A Hot Knife, Cuts Plastic, Rope Opens Welded Plastic Cases.




Introduction: A Hot Knife, Cuts Plastic, Rope Opens Welded Plastic Cases.

About: Im a bit of a geek of all trades. Of late most of my free energy has going into Making sure our hacker/makerspace is awesome! Come check us out!

Manufacturers love to blisterpack, and glue shut, and weld closed all sorts of plastic things.  making it really hard for those of us whom wish to fix or modify or even recycle them!

This trick will get you up and rolling easy and cheaply slicing into plastic "like a hot knife through..... uhm plastic!"

I warn you this is pretty dangerous,  we are talking about a Exacto knife(SHARP), and a soldering iron(HOT).  there is possibility of both burns and cuts!  (and possibly burned cuts!)  Please use it with care! but if using it carefully, I think it could be safer than trying to do the same things with a sharp knife and pressure.

I first ran into mention of such a thing from a guy who rebuilt battery packs for laptops.  he would slice open the old one,  then replace the bad cells, (or all of them) and then glue it shut.

But its useful for cutting vinyl. for cutting rayon or vinyl cloth, plastic coke bottles. packaging.  It could probably also be used to carve plastic or wax ,  but i suspect the end result would be a bit globby/stringy.

Step 1: Parts!

You need one cheap metal jawed Exacto Knife
one pencil type Soldering Iron (between 25 and 25w) Preferably one where the tip can be removed via loosening a screw on the side, (see photos)

That's it.  (might need some wire cutters, pliers, sandpaper, I didnt.)

Step 2: Making It.

I made one of these irons years ago to cut into a bad laptop battery pack, and fix it. it was so handy ive nearly used it up,  Flaws in my first design were that i used a very flimsy iron, and so sometimes i have to bend it back a bit if im cutting with too much pressure. 

I was seeing someone elses instructable the other day and realized i should document how to make a hot knife1

This can be simple or tricky depending on the soldering iron you have. 

this green one i used was not ideal, because the tip was threaded and screwed in,  A better iron  would be one with a single screw on the side that lets you pull the tip out if loosened.  So I had to modify my iron a little. (I will annotate that as I go)

Step one,  unscrew the blade/jaws/gripper from the handle on a cheapo Exacto knife, set the blade off to the side.

Step two, remove your soldering iron tip from the iron.   If it s the kind with a screw on the side loosen the screw and pull it out.  The tip  might be long or short, does not really matter. just pull it out set it aside,  (might want it if you ever want to turn it back)

*If its a threaded tip Soldering iron like the green one in the photos, hopefully its the same threads as the blade holder on the exacto.  mine did not it was smaller, so i had to cut the threads a little bigger.

Step 3, where the irons tip was, insert the blade holder and grip of the exacto into the iron, and slightly tighten the screw (the one that held  in the tip of the iron) 

Step 4, put the blade into the exacto holder,  if its loose, turn the holder clockwise a little bit, so that the threads on the blade holder spin past the screw, once its tight, tighten the screw to keep it in place. 

step 5,  find a nice safe surface to heat it on, (i have spare tiles from a bathroom remodel I can set it on as i plug it in.

Reminder, its now very sharp, and will soon be very hot!

Step 3: Using

The blade will heat up to (183 °C or 361.4 °F) or better which is hot enough to cut most synthetic cloth, rope, and plastics. 

The trick to using it is to be slow and steady.  it will do the work if you let it.  its like a hot knife in butter.  This contraption wont take alot of pressure. so let the heat do the work.

care should be made to not cut yourself,  not burn yourself,  not breathe any smoke or fumes,  not to cut into things that may explode or catch fire, and be careful of your cut depth to avoid damage/danger.

It might be handy to keep around another exacto knife to cut off any plastic that gets stuck to the tip. ]

place it somewhere safe to cool, and then put it away some place safe. I have an old dremmel case i store mine in.

Happy Hacking (into things)
If you enjoy this tip, feel free to support your local Hackerspace, mine is http://quelab.net feel free to drop in and visit us!

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    8 years ago on Introduction

    Looks easy, but it's not. You have to make it strong. Because Xacto knife moves! Also you can't cut pvc, plastic etc. It's so sticky.


    8 years ago

    thats pretty cool I think I might use it for melting paracord ends


    Reply 8 years ago

    would work great for that, one mod ive done since i first made it, ive wound up dulling the blade, and even the point a little, to encourage folks to use the heat not the force to do the work.


    Cool! You made the instructable like I suggested! XD

    Have your tried keeping the blade clean with a brass wire sponge? Wondering how to keep it clean with all that plastic residue.



    Reply 10 years ago on Introduction

    The green handled one here is just 25W it would be better with a 30W, im pretty sure the one i have at home is 30W, heats up faster but has a longer barrell, makes precision slicing harder. Id say pick one with a short barrel (between the point and the handle) and one with the set screw on the side.