Making Plastic Bolts From Polycaprolactone

About: "If you want to build a ship, don't drum up people to collect wood and don't assign them tasks and work, but rather teach them to long for the endless immensity of the sea." - Antoine de Saint Exupéry. I lon...

Polycaprolacton  (also known as InstaMorph, ShapeLock and maybe other brand names) is a plastic that becomes workable at 150 deg. F. It's hard at room temperature. It's a great material for inventing and prototyping. This Instructable shows you how to make plastic bolts. I made a 1/4 inch bolt for these instructions. I think this is close to the smallest bolt you could make with this material and process.

Step 1: Gather Materials

You will need.
  • 1 x 1/4" bolt
  • 1 x 1/4" nut
  • a source of 150 degree heat. (stove, microwave, hot plate)
  • 1/8 - 1/4 cup Polycaprolactone pellets
  • Thermometer (optional, but handy)
  • Waterproof work surface that won't stick to the Polycaprolactone.
  • Sharp knife or razor blade.
You have many choice for a heat source. A pot of water on the stove works well. You can use a microwave, but I find it to be a pain to keep heating the water all the time. I used a $32 electric skillet with a heat control. I was able to "dial in" 150 deg. F. exactly and hold it there. For other projects I have used a heat gun. Be careful if you use a heat gun. It's easy to overheat the Polycaprolactone or get the bolt scorching hot.

You don't have to use a 1/4" bolt exactly. Any nut and bolt that match could work. I don't think you can make a bolt much smaller than 1/4 inch that's very long and works.

Polycaprolactone will stick to plastic, so test a tiny bit on your work surface before you get going. If you overheat Polycaprolactone it gets stickier. That's a useful trick for other projects, but don't let it get over 150 deg. F in this project.

Step 2: Make the Nut

Making a wing nut is very easy. Heat up some of the pellets in the water. Pull them out with a skewer or tongs and just form them around a bolt. As it cools, turn the bolt in and out to make sure you have good threads. Let the nut cool. Use some damp paper towel to prop up the wings if needed. Polycaprolactone won't stick to most things if they are wet.

Step 3: Make the Bottom Mold

Make a box shaped piece of plastic. It should be a bit longer than the part of the bolt you want to copy. Make sure the top is quite flat. You can return it to the water a few times to soften it and then push it on the table to shape it.

Step 4: Take the Bottom Half Impression of the Bolt

This step is critical. You need to push the bolt into the mold exactly half-way. You can eyeball this. Just get it as close as you can.
If you mess with the block it may pull away from the bolt, but you may need to gently work the top surface to get it right. Let the mold cool and make sure you have a good impression and the bolt isn't more than halfway in. Try again if you pushed it in too far or the block pulled away when you worked the top surface.

Step 5: Make the Top Half of the Mold

The top half of the mold is just like the bottom. Make it about the same size and shape.

Step 6: Complete the Mold

Make sure the bottom mold is completely cool before you start this step. Polycaprolactone will weld together only if both parts are hot. If one is cool and has a decent thermal mass, they will come apart. In other projects this is problematic, but it's the key to making Polycaprolactone molds.

Put the top half of the mold into the hot water and let it get warmed to 150 deg. F all the way through. It will be completely clear. Take it out and quickly push it down on top of the bolt and bottom mold while it is still very soft. Push the sides down a bit around the edges to get a good seal. Don't work it too much. Walk away while it cools completely.

Step 7: Open and Inspect the Mold

When both halves have cooled use the bolt as a lever to split them open. It shouldn't take very much force. Inspect the mold to make sure you have a good fit and and close to 50/50 depth. It's normal for the top mold to be a little shallower than the bottom. Don't worry if it's not perfectly 50/50. Test fit the molds together to get a feel for how the fit.

Step 8: Make the Bolt

Add some more Polycaprolactone pellet to the water. Make a snake that is about the size of the bolt you want. You will need it a little bit thicker than the bolt, but not much. Add some wings so you can tighten it easily. Once you have a good shape, put it back in the water and heat it up completely.

Step 9: Cast the Bolt

This step is also critical. Heat the bolt up all the way and work fast. Get the mold ready. Take the bolt out of the water and lay it in the bottom mold half. Get the top half, line it up and push it down hard in one motion. Hold it down for 10 seconds while the bolt starts to cool. Wait for the bolt to cool completely.

When you remove the bolt, there be some extra plastic from the seam in the mold. See the second image. This is also known as a sprue. It's normal and we will fix it next.

Step 10: Clean Up the Bolt

Using a sharp knife, shave off the sprue and sharpen the end of the bolt a little. Use the nut to test the threads. Keep shaving until you can get the nut all the way on. Once the nut is all the way on, or just stick in the middle, soak the threaded end of the bolt in the water for about 10 seconds. Then turn the bolt some more. Get the bolt all the way on and off at least once while barely heating up the threads.

Don't overheat the bolt while touching up the threads. If you do melt it, just go back to the molding step and try again.

Let the bolt cool and make sure the nut can turn all the way on and off.

Step 11: Testing the Bolt

Test the bolt with the metal nut and the plastic nut you made earlier. Chances are the plastic nut will be a super tight fit. You may need to oil the bolt a little to get it to go in and out smoothly. I had to hold the bolt with pliers close to the nut to get the bottom half in. The bolt will twist a little, but you can apply a lot of force to Polycaprolactone. It will bend rather than breaking.

Congratulations. You now have a plastic nut and bolt. The bolt won't be as strong as a nylon bolt of the same diameter. But you can easily make all you need.

Let me know if you find a good use for these bolts. I plan to use them to close project boxes I will also make out of Polycaprolactone.



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    3 Discussions


    6 years ago on Introduction

    Good! I like it!


    You said that it sticks on plastic.

    Does it stick also on Polycaprolactone pieces?

    What I want to know is how do you prevent both parts of the mold to stick together?

    And also, how do you prevent the mold from sticking with your project?

    I have also worked with molds before, and used to make some protrussions on the first mold in order to make the second part attach exactly on the first one.

    In order to achieve this perfect attaching I needed to cover the first mold with some liquid soap or oil to prevent the second mold from sticking with fhe first mold...

    Your molds should be pretty accurate to work well. How do you do this?

    Thank for this great idea!


    1 reply

    Reply 6 years ago on Introduction

    The shapelock (or whatever brand you use) won't stick very well to itself unless both pieces are hot. So if you have a cold piece and a hot piece, they snap apart as soon as it all cools. Thus my mold technique needs no release agent.
    It also won't stick to plastic at 150F (usually). But it will stick like crazy (to plastics) around 180F.

    For registering the molds I just relied on the unevenness of the surface of the first half. They second half snapped right into place.

    The molds were accurate enough for the screw, but I did touch it up at the end with a metal nut.


    6 years ago on Introduction

    Clever. I have never seen this before, very useful.