2 Part Silicone Mold




Introduction: 2 Part Silicone Mold

About: Hey there! I'm recent graduate of Carnegie Mellon University's Industrial Design program and a former Instructables intern. When I'm not working in a studio I enjoy exploring cities by bike and brainstorming n…

This Instructable will teach you how to make a basic 2-part pour mold. I made my mold for a 3D printed stegosaurus because dinosaurs are awesome!

3D printing can be expensive and hard to access. Molds are a great way to make multiples of a 3D printed part cheaper from your own home. If you don't have access to a 3D printer you can outsource your file to a 3D printing company and have them print the part for you. Or make the part yourself physically rather than digitally.

Start by selecting the part you would like to make a mold of.

Check out my mold making introduction Instructable to determine if a 2-part pour mold is right for your part.

Step 1: Making the Mold Box

Once you have selected the object you would like to mold, measure its largest dimensions for height, width and length. You will use these dimensions to build a mold box.

Cut the parts for your mold box, I made mine out of acrylic. Make sure the material you use will not react with the silicone. If you don't have access to acrylic as a quick alternative you can make your box out of Legos.

Make sure the box is water tight or the silicone will leak out of the box and ruin your mold. If you're making your box out of acrylic seal the edges with hot glue. Make sure you can easily break apart your box once the mold has cured and reuse the parts.

Step 2: Preparing Side One

Now that your box is ready for molding. Determine where the part line will fall on your part. The part line is the seam where your mold will be split in two. Make sure you split it is such a way that you avoid undercuts. Try to visualize if you could pull the object out of the mold with the way you have it split. This step is important to ensuring your mold will work or you'll end up with a block of silicone with your part stuck inside.

Embed your part in non-sulfur oil based clay up to the part line I used plastilina clay. Try to make the clay surface as water tight and smooth as possible.

Once the part is embedded insert mold registration keys into the clay where necessary. I used a couple of nuts of different sizes. The keys will help lining up the two halves of your mold and keep them together while casting.

Spray the inside of the box with a mold release agent. This will aid in releasing the part from the silicone.

Step 3: Pouring Side One

Now we're ready to start working with silicone.

I decided to use oomoo 25 from smoothon because it has a convenient 1:1 by volume mix ratio and short cure time.

  • Open part A and mix thoroughly.
  • Measure and pour part A into a mixing container.
  • Open and mix part B with a new stirring rod.
  • Pour and equal volume of part B into a new container.
  • Pour both A and B into a separate container and stir completely.
  • Be careful to mix everything. Scrape the edges of the container and stir until the color is consistent. There should be no marbling effect in the mixture. Don't mix too vigorously or you might create air bubbles which can effect the quality of the mold.
  • Pour the mixed silicone into the mold box slowly starting at the lowest point. Take your time and pour consistently to avoid trapping air bubbles in the mold.
  • If you didn't mix enough silicone like me, don't worry you can mix more and pour it on top. The two layers will cure as one.
  • Once the entire part is covered in silicone above the part about .5" let it set until completely cured.

Step 4: Pouring Side Two

Once the first side has cured break apart the mold box at the edges and remove the clay.

Flip the mold over so the silicone is on the table. This should reveal the part of your clay that was embedded in the clay. Clean the area of clay particles as best as possible.

No matter how tempting it may be, DO NOT REMOVE YOUR PART!

Build the mold box around the silicone and your part as before. Make sure the edges are water tight.

Add a pour spout for you to add material to the mold when casting.

Due to the shape of my part I needed to add my pour spout to the top of my part and attach it with clay.

You may find it easier to build it into the part line and embed it in the clay like the registration keys touching one of the mold walls.

**IMPORTANT! Spray the inside of the box, containing the part and first pour of silicone with release agent! If you skip this step the second pour of silicone will stick to the first leaving you with a block of silicone and no part line. **

I've forgotten this step before and ended up ruining my mold and trapping the original inside. It's no fun I promise.

Follow the same procedure as pouring side one for pouring side two

Once the silicone has completely cured, release it from the mold box and open the two halves of silicone along the part line.

Remove your part and pour spout from the mold and you're ready to start casting!

Step 5: Casting

Close the two halves of the mold, making sure the registration keys are aligned.

Shake parts A and B.

Measure out equal parts by volume of A and B into separate containers.

Pour parts A and B into a separate mixing container and stir them until completely combined.

At this point you may optionally add a mix in to your casting material. There are a variety of color and material finish options available through smoothon. I tried out the glow-in-the-dark mix in for a few casts.

Now SLOWLY pour the mixture into the mold spout. If you pour too quickly you will trap air bubbles in the crevices of your mold. It may help to rock the mold from side to side to allow potential air bubbles to escape.

My first few casts had lots of air bubbles I fixed this by adding more air vents in the problem areas.

Your mold cavity is filled when it spills out the top

Let your cast set to cure. The plastic will heat up as it cures due to a chemical reaction. Wait until the mold and excess plastic are cool to the touch before releasing or you may damage the cast when it is in a fragile curing state.

Once the cast is fully cured open the mold and release your cast.

Step 6: Cleaning Up and Finishing the Cast

Once your cast is free from the mold you may want to clean it up.

Sometimes excess plastic leaks out along the part line creating flash.

This is usually thin enough to be broken off and sanded down.

Feel free to paint or modify your part!

I added magnets to the feet of my dinosaur so he could climb all over the office!

For the casts that didn't go as well I cut off the feet and plan to add new fun attachments like dino-wheels!

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    Question 6 months ago on Step 6

    What is the best way to clean the clay off of the first side of the silicone block?


    Question 2 years ago on Step 6

    I am new to silicone but had a bottle left by a former tenant. Do I need a second part to use the silicone or can I use it as is? The label on bottle has come off and been lost.

    vincent pybis

    hi .....completely new to this and am wondering ..........i have a garden statue would like to create a mold so i can cast it . Its outdoors so needs to be weatherproof , could i use concrete in a rubber mold ?


    8 years ago

    I know no experience on molding... but should the mold release spray on the original? also how much money is need to make a mold this sizze?


    8 years ago on Introduction

    Glad you enjoyed it!

    The mold making material doesn't put off strong fumes but you should do it in a ventilated area. and let it cure at room temperature (the warmer the environment the faster it will cure) all of this information and more can be found in the MSDS.


    Fikjast Scott

    Very nice instructable, i like the detail you provided. I really like the idea of using the glue gun to build the box.


    8 years ago

    This is informative! Just two questions: why do you need two holes in the second mold? And does this work for latex masks instead of silicone figures?


    Reply 8 years ago on Introduction

    According to me one hole is to pour in and second hole is for air to escape from mould while pouring.


    Reply 8 years ago on Introduction

    Nice Instructable. The holes you refer to are called a sprue and vent. The sprue is the main channel that the material flows through to get to the casting. The vent is were the air flow out when material is being poured in.

    For your reference: http://www.ironhandbook.com/sprue-and-vents


    Reply 8 years ago on Introduction

    That's correct the larger hole is to pour your casting material in and the smaller hole is to allow air to escape. I found that I should have actually added more air holes than shown above. (one for each leg).

    I'm not sure what you mean by the second part of your question but I'd love to help!


    8 years ago on Introduction

    Dinosaurs, larges amounts of gooey chemicals, what's not to love? Great work!