Introduction: Water Bottle Cap Karambit

Hey there! This is my first time ever making an instructables, this is actually my end semester exam for one of my college courses. The exam is to make something out of any kind of plastic/polymer, and i chose to do a karambit.



  • around 50 water bottle caps
  • 2 sheets of 15cm x 20 cm 1.5 mm sheet steel
  • 2 blocks of 1.8 cm teak wood, same size as the sheet steel
  • paper
  • random piece off wood, bigger in size than the sheet steel and teak block

Tools (this is based on the tools i have at hand, you can use other tools if you think it'll work):

  • bench grinder with a stone wheel (preferably with a smoother stone wheel )
  • bench grinder with sandpaper
  • metal foot shear (or anything you can use to cut the sheet metal)
  • stove
  • a cooking pot (preferably a cheap one, since you wont ever cook food on it again)
  • cooking oil
  • a large plastic water bottle
  • rotary tool set
  • heat gun
  • pliers
  • clamps
  • cutter/hobby knife
  • pen/pencil
  • sand paper around p320, p500, and p1000

Step 2: Make the Templates

make 2 templates of the karambit, showing the handle and blade.

The ring should have a 1.1 cm radius inner part and 1.7 cm radius for the outer ring. And for the handle it should be 6.5 cm from the outer ring to the start of the blade.

For the blade, it should have a tang (the part of the blade that's embedded in the handle) that reaches all the way to the outer ring radius.

Then cut out the handle part for the first template, and the blade for the second one. So you know will have 2 templates, one for the handle, and one for the blade.

Step 3: Shaping the Blade

  1. Trace the blade template on one of the sheet steel
  2. cut out the rough shape of it. I used a foot shear, but you can use other tools like a hack saw, angle grinder, etc.
  3. use the rough stone wheel stone bench grinder to shape the form of the blade. (you might want to use a plier clamp to do this, since the blade will get really hot from the friction)
  4. then switch to the smoother wheel to clean the grind junk
  5. then using the rotary tool with a cut off disc, make some notches like saw teeth an the tang, make sure that the notches is shaped like in the picture, this will ensure the blade will attach securely to the handle.

Step 4: Hardening the Blade

Like any good knife, the blade should be hardened and i used a portable stove to do this

  1. put the blade on the fire off the stove and wait till the metal became red hot, this might take from 1/2 hour to 2 hours depending on the intensity of the flame.
  2. while waiting, cut the top of the large water bottle and fill it with cooking oil, till the whole blade can be submerged in it
  3. when the blade is all red, dip the hot blade in the oil
  4. the knife is brittle now, and since i used really soft steel, it became harder but not brittle. so i don't need to do any tempering. But if you use high carbon steel, you'll need to temper it first.
  5. now you can go back to the bench grinder and make the cutting edge of the blade (the convex part)
  6. use the smoother stone wheel to grind the cutting edge on an angle on both sides.

Step 5: Prepping the Handle Mold

  1. trace the handle template and the blade with the proper position on to one of the teak wood
  2. on the teak wood, cut out the part of the blade that is NOT in side the handle tracing. But only cut out the outer most layer of the teak wood. Teak wood will have 5 layers: 2 outer layer, 2 middle layer, and the central core. you basically skin the thing.
  3. then draw a rectangle around the handle trace, like the one in the picture.
  4. Now cut out the shaded area, without damaging the unshaded part. i did this by drilling a hole and using a scroll saw in the hole and cutting it from the inside.
  5. do the same thing on the other block BUT mirrored, this means exactly on the same position only reversed left to right.
  6. and remember to keep one of the cut out rectangles, it'll be important soon
  7. now get that other piece of sheet metal

Step 6: Assembling the Mold

  1. get the 2 pieces of teak wood, the blade and the remaining steel sheet
  2. for the first layer, put down the steel sheet
  3. second layer is the first teak wood, with the skinned blade tracing facing up
  4. put the blade in the skinned part.
  5. now sandwich the blade with the other teak wood, and it should fit perfectly with the other skinned part
  6. now secure the stack with clamps
  7. Then make a rig like the last picture, from the big piece of random wood, and add and secure 4 'legs' facing up. I used hammer and nails to do it. the legs should be a bit longer than your heat gun when facing up.
  8. then put the mold (with the steel sheet on the bottom) on the 'legs' and the heat gun nozzle pointing the part of the mold where the hollow spot is

Step 7: Melting the Bottle Caps

  1. first you'd want to cut up the caps to smaller pieces, so it'll melt easier
  2. then preheat the pot on medium heat
  3. then slowly add the cut up bottle caps, and keep stirring the melted plastic, or it'll burn
  4. i used the heat gun to help it melt faster
  5. This goo is really sticky, so be warned

Step 8: Pouring and Casting the Melted Plastic

  1. first preheat the steel bottom of the mold (that is now suspended with the 4 legged wooden rig)
  2. then pour the molten plastic in the mold until its full
  3. Then cap the now filled hole with the previous teakwood rectangle
  4. Press it down, i suggest using a twisting clamp, so you can adjust the pressure easily
  5. press it down till about halfway down the first teak block
  6. now let it the plastic cool and harden
  7. After it cooled down, open up the mold starting from the steel bottom, and you will see a flat bottom on the plastic this will make it much easier to work with
  8. then star opneing the wood parts, an it may get stuck, and you'll have to cut out the mold if that's the case
  9. after you have your form out, you now should have a plastic block with a blade sticking out of it
  10. now again trace the handle template on the plastic block, according to the position of the blade

Step 9: Shaping the Handle

  1. cut out the smaller ring , i used a hole saw drill bit on a drill press since i have a hole saw with the right size
  2. then cut out the outline of the handle according to the tracing, I used a scroll saw
  3. Now using a sandpaper bench sander, make the grip thinner and fillet it until it's comfortable to grip in your hand

Step 10: Finishing and Salvaging

At this point you should be able to tell the condition of your plastic handle. If its a perfectly casted handle, it won't have any airholes. All you need to do now is to manually sand the whole handle progressively from p320, to p500, and finally p1000.

But, if your casting is not perfect like mine, it will have some air holes. This can be caused by a number of reasons, like the mold being to cold, or opening the mold while the plastic has't hardened yet. At this point you should cover the holes using epoxy, specifically plastic-paste form-epoxy. Just mix the 2 compounds of the epoxy and smear it over the holes, and wipe off the excess. The curing time for the epoxy really depends in the brand and type you use, but usually you will have about 1/2 to 1 hour window to apply and manage the epoxy to your liking, and past that the epoxy will be too hard to manage. And it will take from 3 to 24 hours for the epoxy to completely cure. after its cured, do the same as if its perfectly casted, sand it progressively from p320, to p500, and finally p1000.

Step 11: Now You're Done!

OKay now we are done! if you want you can colour the handle, i personally like the original colour, but if you do wanna recolor it, use a primer, dont use regular spray paint, since it wont latch on the plastic surface properly.

and here's s link to an awesome karambit techique to try out (not on a real human of course)