and flip the whole thing over. Carefully pull the clay off the mold and clean off any clay that has stuck to it, as you don't want that detail caught by the silicone. Now you need to add a release agent to avoid the silicone from stick to itself, I just smeared petroleum jelly on the surface of it, but using a spray agent would probably be easier/offer more coverage. Make your walls again and do the same as before, a thin detail coat with no bubbles and a thicker skin coat. Once it cures, pull the whole thing apart and take out the batarang, you can chuck that now if you want, now you should have a proper 2-part mold, ready to use! So the casting keeps it shape, you need something to sandwich the silicone together. I cut out some more foam core sheets (can use wood or something else) and used some rubber bands to keep the 2 halves together.

Now get you resin and mix it up according to its instructions, I added some black tint to mine in part A before mixing them together so the final product was black-grey. What I worked out with some trial casts was that the best way to avoid air bubbles and filling in that hinge pole correctly was to separate the halves, mix up a tiny bit of resin (10-15ml), and brush it across the whole inside for a thin coat (making sure cover the edges and holes with resin). Let that partially cure, you can actually see it happen (with no tint, it mixes clear and changes colour like if you added food dye), and once it is still soft but not fully cured fit the 2 parts together and pour in more resin down the spout. You should see some excess resin come out the exhaust holes, that's fine, and just let it cure for 10-20 minutes.

Step 4: Cleaning the Cast and Middle Hinge Parts

Once it fully cures you can take it out of the mold. It gives off some heat while curing, so you can tell when it's done (seriously, the thicker the resin, the more heat it gives off, you can feel it through the silicone jacket). You should have a replica of the original you made from wood, with some excess resin along the seams and pour in spout. I took an exact-o and cut off most of that, and then used some 180-240-400 grit sandpaper to clean everything up, and cast another copy to have the 2 parts of the batarang. Now we need a middle part, where the blades can hinge on and close up.

Now you can take some wood, trace out a circle and do the same as before (cut, sand, refine, mold), but I couldn't cut out a perfect circle so I was able to do something easier (may not be possible for you though). The lid of my resin tint was the perfect size to act as the middle of the batarang, so I pressed into some clay and just poured in a tiny bit of resin to make a small disc. Let that cure, sanded that down a little and I carved in a smaller circle with a compass and exact-o blade for mine, you could carve in a bat or nothing if you want, and I drilled in some small holes in the inside of it for the blade hinge pole to catch on (the blade is sandwiched between 2 discs and it swings along them). I made a mold of it using the same method as before, took a second copy of it and used some scrap wood/clay to make a wall along the middle of the disc. The blades will stop against this when open and shut, and the thickness will lock the blades together (I explained it badly, you get the point in the photos). I made another mold of that piece, and the is every piece of the batarang molded and casted in resin.

Step 5: Magnets and Paint and Glue, Oh My!

Now this is the last step before you have your own Arkham Style Batarang. I wanted it to stay open on its own and close, so I added some small magnets in the end of each blade (I think 3mm wide, the blade was 6mm thick at this point from filler build up). I drilled a small bit into each blade and superglued in the magnets, and the wall part of the discs stop them from smashing together and breaking/ripping out. I did some test fits before painting to make sure everything worked fine, and started painting.

I sprayed everything with 2 coast of flat black paint, had to hang the blades on some cord to get decent coverage. Once that dried I did something called dry brushing with silver acrylic. Essentially dip a brush into a small amount of paint, wipe most of it off the brush with a paper towel and lightly brush the blade so it looks like it's scratched/damaged. I did the whole blade like that, do it in small amounts so you don't add too much. To do the edges, I masked off the inside blade and just heavily dry brushed it so it was mostly silver with flecks of black.

Cover every piece with some clear coat to protect the paint (I used matt spray, can use gloss or satin), fit everything together and superglue the 2 discs together, and you're done! You now have your very own 'working' batarang replica, it just won't be stopping any criminals any time soon. I got it next to my Dawn of Justice batarang for scale, so with each blade being 19cm, it's a whopping 40cm wide (may have been better to scale it to 15cm per blade). You can scale it down in the initial stages if you want, Batman is just a huge beast in the games.