Mold Making & Casting Class
Lesson 5: Two Part Molds
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Introduction: Two Part Molds

I try as hard as I can to keep all my silicone mold making in the form of flexible block molds. Frankly, they take less time. But once in awhile, you come across a model that you HAVE to reproduce and it demands a two part mold.

Ideal Models for This Kind of Mold

Dome examples of models that require two part molds are those that:

  • Have holes going all the way through them (i.e.: a donut!)
  • Are thin and floppy and can't stand up on end (like a fish)
  • Are thin and top heavy and can't stand up on end (like a mallet)
  • Are completely in the round, with no flat bottom (like a sphere)

Or any combination of those things!

Another reason to use a two part mold is if you want to make a rigid mold of a model with undercuts (i.e.: a plaster mold for ceramic slip casting.) Because there is no give at all in the material, you must find the part line of the shape that will allow the two mold pieces to pull apart and the model or cast to be released, free of undercuts.

A mug with a handle is a good example of this. A mug without a handle (if it has a good draft angle) can be a one part mold, but add a handle and you are introducing a protrusion (the handle itself) with a through-hole, both of which would get stuck in a rigid one part mold. Finding the halfway line (aka part line) of the handle and mug body and casting one half of the mug first and then the other, allows you to easily remove the mug from the mold.

Tools & Materials

Here's a list of what you need to make a two part mold:

  • Mold board (I used a 14" x 14")
  • Mold wall materials (I used pre-cut acrylic/plexiglass pieces)
  • Kraft paper, newsprint, or cardboard (to cover & protect your work surface)
  • Mixing containers
  • Mold making material (I used Smooth-On Oomoo 25 Silicone Rubber*)
  • Casting material (I used casting plaster)
  • Glue gun & glue sticks
  • SOFT sulfur-free sculpting clay
  • Stir sticks (paint style and tongue depressors)
  • Metal scraper
  • Exacto knife
  • Sculpting tools
  • Thin permanent marker
  • Ruler
  • Pencil
  • Notepad
  • Paper towels
  • Mold release - I use Mann Ease Release 200
  • Small disposable brush
  • Disposable latex-free gloves
  • Long sleeve shirt
  • Safety glasses
  • Dust mask (for working with plaster)
  • Apron

*(x2) 2.8 lbs kits for this particular mold.

Two very small additional things you'll need are short dowels:two pieces 5/8" long, one 1/2" in diameter and the other 3/16" diameter. These will act as your sprue and air hole. I will explain what these are and why they are necessary as we make the mold.

NOTE: You can make the sprue and air hole out of clay if you would prefer, they just won't be quite as uniform.

Prepping the Model

I wanted to make a donut trophy of sorts to give to friends.

Just a plain donut trophy on its own would be a good gift, but I decided to up the ante. I 3D printed the words "I donut know what I would do without you!" on a thin ring and attached it to the underside of the donut model.

Now it's going to be a GREAT gift. :)

Here are the steps I followed to attach my word ring to the donut model. (Just in case you also have friends and want to make your own!)

NOTE: If you'd like to learn how to 3D print your own anything, check out JON-A-TRON's free 3D printing class!

Expose the underbelly of the donut model.

Peel off one side of protective paper from a double sided adhesive sheet and place the word ring bottom side down on the adhesive. Use a sharp exacto blade to cut the ring out.

Remove a small section of the other backing.

Adjust the ring until you have it centered on the backside of the donut. Stick down the exposed adhesive. Remove the rest of the backing paper and press the ring firmly in place.

There you have it. One customized donut model!

Now use sculpting clay to fill any holes in your model.

Wipe away any excess clay.

Roll out some small coils of sculpting clay and fill in the outer and inside edges of the word ring. Smooth the clay into place with a finger.

Use a sculpting tool to clean up any stray bits of clay.

Aaaaand, done!

Calculate the Model Volume

Like we did for all previous models, calculate the model volume in fluid ounces and measure its height.

I get 6 fl oz.

And a height of 1 3/8".

Draw Part Line on Model

As we learned in the last lesson, 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 its removal.

With plastic parts like this one, they usually already have a visible part line from being manufactured themselves (in a mold!).

If the one on yours is visible enough, you don't have to draw another line over it. But if, like mine, it's not apparent enough, use a contrasting colored marker to re-draw it.

If you're using a non-plastic model or one you made yourself, you'll have to eyeball the part line. I believe in you!

Building Up Clay Support

In preparation for pouring the first half of the mold, we need to build up clay around the model to the parting line. I've outlined the process in the following steps:

Use a few small pieces of clay to level and secure the model so that the part line is parallel to the mold board surface.

If there's a hole (or holes) in the middle of your model, like mine, go ahead and fill it with clay up to the part line.

Use a pencil and ruler to measure and mark the mold wall lines. For a two part mold, you want the lines to be 3/4" from the model instead of the 1/2" we did in the other lessons. The reason for this is that we need to have room to add keys (more on what keys are in a bit!).

We're now going to start building up clay around the model to fill in the space between the model and the mold wall lines – bringing the clay just up to the part line and no higher.

Use your fingers and a sculpting tool to smooth and flatten the clay surface so that it comes right up to the model at the part line.

You can use coils to crisp up your square.

Adding a Sprue & Airhole

Now for a VERY important part of two part mold making: adding a sprue (and air hole if needed).

A sprue is a canal that you add so you can pour the casting material into the mold. The material that squishes up into it as a result of the pouring gets removed once the cast piece has set and been taken out of the mold.

You want the diameter of the sprue to be as small as possible so that the blemish on the finished cast piece is minimal, but big enough to accommodate the pouring in of material.

A way to help keep the sprue on the smaller side, is to add an even smaller canal as an air hole. This keeps the sprue from getting clogged up by trapped air in the mold and helps the casting material flow in smoothly, filling every nook and cranny of the mold as casting material is able to replace the air that is escaping out through the air hole.

NOTE: If you're making a plaster mold for casting a ceramic cup (as an example), the sprue would be the top opening of the cup and because this would be big enough of an opening for cast material to go in and air to go out at the same time, there would be no need for an air hole.

For this model I used two 3/4" pieces of dowel (remember, you can also make these out of clay), one bigger than the other (air hole = 3/16", sprue = 1/2").

Place the sprue so that the end of it will be flush with one of the mold walls. Put the air hole dowel a short distance away, at a slight angle. Don't worry if the air hole dowel doesn't reach the 'wall' edge, we'll make a clay bridge to help it reach in a bit.

Carve out enough clay so that the dowels sit halfway down into the support clay. Their mid-lines should line up with the surface of the clay, aka the part line.

Roll very thin coils of clay and fill in the gaps between the model and the dowels, as well as along the length of the dowels.

Use a sculpting tool to smooth the coils out.

Add a little clay 'bridge' to the end of the air hole so that it will reach the mold wall.

You're now ready to add the mold walls!

Building Mold Walls

For this mold, I chose to make the wall out of clear acrylic for two reasons:

  1. So it would be easy for you to see the process.
  2. They are reusable and you need to use the mold walls twice for a two part mold = reusable is better!

To calculate the height of the pieces needed:

  • Take the height of the model (In my case 1 3/8")
  • Add the extra 1/2" for both mold halves (1")
  • Add a bit extra to prevent material overflow (1/4")
    • Height: 1 3/8" + 1" + 1/4" = 2 5/8"

To calculate the length of the pieces needed:

  • Take the width of the model (in my case 4 1/4" Diameter)
  • Add the 3/4" on both sides (1 1/2")
  • Add a bit extra for construction and wiggle room (1 1/4")
    • Length: 4 1/4" + 1 1/2" + 1 1/4" = 7"

So I cut (x4) pieces of 2 5/8" x 7" x 3/16" acrylic (3/16" is the thickness of the material).

Start with the piece that will touch the side with the sprue and air hole. Make sure it lines up.

Hot glue it into place. Hold it until it's almost dry so it doesn't fall over.

Trim any excess clay off the model so that the remaining walls will line up with the mold wall lines drawn on the board.

Glue the remaining 3 walls into place.

Make sure to glue the wall connection seams as well as the bottoms.

Sealing & Leveling the Clay

Now we need to fill in any space between the clay base and the walls with more clay.

Make medium sized clay coils and run them all around the inside edge of the mold walls, filling the gaps if any.

Use your fingers and a sculpting tool to smooth the coils until the clay surface is flush with, and perpendicular to, the mold walls.

Don't forget to add tiny coils to the ends of the sprue and air hole that touch the wall to seal them up!

Add Keys

This is a VERY important step. We are going to add what are called keys to the clay surface.

These are depressions that will create positive shapes in this first half of the mold – that will fit perfectly into mirrored negative shapes in the second half.

Without keys -– if the mold halves had just two flat surfaces – there would be no way to accurately align (register) the two mold halves.

To make keys, use a sculpting tool with a rounded end. If none of your sculpting tools have one, look through your kitchen drawers. There's a good chance that you'll have a wooden spoon with a softened end. Or you can always use the tip of a small teaspoon and twist it like a drill bit to create a shallow round depression (think a quarter sphere).

If you're using a rigid material like plaster, one substantial keyhole in each corner will do the trick. But if you're using a flexible mold material (like I am), make several keyholes following the contour of the model, about 1/4" from the model's edge. If you have a hole in your model, add keys to that too (see above image).

A well aligned mold is a happy cast! So three cheers for keys!!

Calculating the Mold Material Volume

Use the formula for a rectangular mold, that we went over in the Flexible Block Mold lesson, to calculate the mold material volume needed for the first half of the mold. REMEMBER to cut the total amount in half since you're only pouring half the mold at a time!

Use a ruler and pen to mark where you want the mold material to come up to – which will be 1/2" higher than highest point of the model.

Spray Mold Release

Spray a fine mist of mold release on the model and the inside of the mold walls. Let dry for 30 minutes before pouring the first batch of mold material.

Pour the First Half of the Mold

Once the mold release has dried for 30 minutes, don all your safety gear and carefully mix and pour the calculated amount of your chosen mold material up to the your marked line. Follow the steps we did in the Simple One Part Mold lesson and the manufacturer's instructions for your material.

I like to mark my mixing container if it's an in-between amount. I'm going for 18 oz total, so I marked 9 oz for my 1:1 needs.

If using silicone rubber, don't forget to stir your ingredients separately before mixing them together!

If you have any fine details on your model, like I do with my slightly raised letters spelling "I DONUT KNOW WHAT I WOULD DO WITHOUT YOU!" :), I recommend using a small, soft brush to apply a thin coat of the mold material onto the model details before pouring the mold.

This ensures that mold material will completely cover the details and that no air bubbles will get trapped in between them.

Note: Video is 600x normal speed. :)

Pour the first half from high up the same way we did for all previous molds, so that the material finds its way into all the key holes and model details without forming air pockets (video is WAY sped up. Please go slowly and take your careful time). Fill it to the marked line. Then give the board a shake and shimmy to help the material settle into place and any trapped air bubbles, rise to the surface.

Then, let the first half fully cure before moving on to the second half. This Oomoo 25 product takes about 1 1/2" hours to dry. Like greased lightning! So you won't have to wait too long.

Clean Up First Mold Half

And we're back!

Once your material has fully cured, use the scraper and exacto to remove the mold walls and the mold from the board.

Use the scraper and exacto to clean up the mold board and walls, removing any clay or glue bits.

Turn the mold upside down so that the clay is exposed. Carefully remove the clay from the mold and model WITHOUT removing the model, sprue, or air hole dowel from the mold.

Use a sculpting tool and your fingers to clear away any lingering bits of clay.

Make a few more tiny clay coils and fill in any gaps between the sprue, air hole, and model like we did for the first half.

Finally, trim off any edge bits that might get in the way when we reconstruct the mold walls to pour the second mold half.

Prep for Casting Second Half of Mold

This part has just a hint of Groundhog Day. We are going to do exactly what we did for the first half of the mold, minus the medium sized coils around the edge.

Rebuild the mold walls, hot gluing them into place.

Fill the gaps between the sprue and air hole and the mold wall with tiny clay coils.

Measure and mark the mold material height line.

Spray & Pour

There's a slightly different protocol for applying mold release to the first half of the model, in preparation for pouring the second half.

This time, spray a thin coat of release over the mold and inner mold walls.

Then take a small disposable brush and gently brush the surface of the first half of the mold and the model, being careful to avoid the small clay bits. NOTE: Video at 400x normal speed.

Spray with one more very thin coat and leave to dry for 30 minutes before proceeding.

Once 30 minutes has passed, put on your safety gear and mix up another batch of your chosen mold material (the same kind as the first mold half) and from on high, carefully pour the second half of the mold.

Set the mold aside and let it cure for its prescribed amount of time.

The Breakdown and the Reveal

After the second half of your mold has fully cured, it's time to break down the walls and see what's inside!

Use an exacto knife to cut the glue and separate the walls.

Lift the mold away from the walls and mold board.

Setting the mold down on the work surface, gently peel up the corners and remove the top half of the mold, revealing the model.

Take the model, sprue, and air hole dowels out of the bottom half of the mold.

Use a tool to remove any remaining clay bits.

Give both halves of the mold a good bath and scrub to remove any clay residue.

You are now ready to cast your mold!!

Casting a Two Part Mold

Now you have your two part mold. That's awesome! And you'd like to cast it in your chosen material. We're almost ready!

There are a couple of things you need before you attempt to fill the mold:

  • Two backing boards (if you made a flexible mold)
  • Mold straps or heavy duty rubber bands

NOTE: The two backing boards are only necessary if you have a flexible mold that needs rigid support in order to not deform when applying the binding straps or bands (see above image).

Backing boards can be made out of any rigid, perfectly flat material. Acrylic/plexiglass, melamine, or thin high quality plywood are all good options.

Measure the back side of one mold half (the other backside will be the same size) and cut two boards to those dimensions.

Next up are the mold bands/straps. If you're using the heavy duty rubber bands, for added size flexibility, I like to cut them into smaller bands and tie knots to adjust their size. Mold straps also work well, but I tend to only use those on larger molds and use the less expensive rubber bands for smaller molds like this one since you need at least 4 per mold.

To assemble your mold in preparation for casting, spray the inside of the two mold halves with the appropriate mold release, line up the two halves using the keys, place a backing board on each side of the mold, and use bands or straps to secure the two halves together. Put at least two bands in each direction like pictured, making sure not to cover the sprue or air hole.

You're now ready to cast your mold!

I chose to cast mine in plaster. Plaster is a paintable material that is cheap and easy to work with making it ideal for my donut trophy as I want to paint it GOLD!

To mix small amounts of plaster, do the following steps:

Fill a small mixing container with slightly less water than you need for your mold. If you'll remember, my donut model took up 6 fl oz of space, so I started with 4 oz of water.

Wearing your dust mask, disposable gloves, and apron, sprinkle plaster a little bit at a time into the water. Let one sprinkle batch soak into the water before adding a second. Repeat this process until the plaster forms little peaks that don't quickly sink into the water. Stop adding plaster and let it sit for 2 minutes before stirring.

Stir gently for 3 minutes.

Pour into the mold until a bit of plaster overflows out of the sprue and air hole.

Let set for 2-6 hours depending not the size of your mold. If you're unsure of how long to leave yours, let it sit overnight before de-molding the cast.

Once the plaster has set, remove the bands, gently pull the two mold halves apart, and remove the cast.

The plaster cast will take anywhere from one to three days to completely dry, depending on the size of the piece. Once it's dry, you can paint it.

Put on Your Smart Pants

Cause it's quiz time!!

    "id": "quiz-1",
    "question": "Which of the following shapes would require a two part mold?",
    "answers": [
            "title": "A medium sized rock",
            "correct": false
            "title": "A mallet",
            "correct": true
    "correctNotice": "That's correct! The long, thin, and top heavy nature of this object would make it very hard (if not impossible) to use a cut flexible block mold.",
    "incorrectNotice": "That's incorrect! Try again."
    "id": "quiz-2",
    "question": "Why are keys important to two part mold making?": [
            "title": "They help the two halves of the mold align with each other.",
            "correct": true
            "title": "They act as a moat to keep the casting material from leaking out.",
            "correct": false
    "correctNotice": "That's correct!",
    "incorrectNotice": "While a well aligned two part mold is less likely to leak, this is incorrect!"

The Finish Line!

You have run a good race!

Now that you're armed with the fundamentals of basic mold making and casting, the sky's the limit!

We've barely scratched the surface of what's possible to do with these techniques, so I encourage you to take your new knowledge and try things! I've added lots of great instructables to the bottom of the main class page and there are tons more on the site.

As always, if you come up with a cool project that you'd like to share with the community, make an instructable!

And if you have any questions for me, please ask away in the question section at the bottom of each lesson or send me a message directly to my profile.

Thanks for spending time with me and I hope you enjoy your new skills!


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