Flexible Block Molds

Introduction: Flexible Block Molds

About: Made in Canada, I grew up crafting, making, and baking. Out of this love for designing and creating, I pursued a BFA in product design from Parsons School of Design in NYC. Since then I've done work for Martha…

Silicone rubber is a wonder of a material! If you have simple models with minor undercuts, the flexibility of silicone makes it possible to avoid making a two part mold – and to just go for it with a one part block mold.

Here's how that magic happens...

Step 1: Ideal Models for This Kind of Mold

Models for flexible block molds can have a surprising amount of undercut and still make successful molds.

The trick is, with only minor undercuts that don't go too far beyond the bottom surface (mold opening) like my gem soap model (top image), the flexibility of silicone allows you to just stretch the mold and pop/push out the cast. Easy Peasy.

With my duck friend just below the gem, he has significantly larger undercuts, but what saves him (and your elbow grease) from a two part mold fate, is that silicone block molds can be cut to remove the model using a sharp exacto blade without creating a very noticeable seam line. So once the mold has cured, we simply cut him out of the mold and the mold becomes a sort of fake two parter.

NOTE: The undercut areas on both models above are designated by red stripes.

Both of these molds are endlessly reusable, like the rhino mold, and I'll demonstrate these modern mold making miracles in this lesson!

P.S. The gem soap model was my very first 3D print, EVER! It's a very simple project, but I'm still super proud. :) I used Fusion 360 and a Fortis PLA printer to make the magic happen.

If you're interested in learning how to 3D print, which will require you to know how to generate 3D models, I highly recommend trying the rendering software Fusion 360. Full disclosure, it's an Autodesk product (Autodesk owns Instructables), but it really is just a super easy-to-use, intuitive software. Plus it's FREE for students, teachers, hobbyists, and start-ups!

Autodesk Fusion 360 (free)

This is a powerful 3D modeling platform that's easy to learn but has endless potential. With it, you can design complex 3D objects for practically any kind of fabrication, digital or otherwise.

Click here to sign up for free as a Hobbyist / Enthusiast / Startup or as a Student or Educator.

  1. Follow one of the links above to download the app (don't use the App Store on Mac).
  2. Enter your email and download the free trial.
  3. Install and setup a free Autodesk ID account.
  4. When you open Fusion, select the Trial Counter in the upper toolbar (it tells you how many days are left on your trial).
  5. In the next dialog box, select "Register for Free Use".
  6. Sign up as a Start-Up or Enthusiast (Free). You can also Sign up as a Student or Educator (Free) if you're a student or educator at a registered institution. This will give you free use of Fusion 360 for as long as you need it (not just a 30-day trial).
  7. Select the "I accept Terms and Conditions" checkbox and click Submit.

A browser-only version is in the works, but hasn't been released yet. It's in beta right now, but check this link after January 20th for an anouncement about a release date: Project Leopard

Once you've downloaded the software, check out my teammate JON-A-TRON's free class on 3D Printing and how to get started using Fusion 360.

The future is now. :)

Step 2: Tools & Materials

Here's a list of what you need to make a flexible silicone rubber block mold, either cut or uncut:

  • Mold board (I used a 10" x 14")
  • Mold wall materials (I used corrugated plastic & a thin polycarbonate sheet)
  • Kraft paper, newsprint, or cardboard (to cover & protect your work surface)
  • Mixing containers
  • Mold making material (I used Smooth-On Oomoo 30 Silicone Rubber*)
  • Casting material (I went for glycerine and shea butter soap)
  • Glue gun & glue sticks
  • Aleene's tacky 2-sided adhesive sheets
  • SOFT sulfur-free sculpting clay
  • Stir sticks (paint style and tongue depressors)
  • Masking tape
  • Metal scraper
  • Exacto knife
  • Sculpting tools
  • Thin permanent marker
  • Ruler
  • Pencil
  • Notepad
  • Paper towels
  • Mold release - I use Mann Ease Release 200
  • Small disposable brushes
  • Disposable latex-free gloves
  • Long sleeve shirt
  • Safety glasses
  • Apron

*(x1) 2.8 lbs kit will do for either mold. For both, get 2 kits.

Step 3: Filling a Hollow Model

In this lesson, we have a different kind of hollow model than we did in the last. This duck has a bottom, but it's filled with air. He's also made of flexible material, so if we don't fill him, there's the chance that he will deform slightly.

The fix for this is to fill him.

First, cut a small hole in the bottom.

Then, insert a funnel and fill him with salt.

And finally, seal up the hole with hot glue. Now he has something more substantial than air inside him to push back with once covered with the mold material to prevent him from deforming.

Step 4: Calculate the Model Volumes

Following the same steps we used in the previous lesson to find the model volume of the rhino, use measuring cups/bowls to find the volumes of the two models. The only difference this time is that we need to do it BEFORE we glue them to the mold boards.

NOTE: I don't like to use the tapioca method for finding the mold material volume on this style of mold because once the models are glued down and the mold walls are in place, there's no way to know for sure that a little rogue tapioca didn't get stuck underneath an undercut where I can't see it.

So, now we know the volume of our models (yours of course will be different):

  • The gem model is 4 fl oz
  • The ducky model is 8 fl oz

Write these numbers down! In a few steps, we'll be using this info to do a bit of simple math in order to find the mold material volumes. This is your notebook's chance to shine!

Step 5: Drawing Part Line on Model

Like we will do in the next lesson for the model of the two part mold, we are going to draw a part line on the ducky model. Because we're going to have to cut him out of the mold (we'll only be cutting the mold material, not the ducky if we're careful!), the line will act as a guideline to follow.

In the case of a two part mold, a part line is the bisecting line through the model where if a mold half was lifted away from that line, there would be no undercuts to impede it's removal.

NOTE: For multi-part molds (3 to an unlimited number of parts), there would be more than one part line drawn on a model. There would be as many as necessary to indicate the areas that can be easily removed without undercuts.

Use a permanent marker to draw a line around the center or midpoint of our duck friend.

Step 6: Marking & Making Mold Wall Lines

Place your model(s) on the mold board with some room to work between them.

Measure, mark and draw the mold lines. Remember that the mold walls should be 1/2" away from the model in all directions.

Step 7: Attaching Models to Mold Board

We're going to use two new materials to attach these models to the mold board:

  • Double-sided adhesive paper
  • Hot glue

The first one is my favorite way to attach a rigid model that has a perfectly flat bottom. Double-sided adhesive paper offers a good, secure stick without having the flat bottom push any globs of glue out the way it would if I used hot glue. I recommend this for attaching any flat bottomed model made of a rigid material.

Here's how to use it:

Remove one side of the adhesive, place the model, and cut close to the model's edge.

It's ok if it's not perfectly lined up with the edge.

Peel off the other side of adhesive backing.

Place it in its spot on the mold board and press down firmly.

For the duck model, I used hot glue. The duck has a mostly flat bottom, but not perfectly flat, AND is made of a flexible material. The hot glue will help fill any irregularities in the bottom's landscape and ensure a complete seal. I recommend using hot glue for any model that has a flexible, irregular, or concave bottom.

Here's how to use hot glue to attach a model:

When the glue gun is hot enough...

Put two concentric lines of glue on the base of the model. One just inside of its surface contact line (bottom's edge) and another in the center.

When placing it on the board, hold it down with gentle pressure until the glue dries so that you get an even seal.

Now we're ready to build our mold walls!

Step 8: Making the Mold Walls

This will be a fairly similar process to making the mold walls in the last lesson. The gem will get the exact same kind of wall as last time. The ducky on the other hand, will get a custom round wall to fit to his form. :)

Here are the how-to's for these walls:

Measure the heights of the models and note them in the notebook.

The Cylindrical Mold Wall

Measure and cut a piece of flexible sheet plastic (styrene or polycarbonate) so that it's at least 1" taller than your model. In my case, the duck.

Pre-cut a piece or two of wide masking tape.

Roll the sheet plastic up until it fits the contour of the mold wall line you drew on the board. Make sure the bottom edge is lined up all the way around the wall and tape it in place. Be sure to put a piece of tape along the entire length of the inside seam, like pictured above.

Put the mold wall in place...

and seal it to the mold board using hot glue.

Whenever I'm casting molds with larger volumes, I like to double seal the walls to the boards by adding and smoothing coils of sculpting clay over the seam of hot glue.

There's nothing worse than doing all this prep work only to have missed a tiny spot with the hot glue and watch mold material come seeping out the bottom! Doubling down on the seal may seem superfluous, but I promise you, it is worth the little bit of extra effort!

Measure and mark your mold material pour line (model height + 1/2").

The Rectangular Mold Wall

Repeat the same steps we did for the Rhino mold wall in the previous lesson:

Measure and cut a strip of corrugated plastic that is at least 1" taller than the height of your model (my gem model is 1" high), and long enough to go all the way around your mold wall line, plus 2" for a sealing flap.

Measure and mark the sides of the rectangle to match the mold wall lines you drew around your model. Then cut ONLY HALF the way though the material to create the corner hinges.

Tape the flap down and the inside corner, covering any cut ends inside as well.

Place mold walls on the drawn lines.

Hot glue the wall in place.

Make sure that there are no gaps or breaks in the glue seal. Remember to add extra glue to the outside corners.

Because it's a small mold, covering the glue seal with clay isn't really necessary. Since I was on a roll, I decided to do it anyway! Remember to measure and mark the mold material line (should be 1/2" above the highest point on your model) which is 1 1/2" for my gem mold.

Step 9: Calculating the Mold Material Volumes

Now for a bit of math! If you don't want to (like me), or can't, use the tapioca method for one reason or another, this is how to calculate the amount of mold material you'll need using math:

Step 1: Find model volumes in fluid ounces. Yay, we've already done this! The gem is 4 fl oz. and the ducky is 8 fl oz.

Before we can know the fluid ounce volumes of the casting areas of the molds, we must calculate the volume in cubic inches. Once that's done, we'll convert those numbers into fluid ounce volumes.

To do this, measure the length and width of the mold walls for a rectangle mold and the diameter of a cylindrical mold. You'll also need the heights of those mold pour marks we made in the last step.

Write down of all the measurements in a notebook. We'll need the radius of the cylindrical mold, so divide the diameter by two to find it.

Follow the above instructions to find the amount of mold material in fluid ounces that you need to mix up to pour your rectangular mold.

NOTE: In Step 1, for the height, when finding out the cubic inch volume of your mold, use the measurement from the mold board up to the fill mark (that will be 1/2" above the highest point on your model) and NOT the height of your mold wall.

Follow the above instructions to find theamount of mold material in fluid ounces that you need to mix up to pour your cylindrical mold.

My rectangle mold totals.

My cylindrical mold totals.

The totals for mixing the same material for both molds at the same time – which is what I decided to do!

Step 10: Spray With Mold Release

As I mentioned before, when using silicone rubber with other materials, it's not necessary to use release. But my motto with mold making, is why take any risks? It's a lot of work and I always choose to do everything I can to support a successful outcome. So I always use release.

For non food-safe molds, I recommend using Mann's Ease Release 200. It is effective with even a very fine coating.

In a well ventilated area, mist a light coating of release onto the model and the inside of the mold cavity. Don't worry about the underside parts of the models that you can't see. Enough will swirl around in there to give even those parts a light coat. Let dry for 30 minutes.

Step 11: Pouring the Mold

Carefully mix the correct amount of your chosen mold material (mine is Smooth-On Oomoo 30) following the manufacturer's instructions. Remember to put on all the proper safety stuff:

  • Latex-free disposable gloves
  • Safety glasses
  • Long sleeve shirt
  • Apron

Pour the material into the molds as we did for the rhino mold – from high up and very slowly.

I stopped short of the height mark on the duck mold to show you how you can add silicone to silicone to make up that height if you miscalculate and don't end up with enough material.

Just wait until the first pour has cured, then mix up another small batch of silicone (I used Oomoo 25 which is blue, to better illustrate the fix), and add it to the cured batch. Problem solved! The silicones will stick together like they were never two separate materials.

Step 12: Freeing the Mold & Model

Once the mold material has fully cured, it's time to remove the molds from the board and the models from the molds. This moment always has a feeling of Christmas for me! It's so exciting to see the fruits of one's labour.

Use a sharp blade and the metal scraper to remove the clay and hot glue from around the molds. Use the sharp blade to cut the corners of the plastic sheet mold walls and the tape along the seam of the cylindrical mold. Remove the mold walls.

Use the scraper to unstick the models and molds from the mold board.

Looking good!

If you like, use the exacto blade to clean up the mold edges.

To remove the model from the uncut mold, gently release the mold away from the model, one corner at a time.

Then turn the mold upside down and flex and press the model out of the mold. It should pop out easily!

One down, one to go!

When you've used a model with undercuts in a flexible block mold, it's necessary to 'free' the model from the mold by cutting it out. Here's where the cut/part line we drew on the ducky come in handy. It gives us a line to follow with our cuts so we know we're staying on course for easy model removal.

Gently start cutting into the mold material following the line.

Use the fingers of your other hand to pull the two side apart while you cut. This makes it easier to keep an eye on the line. Cut about 3/4 of the way down the mold. Repeat this on the opposite side.

Test your cuts to make sure you went far enough down to be able to easily get ahold of the model.

Gently remove the model from the mold.

And that's it!

Take a peek inside. :)

Trim the mold edges.

Now take a step back and admire your work! These molds will be able to make hundreds of casts in almost any material!

And now I can show you proof that silicone cast on silicone, without mold release, becomes like one single material. It really is amazing stuff!

Step 13: Casting the Molds in Soap

As I mentioned in the last lesson, silicone rubber is so heat resistant and durable, that you can cast almost any non-metal material into it! For these two molds, I'm going to use soap to make my casts. To view this process, click on the below link for my soap making instructable:

5 Minute Homemade Soap

If you choose a different material, simply follow the mixing and casting instructions from the manufacturer.

NOTE: We're already a step ahead because we know the volume of the models, which is the same as the amount of casting material we need to make one cast! Mix that amount for each cast.

PRO TIP: To cast the duck mold (or any cut flexible block mold), you will need to bind it to keep the two cut sides together. This mold material is so soft/flexible that if it is bound unevenly, it will deform the shape inside. The best way to gently secure it is to wrap it in masking tape, starting at the bottom and working your way up. All the while making sure that the two halves are lining up as perfectly as possible. You want the tape wraps to be snug, but not too tight. You can do it in multiple single line wraps, or in one continuous wrap.

Once the cut flexible block mold's cast has cured/set, simply remove the tape and peel back the two cut sides to remove the piece from the mold.

Step 14: Check, Check, One, Two

Is this thing on? Ok, good. Now it's time to show off what you've learned in this lesson!

    "id": "quiz-1",
    "question": "Is it ok for models to have undercuts when making a one part mold?",
    "answers": [
            "title": "No",
            "correct": false
            "title": "Yes",
            "correct": true
    "correctNotice": "That's correct! If the mold making material is flexible, like silicone and  urethane rubber, or alginate, it's possible to remove models with undercuts.",
    "incorrectNotice": "That's incorrect! Try again."
    "id": "quiz-2",
    "question": "Is it important to attach models to mold boards before making a mold?": [
            "title": "No",
            "correct": false

            "title": "Yes",
            "correct": true
    "correctNotice": "That's correct! If the model isn't secured, it can float up, ruining the mold." 
    "incorrectNotice": "That's incorrect! Try again."

Step 15: What's Next?

Get playful! Get creative! The possibilities with this style of mold is endless!

Next, I'm going to show you how to make a two part mold for more complicated models.

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