Identical Molds for Rotocasting

Introduction: Identical Molds for Rotocasting

About: Industrial Design Student

This instructable is the result of Kean University's Industrial Designers Society of America (IDSA) student chapter's experiences in low scale toy manufacturing for Five Points Festival NYC 2018.

Some images have image notes for more information.


When casting products, usually one or two molds are made with a flexible silicone rubber. An example of this is a silicone ice cube tray that is in the shape of robots, geometric figures, or even sinking ships. These types of molds require that you fill them up all the way to the top with material such as liquid plastic, water, chocolate, etc. Molds like these are ideal for small items which can give you a large output (think about how many ice cubes are created from one tray). However, for most people, larger items cannot be made this way, especially if you are doing it on your own. For people who want to save on materials by creating a figure that is hollow on the inside, they turn to rotocasting.

What is Rotocasting?

Rotocasting involves creating a multipart closed mold which is filled partially with liquid resin. When the mold is closed it is then rotated on multiple axes so that as the resin solidifies, it creates a shell on the inside of the mold, but has a hollow air pocket in the center. This is beneficial since it allows the manufacturer to save on materials, which can hopefully lower the cost of the overall product.

Now back to the point of this tutorial. Why would you need identical molds, and why would you need to make more than one in the first place?

Well, it is true that you could make one mold and then just manually rotate it by hand while sitting on the couch watching TV (which some people do) but that means you would only get one product for the time spent rotating. In a rotocaster (the machine used for rotocasting pictured) it relies on having two similar molds balanced against each other as the machine rotates. So, minimally you would need to create two molds, but if you want to really manage your time well, you would set up more (at least four, ideally six).

In the case of the rotocaster, a liquid plastic with a relatively quick setting time is used. Some plastics take a few minutes, while others take a day to fully cure. In our case, we used a liquid plastic (Smooth-On 65D) which is suitable for rotocasting and fully cures in about 15 minutes.

Here is the breakdown of why you would want six nearly identical molds.

Time starts when liquid plastic is poured into molds 1 and 2 and are set in the rotocaster to run for 5 minutes.

5-10 minutes: Molds 1 and 2 are taken out and placed to the side. Liquid plastic is poured into molds 3 and 4 and are set in the rotocaster to run for 5 minutes.

10-15 minutes: Molds 3 and 4 are taken out and placed to the side. Liquid plastic is poured into molds 5 and 6 and are set in the rotocaster to run for 5 minutes. Once the rotocaster is almost done, molds 1 and 2 are finally cured and you can then prep them to go in the rotocaster.

This mold rotation helps you keep producing pieces one after another.

So, how do you create the multiple molds?

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Step 1: Materials

So, there are actually quite a few things needed to accomplish this and are broken down into each category. This instructable is assuming you have a rotocast machine (which means you are familiar with molding and casting) and want to increase your productivity, so some advanced methods have been used and will not be explained.

1. Positive

As for every molding and casting project, you need a positive that you will be replicating. Our positive, Otto, was originally created using clay, which was then 3D scanned. From there it was manipulated using ZBrush and Solidworks which are 3D modeling softwares. The final 3D model was then printed using a Form2 printer.

Of course, your positive does not need to be a 3D printed item, it can be a handworked piece as long as it is sealed properly. The only issue is that you may have difficulty later one when I explain how we made the lid of the mold.

2. Mother Mold

So, first off, what is a Mother Mold? A mother mold is typically an outer shell that encompasses and supports the inner flexible mold. This is to help retain the mold's shape during the casting process. In our case, we made a mother mold specifically because Otto had a lot of curvature and undercuts which resulted in a lot of lost material when casting. Our first test mold wasted a lot of silicone so our professor came up with the idea to save on materials by making the mold a shape that was better formed to Otto. This also helped us make sure that all the molds were nearly identical to each other and had relatively the same weight for balancing in the rotocaster. It was also easier to take the finished products out of their molds because the silicone wasn't as thick or block shaped.

3. Mold Making Materials (Some items are not shown in images)

1. Ease Release 200 which is a mold release. You can use whatever brand you like as long as it works for silicone to silicone.

2. Carpet tape. This is to hold the positive down to the base while still being able to take the base off later.

3. Xacto knife / scalpel / extra blades

4. Silicone. We used Smooth-On products Mold Max 14NV and Mold Max 27T. The main body would be the more flexible 14NV while the lip and bottom were made with 27T to give it more rigidity in the rotocaster. We also used Accel-T, which accelerates the curing time of the silicone.

5. Scale. The silicone we used was not a 1:1 ratio and was instead by weight.

6. Newspaper / cardboard

7. Measuring cups / saved containers. This is for mixing and pouring the silicone. Since the ones we used were weighted, it didn't matter that we used saved yogurt containers. Just make sure everything is clean.

8. 91% Isopropyl Alcohol. This is for cleaning things.

9. Electrical tape / rubber bands. This is for holding the mother mold together. Electrical tape doesn't leave an residue on your mother mold and it has some elasticity so you can really make sure things are tight.

10. Popsicle sticks / larger stirring sticks meant for paint cans

11. Paper towels

12. Latex free gloves. I use black Nitrile gloves.

13. Scissors

Step 2: The Positive


There are some things you need to keep in mind when creating a positive for rotocasting. The first is that not every form will work. The reason lies in the nature of the rotocast machine.

Our professor's rotocast machine could fit something approximately 5" x 5" x 5" on his rotocast plate, so this limited the size of our figure. So, before you start, you should measure how much space you have to work with on either side of your rotocast plate before starting on your design.

Now, you also have to take into account that the dimensions you measured, includes the mold. So, we had about 4.5" x 4.5" x 4.5" to work with if we wanted at least 1/2" of mold around our form. We ended up being smaller than that, but you should design your item to the space you have.

Also, it is easier to design an object / toy / product that has a flat bottom. This lets it sit well and also leaves space for any marketing / information that needs to be on it without taking away the aesthetics. Ours was created so that it had Brutherford Industries, Kean, a year, and a logo on the bottom. When our professor added these to the bottom of toy, he made it so there was a small recess with the information extruded out. When the figure was made, all the information was projecting above the recess. If you want the information to looked engraved, you will need to do the opposite. Using 3D software will make this process easy, especially if you are used to 3D printing your models. If you are creating your positive by hand, you will need to engrave around your information.

Besides having a flat bottom, your figure should not have any delicate extremities. Everything should be designed in relief from the main body. You can add as much surface detail as you want, but you can't have thin limbs sticking out or other thin protrusions. This is because as the mold is rotating the liquid plastic needs to get into those gaps to form a shell. If you have a very thin appendage protruding from your figure, such as a limb, horn, or other detail, the plastic might not be able to get into that space or it may go in, but not come out again. This will result in either an unformed part, a part that is barely attached, or a section of your mold that has a bit of cured plastic that you can't pull out. You can still create textures, limbs, clothing, and other details by having the parts together. Just keep in mind that you are making a shell of your figure.

Step 3: The Mother Mold

Making a Mother Mold

Since this instructable is focused on creating identical molds, the focus of this process is the mother mold. Having one that is 3D printed makes sure that everything is nearly identical, while at the same time it does not rely on physical measuring.

I mentioned previously that our positive, Otto, was scanned and manipulated using 3D software. Due to this, it was easy to build a casing around him using Solidworks. The end result was printed using a Form2 printer with a more rigid resin.

Most of you reading this probably do not have the software, but you can create your own using free software such as Fusion 360. If your positive was hand made and you do not have the means to scan it in, create a positive that is roughly the same shape / dimensions and build around that.

The mother mold should have at least 3 pieces (2 sides and a base) and there should be an open side for pouring. You can set the thickness to 1/8" and round all edges.


The base will act as a pedestal and also a key. The base should fit within the dimensions you have and the center should be a piece that is about the same shape as the bottom of your item, but 1/8" smaller. If you are using software, you can do this by offsetting instead of remaking the shape. In the photo provided, you can see that the pedestal to hold our piece is in the center, with key holes around it. When making the key holes, draft them slightly to make pulling them out easier. Make sure the draft inwards when extruding the keys on the base.

The base itself is a shell and a drawing view is provided above to give you more of an idea of what it is like.


We have 2 side pieces that fit together in a way that there is an overlap of material to help catch any silicone that seeps out. You can see it in the above picture where the two side edges are touching and press together. However, there is still some seepage so we ended up with a little bit of flashing.

When you made the mother mold, make it so that it follows the shape of your positive since it results in less wasted material. Silicone can get expensive so you want to save as much as you can. Basically, if you were to imagine a box built around your figure, add walls to reduce empty space. From there you can refine and refine until you get a shell that works for you. The sides should be higher than your positive because you need to have silicone cover the top of it as well.

It is possible, based on your project, to have two pieces (a base and one side piece) but that depends on the shape of your item. In our case, it was not possible.

Step 4: Prepping the Positive

Your positive should be prepped with the appropriate method. Since ours was a resin print, it was cleaned with alcohol, dry / wet sanded, cleaned again, and then rubbed with a little bit of butcher's wax.

The important thing about silicone molds is that they will take in every detail. This is great for when you want to capture intricate details and textures, but that also means that it will pick up any fingerprints on your model. If there is a fingerprint on your positive, the mold will take it, and every cast you do will have the same fingerprint on it. It may be something you want, but in most cases it is not.

So, to prep the positive for molding you need to clean away any fingerprints. This means that you need to put on gloves from here on out.

After you have your gloves on, you can then clean your positive with a paper towel and some alcohol. You also need to make sure you clean the bottom of your item so it properly adheres to the base in the next step.

When your positive is clean, set it aside on a clean surface.

Step 5: Prepping the Base

Since this is showing how to use a mother mold, I will be skipping over detailed instructions of how to mix and pour the silicone.

1. Prep your base. As mentioned, the base acts as both a pedestal for your positive and the key for the lid of your mold. Clean it with some alcohol and paper towels and let air dry for a few minutes. Cut a piece of carpet tape and flatten the sticky side onto the pedestal, pushing out as many air bubbles as you can with your fingers. Cut away the excess tape so that you only have tape on the pedestal and none overhanging. You may need to change your blade if it is not cutting through. I found it easier to set your blade at an angle against the pedestal and cut around the outline of it. It should look like the image above.

2. Spray with mold release. The carpet tape has a protective cover over it, like double sided tape, so before you tear it off, spray the base with mold release. Then when you tear the tape off it will still be sticky but everything else will be sprayed. However, don't tear off the tape until you are ready to attach the positive so that no dust or other particles get in the way of the tape sticking to the positive.

3. Set it aside as you prep everything else.

Note: I mentioned before that you might have difficulty if you made your positive by hand. Wood, clay, or other materials may not adhere properly to the carpet tape, so you might have to create a separate item to help you stick it to the base. Since we used a 3D print, the carpet tape was able to adhere to the smooth, rigid surface but was able to be pulled off without breaking the positive. Unfortunately, I don't have any sure fire way for those of you using other materials.

Step 6: Putting Everything Together

By this point, your positive and base are prepped and ready to go. Before you start assembling everything together, you need to also prep the sides of your mother mold. All you need to do is spray some mold release onto the insides so that it is easier to pull everything apart later. Now, you can put everything together.

Attaching the Positive to the Base

I will warn you that this is probably going to be the most frustrating part of the molding process. You will get better at it, but it will still take you multiple tries.

1. Tear off the protective backing from your carpet tape. When holding the base, hold the sides and use an Xacto knife or scalpel to catch the backing by wedging the blade between the backing and the tape. You can then use a flicking motion to get the backing to lift off the tape in one spot. Then you can use your fingers to pull the backing off completely. If the carpet tape comes up from the base pedestal, just press it down with the blade of your tool.

2. Press the base to the bottom of your positive. When you do this, make sure it is centered. Press down on it until it is secure. You want this to not lift off the base when you are pouring the silicone, so if it comes off very easily, take off the tape and do the entire process again. It is frustrating, but if it didn't stick the first time, it won't stick any more. Between each try, you need to clean the bottom of your positive with alcohol. If you are having a really hard time, try using a heat gun on the tape for 30-50 seconds. You want to make the adhesive stickier by increasing the temperature, but you do not want to distort your base. You know when you are secure when you give the two a slight pull apart (very slight pressure of the fingers), but they remain stuck together.

Once it is good and stuck, spray the entire thing with mold release. This will help it come out of the mold easier.

Put the Mother Mold Together

1. Attach your pieces. If you have two side pieces of your mother mold, attach them around the base.

2. Secure with rubber bands. At this point you should have a piece that is all together and the base does not fall out when you lift the sides. If this is the case, you are going to have a lot of leakage. Put rubber bands around the sides to hold it in place.

3. Secure the seam with electrical tape. In your mother mold, the sides should overlap with the base on the bottom. Tie electrical tape tightly around this overlap to close off the gap. There will be some leakage from here regardless, but if you use an accelerator for your silicone, it should cure before leaking too much. If you don't use an accelerator, then just wait to see how much has leaked. You can always add more silicone to the main body.

Now your mold is ready for the silicone.

Step 7: Pouring the Silicone: Main Body and Bottom

As mentioned, this mold had a main body of Mold Max 14NV and a bottom and lid made from Mold Max 27T. I won't be going over any specifics regarding mixing and pouring silicone.

Main Body

We wanted the main body of the mold to be flexible enough that we could stretch it and grab the end product, peeling the silicone back if we needed to. Mold Max 14NV cures white and had the flexibility we needed. Before you start, make sure the surface you are working on is level.

1. Mix your silicone. Read the instructions and mix. You can either take your best guess on how much silicone you need, or calculate it. If you pour too little, you can always add more, but it is a waste if you make too much. However, you need to leave at least 1/4" of the top of the mold empty for the 27T later on.

2. Pour the silicone. When you pour, you want to tilt the mother mold slightly to reduce air bubbles (you will level it out again halfway through the pouring process). Pour high, slow, and thin. A high, thin stream will reduce the amount of air bubbles that get in the mold. Pour until you have 1/4" of space left at the top of the mother mold.

3. Wait. Depending on your silicone and use of accelerator, the curing time will be stated in your silicone instructions.


Once it is cured slightly and not tacky then you can pour on the other silicone. While the silicones will bond together if you pour one right after another, you want to wait so that the 27T (which is denser than the 14NV) doesn't start sinking through and possibly disturb the rest of your mold. DO NOT put mold release between the two layers of silicone, otherwise they will not stick together.

Step 8: Pouring the Silicone: Lid

So, now that you've gotten this far you can do the lid. It is important that the surface you are working is level because you will be turning everything upside down. On that note, you should make sure that your silicone is fully cured.

Turn Everything Upside Down

Your silicone is fully cured so you can move on to the next steps.

1. Take off the rubber bands and tape. You will need to remove the sides and the base. Also remove any excess silicone that may prevent you from pulling things apart. If you have overpoured, then you will need to cut those parts off. Simply cut around the top of your mold which will release the main portion from the rest. You can then pull off any excess silicone.

2. Pull off the sides of the mother mold. Do this first because the base is still stuck to the positive with tape and you won't be able to get a good grip with the sides in the way. Instead of pulling at the seams, I would start at the top of your mold, where the 27T is and start prying the sides away from there. The mold release you used on the mother mold should help you pull them off easier.

3. Pull off the base. Now, at this point the base should still be stuck on the positive, which is currently in the mold. Take a firm hold of the mold and pull the base off. It might take some strength, but try not to distort the base as you pull.

4. Clean up the edges. Remove the tape from the base pedestal and throw it away. Now, if you look at your mold, there may be some silicone that got through some gaps of your tape and positive. You can rub those away with your gloves, or you can use an Xacto to cut the edges of the interior cleanly. You don't want anything that will be hanging in the way when you take the positive out of the mold.

5. Mold release. Spray mold release on the walls of the mother mold that are at the base. This will help pull it off easier. Spray mold release on the side of the silicone of the main body where it will touch the lid in case some drip, otherwise silicone will stick to itself and you won't be able to take it off. Put the mother mold back around the silicone, just without the base. Secure it with rubber bands. Mold release the top, making sure to get a good coat on the face of the silicone that has the key holes and the bottom of your positive. You don't have to worry about silicone seeping into your mold since the positive is in the way. This is why you made the pedestal slightly smaller so that silicone can't run off the sides of the figure. If you don't spray enough mold release, the 27T that you pour will stick to the 14NV and you will end up have an encased positive and not a lid.

6. Secure everything with rubber bands. This is so that it doesn't come apart while the 27T is curing.


1. Mix your 27T silicone. If you placed any lettering or logo on the bottom of your positive, it is important that you have minimal to no bubbles so make sure to mix slowly. Again, you will need to estimate how much silicone you will need, but it will be a small amount compared to the main body.

2. Pour. Again, you want to pour high and thin. You don't need to tilt the mother mold, just pour directly of the face of the bottom. Having a thin stream will reduce the bubbles.

3. Wait. Make sure you let it fully cure before moving on.

Step 9: Finishing Up

The mold should be all cured at this point, which only leaves getting the positive out.

1. Clean up the lid. If you overpoured the 27T on the lid part, or have excess that has dripped down, cut the lid off by outlining the perimeter with an Xacto. Then you can pull off any excess silicone.

2. Remove rubber bands.

3. Remove the lid. At this point the lid should come off easily. If you have some thin portions stuck, you can pull until they come apart or you might have to cut them apart. Ideally, you want the lid to be a perfect match for the rest of the body.

4. Remove the positive. After the lid is taken off, just pull out the positive in your mold. Depending on the shape, you may have to peel the silicone over the sides of itself which is why the 14NV is a good choice since it is very flexible.

5. Repeat.


You've made one mold using this method, congratulations! Now, you just have to do it again 3-5 more times. The steps are the same and you will get better the more molds you make. It is a good idea to jot down the amount of silicone you used per section of the mold since it will be roughly around the same amount each time.

Step 10: Production

Hopefully this was helpful to you and I wish you luck! This will not raise you up to master level at mold making, but it will help you on your way there.

Again, this instructable is the result of Kean University's Industrial Designers Society of America (IDSA) student chapter's experiences in low scale toy manufacturing for Five Points Festival NYC 2018. As this was a group effort, I would like to acknowledge our professor, Ryan Rutherford, along with my group mates Carly Ciardullo, Imani Pierre, Sara Camacho, Clara Ponce, and Emma Mantell for all their hard work.

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    1 year ago

    Great information about rotocasting!