6. Moulding and casting¶
This week we were split up into 3 different groups for the last 3 topics to distribute the load. I was assigned to the moulding & casting group.
Group Test Cast¶
Firstly, we worked together to research and read data sheets about different casting materials available in Fab Lab Bahrain. You can read about our findings here.
We then did a test cast of each material. Mine was gypsum (standard plaster).
First, we need to find how much cast material is needed to fill the desired mold. To do this we placed the mold on a scale and zeroed it, then we poured water in the mold and measured how much is needed to fill it. The water needed to fill our mold weighed 10g, so the mixture we will be pouring in has to be 10g as well (disregarding the difference in density between plaster and water as a good enough approximation).
The recommended mixture ratio is 1.25kg of plaster for 1L of water, giving a total mixture mass of 2.25kg. So, when the total mixture weight needs to be 10g, the optimal ratio is 4.44g of water for 5.55g of gypsum. We also tried different ratios to compare the results.
To adhere to safety guidelines we made sure to wear protective gloves and an apron (I guess our eyes don’t matter 🤷♂️).
After identifying the ratios, measure each material in a separate container using its own spoon to accurately obtain the correct amount. Do not mix spoons or else you will contaminate the materials.
The instructions of this plaster said to gather the plaster into the water. Usually in mixing you can use a separate container to pour the materials, or simply pour one material into the other’s container and then pour the total mixture in the first container to ensure no material is stuck inside a container.
After pouring the plaster on the water we mixed for a bit (mixing time varies based on cast material) and then the mixture started to dry up, so we poured it into the mold carefully.
We followed the same steps for the other test samples using different ratios.
The ratios used are as follows:
| Water Used | Plaster Used | |
|---|---|---|
| Bottom Left | 5.55g | 4.44g |
| Top Left | 4.44g | 5.55g |
| Bottom Right | 4g | 6g |
| Top Right | Intruders from the resin test |
We left the casts to dry for one day. The results look like this:
Individual Moulding and Casting¶
Creating the Design¶
When we were informed of the constraints we had to follow when designing:
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Minimum detail size of 1.6mm (based on diameter of smallest drill size available)
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Maximum design height of 8mm (based on length of cutting part of the drill bit)
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Maximum design width and height of 60mm (this is to fit multiple people on one piece)
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Design must be 2.5D with no undercuts. This is because the drill cannot reach these places.
I immediately thought of making something simple and small to hold in my hand as a sort of fidget toy. A worry stone to rub between my thumb and index finger seemed like a good idea.
To make the shape, I first drew the outline from the top view then extruded it 8mm. I though I used my thumb’s dimensions to create this profile but it appears I somehow messed this up 😔.
I then created planes and drew estimates of the indentation profile of my thumb, which I then cut using the loft cut feature.
This is the final result:
You can download it here.
To proceed with the next steps you need to save the file as a .stl
Making Process¶
The molding and casting process generally has three steps:
1. Creating the Positive Mold¶
This is done using a fine milling machine shaving away at a block of machinable wax.
Machinable wax is a special type of wax made of a mixture of wax and plastic. It is used because it is soft enough to be machined without damaging the tool, and hard enough to not melt or get stuck to the cutting tool.
The machine used is the Roland SRM-20 Compact Milling Machine:
The software used is the Roland SRP Player. But before we move on to this software, because we are printing multiple’s molds at the same time we used TinkerCAD to import the desired models and export them as a single .stl file.
After exporting the desired .stl file, we import it in SRP Player.
Because the machinable wax is positioned where its long side is on the x axis, we edited the size of the model fom the left to flip the x and y axis values.
The next step we defined the type of milling. We selected the better surface finish instead of the faster cutting time. And because our models had curves we chose the curvy option. We will not be doing a top and bottom cut for a mold, so we only choose to cut the top.
In the next step we define the material used. In this case it is modeling wax with these dimensions. The z axis value of 10mm resulted in the models being a bit too deeply engraved, which is wasteful, so we changed it to 9mm to make the models only be engraved slightly in the wax.
When creating the tool path, we must specify the processes we want it to do and the location where it does them.
There are three processes when milling molds:
- Surfacing: where the machine goes over the surface of the wax to make sure its smooth and even. This is especially important because the wax we use gets melted into its shape, resulting in a lot of imperfections.
- Roughing: The machine shaves away rough details quickly.
- Finishing: The machine takes its time to achieve fine details.
You can see the result of each process below:
The surfacing of the work piece was done prior by our instructor, because that can be done on any piece with no design being required.
Because we were making two molds at the same time, we had to do the roughing and finishing processes separate for each design. So, in total there must be 4 processes.
For each process the cut area must be defined, this is done by clicking on the cut area under each process, and selecting the partial option. The area is then defined by dragging and resizing the red rectangle shown on the work piece. The area for each process should only include its relevant part.
After defining the processes, the tool path can be created. This will allow the machine to make an estimate of the required time. For our two designs, it estimates it will take 1.5h.
This time is affected by the size of the work piece as defined in step 3, and the size of the drill bit. The size of the drill bit is automatically chosen by the machine based on how small the details in the design are.
The machine will remind you to install the correct tool in the spindle.
Ours was deemed to be 3/32 inch bit (2.38125 mm). It looks like this:
This is how to install it using a key:
Next we have to make the coordinates zero. In this machine it is the center of the piece:
To set the z axis coordinates we use this panel.
We use the +z and -z to calibrate the tool to be just above the work piece, it should not touch because the initial impact forces will damage the tool. To get finer adjustment turn on the spindle and adjust the cursor step multiplier.
After setting up everything we can start cutting. It will perform each process sequentially, this allows you to use different tools for each process.
This is the positive mold for my design and another person’s:
2. Creating the Negative Mold¶
To create the negative mold we pour silicon mixture in the positive mold.
This is the silicon we used:
To find the required mass we filled the positive mold with water, it turned out to be 20g of water. So, as an estimate we used 30g of total mixture. Make sure to completely dry out and clean the mold before pouring anything.
The ratio is 1:1 between the base and the hardener.
We used 15 grams of each component, then poured one cup into another and mixed, then poured the mixture into the other cup and finished mixing. Mixing time was around 2-3 minutes.
After preparing the mixture, we poured it into the positive mold.
During pouring, the mixtures overflowed from one design to another. This was fine since it didn’t affect the design, we could simply cut it using a cutter to separate them, and then use scissors to trim any excess of a mold.
After 24 hours, it dried to make this negative mold:
Keep in mind that if you use a flexible material for your final product, you can skip this step. This is because generally we only use flexible materials for a rigid mold and rigid materials for a flexible mold, this is so that the cast is able to come out of its mold (this difficulty will be experienced in the cnc week if you make press fit joints).
3. Casting the Final Product¶
Since I wanted a smooth and rigid product, I elected to use resin.
The ratio is 10:6 as written on the bottles. Additionally, I used a tiny bit of this acrylic blue color, as I thought it matched the ambience of the project.
I used 15g of bottle A and 9g of bottle B, and mixed them using the same method as the silicon.
In this step you have to be careful when pouring, there mustn’t be any air bubbles and the amount has to be just right.
I had some of the resin mixture left over, so I poured it into a some mold that was lying around. I left it for 24 hours to fully dry.
Final Result¶
This is the piece I made with the excess resin.
The larger size does not seem like an issue now that it is in my hand. I thought it would be smoother, but it still functions perfectly fine. I really like the color.
