Mini-Nuclear Power Plant - Make a Urethane Mold to Cast Concrete​




About: Alejandro is an industrial designer who focuses on creating impact through his designs. He has a broad background having worked at a graphic design agency, a furniture manufacturer, founded a successful furn...

This instructable will show you how I made a Urethane 3 Part Mold that I later used to cast concrete.

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Step 1: Materials

I"m going to split up the process into 2 steps.

Urethane Mold:

Casting Concrete:

Step 2: Design, 3D Print and Lasercut

Design Concept:

I really wanted to create a mini-nuclear power plant to use as a pencil-holder for my desk. That I found interesting about the shape was that I would have to make a 3 PART mold to actually cast concrete. I went ahead and designed it using Rhinoceros, a 3D software that I feel really comfortable using. I won't go over the details about the design in this instructable but since the shape of the nuclear plant is by revolution, the main operation we will use in Rhino is REVOLVE. You first decide a 2D line of the profile and then revolve it. I'm including the STL file of the Mini Nuclear Plant.

Once you have the design ready, I went to the 3D Printer and printed it.

Making a box:

While you are printing your mini-nuclear power plant you can start by making an acrylic box where we will pour our urethane. I also included the 2 profiles that will be our parting line in the middle of the object.

Step 3: Hot Glue Model

Once you have your 3D printed model and acrylic ready you need to use a hot glue and glue the base of the model to the bottom of your box. Also, join glue the 2 profiles to the center line of the model.

Continue gluing a square base to the top and then follow doing the 3 sides leaving 1 open to pour, as shown on the 2nd photo.

Step 4: Measuring Volume

Before you start guessing how much volume you need to use to fill the open side of the mold it is a good idea to measure the volume. A good and easy method that I recommend is by using RICE. Pour as much rice need to fill the open side and then use a measuring device and take note of how much was used (800 ml).

Step 5: Spray Mold Release

Now spray the mold release to your model and box making sure you cover all of the surface that will be in contact with the urethane. If you don't do this it will be really hard (impossible) to remove the urethane from the box.

Step 6: Mix Urethane

The Urethane I used for this project was the Smooth-On PMC-746. It is a 2 part mix so we divide 800ml by 3 which gives us roughly 270 ml for 1 component and 540 ml for the 2nd componen5 of the mix. Use a stick to stir well and pour as shown on the video. Try and select the lowest point of the mold and pour all of it at the same location to prevent air traps (bubbles). The urethane will spread slowly across the mold. Wait for it to dry for 24 hours and brake apart the acrylic box.

Step 7: Make Box and Cast the Other Side

Now it is time to make the other half of your 1st mold. For this you need to make a box out of melamine with the same dimensions for the mold to and 3d printed model fit snug.

Spray Mold release to the inside of the melamine box, 3d printed model and over the urethane mold.

Mix the same amount of urethane from the previous step and pour it over. Wait for 24 hours and remove the 2 molds and model.

Step 8: Measuring Inside Volume

Now that you have 2 molds you need to measure the inside volume to make the 3rd mold. Use rice to fill the inside of the model. You also need to add some volume. I ended up using the same amount of volume as in the 1st and 2nd mold (270 ml and 540 ml.)

Step 9: Make a Box and Pour Urethane

Now grab 5 pieces of melamine and create a small box where the 2 molds + the urethane fit snug. Use hot glue to join the melamine pieces together as well as sealing the seams (1st photo).

Spray the Universal Mold Release over the 2 molds, 3d printed model and inside melamine walls.

Mix the 2 part urethane in 270 ml and 540 ml amounts. Stir with a mixing stick for 2 minutes and then pour inside the box. Try to pour it first inside the hole of the nuclear power plant (the lowest point of the mold) (photo 2).

Let it sit for a day and then grab a screwdriver and mallet to break apart the mold (photo 3) and remove the 3rd piece of your mold.

Step 10: Ready to Cast Concrete!

OK, you now have your 3 part mold that might be a little dirty with mold release so it might be a good idea to clean it with soap and water.

After they are dry, separate them as in the 2nd photo and spray a little of the Universal Spray Release so it is a little easier to un-mold the concrete although it is not super necessary.

Then grab a few pieces of scrap plywood and create 2 "L" shaped pieces and clamp them together so you can have a more rigid body around the urethane mold .

Now you are ready to cast concrete.

Step 11: Measure the Volume and Prepare Concrete

Grab a big bucket fill it with water and put in the 3d printed model. Mark the total volume, then remove the model and mark the water level again (photo 1). Do the math and you will have the volume of your 3d printed model.

The concrete I'm using is called Duracal and it is a 4:1 cement-water weight mix ratio. I ended up using 480 grams of Duracal and 120 grams of water (I really recommend using something like 560 g of Duracal and 140 of water) so you have a little extra.

Pour the water over the Duracal and mix non-stopping for 30 seconds with a stick. It is important to get a good and even mix.

Now pour it smoothly over the mold and let it sit for 2 hours.

Step 12: Remove the Mini-nuclear Power Plant

After 1 or 2 hours the concrete should be cured enough to grab and take it off the mold. I recommend waiting 24 hours for it to be fully cured.

Your mold is ready to reuse and cast multiple power plants.

Step 13: Enjoy at Your Desk

Awesome! You made a cool mini nuclear power plant! Fill it with some pencils and use it by your desk.

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    27 Discussions


    4 years ago on Introduction

    Nice! There was a very famous german comedian named Loriot. In Germany, he was as popular as, let´s say Monty Python in the UK. One of his best sketches is about a toy nuclear power plant:

    In the first part, the grandpa buys the building kit as a christmas gift, in the second part it is put together and explodes (as intended) :)


    5 years ago on Step 13

    Buenísimo! Mucho trabajo que completamente vale la pena! Muy original!
    (Disfrutaste bastante el proceso me imagino!)
    Estaría bueno agregarle bajo relieve el símbolo de Radioactivo, algo así como la muestra que hice :D Saludos!

    1 reply

    Reply 5 years ago on Step 13

    Muchas gracias por el saludo. Si, el simbolo quedaria muy bien!


    5 years ago on Introduction

    Great instructable! I have never built anything with Urethane molds but I was wondering why not making the outside urethane mold all at once and then cut it in two pieces?

    Captain Greg

    5 years ago on Introduction

    Love the idea, you cast it well! Couple questions: is the mold material soft enough you could cast it in one piece, with the tower upside down? Also, for those with lathe access, I wonder if it could be turned.. Either way, I'm going to make shot glasses that look like cooling towers, thank you for the inspiration!

    2 replies

    I think the flare in the shape of the top lip would create sufficient undercut to prevent a single piece mould (I assume you mean having the outer mould shell and core as one piece). If you were happy to modify the shape of the design slightly so that the tower tapers uniformly from the base to the lip, you could make that work. It wouldn't quite be accurate compared to a real cooling tower, but it would make the moulding and casting process easier, and eliminate the mould line on the outer wall of the tower casting.

    The mold is soft but I don't think it won't work with the current design if you make a single part mold. As dustbuster7000 mentions you might need to change the design a little bit so it doesn't have any undercuts.

    YES, this is a shape that you can work on the lathe.It would be really cool if you can make this shape in aluminum!


    5 years ago on Step 4

    Brilliant idea the rice. You just solved a problem I was thinking about of 2 or 3 days for a project of mine.

    1 reply

    5 years ago on Step 13

    Cute cooling tower. Any plans to do say, a containment vessel too for paper clips perhaps?

    1 reply

    Reply 5 years ago on Introduction

    Thanks! I don't know what a containment vessel is. Can you send copy a photo here? It would be really cool to expand and make a product line.