Introduction: 3D Printed Molds Casting Tutorial: Complex Shapes

About: I am a University of Edinburgh electronics engineering student.

Here is a very easy technique to cast complex shapes in 3D printed molds.

By complex shapes I mean shapes that are hollow, have channels or thin walls etc. Standard techniques usually involve making a negative shape of your object in sand or other material and then pouring the casting resin inside. This method is not something easy to do in a lab and it does not provide the possibility to easily cast shapes hollow from the inside. It also lacks repeatability and consistency in the final object.

It all starts with an idea - you want to make that minion figure from Despicable Me or a copy of someone's toe (why not haha).. Follow the instructable for a simple method to produce soft silicone casts out of 3D printed molds.

The process in 3 steps:

  • First we create the cast
  • Then, we mix two silicone compounds (usually or whatever you have) and pour them into the cast - gravity takes over and replaces the air in the mold with silicone
  • When the silicone cures we extract it from the cast and it's done!

Issues arise when we want to cast a complex object that has many surfaces or cavities.

What you need for this project:

  • Your design in 3D on CAD
  • 3D Printer
  • Cure silicone for your model (I have used Ecoflex)
  • Other tools such as carving knife, pliers, screwdriver or sandpaper may be needed

Step 1: CAD Model

Develop your model in some sort of CAD application. I find Tinkercad easy, useful and it's free but it's up to you.

What I am showing you is an object shaped like a ring. It has a curvature similar to bicycle tyre on the outside. The wall thickness is roughly 1mm (0.04 inches). The right part of the image shows just a quarter section of the whole model. As you can see the model has two flat rings at each side. Those details will have impact on the positioning of the cast which you will see further.

The finished part is quite delicate suggesting that the method can be applied to a variety of parts. Follow the details when you design your model:

  • Do not overcomplicate the model - the simpler it is, the better the chances your cast will come out nice and without bubbles
  • Do not create filled objects within hollow spaces - the final object has to come out as one piece
  • Do not make the object's walls very thin - this depends on the type of resin you want to use and its viscosity. A high viscosity resin (i.e. like honey) will need wider channels to flow through than a less viscous resin (like water).
  • A symmetric object is easier to manage in the next steps.

At this step you want to have a model of your final object with all its details as you would want it to look like.

Step 2: Hollow or Solid Model?

Depending on whether your model is hollow inside or it's solid, we will proceed in two different ways.

  • The tricky way: we have a hollow object on the inside. We have to make a cast that will support the model on the inside and then we need to remove that filament out of the final piece. This means that your final piece will have a hole somewhere and it will be big enough to allow the supporting piece to come out when the cured resin is stretched.
  • The easy way: Casts for solid objects are easier to construct. They do not require any internal structures. In this case you can jump to step 4.

Step 3: Generate the Inside Sturcure of the Model - for Hollow Models Only

In order to make a cast for a hollow model using my method, we will create an object that fills the model. (Other methods such as rotational casting may not require that).

In order to create the inside structure:

  • Generate a new solid block that is a little smaller than the original model
  • Overlay the original model on top and centre
  • Make the original model a 'Hole' shape
  • Combine the two shapes
  • Add support piece(s) - i.e. we have to have an inside 'hanging in the air'

mold inside

Now we have an exact copy of the insides of the original model. The redundant support bits are added for where this piece will be held to the external cast. This is where your model will 'hang in the air'.

Step 4: Generate a Solid Block Around Your Model

Generate a solid piece of your model if you don't already have one (for hollow models). Now, add a block that is significantly larger than the model. This will form the actual walls of the mold.

Step 5: Split the Block in 2 (or More)

Depending on the complexity of your model you may need to split the block in pieces. Basically, you want to make the mold easy to disassemble once your cast is cured and be able to extract the part from the inside. If, for example, your model looks like a sphere you have to have at least one split in the middle of the sphere so that you can pull the parts apart.

You want to avoid wedges that won't allow you to pull the cast's parts. Because we have a solid block in the cast, we cannot pull it out by squishing the model.

Step 6: Embed the Model in the Solid Blocks

Now, we will combine the model with the blocks. In my case as the model was symmetrical, I placed the model sideways half way into the blocks and combined them.

  • Position your model half way into the blocks
  • Combine the shapes
  • Add alignment pins to make sure the mold halves will meet properly aligned to the model
  • Add a central channel to allow poured resin to flow - make it quite wide. Again, more viscous materials will need a wider channel to flow through.
  • Add a chamber that will contain some extra resin on top of the mold - here think of gravity, it will make the resin flow down into the mold, so the bigger the chamber the more liquid to push down into the channel


Step 7: Print

3D print the design and put it together. The middle piece goes in between the two walls 'floating' because its sides hold it in place. If you look at step 3, the green bits on the sides of the part are the ones holding the piece in place. They snap into the two walls. Another method to achieve similar design is to hang the middle piece on a rod from above (applies to hollow models only).

Step 8: Pouring Silicone

Get your silicone of choice. Mix it and pour it in the mold. Here you can use many things I guess. I have used Ecoflex by mixing parts A and B 1:1 ratio. Check the stuff on their website (smoothon).

I have wrapped the mold with tape to stop the silicone from leaking out between the gaps. A rubber band will hold the mold's walls together.

When pouring I have also used a vacuum chamber where I placed the mold. That's not mandatory if you don't have access to a vacuum chamber but the final result looks better. This technique lowers the pressure in the chamber which starts sucking all the bubbles from the silicone. You end up having a better cast that lacks air bubbles.

Step 9: Ta-da It's Ready!

Let the silicone cure for 24 hours. Open the mold and voilà!

You will need to cut the channel between your part and the mold chamber on top.

As you can see sometimes the cast comes out with bubbles, but we can fix that!

Step 10: Tips and Tricks: Mold Geometry

There is no single rule on how to design a mold to make a perfect cast that will produce good quality parts consistently. Here are a few tips I found out myself that will save you a lot of time:

  • Keep in mind gravity - When you pour silicone into such a mold it flows down into the cavities due to gravity and this means it only goes downwards
  • Position your part in a way that it has only one peak (corner)that connects to the chamber
    • Think of pouring a cube shape
    • It will be positioned sitting on one of its 8 corners so it has a unique height point
  • Avoid flat horizontal surfaces - Those will most likely trap air bubbles which won't escape if your silicone has high viscosity
  • Minimise the channel contact area to the cast part: The connection between your part and the silicone chamber where you pour will have a large contact area. You will need to trim the cured part and this may be visible
  • Design a large volume chamber on top of the mold - The more silicone you have on top of the mold, the greater the force will be to push material down into the cavities

I have tried six different molds to cast the ring and the one that worked best is where the ring is placed vertically and the mold is made out of two sides and an inside piece. Other designs proved to be inconsistent and trap a lot of air bubbles.

Step 11: Tips and Tricks: Trapped Air

Not all casts come out perfect. Often you will see that parts with thin walls will tend to have bubbles all over.Those defects can be avoided using the following set of techniques:

  • Use the tips in the previous step to design your mold structure-flat horizontal surfaces will have bubbles
  • Pour while the silicone hasn't started curing - don't mix it for too long
  • Use a vacuum chamber - this will certainly improve the quality
    • While you mix compounds A and B by hand or using a drill small air bubbles will be captured into the silicone, like for example, when you make cream out of egg whites
    • Put the silicone mix into the vacuum chamber BEFORE pouring it - this will get rid of most of the air in the silicone
    • Now, after you pour the silicone you can place the whole mold again into the vacuum to help the air escape from the inside. As you see on the picture there is lots of air coming out from the mold. This escaping air is replaced by the silicone flowing down.

Of course, not everyone has access to a vacuum chamber but even without it you will be able to get good results.

Step 12: Tutorial End - Share Your Knowledge

Here's how you can put a 3D printer into use and create soft rubbery stuff out of silicone! The method is easy and produces fairly good results in no time. 3D printing can take 6-7 hours, then casting will be another day including the curing. From start to finish you can make your own cast detail in two days.

I hope you found this tutorial useful! Share your thoughts, comments and advice please!

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