Casting Molds Using 3D Printer & Vacuum Forming

A couple of years ago I bought the Mayku Formbox mostly on a whim and partly because I saw potential in the use of vacuum forming to make molds for casting. Over the past years I've experimented a bit and got some good results using 3D prints as countermolds to create detailed castings with concrete, plaster and even chocolate!

This instructable describes my workflow from design to finished product as well as some tips. Even if you don't have access to a 3D printer and/or a vacuum forming machine there may be parts of this workflow that you can apply to your own process.

The key parts of my workflow are as follows:

1. designing the product as a countermold in 3D software
2. printing the countermold using a 3D printer
3. vacuum forming the mold using the printed part(s)
4. casting the product using the mold
5. decorating and finishing the final product

Because this instructable covers my entire process from design to finished product there are a lot of tools, supplies and machines involved. Below I will list the most important ones I've used and there will be more detailed suggestions in the following steps.

1. A computer with design software to create your 3D printable countermold. I used ArchiCAD because that's what I work with everyday, but Tinkercad or Sketchup are good alternatives.
2. A 3D printer to print the countermold. I used an Ultimaker 3 with Cura to slice the model.
3. 3D printing filament. Because the printed model is not the final product, color is not important. So you can use whatever filament is not being used much for anything else.
4. A vacuum forming machine. I used the Mayku Formbox, but there are other machines available and you can also make your own of course. There are several instructables that can tell you how.
5. Plastic sheets suitable for thermoforming. I used PETG clear sheets that came with the Formbox, but there are many other plastics that can be used for thermoforming.
6. Casting material: plaster, concrete, jesmonite, chocolate, or whatever you like to use.
7. Sanding paper to smooth your cast.
8. Your choice of coloring method for decorating your castings.
9. Some sort of varnish or permanent spray to protect your finished product against the elements.

First you need to decide what to make and how big you want to make it. Keep in mind the maximum print volume of your 3D printer as well as the work area of your vacuum forming machine. Another thing to take into account is the amount of casting material that is needed to fill the mold. Large models take a long time to set and cost a lot of material so I suggest you start with a small or flat object to get familiar with the process.

In this instructable I will show a couple of products I've made using this method like trinket dishes, paperweights and chocolate gifts.

The most important thing to remember is how the vacuum former will shape the plastic around your object and how the air passes around and/or through your model. You need to be able to take your model out of the mold after and you want to create as much detail as possible.

A few tips to make sure you get a good result:

1. The model should be wider at the base than at the top so it can be removed from the mold after forming.
2. Add holes or gaps to your model through which the air can be sucked out to create more detailed shapes.
3. Don't make your model too thin. It might collapse or cave in under pressure during vacuum forming.
4. Add a separate model for an inner mold when you're making larger objects. This saves material and reduces setting time.

The models were created with ArchiCAD, which is BIM modelling software used in our architectural firm. Any other (free) 3D modelling software like Tinkercad or SketchUp will work just as well. In the software I used you can set an angle for the edges. A slope of about 85 degrees was enough to get the model out of the mold without problems.

The 3D models are exported as .stl files to be printed on a 3D printer. You will need slicing software to slice your model into layers. Most common 3D printers come with their own compatible software, but there are 3rd party apps as well. I used the Cura software from Ultimaker.

IMPORTANT: Make sure your sliced model is strong enough to withstand the pressing force from the plastic sheet during vacuum forming. Your model will get hot and might deform under pressure when the material gets soft. Use enough infill or print a solid model to maintain the shape of your design.

I used PLA filament to print the models at 0,15mm or 0,1mm layer height ('Fine' setting in Cura). The settings will depend of the level of detail in your model and the type of filament you're using. Because vacuum forming can transfer a high level of detail it is recommended to check your prints and make sure there are no messy bits or jagged edges.

To get the best results you can try to smooth out your printed model to get rid of the layer lines. There are several ways to go about this which I won't get into here, but taking a bit more time preparing your 3D print might save you time later with finishing your castings.

I'm aware not everyone has access to a 3D printer so I just wanted to show you an alternative method of making a countermold. This model was made using cardboard and craft foam. I started with a printed image of a drawing I made and traced in onto a piece of cardboard using a pen. I cut out the shape (which was a real ** in the ** by the way) and placed it on top of a heart-shaped piece of cardboard. Using that I cut the shape out of a 1/2 inch (10mm) thick piece of sturdy foam.

To ensure the vacuum former would create all the details I made several holes with a needle in all of the inner corners.

IMPORTANT: do not use foam only as a base model. The plastic sheet used in vacuum forming gets very hot and will probably melt your foam! The cardboard I used here protected the foam from most of the heat.

The basic steps involved in vacuum forming are these:

1. Plug in your machine and connect a vacuum cleaner hose
2. Wait for the heat element to reach the desired temperature.
3. Heat up a sheet of thermoplastic until its soft and pliable.
4. Place the plastic sheet over the object and use a vacuum to form the plastic around the object.
5. Wait for the plastic to cool down a bit and remove it from the object.

Before you start with the vacuum former it is good to know that your 3D printed model will get hot and might deform and may only be used once. Check your print to find possible weaknesses and try to reinforce these if you can. The cat-shaped print caved in the first time so I added a cardboard piece to get a flat surface in the final mold. With this in mind I added a cardboard piece in the heart-shaped print right away.

Because each vacuum forming machine is different I will not list my settings here. I will give you some tips to get better results:

1. Do not place multiple objects close to eachother because there migth not be enough plastic to stretch around each one, resulting in folds and creases in the plastic sheet and a mold that is not formed neatly around your models.
2. Let the plastic sheet heat up properly and get really soft. This will make it easier to stretch around your models and will minimize the chance at folds at sharp corners in your model.
3. Set your vacuum cleaner at maximum power to get the best results. The plastic will cool down as soon as you place it over your model so you want to get as much suction as possible in the first couple seconds.

Once your molds are formed, let them cool down for a bit and remove your models by blowing air around the edges. Cut off any excess plastic around your mold to make it easier to work with later and not take up as much space. Unless your mold does not have a flat top, in which case it is better to leave some plastic around your mold so you can place it inside a vase and keep it level while pouring your casting material.

The finished plastic molds can be used with various casting materials. For the examples in this instructable I used plaster and concrete. The chocolate castings were done by a friend.

For both plaster and concrete the method is the same. You mix the powder with water in a specified ratio to get a smooth mix that you can pour into your mold. Shake or vibrate the mold carefully to bring any airbubbles to the surface. This prevents holes from forming in your final product. After this you wait for the material to set enough to remove it from the mold. You then leave it to set for another while before it is fully hardened and ready for post-processing and decorating.

The exact water to cement/powder ratio as well as curing times vary between materials and even brands so I won't suggest any specifics. Most casting powders come with an ideal mix ratio and some even have measuring cups that are already made to the required ratio for powder and water.

I also have a couple of tips that might help you along:

1. Mix enough casting material but never more than you can process within a couple minutes. Most mixtures start to set as soon as they are mixed and can get lumpy before you have time to fill your mold.
2. Have everything ready and within reach before you start to mix your casting material. Work quickly so you have enough time to pour and make sure the material gets into all the corners and detailed parts of your mold.
3. Start with the water and add the powder slowly while stirring to get a more consistent mix without lumps.
4. I found that adding a little bit more water worked great when mixing plaster. This gave me a smoother, more liquid mix that produced a cleaner result. Because it was more fluid it levelled itself out better than the ratio proposed on the package.
5. To help the material to get into the smallest corners of your mold you can use something like a needle or a paperclip directly after you fill the mold. Carefully look from the sides or the bottom to see where there are small airbubbles and use the needle to push the material into the corner.
6. When using inner molds for larger objects, get something heavy to press down on it. The liquid will try to push the inner mold up.

Depending on the quality of your mold and the consistency of your casting material you will probably get varying results. Some of the first molds I made had sharp edges where the plastic folded into itself. At these points the concrete or plaster left excess material that created sharp or rough edges. For other models the ridges from the 3D print were visible in the cast.

Fine grit sandpaper works fine for smoothing out plaster casts. For concrete it's a bit more difficult since the material is a lot harder. I used a disk sander to smooth some of the jagged edges.

The best way to avoid this is to spend time on your 3D print and get it as smooth as possible before vacuum forming the mold.

Once your models are fully set and smoothed you can choose to leave them as they are in their natural color or decorate them and make each cast unique.

For the cat-shaped trinket dishes I tried a couple techniques and styles to give each a distinct look. Using Sharpies and Posca Paint markers I created some different modern art versions. I added a few layers of permament spray to protect the final piece against wear and tear and also against fading colors.

The possibilities are literally endless and when you give it a try you will be amazed by how easy and how much fun it is once you get the hang of it.

Let me know what you think or if you have any tips for my next project. Thank you for reading this!